INTERNET-DRAFT Joerg Ott
Helsinki University of Technology
draft-ietf-avt-profile-savpf-06.txt Elisabetta Carrara
KTH
June 2006
Expires December 2006
Extended Secure RTP Profile for RTCP-based Feedback (RTP/SAVPF)
Status of this Memo
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Abstract
An RTP profile (SAVP) is defined for secure real-time
communications, and another profile (AVPF) is specified to provide
timely feedback from the receivers to a sender. This memo defines
the combination of both profiles to enable secure RTP
communications with feedback.
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Table of Contents
1 Introduction..................................................2
1.1 Definitions..............................................3
1.2 Terminology..............................................4
2 SAVPF Rules...................................................4
2.1 Packet Formats...........................................5
2.2 Extensions...............................................5
2.3 Implications from combining AVPF and SAVP................5
3 SDP Definitions...............................................6
3.1 Profile Definition.......................................6
3.2 Attribute Definitions....................................6
3.3 Profile Negotiation......................................6
3.3.1 Offer/Answer-based Negotiation of Session Descriptions.6
3.3.2 RTSP-based Negotiation of Session Descriptions.........7
3.3.3 Announcing Session Descriptions........................8
3.3.4 Describing Alternative Session Profiles................9
3.4 Examples.................................................9
4 Interworking of AVP, SAVP, AVPF, and SAVPF Entities..........14
5 Security Considerations......................................14
6 IANA Considerations..........................................15
7 Acknowledgements.............................................16
8 Authors' Addresses...........................................16
9 Bibliography.................................................16
9.1 Normative references....................................16
9.2 Informative References..................................17
10 IPR Notice...................................................17
11 Disclaimer of Validity.......................................18
12 Full Copyright Statement.....................................18
13 Acknowledgment...............................................18
1 Introduction
The Real-time Transport Protocol (RTP/RTCP) [1] and the associated
profile for audiovisual communications with minimal control [2]
define mechanisms for transmitting time-based media across an IP
network. RTP provides means to preserve timing and detect packet
losses, among other things, and RTP payload formats provide for
proper framing of (continuous) media in a packet-based environment.
RTCP enables receivers to provide feedback on reception quality and
allows all members of an RTP session to learn about each other.
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The RTP specification provides only rudimentary support for
encrypting RTP and RTCP packets. SRTP [4] defines an RTP profile
("SAVP") for secure RTP media sessions, defining methods for proper
RTP and RTCP packet encryption, integrity and replay protection.
The initial negotiation of SRTP and its security parameters needs
to be done out of band, using e.g. the Session Description Protocol
(SDP) [6] together with extensions for conveying keying material
[7][8].
The RTP specification also provides limited support for timely
feedback from receivers to senders, typically by means of reception
statistics reporting in somewhat regular intervals depending on the
group size, the average RTCP packet size, and the available RTCP
bandwidth. The extended RTP profile for RTCP-based feedback
("AVPF") [3] allows session participants statistically to provide
immediate feedback while maintaining the average RTCP data rate for
all senders. As for SAVP, the use of AVPF and its parameters needs
to be negotiated out-of-band by means of SDP [6] and the extensions
defined in [3].
Both SRTP and AVPF are RTP profiles and need to be negotiated.
This implies that either one or the other may be used, but both
profiles cannot be negotiated for the same RTP session (using one
SDP session level description). However, using secure
communications and timely feedback together is desirable.
Therefore, this document specifies a new RTP profile ("SAVPF") that
combines the features of SAVP and AVPF.
As SAVP and AVPF are largely orthogonal, the combination of both is
mostly straightforward. No sophisticated algorithms need to be
specified in this document. Instead, reference is made to both
existing profiles and only the implications of their combination
and possible deviations from rules of the existing profiles are
described as is the negotiation process.
1.1 Definitions
The definitions of [1], [2], [3], and [4] apply.
The following definitions are specifically used in this document:
RTP session:
An association among a set of participants communicating with
RTP as defined in [1].
(SDP) media description:
This term refers to the specification given in a single m=
line in an SDP message. An SDP media description may define
only one RTP session. Grouping of m= lines in SDP may cause
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several SDP session level descriptions to define (alternatives
of) the same RTP session for the same media type.
