Network Working Group F. Andreasen
Request for Comments: 5027 D. Wing
Updates: 3312 Cisco Systems
Category: Standards Track October 2007
Security Preconditions for
Session Description Protocol (SDP) Media Streams
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document defines a new security precondition for the Session
Description Protocol (SDP) precondition framework described in RFCs
3312 and 4032. A security precondition can be used to delay session
establishment or modification until media stream security for a
secure media stream has been negotiated successfully.
Table of Contents
1. Introduction ....................................................2
2. Notational Conventions ..........................................2
3. Security Precondition Definition ................................2
4. Examples ........................................................6
4.1. SDP Security Descriptions Example ..........................6
4.2. Key Management Extension for SDP Example ...................9
5. Security Considerations ........................................11
6. IANA Considerations ............................................13
7. Acknowledgements ...............................................13
8. Normative References ...........................................13
9. Informative References .........................................14
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1. Introduction
The concept of a Session Description Protocol (SDP) [RFC4566]
precondition is defined in [RFC3312] as updated by [RFC4032]. A
precondition is a condition that has to be satisfied for a given
media stream in order for session establishment or modification to
proceed. When a (mandatory) 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.
Media streams can either be provided in cleartext and with no
integrity protection, or some kind of media security can be applied,
e.g., confidentiality and/or message integrity. For example, the
Audio/Video profile of the Real-Time Transfer Protocol (RTP)
[RFC3551] is normally used without any security services whereas the
Secure Real-time Transport Protocol (SRTP) [SRTP] is always used with
security services. When media stream security is being negotiated,
e.g., using the mechanism defined in SDP Security Descriptions
[SDESC], both the offerer and the answerer [RFC3264] need to know the
cryptographic parameters being used for the media stream; the offerer
may provide multiple choices for the cryptographic parameters, or the
cryptographic parameters selected by the answerer may differ from
those of the offerer (e.g., the key used in one direction versus the
other). In such cases, to avoid media clipping, the offerer needs to
receive the answer prior to receiving any media packets from the
answerer. This can be achieved by using a security precondition,
which ensures the successful negotiation of media stream security
parameters for a secure media stream prior to session establishment
or modification.
2. Notational Conventions
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].
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,
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RFC 5027 Security Preconditions October 2007
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 Section 9.5 of [SRTP] discourages the use of SRTP
without message integrity.
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
hence we modify the grammar found in RFC 3312 as follows:
precondition-type = "sec" / "qos" / token
RFC 3312 defines support for two kinds of status types, namely
segmented and end-to-end. The security precondition-type defined
here MUST be used with the end-to-end status type; use of the
segmented status type is undefined.
A security precondition can use the strength-tag "mandatory",
"optional", or "none".
When a security precondition with a strength-tag of "mandatory" is
received in an offer, session establishment or modification MUST be
delayed until the security precondition has been met, i.e., the
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 alerting
of the called party MUST NOT occur and media for which security is
being negotiated MUST NOT be exchanged until the precondition has
been satisfied. In cases where secure media and other non-media data
is multiplexed on a media stream (e.g., when Interactive Connectivity
Establishment [ICE] is being used), the non-media data is allowed to
be exchanged prior to the security precondition being satisfied.
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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
in an offer 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 as usual. The "none" strength-tag
merely indicates that the offerer supports the following 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:
* send: Media stream security negotiation is at a stage where it is
possible to send media packets to the other party and the other
party will be able to process them correctly from a security point
of view, i.e., decrypt and/or integrity check them as necessary.
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
are multiplexed on a given media stream (e.g., when ICE is being
used), the requirement applies to the media packets only.
* recv: Media stream security negotiation is at a stage where it is
possible to receive and correctly process media stream packets
sent by the other party from a security point of view.
The precise criteria for determining when the other party is able to
correctly process media stream packets from a security point of view
depend on the secure media stream protocol being used as well as the
mechanism by which the required cryptographic parameters are
negotiated.
