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Versions: (draft-jones-insipid-session-id) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 RFC 7989

Network Working Group                                           P. Jones
Internet Draft                                              G. Salgueiro
Intended status: Standards Track                               C. Pearce
Expires: March 25, 2016                                    Cisco Systems
                                                      September 25, 2015




         End-to-End Session Identification in IP-Based Multimedia
                          Communication Networks
                     draft-ietf-insipid-session-id-15


Abstract

   This document describes an end-to-end Session Identifier for use in
   IP-based multimedia communication systems that enables endpoints,
   intermediary devices, and management systems to identify a session
   end-to-end, associate multiple endpoints with a given multipoint
   conference, track communication sessions when they are redirected,
   and associate one or more media flows with a given communication
   session.

   This document also describes a backwards compatibility mechanism for
   an existing (RFC 7329) session identifier implementation that is
   sufficiently different from the procedures defined in this document.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html

   This Internet-Draft will expire on March 25, 2016.




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Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction...................................................3
   2. Conventions used in this document..............................3
   3. Session Identifier Requirements and Use Cases..................4
   4. Constructing and Conveying the Session Identifier..............4
      4.1. Constructing the Session Identifier.......................4
      4.2. Conveying the Session Identifier..........................5
   5. The Session-ID Header Field....................................6
   6. Endpoint Behavior..............................................7
   7. Processing by Intermediaries...................................9
   8. Associating Endpoints in a Multipoint Conference..............11
   9. Examples of Various Call Flow Operations......................12
      9.1. Basic Call with 2 UUIDs..................................12
      9.2. Basic Call Transfer using REFER..........................16
      9.3. Basic Call Transfer using re-INVITE......................18
      9.4. Single Focus Conferencing................................19
      9.5. Single Focus Conferencing using WebEx....................21
      9.6. Cascading Conference Bridges.............................22
         9.6.1. Establishing a Cascaded Conference..................22
         9.6.2. Calling into Cascaded Conference Bridges............23
      9.7. Basic 3PCC for two UAs...................................24
      9.8. Handling in 100 Trying SIP Response and CANCEL Request...25
         9.8.1. Handling in a 100 Trying SIP Response...............25
         9.8.2. Handling a CANCEL SIP Request.......................27
      9.9. Out-of-dialog REFER Transaction..........................27
   10. Compatibility with a Previous Implementation.................28
   11. Security Considerations......................................30
   12. IANA Considerations..........................................31
      12.1. Registration of the "Session-ID" Header Field...........31
      12.2. Registration of the "remote" Parameter..................31
   13. Acknowledgments..............................................31
   14. Dedication...................................................31
   15. References...................................................32
      15.1. Normative References....................................32
      15.2. Informative References..................................32



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   Authors' Addresses...............................................34


1. Introduction

   IP-based multimedia communication systems like SIP [RFC3261] and
   H.323 [H.323] have the concept of a "call identifier" that is
   globally unique.  The identifier is intended to represent an end-to-
   end communication session from the originating device to the
   terminating device.  Such an identifier is useful for
   troubleshooting, session tracking, and so forth.

   For several reasons, however, the current call identifiers defined in
   SIP and H.323 are not suitable for end-to-end session identification.
   A fundamental issue in protocol interworking is the fact that the
   syntax for the call identifier in SIP and H.323 is different.  Thus,
   if both protocols are used in a call, it is impossible to exchange
   the call identifier end-to-end.

   Another reason why the current call identifiers are not suitable to
   identify a session end-to-end is that, in real-world deployments,
   devices like session border controllers [RFC7092] often change the
   session signaling as it passes through the device, including the
   value of the call identifier.  While this is deliberate and useful,
   it makes it very difficult to track a session end-to-end.

   This document defines a new identifier for SIP referred to as the
   Session Identifier that is intended to overcome the issues that exist
   with the currently defined call identifiers used in SIP.  The
   procedures specified in this document attempt to comply with the
   requirements specified in [RFC7206].  The procedures also specify
   capabilities not mentioned in [RFC7206], shown in call flows in
   section 9.  Additionally, the specification attempts to account for a
   previous, proprietary version of a SIP Session Identifier header
   [RFC7329], specifying a backwards compatibility approach in section
   10.

2. Conventions used in this document

   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 RFC 2119 [RFC2119]
   when they appear in ALL CAPS.  These words may also appear in this
   document in lower case as plain English words, absent their normative
   meanings.

   The term "Session Identifier" refers to the value of the identifier,
   whereas "Session-ID" refers to the header field used to convey the
   identifier.  The Session Identifier is a set of two Universally
   Unique Identifiers (UUIDs) and each element of that set is simply
   referred to herein as a UUID.



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   Throughout this document, the term "endpoint" refers to a SIP User
   Agent (UA) that either initiates or terminates a SIP session, such as
   a user's mobile phone or a conference server, but excludes entities
   like B2BUAs that are generally located along the call signaling path
   between endpoints.  The term "intermediary" refers to any SIP entity
   along the call signaling path between the aforementioned endpoints,
   including Back-to-Back User Agents (B2BUAs) and SIP proxies.

3. Session Identifier Requirements and Use Cases

   Requirements and use cases for the end-to-end Session Identifier,
   along with a definition of "session identifier" and "communication
   session", can be found in [RFC7206].

   As mentioned in section 6.1 of RFC 7206, the ITU-T undertook a
   parallel effort to define compatible procedures for an H.323 Session
   Identifier.  They are documented in [H.460.27].

4. Constructing and Conveying the Session Identifier

4.1. Constructing the Session Identifier

   The Session Identifier comprises two UUIDs [RFC4122], with each UUID
   representing one of the endpoints participating in the session.

   The version number in the UUID indicates the manner in which the UUID
   is generated, such as using random values or using the MAC address of
   the endpoint.  To satisfy the requirement that no user or device
   information be conveyed, endpoints SHOULD generate version 4 (random)
   or version 5 (SHA-1) UUIDs to address privacy concerns related to use
   of MAC addresses in UUIDs.

   When generating a version 5 UUID, endpoints or intermediaries MUST
   utilize the procedures defined in Section 4.3 of [RFC4122] and employ
   the following "name space ID":

       uuid_t NameSpace_SessionID = {
           /* a58587da-c93d-11e2-ae90-f4ea67801e29 */
           0xa58587da,
           0xc93d,
           0x11e2,
           0xae, 0x90, 0xf4, 0xea, 0x67, 0x80, 0x1e, 0x29
       }

   Further, the "name" to utilize for version 5 UUIDs is the
   concatenation of the Call-ID header-value and the "tag" parameter
   that appears on the "From" or "To" line associated with the device
   for which the UUID is created.  Once an endpoint generates a UUID for
   a session, the UUID never changes, even if values originally used as
   input into its construction change over time.




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   Stateless intermediaries that insert a Session-ID header field into a
   SIP message on behalf of an endpoint MUST utilize version 5 UUIDs to
   ensure that UUIDs for the communication session are consistently
   generated.  If a stateless intermediary does not know the tag value
   for the endpoint (e.g., a new INVITE without a To: tag value or an
   older SIP [RFC2543] implementation that did not include a tag
   parameter), the intermediary MUST NOT attempt to generate a UUID for
   that endpoint.  Note that if an intermediary is stateless and the
   endpoint on one end of the call is replaced with another endpoint due
   to some service interaction, the values used to create the UUID
   should change and, if so, the intermediary will compute a different
   UUID.

4.2. Conveying the Session Identifier

   The SIP User Agent (UA) initially transmitting the SIP request
   ("Alice"), i.e., a User Agent Client (UAC), will create a UUID and
   transmit that to the ultimate destination UA ("Bob").  Likewise, the
   destination UA ("Bob"), i.e., a User Agent Server (UAS), will create
   a UUID and transmit that to the first UA ("Alice").  These two
   distinct UUIDs form what is referred to as the Session Identifier and
   is represented in this document in set notation of the form {A,B},
   where "A" is UUID value created by UA "Alice" and "B" is the UUID
   value created by UA "Bob".  The Session Identifier {A,B} is equal to
   the Session Identifier {B,A}.

   In the case where only one UUID is known, such as when a UA first
   initiates a SIP request, the Session Identifier would be {A,N}, where
   "A" represents the UUID value transmitted by the UA "Alice" and "N"
   is what is referred to as the null UUID (see section 5).

