One document matched: draft-ietf-sip-outbound-13.xml
<?xml version="1.0" encoding="US-ASCII"?>
<!DOCTYPE rfc SYSTEM "http://xml.resource.org/authoring/rfc2629.dtd" [
<!ENTITY rfc2119 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2119.xml">
<!ENTITY rfc2141 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2141.xml">
<!ENTITY rfc2506 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2506.xml">
<!ENTITY rfc3261 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3261.xml">
<!ENTITY rfc3263 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3263.xml">
<!ENTITY rfc3327 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3327.xml">
<!ENTITY rfc3489 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3489.xml">
<!ENTITY rfc3581 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3581.xml">
<!ENTITY rfc3629 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3629.xml">
<!ENTITY rfc3840 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3840.xml">
<!ENTITY rfc3841 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3841.xml">
<!ENTITY rfc3968 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3968.xml">
<!ENTITY rfc3969 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3969.xml">
<!ENTITY rfc3986 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3986.xml">
<!ENTITY rfc4122 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.4122.xml">
<!ENTITY rfc5234 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.5234.xml">
<!ENTITY I-D.ietf-behave-rfc3489bis SYSTEM "http://xml.resource.org/public/rfc/bibxml3/reference.I-D.ietf-behave-rfc3489bis.xml">
<!ENTITY rfc0768 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.0768.xml">
<!ENTITY rfc0793 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.0793.xml">
<!ENTITY rfc2104 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2104.xml">
<!ENTITY rfc2782 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.2782.xml">
<!ENTITY rfc3320 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.3320.xml">
<!ENTITY rfc4346 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.4346.xml">
<!ENTITY rfc4648 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.4648.xml">
<!ENTITY rfc4966 SYSTEM "http://xml.resource.org/public/rfc/bibxml/reference.RFC.4966.xml">
<!ENTITY I-D.ietf-sip-gruu SYSTEM "http://xml.resource.org/public/rfc/bibxml3/reference.I-D.ietf-sip-gruu.xml">
<!ENTITY I-D.ietf-sipping-config-framework SYSTEM "http://xml.resource.org/public/rfc/bibxml3/reference.I-D.ietf-sipping-config-framework.xml">
<!ENTITY I-D.ietf-sipping-nat-scenarios SYSTEM "http://xml.resource.org/public/rfc/bibxml3/reference.I-D.ietf-sipping-nat-scenarios.xml">
]>
<?xml-stylesheet type='text/xsl' href='rfc2629.xslt' ?>
<?rfc toc="yes" ?>
<?rfc symrefs="yes"?>
<?rfc iprnotified="no" ?>
<?rfc strict="yes" ?>
<?rfc compact="yes" ?>
<?rfc sortrefs="yes" ?>
<?rfc colonspace='yes' ?>
<rfc category="std" docName="draft-ietf-sip-outbound-13" ipr="full3978"
updates="3261,3327">
<front>
<title abbrev="Client Initiated Connections in SIP">Managing Client
Initiated Connections in the Session Initiation Protocol (SIP)</title>
<author fullname="Cullen Jennings" initials="C." role="editor"
surname="Jennings">
<organization>Cisco Systems</organization>
<address>
<postal>
<street>170 West Tasman Drive</street>
<street>Mailstop SJC-21/2</street>
<city>San Jose</city>
<region>CA</region>
<code>95134</code>
<country>USA</country>
</postal>
<phone>+1 408 902-3341</phone>
<email>fluffy@cisco.com</email>
</address>
</author>
<author fullname="Rohan Mahy" initials="R." role="editor" surname="Mahy">
<organization>Plantronics</organization>
<address>
<postal>
<street>345 Encincal St</street>
<city>Santa Cruz</city>
<region>CA</region>
<code>95060</code>
<country>USA</country>
</postal>
<email>rohan@ekabal.com</email>
</address>
</author>
<date day="21" month="March" year="2008" />
<abstract>
<t>The Session Initiation Protocol (SIP) allows proxy servers to
initiate TCP connections and send asynchronous UDP datagrams to User
Agents in order to deliver requests. However, many practical
considerations, such as the existence of firewalls and Network Address
Translators (NATs), prevent servers from connecting to User Agents in
this way. This specification defines behaviors for User Agents,
registrars and proxy servers that allow requests to be delivered on
existing connections established by the User Agent. It also defines keep
alive behaviors needed to keep NAT bindings open and specifies the usage
of multiple connections from the User Agent to its Registrar.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>There are many environments for <xref target="RFC3261">SIP</xref>
deployments in which the User Agent (UA) can form a connection to a
Registrar or Proxy but in which connections in the reverse direction to
the UA are not possible. This can happen for several reasons, but the
most likely is a NAT or a firewall in between the SIP UA and the proxy.
Many such devices will only allow outgoing connections. This
specification allows a SIP User Agent behind such a firewall or NAT to
receive inbound traffic associated with registrations or dialogs that it
initiates.</t>
<t>Most IP phones and personal computers get their network
configurations dynamically via a protocol such as DHCP (Dynamic Host
Configuration Protocol). These systems typically do not have a useful
name in the Domain Name System (DNS), and they almost never have a
long-term, stable DNS name that is appropriate for use in the
subjectAltName of a certificate, as required by <xref
target="RFC3261"></xref>. However, these systems can still act as a
Transport Layer Security (TLS) <xref target="RFC4346"></xref> client and
form connections to a proxy or registrar which authenticates with a
server certificate. The server can authenticate the UA using a shared
secret in a digest challenge (as defined in Section 22 of RFC 3261) over
that TLS connection.</t>
<t>The key idea of this specification is that when a UA sends a REGISTER
or a dialog-forming request, the proxy can later use this same network
"flow"--whether this is a bidirectional stream of UDP datagrams, a TCP
connection, or an analogous concept in another transport protocol--to
forward any incoming requests that need to go to this UA in the context
of the registration or dialog.</t>
<t>For a UA to receive incoming requests, the UA has to connect to a
server. Since the server can't connect to the UA, the UA has to make
sure that a flow is always active. This requires the UA to detect when a
flow fails. Since such detection takes time and leaves a window of
opportunity for missed incoming requests, this mechanism allows the UA
to register over multiple flows at the same time. This specification
also defines multiple keep alive schemes. The keep alive mechanism is
used to keep NAT bindings fresh, and to allow the UA to detect when a
flow has failed.</t>
</section>
<section title="Conventions and 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"></xref>.</t>
<section title="Definitions">
<t><list style="hanging">
<t hangText="Authoritative Proxy:">A proxy that handles
non-REGISTER requests for a specific Address-of-Record (AOR),
performs the logical Location Server lookup described in RFC 3261,
and forwards those requests to specific Contact URIs. (In RFC
3261, the role which is authoritative for REGISTER requests for a
specific AOR is a Registration Server.)</t>
<t hangText="Edge Proxy:">An Edge Proxy is any proxy that is
located topologically between the registering User Agent and the
Authoritative Proxy. The "first" edge proxy refers to the first
edge proxy encountered when a UA sends a request.</t>
<t hangText="Flow:">A Flow is a network protocol layer (layer 4)
association between two hosts that is represented by the network
address and port number of both ends and by the protocol. For TCP,
a flow is equivalent to a TCP connection. For UDP a flow is a
bidirectional stream of datagrams between a single pair of IP
addresses and ports of both peers. With TCP, a flow often has a
one to one correspondence with a single file descriptor in the
operating system.</t>
<t hangText="Flow Token:">An identifier which uniquely identifies
a flow which can be included in a SIP URI (Uniform Resource
Identifier).</t>
<t hangText="reg-id:">This refers to the value of a new header
field parameter value for the Contact header field. When a UA
registers multiple times, each concurrent registration gets a
unique reg-id value.</t>
<t hangText="instance-id:">This specification uses the word
instance-id to refer to the value of the "sip.instance" media
feature tag in the Contact header field. This is a Uniform
Resource Name (URN) that uniquely identifies this specific UA
instance.</t>
<t hangText="'ob' Parameter:">The 'ob' parameter is a SIP URI
parameter which has different meaning depending on context. In a
Path header field value it is used by the first edge proxy to
indicate that a flow token was added to the URI. In a Contact or
Route header field value it indicates that the UA would like other
requests in the same dialog routed over the same flow.</t>
<t hangText="outbound-proxy-set:">A set of SIP URIs (Uniform
Resource Identifiers) that represents each of the outbound proxies
(often Edge Proxies) with which the UA will attempt to maintain a
direct flow. The first URI in the set is often referred to as the
primary outbound proxy and the second as the secondary outbound
proxy. There is no difference between any of the URIs in this set,
nor does the primary/secondary terminology imply that one is
preferred over the other.</t>
</list></t>
</section>
</section>
<section title="Overview">
<t>The mechanisms defined in this document are useful in several
scenarios discussed below, including the simple co-located registrar and
proxy, a User Agent desiring multiple connections to a resource (for
redundancy, for example), and a system that uses Edge Proxies.</t>
<t>This entire section is non-normative.</t>
<section title="Summary of Mechanism">
<t>The overall approach is fairly simple. Each UA has a unique
instance-id that stays the same for this UA even if the UA reboots or
is power cycled. Each UA can register multiple times over different
flows for the same SIP Address of Record (AOR) to achieve high
reliability. Each registration includes the instance-id for the UA and
a reg-id label that is different for each flow. The registrar can use
the instance-id to recognize that two different registrations both
correspond to the same UA. The registrar can use the reg-id label to
recognize whether a UA is creating a new flow or refreshing or
replacing an old one, possibly after a reboot or a network
failure.</t>
<t>When a proxy goes to route a message to a UA for which it has a
binding, it can use any one of the flows on which a successful
registration has been completed. A failure to deliver a request on a
particular flow can be tried again on an alternate flow. Proxies can
determine which flows go to the same UA by comparing the instance-id.
Proxies can tell that a flow replaces a previously abandoned flow by
looking at the reg-id.</t>
<t>When sending a dialog-forming request, a UA can also ask its first
edge proxy to route subsequent requests in that dialog over the same
flow. This is necessary whether the UA has registered or not.</t>
<t>UAs use a simple periodic message as a keep alive mechanism to keep
their flow to the proxy or registrar alive. For connection oriented
transports such as TCP this is based on carriage-return and line-feed
sequences (CRLF), while for transports that are not connection
oriented this is accomplished by using a SIP-specific usage profile of
<xref target="I-D.ietf-behave-rfc3489bis">STUN (Session Traversal
Utilities for NAT)</xref>.</t>
</section>
<section anchor="example-single" title="Single Registrar and UA">
<t>In the topology shown below, a single server is acting as both a
registrar and proxy.</t>
<figure>
<artwork><![CDATA[
+-----------+
| Registrar |
| Proxy |
+-----+-----+
|
|
+----+--+
| User |
| Agent |
+-------+
]]></artwork>
</figure>
<t>User Agents which form only a single flow continue to register
normally but include the instance-id as described in <xref
target="section-instance"></xref>. The UA also includes a reg-id
parameter which is used to allow the registrar to detect and avoid
keeping invalid contacts when a UA reboots or reconnects after its old
connection has failed for some reason.</t>
<t>For clarity, here is an example. Bob's UA creates a new TCP flow to
the registrar and sends the following REGISTER request.</t>
<figure>
<artwork><![CDATA[
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/TCP 192.168.0.2;branch=z9hG4bK-bad0ce-11-1036
Max-Forwards: 70
From: Bob <sip:bob@example.com>;tag=d879h76
To: Bob <sip:bob@example.com>
Call-ID: 8921348ju72je840.204
CSeq: 1 REGISTER
Supported: path, outbound
Contact: <sip:line1@192.168.0.2;transport=tcp>; reg-id=1;
;+sip.instance="<urn:uuid:00000000-0000-1000-8000-000A95A0E128>"
Content-Length: 0
]]></artwork>
</figure>
<t>The registrar challenges this registration to authenticate Bob.
