draft-ietf-mmusic-media-loopback-14.txt   draft-ietf-mmusic-media-loopback-15.txt 
Internet Draft K. Hedayat Internet Draft K. Hedayat
Expires: January 27, 2011 EXFO Expires: September 11, 2011 EXFO
N. Venna N. Venna
EXFO Saperix
P. Jones P. Jones
Cisco Systems, Inc. Cisco Systems, Inc.
A. Roychowdhury A. Roychowdhury
Hughes Systique Corp. Hughes Systique Corp.
C. SivaChelvan C. SivaChelvan
Cisco Systems, Inc. Cisco Systems, Inc.
N. Stratton N. Stratton
BlinkMind, Inc. BlinkMind, Inc.
July 27, 2010 March 11, 2011
An Extension to the Session Description Protocol (SDP) for Media An Extension to the Session Description Protocol (SDP) for Media
Loopback Loopback
draft-ietf-mmusic-media-loopback-14 draft-ietf-mmusic-media-loopback-15
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with This Internet-Draft is submitted to IETF in full conformance with
the provisions of BCP 78 and BCP 79. the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
skipping to change at page 1, line 43 skipping to change at page 1, line 43
documents at any time. It is inappropriate to use Internet-Drafts documents at any time. It is inappropriate to use Internet-Drafts
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progress." progress."
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This Internet-Draft will expire on January 27, 2011. This Internet-Draft will expire on Septembre 11, 2011.
Copyright Notice Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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service. The extension defined herein adds new SDP media service. The extension defined herein adds new SDP media
attributes which enables establishment of media sessions where the attributes which enables establishment of media sessions where the
media is looped back to the transmitter. Such media sessions will media is looped back to the transmitter. Such media sessions will
serve as monitoring and troubleshooting tools by providing the serve as monitoring and troubleshooting tools by providing the
means for measurement of more advanced VoIP, Real-time Text and means for measurement of more advanced VoIP, Real-time Text and
Video over IP performance metrics. Video over IP performance metrics.
Table of Contents Table of Contents
1. Introduction .................................................. 3 1. Introduction .................................................. 3
2. Terminology ................................................... 4 1.1 Use Cases Supported ....................................... 4
3. Offering Entity Behavior ...................................... 4 2. Terminology ................................................... 6
4. Answering Entity Behavior ..................................... 4 3. Offering Entity Behavior ...................................... 6
5. SDP Constructs Syntax ......................................... 5 4. Answering Entity Behavior ..................................... 6
5.1 Loopback Type Attribute ................................... 5 5. SDP Constructs Syntax ......................................... 7
5.2 Loopback Mode Attribute ................................... 6 5.1 Loopback Type Attribute ................................... 7
5.3 Payload type mapping for loopback sessions ................ 6 5.2 Loopback Mode Attribute ................................... 7
5.4 Generating the Offer for Loopback Session ................. 7 5.3 Generating the Offer for Loopback Session ................. 8
5.5 Generating the Answer for Loopback Session ................ 8 5.4 Generating the Answer for Loopback Session ................ 9
5.6 Offerer Processing of the Answer .......................... 9 5.5 Offerer Processing of the Answer ......................... 11
5.7 Modifying the Session .................................... 10 5.6 Modifying the Session .................................... 12
5.8 Priming the loopback pump ................................ 10 5.7 Priming the loopback pump ................................ 12
6. RTP Requirements ............................................. 10 6. RTP Requirements ............................................. 13
7. Payload formats for Packet loopback .......................... 11 7. Payload formats for Packet loopback .......................... 13
7.1 Encapsulated Payload format .............................. 12 7.1 Encapsulated Payload format .............................. 14
7.2 Direct loopback RTP payload format ....................... 14 7.2 Direct loopback RTP payload format ....................... 16
8. Payload format for Priming the Loopback Pump ................. 15 8. Payload format for Priming the Loopback Pump ................. 18
8.1 Usage of RTP Header fields ............................... 15 8.1 Usage of RTP Header fields ............................... 18
8.2 Usage of SDP ............................................. 16 8.2 Usage of SDP ............................................. 18
9. RTCP Requirements ............................................ 16 9. RTCP Requirements ............................................ 18
10. Congestion Control .......................................... 16 10. Congestion Control .......................................... 19
11. Examples .................................................... 17 11. Examples .................................................... 19
11.1 Offer for specific media loopback type .................. 17 11.1 Offer for specific media loopback type .................. 19
