Network Working Group J. Arkko
Request for Comments: 4567 F. Lindholm
Category: Standards Track M. Naslund
K. Norrman
Ericsson
E. Carrara
Royal Institute of Technology
July 2006
Key Management Extensions for Session Description
Protocol (SDP) and Real Time Streaming Protocol (RTSP)
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.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document defines general extensions for Session Description
Protocol (SDP) and Real Time Streaming Protocol (RTSP) to carry
messages, as specified 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, their use
is meaningful only when complemented by an appropriate key management
protocol.
General guidelines are also given on how the framework should be used
together with SIP and RTSP. The usage with the Multimedia Internet
KEYing (MIKEY) key management protocol is also defined.
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RFC 4567 Key Management Extensions for SDP and RTSP July 2006
Table of Contents
1. Introduction ....................................................3
1.1. Notational Conventions .....................................4
2. Applicability ...................................................4
3. Extensions to SDP and RTSP ......................................5
3.1. SDP Extensions .............................................5
3.2. RTSP Extensions ............................................6
4. Usage with SDP, SIP, RTSP, and SAP ..............................7
4.1. Use of SDP .................................................8
4.1.1. General Processing ..................................8
4.1.2. Use of SDP with Offer/Answer and SIP ...............10
4.1.3. Use of SDP with SAP ................................13
4.1.4. Bidding-Down Attack Prevention .....................13
4.2. RTSP Usage ................................................14
5. Example Scenarios ..............................................17
5.1. Example 1 (SIP/SDP) .......................................17
5.2. Example 2 (SDP) ...........................................18
5.3. Example 3 (RTSP) ..........................................18
5.4. Example 4 (RTSP) ..........................................20
6. Adding Further Key Management Protocols ........................21
7. Integration of MIKEY ...........................................22
7.1. MIKEY Interface ...........................................22
8. Security Considerations ........................................23
9. IANA Considerations ............................................25
9.1. SDP Attribute Registration ................................25
9.2. RTSP Registration .........................................26
9.3. Protocol Identifier Registration ..........................26
10. Acknowledgements ..............................................27
11. References ....................................................27
11.1. Normative References .....................................27
11.2. Informative References ...................................28
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1. Introduction
There has recently been work to define a security profile for the
protection of real-time applications running over RTP, [SRTP].
However, a security protocol needs a key management solution to
exchange keys and security parameters, manage and refresh keys, etc.
A key management protocol is executed prior to the security
protocol's execution. The key management protocol's main goal is to,
in a secure and reliable way, establish a security association for
the security protocol. This includes one or more cryptographic keys
and the set of necessary parameters for the security protocol, e.g.,
cipher and authentication algorithms 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 set up the session.
The focus in the following sections is to describe a new SDP
attribute and RTSP header extension to support key management, and to
show how these can be integrated within SIP and RTSP. The resulting
framework is completed by one or more key management protocols, which
use the extensions provided.
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 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 setup for each stream, hence implying more delay to the
actual setup of the media session.
* The possibility to negotiate keying material end-to-end without
applying end-to-end protection of the SDP (instead, hop-by-hop
security mechanisms can be used, which may be useful if
intermediate proxies need access to the SDP).
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Currently in SDP [SDPnew], there exists one field to transport keys,
the "k=" field. However, this is not enough for a key management
protocol as there are many more parameters that need to be
transported, and the "k=" field is not extensible. The approach used
is to extend the SDP description through a number of attributes that
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 with SDP, so a new RTSP header is
introduced to convey key management data. [SDES] uses the approach
of extending SDP, to carry the security parameters for the media
streams. However, the mechanism defined in [SDES] requires end-to-
end protection of the SDP by some security protocol such as S/MIME,
in order to get end-to-end protection. The solution described here
focuses only on the end-to-end protection of key management
parameters and as a consequence does not require external end-to-end
protection means. It is important to note though, and we stress this
again, that only the key management parameters are protected.
The document also defines the use of the described framework together
with the key management protocol Multimedia Internet KEYing (MIKEY)
[MIKEY].
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 [RFC2119].
2. Applicability
[SDES] provides similar cryptographic key distribution capabilities,
and it is intended for use when keying material is protected along
with the signaling.
In contrast, this specification expects endpoints to have
preconfigured keys or common security infrastructure. It provides
its own security and is independent of the protection of signaling
(if any). As a result, it can be applied in environments where
signaling protection is not turned on, or used hop-by-hop (i.e.,
scenarios where the SDP is not protected end-to-end). This
specification will, independently of the signaling protection
applied, ensure end-to-end security establishment for the media.
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3. Extensions to SDP and RTSP
This section describes common attributes that can be included in SDP
or RTSP when an integrated key management protocol is used. The
attribute values follow the general SDP and RTSP guidelines (see
[SDPnew] and [RTSP]).
For both SDP and RTSP, the general method of adding the key
management protocol is to introduce new attributes, one identifier to
identify the specific key management protocol, and one data field
where the key management protocol data is placed. 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 protocol.
The key management data SHALL be base64 [RFC3548] encoded and comply
with the base64 grammar as defined in [SDPnew]. The key management
protocol identifier, KMPID, is defined as below in Augmented Backus-
Naur Form grammar (ABNF) [RFC4234].
KMPID = 1*(ALPHA / DIGIT)
Values for the identifier, KMPID, are registered and defined in
accordance to Section 9. Note that the KMPID is case sensitive, and
it is RECOMMENDED that values registered are lowercase letters.
