Internet Engineering Task Force (IETF) P. Jones
Request for Comments: 7206 G. Salgueiro
Category: Informational J. Polk
ISSN: 2070-1721 Cisco Systems
L. Liess
Deutsche Telekom
H. Kaplan
Oracle
May 2014
Requirements for an End-to-End Session Identifier
in IP-Based Multimedia Communication Networks
Abstract
This document specifies the requirements for an end-to-end session
identifier in IP-based multimedia communication networks. This
identifier would enable endpoints, intermediate devices, and
management and monitoring systems to identify a session end-to-end
across multiple SIP devices, hops, and administrative domains.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7206.
Jones, et al. Informational [Page 1]
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Copyright Notice
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document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
2. Conventions Used in This Document ...............................3
3. Terminology .....................................................4
3.1. What Does the Session Identifier Identify? .................4
3.2. Communication Session ......................................5
3.3. End-to-End .................................................6
4. Session Identifier Use Cases ....................................6
4.1. End-to-End Identification of a Communication Session .......6
4.2. Protocol Interworking ......................................6
4.3. Traffic Monitoring .........................................7
4.4. Tracking Transferred Sessions ..............................7
4.5. Session Signal Logging .....................................8
4.6. Identifier Syntax ..........................................8
4.7. 3PCC Use Case ..............................................9
5. Requirements for the End-to-End Session Identifier ..............9
6. Related Work in Other Standards Organizations ..................10
6.1. Coordination with the ITU-T ...............................10
6.2. Requirements within 3GPP ..................................11
7. Security Considerations ........................................11
8. Acknowledgments ................................................12
9. Contributors ...................................................12
10. References ....................................................12
10.1. Normative References .....................................12
10.2. Informative References ...................................12
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1. Introduction
IP-based multimedia communication systems like SIP [1] and H.323 [2]
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.
Unfortunately, there are a number of factors that mean that the
current call identifiers defined in SIP and H.323 are not suitable
for end-to-end session identification. Perhaps most significant is
the fact that the syntax for the call identifier in SIP and H.323 is
different between the two protocols. This important fact makes it
impossible for call identifiers to be exchanged end-to-end when a
network uses both of these session protocols.
Another reason why the current call identifiers are not suitable to
identify the session end-to-end is that in real-world deployments,
devices like Back-to-Back User Agents (B2BUAs) often change the
values as the session signaling passes through. This is true even
when a single session protocol is employed and is not a byproduct of
protocol interworking.
Lastly, identifiers that might have been used to identify a session
end-to-end fail to meet that need when sessions are manipulated
through supplementary service interactions. For example, when a
session is transferred or if a private branch exchange (PBX) joins or
merges two communication sessions together locally, the end-to-end
properties of currently defined identifiers are lost.
This document specifies the requirements for an end-to-end session
identifier in IP-based multimedia communication networks. This
identifier would enable endpoints, intermediate devices, and
management and monitoring systems to identify a session end-to-end
across multiple SIP devices, hops, and administrative domains.
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 [3] 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.
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3. Terminology
3.1. What Does the Session Identifier Identify?
The identifier on which this document places requirements, the
session identifier, identifies a set of signaling messages associated
with exactly two endpoints that, from each endpoint's perspective,
are related to a single invocation of a communication application.
How the endpoints determine which signaling messages share a given
identifier (that is, what constitutes a single invocation of a
communication application) is intentionally left loosely defined.
The term "call" is often used as an example of such an invocation for
voice and video communication, but different protocols and
deployments define the scope of a "call" in different ways. For
instance, some systems would associate all of the activity between
all three parties involved in a transfer as a single "call".
Similarly, the term "session" is often used as an example of such an
invocation, but this term is overloaded to describe both signaling
and media-level interaction. A single invocation of the
communication application, as described above, may involve multiple
RTP "sessions" as described by RFC 3550 [4], and possibly even
multiple concurrent sessions.
In this document, unless otherwise qualified, the term "communication
session", or simply "session", will refer only to the set of
signaling messages identified by the common session identifier. That
is, a "session" is a set of signaling messages associated with
exactly two endpoints that, from each endpoint's perspective, are
related to a single invocation of a communication application.
The requirements in this document put some constraints on what an
endpoint will consider the same, or a different, invocation of a
communication session. They also ensure that related sessions (as
this document is using the term) can be correlated using only the
session identifiers for each session. Again, what constitutes a
"related" session is intentionally left loosely defined.