11.2 Offer for choice of media loopback type ................. 17 11.2 Offer for choice of media loopback type ................. 20
11.3 Offer for choice of media loopback type with loopback 11.3 Offer for choice of media loopback type with loopback
primer ....................................................... 18 primer ....................................................... 21
11.4 Response to INVITE request rejecting loopback media ..... 19 11.4 Response to INVITE request rejecting loopback media ..... 22
11.5 Response to INVITE request rejecting loopback media with 11.5 Response to INVITE request rejecting loopback media with
loopback primer payload type ................................. 20 loopback primer payload type ................................. 23
12. Security Considerations ..................................... 21 12. Security Considerations ..................................... 23
13. Implementation Considerations ............................... 21 13. Implementation Considerations ............................... 24
14. IANA Considerations ......................................... 22 14. IANA Considerations ......................................... 24
14.1 SDP Attributes .......................................... 22 14.1 SDP Attributes .......................................... 24
14.2 MIME Types .............................................. 23 14.2 MIME Types .............................................. 25
15. Acknowledgements ............................................ 36 15. Additional Authors and Acknowledgements ..................... 39
1. Introduction 1. Introduction
The overall quality, reliability, and performance of VoIP, The overall quality, reliability, and performance of VoIP,
Real-time Text and Video over IP services rely on the performance Real-time Text and Video over IP services rely on the performance
and quality of the media path. In order to assure the quality of and quality of the media path. In order to assure the quality of
the delivered media there is a need to monitor the performance of the delivered media there is a need to monitor the performance of
the media transport. One method of monitoring and managing the the media transport. One method of monitoring and managing the
overall quality of VoIP, Real-time Text and Video over IP Services overall quality of VoIP, Real-time Text and Video over IP Services
is through monitoring the quality of the media in an active is through monitoring the quality of the media in an active
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of testing a specific endpoint that may be exhibiting problems. of testing a specific endpoint that may be exhibiting problems.
The extension defined in this memo introduces new SDP media The extension defined in this memo introduces new SDP media
attributes that enable establishment of media sessions where the attributes that enable establishment of media sessions where the
media is looped back to the transmitter. The offer/answer model media is looped back to the transmitter. The offer/answer model
[RFC3264] is used to establish a loopback connection. Furthermore, [RFC3264] is used to establish a loopback connection. Furthermore,
this extension provides guidelines on handling RTP [RFC3550], as this extension provides guidelines on handling RTP [RFC3550], as
well as usage of RTCP [RFC3550] and RTCP XR [RFC3611] for reporting well as usage of RTCP [RFC3550] and RTCP XR [RFC3611] for reporting
media related measurements. media related measurements.
2. Terminology 1.1 Use Cases Supported
As a matter of terminology in this document, packets flow from one
peer acting as a "loopback source", to the other peer acting as a
"loopback mirror", which in turn returns packets to the loopback
source. In advance of the session, the peers negotiate to determine
which one acts in which role. The negotiation also includes details
such as the type of loopback to be used.
This specification supports three use cases: "encapsulated packet
loopback", "direct loopback", and "media loopback". These are
distinguished by the treatment of incoming RTP packets at the
loopback mirror.
As a supplement to these use cases, this specification also allows
the loopback source to request the loopback mirror to begin sending
a media stream to the loopback source, ending when the mirror
begins to receive packets from the source. This facility is needed
in some circumstances to establish the media path through
middleboxes lying between the peers.
1.1.1 Encapsulated Packet Loopback
In the encapsulated packet loopback case, the entire incoming RTP
packet is encapsulated as payload within an outer payload type that
is specific to this use case and specified below (Section 7.1).
The encapsulated packet is returned to the loopback source. The
loopback source can generate statistics for one-way path
performance up to the RTP level for each direction of travel by
examining sequence numbers and timestamps in the outer header and
the encapsulated RTP packet payload. The loopback source can also
play back the returned media content for evaluation.
Because the encapsulating payload extends the packet size, it could
encounter difficulties in an environment where the original RTP
packet size is close to the path MTU size. The encapsulating
payload type therefore offers the possibility of RTP-level
fragmentation of the returned packets. The use of this facility
could affect statistics derived for the return path. In addition,
the increased bit rate required in the return direction may affect
these statistics more directly in a restricted-bandwidth situation.