   Since SIP sessions are subject to any number of service interactions,
   SIP INVITE messages might be forked as sessions are established, and
   since conferences might be established or expanded with endpoints
   calling in or the conference focus calling out, the construction of
   the Session Identifier as a set of UUIDs is important.

   To understand this better, consider that an endpoint participating in
   a communication session might be replaced with another, such as the
   case where two "legs" of a call are joined together by a PBX.
   Suppose "Alice" and "Bob" both call UA C ("Carol").  There would be
   two distinctly identifiable Session Identifiers, namely {A,C} and
   {B,C}.  Then suppose that "Carol" uses a local PBX function to join
   the call between herself and "Alice" with the call between herself
   and "Bob", resulting in a single remaining call between "Alice" and
   "Bob".  This merged call can be identified using two UUID values
   assigned by each entity in the communication session, namely {A,B} in
   this example.

   In the case of forking, "Alice" might send an INVITE that gets forked
   to several different endpoints.  A means of identifying each of these



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   separate communication sessions is needed and, since each of the
   destination UAs will create its own UUID, each communication session
   would be uniquely identified by the values {A, B1}, {A, B2}, {A, B3},
   and so on, where each of the Bn values refers to the UUID created by
   the different UAs to which the SIP session is forked.

   For conferencing scenarios, it is also useful to have a two-part
   Session Identifier where the conference focus specifies the same UUID
   for each conference participant.  This allows for correlation among
   the participants in a single conference.  For example, in a
   conference with three participants, the Session Identifiers might be
   {A,M}, {B,M}, and {C,M}, where "M" is assigned by the conference
   focus.  Only a conference focus will purposely utilize the same UUID
   for more than one SIP session and, even then, such reuse MUST be
   restricted to the participants in the same conference.

   How a device acting on Session Identifiers stores, processes, or
   utilizes the Session Identifier is outside the scope of this
   document.

5. The Session-ID Header Field

   The syntax specified here replaces the Session-ID header field syntax
   defined in RFC 7329 [RFC7329].

   Each endpoint participating in a communication session has a
   distinct, preferably locally-generated, UUID associated with it.  The
   endpoint's UUID value remains unchanged throughout the duration of
   the communication session.  An intermediary MAY generate a UUID on
   behalf of an endpoint that did not include a UUID of its own.

   The UUID values for each endpoint are inserted into the "Session-ID"
   header field of all transmitted SIP messages.  The Session-ID header
   field has the following ABNF [RFC5234] syntax:

     session-id          = "Session-ID" HCOLON session-id-value

     session-id-value    = local-uuid *(SEMI sess-id-param)

     local-uuid          = sess-uuid / null

     remote-uuid         = sess-uuid / null

     sess-uuid           = 32(DIGIT / %x61-66)  ;32 chars of [0-9a-f]

     sess-id-param       = remote-param / generic-param

     remote-param        = "remote" EQUAL remote-uuid

     null                = 32("0")




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   The productions "SEMI", "EQUAL", and "generic-param" are defined in
   [RFC3261].  The production DIGIT is defined in [RFC5234].

   The Session-ID header field MUST NOT have more than one "remote"
   parameter.  In the case where an entity compliant with this
   specification is interworking with an entity that implemented
   [RFC7329], the "remote" parameter may be absent, but otherwise the
   remote parameter MUST be present.  The details under which those
   conditions apply are described in Section 10.  Except for backwards
   compatibility with [RFC7329], the "remote" parameter MUST be present.

   A special null UUID value composed of 32 zeros is required in certain
   situations.  A null UUID is expected as the "remote-uuid" of every
   initial standard SIP request since the initiating endpoint would not
   initially know the UUID value of the remote endpoint.  This null
   value will get replaced by the ultimate destination UAS when that UAS
   generates response message.  One caveat is explained in Section 10
   for a possible backwards compatibility case.  A null UUID value is
   also returned by some intermediary devices that send provisional
   responses as the "local-uuid" component of the Session-ID header
   field value, as described in Section 7.

   The "local-uuid" in the Session-ID header field represents the UUID
   value of the endpoint transmitting a message and the "remote-uuid" in
   the Session-ID header field represents the UUID of the endpoint's
   peer.  For example, a Session-ID header field might appear like this:

     Session-ID: ab30317f1a784dc48ff824d0d3715d86;
                 remote=47755a9de7794ba387653f2099600ef2

   While this is the general form of the Session-ID header field,
   exceptions to syntax and procedures are detailed in subsequent
   sections.

   The UUID values are presented as strings of lower-case hexadecimal
   characters, with the most significant octet of the UUID appearing
   first.

   The Session-ID header field value is technically case-INSENSITIVE,
   but only lowercase characters are allowed in the sess-uuid
   components.  Receiving entities MUST treat sess-uuid components as
   case-insensitive and not produce an error if an uppercase hexadecimal
   character is received.

6. Endpoint Behavior

   To comply with this specification, endpoints (non-intermediaries)
   MUST include a Session-ID header field value in all SIP messages
   transmitted as a part of a communication session.  The locally-
   generated UUID of the transmitter of the message MUST appear in the
   "local-uuid" portion of the Session-ID header field value.  The UUID



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   of the peer device, if known, MUST appear as the "remote" parameter
   following the transmitter's UUID.  The null UUID value MUST be used
   if the peer device's UUID is not known.

   Once an endpoint allocates a UUID value for a communication session,
   the endpoint MUST NOT change that UUID value for the duration of the
   session, including when

      - communication attempts are retried due to receipt of 4xx
        messages or request timeouts;

      - the session is redirected in response to a 3xx message; or

      - a session is transferred via a REFER message [RFC3515]; or

      - a SIP dialog is replaced via an INVITE with Replaces [RFC3891].

   An endpoint that receives a Session-ID header field MUST take note of
   any non-null "local-uuid" value that it receives and assume that is
   the UUID of the peer endpoint within that communications session.
   Endpoints MUST include this received UUID value as the "remote"
   parameter when transmitting subsequent messages, making sure not to
   change this UUID value in the process of moving the value internally
   from the "local-uuid" field to the "remote-uuid" field.

   If an endpoint receives a 3xx or 4xx message, receives a REFER that
   directs the endpoint to a different peer, or receives an INVITE with
   Replaces that also potentially results in communicating with a new
   peer, the endpoint MUST complete any message exchanges with its
   current peer using the existing Session Identifier, but MUST NOT use
   the current peer's UUID value when sending the first message to what
   it believes may be a new peer endpoint (even if the exchange results
   in communicating with the same physical or logical entity).  The
   endpoint MUST retain its own UUID value, however, as described above.

   It should be noted that messages received by an endpoint might
   contain a "local-uuid" value that does not match what the endpoint
   expected its peer's UUID to be.  It is also possible for an endpoint
   to receive a "remote-uuid" value that does not match its generated
   UUID for the session.  Either might happen as a result of service
   interactions by intermediaries and MUST NOT negatively affect the
   communication session.  However, the endpoint may log this event for
   the purposes of troubleshooting.

   An endpoint MUST assume that the UUID value of the peer endpoint MAY
   change at any time due to service interactions.  If the UUID value of
   the peer changes, the endpoint MUST accept the new UUID as the peer's
   UUID and include this new UUID as the "remote" parameter in any
   subsequent messages.





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   It is also important to note that if an intermediary in the network
   forks a session, the endpoint initiating a session may receive
   multiple responses back from different endpoints, each of which
   contains a different UUID ("local-uuid") value.  Endpoints MUST take
   care to ensure that the correct UUID value is returned in the
   "remote" parameter when interacting with each endpoint.  The one
   exception is when the endpoint sends a CANCEL message, in which case
   the Session-ID header field value MUST be identical to the Session-ID
   header field value sent in the original INVITE.

   If an endpoint receives a message that does not contain a Session-ID
   header field, that message MUST have no effect on what the endpoint
   believes is the UUID value of the remote endpoint.  That is, the
   endpoint MUST NOT change the internally maintained "remote-uuid"
   value for the peer.

   A Multipoint Control Unit (MCU) is a special type of conferencing
   endpoint and is discussed in Section 8.

7. Processing by Intermediaries

   The following applies only to an intermediary that wishes to comply
   with this specification and does not impose a conformance requirement
   on intermediaries that elect to not provide any special treatment for
   the Session-ID header field.