When the registrar adds an entry for this contact under the AOR for
Bob, the registrar also keeps track of the connection over which it
received this registration.</t>
<t>The registrar saves the instance-id
("urn:uuid:00000000-0000-1000-8000-000A95A0E128") and reg-id ("1")
along with the rest of the Contact header field. If the instance-id
and reg-id are the same as a previous registration for the same AOR,
the registrar replaces the old Contact URI and flow information. This
allows a UA that has rebooted to replace its previous registration for
each flow with minimal impact on overall system load.</t>
<t>When Alice sends a request to Bob, his authoritative proxy selects
the target set. The proxy forwards the request to elements in the
target set based on the proxy's policy. The proxy looks at the target
set and uses the instance-id to understand if two targets both end up
routing to the same UA. When the proxy goes to forward a request to a
given target, it looks and finds the flows over which it received the
registration. The proxy then forwards the request over an existing
flow, instead of resolving the Contact URI using the procedures in
<xref target="RFC3263"> </xref> and trying to form a new flow to that
contact.</t>
<t>As described in the next section, if the proxy has multiple flows
that all go to this UA, the proxy can choose any one of the
registration bindings for this AOR that has the same instance-id as
the selected UA.</t>
</section>
<section anchor="multiple"
title="Multiple Connections from a User Agent">
<t>There are various ways to deploy SIP to build a reliable and
scalable system. This section discusses one such design that is
possible with the mechanisms in this specification. Other designs are
also possible.</t>
<t>In the example system below, the logical outbound proxy/registrar
for the domain is running on two hosts that share the appropriate
state and can both provide registrar and outbound proxy functionality
for the domain. The UA will form connections to two of the physical
hosts that can perform the authoritative proxy/registrar function for
the domain. Reliability is achieved by having the UA form two TCP
connections to the domain.</t>
<figure>
<artwork><![CDATA[
+-------------------+
| Domain |
| Logical Proxy/Reg |
| |
|+-----+ +-----+|
||Host1| |Host2||
|+-----+ +-----+|
+---\------------/--+
\ /
\ /
\ /
\ /
+------+
| User |
| Agent|
+------+
]]></artwork>
</figure>
<t>The UA is configured with multiple outbound proxy registration
URIs. These URIs are configured into the UA through whatever the
normal mechanism is to configure the proxy address and AOR in the UA.
If the AOR is alice@example.com, the outbound-proxy-set might look
something like "sip:primary.example.com" and
"sip:secondary.example.com". Note that each URI in the
outbound-proxy-set could resolve to several different physical hosts.
The administrative domain that created these URIs should ensure that
the two URIs resolve to separate hosts. These URIs are handled
according to normal SIP processing rules, so mechanisms like
<xref target="RFC2782">DNS SRV</xref> can
be used to do load balancing across a proxy farm. The
approach in this document does not prevent future extensions, such
as <xref target="I-D.ietf-sipping-config-framework">the SIP UA
configuration framework</xref>, from adding other ways for a User
Agent to discover its outbound-proxy-set.</t>
<t>The domain also needs to ensure that a request for the UA sent to
host1 or host2 is then sent across the appropriate flow to the UA. The
domain might choose to use the Path header approach (as described in
the next section) to store this internal routing information on host1
or host2.</t>
<t>When a single server fails, all the UAs that have a flow through it
will detect a flow failure and try to reconnect. This can cause large
loads on the server. When large numbers of hosts reconnect nearly
simultaneously, this is referred to as the avalanche restart problem,
and is further discussed in <xref target="recovery"></xref>. The
multiple flows to many servers help reduce the load caused by the
avalanche restart. If a UA has multiple flows, and one of the servers
fails, the UA delays a recommended amount of time before trying to form a new
connection to replace the flow to the server that failed. By spreading
out the time used for all the UAs to reconnect to a server, the load
on the server farm is reduced.</t>
<t>Scalability is achieved by using <xref target="RFC2782">DNS
SRV</xref> to load balance the primary connection across a set of
machines that can service the primary connection, and also using DNS
SRV to load balance across a separate set of machines that can service
the secondary connection. The deployment here requires that DNS is
configured with one entry that resolves to all the primary hosts and
another entry that resolves to all the secondary hosts. While this
introduces additional DNS configuration, the approach works and
requires no additional SIP extensions.</t>
<t>Another motivation for maintaining multiple flows between the UA
and its registrar is related to multihomed UAs. Such UAs can benefit
from multiple connections from different interfaces to protect against
the failure of an individual access link.</t>
</section>
<section title="Edge Proxies">
<t>Some SIP deployments use edge proxies such that the UA sends the
REGISTER to an Edge Proxy that then forwards the REGISTER to the
Registrar. There could be a NAT or firewall between the UA and the
Edge Proxy.</t>
<figure>
<artwork><![CDATA[ +---------+
|Registrar|
|Proxy |
+---------+
/ \
/ \
/ \
+-----+ +-----+
|Edge1| |Edge2|
+-----+ +-----+
\ /
\ /
----------------------------NAT/FW
\ /
\ /
+------+
|User |
|Agent |
+------+
]]></artwork>
</figure>
<t>The Edge Proxy includes a <xref target="RFC3327">Path header</xref>
so that when the registrar later forwards a request to this UA, the
request is routed through the Edge Proxy.</t>
<t>These systems can use effectively the same mechanism as described
in the previous sections but need to use the Path header. When the
Edge Proxy receives a registration, it needs to create an identifier
value that is unique to this flow (and not a subsequent flow with the
same addresses) and put this identifier in the Path header URI. This
identifier has two purposes. First, it allows the Edge Proxy to map
future requests back to the correct flow. Second, because the
identifier will only be returned if the user authenticates with the
registrar successfully, it allows the Edge Proxy to indirectly check
the user's authentication information via the registrar. The
identifier is placed in the user portion of a loose route in the Path
header. If the registration succeeds, the Edge Proxy needs to map
future requests that are routed to the identifier value from the Path
header, to the associated flow.</t>
<t>The term Edge Proxy is often used to refer to deployments where the
Edge Proxy is in the same administrative domain as the Registrar.
However, in this specification we use the term to refer to any proxy
between the UA and the Registrar. For example the Edge Proxy may be
inside an enterprise that requires its use and the registrar could be
from a service provider with no relationship to the enterprise.
Regardless if they are in the same administrative domain, this
specification requires that Registrars and Edge proxies support the
Path header mechanism in <xref target="RFC3327"></xref>.</t>
</section>
<section title="Keep alive Technique">
<t>This document describes two keep alive mechanisms: a CRLF keep
alive and a STUN keep alive. Each of these mechanisms uses a
client-to-server "ping" keep alive and a corresponding
server-to-client "pong" message. This ping-pong sequence allows the
client, and optionally the server, to tell if its flow is still active
and useful for SIP traffic. The server responds to pings by sending
pongs. If the client does not receive a pong in response to its ping,
it declares the flow dead and opens a new flow in its place.</t>
<t>This document also suggests timer values for these client
keep alive mechanisms. These timer values were chosen to keep most NAT
and firewall bindings open, to detect unresponsive servers within 2
minutes, and to prevent the avalanche restart problem. However, the
client may choose different timer values to suit its needs, for
example to optimize battery life. In some environments, the server can
also keep track of the time since a ping was received over a flow to
guess the likelihood that the flow is still useful for delivering SIP
messages.</t>
<t>When the UA detects that a flow has failed or that the flow
definition has changed, the UA needs to re-register and will use the
back-off mechanism described in <xref target="recovery"></xref> to
provide congestion relief when a large number of agents simultaneously
reboot.</t>
<t>A keep alive mechanism needs to keep NAT bindings refreshed; for
connections, it also needs to detect failure of a connection; and for
connectionless transports, it needs to detect flow failures including
changes to the NAT public mapping. For connection oriented transports
such as TCP <xref target="RFC0793"></xref> and SCTP <xref
target="RFC4966"></xref>, this specification describes a keep alive
approach based on sending CRLFs. For connectionless transport, such as
UDP <xref target="RFC0768"></xref>, this specification describes using
<xref target="I-D.ietf-behave-rfc3489bis">STUN</xref> over the same
flow as the SIP traffic to perform the keepalive.</t>
<t>UAs and Proxies are also free to use native transport keep alives,
however the application may not be able to set these timers on a
per-connection basis, and the server certainly cannot make any
assumption about what values are used. Use of native transport keep
alives is outside the scope of this document.</t>
<section title="CRLF Keep alive Technique">
<t>This approach can only be used with connection-oriented
transports such as TCP or SCTP. The client periodically sends a
double-CRLF (the "ping") then waits to receive a single CRLF (the
"pong"). If the client does not receive a "pong" within an
appropriate amount of time, it considers the flow failed.</t>
<t><list>
<t>Sending a CRLF over a connection-oriented transport is
backwards compatible (because of requirements in Section 7.5 of
RFC 3261), but only implementations which support this
specification will respond to a "ping" with a "pong".</t>
</list></t>
</section>
<section title="STUN Keep alive Technique">
<t>This approach can only be used for connection-less transports,
such as UDP.</t>
<t>For connection-less transports, a flow definition could change
because a NAT device in the network path reboots and the resulting
public IP address or port mapping for the UA changes. To detect
this, STUN requests are sent over the same flow that is being used
for the SIP traffic. The proxy or registrar acts as a <xref
target="I-D.ietf-behave-rfc3489bis">Session Traversal Utilities for
NAT (STUN)</xref> server on the SIP signaling port.</t>
<t><list>
<t>Note: The STUN mechanism is very robust and allows the
detection of a changed IP address. Many other options were
considered, but the SIP Working Group selected the STUN-based
approach. Approaches using SIP requests were abandoned because
many believed that good performance and full backwards
compatibility using this method were mutually exclusive.</t>
</list></t>
</section>
</section>
</section>
<section anchor="mech-ua" title="User Agent Mechanisms">
<section anchor="section-instance" title="Instance ID Creation">
<t>Each UA MUST have an Instance Identifier <xref
target="RFC2141">Uniform Resource Name (URN)</xref> that uniquely
identifies the device. Usage of a URN provides a persistent and unique
name for the UA instance. It also provides an easy way to guarantee
uniqueness within the AOR. This URN MUST be persistent across power
cycles of the device. The Instance ID MUST NOT change as the device
moves from one network to another.</t>
<t>A UA SHOULD create a UUID URN <xref target="RFC4122"></xref> as its
instance-id. The UUID URN allows for non-centralized computation of a
URN based on time, unique names (such as a MAC address), or a random
number generator.</t>
<t><list style="empty">
<t>A device like a soft-phone, when first installed, can generate
a <xref target="RFC4122">UUID</xref> and then save this in
persistent storage for all future use. For a device such as a hard
phone, which will only ever have a single SIP UA present, the UUID
can include the MAC address and be generated at any time because
it is guaranteed that no other UUID is being generated at the same
time on that physical device. This means the value of the time
component of the UUID can be arbitrarily selected to be any time
less than the time when the device was manufactured. A time of 0
(as shown in the example in <xref target="example-single"></xref>)
is perfectly legal as long as the device knows no other UUIDs were
generated at this time on this device.</t>
</list></t>
<t>If a URN scheme other than UUID is used, the UA MUST only use URNs
for which an IETF consensus RFC defines how the specific URN needs to
be constructed and used in the sip.instance Contact parameter for
outbound behavior.</t>
<t>To convey its instance-id in both requests and responses, the UA
includes a "sip.instance" media feature tag as a UA characteristic
<xref target="RFC3840"></xref> . As described in <xref
target="RFC3840"></xref>, this media feature tag will be encoded in
the Contact header field as the "+sip.instance" Contact header field
parameter. One case where a UA may not want to include the
sip.instance media feature tag at all is when it is making an
anonymous request or some other privacy concern requires that the UA
not reveal its identity.</t>
<t><list style="empty">
<t><xref target="RFC3840"></xref> defines equality rules for
callee capabilities parameters, and according to that
specification, the "sip.instance" media feature tag will be
compared by case-sensitive string comparison. This means that the
URN will be encapsulated by angle brackets ("<" and ">")
when it is placed within the quoted string value of the
+sip.instance Contact header field parameter. The case-sensitive
matching rules apply only to the generic usages defined in the
<xref target="RFC3841">callee capabilities</xref> and the <xref
target="RFC3841">caller preferences</xref> specifications. When
the instance ID is used in this specification, it is effectively
"extracted" from the value in the "sip.instance" media feature
tag. Thus, equality comparisons are performed using the rules for
URN equality that are specific to the scheme in the URN. If the
element performing the comparisons does not understand the URN
scheme, it performs the comparisons using the lexical equality
rules defined in <xref target="RFC2141"></xref>. Lexical equality
