draft-ietf-mmusic-securityprecondition-01.txt   draft-ietf-mmusic-securityprecondition-02.txt 
Internet Engineering Task Force Flemming Andreasen Internet Engineering Task Force Flemming Andreasen
MMUSIC Working Group Dan Wing MMUSIC Working Group Dan Wing
Internet-Draft Internet-Draft
Expires: April 2006 Cisco Systems Expires: December 2006 Cisco Systems
October, 2005 June, 2006
Security Preconditions for Security Preconditions for
Session Description Protocol Media Streams Session Description Protocol (SDP) Media Streams
<draft-ietf-mmusic-securityprecondition-01.txt> <draft-ietf-mmusic-securityprecondition-02.txt>
Status of this memo Status of this memo
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved. Copyright (C) The Internet Society (2006). All Rights Reserved.
Abstract Abstract
This document defines a new security precondition for the Session This document defines a new security precondition for the Session
Description Protocol precondition framework described in RFCs 3312 Description Protocol (SDP) precondition framework described in RFCs
and 4032. A security precondition can be used to delay session 3312 and 4032. A security precondition can be used to delay session
establishment or modification until media stream security has been establishment or modification until media stream security for a
negotiated successfully. secure media stream has been negotiated successfully.
1 Notational Conventions............................................2 1 Notational Conventions............................................2
2 Introduction......................................................2 2 Introduction......................................................2
3 Security Precondition Definition..................................3 3 Security Precondition Definition..................................3
4 Examples..........................................................5 4 Examples..........................................................6
4.1 SDP Security Descriptions Example.............................5 4.1 SDP Security Descriptions Example.............................6
4.2 Key Management Extension for SDP Example......................8 4.2 Key Management Extension for SDP Example......................8
5 Security Considerations..........................................10 5 Security Considerations..........................................11
6 IANA Considerations..............................................11 6 IANA Considerations..............................................12
7 Acknowledgements.................................................11 7 Acknowledgements.................................................12
8 Authors' Addresses...............................................11 8 Authors' Addresses...............................................13
9 Normative References.............................................12 9 Normative References.............................................13
10 Informative References.........................................12 10 Informative References.........................................13
11 Intellectual Property Statement................................14 11 Intellectual Property Statement................................15
1 Notational Conventions 1 Notational Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT", The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
2 Introduction 2 Introduction
The concept of a Session Description Protocol (SDP) [SDP] The concept of a Session Description Protocol (SDP) [SDP]
precondition is defined in [RFC3312] as updated by [RFC4032]. A precondition is defined in [RFC3312] as updated by [RFC4032]. A
precondition is a condition that has to be satisfied for a given precondition is a condition that has to be satisfied for a given
media stream in order for session establishment or modification to media stream in order for session establishment or modification to
proceed. When the precondition is not met, session progress is proceed. When a (mandatory) precondition is not met, session
delayed until the precondition is satisfied or the session progress is delayed until the precondition is satisfied or the
establishment fails. For example, RFC 3312 defines the Quality of session establishment fails. For example, RFC 3312 defines the
Service precondition, which is used to ensure availability of Quality of Service precondition, which is used to ensure
network resources prior to establishing (i.e. alerting) a call. availability of network resources prior to establishing (i.e.
alerting) a call.
Media streams can either be provided in cleartext and with no Media streams can either be provided in cleartext and with no
integrity protection, or some kind of media security can be applied, integrity protection, or some kind of media security can be applied,
e.g., confidentiality and/or message integrity. For example, the e.g., confidentiality and/or message integrity. For example, the
Audio/Video profile of the Real-Time Transfer protocol (RTP) Audio/Video profile of the Real-Time Transfer protocol (RTP)
[RFC3551] is normally used without any security services whereas the [RFC3551] is normally used without any security services whereas the
Secure Real-time Transport Protocol (SRTP) [SRTP] is always used Secure Real-time Transport Protocol (SRTP) [SRTP] is always used
with security services. When media stream security is being with security services. When media stream security is being
negotiated, e.g., using the mechanism defined in SDP Security negotiated, e.g., using the mechanism defined in SDP Security
Descriptions [SDESC], both the offerer and the answerer need to know Descriptions [SDESC], both the offerer and the answerer need to know
the cryptographic parameters being used for the media stream; the the cryptographic parameters being used for the media stream; the
offerer may provide multiple choices for the cryptographic offerer may provide multiple choices for the cryptographic
parameters, or the cryptographic parameters selected by the answerer parameters, or the cryptographic parameters selected by the answerer
may differ from those of the offerer (e.g. the key used in one may differ from those of the offerer (e.g. the key used in one
direction versus the other). In such cases, to avoid media direction versus the other). In such cases, to avoid media
clipping, the offerer must receive the answer prior to receiving any clipping, the offerer needs to receive the answer prior to receiving
media packets from the answerer. This can be achieved by using a any media packets from the answerer. This can be achieved by using
security precondition, which ensures the successful negotiation of a security precondition, which ensures the successful negotiation of
media stream security parameters prior to session establishment or media stream security parameters for a secure media stream prior to
modification. session establishment or modification.
