One document matched: draft-ietf-core-coap-11.xml
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<front>
<title>Constrained Application Protocol (CoAP)</title>
<author fullname="Zach Shelby" initials="Z." surname="Shelby">
<organization>Sensinode</organization>
<address>
<postal>
<street>Kidekuja 2</street>
<city>Vuokatti</city>
<code>88600</code>
<country>Finland</country>
</postal>
<phone>+358407796297</phone>
<email>zach@sensinode.com</email>
</address>
</author>
<author initials="K." surname="Hartke" fullname="Klaus Hartke">
<organization>Universitaet Bremen TZI</organization>
<address>
<postal>
<street>Postfach 330440</street>
<city>Bremen</city>
<code>D-28359</code>
<country>Germany</country>
</postal>
<phone>+49-421-218-63905</phone>
<facsimile>+49-421-218-7000</facsimile>
<email>hartke@tzi.org</email>
</address>
</author>
<author initials="C." surname="Bormann" fullname="Carsten Bormann">
<organization>Universitaet Bremen TZI</organization>
<address>
<postal>
<street>Postfach 330440</street>
<city>Bremen</city>
<code>D-28359</code>
<country>Germany</country>
</postal>
<phone>+49-421-218-63921</phone>
<facsimile>+49-421-218-7000</facsimile>
<email>cabo@tzi.org</email>
</address>
</author>
<author fullname="Brian Frank" initials="B." surname="Frank">
<organization>SkyFoundry</organization>
<address>
<postal>
<street></street>
<city>Richmond, VA</city>
<code></code>
<country>USA</country>
</postal>
<phone></phone>
<email>brian@skyfoundry.com</email>
</address>
</author>
<date year="2012" />
<area>Applications</area>
<workgroup>CoRE Working Group</workgroup>
<keyword>CoAP</keyword>
<keyword>Constrained Application Protocol</keyword>
<keyword>REST</keyword>
<abstract>
<t>The Constrained Application Protocol (CoAP) is a specialized web
transfer protocol for use with constrained nodes and
constrained (e.g., low-power, lossy)
networks. The nodes often have 8-bit microcontrollers with small
amounts of ROM and RAM, while constrained networks such as 6LoWPAN often have high
packet error rates and a typical throughput of 10s of kbit/s. The
protocol is designed for machine-to-machine (M2M) applications such as
smart energy and building automation.</t>
<t>CoAP provides a request/response interaction model between application
end-points, supports built-in discovery of services and resources, and
includes key concepts of the Web such as URIs and Internet media types.
CoAP easily interfaces with HTTP for integration with the Web while
meeting specialized requirements such as multicast support, very low
overhead and simplicity for constrained environments.</t>
</abstract>
</front>
<middle>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section anchor="introduction" title="Introduction">
<t>The use of web services on the Internet has become ubiquitous in most
applications, and depends on the fundamental Representational State
Transfer <xref target="REST"/> architecture of the web.</t>
<t>The Constrained RESTful Environments (CoRE) work aims at
realizing the REST architecture in a suitable form for the most
constrained nodes (e.g. 8-bit microcontrollers with limited RAM and ROM)
and networks (e.g. 6LoWPAN, <xref target="RFC4944"/>). Constrained networks like 6LoWPAN support
the expensive fragmentation of IPv6 packets into small link-layer
frames. One design goal of CoAP has been to keep message overhead small,
thus limiting the use of fragmentation.</t>
<t>One of the main goals of CoAP is to design a generic web protocol for
the special requirements of this constrained environment, especially
considering energy, building automation and other machine-to-machine (M2M) applications. The
goal of CoAP is not to blindly compress HTTP <xref target="RFC2616"/>,
but rather to realize a
subset of REST common with HTTP but optimized for M2M applications.
Although CoAP could be used for compressing simple HTTP interfaces, it
more importantly also offers features for M2M such as built-in
discovery, multicast support and asynchronous message exchanges.</t>
<t>This document specifies the Constrained Application Protocol (CoAP),
which easily translates to HTTP for integration with the existing web
while meeting specialized requirements such as multicast support, very
low overhead and simplicity for constrained environments and M2M
applications.</t>
<section anchor="features" title="Features">
<t>CoAP has the following main features:
<list style="symbols">
<t>Constrained web protocol fulfilling M2M requirements.</t>
<t>UDP binding with optional reliability supporting unicast and multicast requests.</t>
<t>Asynchronous message exchanges.</t>
<t>Low header overhead and parsing complexity.</t>
<t>URI and Content-type support.</t>
<t>Simple proxy and caching capabilities.</t>
<t>A stateless HTTP mapping, allowing proxies to be built providing
access to CoAP resources via HTTP in a uniform way or for HTTP
simple interfaces to be realized alternatively over CoAP.</t>
<t>Security binding to Datagram Transport Layer Security (DTLS).</t>
</list></t>
</section>
<section anchor="terminology" title="Terminology">
<t>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref
target="RFC2119"/> when they
appear in ALL CAPS. These words may also appear in this document in
lower case as plain English words, absent their normative meanings.
</t>
<t>This specification requires readers to be familiar with all the terms
and concepts that are discussed in <xref target="RFC2616"/>. In addition,
this specification defines the following terminology:
<list style="hanging">
<t hangText="Endpoint"><vspace/>An entity participating in
the CoAP protocol. Colloquially, an endpoint lives on a "Node",
although "Host" would be more consistent with Internet
standards usage, and is further identified by transport
layer multiplexing information that can include a UDP port number and a
security association (<xref target="messages-and-endpoints"/>).</t>
<t hangText="Sender"><vspace/>The originating endpoint of a
message. When the aspect of identification of the specific
sender is in focus, also "source endpoint".</t>
<t hangText="Recipient"><vspace/>The destination endpoint
of a message. When the aspect of identification of the
specific recipient is in focus, also "destination endpoint".</t>
<t hangText="Client"><vspace/>The originating endpoint of a
request; the destination endpoint of a response.</t>
<t hangText="Server"><vspace/>The destination endpoint of a
request; the originating endpoint of a response.</t>
<t hangText="Origin Server"><vspace/>
The server on which a given resource resides or is to be created.
</t>
<t hangText="Intermediary"><vspace />A CoAP endpoint that
acts both as a server and as a client towards (possibly via
further intermediaries) an origin server.
There are two common forms of
intermediary: proxy and reverse proxy. In some cases, a single
endpoint might act as an origin server, proxy, or reverse proxy,
switching behavior based on the nature of each request.</t>
<t hangText="Proxy"><vspace />A "proxy" is an endpoint
selected by a client,
usually via local configuration rules, to perform requests on behalf
of the client, doing any necessary translations. Some translations
are minimal, such as for proxy requests for "coap" URIs, whereas
other requests might require translation to and from entirely
different application-layer protocols.</t>
<t hangText="Reverse Proxy"><vspace />A "reverse proxy" is
an endpoint that
acts as a layer above some other server(s) and satisfies requests
on behalf of these, doing any necessary translations. Unlike a
proxy, a reverse proxy receives requests as if it was the origin
server for the target resource; the requesting client will
not be aware that it is communicating with a reverse proxy.</t>
<t hangText="Confirmable Message"><vspace />
Some messages require an acknowledgement. These messages are
called "Confirmable". When no packets are lost, each confirmable
message elicits exactly one return message of type Acknowledgement
or type Reset.</t>
<t hangText="Non-Confirmable Message"><vspace/>
Some other messages do not require an acknowledgement. This is
particularly true for messages that are repeated regularly for
application requirements, such as repeated readings from a sensor
where eventual success is sufficient.</t>
<t hangText="Acknowledgement Message"><vspace/>
An Acknowledgement message acknowledges that a specific Confirmable
Message arrived. It does not indicate success or failure of any
encapsulated request.</t>
<t hangText="Reset Message"><vspace/>
A Reset message indicates that a specific message (confirmable or non-confirmable) was
received, but some context is missing to properly process it. This
condition is usually caused when the receiving node has rebooted and
has forgotten some state that would be required to interpret the
message.</t>
<t hangText="Piggy-backed Response"><vspace /> <!--XML2RFC is stupid-->
A &Pb; Response is included right in a CoAP
Acknowledgement (ACK) message that is sent to acknowledge receipt of
the Request for this Response (<xref target="pb"/>).</t>
<t hangText="Separate Response"><vspace /> <!--XML2RFC is stupid-->
When a Confirmable message carrying a Request is
acknowledged with an empty message (e.g., because the server doesn't
have the answer right away), a &Npb; Response is sent in a
separate message exchange (<xref target="npb"/>). </t>
<t hangText="Critical Option"><vspace />
An option that would need to be understood by the endpoint
receiving the message in order to properly process the message (<xref
target="critical-elective"/>). Note that the implementation
of critical options is, as the name "Option" implies,
generally optional: unsupported critical options lead to
rejection of the message.
</t>
<t hangText="Elective Option"><vspace />
An option that is intended to be ignored by an endpoint that
does not understand it. Processing the message even without understanding the option is acceptable (<xref target="critical-elective"/>).
</t>
<t hangText="Resource Discovery"><vspace />
The process where a CoAP client queries a server for its list
of hosted resources (i.e., links, <xref target="discovery"/>).
</t>
<!-- Additional terminology may be added as found necessary
in the document
<t hangText="Node"><vspace/>TODO</t>
-->
</list>
</t>
<t>In this specification, the term "byte" is used in its now customary
sense as a synonym for "octet".</t>
<t>In this specification, the operator "^"
stands for exponentiation.</t>
</section>
</section>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section anchor="protocol" title="Constrained Application Protocol">
<t>The interaction model of CoAP is similar to the client/server model
of HTTP. However, machine-to-machine interactions typically result in
a CoAP implementation acting in both client and server roles.
A CoAP request is equivalent to that of HTTP, and is
sent by a client to request an action (using a method code) on a
resource (identified by a URI) on a server. The server then
sends a response with a response code; this response may include a resource representation.</t>
<t>Unlike HTTP, CoAP deals with these interchanges asynchronously over
a datagram-oriented transport such as UDP. This is done logically using a layer of messages that supports optional reliability (with exponential back-off). CoAP defines four types of messages: Confirmable, Non-Confirmable,
Acknowledgement, Reset; method codes and response codes included in
some of these messages make them carry requests or responses.
The basic exchanges of the four types of messages
are somewhat orthogonal to the request/response interactions;
requests can be carried in Confirmable and Non-Confirmable
messages, and responses can be carried in these as well as
piggy-backed in acknowledgements.</t>
<t>One could think of CoAP logically as using a two-layer approach, a
CoAP messaging layer used to deal with UDP and the asynchronous nature
of the interactions, and the request/response interactions using
Method and Response codes (see <xref target="fig-layers"/>). CoAP is however a single protocol, with messaging and request/response just
features of the CoAP header.</t>
<figure anchor="fig-layers" title="Abstract layering of CoAP">
<artwork align="center"><![CDATA[
+----------------------+
| Application |
+----------------------+
+----------------------+
| Requests/Responses |
|----------------------| CoAP
| Messages |
+----------------------+
+----------------------+
| UDP |
+----------------------+
]]></artwork>
</figure>
<section title="Messaging Model">
<t>The CoAP messaging model is based on the exchange of messages over UDP between endpoints.</t>
<t>CoAP uses a short fixed-length binary header
(4 bytes) that may be followed by compact binary options and a payload. This message
format is shared by requests and responses.
The CoAP message format is specified in <xref target="syntax"/>.
Each message contains a Message ID used to detect duplicates and for optional reliability.</t>
<t>Reliability is provided by marking a message as Confirmable (CON). A Confirmable message is
retransmitted using a default timeout and
exponential back-off between retransmissions, until the
recipient sends an Acknowledgement message (ACK)
with the same Message ID (for example, 0x7d34) from the corresponding endpoint; see <xref
target="fig-reliable"/>.
When a recipient is not able to process a Confirmable
message, it replies with a Reset message (RST) instead
of an Acknowledgement (ACK). </t>
<figure anchor="fig-reliable" title="Reliable message transmission">
<artwork align="center"><![CDATA[
Client Server
| |
| CON [0x7d34] |
+----------------->|
| |
| ACK [0x7d34] |
|<-----------------+
| |
]]></artwork>
</figure>
<t>A message that does not require reliable transmission, for
example each single measurement out of a stream of sensor
data, can be sent as a Non-confirmable message (NON).
These are not acknowledged, but still have a Message ID for
duplicate detection; see <xref target="fig-unreliable"/>.
When a recipient is not able to process a Non-confirmable
message, it may reply with a Reset message (RST).</t>
<figure anchor="fig-unreliable" title="Unreliable message transmission">
<artwork align="center"><![CDATA[
Client Server
| |
| NON [0x01a0] |
+----------------->|
| |
]]></artwork>
</figure>
<t>See <xref target="messages"/> for details of CoAP messages.</t>
<t>
As CoAP is based on UDP, it also supports the use of multicast IP destination addresses,
enabling multicast CoAP requests. <xref target="multicast"/> discusses the proper use
of CoAP messages with multicast addresses and precautions for avoiding response congestion.</t>
<t>Several security modes are defined for CoAP in <xref target="securing-coap"/> ranging
from no security to certificate-based security. The use of IPsec along with a binding to DTLS are specified for securing the protocol.</t>
</section>
<section title="Request/Response Model">
<t>CoAP request and response semantics are carried in CoAP messages, which include
either a method code or response code, respectively. Optional (or default) request and response information,
such as the URI and payload content-type are carried as CoAP options. A Token Option
is used to match responses to requests independently from the
underlying messages (<xref target="response-matching"/>).</t>
<t>A request is carried in a Confirmable (CON) or
Non-confirmable (NON)
message, and if immediately available, the response to a
request carried in a Confirmable message is carried in the resulting
Acknowledgement (ACK) message. This is called a &pb; response, detailed in
<xref target="pb"/>. Two examples for a basic GET request with &pb; response are shown
in <xref target="example-pb"/>, one successful, one
resulting in a 4.04 (Not Found) response.</t>
<figure anchor="example-pb"
title="Two GET requests with piggy-backed responses"> <!--XML2RFC is stupid-->
<artwork align="center"><![CDATA[
Client Server Client Server
| | | |
| CON [0xbc90] | | CON [0xbc91] |
| GET /temperature | | GET /temperature |
| (Token 0x71) | | (Token 0x72) |
+----------------->| +----------------->|
| | | |
| ACK [0xbc90] | | ACK [0xbc91] |
| 2.05 Content | | 4.04 Not Found |
| (Token 0x71) | | (Token 0x72) |
| "22.5 C" | | "Not found" |
|<-----------------+ |<-----------------+
| | | |
]]></artwork>
</figure>
<t>If the server is not able to respond immediately to a
request carried in a Confirmable message, it simply responds with an empty Acknowledgement
message so that the client can stop retransmitting the request. When the
response is ready, the server sends it in a new Confirmable
message (which then in turn needs to be
acknowledged by the client). This is called a &npb;
response, as illustrated in <xref target="example-npb"/> and
described in more detail in <xref target="npb"/>.</t>
<figure anchor="example-npb" title="A GET request with a separate response"> <!--XML2RFC is stupid-->
<artwork align="center"><![CDATA[
Client Server
| |
| CON [0x7a10] |
| GET /temperature |
| (Token 0x73) |
+----------------->|
| |
| ACK [0x7a10] |
|<-----------------+
| |
... Time Passes ...
| |
| CON [0x23bb] |
| 2.05 Content |
| (Token 0x73) |
| "22.5 C" |
|<-----------------+
| |
| ACK [0x23bb] |
+----------------->|
| |
]]></artwork>
</figure>
<t>Likewise, if a request is sent in a Non-Confirmable message, then the response is usually
sent using a new Non-Confirmable message, although the server may send a Confirmable message.
This type of exchange is illustrated in
<xref target="example-non"/>.
</t>
<figure anchor="example-non" title="A NON request and response">
<artwork align="center"><![CDATA[
Client Server
| |
| NON [0x7a11] |
| GET /temperature |
| (Token 0x74) |
+----------------->|
| |
| NON [0x23bc] |
| 2.05 Content |
| (Token 0x74) |
| "22.5 C" |
|<-----------------+
| |
]]></artwork>
</figure>
<t>CoAP makes use of GET, PUT, POST and DELETE methods in a similar manner to HTTP,
with the semantics specified in <xref target="methods"/>.
(Note that the detailed semantics of CoAP methods are "almost,
but not entirely unlike" those of HTTP methods: Intuition taken
from HTTP experience generally does apply well, but there are enough
differences that make it worthwhile to actually read the present
specification.)</t>
<t>
URI support in a server is
simplified as the client already parses the URI and splits it into
host, port, path and query components, making use of default values for efficiency. Response
codes correspond to a small subset of HTTP response codes with
a few CoAP specific
codes added, as defined in <xref target="response-codes"/>.</t>
</section>
<section title="Intermediaries and Caching">
<t>The protocol supports the caching of responses in order to efficiently fulfill
requests. Simple caching is enabled using freshness and validity information carried with
CoAP responses. A cache could be located in an endpoint or an intermediary. Caching
functionality is specified in <xref target="caching"/>.</t>
<t>Proxying is useful in constrained networks for several reasons, including network
traffic limiting, to improve performance, to access resources of sleeping devices or
for security reasons. The proxying of requests on behalf of another CoAP endpoint is supported in
the protocol. When using a proxy, the URI of the resource to request is included in the request, while
the destination IP address is set to the address of the proxy. See <xref target="proxying"/> for more
information on proxy functionality. </t>
<t>As CoAP was designed according to the REST architecture and thus
exhibits functionality similar to that of the HTTP protocol,
it is quite
straightforward to map from CoAP to HTTP and from HTTP to CoAP. Such a mapping may be used to
realize an HTTP REST interface using CoAP, or for converting between HTTP and CoAP. This
conversion can be
carried out by a proxy, which converts the method or response code, content-type,
and options to the corresponding HTTP feature. <xref
target="http"/> provides more detail about HTTP
mapping. </t>
</section>
<section title="Resource Discovery">
<t>Resource discovery is important for machine-to-machine interactions, and is supported
using the CoRE Link Format <xref target="I-D.ietf-core-link-format"/> as discussed in
<xref target="discovery"/>.</t>
</section>
</section>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section anchor="syntax" title="Message Format">
<t>CoAP is based on the exchange of short messages which, by default,
are transported over UDP (i.e. each CoAP message occupies the data
section of one UDP datagram). CoAP may be used with Datagram
Transport Layer Security (DTLS) (see <xref target="dtls"/>). It could
also be used over other transports such as TCP or SCTP, the specification
of which is out of this document's scope.</t>
<t>CoAP messages are encoded in a simple binary format. A message
consists of a fixed-sized CoAP Header followed by options in
Type-Length-Value (TLV) format and a payload. The number of options
is determined by the header. The payload is made up of the bytes
after the options, if any; its length is calculated from the datagram
length.</t>
<figure anchor="fig-message-format" title="Message Format">
<artwork type="drawing"><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver| T | OC | Code | Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<section anchor="message-format" title="Header Format">
<t>
The fields in the header are defined as follows:
<list style="hanging">
<t hangText="Version (Ver):">2-bit unsigned integer. Indicates the
CoAP version number. Implementations of this specification MUST set
this field to 1. Other values are reserved for future
versions.</t>
<t hangText="Type (T):">2-bit unsigned integer. Indicates
if this message is of type Confirmable (0), Non-Confirmable (1),
Acknowledgement (2) or Reset (3). See <xref target="messages"/> for
the semantics of these message types.</t>
<t hangText="Option Count (OC):">4-bit unsigned integer. Indicates
the number of options after the header (0-14). If set to 0, there are no options and the payload (if any) immediately follows the header. If set to 15, then an end-of-options marker is used to indicate the end of options and the start of the payload. The format of options is defined below.</t>
<t hangText="Code:">8-bit unsigned integer. Indicates if the message
carries a request (1-31) or a response (64-191), or is empty (0).
(All other code values are reserved.)
In case of a request, the Code field indicates the Request Method;
in case of a response a Response Code. Possible values are maintained
in the <xref target="coap-code-registry">CoAP Code Registry</xref>.
See <xref target="requests-responses"/> for the semantics of requests
and responses.</t>
<t hangText="Message ID:">16-bit unsigned integer in network
byte order. Used for the
detection of message duplication, and to match messages of type
Acknowledgement/Reset and messages of type Confirmable/Non-confirmable. See <xref
target="messages"/> for Message ID generation rules and how
messages are matched.</t>
</list>
</t>
</section>
<section anchor="option-format" title="Option Format">
<t>Options MUST appear in order of their Option Number (see <xref
target="option-numbers"/>). A delta encoding is used between
options: The Option Number for each Option is calculated as
the sum of its Option Delta field and the Option Number of the
preceding Option in the message, if any. For the first Option
in the message, the Option Delta becomes the Option Number
(i.e., an implementation can simply initialize the number variable as zero).
Multiple options with the same Option Number can be included
by using an Option Delta of zero.
Following the Option Delta, each option has a Length field which
specifies the length of the Option Value, in bytes. The Length field can be
extended by one byte for options with values longer than 14 bytes.
The Option Value immediately follows the Length field.</t>
<figure anchor="fig-option-format" title="Option Format">
<artwork type="drawing"><![CDATA[
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Option Delta | Length | for 0..14
+---+---+---+---+---+---+---+---+
| Option Value ...
+---+---+---+---+---+---+---+---+
for 15..270:
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| Option Delta | 1 1 1 1 | Length - 15 |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| Option Value ...
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
]]></artwork>
</figure>
<t>The fields in an option are defined as follows:
<list style="hanging">
<t hangText="Option Delta:">4-bit unsigned integer.
Indicates the difference between the Option Number of this option
and the previous option (or zero for the first option). In other
words, the Option Number is calculated by simply summing the
Option Delta fields of this and previous options before it.
If a delta larger than 14 is needed, the Option Numbers that
are non-zero multiples of 14 (i.e., 14,
28, 42, ...) can be used with the Length field set to 0 as "fenceposts".
The Option Delta 15 is reserved for the the end-of-options marker (see below).</t>
<t hangText="Length:">Indicates the length of the Option Value, in bytes.
