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Versions: (draft-kaplan-dispatch-session-id) 00 01 02 03 04

INSIPID Working Group                                         H. Kaplan
Internet Draft                                                   Oracle
Intended status: Informational
Expires: February 1, 2014                                August 1, 2013


      A Session Identifier for the Session Initiation Protocol (SIP)
                    draft-kaplan-insipid-session-id-02

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   is published for the historical record.

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   This Internet-Draft will expire on February 1, 2014.

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Abstract

   This RFC, which contains the text of an individual Internet Draft
   that was submitted originally to the DISPATCH Working Group, is
   being published now as an Informational document to provide a
   reference for later RFCs.  The mechanism defined in this document
   has been widely deployed, and is being followed in a backward-
   compatible fashion for a new Standards Track RFC in the INSIPID
   Working Group.  The original Abstract follows.

   There is a need for having a globally unique session identifier for
   the same SIP session, which can be consistently maintained across
   Proxies, B2BUAs and other SIP middle-boxes, for the purpose of
   Troubleshooting.  This draft proposes a new SIP header to carry such
   a value: Session-ID.

Table of Contents

   1. Introduction..................................................3
      1.1. Requirements.............................................4
   2. Terminology...................................................4
   3. Overview of Operation.........................................4
   4. Session-ID Behavior...........................................5
      4.1. Generating a Session-ID value............................5
      4.2. UAC Behavior.............................................6
      4.3. UAS Behavior.............................................6
      4.4. Proxy Behavior...........................................6
      4.5. B2BUA Behavior...........................................7
         4.5.1 B2BUA Generation of New Session-ID  7
         4.5.2 B2BUA Insertion of Saved Session-ID 8
   5. Handling SIP Transfer Scenarios...............................8
      5.1. Out-of-Dialog REFER......................................9
      5.2. Refer-To URI.............................................9
      5.3. Out-of-Dialog INVITE with Replaces.......................9
   6. Session-ID Migration and Failure Scenarios...................10
   7. New 'Session-ID' Header......................................10
      7.1. Augmented BNF Definitions...............................10
   8. Example Exchange.............................................11
   9. Security Considerations......................................11
      9.1. Security considerations for administrators..............11
      9.2. Security considerations for Session-ID extensions.......11
   10. IANA Considerations.........................................12
   11. Acknowledgments.............................................12
   12. References..................................................13
      12.1. Normative References...................................13
   Author's Address.................................................13
   Appendix A. Use-cases not in scope for Session-ID...............13
      A.1. Dialog Correlation for SIP..............................14


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1. Introduction

   This RFC, which contains the text of an individual Internet Draft
   that was submitted originally to the DISPATCH Working Group, is
   being published now as an Informational document to provide a
   reference for later RFCs.  The mechanism defined in this document
   has been widely deployed, and is being followed in a backward-
   compatible fashion for a new Standards Track RFC in the INSIPID
   Working Group.

   The SIP [RFC3261] Call-ID header value is a globally unique
   identifier, mandatory in all requests/responses, which identifies
   SIP messages belonging to the same dialog or registration.  It
   provides a portion of the SIP message dialog-matching criteria, and
   is used in such things as [Replaces] headers and [dialog-events]
   package for matching to dialogs, and in [SIP-Identity] and
   [Connected-identity] as one of the inputs for signing.

   In practice, the Call-ID is often changed by B2BUAs and other such
   middle-boxes in the logical end-to-end message path.  A B2BUA
   logically represents a UAS and UAC, and as such generates a new
   Call-ID value for the dialog it creates on its UAC side; in fact for
   some B2BUA scenarios the Call-ID *must* be changed for SIP to
   function properly.

   At the same time, there is a need for a unique, common, consistent
   end-to-end identifier to help troubleshoot SIP sessions and message-
   flows as they cross SIP nodes.  Troubleshooting is more complicated
   if multiple legs of the session are on different sides of B2BUAs,
   due to the lack of a common identifier such as a Call-ID to tie the
   legs together.  Proprietary mechanisms are currently used to achieve
   this goal.