   The Call-ID often reveals personal, device, domain or other sensitive
   information associated with a user, which is why intermediaries, such
   as session border controllers, sometimes alter the Call-ID.  In order
   to ensure the integrity of the end-to-end Session Identifier, it is
   constructed in a way which does not reveal such information, removing
   the need for intermediaries to alter it.

   When an intermediary receives messages from one endpoint in a
   communication session that causes the transmission of one or more
   messages toward the second endpoint in a communication session, the
   intermediary MUST include the Session-ID header field in the
   transmitted messages with the same UUID values found in the received
   message, except as outlined in this section.

   If the intermediary aggregates several responses from different
   endpoints, as described in Section 16.7 of [RFC3261], the
   intermediary MUST set the local-uuid field to the null UUID value
   when forwarding the aggregated response to the endpoint since the
   true UUID value of the peer is undetermined at that point.

   Intermediary devices that transfer a call, such as by joining
   together two different "call legs", MUST properly construct a
   Session-ID header field that contains the correct UUID values and
   correct placement of those values.  As described in Section 6, the




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   endpoint receiving a message transmitted by the intermediary will
   assume that the first UUID value belongs to its peer endpoint.

   If an intermediary receives a SIP message from an endpoint without a
   Session-ID header field or valid header field value, the intermediary
   MAY assign a "local-uuid" value to represent that endpoint and,
   having done so, MUST insert that assigned value into all signaling
   messages on behalf of the endpoint for that dialog.  If the
   intermediary is aware of the UUID value that identifies the endpoint
   to which a message is directed, it MUST insert that UUID value into
   the Session-ID header field value as the "remote-uuid" value.  If the
   intermediary is unaware of the UUID value that identifies the
   receiving endpoint, it MUST use the null UUID value as the "remote-
   uuid" value.

   Whenever there is an endpoint communicating through a B2BUA that does
   not implement this specification, the B2BUA MAY become dialog
   stateful and insert a UUID value into the Session-ID header field
   value on behalf of the endpoint, at which point it MUST follow the
   endpoint procedures in Section 6 with respect to Session-ID header
   field value treatment with itself acting as the endpoint (for the
   purposes of the Session-ID header field) for which it inserted a
   component into the Session-ID header field value.

   When an intermediary originates a response, such as a provisional
   response or a response to a CANCEL request, the "remote-uuid" field
   will contain the UUID value of the receiving endpoint.  When the UUID
   of the peer endpoint is known, the intermediary MUST insert the UUID
   of the peer endpoint in the "local-uuid" field of the header value.
   Otherwise, the intermediary MAY set the "local-uuid" field of the
   header value to the "null" UUID value.

   When an intermediary originates a request message without first
   having received a SIP message that triggered the transmission of the
   message (e.g., sending a BYE message to terminate a call for policy
   reasons), the intermediary MUST, if it has knowledge of the UUID
   values for the two communicating endpoints, insert a Session-ID
   header field with the "remote-uuid" field of the header value set to
   the UUID value of the receiving endpoint and the "local-uuid" field
   of the header value set to the UUID value of the other endpoint.
   When the intermediary does not have knowledge of the UUID value of am
   endpoint in the communication session, the intermediary SHOULD set
   the unknown UUID value(s) to the "null" UUID value.  (If both are
   unknown, the Session-ID header value SHOULD NOT be included at all,
   since it would have no practical value.)

   With respect to the previous two paragraphs, note that if an
   intermediary transmits a "null" UUID value, the receiving endpoint
   might use that value in subsequent messages it sends.  This
   effectively violates the requirement of maintaining an end-to-end
   Session Identifier value for the communication session if a UUID for



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   the peer endpoint had been previously conveyed.  Therefore, an
   intermediary MUST only send the "null" UUID when the intermediary has
   not communicated with the peer endpoint to learn its UUID.  This
   means that intermediaries SHOULD maintain state related to the UUID
   values for both ends of a communication session if it intends to
   originate messages (versus merely conveying messages).  An
   intermediary that does not maintain this state and that originates a
   message as described in the previous two paragraph MUST NOT insert a
   Session-ID header field in order to avoid unintended, incorrect
   reassignment of a UUID value.

   The Session-ID header field value included in a CANCEL request MUST
   be identical to the Session-ID header field value included in the
   corresponding INVITE.

   If a SIP intermediary initiates a dialog between two endpoints in a
   3PCC [RFC3725] scenario, the SIP request in the initial INVITE will
   have a non-null, locally-frabricated "local-uuid" value; call this
   temporary UUID X.  The request will still have a null "remote-uuid"
   value; call this value N.  The SIP server MUST be transaction
   stateful.  The UUID pair in the INVITE will be {X,N}.  A non-
   redirected or rejected response will have a UUID pair {A,X}.  This
   transaction stateful, dialog initiating SIP server MUST replace its
   own UUID, i.e., X, with a null UUID (i.e., {A,N}) as expected by
   other UAS (see Section 9.7 for an example).

   Intermediaries that manipulate messages containing a Session-ID
   header field SHOULD be aware of what UUID values it last sent towards
   an endpoint and, following any kind of service interaction initiated
   or affected by the intermediary, of what UUID values the receiving
   endpoint should have knowledge to ensure that both endpoints in the
   session have the correct and same UUID values.  If an intermediary
   can determine that an endpoint might not have received a current,
   correct Session-ID field, the Intermediary SHOULD attempt to provide
   the correct Session-ID header field to the endpoint such as by
   sending a re-INVITE message.

8. Associating Endpoints in a Multipoint Conference

   Multipoint Control Units (MCUs) group two or more sessions into a
   single multipoint conference and have a conference Focus responsible
   for maintaining the dialogs connected to it [RFC4353].  MCUs,
   including cascaded MCUs, MUST utilize the same UUID value ("local-
   uuid" portion of the Session-ID header field value) with all
   participants in the conference.  In so doing, each individual session
   in the conference will have a unique Session Identifier (since each
   endpoint will create a unique UUID of its own), but will also have
   one UUID in common with all other participants in the conference.

   When creating a cascaded conference, an MCU MUST convey the UUID
   value to utilize for a conference via the "local-uuid" portion of the



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   Session-ID header field value in an INVITE to a second MCU when using
   SIP to establish the cascaded conference.  A conference bridge, or
   MCU, needs a way to identify itself when contacting another MCU.
   [RFC4579] defines the "isfocus" Contact header field value parameter
   just for this purpose.  The initial MCU MUST include the UUID of that
   particular conference in the "local-uuid" of an INVITE to the other
   MCU(s) participating in that conference.  Also included in this
   INVITE is an "isfocus" Contact header field value parameter
   identifying that this INVITE is coming from an MCU and that this UUID
   is to be given out in all responses from endpoints into those MCUs
   participating in this same conference.  This ensures a single UUID is
   common across all participating MCUs of the same conference, but is
   unique between different conferences.

   Intermediary devices or network diagnostics equipment might assume
   that when they see two or more sessions with different Session
   Identifiers, but with one UUID in common, that the sessions are part
   of the same conference.  However, the assumption that two sessions
   having one common UUID being part of the same conference is not
   always correct.  In a SIP forking scenario, for example, there might
   also be what appears to be multiple sessions with a shared UUID
   value; this is intended.  The desire is to allow for the association
   of related sessions, regardless of whether a session is forked or
   part of a conference.

9. Examples of Various Call Flow Operations

   Seeing something frequently makes understanding easier.  With that in
   mind, this section includes several call flow examples with the
   initial UUID and the complete Session Identifier indicated per
   message, as well as when the Session Identifier changes according to
   the rules within this document during certain operations/functions.

   This section is for illustrative purposes only and is non-normative.
   In the following flows, RTP refers to the Real-time Transport
   Protocol [RFC3550].

   In the examples in this section, "N" represents a null UUID and other
   letters represents the unique UUID values corresponding to endpoints
   or MCUs.