Normally Length is a 4-bit unsigned integer allowing value lengths
of 0-14 bytes. When the Length field is set to 15, another byte is
added as an 8-bit unsigned integer whose value is added to
the 15, allowing option value lengths of 15-270 bytes.</t>
<t hangText="Value:">The length and format of the Option Value
depends on the respective option, which MAY define variable
length values. See <xref target="option-value-formats"/> for the
formats the options defined in this document make use of; other
options MAY make use of other option value formats.</t>
</list>
</t>
<t>If the Option Count field in the header is 15 and the Option Delta
is 15, the option is interpreted as the end-of-options marker instead
of the option with the resulting Option Number. A sender MUST NOT
include a value with the marker (i.e., the option length is 0) and a recipient MUST ignore any value
of the marker. When this marker is encountered, it is immediately followed by the payload (if any).
(Note that, by this special meaning, the Option Delta of 15 is made
special, not any specific Option Number.)
The sender MUST NOT include the Option Delta of 15 in a
message with an Option Count other than 15.
</t>
<t>Option Numbers are maintained in the <xref
target="option-number-registry">CoAP Option Number Registry</xref>.
See <xref target="options"/> for the semantics of the options defined
in this document.</t>
</section>
<section title="Option Value Formats" anchor="option-value-formats">
<t>The options defined in
this document make use of the following option value formats.
</t>
<section title="uint">
<t>A non-negative integer which is represented in
network byte order using the given number of bytes. An
option definition may
specify a range of permissible numbers of bytes; if it has a
choice, a sender
SHOULD represent the integer with as few bytes as possible,
i.e., without leading zeros. A recipient MUST be prepared to process
values with leading zeros.</t>
<t>
<list style="hanging">
<t hangText="Implementation Note:">The exceptional
behavior permitted above is for highly
constrained templated implementations (e.g. hardware implementations)
that use fixed size options in the templates.</t>
</list>
</t>
<figure>
<artwork type="drawing"><![CDATA[
Length = 0 (implies value of 0)
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
Length = 1 | 0-255 |
+-+-+-+-+-+-+-+-+
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Length = 2 | 0-65535 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Length = 3 is 24 bits, Length = 4 is 32 bits etc.
]]></artwork>
</figure>
</section>
<section title="string">
<t>A Unicode string which is encoded using
<xref target="RFC3629">UTF-8</xref> in
<xref target="RFC5198">Net-Unicode form</xref>.
Note that here and in all other places where UTF-8 encoding
is used in the CoAP protocol, the intention is that the
encoded strings can be directly used and compared as opaque byte
strings by CoAP protocol implementations. There is no
expectation and no need to perform normalization within a
CoAP implementation unless Unicode strings that are not
known to be normalized are imported from sources outside the
CoAP protocol. Note also that ASCII strings (that do not
make use of special control characters) are always
valid UTF-8 Net-Unicode strings.
</t>
</section>
<section title="opaque">
<t>An opaque sequence of bytes.</t>
</section>
<section title="empty">
<t>A zero-length sequence of bytes.</t>
</section>
</section>
</section>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section anchor="messages" title="Message Transmission">
<t>CoAP messages are exchanged asynchronously between CoAP endpoints.
They are used to transport CoAP requests and responses, the semantics
of which are defined in <xref target="requests-responses"/>.</t>
<t>As CoAP is bound to non-reliable transports
such as UDP, CoAP messages may arrive out of
order, appear duplicated, or go missing without notice. For this
reason, CoAP implements a lightweight reliability mechanism, without
trying to re-create the full feature set of a transport like TCP.
It has the following features:
<list style="symbols">
<t>Simple stop-and-wait retransmission reliability with exponential
back-off for "confirmable" messages.</t>
<t>Duplicate detection for both "confirmable" and "non-confirmable"
messages.</t>
</list>
</t>
<section anchor="messages-and-endpoints" title="Messages and Endpoints">
<t>A CoAP endpoint is the source or destination of a CoAP message. It
is identified depending on the security mode used (see <xref
target="securing-coap"/>): With no security, the endpoint is
solely identified by an IP address and a UDP port number. With
other security modes, the endpoint is identified
as defined by the security mode.</t>
<t>There are different types of messages. The type of
a message is specified by the T field of the CoAP header.</t>
<t>Separate from the message type, a message may carry a
request, a response, or be empty. This is
signaled by the Code field in the CoAP header and is relevant to
the request/response model.
Possible values for the Code field are
maintained by the <xref target="coap-code-registry"> CoAP Code
Registry</xref>.</t>
<t>An empty message has the Code field set to 0. The OC field SHOULD
be set to 0 and no bytes SHOULD be present after the Message ID
field. <!--TODO: really? diagnostics! --> The OC field and any
bytes trailing the header MUST be ignored by any
recipient.</t>
</section>
<section anchor="reliable" title="Messages Transmitted Reliably">
<t>The reliable transmission of a message is initiated by marking
the message as "confirmable" in the CoAP header. A confirmable message
always carries either a request or response and MUST NOT be empty.
A recipient MUST
acknowledge such a message with an acknowledgement message or, if
it lacks context to process the message properly, MUST reject it
with a reset message. The acknowledgement message MUST echo the Message
ID of the confirmable message, and MUST carry a response or be empty (see
<xref target="pb"/> and <xref target="npb"/>).
The reset message MUST echo the Message ID of the confirmable
message, and MUST be empty.</t>
<t>The sender retransmits the confirmable
message at exponentially increasing intervals, until it receives
an acknowledgement (or reset message), or runs out of attempts.</t>
<t>Retransmission is controlled by two things that a CoAP endpoint
MUST keep track of for each confirmable message it sends while waiting
for an acknowledgement (or reset): a timeout and a
retransmission counter. For a new
confirmable message, the initial timeout is set to a random number between
ACK_TIMEOUT and (ACK_TIMEOUT * ACK_RANDOM_FACTOR) (see <xref target="constants"/>),
and the retransmission counter is set to 0. When the timeout is triggered
and the retransmission counter is less than MAX_RETRANSMIT, the message
is retransmitted, the retransmission counter is incremented, and the
timeout is doubled. If the retransmission counter reaches
MAX_RETRANSMIT on a timeout, or if the endpoint receives a reset
message, then the attempt to transmit the message is canceled and
the application process informed of failure. On the other hand, if
the endpoint receives an acknowledgement message in time,
transmission is considered successful.</t>
<t>A CoAP endpoint that sent a confirmable message
MAY give up in attempting to obtain an ACK even before the
MAX_RETRANSMIT counter value is reached: E.g., the
application has canceled the request as it no longer needs a
response, or there is some other indication that the CON
message did arrive. In particular, a CoAP request message
may have elicited a separate response, in which case it is
clear to the requester that only the ACK was lost and a
retransmission of the request would serve no purpose.
However, a responder MUST NOT in turn rely on this
cross-layer behavior from a requester, i.e. it SHOULD retain
the state to create the ACK for the request, if needed, even
if a confirmable response was already acknowledged by the
requester.</t>
</section>
<section anchor="unreliable" title="Messages Transmitted Without
Reliability">
<t>Some messages do not require an acknowledgement. This is
particularly true for messages that are repeated regularly for
application requirements, such as repeated readings from a sensor
where eventual success is sufficient.</t>
<t>As a more lightweight alternative, a message can be transmitted
less reliably by marking the message as "non-confirmable". A
non-confirmable message always carries either a request or
response and MUST NOT be empty. A
non-confirmable message MUST NOT be acknowledged by the recipient.
If a recipient lacks context to process the message properly, it MAY
reject the message with a reset message or otherwise MUST
silently ignore
it.</t>
<t>At the CoAP level, there is no way for the sender to detect
if a non-confirmable message was received
or not. A sender MAY choose to transmit a
non-confirmable message multiple times, or the network may
duplicate the message in transit. To enable the receiver to
act only once on the message,
non-confirmable messages specify a Message ID as well.
(This Message ID is drawn from the same number space as the
Message IDs for confirmable messages.)
</t>
<!-- ... as we only have one type of RST. -->
</section>
<section title="Message Correlation" anchor="message-correlation">
<t>An acknowledgement or reset message is related to a confirmable
message or non-confirmable message by means of a Message ID along with additional address information of the corresponding endpoint. The Message ID is a 16-bit
unsigned integer that is generated by the sender of a confirmable or non-confirmable
message and included in the CoAP header. The Message ID MUST be
echoed in the acknowledgement or reset message by the recipient.</t>
<t>The same Message ID MUST NOT
be re-used (per Message ID variable) within the
EXCHANGE_LIFETIME (<xref target="derived-values"/>).</t>
<t>
<list style="hanging">
<t hangText="Implementation Note:">Several implementation strategies can be employed for generating Message IDs. In the simplest case a CoAP endpoint generates Message IDs by keeping a single Message ID variable, which is changed each time a new confirmable or non-confirmable message is sent regardless of the destination address or port. Endpoints dealing with large numbers of transactions could keep multiple Message ID variables, for example per prefix or destination address. The initial variable value should be randomized.</t>
</list>
</t>
<!--TODO: TIME_WAIT2, see also 11-->
<t>For an acknowledgement or reset message to match a confirmable or
non-confirmable message, the Message ID and source endpoint of the
acknowledgement or reset message MUST match the Message ID and
destination endpoint of the confirmable or non-confirmable
message.</t>
</section>
<section title="Message Deduplication" anchor="message-deduplication">
<t>A recipient MUST be prepared to receive the same confirmable message
(as indicated by the Message ID and source endpoint) multiple
times within the EXCHANGE_LIFETIME (<xref target="derived-values"/>), for example, when
its acknowledgement went missing or didn't reach the original sender
before the first timeout. The
recipient SHOULD acknowledge each
duplicate copy of a confirmable message using the same
acknowledgement or reset message, but SHOULD process any
request or response in the message only once.
This rule MAY be relaxed in case the confirmable message transports a
request that is idempotent (see <xref
target="request-semantics"/>) or can be handled in an idempotent fashion.
Examples for relaxed message deduplication:
<list style='symbols'>
<t>A server MAY relax the requirement to answer all
retransmissions of an idempotent request with the same
response (<xref target="reliable" />), so that it does not
have to maintain state for Message IDs. For example, an
implementation might want to process duplicate transmissions
of a GET, PUT or DELETE request as separate requests if the
effort incurred by duplicate processing is less expensive
than keeping track of previous responses would be.</t>
<t>A constrained server
MAY even want to relax this requirement for certain
non-idempotent requests if the application semantics make this
trade-off favorable. For example, if the result of a POST request is
just the creation of some short-lived state at the server,
it may be less expensive to incur this effort multiple times
for a request than keeping track of whether a previous
transmission of the same request already was processed.</t>
</list>
</t>
<t>A recipient MUST be prepared to receive the same non-confirmable
message (as indicated by the Message ID and source endpoint)
multiple times within NON_LIFETIME (<xref target="derived-values"/>). As a general rule that may be
relaxed based on the specific semantics of a message, the
recipient SHOULD silently ignore any duplicated non-confirmable
message, and SHOULD process any request or response in the message
only once.</t>
</section>
<section title="Message Size" anchor="message-size">
<t>While specific link layers make it beneficial to keep CoAP messages
small enough to fit into their link layer packets (see
<xref target="introduction"/>), this is a matter of implementation
quality. The CoAP specification itself provides only an upper bound to
the message size. Messages larger than an IP fragment result in undesired packet fragmentation.
A CoAP message, appropriately encapsulated, SHOULD
fit within a single IP packet (i.e., avoid IP fragmentation)
and (by fitting into one UDP payload) obviously MUST fit within a single IP datagram. If the Path MTU is not known
for a destination, an IP MTU of 1280 bytes SHOULD be assumed; if
nothing is known about the size of the headers, good upper
bounds are 1152 bytes for the message size and 1024 bytes for
the payload size. </t>
<t><list style="hanging">
<t hangText="Implementation Note:">
CoAP's choice of message size parameters works
well with IPv6 and with most of today's IPv4 paths. (However,
with IPv4, it is harder to absolutely ensure that there is no
IP fragmentation. If IPv4 support on unusual networks is a
consideration, implementations may want to limit themselves to
more conservative IPv4 datagram sizes such as 576 bytes;
worse, the absolute minimum value of the IP MTU for IPv4 is as
low as 68 bytes, which would leave only 40 bytes minus
security overhead for a UDP payload. Implementations
extremely focused on this problem set might also set the IPv4
DF bit and perform some form of path MTU discovery; this
should generally be unnecessary in most realistic use cases
for CoAP, however.) A more important kind of fragmentation in
many constrained networks is that on the adaptation layer
(e.g., 6LoWPAN L2 packets are limited to 127 bytes including
various overheads); this may motivate implementations to be
frugal in their packet sizes and to move to block-wise
transfers <xref target="I-D.ietf-core-block"/> when
approaching three-digit message sizes.
</t>
<t>Message sizes are also of considerable importance
to implementations on constrained nodes. Many implementations
will need to allocate a buffer for incoming messages. If an
implementation is too constrained to allow for allocating the
above-mentioned upper bound, it could apply the following
implementation strategy: Implementations receiving a datagram
into a buffer that is too small are usually able to determine
if the trailing portion of a datagram was discarded and to
retrieve the initial portion. So, if not all of the payload,
at least the CoAP header and options are likely to fit within
the buffer. A server can thus fully interpret a request and
return a 4.13 (Request Entity Too Large) response code if the
payload was truncated. A client sending an idempotent request
and receiving a response larger than would fit in the buffer
can repeat the request with a suitable value for the Block
Option <xref target="I-D.ietf-core-block"/>.
</t>
</list></t>
</section>
<section anchor="congestion" title="Congestion Control">
<t>Basic congestion control for CoAP is provided by the
exponential back-off mechanism in <xref
target="reliable"/>. </t>
<t>
In order not to cause congestion, Clients (including proxies)
SHOULD strictly limit the number of simultaneous outstanding
interactions that they maintain to a given server (including
proxies). An outstanding interaction is either a CON for
which an ACK has not yet been received but is still expected
(message layer) or a request for which a response has not yet
been received but is still expected (which may both occur at
the same time, counting as one outstanding interaction). A
good value for this limit is the number 1. (Note that <xref target="RFC2616"/>, in trying to achieve a similar
objective, did specify a specific number of simultaneous
connections as a ceiling. While revising <xref target="RFC2616"/>, this
was found to be impractical for many applications
<xref target="I-D.ietf-httpbis-p1-messaging"/>. For the same
considerations, this specification does not mandate a
particular maximum number of outstanding interactions, but
instead encourages clients to be conservative when initiating
interactions.)
</t>
<t>Further congestion control optimizations and
considerations are expected in the future, which may for example
provide automatic initialization of the CoAP transmission parameters defined in
<xref target="constants"/>.</t>
</section>
<section anchor="constants" title="Transmission Parameters">
<t>Message transmission is controlled by the following parameters:</t>
<texttable>
<ttcol align='left'>name</ttcol>
<ttcol align='left'>default value</ttcol>
<c>ACK_TIMEOUT</c>
<c>2 seconds</c>
<c>ACK_RANDOM_FACTOR</c>
<c>1.5</c>
<c>MAX_RETRANSMIT</c>
<c>4</c>
</texttable>
<section anchor="constant_changes" title="Changing The Parameters">
<t>The values for ACK_TIMEOUT, ACK_RANDOM_FACTOR,
and MAX_RETRANSMIT may be configured to values specific to the
application environment, however the configuration method is
out of scope of this document. It is recommended that an
application environment use consistent values for these
parameters.</t>
<t>The transmission parameters have been chosen to achieve a behavior in the
presence of congestion that is safe in the Internet. If a
configuration desires to use different values, the onus is on the
configuration to ensure these congestion control properties are not
violated. In particular, a decrease of ACK_TIMEOUT below 1
second would violate the guidelines of <xref target="RFC5405"/>.
(<xref target="I-D.allman-tcpm-rto-consider"/> provides some additional
background.) CoAP was designed to enable implementations that do
not maintain round-trip-time (RTT) measurements. However, where it is
desired to decrease the ACK_TIMEOUT significantly, this can
only be done safely when maintaining such measurements.
Configurations MUST NOT decrease ACK_TIMEOUT without using
mechanisms that ensure congestion control safety, either defined in
the configuration or in future standards documents.</t>
<t>ACK_RANDOM_FACTOR MUST NOT be decreased below 1.0, and it
SHOULD have a value that is sufficiently different from 1.0 to
provide some protection from synchronization effects.</t>
<t>MAX_RETRANSMIT can be freely adjusted, but a too small value will
reduce the probability that a confirmable message is actually
received, while a larger value will require further adjustments in
the time values (see discussion below).</t>
<t>If the choice of transmission parameters leads to an increase of derived
time values (see below), the configuration mechanism MUST ensure
the adjusted value is available to the corresponding end-points,
too.</t>
</section>
<section anchor="derived-values" title="Time Values derived from Transmission Parameters">
<t>The combination of ACK_TIMEOUT, ACK_RANDOM_FACTOR and
MAX_RETRANSMIT influences the timing of retransmissions, which in
turn influences how long certain information items need to be kept by
an implementation. To be able to unambiguously reference these
derived time values, we give them names as follows:</t>
<t><list style='symbols'>
<t>MAX_TRANSMIT_SPAN is the maximum time from the first transmission
of a confirmable message to its last retransmission. For the default
transmission parameters, the value is (2+4+8+16)*1.5 = 45 seconds, or
more generally: <list style='empty'>
<t>ACK_TIMEOUT * (2 ** MAX_RETRANSMIT - 1) * ACK_RANDOM_FACTOR</t>
</list></t>
<t>MAX_TRANSMIT_WAIT is the maximum time from the first transmission
of a confirmable message to the time when the sender gives up on
receiving an acknowledgement or reset. For the default
transmission parameters, the value is (2+4+8+16+32)*1.5 = 93 seconds, or
more generally: <list style='empty'>
<t>ACK_TIMEOUT * (2 ** (MAX_RETRANSMIT + 1) - 1) * ACK_RANDOM_FACTOR</t>
</list></t>
</list></t>
<t>In addition, some assumptions need to be made on the characteristics
of the network and the nodes.</t>
<t><list style='symbols'>
<t>MAX_LATENCY is the maximum time a datagram is expected to take from
the start of its transmission to the completion of its reception.
This constant is related to the MSL (Maximum Segment Lifetime) of
<xref target="RFC0793"/>, which is "arbitrarily defined to be 2 minutes"
(<xref target="RFC0793"/> glossary, page 81).
Note that this is not necessarily smaller than MAX_TRANSMIT_WAIT,
as MAX_LATENCY is not intended to describe a situation when the
protocol works well, but the worst case situation against which the
protocol has to guard.
We, also arbitrarily, define MAX_LATENCY to be 100 seconds. Apart
from being reasonably realistic for the bulk of configurations as
well as close to the historic choice for TCP, this value also allows
message ID lifetime timers to be represented in 8 bits (when
measured in seconds).
In these calculations, there is no assumption that the direction of
the transmission is irrelevant (i.e. that the network is
symmetric), just that the same value can reasonably be used as a maximum
value for both directions. If that is not the case, the following
calculations become only slightly more complex.</t>
<t>PROCESSING_DELAY is the time a node takes to turn around a
confirmable message into an acknowledgement. We assume the node will
attempt to send an ACK before having the sender time out, so as a
conservative assumption we set it equal to ACK_TIMEOUT.</t>
<t>MAX_RTT is the maximum round-trip time, or: <list style='empty'>
<t>2 * MAX_LATENCY + PROCESSING_DELAY</t>
</list></t>
</list></t>
<t>From these values, we can derive the following values relevant to
the protocol operation:</t>
<t><list style='symbols'>
<t>EXCHANGE_LIFETIME is the time from starting to send a confirmable
message to the time when an acknowledgement is no longer expected,
i.e. message layer information about the message exchange can be purged.
EXCHANGE_LIFETIME includes a MAX_TRANSMIT_SPAN, a MAX_LATENCY
forward, PROCESSING_DELAY, and a MAX_LATENCY for the way back.
Note that there is no need to consider MAX_TRANSMIT_WAIT if the
configuration is chosen such that the last waiting period
(ACK_TIMEOUT * (2 ** MAX_RETRANSMIT) or the difference
between MAX_TRANSMIT_SPAN and MAX_TRANSMIT_WAIT) is less than
MAX_LATENCY -- which is a likely choice, as MAX_LATENCY is a worst
case value unlikely to be met in the real world.
In this case, EXCHANGE_LIFETIME simplifies to: <list style='empty'>
<t>(ACK_TIMEOUT * (2 ** MAX_RETRANSMIT - 1) *
ACK_RANDOM_FACTOR) + (2 * MAX_LATENCY) + PROCESSING_DELAY</t>
</list>
or 248 seconds with the default transmission parameters.</t>
<t>NON_LIFETIME is the time from sending a non-confirmable message to
the time its message-ID can be safely reused. If multiple
transmission of a NON message is not used, its value is
MAX_LATENCY, or 100 seconds. However, a CoAP sender might send a
NON message multiple times, in particular for multicast
applications. While the period of re-use is not bounded by the
specification, an expectation of reliable detection of duplication
at the receiver is in the timescales of MAX_TRANSMIT_SPAN.
Therefore, for this purpose, it is safer to use the value: <list style='empty'>
<t>MAX_TRANSMIT_SPAN + MAX_LATENCY</t>
</list>
or 145 seconds with the default transmission parameters; however, an
implementation that just wants to use a single timeout value for
retiring message-IDs can safely use the larger value for
EXCHANGE_LIFETIME.</t>
</list></t>
</section>
</section>
</section>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section anchor="requests-responses" title="Request/Response Semantics">
<t>CoAP operates under a similar request/response model as HTTP: a CoAP
endpoint in the role of a "client" sends one or more CoAP requests to
a "server", which services the requests by sending CoAP responses.
Unlike HTTP, requests and responses are not sent over a previously
established connection, but exchanged asynchronously over CoAP
messages.</t>
<section anchor="request-semantics" title="Requests">
<t>A CoAP request consists of the method to be applied to the resource,
the identifier of the resource, a payload and Internet media type
(if any), and optional meta-data about the request.</t>
<t>CoAP supports the basic methods of GET, POST, PUT, DELETE, which
are easily mapped to HTTP. They have the same properties of safe (only
retrieval) and idempotent (you can invoke it multiple times with the
same effects) as HTTP (see Section 9.1 of <xref target="RFC2616"/>).