3 Security Precondition Definition 3 Security Precondition Definition
The semantics for a security precondition are that the relevant
cryptographic parameters (cipher, key, etc.) for a secure media
stream are known to have been negotiated in the direction(s)
required. If the security precondition is used with a non-secure
media stream, the security precondition is by definition satisfied.
A secure media stream is here defined as a media stream that uses
some kind of security service, e.g. message integrity,
confidentiality or both, regardless of the cryptographic strength of
the mechanisms being used.
As an extreme example of this, Secure RTP (SRTP) using the NULL
encryption algorithm and no message integrity would be considered
a secure media stream whereas use of plain RTP would not. Note
though, that use of SRTP without authentication is discouraged.
Security preconditions do not guarantee that an established media
stream will be secure. They merely guarantee that the recipient of
the media stream packets will be able to perform any relevant
decryption and integrity checking on those media stream packets.
Please refer to Section 5 for further security considerations.
The security precondition type is defined by the string "sec" and The security precondition type is defined by the string "sec" and
hence we modify the grammar found in RFC 3312 as follows: hence we modify the grammar found in RFC 3312 as follows:
precondition-type = "sec" | "qos" | token precondition-type = "sec" | "qos" | token
RFC 3312 defines support for two kinds of status types, namely RFC 3312 defines support for two kinds of status types, namely
segmented and end-to-end. The security precondition-type defined segmented and end-to-end. The security precondition-type defined
here MUST be used with the end-to-end status type; use of the here MUST be used with the end-to-end status type; use of the
segmented status type is undefined. segmented status type is undefined.
An entity that wishes to delay session establishment or modification A security preconditions can use the strength-tag "mandatory",
until media stream security has been established uses the security "optional" or "none".
precondition-type in an offer. When a mandatory security
precondition is received in an offer, session establishment or
modification MUST be delayed until the security precondition has
been met, i.e. cryptographic parameters (cipher, key, etc.) for a
secure media stream are known to have been negotiated in the
direction(s) required. A secure media stream is here defined as a
media stream that uses some kind of security service, e.g. message
integrity, confidentiality or both, regardless of the cryptographic
strength of the mechanisms being used.
As an extreme example of this, Secure RTP (SRTP) using the NULL When a security precondition with a strength-tag of "mandatory" is
encryption algorithm and no message integrity would satisfy the received in an offer, session establishment or modification MUST be
above whereas use of plain RTP would not. Note though, that use delayed until the security precondition has been met, i.e. the
of SRTP without authentication is discouraged. relevant cryptographic parameters (cipher, key, etc.) for a secure
media stream are known to have been negotiated in the direction(s)
required. When a mandatory security precondition is offered, and
the answerer cannot satisfy the security precondition, e.g. because
the offer was for a secure media stream, but it did not include the
necessary parameters to establish the secure media stream (keying
material for example), the offered media stream MUST be rejected as
described in RFC 3312.
The delay of session establishment defined here implies that The delay of session establishment defined here implies that
alerting of the called party MUST NOT occur and media for which alerting of the called party MUST NOT occur and media for which
security is being negotiated MUST NOT be exchanged until the security is being negotiated MUST NOT be exchanged until the
precondition has been satisfied. In cases where secure media and precondition has been satisfied. In cases where secure media and
other non-secure data is multiplexed on a media stream, e.g. when other non-secure data is multiplexed on a media stream, e.g. when
Interactive Connectivity Establishment [ICE] is being used, the non- Interactive Connectivity Establishment [ICE] is being used, the non-
secure data is allowed to be exchanged prior to the security secure data is allowed to be exchanged prior to the security
precondition being satisfied. precondition being satisfied.