could result in two URNs being considered unequal when they are
actually equal. In this specific usage of URNs, the only element
which provides the URN is the SIP UA instance identified by that
URN. As a result, the UA instance MUST provide lexically
equivalent URNs in each registration it generates. This is likely
to be normal behavior in any case; clients are not likely to
modify the value of the instance ID so that it remains
functionally equivalent yet lexicographically different from
previous registrations.</t>
</list></t>
</section>
<section anchor="reg" title="Registrations">
<section title="Initial Registrations">
<t>At configuration time, UAs obtain one or more SIP URIs
representing the default outbound-proxy-set. This specification
assumes the set is determined via any of a number of configuration
mechanisms, and future specifications can define additional
mechanisms such as using DNS to discover this set. How the UA is
configured is outside the scope of this specification. However, a UA
MUST support sets with at least two outbound proxy URIs and SHOULD
support sets with up to four URIs.</t>
<t>For each outbound proxy URI in the set, the UAC SHOULD send a
REGISTER request using this URI as the
default outbound proxy. (The UA could limit the number of flows
formed to conserve battery power, for example). UAs that support
this specification MUST include the outbound option tag in a
Supported header field in a REGISTER request. Each of these REGISTER
requests will use a unique Call-ID. Forming the route set for the
request is outside the scope of this document, but typically results
in sending the REGISTER such that the topmost Route header field
contains a loose route to the outbound proxy URI.</t>
<t>Registration requests, other than those described in <xref
target="third-party-reg"></xref>, MUST include an instance-id media
feature tag as specified in <xref
target="section-instance"></xref>.</t>
<t>These registration requests include a distinct reg-id
parameter in the Contact header field. Each one of these
registrations will form a new flow from the UA to the proxy. The
sequence of reg-id values does not have to be sequential but MUST be
exactly the same sequence of reg-id values each time the UA instance
power cycles or reboots so that the reg-id values will collide with
the previously used reg-id values. This is so the registrar can
replace the older registrations.</t>
<t><list>
<t>The UAC can situationally decide whether to request outbound
behavior by including or omitting the 'reg-id' parameter. For
example, imagine the outbound-proxy-set contains two proxies in
different domains, EP1 and EP2. If an outbound-style
registration succeeded for a flow through EP1, the UA might
decide to include 'outbound' in its Require header field when
registering with EP2, in order to insure consistency. Similarly,
if the registration through EP1 did not support outbound, the UA
might not register with EP2 at all.</t>
</list></t>
<t>The UAC MUST indicate that it supports the <xref
target="RFC3327">Path header</xref> mechanism, by including the
'path' option-tag in a Supported header field value in its REGISTER
requests. Other than optionally examining the Path vector in the
response, this is all that is required of the UAC to support
Path.</t>
<t>The UAC examines successful registration responses for the
presence of an 'outbound' option-tag in a Require header field
value. Presence of this option-tag indicates that the registrar is
compliant with this specification, and that any edge proxies which
needed to participate are also compliant. If the registrar did not
support outbound, the UA may have unintentionally registered an
unroutable contact. It is the responsiblity of the UA to remove any
inappropriate Contacts.</t>
<t>If outbound registration succeeded, as indicated by the presence
of the outbound option-tag in the Require header field of a
successful registration response, the UA begins sending keepalives
as described in <xref target="detect-fail"></xref>.</t>
<t>Note that the UA needs to honor 503 (Service Unavailable)
responses to registrations as described in <xref
target="RFC3261"></xref> and <xref target="RFC3263"></xref>. In
particular, implementors should note that when receiving a 503
(Service Unavailable) response with a Retry-After header field, the
UA is expected to wait the indicated amount of time and retry the
registration. A Retry-After header field value of 0 is valid and
indicates the UA is expected to retry the REGISTER immediately.
Implementations need to ensure that when retrying the REGISTER, they
revisit the DNS resolution results such that the UA can select an
alternate host from the one chosen the previous time the URI was
resolved.</t>
<t>If the registering UA receives a 439 (First Hop Lacks Outbound
Support) response to a REGISTER request, it MAY re-attempt
registration without an outbound proxy (subject to local policy at
the client). If the client has one or more alternate outbound
proxies available, it MAY re-attempt registration through such
outbound proxies. See <xref target="bad-first-hop"></xref> for more
information on the 439 response code.</t>
</section>
<section title="Subsequent REGISTER requests">
<t>Re-registrations and single Contact de-registrations use the same
instance-id and reg-id values as the corresponding initial
registration. Re-registrations which merely refresh an existing
valid registration are sent over the same flow as the original
registration.</t>
<t>If a re-registration is rejected with a recoverable error
response, for example by a 503 (Service Unavailable) containing a
Retry-After header, the UAC SHOULD NOT tear down the corresponding
flow if the flow uses a connection-oriented transport such as TCP.
As long as "pongs" are received in response to "pings", the flow
SHOULD be kept active until a non-recoverable error response is
received. This prevents unnecessary closing and opening of
connections.</t>
</section>
<section anchor="third-party-reg" title="Non Outbound Registrations">
<t>In an initial registration, a User Agent MUST NOT include a
reg-id header parameter in the Contact header field if the
registering UA is not the same instance as the UA referred to by the
target Contact header field. (This practice is occasionally used to
install forwarding policy into registrars.)</t>
<t>A UAC also MUST NOT include an instance-id or reg-id parameter in
a request to unregister all Contacts (a single Contact header field
value with the value of "*").</t>
</section>
</section>
<section anchor="send" title="Sending Non-REGISTER Requests">
<t>When a UAC is about to send a request, it first performs normal
processing to select the next hop URI. The UA can use a variety of
techniques to compute the route set and accordingly the next hop URI.
Discussion of these techniques is outside the scope of this document.
UAs that support this specification SHOULD include the outbound option
tag in a Supported header field in a non-Register request.</t>
<t>The UAC performs normal DNS resolution on the next hop URI (as
described in <xref target="RFC3263"></xref>) to find a protocol, IP
address, and port. For protocols that don't use TLS, if the UAC has an
existing flow to this IP address, and port with the correct protocol,
then the UAC MUST use the existing connection. For TLS protocols,
there MUST also be a match between the host production in the next hop
and one of the URIs contained in the subjectAltName in the peer
certificate. If the UAC cannot use one of the existing flows, then it
SHOULD form a new flow by sending a datagram or opening a new
connection to the next hop, as appropriate for the transport
protocol.</t>
<t>If the UAC is sending a dialog-forming request, and wants all
subsequent requests in the dialog to arrive over the same flow, the
UAC adds an 'ob' parameter to its Contact header. Typically this is
desirable, but it is not necessary for example if the Contact is a
<xref target="I-D.ietf-sip-gruu">GRUU</xref>. The flow used for the
request is typically the same flow the UA registered over, but it
could be a new flow, for example the initial subcription dialog for
the <xref target="I-D.ietf-sipping-config-framework">configuration
framework</xref> needs to exist before registration.</t>
<t><list>
<t>Note that if the UAC wants a UDP flow to work through NATs or
firewalls it still needs to put the 'rport' parameter <xref
target="RFC3581"></xref> in its Via header field value, and send
from the port it is prepared to receive on. More general
information about NAT traversal in SIP is described in <xref
target="I-D.ietf-sipping-nat-scenarios"></xref>.</t>
</list></t>
</section>
<section anchor="detect-fail"
title="Keep-alives and Detecting Flow Failure">
<t>Keepalives are used for refreshing NAT/firewall bindings and
detecting flow failure. Flows can fail for many reasons including NATs
rebooting and Edge Proxies crashing.</t>
<t>As described in <xref target="reg"></xref>, a UA that registers
will begin sending keepalives after an appropriate registration
response. A UA that does not register (for example, a PSTN gateway
behind a firewall) can also send keepalives under certain
circumstances.</t>
<t>
Under specific circumstances, a UAC might be allowed to send STUN keep
alives even if the procedures in <xref target="reg"/> were not completed, provided that
there is an explicit indication that the target first hop SIP note supports
STUN keep alives. This applies for example to a non-registering
UA or to a case where the UA registration succeeded, but the response did not include
the outbound option-tag in the Require header field.
</t><t>
Note that a UA can "always" send a double CRLF (a "ping") over
connection-oriented transports as this is already allowed by [RFC3261].
Section 7.5, however a UA that did not register using outbound registration
cannot expect a CRLF in
response (a "pong") unless the UA has an explicit indication that
CRLF keepalives are supported as described in this section.
Likewise, a UA that did not successfully register with outbound procedures
needs explicit indication that the target
first hop SIP node supports STUN keepalives before it can send any
STUN messages. </t>
<t>Configuration indicating keepalive support for a specific target is
considered an explicit indication. If these conditions are satisfied,
the UA sends its keepalives according to the same guidelines described
in the rest of this section as UAs which register.</t>
<t>The UA needs to detect when a specific flow fails. The UA actively
tries to detect failure by periodically sending keep alive messages
using one of the techniques described in <xref
target="keepcrlf"></xref> or <xref target="keepstun"></xref>. If a
flow with a registration has failed, the UA follows the procedures in
<xref target="reg"></xref> to form a new flow to replace the failed
one.</t>
<t>When a successful registration response contains the Flow-Timer
header field, the value of this header field is the number of seconds
the server is prepared to wait without seeing keepalives before it
considers the corresponding flow dead. The UA MUST send keepalives at
least as often as this number of seconds. If the UA uses the server
recommended keepalive frequency it SHOULD send its keepalives so that
the interval between each keepalive is randomly distributed between
80% and 100% of the server provided time. For example, if the server
suggests 120 seconds, the UA would send each keepalive with a
different frequency between 95 and 120 seconds.</t>
<t>If no Flow-Timer header field was present in a register response
for this flow, the UA can send keepalives at its discretion. The
sections below provide RECOMMENDED default values for these
keepalives.</t>
<t>The client needs to perform normal <xref target="RFC3263"></xref>
SIP DNS resolution on the URI from the outbound-proxy-set to pick a
transport. Once a transport is selected, the UA selects the keep alive
approach that is recommended for that transport.</t>
<section anchor="keepcrlf" title="Keep alive with CRLF">
<t>This approach MUST only be used with connection oriented
transports such as TCP or SCTP.</t>
<t>A User Agent that forms flows, checks if the configured URI to
which the UA is connecting resolves to a stream-based transport (ex:
TCP and TLS over TCP).</t>
<t>For this mechanism, the client "ping" is a double-CRLF sequence,
and the server "pong" is a single CRLF, as defined in the ABNF
below:</t>
<figure>
<artwork><![CDATA[
CRLF = CR LF
double-CRLF = CR LF CR LF
CR = 0x0d
LF = 0x0a]]></artwork>
</figure>
<t>The ping and pong need to be sent between SIP messages and cannot
be sent in the middle of a SIP message. If sending over TLS, the
CRLFs are sent inside the TLS protected channel. If sending over a
<xref target="RFC3320">SigComp</xref> compressed data stream, the
CRLF keep alives are sent inside the compressed stream. The double
CRLF is considered a single SigComp message. The specific mechanism
for representing these characters is an implementation specific
matter to be handled by the SigComp compressor at the sending
end.</t>
<t>If a pong is not received within 10 seconds then the client MUST
treat the flow as failed. Clients MUST support this CRLF keep
alive.</t>
<t>The proper selection of keepalive frequency is primarily a
trade-off between battery usage and availability. For devices where
power is not a significant concern, the UA SHOULD select a random
number between 95 and 120 seconds between keepalives. When battery
power is a concern, the UA SHOULD select a random number between 672
and 840 seconds (14 minutes). These times MAY be configurable. To
clarify, the random number will be different for each keepalive
ping.</t>
<t><list>
<t>Note on selection of time values: the 120 seconds upper bound
was chosen based on the idea that for a good user experience,
failures normally will be detected in this amount of time and a
new connection set up. The 14 minute upper-bound for
battery-powered devices was selected based on NATs with TCP
timeouts as low as 15 minutes. Operators that wish to change the
relationship between load on servers and the expected time that
a user might not receive inbound communications will probably
adjust this time. The 95 seconds lower bound was chosen so that
the jitter introduced will result in a relatively even load on
the servers after 30 minutes.</t>
</list></t>
</section>
<section anchor="keepstun" title="Keep alive with STUN">
<t>This approach MUST only be used with connection-less transports,
such as UDP.</t>
<t>A User Agent that forms flows, checks if the configured URI to
which the UA is connecting resolves to use the UDP transport. The UA
can periodically perform keep alive checks by sending <xref
target="I-D.ietf-behave-rfc3489bis">STUN</xref> Binding Requests
over the flow as described in <xref target="stunkeep"></xref>.