The GET method is safe, therefore it MUST NOT take any other action
on a resource other than retrieval. The GET, PUT and DELETE methods
MUST be performed in such a way that they are idempotent. POST is not
idempotent, because its effect is determined by the origin server and
dependent on the target resource; it usually results in a new resource
being created or the target resource being updated.</t>
<t>A request is initiated by setting the Code field in the CoAP header
of a confirmable or a non-confirmable message to a Method Code and
including request information.</t>
<t>The methods used in requests are described in detail in
<xref target="methods"/>.</t>
</section>
<section anchor="response-semantics" title="Responses">
<t>After receiving and interpreting a request, a server responds
with a CoAP response, which is matched to the request by means
of a client-generated token.</t>
<t>A response is identified by the Code field in the CoAP
header being set
to a Response Code. Similar to the HTTP Status Code, the CoAP Response
Code indicates the result of the attempt to understand and satisfy the
request. These codes are fully defined in
<xref target="response-codes"/>. The Response Code numbers to be set
in the Code field of the CoAP header are maintained in the
<xref target="coap-code-registry-responses">CoAP Response Code
Registry</xref>.</t>
<figure title="Structure of a Response Code" anchor="response-code">
<artwork type="drawing" align="center"><![CDATA[
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|class| detail |
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>The upper three bits of the 8-bit Response Code number define the
class of response. The lower five bits do not have any categorization
role; they give additional detail to the overall class (<xref
target="response-code"/>). There are 3 classes:
<list style="hanging">
<t hangText="2 - Success:">The request was successfully received,
understood, and accepted.</t>
<t hangText="4 - Client Error:">The request contains bad syntax or
cannot be fulfilled.</t>
<t hangText="5 - Server Error:">The server failed to fulfill an
apparently valid request.</t>
</list>
The response codes are designed to be extensible: Response Codes in
the Client Error and Server Error class that are unrecognized by an
endpoint MUST be treated as being equivalent to the generic Response
Code of that class (4.00 and 5.00, respectively). However, there is no generic Response Code
indicating success, so a Response Code in the Success class that is
unrecognized by an endpoint can only be used to determine that the
request was successful without any further details.</t>
<t>As a human readable notation for specifications and
protocol diagnostics, the numeric value of a response
code is indicated
by giving the upper three bits in decimal, followed by a
dot and then the lower five bits in a two-digit decimal. E.g., "Not
Found" is written as 4.04 -- indicating a value of hexadecimal
0x84 or decimal 132. In other words, the dot "." functions as
a short-cut for "*32+".</t>
<t>The possible response codes are described in detail in
<xref target="response-codes"/>.</t>
<t>Responses can be sent in multiple ways, which are defined below.</t>
<section anchor="pb" title="Piggy-backed"> <!--XML2RFC is stupid-->
<t>In the most basic case, the response is carried directly in the
acknowledgement message that acknowledges the request (which
requires that
the request was carried in a confirmable message). This is called a
"&Pb;" Response.</t>
<t>The response is returned in the acknowledgement message
independent of whether the response indicates success or failure.
In effect, the response is piggy-backed on the acknowledgement
message, so no separate message is required to both acknowledge
that the request was received and return the response.</t>
<t>Implementation note: The protocol leaves the decision whether to piggy-back a response or not (i.e., send a separate response) to the server. The client MUST be prepared to receive either. On the quality of implementation level, there is a strong expectation that servers will implement code to piggy-back whenever possible -- saving resources in the network and both at the client and at the server.</t>
</section>
<section anchor="npb" title="Separate"> <!--XML2RFC is stupid-->
<t>It may not be possible to return a &pb; response in all
cases. For example, a server might need longer to obtain the
representation of the resource requested than it can wait sending
back the acknowledgement message, without risking the client to
repeatedly retransmit the request message. Responses to requests carried in
a Non-Confirmable message are always sent separately (as there is no
acknowledgement message).</t>
<t>The server maybe initiates the attempt to obtain the resource representation
and times out an acknowledgement timer, or it immediately sends an
acknowledgement knowing in advance that there will be no &pb;
response. The acknowledgement effectively is a promise
that the request will be acted upon.</t>
<t>When the server finally has obtained the resource representation,
it sends the response. To ensure that this message is not lost, it
is again sent as a confirmable message and answered by the client
with an acknowledgement, echoing the new Message ID chosen by
the server.</t>
<t>(Implementation notes: Note that, as the underlying
datagram transport may not be sequence-preserving, the
confirmable message carrying the response may actually
arrive before or after the acknowledgement message for the
request. Note also that, while the CoAP protocol itself
does not make any specific demands here, there is an
expectation that the response will come within a time frame
that is reasonable from an application point of view; as
there is no underlying transport protocol that could be
instructed to run a keep-alive mechanism, the requester MAY
want to set up a timeout that is unrelated to CoAP's
retransmission timers in case the server is destroyed or
otherwise unable to send the response.)</t>
<t>For a &npb; exchange, both the acknowledgement to the confirmable request and the
acknowledgement to the confirmable response MUST be an empty message,
i.e. one that carries neither a request nor a response.</t>
</section>
<section anchor="non-confirmable-responses" title="Non-Confirmable">
<t>If the request message is non-confirmable, then the response
SHOULD be returned in a non-confirmable message as well. However,
an endpoint MUST be prepared to receive a non-confirmable response
(preceded or followed by an empty acknowledgement message) in reply to a confirmable
request, or a confirmable response in reply to a non-confirmable
request.</t>
</section>
</section>
<section anchor="response-matching" title="Request/Response Matching">
<t>Regardless of how a response is sent, it is matched to the
request by means of a token that is included by the client in the
request as one of the options along with additional address information of the corresponding endpoint. The token MUST be echoed by the server in any resulting response without modification.</t>
<t>The exact rules for matching a response to a request are as
follows:
<list style="numbers">
<t>The source endpoint of the
response MUST be the same as the destination endpoint of the
original request.</t>
<t>In a &pb; response, both the Message ID of the
confirmable request and the acknowledgement, and the token of the
response and original request MUST match. In a &npb; response,
just the token of the response and original request MUST match.</t>
</list>
</t>
<t>The client SHOULD generate tokens in a way that tokens currently in
use for a given source/destination pair are unique. (Note that a client
can use the same token for any request if it uses a different source
port number each time.)</t>
<t>An endpoint that did not generate a token MUST treat it as opaque and make no
assumptions about its format. (Note that there is a default value for
the Token Option, so every message carries a token, even if it is not
explicitly expressed in a CoAP option.)</t>
<t>In case a message carrying a response is unexpected (i.e. the client is
not waiting for a response with the specified address and/or token), the
response SHOULD be rejected with a reset message and MUST NOT be
acknowledged.</t> <!--TODO: This is the only place where
we break strict layering? -->
</section>
<section anchor="option-semantics" title="Options">
<t>Both requests and responses may include a list of one or more
options. For example, the URI in a request is transported in several
options, and meta-data that would be carried in an HTTP header in
HTTP is supplied as options as well.</t>
<t>CoAP defines a single set of options that are used in both requests
and responses:
<list style="symbols">
<t>Content-Type</t>
<t>ETag</t>
<t>Location-Path</t>
<t>Location-Query</t>
<t>Max-Age</t>
<t>Proxy-Uri</t>
<t>Token</t>
<t>Uri-Host</t>
<t>Uri-Path</t>
<t>Uri-Port</t>
<t>Uri-Query</t>
<t>Accept</t>
<t>If-Match</t>
<t>If-None-Match</t>
</list>
The semantics of these options along with their properties are defined
in detail in <xref target="options"/>.</t>
<t>Not all options are defined for use with all methods and response codes.
The possible options for methods and response codes are defined in
<xref target="methods"/> and <xref target="response-codes"/>
respectively. In case an option is not defined for a method or
response code, it MUST NOT be included by a sender and MUST be
treated like an unrecognized option by a recipient.</t>
<section anchor="critical-elective" title="Critical/Elective">
<t>Options fall into one of two classes: "critical" or "elective".
The difference between these is how an option unrecognized by an
endpoint is handled:
<list style="symbols">
<t>Upon reception, unrecognized options of class "elective" MUST
be silently ignored.</t>
<t>Unrecognized options of class "critical" that occur in a
confirmable request MUST cause the return of a 4.02 (Bad
Option) response. This response SHOULD include a
diagnostic message describing the unrecognized
option(s) (see <xref target="diagnostic-message-payload"/>).</t>
<t>Unrecognized options of class "critical" that occur in a
confirmable response SHOULD cause the response to be rejected
with a reset message.</t> <!--TODO: DIAGNOSTICS -->
<t>Unrecognized options of class "critical" that occur in a
non-confirmable message MUST cause the message to be silently
ignored. The response MAY be rejected with a reset message.</t>
</list>
</t>
<t>Note that, whether critical or elective, an option is never
"mandatory" (it is always optional): These rules are defined in
order to enable implementations to reject options they do not
understand or implement.</t>
</section>
<section anchor="option-length" title="Length">
<t>Option values are defined to have a specific length, often in the form
of an upper and lower bound. If the length of an option value in a request
is outside the defined range, that option MUST be treated like an
unrecognized option (see <xref target="critical-elective"/>).</t>
</section>
<section anchor="option-defaults" title="Default Values">
<t>Options may be defined to have a default value. If the
value of option is intended to be this default value, the
option SHOULD NOT be included in the message.
If the option is not present, the default value MUST be assumed.</t>
<t>Where a critical option has a default value, this is chosen in such
a way that the absence of the option in a message can be processed
properly both by implementations unaware of the critical option and by
implementations that interpret this absence as the presence of the
default value for the option.
</t>
</section>
<section anchor="repeatable-options" title="Repeatable Options">
<t>The definition of an option MAY specify the option to be
repeatable. An option that is repeatable MAY be included one or
more times in a message. An option that is not repeatable MUST NOT
be included more than once in a message.</t>
<t>If a message includes an option with more occurrences than the
option is defined for, the additional option occurrences MUST be
treated like an unrecognized option (see <xref
target="critical-elective"/>).</t>
</section>
<section anchor="option-numbers" title="Option Numbers">
<t>Options are identified by an option number. Odd numbers indicate
a critical option, while even numbers indicate an elective option.
(Note that this is not just a convention, it is a feature of
the protocol: Whether an option is elective or critical is
entirely determined by whether its option number is even or odd.)
</t>
<t>The numbers that are non-zero multiples of 14 are used in conjunction with "fenceposting", as
described in <xref target="option-format"/>. Options with these numbers MUST
have a zero-length default value.</t>
<t>The option numbers for the options defined in this document are
listed in the <xref target="option-number-registry">CoAP Option
Number Registry</xref>.</t>
</section>
</section>
<section anchor="payload-semantics" title="Payload">
<t>Both requests and responses may include payload, depending on the
method or response code respectively. If a method or response code is
not defined to have a payload, then a sender MUST NOT include one,
and a recipient MUST ignore it.</t>
<section title="Representation" anchor="representation-payload">
<t>The payload of requests or of responses indicating success is
typically a representation of a resource or the result of the
requested action. Its format is specified by the Internet media
type given by the Content-Type Option. In the absence of this
option, no default value is assumed and the format must be inferred
by the application (e.g., from the application context or by
"sniffing" the payload).</t>
</section>
<section title="Diagnostic Message" anchor="diagnostic-message-payload">
<t>The payload of responses indicating a client or server error is a
brief human-readable diagnostic message, explaining the error
situation. This diagnostic message MUST be encoded using <xref
target="RFC3629">UTF-8</xref>, more specifically using <xref
target="RFC5198">Net-Unicode form</xref>. The Content-Type Option
MUST NOT be included by the sender and MUST be treated like an
unrecognized option by the recipient.</t>
<t>The message is similar to the Reason-Phrase on an HTTP status
line. It is not intended for end-users but for software engineers
that during debugging need to interpret it in the context of the
present, English-language specification; therefore no mechanism for
language tagging is needed or provided.</t>
</section>
</section>
<section anchor="caching" title="Caching">
<t>CoAP endpoints MAY cache responses in order to reduce the response
time and network bandwidth consumption on future, equivalent
requests.</t>
<t>The goal of caching in CoAP is to reuse a prior response message
to satisfy a current request. In some cases, a stored response can
be reused without the need for a network request, reducing latency
and network round-trips; a "freshness" mechanism is used for this
purpose (see <xref target="freshness-model"/>).
Even when a new request is required, it is often possible to reuse
the payload of a prior response to satisfy the request, thereby
reducing network bandwidth usage; a "validation" mechanism is
used for this purpose (see <xref target="validation-model"/>).</t>
<!-- Response Cacheability -->
<t>Unlike HTTP, the cacheability of CoAP responses does not depend
on the request method, but the Response Code. The cacheability of
each Response Code is defined along the Response Code definitions
in <xref target="response-codes"/>. Response Codes that indicate
success and are unrecognized by an endpoint MUST NOT be cached.</t>
<!-- Constructing Responses from Caches -->
<t>For a presented request, a CoAP endpoint MUST NOT use a stored
response, unless:
<list style="symbols">
<t>the presented request method and that used to obtain the stored response
match,</t>
<t>all options match between those in the presented request
and those of the request
used to obtain the stored response (which includes the request
URI), except that there is no need for a match of the Token,
Max-Age, or ETag request option(s), and</t>
<t>the stored response is either fresh or successfully
validated as defined below.</t>
</list>
</t>
<section anchor="freshness-model" title="Freshness Model">
<t>When a response is "fresh" in the cache, it can be used to
satisfy subsequent requests without contacting the origin server,
thereby improving efficiency.</t>
<t>The mechanism for determining freshness is for an origin server
to provide an explicit expiration time in the future, using the
Max-Age Option (see <xref target="max-age"/>). The Max-Age Option
indicates that the response is to be considered not fresh after its
age is greater than the specified number of seconds.</t>
<t>The Max-Age Option defaults to a value of 60. Thus, if it is
not present in a cacheable response,
then the response is considered not fresh after its age is greater
than 60 seconds. If an origin server wishes to prevent caching,
it MUST explicitly include a Max-Age Option with a value of zero
seconds.</t>
</section>
<section anchor="validation-model" title="Validation Model">
<t>When an endpoint has one or more stored responses for a GET
request, but cannot use any of them (e.g., because they are not
fresh), it can use the ETag Option (<xref target="etag"/>) in the GET request to give the
origin server an opportunity to both select a stored response to
be used, and to update its freshness. This process is known as
"validating" or "revalidating" the stored response.</t>
<t>When sending such a request, the endpoint SHOULD add an ETag
Option specifying the entity-tag of each stored response that
is applicable.</t>
<t>A 2.03 (Valid) response
indicates the stored response identified by the entity-tag given
in the response's ETag Option can be reused, after updating its
freshness with the value of the Max-Age Option that is included with
the response (see <xref target="valid"/>).</t>
<t>Any other response code indicates that none of the stored
responses nominated in the request is suitable. Instead, the
response SHOULD be used to satisfy the request and MAY replace
the stored response.</t>
</section>
</section>
<section anchor="proxying" title="Proxying">
<t>CoAP distinguishes between requests to an origin server and a
request made through a proxy. A proxy is a CoAP endpoint that can
be tasked by CoAP clients to perform requests on their behalf.
This may be useful, for example, when the request could otherwise
not be made, or
to service the response from a cache in order to reduce response
time and network bandwidth or energy consumption.</t>
<t>CoAP requests to a proxy are made as normal confirmable or
non-confirmable requests to the proxy endpoint, but specify the
request URI in a different way: The request URI in a proxy request
is specified as a string in the Proxy-Uri Option (see <xref
target="proxy-uri"/>), while the request URI in a request to an origin
server is split into the Uri-Host, Uri-Port, Uri-Path and Uri-Query
Options (see <xref target="uri-options"/>).</t>
<t>When a proxy request is made to an endpoint and the endpoint is
unwilling or unable to act as proxy for the request URI, it MUST
return a 5.05 (Proxying Not Supported) response. If the authority
(host and port) is recognized as identifying the proxy endpoint,
then the request MUST be treated as a local request.</t>
<t>Unless a proxy is configured to forward the proxy request
to another proxy,
it MUST translate the request as follows: The origin server's
IP address and port are determined by the authority component of the
request URI, and the request URI is decoded and split into the
Uri-Host, Uri-Port,
Uri-Path and Uri-Query Options.</t>
<t>All options present in a proxy request MUST be processed at
the proxy. Critical options in a request that are not recognized
by the proxy MUST lead to a 4.02 (Bad Option) response being
returned by the proxy. Elective options not recognized by the
proxy MUST NOT be forwarded to the origin server. Similarly,
critical options in a response that are not recognized by the
proxy server MUST lead to a 5.02 (Bad Gateway) response.
Again, elective options that are not recognized MUST NOT be
forwarded.</t>
<t>If the proxy does not employ a cache, then it simply forwards the
translated request to the determined destination. Otherwise, if it does employ a cache but
does not have a stored response that matches the translated request and
is considered fresh, then it needs to refresh its cache according
to <xref target="caching"/>.</t>
<t>If the request to the destination times out, then a 5.04 (Gateway
Timeout) response MUST be returned. If the request to the destination
returns an response that cannot be processed by the proxy,
then a 5.02 (Bad Gateway) response MUST be
returned. Otherwise, the proxy returns the response to the client.</t>
<t>If a response is generated out of a cache, it MUST be
generated with a Max-Age Option that does not extend the max-age
originally set by the server, considering the time the
resource representation spent in the cache.
E.g., the Max-Age Option could be adjusted by the proxy for each
response using the formula: proxy-max-age = original-max-age -
cache-age. For example if a request is made to a proxied resource that
was refreshed 20 seconds ago and had an original Max-Age of 60 seconds,
then that resource's proxied max-age is now 40 seconds.</t>
</section>
<section anchor="methods" title="Method Definitions">
<t>In this section each method is defined along with its behavior.
A request with an unrecognized or unsupported Method Code MUST generate
a 4.05 (Method Not Allowed) response.</t>
<!-- should be &pb; or at least not &npb;? -->
<section anchor="get" title="GET">
<t>The GET method retrieves a representation for the
information that currently corresponds to the resource
identified by the request URI. If the request includes one or more Accept Options, they indicate the preferred content-type of a response. If the request includes an ETag Option, the GET method requests that ETag be validated and that the representation be transferred only if validation failed. Upon success a 2.05 (Content) or 2.03 (Valid) response SHOULD be sent.</t>
<t>The GET method is safe and idempotent.</t>
</section>
<section anchor="post" title="POST">
<t>The POST method requests that the representation enclosed in the
request be processed. The actual function performed by the POST
method is determined by the origin server and dependent on the
target resource. It usually results in a new resource being created
or the target resource being updated.</t>
<t>If a resource has been created on the server, the response returned by the server SHOULD have a 2.01 (Created)
response code and SHOULD include the URI of the new resource in a
sequence of one or more Location-Path and/or Location-Query
Options (<xref target="location-options"/>). If the POST succeeds but does not result
in a new resource being created on the server, the response SHOULD have a 2.04 (Changed)
response code. If the POST succeeds and results in
the target resource being deleted, the response SHOULD have a 2.02 (Deleted)
response code.</t>
<t>POST is neither safe nor idempotent.</t>
</section>
<section anchor="put" title="PUT">
<t>The PUT method requests that the resource identified by the
request URI be updated or created with the enclosed representation.
The representation format is specified by the media type given in
the Content-Type Option.</t>
<t>If a resource exists at the request URI the enclosed
representation SHOULD be considered a modified version of that
resource, and a 2.04 (Changed) response SHOULD be returned.
If no resource exists then the server MAY create a new resource
with that URI, resulting in a 2.01 (Created) response.
If the resource could not be created or modified, then an
appropriate error response code SHOULD be sent.</t>
<t>Further restrictions to a PUT can be made by including the If-Match (see <xref target="if-match"/>) or If-None-Match (see <xref target="if-none-match"/>) options in the request.</t>
<t>PUT is not safe, but is idempotent.</t>
</section>
<section anchor="delete" title="DELETE">
<t>The DELETE method requests that the resource identified by the
request URI be deleted. A 2.02 (Deleted) response SHOULD be
sent on success or in case the resource did not exist before the
request.</t> <!-- this is different from HTTP! -->
<t>DELETE is not safe, but is idempotent.</t>
</section>
</section>
<section anchor="response-codes" title="Response Code Definitions">
<t>Each response code is described below, including any options
required in the response. Where appropriate, some of the codes will be specified
in regards to related response codes in HTTP <xref
target="RFC2616"/>; this does not mean that any such
relationship modifies the HTTP mapping specified in <xref target="http"/>.</t>
<section anchor="success" title="Success 2.xx">
<t>This class of status code indicates that the clients request
was successfully received, understood, and accepted.</t>
<section anchor="created" title="2.01 Created">
<t>Like HTTP 201 "Created", but only used in response to POST and PUT requests. The payload returned with the response, if any, is a representation of the action result.</t>
<t>If the response includes one or more Location-Path
and/or Location-Query Options, the values of
these options specify the location at which the resource was
created. Otherwise, the resource was created at the request
URI. A cache MUST mark any stored response for the created
resource as not fresh.</t>
<t>This response is not cacheable.</t>
</section>
<section anchor="deleted" title="2.02 Deleted">
<t>Like HTTP 204 "No Content", but only used in response to DELETE
requests. The payload returned with the response, if any, is a representation of the action result.</t>
<t>This response is not cacheable. However, a cache SHOULD mark any
stored response for the deleted resource as not fresh.</t>
</section>
<section anchor="valid" title="2.03 Valid">
<t>Related to HTTP 304 "Not Modified", but only used to
indicate that the response
identified by the entity-tag identified by the included ETag Option
is valid. Accordingly, the response MUST include
an ETag Option.</t>
<t>When a cache receives a 2.03 (Valid) response, it MUST update
the stored response with the value of the Max-Age Option included
in the response (see <xref target="validation-model"/>).</t>
</section>
<section anchor="changed" title="2.04 Changed">
<t>Like HTTP 204 "No Content", but only used in response to POST and PUT requests. The payload returned with the response, if any, is a representation of the action result.</t>
<t>This response is not cacheable. However, a cache MUST mark any
stored response for the changed resource as not fresh.</t>
</section>
<section anchor="content" title="2.05 Content">
<t>Like HTTP 200 "OK", but only used in response to GET requests.</t>
<t>The payload returned with the response is a representation
of the target resource.</t>
<t>This response is cacheable: Caches can use the Max-Age Option
to determine freshness (see <xref target="freshness-model"/>)
and (if present) the ETag Option for validation (see <xref
target="validation-model"/>).</t>
</section>
</section>
<section anchor="client-error" title="Client Error 4.xx">
<t>This class of response code is intended for cases in which the
client seems to have erred. These response codes are applicable to
any request method.</t>
<t>The server SHOULD include a diagnostic message
as detailed in <xref target="diagnostic-message-payload"/>.</t>
<t>Responses of this class are cacheable: Caches can use the
Max-Age Option to determine freshness (see <xref
target="freshness-model"/>). They cannot be validated.</t>
<section anchor="bad-request" title="4.00 Bad Request">
<t>Like HTTP 400 "Bad Request".</t>
</section>
<section anchor="unauthorized" title="4.01 Unauthorized">
<t>The client is not authorized to perform the requested action.