When a security precondition with a strength-tag of "optional" is
received in an offer, the answerer MUST generate its answer SDP as
soon as possible. Since session progress is not delayed in this
case, the answerer does not know when the offerer is able to process
secure media stream packets and hence clipping may occur. If the
answerer wants to avoid clipping and delay session progress until he
knows the offerer has received the answer, the answerer MUST
increase the strength of the security precondition by using a
strength-tag of "mandatory" in the answer. Note that use of a
mandatory precondition requires the presence of a SIP "Require"
header field containing the option tag "precondition": Any SIP UA
that does not support a mandatory precondition will consequently
reject such requests (which also has unintended ramifications for
SIP forking that are known as the Heterogeneous Error Response
Forking Problem (see e.g. [HERFP]). To get around this, an optional
security precondition and the SIP "Supported" header field
containing the option tag "precondition" can be used instead.
When a security precondition with a strength-tag of "none" is
received, processing continues us usual. The "none" strength-tag
merely indicates that the offerer supports the security precondition
- the answerer MAY upgrade the strength-tag in the answer as
described in [RFC3312].
The direction tags defined in RFC 3312 are interpreted as follows: The direction tags defined in RFC 3312 are interpreted as follows:
* send: Media stream security negotiation is at a stage where it is * send: Media stream security negotiation is at a stage where it is
possible to send secure media packets to the other party and the possible to send media packets to the other party and the other
other party will be able to process them correctly. The party will be able to process them correctly from a security point
definition of "media packets" includes all packets that make up of view, i.e. decrypt and/or integrity check them as necessary.
the media stream. In the case of Secure RTP for example, it The definition of "media packets" includes all packets that make
up the media stream. In the case of Secure RTP for example, it
includes SRTP as well as SRTCP. When media and non-media packets includes SRTP as well as SRTCP. When media and non-media packets
are multiplexed on a given media stream, e.g. when ICE is being are multiplexed on a given media stream, e.g. when ICE is being
used, the requirement applies to the media packets only. used, the requirement applies to the media packets only.
* recv: Media stream security negotiation is at a stage where it is * recv: Media stream security negotiation is at a stage where it is
possible to receive and correctly process secure media stream possible to receive and correctly process media stream packets
packets sent by the other party. sent by the other party from a security point of view.
The precise criteria for determining when the other party is able to The precise criteria for determining when the other party is able to
correctly process secure media stream packets depends on the secure correctly process media stream packets from a security point of view
media stream protocol being used as well as the mechanism by which depend on the secure media stream protocol being used as well as the
the required cryptographic parameters are negotiated. mechanism by which the required cryptographic parameters are
negotiated.
We here provide details for SRTP negotiated through SDP security We here provide details for SRTP negotiated through SDP security
descriptions as defined in [SDESC]: descriptions as defined in [SDESC]:
* When the offerer requests the "send" security precondition, it * When the offerer requests the "send" security precondition, it
needs to receive the answer before the security precondition is needs to receive the answer before the security precondition is
satisfied. The reason for this is twofold. First, the offerer satisfied. The reason for this is twofold. First, the offerer
needs to know where to send the media to. Secondly, in the case needs to know where to send the media to. Secondly, in the case
where alternative cryptographic parameters are offered, the where alternative cryptographic parameters are offered, the
offerer needs to know which set was selected. The answerer does offerer needs to know which set was selected. The answerer does
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answerer needs to use the confirm-status attribute [RFC3312]. answerer needs to use the confirm-status attribute [RFC3312].
This will make the offerer generate a new offer showing the This will make the offerer generate a new offer showing the
updated status of the precondition. updated status of the precondition.
* When the offerer requests the "recv" security precondition, it * When the offerer requests the "recv" security precondition, it
also needs to receive the answer before the security precondition also needs to receive the answer before the security precondition
is satisfied. The reason for this is straightforward: The answer is satisfied. The reason for this is straightforward: The answer
contains the cryptographic parameters that will be used by the contains the cryptographic parameters that will be used by the
answerer for sending media to the offerer; prior to receipt of answerer for sending media to the offerer; prior to receipt of
these cryptographic parameters the offerer is unable to these cryptographic parameters the offerer is unable to
authenticate or decrypt media. authenticate or decrypt such media.