1.1.2 Direct Loopback
In the direct loopback case, the loopback mirror copies the payload
of the incoming RTP packet into a new packet, the payload type of
which is again specific to this use case and specified below
(Section 7.2). The loopback mirror returns the new packet to the
packet source. There is no provision in this case for RTP-level
fragmentation.
This use case has the advantage of keeping the packet size the same
in both directions. The packet source can compute only two-way
path statistics from the direct loopback packet header, but can
play back the returned media content.
It has been suggested that the loopback source, knowing that the
incoming packet will never be passed to a decoder, can store a
timestamp and sequence number inside the payload of the packet it
sends to the mirror, then extract that information from the
returned direct loopback packet and compute one-way path statistics
as in the previous case. Obviously, playout of returned content is
no longer possible if this is done.
1.1.3 Media Loopback
In the media loopback case, the loopback mirror submits the
incoming packet to a decoder appropriate to the incoming payload
type. The packet is taken as close as possible to the analog level,
then reencoded according to an outgoing format determined by
negotiation. The reencoded content is returned to the loopback
source as an RTP packet with payload type corresponding to the
reencoding format.
This usage allows trouble-shooting at the codec level. The
capability for path statistics is limited to what is available from
RTCP reports.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119. this document are to be interpreted as described in RFC 2119.
3. Offering Entity Behavior 3. Offering Entity Behavior
An offering entity compliant to this memo and attempting to An offering entity compliant to this memo and attempting to
establish a media session with media loopback MUST include establish a media session with media loopback MUST include
"loopback" media attributes for each individual media description "loopback" media attributes for each individual media description
in the offer message. The offering entity MUST look for the in the offer message. The offering entity MUST look for the
"loopback" media attributes in the media description(s) of the "loopback" media attributes in the media description(s) of the
response from the answering entity for confirmation that the response from the answering entity for confirmation that the
request is accepted. request is accepted.
4. Answering Entity Behavior 4. Answering Entity Behavior
An answering entity compliant to this specification and receiving An answering entity compliant to this specification and receiving
an offer containing media descriptions with the "loopback" media an offer containing media descriptions with the "loopback" media
attributes, MUST acknowledge the request by including the received attributes MUST acknowledge the request by including the received
"loopback" media attributes for each media description in its "loopback" media attributes for each media description in its
response. The server MAY reject the "loopback" request for response if it agrees to do the loopback. If the answerer does not
specific media types as defined in section 5.5.1 of this want to do loopback or wants to reject the "loopback" request for
specification. specific media types, it MAY do so as defined in section 5.4.1 of
this specification.
An answering entity that is not compliant to this specification and An answering entity that is not compliant to this specification and
which receives an offer with the "loopback" media attributes MAY which receives an offer with the "loopback" media attributes MAY
ignore the attribute and treat the incoming offer as a normal ignore the attribute and treat the incoming offer as a normal
request. request.
5. SDP Constructs Syntax 5. SDP Constructs Syntax
Two new media attributes are defined: one indicates the type of Two new media attributes are defined: one indicates the type of
loopback and the other indicates the mode of the loopback. loopback and the other indicates the mode of the loopback.
5.1 Loopback Type Attribute 5.1 Loopback Type Attribute
The loopback type is a property media attribute with the following The loopback type is a property media attribute with the following
syntax: syntax:
a=loopback:<loopback-type> a=loopback:<loopback-type>
Following is the Augmented BNF [RFC5234] for loopback-type: Following is the Augmented BNF [RFC5234] for loopback-type:
loopback-type = loopback-choice [1*SP loopback-choice] loopback-type = loopback-choice [1*SP loopback-choice]
loopback-choice = loopback-type-pkt / loopback-type-media loopback-choice = loopback-type-pkt / loopback-type-media
loopback-type-pkt = "rtp-pkt-loopback" loopback-type-pkt = "rtp-pkt-loopback"
loopback-type-media = "rtp-media-loopback" loopback-type-media = "rtp-media-loopback"
The loopback type is used to indicate the type of loopback. The The loopback type is used to indicate the type of loopback. The
loopback-type values are rtp-pkt-loopback, and rtp-media-loopback. loopback-type values are rtp-pkt-loopback, and rtp-media-loopback.