The client SHOULD NOT repeat the request without
previously improving its
authentication status to the server. Which specific mechanism
can be used for this is outside
this document's scope; see also <xref target="securing-coap"/>.</t>
</section>
<section anchor="bad-option" title="4.02 Bad Option">
<t>The request could not be understood by the server due to one
or more unrecognized or malformed critical options. The client
SHOULD NOT repeat the request without modification.</t>
</section>
<section anchor="forbidden" title="4.03 Forbidden">
<t>Like HTTP 403 "Forbidden".</t>
</section>
<section anchor="not-found" title="4.04 Not Found">
<t>Like HTTP 404 "Not Found".</t>
</section>
<section anchor="method-not-allowed" title="4.05 Method Not Allowed">
<t>Like HTTP 405 "Method Not Allowed", but with no
parallel to the "Allow" header field.</t>
</section>
<section anchor="not-acceptable" title="4.06 Not Acceptable">
<t>Like HTTP 406 "Not Acceptable", but with no response entity.</t>
</section>
<section anchor="precondition-failed" title="4.12 Precondition Failed">
<t>Like HTTP 412 "Precondition Failed".</t>
</section>
<section anchor="request-entity-too-large" title="4.13 Request Entity Too Large">
<t>Like HTTP 413 "Request Entity Too Large".</t>
</section>
<section anchor="unsupported-media-type" title="4.15 Unsupported Media Type">
<t>Like HTTP 415 "Unsupported Media Type".</t>
</section>
</section>
<section anchor="server-error" title="Server Error 5.xx">
<t>This class of response code indicates cases in which the server
is aware that it has erred or is incapable of performing the
request. These response codes are applicable to any request
method.</t>
<t>The server SHOULD include a diagnostic message
as detailed in <xref target="diagnostic-message-payload"/>.</t>
<t>Responses of this class are cacheable: Caches can use the
Max-Age Option to determine freshness (see <xref
target="freshness-model"/>). They cannot be validated.</t>
<section anchor="internal-server-error" title="5.00 Internal Server Error">
<t>Like HTTP 500 "Internal Server Error".</t>
</section>
<section anchor="not-implemented" title="5.01 Not Implemented">
<t>Like HTTP 501 "Not Implemented".</t>
</section>
<section anchor="bad-gateway" title="5.02 Bad Gateway">
<t>Like HTTP 502 "Bad Gateway".</t>
</section>
<section anchor="service-unavailable" title="5.03 Service Unavailable">
<t>Like HTTP 503 "Service Unavailable", but using the Max-Age Option in place of the
"Retry-After" header field.</t>
</section>
<section anchor="gateway-timeout" title="5.04 Gateway Timeout">
<t>Like HTTP 504 "Gateway Timeout".</t>
</section>
<section anchor="proxying-not-supported" title="5.05 Proxying Not Supported">
<t>The server is unable or unwilling to act as a proxy for
the URI specified in the Proxy-Uri Option (see <xref
target="proxy-uri"/>).</t>
</section>
</section>
</section>
<section anchor="options" title="Option Definitions">
<t>The individual CoAP options are summarized in <xref
target="tab-options"/> and explained below.</t>
<texttable anchor="tab-options" title="Options">
<ttcol align="right">No.</ttcol>
<ttcol align="left">C</ttcol>
<ttcol align="left">R</ttcol>
<ttcol align="left">Name</ttcol>
<ttcol align="left">Format</ttcol>
<ttcol align="left">Length</ttcol>
<ttcol align="left">Default</ttcol>
<c> 1</c><c>x</c><c> </c><c>Content-Type </c><c>uint </c><c>0-2 B </c><c>(none) </c>
<c> 2</c><c> </c><c> </c><c>Max-Age </c><c>uint </c><c>0-4 B </c><c>60 </c>
<c> 3</c><c>x</c><c>x</c><c>Proxy-Uri </c><c>string</c><c>1-270 B</c><c>(none) </c>
<c> 4</c><c> </c><c>x</c><c>ETag </c><c>opaque</c><c>1-8 B </c><c>(none) </c>
<c> 5</c><c>x</c><c> </c><c>Uri-Host </c><c>string</c><c>1-270 B</c><c>(see below)</c>
<c> 6</c><c> </c><c>x</c><c>Location-Path </c><c>string</c><c>0-270 B</c><c>(none) </c>
<c> 7</c><c>x</c><c> </c><c>Uri-Port </c><c>uint </c><c>0-2 B </c><c>(see below)</c>
<c> 8</c><c> </c><c>x</c><c>Location-Query</c><c>string</c><c>0-270 B</c><c>(none) </c>
<c> 9</c><c>x</c><c>x</c><c>Uri-Path </c><c>string</c><c>0-270 B</c><c>(none) </c>
<c>11</c><c>x</c><c> </c><c>Token </c><c>opaque</c><c>1-8 B </c><c>(empty) </c>
<c>12</c><c> </c><c>x</c><c>Accept </c><c>uint </c><c>0-2 B </c><c>(none) </c>
<c>13</c><c>x</c><c>x</c><c>If-Match </c><c>opaque</c><c>0-8 B </c><c>(none) </c>
<c>15</c><c>x</c><c>x</c><c>Uri-Query </c><c>string</c><c>0-270 B</c><c>(none) </c>
<c>21</c><c>x</c><c> </c><c>If-None-Match </c><c>empty </c><c>0 B </c><c>(none) </c>
<postamble>C=Critical, R=Repeatable</postamble>
</texttable>
<section anchor="token" title="Token">
<t>The Token Option is used to match a response with a request.
Every request has a client-generated token which the server MUST
echo in any response. A default value of a zero-length token is assumed in the absence of the option. Thus when the token value is empty, the Token Option SHOULD be elided for efficiency. </t>
<t>A token is intended for use as a client-local identifier for
differentiating between concurrent requests (see <xref target="response-matching"/>). A client SHOULD
generate tokens in a way that tokens currently in use for a given
source/destination pair are unique. An empty token value is appropriate e.g. when no other tokens are in use to a destination, or when requests are made serially per destination. There are however multiple possible implementation strategies to fulfill this. An endpoint receiving a token MUST treat it as opaque and make
no assumptions about its format.</t>
</section>
<section anchor="uri-options" title="Uri-Host, Uri-Port, Uri-Path and Uri-Query">
<t>The Uri-Host, Uri-Port, Uri-Path and Uri-Query Options are used
to specify the target resource of a request to a CoAP origin server. The
options encode the different components of the request URI in a way
that no percent-encoding is visible in the option values
and that the full URI can be
reconstructed at any involved endpoint. The syntax of CoAP URIs is
defined in <xref target="uri"/>.</t>
<t>The steps for parsing URIs into options is defined in
<xref target="uri-parsing"/>. These steps result in zero or more
Uri-Host, Uri-Port, Uri-Path and Uri-Query Options being included
in a request, where each option holds the following values:
<list style="symbols">
<t>the Uri-Host Option specifies the Internet host of the resource
being requested,</t>
<t>the Uri-Port Option specifies the transport layer port number of the resource,</t>
<t>each Uri-Path Option specifies one segment of the absolute path
to the resource, and</t>
<t>each Uri-Query Option specifies one argument parameterizing the
resource.</t>
</list>
Note: Fragments (<xref target="RFC3986"/>, Section 3.5) are not
part of the request URI and thus will not be transmitted in a CoAP
request.</t>
<t>The default value of the Uri-Host Option is the IP literal
representing the destination IP address of the request message.
Likewise, the default value of the Uri-Port Option is the
destination UDP port. The default values for the Uri-Host and Uri-Port
Options are sufficient for requests to most servers.
Explicit Uri-Host and Uri-Port Options are typically used when an endpoint hosts multiple virtual servers. </t>
<t>The Uri-Path and Uri-Query Option can contain any character sequence. No
percent-encoding is performed. The value of a Uri-Path Option MUST NOT be "." or ".."
(as the request URI must be resolved before parsing it into
options).</t>
<t>The steps for constructing the request URI from the options are
defined in <xref target="uri-constructing"/>. Note that an
implementation does not necessarily have to construct the URI;
it can simply look up the target resource by looking at the
individual options.</t>
<t>Examples can be found in <xref target="uri-examples"/>.</t>
</section>
<section anchor="proxy-uri" title="Proxy-Uri">
<t>The Proxy-Uri Option is used to make a request to a proxy (see
<xref target="proxying"/>). The proxy is requested to forward the
request or service it from a valid cache, and return the
response.</t>
<t>The option value is an absolute-URI (<xref target="RFC3986"/>,
Section 4.3). In case the absolute-URI doesn't fit within a single
option, the Proxy-Uri Option MAY be included multiple times in a
request such that the concatenation of the values results in the
single absolute-URI.</t>
<t>All but the last instance of the Proxy-Uri Option MUST have a
value with a length of 270 bytes, and the last instance MUST NOT
be empty.</t>
<t>Note that the proxy MAY forward the request on to another proxy
or directly to the server specified by the absolute-URI.
In order to avoid request loops, a proxy MUST be able to recognize
all of its server names, including any aliases, local variations,
and the numeric IP addresses.</t>
<t>An endpoint receiving a request with a Proxy-Uri Option that
is unable or unwilling to act as a proxy for the request MUST
cause the return of a 5.05 (Proxying Not Supported) response.</t>
<t>The Proxy-Uri Option MUST take precedence over any of the
Uri-Host, Uri-Port, Uri-Path or Uri-Query options (which MUST NOT
be included at the same time).</t>
</section>
<section anchor="content-type" title="Content-Type">
<t>The Content-Type Option indicates the representation format
of the message payload. The representation format is given as a
numeric media type identifier that is defined in the <xref
target="media-type-registry">CoAP Media Type registry</xref>.
No default value is assumed in the absence of the option.</t>
</section>
<section anchor="accept" title="Accept">
<t>
The CoAP Accept option indicates when included one or more times in a request, one or more media types, each of which is an acceptable media type for the client, in the order of preference (most preferred first). The representation format is given as a numeric media type identifier that is defined in the <xref target="media-type-registry">CoAP Media Type registry</xref>. If no Accept options are given, the client does not express a preference (thus no default value is assumed). The client prefers the representation returned by the server to be in one of the media types indicated. The server SHOULD return one of the preferred media types if available. If none of the preferred media types can be returned, then a 4.06 "Not Acceptable" SHOULD be sent as a response.</t>
<t>Note that as a server might not support the Accept option (and thus would ignore it as it is elective), the client needs to be prepared to receive a representation in a different media type. The client can simply discard a representation it can not make use of.
</t>
</section>
<section anchor="max-age" title="Max-Age">
<t>The Max-Age Option indicates the maximum time a response may be
cached before it MUST be considered not fresh (see <xref
target="freshness-model"/>).</t>
<t>The option value is an integer number of seconds between 0 and
2^32-1 inclusive (about 136.1 years). A default value of 60 seconds
is assumed in the absence of the option in a response.</t>
</section>
<section anchor="etag" title="ETag">
<t>The ETag Option in a response provides the current value of the
entity-tag for the enclosed representation of the target
resource.</t>
<t>An entity-tag is intended for use as a resource-local identifier
for differentiating between representations of the same resource
that vary over time. It may be generated in any number of ways
including a version, checksum, hash or time. An endpoint receiving
an entity-tag MUST treat it as opaque and make no assumptions about
its format. (Endpoints generating an entity-tag are
encouraged to use the most compact representation possible,
in particular in regards to clients and intermediaries that
may want to store multiple ETag values.)</t>
<t>An endpoint that has one or more representations previously
obtained from the resource can specify the ETag Option in a request
for each stored response to determine if any of those representations
is current (see <xref target="validation-model"/>).</t>
<t>The ETag Option MUST NOT occur more than once in a
response, and MAY occur one or more times in a request.</t>
</section>
<section anchor="location-options" title="Location-Path and Location-Query">
<t>The Location-Path and Location-Query Options together indicate a
relative URI that consists either of an absolute path, a
query string or both. A combination of these options is
included in a 2.01 (Created) response to indicate the location of the a resource
created as the result of a POST request (see <xref target="post"/>).
The location is resolved relative to the request URI.</t>
<t>If a response with one or more Location-Path and/or Location-Query Options
passes through a cache and the implied URI identifies one or more
currently stored responses, those entries SHOULD be marked as not
fresh.
</t>
<t>Each Location-Path Option specifies one segment of the absolute
path to the resource, and each Uri-Location Option specifies one
argument parameterizing the resource. The Location-Path and
Location-Query Option can contain any character sequence. No
percent-encoding is performed. The value of a Location-Path Option
MUST NOT be "." or "..".</t>
<t>The steps for constructing the location URI from the options are
analogous to <xref target="uri-constructing"/>, except that the
first five
<!-- BRITTLE - - need to recheck -->
steps are skipped and the result is a relative
URI-reference.</t>
<t>More Location-* options may be defined in the future, and have
been reserved option numbers 44, 46 and 48. If any of these reserved
option numbers occurs in addition to Location-Path and/or
Location-Query and are not supported, then a 4.02 (Bad Option) error
MUST be returned.</t>
</section>
<section anchor="if-match" title="If-Match">
<t>The If-Match Option MAY be used to make a request conditional on the current existence or value of an ETag for one or more representations of the target resource. If-Match is generally useful for resource update requests, such as PUT requests, as a means for protecting against accidental overwrites when multiple clients are acting in parallel on the same resource (i.e., the "lost update" problem).</t>
<t>The value of an If-Match option is either an ETag or the empty string. An empty string places the precondition on the existence of any current representation for the target resource.</t>
<t>The If-Match Option can occur multiple times. If any of the ETags given as an option value match the ETag of the current representation for the target resource, or if an If-Match Option with an empty string as option value is given and any current representation exists for the target resource, then the server MAY perform the request method as if the If-Match Option was not present.</t>
<t>If none of the ETags match and, if an empty string is given, no current representation exists at all, the server MUST NOT perform the requested method. Instead, the server MUST respond with the 4.12 (Precondition Failed) response code.</t>
<t>If the request would, without the If-Match Options, result in anything other than a 2.xx or 4.12 response code, then any If-Match Options MUST be ignored.</t>
</section>
<section anchor="if-none-match" title="If-None-Match">
<t>The If-None-Match Option MAY be used to make a request conditional on the non-existence of the target resource. If-None-Match is useful for resource creation requests, such as PUT requests, as a means for protecting against accidental overwrites when multiple clients are acting in parallel on the same resource. The If-None-Match Option carries no value.</t>
<t>If the target resource does exist, then the server MUST NOT perform the requested method. Instead, the server MUST respond with the 4.12 (Precondition Failed) response code.</t>
</section>
</section>
</section>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section anchor="uri" title="CoAP URIs">
<t>CoAP uses the "coap" and "coaps" URI schemes for identifying CoAP resources and
providing a means of locating the resource. Resources are organized
hierarchically and governed by a potential CoAP origin server listening
for CoAP requests ("coap") or DTLS-secured CoAP requests ("coaps") on a
given UDP port. The CoAP server is identified via the
generic syntax's authority component, which includes a host identifier
and optional UDP port number. The remainder of the URI is considered to be
identifying a resource which can be operated on by the methods defined
by the CoAP protocol. The "coap" and "coaps" URI schemes can thus be
compared to the "http" and "https" URI schemes respectively.</t>
<t>The syntax of the "coap" and "coaps" URI schemes is specified below in
Augmented Backus-Naur Form (ABNF) <xref target="RFC5234"/>.
The definitions of "host", "port", "path-abempty", "query",
"segment", "IP-literal", "IPv4address" and "reg-name"
are adopted from <xref target="RFC3986"/>.</t>
<section anchor="uri-coap" title="coap URI Scheme">
<!-- FIXME: any valid "coap" URI matches this grammar, but not all
URIs matching this grammar are valid "coap" URIs
-->
<figure>
<artwork type="abnf"><![CDATA[
coap-URI = "coap:" "//" host [ ":" port ] path-abempty [ "?" query ]
]]></artwork>
</figure>
<!-- TODO: check this with an ABNF compiler -->
<t>If host is provided as an IP-literal or IPv4address, then the
CoAP server can be reached at that IP address.
If host is a registered name, then that name is considered an indirect
identifier and the endpoint might use a name resolution service, such
as DNS,
<!-- TODO: can/should we wed ourselves to DNS? -->
to find the address of that host. The host MUST NOT be empty.
The port subcomponent indicates the UDP port at which the CoAP
server is located. If it is
empty or not given, then the default port
&PORT; is assumed.</t>
<t>The path identifies a resource within the scope of the host and
port. It consists of a sequence of path segments separated by a slash
character (U+002F SOLIDUS "/").</t>
<t>The query serves to further parameterize the resource. It consists
of a sequence of arguments separated by an ampersand character
(U+0026 AMPERSAND "&"). An argument is often in the form of a
"key=value" pair.</t>
<t>The "coap" URI scheme supports the path prefix "/.well-known/"
defined by <xref target="RFC5785"/> for "well-known locations" in the
name-space of a host. This enables discovery of policy or other
information about a host ("site-wide metadata"), such as hosted
resources (see <xref target="discovery"/>).</t>
<t>Application designers are encouraged to make use of short, but
descriptive URIs. As the environments that CoAP is used in
are usually constrained for bandwidth and energy, the
trade-off between these two qualities should lean towards the
shortness, without ignoring descriptiveness.</t>
</section>
<section anchor="uri-coaps" title="coaps URI Scheme">
<!-- FIXME: any valid "coaps" URI matches this grammar, but not all
URIs matching this grammar are valid "coaps" URIs
-->
<figure>
<artwork type="abnf"><![CDATA[
coaps-URI = "coaps:" "//" host [ ":" port ] path-abempty
[ "?" query ]
]]></artwork>
</figure>
<!-- TODO: check this with an ABNF compiler -->
<t>All of the requirements listed above for the "coap" scheme are also
requirements for the "coaps" scheme, except that a default UDP port
of &PORTS; is assumed if the port subcomponent is empty or not given,
and the UDP datagrams MUST be secured for privacy through the use of
DTLS as described in <xref target="dtls"/>.</t>
<t>Unlike the "coap" scheme, responses to "coaps" identified requests
are never "public" and thus MUST NOT be reused for shared caching. They
can, however, be reused in a private cache if the message is cacheable
by default in CoAP.</t>
<t>Resources made available via the "coaps" scheme have no shared
identity with the "coap" scheme even if their resource identifiers
indicate the same authority (the same host listening to the same UDP
port). They are distinct name spaces and are considered to be distinct
origin servers.</t>
</section>
<section anchor="uri-normalization" title="Normalization and Comparison Rules">
<t>Since the "coap" and "coaps" schemes conform to the URI generic syntax,
such URIs are normalized and compared according to the algorithm
defined in <xref target="RFC3986"/>, Section 6, using the defaults
described above for each scheme.</t>
<t>If the port is equal to the default port for a scheme, the normal form
is to elide the port subcomponent. Likewise, an empty path component
is equivalent to an absolute path of "/", so the normal form is to
provide a path of "/" instead. The scheme and host are
case-insensitive and normally provided in lowercase;
IP-literals are in recommended form <xref target="RFC5952"/>; all other
components are compared in a case-sensitive manner. Characters
other than those in the "reserved" set are equivalent to their
percent-encoded octets (see <xref target="RFC3986"/>, Section
2.1): the normal form is to not encode them.</t>
<t>For example, the following three URIs are equivalent, and
cause the same options and option values to appear in the CoAP
messages:</t>
<figure>
<artwork type="example"><![CDATA[
coap://example.com:5683/~sensors/temp.xml
coap://EXAMPLE.com/%7Esensors/temp.xml
coap://EXAMPLE.com:/%7esensors/temp.xml
]]></artwork>
</figure>
</section>
<!-- The ugly /url/ stuff will be fixed by the RFC editor :-) -->
<section anchor="uri-parsing" title="Decomposing URIs into Options">
<t>The steps to parse a request's options from a string /url/ are as
follows. These steps either result in zero or more of the Uri-Host,
Uri-Port, Uri-Path and Uri-Query Options being included in the
request, or they fail.
<list style="numbers">
<t>If the /url/ string is not an absolute URI (<xref
target="RFC3986"/>), then fail this algorithm.</t>
<t>Resolve the /url/ string using the process of reference
resolution defined by <xref target="RFC3986"/>, with the URL
character encoding set to <xref target="RFC3629">UTF-8</xref>.
<vspace blankLines="1"/>
NOTE: It doesn't matter what it is resolved relative to, since
we already know it is an absolute URL at this point.</t>
<t>If /url/ does not have a <scheme> component whose value,
when converted to ASCII lowercase, is "coap" or "coaps", then fail this
algorithm.</t>
<t>If /url/ has a <fragment> component, then fail this
algorithm.</t>
<t>If the <host> component of /url/ does not represent the
request's destination IP address as an IP-literal or IPv4address,
include a Uri-Host Option and
let that option's value be the value of the <host> component
of /url/, converted to ASCII lowercase, and then converting
all percent-encodings ("%" followed by
two hexadecimal digits) to the corresponding characters.
