Internet Engineering Task Force Flemming Andreasen
MMUSIC Working Group Dave Oran
INTERNET-DRAFT Dan Wing
EXPIRES: April 2005 Cisco Systems
October, 2004
Connectivity Preconditions for
Session Description Protocol Media Streams
<draft-andreasen-mmusic-connectivityprecondition-01.txt>
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Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document defines a new connectivity precondition for the
Session Description Protocol precondition framework described in RFC
3312. A connectivity precondition can be used to delay session
establishment or modification until media stream connectivity has
been verified successfully.
INTERNET-DRAFT Connectivity Preconditions October, 2004
1. Notational Conventions..........................................2
2. Introduction....................................................2
3. Connectivity Precondition Definition............................2
4. Examples........................................................3
5. Security Considerations.........................................6
6. IANA Considerations.............................................7
7. Acknowledgements................................................7
8. Authors' Addresses..............................................7
9. Normative References............................................7
10. Informative References..........................................7
Intellectual Property Statement......................................8
1. Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Introduction
RFC 3312 defines the concept of a Session Description Protocol (SDP)
[SDP] precondition, which is a condition that has to be satisfied
for a given media stream in order for session establishment or
modification to proceed. When the precondition is not met, session
progress is delayed until the precondition is satisfied, or the
session establishment fails. For example, RFC 3312 defines the
Quality of Service precondition, which is used to ensure
availability of network resources prior to establishing (i.e.
alerting) a call.
SIP sessions are typically established in order to setup one or more
media streams. Even though a media stream may be negotiated
successfully, the actual media stream itself may fail. For example,
when there is one or more Network Address Translators (NATs) or
firewalls in the media path, the media stream may not be received by
the far end. The connectivity precondition defined in this document
ensures, that session progress is delayed until media stream
connectivity has been verified, or the session itself is abandoned.
3. Connectivity Precondition Definition
The connectivity precondition type is defined by the string "con"
and hence we modify the grammar found in RFC 3312 as follows:
precondition-type = "con" | "qos" | token
RFC 3312 defines support for two kinds of status types, namely
segmented and end-to-end. The connectivity precondition-type
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defined here MUST be used with the end-to-end status type; use of
the segmented status type is undefined.
An entity that wishes to delay session establishment or modification
until media stream connectivity has been established uses this
precondition-type in an offer. When a connectivity precondition is
received in an offer, session establishment or modification MUST be
delayed until the connectivity precondition has been met, i.e. media
stream connectivity has been established in the desired
direction(s).
The direction attributes are interpreted as follows:
* send: The offerer/answerer is sending media stream packets to the
other party, and the offerer/answer knows the other party has
received at least one of those media stream packets, i.e., there
is connectivity in the forward (sending) direction.
* recv: The offerer/answerer knows that the other party has
ascertained media stream connectivity to it, i.e., there is
connectivity in the backwards (receiving) direction, and it is
know that the other side has determined this.
Note that a "send" connectivity precondition from the offerer's
point of view corresponds to a "recv" connectivity precondition from
the answerer's point of view, and vice versa. If media stream
connectivity in both directions is required before session
establishment continues, the desired status must be set to
"sendrecv".
Media stream connectivity can be ascertained in different ways and
this document does not mandate any particular mechanism for doing
so. It is however RECOMMENDED that the No-Op RTP payload format
defined in [no-op] is supported by entities that support
connectivity preconditions. This will ensure that all entities that
support the connectivity preconditions have at least one common way
of ascertaining connectivity.
4. Examples
The call flow of Figure 1 shows a basic session establishment with
the Session Initiation Protocol using SDP connectivity preconditions
and RTP no-op. Note that not all SDP details are provided in the
following.
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A B
| |
|-------------(1) INVITE SDP1--------------->|
| |
|<------(2) 183 Session Progress SDP2--------|
| |
|<~~~~~ Connectivity check to A ~~~~~~~~~~~~~|
| |
|----------------(3) PRACK------------------>|
| |
|~~~~~ Connectivity to A OK ~~~~~~~~~~~~~~~~>|
| |
|<-----------(4) 200 OK (PRACK)--------------|
| |
|~~~~~ Connectivity check to B ~~~~~~~~~~~~~>|
|<~~~~ Connectivity to B OK ~~~~~~~~~~~~~~~~~|
| |
|-------------(5) UPDATE SDP3--------------->|
| |
|<--------(6) 200 OK (UPDATE) SDP4-----------|
| |
|<-------------(7) 180 Ringing---------------|
| |
| |
| |
Figure 1: Example using the connectivity precondition
SDP1: A includes the end-to-end connectivity precondition with a
desired status of "sendrecv"; this will ensure media stream
connectivity in both directions before continuing with the session
setup. Since media stream connectivity in either direction is
unknown at this point, the current status is set to "none". A's
local status table (see RFC 3312) for the connectivity precondition
is as follows:
Direction | Current | Desired Strength | Confirm
-----------+----------+------------------+----------
send | no | mandatory | no
recv | no | mandatory | no
and the resulting offer SDP is:
m=audio 20000 RTP/AVP 0 96
c=IN IP4 192.0.2.1
a=rtpmap:96 no-op/8000
a=curr:con e2e none
a=des:con mandatory e2e sendrecv
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SDP2: When B receives the offer, B sees the bidirectional
connectivity preconditions. B can ascertain connectivity to A
("send" from B's point of view) by use of the RTP no-op, however B
needs A to inform it about connectivity in the other direction
("recv" from B's point of view). B's local status table therefore
looks as follows:
Direction | Current | Desired Strength | Confirm
-----------+----------+------------------+----------
send | no | mandatory | no
recv | no | mandatory | no
Since B needs to ask A for confirmation about the "recv" (from B's
point of view) connectivity precondition, the resulting answer SDP
becomes:
m=audio 30000 RTP/AVP 0 96
a=rtpmap:96 no-op/8000
c=IN IP4 192.0.2.4
a=curr:con e2e none
a=des:con mandatory e2e sendrecv
a=conf:con e2e recv
Meanwhile, B performs a connectivity check to A, which succeeds and
hence B's local status table is updated as follows:
Direction | Current | Desired Strength | Confirm
-----------+----------+------------------+----------
send | yes | mandatory | no
recv | no | mandatory | no
Since the "send" connectivity precondition is still not satisfied,
session establishment remains suspended.
