draft-ietf-sipping-transc-3pcc-02.txt   rfc4117.txt 
Internet Engineering Task Force SIP WG Network Working Group G. Camarillo
Internet Draft G. Camarillo Request for Comments: 4117 Ericsson
Ericsson Category: Informational E. Burger
E. Burger
Brooktrout Brooktrout
H. Schulzrinne H. Schulzrinne
Columbia University Columbia University
A. van Wijk A. van Wijk
Viataal Viataal
draft-ietf-sipping-transc-3pcc-02.txt June 2005
September 17, 2004
Expires: March, 2005
Transcoding Services Invocation in the Session Initiation
Protocol (SIP) Using Third Party Call Control (3pcc)
STATUS OF THIS MEMO
By submitting this Internet-Draft, I certify that any applicable Transcoding Services Invocation in
patent or other IPR claims of which I am aware have been disclosed, the Session Initiation Protocol (SIP)
and any of which I become aware will be disclosed, in accordance with Using Third Party Call Control (3pcc)
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering Status of This Memo
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time. It is inappropriate to use Internet-Drafts as reference memo is unlimited.
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at Copyright Notice
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at Copyright (C) The Internet Society (2005).
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Abstract Abstract
This document describes how to invoke transcoding services using SIP This document describes how to invoke transcoding services using
and third party call control. This way of invocation meets the Session Initiation Protocol (SIP) and third party call control. This
requirements for SIP regarding transcoding services invocation to way of invocation meets the requirements for SIP regarding
support deaf, hard of hearing and speech-impaired individuals. transcoding services invocation to support deaf, hard of hearing and
speech-impaired individuals.
Table of Contents Table of Contents
1 Introduction ........................................ 3 1. Introduction ....................................................2
2 General Overview .................................... 3 2. General Overview ................................................2
3 Third Party Call Control Flows ...................... 3 3. Third Party Call Control Flows ..................................2
3.1 Terminology ......................................... 4 3.1. Terminology ................................................3
3.2 Callee's Invocation ................................. 4 3.2. Callee's Invocation ........................................3
3.3 Caller's Invocation ................................. 9 3.3. Caller's Invocation ........................................8
3.4 Receiving the Original Stream ....................... 9 3.4. Receiving the Original Stream ..............................8
3.5 Transcoding Services in Parallel .................... 11 3.5. Transcoding Services in Parallel ..........................10
3.6 Transcoding Services in Serial ...................... 15 3.6. Multiple Transcoding Services in Series ...................14
4 Security Considerations ............................. 17 4. Security Considerations ........................................16
5 IANA Considerations ................................. 17 5. Normative References ...........................................17
6 Authors' Addresses .................................. 17 6. Informative References .........................................17
7 Normative References ................................ 17
8 Informative References .............................. 18
1 Introduction 1. Introduction
The framework for transcoding with SIP [4] describes how two SIP [1] The framework for transcoding with SIP [4] describes how two SIP [1]
UAs (User Agents) can discover imcompatibilities that prevent them UAs (User Agents) can discover incompatibilities that prevent them
from establishing a session (e.g., lack of support for a common codec from establishing a session (e.g., lack of support for a common codec
or for a common media type). When such incompatibilities are found, or common media type). When such incompatibilities are found, the
the UAs need to invoke transcoding services to successfully establish UAs need to invoke transcoding services to successfully establish the
the session. 3pcc (third party call control) [2] is one way to session. 3pcc (third party call control) [2] is one way to perform
perform such invocation. such invocation.
2 General Overview 2. General Overview
In the 3pcc model for transcoding invocation, a transcoding server In the 3pcc model for transcoding invocation, a transcoding server
that provides a particular transcoding service (e.g., speech-to-text) that provides a particular transcoding service (e.g., speech-to-text)
is identified by a URI. A UA that wishes to invoke that service sends is identified by a URI. A UA that wishes to invoke that service
an INVITE request to that URI establishing a number of media streams. sends an INVITE request to that URI establishing a number of media
The way the transcoder manipulates and manages the contents of those streams. The way the transcoder manipulates and manages the contents
media streams (e.g., the text received over the text stream is of those media streams (e.g., the text received over the text stream
transformed into speech and sent over the audio stream) is service is transformed into speech and sent over the audio stream) is service
specific. specific.