<vspace blankLines="1"/>
NOTE: In the usual case where the request's destination IP
address is derived from the host part, this ensures that
Uri-Host Options are only used for host parts of the form
reg-name.</t>
<t>If /url/ has a <port> component, then let /port/ be
that component's value interpreted as a decimal integer;
otherwise, let /port/ be the default port for the scheme.</t>
<t>If /port/ does not equal the request's destination UDP port,
include a Uri-Port Option and let that option's value be
/port/.</t>
<t>If the value of the <path> component of /url/ is empty or
consists of a single slash character (U+002F SOLIDUS "/"), then move
to the next step.<vspace blankLines="1"/>
Otherwise, for each segment in the <path> component,
include a Uri-Path Option and let that option's value be the
segment (not including the delimiting slash characters)
after converting all percent-encodings ("%" followed by
two hexadecimal digits) to the corresponding characters.</t>
<t>If /url/ has a <query> component, then,
for each argument in the <query> component,
include a Uri-Query Option and let that option's value be the
argument (not including the question mark and the delimiting ampersand
characters) after converting all percent-encodings to the
corresponding characters.</t>
</list></t>
<t>Note that these rules completely resolve any percent-encoding.
</t>
</section>
<section anchor="uri-constructing" title="Composing URIs from Options">
<t>The steps to construct a URI from a request's options are
as follows. These steps either result in a URI, or they fail.
In these steps, percent-encoding a character means replacing each of its
(UTF-8 encoded) bytes by a
"%" character followed by two hexadecimal digits representing the
byte, where the digits A-F are in upper case (as defined in <xref
target="RFC3986"/> Section 2.1; to reduce variability, the hexadecimal
notation for percent-encoding in CoAP URIs MUST use uppercase letters).
The definitions of "unreserved" and "sub-delims" are adopted from
<xref target="RFC3986"/>.
<list style="numbers">
<t>If the request is secured using DTLS, let /url/ be the string
"coaps://". Otherwise, let /url/ be the string "coap://".</t>
<t>If the request includes a Uri-Host Option, let /host/ be
that option's value, where any non-ASCII characters are
replaced by their corresponding percent-encoding. If /host/ is not a valid
reg-name or IP-literal or IPv4address, fail the algorithm.
Otherwise, let /host/ be the IP-literal (making
use of the conventions of <xref target="RFC5952"/>)
or IPv4address
representing the request's destination IP address.</t>
<t>Append /host/ to /url/.</t>
<t>If the request includes a Uri-Port Option, let /port/ be
that option's value. Otherwise, let /port/ be the request's
destination UDP port.</t>
<t>If /port/ is not the default port for the scheme, then append a
single U+003A COLON character (:) followed by the decimal
representation of /port/ to /url/.</t>
<!-- BRITTLE: location-options refers to "first five steps" -->
<t>Let /resource name/ be the empty string. For each Uri-Path
Option in the request, append a single character U+002F SOLIDUS
(/) followed by the option's value to /resource name/, after
converting any character that is not either in the
"unreserved" set, "sub-delims"
set, a U+003A COLON (:) or U+0040 COMMERCIAL AT (@) character,
to its percent-encoded form.</t>
<t>If /resource name/ is the empty string, set it to a single
character U+002F SOLIDUS (/).</t>
<t>For each Uri-Query Option in the request, append a single character
U+003F QUESTION MARK (?) (first option) or U+0026 AMPERSAND (&)
(subsequent options) followed by the option's value to /resource
name/, after converting any character that is not either in the
"unreserved" set, "sub-delims"
set (except U+0026 AMPERSAND (&)), a U+003A COLON (:), U+0040 COMMERCIAL AT (@), U+002F SOLIDUS (/)
or U+003F QUESTION MARK (?) character,
to its percent-encoded form.</t>
<t>Append /resource name/ to /url/.</t>
<t>Return /url/.</t>
</list></t>
<t>Note that these steps have been designed to lead to a URI
in normal form (see <xref target="uri-normalization"/>).
</t>
</section>
</section>
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<section anchor="discovery" title="Discovery">
<section title="Service Discovery" anchor="service-discovery">
<t>A server is discovered by a client by the client knowing or learning
a URI that references a resource in the namespace of the server.
Alternatively, clients can use Multicast CoAP (see <xref
target="multicast"/>) and the "All CoAP Nodes" multicast address to
find CoAP servers.</t>
<t>Unless the port subcomponent in a "coap" or "coaps" URI indicates
the UDP port at which the CoAP server is located, the server is
assumed to be reachable at the default port.</t>
<t>The CoAP default port number &PORT; MUST be supported by a
server for resource discovery (see <xref target="resource-discovery"/> below)
and SHOULD be supported for providing access to other resources. The default port number &PORTS; for DTLS-secured CoAP MAY be supported by a server for resource discovery and for providing access to other resources. In addition other endpoints may be hosted in the dynamic port space.</t>
<t>When a CoAP server is hosted by a 6LoWPAN node, it SHOULD also
support a port number in the 61616-61631 compressed UDP port space defined
in <xref target="RFC4944"/> (note that, as its
UDP port differs from the default port, it is a different
endpoint from the server at the default port).
So if the default port number does not work and a client knows that
the CoAP server is hosted by a 6LoWPAN node, the client MAY try to
contact the CoAP server at a port number in the 61616-61631 space.
</t>
</section>
<section anchor="resource-discovery" title="Resource Discovery">
<t>The discovery of resources offered by a CoAP endpoint is extremely
important in machine-to-machine applications where there are no humans
in the loop and static interfaces result in fragility. A CoAP endpoint
SHOULD support the CoRE Link Format of discoverable resources as
described in <xref target="I-D.ietf-core-link-format"/>. It is up to the server which resources are made discoverable (if any). </t>
<section title="'ct' Attribute">
<t>
This section defines a new Web Linking <xref target="RFC5988"/> attribute for use with <xref target="I-D.ietf-core-link-format"/>.
The Content-type code "ct" attribute provides a hint about the Internet media type(s) this resource returns. Note that this is only a hint, and does not override the Content-type Option of a CoAP response obtained by actually following the link. The value is in the CoAP identifier code format as a decimal ASCII integer and MUST be in the range of 0-65535 (16-bit unsigned integer). For example application/xml would be indicated as "ct=41". If no Content-type code attribute is present then nothing about the type can be assumed. The Content-type code attribute MAY appear more than once in a link, indicating that multiple content-types are available.
</t>
<!-- Zach: This text is no longer relvant in the core CoAP spec...
<t>
Alternatively, the "type" attribute MAY be used to indicate an Internet media type as a quoted-string <xref target="RFC5988"/>. It is not however expected that constrained implementations are able to parse quoted-string Content-type values. A link MAY include either a ct attribute or a type attribute, but MUST NOT include both.
</t>
-->
<figure>
<artwork><![CDATA[
link-extension = <Defined in RFC5988>
link-extension = ( "ct" "=" cardinal ) ; Range of 0-65535
cardinal = "0" / %x31-39 *DIGIT
]]></artwork>
</figure>
</section>
</section>
</section>
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<section anchor="multicast" title="Multicast CoAP">
<t>CoAP supports making requests to a IP multicast group. This is defined
by a series of deltas to Unicast CoAP.</t>
<section title="Messaging Layer">
<t>A multicast request is characterized by being transported in a CoAP
message that is addressed to an IP multicast address instead of a CoAP end-point. Such multicast
requests MUST be Non-Confirmable.</t>
<t>Some mechanisms for avoiding
congestion from multicast requests have been considered in <xref
target="I-D.eggert-core-congestion-control"/>.</t>
<t><!--To reduce response congestion, -->A server SHOULD be aware
that a request arrived via multicast, e.g. by making use of modern APIs such
as IPV6_RECVPKTINFO <xref target="RFC3542"/>, if
available.</t>
<t>When a server is aware that a request arrived via multicast, it MUST
NOT return a RST in reply to NON. If it is not aware, it MAY return a
RST in reply to NON as usual.</t>
</section>
<section title="Request/Response Layer">
<t>When a server is aware that a request arrived via
multicast, the server MAY always pretend it did not receive
the request, in particular if it doesn't have anything useful
to respond (e.g., if it only has an empty payload or an error
response). The decision for this may depend on the
application. (For example, in <xref
target="I-D.ietf-core-link-format"/> query filtering, a server
should not respond to a multicast request if the filter does
not match.) </t>
<t>If a server does decide to respond to a multicast request,
it should not respond immediately. Instead, it should pick a
duration for the period of time during which it intends to
respond. For purposes of this exposition, we call the length of this period
the Leisure. The specific value of this Leisure may depend on
the application, or MAY be derived as described below.
The server SHOULD then pick a random point of time within the
chosen Leisure period to send back the unicast response to the
multicast request.</t>
<t>To compute a value for Leisure, the server should have a group size estimate G,
a target rate R (which both should be chosen conservatively) and an
estimated response size S; a rough lower bound for Leisure can
then be computed as</t>
<figure><artwork align="center"><![CDATA[lb_Leisure = S * G / R]]></artwork></figure>
<t>E.g., for a multicast request with link-local scope on an 2.4 GHz IEEE
802.15.4 (6LoWPAN) network, G could be (relatively conservatively) set
to 100, S to 100 bytes, and the target rate to a conservative 8 kbit/s = 1 kB/s. The
resulting lower bound for the Leisure is 10 seconds.</t>
<t>When matching a response to a multicast request, only the token MUST
match; the source endpoint of the response does not need to (and
will not) be the same as the destination endpoint of the original
request.</t>
<section title="Caching">
<t>When a client makes a multicast request, it always makes a new
request to the multicast group (since there may be new group
members that joined meanwhile or ones that did not get the previous
request). It MAY update the cache with the received responses. Then
it uses both cached-still-fresh and 'new' responses as the result
of the request.</t>
<t>A response received in reply to a GET request to a multicast group
MAY be used to satisfy a subsequent request on the related unicast
request URI. The unicast request URI is obtained by replacing the
authority part of the request URI with the transport layer source
address of the response message.</t>
<t>A cache MAY revalidate a response by making a GET request on the
related unicast request URI.</t>
<t>A GET request to a multicast group MUST NOT contain an ETag
option. A mechanism to suppress responses the client already has is
left for further study.</t>
</section>
<section title="Proxying">
<t>When a forward proxy receives a request with a Proxy-Uri that
indicates a multicast address, the proxy obtains
a set of responses as described above and sends all responses (both
cached-still-fresh and new) back to the original client.</t>
</section>
<!-- TODO: Security Considerations -->
</section>
</section>
<!-- **************************************************************** -->
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<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section anchor="securing-coap" title="Securing CoAP">
<t>This section defines the DTLS binding for CoAP, and the alternative use of IPsec.
</t>
<t>During the provisioning phase, a CoAP device is provided
with the security information that it needs, including keying
materials and access control lists. This specification defines provisioning for the RawPublicKey mode in <xref target="rawpublickey-provisioning"/>. At
the end of the provisioning phase, the device will be in one of four security modes with the following information for the given
mode. The NoSec and RawPublicKey modes are mandatory to implement for this specification. </t>
<t><list style="hanging">
<t hangText="NoSec:">There is no protocol level security (DTLS is disabled). Alternative techniques to provide lower layer security SHOULD be used when appropriate. The use of
IPsec is discussed in <xref target="ipsec"/>.</t>
<t hangText="PreSharedKey:">DTLS is enabled and there is a list of pre-shared keys <xref target="RFC4279"/> and each key includes a list of which nodes it can be used to communicate with as described in <xref target="presharedkey"/>. At the extreme there may be one key for each node this CoAP node needs to communicate with (1:1 node/key ratio). </t>
<t hangText="RawPublicKey:">DTLS is enabled and the device has a raw public key certificate that is validated using an out-of-band mechanism <xref target="I-D.ietf-tls-oob-pubkey"/> as described in <xref target="rawpublickey"/>. The device also has an identity calculated from the public key and a list of identities of the nodes it can communicate with.
</t>
<t hangText="Certificate:">DTLS is enabled and the device has an asymmetric key pair
with an X.509 certificate <xref target="RFC5280"/> that binds it
to its Authority Name and is signed by some common trust root as described in <xref target="certificate"/>. The
device also has a list of root trust anchors that can be used for
validating a certificate. </t>
</list>
</t>
<t>In the "NoSec" mode, the system simply sends the packets over normal
UDP over IP and is indicated by the "coap" scheme and the CoAP default port. The system is secured only by keeping attackers
from being able to send or receive packets from the network with the
CoAP nodes; see <xref target="cross-protocol-attacks"/> for an additional
complication with this approach.</t>
<t>
The other three security modes are achieved using DTLS and are indicated by the "coaps" scheme and DTLS-secured CoAP default port. The
result is a security association that can be used to
authenticate (within the limits of the security model) and,
based on this authentication, authorize the communication partner.
CoAP itself does not
provide protocol primitives for authentication or authorization;
where this is required, it can either be provided by
communication security (i.e., IPsec or DTLS) or by object
security (within the payload). Devices that require
authorization for certain operations are expected to require one
of these two forms of security. Necessarily, where an
intermediary is involved, communication security only works when
that intermediary is part of the trust relationships; CoAP does
not provide a way to forward different levels of authorization
that clients may have with an intermediary to further
intermediaries or origin servers -- it therefore may be required
to perform all authorization at the first intermediary.</t>
<section anchor="dtls" title="DTLS-secured CoAP">
<t>Just as HTTP is secured using Transport Layer Security (TLS)
over TCP, CoAP is secured using Datagram TLS (DTLS) <xref
target="RFC6347"/> over UDP (see <xref target="fig-layers-secured"/>). This section defines the CoAP binding to DTLS, along with the minimal mandatory-to-implement configurations
appropriate for constrained environments. The binding is defined by a series of deltas to Unicast CoAP. DTLS is in practice TLS with
added features to deal with the unreliable nature of the UDP
transport.</t>
<figure anchor="fig-layers-secured" title="Abstract layering of DTLS-secured CoAP">
<artwork align="center"><![CDATA[
+----------------------+
| Application |
+----------------------+
+----------------------+
| Requests/Responses |
|----------------------| CoAP
| Messages |
+----------------------+
+----------------------+
| DTLS |
+----------------------+
+----------------------+
| UDP |
+----------------------+
]]></artwork>
</figure>
<t>In some constrained nodes (limited flash and/or RAM) and networks
(limited bandwidth or high scalability requirements), and
depending on the specific cipher suites in use, DTLS may not be
applicable. Some of DTLS' cipher suites can add significant
implementation complexity as well as some initial handshake
overhead needed when setting up the security association.
Once the initial handshake is completed, DTLS adds a limited
per-datagram overhead of approximately 13 bytes, not including
any initialization vectors/nonces (e.g., 8 bytes with
TLS_PSK_WITH_AES_128_CCM_8 <xref target="I-D.mcgrew-tls-aes-ccm"/>), integrity check values (e.g., 8 bytes with
TLS_PSK_WITH_AES_128_CCM_8 <xref target="I-D.mcgrew-tls-aes-ccm"/>) and padding required by the cipher suite.
Whether and which mode of using DTLS is applicable for a CoAP-based application should be carefully weighed
considering the specific cipher suites that may be applicable,
and whether the session maintenance makes it compatible with application flows and sufficient resources are available on the constrained nodes and for the added network overhead. DTLS is not applicable to group keying (multicast communication); however, it may be a component in a future group key management protocol.</t>
<section title="Messaging Layer">
<!-- borrowed from rfc2818 -->
<t>The endpoint acting as the CoAP client should also act as the DTLS
client. It should initiate a session to the server on the
appropriate port. When the DTLS handshake has finished, the client
may initiate the first CoAP request. All CoAP messages MUST be sent
as DTLS "application data".</t>
<t>The following rules are added for matching an ACK or RST to a CON
message or a RST to a NON message are as follows: The DTLS session
MUST be the same and the epoch MUST be the same.</t>
<t>A message is the same when it is sent within the same DTLS session and same epoch
and has the same Message ID.</t>
<t>Note: When a confirmable message is retransmitted, a new DTLS
sequence_number is used for each attempt, even though the CoAP
Message ID stays the same. So a recipient still has to perform
deduplication as described in <xref target="message-deduplication"
/>. Retransmissions MUST NOT be performed across epochs.</t>
<t>DTLS connections in RawPublicKey and Certificate mode are set up using mutual
authentication so they can remain up and be reused for future
message exchanges in either direction. Devices can close a DTLS connection when they need to recover resources but in general they should keep the connection up for as long as possible. Closing the DTLS connection after every CoAP message exchange is very inefficient.</t>
</section>
<section title="Request/Response Layer">
<t>The following rules are added for matching a response to a request:
The DTLS session MUST be the same and the epoch MUST be the same.</t>
<section title="Caching">
<t>The following rules are added for using a response that was
obtained using DTLS-secured CoAP: For a presented request, a CoAP
endpoint MUST NOT use a stored response, unless the identity is
the same.</t>
</section>
<section title="Proxying">
<t>Responses to "coaps" identified requests are never "public" and
thus MUST NOT be reused for shared caching. They can, however, be
reused in a private cache if the message is cacheable by default in
CoAP.</t>
</section>
</section>
<section title="Endpoint Identity">
<t>Devices SHOULD support the Server Name Indication (SNI) to indicate
their Authority Name in the SNI HostName field as defined in Section 3
of <xref target="RFC6066"/>. This is needed so that when a host that
acts as a virtual server for multiple Authorities receives a new DTLS
connection, it knows which keys to use for the DTLS session.</t>
<!-- TODO: explain how a client can find out the right
authentication realm (i.e., which client certificate/identity to send,
and also whether DTLS security is desired at all) from a CoAP
URI.
-->
<section anchor="presharedkey" title="Pre-Shared Keys">
<t>When forming a connection to a new node, the system selects an
appropriate key based on which nodes it is trying to reach and then forms
a DTLS session using a PSK (Pre-Shared Key) mode of DTLS.
Implementations in these modes MUST support the mandatory to implement cipher suite TLS_PSK_WITH_AES_128_CCM_8 as specified in <xref target="I-D.mcgrew-tls-aes-ccm"/>.
</t>
<t>The security considerations of <xref target="RFC4279"/>
(Section 7) apply. In particular, applications should
carefully weigh whether they need Perfect Forward Secrecy
(PFS) or not and select an appropriate cipher suite (7.1).
The entropy of the PSK must be sufficient to mitigate against
brute-force and (where the PSK is not chosen randomly but by a
human) dictionary attacks (7.2). The cleartext communication
of client identities may leak data or compromise privacy (7.3).
</t>
</section>
<section anchor="rawpublickey" title="Raw Public Key Certificates">
<t>In this mode the device has an asymmetric key pair but without an X.509 certificate (called a raw public key). A device MAY be configured with multiple raw public keys. The type and length of the raw public key depends on the cipher suite used. Implementations in RawPublicKey mode MUST support the mandatory to implement cipher suite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 as specified in <xref target="I-D.mcgrew-tls-aes-ccm-ecc"/>, <xref target="RFC5246"/>, <xref target="RFC4492"/>. The mechanism for using raw public keys with TLS is specified in <xref target="I-D.ietf-tls-oob-pubkey"/>. </t>
<section anchor="rawpublickey-provisioning" title="Provisioning">
<t>The RawPublicKey mode was designed to be easily provisioned in M2M deployments. It is assumed that each device has an appropriate asymmetric public key pair installed. An identifier is calculated from the public key as described in Section 2 of <xref target="I-D.farrell-decade-ni"/>. All implementations that support checking RawPublicKey identities MUST support at least the sha-256-120 mode (SHA-256 truncated to 120 bits). Implementations SHOULD support also longer length identifiers and MAY support shorter lengths. Note that the shorter lengths provide less security against attacks and their use is NOT RECOMMENDED.</t>
<t>Depending on how identifiers are given to the system that verifies them, support for URI, binary, and/or human-speakable format <xref target="I-D.farrell-decade-ni"/> needs to be implemented. All implementations SHOULD support the binary mode and implementations that have a user interface SHOULD also support the human-speakable format.</t>
<t>During provisioning, the identifier of each node is collected, for example by reading a barcode on the outside of the device or by obtaining a pre-compiled list of the identifiers. These identifiers are then installed in the corresponding endpoint, for example an M2M data collection server. The identifier is used for two purposes, to associate the endpoint with further device information and to perform access control. During provisioning, an access control list of identifiers the device may start DTLS sessions with SHOULD also be installed.</t>
</section>
</section>
<section anchor="certificate" title="X.509 Certificates">
<t>Implementations in Certificate Mode MUST support
the mandatory to implement cipher suite TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8
as specified in <xref target="RFC5246"/>.</t>
<t>
The Authority Name in the certificate is the name that would be
used in the Authority part of a CoAP URI. It is worth noting that
this would typically not be either an IP address or DNS name but would
instead be a long term unique identifier for the device such as the
EUI-64 <xref target="EUI64"/>. The discovery process used in the system would build up the
mapping between IP addresses of the given devices and the Authority Name
for each device. Some devices could have more than one Authority and
would need more than a single certificate.</t>
<t>When a new connection is formed, the certificate from the remote
device needs to be verified. If the CoAP node has a source of absolute
time, then the node SHOULD check the validity dates are of the
certificate are within range. The certificate MUST also be signed by
an appropriate chain of trust. If the certificate contains a
SubjectAltName, then the Authority Name MUST match at least one of the
authority names of any CoAP URI found in a URI type fields in the
SubjectAltName set. If there is no SubjectAltName in the certificate,
then the Authoritative Name must match the CN found in the certificate
using the matching rules defined in <xref target="RFC2818"/>
with the exception that certificates with wildcards are not
allowed. </t>
<t>If the system has a shared key in addition to the certificate, then
a cipher suite that includes the shared key such as
TLS_RSA_PSK_WITH_AES_128_CBC_SHA <xref target="RFC4279"/> SHOULD be used. </t>
</section>
</section>
</section>
<section anchor="ipsec" title="Using CoAP with IPsec">
<t>One mechanism to secure CoAP in constrained environments is the
IPsec Encapsulating Security Payload (ESP) <xref
target="RFC4303"/> when CoAP is used without DTLS in NoSec Mode. Using IPsec ESP with the appropriate
configuration, it is possible for many constrained devices to support
encryption with built-in link-layer encryption hardware. For example,
some IEEE 802.15.4 radio chips are compatible with AES-CBC (with
128-bit keys) <xref target="RFC3602"/> as defined for use with
IPsec in <xref target="RFC4835"/>. Alternatively,
particularly on more common IEEE 802.15.4 hardware that supports AES
encryption but not decryption, and to avoid the need for padding, nodes
could directly use the more widely supported AES-CCM as
defined for use with IPsec in
<xref target="RFC4309"/>, if the security considerations in
Section 9 of that specification can be fulfilled.</t>
<t>
Necessarily for AES-CCM, but much preferably also for AES-CBC,
static keying should be avoided and the initial keying
material be derived into transient session keys, e.g. using a
low-overhead mode of IKEv2 <xref target="RFC5996"/> as described in
<xref target="I-D.kivinen-ipsecme-ikev2-minimal"/>; such a
protocol for managing keys and sequence numbers is
also the only way to achieve anti-replay capabilities.
