Internet Engineering Task Force                                 J. Arkko
MMUSIC Working Group                                          E. Carrara
INTERNET-DRAFT                                               F. Lindholm
Expires: August 2003                                          M. Naslund
                                                              K. Norrman
                                                          February, 2003

                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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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at

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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................................................4
   3. Usage with SIP and RTSP..........................................5
   3.1. General SDP processing.........................................5
   3.2. SIP usage......................................................6
   3.3. RTSP usage.....................................................7
   3.4. Example scenarios..............................................7
   4. Adding further Key management protocols.........................10
   5. Security Considerations.........................................10
   6. IANA Considerations.............................................11
   7. Conclusions.....................................................11
   8. Acknowledgments.................................................11
   9. Author's Addresses..............................................11
   10. References.....................................................12
   10.1. Normative References.........................................12
   10.2. Informative 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 and authentication algorithm to be used. The key management
   protocol has similarities with, e.g., SIP [SIP] and RTSP [RTSP] in
   the sense that it negotiates necessary information in order to be
   able to setup the session.

   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. Such a framework will need to be
   completed by one or more key management protocols, to describe how
   the framework is used, e.g. which is the data to be carried in the

   Some of the motivations to create a framework with the possibility to
   include the key management in the session establishment are:

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   * 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.

   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 as there are many more parameters that need to be
   transported. 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. However, RTSP [RTSP]
   does not use the offer/answer model. This makes it impossible to send
   back an answer to the server. To solve this, a new header is
   introduced in which the key management data can be included.

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. The attribute values MUST follow the
   general SDP or RTSP guidelines.

   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 following SDP attribute is defined:

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   key-mgmt:<identifier> <opaque-data>

   <identifier> is the name of the key management protocol and the
   opaque-data is a field to transport the key management protocol 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. So, if the key management fails, the offer must be rejected.

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

   One new attribute for SDP is defined:

   key-mgmt     = "key-mgmt: " prtcl-id keymgmt-data

   prtcl-id     = non-ws-string
                  ; e.g. "mikey"

   keymgmt-data = text

   where non-ws-string and text are as defined in SDP [SDPnew]. The
   attribute may be used at session level, media level or at both
   levels. An attribute defined at media level overrides an attribute
   defined at session level.

2.2. RTSP Extensions

   To support the needed attribute described, the following RTSP header
   is defined:

   KeyMgmt = "keymgmt" ":" 1#key-mgmt-spec

   key-mgmt-spec = "prot" "=" token ";" "data" "=" quoted-string

   token and quoted-string are as defined in the RTSP specification

   The KeyMgmt header should be possible to use in the messages
   described in the table below.

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   Method      Direction     Requirement
   DESCRIBE    C->S          required
   SETUP       C->S          required
   ANNOUNCE    C->S, S->C    optional (required: if re-key should
                                       be supported)

3. Usage with SIP and RTSP

   This section gives recommendations of how/when to include the defined
   key management attribute when SIP and/or RTSP are used together with

   When a key management protocol is integrated with SIP/SDP and RTSP,
   the following requirements are put on the key management:

   * It MUST be possible to execute the key management protocol in at
      most one roundtrip in case the answerer accepts the offer.

   * It MUST be possible from the SIP/SDP and RTSP application, using
      the key management API, to receive key management data, and
      information of whether a message is accepted or not.

   Today, the MIKEY protocol [MIKEY] has adopted the key management
   extensions to work together with SIP and RTSP. Other protocols MAY
   use the described attribute and header, e.g. Kerberos [KERB].

3.1. General SDP processing

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

   * The identifier of the key management protocol used (e.g. MIKEY or
      Kerberos) MUST be put in the prtcl-id field.

   * The keymgmt-data field MUST be created with the data received from
      the key management protocol (this data MUST be base64 encoded).
      The data may e.g. be a MIKEY message or Kerberos ticket.

   A received SDP message that contains the key management attributes
   SHOULD process these attributes in the following manner:

   * The key management protocol used MUST be identified by checking the
      prtcl-id field in the key management attribute.

   * The key management data from the keymgmt-data field MUST be
      extracted and given to the key management protocol. 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.

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   * Depending on the outcome of the key management processing (i.e.
      whether it was accepted or not), the processing can proceed
      according to normal processing (e.g. according to the offer/answer
      model, see also Section 3.2).

   Note that the attribute MAY be repeated more than once (e.g., one at
   session level and one at media level). Consequently, the process is
   repeated for each key management attribute detected.

   If more than one key management protocol is supported, multiple
   instances of the key management attribute MAY be included in the
   initial offer, each transporting a different key management data,
   thus indicating alternatives supported.

   If the sender includes more than one key management protocol
   attributes at session level (analogous for the media level), these
   SHOULD be listed in order of preference (with the first being the
   preferred). The receiver chooses the key management protocol it
   supports. When answering, only the accepted key management protocol
   attribute MUST be included. If the receiver does not support any of
   the sender's suggested key management protocols, the receiver answers
   with an error message (see SIP and RTSP), whereby the sender MUST put
   down the current setup procedure.