   Therefore, in order to provide an identifier that will not be
   modified/replaced by B2BUAs, this draft proposes a new SIP Header
   "Session-ID", and mandatory rules for the value of such a header.
   The rules are designed to be such that the value in the Session-ID
   header is not considered unsafe, private, or have any property that
   would cause B2BUAs to change it.  The goal of this draft is to
   enable troubleshooting by providing a unique identifier for a given
   session which can successfully cross B2BUAs, such as Application
   Servers, Softswitches, PBXs, SBCs, Feature Servers, etc.







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1.1. Requirements

   The following requirements drive the need for Session-ID:

   REQ1: It must be possible for an administrator to use the identifier
   to identify a set of dialogs which have a direct correlation with
   each other such that they represent the same SIP session, with as
   high a probability as possible.

   REQ2: It must be possible to pass the identifier through SIP B2BUAs,
   with as high a probability as possible.  This requirement drives the
   following requirements:

   REQ2a: The identifier must not reveal any information related to any
   SIP device or domain identity, including IP Address, port, hostname,
   domain name, username, Address-of-Record, MAC address, IP address
   family, transport type, etc.

   REQ2b: The identifier must not reveal to the receiver of it that the
   Call-ID, tags, or any other SIP header or body portion have been
   changed by middle-boxes, with as high a probability as possible.

   REQ2c: The identifier must not be used for anything at a SIP layer
   to change the behavior of the SIP protocol.

2. Terminology

   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.  The
   terminology in this document conforms to RFC 2828, "Internet
   Security Glossary".

   This document uses the terms "header field" and "header field value"
   following the definition of those terms in [RFC3261], which are not
   interchangeable.  The "header field" is the entire SIP header's
   contents, including any parameters.  The "header field value" is
   only the field-value portion, which does not include header
   parameters.

3. Overview of Operation

   The general concept is that the UAC generating an out-of-dialog
   request generates a new, pseudo-random, unique value that remains
   constant for the duration of the transaction, any dialog created
   from that request, or for a registration.  The value is inserted in
   a new Session-ID header field defined in this draft.  The UAC and
   UAS then reflect this header field value in all messages for the
   duration of the dialog.  In other words, the Session-ID provides a


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   value similar in nature to Call-ID, except one which crosses B2BUAs
   and which has no sensitive information in it.

   To aid in migration of deployments, a B2BUA or Proxy MAY also
   generate and/or insert the value on behalf of a UAC or UAS, if one
   or the other does not support this document's mechanism.

   Although the Session-ID concept is similar to the Call-ID, it is not
   used for message dialog-matching rules in RFC 3261, nor does it
   change the Call-ID usage, nor does it replace the Call-ID value.
   Instead this new header field provides an identifier for
   troubleshooting uses only.

   The format of the Session-ID value is restricted, both to avoid
   detection of the system type which generated it, and to keep it a
   hexadecimal representation such that it can be stored as a 128-bit
   binary value in log records.

4. Session-ID Behavior

4.1. Generating a Session-ID value

   This draft proposes the Session-ID header value be generated based
   on a defined hash mechanism for creating a 128-bit pseudo-random
   value, and encode it as its lower-case hex representation.  The
   reason for specifying the mechanism is two-fold: to make it
   impossible to determine the manufacturer of the device which
   generated it by looking at its format or value, and to allow devices
   to generate the same value if they have the same private key.

   The Session-ID value is generated by taking the Call-ID header
   value, and SHA-1 hashing it based on [RFC2104] HMAC using a locally
   generated pseudo-random 128-bit system secret key, to create a 128-
   bit resultant HMAC value.  The secret key makes the resultant HMAC
   value not re-creatable by other parties, which is necessary to
   prevent detection of Call-IDs being changed, as required by Req-2b.
   Otherwise, middle-boxes may have motivation to remove the Session-ID
   in order to hide the fact that they changed the Call-ID.

   Per [RFC2104], the algorithm is thus HMAC-SHA-1-128(Call-ID_value,
   secret_key), and the 128-bit result is encoded using lowercase
   alphanumeric hex representation, as defined in the ABNF section of
   this document.

   In order to enable trouble-shooting of in-dialog messages, a
   generator needs to remember or re-create the same Session-ID value
   for the duration of a given dialog(s).  This is described in more
   detail in following sections of this document.