However, no
recommendation can be made at this point on how to manage
group keys (i.e., for multicast) in a constrained environment.
Once any initial setup is completed, IPsec ESP adds a limited
overhead of approximately 10 bytes per packet, not including
initialization vectors, integrity check values and padding required by the cipher suite.
</t>
<t>
When using IPsec to secure CoAP, both authentication and confidentiality SHOULD be applied as
recommended in <xref target="RFC4303"/>. The use of IPsec
between CoAP endpoints is transparent to the application layer and
does not require special consideration for a CoAP implementation. </t>
<t>IPsec may not be appropriate for all environments. For example,
IPsec support is not available for many embedded IP stacks and even in
full PC operating systems or on back-end web servers, application
developers may not have sufficient access to configure or enable IPsec
or to add a security gateway to the infrastructure. Problems with
firewalls and NATs may furthermore limit the use of IPsec.</t>
</section>
</section>
<!-- **************************************************************** -->
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<!-- **************************************************************** -->
<section anchor="http" title="Cross-Protocol Proxying between CoAP and HTTP">
<t>CoAP supports a limited subset of HTTP functionality, and thus cross-protocol
proxying to HTTP is straightforward. There might be several reasons
for proxying between CoAP and HTTP, for example when designing a web
interface for use over either protocol or when realizing a CoAP-HTTP
proxy. Likewise, CoAP could equally be proxied to other protocols such
as XMPP <xref target="RFC6120"/> or SIP <xref target="RFC3264"/>; the
definition of these mechanisms is out of scope of this specification.</t>
<t>There are two possible directions to access a resource via a forward proxy:
<list style="hanging">
<t hangText="CoAP-HTTP Proxying:">Enables CoAP clients to access
resources on HTTP servers through an intermediary.
This is initiated by including the Proxy-Uri Option with an "http" or "https" URI in a CoAP request to a CoAP-HTTP proxy.</t>
<t hangText="HTTP-CoAP Proxying:">Enables HTTP clients to access
resources on CoAP servers through an intermediary.
This is initiated by specifying a "coap" or "coaps" URI in the Request-Line of an HTTP request to an HTTP-CoAP proxy.</t>
</list>
</t>
<!-- TODO: s/mapping/proxying/ -->
<t>Either way, only the Request/Response model of CoAP is mapped to
HTTP. The underlying model of confirmable or non-confirmable messages,
etc., is invisible and MUST have no effect on a proxy function. The following
sections describe the handling of requests to a forward proxy. Reverse
proxies are not specified as the proxy function is transparent to the client with the proxy acting as if it was the origin server.</t>
<section anchor="coap-http" title="CoAP-HTTP Mapping">
<t>If a request contains a Proxy-URI Option with an 'http' or 'https'
URI <xref target="RFC2616"/>, then the receiving CoAP endpoint (called
"the proxy" henceforth) is requested to perform the operation specified
by the request method on the indicated HTTP resource and return the
result to the client.</t>
<t>This section specifies for any CoAP request the CoAP response that the
proxy should return to the client. How the proxy actually satisfies the
request is an implementation detail, although the typical case is
expected to be the proxy translating and forwarding the request to an
HTTP origin server.</t>
<t>Since HTTP and CoAP share the basic set of request methods, performing
a CoAP request on an HTTP resource is not so different from performing it
on a CoAP resource. The meanings of the individual CoAP methods when
performed on HTTP resources are explained below.</t>
<t>If the proxy is unable or unwilling to service a request with an HTTP
URI, a 5.05 (Proxying Not Supported) response SHOULD be returned to the
client. If the proxy services the request by interacting with a third
party (such as the HTTP origin server) and is unable to obtain a result
within a reasonable time frame, a 5.04 (Gateway Timeout) response SHOULD be returned; if a result can be obtained but is not understood, a 5.02 (Bad Gateway) response SHOULD be returned.</t>
<section anchor="coap-http-get" title="GET">
<t>The GET method requests the proxy to return a representation of
the HTTP resource identified by the request URI.</t>
<t>Upon success, a 2.05 (Content) response SHOULD be returned. The payload
of the response MUST be a representation of the target HTTP resource,
and the Content-Type Option be set accordingly. The response MUST
indicate a Max-Age value that is no greater than the remaining time
the representation can be considered fresh. If the HTTP entity has an
entity tag, the proxy SHOULD include an ETag Option in the response
and process ETag Options in requests as described below.</t>
<t>A client can influence the processing of a GET request by including
the following option:
<list style="hanging">
<t hangText="Accept:">
The request MAY include one or more Accept Options, identifying the preferred response content-type.
</t>
<t hangText="ETag:">The request MAY include one or more ETag Options,
identifying responses that the client has stored. This requests
the proxy to send a 2.03 (Valid) response whenever it would send a
2.05 (Content) response with an entity tag in the requested set otherwise.
</t>
</list>
</t>
</section>
<section anchor="coap-http-put" title="PUT">
<t>The PUT method requests the proxy to update or create the HTTP
resource identified by the request URI with the enclosed
representation.</t>
<t>If a new resource is created at the request URI, a 2.01 (Created)
response MUST be returned to the client. If an existing resource is
modified, a 2.04 (Changed) response MUST be returned to indicate
successful completion of the request.</t>
<!--
<t>No further options are defined in this specification to enable the client to influence
the processing of a PUT request.</t>
-->
<!-- TODO: What about If-Match and If-None-Match? -->
</section>
<section anchor="coap-http-delete" title="DELETE">
<t>The DELETE method requests the proxy to delete the HTTP resource
identified by the request URI at the HTTP origin server.</t>
<t>A 2.02 (Deleted) response MUST be returned to client upon success
or if the resource does not exist at the time of the request.</t>
<!--
<t>No further options are defined in this specification to enable the client to influence
the processing of a DELETE request.</t>
-->
</section>
<section anchor="coap-http-post" title="POST">
<t>The POST method requests the proxy to have the representation
enclosed in the request be processed by the HTTP origin server. The
actual function performed by the POST method is determined by the
origin server and dependent on the resource identified by the request
URI.</t>
<t>If the action performed by the POST method does not result in a
resource that can be identified by a URI, a 2.04 (Changed) response
MUST be returned to the client. If a resource has been created on
the origin server, a 2.01 (Created) response MUST be returned.</t>
<!--
<t>No further options are defined in this specification to enable the client to influence
the processing of a POST request.</t>
-->
</section>
</section>
<section anchor="http-coap" title="HTTP-CoAP Mapping">
<t>If an HTTP request contains a Request-URI with a 'coap' or 'coaps'
URI, then the receiving HTTP endpoint (called
"the proxy" henceforth) is requested to perform the operation specified
by the request method on the indicated CoAP resource and return the
result to the client.</t>
<t>This section specifies for any HTTP request the HTTP response that the
proxy should return to the client. How the proxy actually satisfies the
request is an implementation detail, although the typical case is
expected to be the proxy translating and forwarding the request to a CoAP
origin server. The meanings of the individual HTTP methods when
performed on CoAP resources are explained below.</t>
<t>If the proxy is unable or unwilling to service a request with a CoAP
URI, a 501 (Not Implemented) response SHOULD be returned to the
client. If the proxy services the request by interacting with a third
party (such as the CoAP origin server) and is unable to obtain a result
within a reasonable time frame, a 504 (Gateway Timeout) response SHOULD be
returned; if a result can be obtained but is not understood, a 502 (Bad Gateway)
response SHOULD be returned.</t>
<section anchor="http-coap-options-trace" title="OPTIONS and TRACE">
<t>
As the OPTIONS and TRACE methods are not supported in CoAP a 501 (Not Implemented) error MUST be returned to the client.
</t>
</section>
<section anchor="http-coap-get" title="GET">
<t>The GET method requests the proxy to return a representation of
the CoAP resource identified by the Request-URI.</t>
<t>Upon success, a 200 (OK) response SHOULD be returned. The payload
of the response MUST be a representation of the target CoAP resource,
and the Content-Type Option be set accordingly. The response MUST
indicate a Max-Age value that is no greater than the remaining time
the representation can be considered fresh. If the CoAP entity has an
entity tag, the proxy SHOULD include an ETag Option in the response.</t>
<t>A client can influence the processing of a GET request by including
the following option:
<list style="hanging">
<t hangText="Accept:">
Each individual Media-type of the HTTP Accept header in a request is mapped to a CoAP Accept option. HTTP Accept Media-type ranges, parameters and extensions are not supported by the CoAP Accept option. If the proxy cannot send a response which is acceptable according to the combined Accept field value, then the proxy SHOULD send a 406 (not acceptable) response.
</t>
<t hangText="Conditional GETs:">
Conditional HTTP GET requests
that include an "If-Match" or "If-None-Match" request-header
field can be mapped to a corresponding CoAP request. The "If-Modified-Since" and "If-Unmodified-Since"
request-header fields are not directly supported by CoAP, but
SHOULD be implemented locally by a caching proxy.
</t>
<!--
<t hangText="If-Range:">
The "If-Range" request-header field is not directly supported by CoAP, but
SHOULD be implemented locally by a caching proxy.
</t>
-->
</list>
</t>
</section>
<section anchor="http-coap-head" title="HEAD">
<t>
The HEAD method is identical to GET except that the server MUST NOT
return a message-body in the response.
</t>
<t>
Although there is no direct equivalent of HTTP's HEAD method in CoAP, an HTTP-CoAP
proxy responds to HEAD requests for CoAP resources, and the HTTP headers are returned without
a message-body.
</t>
</section>
<section anchor="http-coap-post" title="POST">
<t>The POST method requests the proxy to have the representation
enclosed in the request be processed by the CoAP origin server. The
actual function performed by the POST method is determined by the
origin server and dependent on the resource identified by the request
URI.</t>
<t>If the action performed by the POST method does not result in a
resource that can be identified by a URI, a 200 (OK) or 204 (No Content)
response MUST be returned to the client. If a resource has been created on
the origin server, a 201 (Created) response MUST be returned.</t>
</section>
<section anchor="http-coap-put" title="PUT">
<t>The PUT method requests the proxy to update or create the CoAP
resource identified by the Request-URI with the enclosed
representation.</t>
<t>If a new resource is created at the Request-URI, a 201 (Created)
response MUST be returned to the client. If an existing resource is
modified, either the 200 (OK) or 204 (No Content) response codes SHOULD be sent
to indicate successful completion of the request.</t>
<!--
<t>A client can influence the processing of a PUT request by including
the following option:
<list style="hanging">
<t hangText="Content-*:">
The proxy MUST return a 501 (Not Implemented) error in response to any
Content-* (e.g. Content-Range) headers that it can not internally handle or
that CoAP does not support.
</t>
</list>
</t>
-->
</section>
<section anchor="http-coap-delete" title="DELETE">
<t>The DELETE method requests the proxy to delete the CoAP resource
identified by the Request-URI at the CoAP origin server.</t>
<t>A successful response SHOULD be 200 (OK) if the response includes an
entity describing the status or 204 (No Content) if the action has been enacted but the response does not include an entity.
</t>
</section>
<section anchor="http-coap-connect" title="CONNECT">
<t>
This method can not currently be satisfied by an HTTP-CoAP proxy
function as TLS to DTLS tunneling has not been specified. It is however expected that such a tunneling mapping will be defined in the future. A 501 (Not Implemented) error SHOULD be returned to the client.
</t>
</section>
</section>
</section>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section anchor="security" title="Security Considerations">
<t>This section analyzes the possible threats to the protocol.
It is meant to inform protocol and application developers
about the security limitations of CoAP as described in this document.
As CoAP realizes a subset of the features in HTTP/1.1, the security
considerations in Section 15 of <xref target="RFC2616"/> are
also pertinent to CoAP. This section concentrates on describing
limitations specific to CoAP.</t>
<section anchor="protocol-parsing-processing-uris" title="Protocol Parsing, Processing URIs">
<t>A network-facing application can exhibit vulnerabilities
in its processing logic for incoming packets.
Complex parsers are well-known as a likely source of such
vulnerabilities, such as the ability to remotely crash a node, or
even remotely execute arbitrary code on it.
CoAP attempts to narrow the opportunities for introducing
such vulnerabilities by reducing parser complexity, by giving the
entire range of encodable values a meaning where possible, and by
aggressively reducing complexity that is often caused by unnecessary
choice between multiple representations that mean the same thing.
Much of the URI processing has been moved to the clients,
further reducing the opportunities for introducing
vulnerabilities into the servers.
Even so, the URI processing code in CoAP implementations
is likely to be a large source of remaining
vulnerabilities and should be implemented with special care.
The most complex parser remaining could be the one for the
link-format, although this also has been designed with a goal of
reduced implementation complexity <xref
target="I-D.ietf-core-link-format"/>.
(See also section
15.2 of <xref target="RFC2616"/>.)</t>
</section>
<section title="Proxying and Caching">
<t>
As mentioned in 15.7 of <xref target="RFC2616"/>,
proxies are by their very nature men-in-the-middle,
breaking any IPsec or DTLS protection that a direct CoAP
message exchange might have. They are therefore interesting
targets for breaking confidentiality or integrity of CoAP
message exchanges. As noted in <xref target="RFC2616"/>, they
are also interesting targets for breaking availability.
</t>
<t>
The threat to confidentiality and integrity of request/response
data is amplified where proxies also cache. Note that
CoAP does not define any of the cache-suppressing
Cache-Control options that HTTP/1.1 provides to better
protect sensitive data.
</t>
<t>
Finally, a proxy that fans out &Npb; Responses (as opposed
to &Pb; Responses) to
multiple original requesters may provide additional
amplification (see below).
</t>
</section>
<!--
<section title="Attacks on Message IDs">
<t>TODO. CoAP implementations should be tested against the
reception of unexpected Message IDs.</t>
</section>
-->
<section anchor="amplification" title="Risk of amplification">
<t>
CoAP servers generally reply to a request packet with a
response packet. This response packet may be
significantly larger than the request packet.
An attacker might use CoAP nodes to turn a small attack
packet into a larger attack packet, an approach known as
amplification.
There is therefore a danger that CoAP nodes could become
implicated in denial of service (DoS) attacks by using the
amplifying properties of the protocol: An attacker that
is attempting to overload a victim but is
limited in the amount of traffic it can generate, can use
amplification to generate a larger amount of traffic.
</t>
<t>
This is particularly a problem in nodes that enable NoSec
access, that are accessible from an attacker and can
access potential victims (e.g. on the general Internet),
as the UDP protocol provides no way to verify the source
address given in the request packet. An attacker need
only place the IP address of the victim in the source
address of a suitable request packet to generate a larger
packet directed at the victim.
</t>
<t>
As a mitigating factor, many constrained networks will only
be able to generate a small amount of traffic, which may
make CoAP nodes less attractive for this attack. However,
the limited capacity of the constrained network makes the network
itself a likely victim of an amplification attack.
</t>
<t>
A CoAP server can reduce the amount of amplification it
provides to an attacker by using slicing/blocking modes of
CoAP <xref target="I-D.ietf-core-block"/> and offering large resource
representations only in relatively small slices. E.g., for
a 1000 byte resource, a 10-byte request might result in an
80-byte response (with a 64-byte block) instead of a
1016-byte response, considerably reducing the amplification
provided.
</t>
<t>
CoAP also supports the use of multicast IP addresses in
requests, an important requirement for M2M. Multicast CoAP
requests may be the source of accidental or deliberate
denial of service attacks, especially over constrained
networks. This specification attempts to reduce the
amplification effects of multicast requests by limiting
when a response is returned. To limit the possibility of
malicious use, CoAP servers SHOULD NOT accept multicast
requests that can not be authenticated. If possible a
CoAP server SHOULD limit the support for multicast
requests to specific resources where the feature is
required.
</t>
<t>
On some general purpose operating systems providing a
Posix-style API, it is not straightforward to find out
whether a packet received was addressed to a multicast
address. While many implementations will know whether they have
joined a multicast group,
this creates a problem for packets addressed to
multicast addresses of the form FF0x::1, which are
received by every IPv6 node.
Implementations SHOULD make use of modern APIs such
as IPV6_RECVPKTINFO <xref target="RFC3542"/>, if
available, to make this determination.
</t>
</section>
<section anchor="address-spoofing-attacks" title="IP Address Spoofing Attacks">
<t>Due to the lack of a handshake in UDP, a rogue endpoint which is free to read and write messages carried by the constrained network (i.e. NoSec or PreSharedKey deployments with nodes/key ratio > 1:1), may easily attack a single endpoint, a group of endpoints, as well as the whole network e.g. by:
<list style="numbers">
<t>spoofing RST in response to a CON or NON message, thus making an endpoint "deaf"; or</t>
<t>spoofing the entire response with forged payload/options (this has different levels of impact: from single response disruption, to much bolder attacks on the supporting infrastructure, e.g. poisoning proxy caches, or tricking validation / lookup interfaces in resource directories and, more generally, any component that stores global network state and uses CoAP as the messaging facility to handle state set/update's is a potential target.); or</t>
<t>spoofing a multicast request for a target node which may result in both network congestion/collapse and victim DoS'ing / forced wakeup from sleeping; or</t>
<t>spoofing observe messages, etc.</t>
</list></t>
<t>In principle, spoofing can be detected by CoAP only in case CON semantics is used, because of unexpected ACK/RSTs coming from the deceived endpoint. But this imposes keeping track of the used Message IDs which is not always possible, and moreover detection becomes available usually after the damage is already done. This kind of attack can be prevented using security modes other than NoSec.</t>
</section>
<section anchor="cross-protocol-attacks" title="Cross-Protocol Attacks">
<t>The ability to incite a CoAP endpoint to send packets to a fake
source address can be used not only for amplification, but also for
cross-protocol attacks:</t>
<t><list style='symbols'>
<t>the attacker sends a message to a CoAP endpoint with a fake source address,</t>
<t>the CoAP endpoint replies with a message to the given source address,</t>
<t>the victim at the given source address receives a UDP packet that it
interprets according to the rules of a different protocol.</t>
</list></t>
<t>This may be used to circumvent firewall rules that prevent direct
communication from the attacker to the victim, but happen to allow
communication from the CoAP endpoint (which may also host a valid
role in the other protocol) to the victim.</t>
<t>Also, CoAP endpoints may be the victim of a cross-protocol attack
generated through an endpoint of another UDP-based protocol such as
DNS. In both cases, attacks are possible if the security properties of
the endpoints rely on checking IP addresses (and firewalling off
direct attacks sent from outside using fake IP addresses). In
general, because of their lack of context, UDP-based protocols are
relatively easy targets for cross-protocol attacks.</t>
<t>Finally, CoAP URIs transported by other means could be used to incite
clients to send messages to endpoints of other protocols.</t>
<t>One mitigation against cross-protocol attacks is strict checking of
the syntax of packets received, combined with sufficient
difference in syntax. As an example, it might help if it
were difficult to incite a DNS server to send a DNS response that
would pass the checks of a CoAP endpoint. Unfortunately, the first
two bytes of a DNS reply are an ID that can be chosen by the attacker,
which map into the interesting part of the CoAP header, and the next
two bytes are then interpreted as CoAP's Message ID (i.e., any
value is acceptable). The DNS count words may be interpreted as
multiple instances of a (non-existent, but elective) CoAP option 0.
The echoed query finally may be manufactured by the attacker to
achieve a desired effect on the CoAP endpoint; the response added by
the server (if any) might then just be interpreted as added payload.</t>
<figure title="DNS Header vs. CoAP Message" anchor="dns-header">
<artwork type="drawing"><![CDATA[
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ID | T, OC, code
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR| Opcode |AA|TC|RD|RA| Z | RCODE | message id
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| QDCOUNT | (options 0)
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ANCOUNT | (options 0)
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| NSCOUNT | (options 0)
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ARCOUNT | (options 0)
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
]]></artwork>
</figure>
<t>In general, for any pair of protocols, one of the protocols can very
well have been designed in a way that enables an attacker to cause the
generation of replies that look like messages of the other protocol.
It is often much harder to ensure or prove the absence of viable
attacks than to generate examples that may not yet completely enable
an attack but might be further developed by more creative minds.