All the call flows in this document use SDP. The same call All the call flows in this document use SDP. The same call flows
flows could be used with another session description could be used with another session description protocol that provides
protocol that provided similar session description similar session description capabilities.
capabilities.
3 Third Party Call Control Flows 3. Third Party Call Control Flows
Given two UAs (A and B) and a transcoding server (T), the invocation Given two UAs (A and B) and a transcoding server (T), the invocation
of a transcoding service consists of establishing two sessions; A-T of a transcoding service consists of establishing two sessions; A-T
and T-B. How these sessions are established depends on which party, and T-B. How these sessions are established depends on which party,
the caller (A) or the callee (B), invokes the transcoding services. the caller (A) or the callee (B), invokes the transcoding services.
Section 3.2 deals with callee invocation and Section 3.3 deals with Section 3.2 deals with callee invocation and Section 3.3 deals with
caller invocation. caller invocation.
In all our 3pcc flows we have followed a general principle; a 200 In all our 3pcc flows we have followed the general principle that a
(OK) response from the transcoding service has to be received before 200 (OK) response from the transcoding service has to be received
contacting the callee. This tries to ensure that the transcoding before contacting the callee. This tries to ensure that the
service will be available when the callee accepts the session. transcoding service will be available when the callee accepts the
session.
Still, the transcoding service does not know the exact type of Still, the transcoding service does not know the exact type of
transcoding it will be performing until the callee accepts the transcoding it will be performing until the callee accepts the
session. So, there are always chances of failing to provide session. So, there is always the chance of failing to provide
transcoding services after the callee has accepted the session. A transcoding services after the callee has accepted the session. A
system with tough requirements could use preconditions to avoid this system with more stringent requirements could use preconditions to
situation. When preconditions are used, the callee is not alerted avoid this situation. When preconditions are used, the callee is not
until everything is ready for the session. alerted until everything is ready for the session.
3.1 Terminology 3.1. Terminology
All the flows in this document follow the naming convention below: All the flows in this document follow the naming convention below:
SDP A: A session description generated by A. It contains, among SDP A: A session description generated by A. It contains, among
other things, the transport address/es (IP address and port other things, the transport address/es (IP address and
number) where A wants to receive media for each particular port number) where A wants to receive media for each
stream. particular stream.
SDP B: A session description generated by B. It contains, among SDP B: A session description generated by B. It contains, among
other things, the transport address/es where B wants to other things, the transport address/es where B wants to
receive media for each particular stream. receive media for each particular stream.
SDP A+B: A session description that contains, among other SDP A+B: A session description that contains, among other things,
things, the transport address/es where A wants to receive the transport address/es where A wants to receive media
media and the transport address/es where B wants to receive and the transport address/es where B wants to receive
media. media.
SDP TA: A session description generated by T and intended for A. SDP TA: A session description generated by T and intended for A.
It contains, among other things, the transport address/es It contains, among other things, the transport address/es
where T wants to receive media from A. where T wants to receive media from A.
SDP TB: A session description generated by T and intended for B. SDP TB: A session description generated by T and intended for B.
It contains, among other things, the transport address/es It contains, among other things, the transport address/es
where T wants to receive media from B. where T wants to receive media from B.
SDP TA+TB: A session description generated by T that contains, SDP TA+TB: A session description generated by T that contains, among
among other things, the transport address/es where T wants other things, the transport address/es where T wants to
to receive media from A and the transport address/es where receive media from A and the transport address/es where T
T wants to receive media from B. wants to receive media from B.
3.2 Callee's Invocation 3.2. Callee's Invocation
In this scenario, B receives an INVITE from A and B decides to In this scenario, B receives an INVITE from A, and B decides to
introduce T in the session. Figure 1 shows the call flow for this introduce T in the session. Figure 1 shows the call flow for this
scenario. scenario.
In Figure 1, A can both hear and speak and B is a deaf user with a In Figure 1, A can both hear and speak, and B is a deaf user with a
speech impairment. A proposes to establish a session that consists of speech impairment. A proposes to establish a session that consists
an audio stream (1). B wants to send and receive only text, so it of an audio stream (1). B wants to send and receive only text, so it
invokes a transcoding service T that will perform both speech-to-text invokes a transcoding service T that will perform both speech-to-text
and text-to-speech conversions (2). The session descriptions of and text-to-speech conversions (2). The session descriptions of
Figure 1 are partially shown below. Figure 1 are partially shown below.
