Internet Engineering Task Force Flemming Andreasen
MMUSIC Working Group Dan Wing
INTERNET-DRAFT
EXPIRES: August 2005 Cisco Systems
February, 2005
Security Preconditions for
Session Description Protocol Media Streams
<draft-ietf-mmusic-securityprecondition-00.txt>
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Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved.
Abstract
This document defines a new security precondition for the Session
Description Protocol precondition framework described in RFC 3312.
A security precondition can be used to delay session establishment
or modification until media stream security has been negotiated
successfully.
INTERNET-DRAFT Security Preconditions February, 2005
1 Notational Conventions............................................2
2 Introduction......................................................2
3 Security Precondition Definition..................................3
4 Examples..........................................................5
4.1 SDP Security Descriptions Example.............................5
4.2 Key Management Extension for SDP Example......................7
5 Security Considerations..........................................10
6 IANA Considerations..............................................10
7 Acknowledgements.................................................10
8 Authors' Addresses...............................................11
9 Normative References.............................................11
10 Informative References.........................................11
11 Intellectual Property Statement................................13
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
The concept of a Session Description Protocol (SDP) [SDP]
precondition is defined in [RFC3312] and [RFC3312upd]. 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 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.
Media streams can either be provided in cleartext and with no
integrity checks, 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 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 clipping, the
offerer must receive the answer prior to receiving any media packets
from the answerer. This can be achieved by using a security
precondition, which is used to ensure the successful negotiation of
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media stream security prior to session establishment or
modification.
3 Security Precondition Definition
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.
An entity that wishes to delay session establishment or modification
until media stream security has been established uses the security
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. parameters 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
encryption algorithm and no message authentication/integrity would
satisfy the above whereas use of plain RTP would not. Note
though, that use of SRTP without authentication is discouraged.
The delay of session establishment defined here implies that 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-secure data is multiplexed on a media stream, e.g. when
Interactive Connectivity Establishment [ICE] is being used, the non-
secure data is allowed to be exchanged irrespective of the status of
the security precondition.
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 secure media packets to the other party and the
other party will be able to process them correctly. 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. as when ICE is being
used, the requirement applies to the media packets only.
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* recv: Media stream security negotiation is at a stage where it is
possible to receive and correctly process secure media stream
packets sent by the other party.
The precise criteria for determining when the other party is able to
correctly process secure media stream packets depends on the secure
media stream protocol being used as well as the mechanism by which
the required cryptographic parameters are negotiated.
We here provide details for SRTP negotiated through SDP security
descriptions [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 to. 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.
When security preconditions are used with the Key Management
Extensions for Session Description Protocol (SDP) [KMGMT], the
details depend on the actual key management protocol being used.
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
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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
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."
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
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:
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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. However, A
does not know B's security parameters, so the current status of B's
"send" security precondition (which equal A's "recv" security
precondition) is "no". Similarly, A does not know any of B's SDP
information, so B's "send" security precondition is also "no". B's
local status table therefore looks as follows:
Direction | Current | Desired Strength | Confirm
-----------+----------+------------------+----------
send | no | mandatory | no
recv | no | 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 none
a=des:sec mandatory e2e sendrecv
a=conf:sec e2e sendrecv
a=crypto:bar...
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
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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=crypto:foo...
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) which 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.
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---------|
| |
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|<-------------(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
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 (which equal A's "recv" security precondition) is "no".
Similarly, A does not know any of B's SDP information, so B's "recv"
security precondition is also "no". B's local status table
therefore looks as follows:
Direction | Current | Desired Strength | Confirm
-----------+----------+------------------+----------
send | no | mandatory | no
recv | no | mandatory | no
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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 none
a=des:sec mandatory e2e sendrecv
a=conf:sec e2e sendrecv
a=key-mgmt:mikey AQAFgM0X...
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
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) which 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...
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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 for preconditions provided in
RFC 3312, the following security issues, which are specific to
security preconditions, should be considered.
Security preconditions delay session establishment until
cryptographic parameters required to send and/or receive media 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
intermediaries can remove security preconditions or downgrade the
strength from an offer/answer exchange, they can thereby cause user
alerting for a session that may have no functioning media, which is
likely to cause inconvenience to the called party. Similarly,
security preconditions can 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 intermediaries can
remove or downgrade the strength of security preconditions from an
offer/answer exchange, they can cause clipping to occur in the
associated secure media stream.
Conversely, intermediaries may also add security preconditions to
offers that do not contain them or increase their strength. This in
turn may lead to session failure or delayed session establishment
that was not desired.
Use of integrity mechanisms can prevent all of the above problems.
Where intermediaries on the signaling path are trusted, it is
sufficient to only use hop-by-hop integrity protection, e.g. IPSec
or TLS. In all other cases, end-to-end integrity protection, e.g.
S/MIME, MUST be used.
6 IANA Considerations
IANA is hereby requested to register a 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 draft versions of
RFC 3312. RFC 3312 contains an extensive list of people who worked
on those earlier draft versions which are acknowledged here as well.
The authors would like to thank Mark Baugher, Gonzalo Camarillo, and
Paul Kyzivat for their comments on this document.
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8 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
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.
[SIP] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J.
Peterson, R. Sparks, M. Handley, E. Schooler, "SIP: Session
Initiation Protocol", RFC 3261, June 2002.
10 Informative References
[SDESC] F. Andreasen, M. Baugher, and D. Wing, "SDP Security
Descriptions for Media Streams", work in progress
[RFC3551] H. Schulzrinne, and S. Casner "RTP Profile for Audio and
Video Conferences with Minimal Control", RFC 3550, July 2003.
[SRTP] M. Baugher, D. McGrew, M. Naslund, E. Carrara, K. Norrman,
"The Secure Real-time Transport Protocol", RFC 3711, March 2004.
[RFC3312upd] G. Camarillo and P. Kyzivat, "Update to the Session
Initiation Protocol (SIP) Preconditions Framework", IETF, work-in-
progress.
[ICE] J. Rosenberg, "Interactive Connectivity Establishment (ICE): A
Methodology for Network Address Translator (NAT) Traversal for
Multimedia Session Establishment Protocols", IETF, work-in-progress.
[KMGMT] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K.
Norrman, "Key Management Extensions for Session Description Protocol
(SDP) and Real Time Streaming Protocol (RTSP)", IETF, work-in-
progress.
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[MIKEY] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830, August 2004.
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11 Intellectual Property Statement
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The IETF invites any interested party to bring to its attention any
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this standard. Please address the information to the IETF at
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Disclaimer of Validity
This document and the information contained herein are provided on
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Copyright Statement
Copyright (C) The Internet Society (2005). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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