Cross-protocol attacks can therefore only be completely mitigated if
endpoints don't authorize actions desired by an attacker just based
on trusting the source IP address of a packet. Conversely, a NoSec
environment that completely relies on a firewall for CoAP security not
only needs to firewall off the CoAP endpoints but also all other
endpoints that might be incited to send UDP messages to CoAP
endpoints using some other UDP-based protocol.</t>
<t>In addition to the considerations above, the security considerations
for DTLS with respect to cross-protocol attacks apply. E.g., if the
same DTLS security association ("connection") is used to carry data of
multiple protocols, DTLS no longer provides protection against
cross-protocol attacks between these protocols.</t>
</section>
</section>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section anchor="iana-considerations" title="IANA Considerations">
<!-- Based on RFC5226, RFC4395 and I-D.ietf-tsvwg-iana-ports-10. -->
<section anchor="coap-code-registry" title="CoAP Code Registry">
<t>This document defines a registry for the values of the Code field
in the CoAP header. The name of the registry is "CoAP Codes".</t>
<t>All values are assigned by sub-registries according to the following
ranges:
<list style="hanging" hangIndent="10">
<t hangText="0">Indicates an empty message (see <xref
target="messages-and-endpoints"/>).</t>
<t hangText="1-31">Indicates a request. Values in this range are
assigned by the "CoAP Method Codes" sub-registry
(see <xref target="coap-code-registry-methods"/>).</t>
<t hangText="32-63">Reserved</t>
<t hangText="64-191">Indicates a response. Values in this range are
assigned by the "CoAP Response Codes" sub-registry
(see <xref target="coap-code-registry-responses"/>).</t>
<t hangText="192-255">Reserved</t>
</list>
</t>
<section anchor="coap-code-registry-methods" title="Method Codes">
<!-- Name of the sub-registry -->
<t>The name of the sub-registry is "CoAP Method Codes".</t>
<!-- Size, format and syntax of registry entries -->
<t>Each entry in the sub-registry must include the Method Code
in the range 1-31, the name of the method, and a reference to
the method's documentation.</t>
<!-- Initial assignments and reservations -->
<t>Initial entries in this sub-registry are as follows:</t>
<texttable anchor="tab-method-code-registry" title="CoAP Method Codes">
<ttcol align="right">Code</ttcol>
<ttcol align="left">Name</ttcol>
<ttcol align="left">Reference</ttcol>
<c> 1</c><c>GET </c><c>&SELF;</c>
<c> 2</c><c>POST </c><c>&SELF;</c>
<c> 3</c><c>PUT </c><c>&SELF;</c>
<c> 4</c><c>DELETE</c><c>&SELF;</c>
</texttable>
<t>All other Method Codes are Unassigned.</t>
<!-- Review process -->
<t>The IANA policy for future additions to this registry is
"IETF Review" as described in <xref target="RFC5226"/>.</t>
<!-- Review guidelines -->
<t>The documentation of a method code should specify the semantics
of a request with that code, including the following properties:
<list style="symbols">
<t>The response codes the method returns in the success case.</t>
<t>Whether the method is idempotent, safe, or both.</t>
</list>
</t>
</section>
<section anchor="coap-code-registry-responses" title="Response Codes">
<!-- Name of the sub-registry -->
<t>The name of the sub-registry is "CoAP Response Codes".</t>
<!-- Size, format and syntax of registry entries -->
<t>Each entry in the sub-registry must include the Response Code
in the range 64-191, a description of the Response Code, and a
reference to the Response Code's documentation.</t>
<!-- Initial assignments and reservations -->
<t>Initial entries in this sub-registry are as follows:</t>
<texttable anchor="tab-response-code-registry" title="CoAP Response Codes">
<ttcol align="right">Code</ttcol>
<ttcol align="left">Description</ttcol>
<ttcol align="left">Reference</ttcol>
<!-- Success 64-95 -->
<c> 65</c><c>2.01 Created </c><c>&SELF;</c>
<c> 66</c><c>2.02 Deleted </c><c>&SELF;</c>
<c> 67</c><c>2.03 Valid </c><c>&SELF;</c>
<c> 68</c><c>2.04 Changed </c><c>&SELF;</c>
<c> 69</c><c>2.05 Content </c><c>&SELF;</c>
<!-- Client Error 128-159 -->
<c>128</c><c>4.00 Bad Request </c><c>&SELF;</c>
<c>129</c><c>4.01 Unauthorized </c><c>&SELF;</c>
<c>130</c><c>4.02 Bad Option </c><c>&SELF;</c>
<c>131</c><c>4.03 Forbidden </c><c>&SELF;</c>
<c>132</c><c>4.04 Not Found </c><c>&SELF;</c>
<c>133</c><c>4.05 Method Not Allowed </c><c>&SELF;</c>
<c>134</c><c>4.06 Not Acceptable </c><c>&SELF;</c>
<c>140</c><c>4.12 Precondition Failed </c><c>&SELF;</c>
<c>141</c><c>4.13 Request Entity Too Large</c><c>&SELF;</c>
<c>143</c><c>4.15 Unsupported Media Type </c><c>&SELF;</c>
<!-- Server Error 160-191 -->
<c>160</c><c>5.00 Internal Server Error </c><c>&SELF;</c>
<c>161</c><c>5.01 Not Implemented </c><c>&SELF;</c>
<c>162</c><c>5.02 Bad Gateway </c><c>&SELF;</c>
<c>163</c><c>5.03 Service Unavailable </c><c>&SELF;</c>
<c>164</c><c>5.04 Gateway Timeout </c><c>&SELF;</c>
<c>165</c><c>5.05 Proxying Not Supported </c><c>&SELF;</c>
</texttable>
<t>The Response Codes 96-127 are Reserved for future use. All other
Response Codes are Unassigned.</t>
<!-- Review process -->
<t>The IANA policy for future additions to this registry is
"IETF Review" as described in <xref target="RFC5226"/>.</t>
<!-- Review guidelines -->
<t>The documentation of a response code should specify the semantics
of a response with that code, including the following properties:
<list style="symbols">
<t>The methods the response code applies to.</t>
<t>Whether payload is required, optional or not allowed.</t>
<t>The semantics of the payload. For example, the payload of a 2.05
(Content) response is a representation of the target resource; the
payload in an error response is a human-readable diagnostic
message.</t>
<t>The format of the payload. For example, the format in a
2.05 (Content) response is indicated by the Content-Type Option;
the format of the payload in an error response is always
Net-Unicode text.</t>
<t>Whether the response is cacheable according to the freshness
model.</t>
<t>Whether the response is validatable according to the validation
model.</t>
<t>Whether the response causes a cache to mark responses stored
for the request URI as not fresh.</t>
</list>
</t>
</section>
</section>
<section anchor="option-number-registry" title="Option Number Registry">
<!-- Name of the registry -->
<t>This document defines a registry for the Option Numbers used in
CoAP options. The name of the registry is "CoAP Option Numbers".</t>
<!-- Size, format and syntax of registry entries -->
<t>Each entry in the registry must include the Option Number, the name
of the option and a reference to the option's documentation.</t>
<!-- Initial assignments and reservations -->
<t>Initial entries in this registry are as follows:</t>
<texttable anchor="tab-option-registry" title="CoAP Option Numbers">
<ttcol align="right">Number</ttcol>
<ttcol align="left">Name</ttcol>
<ttcol align="left">Reference</ttcol>
<c> 0</c><c>(Reserved) </c><c></c>
<c> 1</c><c>Content-Type </c><c>&SELF;</c>
<c> 2</c><c>Max-Age </c><c>&SELF;</c>
<c> 3</c><c>Proxy-Uri </c><c>&SELF;</c>
<c> 4</c><c>ETag </c><c>&SELF;</c>
<c> 5</c><c>Uri-Host </c><c>&SELF;</c>
<c> 6</c><c>Location-Path </c><c>&SELF;</c>
<c> 7</c><c>Uri-Port </c><c>&SELF;</c>
<c> 8</c><c>Location-Query</c><c>&SELF;</c>
<c> 9</c><c>Uri-Path </c><c>&SELF;</c>
<c>10</c><c>(Unassigned) </c><c></c>
<c>11</c><c>Token </c><c>&SELF;</c>
<c>12</c><c>Accept </c><c>&SELF;</c>
<c>13</c><c>If-Match </c><c>&SELF;</c>
<c>14</c><c>(Unassigned) </c><c></c>
<c>15</c><c>Uri-Query </c><c>&SELF;</c>
<c>16-20</c><c>(Unassigned) </c><c></c>
<c>21</c><c>If-None-Match </c><c>&SELF;</c>
<c>22-43</c><c>(Unassigned) </c><c></c>
<c>44</c><c>(Reserved) </c><c></c>
<c>45</c><c>(Unassigned) </c><c></c>
<c>46</c><c>(Reserved) </c><c></c>
<c>47</c><c>(Unassigned) </c><c></c>
<c>48</c><c>(Reserved) </c><c></c>
<c>49-</c><c>(Unassigned) </c><c></c>
</texttable>
<!-- Review process -->
<t>The IANA policy for future additions to this registry is
"IETF Review" as described in <xref target="RFC5226"/>.</t>
<!-- Review guidelines -->
<t>The documentation of an Option Number should specify the semantics
of an option with that number, including the following properties:
<list style="symbols">
<t>The meaning of the option in a request.</t>
<t>The meaning of the option in a response.</t>
<t>Whether the option is critical or elective, as determined by the
Option Number.</t>
<t>The format and length of the option's value.</t>
<t>Whether the option must occur at most once or whether it can occur
multiple times.</t>
<t>The default value, if any.
For a critical option with a default value, a discussion on how the default value enables processing by implementations not implementing the critical option (<xref target="option-defaults"/>).
For options with numbers that are a multiple of 14, the default value MUST be empty.</t>
</list>
</t>
</section>
<section anchor="media-type-registry" title="Media Type Registry">
<!-- Name of the registry -->
<t>Media types are identified by a string, such as "application/xml"
<xref target="RFC2046"/>. In order to minimize the overhead of
using these media types to indicate the format of payloads, this
document defines a registry for a subset of Internet media types to
be used in CoAP and assigns each a numeric identifier. The name of
the registry is "CoAP Media Types".</t>
<!-- Size, format and syntax of registry entries -->
<t>Each entry in the registry must include the media type registered
with IANA, the numeric identifier in the range 0-65535 to be used for
that media type in CoAP, the content-encoding associated with this identifier, and a reference to a document describing what
a payload with that media type means semantically.</t>
<t>CoAP does not include a way to convey content-encoding information with a request or response, and for that reason the content-encoding is also specified for each identifier (if any). If multiple content-encodings will be used with a media type, then a separate identifier for each is to be registered.</t>
<!-- Initial assignments and reservations -->
<t>Initial entries in this registry are as follows:</t>
<texttable anchor="tab-mediatype" title="CoAP Media Types">
<ttcol align="left">Media type</ttcol>
<ttcol align="left">Encoding</ttcol>
<ttcol align="right">Id.</ttcol>
<ttcol align="left">Reference</ttcol>
<!-- text 0-19 -->
<c>text/plain; charset=utf-8</c> <c>-</c> <c> 0</c><c><xref target="RFC2046"/><xref target="RFC3676"/><xref target="RFC5147"/></c>
<!-- image, audio, video 20-39-->
<!-- application 40-200 -->
<c>application/ link-format</c> <c>-</c> <c>40</c><c><xref target="I-D.ietf-core-link-format"/></c>
<c>application/xml</c> <c>-</c> <c>41</c><c><xref target="RFC3023"/></c>
<c>application/ octet-stream</c> <c>-</c> <c>42</c><c><xref target="RFC2045"/><xref target="RFC2046"/></c>
<c>application/exi</c> <c>-</c> <c>47</c><c><xref target="EXIMIME"/></c>
<c>application/json</c> <c>-</c> <c>50</c><c><xref target="RFC4627"/></c>
</texttable>
<t>The identifiers between 201 and 255 inclusive are reserved for
Private Use.
All other identifiers are Unassigned.</t>
<!-- Review process -->
<t>Because the name space of single-byte identifiers is so small, the IANA policy for future
additions in the range 0-200 inclusive to the registry is "Expert Review" as described in
<xref target="RFC5226"/>.
The IANA policy for additions in the range 256-65535 inclusive is
"First Come First Served" as described in <xref target="RFC5226"/>.</t>
<!-- Review guidelines -->
<t>
In machine to machine applications, it is not expected that generic Internet
media types such as text/plain, application/xml or application/octet-stream
are useful for real applications in the long term. It is recommended that
M2M applications making use of
CoAP will request new Internet media types from IANA indicating semantic
information about how to create or parse a payload. For example, a Smart Energy
application payload carried as XML might request a more specific type like
application/se+xml or application/se+exi.
</t>
</section>
<section anchor="uri-scheme-registration" title="URI Scheme Registration">
<t>This document requests the registration of the Uniform Resource
Identifier (URI) scheme "coap". The registration request complies with
<xref target="RFC4395"/>.
<list style="hanging">
<t hangText="URI scheme name."><vspace/>
coap</t>
<t hangText="Status."><vspace/>
Permanent.</t>
<t hangText="URI scheme syntax."><vspace/>
Defined in <xref target="uri-coap"/> of &SELF;.</t>
<t hangText="URI scheme semantics."><vspace/>
The "coap" URI scheme provides a way to identify resources that are
potentially accessible over the Constrained Application Protocol
(CoAP). The resources can be located by contacting the governing
CoAP server and operated on by sending CoAP requests to the server.
This scheme can thus be compared to the "http" URI scheme
<xref target="RFC2616"/>. See <xref target="uri"/> of &SELF; for the
details of operation.</t>
<t hangText="Encoding considerations."><vspace/>
The scheme encoding conforms to the encoding rules established for
URIs in <xref target="RFC3986"/>, i.e. internationalized and reserved
characters are expressed using UTF-8-based percent-encoding.</t>
<t hangText="Applications/protocols that use this URI scheme name.">
<vspace/>
The scheme is used by CoAP endpoints to access CoAP resources.</t>
<t hangText="Interoperability considerations."><vspace/>
None.</t>
<t hangText="Security considerations."><vspace/>
See <xref target="protocol-parsing-processing-uris"/> of &SELF;.</t>
<t hangText="Contact."><vspace/>
IETF Chair <chair@ietf.org></t>
<t hangText="Author/Change controller."><vspace/>
IESG <iesg@ietf.org></t>
<t hangText="References."><vspace/>
&SELF;</t>
</list>
</t>
</section>
<section anchor="coaps-uri-scheme-registration" title="Secure URI Scheme Registration">
<t>This document requests the registration of the Uniform Resource
Identifier (URI) scheme "coaps". The registration request complies with
<xref target="RFC4395"/>.
<list style="hanging">
<t hangText="URI scheme name."><vspace/>
coaps</t>
<t hangText="Status."><vspace/>
Permanent.</t>
<t hangText="URI scheme syntax."><vspace/>
Defined in <xref target="uri-coaps"/> of &SELF;.</t>
<t hangText="URI scheme semantics."><vspace/>
The "coaps" URI scheme provides a way to identify resources that are
potentially accessible over the Constrained Application Protocol
(CoAP) using Datagram Transport Layer Security (DTLS) for transport security. The resources can be located by contacting the governing CoAP server and operated on by sending CoAP requests to the server.
This scheme can thus be compared to the "https" URI scheme
<xref target="RFC2616"/>. See <xref target="uri"/> of &SELF; for the
details of operation.</t>
<t hangText="Encoding considerations."><vspace/>
The scheme encoding conforms to the encoding rules established for
URIs in <xref target="RFC3986"/>, i.e. internationalized and reserved
characters are expressed using UTF-8-based percent-encoding.</t>
<t hangText="Applications/protocols that use this URI scheme name.">
<vspace/>
The scheme is used by CoAP endpoints to access CoAP resources using DTLS.</t>
<t hangText="Interoperability considerations."><vspace/>
None.</t>
<t hangText="Security considerations."><vspace/>
See <xref target="protocol-parsing-processing-uris"/> of &SELF;.</t>
<t hangText="Contact."><vspace/>
IETF Chair <chair@ietf.org></t>
<t hangText="Author/Change controller."><vspace/>
IESG <iesg@ietf.org></t>
<t hangText="References."><vspace/>
&SELF;</t>
</list>
</t>
</section>
<section anchor="port-registration" title="Service Name and Port Number Registration">
<t>One of the functions of CoAP is resource discovery: a CoAP
client can ask a CoAP server about the resources offered by it
(see <xref target="discovery"/>). To enable resource discovery
just based on the knowledge of an IP address, the CoAP port for
resource discovery needs to be standardized.</t>
<t>IANA has assigned the port number &PORT;
and the service name "coap", in accordance with
<xref target="RFC6335"/>.</t>
<t>Besides unicast, CoAP can be used with both multicast and anycast.
<list style="hanging">
<t hangText="Service Name."><vspace/>
coap</t>
<t hangText="Transport Protocol."><vspace/>
UDP</t>
<t hangText="Assignee."><vspace/>
IESG <iesg@ietf.org></t>
<t hangText="Contact."><vspace/>
IETF Chair <chair@ietf.org></t>
<t hangText="Description."><vspace/>
Constrained Application Protocol (CoAP)</t>
<t hangText="Reference."><vspace/>
&SELF;</t>
<t hangText="Port Number."><vspace/>
&PORT;</t>
</list>
</t>
</section>
<section anchor="secure-port-registration" title="Secure Service Name and Port Number Registration">
<t>CoAP resource discovery may also be provided using the DTLS-secured CoAP "coaps" scheme. Thus the CoAP port for
secure resource discovery needs to be standardized.</t>
<t>This document requests the assignment of the port number &PORTS;
and the service name "coaps", in accordance with
<xref target="RFC6335"/>.</t>
<t>Besides unicast, DTLS-secured CoAP can be used with anycast.
<list style="hanging">
<t hangText="Service Name."><vspace/>
coaps</t>
<t hangText="Transport Protocol."><vspace/>
UDP</t>
<t hangText="Assignee."><vspace/>
IESG <iesg@ietf.org></t>
<t hangText="Contact."><vspace/>
IETF Chair <chair@ietf.org></t>
<t hangText="Description."><vspace/>
DTLS-secured CoAP</t>
<t hangText="Reference."><vspace/>
&SELF;</t>
<t hangText="Port Number."><vspace/>
&PORTS;</t>
</list>
</t>
</section>
<section anchor="multicast-addresses" title="Multicast Address Registration">
<t><xref target="multicast"/>, "Multicast CoAP", defines the use of
multicast. This document requests the assignment of the following
multicast addresses for use by CoAP nodes:<list style="hanging">
<t hangText="IPv4">-- "All CoAP Nodes" address
&MULTICASTv4;, from the IPv4 Multicast Address Space
Registry.
As the address is used for discovery that may span beyond
a single network, it should come from the Internetwork
Control Block (224.0.1.x, RFC 5771).</t>
<t hangText="IPv6">-- "All CoAP Nodes" address
&MULTICASTv6;, from the IPv6 Multicast Address Space
Registry, in the Variable Scope Multicast Addresses
space (RFC3307). Note that there is a distinct multicast address
for each scope that interested CoAP nodes should listen to.</t>
</list></t>
<t>[The explanatory text to be removed upon allocation of the
addresses, except for the note about the distinct multicast addresses.]</t>
</section>
</section>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section title="Acknowledgements">
<t>Special thanks to Peter Bigot, Esko Dijk and Cullen Jennings
for substantial contributions to the ideas and text in the document, along
with countless detailed reviews and discussions.</t>
<t>Thanks to
Ed Beroset, Angelo P. Castellani, Gilbert Clark, Robert Cragie, Esko Dijk,
Lisa Dussealt, Thomas Fossati, Tom Herbst, Richard Kelsey, Ari Keranen,
Matthias Kovatsch, Salvatore Loreto, Kerry Lynn, Alexey Melnikov, Guido
Moritz, Petri Mutka, Colin O'Flynn, Charles Palmer, Adriano Pezzuto, Robert
Quattlebaum, Akbar Rahman, Eric Rescorla, David Ryan, Szymon Sasin, Michael
Scharf, Dale Seed, Robby Simpson, Peter van der Stok, Michael Stuber, Linyi
Tian, Gilman Tolle, Matthieu Vial
and
Alper Yegin
for helpful comments and discussions that have shaped the document.</t>
<t>Some of the text has been
lifted from the working documents of the IETF httpbis working group.</t>
</section>
</middle>
<back>
<references title="Normative References">
&RFC2045;
&RFC2046;
&RFC2616;
&RFC2119;
&RFC4279;
&RFC4303;
&RFC5996;
&RFC4309;
&RFC3023;
&RFC3676;
&RFC3602;
&RFC3629;
&RFC3986;
&RFC6347;
&RFC4395;
&RFC4835;
&RFC5147;
&RFC5198;
&RFC5226;
&RFC5234;
&RFC5246;
&RFC5280;
&RFC5785;
&RFC5952;
&RFC5988;
&RFC6066;
&I-D.ietf-core-link-format;
&I-D.ietf-tls-oob-pubkey;
&I-D.farrell-decade-ni;
</references>
<references title="Informative References">
&RFC2818;
&RFC4492;
&RFC4627;
<reference anchor="REST" target="http://www.ics.uci.edu/~fielding/pubs/dissertation/fielding_dissertation.pdf">
<front>
<title>Architectural Styles and the Design of Network-based Software Architectures</title>
<author initials="R." surname="Fielding" fullname="Roy Fielding">
<organization>University of California, Irvine</organization>
</author>
<date year="2000"/>
</front>
<seriesInfo name="Ph.D." value="Dissertation, University of California, Irvine"/>
<format type="PDF" target="http://www.ics.uci.edu/~fielding/pubs/dissertation/fielding_dissertation.pdf"/>
</reference>
&RFC3264;
&RFC3542;
&RFC6120;
&RFC4944;
&I-D.kivinen-ipsecme-ikev2-minimal;
&I-D.eggert-core-congestion-control;
&I-D.ietf-core-block;
&I-D.ietf-httpbis-p1-messaging;
<reference anchor="EUI64" target="http://standards.ieee.org/regauth/oui/tutorials/EUI64.html">
<front>
<title>GUIDELINES FOR 64-BIT GLOBAL IDENTIFIER (EUI-64) REGISTRATION AUTHORITY</title>
<author><organization/></author><date month='April' day='23' year='2010' />
</front>
</reference>
<reference anchor="EXIMIME" target="http://www.w3.org/TR/2009/CR-exi-20091208/#mediaTypeRegistration">
<front>
<title>Efficient XML Interchange (EXI) Format 1.0</title>
<author><organization/></author><date month='December' day='08' year='2009' />
</front>
</reference>
&I-D.mcgrew-tls-aes-ccm;
&I-D.mcgrew-tls-aes-ccm-ecc;
&RFC6335;
&RFC0793;
&RFC5405;
&I-D.allman-tcpm-rto-consider;
</references>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section anchor="examples" title="Examples">
<t>This section gives a number of short examples with message flows
for GET requests. These examples demonstrate the basic operation, the
operation in the presence of retransmissions, and multicast.</t>
<t><xref target="fig-example-1"/> shows a basic GET
request causing a &pb; response: The client sends a Confirmable
GET request for the resource coap://server/temperature to the server
with a Message ID of 0x7d34. The request includes one Uri-Path Option
(Delta 0 + 9 = 9, Length 11, Value "temperature"); the Token is
left at its default value (empty). This request is a total of 16
bytes long. A 2.05 (Content) response is returned in the Acknowledgement
message that acknowledges the Confirmable request, echoing both the
Message ID 0x7d34 and the (implicitly empty) Token value. The response
includes a Payload of "22.3 C" and is 10 bytes long.</t>
<figure anchor="fig-example-1" title="Confirmable request; piggy-backed response"> <!--XML2RFC is stupid-->
<artwork type="example"><![CDATA[
Client Server
| |
| |
+----->| Header: GET (T=CON, Code=1, MID=0x7d34)
| GET | Uri-Path: "temperature"
| |
| |
|<-----+ Header: 2.05 Content (T=ACK, Code=69, MID=0x7d34)
| 2.05 | Payload: "22.3 C"
| |
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | 0 | 1 | GET=1 | MID=0x7d34 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 9 | 11 | "temperature" (11 B) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | 2 | 0 | 2.05=69 | MID=0x7d34 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| "22.3 C" (6 B) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>
<xref target="fig-example-1a"/> shows a similar example, but with the
inclusion of an explicit Token Option (Delta 9 + 2 = 11, Length 1,
Value 0x20) in the request and (Delta 11 + 0 = 11) in the response,
increasing the sizes to 18 and 12 bytes, respectively.