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4.2. UAC Behavior

   The rules for when a UAC generates a new Session-ID value are
   similar as those for Call-ID value: a UAC supporting this document's
   mechanism MUST generate a new unique Session-ID value whenever it
   generates an out-of-dialog request, or for a new Registration.  The
   exception to this rule is for out-of-dialog REFER requests, or
   INVITE with [RFC3891] Replaces header field, as described in section
   5.

   The UAC MUST re-use the same Session-ID value for in-dialog messages
   as that of the original dialog-creating request, and for any out-of-
   dialog request it retransmits or re-generates in response to a 3xx,
   or it re-formulates due to failure responses.  This follows the
   rules in [RFC3261] for Call-ID generation.

   Session-ID values in Registration "refreshes" - REGISTER requests
   which are used to update the expiry time but not to register a new
   contact - MUST use the same Session-ID value as previous REGISTER
   requests.  New Registrations, which add or change the Contact URI
   for the AoR, but not simply to delete them, MUST use a new Session-
   ID value.  This follows the behavior of Call-ID per RFC 3261; it is
   re-iterated here because some devices incorrectly change their Call-
   ID value for every re-Registration, and MUST NOT do the same to the
   Session-ID.

   The UAC MUST include the Session-ID header field in every SIP
   message it transmits.

4.3. UAS Behavior

   A UAS compliant with this document MUST copy a received Session-ID
   header field in a request, into responses and subsequent upstream
   requests sent within the dialog.

   If an out-of-dialog request is received without a Session-ID header
   field, the UAS SHOULD generate a new one for subsequent use in the
   transaction and dialog, as defined for a UAC, and use the same value
   in all responses and upstream in-dialog requests for the same
   dialog.

4.4. Proxy Behavior

   A Proxy MUST NOT remove or modify the Session-ID header field it
   receives, if one is in the message.  By definition, an RFC 3261
   compliant Proxy would not modify or remove such a header.

   If the Proxy forks a request, it MUST copy the same Session-ID
   header field into all the forked request copies.  If the Proxy


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   recurses requests due to 3xx redirection, or regenerates requests
   due to failures, it MUST use the same Session-ID header field as the
   original request, just as the UAC does.

   If the Proxy locally generates any response or request based on a
   received request, including 100 Trying, it MUST insert any received
   Session-ID header field from the original request into the response
   message it locally creates.  This is necessary for troubleshooting
   purposes.

   A Proxy compliant with this draft MAY generate a new Session-ID or
   insert a previously saved one, if and only if none existed in a
   received message, following the rules for doing so as a B2BUA
   defined later.

4.5. B2BUA Behavior

   A B2BUA compliant with this document MUST copy the Session-ID header
   field it receives in requests as a UAS into the related requests it
   generates as a UAC; and any Session-ID field it receives in
   responses as a UAC into the correlated responses it generates as a
   UAS.

   If the B2BUA forks or creates multiple requests as a UAC, from a
   request it received as a UAS, the B2BUA MUST copy the same Session-
   ID header field it received into all the forks/requests.  If the
   B2BUA recurses requests due to 3xx redirection, or regenerates
   requests due to failures, it MUST use the same Session-ID field,
   just as the UAC does.

   If the B2BUA locally generates any response or request based on a
   received request, including 100 Trying, it MUST insert any received
   Session-ID field from the original request into the response message
   it locally creates.

   A B2BUA MAY remember the received Session-ID value for the duration
   of the transaction and dialog, for the purpose of re-insertion, in
   case the far-end does not support this draft.

   In all cases, if the SIP message received by a B2BUA contained a
   Session-ID header field, a B2BUA compliant with this document MUST
   NOT remove, modify or replace it as it "forwards" the message on the
   other logical UA "side" of itself.