9.1. Basic Call with 2 UUIDs

      Session-ID
         ---     Alice            B2BUA             Bob            Carol
        {A,N}      |---INVITE F1--->|                |
        {A,N}      |                |---INVITE F2--->|
        {B,A}      |                |<---200 OK F3---|
        {B,A}      |<---200 OK F4---|                |
        {A,B}      |-----ACK F5---->|                |




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        {A,B}      |                |-----ACK F6---->|
                   |<==============RTP==============>|

             Figure 1 - Session-ID Creation when Alice calls Bob

   General operation of this example:

      o UA-Alice populates the "local-uuid" portion of the Session-ID
        header field value.

      o UA-Alice sends its UUID in the SIP INVITE, and populates the
        "remote" parameter with a null value (32 zeros).

      o B2BUA receives an INVITE with both a "local-uuid" portion of
        the Session-ID header field value from UA-Alice as well as the
        null "remote-uuid" value, and transmits the INVITE towards UA-
        Bob with an unchanged Session-ID header field value.

      o UA-Bob receives Session-ID and generates its "local-uuid"
        portion of the Session-ID header field value UUID to construct
        the whole/complete Session-ID header field value, at the same
        time transferring Alice's UUID unchanged to the "remote-uuid"
        portion of the Session-ID header field value in the 200 OK SIP
        response.

      o B2BUA receives the 200 OK response with a complete Session-ID
        header field value from UA-Bob, and transmits 200 OK towards
        UA-Alice with an unchanged Session-ID header field value.

      o UA-Alice, upon reception of the 200 OK from the B2BUA,
        transmits the ACK towards the B2BUA. The construction of the
        Session-ID header field in this ACK is that of Alice's UUID is
        the "local-uuid", and Bob's UUID populates the "remote-uuid"
        portion of the header-value.

      o B2BUA receives the ACK with a complete Session-ID header field
        from UA-Alice, and transmits ACK towards UA-Bob with an
        unchanged Session-ID header field value.

   Below is a complete SIP message exchange illustrating proper use of
   the Session-ID header field. For the sake of brevity, non-essential
   headers and message bodies are omitted.



   F1 INVITE Alice -> B2BUA



   INVITE sip:bob@biloxi.com SIP/2.0
   Via: SIP/2.0/UDP pc33.atlanta.example.com



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    ;branch=z9hG4bK776asdhds
   Max-Forwards: 70
   To: Bob <sip:bob@biloxi.example.com>
   From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710@pc33.atlanta.example.com
   Session-ID: ab30317f1a784dc48ff824d0d3715d86
    ;remote=00000000000000000000000000000000
   CSeq: 314159 INVITE
   Contact: <sip:alice@pc33.atlanta.example.com>
   Content-Type: application/sdp
   Content-Length: 142

   (Alice's SDP not shown)



   F2 INVITE B2BUA -> Bob



   INVITE sip:bob@192.168.10.20 SIP/2.0
   Via: SIP/2.0/UDP server10.biloxi.example.com
    ;branch=z9hG4bK4b43c2ff8.1
   Via: SIP/2.0/UDP pc33.atlanta.example.com
    ;branch=z9hG4bK776asdhds;received=10.1.3.33
   Max-Forwards: 69
   To: Bob <sip:bob@biloxi.example.com>
   From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710@pc33.atlanta.example.com
   Session-ID: ab30317f1a784dc48ff824d0d3715d86
    ;remote=00000000000000000000000000000000
   CSeq: 314159 INVITE
   Contact: <sip:alice@pc33.atlanta.example.com>
   Record-Route: <sip:server10.biloxi.example.com;lr>
   Content-Type: application/sdp
   Content-Length: 142

   (Alice's SDP not shown)



   F3 200 OK Bob -> B2BUA



   SIP/2.0 200 OK
   Via: SIP/2.0/UDP server10.biloxi.example.com
    ;branch=z9hG4bK4b43c2ff8.1;received=192.168.10.1
   Via: SIP/2.0/UDP pc33.atlanta.example.com
    ;branch=z9hG4bK776asdhds;received=10.1.3.33
   To: Bob <sip:bob@biloxi.example.com>;tag=a6c85cf



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   From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710@pc33.atlanta.example.com
   Session-ID: 47755a9de7794ba387653f2099600ef2
    ;remote=ab30317f1a784dc48ff824d0d3715d86
   CSeq: 314159 INVITE
   Contact: <sip:bob@192.168.10.20>
   Record-Route: <sip:server10.biloxi.example.com;lr>
   Content-Type: application/sdp
   Content-Length: 131

   (Bob's SDP not shown)



   F4 200 OK B2BUA -> Alice



   SIP/2.0 200 OK
   Via: SIP/2.0/UDP pc33.atlanta.example.com
    ;branch=z9hG4bK776asdhds;received=10.1.3.33
   To: Bob <sip:bob@biloxi.example.com>;tag=a6c85cf
   From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710@pc33.atlanta.example.com
   Session-ID: 47755a9de7794ba387653f2099600ef2
    ;remote=ab30317f1a784dc48ff824d0d3715d86
   CSeq: 314159 INVITE
   Contact: <sip:bob@192.168.10.20>
   Record-Route: <sip:server10.biloxi.example.com;lr>
   Content-Type: application/sdp
   Content-Length: 131

   (Bob's SDP not shown)



   F5 ACK Alice -> B2BUA



   ACK sip:bob@192.168.10.20 SIP/2.0
   Via: SIP/2.0/UDP pc33.atlanta.example.com
    ;branch=z9hG4bKnashds8
   Route: <sip:server10.biloxi.example.com;lr>
   Max-Forwards: 70
   To: Bob <sip:bob@biloxi.example.com>;tag=a6c85cf
   From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710@pc33.atlanta.example.com
   Session-ID: ab30317f1a784dc48ff824d0d3715d86
    ;remote=47755a9de7794ba387653f2099600ef2
   CSeq: 314159 ACK



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   Content-Length: 0



   F6 ACK B2BUA -> Bob



   ACK sip:bob@192.168.10.20 SIP/2.0
   Via: SIP/2.0/UDP server10.biloxi.example.com
    ;branch=z9hG4bK4b43c2ff8.2
   Via: SIP/2.0/UDP pc33.atlanta.example.com
    ;branch=z9hG4bKnashds8;received=10.1.3.33
   Max-Forwards: 70
   To: Bob <sip:bob@biloxi.example.com>;tag=a6c85cf
   From: Alice <sip:alice@atlanta.example.com>;tag=1928301774
   Call-ID: a84b4c76e66710@pc33.atlanta.example.com
   Session-ID: ab30317f1a784dc48ff824d0d3715d86
    ;remote=47755a9de7794ba387653f2099600ef2
   CSeq: 314159 ACK
   Content-Length: 0



   The remaining examples in this Section do not display the complete
   SIP message exchange.  Instead, they simply use the set notation
   described in Section 4.2 to show the Session Identifier exchange
   throughout the particular call flow being illustrated.

9.2. Basic Call Transfer using REFER

   From the example built within Section 9.1, we proceed to this 'Basic
   Call Transfer using REFER' example.  Note that this is a mid-dialog
   REFER in contrast with the out-of-dialog REFER in Section 9.9.

      Session-ID
         ---     Alice            B2BUA             Bob            Carol
                   |                |                |               |
                   |<==============RTP==============>|               |
        {B,A}      |                |<---re-INVITE---|               |
        {B,A}      |<---re-INVITE---| (puts Alice on Hold)           |
        {A,B}      |-----200 OK---->|                |               |
        {A,B}      |                |-----200 OK---->|               |
        {B,A}      |                |<-----ACK-------|               |
        {B,A}      |<-----ACK-------|                |               |
                   |                |                |               |
        {B,A}      |                |<----REFER------|               |
        {B,A}      |<----REFER------|                |               |
        {A,B}      |-----200 OK---->|                |               |
        {A,B}      |                |-----200 OK---->|               |
        {A,B}      |-----NOTIFY---->|                |               |



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        {A,B}      |                |-----NOTIFY---->|               |
        {B,A}      |                |<----200 OK-----|               |
        {B,A}      |<----200 OK-----|                |               |
                   |                |                |               |
        {A,N}      |-----INVITE---->|                                |
        {A,N}      |                |-----INVITE-------------------->|
        {C,A}      |                |<----200 OK---------------------|
        {C,A}      |<----200 OK-----|                                |
        {A,C}      |------ACK------>|                                |
        {A,C}      |                |------ACK---------------------->|
                   |                |                |               |
                   |<======================RTP======================>|
                   |                |                |               |
        {A,B}      |-----NOTIFY---->|                |               |
        {A,B}      |                |-----NOTIFY---->|               |
        {B,A}      |                |<----200 OK-----|               |
        {B,A}      |<----200 OK-----|                |               |
        {B,A}      |                |<-----BYE-------|               |
        {B,A}      |<-----BYE-------|                |               |
        {A,B}      |-----200 OK---->|                |               |
        {A,B}      |                |-----200 OK---->|               |
                   |                |                |               |