Clients MUST support STUN based keep alives.</t>
<t>The time between each keep alive request SHOULD be a random
number between 24 and 29 seconds.</t>
<t><list>
<t>Note on selection of time values: the upper bound of 29
seconds was selected, as many NATs have UDP timeouts as low as
30 seconds. The 24 second lower bound was selected so that after
10 minutes the jitter introduced by different timers will make
the keep alive requests unsynchronized to evenly spread the load
on the servers.</t>
</list></t>
<t>If a STUN Binding Error Response is received, or if no Binding
Response is received after 7 retransmissions (16 times the STUN
"RTO" timer--RTO is an estimate of round-trip time), the UA
considers the flow failed. If the XOR-MAPPED-ADDRESS in the STUN
Binding Response changes, the UA MUST treat this event as a failure
on the flow.</t>
</section>
</section>
<section anchor="recovery" title="Flow Recovery">
<t>When a flow used for registration (through a particular URI in the
outbound-proxy-set) fails, the UA needs to form a new flow to replace
the old flow and replace any registrations that were previously sent
over this flow. Each new registration MUST have the same reg-id as the
registration it replaces. This is done in much the same way as forming
a brand new flow as described in <xref target="reg"></xref>; however,
if there is a failure in forming this flow, the UA needs to wait a
certain amount of time before retrying to form a flow to this
particular next hop.</t>
<t>The amount of time to wait depends if the previous attempt at
establishing a flow was successful. For the purposes of this section,
a flow is considered successful if outbound registration succeeded,
and if keep alives are in use on this flow, at least one subsequent
keep alive response was received.</t>
<t>The number of seconds to wait is computed in the following way. If
all of the flows to every URI in the outbound proxy set have failed,
the base-time is set to 30 seconds; otherwise, in the case where at
least one of the flows has not failed, the base-time is set to 90
seconds. The wait time is computed by taking two raised to the power
of the number of consecutive registration failures for that URI, and
multiplying this by the base time, up to a maximum of 1800
seconds.</t>
<figure>
<artwork><![CDATA[
wait-time = min( max-time, (base-time * (2 ^ consecutive-failures)))]]></artwork>
</figure>
<t>These times MAY be configurable in the UA. The three times are:
<list style="symbols">
<t>max-time with a default of 1800 seconds</t>
<t>base-time (if all failed) with a default of 30 seconds</t>
<t>base-time (if all have not failed) with a default of 90
seconds</t>
</list> For example, if the base time is 30 seconds, and there were
three failures, then the wait time is min(1800,30*(2^3)) or 240
seconds. The delay time is computed by selecting a uniform random time
between 50 and 100 percent of the wait time. The UA MUST wait for the
value of the delay time before trying another registration to form a
new flow for that URI.</t>
<t>To be explicitly clear on the boundary conditions: when the UA
boots it immediately tries to register. If this fails and no
registration on other flows succeed, the first retry happens somewhere
between 30 and 60 seconds after the failure of the first registration
request. If the number of consecutive-failures is large enough that
the maximum of 1800 seconds is reached, the UA will keep trying
indefinitely with a random time of 15 to 30 minutes between each
attempt.</t>
</section>
</section>
<section title="Edge Proxy Mechanisms">
<section anchor="edge" title="Processing Register Requests">
<t>When an Edge Proxy receives a registration request with a reg-id
header parameter in the Contact header field, it needs to determine if
it (the edge proxy) will have to be visited for any subsequent
requests sent to the user agent identified in the Contact header
field, or not. If the edge proxy is the first hop, as indicated by the
Via header field, it always inserts its URI in a Path header field
value as described in <xref target="RFC3327"></xref>. If it is not the
first hop, it might still decide to add itself to the Path header
based on local policy. In addition, if the Edge Proxy is the first SIP
node after the UAC, the edge proxy either MUST store a "flow token"
(containing information about the flow from the previous hop) in its
Path URI or reject the request. The flow token MUST be an identifier
that is unique to this network flow. The flow token MAY be placed in
the userpart of the URI. In addition, the first node MUST include an
'ob' URI parameter in its Path header field value. If the Edge Proxy
is not the first SIP node after the UAC it MUST NOT place an 'ob' URI
parameter in a Path header field value. The Edge Proxy can determine
if it is the first hop by examining the Via header field.</t>
</section>
<section anchor="flowtokens" title="Generating Flow Tokens">
<t>A trivial but impractical way to satisfy the flow token requirement
in <xref target="edge"></xref> involves storing a mapping between an
incrementing counter and the connection information; however this
would require the Edge Proxy to keep an infeasible amount of state. It
is unclear when this state could be removed and the approach would
have problems if the proxy crashed and lost the value of the counter.
A stateless example is provided below. A proxy can use any algorithm
it wants as long as the flow token is unique to a flow, the flow can
be recovered from the token, and the token cannot be modified by
attackers.</t>
<t><list style="hanging">
<t hangText="Example Algorithm:">When the proxy boots it selects a
20-octet crypto random key called K that only the Edge Proxy
knows. A byte array, called S, is formed that contains the
following information about the flow the request was received on:
an enumeration indicating the protocol, the local IP address and
port, the remote IP address and port. The HMAC of S is computed
using the key K and the HMAC-SHA1-80 algorithm, as defined in
<xref target="RFC2104"></xref>. The concatenation of the HMAC and
S are base64 encoded, as defined in <xref
target="RFC4648"></xref>, and used as the flow identifier. When
using IPv4 addresses, this will result in a 32-octet
identifier.</t>
</list></t>
</section>
<section title="Forwarding Non-REGISTER Requests">
<t>When an Edge Proxy receives a request, it applies normal routing
procedures with the following additions. If the Edge Proxy receives a
request where the edge proxy is the host in the topmost Route header
field value, and the Route header field value contains a flow token,
the proxy may need to do additional processing described in the rest
of this section. Otherwise the edge proxy skips the procedures in this
section, removes itself from the Route header field, and continues
processing the request.</t>
<t>The proxy decodes the flow token and compares the flow in the flow
token with the source of the request to determine if this is an
"incoming" or "outgoing" request.</t>
<t>If the flow in the flow token identified by the topmost Route
header field value matches the source IP address and port of the
request, the request is an "outgoing" request, otherwise, it is an
"incoming" request.</t>
<section title="Processing Incoming Requests">
<t>If the Route header value contains an 'ob' parameter, the Route
header was probably copied from the Path header in a registration. If
the Route header value contains an 'ob' parameter, and the request is
a new dialog-forming request, the proxy needs to adjust the route set
to insure that subsequent requests in the dialog can be delivered over
a valid flow to the UA instance identified by the flow token.</t>
<t><list>
<t>A simple approach to satisfy this requirement is for the proxy
to add a Record-Route header field value that contains the
flow-token, by copying the URI in the Route header minus the 'ob'
parameter.</t>
</list></t>
<t>Next, whether the Route header field contained an 'ob' parameter or
not, the proxy removes the Route header field
value and forwards the request over the 'logical flow' identified by
the flow token, that is known to deliver data to the specific target
UA instance. If the flow token has been tampered with, the proxy SHOULD
send a 403 (Forbidden) response. If the flow no longer
exists the proxy SHOULD send a 430 (Flow Failed) response to the
request.</t>
<t>Proxies which used the example algorithm described in <xref
target="flowtokens"></xref> to form a flow token follow the procedures
below to determine the correct flow. To decode the flow token, take
the flow identifier in the user portion of the URI and base64 decode
it, then verify the HMAC is correct by recomputing the HMAC and
checking that it matches. If the HMAC is not correct, the request has
been tampered with.</t>
</section>
<section title="Processing Outgoing Requests">
<t>For mid-dialog requests to work with outbound UAs, the requests
need to be forwarded over some valid flow to the appropriate UA
instance. If the Edge Proxy receives an outgoing dialog-forming request, the
Edge Proxy can use the presence of the "ob" parameter in the
UAC's Contact URI (or topmost Route header field) to determine if
the Edge Proxy needs to assist in mid-dialog request routing.
</t>
<t><list style="hanging">
<t hangText="Implementation note:">Specific procedures at the edge
proxy to ensure that mid-dialog requests are routed over an
existing flow are not part of this specification. However, an
approach such as having the Edge Proxy add a Record-Route header
with a flow token is one way to ensure that mid-dialog requests
are routed over the correct flow. </t>
</list></t>
</section>
</section>
<section anchor="edgekeep" title="Edge Proxy Keep alive Handling">
<t>All edge proxies compliant with this specification MUST implement
support for STUN NAT Keep alives on its SIP UDP ports as described in
<xref target="stunkeep"></xref>.</t>
<t>When a server receives a double CRLF sequence between SIP messages on a connection
oriented transport such as TCP or SCTP, it MUST immediately respond
with a single CRLF over the same connection.</t>
<t>The last proxy to forward a successful registration response to a
UA MAY include a Flow-Timer header field if the response contains the
outbound option-tag in a Require header field value in the
response.</t>
</section>
</section>
<section anchor="registrar" title="Registrar Mechanisms">
<t>This specification updates the definition of a binding in <xref
target="RFC3261"></xref> Section 10 and <xref target="RFC3327"></xref>
Section 5.3.</t>
<t>Registrars which implement this specification MUST support the Path
header mechanism <xref target="RFC3327"></xref>.</t>
<t>When receiving a REGISTER request, the registrar MUST check from its
Via header field if the registrar is the first hop or not. If the
registrar is not the first hop, it MUST examine the Path header of the
request. If the Path header field is missing or it exists but the first
URI does not have an 'ob' URI parameter, then outbound processing MUST
NOT be applied to the registration. In this case, the following
processing applies: if the REGISTER request contains the "outbound"
option tag in a "Require" header field, then the registrar MUST respond
to the REGISTER request with a 439 (First Hop Lacks Outbound Support)
response; otherwise, the registrar MUST ignore the reg-id parameter of
the Contact header. See <xref target="bad-first-hop"></xref> for more
information on the 439 response code.</t>
<t>A Contact header field value with an instance-id but no reg-id is
valid (this combination can be used in the <xref
target="I-D.ietf-sip-gruu">GRUU</xref> specification), but one with a
reg-id but no instance-id is not. If the registrar processes a Contact
header field value with a reg-id but no instance-id, it simply ignores
the reg-id parameter.</t>
<t>A registration containing a reg-id parameter and a non-zero
expiration is used to register a single UA instance over a single flow,
and can also de-register any Contact header fields with zero expiration.