Media session:
A media session refers to a collection of SDP media
descriptions that are semantically grouped to represent
alternatives of the same communications means. Out of such a
group, one will be negotiated or chosen for a communication
relationship and the corresponding RTP session will be
instantiated. Or the media session will be rejected.
In the simplest case, a media session is equivalent to an SDP
media description and equivalent to an RTP session.
1.2 Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
in this document are to be interpreted as described in RFC 2119
[5].
2 SAVPF Rules
SAVP is defined as an intermediate layer between RTP (following the
regular RTP profile AVP) and the transport layer (usually UDP).
This yields a two layer hierarchy within the Real-time Transport
Protocol. In SAVPF, the upper (AVP) layer is replaced by the
extended RTP profile for feedback (AVPF).
AVPF modifies timing rules for transmitting RTCP packets and adds
extra RTCP packet formats specific to feedback. These functions
are independent of whether or not RTCP packets are subsequently
encrypted and/or integrity protected. The functioning of the AVPF
layer remains unchanged in SAVPF.
The AVPF profile derives from [1] the (optional) use of the
encryption prefix for RTCP. The encryption prefix MUST NOT be used
within the SAVPF profile (it is not used in SAVP, as it is only
applicable to the encryption method specified in [1]).
The SAVP part uses extra fields added to the end of RTP and RTCP
packets and executes cryptographic transforms on (some of) the
RTP/RTCP packet contents. This behavior remains unchanged in
SAVPF. The average RTCP packet size calculation done by the AVPF
layer for timing purposes MUST take into account the fields added
by the SAVP layer.
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The SRTP part becomes only active whenever the RTP or RTCP was
scheduled by the "higher" AVPF layer or received from the transport
protocol, irrespective of its timing and contents.
2.1 Packet Formats
AVPF defines extra packet formats to provide feedback information.
Those extra packet formats defined in [3] (and further ones defined
elsewhere for use with AVPF) MAY be used with SAVPF.
SAVP defines a modified packet format for SRTP and SRTCP packets
that essentially consists of the RTP/RTCP packet formats plus some
trailing protocol fields for security purposes. For SAVPF, all
RTCP packets MUST be encapsulated as defined in section 3.4 of [4].
2.2 Extensions
Extensions to AVPF RTCP feedback packets defined elsewhere MAY be
used with the SAVPF profile provided that those extensions are in
conformance with the extension rules of [3].
Additional extensions (e.g., transforms) defined for SAVP following
the rules of section 6 of [4] MAY also be used with the SAVPF
profile. The overhead per RTCP packet depends on the extensions
and transforms chosen. New extensions and transforms added in the
future MAY introduce yet unknown further per-packet overhead.
Finally, further extensions specifically to SAVPF MAY be defined
elsewhere.
2.3 Implications from combining AVPF and SAVP
The AVPF profile aims at -- statistically -- allowing receivers to
provide timely feedback to senders. The frequency at which
receivers are, on average, allowed to send feedback information
depends on the RTCP bandwidth, the group size, and the average size
of an RTCP packet. SRTCP (see Section 3.4 of [4]) adds extra
fields (some of which are of configurable length) at the end of
each RTCP packet that are probably at least some 10 to 20 bytes in
size (14 bytes as default). Note that extensions and transforms
defined in the future, as well as the configuration of each field
length, MAY add greater overhead. By using SRTP, the average size
of an RTCP packet will increase and thus reduce the frequency at
which (timely) feedback can be provided. Application designers
need to be aware of this, and take precautions so that the RTCP
bandwidth shares are maintained. This MUST be done by adjusting
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the RTCP variable "avg_rtcp_size" to reflect the size of the SRTCP
packets.
3 SDP Definitions
3.1 Profile Definition
The AV profiles defined in [2], [3], and [4] are referred to as
"AVP", "AVPF", and "SAVP", respectively, in the context of e.g. the
Session Description Protocol (SDP) [3]. The combined profile
specified in this document is referred to as "SAVPF".
3.2 Attribute Definitions
SDP attributes for negotiating SAVP sessions are defined in [7] and
[8]. Those attributes MAY also be used with SAVPF. The rules
defined in [7] and [8] apply.
SDP attributes for negotiating AVPF sessions are defined in [3].
Those attributes MAY also be used with SAVPF. The rules defined in
[3] apply.
