SIP Working Group H. Kaplan
Internet Draft Acme Packet
Intended status: Standards Track
Expires: September 8, 2009 March 8, 2009
A Session Identifier for the Session Initiation Protocol (SIP)
draft-kaplan-sip-session-id-02
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Abstract
There are several reasons for having a globally unique session
identifier for the same SIP session, which can be maintained across
B2BUA's and other SIP middle-boxes. This draft proposes a new SIP
header to carry such a value: Session-ID.
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Table of Contents
1. Introduction................................................2
1.1. Requirements............................................3
1.2. The use-case for Session-ID.............................4
2. Terminology.................................................4
3. Applicability...............................................4
4. Overview of Operation.......................................4
5. Session-ID Behavior.........................................5
5.1. Generating a Session-ID value...........................5
5.2. UAC Behavior............................................5
5.3. UAS Behavior............................................6
5.4. Proxy Behavior..........................................6
5.5. B2BUA Behavior..........................................6
5.5.1 B2BUA Generation of New Session-ID....................7
5.5.2 B2BUA Insertion of Saved Session-ID...................7
6. Session-ID Migration and Failure Scenarios..................8
7. New Header..................................................8
7.1. "Session-ID" header.....................................8
7.2. Augmented BNF Definitions...............................9
8. Example Exchange............................................9
9. Security Considerations.....................................9
9.1. Security considerations for B2BUA vendors and operators10
9.2. Security considerations for extensions to the Session-ID10
10. IANA Considerations........................................11
11. Acknowledgments............................................11
12. References.................................................11
12.1. Normative References...................................11
12.2. Informative References.................................11
Author's Address.................................................11
1. Introduction
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.
Unfortunately, the Call-ID is often changed by B2BUA's and other
such middle-boxes in the end-to-end message path. A B2BUA logically
represents a UAS and UAC, and as such may use a new Call-ID value
for the dialog it creates on its UAC half; but there are several
use-cases for having a common, consistent end-to-end identifier, as
described later in this draft.
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There are several reasons the Call-ID value is changed by B2BUA's.
There are security and privacy reasons, since Call-ID values
typically contain UA IP Addresses; some B2BUA's need to change them
to keep track of spiraling dialogs; and some need to change them to
keep track of separate forks. In fact, some people have argued a
B2BUA has no choice but to create a new one, in order to strictly
comply with RFC 3261 as a UAC. In general, B2BUA's modify the Call-
ID value in both directions, "fixing" it to be what each side of the
B2BUA would expect. This works fine if the B2BUA is in the message
path, and knows all SIP message or body contents which use or
reference the value. However for subsequent out-of-dialog requests,
or new SIP uses, a B2BUA often does not or cannot "fix" the value
correctly, for example if it is not traversed.
Therefore, in order to provide an identifier which will not be
modified/replaced by B2BUA's, 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 which
would cause B2BUA's to change it. The goal of this draft is to
enable use-cases which need a unique identifier for a given session
which can successfully cross B2BUA's.
1.1. Requirements
The following requirements drive the need for Session-ID:
REQ1: It must be possible 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 B2BUA's,
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.
In a previous version of this draft, an additional requirement was
proposed that the identifier be usable in out-of-dialog requests for
matching purposes, similar to the Call-ID usage today. The
motivation for this was to enable SIP use-cases to work that
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currently do not work, or may not work in the future as SIP domains
connect together. Concerns were raised that the solution would not
solve all possible use-cases, in all possible scenarios, and thus
would not be a complete solution to the problem. Therefore, in
order to progress this draft for troubleshooting uses, that
additional requirement and its associated solution behavior has been
removed from this draft.
1.2. The use-case for Session-ID
The need for a unique identifier is driven by the need to
troubleshoot SIP sessions as they cross SIP nodes. Troubleshooting
is more complicated if multiple legs of the session are on different
sides of B2BUA's, due to the lack of a common identifier to tie the
legs together. Currently proprietary mechanisms are used to achieve
this.
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".
3. Applicability
This draft proposes a new SIP header for all requests and responses.
4. Overview of Operation
The general concept is that the UAC generating an out-of-dialog
request generates a new, pseudo-random, unique value which remains
constant for the duration of the transaction, any dialog created
from the request, or a registration. The value is based on the
rules for creating a fixed-length pseudo-random value, and is
inserted in a new Session-ID header defined in this draft. The UAC
and UAS then reflect this value in all messages for the duration of
the dialog.
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.
This Session-ID 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 value provides an identifier
for troubleshooting uses only.
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5. Session-ID Behavior
5.1. 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, and its length, is to make it
impossible to determine the manufacturer of the device which
generated it by looking at its format or value. For example, the
theoretically random "session id" value in SDP origin line has been
found to be fairly vendor-specific in nature, and one can narrow the
vendor that generated the SDP simply by the origin session id value.