</t>
<figure anchor="fig-example-1a" title="Confirmable request; piggy-backed response"> <!--XML2RFC is stupid-->
<artwork type="example"><![CDATA[
Client Server
| |
| |
+----->| Header: GET (T=CON, Code=1, MID=0x7d35)
| GET | Token: 0x20
| | Uri-Path: "temperature"
| |
| |
|<-----+ Header: 2.05 Content (T=ACK, Code=69, MID=0x7d35)
| 2.05 | Token: 0x20
| | Payload: "22.3 C"
| |
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | 0 | 2 | GET=1 | MID=0x7d35 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 9 | 11 | "temperature" (11 B) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 1 | 0x20 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | 2 | 1 | 2.05=69 | MID=0x7d35 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 11 | 1 | 0x20 | "22.3 C" (6 B) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
<t>In <xref target="fig-example-2"/>, the
Confirmable GET request is lost. After ACK_TIMEOUT seconds,
the client retransmits the request, resulting in a &pb; response
as in the previous example.</t>
<figure anchor="fig-example-2" title="Confirmable request (retransmitted); piggy-backed response"> <!--XML2RFC is stupid-->
<artwork type="example"><![CDATA[
Client Server
| |
| |
+----X | Header: GET (T=CON, Code=1, MID=0x7d36)
| GET | Token: 0x31
| | Uri-Path: "temperature"
TIMEOUT |
| |
+----->| Header: GET (T=CON, Code=1, MID=0x7d36)
| GET | Token: 0x31
| | Uri-Path: "temperature"
| |
| |
|<-----+ Header: 2.05 Content (T=ACK, Code=69, MID=0x7d36)
| 2.05 | Token: 0x31
| | Payload: "22.3 C"
| |
]]></artwork>
</figure>
<t>In <xref target="fig-example-3"/>, the first
Acknowledgement message from the server to the client is lost. After
ACK_TIMEOUT seconds, the client retransmits the request.</t>
<figure anchor="fig-example-3" title="Confirmable request; piggy-backed response (retransmitted)"> <!--XML2RFC is stupid-->
<artwork type="example"><![CDATA[
Client Server
| |
| |
+----->| Header: GET (T=CON, Code=1, MID=0x7d37)
| GET | Token: 0x42
| | Uri-Path: "temperature"
| |
| |
| X----+ Header: 2.05 Content (T=ACK, Code=69, MID=0x7d37)
| 2.05 | Token: 0x42
| | Payload: "22.3 C"
TIMEOUT |
| |
+----->| Header: GET (T=CON, Code=1, MID=0x7d37)
| GET | Token: 0x42
| | Uri-Path: "temperature"
| |
| |
|<-----+ Header: 2.05 Content (T=ACK, Code=69, MID=0x7d37)
| 2.05 | Token: 0x42
| | Payload: "22.3 C"
| |
]]></artwork>
</figure>
<t>In <xref target="fig-example-4"/>, the server
acknowledges the Confirmable request and sends a 2.05 (Content) response
separately in a Confirmable message. Note that the Acknowledgement
message and the Confirmable response do not necessarily arrive in the same
order as they were sent. The client acknowledges the Confirmable
response.</t>
<figure anchor="fig-example-4" title="Confirmable request; separate response"> <!--XML2RFC is stupid-->
<artwork type="example"><![CDATA[
Client Server
| |
| |
+----->| Header: GET (T=CON, Code=1, MID=0x7d38)
| GET | Token: 0x53
| | Uri-Path: "temperature"
| |
| |
|<- - -+ Header: (T=ACK, Code=0, MID=0x7d38)
| |
| |
|<-----+ Header: 2.05 Content (T=CON, Code=69, MID=0xad7b)
| 2.05 | Token: 0x53
| | Payload: "22.3 C"
| |
| |
+- - ->| Header: (T=ACK, Code=0, MID=0xad7b)
| |
]]></artwork>
</figure>
<t><xref target="fig-example-5"/> shows an example
where the client loses its state (e.g., crashes and is rebooted) right after
sending a Confirmable request,
so the &npb; response arriving some time later comes unexpected. In
this case, the client rejects the Confirmable response with a Reset
message. Note that the unexpected ACK is silently ignored.</t>
<figure anchor="fig-example-5" title="Confirmable request; separate response (unexpected)"> <!--XML2RFC is stupid-->
<artwork type="example"><![CDATA[
Client Server
| |
| |
+----->| Header: GET (T=CON, Code=1, MID=0x7d39)
| GET | Token: 0x64
| | Uri-Path: "temperature"
CRASH |
| |
|<- - -+ Header: (T=ACK, Code=0, MID=0x7d39)
| |
| |
|<-----+ Header: 2.05 Content (T=CON, Code=69, MID=0xad7c)
| 2.05 | Token: 0x64
| | Payload: "22.3 C"
| |
| |
+- - ->| Header: (T=RST, Code=0, MID=0xad7c)
| |
]]></artwork>
</figure>
<t><xref target="fig-example-6"/> shows a basic GET
request where the request and the response are non-confirmable, so both may
be lost without notice.</t>
<figure anchor="fig-example-6" title="Non-confirmable request; Non-confirmable response">
<artwork type="example"><![CDATA[
Client Server
| |
| |
+----->| Header: GET (T=NON, Code=1, MID=0x7d40)
| GET | Token: 0x75
| | Uri-Path: "temperature"
| |
| |
|<-----+ Header: 2.05 Content (T=NON, Code=69, MID=0xad7d)
| 2.05 | Token: 0x75
| | Payload: "22.3 C"
| |
]]></artwork>
</figure>
<t>In <xref target="fig-example-7"/>, the client
sends a Non-confirmable GET request to a multicast address: all nodes
in link-local scope. There are 3 servers on the link: A, B and C.
Servers A and B have a matching resource, therefore they send back a
Non-confirmable 2.05 (Content) response. The response sent by B is lost.
C does not have matching response, therefore it sends a Non-confirmable
4.04 (Not Found) response.</t>
<figure anchor="fig-example-7" title="Non-confirmable request (multicast); Non-confirmable response">
<artwork type="example"><![CDATA[
Client ff02::1 A B C
| | | | |
| | | | |
+------>| | | | Header: GET (T=NON, Code=1, MID=0x7d41)
| GET | | | | Token: 0x86
| | | | Uri-Path: "temperature"
| | | |
| | | |
|<------------+ | | Header: 2.05 (T=NON, Code=69, MID=0x60b1)
| 2.05 | | | Token: 0x86
| | | | Payload: "22.3 C"
| | | |
| | | |
| X------------+ | Header: 2.05 (T=NON, Code=69, MID=0x01a0)
| 2.05 | | | Token: 0x86
| | | | Payload: "20.9 C"
| | | |
| | | |
|<------------------+ Header: 4.04 (T=NON, Code=132, MID=0x952a)
| 4.04 | | | Token: 0x86
| | | |
]]></artwork>
</figure>
</section>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section anchor="uri-examples" title="URI Examples">
<t>The following examples demonstrate different sets of Uri options, and
the result after constructing an URI from them.
<list style="symbols">
<t>coap://[2001:db8::2:1]/
<list style="empty">
<t>Destination IP Address = [2001:db8::2:1]</t>
<t>Destination UDP Port = &PORT;</t>
</list>
</t>
<t>coap://example.net/
<list style="empty">
<t>Destination IP Address = [2001:db8::2:1]</t>
<t>Destination UDP Port = &PORT;</t>
<t>Uri-Host = "example.net"</t>
</list>
</t>
<t>coap://example.net/.well-known/core
<list style="empty">
<t>Destination IP Address = [2001:db8::2:1]</t>
<t>Destination UDP Port = &PORT;</t>
<t>Uri-Host = "example.net"</t>
<t>Uri-Path = ".well-known"</t>
<t>Uri-Path = "core"</t>
</list>
</t>
<t>coap://xn--18j4d.example/%E3%81%93%E3%82%93%E3%81%AB%E3%81%A1%E3%81%AF
<list style="empty">
<t>Destination IP Address = [2001:db8::2:1]</t>
<t>Destination UDP Port = &PORT;</t>
<t>Uri-Host = "xn--18j4d.example"</t>
<t>Uri-Path = the string composed of the Unicode characters
U+3053 U+3093 U+306b U+3061 U+306f, usually represented in UTF-8 as
E38193E38293E381ABE381A1E381AF hexadecimal</t>
</list>
</t>
<t>coap://198.51.100.1:61616//%2F//?%2F%2F&?%26
<list style="empty">
<t>Destination IP Address = 198.51.100.1</t>
<t>Destination UDP Port = 61616</t>
<t>Uri-Path = ""</t>
<t>Uri-Path = "/"</t>
<t>Uri-Path = ""</t>
<t>Uri-Path = ""</t>
<t>Uri-Query = "//"</t>
<t>Uri-Query = "?&"</t>
</list>
</t>
</list>
</t>
</section>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section title="Changelog">
<t>Changed from ietf-10 to ietf-11:
<list style="symbols">
<t>Expanded section 4.8 on Transmission Parameters, and used the
derived values defined there (#201). Changed parameter names to
be shorter and more to the point.</t>
<t>Several more small editorial changes, clarifications and improvements have been made.</t>
</list>
</t>
<t>Changed from ietf-09 to ietf-10:
<list style="symbols">
<t>Option deltas are restricted to 0 to 14; the option delta 15 is used exclusively for the end-of-options marker (#239).</t>
<t>Option numbers that are a multiple of 14 are not reserved, but are required to have an empty default value (#212).</t>
<t>Fixed misleading language that was introduced in 5.10.2 in coap-07 re Uri-Host and Uri-Port (#208).</t>
<t>Segments and arguments can have a length of zero characters (#213).</t>
<t>The Location-* options describe together describe one location. The location is a relative URI, not an "absolute path URI" (#218).</t>
<t>The value of the Location-Path Option must not be '.' or '..' (#218).</t>
<t>Added a sentence on constructing URIs from Location-* options (#231).</t>
<t>Reserved option numbers for future Location-* options (#230).</t>
<t>Fixed response codes with payload inconsistency (#233).</t>
<t>Added advice on default values for critical options (#207).</t>
<t>Clarified use of identifiers in RawPublicKey Mode Provisioning (#222).</t>
<t>Moved "Securing CoAP" out of the "Security Considerations" (#229).</t>
<t>Added "All CoAP Nodes" multicast addresses to "IANA Considerations" (#216).</t>
<t>Over 100 small editorial changes, clarifications and improvements have been made.</t>
</list>
</t>
<t>Changed from ietf-08 to ietf-09:
<list style="symbols">
<t>Improved consistency of statements about RST on NON: RST is a valid response to a NON message (#183).</t>
<t>Clarified that the protocol constants can be configured for specific application environments.</t>
<t>Added implementation note recommending piggy-backing whenever possible (#182). </t>
<t>Added a content-encoding column to the media type registry (#181).</t>
<t>Minor improvements to Appendix D.</t>
<t>Added text about multicast response suppression (#177).</t>
<t>Included the new End-of-options Marker (#176).</t>
<t>Added a reference to draft-ietf-tls-oob-pubkey and updated the RPK text accordingly.</t>
</list>
</t>
<t>Changed from ietf-07 to ietf-08:
<list style="symbols">
<t>Clarified matching rules for messages (#175)</t>
<t>Fixed a bug in Section 8.2.2 on Etags (#168)</t>
<t>Added an IP address spoofing threat analysis contribution (#167)</t>
<t>Re-focused the security section on raw public keys (#166)</t>
<t>Added an 4.06 error to Accept (#165)</t>
</list>
</t>
<t>Changed from ietf-06 to ietf-07:
<list style="symbols">
<t>application/link-format added to Media types registration (#160)</t>
<t>Moved content-type attribute to the document from link-format.</t>
<t>Added coaps scheme and DTLS-secured CoAP default port (#154)</t>
<t>Allowed 0-length Content-type options (#150)</t>
<t>Added congestion control recommendations (#153)</t>
<t>Improved text on PUT/POST response payloads (#149)</t>
<t>Added an Accept option for content-negotiation (#163)</t>
<t>Added If-Match and If-None-Match options (#155)</t>
<t>Improved Token Option explanation (#147)</t>
<t>Clarified mandatory to implement security (#156)</t>
<t>Added first come first server policy for 2-byte Media type codes (#161)</t>
<t>Clarify matching rules for messages and tokens (#151)</t>
<t>Changed OPTIONS and TRACE to always return 501 in HTTP-CoAP mapping (#164)</t>
</list>
</t>
<t>Changed from ietf-05 to ietf-06:
<list style="symbols">
<t>HTTP mapping section improved with the minimal protocol standard text for CoAP-HTTP
and HTTP-CoAP forward proxying (#137).</t>
<t>Eradicated percent-encoding by including one Uri-Query Option per &-delimited argument in a query.</t>
<t>Allowed RST message in reply to a NON message with unexpected token (#135).</t>
<t>Cache Invalidation only happens upon successful responses (#134).</t>
<t>50% jitter added to the initial retransmit timer (#142).</t>
<t>DTLS cipher suites aligned with ZigBee IP, DTLS clarified as default CoAP security mechanism (#138, #139)</t>
<t>Added a minimal reference to draft-kivinen-ipsecme-ikev2-minimal (#140).</t>
<t>Clarified the comparison of UTF-8s (#136).</t>
<t>Minimized the initial media type registry (#101).</t>
</list>
</t>
<t>Changed from ietf-04 to ietf-05:
<list style="symbols">
<t>Renamed Immediate into &Pb; and Deferred into &Npb; --
should finally end the confusion on what this is about.</t>
<t>GET requests now return a 2.05 (Content) response instead of 2.00 (OK) response (#104).</t>
<t>Added text to allow 2.02 (Deleted) responses in reply to POST requests (#105).</t>
<t>Improved message deduplication rules (#106).</t>
<t>Section added on message size implementation considerations (#103).</t>
<t>Clarification made on human readable error payloads (#109).</t>
<t>Definition of CoAP methods improved (#108).</t>
<t>Max-Age removed from requests (#107).</t>
<t>Clarified uniqueness of tokens (#112).</t>
<t>Location-Query Option added (#113).</t>
<t>ETag length set to 1-8 bytes (#123).</t>
<t>Clarified relation between elective/critical and option numbers (#110).</t>
<t>Defined when to update Version header field (#111).</t>
<t>URI scheme registration improved (#102).</t>
<t>Added review guidelines for new CoAP codes and numbers.</t>
</list>
</t>
<t>Changes from ietf-03 to ietf-04:
<list style="symbols">
<t>Major document reorganization (#51, #63, #71, #81).</t>
<t>Max-age length set to 0-4 bytes (#30).</t>
<t>Added variable unsigned integer definition (#31).</t>
<t>Clarification made on human readable error payloads (#50).</t>
<t>Definition of POST improved (#52).</t>
<t>Token length changed to 0-8 bytes (#53).</t>
<t>Section added on multiplexing CoAP, DTLS and STUN (#56).</t>
<t>Added cross-protocol attack considerations (#61).</t>
<t>Used new Immediate/Deferred response definitions (#73).</t>
<t>Improved request/response matching rules (#74).</t>
<t>Removed unnecessary media types and added recommendations for their use in M2M (#76).</t>
<t>Response codes changed to base 32 coding, new Y.XX naming (#77).</t>
<t>References updated as per AD review (#79).</t>
<t>IANA section completed (#80).</t>
<t>Proxy-Uri Option added to disambiguate between proxy and non-proxy requests (#82).</t>
<t>Added text on critical options in cached states (#83).</t>
<t>HTTP mapping sections improved (#88).</t>
<t>Added text on reverse proxies (#72).</t>
<t>Some security text on multicast added (#54).</t>
<t>Trust model text added to introduction (#58, #60).</t>
<t>AES-CCM vs. AES-CCB text added (#55).</t>
<t>Text added about device capabilities (#59).</t>
<t>DTLS section improvements (#87).</t>
<t>Caching semantics aligned with RFC2616 (#78).</t>
<t>Uri-Path Option split into multiple path segments.</t>
<t>MAX_RETRANSMIT changed to 4 to adjust for RESPONSE_TIME = 2.</t>
</list>
</t>
<t>Changes from ietf-02 to ietf-03:
<list style="symbols">
<t>Token Option and related use in asynchronous requests added (#25).</t>
<t>CoAP specific error codes added (#26).</t>
<t>Erroring out on unknown critical options changed to a MUST (#27).</t>
<t>Uri-Query Option added.</t>
<t>Terminology and definitions of URIs improved. </t>
<t>Security section completed (#22).</t>
</list>
</t>
<t>Changes from ietf-01 to ietf-02:
<list style="symbols">
<t>Sending an error on a critical option clarified (#18).</t>
<t>Clarification on behavior of PUT and idempotent operations (#19).</t>
<t>Use of Uri-Authority clarified along with server processing rules; Uri-Scheme Option removed (#20, #23).</t>
<t>Resource discovery section removed to a separate CoRE Link Format draft (#21).</t>
<t>Initial security section outline added.</t>
</list>
</t>
<t>Changes from ietf-00 to ietf-01:
<list style="symbols">
<t>New cleaner transaction message model and header (#5).</t>
<t>Removed subscription while being designed (#1).</t>
<t>Section 2 re-written (#3).</t>
<t>Text added about use of short URIs (#4).</t>
<t>Improved header option scheme (#5, #14).</t>
<t>Date option removed whiled being designed (#6).</t>
<t>New text for CoAP default port (#7).</t>
<t>Completed proxying section (#8).</t>
<t>Completed resource discovery section (#9).</t>
<t>Completed HTTP mapping section (#10).</t>
<t>Several new examples added (#11).</t>
<t>URI split into 3 options (#12).</t>
<t>MIME type defined for link-format (#13, #16).</t>
<t>New text on maximum message size (#15).</t>
<t>Location Option added.</t>
</list>
</t>
<t>Changes from shelby-01 to ietf-00:
<list style="symbols">
<t>Removed the TCP binding section, left open for the future.</t>
<t>Fixed a bug in the example.</t>
<t>Marked current Sub/Notify as (Experimental) while under WG discussion.</t>
<t>Fixed maximum datagram size to 1280 for both IPv4 and IPv6 (for CoAP-CoAP proxying to work).</t>
<t>Temporarily removed the Magic Byte header as TCP is no longer included as a binding.</t>
<t>Removed the Uri-code Option as different URI encoding schemes are being discussed.</t>
<t>Changed the rel= field to desc= for resource discovery.</t>
<t>Changed the maximum message size to 1024 bytes to allow for IP/UDP headers.</t>
<t>Made the URI slash optimization and method idempotence MUSTs</t>
<t>Minor editing and bug fixing.</t>
</list>
</t>
<t>Changes from shelby-00 to shelby-01:
<list style="symbols">
<t>Unified the message header and added a notify message type.</t>
<t>Renamed methods with HTTP names and removed the NOTIFY method.</t>
<t>Added a number of options field to the header.</t>
<t>Combines the Option Type and Length into an 8-bit field.</t>
<t>Added the magic byte header.</t>
<t>Added new ETag Option.</t>
<t>Added new Date Option.</t>
<t>Added new Subscription Option.</t>
<t>Completed the HTTP Code - CoAP Code mapping table appendix.</t>
<t>Completed the Content-type Identifier appendix and tables.</t>
<t>Added more simplifications for URI support.</t>
<t>Initial subscription and discovery sections.</t>
<t>A Flag requirements simplified.</t>
</list>
</t>
</section>
</back>
</rfc>
<!-- LocalWords: CoAP IP URI URIs CoRE datagram confirmable Confirmable TLV DTLS
-->
<!-- LocalWords: MTU IPsec microcontrollers LoWPAN kbit RESTful IPv
-->
<!-- LocalWords: unicast ACK MID RST cacheable ETag IEEE AES CCM
-->
<!-- LocalWords: NATs TLS PSK requesters proxying proxied backoff
-->
<!-- LocalWords: subcomponent acknowledgement acknowledgements SCTP
-->
<!-- LocalWords: retransmission responder retransmits RETRANSMIT
-->
<!-- LocalWords: retransmitted retransmissions retransmit cleartext
-->
<!-- LocalWords: scalability wildcards parsers encodable Uri UTF
-->
<!-- LocalWords: parameterizing metadata parameterize fenceposts
-->
<!-- LocalWords: retransmitting optimizations cacheability NoSec
-->
<!-- LocalWords: revalidating encodings discoverable demultiplexing
-->
<!-- LocalWords: datagrams unescaped API APIs firewalling DNS IANA
-->
<!-- LocalWords: validatable fenceposting Interoperability Kidekuja
-->
<!-- LocalWords: Universitaet Sensinode Vuokatti Hartke TZI Bormann
-->
<!-- LocalWords: Postfach Carsten lossy multicast UDP coap Zach Acknowledgement
-->
<!-- LocalWords: TCP templated CoAP's checksum coaps ABNF namespace
-->
<!-- LocalWords: revalidate RawPublicKey nonces SNI speakable XMPP
-->
<!-- LocalWords: barcode SubjectAltName RECVPKTINFO Posix lookup
-->
<!-- LocalWords: PreSharedKey wakeup IETF Internetwork
-->
| PAFTECH AB 2003-2026 | 2026-04-23 03:38:02 |