rtp-pkt-loopback: In this mode, the RTP packets are looped back to rtp-pkt-loopback: In this mode, the RTP packets are looped back to
the sender at a point before the encoder/decoder function in the the sender at a point before the encoder/decoder function in the
receive direction to a point after the encoder/decoder function in receive direction to a point after the encoder/decoder function in
the send direction. This effectively re-encapsulates the RTP the send direction. This effectively re-encapsulates the RTP
payload with the RTP/UDP/IP overheads appropriate for sending it in payload with the RTP/UDP/IP headers appropriate for sending it in
the reverse direction. Any type of encoding related functions, the reverse direction. Any type of encoding related functions,
such as packet loss concealment, MUST NOT be part of this type of such as packet loss concealment, MUST NOT be part of this type of
loopback path. In this mode the RTP packets are looped back with a loopback path. In this mode the RTP packets are looped back with a
new payload type and format. Section 7 describes the payload new payload type and format. Section 7 describes the payload
formats that MUST be used for this type of loopback. formats that MUST be used for this type of loopback.
rtp-media-loopback: This loopback is activated as close as possible rtp-media-loopback: This loopback is activated as close as possible
to the analog interface and after the decoder so that the RTP to the analog interface and after the decoder so that the RTP
packets are subsequently re-encoded prior to transmission back to packets are subsequently re-encoded prior to transmission back to
the sender. the sender.
5.2 Loopback Mode Attribute 5.2 Loopback Mode Attribute
The loopback mode is a value media attribute that is used to The loopback mode is a value media attribute that is used to
indicate the mode of the loopback. These attributes are additional indicate the mode of the loopback. These attributes are additional
mode attributes like sendonly, recvonly, etc. The syntax of the mode attributes like sendonly, recvonly, etc. The syntax of the
loopback mode media attribute is: loopback mode media attribute is:
a=<loopback-mode>:<fmt>... a=<loopback-mode>:<fmt>...
The loopback-mode values are loopback-source and loopback-mirror. The loopback-mode values are loopback-source and loopback-mirror.
loopback-source: This attribute specifies that the sender is the loopback-source: This attribute specifies that the entity that
media source and expects the receiver to act as a loopback-mirror. generated the SDP is the media source and expects the receiver of
the SDP message to act as a loopback-mirror.
loopback-mirror: This attribute specifies that the receiver will loopback-mirror: This attribute specifies that the entity that
mirror (echo) all received media back to the sender of the RTP generated the SDP will mirror (echo) all received media back to the
stream. No media is generated locally by the receiver for sender of the RTP stream. No media is generated locally by the
transmission in the mirrored stream unless the loopback primer looping back entity for transmission in the mirrored stream unless
payload type (described in Section 8 of this document) is requested the loopback primer payload type (described in Section 8 of this
by the loopback-source or included in the response by document) is requested by the loopback-source or included in the
loopback-mirror. response by loopback-mirror.
<fmt> is a media format description. The format description has the <fmt> is a media format description. The format description has the
semantics as defined in section 5.14 of RFC 4566[RFC4566]. When semantics as defined in section 5.14 of RFC 4566[RFC4566]. When
loopback-mode is specified as loopback-source, the media format loopback-mode is specified as loopback-source, the media format
corresponds to the RTP payload types the source is willing to send. corresponds to the RTP payload types the entity that generated the
When loopback-mode is specified as loopback-mirror, the media SDP is willing to send. When loopback-mode is specified as
format corresponds to the RTP payload types the mirror is willing loopback-mirror, the media format corresponds to the RTP payload
to receive. The payload types specified in "m=" line for a types the mirror is willing to receive. The "m=" line in the SDP
loopback-source specify the payloads the source is willing to MUST include all the payload types that will be used during the
receive. Similarly, for the loopback-mirror these payload types loopback session including those specified in the loopback-mode
specify the payloads it is willing to send. This separation of attribute line. The complete payload space for the call is
payload types allow receivers that do not understand media loopback specified in the "m=" line and the rtpmap attribute is used to map
to reject the call. from the payload type number to an encoding name denoting the
payload format to be used.