Therefore if the Contact header field contains more than one header
field value with a non-zero expiration and any of these header field
values contain a 'reg-id' parameter, the entire registration SHOULD be
rejected with a 400 (Bad Request) response. The justification for
recommending rejection versus making it mandatory is that the receiver
is allowed by <xref target="RFC3261"></xref> to squelch (not respond to)
excessively malformed or malicious messages.</t>
<t>If the Contact header did not contain a 'reg-id' parameter or if that
parameter was ignored (as described above) the registrar MUST NOT
include the 'outbound' option-tag in the Require header field of its
response.</t>
<t>The registrar MUST be prepared to receive, simultaneously for the
same AOR, some registrations that use instance-id and reg-id and some
registrations that do not. The Registrar MAY be configured with local
policy to reject any registrations that do not include the instance-id
and reg-id, or with Path header field values that do not contain the
'ob' parameter. If the Contact header field does not contain a
'+sip.instance' media feature parameter, the registrar processes the
request using the Contact binding rules in <xref
target="RFC3261"></xref>.</t>
<t>When a '+sip.instance' media feature parameter is present in a
Contact header field of a REGISTER request (after the Contact header
validation as described above), the corresponding binding is between an
AOR and the combination of the instance-id (from the +sip.instance media
feature parameter) and the value of reg-id parameter if it is present.
The registrar MUST store in the binding the Contact URI, all the Contact
head field parameters, and any Path header field values. (Even though
the Contact URI is not used for binding comparisons, it is still needed
by the authoritative proxy to form the target set.) The Registrar MUST
include the 'outbound' option-tag (defined in <xref
target="iana-reg-id"></xref>) in a Require header field value in its
response to the REGISTER request.</t>
<t>If the UAC has a direct flow with the registrar, the registrar MUST
store enough information to uniquely identify the network flow over
which the request arrived. For common operating systems with TCP, this
would typically just be the handle to the file descriptor where the
handle would become invalid if the TCP session was closed. For common
operating systems with UDP this would typically be the file descriptor
for the local socket that received the request, the local interface, and
the IP address and port number of the remote side that sent the request.
The registrar MAY store this information by adding itself to the Path
header field with an appropriate flow token.</t>
<t>If the registrar receives a re-registration for a specific
combination of AOR, instance-id and reg-id values, the registrar MUST
update any information that uniquely identifies the network flow over
which the request arrived if that information has changed, and SHOULD
update the time the binding was last updated.</t>
<t>To be compliant with this specification, registrars which can receive
SIP requests directly from a UAC without intervening edge proxies MUST
implement the same keep alive mechanisms as Edge Proxies (<xref
target="edgekeep"></xref>). Registrars with a direct flow with a UA MAY
include a Flow-Timer header in a 200-class registration response which
includes the outbound option-tag in the Require header.</t>
</section>
<section title="Authoritative Proxy Mechanisms: Forwarding Requests">
<t>When a proxy uses the location service to look up a registration
binding and then proxies a request to a particular contact, it selects a
contact to use normally, with a few additional rules:</t>
<t><list style="symbols">
<t>The proxy MUST NOT populate the target set with more than one
contact with the same AOR and instance-id at a time.</t>
<t>If a request for a particular AOR and instance-id fails with a
430 (Flow Failed) response, the proxy SHOULD replace the failed
branch with another target (if one is available) with the same AOR
and instance-id, but a different reg-id.</t>
<t>If the proxy receives a final response from a branch other than a
408 (Request Timeout) or a 430 (Flow Failed) response, the proxy
MUST NOT forward the same request to another target representing the
same AOR and instance-id. The targeted instance has already provided
its response.</t>
</list></t>
<t>The proxy uses the next-hop target of the message and the value of
any stored Path header field vector in the registration binding to
decide how to forward and populate the Route header in the request. If
the proxy is colocated with the registrar and stored information about
the flow to the UA that created the binding, then the proxy MUST send
the request over the same 'logical flow' saved with the binding, since
that flow is known to deliver data to the specific target UA instance's
network flow that was saved with the binding.</t>
<t><list style="hanging">
<t hangText="Implementation Note:">Typically this means that for
TCP, the request is sent on the same TCP socket that received the
REGISTER request. For UDP, the request is sent from the same local
IP address and port over which the registration was received, to the
same IP address and port from which the REGISTER was received.</t>
</list></t>
<t>If a proxy or registrar receives information from the network that
indicates that no future messages will be delivered on a specific flow,
then the proxy MUST invalidate all the bindings in the target set that
use that flow (regardless of AOR). Examples of this are a TCP socket
closing or receiving a destination unreachable ICMP error on a UDP flow.
Similarly, if a proxy closes a file descriptor, it MUST invalidate all
the bindings in the target set with flows that use that file
descriptor.</t>
</section>
<section anchor="stunkeep" title="STUN Keep alive Processing">
<t>This section describes changes to the SIP transport layer that allow
SIP and <xref target="I-D.ietf-behave-rfc3489bis">STUN</xref> Binding
Requests to be mixed over the same flow. This constitues a new STUN
usage. The STUN messages are used to verify that connectivity is still
available over a UDP flow, and to provide periodic keep alives. Note
that these STUN keep alives are always sent to the next SIP hop. STUN
messages are not delivered end-to-end.</t>
<t>The only STUN messages required by this usage are Binding Requests,
Binding Responses, and Binding Error Responses. The UAC sends Binding
Requests over the same UDP flow that is used for sending SIP messages.
These Binding Requests do not require any STUN attributes except the
XOR-MAPPED-ADDRESS and never use any form of authentication. The UAS,
proxy, or registrar responds to a valid Binding Request with a Binding
Response which MUST include the XOR-MAPPED-ADDRESS attribute.</t>
<t>If a server compliant to this section receives SIP requests on a
given interface and UDP port, it MUST also provide a limited version of
a STUN server on the same interface and UDP port.</t>
<t><list>
<t>It is easy to distinguish STUN and SIP packets sent over UDP,
because the first octet of a STUN Binding method has a value of 0 or
1 while the first octet of a SIP message is never a 0 or 1.</t>
</list></t>
<t>Because sending and receiving binary STUN data on the same ports used
for SIP is a significant and non-backwards compatible change to RFC
3261, this section requires a number of checks before sending STUN
messages to a SIP node. If a SIP node sends STUN requests (for example
due to incorrect configuration) despite these warnings, the node could
be blacklisted for UDP traffic.</t>
<t>A SIP node MUST NOT send STUN requests over a flow unless it has an
explicit indication that the target next hop SIP server claims to
support this specification. Note that UACs MUST NOT use an ambiguous
configuration option such as "Work through NATs?" or "Do Keep alives?"
to imply next hop STUN support. A UAC MAY use the presence of an 'ob'
parameter in the Path header in a registration response as an indication
that its first edge proxy supports the keep alives defined in this
document.</t>
<t><list>
<t>Typically, a SIP node first sends a SIP request and waits to
receive a 200-class response over a flow to a new target
destination, before sending any STUN messages. When scheduled for
the next NAT refresh, the SIP node sends a STUN request to the
target.</t>
</list></t>
<t>Once a flow is established, failure of a STUN request (including its
retransmissions) is considered a failure of the underlying flow. For SIP
over UDP flows, if the XOR-MAPPED-ADDRESS returned over the flow
changes, this indicates that the underlying connectivity has changed,
and is considered a flow failure.</t>
<t>The SIP keep alive STUN usage requires no backwards compatibility
with <xref target="RFC3489"></xref>.</t>
<section title="Use with Sigcomp">
<t>When STUN is used together with <xref
target="RFC3320">SigComp</xref> compressed SIP messages over the same
flow,the STUN messages are simply sent uncompressed, "outside" of
SigComp. This is supported by multiplexing STUN messages with SigComp
messages by checking the two topmost bits of the message. These bits
are always one for SigComp, or zero for STUN.</t>
<t><list>
<t>All SigComp messages contain a prefix (the five
most-significant bits of the first byte are set to one) that does
not occur in <xref target="RFC3629">UTF-8</xref> encoded text
messages, so for applications which use this encoding (or ASCII
encoding) it is possible to multiplex uncompressed application
messages and SigComp messages on the same UDP port.</t>
<t>The most significant two bits of every STUN Binding method are
both zeroes. This, combined with the magic cookie, aids in
differentiating STUN packets from other protocols when STUN is
multiplexed with other protocols on the same port.</t>
</list></t>
</section>
</section>
<section title="Example Message Flow">
<t>Below is an example message flow illustrating most of the concepts
discussed in this specification. In many cases, Via, Content-Length and
Max-Forwards headers are omitted for brevity and readability.</t>
<t>The section is subdivided into independent calls flows: however, they
are structured in sequential order of an hypothetical sequence of call
flows.</t>
<section title="Subscription to configuration package">
<t>If the outbound proxy set is already configured on Bob's UA, then
this subsection can be skipped. Otherwise, if the outbound proxy set
is learned through the configuration package, Bob's UA sends a
SUBSCRIBE request for the UA profile configuration package <xref
target="I-D.ietf-sipping-config-framework"></xref>. This request is a
poll (Expires is zero). After receiving the NOTIFY request, Bob's UA
fetches the external configuration using HTTPS (not shown) and obtains
a configuration file which contains the outbound-proxy-set
"sip:ep1.example.com;lr" and "sip:ep2.example.com;lr.</t>
<figure>
<artwork><![CDATA[
[----example.com domain-------------------------]
Bob EP1 EP2 Proxy Config
| | | | |
1)|SUBSCRIBE->| | | |
2)| |---SUBSCRIBE Event: ua-profile ->|
3)| |<--200 OK -----------------------|
4)|<--200 OK--| | | |
5)| |<--NOTIFY------------------------|
6)|<--NOTIFY--| | | |
7)|---200 OK->| | | |
8)| |---200 OK ---------------------->|
| | | | |
Example Message #1:
SUBSCRIBE sip:00000000-0000-1000-8000-AABBCCDDEEFF@example.com SIP/2.0
Via: SIP/2.0/TCP 192.168.1.2;branch=z9hG4bKnlsdkdj2
Max-Forwards: 70
From: <anonymous@example.com>;tag=23324
To: <sip:00000000-0000-1000-8000-AABBCCDDEEFF@example.com>
Call-ID: nSz1TWN54x7My0GvpEBj
CSeq: 1 SUBSCRIBE
Event: ua-profile ;profile-type=device
;vendor="example.com";model="uPhone";version="1.1"
Expires: 0
Supported: path, outbound
Accept: message/external-body, application/x-uPhone-config
Contact: <sip:192.168.1.2;transport=tcp;ob>
;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>"
Content-Length: 0]]></artwork>
</figure>
<t>In message #2, EP1 adds the following Record-Route header:</t>
<figure>
<artwork><![CDATA[Record-Route:
<sip:GopIKSsn0oGLPXRdV9BAXpT3coNuiGKV@ep1.example.com;lr>]]></artwork>
</figure>
<t>In message #5, the configuration server sends a NOTIFY with an
external URL for Bob to fetch his configuration. The NOTIFY has a
Subscription-State header that ends the subscription.</t>
<figure>
<artwork><![CDATA[
Message #5
NOTIFY sip:192.168.1.2;transport=tcp;ob SIP/2.0
Via: SIP/2.0/TCP 192.0.2.5;branch=z9hG4bKn81dd2
Max-Forwards: 70
To: <anonymous@example.com>;tag=23324
From: <sip:00000000-0000-1000-8000-AABBCCDDEEFF@example.com>;tag=0983
Call-ID: nSz1TWN54x7My0GvpEBj
CSeq: 1 NOTIFY
Route: <sip:GopIKSsn0oGLPXRdV9BAXpT3coNuiGKV@ep1.example.com;lr>
Subscription-State: terminated;reason=timeout
Event: ua-profile
Content-Type: message/external-body; access-type="URL"
;expiration="Thu, 01 Jan 2009 09:00:00 UTC"
;URL="http://example.com/uPhone.cfg"
;size=9999;hash=10AB568E91245681AC1B
Content-Length: 0
]]></artwork>
</figure>
<t>EP1 receives this NOTIFY request, strips off the Route header,
extracts the flow-token, calculates the correct flow and forwards the
request (Message #6) over that flow to Bob.</t>
<t>Bob's UA fetches the configuration file and learns the outbound
proxy set.</t>
</section>
<section title="Registration">
<t>Now that Bob's UA is configured with the outbound-proxy-set whether
through configuration or using the configuration framework procedures
of the previous section, Bob's UA sends REGISTER requests through each
edge proxy in the set. Once the registrations succeed, Bob's UA begins
sending CRLF keepalives about every 2 minutes.</t>
<figure>
<artwork><![CDATA[
Bob EP1 EP2 Proxy Alice
| | | | |
9)|-REGISTER->| | | |
10)| |---REGISTER-->| |
11)| |<----200 OK---| |
12)|<-200 OK---| | | |
13)|----REGISTER---->| | |
14)| | |--REG-->| |
15)| | |<-200---| |
16)|<----200 OK------| | |
| | | | |
| about 120 seconds later... |
| | | | |
17)|--2CRLF--->| | | |
18)|<--CRLF----| | | |
19)|------2CRLF----->| | |
20)|<------CRLF------| | |
| | | | |
]]></artwork>
</figure>
<t>In message #9, Bob's UA sends its first registration through the
first edge proxy in the outbound-proxy-set by including a loose route.