3.3 Profile Negotiation
Session descriptions for RTP sessions may be conveyed using
protocols dedicated for multimedia communications such as the SDP
offer/answer model [10] used with SIP, RTSP [11], or SAP [12] but
may also be distributed using email, NetNews, web pages, etc.
The offer/answer model allows the resulting session parameters to
be negotiated using the SDP attributes defined in [7] and [8]. In
the following subsection, the negotiation process is described in
terms of the offer/answer model.
Afterwards, the cases that do not use the offer/answer model are
addressed: RTSP-specific negotiation support is provided by [7] as
discussed in subsection 3.3.2 and support for SAP announcements
(with no negotiation at all) is addressed in subsection 3.3.3.
3.3.1 Offer/Answer-based Negotiation of Session Descriptions
Negotiations (e.g. of RTP profiles, codecs, transport addresses,
etc.) are carried out on a per-media session basis (i.e. per m=
line in SDP). If negotiating one media session fails, others MAY
still succeed.
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Different RTP profiles MAY be used in different media sessions.
For negotiation of a media description, the four profiles AVP,
AVPF, SAVP, and SAVPF are mutually exclusive. Note, however, that
SAVP and SAVPF entities MAY be mixed in a single RTP session (see
section 4). Also, the offer/answer mechanism MAY be used to offer
alternatives for the same media session (e.g. using the same
transport parameters) and allow the answered to choose one of the
profiles.
An offerer that is capable of supporting multiple of these profiles
for a certain media session SHOULD always offer all alternatives
acceptable in a certain situation. The alternatives SHOULD be
listed in order of preference and the offerer SHOULD prefer secure
profiles over non-secure ones. The offer SHOULD NOT include both a
secure alternative (SAVP and SAVPF) and an insecure alternative
(e.g. AVP and AVPF) in the same offer (as this may open up for
bidding down attacks) unless the signaling channel is protected
against modification of SDP messages.
If an offer contains multiple alternative profiles the answerer
MUST select exactly only one and reject the other(s), e.g., by
setting the port number in the respect m= line to 0. If supported,
the answerer SHOULD prefer a secure profile over non-secure ones.
Among the secure or insecure profiles, the answerer SHOULD select
the first acceptable alternative to respect the preference of the
offerer.
If a media description in an offer uses SAVPF and the answerer does
not support SAVPF, the media session MUST be rejected.
If a media description in an offer does not use SAVPF but the
answerer wants to use SAVPF, the answerer MUST reject the media
session. The answerer MAY provide a counter-offer with a media
description indicating SAVPF in a subsequently initiated
offer/answer exchange.
3.3.2 RTSP-based Negotiation of Session Descriptions
RTSP [11] does not support the offer/answer model. However, RTSP
supports exchanging media session parameters (including profile and
address information) by means of the "Transport:" header. SDP-
based key management as defined in [7] adds an RTSP header
(KeyMgmt:) to support conveying a key management protocol
(including keying material).
The RTSP "Transport:" header MAY be used to determine the profile
for the media session. Conceptually, the rules defined in section
3.3.1 apply accordingly. Detailed operation is as follows:
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An SDP description (e.g., retrieved from the RTSP server by means
of DESCRIBE) contains the description of the media streams of the
particular RTSP resource. To offer two or more alternative RTP
profiles for a particular media stream, the SDP description MUST
contain exactly one m= line for this media stream for each profile
and all the same transport address (IP address and port number).
The profiles SHOULD be listed in the order of preference. This is
similar to formulating an offer per section 3.3.1.
The RTSP client MUST select exactly one of the profiles per media
stream it wants to receive. It MUST do so in the SETUP request.
The RTSP client MUST indicate the chosen RTP profile by indicating
the profile and the full server transport address (IP address and
port) in the Transport: header included in the SETUP request. The
RTSP server's response to the client's SETUP message MUST confirm
this profile selection or refuse the SETUP request (the latter of
which it should not do after offering the profiles in the first
place).
Note: To change any of the profiles in use, the client needs
tear down the RTSP session (using the TEARDOWN method) and re-
establish it using SETUP. This may change as soon as media
updating (similar to a SIP UPDATE or re-INVITE) becomes
specified.