A T B A T B
| | | | | |
|--------------------(1) INVITE SDP A-------------------->| |--------------------(1) INVITE SDP A-------------------->|
| | | | | |
| |<---(2) INVITE SDP A+B------| | |<---(2) INVITE SDP A+B------|
skipping to change at page 5, line 25 skipping to change at page 4, line 25
| | | | | |
|<-------------------(5) 200 OK SDP TA--------------------| |<-------------------(5) 200 OK SDP TA--------------------|
| | | | | |
|------------------------(6) ACK------------------------->| |------------------------(6) ACK------------------------->|
| | | | | |
| ************************** | ************************** | | ************************** | ************************** |
|* MEDIA *|* MEDIA *| |* MEDIA *|* MEDIA *|
| ************************** | ************************** | | ************************** | ************************** |
| | | | | |
Figure 1: Callee's invocation of a transcoding service Figure 1: Callee's Invocation of a Transcoding Service
(1) INVITE SDP A (1) INVITE SDP A
m=audio 20000 RTP/AVP 0 m=audio 20000 RTP/AVP 0
c=IN IP4 A.example.com c=IN IP4 A.example.com
(2) INVITE SDP A+B (2) INVITE SDP A+B
m=audio 20000 RTP/AVP 0 m=audio 20000 RTP/AVP 0
c=IN IP4 A.example.com c=IN IP4 A.example.com
skipping to change at page 6, line 20 skipping to change at page 5, line 15
established at this point: established at this point:
1. Audio from A to T.example.com:30000 1. Audio from A to T.example.com:30000
2. Text from T to B.example.com:40000 2. Text from T to B.example.com:40000
3. Text from B to T.example.com:30002 3. Text from B to T.example.com:30002
4. Audio from T to A.example.com:20000 4. Audio from T to A.example.com:20000
When either A or B decide to terminate the session, they send a BYE When either A or B decides to terminate the session, it sends a BYE
indicating that the session is over. indicating that the session is over.
If the first INVITE (1) received by B is empty (no session If the first INVITE (1) received by B is empty (no session
description), the call flow is slightly different. Figure 2 shows the description), the call flow is slightly different. Figure 2 shows
messages involved. the messages involved.
B may have different reasons for invoking T before knowing A's B may have different reasons for invoking T before knowing A's
session description. B may want to hide its capabilities, and session description. B may want to hide its lack of native
therefore it wants to return a session description with all the capabilities, and therefore wants to return a session description
codecs B supports plus all the codecs T supports. Or T may provide with all the codecs that B supports, plus all the codecs that T
recording services (besides transcoding), and B wants T to record the supports. Or T may provide recording services (besides transcoding),
conversation, regardless of whether or not transcoding is needed. and B wants T to record the conversation, regardless of whether
transcoding is needed.
This scenario (Figure 2) is a bit more complex than the previous one. This scenario (Figure 2) is a bit more complex than the previous one.
In INVITE (2), B still does not have SDP A, so it cannot provide T In INVITE (2), B still does not have SDP A, so it cannot provide T
with that information. When B finally receives SDP A in (6), it has with that information. When B finally receives SDP A in (6), it has
to send it to T. B sends an empty INVITE to T (7) and gets a 200 OK to send it to T. B sends an empty INVITE to T (7) and gets a 200 OK
with SDP TA+TB (8). In general, this SDP TA+TB can be different than with SDP TA+TB (8). In general, this SDP TA+TB can be different than
the one that was sent in (3). That is why B needs to send the updated the one sent in (3). That is why B needs to send the updated SDP TA
SDP TA to A in (9). A then sends a possibly updated SDP A (10) and B to A in (9). A then sends a possibly updated SDP A (10) and B sends
sends it to T in (12). On the other hand, if T happens to return the it to T in (12). On the other hand, if T happens to return the same
same SDP TA+TB in (8) as in (3), B can skip messages (9), (10), and SDP TA+TB in (8) as in (3), B can skip messages (9), (10), and (11).
