4. User Agent Procedures
4.1. Instance ID Creation
Each UA MUST have an Instance Identifier Uniform Resource Name (URN)
[RFC2141] 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
A UA SHOULD create a Universally Unique Identifier (UUID) URN
[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.
Note: A device like a "soft phone", when first installed, can
generate a UUID [RFC4122] 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 Section 3.2) is perfectly legal as
long as the device knows no other UUIDs were generated at this
time on this device.
If a URN scheme other than UUID is used, the UA MUST only use URNs
for which an RFC (from the IETF stream) defines how the specific URN
needs to be constructed and used in the "+sip.instance" Contact
header field parameter for outbound behavior.
To convey its instance-id in both requests and responses, the UA
includes a "sip.instance" media feature tag as a UA characteristic
[RFC3840]. This media feature tag is encoded in the Contact header
field as the "+sip.instance" Contact header field parameter. One
case where a UA could prefer to omit the "sip.instance" media feature
tag is when it is making an anonymous request or some other privacy
concern requires that the UA not reveal its identity.
Note: [RFC3840] defines equality rules for callee capabilities
parameters, and according to that specification, the
"sip.instance" media feature tag will be compared by case-
sensitive string comparison. This means that the URN will be
encapsulated by angle brackets ("<" and ">") when it is placed
within the quoted string value of the "+sip.instance" Contact
header field parameter. The case-sensitive matching rules apply
only to the generic usages defined in the callee capabilities
[RFC3840] and the caller preferences [RFC3841] specifications.
When the instance ID is used in this specification, it is
"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 [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 that provides the
URN is the SIP UA instance identified by that URN. As a result,
the UA instance has to 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 to (yet
lexicographically different from) previous registrations.
4.2.1. Initial Registrations
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
For each outbound proxy URI in the set, the User Agent Client (UAC)
SHOULD send a REGISTER request using this URI as the default outbound
proxy. (Alternatively, the UA could limit the number of flows formed
to conserve battery power, for example). If the set has more than
one URI, the UAC MUST send a REGISTER request to at least two of the
default outbound proxies from the set. 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.
REGISTER requests, other than those described in Section 4.2.3, MUST
include an instance-id media feature tag as specified in Section 4.1.
A UAC conforming to this specification MUST include in the Contact
header field, a "reg-id" parameter that is distinct from other
"reg-id" parameters used in other registrations that use the same
"+sip.instance" Contact header field parameter and AOR. 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.
Note: The UAC can situationally decide whether to request outbound
behavior by including or omitting the "reg-id" Contact header
field 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 ensure consistency.
Similarly, if the registration through EP1 did not support
outbound, the UA might not register with EP2 at all.
The UAC MUST support the Path header [RFC3327] mechanism, and
indicate its support 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.
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 has potentially registered an un-routable contact.
It is the responsibility of the UA to remove any inappropriate
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 keep-alives as described
in Section 4.4.
Note: The UA needs to honor 503 (Service Unavailable) responses to
registrations as described in [RFC3261] and [RFC3263]. 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 request
immediately. Implementations need to ensure that when retrying
the REGISTER request, 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.
If the registering UA receives a 439 (First Hop Lacks Outbound
Support) response to a REGISTER request, it MAY re-attempt
registration without using the outbound mechanism (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 Section 11.6 for more information on the
439 response code.
4.2.2. Subsequent REGISTER Requests
Registrations for refreshing a binding and for removing a binding use
the same instance-id and reg-id values as the corresponding initial
registration where the binding was added. Registrations that merely
refresh an existing binding are sent over the same flow as the
original registration where the binding was added.
If a re-registration is rejected with a recoverable error response,
for example by a 503 (Service Unavailable) containing a Retry-After
header, the UAC SHOULD NOT tear down the corresponding flow if the
flow uses a connection-oriented transport such as TCP. As long as
"pongs" are received in response to "pings", the flow SHOULD be kept
active until a non-recoverable error response is received. This
prevents unnecessary closing and opening of connections.
4.2.3. Third-Party Registrations
In an initial registration or re-registration, a UA MUST NOT include
a "reg-id" header field parameter in the Contact header field if the
registering UA is not the same instance as the UA referred to by the
target Contact header field. (This practice is occasionally used to
install forwarding policy into registrars.)