5.3 Payload type mapping for loopback sessions
As specified in RFC 4566[RFC4566], the rtpmap attribute maps from
an RTP payload type number (as used in an "m=" line) to an encoding
name denoting the payload format to be used. For loopback sessions,
media formats are specified both in the "m=" line and the loopback-
mode attribute line as described in Section 5.2. This specification
extends the usage of rtpmap attribute to media formats specified in
the loopback-mode attribute line in addition to the "m=" line.
5.4 Generating the Offer for Loopback Session 5.3 Generating the Offer for Loopback Session
If an offerer wishes to make a loopback request, it MUST include If an offerer wishes to make a loopback request, it MUST include
both the loopback-type and loopback-mode attribute in a valid SDP both the loopback-type and loopback-mode parameters in a valid SDP
offer: offer:
Example: m=audio 41352 RTP/AVP 0 8 100 Example: m=audio 41352 RTP/AVP 0 8 100
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-source:0 8 100 a=loopback-source:0 8 100
a=rtpmap:0 pcmu/8000 a=rtpmap:0 pcmu/8000
a=rtpmap:8 pcma/8000 a=rtpmap:8 pcma/8000
a=rtpmap:100 G7221/16000/1 a=rtpmap:100 G7221/16000/1
Note: A loopback offer in a given media description MUST NOT Note: A loopback offer in a given media description MUST NOT
contain the standard mode attributes sendonly, recvonly, sendrecv contain the standard mode attributes sendonly, recvonly, sendrecv,
or inactive. The loopback-mode attributes (loopback-source and or inactive. The loopback-mode attributes (loopback-source and
loopback-mirror) replace the standard attributes. loopback-mirror) replace the standard attributes.
The offerer may offer more than one loopback-type in the SDP offer. The offerer may offer more than one loopback-type in the SDP offer.
In this case the answer MUST include only one of the loopback types The port number and the address in the offer (m= line) indicate
that are accepted by the answerer. The answerer SHOULD give where the offerer would like to send and receive the media stream.
preference to the first loopback-type in the SDP offer. The payload type numbers indicate the value of the payload the
offerer expects to send and receive. If the offerer is the
For loopback-source media (e.g. audio) streams, the port number and loopback-source, the subset of payload types indicated in the
the address in the offer (m= line) indicate where the offerer would a=loopback-source line are the payload types for the codecs the
like to receive the media stream. The payload type numbers offerer is willing to send. However, the answer might indicate a
indicate the value of the payload the offerer expects to receive. different payload type number for the same codec in the loopback-
The RTP payload types indicated in the a=loopback-source line are mirror line. In that case, the offerer MUST send the payload type
the payload types for the codecs the offerer is willing to send. received in the answer. If the offerer is the loopback-mirror, the
However, the answer might indicate a different payload type number subset of payload types indicated in the a=loopback-mirror line are
for the same codec. In that case, the offerer MUST send the the payload types for the codecs the offerer is willing to receive.
payload type received in the answer.
If loopback-type is rtp-pkt-loopback, the loopback-mirror MUST send If loopback-type is rtp-pkt-loopback, the loopback-mirror MUST send
and the loopback-source MUST receive the looped back packets and the loopback-source MUST receive the looped back packets
encoded in one of the two payload formats (encapsulated RTP or encoded in one of the two payload formats (encapsulated RTP or
payload loopback) as defined in section 7. payload loopback) as defined in section 7.
Example: m=audio 41352 RTP/AVP 112 Example: m=audio 41352 RTP/AVP 0 8 112
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-source:0 8 a=loopback-source:0 8
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
Example: m=audio 41352 RTP/AVP 112 Example: m=audio 41352 RTP/AVP 0 8 112
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-source:0 8 a=loopback-source:0 8
a=rtpmap:112 rtploopback/8000 a=rtpmap:112 rtploopback/8000
5.5 Generating the Answer for Loopback Session 5.4 Generating the Answer for Loopback Session
As with the offer, a loopback answer in a given media description
MUST NOT contain the standard mode attributes sendonly, recvonly,
sendrecv, or inactive. The loopback-mode attributes (loopbackThe
port number and the address in the answer (m= line) indicate where
the answerer would like to receive the media stream. The payload
type numbers indicate the value of the payload types the answerer
expects to send and receive. If the offerer is the loopback-
source, the answerer MUST be a loopback-mirror and the subset of
payload types indicated in the a=loopback-mirror line are the
payload types for the codecs the answerer is willing to receive.