When security preconditions are used with the Key Management When security preconditions are used with the Key Management
Extensions for Session Description Protocol (SDP) [KMGMT], the Extensions for Session Description Protocol (SDP) [KMGMT], the
details depend on the actual key management protocol being used. details depend on the actual key management protocol being used.
After an initial offer/answer sequence in which the security After an initial offer/answer exchange in which the security
precondition is requested, any subsequent offer/answer sequence for precondition is requested, any subsequent offer/answer sequence for
the purpose of updating the status of the precondition SHOULD use the purpose of updating the status of the precondition for a secure
the same key material as the initial offer/answer sequence. This media stream SHOULD use the same key material as the initial
means that the key-mgmt attribute lines [KMGMT] or crypto attribute offer/answer exchange. This means that the key-mgmt attribute lines
lines [SDESC] in SDP offers that are sent in response to SDP answers [KMGMT] or crypto attribute lines [SDESC] in SDP offers, that are
containing a confirm-status field [RFC3312] SHOULD repeat the same sent in response to SDP answers containing a confirm-status field
data as that sent in the previous SDP offer. If applicable to the [RFC3312], SHOULD repeat the same data as that sent in the previous
key management protocol or SDP security description, the SDP answers SDP offer. If applicable to the key management protocol or SDP
to these SDP offers SHOULD repeat the same data in the key-mgmt security description, the SDP answers to these SDP offers SHOULD
attribute lines [KMGMT] or crypto attribute lines [SDESC] as that repeat the same data in the key-mgmt attribute lines [KMGMT] or
sent in the previous SDP answer. crypto attribute lines [SDESC] as that sent in the previous SDP
answer.
Of course, this duplication of key exchange during precondition Of course, this duplication of key exchange during precondition
establishment is not to be interpreted as a replay attack. This establishment is not to be interpreted as a replay attack. This
issue may be solved if, e.g. the SDP implementation recognizes that issue may be solved if, e.g., the SDP implementation recognizes that
the key management protocol data is identical in the second the key management protocol data is identical in the second
offer/answer exchange and avoids forwarding the information to the offer/answer exchange and avoids forwarding the information to the
security layer for further processing. security layer for further processing.
Security preconditions may have a strength-tag of either "mandatory"
or "optional". When a mandatory security precondition is offered,
and the answerer cannot satisfy the security precondition, e.g.
because the offer does not include any parameters related to
establishing a secure media stream, the offer MUST be rejected as
described in RFC 3312. When an optional security precondition is
offered, the answerer MUST generate its answer SDP as soon as
possible; since session progress is not delayed in this case,
clipping may occur. If the answerer wants to avoid clipping and
delay session progress until the offerer has received the answer,
the answerer MUST increase the strength of the security precondition
by using a strength-tag of "mandatory" in the answer.
Note that use of a "mandatory" precondition requires the presence
of a SIP "Require" header with the option tag "precondition": Any
SIP UA that does not support a mandatory precondition will
consequently reject such requests. To get around this issue, an
optional security precondition and the SIP "Supported" header with
the option tag "precondition" can be used instead.
Offers with security preconditions in re-INVITEs or UPDATEs follow Offers with security preconditions in re-INVITEs or UPDATEs follow
the rules given in Section 6 of RFC 3312, i.e.: the rules given in Section 6 of RFC 3312, i.e.:
"Both user agents SHOULD continue using the old session parameters "Both user agents SHOULD continue using the old session parameters
until all the mandatory preconditions are met. At that moment, until all the mandatory preconditions are met. At that moment,
the user agents can begin using the new session parameters." the user agents can begin using the new session parameters."
4 Examples 4 Examples
4.1 SDP Security Descriptions Example 4.1 SDP Security Descriptions Example
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a=curr:sec e2e sendrecv a=curr:sec e2e sendrecv
a=des:sec mandatory e2e sendrecv a=des:sec mandatory e2e sendrecv
a=key-mgmt:mikey AQAFgM0X... a=key-mgmt:mikey AQAFgM0X...