The UA includes an instance-id and reg-id in its Contact header field
value. Note the option-tags in the Supported header.</t>
<figure>
<artwork><![CDATA[
Message #9
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/TCP 192.168.1.2;branch=z9hG4bKnashds7
Max-Forwards: 70
From: Bob <sip:bob@example.com>;tag=7F94778B653B
To: Bob <sip:bob@example.com>
Call-ID: 16CB75F21C70
CSeq: 1 REGISTER
Supported: path, outbound
Route: <sip:ep1.example.com;lr>
Contact: <sip:bob@192.168.1.2;transport=tcp>;reg-id=1
;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>"
Content-Length: 0
]]></artwork>
</figure>
<t>Message #10 is similar. EP1 removes the Route header field value,
decrements Max-Forwards, and adds its Via header field value. Since
EP1 is the first edge proxy, it adds a Path header with a flow token
and includes the 'ob' parameter.</t>
<figure>
<artwork><![CDATA[
Path: <sip:VskztcQ/S8p4WPbOnHbuyh5iJvJIW3ib@ep1.example.com;lr;ob>
]]></artwork>
</figure>
<t>Since the response to the REGISTER (message #11) contains the
outbound option-tag in the Require header field, Bob's UA will know
that the registrar used outbound binding rules. The response also
contains the currently active Contacts, the Path for the current
registration.</t>
<figure>
<artwork><![CDATA[
SIP/2.0 200 OK
Via: SIP/2.0/TCP 192.0.2.15;branch=z9hG4bKnuiqisi
Via: SIP/2.0/TCP 192.168.1.2;branch=z9hG4bKnashds7
From: Bob <sip:bob@example.com>;tag=7F94778B653B
To: Bob <sip:bob@example.com>;tag=6AF99445E44A
Call-ID: 16CB75F21C70
CSeq: 1 REGISTER
Supported: path, outbound
Require: outbound
Contact: <sip:bob@192.168.1.2;transport=tcp>;reg-id=1;expires=3600
;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>"
Path: <sip:VskztcQ/S8p4WPbOnHbuyh5iJvJIW3ib@ep1.example.com;lr;ob>
Content-Length: 0
]]></artwork>
</figure>
<t>The second registration through EP2 (message #13) is similar other
than the Call-ID has changed, the reg-id is 2, and the Route header
goes through EP2.</t>
<figure>
<artwork><![CDATA[
REGISTER sip:example.com SIP/2.0
Via: SIP/2.0/TCP 192.168.1.2;branch=z9hG4bKnqr9bym
Max-Forwards: 70
From: Bob <sip:bob@example.com>;tag=755285EABDE2
To: Bob <sip:bob@example.com>
Call-ID: E05133BD26DD
CSeq: 1 REGISTER
Supported: path, outbound
Route: <sip:ep2.example.com;lr>
Contact: <sip:bob@192.168.1.2;transport=tcp>;reg-id=2
;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>"
Content-Length: 0
]]></artwork>
</figure>
<t>Likewise in message #14, EP2 adds a Path header with flow token and
'ob' parameter.</t>
<figure>
<artwork><![CDATA[
Path: <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr;ob>
]]></artwork>
</figure>
<t>Message #16 tells Bob's UA that outbound registration was
successful, and shows both Contacts. Note that only the Path
corresponding to the current registration is returned.</t>
<figure>
<artwork><![CDATA[
SIP/2.0 200 OK
Via: SIP/2.0/TCP 192.168.1.2;branch=z9hG4bKnqr9bym
From: Bob <sip:bob@example.com>;tag=755285EABDE2
To: Bob <sip:bob@example.com>;tag=49A9AD0B3F6A
Call-ID: E05133BD26DD
Supported: path, outbound
Require: outbound
CSeq: 1 REGISTER
Contact: <sip:bob@192.168.1.2;transport=tcp>;reg-id=1;expires=3600
;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>"
Contact: <sip:bob@192.168.1.2;transport=tcp>;reg-id=2;expires=3600
;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>"
Path: <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr;ob>
Content-Length: 0
]]></artwork>
</figure>
</section>
<section title="Incoming call and proxy crash">
<t>In this example, after registration, EP1 crashes and reboots.
Before Bob's UA notices that its flow to EP1 is no longer responding,
Alice calls Bob. Bob's authoritative proxy first tries the flow to
EP1, but EP1 no longer has a flow to Bob so it responds with a 430
Flow Failed response. The proxy removes the stale registration and
tries the next binding for the same instance.</t>
<figure>
<artwork><![CDATA[
Bob EP1 EP2 Proxy Alice
| | | | |
| CRASH X | | |
| Reboot | | |
| | | | |
21)| | | |<-INVITE-|
22)| |<---INVITE----| |
23)| |----430------>| |
24)| | |<-INVITE| |
25)|<---INVITE-------| | |
26)|----200 OK------>| | |
27)| | |200 OK->| |
28)| | | |-200 OK->|
29)| | |<----------ACK----|
30)|<---ACK----------| | |
| | | | |
31)| | |<----------BYE----|
32)|<---BYE----------| | |
33)|----200 OK------>| | |
34)| | |--------200 OK--->|
| | | | |
]]></artwork>
</figure>
<t></t>
<figure>
<artwork><![CDATA[
Message #21
INVITE sip:bob@example.com SIP/2.0
To: Bob <sip:bob@example.com>
From: Alice <sip:alice@a.example>;tag=02935
Call-ID: klmvCxVWGp6MxJp2T2mb
CSeq: 1 INVITE
]]></artwork>
</figure>
<t>Bob's proxy rewrites the Request-URI to the Contact URI used in
Bob's registration, and places the path for one of the registrations
towards Bob's UA instance into a Route header field. This Route goes
through EP1.</t>
<figure>
<artwork><![CDATA[
Message #22
INVITE sip:bob@192.168.1.2;transport=tcp SIP/2.0
To: Bob <sip:bob@example.com>
From: Alice <sip:alice@a.example>;tag=02935
Call-ID: klmvCxVWGp6MxJp2T2mb
CSeq: 1 INVITE
Route: <sip:VskztcQ/S8p4WPbOnHbuyh5iJvJIW3ib@ep1.example.com;lr;ob>
]]></artwork>
</figure>
<t>Since EP1 just rebooted, it does not have the flow described in the
flow token. It returns a 430 Flow Failed response.</t>
<figure>
<artwork><![CDATA[
Message #23
SIP/2.0 430 Flow Failed
To: Bob <sip:bob@example.com>
From: Alice <sip:alice@a.example>;tag=02935
Call-ID: klmvCxVWGp6MxJp2T2mb
CSeq: 1 INVITE
]]></artwork>
</figure>
<t>The proxy deletes the binding for this path and tries to forward
the INVITE again, this time with the path through EP2.</t>
<figure>
<artwork><![CDATA[
Message #24
INVITE sip:bob@192.168.1.2;transport=tcp SIP/2.0
To: Bob <sip:bob@example.com>
From: Alice <sip:alice@a.example>;tag=02935
Call-ID: klmvCxVWGp6MxJp2T2mb
CSeq: 1 INVITE
Route: <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr;ob>
]]></artwork>
</figure>
<t>In message #25, EP2 needs to add a Record-Route header field value,
so that any subsequent in-dialog messages from Alice's UA arrive at
Bob's UA. EP2 can determine it needs to Record-Route since the request
is a dialog-forming request and the Route header contained a flow
token and an 'ob' parameter. This Record-Route information is passed
back to Alice's UA in the responses (messages #26, 27, and 28)</t>
<figure>
<artwork><![CDATA[
Message #25
INVITE sip:bob@192.168.1.2;transport=tcp SIP/2.0
To: Bob <sip:bob@example.com>
From: Alice <sip:alice@a.example>;tag=02935
Call-ID: klmvCxVWGp6MxJp2T2mb
CSeq: 1 INVITE
Record-Route: <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr>
Message #26
SIP/2.0 200 OK
To: Bob <sip:bob@example.com>;tag=skduk2
From: Alice <sip:alice@a.example>;tag=02935
Call-ID: klmvCxVWGp6MxJp2T2mb
CSeq: 1 INVITE
Record-Route: <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr>
]]></artwork>
</figure>
<t>At this point, both UAs have the correct route-set for the dialog.