3.1. SDP Extensions
This section provides an 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
The ABNF for the key management extensions (conforming to the
att-field and att-value) are as follows:
key-mgmt-attribute = key-mgmt-att-field ":" key-mgmt-att-value
key-mgmt-att-field = "key-mgmt"
key-mgmt-att-value = 0*1SP prtcl-id SP keymgmt-data
prtcl-id = KMPID
; e.g., "mikey"
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keymgmt-data = base64
SP = %x20
where KMPID is as defined in Section 3 of this memo, and base64 is as
defined in SDP [SDPnew]. Prtcl-id refers to the set of values
defined for KMPID in Section 9.
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. In other words, if the media-level
attribute is present, the session level attribute MUST be ignored for
this media. Section 4.1 describes in detail how the attributes are
used and how the SDP is handled in different usage scenarios. The
choice of the level depends, for example, on the particular key
management protocol. Some protocols may not be able to derive enough
key material for all the sessions; furthermore, possibly a different
protection to each session could be required. The particular
protocol might achieve this only by specifying it at the media level.
Other protocols, such as MIKEY, have instead those capabilities (as
it can express multiple security policies and derive multiple keys),
so it may use the session level.
3.2. RTSP Extensions
To support the key management attributes, the following RTSP header
is defined:
KeyMgmt = "KeyMgmt" ":" key-mgmt-spec 0*("," key-mgmt-spec)
key-mgmt-spec = "prot" "=" KMPID ";" ["uri" "=" %x22 URI %x22 ";"]
where KMPID is as defined in Section 3 of this memo, "base64" as
defined in [SDPnew], and "URI" as defined in Section 3 of [RFC3986].
The "uri" parameter identifies the context for which the key
management data applies, and the RTSP URI SHALL match a (session or
media) URI present in the description of the session. If the RTSP
aggregated control URI is included, it indicates that the key
management message is on session level (and similarly the RTSP media
control URI that it applies to the media level). If no "uri"
parameter is present in a key-mgmt-spec the specification applies to
the context identified by the RTSP request URI.
The KeyMgmt header MAY be used in the messages and directions
described in the table below.
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Method | Direction | Requirement
---------------------------------------------
DESCRIBE response | S->C | RECOMMENDED
SETUP | C->S | REQUIRED
SETUP Response | S->C | REQUIRED (error)
Note: Section 4.2 describes in detail how the RTSP extensions are
used.
We define one new RTSP status code to report error due to any failure
during the key management processing (Section 4.2):
Status-Code = "463" ; Key management failure
A 463 response MAY contain a KeyMgmt header with a key management
protocol message that further indicates the nature of the error.
4. Usage with SDP, SIP, RTSP, and SAP
This section gives rules and recommendations of how/when to include
the defined key management attribute when SIP and/or RTSP are used
together with SDP.
When a key management protocol is integrated with SIP/SDP and RTSP,
the following general requirements are placed on the key management:
* At the current time, it MUST be possible to execute the key
management protocol in at most one request-response message
exchange. Future relaxation of this requirement is possible but
would introduce significant complexity for implementations
supporting multi-roundtrip mechanisms.
* 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 or not a message is accepted.
The content of the key management messages depends on the key
management protocol that is used. However, the content of such key
management messages might be expected to be roughly as follows: the
key management Initiator (e.g., the offerer) includes the key
management data in a first message, containing the media description
it should apply to. This data in general consists of the security
parameters (including key material) needed to secure the
communication, together with the necessary authentication information
(to ensure that the message is authentic).
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At the Responder's side, the key management protocol checks the
validity of the key management message, together with the
availability of the parameters offered, and then provides the key
management data to be included in the answer. This answer may
typically authenticate the Responder to the Initiator, and also state
if the initial offer was accepted or not. Certain protocols might
require the Responder to include a selection of the security
parameters that he is willing to support. Again, the actual content
of such responses is dependent on the particular key management
protocol.
Section 7 describes a realization of the MIKEY protocol using these
mechanisms. Procedures to be used when mapping new key management
protocols onto this framework are described in Section 6.
4.1. Use of SDP
This section describes the processing rules for the different
applications that use SDP for the key management.
4.1.1. General Processing
The processing when SDP is used is slightly different according to
the way SDP is transported, and if it uses an offer/answer or
announcement. The processing can be divided into four different
steps:
1) How to create the initial offer.
2) How to handle a received offer.
3) How to create an answer.
4) How to handle a received answer.
It should be noted that the last two steps may not always be
applicable, as there are cases where an answer cannot or will not be
sent back.
The general processing for creating an initial offer SHALL follow the
following actions:
* The identifier of the key management protocol used MUST be placed
in the prtcl-id field of SDP. A table of legal protocols
identifiers is maintained by IANA (see Section 9).
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* The keymgmt-data field MUST be created as follows: the key
management protocol MUST be used to create the key management
message. This message SHALL be base64 encoded [RFC3548] by the SDP
application and then encapsulated in the keymgmt-data attribute.
Note though that the semantics of the encapsulated message is
dependent on the key management protocol that is used.
The general processing for handling a received offer SHALL follow the
following actions:
* The key management protocol is identified according to the prtcl-id
field. A table of legal protocols identifiers is maintained by
IANA (Section 9).
* The key management data from the keymgmt-data field MUST be
extracted, base64 decoded to reconstruct the original message, and
then passed to the key management protocol for processing. Note
that depending on key management protocol, some extra parameters
might also be requested by the specific API, such as the
source/destination network address/port(s) for the specified media
(however, this will be implementation specific depending on the
actual API). The extra parameters that a key management protocol
might need (other than the ones defined here) MUST be documented,
describing their use, as well as the interaction of that key
management protocol with SDP and RTSP.