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We here provide details for SRTP negotiated through SDP security
descriptions as defined in [SDESC]:
* When the offerer requests the "send" security precondition, it
needs to receive the answer before the security precondition is
satisfied. The reason for this is twofold. First, the offerer
needs to know where to send the media. Secondly, in the case
where alternative cryptographic parameters are offered, the
offerer needs to know which set was selected. The answerer does
not know when the answer is actually received by the offerer
(which in turn will satisfy the precondition), and hence the
answerer needs to use the confirm-status attribute [RFC3312].
This will make the offerer generate a new offer showing the
updated status of the precondition.
* When the offerer requests the "recv" security precondition, it
also needs to receive the answer before the security precondition
is satisfied. The reason for this is straightforward: The answer
contains the cryptographic parameters that will be used by the
answerer for sending media to the offerer; prior to receipt of
these cryptographic parameters, the offerer is unable to
authenticate or decrypt such media.
When security preconditions are used with the Key Management
Extensions for the Session Description Protocol (SDP) [KMGMT], the
details depend on the actual key management protocol being used.
After an initial offer/answer exchange in which the security
precondition is requested, any subsequent offer/answer sequence for
the purpose of updating the status of the precondition for a secure
media stream SHOULD use the same key material as the initial
offer/answer exchange. This means that the key-mgmt attribute lines
[KMGMT], or crypto attribute lines [SDESC] in SDP offers, that are
sent in response to SDP answers containing a confirm-status field
[RFC3312] SHOULD repeat the same data as that sent in the previous
SDP offer. If applicable to the key management protocol or SDP
security description, the SDP answers to these SDP offers SHOULD
repeat the same data in the key-mgmt attribute lines [KMGMT] or
crypto attribute lines [SDESC] as that sent in the previous SDP
answer.
Of course, this duplication of key exchange during precondition
establishment is not to be interpreted as a replay attack. This
issue may be solved if, e.g., the SDP implementation recognizes that
the key management protocol data is identical in the second
offer/answer exchange and avoids forwarding the information to the
security layer for further processing.
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Offers with security preconditions in re-INVITEs or UPDATEs follow
the rules given in Section 6 of RFC 3312, i.e.:
"Both user agents SHOULD continue using the old session parameters
until all the mandatory preconditions are met. At that moment,
the user agents can begin using the new session parameters."
At that moment, we furthermore require that user agents MUST start
using the new session parameters for media packets being sent. The
user agents SHOULD be prepared to process media packets received with
either the old or the new session parameters for a short period of
time to accommodate media packets in transit. Note that this may
involve iterative security processing of the received media packets
during that period of time. Section 8 in [RFC3264] lists several
techniques to help alleviate the problem of determining when a
received media packet was generated according to the old or new
offer/answer exchange.
4. Examples
4.1. SDP Security Descriptions Example
The call flow of Figure 1 shows a basic session establishment using
the Session Initiation Protocol [SIP] and SDP security descriptions
[SDESC] with security descriptions for the secure media stream (SRTP
in this case).
A B
| |
|-------------(1) INVITE SDP1--------------->|
| |
|<------(2) 183 Session Progress SDP2--------|
| |
|----------------(3) PRACK SDP3------------->|
| |
|<-----------(4) 200 OK (PRACK) SDP4---------|
| |
|<-------------(5) 180 Ringing---------------|
| |
| |
| |
Figure 1: Security Preconditions with SDP Security
Descriptions Example
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The SDP descriptions of this example are shown below - we have
omitted the details of the SDP security descriptions as well as any
SIP details for clarity of the security precondition described here:
SDP1: A includes a mandatory end-to-end security precondition for
both the send and receive direction in the initial offer as well as a
"crypto" attribute (see [SDESC]), which includes keying material that
can be used by A to generate media packets. Since B does not know
any of the security parameters yet, the current status (see RFC 3312)
is set to "none". A's local status table (see RFC 3312) for the
security 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/SAVP 0
c=IN IP4 192.0.2.1
a=curr:sec e2e none
a=des:sec mandatory e2e sendrecv
a=crypto:foo...