Similarly, if the offerere is the loopback-mirror, the answerer
MUST be aloopback-source and the subset of payload types indicated
in the a=loopback-source line are the payload types for the codecs
the answerer is willing to send.
If an answerer wishes to accept the loopback request it MUST If an answerer wishes to accept the loopback request it MUST
include both the loopback mode and loopback type attribute in the include both the loopback mode and loopback type attribute in the
answer. When a stream is offered with the loopback-source answer. When a stream is offered with the loopback-source
attribute, the corresponding stream in the response MUST be attribute, the corresponding stream in the response MUST be
loopback-mirror and vice versa, provided that answerer is capable loopback-mirror and vice versa, provided that answerer is capable
of supporting the requested loopback-type. of supporting the requested loopback-type.
For example, if the offer contains the loopback-source attribute: For example, if the offer contains the loopback-source attribute:
skipping to change at page 8, line 29 skipping to change at page 10, line 30
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-source:0 8 a=loopback-source:0 8
The answer that is capable of supporting the offer MUST contain the The answer that is capable of supporting the offer MUST contain the
loopback-mirror attribute: loopback-mirror attribute:
m=audio 41352 RTP/AVP 0 8 m=audio 41352 RTP/AVP 0 8
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-mirror:0 8 a=loopback-mirror:0 8
As previously stated if a stream is offered with multiple loopback If a stream is offered with multiple loopback type attributes, the
type attributes, the corresponding stream MUST contain only one answer MUST include only one of the loopback types that are
loopback type attribute selected by the answerer. accepted by the answerer. The answerer SHOULD give preference to
the first loopback-type in the SDP offer.
For example, if the offer contains: For example, if the offer contains:
m=audio 41352 RTP/AVP 0 8 112 m=audio 41352 RTP/AVP 0 8 112
a=loopback:rtp-media-loopback rtp-pkt-loopback a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source:0 8 a=loopback-source:0 8
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
The answer that is capable of supporting the offer and chooses to The answer that is capable of supporting the offer and chooses to
loopback the media using the rtp-media-loopback type MUST contain: loopback the media using the rtp-media-loopback type MUST contain:
skipping to change at page 9, line 9 skipping to change at page 11, line 9
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-mirror:0 8 a=loopback-mirror:0 8
As specified in section 7, if the loopback-type is As specified in section 7, if the loopback-type is
rtp-pkt-loopback, either the encapsulated RTP payload format or rtp-pkt-loopback, either the encapsulated RTP payload format or
direct loopback RTP payload format MUST be used for looped back direct loopback RTP payload format MUST be used for looped back
packets. packets.
For example, if the offer contains: For example, if the offer contains:
m=audio 41352 RTP/AVP 112 113 m=audio 41352 RTP/AVP 0 8 112 113
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-source:0 8 a=loopback-source:0 8
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
a=rtpmap:113 rtploopback/8000 a=rtpmap:113 rtploopback/8000
The answer that is capable of supporting the offer MUST contain one The answer that is capable of supporting the offer must contain one
of the following: of the following:
m=audio 41352 RTP/AVP 112 m=audio 41352 RTP/AVP 0 8 112
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-mirror:0 8 a=loopback-mirror:0 8
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
m=audio 41352 RTP/AVP 113 m=audio 41352 RTP/AVP 0 8 113
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-mirror:0 8 a=loopback-mirror:0 8
a=rtpmap:113 rtploopback/8000 a=rtpmap:113 rtploopback/8000
5.5.1 Rejecting the Loopback Offer 5.4.1 Rejecting the Loopback Offer
An offered stream with loopback-source MAY be rejected if the An offered stream (either with loopback-source or loopback-mirror)
loopback-type is not specified, the specified loopback-type is not MAY be rejected if the loopback-type is not specified, the
supported, or the endpoint cannot honor the offer for any other specified loopback-type is not supported, or the endpoint cannot
reason. The Loopback request may be rejected by setting the media honor the offer for any other reason. The Loopback request may be
port number to zero in the answer as per RFC 3264 [RFC3264]. rejected by setting the media port number to zero in the answer as
per RFC 3264 [RFC3264].