(11). So, implementors of transcoding services are encouraged to So, implementors of transcoding services are encouraged to return the
return the same session description in (8) as in (3) in this type of same session description in (8) as in (3) in this type of scenario.
scenario. The session descriptions of this flow are shown below: The session descriptions of this flow are shown below:
A T B A T B
| | | | | |
|----------------------(1) INVITE------------------------>| |----------------------(1) INVITE------------------------>|
| | | | | |
| |<-----(2) INVITE SDP B------| | |<-----(2) INVITE SDP B------|
| | | | | |
| |---(3) 200 OK SDP TA+TB---->| | |---(3) 200 OK SDP TA+TB---->|
| | | | | |
skipping to change at page 9, line 19 skipping to change at page 8, line 15
established at this point: established at this point:
1. Audio from A to T.example.com:30004 1. Audio from A to T.example.com:30004
2. Text from T to B.example.com:40000 2. Text from T to B.example.com:40000
3. Text from B to T.example.com:30006 3. Text from B to T.example.com:30006
4. Audio from T to A.example.com:20002 4. Audio from T to A.example.com:20002
3.3 Caller's Invocation 3.3. Caller's Invocation
In this scenario, A wishes to establish a session with B using a In this scenario, A wishes to establish a session with B using a
transcoding service. A uses 3pcc to set up the session between T and transcoding service. A uses 3pcc to set up the session between T and
B. The call flow we provide here is slightly different than the ones B. The call flow we provide here is slightly different than the ones
in [2]. In [2], the controller establishes a session between two user in [2]. In [2], the controller establishes a session between two
agents, which are the ones deciding the characteristics of the user agents, which are the ones deciding the characteristics of the
streams. Here, A wants to establish a session between T and B, but A streams. Here, A wants to establish a session between T and B, but A
wants to decide how many and which types of streams are established. wants to decide how many and which types of streams are established.
That is why A sends its session description in the first INVITE (1) That is why A sends its session description in the first INVITE (1)
to T, as opposed to the media-less initial INVITE recommended by [2]. to T, as opposed to the media-less initial INVITE recommended by [2].
Figure 3 shows the call flow for this scenario. Figure 3 shows the call flow for this scenario.
We do not include the session descriptions of this flow, since they We do not include the session descriptions of this flow, since they
are very similar to the ones in Figure 2. In this flow, if T returns are very similar to those in Figure 2. In this flow, if T returns
the same SDP TA+TB in (8) as in (2), messages (9), (10), and (11) can the same SDP TA+TB in (8) as in (2), messages (9), (10), and (11) can
be skipped. be skipped.
3.4 Receiving the Original Stream 3.4. Receiving the Original Stream
Sometimes, as pointed out in the requirements for SIP in support of Sometimes, as pointed out in the requirements for SIP in support of
deaf, hard of hearing, and speech-impaired individuals [5], a user deaf, hard of hearing, and speech-impaired individuals [5], a user
wants to receive both the original stream (e.g., audio) and the wants to receive both the original stream (e.g., audio) and the
transcoded stream (e.g., the output of the speech-to-text transcoded stream (e.g., the output of the speech-to-text
conversion). There are various possible solutions for this problem. conversion). There are various possible solutions for this problem.
One solution consists of using the SDP group attribute with FID One solution consists of using the SDP group attribute with Flow
semantics [3]. FID allows requesting that a stream is sent to two Identification (FID) semantics [3]. FID allows requesting that a
different transport addresses in parallel, as shown below: stream is sent to two different transport addresses in parallel, as
shown below:
A T B A T B
| | | | | |
|-------(1) INVITE SDP A---->| | |-------(1) INVITE SDP A---->| |
| | | | | |
|<----(2) 200 OK SDP TA+TB---| | |<----(2) 200 OK SDP TA+TB---| |
| | | | | |
|----------(3) ACK---------->| | |----------(3) ACK---------->| |
| | | | | |
skipping to change at page 10, line 48 skipping to change at page 9, line 48
a=group:FID 1 2 a=group:FID 1 2
m=audio 20000 RTP/AVP 0 m=audio 20000 RTP/AVP 0
c=IN IP4 A.example.com c=IN IP4 A.example.com
a=mid:1 a=mid:1
m=audio 30000 RTP/AVP 0 m=audio 30000 RTP/AVP 0
c=IN IP4 T.example.com c=IN IP4 T.example.com
a=mid:2 a=mid:2
The problem with this solution is that the majority of the SIP user The problem with this solution is that the majority of the SIP user
agents do not support FID. Moreover, only a small fraction of the few agents do not support FID. Moreover, only a small fraction of the
UAs that do support FID, support sending simultaneous copies of the few UAs that support FID, also support sending simultaneous copies of
same media stream at the same time. In addition, FID forces both the same media stream at the same time. In addition, FID forces both
copies of the stream to use the same codec. copies of the stream to use the same codec.