4.5.1     B2BUA Generation of New Session-ID

   If an out-of-dialog request is received by a B2BUA compliant with
   this document, and the request does *not* contain a Session-ID
   header field, the B2BUA MAY generate a new one.  The new Session-ID


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   value MUST be calculated based on the received Call-ID of the
   received request, even if the B2BUA uses a different Call-ID value
   for requests generated on its other "side(s)".  It MUST then insert
   it in any requests or responses it generates, as if it had actually
   received the new Session-ID from the UAC, following the rules
   previously defined for a B2BUA.  This allows for a B2BUA to provide
   a migration to Session-ID deployment, on behalf of upstream nodes
   that do not yet support it.

   As defined previously, if any received message already had a
   Session-ID, a B2BUA compliant with this document would not replace
   it.

4.5.2     B2BUA Insertion of Saved Session-ID

   If a Session-ID was received in an out-of-dialog request, or the
   B2BUA locally generated one because none existed, the B2BUA SHOULD
   insert the same Session-ID field into all responses and upstream in-
   dialog requests if and only if a Session-ID is not already in them.
   This allows for a B2BUA to provide a migration to Session-ID
   deployment, on behalf of downstream nodes that do not yet support
   it.

5. Handling SIP Transfer Scenarios

   The transfer or movement of SIP sessions represents a complication
   for a Session-ID type mechanism.  On the one hand, the replacement
   SIP session represents a new one, and could reasonably be expected
   to use a new Session-ID value; on the other hand, from a
   troubleshooting and human user perspective, it is clearly related
   to, if not just a continuation of, the previous session.  Since the
   purpose of this document's mechanism is to aid monitoring and
   troubleshooting, and not used for actual SIP protocol mechanics, the
   behavior defined in this section is to re-use the same Session-ID
   value for the replacement SIP session.

   In order to do so, the Session-ID of the "original" session is
   transferred as well, in the Refer-To URI of a [RFC3515] REFER
   request.  Furthermore, out-of-dialog REFER and INVITE with [RFC3891]
   Replaces requests use the appropriate Session-ID values.  This
   assumes, of course, that the UAs involved support the Session-ID
   mechanism.  If they do not, then it is possible for the Session-ID
   to not be "carried forward" to the new SIP dialog.  Unfortunately,
   this means troubleshooting such dialogs is not improved/aided by
   this document's mechanism; but it would not "break" anything at a
   SIP layer.

   It should also be noted that using the same Session-ID for the
   transferred-to dialog means the same Session-ID now exists in two


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   independent dialogs, because the original one may well continue due
   to the implicit Subscription usage created by a REFER - that
   implicit Subscription based usage will continue to use the same
   Session-ID as the new dialog created to the transferred-to party.

   For the following sub-sections, the term "UA" is used for User
   Agent.  The language applies to the SIP device which creates the
   request, whether it be a UA or B2BUA.

5.1. Out-of-Dialog REFER

   A UA compliant with this document MUST use the same Session-ID
   header field value for an out-of-dialog REFER request it generates,
   as the original dialog the REFER is targeted to (i.e., as if the
   REFER had been in-dialog).  For example, if UA Bob has a SIP dialog
   X to Alice, and Bob sends an out-of-dialog REFER to Alice to refer
   her to Charlie, the Session-ID header field value of the REFER
   request would be the same as that used in dialog X.

5.2. Refer-To URI

   A UA compliant with this document MUST add the Session-ID header
   field as an embedded header in the Refer-To header field URI of any
   REFER request it generates, using the value of the session it is
   referring to.  For example, if UA Bob has a SIP dialog X to Alice
   and dialog Y to Charlie, and Bob sends a REFER request to Alice to
   refer her to Charlie, the Session-ID header field value embedded in
   the Refer-To URI of the REFER request would be the same as that used
   in dialog Y.

5.3. Out-of-Dialog INVITE with Replaces

   When generating an out-of-dialog INVITE with [RFC3891] Replaces
   header field, a UA compliant with this document MUST use the same
   Session-ID header field value for the INVITE request as that used
   for the dialog it is replacing, if it knows the value.  Typically it
   would know the value by having received it in the Refer-To header
   field of a REFER, as described previously.  For example, if UA Bob
   has a SIP dialog X to Alice and dialog Y to Charlie, and Bob sends a
   REFER request to Alice to refer her to Charlie, the Session-ID
   header field value embedded in the Refer-To URI of the REFER request
   would be the one used in dialog Y, which Alice would use as the
   Session-ID header field value for her INVITE to Charlie.

