One document matched: draft-ietf-sip-outbound-12.xml
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<rfc category="std" docName="draft-ietf-sip-outbound-12" 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="24" month="February" 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.
Connections to the UA can be blocked by a firewall device between the UA
and the proxy or registrar, which will only allow new connections in the
direction of the UA to the Proxy. Similarly a NAT could be present,
which is only capable of allowing new connections from the private
address side to the public side. 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"/>. However, these systems can still act as a
Transport Layer Security (TLS) <xref target="RFC4346"/> 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 of 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">RFC 2119</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.</t>
<t hangText="Edge Proxy:">An Edge Proxy is any proxy that is
located topologically between the registering User Agent and the
Authoritative Proxy.</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>
<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
connections 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
reach 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 can 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>
<t>The UA can also ask its first hop proxy to use an specific flow for
subsequent messages when sending a dialog-forming request. This allows
the UA to setup a subscription dialog for the <xref target="I-D.ietf-sipping-config-framework">SIP configuration
package</xref> before the UA registers.</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"/>.
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 on that existing
flow, instead of resolving the Contact URI using the procedures in
<xref target="RFC3263">RFC 3263</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 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>
<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>
<list>
<t>Note: Approaches which select multiple connections from a
single DNS SRV set were also considered, but cannot prevent two
connections from accidentally resolving to the same host. 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>
</list>
</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 SRV can
be used to do load balancing across a proxy farm.</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"/>. 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 the specified 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>When used in this fashion to achieve high reliability, the operator
will need to configure DNS such that the various URIs in the outbound
proxy set do not resolve to the same host.</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. 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. 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>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">RFC 3327</xref>.</t>
</section>
<section title="Keep alive Technique">
<t>This document describes three keep alive mechanisms. 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 two of 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="mech-ua"/> 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 and SCTP, this specification describes a keep alive
approach based on sending CRLFs. For connectionless transport, such as
UDP, 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 are also free to use native transport keep alives, however the
UA 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 therefore
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"/> 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"/>)
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"/> . As described in <xref target="RFC3840"/>,
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">RFC 3840</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 <xref target="RFC3840">RFC 3840</xref> and in the caller preferences
specification <xref target="RFC3841"/>. 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 RFC
2141 <xref target="RFC2141"/>. 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 UA SHOULD send a
unique REGISTER in the normal way 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"/>, MUST include an instance-id media
feature tag as specified in <xref target="section-instance"/>.</t>
<t>These ordinary 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"/>.
</t>
<t>Note that the UA needs to honor 503 (Service Unavailable) responses
to registrations as described in RFC 3261 and <xref target="RFC3263">RFC 3263</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"/> 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 SHOULD be sent over the same flow as the original
registration.</t>
</section>
<section anchor="third-party-reg" title="Non Outbound Registrations">
<t>A User Agent MUST NOT include a reg-id header parameter in the
Contact header field of a registration with a non-zero expiration,
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 UA 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 REGISTER request.</t>
<t>The UA performs normal DNS resolution on the next hop URI (as
described in <xref target="RFC3263">RFC 3263</xref>) to find a
protocol, IP address, and port. For protocols that don't use TLS, if
the UA has an existing flow to this IP address, and port with the
correct protocol, then the UA 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 UA 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 UA is sending a dialog-forming request, and wants all
subsequent requests in the dialog to arrive over the same flow, the UA
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 UA wants a UDP flow to work through NATs or
firewalls it still needs to put the 'rport' parameter <xref target="RFC3581"/>
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"/>.</t>
</list>
</t>
</section>
<section anchor="detect-fail" title="Keep-alives and Detecting Flow Failure">
<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"/> or
<xref target="keepstun"/>. If a
flow has failed, the UA follows the procedures in <xref target="reg"/>
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, it 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 rest of this paragraph provides RECOMMENDED
default values for these keepalives.
The time between each keep alive request when using non connection
based transports such as UDP SHOULD be a random number between 24 and
29 seconds.
For connection based transports such as TCP, correct 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: For UDP, 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. For TCP, 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>
<t>The client needs to perform normal <xref target="RFC3263">RFC
3263</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>
</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 resolve 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"/>.