So, we recommend that T (instead of one of the user agent) replicates Therefore, we recommend that T (instead of a user agent) replicates
the media stream. The transcoder T receiving the following session the media stream. The transcoder T receiving the following session
description performs speech-to-text and text-to-speech conversions description performs speech-to-text and text-to-speech conversions
between the first audio stream and the text stream. In addition, T between the first audio stream and the text stream. In addition, T
copies the first audio stream to the second audio stream and sends it copies the first audio stream to the second audio stream and sends it
to A. to A.
m=audio 40000 RTP/AVP 0 m=audio 40000 RTP/AVP 0
c=IN IP4 B.example.com c=IN IP4 B.example.com
m=audio 20000 RTP/AVP 0 m=audio 20000 RTP/AVP 0
c=IN IP4 A.example.com c=IN IP4 A.example.com
a=recvonly a=recvonly
m=text 20002 RTP/AVP 96 m=text 20002 RTP/AVP 96
c=IN IP4 A.example.com c=IN IP4 A.example.com
a=rtpmap:96 t140/1000 a=rtpmap:96 t140/1000
3.5 Transcoding Services in Parallel 3.5. Transcoding Services in Parallel
Transcoding services sometimes consist of human relays (e.g., a Transcoding services sometimes consist of human relays (e.g., a
person performing speech-to-text and text-to-speech conversions for a person performing speech-to-text and text-to-speech conversions for a
session). If the same person is involved in both conversions (i.e., session). If the same person is involved in both conversions (i.e.,
from A to B and from B to A), he or she has access to all the from A to B and from B to A), he or she has access to all of the
conversation. In order to provide some degree of privacy, sometimes conversation. In order to provide some degree of privacy, sometimes
two different persons are allocated to do the job (i.e., one person two different persons are allocated to do the job (i.e., one person
handles A->B and the other B->A). This type of disposition is also handles A->B and the other B->A). This type of disposition is also
useful for automated transcoding services, where one machine converts useful for automated transcoding services, where one machine converts
text to synthetic speech (text-to-speech) and a different machine text to synthetic speech (text-to-speech) and another performs voice
performs voice recognition (speech-to-text). recognition (speech-to-text).
The scenario just described involves four different sessions; A-T1, The scenario described above involves four different sessions: A-T1,
T1-B, B-T2 and T2-A. Figure 4 shows the call flow where A invokes T1 T1-B, B-T2 and T2-A. Figure 4 shows the call flow where A invokes T1
and T2. and T2.
Note this example uses unidirectional media streams (i.e., sendonly
or recvonly) to clearly identify which transcoder handles media in
which direction. Nevertheless, nothing precludes the use of
bidirectional streams in this scenario. They could be used, for
example, by a human relay to ask for clarifications (e.g., I did not
get that, could you repeat, please?) to the party he or she is
receiving media from.
(1) INVITE SDP AT1 (1) INVITE SDP AT1
m=text 20000 RTP/AVP 96 m=text 20000 RTP/AVP 96
c=IN IP4 A.example.com c=IN IP4 A.example.com
a=rtpmap:96 t140/1000 a=rtpmap:96 t140/1000
a=sendonly a=sendonly
m=audio 20000 RTP/AVP 0 m=audio 20000 RTP/AVP 0
c=IN IP4 0.0.0.0 c=IN IP4 0.0.0.0
a=recvonly a=recvonly
(2) INVITE SDP AT2 (2) INVITE SDP AT2
m=text 20002 RTP/AVP 96 m=text 20002 RTP/AVP 96
c=IN IP4 A.example.com c=IN IP4 A.example.com
a=rtpmap:96 t140/1000 a=rtpmap:96 t140/1000
a=recvonly a=recvonly
m=audio 20000 RTP/AVP 0 m=audio 20000 RTP/AVP 0
c=IN IP4 0.0.0.0 c=IN IP4 0.0.0.0
a=sendonly a=sendonly
skipping to change at page 14, line 33 skipping to change at page 13, line 33
(12) 200 OK SDP T2A+T2B (12) 200 OK SDP T2A+T2B
m=text 40000 RTP/AVP 96 m=text 40000 RTP/AVP 96
c=IN IP4 T2.example.com c=IN IP4 T2.example.com
a=rtpmap:96 t140/1000 a=rtpmap:96 t140/1000
a=sendonly a=sendonly
m=audio 40002 RTP/AVP 0 m=audio 40002 RTP/AVP 0
c=IN IP4 T2.example.com c=IN IP4 T2.example.com
a=recvonly a=recvonly
Since T1 have returned the same SDP in (11) as in (3) and T2 has Since T1 have returned the same SDP in (11) as in (3), and T2 has
returned the same SDP in (12) as in (5), messages (13), (14) and (15) returned the same SDP in (12) as in (5), messages (13), (14) and (15)
can be skipped. can be skipped.