* If errors occur, or the key management offer is rejected, the
session SHALL be aborted. Possible error messages are dependent on
the specific session establishment protocol.
At this stage, the key management will have either accepted or
rejected the offered parameters. This MAY cause a response message
to be generated, depending on the key management protocol and the
application scenario.
If an answer is to be generated, the following general actions SHALL
be performed:
* The identifier of the key management protocol used MUST be placed
in the prtcl-id field.
* The keymgmt-data field MUST be created as follows. The key
management protocol MUST be used to create the key management
message. This message SHALL be base64 encoded [RFC3548] by the SDP
application and then encapsulated in the keymgmt-data attribute.
The semantics of the encapsulated message is dependent on the key
management protocol that is used.
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The general processing for handling a received answer SHALL follow
the following actions:
* The key management protocol is identified according to the prtcl-id
field.
* The key management data from the keymgmt-data field MUST be
extracted, base64 decoded to reconstruct the original message, and
then passed to the key management protocol for processing.
* If the key management offer is rejected and the intent is to re-
negotiate it, it MUST be done through another Offer/Answer
exchange. It is RECOMMENDED to NOT abort the session in that case,
but to re-negotiate using another Offer/Answer exchange. For
example, in [SIP], the "security precondition" as defined in
[SPREC] solves the problem for a session initiation. The
procedures in [SPREC] are outside the scope of this document. In
an established session, an additional Offer/Answer exchange using a
re-INVITE or UPDATE as appropriate MAY be used
* If errors occur, or the key management offer is rejected and there
is no intent to re-negotiate it, the session SHALL be aborted. If
possible, an error message indicating the failure SHOULD be sent
back.
Otherwise, if all the steps are successful, the normal setup
proceeds.
4.1.2. Use of SDP with Offer/Answer and SIP
This section defines additional processing rules, to the general
rules defined in Section 4.1.1, applicable only to applications using
SDP with the offer/answer model [OAM] (and in particular SIP).
When an initial offer is created, the following offer/answer-specific
procedure SHALL be applied:
* Before creating the key management data field, the list of protocol
identifiers MUST be provided by the SDP application to (each) key
management protocol, as defined in Section 4.1.4 (to defeat
bidding-down attacks).
For a received SDP offer that contains the key management attributes,
the following offer/answer-specific procedure SHALL be applied:
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* Before, or in conjunction with, passing the key management data to
the key management protocol, the complete list of protocol
identifiers from the offer message is provided by the SDP
application to the key management protocol (as defined in Section
4.1.4).
When an answer is created, the following offer/answer-specific
procedure SHALL be applied:
* If the key management rejects the offer and the intent is to re-
negotiate it, the Answer SHOULD include the cause of failure in an
included message from the key management protocol. The
renegotiation MUST be done through another Offer/Answer exchange
(e.g., using [SPREC]). In an established session, it can also be
done through a re-INVITE or UPDATE as appropriate.
* If the key management rejects the offer and the session needs to be
aborted, the answerer SHOULD return a "488 Not Acceptable Here"
message, optionally also including one or more Warning headers (a
"306 Attribute not understood" when one of the parameters is not
supported, and a "399 Miscellaneous warning" with arbitrary
information to be presented to a human user or logged; see Section
20.43 in [SIP]). Further details about the cause of failure MAY be
described in an included message from the key management protocol.
The session is then aborted (and it is up to local policy or end
user to decide how to continue).
Note that the key management attribute (related to the same key
management protocol) MAY be present both at session level and at
media level. Consequently, the process SHALL be repeated for each
such key management attribute detected. In case the key management
processing of any such attribute does not succeed (e.g.,
authentication failure, parameters not supported, etc.), on either
session or media level, the entire session setup SHALL be aborted,
including those parts of the session that successfully completed
their part of the key management.
If more than one key management protocol is supported, multiple
instances of the key management attribute MAY be included in the
initial offer when using the offer/answer model, each transporting a
different key management protocol, thus indicating supported
alternatives.
If the offerer includes more than one key management protocol
attribute at session level (analogous for the media level), these
SHOULD be listed in order of preference (the first being the
preferred). The answerer selects the key management protocol it
wishes to use, and processes only it, on either session or media
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level, or on both, according to where located. If the answerer does
not support any of the offerer's suggested key management protocols,
the answerer indicates this to the offerer so a new Offer/Answer can
be triggered; alternatively, it may return a "488 Not Acceptable
Here" error message, whereby the sender MUST abort the current setup
procedure.
Note that the placement of 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.
Unless the guidelines of Section 4.1.4 are followed, multiple lines
may open up bidding-down attacks. Note also that the multiple-offer
option has been added to optimize signaling overhead in case the
Initiator knows some key (e.g., a public key) that the Responder has,
but is unsure of what protocol the Responder supports. The mechanism
is not intended to negotiate options within one and the same
protocol.
The offerer MUST include the key management data within an offer that
contains the media description it applies to.
Re-keying MUST be handled as a new offer, with the new proposed
parameters. The answerer treats this as a new offer where the key
management is the issue of change. The re-keying exchange MUST be
finalized before the security protocol can change the keys. The same
key management protocol used in the original offer SHALL also be used
in the new offer carrying re-keying. If the new offer carrying re-
keying fails (e.g., the authentication verification fails), the
answerer SHOULD send a "488 Not Acceptable Here" message, including
one or more Warning headers (at least a 306). The offerer MUST then
abort the session.