B's local status table indicates that all mandatory preconditions B's local status table indicates that all mandatory preconditions
have been satisfied, and hence session establishment resumes; B have been satisfied, and hence session establishment resumes; B
returns a 180 (Ringing) response (5) to indicate alerting. returns a 180 (Ringing) response (5) to indicate alerting.
5 Security Considerations 5 Security Considerations
In addition to the general security for preconditions provided in In addition to the general security considerations for preconditions
RFC 3312, the following security issues, which are specific to provided in RFC 3312, the following security issues should be
security preconditions, should be considered. considered.
Security preconditions delay session establishment until Security preconditions delay session establishment until
cryptographic parameters required to send and/or receive media have cryptographic parameters required to send and/or receive media for a
been negotiated. Negotiation of such parameters can fail for a media stream have been negotiated. Negotiation of such parameters
variety of reasons, including policy preventing use of certain can fail for a variety of reasons, including policy preventing use
cryptographic algorithms, keys, and other security parameters. If of certain cryptographic algorithms, keys, and other security
intermediaries can remove security preconditions or downgrade the parameters. If an attacker can remove security preconditions or
strength from an offer/answer exchange, they can thereby cause user downgrade the strength-tag from an offer/answer exchange, the
alerting for a session that may have no functioning media, which is attacker can thereby cause user alerting for a session that may have
likely to cause inconvenience to the called party. Similarly, no functioning media. This is likely to cause inconvenience to both
security preconditions can be used to prevent clipping due to race the offerer and the answerer. Similarly, security preconditions can
conditions between an offer/answer exchange and secure media stream be used to prevent clipping due to race conditions between an
packets based on that offer/answer exchange. If intermediaries can offer/answer exchange and secure media stream packets based on that
remove or downgrade the strength of security preconditions from an offer/answer exchange. If an attacker can remove or downgrade the
offer/answer exchange, they can cause clipping to occur in the strength-tag of security preconditions from an offer/answer
associated secure media stream. exchange, the attacker can cause clipping to occur in the associated
secure media stream.
Conversely, intermediaries may also add security preconditions to Conversely, an attacker might add security preconditions to offers
offers that do not contain them or increase their strength. This in that do not contain them or increase their strength-tag. This in
turn may lead to session failure or delayed session establishment turn may lead to session failure (e.g. if the answerer does not
that was not desired. support it), heterogeneous error response forking problems, or a
delay in session establishment that was not desired.
Use of integrity mechanisms can prevent all of the above problems. Use of signaling integrity mechanisms can prevent all of the above
Where intermediaries on the signaling path are trusted, it is problems. Where intermediaries on the signaling path (e.g. SIP
sufficient to only use hop-by-hop integrity protection, e.g. IPSec proxies) are trusted, it is sufficient to use only hop-by-hop
or TLS. In all other cases, end-to-end integrity protection, e.g. integrity protection of signaling, e.g., IPSec or TLS. In all other
S/MIME, MUST be used. cases, end-to-end integrity protection of signaling, e.g. S/MIME,
MUST be used. Note that the end-to-end integrity protection MUST
cover not only the message body, which contains the security
preconditions, but also the SIP "Supported" and "Require" headers,
which may contain the "precondition" option tag. If only the
message body were integrity protected, removal of the "precondition"
option tag could lead to clipping (when a security precondition was
otherwise to be used), whereas addition of the option tag could lead
to session failure (if the other side does not support
preconditions).
As specified in Section 3, security preconditions do not guarantee
that an established media stream will be secure. They merely
guarantee that the recipient of the media stream packets will be
able to perform any relevant decryption and integrity checking on
those media stream packets. If an offer includes a secure and a
non-secure media stream as alternatives, this may lead to additional
security issues. It is important to understand how security
preconditions interact with those.
SDP and the offer/answer model currently do not define how such
alternatives could be negotiated, however there is work in
progress to address that (see e.g. [SDPCN]). Below, we provide
general security considerations for security preconditions with
such mechanisms.
If the offer were to include secure and non-secure media streams as
alternative offers, and media for either alternative may be received
prior to the answer, then the offerer may not know if the answerer
accepted the secure alternative. An active attacker thus may be
able to inject malicious media stream packets until the answer is
received. Use of security preconditions would not address this
vulnerability since security preconditions do not guarantee that a
media stream established is secure, even if the strength-tag is
"mandatory".