A UAC also MUST NOT include an instance-id feature tag or "reg-id"
Contact header field parameter in a request to un-register all
Contacts (a single Contact header field value with the value of "*").
4.3. Sending Non-REGISTER Requests
When a UAC is about to send a request, it first performs normal
processing to select the next hop URI. The UA can use a variety of
techniques to compute the route set and accordingly the next hop URI.
Discussion of these techniques is outside the scope of this document.
UAs that support this specification SHOULD include the outbound
option tag in a Supported header field in a request that is not a
The UAC performs normal DNS resolution on the next hop URI (as
described in [RFC3263]) to find a protocol, IP address, and port.
For protocols that don't use TLS, if the UAC has an existing flow to
this IP address, and port with the correct protocol, then the UAC
MUST use the existing connection. For TLS protocols, there MUST also
be a match between the host production in the next hop and one of the
URIs contained in the subjectAltName in the peer certificate. If the
UAC cannot use one of the existing flows, then it SHOULD form a new
flow by sending a datagram or opening a new connection to the next
hop, as appropriate for the transport protocol.
Typically, a UAC using the procedures of this document and sending a
dialog-forming request will want all subsequent requests in the
dialog to arrive over the same flow. If the UAC is using a Globally
Routable UA URI (GRUU) [RFC5627] that was instantiated using a
Contact header field value that included an "ob" parameter, the UAC
sends the request over the flow used for registration, and subsequent
requests will arrive over that same flow. If the UAC is not using
such a GRUU, then the UAC adds an "ob" parameter to its Contact
header field value. This will cause all subsequent requests in the
dialog to arrive over the flow instantiated by the dialog-forming
request. This case is typical when the request is sent prior to
registration, such as in the initial subscription dialog for the
configuration framework [CONFIG-FMWK].
Note: If the UAC wants a UDP flow to work through NATs or
firewalls, it still needs to put the 'rport' parameter [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 [NAT-SCEN].
4.4. Keep-Alives and Detecting Flow Failure
Keep-alives are used for refreshing NAT/firewall bindings and
detecting flow failure. Flows can fail for many reasons including
the rebooting of NATs and the crashing of edge proxies.
As described in Section 4.2, a UA that registers will begin sending
keep-alives after an appropriate registration response. A UA that
does not register (for example, a PSTN gateway behind a firewall) can
also send keep-alives under certain circumstances.
Under specific circumstances, a UAC might be allowed to send STUN
keep-alives even if the procedures in Section 4.2 were not completed,
provided that there is an explicit indication that the target first-
hop SIP node supports STUN keep-alives. For example, this applies to
a non-registering UA or to a case where the UA registration
succeeded, but the response did not include the outbound option-tag
in the Require header field.
Note: A UA can "always" send a double CRLF (a "ping") over
connection-oriented transports as this is already allowed by
Section 7.5 of [RFC3261]. However a UA that did not register
using outbound registration cannot expect a CRLF in response (a
"pong") unless the UA has an explicit indication that CRLF keep-
alives are supported as described in this section. Likewise, a UA
that did not successfully register with outbound procedures needs
explicit indication that the target first-hop SIP node supports
STUN keep-alives before it can send any STUN messages.
A configuration option indicating keep-alive support for a specific
target is considered an explicit indication. If these conditions are
satisfied, the UA sends its keep-alives according to the same
guidelines as those used when UAs register; these guidelines are
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 Sections 4.4.1 or 4.4.2. If
a flow with a registration has failed, the UA follows the procedures
in Section 4.2 to form a new flow to replace the failed one.
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 keep-alives before it
could consider the corresponding flow dead. Note that the server
would wait for an amount of time larger than the Flow-Timer in order
to have a grace period to account for transport delay. The UA MUST
send keep-alives at least as often as this number of seconds. If the
UA uses the server-recommended keep-alive frequency it SHOULD send
its keep-alives so that the interval between each keep-alive is
randomly distributed between 80% and 100% of the server-provided
time. For example, if the server suggests 120 seconds, the UA would
send each keep-alive with a different frequency between 95 and 120
If no Flow-Timer header field was present in a register response for
this flow, the UA can send keep-alives at its discretion. The
sections below provide RECOMMENDED default values for these keep-
The client needs to perform normal [RFC3263] 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.
Section 4.4.1 describes a keep-alive mechanism for connection-
oriented transports such as TCP or SCTP. Section 4.4.2 describes a
keep-alive mechanism for connection-less transports such as UDP.