When using the SDP key management [7], the keymgmt: header MUST be
included in the appropriate RTSP messages if a secure profile is
chosen. If different secure profiles are offered in the SDP
description (e.g., SAVP and SAVPF) and different keying material is
provided for these, after choosing one profile in the SETUP
message, only the keymgmt: header for the chosen one MUST be
provided. The rules for matching keymgmt: headers to media streams
according to [7] apply.
3.3.3 Announcing Session Descriptions
Protocols that do not allow negotiate session descriptions
interactively (e.g. SAP [12], descriptions posted on a web page or
sent by mail) pose the responsibility for adequate access to the
media sessions on the initiator of a session.
The initiator SHOULD provide alternative session descriptions for
multiple RTP profiles as far as acceptable to the application and
the purpose of the session. If security is desired, SAVP may be
offered as alternative to SAVPF -- but AVP or AVPF sessions SHOULD
NOT be announced unless other security means not relying on SRTP
are employed.
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The SDP attributes defined in [7] and [8] may also be used for the
security parameter distribution of announced session descriptions.
The security scheme description defined in [8] requires a secure
communications channel to prevent third parties from eavesdropping
on the keying parameters and manipulation. Therefore, SAP security
(as defined in [12]), S/MIME [13], HTTPS [14], or other suitable
mechanisms SHOULD be used for distributing or accessing these
session descriptions.
3.3.4 Describing Alternative Session Profiles
SAVP and SAVPF entities MAY be mixed in the same RTP session (see
also section 4) and so MAY AVP and AVPF entities. Other
combinations -- i.e. between secure and insecure profiles -- in the
same RTP session are incompatible and MUST NOT be used together.
If negotiation between the involved peers is possible (as with the
offer/answer model per section 3.3.1 or RTSP per section 3.3.2),
alternative (secure and non-secure) profiles MAY be specified by
one entity (e.g., the offerer) and a choice of one profile MUST be
made by the other. If no such negotiation is possible (e.g., with
SAP as per section 3.3.3) incompatible profiles MUST NOT be
specified as alternatives.
If alternative RTP profiles are to be specified for a single RTP
session, each alternative MUST be included on a separate media line
in SDP. With basic SDP, for the same RTP session, all these media
lines SHOULD use the same address and port numbers to indicate that
they are alternatives. It is RECOMMENDED, however, that these
media sessions be grouped by using the mechanism defined in [9] to
indicate semantic equivalence between the individual sessions and
ensure that any receiver only joins one of them.
Editor's note: An appropriate grouping mechanism still has to
be defined as FID [9] does not really match the semantics, nor
does ANAT. An Alternative Profile (AP) might be the correct
approach. This could be defined in a separate draft. So far,
we rely exclusively on the implicit grouping.
RTP profiles MAY be mixed arbitrarily across different RTP
sessions.
3.4 Examples
Example 1: The following session description indicates a secure
session made up from audio and DTMF for point-to-point
communication in which the DTMF stream uses Generic NACKs. The key
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management protocol indicated is MIKEY. This session description
(the offer) could be contained in a SIP INVITE or 200 OK message to
indicate that its sender is capable of and willing to receive
feedback for the DTMF stream it transmits. The corresponding
answer may be carried in a 200 OK or an ACK. The parameters for
the security protocol are negotiated as described by the SDP
extensions defined in [7].
v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Media with feedback
t=0 0
c=IN IP4 host.example.com
m=audio 49170 RTP/SAVPF 0 96
a=rtpmap:0 PCMU/8000
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-16
a=rtcp-fb:96 nack
a=key-mgmt:mikey uiSDF9sdhs727ghsd/dhsoKkdOokdo7eWsnDSJD...
Example 2: This example shows the same feedback parameters as
example 1 but uses the secure descriptions syntax [8]. Note that
the key part of the a=crypto attribute is not protected against
eavesdropping and thus the session description needs to be
exchanged over a secure communication channel.
v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Media with feedback
t=0 0
c=IN IP4 host.example.com
m=audio 49170 RTP/SAVPF 0 96
a=rtpmap:0 PCMU/8000
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-16
a=rtcp-fb:96 nack
a=crypto:AES_CM_128_HMAC_SHA1_32
inline:d/16/14/NzB4d1BINUAvLEw6UzF3WSJ+PSdFcGdUJShpX1Zj/2^20/1
:32
Example 3: This example is replicated from example 1 above but
shows the interaction between the offerer and the answered in an
offer/answer exchange, again using MIKEY to negotiate the keying
material:
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Offer:
v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Media with feedback
t=0 0
c=IN IP4 host.example.com
a=key-mgmt:mikey uiSDF9sdhs727ghsd/dhsoKkdOokdo7eWsnDSJD...