   If the UA does not know the Session-ID of the dialog it is
   replacing, for example because it is not embedded in the Refer-To
   URI of a received REFER, then it MUST use a new Session-ID value,
   calculated using the mechanism as defined in section 4.1 with the
   Call-ID of the INVITE.


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6. Session-ID Migration and Failure Scenarios

   SIP is already widely deployed on the Internet, and it is
   impractical to expect all UAs to be upgraded to support this
   document's mechanism in the near future.  A solution for gradual
   migration is necessary, which this document provides by allowing
   B2BUAs or Proxies to perform the Session-ID generator and inserter
   role.  Even within those device types, it is impractical to expect
   all B2BUAs to support this mechanism all at once, or any time in the
   near future.  Therefore, it is expected that some B2BUAs and/or UAs
   will support generating and inserting Session-ID, while others will
   not support Session-ID at all.

   Due to the varying types of B2BUAs, such as PBXs, SBCs, Application
   Servers, Feature Servers, and Softswitches of various flavors, and
   the numerous SIP deployment models in use, there are going to be
   cases in which Session-ID will fail to be a consistent value for all
   related dialogs, or fail to successfully match.  The goal of this
   draft is to improve troubleshooting of current deployments as much
   as possible - and in this author's opinion that is the best that can
   be done given the constraints.

   One example is for forked requests: if a UAC which does not support
   this mechanism sends a request to a Proxy or B2BUA which also does
   not support this mechanism, each fork could reach B2BUAs or UASs
   which *do* support this mechanism.  In such a case, each of those
   forked-to B2BUA/UAS will generate unique Session-IDs and put them in
   their responses, temporarily leading to multiple, different Session-
   ID values for the same related early dialogs.  Typically, the UAC
   would eventually only accept one of the dialogs, and only one
   Session-ID would remain.

7. New 'Session-ID' Header

   This draft adds the "Session-ID" token to the definition of the
   element "message-header" in the SIP message grammar.  The Session-ID
   header is a single-instance header.

7.1. Augmented BNF Definitions

   Session-ID           =  "Session-ID" HCOLON sess-id
                           *( SEMI generic-param )
   sess-id              =  32(DIGIT / %x61-66)  ;32 chars of [0-9a-f]

   NOTE: The sess-id value is technically case-INSENSITIVE, but note
   that only lower-case characters are allowed.




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   See the Security Considerations section for discussion about using
   header parameters in Session-ID header fields.

8. Example Exchange

   In the following example, Alice initiates a call to Bob.  Alice
   generates a Session-ID header in the out-of-dialog INVITE.

   Alice generates the following: (note: much has been left out for
   simplicity)
      INVITE sip:bob@example.com SIP/2.0
      Via: SIP/2.0/UDP 192.0.2.1:5060;branch=z9hG4bKnashds10
      From: Alice <sip:alice@example.net>;tag=1234567
      To: Bob <sip:bob@example.com>
      Call-Id: 123456mcmxcix@1.2.3.4
      Session-ID: f81d4fae7dec11d0a76500a0c91e6bf6
      CSeq: 1 INVITE
      Contact: <sip:alice@192.168.1.1>

9. Security Considerations

   There are several security considerations surrounding this
   document's mechanism.

   The Session-ID generation algorithm should provide a reasonably
   random 32-character Session-ID value, to avoid collisions, and would
   not let one re-create the original Call-ID.

   There is no known security issue with viewing or modifying the
   Session-ID, other than to hamper troubleshooting efforts.

9.1. Security considerations for administrators

   The requirement for the Session-ID is to be an identifier which
   cannot be used by a recipient to identify if the Call-ID has been
   changed by middle-boxes.  As such, a UAS/UAC cannot detect the
   original Call-ID, nor whether it has been changed, and thus should
   not cause administrators concern to be "passed through".

9.2. Security considerations for Session-ID extensions

   The Session-ID's value is created from the Call-ID using a hashing
   mechanism based on [RFC2104], using SHA-1 and a secret key known
   only to the system generating the Session-ID.  Because the algorithm
   is defined in this document, it should be fairly secure from
   detecting the generator of the Session-ID, in terms of manufacturer
   or code base.