Any subsequent requests in this dialog will route correctly. For
example, the ACK request in message #29 is sent form Alice's UA
directly to EP2. The BYE request in message #31 uses the same
route-set.</t>
<figure>
<artwork><![CDATA[
Message #29
ACK sip:bob@192.168.1.2;transport=tcp SIP/2.0
To: Bob <sip:bob@example.com>;tag=skduk2
From: Alice <sip:alice@a.example>;tag=02935
Call-ID: klmvCxVWGp6MxJp2T2mb
CSeq: 1 ACK
Route: <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr>
Message #31
BYE sip:bob@192.168.1.2;transport=tcp SIP/2.0
To: Bob <sip:bob@example.com>;tag=skduk2
From: Alice <sip:alice@a.example>;tag=02935
Call-ID: klmvCxVWGp6MxJp2T2mb
CSeq: 2 BYE
Route: <sip:wazHDLdIMtUg6r0I/oRZ15zx3zHE1w1Z@ep2.example.com;lr>
]]></artwork>
</figure>
</section>
<section title="Re-registration">
<t>Somewhat later, Bob's UA sends keepalives to both its edge proxies,
but it discovers that the flow with EP1 failed. Bob's UA re-registers
through EP1 using the same reg-id and Call-ID it previously used.</t>
<figure>
<artwork><![CDATA[
Bob EP1 EP2 Proxy Alice
| | | | |
35)|------2CRLF----->| | |
36)|<------CRLF------| | |
37)|--2CRLF->X | | | |
| | | | |
38)|-REGISTER->| | | |
39)| |---REGISTER-->| |
40)| |<----200 OK---| |
41)|<-200 OK---| | | |
| | | | |
Message #38
REGISTER sip:example.com SIP/2.0
From: Bob <sip:bob@example.com>;tag=7F94778B653B
To: Bob <sip:bob@example.com>
Call-ID: 16CB75F21C70
CSeq: 2 REGISTER
Supported: path, outbound
Route: <sip:ep1.example.com;lr>
Contact: <sip:bob@192.168.1.2;transport=tcp>;reg-id=1
;+sip.instance="<urn:uuid:00000000-0000-1000-8000-AABBCCDDEEFF>"
]]></artwork>
</figure>
<t>In message #39, EP1 inserts a Path header with a new flow
token:</t>
<figure>
<artwork><![CDATA[
Path: <sip:3yJEbr1GYZK9cPYk5Snocez6DzO7w+AX@ep1.example.com;lr;ob>
]]></artwork>
</figure>
</section>
<section title="Outgoing call">
<t>Finally, Bob makes an outgoing call to Alice. Bob's UA includes an
'ob' parameter in its Contact URI in message #42. EP1 adds a
Record-Route with a flow-token in message #43. The route-set is
returned to Bob in the response (messages #45, 46, and 47) and either
Bob or Alice can send in-dialog requests.</t>
<figure>
<artwork><![CDATA[
Bob EP1 EP2 Proxy Alice
| | | | |
42)|--INVITE-->| | | |
43)| |---INVITE---->| |
44)| | | |-INVITE->|
45)| | | |<--200---|
46)| |<----200 OK---| |
47)|<-200 OK---| | | |
48)|--ACK----->| | | |
49)| |-----ACK--------------->|
| | | | |
50)|-- BYE---->| | | |
51)| |-----------BYE--------->|
52)| |<----------200 OK-------|
53)|<--200 OK--| | | |
| | | | |
Message #42
INVITE sip:alice@a.example SIP/2.0
From: Bob <sip:bob@example.com>;tag=ldw22z
To: Alice <sip:alice@a.example>
Call-ID: 95KGsk2V/Eis9LcpBYy3
CSeq: 1 INVITE
Route: <sip:ep1.example.com;lr>
Contact: <sip:bob@192.168.1.2;transport=tcp;ob>
]]></artwork>
</figure>
<t>In message #43, EP1 adds the following Record-Route header.</t>
<figure>
<artwork><![CDATA[
Record-Route: <sip:3yJEbr1GYZK9cPYk5Snocez6DzO7w+AX@ep1.example.com;lr>
]]></artwork>
</figure>
<t>When EP1 receives the BYE (message #50) from Bob's UA, it can tell
that the request is an "outgoing" request (since the source of the
request matches the flow in the flow token) and simply deletes its
Route header field value and forwards the request on to Alice's
UA.</t>
<figure>
<artwork><![CDATA[
Message #50
BYE sip:alice@a.example SIP/2.0
From: Bob <sip:bob@example.com>;tag=ldw22z
To: Alice <sip:alice@a.example>;tag=plqus8
Call-ID: 95KGsk2V/Eis9LcpBYy3
CSeq: 2 BYE
Route: <sip:3yJEbr1GYZK9cPYk5Snocez6DzO7w+AX@ep1.example.com;lr>
Contact: <sip:bob@192.168.1.2;transport=tcp;ob>
]]></artwork>
</figure>
</section>
</section>
<section anchor="grammar" title="Grammar">
<t>This specification defines a new header field, new Contact header
field parameters, reg-id and +sip.instance. The grammar includes the
definitions from <xref target="RFC3261">RFC 3261</xref> and includes the
definition of uric from <xref target="RFC3986">RFC 3986</xref>.</t>
<t><list>
<t>Note: The "=/" syntax used in this ABNF indicates an extension of
the production on the left hand side.</t>
</list></t>
<t>The ABNF<xref target="RFC5234"></xref> is:</t>
<figure>
<artwork type="abnf"><![CDATA[
message-header =/ Flow-Timer
Flow-Timer = "Flow-Timer" HCOLON 1*DIGIT
contact-params =/ c-p-reg / c-p-instance
c-p-reg = "reg-id" EQUAL 1*DIGIT ; 1 to (2**31 - 1)
c-p-instance = "+sip.instance" EQUAL
LDQUOT "<" instance-val ">" RDQUOT
instance-val = *uric ; defined in RFC 3986
]]></artwork>
</figure>
<t>The value of the reg-id MUST NOT be 0 and MUST be less than
2**31.</t>
</section>
<section anchor="iana" title="IANA Considerations">
<t></t>
<section anchor="iana-flow-timer" title="Flow-Timer Header Field">
<t>This specification defines a new SIP header field "Flow-Timer"
whose syntax is defined in <xref target="grammar"></xref>.</t>
<figure>
<artwork><![CDATA[
RFC Number: RFC XXXX
Header Field Name: Flow-Timer
Compact Form: none
[NOTE TO RFC Editor: Please replace AAAA with
the RFC number of this specification.]
]]></artwork>
</figure>
</section>
<section anchor="iana-reg-id"
title="'reg-id' Contact Header Field Parameter">
<t>This specification defines a new Contact header field parameter
called reg-id in the "Header Field Parameters and Parameter Values"
sub-registry as per the registry created by <xref
target="RFC3968"></xref>. The syntax is defined in <xref
target="grammar"></xref>. The required information is:</t>
<figure>
<artwork><![CDATA[
Header Field Parameter Name Predefined Reference
Values
____________________________________________________________________
Contact reg-id No [RFC AAAA]
[NOTE TO RFC Editor: Please replace AAAA with
the RFC number of this specification.]
]]></artwork>
</figure>
</section>
<section anchor="iana-keepalive" title="SIP/SIPS URI Parameters">
<t>This specification augments the "SIP/SIPS URI Parameters"
sub-registry as per the registry created by <xref
target="RFC3969"></xref>. The required information is:</t>
<figure>
<artwork><![CDATA[
Parameter Name Predefined Values Reference
____________________________________________
ob No [RFC AAAA]
[NOTE TO RFC Editor: Please replace AAAA with
the RFC number of this specification.]
]]></artwork>
</figure>
</section>
<section anchor="iana-outbound" title="SIP Option Tag">
<t>This specification registers a new SIP option tag, as per the
guidelines in Section 27.1 of RFC 3261.</t>
<t><list style="hanging">
<t hangText="Name:">outbound</t>
<t hangText="Description:">This option-tag is used to identify UAs
and Registrars which support extensions for Client Initiated
Connections. A UA places this option in a Supported header to
communicate its support for this extension. A Registrar places
this option-tag in a Require header to indicate to the registering
User Agent that the Registrar used registrations using the binding
rules defined in this extension.</t>
</list></t>
</section>
<section title="430 (Flow Failed) Response Code">
<t>This document registers a new SIP response code (430 Flow Failed),
as per the guidelines in Section 27.4 of <xref
target="RFC3261"></xref>. This response code is used by an Edge Proxy
to indicate to the Authoritative Proxy that a specific flow to a UA
instance has failed. Other flows to the same instance could still
succeed. The Authoritative Proxy SHOULD attempt to forward to another
target (flow) with the same instance-id and AOR. This response code is
defined by the following information, which has been added to the
method and response-code sub-registry under
http://www.iana.org/assignments/sip-parameters.<list style="hanging">
<t hangText="Response Code Number:">430</t>
<t hangText="Default Reason Phrase:">Flow Failed</t>
</list></t>
</section>
<section anchor="bad-first-hop"
title="439 (First Hop Lacks Outbound Support) Response Code">
<t>This document registers a new SIP response code (439 First Hop
Lacks Outbound Support), as per the guidelines in Section 27.4 of
<xref target="RFC3261"></xref>. This response code is used by a
registrar to indicate that it supports the 'outbound' feature
described in this specifcation, but that the first outbound proxy that
the user is attempting to register through does not. Note that this
response code is only appropriate in the case that the registering
user agent is mandating outbound processing by including the
'outbound' option tag in a 'Require' header field. Proxies MUST NOT
send a 439 response to any requests that don't contain an 'outbound'
option tag in a 'Require' header field. This response code is defined
by the following information, which has been added to the method and
response-code sub-registry under
http://www.iana.org/assignments/sip-parameters.<list style="hanging">
<t hangText="Response Code Number:">439</t>
<t hangText="Default Reason Phrase:">First Hop Lacks Outbound
Support</t>
</list></t>
</section>
<section title="Media Feature Tag">
<t>This section registers a new media feature tag, per the procedures
defined in <xref target="RFC2506"></xref>. The tag is placed into the
sip tree, which is defined in <xref target="RFC3840"></xref>.</t>
<t>Media feature tag name: sip.instance</t>
<t>ASN.1 Identifier: New assignment by IANA.</t>
<t>Summary of the media feature indicated by this tag: This feature
tag contains a string containing a URN that indicates a unique
identifier associated with the UA instance registering the
Contact.</t>
<t>Values appropriate for use with this feature tag: String.</t>
<t>The feature tag is intended primarily for use in the following
applications, protocols, services, or negotiation mechanisms: This
feature tag is most useful in a communications application, for
describing the capabilities of a device, such as a phone or PDA.</t>
<t>Examples of typical use: Routing a call to a specific device.</t>
<t>Related standards or documents: RFC XXXX</t>
<t>[[Note to IANA: Please replace XXXX with the RFC number of this
specification.]]</t>
<t>Security Considerations: This media feature tag can be used in ways
which affect application behaviors. For example, the <xref
target="RFC3841">SIP caller preferences extension</xref> allows for
call routing decisions to be based on the values of these parameters.
Therefore, if an attacker can modify the values of this tag, they
might be able to affect the behavior of applications. As a result,
applications which utilize this media feature tag SHOULD provide a
means for ensuring its integrity. Similarly, this feature tag should
only be trusted as valid when it comes from the user or user agent
described by the tag. As a result, protocols for conveying this
feature tag SHOULD provide a mechanism for guaranteeing
authenticity.</t>
</section>
</section>
<section title="Security Considerations">
<t>One of the key security concerns in this work is making sure that an
attacker cannot hijack the sessions of a valid user and cause all calls
destined to that user to be sent to the attacker. Note that the intent
is not to prevent existing active attacks on SIP UDP and TCP traffic,
but to insure that no new attacks are added by introducing the outbound
mechanism.</t>
<t>The simple case is when there are no edge proxies. In this case, the
only time an entry can be added to the routing for a given AOR is when
the registration succeeds. SIP already protects against attackers being
able to successfully register, and this scheme relies on that security.
Some implementers have considered the idea of just saving the
instance-id without relating it to the AOR with which it registered.