The definition considers messages associated with exactly two
endpoints instead of messages sent between two endpoints to allow for
intermediaries that create messages on an endpoint's behalf. It is
possible that an endpoint may not see all of the messages in a
session (as this document is using the term) associated with it.
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This definition, along with the constraints imposed by the
requirements in this document, facilitates specifying an identifier
that allows the two endpoints to use two entirely different protocols
(and hence to potentially have different ideas of what a single
invocation means) or use two applications that have a different idea
of what a single invocation means.
3.2. Communication Session
A communication session may exist between two SIP User Agents (UAs)
and may pass through one or more intermediary devices, including
B2BUAs or SIP proxies. For example:
UA A Middlebox(es) UA B
SIP message(s) -------[]---[]-------> SIP message(s)
SIP message(s) <-----[]---[]------- SIP message(s)
Figure 1: Communication Session through Middlebox(es)
The following are examples of acceptable communication sessions as
described in Section 3.1 and are not exhaustive:
o A call directly between two user agents
o A call between two user agents with one or more SIP middleboxes in
the signaling path
o A call between two user agents that was initiated using third-
party call control (3PCC) [5]
o A call between two user agents (e.g., between Alice and Carol)
that results from a different communication session (e.g., Alice
and Bob) wherein one of those user agents (Alice) is transferred
to another user agent (Carol) using a REFER request or a re-INVITE
request
The following are not considered communication sessions:
o A call between any two user agents wherein two or more user agents
are engaged in a conference call via a conference focus:
- each call between the user agent and the conference focus would
be a communication session, and
- each of these is a distinct communication session.
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o A call between three user agents (e.g., Alice, Bob, and Carol)
wherein the first user agent (Alice) ad hoc conferences the other
two user agents (Bob and Carol):
- The call between Alice and Bob would be one communication
session.
- The call between Alice and Carol would be a different
communication session.
3.3. End-to-End
The term "end-to-end" in this document means the communication
session from the point of origin, passing through any number of
intermediaries, to the ultimate point of termination. It is
recognized that legacy devices may not support the end-to-end session
identifier. Since such an endpoint will not create a session
identifier, an intermediary device that supports this identifier can
inject an identifier into the session signaling.
4. Session Identifier Use Cases
4.1. End-to-End Identification of a Communication Session
For SIP messaging that either does not involve SIP servers or only
involves SIP proxies, the Call-ID header field value sufficiently
identifies each SIP message within a transaction (see Section 17 of
[1]) or dialog (see Section 12 of [1]). This is not the case when
either B2BUAs or Session Border Controllers (SBCs) [6] are in the
signaling path between User Agents (UAs). Therefore, we need the
ability to identify each communication session through a single SIP
header field, regardless of which types of SIP servers are in the
signaling path between UAs. For messages that create a dialog, each
message within the same dialog MUST use the same session identifier.
Derived Requirements: All Requirements in Section 5.
4.2. Protocol Interworking
A communication session might originate on an H.323 [2] endpoint and
pass through an SBC before ultimately reaching a terminating SIP user
agent. Likewise, a call might originate on a SIP user agent and
terminate on an H.323 endpoint. It MUST be possible to identify such
sessions end-to-end across the plurality of devices, networks, or
administrative domains.
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It is anticipated that the ITU-T will define protocol elements for
H.323 to make the end-to-end signaling possible.
Derived Requirements: REQ5, REQ7 (Section 5).
4.3. Traffic Monitoring
UA A and UA B communicate using SIP messaging with a SIP B2BUA acting
as a middlebox that belongs to a SIP service provider. For privacy
reasons, the B2BUA changes the SIP header fields that reveal
information related to the SIP users, devices, or domain identities.
The service provider uses an external device to monitor and log all
SIP traffic coming to and from the B2BUA. In the case of failures
reported by the customer or when security issues arise (e.g., theft
of service), the service provider has to analyze the logs from the
past several days or weeks and then correlates those messages that
were messages for a single end-to-end SIP session.
For this scenario, we must consider three particular use cases:
a) UAs A and B support the end-to-end session identifier.
Derived Requirements: REQ1, REQ3, REQ4, REQ6.
b) Only UA A supports the end-to-end session identifier; UA B
does not.
Derived Requirements: REQ1, REQ3, REQ4, REQ5, REQ6.
c) UAs A and B do not support the end-to-end session identifier.