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   The Session-ID generation algorithm should provide a reasonably
   random 128-bit Session-ID value, to avoid collisions, and would not
   let one re-create the original Call-ID.  The secret key MUST only be
   used for the Session-ID mechanism, in case a weakness is found which
   reveals the key.  One such weakness may be that a UAC generates one
   or more Call-IDs which have a property that makes determining the
   key more likely.

   In general, B2BUA behavior cannot be dictated by standards.  They do
   whatever their owners/operators wish them to do, or whatever is
   necessary to make their applications work.  This document attempts
   to normatively specify some B2BUA behavior, by creating a SIP header
   value for which the properties are such that B2BUAs should have no
   legitimate reason to interfere.  This effectively creates a
   "promise" that future uses of this Session-ID header field,
   including its value *and* any future defined parameters, maintain
   this benign property.  Any future extensions to the Session-ID
   mechanism and header field MUST maintain this property, or else
   B2BUAs will begin to modify it again or remove it, and its value
   will be lost.

   Manufacturers of SIP devices should note that there is no guarantee
   that a B2BUA will not inspect the Session-ID header field, and
   remove it if it does not comply with this document's value
   restrictions.  Any Session-ID header parameters MUST be registered
   with IANA and documented in IETF RFCs, pursuant to the requirements
   of [RFC3968].

10.  IANA Considerations

   This document asks IANA to register a new SIP header field named
   'Session-ID', pursuant to the registration policies for such in
   [RFC5727].  This is a single-instance header field, and is
   appropriate for any SIP message, of any Method type, in any request
   or response.

   The ABNF rules for this new header allow for header parameters,
   however they must be registered following the rules of [RFC3968], as
   required by [RFC5727].

11.  Acknowledgments

   Thanks to Raphael Coeffic, Bob Penfield, Dale Worley, Paul Kyzivat,
   Ian Elz, Marco Stura, Martin Dolly, Martin Huelsemann, Laura Liess
   and Adam Roach for their input.






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12.  References

12.1.     Normative References

    [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
         A., Peterson, J., Sparks, R., Handley, M., and E. Schooler,
         "SIP:  Session Initiation Protocol", RFC 3261, June 2002.

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

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

    [RFC3968]  Camarillo, G., "The Internet Assigned Number Authority
         (IANA) Header Field Parameter Registry for the Session
         Initiation Protocol (SIP)", RFC 3968, December 2004.

    [RFC4538]  Rosenberg, J., "Request Authorization through Dialog
         Identification in the Session Initiation Protocol (SIP)", RFC
         4538, June 2006.

    [RFC5727]  Peterson, J., Jennings, C., Sparks, R., "Change Process
         for the Session Initiation Protocol (SIP) and the Real-time
         Applications and Infrastructure Area", RFC 5727, March 2010.


Author's Address

   Hadriel Kaplan
   Oracle
   Email: hadriel.kaplan@oracle.com


Appendix A.    Use-cases not in scope for Session-ID

   It is very tempting to use a header field value such as that
   provided by Session-ID, for other purposes than troubleshooting.  In
   a previous draft for this same Session-ID concept, the proposal
   included other uses; but these were removed because any use-case
   other than troubleshooting can easily lead to a B2BUA needing to
   change the value, in certain cases.  That would defeat the
   troubleshooting value of Session-ID.  This section discusses such
   use-cases and explains why they are potentially harmful.






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A.1. Dialog Correlation for SIP

   Although Session-ID does provide a means to correlate separate SIP
   dialogs, messages, and transactions, it does so at a higher layer
   than SIP.  It does not replace the mechanics of SIP using the Call-
   ID and To/From tags of SIP messages to correlate SIP dialogs, nor in
   other uses such as Replaces headers or dialog-event packages.  It is
   tempting, however, to use it for exactly that purpose in certain
   cases.