m=audio 49170 RTP/SAVPF 0 96
a=rtpmap:0 PCMU/8000
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-16
a=rtcp-fb:96 nack
Answer:
v=0
o=alice 3203093521 3203093521 IN IP4 host.another.example.com
s=Media with feedback
t=0 0
c=IN IP4 host.another.example.com
a=key-mgmt:mikey ushdgfdhgfuiweyfhjsgdkj2837do7eWsnDSJD...
m=audio 53012 RTP/SAVPF 0 96
a=rtpmap:0 PCMU/8000
a=rtpmap:96 telephone-event/8000
a=fmtp:96 0-16
a=rtcp-fb:96 nack
Example 4: This example shows the exchange for video streaming
controlled via RTSP. A client inquires a media description from a
server using DESCRIBE and obtains a static SDP description without
any keying parameters but the media description shows that both
secure and non-secure media sessions using (S)AVPF are available.
Both are marked as alternatives by using the same port number
(different from 0). The client then issues a SETUP request and
indicates its choice by including the respective profile in the
Transport parameter. Furthermore, the client includes a KeyMgmt:
header to convey its security parameters which is matched by a
corresponding KeyMgmt header from the server in the response. Only
a single media session is chosen so that the aggregate RTSP URI is
sufficient for identification; if several media sessions shall use
different keying material, the a=control: attribute needs to be
included in the SDP description.
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RTSP DESCRIBE request-response pair (optional):
DESCRIBE rtsp://movies.example.org/example RTSP/2.0
CSeq: 314
Accept: application/sdp
200 OK
CSeq: 314
Date: 25 Nov 2005 22:09:35 GMT
Content-Type: application/sdp
Content-Length: 316
v=0
o=alice 3203093520 3203093520 IN IP4 movies.example.com
s=Media with feedback
t=0 0
c=IN IP4 0.0.0.0
m=video 49170 RTP/SAVPF 96
a=rtpmap:96 H263-2000/90000
a=rtcp-fb:96 nack
m=video 49170 RTP/AVPF 96
a=rtpmap:96 H263-2000/90000
a=rtcp-fb:96 nack
RTSP SETUP request-response pair
SETUP rtsp://movies.example.org/example RTSP/2.0
CSeq: 315
Transport: RTP/SAVPF;unicast;dest_addr=":53012"/":53013"
KeyMgmt: prot=mikey;url="rtsp://movies.example.org/example";
data="uiSDF9sdhs727ghsd/dhsoKkdOokdo7eWsnD..."
200 OK
CSeq: 315
Date: 25 Nov 2005 22:09:36 GMT
Session: 4711
Transport: RTP/SAVPF;unicast;dest_addr=":53012"/":53013";
src_addr="10.0.12.15:60000"/"10.0.12.15:60001"
KeyMgmt: prot=mikey;url="rtsp://movies.example.org/example";
data="ushdgfdhgfuiweyfhjsgdkj2837do7eWsnDSJD..."
Accept-Ranges: NPT, SMPTE
Example 5: The following session description indicates a multicast
audio/video session (using PCMU for audio and either H.261 or
H.263+) with the video source accepting Generic NACKs for both
codecs and Reference Picture Selection for H.263. The parameters
for the security protocol are negotiated as described by the SDP
extensions defined in [7], used at the session level. Such a
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description may have been conveyed using the Session Announcement
Protocol (SAP).
v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Multicast video with feedback
t=3203130148 3203137348
a=key-mgmt:mikey uiSDF9sdhs7494ghsd/dhsoKkdOokdo7eWsnDSJD...