                    Figure 2 - Call Transfer using REFER

   General operation of this example:

   Starting from the existing Alice/Bob call described in Figure 1 of
   this document, which established an existing Session-ID header field
   value:

      o UA-Bob requests Alice to call Carol, using a REFER transaction,
        as described in [RFC3515]. UA-Alice is initially put on hold,
        then told in the REFER who to contact with a new INVITE, in
        this case UA-Carol. This Alice-to-Carol dialog will have a new
        Call-ID, therefore it requires a new Session-ID header field
        value.  The wrinkle here is we can, and will, use Alice's UUID
        from her existing dialog with Bob in the new INVITE to Carol.

      o UA-Alice retains her UUID from the Alice-to-Bob call {A} when
        requesting a call with UA-Carol. This is placed in the "local-
        uuid" portion of the Session-ID header field value, at the same
        time inserting a null "remote-uuid" value (because Carol's UA
        has not yet received the UUID value). This same UUID traverses
        the B2BUA unchanged.

      o UA-Carol receives the INVITE with a Session Identifier UUID
        {A,N}, replaces the A UUID value into the "remote-uuid" portion
        of the Session-ID header field value and creates its own UUID
        {C} and places this value in the "local-uuid" portion of the
        Session-ID header field value, thereby removing the N (null)



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        value altogether. This combination forms a full Session
        Identifier {C,A} in the 200 OK to the INVITE.  This Session-ID
        header field traverses the B2BUA unchanged towards UA-Alice.

      o UA-Alice receives the 200 OK with the Session Identifier {C,A}
        and responds to UA-Carol with an ACK (just as in Figure 1 -
        switches places of the two UUID fields), and generates a NOTIFY
        to Bob with a Session Identifier {A,B} indicating the call
        transfer was successful.

      o It does not matter which UA terminates the Alice-to-Bob call;
        Figure 2 shows UA-Bob doing this transaction.

9.3. Basic Call Transfer using re-INVITE

   From the example built within Section 9.1, we proceed to this 'Basic
   Call Transfer using re-INVITE' example.

   Alice is talking to Bob. Bob pushes a button on his phone to transfer
   Alice to Carol via the B2BUA (using re-INVITE).

      Session-ID
         ---     Alice            B2BUA             Bob            Carol
                   |                |                |               |
                   |<==============RTP==============>|               |
                   |                |                |               |
                   |                | <--- (non-standard signaling)  |
        {A,B}      |                |---re-INVITE--->|               |
        {B,A}      |                |<-----200 OK----|               |
        {B,A}      |                |-----ACK------->|               |
                   |                |                |               |
        {A,N}      |                |-----INVITE-------------------->|
        {C,A}      |                |<----200 OK---------------------|
        {A,C}      |                |------ACK---------------------->|
                   |                |                |               |
                   |<======================RTP======================>|
                   |                |                |               |
        {A,B}      |                |------BYE------>|               |
        {B,A}      |                |<----200 OK-----|               |
                   |                |                |               |
                   | (Suppose Alice modifies the session)            |
        {A,B}      |---re-INVITE--->|                |               |
        {A,B}      |                |---re-INVITE------------------->|
        {C,A}      |                |<---200 OK----------------------|
        {C,A}      |<---200 OK------|                |               |
        {A,C}      |------ACK------>|                |               |
        {A,C}      |                |------ACK---------------------->|
                   |                |                |               |

                  Figure 3 - Call transfer using re-INVITE




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   General operation of this example:

      o We assume the call between Alice and Bob from Section 9.1 is
        operational with Session Identifier {A,B}.

      o Bob uses non-standard signaling to the B2BUA to initiate a call
        transfer from Alice to Carol.  This could also be initiated via
        a REFER message from Bob, but the signaling that follows might
        still be similar to the above flow.  In either case, Alice is
        completely unaware of the call transfer until a future point in
        time when Alice receives a message from Carol.

      o The B2BUA sends a new INVITE with Alice's UUID {"local-uuid" =
        "A"} to Carol.

      o Carol receives the INVITE and accepts the request and adds her
        UUID {C} to the Session Identifier for this session {"local-
        uuid" = "C", "remote-uuid" = "A"}.

      o The B2BUA then terminates the call to Bob with a BYE using the
        Session Identifier {"local-uuid" = "A", "remote-uuid" = "B"}.

      o Since Alice never received Carol's UUID from the B2BUA, when
        Alice later attempts to modify the session with a re-INVITE,
        Alice would send the "remote-uuid" = "B" toward Carol.  Carol
        replies with the "local-uuid" = "A", "remote-uuid" = "A" to
        reflect what was received in the INVITE (which Carol already
        knew from previous exchanges with the B2BUA).  Alice then
        includes "remote-uuid" = "C" in the following ACK message.

9.4. Single Focus Conferencing

   Multiple users call into a conference server (say, an MCU) to attend
   one of many conferences hosted on or managed by that server. Each
   user has to identify which conference they want to join, but this
   information is not necessarily in the SIP messaging.  It might be
   done by having a dedicated address for the conference or via an IVR,
   as assumed in this example and depicted with the use of M1, M2, and
   M3. Each user in this example goes through a two-step process of
   signaling to gain entry onto their conference call, which the
   conference focus identifies as M'.

      Session-ID                Conference
         ---     Alice            Focus             Bob            Carol
                   |                |                |               |
                   |                |                |               |
        {A,N}      |----INVITE----->|                |               |
        {M1,A}     |<---200 OK------|                |               |
        {A,M1}     |-----ACK------->|                |               |
                   |<====RTP=======>|                |               |
        {M',A}     |<---re-INVITE---|                |               |



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        {A,M'}     |-----200 OK---->|                |               |
        {M',A}     |<-----ACK-------|                |               |
                   |                |                |               |
                   |                |                |               |
        {B,N}      |                |<----INVITE-----|               |
        {M2,B}     |                |-----200 OK---->|               |
        {B,M2}     |                |<-----ACK-------|               |
                   |                |<=====RTP======>|               |
        {M',B}     |                |---re-INVITE--->|               |
        {B,M'}     |                |<----200 OK-----|               |
        {M',B}     |                |------ACK------>|               |
                   |                |                |               |
                   |                |                |               |
        {C,N}      |                |<--------------------INVITE-----|
        {M3,C}     |                |---------------------200 OK---->|
        {C,M3}     |                |<---------------------ACK-------|
                   |                |<=====================RTP======>|
        {M',C}     |                |-------------------re-INVITE--->|
        {C,M'}     |                |<--------------------200 OK-----|
        {M',C}     |                |----------------------ACK------>|

                  Figure 4 - Single Focus Conference Bridge

   General operation of this example:

   Alice calls into a conference server to attend a certain conference.
   This is a two-step operation since Alice cannot include the
   conference ID at this time and/or any passcode in the INVITE request.
   The first step is Alice's UA calling another UA to participate in a
   session.  This will appear to be similar as the call-flow in Figure 1
   (in section 9.1). What is unique about this call is the second step:
   the conference server sends a re-INVITE request with its second UUID,
   but maintaining the UUID Alice sent in the first INVITE. This
   subsequent UUID from the conference server will be the same for each
   UA that calls into this conference server participating in this same
   conference bridge/call, which is generated once Alice typically
   authenticates and identifies which bridge she wants to participate
   on.

      o Alice sends an INVITE to the conference server with her UUID
        {A} and a "remote-uuid" = N.

      o The conference server responds with a 200 OK response which
        replaces the N UUID with a temporary UUID ("M1") as the "local-
        uuid" and a "remote-uuid" = "A".

     NOTE: this 'temporary' UUID is a real UUID; it is only temporary
     to the conference server because it knows that it is going to
     generate another UUID to replace the one just send in the 200 OK.