Derived Requirements: REQ1, REQ3, REQ4, REQ5, REQ6.
4.4. Tracking Transferred Sessions
It is difficult to track which SIP messages were involved in the same
call across transactions, especially when invoking supplementary
services such as call transfer or call join. There exists a need for
the ability to track communication sessions as they are transferred,
one side at a time, until completion of the session (i.e., until a
BYE is sent).
Derived Requirements: REQ1, REQ2, REQ9.
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4.5. Session Signal Logging
An after-the-fact search of SIP messages to determine which messages
were part of the same transaction or call is difficult when B2BUAs
and SBCs are involved in the signaling between UAs. Mapping more
than one Call-ID together can be challenging because all of the
values in SIP header fields on one side of the B2BUA or SBC will
likely be different than those on the other side. If multiple B2BUAs
and/or SBCs are in the signaling path, more than two sets of header
field values will exist, creating more of a challenge. Creating a
common header field value through all SIP entities will greatly
reduce any challenge for the purposes of debugging, communication
tracking (such as for security purposes in case of theft of
service), etc.
Derived Requirements: REQ1, REQ3, REQ5, REQ6.
4.6. Identifier Syntax
A syntax that is too lax (e.g., one that allows special characters or
a very long identifier) would make it difficult to encode the
identifier in other protocols. Therefore, the syntax of the
identifier should be reasonably constrained.
Derived Requirement: REQ8.
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4.7. 3PCC Use Case
Third-party call control refers to the ability of an entity to create
a call in which communication is actually between two or more parties
other than the one setting up the call. For example, a B2BUA acting
as a third-party controller could establish a call between two SIP
UAs using 3PCC procedures as described in Section 4.1 of RFC 3725
[5], the flow for which is reproduced below.
A Controller B
|(1) INVITE no SDP | |
|<------------------| |
|(2) 200 offer1 | |
|------------------>| |
| |(3) INVITE offer1 |
| |------------------>|
| |(4) 200 OK answer1 |
| |<------------------|
| |(5) ACK |
| |------------------>|
|(6) ACK answer1 | |
|<------------------| |
|(7) RTP | |
|.......................................|
Figure 2: Session Identifier 3PCC Scenario
Such a flow must result in a single session identifier being used for
the communication session between UA A and UA B. This use case does
not extend to three SIP UAs.
Derived Requirement: REQ9.
5. Requirements for the End-to-End Session Identifier
The following requirements are derived from the use cases and
additional constraints regarding the construction of the identifier.
REQ1: It MUST be possible for an administrator or an external device
that monitors the SIP traffic to use the identifier to identify
those dialogs, transactions, and messages that were at some point
in time components of a single end-to-end SIP session (e.g., parts
of the same call).
REQ2: It MUST be possible to correlate two end-to-end sessions when a
session is transferred or if two different sessions are joined
together via an intermediary (e.g., a PBX).
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REQ3: The solution MUST require that the identifier, if present, pass
unchanged through SIP B2BUAs or other intermediaries.
REQ4: The identifier MUST NOT reveal any information related to any
SIP user, device, or domain identity. Additionally, it MUST NOT
be possible to correlate a set of session identifiers produced
over a period of time with one another, or with a particular user
or device. This includes any IP address, port, hostname, domain
name, username, Address-of-Record, Media Access Control (MAC)
address, IP address family, transport type, subscriber ID,
Call-ID, tags, or other SIP header field or body parts.
REQ5: It MUST be possible to identify SIP traffic with an end-to-end
session identifier from and to end devices that do not support
this new identifier, such as by allowing an intermediary to inject
an identifier into the session signaling.
REQ6: The identifier SHOULD be unique in time and space, similar to
the Call-ID.
REQ7: The identifier SHOULD be constructed in such a way as to make
it suitable for transmission in SIP [1] and H.323 [2].
REQ8: The identifier SHOULD use a restricted syntax and length so as
to allow the identifier to be used in other protocols.
REQ9: It MUST be possible to correlate two end-to-end sessions when
the sessions are created by a third-party controller using 3PCC
procedures as shown in Figure 1 of RFC 3725 [5].
6. Related Work in Other Standards Organizations
6.1. Coordination with the ITU-T
IP multimedia networks are often comprised of a mix of session
protocols like SIP [1] and H.323 [2]. A benefit of the session
identifier is that it uniquely identifies a communication session
end-to-end across session protocol boundaries. Therefore, the need
for coordinated standardization activities across Standards
Development Organizations (SDOs) is imperative.
