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RFC 5626


Managing Client-Initiated Connections in the Session Initiation Protocol (SIP)

Part 2 of 3, p. 13 to 30
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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.

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   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.  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

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   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

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   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.

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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
   REGISTER request.

   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].

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      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
   described below.

   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.

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   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:

   double-CRLF = CR LF CR LF
   CR = %x0D
   LF = %x0A

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   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

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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.

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   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

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   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.

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   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.

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      Implementation note: Specific procedures at the edge proxy to
      ensure that mid-dialog requests are routed over an existing flow
      are not part of this specification.  However, an approach such as
      having the edge proxy add a Record-Route header with a flow token
      is one way to ensure that mid-dialog requests are routed over the
      correct flow.

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.

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   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
   malicious messages.

   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

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   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.

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   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.

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      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
   with [RFC3489].

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

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      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.

(page 30 continued on part 3)

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