Support for other transports such as DCCP [RFC4340] is for further
4.4.1. Keep-Alive with CRLF
This approach MUST only be used with connection oriented transports
such as TCP or SCTP; it MUST NOT be used with connection-less
transports such as UDP.
A User Agent that forms flows checks if the configured URI to which
the UA is connecting resolves to a connection-oriented transport
(e.g., TCP and TLS over TCP).
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:
CRLF = CR LF
double-CRLF = CR LF CR LF
CR = %x0D
LF = %x0A
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
SigComp [RFC3320] 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.
If a pong is not received within 10 seconds after sending a ping (or
immediately after processing any incoming message being received when
that 10 seconds expires), then the client MUST treat the flow as
failed. Clients MUST support this CRLF keep-alive.
Note: This value of 10-second timeout was selected to be long
enough that it allows plenty of time for a server to send a
response even if the server is temporarily busy with an
administrative activity. At the same time, it was selected to be
small enough that a UA registered to two redundant servers with
unremarkable hardware uptime could still easily provide very high
levels of overall reliability. Although some Internet protocols
are designed for round-trip times over 10 seconds, SIP for real-
time communications is not really usable in these type of
environments as users often abandon calls before waiting much more
than a few seconds.
When a Flow-Timer header field is not provided in the most recent
success registration response, the proper selection of keep-alive
frequency is primarily a trade-off between battery usage and
availability. The UA MUST select a random number between a fixed or
configurable upper bound and a lower bound, where the lower bound is
20% less then the upper bound. The fixed upper bound or the default
configurable upper bound SHOULD be 120 seconds (95 seconds for the
lower bound) where battery power is not a concern and 840 seconds
(672 seconds for the lower bound) where battery power is a concern.
The random number will be different for each keep-alive "ping".
Note on selection of time values: the 120-second 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 will be 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-second lower bound was chosen so that the jitter introduced
will result in a relatively even load on the servers after 30
4.4.2. Keep-Alive with STUN
This approach MUST only be used with connection-less transports, such
as UDP; it MUST NOT be used for connection-oriented transports such
as TCP and SCTP.
A User Agent that forms flows checks if the configured URI to which
the UA is connecting resolves to use the UDP transport. The UA can
periodically perform keep-alive checks by sending STUN [RFC5389]
Binding Requests over the flow as described in Section 8. Clients
MUST support STUN-based keep-alives.
When a Flow-Timer header field is not included in a successful
registration response, the time between each keep-alive request
SHOULD be a random number between 24 and 29 seconds.
Note on selection of time values: the upper bound of 29 seconds
was selected, as many NATs have UDP timeouts as low as 30 seconds.
The 24-second lower bound was selected so that after 10 minutes
the jitter introduced by different timers will make the keep-alive
requests unsynchronized to evenly spread the load on the servers.
Note that the short NAT timeouts with UDP have a negative impact
on battery life.
If a STUN Binding Error Response is received, or if no Binding
Response is received after 7 retransmissions (16 times the STUN "RTO"
timer -- where 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.
4.5. Flow Recovery
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
value as the registration it replaces. This is done in much the same
way as forming a brand new flow as described in Section 4.2; 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.
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.
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 a lower value (with a default of 30 seconds);
otherwise, in the case where at least one of the flows has not
failed, the base-time is set to a higher value (with a default of 90
seconds). The upper-bound wait time (W) 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
configurable maximum time (with a default of 1800 seconds).
W = min (max-time, (base-time * (2 ^ consecutive-failures)))
These times MAY be configurable in the UA. The three times are:
o max-time with a default of 1800 seconds
o base-time (if all failed) with a default of 30 seconds
o base-time (if all have not failed) with a default of 90 seconds
For example, if the base-time is 30 seconds, and there were three
failures, then the upper-bound wait time is min(1800, 30*(2^3)) or
240 seconds. The actual amount of time the UA waits before retrying
registration (the retry delay time) is computed by selecting a
uniform random time between 50 and 100% of the upper-bound wait time.
The UA MUST wait for at least the value of the retry delay time
before trying another registration to form a new flow for that URI (a
503 response to an earlier failed registration attempt with a Retry-
After header field value may cause the UA to wait longer).