To facilitate this, a parallel effort is underway in the ITU-T to
introduce the session identifier for H.323 in such a way as to be
interoperable with the procedures defined by the IETF.
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6.2. Requirements within 3GPP
The Third Generation Partnership Project (3GPP) identified in their
Release 9 the need for a session identifier for operation and
maintenance purposes to correlate flows in an end-to-end
communication session. 3GPP TS24.229 [7] points to the fact that the
session identifier can be used to correlate SIP messages belonging to
the same session. In the case where signaling passes through SIP
entities like B2BUAs, the end-to-end session identifier indicates
that these dialogs belong to the same end-to-end SIP communication
session.
7. Security Considerations
The security vulnerabilities, attacks, and threat models affecting
other similar SIP identifiers are well documented in RFC 3261 [1] and
are equally applicable to the end-to-end session identifier and
subject to the same mitigating security best practices. Further,
storage of the session identifier in a log file is also subject to
the security considerations specified in RFC 6872 [8].
An end-to-end identifier, if not properly constructed, could provide
confidential information that would allow one to identify the
individual, device, or domain initiating or terminating a
communication session. In adhering to REQ4, the solution produced in
accordance with these requirements MUST take appropriate measures to
properly secure and obfuscate sensitive or private information that
might allow one to identify a person, device, or domain. This means
that the end-to-end session identifier MUST NOT reveal information
elements such as the MAC address or IP address. It is outside the
scope of this document to specify the implementation details of such
security and privacy measures. Those details may vary with the
specific construction mechanism selected for the end-to-end session
identifier and therefore will be discussed in the document specifying
the actual end-to-end identifier.
A key security consideration is to ensure that an attacker cannot
surreptitiously spoof the identifier and effectively render it
useless to diagnostic equipment that cannot properly correlate
signaling messages due to the duplicate session identifiers that
exist in the same space and time. In accordance with REQ6, this
end-to-end identifier MUST be sufficiently long and random to prevent
it from being guessable as well as avoid collision with another
identifier. The secure transport of the identifier, need for
authentication, encryption, etc. should be appropriately evaluated
based on the network infrastructure, transport domain, and usage
scenarios for the end-to-end session identifier.
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8. Acknowledgments
The authors would like to acknowledge Paul Kyzivat, Christer
Holmberg, Charles Eckel, Andy Hutton, Salvatore Loreto, Keith Drage,
and Chris Pearce for their contribution and collaboration in
developing this document.
9. Contributors
Roland Jesske and Parthasarathi Ravindran provided substantial
contributions to this document during its initial creation.
10. References
10.1. Normative References
[1] 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.
[2] Recommendation ITU-T H.323, "Packet-based multimedia
communications systems", December 2009.
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
10.2. Informative References
[4] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[5] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
"Best Current Practices for Third Party Call Control (3pcc) in
the Session Initiation Protocol (SIP)", BCP 85, RFC 3725,
April 2004.
[6] Hautakorpi, J., Ed., Camarillo, G., Penfield, R., Hawrylyshen,
A., and M. Bhatia, "Requirements from Session Initiation
Protocol (SIP) Session Border Control (SBC) Deployments",
RFC 5853, April 2010.
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[7] 3GPP TS 24.229, "IP multimedia call control protocol based on
Session Initiation Protocol (SIP) and Session Description
Protocol (SDP); Stage 3".
[8] Gurbani, V., Ed., Burger, E., Ed., Anjali, T., Abdelnur, H., and
O. Festor, "The Common Log Format (CLF) for the Session
Initiation Protocol (SIP): Framework and Information Model",
RFC 6872, February 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
James Polk
Cisco Systems, Inc.
3913 Treemont Circle
Colleyville, TX
USA
Phone: +1 817 271 3552
EMail: jmpolk@cisco.com
IM: xmpp:jmpolk@cisco.com
Laura Liess
Deutsche Telekom NP
64295 Darmstadt
Heinrich-Hertz-Str. 3-7
Germany
Phone: +49 6151 268 2761
EMail: laura.liess.dt@gmail.com
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Hadriel Kaplan
Oracle
71 Third Ave.
Burlington, MA 01803
USA
EMail: hadriel.kaplan@oracle.com
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