Internet Engineering Task Force J. Arkko
MMUSIC Working Group E. Carrara
INTERNET-DRAFT F. Lindholm
Expires: August 2002 M. Naslund
K. Norrman
Ericsson
February, 2002
Key Management Extensions for SDP and RTSP
<draft-ietf-mmusic-kmgmt-ext-02.txt>
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This document defines general extensions for SDP and RTSP to carry
the security information needed by a key management protocol, in
order to secure the media. These extensions are presented as a
framework, to be used by one or more key management protocols. As
such, its use is meaningful only when it is completed by the key
management protocol in use.
General guidelines are also given on how the framework should be used
together with SIP and RTSP.
TABLE OF CONTENTS
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1. Introduction.................................................. 2
1.1. Notational Conventions...................................... 3
2. Extensions to SDP and RTSP.................................... 3
2.1. SDP Extensions.............................................. 4
2.2. RTSP Extensions............................................. 4
3. Usage with SIP and RTSP....................................... 5
3.1. General SDP processing...................................... 5
3.2. SIP usage................................................... 6
3.3. RTSP usage.................................................. 6
3.4. Example scenarios........................................... 7
4. Security Considerations....................................... 9
5. IANA Considerations...........................................10
6. Conclusions...................................................10
7. Acknowledgments...............................................10
8. Author's Addresses............................................10
9. References....................................................11
1. Introduction
There has recently been work to define a security framework for the
protection of real-time applications running over RTP, [SRTP].
However, a security protocol needs a key management infrastructure to
exchange keys and security parameters, managing and refreshing keys,
etc.
The focus in the following sections is to describe SDP attribute
extensions and RTSP header extensions to support key management, and
a possible integration within SIP and RTSP. A framework is therefore
described in the following, that will need to be accompanied by one
or more key management protocols.
Some of the motivations to create a framework with the possibility to
include the key management in the session establishment are:
* Just as the codec information is a description of how to encode
and decode the audio (or video) stream, the key management data
is a description of how to encrypt and decrypt the data.
* The possibility to negotiate the security for the entire
multimedia session at the same time.
* The knowledge of the media at the session establishment makes it
easy to tie the key management to the multimedia sessions.
* This approach may be more efficient than setting up the security
later, as that approach might force extra roundtrips, possibly
also a separate set-up for each stream, hence implying more delay
to the actual setup of the media session.
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Currently in SDP [SDP], one field exists to transport keys, i.e. the
"key=" field. However, this is not enough for a key management
protocol. The approach here is to use and extend the SDP description
to transport the key management offer/answer and also to associate it
with the media sessions. SIP uses the offer/answer model [OAM]
whereby extensions to SDP will be enough. An extra RTSP header is
also defined.
1.1. 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 RFC-2119.
2. Extensions to SDP and RTSP
This section describes common attributes that are to be included in
an SDP description or in an RTSP header when an integrated key
management protocol is used. All attribute values MUST follow the
general SDP or RTSP guideline.
For the SDP description, the key management attributes may be defined
at session level (i.e. before the media descriptor lines) and/or at
media level. If the key management attributes are defined at media
level, they will only apply to that specific media. If the key
management attributes are defined at both session and media level,
the media level definition overrides the session level definition for
that specific media.
The extensions were defined in a way to:
* give a minimal impact on current SDP implementations, i.e. only
minimal modifications MUST be built into an existing SDP stack in
order to support the key management.
* make it easy to use another key management protocol, or a new
version, without having to redefine the attributes or add new
ones.
The following set of attributes have been identified as necessary to
support:
* key management protocol identifier, to indicate the key
management protocol used ("keymgmt-prtcl")
* key management raw data field, to transport the key management
protocol data ("keymgmt-data")
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* in the case of SDP, an extra (optional) authentication attribute
to be able to tie the key management data to the surrounding
media definitions ("key-extra-auth").
2.1. SDP Extensions
This section provides an Augmented Backus-Naur Form (ABNF) grammar
(as used in [SDP]) for the key management extensions to SDP.