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      o Once Alice, the user, gains access to the IVR for this
        conference server, she enters a specific conference ID and
        whatever passcode (if needed) to enter a specific conference
        call.

      o Once the conference server is satisfied Alice has identified
        which conference she wants to attend (including any passcode
        verification), the conference server re-INVITEs Alice to the
        specific conference and includes the Session-ID header field
        value component "local-uuid" = "M'" (and "remote-uuid" = "A")
        for that conference. All valid participants in the same
        conference will receive this same UUID for identification
        purposes and to better enable monitoring, and tracking
        functions.

      o Bob goes through this two-step process of an INVITE
        transaction, followed by a re-INVITE transaction to get this
        same UUID ("M'") for that conference.

      o In this example, Carol (and each additional user) goes through
        the same procedures and steps as Alice and Bob to get on this
        same conference.

9.5. Single Focus Conferencing using WebEx

   Alice, Bob and Carol call into same WebEx conference.

      Session-ID                Conference
         ---     Alice            Focus             Bob            Carol
                   |                |                |               |
                   |<** HTTPS *****>|                |               |
                   |  Transaction   |                |               |
                   |                |                |               |
        {M,N}      |<----INVITE-----|                |               |
        {A,M}      |-----200 OK---->|                |               |
        {M,A}      |<-----ACK-------|                |               |
                   |<=====RTP======>|                |               |
                   |                |                |               |
                   |                |<** HTTPS *****>|               |
                   |                |  Transaction   |               |
                   |                |                |               |
        {M,N}      |                |-----INVITE---->|               |
        {B,M}      |                |<----200 OK-----|               |
        {M,B}      |                |------ACK------>|               |
                   |                |<=====RTP======>|               |
                   |                |                |               |
                   |                |<****************** HTTPS *****>|
                   |                |                   Transaction  |
                   |                |                |               |
        {M,N}      |                |--------------------INVITE----->|
        {C,M}      |                |<-------------------200 OK------|



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        {M,C}      |                |---------------------ACK------->|
                   |                |<====================RTP=======>|

                  Figure 5 - Single Focus WebEx Conference

   General operation of this example:

      o Alice communicates with WebEx server with desire to join a
        certain meeting, by meeting number; also includes UA-Alice's
        contact information (phone number, URI and/or IP address, etc.)
        for each device she wants for this conference call.  For
        example, the audio and video play-out devices could be separate
        units.

      o Conference Focus server sends INVITE (Session-ID header field
        value components "local-uuid" = M and a remote UUID of N, where
        M equals the "local-uuid" for each participant on this
        conference bridge) to UA-Alice to start session with that
        server for this A/V conference call.

      o Upon receiving the INVITE request from the conference focus
        server, Alice responds with a 200 OK. Her UA moves the "local-
        uuid" unchanged into the "remote-uuid" field, and generates her
        own UUID and places that into the "local-uuid" field to
        complete the Session-ID construction.

      o Bob and Carol perform same function to join this same A/V
        conference call as Alice.

9.6. Cascading Conference Bridges

9.6.1. Establishing a Cascaded Conference

   To expand conferencing capabilities requires cascading conference
   bridges. A conference bridge, or MCU, needs a way to identify itself
   when contacting another MCU. RFC 4579 [RFC4579] defines the 'isfocus'
   Contact: header parameter just for this purpose.

      Session-ID
         ---     MCU-1            MCU-2            MCU-3           MCU-4
                   |                |                |               |
        {M',N}     |----INVITE----->|                |               |
        {J,M'}     |<---200 OK------|                |               |
        {M',J}     |-----ACK------->|                |               |

      Figure 6 - MCUs Communicating Session Identifier UUID for Bridge

   Regardless of which MCU (1 or 2) a UA contacts for this conference,
   once the above exchange has been received and acknowledged, the UA
   will get the same {M',N} UUID pair from the MCU for the complete
   Session Identifier.



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   A more complex form would be a series of MCUs all being informed of
   the same UUID to use for a specific conference. This series of MCUs
   can either be informed

      o All by one MCU (that initially generates the UUID for the
        conference).

      o The MCU that generates the UUID informs one or several MCUs of
        this common UUID, and they inform downstream MCUs of this
        common UUID that each will be using for this one conference.

      Session-ID
         ---     MCU-1            MCU-2            MCU-3           MCU-4
                   |                |                |               |
        {M',N}     |----INVITE----->|                |               |
        {J,M'}     |<---200 OK------|                |               |
        {M',J}     |-----ACK------->|                |               |
                   |                |                |               |
        {M',N}     |---------------------INVITE----->|               |
        {K,M'}     |<--------------------200 OK------|               |
        {M',K}     |----------------------ACK------->|               |
                   |                |                |               |
        {M',N}     |-------------------------------------INVITE----->|
        {L,M'}     |<------------------------------------200 OK------|
        {M',L}     |--------------------------------------ACK------->|

     Figure 7 - MCU Communicating Session Identifier UUID to More than
                                  One MCU

   General operation of this example:

      o The MCU generating the Session Identifier UUID communicates
        this in a separate INVITE, having a Contact header with the
        'isfocus' header parameter. This will identify the MCU as what
        RFC 4579 calls a conference-aware SIP entity.

      o An MCU that receives this {M',N} UUID pair in an inter-MCU
        transaction can communicate the M' UUID in a manner in which it
        was received to construct a hierarchical cascade (though this
        time this second MCU would be the UAC MCU).

      o Once the conference is terminated, the cascaded MCUs will
        receive a BYE message to terminate the cascade.

9.6.2. Calling into Cascaded Conference Bridges

   Here is an example of how a UA, say Robert, calls into a cascaded
   conference focus. Because MCU-1 has already contacted MCU-3, the MCU
   where Robert is going to join the conference, MCU-3 already has the
   Session-ID (M') for this particular conference call.




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      Session-ID
         ---     MCU-1            MCU-2            MCU-3          Robert
                   |                |                |               |
        {M',N}     |----INVITE----->|                |               |
        {J,M'}     |<---200 OK------|                |               |
        {M',J}     |-----ACK------->|                |               |
                   |                |                |               |
        {M',N}     |---------------------INVITE----->|               |
        {K,M'}     |<--------------------200 OK------|               |
        {M',K}     |----------------------ACK------->|               |
                   |                |                |               |
        {R,N}      |                |                |<---INVITE-----|
        (M',R}     |                |                |----200 OK---->|
        {R,M'}     |                |                |<----ACK-------|

              Figure 8 - A UA Calling into a Cascaded MCU UUID

   General operation of this example:

      o The UA, Robert in this case, INVITEs the MCU to join a
        particular conference call. Robert's UA does not know anything
        about whether this is the main MCU of the conference call, or a
        cascaded MCU. Robert likely does not know MCUs can be cascaded,
        he just wants to join a particular call. Like as with any
        standard implementation, he includes a null "remote-uuid".

      o The cascaded MCU, upon receiving this INVITE from Robert,
        replaces the null UUID with the UUID value communicated from
        MCU-1 for this conference call as the "local-uuid" in the SIP
        response. Thus, moving Robert's UUID "R" to the "remote-uuid"
        value.

      o The ACK has the Session-ID {R,M'}, completing the 3-way
        handshake for this call establishment. Robert has now joined
        the conference call originated from MCU-1.

      o Once the conference is terminated, the cascaded MCUs will
        receive a BYE message to terminate the cascade.

9.7. Basic 3PCC for two UAs

   External entity sets up call to both Alice and Bob for them to talk
   to each other.

      Session-ID
         ---     Alice            B2BUA             Bob            Carol
                   |                |                |
        {X,N}      |<----INVITE-----|                |
        {A,X}      |-----200 OK---->|                |
        {A,N}      |                |----INVITE----->|
        {B,A}      |                |<---200 OK------|



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        {B,A}      |<-----ACK-------|                |
        {A,B}      |                |------ACK------>|
                   |<==============RTP==============>|

            Figure 9 - 3PCC initiated call between Alice and Bob

   General operation of this example:

      o Some out of band procedure directs a B2BUA (or other SIP
        server) to have Alice and Bob talk to each other. In this case,
        the SIP server has to be transaction stateful, if not dialog
        stateful.

      o The SIP server INVITEs Alice to a session and uses a temporary
        UUID {X} and a null UUID pairing.

      o Alice receives and accepts this call set-up and replaces the
        null UUID with her UUID {A} in the Session Identifier, now
        {A,X}.

      o The transaction stateful SIP server receives Alice's UUID {A}
        in the local UUID portion and keeps it there, and discards its
        own UUID {X}, replacing this with a null UUID value in the
        INVITE to Bob as if this came from Alice originally.

      o Bob receives and accepts this INVITE and adds his own UUID {B}
        to the Session Identifier, now {B,A} for the response.

      o And the session is established.