SDP2: When B receives the offer and generates an answer, B knows the
(send and recv) security parameters of both A and B. From a security
perspective, B is now able to receive media from A, so B's "recv"
security precondition is "yes". However, A does not know any of B's
SDP information, so B's "send" security precondition is "no". B's
local status table therefore looks as follows:
Direction | Current | Desired Strength | Confirm
-----------+----------+------------------+----------
send | no | mandatory | no
recv | yes | mandatory | no
B requests A to confirm when A knows the security parameters used in
the send and receive direction (it would suffice for B to ask for
confirmation of A's send direction only) and hence the resulting
answer SDP becomes:
m=audio 30000 RTP/SAVP 0
c=IN IP4 192.0.2.4
a=curr:sec e2e recv
a=des:sec mandatory e2e sendrecv
a=conf:sec e2e sendrecv
a=crypto:bar...
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SDP3: When A receives the answer, A updates its local status table
based on the rules in RFC 3312. A knows the security parameters of
both the send and receive direction and hence A's local status table
is updated as follows:
Direction | Current | Desired Strength | Confirm
-----------+----------+------------------+----------
send | yes | mandatory | yes
recv | yes | mandatory | yes
Since B requested confirmation of the send and recv security
preconditions, and both are now satisfied, A immediately sends an
updated offer (3) to B showing that the security preconditions are
satisfied:
m=audio 20000 RTP/SAVP 0
c=IN IP4 192.0.2.1
a=curr:sec e2e sendrecv
a=des:sec mandatory e2e sendrecv
a=crypto:foo...
Note that we here use PRACK [RFC3262] instead of UPDATE [RFC3311]
since the precondition is satisfied immediately, and the original
offer/answer exchange is complete.
SDP4: Upon receiving the updated offer, B updates its local status
table based on the rules in RFC 3312, which yields the following:
Direction | Current | Desired Strength | Confirm
-----------+----------+------------------+----------
send | yes | mandatory | no
recv | yes | mandatory | no
B responds with an answer (4) that contains the current status of the
security precondition (i.e., sendrecv) from B's point of view:
m=audio 30000 RTP/SAVP 0
c=IN IP4 192.0.2.4
a=curr:sec e2e sendrecv
a=des:sec mandatory e2e sendrecv
a=crypto:bar...
B's local status table indicates that all mandatory preconditions
have been satisfied, and hence session establishment resumes; B
returns a 180 (Ringing) response (5) to indicate alerting.
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4.2. Key Management Extension for SDP Example
The call flow of Figure 2 shows a basic session establishment using
the Session Initiation Protocol [SIP] and Key Management Extensions
for SDP [KMGMT] with security descriptions for the secure media
stream (SRTP in this case):
A B
| |
|-------------(1) INVITE SDP1--------------->|
| |
|<------(2) 183 Session Progress SDP2--------|
| |
|----------------(3) PRACK SDP3------------->|
| |
|<-----------(4) 200 OK (PRACK) SDP4---------|
| |
|<-------------(5) 180 Ringing---------------|
| |
| |
| |
Figure 2: Security Preconditions with Key Management
Extensions for SDP Example
The SDP descriptions of this example are shown below - we show an
example use of MIKEY [MIKEY] with the Key Management Extensions,
however we have omitted the details of the MIKEY parameters as well
as any SIP details for clarity of the security precondition described
here:
SDP1: A includes a mandatory end-to-end security precondition for
both the send and receive direction in the initial offer as well as a
"key-mgmt" attribute (see [KMGMT]), which includes keying material
that can be used by A to generate media packets. Since B does not
know any of the security parameters yet, the current status (see RFC
3312) is set to "none". A's local status table (see RFC 3312) for
the security precondition is as follows:
Direction | Current | Desired Strength | Confirm
-----------+----------+------------------+----------
send | no | mandatory | no
recv | no | mandatory | no
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RFC 5027 Security Preconditions October 2007
and the resulting offer SDP is:
m=audio 20000 RTP/SAVP 0
c=IN IP4 192.0.2.1
a=curr:sec e2e none
a=des:sec mandatory e2e sendrecv
a=key-mgmt:mikey AQAFgM0X...