In the above scenario with secure and non-secure media streams as
alternatives, the offerer may also be concerned about a passive
attacker performing eavesdropping on the media stream. Security
preconditions can help here by ensuring that clipping will not occur
if the answerer supports the secure media stream and furthermore
accepts it. Still, they would not guarantee that the media stream
established is secure and hence by themselves would not protect
against eavesdropping.
6 IANA Considerations 6 IANA Considerations
IANA is hereby requested to register a RFC 3312 precondition type IANA is hereby requested to register a RFC 3312 precondition type
called "sec" with the name "Security precondition". The reference called "sec" with the name "Security precondition". The reference
for this precondition type is the current document. for this precondition type is the current document.
7 Acknowledgements 7 Acknowledgements
The security precondition was defined in earlier draft versions of The security precondition was defined in earlier draft versions of
skipping to change at page 12, line 38 skipping to change at page 13, line 47
Initiation Protocol", RFC 3261, June 2002. Initiation Protocol", RFC 3261, June 2002.
10 Informative References 10 Informative References
[SDESC] F. Andreasen, M. Baugher, and D. Wing, "SDP Security [SDESC] F. Andreasen, M. Baugher, and D. Wing, "SDP Security
Descriptions for Media Streams", work in progress Descriptions for Media Streams", work in progress
[RFC3551] H. Schulzrinne, and S. Casner "RTP Profile for Audio and [RFC3551] H. Schulzrinne, and S. Casner "RTP Profile for Audio and
Video Conferences with Minimal Control", RFC 3550, July 2003. Video Conferences with Minimal Control", RFC 3550, July 2003.
[SRTP] M. Baugher, D. McGrew, M. Naslund, E. Carrara, K. Norrman, [SRTP] M. Baugher, D. McGrew, M. Naslund, E. Carrara, K.
"The Secure Real-time Transport Protocol", RFC 3711, March 2004. Norrman, "The Secure Real-time Transport Protocol", RFC 3711, March
2004.
[ICE] J. Rosenberg, "Interactive Connectivity Establishment (ICE): A [ICE] J. Rosenberg, "Interactive Connectivity Establishment
Methodology for Network Address Translator (NAT) Traversal for (ICE): A Methodology for Network Address Translator (NAT) Traversal
Multimedia Session Establishment Protocols", IETF, work-in-progress. for Multimedia Session Establishment Protocols", IETF, work-in-
progress.
[KMGMT] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K. [KMGMT] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K.
Norrman, "Key Management Extensions for Session Description Protocol Norrman, "Key Management Extensions for Session Description Protocol
(SDP) and Real Time Streaming Protocol (RTSP)", IETF, work-in- (SDP) and Real Time Streaming Protocol (RTSP)", IETF, work-in-
progress. progress.
[MIKEY] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K. [MIKEY] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830, August 2004. Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830, August 2004.
[RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of [RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol (SIP)", RFC Provisional Responses in Session Initiation Protocol (SIP)", RFC
3262, June 2002. 3262, June 2002.
[RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP) [RFC3311] J. Rosenberg, "The Session Initiation Protocol (SIP)
UPDATE Method," RFC 3311, September 2002. UPDATE Method," RFC 3311, September 2002.
[HERFP] R. Mahy, "A Solution to the Heterogeneous Error Response
Forking Problem (HERFP) in the Session Initiation Problem (SIP)",
Work in Progress, March 2006.
[SDPCN] F. Andreasen, "SDP Capability Negotiation", Work in
Progress, June 2006.
11 Intellectual Property Statement 11 Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed Intellectual Property Rights or other rights that might be claimed
to pertain to the implementation or use of the technology described to pertain to the implementation or use of the technology described
in this document or the extent to which any license under such in this document or the extent to which any license under such
rights might or might not be available; nor does it represent that rights might or might not be available; nor does it represent that
it has made any independent effort to identify any such rights. it has made any independent effort to identify any such rights.
Information on the IETF's procedures with respect to rights in IETF Information on the IETF's procedures with respect to rights in IETF
Documents can be found in BCP 78 and BCP 79. Documents can be found in BCP 78 and BCP 79.
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This document and the information contained herein are provided on This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject Copyright (C) The Internet Society (2006). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights. except as set forth therein, the authors retain all their rights.
Acknowledgment Acknowledgment
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is currently provided by the
Internet Society. Internet Society.
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