Note that, in multicast scenarios, unlike unicast, there is only a
single view of the stream [OAM], hence there MUST be a uniform
agreement of the security parameters.
After the offer is issued, the offerer SHOULD be prepared to receive
media, as the media may arrive prior to the answer. However, this
brings issues, as the offerer does not know yet the answerer's choice
in terms of, e.g., algorithms, or possibly the key is known. This
can cause delay or clipping can occur; if this is unacceptable, the
offerer SHOULD use mechanisms outside the scope of this document,
e.g., the security preconditions for SIP [SPREC].
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4.1.3. Use of SDP with SAP
There are cases where SDP is used without conforming to the
offer/answer model; instead, it is a one-way SDP distribution (i.e.,
without back channel), such as when used with SAP and HTTP.
The processing follows the two first steps of the general SDP
processing (see Section 4.1.1). It can be noted that the processing
in this case differs from the offer/answer case in that only one key
management protocol SHALL be offered (i.e., no negotiation will be
possible). This implies that the bidding-down attack is not an
issue; therefore, the countermeasure is not needed. The key
management protocol used MUST support one-way messages.
4.1.4. Bidding-Down Attack Prevention
The possibility to support multiple key management protocols may,
unless properly handled, introduce bidding-down attacks.
Specifically, a man-in-the-middle could "peel off" cryptographically
strong offers (deleting the key management lines from the message),
leaving only weaker ones as the Responder's choice. To avoid this,
the list of identifiers of the proposed key management protocols MUST
be authenticated. The authentication MUST be done separately by each
key management protocol.
Accordingly, it MUST be specified (in the key management protocol
specification itself or in a companion document) how the list of key
management protocol identifiers can be processed to be authenticated
from the offerer to the answerer by the specific key management
protocol. Note that even if only one key management protocol is
used, that still MUST authenticate its own protocol identifier.
The list of protocol identifiers MUST then be given to each of the
selected (offered) key management protocols by the application with
";" separated identifiers. All the offered protocol identifiers MUST
be included, in the same order as they appear in the corresponding
SDP description.
The protocol list can formally be described as
prtcl-list = KMPID *(";" KMPID)
where KMPID is as defined in Section 3.
For example, if the offered protocols are MIKEY and two yet-to-be-
invented protocols KEYP1, KEYP2, the SDP is:
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v=0
o=alice 2891092738 2891092738 IN IP4 lost.example.com
s=Secret discussion
t=0 0
c=IN IP4 lost.example.com
a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyO...
a=key-mgmt:keyp1 727gkdOshsuiSDF9sdhsdKnD/dhsoSJokdo7eWD...
a=key-mgmt:keyp2 DFsnuiSDSh9sdh Kksd/dhsoddo7eOok727gWsJD...
m=audio 39000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
m=video 42000 RTP/SAVP 31
a=rtpmap:31 H261/90000
The protocol list, "mikey;keyp1;keyp2", would be generated from the
SDP description and used as input to each specified key management
protocol (together with the data for that protocol). Each of the
three protocols includes this protocol identifier list in its
authentication coverage (according to its protocol specification).
If more than one protocol is supported by the offerer, it is
RECOMMENDED that all acceptable protocols are included in the first
offer, rather than making single, subsequent alternative offers in
response to error messages; see "Security Considerations".
End-to-end integrity protection of the key-mgmt attributes
altogether, provided externally to the key management itself, also
protects against this bidding-down attack. This is, for example, the
case if SIP uses S/MIME [RFC3851] to end-to-end integrity protect the
SDP description. However, as this end-to-end protection is not an
assumption of the framework, the mechanisms defined in this section
SHALL be applied.
4.2. RTSP Usage
RTSP does not use the offer/answer model, as SIP does. This causes
some problems, as it is not possible (without modifying RTSP) to send
back an answer. To solve this, a new header has been introduced
(Section 3.2). 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 server SHALL be the Initiator of the key management exchange for
sessions in PLAY mode, i.e., transporting media from server to
client. The below text describes the behavior for PLAY mode. For
any other mode, the behavior is not defined in this specification.
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To obtain a session description, the client initially contacts the
server via a DESCRIBE message. The initial key management message
from the RTSP server is sent to the client in the SDP of the 200 OK
in response to the DESCRIBE. Note that only one key management
protocol SHALL be used per session/media level. A server MAY allow
the SDP with key management attribute(s) to be distributed to the
client through other means than RTSP, although this is not specified
here.
The "uri" parameter of the KeyMgmt header is used to indicate for the
key management protocol on what context the carried message applies.
For key management messages on the SDP session level, the answer MUST
contain the RTSP aggregated control URL to indicate this. For key
management messages initially on SDP media level, the key management
response message in the KeyMgmt header MAY use the RTSP media-level
URL. For RTSP sessions not using aggregated control, i.e., no
session-level control URI is defined, the key management protocol
SHALL only be invoked on individual media streams. In this case
also, the key management response SHALL be on individual media
streams (i.e., one RTSP key management header per media).
When responding to the initial key management message, the client
uses the new RTSP header (KeyMgmt) to send back an answer. How this
is done depends on the usage context:
* Key management protocol responses for the initial establishment of
security parameters for an aggregated RTSP session SHALL be sent in
the first SETUP of the session. This means that if the key
management is declared for the whole session but is set up in non-
aggregated fashion (i.e., one media per RTSP session), each SETUP
MUST carry the same response for the session-level context. When
performing a setup of the second or any subsequent media in an RTSP
session, the same key management parameters as established for the
first media also apply to these setups.