This idea will not work because an attacker's UA can impersonate a valid
user's instance-id and hijack that user's calls.</t>
<t>The more complex case involves one or more edge proxies. When a UA
sends a REGISTER request through an Edge Proxy on to the registrar, the
Edge Proxy inserts a Path header field value. If the registration is
successfully authenticated, the registrar stores the value of the Path
header field. Later when the registrar forwards a request destined for
the UA, it copies the stored value of the Path header field into the
Route header field of the request and forwards the request to the Edge
Proxy.</t>
<t>The only time an Edge Proxy will route over a particular flow is when
it has received a Route header that has the flow identifier information
that it has created. An incoming request would have gotten this
information from the registrar. The registrar will only save this
information for a given AOR if the registration for the AOR has been
successful; and the registration will only be successful if the UA can
correctly authenticate. Even if an attacker has spoofed some bad
information in the Path header sent to the registrar, the attacker will
not be able to get the registrar to accept this information for an AOR
that does not belong to the attacker. The registrar will not hand out
this bad information to others, and others will not be misled into
contacting the attacker.</t>
<t>The Security Considerations discussed in <xref
target="RFC3261"></xref> and <xref target="RFC3327"></xref> are also
relevant to this document. For the security considerations of generating
flow tokens, please also see <xref target="flowtokens"></xref>. A
discussion of preventing the avalanche restart problem is in <xref
target="recovery"></xref>.</t>
<t>This document does not change the mandatory to implement security
mechanisms in SIP. User Agents are already required to implement Digest
authentication while support of TLS is recommended; proxy servers are
already required to implement Digest and TLS.</t>
</section>
<section title="Operational Notes on Transports">
<t>This entire section is non-normative.</t>
<t>RFC 3261 requires proxies, registrars, and User Agents to implement
both TCP and UDP but deployments can chose which transport protocols
they want to use. Deployments need to be careful in choosing what
transports to use. Many SIP features and extensions, such as large
presence notification bodies, result in SIP requests that can be too
large to be reasonably transported over UDP. RFC 3261 states that when a
request is too large for UDP, the device sending the request attempts to
switch over to TCP. No known deployments currently use this feature but
it is important to note that when using outbound, this will only work if
the UA has formed both UDP and TCP outbound flows. This specification
allows the UA to do so but in most cases it will probably make more
sense for the UA to form a TCP outbound connection only, rather than
forming both UDP and TCP flows. One of the key reasons that many
deployments choose not to use TCP has to do with the difficulty of
building proxies that can maintain a very large number of active TCP
connections. Many deployments today use SIP in such a way that the
messages are small enough that they work over UDP but they can not take
advantage of all the functionality SIP offers. Deployments that use only
UDP outbound connections are going to fail with sufficiently large SIP
messages.</t>
</section>
<section title="Requirements">
<t>This specification was developed to meet the following
requirements:</t>
<t><list style="numbers">
<t>Must be able to detect that a UA supports these mechanisms.</t>
<t>Support UAs behind NATs.</t>
<t>Support TLS to a UA without a stable DNS name or IP address.</t>
<t>Detect failure of a connection and be able to correct for
this.</t>
<t>Support many UAs simultaneously rebooting.</t>
<t>Support a NAT rebooting or resetting.</t>
<t>Minimize initial startup load on a proxy.</t>
<t>Support architectures with edge proxies.</t>
</list></t>
</section>
<section title="Changes">
<t>Note to RFC Editor: Please remove this whole section.</t>
<section title="Changes from 12 Version">
<t>Lots of editorials comments. Allows for sending keep-alive based on
presence of ob parameter and/or through explicit configuration.
Clarified treatment of recoverable error responses to REGISTER
regarding keeping flows active.</t>
</section>
<section title="Changes from 11 Version">
<t>Added 439 response code to handle "Require: outbound" with first
outbound proxy that doesn't insert ";ob".</t>
</section>
<section title="Changes from 09 Version">
<t>Make outbound consistent with the latest version of STUN 3489bis
draft. The STUN keepalive section of outbound is now a STUN usage
(much less formal).</t>
<t>Fixed references.</t>
</section>
<section title="Changes from 08 Version">
<t>UAs now include the 'ob' parameter in their Contact header for
non-REGISTER requests, as a hint to the Edge Proxy (so the EP can
Record-Route with a flow-token for example).</t>
<t>Switched to CRLF for keepalives of connection-oriented transports
after brutal consensus at IETF 68.</t>
<t>Added timed-keepalive parameter and removed the unnecessary
keep-tcp param, per consensus at IETF68.</t>
<t>Removed example "Algorithm 1" which only worked over SIPS, per
consensus at IETF68.</t>
<t>Deleted text about probing and validating with options, per
consensus at IETF68.</t>
<t>Deleted provision for waiting 120 secs before declaring flow
stable, per consensus at IETF68.</t>
<t>fixed example UUIDs</t>
</section>
<section title="Changes from 07 Version">
<t>Add language to show the working group what adding CRLF keepalives
would look like.</t>
<t>Changed syntax of keep-alive=stun to keep-stun so that it was
easier to support multiple tags in the same URI.</t>
</section>
<section title="Changes from 06 Version">
<t>Added the section on operational selection of transports.</t>
<t>Fixed various editorial typos.</t>
<t>Put back in requirement flow token needs to be unique to flow as it
had accidentally been dropped in earlier version. This did not change
any of the flow token algorithms.</t>
<t>Reordered some of the text on STUN keepalive validation to make it
clearer to implementors. Did not change the actual algorithm or
requirements. Added note to explain how if the proxy changes, the
revalidation will happen.</t>
</section>
<section title="Changes from 05 Version">
<t>Mention the relevance of the 'rport' parameter.</t>
<t>Change registrar verification so that only first-hop proxy and the
registrar need to support outbound. Other intermediaries in between do
not any more.</t>
<t>Relaxed flow-token language slightly. Instead of flow-token saving
specific UDP address/port tuples over which the request arrived, make
language fuzzy to save token which points to a 'logical flow' that is
known to deliver data to that specific UA instance.</t>
<t>Added comment that keep-stun could be added to Path.</t>
<t>Added comment that battery concerns could motivate longer TCP
keepalive intervals than the defaults.</t>
<t>Scrubbed document for avoidable lowercase may, should, and
must.</t>
<t>Added text about how Edge Proxies could determine they are the
first hop.</t>
</section>
<section title="Changes from 04 Version">
<t>Moved STUN to a separate section. Reference this section from
within the relevant sections in the rest of the document.</t>
<t>Add language clarifying that UA MUST NOT send STUN without an
explicit indication the server supports STUN.</t>
<t>Add language describing that UA MUST stop sending STUN if it
appears the server does not support it.</t>
<t>Defined a 'sip-stun' option tag. UAs can optionally probe servers
for it with OPTIONS. Clarified that UAs SHOULD NOT put this in a
Proxy-Require. Explain that the first-hop MUST support this
option-tag.</t>
<t>Clarify that SIP/STUN in TLS is on the "inside". STUN used with
Sigcomp-compressed SIP is "outside" the compression layer for UDP, but
wrapped inside the well-known shim header for TCP-based
transports.</t>
<t>Clarify how to decide what a consecutive registration timer is.
Flow must be up for some time (default 120 seconds) otherwise previous
registration is not considered successful.</t>
<t>Change UAC MUST-->SHOULD register a flow for each member of
outbound-proxy-set.</t>
<t>Reworded registrar and proxy in some places (introduce the term
"Authoritative Proxy").</t>
<t>Loosened restrictions on always storing a complete Path vector back
to the registrar/authoritative proxy if a previous hop in the path
vector is reachable.</t>
<t>Added comment about re-registration typically happening over same
flow as original registration.</t>
<t>Changed 410 Gone to new response code 430 Flow Failed. Was going to
change this to 480 Temporarily Unavailable. Unfortunately this would
mean that the authoritative proxy deletes all flows of phones who use
480 for Do Not Disturb. Oops!</t>
<t>Restored sanity by restoring text which explains that registrations
with the same reg-id replace the old registration.</t>
<t>Added text about the 'ob' parameter which is used in Path header
field URIs to make sure that the previous proxy that added a Path
understood outbound processing. The registrar doesn't include
Supported: outbound unless it could actually do outbound processing
(ex: any Path headers have to have the 'ob' parameter).</t>
<t>Added some text describing what a registration means when there is
an instance-id, but no reg-id.</t>
</section>
<section title="Changes from 03 Version">
<t>Added non-normative text motivating STUN vs. SIP PING, OPTIONS, and
Double CRLF. Added discussion about why TCP Keepalives are not always
available.</t>
<t>Explained more clearly that outbound-proxy-set can be "configured"
using any current or future, manual or automatic
configuration/discovery mechanism.</t>
<t>Added a sentence which prevents an Edge Proxy from forwarding back
over the flow over which the request is received if the request
happens to contain a flow token for that flow. This was an
oversight.</t>
<t>Updated example message flow to show a fail-over example using a
new dialog-creating request instead of a mid-dialog request. The old
scenario was leftover from before the outbound / gruu
reorganization.</t>
<t>Fixed tags, Call-IDs, and branch parameters in the example
messages.</t>
<t>Made the ABNF use the "=/" production extension mechanism
recommended by Bill Fenner.</t>
<t>Added a table in an appendix expanding the default flow recovery
timers.</t>
<t>Incorporated numerous clarifications and rewordings for better
comprehension.</t>
<t>Fixed many typos and spelling steaks.</t>
</section>
<section title="Changes from 02 Version">
<t>Removed Double CRLF Keepalive</t>
<t>Changed ;sip-stun syntax to ;keepalive=stun</t>
<t>Fixed incorrect text about TCP keepalives.</t>
</section>
<section title="Changes from 01 Version">
<t>Moved definition of instance-id from GRUU<xref
target="I-D.ietf-sip-gruu"></xref> draft to this draft.</t>
<t>Added tentative text about Double CRLF Keepalive</t>
<t>Removed pin-route stuff</t>
<t>Changed the name of "flow-id" to "reg-id"</t>
<t>Reorganized document flow</t>
<t>Described the use of STUN as a proper STUN usage</t>
<t>Added 'outbound' option-tag to detect if registrar supports
outbound</t>
</section>
<section title="Changes from 00 Version">
<t>Moved TCP keepalive to be STUN.</t>
<t>Allowed SUBSCRIBE to create flow mappings. Added pin-route option
tags to support this.</t>
<t>Added text about updating dialog state on each usage after a
connection failure.</t>
</section>
</section>
<section title="Acknowledgments">
<t>François Audet acted as document shepherd for this draft,
tracking hundreds of comments and incorporating many grammatical fixes
as well as prodding the editors to "get on with it". Jonathan Rosenberg,
Erkki Koivusalo, and Byron Campben provided many comments and useful
text. Dave Oran came up with the idea of using the most recent
registration first in the proxy. Alan Hawrylyshen co-authored the draft
that formed the initial text of this specification. Additionally, many
of the concepts here originated at a connection reuse meeting at IETF 60
that included the authors, Jon Peterson, Jonathan Rosenberg, Alan
Hawrylyshen, and Paul Kyzivat. The TCP design team consisting of Chris
Boulton, Scott Lawrence, Rajnish Jain, Vijay K. Gurbani, and Ganesh
Jayadevan provided input and text. Nils Ohlmeier provided many fixes and
initial implementation experience. In addition, thanks to the following
folks for useful comments: François Audet, Flemming Andreasen,
Mike Hammer, Dan Wing, Srivatsa Srinivasan, Dale Worely, Juha Heinanen,
Eric Rescorla, Lyndsay Campbell, Christer Holmberg, Kevin Johns, Jeroen
van Bemmel, and Derek MacDonald.</t>
</section>
</middle>
<back>
<references title="Normative References">
&rfc2119;
&rfc2141;
&rfc2506;
&rfc3261;
&rfc3263;
&rfc3327;
&rfc3489;
&rfc3581;
&rfc3629;
&rfc3840;
&rfc3841;
&rfc3968;
&rfc3969;
&rfc3986;
&rfc4122;
&rfc5234;
&I-D.ietf-behave-rfc3489bis;
</references>
<references title="Informational References">
&rfc0768;
&rfc0793;
&rfc2104;
&rfc2782;
&rfc3320;
&rfc4346;
&rfc4648;
&rfc4966;
&I-D.ietf-sip-gruu;
&I-D.ietf-sipping-config-framework;
&I-D.ietf-sipping-nat-scenarios;
</references>
<section title="Default Flow Registration Backoff Times">
<t>The base-time used for the flow re-registration backoff times
described in <xref target="recovery"></xref> are configurable. If the
base-time-all-fail value is set to the default of 30 seconds and the
base-time-not-failed value is set to the default of 90 seconds, the
following table shows the resulting delay values.</t>
<texttable>
<ttcol># of reg failures</ttcol>
<ttcol>all flows unusable</ttcol>
<ttcol>>1 non-failed flow</ttcol>
<c>0</c>
<c>0 secs</c>
<c>0 secs</c>
<c>1</c>
<c>30-60 secs</c>
<c>90-180 secs</c>
<c>2</c>
<c>1-2 mins</c>
<c>3-6 mins</c>
<c>3</c>
<c>2-4 mins</c>
<c>6-12 mins</c>
<c>4</c>
<c>4-8 mins</c>
<c>12-24 mins</c>
<c>5</c>
<c>8-16 mins</c>
<c>15-30 mins</c>
<c>6 or more</c>
<c>15-30 mins</c>
<c>15-30 mins</c>
</texttable>
</section>
</back>
</rfc>| PAFTECH AB 2003-2026 | 2026-04-22 15:57:47 |