Clients MUST support STUN based keep alives.</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"/>; 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 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
keepalives have not timed out for 120
seconds after a registration. For STUN-based keepalives, this typically means
three successful STUN transactions over UDP or one successful STUN
transaction over TCP. If a flow is established and is alive after
this amount of time, the number of consecutive registration failures
is set to zero. Each time a flow fails before two minutes, the number
of consecutive registration failures is incremented by one. Note that
a failure during the initial STUN validation does not count against
the number of consecutive registration failures. </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 determines that this is the case, it
inserts its URI in a Path header field value as described in
<xref target="RFC3327">RFC 3327</xref>. If the Edge Proxy is the first SIP
node after the UAC, it 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"/> 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"/>. The concatenation of the HMAC and
S are base64 encoded, as defined in <xref target="RFC4648"/>,
and used as the flow identifier. When
using IPv4 addresses, this will result in a 32-octet
identifier.</t>
<!-- we don't need time or file descriptor since we migrated to full 5-tuple -->
</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.
</t><t>
If the Route header value
contains an 'ob' parameter, the Route header was probably copied from the Path
header in a registration. If this is the case 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>
Whether the Route header contained an 'ob' parameter or not, next
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 in the topmost Route header field
value matches the source IP address and port of the request, the request is an "outgoing"
request. For an "outgoing" request, the edge proxy just removes the
Route header and continues processing the request. Otherwise, this is
an "incoming" request. For an incoming request, 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. For connection-oriented transports,
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 this document
to form a flow token follow the procedures below to determine the
correct flow.</t>
<t>
<list style="hanging">
<t hangText="Example Algorithm:">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
proxy SHOULD send a 403 (Forbidden) response. (The proxy could
ignore such requests for prevention of denial of service attacks.)
If the HMAC is correct then the proxy SHOULD forward the request
on the flow that was specified by the information in the flow
identifier. If this flow no longer exists, the proxy SHOULD send a
430 (Flow Failed) response to the request.</t>
</list>
</t>
<t>Note that this specification needs mid-dialog requests to be routed
over the same flows as those stored in the Path vector from the
initial registration, but 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.
The Edge Proxy can use the presence of the "ob" parameter in
dialog-forming requests in the UAC's Contact URI (or topmost Route
header field) to determine if it should add a flow token.</t>
</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"/>.</t>
<t>When a server receives a double CRLF sequence 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: Processing REGISTER Requests">
<t>This specification updates the definition of a binding in <xref target="RFC3261">RFC 3261</xref> Section 10 and <xref target="RFC3327">RFC 3327</xref> Section 5.3.</t>
<t>Registrars which implement this specification MUST support the Path
header mechanism <xref target="RFC3327">RFC 3327</xref>.</t>
<t>When receiving a REGISTER request, the registrar first checks from
its Via header field if the registrar is the first hop or not. If the
registrar is not the first hop, it examines 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 cannot
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"/> 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. If the Contact header contains more than one
header field value with a non-zero expiration and a 'reg-id' parameter,
the entire registration SHOULD be rejected with a 400 Bad Request
response. If the Contact header did not contain a 'reg-id' parameter or
if that parameter became 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">RFC
3261</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"/>)
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"/>).</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>
<t>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 SIP node MAY also probe
the next hop using a SIP OPTIONS request to check for support of the
'sip-stun' option tag in a Supported header field. -->
</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.<!-- A 408 response to a next-hop OPTIONS probe is also considered
a flow failure. -->
</t>
<t>The SIP keep alive STUN usage requires no backwards compatibility with
<xref target="RFC3489">RFC 3489</xref>.</t>
<!-- <section anchor="cartman-probe" title="Explicit Option Probes">
<t>This section defines a new SIP option-tag called 'sip-stun'.
Advertising this option-tag indicates that the server can receive SIP
messages and STUN messages as part of the NAT Keepalive usage on the
same port. Clients that want to probe a SIP server to determine support
for STUN, can send an OPTIONS request to the next hop by setting the
Max-Forwards header field to zero or addressing the request to that
server. The OPTIONS response will contain a Supported header field with
a list of the server's supported option-tags.