(16) ACK SDP AT1+BT1 (16) ACK SDP AT1+BT1
m=text 20000 RTP/AVP 96 m=text 20000 RTP/AVP 96
c=IN IP4 A.example.com c=IN IP4 A.example.com
a=rtpmap:96 t140/1000 a=rtpmap:96 t140/1000
a=sendonly a=sendonly
m=audio 50000 RTP/AVP 0 m=audio 50000 RTP/AVP 0
skipping to change at page 15, line 25 skipping to change at page 14, line 25
1. Text from A to T1.example.com:30000 1. Text from A to T1.example.com:30000
2. Audio from T1 to B.example.com:50000 2. Audio from T1 to B.example.com:50000
3. Audio from B to T2.example.com:40002 3. Audio from B to T2.example.com:40002
4. Text from T2 to A.example.com:20002 4. Text from T2 to A.example.com:20002
Note that B, the user agent server, needs to support two media Note that B, the user agent server, needs to support two media
streams; one sendonly and the other recvonly. At present, some user streams: sendonly and recvonly. At present, some user agents,
agents, although they support a single sendrecv media stream, they do although they support a single sendrecv media stream, do not support
not support a different media line per direction. Implementers are a different media line per direction. Implementers are encouraged to
encouraged to build support for this feature. build support for this feature.
3.6 Transcoding Services in Serial 3.6. Multiple Transcoding Services in Series
In a distributed environment, a complex transcoding service (e.g., In a distributed environment, a complex transcoding service (e.g.,
English text to Spanish speech) is often provided by several servers. English text to Spanish speech) is often provided by several servers.
For example, one server performs English text to Spanish text For example, one server performs English text to Spanish text
translation, and its output is feed into a server that performs translation, and its output is fed into a server that performs text-
text-to-speech conversion. The flow in Figure 5 shows how A invokes to-speech conversion. The flow in Figure 5 shows how A invokes T1
T1 and T2. and T2.
A T1 T2 B A T1 T2 B
| | | | | | | |
|----(1) INVITE SDP A-----> | | | |----(1) INVITE SDP A-----> | | |
| | | | | | | |
|<-(2) 200 OK SDP T1A+T1T2- | | | |<-(2) 200 OK SDP T1A+T1T2- | | |
| | | | | | | |
|----------(3) ACK--------> | | | |----------(3) ACK--------> | | |
| | | | | | | |
skipping to change at page 17, line 5 skipping to change at page 16, line 5
| | | | | | | |
|----------------------------(18) ACK-------------------------->| |----------------------------(18) ACK-------------------------->|
| | | | | | | |
| ************************* | ******************* *********** | | ************************* | ******************* *********** |
|* MEDIA *|* MEDIA *|* MEDIA *| |* MEDIA *|* MEDIA *|* MEDIA *|
| ************************* | ******************* | *********** | | ************************* | ******************* | *********** |
| | | | | | | |
Figure 5: Transcoding services in serial Figure 5: Transcoding services in serial
4 Security Considerations 4. Security Considerations
This document describes how to use third party call control to invoke RFC 3725 [2] discusses security considerations which relate to the
transcoding services. It does not introduce new security use of third party call control in SIP. These considerations apply
considerations besides the ones discussed in [2]. to this document, since it describes how to use third party call
control to invoke transcoding service.
5 IANA Considerations In particular, RFC 3725 states that end-to-end media security is
based on the exchange of keying material within SDP and depends on
the controller behaving properly. That is, the controller should not
try to disable the security mechanisms offered by the other parties.
As a result, it is trivially possible for the controller to insert
itself as an intermediary on the media exchange, if it should so
desire.