(In fact, this drove some SBC's to modify that SDP field for
"anonymization" purposes)
In order to enable trouble-shooting of in-dialog messages, a
generator needs to remember the Session-ID for a given dialog(s).
This is described in more detail in following sections of this
document.
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-ID's being changed, as required by Req-3b.
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.
5.2. 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 draft MUST
generate a *new* unique Session-ID value whenever it generates an
out-of-dialog request, or for a new Registration. The UAC MUST re-
use the same Session-ID for in-dialog messages, 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
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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 and
thus the hash mechanism should by definition produce the correct
Session-ID; 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 value in every SIP
message it transmits. This serves both a troubleshooting purpose,
and may be used in specific identity verification mechanisms which
are beyond the scope of this draft.
5.3. UAS Behavior
A UAS compliant with this document MUST copy a received Session-ID
value 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 value in
all responses and upstream in-dialog requests.
5.4. Proxy Behavior
A Proxy MUST NOT remove or modify the Session-ID header values it
receives, if one is in the message. By definition, an RFC 3261
compliant Proxy would not modify or remove such a header.
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
message, following the rules for doing so as a B2BUA defined later.
If the Proxy forks a request, it MUST copy the same Session-ID value
into all the forked request copies. If the Proxy recurses requests
due to 3xx redirection, or regenerates requests due to failures, it
MUST use the same Session-ID value 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 value from the original request into the response message
it locally creates. This is necessary for troubleshooting purposes.
5.5. B2BUA Behavior
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A B2BUA compliant with this document MUST copy the Session-ID it
receives in requests as a UAS into the related requests it generates
as a UAC; and any Session-ID value 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 value 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 value,
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 value from the original request into the response message
it locally creates. A B2BUA MUST 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 the header value.
5.5.1 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 MUST generate a new Session-ID. 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.
5.5.2 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 value 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.
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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 UA's 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
B2BUA's or Proxies to perform the Session-ID generator and inserter
role. Even within those device types, it is impractical to expect
all B2BUA's to support this mechanism all at once, or any time in
the near future. Therefore, it is expected that some B2BUA's and/or
UA's 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 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 current deployments as much as possible - not to
cover all possible scenarios - 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 B2BUA's or UAS's
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, leading to two different Session-ID values for the
same related dialogs temporarily. Eventually the UAC would only
accept one of the dialogs (typically), and only one Session-ID would
remain.
7. New Header
The following table updates Table 2 in [RFC3261] and other defined
extensions.
Hdr-field when ACK BYE CAN INV OPT REG PRA INF REF UPD SUB NOT MSG
-----------------------------------------------------------------
Session-ID R m m m m m m m m m m m m m
Session-ID r m m m m m m m m m m m m m
7.1. "Session-ID" header
This draft proposes "Session-ID" to be added to the definition of
the element "message-header" in the SIP message grammar.
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The Session-ID header is a single-instance header.
The compact form of the header is requested to be: h
[for "Help us help you"]
7.2. Augmented BNF Definitions
Session-ID = "Session-ID" HCOLON sess-id
*( SEMI generic-param )
sess-id = 32(DIGIT / %x61-7A) ; 32 chars of [0-9a-z]
NOTE: The sess-id value is case-SENSITIVE, just as Call-ID is,
however only lower-case characters are defined and allowed.
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'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.
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
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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-ID's which have a property that makes determining the
key more likely.
9.1. Security considerations for B2BUA vendors and operators
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.
There is no known security issue with viewing or modifying the
Session-ID, other than to hamper Troubleshooting efforts.
9.2. Security considerations for extensions to the Session-ID
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 B2BUA behavior, by creating a SIP header
value for which the properties are such that B2BUA's 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* future defined parameters, maintain this
benign property. Any future extensions to the Session-ID mechanism
and header field MUST maintain this property, or else B2BUA's 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. Because of this, any uses for Session-ID header
parameters MUST be documented in RFCs.
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10. IANA Considerations
This document asks IANA for a new SIP header field, in long and
compact form.
11. Acknowledgments
Thanks to Raphael Coeffic, Bob Penfield, Dale Worley, Paul Kyzivat,
and Ian Elz for their input.
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.
[RFC2104] Krawczyk, H., Bellare, M., Canetti, R., "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104, February 1997.
12.2. Informative References
[draft-transparent-b2bua] Marjou, X., Elz, I., Musgrave, P., "Best
Current Practices for a Session Initiation Protocol (SIP)
Transparent Back-To-Back User-Agent (B2BUA)", draft-marjou-
sipping-b2bua-01, July 2007.
Author's Address
Hadriel Kaplan
Acme Packet
71 Third Ave.
Burlington, MA 01803, USA
Email: hkaplan@acmepacket.com
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