Note that the new definitions are compliant with the definition of an
attribute field, i.e.
attribute = (att-field ":" att-value) | att-field
Three new attributes are defined, keymgmt-prtcl, keymgmt-data, and
key-extra-auth.
keymgmt-prtcl = "keymgmt-prot:" prtcl-name
prtcl-name = 1*(safe)
; e.g. "MIKEY"
keymgmt-data = "keymgmt-data:" byte-string
key-extra-auth = "keymgmt-auth:" byte-string
where safe and byte-string are as defined in SDP [SDP].
2.2. RTSP Extensions
To support the three different attributes described, the following
RTSP header is defined:
KeyMgmt = "KeyMgmt" ":" [stream-url] protocol data
stream-url = "url" "=" url ";"
protocol = "Prot" "=" token ";"
data = "Data" "=" quoted-string
url, token and quoted-string are as defined in the RTSP specification
[RTSP]. The url indicates the stream URL, which the parameters
correspond to.
The KeyMgmt header should be possible to use in both request and
response messages of the following methods:
* DESCRIBE
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* ANNOUNCE
* SETUP
* PLAY
* RECORD
* SET_PARAMETER
* GET_PARAMETER
* OPTIONS
3. Usage with SIP and RTSP
This section gives recommendations of how/when to include the defined
key management attributes when SIP and/or RTSP are used together with
SDP.
Some general requirements MUST be set on the key management protocol
if it has to be suitable to work together with SIP and RTSP:
* It MUST be possible to execute the key management protocol in at
most one roundtrip in case the answerer accepts the offer.
* The protocol MUST return, to SDP/RTSP, a valid answer whether the
provided offer was accepted or not.
* There MUST be a possibility for the key management protocol to
tie the media sessions to the negotiated parameters (i.e. an
interface between the key management and the SDP and RTSP
implementation MUST exist).
Today, the MIKEY protocol has adopted the key management extensions
to work together with SIP and RTSP. Other protocols may use the
described attributes and header, e.g. Kerberos.
3.1. General SDP processing
When an SDP message is created, the following procedure should be
applied:
* The "keymgmt-prot" attribute is filled in with the identifier of
the key management protocol used (e.g. MIKEY or Kerberos).
* The "keymgmt-data" attribute is filled in by using the key
management protocol interface to obtain key management data. The
data may e.g. be a MIKEY message or Kerberos ticket.
* If used, the "keymgmt-auth" attribute is filled in by using the
key management protocol interface to obtain key management data.
Note that what data from SDP are supposed to be provided to the
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interface MUST be specified by the key management protocol
itself.
A received SDP message that contains the key management attributes
SHOULD process these attributes in the following manner:
* Detect the key management protocol used by parsing the "keymgmt-
prot" attribute.
* Extract the key management data from the "keymgmt-data" attribute
and call the key management interface with the extracted data.
Note that depending on key management protocol, some extra
parameters might of course be requested, such as the
source/destination network address/port(s) for the specified
media.
* Depending on the outcome of the key management processing (i.e.
whether it was accepted or not), SDP processing can proceed
according to normal processing (e.g. according to the
offer/answer model, see also Section 3.2.).
* If the optional "keymgmt-auth" attribute is included, this is
processed as the "keymgmt-data".
3.2. SIP usage
The offerer should include the key management data within an offer
that contains the media description it should apply to. The answerer
MUST check with the key management protocol if the attribute values
are valid, and then obtain from the key management the data to
include in the answer. If the offer is not accepted, the answerer
returns a notification message and the offerer may go out with a new
(different) offer, depending on the local security policy.
Re-keying should be handled as a new offer, i.e. a re-INVITE should
be sent with the new proposed parameters. The answerer treats this as
a new offer where the key management is the issue of change.
3.3. RTSP usage
RTSP does not use the offer/answer model, as SIP does. This causes
some problems as it is not possible (without abusing RTSP) to send
back an answer to the server (as the server will in most cases be the
one initiating the security parameter exchange). To solve this, a new
header has been introduced (Section 2.2).