SDP2: When B receives the offer and generates an answer, B knows the
(send and recv) security parameters of both A and B. B generates
keying material for sending media to A, however, A does not know B's
keying material, so the current status of B's "send" security
precondition is "no". B does know A's SDP information, so B's "recv"
security precondition is "yes". B's local status table therefore
looks as follows:
Direction | Current | Desired Strength | Confirm
-----------+----------+------------------+----------
send | no | mandatory | no
recv | yes | mandatory | no
B requests A to confirm when A knows the security parameters used in
the send and receive direction and hence the resulting answer SDP
becomes:
m=audio 30000 RTP/SAVP 0
c=IN IP4 192.0.2.4
a=curr:sec e2e recv
a=des:sec mandatory e2e sendrecv
a=conf:sec e2e sendrecv
a=key-mgmt:mikey AQAFgM0X...
Note that the actual MIKEY data in the answer differs from that in
the offer; however, we have only shown the initial and common part of
the MIKEY value in the above.
SDP3: When A receives the answer, A updates its local status table
based on the rules in RFC 3312. A now knows all the security
parameters of both the send and receive direction and hence A's local
status table is updated as follows:
Direction | Current | Desired Strength | Confirm
-----------+----------+------------------+----------
send | yes | mandatory | yes
recv | yes | mandatory | yes
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Since B requested confirmation of the send and recv security
preconditions, and both are now satisfied, A immediately sends an
updated offer (3) to B showing that the security preconditions are
satisfied:
m=audio 20000 RTP/SAVP 0
c=IN IP4 192.0.2.1
a=curr:sec e2e sendrecv
a=des:sec mandatory e2e sendrecv
a=key-mgmt:mikey AQAFgM0X...
SDP4: Upon receiving the updated offer, B updates its local status
table based on the rules in RFC 3312, which yields the following:
Direction | Current | Desired Strength | Confirm
-----------+----------+------------------+----------
send | yes | mandatory | no
recv | yes | mandatory | no
B responds with an answer (4) that contains the current status of the
security precondition (i.e., sendrecv) from B's point of view:
m=audio 30000 RTP/SAVP 0
c=IN IP4 192.0.2.4
a=curr:sec e2e sendrecv
a=des:sec mandatory e2e sendrecv
a=key-mgmt:mikey AQAFgM0X...
B's local status table indicates that all mandatory preconditions
have been satisfied, and hence session establishment resumes; B
returns a 180 (Ringing) response (5) to indicate alerting.
5. Security Considerations
In addition to the general security considerations for preconditions
provided in RFC 3312, the following security issues should be
considered.
Security preconditions delay session establishment until
cryptographic parameters required to send and/or receive media for a
media stream have been negotiated. Negotiation of such parameters
can fail for a variety of reasons, including policy preventing use of
certain cryptographic algorithms, keys, and other security
parameters. If an attacker can remove security preconditions or
downgrade the strength-tag from an offer/answer exchange, the
attacker can thereby cause user alerting for a session that may have
no functioning media. This is likely to cause inconvenience to both
the offerer and the answerer. Similarly, security preconditions can
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RFC 5027 Security Preconditions October 2007
be used to prevent clipping due to race conditions between an
offer/answer exchange and secure media stream packets based on that
offer/answer exchange. If an attacker can remove or downgrade the
strength-tag of security preconditions from an offer/answer exchange,
the attacker can cause clipping to occur in the associated secure
media stream.
Conversely, an attacker might add security preconditions to offers
that do not contain them or increase their strength-tag. This in
turn may lead to session failure (e.g., if the answerer does not
support it), heterogeneous error response forking problems, or a
delay in session establishment that was not desired.