m=audio 49170 RTP/SAVP 0
c=IN IP4 224.2.1.183
a=rtpmap:0 PCMU/8000
m=video 51372 RTP/SAVPF 98 99
c=IN IP4 224.2.1.184
a=rtpmap:98 H263-1998/90000
a=rtpmap:99 H261/90000
a=rtcp-fb:* nack
a=rtcp-fb:98 nack rpsi
Example 5: The following session description defines the same media
session as example 4 but allows for mixed mode operation of SAVP
and SAVPF RTP entities (see also next section). Two m= lines are
needed to convey information about both applicable RTP profiles
which bears the risk for misinterpretation by the receiver (see
below). The parameters for the security protocol are negotiated as
described by SDP extensions defined in [7], used at the session
level.
v=0
o=alice 3203093520 3203093520 IN IP4 host.example.com
s=Multicast video with feedback
t=3203130148 3203137348
a=key-mgmt:mikey uiSDF9sdhs7854ghsd/dhsoKkdOokdo7eWsnDSJD...
m=audio 49170 RTP/SAVP 0
c=IN IP4 224.2.1.183
a=rtpmap:0 PCMU/8000
m=video 51372 RTP/SAVP 98 99
c=IN IP4 224.2.1.184
a=rtpmap:98 H263-1998/90000
a=rtpmap:99 H261/90000
m=video 51374 RTP/SAVPF 98 99
c=IN IP4 224.2.1.184
a=rtpmap:98 H263-1998/90000
a=rtpmap:99 H261/90000
a=rtcp-fb:* nack
a=rtcp-fb:98 nack rpsi
Note that this session description does not explicitly convey that
the two video streams are meant as alternatives (rather than coming
from two different complementary video sources). The two m= lines
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SHOULD be grouped by some appropriate mechanism to indicate that
both are alternatives actually conveying the same contents. The
basis for sample mechanism by which this can be achieved is defined
in [9]; however, specific semantics for indicating alternative
profiles need to be defined in a separate document which is
presently developed in the MMUSIC WG.
4 Interworking of AVP, SAVP, AVPF, and SAVPF Entities
The SAVPF profile defined in this document is a combination of the
SAVP profile [4] and the AVPF profile [3](which in turn is an
extension of the RTP profile as defined in [2]).
SAVP and SAVPF use SRTP [4] to achieve security. AVP and AVPF use
plain RTP [1] and hence do not provide security (unless external
security mechanisms are applied as discussed in section 9.1 of
[1]). SRTP and RTP are not meant to interoperate, the respective
protocol entities are not supposed to be part of the same RTP
session. Hence, AVP and AVPF on one side and SAVP and SAVPF on the
other MUST NOT be mixed.
RTP entities using the SAVP and the SAVPF profiles MAY be mixed in
a single RTP session. The interworking considerations defined in
section 5 of [3] apply.
5 Security Considerations
The SAVPF profile inherits its security properties from the SAVP
profile; therefore it is subject to the security considerations
discussed in [4]. The SAVP profile does not add, nor take away,
any security services compared to SAVP.
There is a desire to support security for media streams and, at the
same time, for backward compatibility with non-SAVP(F) nodes.
Application designers should be aware that security SHOULD NOT be
traded for interoperability. If information is to be distributed
to closed groups (i.e. confidentially protected), it is RECOMMENDED
not to offer alternatives for a media session other than SAVP and
SAVPF as described in sections 3.3 and 3.4, unless other security
mechanisms will be used, e.g. the ones described in Section 9.1 of
[1]. Similarly, if integrity protection is considered important, it
is RECOMMENDED not to offer the alternatives other than SAVP and
SAVPF, unless other mechanisms are known to be in place that can
guarantee it, e.g. lower-layer mechanisms as described in Section 9
of [1].
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Offering secure and insecure profiles simultaneously may open to
bidding down attacks. Therefore, such a mix of profile offer SHOULD
NOT be made.
Note that the rules for sharing master keys apply as described in
[4] (e.g., Section 9.1). In particular, the same rules for avoiding
the two-time pad (keystream reuse) apply: a master key MUST NOT be
shared among different RTP sessions if the SSRCs used are unique
across all the RTP streams of the RTP sessions that share the same
master key.
The key management MUST be called to provide new master key(s)
(previously stored and used keys MUST NOT be used again), or the
session MUST be terminated, when 2^48 SRTP packets or 2^31 SRTCP
packets have been secured with the same key (whichever occurs
before).
Different media sessions may use a mix of different profiles,
particularly including a secure profile and an insecure profile.