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
5. Edge Proxy Procedures
5.1. Processing Register Requests
When an edge proxy receives a registration request with a "reg-id"
header field parameter in the Contact header field, it needs to
determine if it (the edge proxy) will have to be visited for any
subsequent requests sent to the User Agent identified in the Contact
header field, or not. If the edge proxy is the first hop, as
indicated by the Via header field, it MUST insert its URI in a Path
header field value as described in [RFC3327]. If it is not the first
hop, it might still decide to add itself to the Path header based on
local policy. In addition, if the edge proxy is the first SIP node
after the UAC, the edge proxy either MUST store a "flow token"
(containing information about the flow from the previous hop) in its
Path URI or reject the request. The flow token MUST be an identifier
that is unique to this network flow. The flow token MAY be placed in
the userpart of the URI. In addition, the first node MUST include an
"ob" URI parameter in its Path header field value. If the edge proxy
is not the first SIP node after the UAC it MUST NOT place an "ob" URI
parameter in a Path header field value. The edge proxy can determine
if it is the first hop by examining the Via header field.
5.2. Generating Flow Tokens
A trivial but impractical way to satisfy the flow token requirement
in Section 5.1 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
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 [RFC2104]. The
concatenation of the HMAC and S are base64 encoded, as defined in
[RFC4648], and used as the flow identifier. When using IPv4
addresses, this will result in a 32-octet identifier.
5.3. Forwarding Non-REGISTER Requests
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 follows the procedures of this section. Otherwise
the edge proxy skips the procedures in this section, removes itself
from the Route header field, and continues processing the request.
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.
If the flow in the flow token identified by the topmost Route header
field value matches the source IP address and port of the request,
the request is an "outgoing" request; otherwise, it is an "incoming"
5.3.1. Processing Incoming Requests
If the Route header value contains an "ob" URI parameter, the Route
header was probably copied from the Path header in a registration.
If the Route header value contains an "ob" URI parameter, and the
request is a new dialog-forming request, the proxy needs to adjust
the route set to ensure that subsequent requests in the dialog can be
delivered over a valid flow to the UA instance identified by the flow
Note: 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"
Next, whether the Route header field contained an "ob" URI parameter
or not, the proxy removes the Route header field value and forwards
the request over the 'logical flow' identified by the flow token,
that is known to deliver data to the specific target UA instance. If
the flow token has been tampered with, the proxy SHOULD send a 403
(Forbidden) response. If the flow no longer exists, the proxy SHOULD
send a 430 (Flow Failed) response to the request.
Proxies that used the example algorithm described in Section 5.2 to
form a flow token follow the procedures below to determine the
correct flow. To decode the flow token, take the flow identifier in
the user portion of the URI and base64 decode it, then verify the
HMAC is correct by recomputing the HMAC and checking that it matches.
If the HMAC is not correct, the request has been tampered with.
5.3.2. Processing Outgoing Requests
For mid-dialog requests to work with outbound UAs, the requests need
to be forwarded over some valid flow to the appropriate UA instance.
If the edge proxy receives an outgoing dialog-forming request, the
edge proxy can use the presence of the "ob" URI parameter in the
UAC's Contact URI (or topmost Route header field) to determine if the
edge proxy needs to assist in mid-dialog request routing.
Implementation note: Specific procedures at the edge proxy to
ensure that mid-dialog requests are routed over an existing flow
are not part of this specification. However, an approach such as
having the edge proxy add a Record-Route header with a flow token
is one way to ensure that mid-dialog requests are routed over the
5.4. Edge Proxy Keep-Alive Handling
All edge proxies compliant with this specification MUST implement
support for STUN NAT keep-alives on their SIP UDP ports as described
in Section 8.
When a server receives a double CRLF sequence between SIP messages on
a connection-oriented transport such as TCP or SCTP, it MUST
immediately respond with a single CRLF over the same connection.
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.
The reason a proxy would send a Flow-Timer is if it wishes to detect
flow failures proactively and take appropriate action (e.g., log
alarms, provide alternative treatment if incoming requests for the UA
are received, etc.). The server MUST wait for an amount of time
larger than the Flow-Timer in order to have a grace period to account
for transport delay.
6. Registrar Procedures
This specification updates the definition of a binding in [RFC3261],
Section 10 and [RFC3327], Section 5.3.
Registrars that implement this specification MUST support the Path
header mechanism [RFC3327].