</t>
<t><list><t>A UAC SHOULD NOT include the 'sip-stun' option-tag in a
Proxy-Require header. This is because a request with this header will
fail in some topologies where the first proxy support sip-stun, but a
subsequent proxy does not. Note that RFC 3261 does not allow proxies to
remove option-tags from a Proxy-Require header field.
</t></list></t>
</section> -->
<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>
In the first part of this example message flow, Bob's UA sends a
SUBSCRIBE request for the UA profile configuration package. 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. Now that Bob's UA is configured with
the outbound-proxy-set, 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>
<t>
A bit later, 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>
<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>
<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 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"/> 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>
<!--
other-tags = sip-instance / "+" ftag-name
sip-instance = "+sip.instance"
-->
</figure>
<t>The value of the reg-id MUST NOT be 0 and MUST be less than
2**31.</t>
</section>
<section title="New Response Codes">
<section title="Definition of 430 Flow Failed response code">
<t>This specification defines a new SIP response code '430 Flow
Failed'. 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.</t>
</section>
<section anchor="bad-first-hop" title="Definition of 439 First Hop Lacks Outbound Support response">
<t>This specification defines a new SIP response code '439 First Hop
Lacks Outbound Support'. 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.</t>
</section>
</section>
<section anchor="iana" title="IANA Considerations">
<section anchor="iana-flow-timer" title="Flow-Timer Header Field">
<t>
This specification defines a new SIP header field "Flow-Timer".
</t>
<figure><artwork>
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"/>.
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"/>. 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>
<!-- <list style="hanging">
<t hangText="Name:">sip-stun</t>
<t hangText="Description:">This option-tag is used to identify
SIP servers which can receive STUN requests described in the STUN
NAT Keepalive usage on the same ports they use to receive SIP messages.
</t>
</list> -->
</t>
</section>
<section title="Response Codes">
<t>This section registers two new SIP Response Codes, as per the
guidelines in Section 27.4 of RFC 3261.</t>
<section title="430 Response Code">
<t/>
<t>
<list style="hanging">
<t hangText="Code:">430</t>
<t hangText="Default Reason Phrase:">Flow Failed</t>
<t hangText="Reference:">This document</t>
</list>
</t>
</section>
<section title="439 Response Code">
<t/>
<t>
<list style="hanging">
<t hangText="Code:">439</t>
<t hangText="Default Reason Phrase:">First Hop Lacks Outbound
Support</t>
<t hangText="Reference:">This document</t>
</list>
</t>
</section>
</section>
<section title="Media Feature Tag">
<t>This section registers a new media feature tag, per the procedures
defined in <xref target="RFC2506">RFC 2506</xref>. The tag is placed
into the sip tree, which is defined in <xref target="RFC3840">RFC
3840</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.<!-- Note to Cullen: This last sentence is in response to a comment from
Fredrik Thulin. -->
</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"/> and <xref target="RFC3327"/> are also
relevant to this document. For the security considerations of generating
flow tokens, please also see <xref target="flowtokens"/>. A
discussion of preventing the avalanche restart problem is in <xref target="recovery"/>.</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 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"/> 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 = "Changes from 01 Version" >
<t>
Changed the algorithm and timing for retries of re-registrations. </t>
<t>
Changed to using sigcomp style URI parameter to detect it - UA should attempt
STUN to proxy. </t>
<t>
Changed to use a configured set of secondary proxies instead of playing DNS games
to try and figure out what secondary proxies to use. </t>
</section>
<section title = "Changes from 00 Version" >
<t>
Changed the behavior of the proxy so that it does not automatically remove
registrations with the same instance-id and reg-id but instead just uses the
most recently created registration first. </t>
<t>
Changed the connection-id to reg-id. </t>
<t>
Fixed expiry of edge proxies and rewrote mechanism section to be clearer. </t>
</section>
-->
<section title="Acknowledgments">
<t>Francois 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: Francois
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">
&rfc2104;
&rfc2782;
&rfc3320;
&rfc4346;
&rfc4648;
&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"/> 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>
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