Internet Engineering Task Force                                 J. Arkko
MMUSIC Working Group                                          E. Carrara
INTERNET-DRAFT                                               F. Lindholm
Expires: August 2002                                          M. Naslund
                                                              K. Norrman
                                                          February, 2002

               Key Management Extensions for SDP and RTSP

Status of this memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   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.

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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............................................. 5
   3. Usage with SIP and RTSP....................................... 5
   3.1. General SDP processing...................................... 6
   3.2. SIP usage................................................... 6
   3.3. RTSP usage.................................................. 7
   3.4. Example scenarios........................................... 7
   4. Adding a Key management protocol..............................10
   5. Security Considerations.......................................10
   6. IANA Considerations...........................................11
   7. Conclusions...................................................11
   8. Acknowledgments...............................................11
   9. Author's Addresses............................................11
   10. References...................................................12

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,

   A key management protocol is executed prior to the security protocol
   execution. The key management protocol's main goal is to, in a secure
   and reliable way, establish a so called security association for the
   security protocol. This includes one or several cryptographic keys
   and a set of necessary parameters for the security protocol, e.g.,
   cipher or authentication algorithm to be used. The key management
   protocol has similarities with, e.g., SIP and RTSP in the sense that
   it negotiates necessary information in order to be able to setup the

   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.

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   * 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.

   Currently in SDP [SDPnew], 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",
   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.

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   The following set of attributes have been identified as necessary to

   * 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"). The key management protocol data
    contains the necessary information to establish the security
    protocol, e.g., keys and cryptographic parameters. All parameters
    and keys are protected by the key management. Note that if the key
    management protocol fails, e.g., the receiver does not accept any
    of the proposed security parameters, or simply does not understand
    the key management protocol, the security setup will fail.
    Consequently, it is impossible to establish a secure session. This
    is very similar to the normal SIP/SDP behavior; if the sender only
    supports two codecs, and the receiver do not support any of them,
    it will be problematic to set up a session.

   * 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 [SDPnew]) 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

   keymgmt-prtcl  = "keymgmt-prot:" prtcl-name

   prtcl-name     = non-ws-string
                    ; e.g. "MIKEY"

   keymgmt-data   = "keymgmt-data:" byte-string

   key-extra-auth = "keymgmt-auth:" byte-string

   where non-ws-string and byte-string are as defined in SDP [SDPnew].

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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:
   * SETUP
   * PLAY

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

   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

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   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 [KERB].

3.1. General SDP processing

   When an SDP message is created, the following procedure should be

   * 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
    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

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   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
   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:

   o=alice 2891092738 2891092738 IN IP4
   s=Cool stuff
   t=0 0
   c=IN IP4

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   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:

   o=bob 2891092897 2891092897 IN IP4
   s=Cool stuff
   t=0 0
   c=IN IP4
   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.

   o=alice 2891092738 2891092738 IN IP4
   s=Cool stuff
   t=0 0
   c=IN IP4
   m=audio 49000 RTP/SAVP 98
   a=rtpmap:98 AMR/8000
   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.

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   DESCRIBE rtsp:// RTSP/1.0
   CSeq: 312
   Accept: application/sdp

   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
   o=actionmovie 2891092738 2891092738 IN IP4
   s=Action Movie
   t=0 0
   c=IN IP4
   m=audio 0 RTP/SAVP 98
   a=rtpmap:98 AMR/8000
   m=video 0 RTP/SAVP 31
   a=rtpmap:31 H261/90000

   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:// 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;

   The RTSP is then proceeded as usual (with e.g. a SETUP message for
   the video followed by a PLAY message).

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4. Adding a Key management protocol

   This framework can not be used with all key management protocols. The
   key management protocol MUST comply to the requirements described in
   Section 3. To be able to use a key management protocol with this
   framework, the following MUST be specified:

   * the key management protocol name that should be used in the
    "keymgmt-prot" attribute (e.g. "MIKEY" for MIKEY).

   * the information the key management needs from SDP and RTSP (Section
    3. gives a guideline of what SDP and RTSP needs from the key
    management). The exact interface is implementation specific, but it
    SHOULD at least support to exchange the specified information.

   * if the "keymgmt-auth" attribute is needed by the key management, it
    should also specify the information needed for this. This also
    includes a precise list of the SDP lines it covers.

   The encoding of the data for both "keymgmt-data" and "keymgmt-auth"
   MUST be specified for each key management protocol and comply with
   the SDP and RTSP definitions. For most protocols, base64 encoding
   will be most appropriate.

5. 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.

   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
   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.

   If the SDP messages are not sent authenticated between the parties,
   it is possible for an attacker to change attributes without being
   detected. The "keymgmt-data" is protected by itself, but as it relies
   on the session information from SDP, an attack on SDP may give
   indirect consequences on the key management. The result of the
   attacks is not that severe, as it mainly will result in re-direction
   of the streams, or other DoS attacks. To circumvent the problem, the
   "keymgmt-auth" attribute may (optionally) be used to guarantee an

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   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 exact coverage of such "keymgmt-auth" attribute, i.e.,
   a precise list of which SDP lines are covered. Note however, the
   "keymgmt-auth" cannot work when NATs are present (as this may require
   that fields in the SDP are changed).

6. IANA Considerations

   Three new attributes fields for SDP (see Section 2.1) and one new
   RTSP header are registered (see Section 2.2).

7. 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

8. 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.

9. Author's Addresses

     Jari Arkko
     02420 Jorvas             Phone:  +358 40 5079256
     Finland                  Email:

     Elisabetta Carrara
     Ericsson Research
     SE-16480 Stockholm       Phone:  +46 8 50877040
     Sweden                   EMail:

     Fredrik Lindholm
     Ericsson Research
     SE-16480 Stockholm       Phone:  +46 8 58531705
     Sweden                   EMail:

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     Mats Naslund
     Ericsson Research
     SE-16480 Stockholm       Phone:  +46 8 58533739
     Sweden                   EMail:

     Karl Norrman
     Ericsson Research
     SE-16480 Stockholm       Phone:  +46 8 4044502
     Sweden                   EMail:

10. References

   [KERB] Kohl, J., Neuman, C., "The Kerberos Network Authentication
   Service (V5)", IETF, RFC 1510.

   [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)", IETF, RFC 2326.

   [SDPnew] Handley, M., Jacobson, V., and Perkins, C., "SDP: Session
   Description Protocol", Internet Draft, IETF, Work in progress

   [SIP] Handley, M., Schulzrinne, H., Schooler, E., and Rosenberg, J.,
   "SIP: Session Initiation Protocol", IETF, RFC 2543.

   [SRTP] Baugher, M., 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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