5.6 Offerer Processing of the Answer 5.5 Offerer Processing of the Answer
The answer to a loopback-source MUST be loopback-mirror. The The answer to a loopback-source MUST be loopback-mirror. The
answer to a loopback-mirror MUST be loopback-source. The answer to a loopback-mirror MUST be loopback-source. The
loopback-mode line MUST contain at least one codec the answerer is loopback-mode line MUST contain at least one codec the answerer is
willing to send or receive depending on whether it is the loopback- willing to send or receive depending on whether it is the loopback-
source or the loopback-mirror. In addition, the "m=" line MUST source or the loopback-mirror. In addition, the "m=" line MUST
contain at least one codec that the answerer is willing to send or contain at least one codec that the answerer is willing to send or
receive depending on whether it is the loopback-mirror or the receive depending on whether it is the loopback-mirror or the
loopback-source. loopback-source.
If the answer does not contain a=loopback-mirror or If the answer does not contain a=loopback-mirror or
a=loopback-source, it is assumed that the loopback extensions are a=loopback-source, it is assumed that the loopback extensions are
not supported by the target UA. not supported by the target UA.
5.7 Modifying the Session 5.6 Modifying the Session
At any point during the loopback session, either participant may At any point during the loopback session, either participant may
issue a new offer to modify the characteristics of the previous issue a new offer to modify the characteristics of the previous
session. In case of SIP this is defined in section 8 of RFC 3264 session. In case of SIP this is defined in section 8 of RFC 3264
[RFC3264]. This also includes transitioning from a normal media [RFC3264]. This also includes transitioning from a normal media
processing mode to loopback mode, and vice a versa. processing mode to loopback mode, and vice a versa.
5.8 Priming the loopback pump 5.7 Priming the loopback pump
In certain scenarios it is possible that the media transmitted by ICE/STUN/TURN provide a general solution to establishing media
the loopback-source is blocked by a network element until the sessions between entities that are behind NATs. Loopback sessions
loopback-mirror starts transmitting packets. One example of this that involve one or more end points behind NATs SHOULD use these
scenario is the presence of an RTP relay in the path of the media. general solutions wherever possible. In scenarios where
RTP relays exist in VoIP networks for purpose of NAT and Firewall ICE/STUN/TURN is not available and where the loopback-mirror is
traversal. If an RTP relay is present, the loopback-source's behind a NAT and the loopback-source has a public IP address, the
packets are dropped by the RTP relay until the loopback-mirror has following solution MAY be adapted. Note that this solution
started transmitting media and the media state within the RTP relay addresses only a small subset of all possible use cases but
is established. This results in a chicken and egg scenario as the addresses the expected dominant use case for the loopback
looback-mirror cannot transmit any media until it receives the application.
media packets from the loopback-source but for the loopback-mirror
to receive any packets it needs to send one first. In order to
resolve this dilemma, Section 8 introduces a new payload type whose
sole purpose is to establish the media state in the intermediate
devices. In the presence of this payload type, the loopback-mirror
will transmit media according to the payload description until it
receives media from the loopback-source. The loopback-mirror MAY
include this payload type in the answer if it is not present in the
offer. This may be necessary if the loopback-mirror is aware of
NAT's, firewalls, or RTP relays on the path of the call. In this
case the loopback-source MUST accept media corresponding to this
payload type. After the first media packet is received from the
loopback-source, the loopback-mirror MUST terminate the
transmission of media for this payload type and MUST start looping
back media as defined by the other loopback attributes present in
the offer.
6. RTP Requirements If only the loopback-mirror is behind a NAT, it is possible that
An answering entity that is compliant to this specification and the media transmitted by the loopback-source is blocked by a
network element until the loopback-mirror starts transmitting
packets. One example of this scenario is the presence of an RTP
relay in the path of the media. RTP relays exist in VoIP networks
for purpose of NAT and Firewall traversal. If an RTP relay is
present, the loopback-source's packets are dropped by the RTP relay
until the loopback-mirror has started transmitting media and the
media state within the RTP relay is established. This results in a
chicken and egg scenario as the looback-mirror cannot transmit any
media until it receives the media packets from the loopback-source
but for the loopback-mirror to receive any packets it needs to send
one first. In order to resolve this dilemma, Section 8 introduces a
new media format whose sole purpose is to establish the media state
in the intermediate devices. In the presence of this media format,
the loopback-mirror will transmit media according to the payload
description until it receives media from the loopback-source. The
loopback-mirror MAY include this media format in the answer if it
is not present in the offer. This may be necessary if the
loopback-mirror is aware of NATs, firewalls, or RTP relays on the
path of the call. In this case the loopback-source MUST accept
media corresponding to this media format. After the first media
packet is received from the loopback-source, the loopback-mirror
MUST terminate the transmission of media for this payload type and
MUST start looping back media as defined by the other loopback
attributes present in the offer.