9.8. Handling in 100 Trying SIP Response and CANCEL Request

   The following two subsections show examples of the Session Identifier
   for a 100 Trying response and a CANCEL request in a single call-flow.

9.8.1. Handling in a 100 Trying SIP Response

   The following 100 Trying response is taken from an existing RFC, from
   [RFC5359] Section 2.9 ("Call Forwarding - No Answer").

    Session-ID   Alice         SIP Server        Bob-1            Bob-2
                   |                |              |                |
      {A,N}        |----INVITE----->|              |                |
      {A,N}        |                |---INVITE---->|                |
      {N,A}        |<--100 Trying---|              |                |
      {B1,A}       |                |<-180 Ringing-|                |
      {B1,A}       |<--180 Ringing--|              |                |
                   |                |              |                |
                   |                *Request Timeout*               |
                   |                |              |                |
      {A,N}        |                |---CANCEL---->|                |



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      {B1,A}       |                |<--200 OK-----|                |
      {B1,A}       |                |<---487-------|                |
      {A,B1}       |                |---- ACK ---->|                |
                   |                |              |                |
      {N,A}        |<-181 Call Fwd--|              |                |
                   |                |              |                |
      {A,N}        |                |------------------INVITE------>|
      {B2,A}       |                |<----------------180 Ringing---|
      {B2,A}       |<-180 Ringing---|              |                |
      {B2,A}       |                |<-----------------200 OK ------|
      {B2,A}       |<--200 OK-------|              |                |
      {A,B2}       |----ACK-------->|              |                |
      {A,B2}       |                |------------------ACK--------->|
                   |                |              |                |
                   |<=========== Both way RTP Established =========>|
                   |                |              |                |
      {A,B2}       |----BYE-------->|              |                |
      {A,B2}       |                |--------------------BYE------->|
      {B2,A}       |                |<------------------200 OK------|
      {B2,A}       |<--200 OK-------|              |                |
                   |                |              |                |

   Figure 10 - Session Identifier in the 100 Trying and CANCEL Messaging

   Below is the explanatory text from RFC 5359 Section 2.9 detailing
   what the desired behavior is in the above call flow (i.e., what the
   call-flow is attempting to achieve).

     "Bob wants calls to B1 forwarded to B2 if B1 is not answered
     (information is known to the SIP server).  Alice calls B1 and no
     one answers.  The SIP server then places the call to B2."

   General operation of this example:

      o Alice generates an INVITE request because she wants to invite
        Bob to join her session.  She creates a UUID as described in
        section 9.1, and places that value in the "local-uuid" field of
        the Session-ID header field value.  Alice also generates a
        "remote-uuid" of null and sends this along with the "local-
        uuid".

      o The SIP server (imagine this is a B2BUA), upon receiving
        Alice's INVITE, generates the optional provisional response 100
        Trying.  Since the SIP server has no knowledge Bob's UUID for
        his part of the Session Identifier value, it cannot include his
        "local-uuid". Rather, any 100 Trying response includes Alice's
        UUID in the "remote-uuid" portion of the Session-ID header-
        value with a null "local-uuid" value in the response.  This is
        consistent with what Alice's UA expects to receive in any SIP
        response containing this UUID.




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9.8.2. Handling a CANCEL SIP Request

   In the same call-flow example as the 100 Trying response is a CANCEL
   request.  Please refer to Figure 10 for the CANCEL request example.

   General operation of this example:

      o In Figure 10 above, Alice generates an INVITE with her UUID
        value in the Session-ID header field.

      o Bob-1 responds to this INVITE with a 180 Ringing.  In that
        response, he includes his UUID in the Session-ID header field
        value (i.e., {B1,A}); thus completing the Session-ID header
        field for this session, even though no final response has been
        generated by any of Bob's UAs.

      o While this means that if the SIP server were to generate a SIP
        request within this session it could include the complete
        SessionID, the server sends a CANCEL and a CANCEL always uses
        the same Session-ID header field as the original INVITE.  Thus,
        the CANCEL would have a Session Identifier with the "local-
        uuid" = "A", and the "remote-uuid" = "N".

      o As it happens with this CANCEL, the SIP server intends to
        invite another UA of Bob (i.e., B2) for Alice to communicate
        with.

      o In this example call-flow, taken from RFC 5359, Section 2.9, a
        181 (Call is being Forwarded) response is sent to Alice.  Since
        the SIP server generated this SIP request, and has no knowledge
        of Bob-2's UUID value, it cannot include that value in this
        181. Thus, and for the exact reasons the 100 Trying including
        the Session Identifier value, only Alice's UUID is included in
        the remote-uuid component of the Session-ID header field value,
        with a null UUID present in the "local-uuid" component.

9.9. Out-of-dialog REFER Transaction

   The following call-flow was extracted from Section 6.1 of [RFC5589]
   ("Successful Transfer"), with the only changes being the names of the
   UAs to maintain consistency within this document.

         Alice is the transferee
         Bob is the transferer
         and Carol is the transfer-target

     Session-ID     Bob                 Alice                 Carol
                     |                    |                     |
        {A,N}        |<-----INVITE--------|                     |
        {B,A}        |------200 OK------->|                     |
        {A,B}        |<------ACK----------|                     |



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                     |                    |                     |
        {B,A}        |--INVITE {hold}---->|                     |
        {A,B}        |<-200 OK------------|                     |
        {B,A}        |--- ACK ----------->|                     |
                     |                    |                     |
        {B,A}        |--REFER------------>|(Refer-To:Carol)     |
        {A,B}        |<-202 Accepted------|                     |
                     |                    |                     |
        {A,B}        |<NOTIFY {100 Trying}|                     |
        {B,A}        |-200 OK------------>|                     |
                     |                    |                     |
        {A,N}        |                    |--INVITE------------>|
        {C,A}        |                    |<-200 OK-------------|
        {A,C}        |                    |---ACK-------------->|
                     |                    |                     |
        {A,B}        |<--NOTIFY {200 OK}--|                     |
        {B,A}        |---200 OK---------->|                     |
                     |                    |                     |
        {B,A}        |--BYE-------------->|                     |
        {A,B}        |<-200 OK------------|                     |
        {C,A}        |                    |<------------BYE-----|
        {A,C}        |                    |-------------200 OK->|

                  Figure 11: Out-Of-Dialog Call Transfer

   General operation of this example:

      o Just as in Section 9.2, Figure 2, Alice invites Bob to a
        session, and Bob eventually transfers Alice to communicate with
        Carol.

      o What is different about the call-flow in Figure 11 is that
        Bob's REFER is not in-dialog.  Even so, this is treated as part
        of the same communication session and, thus, the Session
        Identifier in those messages is {A,B}.

      o Alice will use her existing UUID and the null UUID ({A,N}) in
        the INVITE towards Carol (who generates UUID "C" for this
        session), thus maintaining the common UUID within the Session
        Identifier for this new Alice-to-Carol session.

10. Compatibility with a Previous Implementation

   There is a much earlier and proprietary document that specifies the
   use of a Session-ID header field (namely, [RFC7329]) that we will
   herewith attempt to achieve backwards compatibility.  Neither
   Session-ID header field has any versioning information, so merely
   adding that this document describes "version 2" is insufficient.
   Here are the set of rules for compatibility between the two
   specifications. For the purposes of this discussion, we will label




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   the proprietary specification of the Session-ID as the "old" version
   and this specification as the "new" version of the Session-ID.

   The previous (i.e., "old") version only has a single UUID value as a
   Session-ID header field value, but has a generic-parameter value that
   can be of use.

   In order to have an "old" version talk to an "old" version
   implementation, nothing needs to be done as far as the IETF is
   concerned.

   In order to have a "new" version talk to a "new" version
   implementation, both implementations need to follow this document (to
   the letter) and everything should be just fine.

   But that is where compatibility is not ensured, given the unknowns
   related to the behavior of entities implementing the pre-standard
   implementation.  For this "new" implementation to work with the "old"
   implementation and an "old" implementation to work with "new"
   implementations, there needs to be a set of rules that all "new"
   implementations MUST follow.