* Key management responses for the initial establishment of security
parameters for an individual media SHALL only be included in SETUP
for the corresponding media stream.
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.
When the server creates an initial SDP message, the procedure SHALL
be the same as described in Section 4.1.1.
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The client processing of the initial SDP message from the server
SHALL follow the same procedures as described in Section 4.1.1,
except that, if there is an error, the session is aborted (no error
is sent back).
The client SHALL create the response, using the key management header
in RTSP, as follows:
* The identifier of the key management protocol used (e.g., MIKEY)
MUST be placed in the "prot" field of the header. The prot values
are maintained by IANA (Section 9).
* The keymgmt-data field MUST be created as follows: the key
management protocol MUST be used to create the key management
message. This message SHALL be base64 encoded by the RTSP
application and then encapsulated in the "data" field of the
header. The semantics of the encapsulated message is dependent on
the key management protocol that is used.
* Include, if necessary, the URL to indicate the context in the "uri"
parameter.
The server SHALL process a received key management header in RTSP as
follows:
* The key management protocol is identified according to the "prot"
field.
* The key management data from the "data" field MUST be extracted,
base64 decoded to reconstruct the original message, and then passed
to the key management protocol for processing.
* If the key management protocol is successful, the processing can
proceed according to normal rules.
* Otherwise, if the key management fails (e.g., due to authentication
failure or parameter not supported), an error is sent back as the
SETUP response using RTSP error code 463 (see Section 3.2) and the
session is aborted. It is up to the key management protocol to
specify (within the RTSP status code message or through key
management messages) details about the type of error that occurred.
Re-keying within RTSP is for further study, given that media updating
mechanisms within RTSP are unspecified at the time this document was
written.
Arkko, et al. Standards Track [Page 16]
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5. Example Scenarios
The following examples utilize MIKEY [MIKEY] as the key management
protocol to be integrated into SDP and RTSP.
5.1. Example 1 (SIP/SDP)
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 w-land.example.com
s=Cool stuff
e=alice@w-land.example.com
t=0 0
c=IN IP4 w-land.example.com
a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAyONQ6gAAAAAGEEoo2pee4hp2
UaDX8ZE22YwKAAAPZG9uYWxkQGR1Y2suY29tAQAAAAAAAQAk0JKpgaVkDaawi9whVBtBt
0KZ14ymNuu62+Nv3ozPLygwK/GbAV9iemnGUIZ19fWQUOSrzKTAv9zV
m=audio 49000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
m=video 52230 RTP/SAVP 31
a=rtpmap:31 H261/90000
That is, Alice proposes to set up one audio stream and one video
stream that run over SRTP (signaled by the use of the SAVP profile).
She uses MIKEY to set up the security parameters for SRTP (Section
7). The MIKEY message contains the security parameters, together
with the necessary key material. Note that MIKEY is exchanging the
crypto suite for both streams, as it is placed at the session level.
Also, MIKEY provides its own security, i.e., when Bob processes
Alice's MIKEY message, he will also find the signaling of the
security parameters used to secure the MIKEY exchange. Alice's
endpoint's authentication information is also carried within the
MIKEY message, to prove that the message is authentic. The above
MIKEY message is an example of message when the pre-shared method
MIKEY is used.
Upon receiving the offer, Bob checks the validity of the received
MIKEY message, and, in case of successful verification, he accepts
the offer and sends an answer back to Alice (with his authentication
information, and, if necessary, also some key material from his
side):
v=0
o=bob 2891092897 2891092897 IN IP4 foo.example.com
s=Cool stuff
e=bob@foo.example.com
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t=0 0
c=IN IP4 foo.example.com
a=key-mgmt:mikey AQEFgM0XflABAAAAAAAAAAAAAAYAyONQ6gAAAAAJAAAQbWlja2
V5QG1vdXNlLmNvbQABn8HdGE5BMDXFIuGEga+62AgY5cc=
m=audio 49030 RTP/SAVP 98
a=rtpmap:98 AMR/8000
m=video 52230 RTP/SAVP 31
a=rtpmap:31 H261/90000
Upon receiving the answer, Alice verifies the correctness of it. In
case of success, at this point Alice and Bob share the security
parameters and the keys needed for a secure RTP communication.
5.2. Example 2 (SDP)
This example shows what Alice would have done if she wished to
protect only the audio stream. She would have placed the MIKEY line
at media level for the audio stream only (also specifying the use of
the SRTP profile there, SAVP). The semantics of the MIKEY messages
is as in the previous case, but applies only to the audio stream.
v=0
o=alice 2891092738 2891092738 IN IP4 w-land.example.com
s=Cool stuff
e=alice@w-land.example.com
t=0 0
c=IN IP4 w-land.example.com
m=audio 49000 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAy...
m=video 52230 RTP/AVP 31
a=rtpmap:31 H261/90000
Bob would then act as described in the previous example, including
the MIKEY answer at the media level for the audio stream (as Alice
did).
Note that even if the key management attribute were specified at the
session level, the video part would not be affected by this (as a
security profile is not used, instead the RTP/AVP profile is
signaled).
5.3. 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
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CSeq: 312
Accept: application/sdp
From: user@example.com
The server sends back an OK message including an SDP description,
together with the MIKEY message. The MIKEY message contains the
necessary security parameters that the server is willing to offer to
the client, together with authentication information (to prove that
the message is authentic) and the key material. The SAVP profile
also signals the use of SRTP for securing the media sessions.