   For example, suppose a call transfer case where Alice calls Bob
   through B2BUA-1.  Bob then calls Charlie, and sends Charlie a REFER
   with embedded Replaces, to make Charlie send an INVITE with Replaces
   header to Alice, to replace the Alice-Bob session.  If Charlie uses
   a different B2BUA-2 to reach Alice, the INVITE with Replaces will
   fail, because the Call-ID/tags won't match anything B2BUA-2 or Alice
   knows about.

    +-----+     +-------+    +-------+    +-----+     +-------+
    |Alice|     |B2BUA-1|    |B2BUA-2|    | Bob |     |Charlie|
    +-----+     +-------+    +-------+    +-----+     +-------+
       |            |            |           |            |
       |INVITE      |            |           |            |
       |callid:1a   |callid:1b   |           |            |
       |----------->|----------------------->|INVITE      |
       |sessid:1    |sessid:1    |           |callid:2a   |
       |            |            |           |----------->|
       |            |            |           |sessid:2    |
       |            |            |           |            |
       |            |            |           |REFER       |
       |            |            |           |referto:1b  |
       |            |            |           |----------->|
       |            |            |           |            |
       |            |            |           |      INVITE|
       |            |            |           | replaces:1b|
       |            |            X<-----------------------|
       |            |      INVITE|           |    sessid:1|
       |            | replaces:1b|           |            |
       X<------------------------|           |            |
       |            |    sessid:1|           |            |
                  Example-1: call transfer case

   If, on the other hand, Alice were to use the Session-ID value for
   correlation, she would see it matches her dialog with Bob (assuming
   the Session-ID were passed along in the Refer-To and Replaces info).

   There are problems with this approach, however.  The first problem
   is, by not sending the INVITE with Replaces to B2BUA-1, B2BUA-1 is


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   in an incorrect state; the dialog is getting replaced, and the B2BUA
   doesn't know it.

   A second issue is the Session-ID doesn't identify enough information
   to replace a dialog.  Imagine there were a third B2BUA, such as a
   SoftSwitch, between Alice and B2BUA-1 and B2BUA-2, and the INVITE
   with Replaces reached the SoftSwitch before Alice.  The SoftSwitch
   won't know which "side" the INVITE is replacing.  The To/From tags
   no longer match anything the SoftSwitch knows about, so it can't
   figure out if the INVITE with Replaces is replacing the dialog from
   SoftSwitch to Alice, or the one to Bob.  If we try to fix this by
   creating a tag-type value pair for Session-ID, we're back to devices
   changing those tag values and defeating the matching property.


   Another example is based on 3GPP 24.605 annex A.2.2.  Alice has a
   call with Bob through multiple B2BUA's and an Application Server.
   The dialogs of that call all have the same session-id, but unique
   call-id/tags.

   Alice wants to invoke a 3rd party conference facility in the AS, and
   reference the call she has with Bob for that.  In this particular
   3gpp scenario, to do that Alice sends a new INVITE to the AS with a
   resource-list body (a la RFC 5366) containing the call information
   for the original call.  This is the "RL<sessid:1>" piece in the
   diagram.  It has the calli-d/tags as well, but they'll be wrong when
   received at the AS.

   The AS processes that list, can't match the callid/tags in the
   resource-lit but does match the session-id, and sends a re-INVITE to
   party B within the original call's dialog.




















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    +-----+     +-------+      +----+    +-------+     +-----+
    |Alice|     |B2BUA-1|      | AS |    |B2BUA-2|     | Bob |
    +-----+     +-------+      +----+    +-------+     +-----+
       |            |            |           |            |
       |INVITE      |            |           |            |
       |callid:1a   |callid:1b   |callid:1c  |callid:1d   |
       |----------->|----------->|---------->|----------->|
       |sessid:1    |sessid:1    |sessid:1   |sessid:1    |
       |            |            |           |            |
       |INVITE      |            |           |            |
       |callid:2a   |callid:2b   |           |            |
       |----------->|----------->|           |            |
       |sessid:2    |sessid:2    |re-INVITE  |            |
       |RL<sessid:1>|RL<sessid:1>|callid:1c  |callid:1d   |
       |            |            |---------->|----------->|
       |            |            |sessid:1   |sessid:1    |
       |            |            |           |            |
                         Example-2: Resource List

































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