However, mixing secure and insecure media sessions may reveal
information to third parties and thus the decision to do so MUST be
in line with a local security policy. For example, the local
policy MUST specify whether it is acceptable to have e.g. the audio
stream not secured and the related video secured.
The security considerations in [3] are valid too. Note in
particular, applying the SAVPF profile implies mandatory integrity
protection on RTCP. While this solves the problem of false packets
from members not belonging to the group, it does not solve the
issues related to a malicious member acting improperly.
6 IANA Considerations
The following contact information shall be used for all
registrations included here:
Contact: Joerg Ott
mailto:jo@acm.org
tel:+358-9-451-2460
The secure RTP feedback profile as a combination of Secure RTP and
the feedback profile needs to be registered for the Session
Description Protocol (specifically the type "proto"): "RTP/SAVPF".
SDP Protocol ("proto"):
Name: RTP/SAVPF
Long form: Secure RTP Profile with RTCP-based Feedback
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Type of name: proto
Type of attribute: Media level only
Purpose: RFC XXXX
Reference: RFC XXXX
All the SDP attribute defined for RTP/SAVP and RTP/AVPF are valid
for RTP/SAVPF, too.
NOTE TO THE RFC EDITOR: Please replace all occurrences of RFC XXXX by
the RFC number assigned to this document.
7 Acknowledgements
This document is a product of the Audio-Visual Transport (AVT)
Working Group of the IETF. The authors would like to thank Magnus
Westerlund and Colin Perkins for their comments.
8 Authors' Addresses
Joerg Ott {sip,mailto}:jo@netlab.hut.fi
Helsinki University of Technology tel:+358-9-451-2460
Otakaari 5A
FI-02150 Espoo
Finland
Elisabetta Carrara mailto: carrara@kth.se
Royal Institute of Technology
Stockholm
Sweden
9 Bibliography
9.1 Normative references
[1] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP
- A Transport Protocol for Real-time Applications," RFC 3550
(STD0064), July 2003.
[2] H. Schulzrinne and S. Casner, "RTP Profile for Audio and Video
Conferences with Minimal Control," RFC 3551 (STD0065), March
2003.
[3] J. Ott, S. Wenger, N. Sato, C. Burmeister, J. Rey, "Extended
RTP Profile for RTCP-based Feedback (RTP/AVPF), "Internet
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Draft draft-ietf-avt-rtcp-feedback-09.txt, Work in Progress,
July 2004.
[4] M. Baugher, D. McGrew, M. Naslund, E. Carrara, K. Norrman,
"The Secure Real-time Transport Protocol", RFC 3711, March
2004.
[5] S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels," RFC 2119, March 1997.
[6] M. Handley, V. Jacobson, and Colin Perkins, "SDP: Session
Description Protocol", Internet Draft draft-ietf-mmusic-sdp-
new-25.txt, July 2005.
[7] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K. Norrman,
"Key Management Extensions for Session Description Protocol
(SDP) and Real Time Streaming Protocol (RTSP)," Internet Draft
draft-ietf-mmusic-kmgmt-ext-15.txt, Work in Progress, June
2005.
[8] F. Andreassen, M. Baugher, and D. Wing, "Session Description
Protocol Security Descriptions for Media Streams," Internet
Draft draft-ietf-mmusic-sdescriptions-12.txt, Work in
Progress, September 2005.
[9] G. Camarillo, J. Holler, G. Eriksson, H. Schulzrinne,
"Grouping of media lines in SDP," RFC 3388, December 2002.
[10] J. Rosenberg and H. Schulzrinne, "An offer/answer model with
SDP," RFC 3264, June 2002.
[11] H. Schulzrinne, A. Rao, and R. Lanphier, "Real Time Streaming
Protocol (RTSP)," RFC 2326, April 1998.
9.2 Informative References
[12] M. Handley, C. Perkins, and E. Whelan, "Session Announcement
Protocol," RFC 2974, October 2000.
[13] B. Ramsdell (ed.), " S/MIME Version 3 Message Specification,"
RFC 2633, June 1999.
[14] E.Rescorla, "HTTP Over TLS," RFC 2818, May 2000.
10 IPR Notice
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This document and the information contained herein are provided on
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12 Full Copyright Statement
Copyright (C) The Internet Society (2006). This document is
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13 Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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