   Note that by placing multiple key management offers in a single
   message has the disadvantage that the message expands and the
   computational workload for the offerer will increase drastically. The
   possibility to support multiple key management protocols may
   introduce bidding down attacks. To avoid this, the list of
   identifiers of the proposed key management protocols MUST be
   authenticated, which MUST be done by each key management. This puts
   the requirement that it MUST be specified (in the key management
   protocol itself or in a companion document) how the protocol
   identifiers could be authenticated from the offerer to the responder
   by the use of the specific key management protocol. Note that even if
   only one key management protocol is used, that still must
   authenticate its own protocol identifier.

   If more than one protocol is supported by the offerer, it is
   RECOMMENDED that he offers all to him acceptable protocols in the
   first offer, rather than making single, subsequent alternative offers
   in response to error messages, see "Security Considerations".

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

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   are valid, and then obtain from the key management the data to
   include in the answer.

   If the offer is not accepted, the answerer SHOULD return a "606 Not
   Acceptable" message, including one or more Warning headers (at least
   a 306). The offer MUST then abort the security setup.

   Re-keying can 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. In
   general, the re-INVITE (and the key exchange) must be finalized
   before the security protocol can change the keys. The synchronization
   method used when changing keys are dependent on the security and key
   management protocol used.

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). This also assumes that the
   key management also has some kind of binding to the media, so that
   the response to the server will be processed as required.

   The processing of a key management header in RTSP should be done
   analogous of the SDP message processing. 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 the SETUP message). If a server receives a
   SETUP message in which it expects a key management message, but none
   is included, a 403 Forbidden SHOULD be returned to the client,
   whereby the current setup MUST be aborted.

   The server MAY provide re-keying/updating facilities by sending a new
   key management message in an ANNOUNCE messages. The ANNOUNCE message
   contains an SDP message including the key management parameters. The
   response message is put in the new RTSP header in the response from
   the client to the server. Note that the ANNOUNCE messages MUST be
   supported if this feature is to be used.

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

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   s=Cool stuff
   t=0 0
   c=IN IP4
   a=key-mgmt:mikey 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:

   o=bob 2891092897 2891092897 IN IP4
   s=Cool stuff
   t=0 0
   c=IN IP4
   a=key-mgmt:mikey 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.

   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
   a=key-mgmt:mikey uiSDF9sdhs727ghsd/dhsoKkdOokdo7eWsnDSJD...
   m=video 52230 RTP/AVP 31
   a=rtpmap:31 H261/90000

   Note that even if the key management attribute is specified at
   session level, the video part will not be affected by this (as a
   security profile is not used).

   Example 3 (RTSP)

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   A client wants to set up a streaming session and requests a media
   description from the streaming server.

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

   The server sends back an OK message including an SDP description.

   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
   a=key-mgmt:mikey uiSDF9sdhs727ghsd/dhsoKkdOokdo7eWsnDSJD...
   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 then proceeds as usual (with e.g. a SETUP message for the
   video followed by a PLAY message).

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4. Adding further Key management protocols

   This framework cannot be used with all key management protocols. The
   key management protocol needs to comply with 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 identifier that should be used in the
      protocol identifier fields in both SDP and RTSP (e.g. "mikey" for

   * 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 API is implementation specific, but it
      SHOULD at least support to exchange the specified information.
      Note that in particular, the key management MUST always be given
      the protocol identifier(s) of the key management protocol(s)
      included in the offer in the correct order as they appear.

   The key management data MUST be base64 encoded in the SDP and RTSP
   fields. Therefore, considerations of possible conversion from the
   normal key management representation to base64 SHOULD be taken into

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 a primary
   intention of this document to describe possible security solutions 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, if used as specified.

   Note that the purpose of the key management fields is to provide
   information to secure the media streams. Under the assumption that
   the key management schemes are secure, the SDP can be passed along
   unprotected without affecting the key management, and the media
   streams will still be secure even if some attackers gained knowledge
   of the SDP contents.

   However, if the SDP messages are not sent authenticated between the
   parties, it is possible for an active attacker to change attributes
   without being detected. As the key management protocol may
   (indirectly) rely on some of the session information from SDP (e.g.,
   address information), an attack on SDP may have indirect consequences
   on the key management. In general, it is therefore a good thing, not
   only to try to secure the session, but also to secure the session

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   Note that it is impossible to assure the authenticity of a declined
   offer, since even if it comes from the true respondent, the fact that
   he/she declines the offer usually means that he/she does not support
   the protocol(s) offered, and consequently cannot be expected to
   authenticate the response either. This means that if the initiator is
   unsure of which protocol(s) the responder supports, we RECOMMEND that
   the initiator offers all acceptable protocols in a single offer. If
   not, this opens up the possibility for a "man-in-the-middle" (MITM)
   to affect the outcome of the eventually agreed upon protocol, by
   faking unauthenticated error messages until the initiator eventually
   offers a protocol "to the liking" of the MITM. This is not really a
   security problem, but rather a mild form of denial of service that
   can be avoided by following the above recommendation.

6. IANA Considerations

   New attribute fields for SDP (see Section 2.1) and 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

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     SE-16480 Stockholm       Phone:  +46 8 50877040
     Sweden                   EMail:

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

     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

10.1. Normative References

   [OAM] Rosenberg, J. and Schulzrinne, H., "An Offer/Answer Model with
   the Session Description Protocol (SDP)", IETF, 3264.

   [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.

10.2. Informative 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).

   [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 2003.

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