SDP3: When A receives the answer SDP, A notes that confirmation was
requested for B's "recv" connectivity precondition, which is the
"send" precondition from A's point of view. A performs a
connectivity check to B, which succeeds, and A's local status table
becomes:
Direction | Current | Desired Strength | Confirm
-----------+----------+------------------+----------
send | yes | mandatory | yes
recv | no | mandatory | no
Since B had asked for confirmation about the "send" connectivity
(from A's point of view), A now sends an UPDATE (5) to B to confirm
the connectivity from A to B:
m=audio 20000 RTP/AVP 0 96
a=rtpmap:96 no-op/8000
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INTERNET-DRAFT Connectivity Preconditions October, 2004
c=IN IP4 192.0.2.1
a=curr:con e2e send
a=des:con mandatory e2e sendrecv
SDP4: Upon receiving the updated offer, B now knows that there is
connectivity from A to B and updates the local status table as
follows ("send" from A corresponds to "recv" from B's point of
view):
Direction | Current | Desired Strength | Confirm
-----------+----------+------------------+----------
send | yes | mandatory | no
recv | yes | mandatory | no
B responds with an answer (6) which contains the current status of
the connectivity precondition (i.e., sendrecv) from B's point of
view:
m=audio 30000 RTP/AVP 0 96
a=rtpmap:96 no-op/8000
c=IN IP4 192.0.2.4
a=curr:con e2e sendrecv
a=des:con mandatory e2e sendrecv
At this point in time, session establishment resumes and B returns a
180 (Ringing) response (7).
5. Security Considerations
In addition to the general security for preconditions provided in
RFC 3312, the following security issues, which are specific to
connectivity preconditions, should be considered.
Connectivity preconditions rely on mechanisms beyond SDP, e.g. RTP
No-Op [no-op] or STUN [stun], to establish and verify connectivity
between an offerer and an answerer. An attacker that prevents those
mechanism from succeeding can prevent media sessions from being
established and hence it is RECOMMENDED that such mechanisms are
adequately secured by message authentication and integrity
protection. Also, the mechanisms SHOULD consider how to prevent
denial of service attacks. Similarly, an attacker that can forge
packets for these mechanisms can enable sessions to be established
when there in fact is no media connectivity, which may lead to a
poor user experience. Authentication and integrity protection of
such mechanisms can prevent this type of attacks and hence use of it
is RECOMMENDED.
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6. IANA Considerations
IANA is hereby requested to register a RFC 3312 precondition type
called "con" with the name "Connectivity precondition". The
reference for this precondition type is the current document.
7. Acknowledgements
The concept of a "connectivity precondition" is the result of
discussions with numerous people over a long period of time; the
authors greatly appreciate these contributions.
8. Authors' Addresses
Flemming Andreasen
Cisco Systems, Inc.
499 Thornall Street, 8th Floor
Edison, New Jersey 08837 USA
EMail: fandreas@cisco.com
David Oran
Cisco Systems, Inc.
7 Ladyslipper Lane
Acton, MA 01720 USA
EMail: oran@cisco.com
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134 USA
EMail: dwing@cisco.com
9. Normative References
[RFC3312] G. Camarillo, W. Marshall, J. Rosenberg, "Integration of
Resource Management and Session Initiation Protocol (SIP)", RFC
3312, October 2002.
[RFC2327] M. Handley and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
10. Informative References
[RFC3551] H. Schulzrinne, and S. Casner "RTP Profile for Audio and
Video Conferences with Minimal Control", RFC 3550, July 2003.
[no-op] F. Andreasen, D. Oran, and D. Wing, "RTP No-Op Payload
Format", Work in Progress
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[stun] J. Rosenberg, J. Weinberger, C. Huitema, R. Mahy, "STUN -
Simple Traversal of User Datagram Protocol (UDP) Through Network
Address Translators (NATs)", RFC 3489, March 2003.
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