The processing of a key management header in RTSP should be done
analogous of the SDP message processing. The only differences will be
that the url has been introduced and that there is no corresponding
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parameter for the extra authentication. The url should be processed
as a standard RTSP stream url, i.e. to identify the session. It is
however not mandatory to use.
The initial key management message from a server should be sent to
the client using SDP. When responding to this, the client uses the
new RTSP header to send back an answer (included in either the SETUP
or the PLAY message).
The server may provide re-keying facilities by sending a new key
management message in a SET_PARAMETER or ANNOUNCE messages. In the
latter, the RTSP header is not used if the ANNOUNCE message includes
a SDP description where the data can be provided in. The response
message is then put in the new RTSP header in the response message
from the client to the server. Note that the SET PARAMETER and the
ANNOUNCE messages are not mandatory to support for the servers.
3.4. Example scenarios
Example 1 (SIP)
A SIP call is taking place between Alice and Bob. Alice sends an
Invite message consisting of the following offer:
v=0
o=alice 2891092738 2891092738 IN IP4 lost.somewhere.com
s=Cool stuff
e=alice@w-land.org
t=0 0
c=IN IP4 lost.somewhere.com
a=keymgmt-prot:MIKEY
a=keymgmt-data:uiSDF9sdhs727ghsd/dhsoKkdOokdo7eWsnDSJD...
m=audio 49000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
m=video 52230 RTP/SAVP 31
a=rtpmap:31 H261/90000
i.e. Alice proposes to set up one audio stream and one video stream
that run over SRTP. To set up the security parameters for SRTP, she
uses MIKEY. Note that MIKEY is negotiating the crypto suite for both
streams (as it is placed at the session level).
Bob accepts the offer and sends an answer back to Alice:
v=0
o=bob 2891092897 2891092897 IN IP4 found.somewhere.com
s=Cool stuff
e=bob@null.org
t=0 0
c=IN IP4 found.somewhere.com
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a=keymgmt-prot:MIKEY
a=keymgmt-data:skaoqDeMkdwRW278HjKVB...
m=audio 49030 RTP/SAVP 98
a=rtpmap:98 AMR/8000
m=video 52230 RTP/SAVP 31
a=rtpmap:31 H261/90000
Example 2 (SDP)
This example shows how Alice would have done in the previous example
if she wished to protect only the audio stream.
v=0
o=alice 2891092738 2891092738 IN IP4 lost.somewhere.com
s=Cool stuff
e=alice@w-land.org
t=0 0
c=IN IP4 lost.somewhere.com
m=audio 49000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=keymgmt-prot:MIKEY
a=keymgmt-data:uiSDF9sdhs727ghsd/dhsoKkdOokdo7eWsnDSJD...
m=video 52230 RTP/AVP 31
a=rtpmap:31 H261/90000
Example 3 (RTSP)
A client wants to set up a streaming session and requests a media
description from the streaming server.
DESCRIBE rtsp://server.example.com/fizzle/foo RTSP/1.0
CSeq: 312
Accept: application/sdp
From: user@client.com
The server sends back an OK message including a SDP description. As
the server
RTSP/1.0 200 OK
CSeq: 312
Date: 23 Jan 1997 15:35:06 GMT
Content-Type: application/sdp
v=0
o=actionmovie 2891092738 2891092738 IN IP4 movie.somewhere.com
s=Action Movie
e=action@movie.somewhere.com
t=0 0
c=IN IP4 movie.somewhere.com
a=keymgmt-prot:MIKEY
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a=keymgmt-data:uiSDF9sdhs727ghsd/dhsoKkdOokdo7eWsnDSJD...
m=audio 0 RTP/SAVP 98
a=rtpmap:98 AMR/8000
control:rtsp://movie.somewhere.com/action/audio
m=video 0 RTP/SAVP 31
a=rtpmap:31 H261/90000
control:rtsp://movie.somewhere.com/action/video
The client is now ready to setup the sessions. It includes the key
management data in the first message going back to the server (i.e.
the SETUP message).
SETUP rtsp://movie.somewhere.com/action/audio RTSP/1.0
CSeq: 313
Transport: RTP/SAVP/UDP;unicast;client_port=3056-3057
KeyMgmt: Prot=MIKEY;Data="skaoqDeMkdwRW278HjKVB..."