RTSP/1.0 200 OK
CSeq: 312
Date: 23 Jan 1997 15:35:06 GMT
Content-Type: application/sdp
Content-Length: 478
v=0
o=actionmovie 2891092738 2891092738 IN IP4 movie.example.com
s=Action Movie
e=action@movie.example.com
t=0 0
c=IN IP4 movie.example.com
a=control:rtsp://movie.example.com/action
a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAAAsAy...
m=audio 0 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=control:rtsp://movie.example.com/action/audio
m=video 0 RTP/SAVP 31
a=rtpmap:31 H261/90000
a=control:rtsp://movie.example.com/action/video
The client checks the validity of the received MIKEY message, and, in
case of successful verification, it accept the message. The client
then includes its key management data in the SETUP request going back
to the server, the client authentication information (to prove that
the message is authentic), and, if necessary, some key material.
SETUP rtsp://movie.example.com/action/audio RTSP/1.0
CSeq: 313
Transport: RTP/SAVP/UDP;unicast;client_port=3056-3057
keymgmt: prot=mikey; uri="rtsp://movie.example.com/action";
data="AQEFgM0XflABAAAAAAAAAAAAAAYAyONQ6g..."
The server processes the request including checking the validity of
the key management header.
Arkko, et al. Standards Track [Page 19]
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RTSP/1.0 200 OK
CSeq: 313
Session: 12345678
Transport: RTP/SAVP/UDP;unicast;client_port=3056-3057;
server_port=5000-5001
Note that in this case the key management line was specified at the
session level, and the key management information only goes into the
SETUP related to the first stream. The "uri" indicates to the server
that the context is for the whole aggregated session the key
management applies. The RTSP client then proceeds setting up the
second media (video) in aggregation with the audio. As the two media
are run in aggregation and the key context was established in the
first exchange, no more key management messages are needed.
5.4. Example 4 (RTSP)
The use of the MIKEY message at the media level would change the
previous example as follows.
The 200 OK would contain the two distinct SDP attributes for MIKEY at
the media level:
RTSP/1.0 200 OK
CSeq: 312
Date: 23 Jan 1997 15:35:06 GMT
Content-Type: application/sdp
Content-Length: 561
v=0
o=actionmovie 2891092738 2891092738 IN IP4 movie.example.com
s=Action Movie
e=action@movie.example.com
t=0 0
c=IN IP4 movie.example.com
a=control:rtsp://movie.example.com/action
m=audio 0 RTP/SAVP 98
a=rtpmap:98 AMR/8000
a=key-mgmt:mikey AQAFgM0XflABAAAAAAAAAAAAA...
a=control:rtsp://movie.example.com/action/audio
m=video 0 RTP/SAVP 31
a=rtpmap:31 H261/90000
a=key-mgmt:mikey AQAFgM0AdlABAAAAAAAAAAAAA...
a=control:rtsp://movie.example.com/action/video
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Each RTSP header is inserted in the SETUP related to the audio and
video separately:
SETUP rtsp://movie.example.com/action/audio RTSP/1.0
CSeq: 313
Transport: RTP/SAVP/UDP;unicast;client_port=3056-3057
keymgmt: prot=mikey; uri="rtsp://movie.example.com/action/audio";
data="AQEFgM0XflABAAAAAAAAAAAAA..."
and similarly for the video session:
SETUP rtsp://movie.example.com/action/video RTSP/1.0
CSeq: 315
Transport: RTP/SAVP/UDP;unicast;client_port=3058-3059
keymgmt: prot=mikey; uri="rtsp://movie.example.com/action/video";
data="AQEFgM0AdlABAAAAAAAAAAAAAA..."
Note: The "uri" parameter could be excluded from the two SETUP
messages in this example.
6. 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 4. In addition to this, the following needs to
be defined:
* The key management protocol identifier to be used as the protocol
identifier should be registered at IANA according to Section 9.
* The information that the key management needs from SDP and RTSP,
and vice versa, as described in Section 4. The exact API is
implementation specific, but it MUST at least support the exchange
of the specified information.
* The key management protocol to be added MUST be such that the
processing in Section 4 (describing its interactions with SDP and
RTSP) can be applied. Note in particular, Section 4.1.4 requires
each key management protocol to specify how the list of protocol
identifiers is authenticated inside that key management protocol.
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.
Finally, it is obviously crucial to analyze possible security
implications induced by the introduction of a new key management
protocol in the described framework.
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Today, the MIKEY protocol [MIKEY] has adopted the key management
extensions to work together with SIP and RTSP (see Section 7). Other
protocols MAY use the described attribute and header, e.g., Kerberos
[KERB]; however, this is subject to future standardization.
7. Integration of MIKEY
[MIKEY] describes a key management protocol for real-time
applications (both for peer-to-peer communication and group
communication). MIKEY carries the security parameters needed for
setting up the security protocol (e.g., SRTP) protecting the media
stream. MIKEY can be integrated within SDP and RTSP, following the
rules and guidelines described in this document.
MIKEY satisfies the requirements described in Section 4. The MIKEY
message is formed as defined in [MIKEY], then passed from MIKEY to
the SDP application that base64 encodes it, and encapsulates it in
the keymgmt-data attribute. The examples in Section 5 use MIKEY,
where the semantics of the exchange is also briefly explained.
The key management protocol identifier (KMPID) to be used as the
protocol identifier SHALL be "mikey" and is registered at IANA; see
Section 9 for details.