This document has no actions for IANA. In this document, the controller is the UA invoking the transcoder,
and there is a media session established using third party call
control between the remote UA and the transcoder. Consequently, the
attack described in RFC 3725 does not constitute a threat because the
controller is the UA invoking the transcoding service and it has
access to the media anyway by definition. So, it seems unlikely that
a UA would attempt to launch an attack against its own session by
disabling security between the transcoder and the remote UA.
6 Authors' Addresses Regarding end-to-end media security from the UAs' point of view, the
transcoder needs access to the media in order to perform its
function. So, by definition, the transcoder behaves as a man in the
middle. UAs that do not want a particular transcoder to have access
to all the media exchanged between them can use a different
transcoder for each direction. In addition, UAs can use different
transcoders for different media types.
5. Normative References
[1] 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.
[2] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo,
"Best Current Practices for Third Party Call Control (3pcc) in
the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April
2004.
[3] Camarillo, G., Eriksson, G., Holler, J., and H. Schulzrinne,
"Grouping of Media Lines in the Session Description Protocol
(SDP)", RFC 3388, December 2002.
6. Informative References
[4] Camarillo, G., "Framework for transcoding with the session
initiation protocol", August 2003, Work in Progress.
[5] Charlton, N., Gasson, M., Gybels, G., Spanner, M., and A. van
Wijk, "User Requirements for the Session Initiation Protocol
(SIP) in Support of Deaf, Hard of Hearing and Speech-impaired
Individuals", RFC 3351, August 2002.
Authors' Addresses
Gonzalo Camarillo Gonzalo Camarillo
Ericsson Ericsson
Advanced Signalling Research Lab. Advanced Signalling Research Lab.
FIN-02420 Jorvas FIN-02420 Jorvas
Finland Finland
electronic mail: Gonzalo.Camarillo@ericsson.com
EMail: Gonzalo.Camarillo@ericsson.com
Eric Burger Eric Burger
Brooktrout Technology, Inc. Brooktrout Technology, Inc.
18 Keewaydin Way 18 Keewaydin Way
Salem, NH 03079 Salem, NH 03079
USA USA
electronic mail: eburger@ieee.org
EMail: eburger@brooktrout.com
Henning Schulzrinne Henning Schulzrinne
Dept. of Computer Science Dept. of Computer Science
Columbia University 1214 Amsterdam Avenue, MC 0401 Columbia University
1214 Amsterdam Avenue, MC 0401
New York, NY 10027 New York, NY 10027
USA USA
electronic mail: schulzrinne@cs.columbia.edu
EMail: schulzrinne@cs.columbia.edu
Arnoud van Wijk Arnoud van Wijk
Viataal Viataal
Research & Development Research & Development
Afdeling RDS Afdeling RDS
Theerestraat 42 Theerestraat 42
5271 GD Sint-Michielsgestel 5271 GD Sint-Michielsgestel
The Netherlands The Netherlands
electronic mail: a.vwijk@viataal.nl
7 Normative References EMail: a.vwijk@viataal.nl
[1] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. R. Johnston, J.
Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP: session
initiation protocol," RFC 3261, Internet Engineering Task Force, June
2002.
[2] J. Rosenberg, J. Peterson, H. Schulzrinne, and G. Camarillo,
"Best current practices for third party call control (3pcc) in the
session initiation protocol (SIP)," RFC 3725, Internet Engineering
Task Force, Apr. 2004.
[3] G. Camarillo, G. Eriksson, J. Holler, and H. Schulzrinne, Full Copyright Statement
"Grouping of media lines in the session description protocol (SDP),"
RFC 3388, Internet Engineering Task Force, Dec. 2002.
8 Informative References Copyright (C) The Internet Society (2005).
[4] G. Camarillo, "Framework for transcoding with the session This document is subject to the rights, licenses and restrictions
initiation protocol," Internet Draft draft-camarillo-sipping-transc- contained in BCP 78, and except as set forth therein, the authors
framework-00, Internet Engineering Task Force, Aug. 2003. Work in retain all their rights.
progress.
[5] N. Charlton, M. Gasson, G. Gybels, M. Spanner, and A. van Wijk, This document and the information contained herein are provided on an
"User requirements for the session initiation protocol (SIP) in "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
support of deaf, hard of hearing and speech-impaired individuals," OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
RFC 3351, Internet Engineering Task Force, Aug. 2002. ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
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 End of changes. 

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