Internet DRAFT - draft-kaplan-insipid-session-id
draft-kaplan-insipid-session-id
INSIPID Working Group H. Kaplan
Internet Draft Oracle
Intended status: Informational
Expires: September 10, 2014 March 10, 2014
A Session Identifier for the Session Initiation Protocol (SIP)
draft-kaplan-insipid-session-id-04
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
SIP Proxies, Back-to-Back User Agents (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.
Status of this Memo
This document is not an Internet Standards Track specification; it
is published for the historical record.
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
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Internet-Drafts are draft documents valid for a maximum of six
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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.
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This Internet-Draft will expire on September 10, 2014.
Copyright and License Notice
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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...........................................5
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............................................13
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 SIP Back-to-Back User
Agents (B2BUAs) and other such middle-boxes in the logical end-to-
end message path. A B2BUA logically represents a SIP User Agent
Server (UAS) and User Agent Client (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, Private Branch Exchanges (PBXs), Session
Border Controllers (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 [RFC2119].
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
value similar in nature to Call-ID, except one which crosses B2BUAs
and which has no sensitive information in it.
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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.
4.2. UAC Behavior
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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
recurses requests due to 3xx redirection, or regenerates requests
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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
value MUST be calculated based on the received Call-ID of the
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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
independent dialogs, because the original one may well continue due
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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].
This registration is intended to be temporary. The author expects
that a standards-track definition of Session-ID will be published at
a future date. Assuming such a document is published, it will
replace this registration with a reference to itself, at which point
this document will no longer be referenced by IANA.
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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.
12. References
12.1. Normative References
[RFC2104] Krawczyk, H., Bellare, M., Canetti, R., "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[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.
[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
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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.
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
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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
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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