The information that the key management needs from SDP and RTSP, and
vice versa, follows Section 4. To avoid bidding-down attacks, the
directives in Section 4.1.4 are followed. The list of protocol
identifiers is authenticated within MIKEY by placing the list in a
General Extension Payload (of type "SDP IDs", [MIKEY]), which then
automatically will be integrity protected/signed. The receiver SHALL
then match the list in the General Extension Payload with the list
included in SDP and SHOULD (according to policy) if they differ, or
if integrity/signature verification fails, reject the offer.
The server will need to be able to know the identity of the client
before creating and sending a MIKEY message. To signal the (MIKEY)
identity of the client to the server in the DESCRIBE, it is
RECOMMENDED to include the From header field in RTSP. Other methods
to establish the identity could be using the IP address or retrieving
the identity from the RTSP authentication if used.
7.1. MIKEY Interface
This subsection describes some aspects, which implementers SHOULD
consider. If the MIKEY implementation is separate from the
SDP/SIP/RTSP, an application programming interface (API) between
MIKEY and those protocols is needed with certain functionality
(however, exactly what it looks like is implementation dependent).
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The following aspects need to be considered:
* the possibility for MIKEY to receive information about the sessions
negotiated. This is to some extent implementation dependent. But
it is RECOMMENDED that, in the case of SRTP streams, the number of
SRTP streams is included (and the direction of these). It is also
RECOMMENDED to provide the destination addresses and ports to
MIKEY. When referring to streams described in SDP, MIKEY SHALL
allocate two consecutive numbers for the related Crypto Session
indexes (as each stream can be bi-directional). An example: if the
SDP contains two m lines (specifying whatever direction of the
streams), and MIKEY is at the session level, then MIKEY allocates,
e.g., the Crypto Sessions Identifiers (CS IDs; see [MIKEY]) '1' and
'2' for the first m line, and '3' and '4' for the second m line.
* the possibility for MIKEY to receive incoming MIKEY messages and
return a status code from/to the SIP/RTSP application.
* the possibility for the SIP or RTSP applications to receive
information from MIKEY. This would typically include the receiving
of the Crypto Session Bundle Identifier (CSB ID; see [MIKEY], to
later be able to identify the active MIKEY session), and the SSRCs
and the rollover counter (ROC; see [SRTP]) for SRTP usage. It is
also RECOMMENDED that extra information about errors can be
received.
* the possibility for the SIP or RTSP application to receive outgoing
MIKEY messages.
* the possibility to tear down a MIKEY CSB (e.g., if the SIP session
is closed, the CSB SHOULD also be closed).
8. Security Considerations
The framework for transfer of key management data as described here
is intended to provide the security parameters for the end-to-end
protection of the media session. It is furthermore good practice to
secure the session setup (e.g., SDP, SIP, RTSP, SAP). However, it
might be that the security of the session setup is not possible to
achieve end-to-end, but only hop-by-hop. For example, SIP requires
intermediate proxies to have access to part of the SIP message, and
sometimes also to the SDP description (cf. [E2M]), although end-to-
end confidentiality can hide bodies from intermediaries. General
security considerations for the session setup can be found in SDP
[SDPnew], SIP [SIP], and RTSP [RTSP]. The framework defined in this
memo is useful when the session setup is not protected in an end-to-
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end fashion, but the media streams need to be end-to-end protected;
hence the security parameters (such as keys) are not wanted revealed
to or manipulated by intermediaries.
The security will also depend on the level of security the key
management protocol offers. It follows that, under the assumption
that the key management schemes are secure, the SDP can be passed
along unencrypted without affecting the key management as such, and
the media streams will still be secure even if some attackers gained
knowledge of the SDP contents. Further security considerations can
be found for each key management protocol (for MIKEY these can be
found in [MIKEY]). However, if the SDP messages are not sent
integrity protected 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. Even if the
key management protocol does not rely on parameters of SDP and will
not be affected by manipulation of these, different denial-of-service
(DoS) attacks aimed at SDP may lead to undesired interruption in the
setup. See also the attacks described at the end of this section.
The only integrity-protected attribute of the media stream is, in the
framework proposed here, the set of key management protocols. For
instance, it is possible to (1) swap key management offers across SDP
messages, or (2) inject a previous key management offer into a new
SDP message. Making the (necessary) assumption that all involved key
management protocols are secure, the second attack will be detected
by replay protection mechanisms of the key management protocol(s).
Making the further assumption that, according to normal best current
practice, the production of each key management offer is done with
independent (pseudo)random choices (for session keys and other
parameters), the first attack will either be detected in the
Responder's (now incorrect) verification reply message (if such is
used) or be a pure DoS attack, resulting in Initiator and Responder
using different keys.
It is RECOMMENDED for the identity at the SPD level to be the one
authenticated at the key management protocol level. However, this
might need to keep into consideration privacy aspects, which are out
of scope for this framework.
The use of multiple key management protocols in the same offer may
open up the possibility of a bidding-down attack, as specified in
Section 4.1.4. To exclude such possibility, the authentication of
the protocol identifier list is used. Note though, that the security
level of the authenticated protocol identifier will be as high (or
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low), as the "weakest" protocol. Therefore, the offer MUST NOT
contain any security protocols (or configurations thereof) weaker
than permitted by local security policy.
Note that it is impossible to ensure the authenticity of a declined
offer, since even if it comes from the true respondent, the fact that
the answerer declines the offer usually means that he 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. Note also that the declined offer could be the
result of an attacker who sits on the path and removes all the key
management offers. The bidding-down attack prevention, as described
above, would not work in this case (as the answerer receives no key
management attribute). Also, here it is impossible to ensure the
authenticity of a declined offer, though here the reason is the
"peeling-off" attack. It is up to the local policy to decide the
behavior in the case that the response declines any security
(therefore, there is impossibility of authenticating it). For
example, if the local policy requires a secure communication and
cannot accept an unsecured one, then the session setup SHALL be
aborted.