When receiving a REGISTER request, the registrar MUST check from its
Via header field if the registrar is the first hop or not. If the
registrar is not the first hop, it MUST examine the Path header of
the request. If the Path header field is missing or it exists but
the first URI does not have an "ob" URI parameter, then outbound
processing MUST NOT be applied to the registration. In this case,
the following processing applies: if the REGISTER request contains
the reg-id and the outbound option tag in a Supported 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
Section 11.6 for more information on the 439 response code.
A Contact header field value with an instance-id media feature tag
but no "reg-id" header field parameter is valid (this combination
will result in the creation of a GRUU, as described in the GRUU
specification [RFC5627]), but one with a reg-id but no instance-id is
not valid. If the registrar processes a Contact header field value
with a reg-id but no instance-id, it simply ignores the reg-id
A registration containing a "reg-id" header field parameter and a
non-zero expiration is used to register a single UA instance over a
single flow, and can also de-register any Contact header fields with
zero expiration. Therefore, if the Contact header field contains
more than one header field value with a non-zero expiration and any
of these header field values contain a "reg-id" Contact header field
parameter, the entire registration SHOULD be rejected with a 400 (Bad
Request) response. The justification for recommending rejection
versus making it mandatory is that the receiver is allowed by
[RFC3261] to squelch (not respond to) excessively malformed or
If the Contact header did not contain a "reg-id" Contact header field
parameter or if that parameter was ignored (as described above), the
registrar MUST NOT include the outbound option-tag in the Require
header field of its response.
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" URI parameter. If the Contact header field does not
contain a "+sip.instance" Contact header field parameter, the
registrar processes the request using the Contact binding rules in
When a "+sip.instance" Contact header field parameter and a "reg-id"
Contact header field parameter are 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" Contact
header parameter) and the value of "reg-id" Contact header field
parameter parameter. The registrar MUST store in the binding the
Contact URI, all the Contact header 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.) Provided that the UAC had included an outbound
option-tag (defined in Section 11.4) in a Supported header field
value in the REGISTER request, the registrar MUST include the
outbound option-tag in a Require header field value in its response
to that REGISTER request.
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 be just 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.
If the registrar receives a re-registration for a specific
combination of AOR, and 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.
To be compliant with this specification, registrars that can receive
SIP requests directly from a UAC without intervening edge proxies
MUST implement the same keep-alive mechanisms as edge proxies
(Section 5.4). Registrars with a direct flow with a UA MAY include a
Flow-Timer header in a 2xx class registration response that includes
the outbound option-tag in the Require header.
7. Authoritative Proxy Procedures: Forwarding Requests
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:
o The proxy MUST NOT populate the target set with more than one
contact with the same AOR and instance-id at a time.
o 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.
o 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.
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 co-located 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.
Implementation note: Typically this means that for TCP, the
request is sent on the same TCP socket that received the REGISTER
request. For UDP, the request is sent from the same local IP
address and port over which the registration was received, to the
same IP address and port from which the REGISTER was received.
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.
8. STUN Keep-Alive Processing
This section describes changes to the SIP transport layer that allow
SIP and STUN [RFC5389] Binding Requests to be mixed over the same
flow. This constitutes 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. These STUN keep-alives are always
sent to the next SIP hop. STUN messages are not delivered end-to-
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.
The corresponding Binding Responses do not require any STUN
attributes except the XOR-MAPPED-ADDRESS. The UAS, proxy, or
registrar responds to a valid Binding Request with a Binding Response
that MUST include the XOR-MAPPED-ADDRESS attribute.
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.
Note: 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
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.
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. UACs MUST NOT use an ambiguous
configuration option such as "Work through NATs?" or "Do keep-
alives?" to imply next-hop STUN support. A UAC MAY use the presence
of an "ob" URI parameter in the Path header in a registration
response as an indication that its first edge proxy supports the
keep-alives defined in this document.
Note: Typically, a SIP node first sends a SIP request and waits to
receive a 2xx 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
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.
The SIP keep-alive STUN usage requires no backwards compatibility
8.1. Use with SigComp
When STUN is used together with SigComp [RFC3320] 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.
Note: All SigComp messages contain a prefix (the five most
significant bits of the first byte are set to one) that does not
occur in UTF-8 [RFC3629] encoded text messages, so for
applications that use this encoding (or ASCII encoding) it is
possible to multiplex uncompressed application messages and
SigComp messages on the same UDP port. 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.