Use of signaling integrity mechanisms can prevent all of the above
problems. Where intermediaries on the signaling path (e.g., SIP
proxies) are trusted, it is sufficient to use only hop-by-hop
integrity protection of signaling, e.g., IPSec or TLS. In all other
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.
Current SDP [RFC4566] and associated offer/answer procedures
[RFC3264] allows only a single type of transport protocol to be
negotiated for a given media stream in an offer/answer exchange.
Negotiation of alternative transport protocols (e.g., plain and
secure RTP) is currently not defined. Thus, if the transport
protocol offered (e.g., secure RTP) is not supported, the offered
media stream will simply be rejected. There is however work in
progress to address that. For example, the SDP Capability
Negotiation framework [SDPCN] defines a method for negotiating the
use of a secure or a non-secure transport protocol by use of SDP and
the offer/answer model with various extensions.
Such a mechanism introduces a number of security considerations in
general, however use of SDP Security Preconditions with such a
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RFC 5027 Security Preconditions October 2007
mechanism introduces the following security precondition specific
security considerations:
A basic premise of negotiating secure and non-secure media streams as
alternatives is that the offerer's security policy allows for non-
secure media. 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 (indicating the chosen secure alternative) is received. From
a security point of view, it is important to note that use of
security preconditions (even with a mandatory strength-tag) would not
address this vulnerability since security preconditions would
effectively apply only to the secure media stream alternatives. If
the non-secure media stream alternative was selected by the answerer,
the security precondition would be satisfied by definition, the
session could progress and (non-secure) media could be received prior
to the answer being received.
6. IANA Considerations
IANA has registered an RFC 3312 precondition type called "sec" with
the name "Security precondition". The reference for this
precondition type is the current document.
7. Acknowledgements
The security precondition was defined in earlier versions of RFC
3312. RFC 3312 contains an extensive list of people who worked on
those earlier versions, which are acknowledged here as well. The
authors would additionally like to thank David Black, Mark Baugher,
Gonzalo Camarillo, Paul Kyzivat, and Thomas Stach for their comments
on this document.
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3312] Camarillo, G., Ed., Marshall, W., Ed., and J. Rosenberg,
"Integration of Resource Management and Session Initiation
Protocol (SIP)", RFC 3312, October 2002.
[RFC4032] Camarillo, G. and P. Kyzivat, "Update to the Session
Initiation Protocol (SIP) Preconditions Framework", RFC
4032, March 2005.
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RFC 5027 Security Preconditions October 2007
[SIP] 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.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June
2002.
9. Informative References
[SDESC] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, July 2006.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Video Conferences with Minimal Control", STD 65, RFC 3551,
July 2003.
[SRTP] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[ICE] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Methodology for Network Address Translator (NAT)
Traversal for Multimedia Session Establishment Protocols",
Work in Progress, September 2007.
[KMGMT] Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
Carrara, "Key Management Extensions for Session Description
Protocol (SDP) and Real Time Streaming Protocol (RTSP)",
RFC 4567, July 2006.
[MIKEY] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
[RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, June 2002.
[RFC3311] Rosenberg, J., "The Session Initiation Protocol (SIP)
UPDATE Method", RFC 3311, October 2002.
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RFC 5027 Security Preconditions October 2007
[HERFP] Mahy, R., "A Solution to the Heterogeneous Error Response
Forking Problem (HERFP) in the Session Initiation Protocol
(SIP)", Work in Progress, March 2006.
[SDPCN] Andreasen, F., "SDP Capability Negotiation", Work in
Progress, July 2007.
Authors' Addresses
Flemming Andreasen
Cisco Systems, Inc.
499 Thornall Street, 8th Floor
Edison, New Jersey 08837 USA
EMail: fandreas@cisco.com
Dan Wing
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134 USA
EMail: dwing@cisco.com
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Andreasen & Wing Standards Track [Page 16]
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