9. IANA Considerations
9.1. SDP Attribute Registration
The IANA has created a new subregistry for the purpose of key
management protocol integration with SDP.
SDP Attribute Field ("att-field"):
Name: key-mgmt-att-field
Long form: key management protocol attribute field
Type of name: att-field
Type of attribute: Media and session level
Purpose: See RFC 4567, Section 3.
Reference: RFC 4567, Section 3.1
Values: See RFC 4567, Sections 3.1 and 9.3.
Arkko, et al. Standards Track [Page 25]
RFC 4567 Key Management Extensions for SDP and RTSP July 2006
9.2. RTSP Registration
The IANA has created a new subregistry for the purpose of key
management protocol integration with RTSP.
Following the guidelines of [RTSP], the registration is defined as
follows:
Header name: keymgmt
Header syntax: see RFC 4567, Section 3.2
Intended usage: see RFC 4567, Section 3.2
Proxy treatment: Proxies SHALL NOT add, change, or delete the
header. The proxy does not need to read this
header.
Purpose: see RFC 4567, Section 3
The RTSP Status-Code "463" (RFC 4567), with the default string "Key
management failure", needs to be registered.
9.3. Protocol Identifier Registration
This document defines one new name space, the "SDP/RTSP key
management protocol identifier", associated with the protocol
identifier, KMPID, defined in Section 3 to be used with the above
registered attributes in SDP and RTSP.
The IANA has created a new subregistry for the KMPID parameter, with
the following registration created initially: "mikey".
Value name: mikey
Long name: Multimedia Internet KEYing
Purpose: Usage of MIKEY with the key-mgmt-att-field
attribute and the keymgmt RTSP header
Reference: Section 7 in RFC 3830
Note that this registration implies that the protocol identifier,
KMPID, name space will be shared between SDP and RTSP.
Further values may be registered according to the "Specification
Required" policy as defined in [RFC2434]. Each new registration
needs to indicate the parameter name, and register it with IANA.
Note that the parameter name is case sensitive, and it is RECOMMENDED
that the name be in lowercase letters. For each new registration, it
is mandatory that a permanent, stable, and publicly accessible
document exists that specifies the semantics of the registered
parameter and the requested details of interaction between the key
management protocol and SDP, as specified in RFC 4567.
Arkko, et al. Standards Track [Page 26]
RFC 4567 Key Management Extensions for SDP and RTSP July 2006
New values MUST be registered with IANA. Registrations SHALL include
the following information:
* Contact: the contact name and email address
* Value name: the name of the value being registered (which MUST
comply with the KMPID as defined in Section 3)
* Long Name: long-form name in English
* Purpose: short explanation of the purpose of the registered name.
* Reference: a reference to the specification (e.g., RFC number)
providing the usage guidelines in accordance to Section 6 (and also
complying to the specified requirements).
10. Acknowledgements
The authors would like to thank Francois Audet, Rolf Blom, Johan
Bilien, Magnus Brolin, Erik Eliasson, Martin Euchner, Steffen Fries,
Joerg Ott, Jon Peterson, and Jon-Olov Vatn. A special thanks to
Colin Perkins and Magnus Westerlund, who contributed in many
sections.
11. References
11.1. Normative References
[MIKEY] Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
August 2004.
[OAM] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264, June
2002.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RFC4234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 4234, October 2005.
Arkko, et al. Standards Track [Page 27]
RFC 4567 Key Management Extensions for SDP and RTSP July 2006
[RTSP] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[SDPnew] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[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.
11.2. Informative References
[E2M] Ono, K. and S. Tachimoto, "Requirements for End-to-Middle
Security for the Session Initiation Protocol (SIP)", RFC
4189, October 2005.
[KERB] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
[RFC3851] Ramsdell, B., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, July 2004.
[SDES] Andreasen, F., Baugher, M., and D. Wing, "Session
Description Protocol (SDP) Security Descriptions for Media
Streams", RFC 4568, July 2006.
[SPREC] Andreasen, F., Baugher, M., and Wing, D., "Security
Preconditions for Session Description Protocol Media
Streams", Work in Progress, October 2005.
[SRTP] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
Arkko, et al. Standards Track [Page 28]
RFC 4567 Key Management Extensions for SDP and RTSP July 2006
Authors' Addresses
Jari Arkko
Ericsson
02420 Jorvas
Finland
Phone: +358 40 5079256
EMail: jari.arkko@ericsson.com
Elisabetta Carrara
Royal Institute of Technology
Stockholm
Sweden
EMail: carrara@kth.se
Fredrik Lindholm
Ericsson
SE-16480 Stockholm
Sweden
Phone: +46 8 58531705
EMail: fredrik.lindholm@ericsson.com
Mats Naslund
Ericsson Research
SE-16480 Stockholm
Sweden
Phone: +46 8 58533739
EMail: mats.naslund@ericsson.com
Karl Norrman
Ericsson Research
SE-16480 Stockholm
Sweden
Phone: +46 8 4044502
EMail: karl.norrman@ericsson.com
Arkko, et al. Standards Track [Page 29]
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Full Copyright Statement
Copyright (C) The Internet Society (2006).
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.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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Arkko, et al. Standards Track [Page 30]