   - Since no option tags or feature tags are to be used for
     distinguishing versions, the presence and order of any "remote-
     uuid" value within the Session-ID header field value is to be used
     to distinguish implementation versions.

   - If a SIP request has a "remote-uuid" value, this comes from a
     standard implementation, and not a pre-standard one.

   - If a SIP request has no "remote-uuid" value, this comes from a pre-
     standard implementation, and not a standard one.  In this case, one
     UUID is used to identify this dialog, even if the responder is a
     standard implementation of this specification.

   - If a SIP response has a non-null "local-uuid" that is 32 octets
     long and differs from the endpoint's own UUID value, this response
     comes from a standard implementation.

   - If a SIP response has a non-null "local-uuid" that is not 32 octets
     long, this response comes from a misbehaving implementation, and
     its Session-ID header field MUST be discarded.  That said, the
     response might still be valid according to the rules within SIP
     [RFC3261], and SHOULD be checked further.

   - If a SIP response arrives that has the same value of Session-ID
     UUIDs in the same order as was sent, this comes from a pre-standard
     implementation, and MUST NOT be discarded for not altering the null
     "remote-uuid".  In this case, any new transaction within this
     dialog MUST preserve the order of the two UUIDs within all Session-




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     ID header field, including the ACK, until this dialog is
     terminated.

   - If a SIP response only contains the "local-uuid" that was sent
     originally, this comes from a pre-standard implementation and MUST
     NOT be discarded for removing the null "remote-uuid".  In this
     case, all future transactions within this dialog MUST contain only
     the UUID received in the first SIP response.  Any new transaction
     starting a new dialog from the standard Session-ID implementation
     MUST include a "local-uuid" and a null "remote-uuid", even if that
     new dialog is between the same two UAs.

   - Standard implementations SHOULD NOT expect pre-standard
     implementations to be consistent in their implementation, even
     within the same dialog.  For example, perhaps the first, third and
     tenth responses contain a "remote-uuid", but all the others do not.
     This behavior MUST be allowed by implementations of this
     specification.

   - The foregoing does not apply to other, presently unknown parameters
     that might be defined in the future.  They are ignored for the
     purposes of interoperability with previous implementations.

11. Security Considerations

   When creating a UUID value, UAs MUST ensure that there is no user or
   device-identifying information contained within the UUID.  In
   particular, this means that a UUID MUST NOT be constructed using a
   MAC address on the host.

   The Session Identifier might be utilized for logging or
   troubleshooting, but MUST NOT be used for billing purposes.

   The Session Identifier could be misused to discover relationships
   between two or more parties.  For example, suppose that Alice calls
   Bob and Bob, via his PBX, forwards or transfers the call to Carol.
   Without use of the Session Identifier, an unauthorized third party
   that is observing the communications between Alice and Bob might not
   know that Alice is actually communicating with Carol.  If Alice, Bob,
   and Carol include the Session Identifier as a part of the signaling
   messages, it is possible for the third party to observe that the UA
   associated with Bob changed to some other UA.  If the third party
   also has access to signaling messages between Bob and Carol, the
   third party can then discover that Alice is communicating with Carol.
   This would be true even if all other information relating to the
   session is changed by the PBX, including both signaling information
   and media address information.







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12. IANA Considerations

12.1. Registration of the "Session-ID" Header Field

   The following is the registration for the 'Session-ID' header field
   to the "Header Name" registry at
   http://www.iana.org/assignments/sip-parameters:

   RFC number: RFC XXXX

   Header name: 'Session-ID'

   Compact form: none

   Note: This document replaces the "Session-ID" header originally
         registered via [RFC7329].

   [RFC Editor: Please replace XXXX in this section and the next with
   the this RFC number of this document.]

12.2. Registration of the "remote" Parameter

   The following parameter is to be added to the "Header Field
   Parameters and Parameter Values" section of the SIP parameter
   registry:

   +------------------+----------------+-------------------+-----------+
   | Header Field     | Parameter Name | Predefined Values | Reference |
   +------------------+----------------+-------------------+-----------+
   | Session-ID       | remote         | No                | [RFCXXXX] |
   +------------------+----------------+-------------------+-----------+

13. Acknowledgments

   The authors would like to thank Robert Sparks, Hadriel Kaplan,
   Christer Holmberg, Paul Kyzivat, Brett Tate, Keith Drage, Mary
   Barnes, Charles Eckel, Peter Dawes, Andrew Hutton, Arun Arunachalam,
   Adam Gensler, Roland Jesske, and Faisal Siyavudeen for their
   invaluable comments during the development of this document.

14. Dedication

   This document is dedicated to the memory of James Polk, a long-time
   friend and colleague.  James made important contributions to this
   specification, including being one of its primary editors.  The IETF
   global community mourns his loss and he will be missed dearly.








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15. References

15.1. Normative References

   [RFC3261]   Rosenberg, J., et al., "SIP: Session Initiation
               Protocol", RFC 3261, June 2002.

   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4122]   Leach, P., Mealling, M., Salz, R., "A Universally Unique
               IDentifier (UUID) URN Namespace", RFC 4122, July 2005.

   [RFC5234]   Crocker, D., Overell, P, "Augmented BNF for Syntax
               Specifications: ABNF", RFC 5234, January 2008.

   [RFC4579]   Johnston, A., Levin, O., "Session Initiation Protocol
               (SIP) Call Control - Conferencing for User Agents", RFC
               4579, August 2006.

   [RFC3891]   Mahy, R., Biggs, B., Dean, R., 'The Session Initiation
               Protocol (SIP) "Replaces" Header', RFC 3891, September
               2004.

   [RFC3515]   Sparks, R., "The Session Initiation Protocol (SIP) Refer
               Method", RFC 3515, April 2003.

   [RFC7329]   Kaplan, H., "A Session Identifier for the Session
               Initiation Protocol (SIP)", RFC 7329, August 2014.

15.2. Informative References

   [H.323]     Recommendation ITU-T H.323, "Packet-based multimedia
               communications systems", December 2009.

   [RFC3550]   Schulzrinne, H., et al., "RTP: A Transport Protocol for
               Real-Time Applications", RFC 3550, July 2003.

   [RFC7206]   Jones, et al., "Requirements for an End-to-End Session
               Identification in IP-Based Multimedia Communication
               Networks", RFC 7206, May 2014.

   [RFC5359]   Johnston, A., et al., "Session Initiation Protocol
               Service Examples", RFC 5359, October 2008.

   [RFC5589]   Sparks, R., Johnston, A., and D. Petrie, "Session
               Initiation Protocol (SIP) Call Control - Transfer", RFC
               5589, June 2009.






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   [RFC2543]   Handley, M., Schulzrinne, H., Schooler, E. and J.
               Rosenberg, "SIP: Session Initiation Protocol", RFC 2543,
               March 1999.

   [H.460.27]  Recommendation ITU-T H.460.27, "End-to-End Session
               Identifier in for H.323 Systems", Work in Progress {RFC
               Editor: A month and year should be available when the RFC
               is published}.

   [RFC3725]   Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
               Camarillo, "Best Current Practices for Third Party Call
               Control (3pcc) in the Session Initiation Protocol (SIP)",
               RFC 3725, April 2004.

   [RFC4353]   Rosenberg, J., "A Framework for Conferencing with the
               Session Initiation Protocol (SIP)", RFC 4353, February
               2006.

   [RFC7092]   Kaplan, H. and V. Pascual, "A Taxonomy of Session
               Initiation Protocol (SIP) Back-to-Back User Agents", RFC
               7092, December 2013.

































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Authors' Addresses

   Paul E. Jones
   Cisco Systems, Inc.
   7025 Kit Creek Rd.
   Research Triangle Park, NC 27709
   USA

   Phone: +1 919 476 2048
   Email: paulej@packetizer.com
   IM: xmpp:paulej@packetizer.com


   Gonzalo Salgueiro
   Cisco Systems, Inc.
   7025 Kit Creek Rd.
   Research Triangle Park, NC 27709
   USA

   Phone: +1 919 392 3266
   Email: gsalguei@cisco.com
   IM: xmpp:gsalguei@cisco.com


   Chris Pearce
   Cisco Systems, Inc.
   2300 East President George Bush Highway
   Richardson, TX 75082
   USA

   Phone: +1 972 813 5123
   Email: chrep@cisco.com
   IM: xmpp:chrep@cisco.com





















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