The server processes the request including checking the validity of
the key management header.
RTSP/1.0 200 OK
CSeq: 313
Session: 12345678
Transport: RTP/SAVP/UDP;unicast;client_port=3056-3057;
server_port=5000-5001
The RTSP is then proceeded as usual (with e.g. a SETUP message for
the video followed by a PLAY message).
4. Security Considerations
The nature of this document is to allow SDP and RTSP to support
security of the media sessions. It is therefore not the intention of
this document to describe possible security solution or to define
possible security problems. The defined SDP and RTSP extensions are
not believed to introduce any new security risks to SDP and RTSP.
The "keymgmt-auth" attribute may be (optionally) used to guarantee an
authenticated binding between the session(s) and the security
parameters, e.g. authenticating both the key management lines and
(parts of) the surrounding SDP description. Each key management
specifies the coverage of such "keymgmt-auth" attribute. A denial-of-
service attack may be open if such authenticated binding is not
provided. Note however, the "keymgmt-auth" cannot work when NATs are
present.
Note that the purpose of the key management fields is to secure the
media streams themselves. Under the assumption that the key
management schemes are secure, the SDP payloads can be passed along
unprotected, and the media streams will still be secure even if some
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attackers gained knowledge of the SDP contents. There may however, be
other reasons to protect the SDP payloads such as preventing
attackers from gaining any information regarding the session or the
used equipment.
5. IANA Considerations
Three new attributes fields for SDP (see Section 2.1) and one new
RTSP header are registered (see Section 2.2).
6. Conclusions
A security solution for real-time applications needs a key management
infrastructure. Integrating the key management scheme with the
session establishment protocol could be done efficiently in most of
the scenarios. This draft proposes a framework that integrates a key
management protocol (e.g. MIKEY) into SIP and RTSP, and which can be
accompanied by different key management protocols. A set of new
attributes and headers has been defined in SDP and RTSP to support
this.
7. Acknowledgments
Thanks to: Rolf Blom, Magnus Westerlund, and the rest involved in the
MMUSIC WG and the MSEC WG.
A special thanks to Joerg Ott and Colin Perkins.
8. Author's Addresses
Jari Arkko
Ericsson
02420 Jorvas Phone: +358 40 5079256
Finland Email: jari.arkko@ericsson.com
Elisabetta Carrara
Ericsson Research
SE-16480 Stockholm Phone: +46 8 50877040
Sweden EMail: elisabetta.carrara@era.ericsson.se
Fredrik Lindholm
Ericsson Research
SE-16480 Stockholm Phone: +46 8 58531705
Sweden EMail: fredrik.lindholm@era.ericsson.se
Mats Naslund
Ericsson Research
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SE-16480 Stockholm Phone: +46 8 58533739
Sweden EMail: mats.naslund@era.ericsson.se
Karl Norrman
Ericsson Research
SE-16480 Stockholm Phone: +46 8 4044502
Sweden EMail: karl.norrman@era.ericsson.se
9. References
[MIKEY] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and
Norrman, K., "MIKEY: Multimedia Internet KEYing", Internet Draft,
IETF, Work in progress (MSEC).
[OAM] Rosenberg, J. and Schulzrinne, H., "An Offer/Answer Model with
SDP", Internet Draft, IETF, Work in progress (MMUSIC).
[RTSP] Schulzrinne, H., Rao, A., and Lanphier, R., "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[SDP] Handley, M., and Jacobson, V., "Session Description Protocol
(SDP)", IETF, RFC2327
[SIP] Handley, M., Schulzrinne, H., Schooler, E., and Rosenberg, J.,
"SIP: Session Initiation Protocol", IETF, RFC2543.
[SRTP] Blom, R., Carrara, E., McGrew, D., Naslund, M, Norrman, K.,
and Oran, D., "The Secure Real Time Transport Protocol", Internet
Draft, IETF, Work in Progress (AVT).
This Internet-Draft expires in August 2002.
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