6. RTP Requirements
A loopback-mirror that is compliant to this specification and
accepting a media with rtp-pkt-loopback loopback-type MUST loopback accepting a media with rtp-pkt-loopback loopback-type MUST loopback
the incoming RTP packets using either the encapsulated RTP payload the incoming RTP packets using either the encapsulated RTP payload
format or the direct loopback RTP payload format as defined in format or the direct loopback RTP payload format as defined in
section 7 of this specification. section 7 of this specification.
An answering entity that is compliant to this specification and An answering entity that is compliant to this specification and
accepting a media with the loopback type rtp-media-loopback MUST accepting a media with the loopback type rtp-media-loopback MUST
transmit all received media back to the sender. The incoming media transmit all received media back to the sender. The incoming media
MUST be treated as if it were to be played (e.g. the media stream MUST be treated as if it were to be played (e.g. the media stream
MAY receive treatment from PLC algorithms). The answering entity MAY receive treatment from PLC algorithms). The answering entity
MUST re-generate all the RTP header fields as it would when MUST re-generate all the RTP header fields as it would when
transmitting media. The answering entity MAY choose to encode the transmitting media. The answering entity MAY choose to encode the
loopback media according to any of the media descriptions supported loopback media according to any of the media descriptions supported
by the offering entity. Furthermore, in cases where the same media by the offering entity. Furthermore, in cases where the same media
type is looped back, the answering entity MAY choose to preserve type is looped back, the answering entity MAY choose to preserve
number of frames/packet and bitrate of the encoded media according number of frames/packet and bitrate of the encoded media according
to the received media. to the received media.
7. Payload formats for Packet loopback 7. Payload formats for Packet loopback
The payload formats described in this section MUST be used by a The payload formats described in this section MUST be used by a
loopback-mirror when rtp-pkt-loopback is the specified loopback-mirror when rtp-pkt-loopback is the specified
loopback-type. Two different formats are specified here - an loopback-type. Two different formats are specified here - an
encapsulated RTP payload format and a direct loopback RTP payload encapsulated RTP payload format and a direct loopback RTP payload
format. The encapsulated RTP payload format should be used when format. The encapsulated RTP payload format should be used when
the incoming RTP header information needs to be preserved during the incoming RTP header information needs to be preserved during
the loopback operation. This is useful in cases where loopback the loopback operation. This is useful in cases where loopback
source needs to measure performance metrics in both directions. source needs to measure performance metrics in both directions.
However, this comes at the expense of increased packet size as However, this comes at the expense of increased packet size as
skipping to change at page 12, line 5 skipping to change at page 14, line 18
loopback-mirror. As described in RFC 3550 [RFC3550], sequence loopback-mirror. As described in RFC 3550 [RFC3550], sequence
numbers and timestamps in the RTP header are generated with initial numbers and timestamps in the RTP header are generated with initial
random values for security reasons. If this were not mandated and random values for security reasons. If this were not mandated and
the source payload is sequence number aware, the loopback-mirror the source payload is sequence number aware, the loopback-mirror
will be required to understand that payload format to generate will be required to understand that payload format to generate
looped back packets that do not violate RFC 3550 [RFC3550]. looped back packets that do not violate RFC 3550 [RFC3550].
Requiring looped back packets to be in one of the two formats means Requiring looped back packets to be in one of the two formats means
loopback-mirror does not have to look into the actual payload loopback-mirror does not have to look into the actual payload
received before generating the loopback packets. received before generating the loopback packets.
7.1 Encapsulated Payload format 7.1 Encapsulated Payload format
A received RTP packet is encapsulated in the payload section of the A received RTP packet is encapsulated in the payload section of the
RTP packet generated by a loopback-mirror. Each received packet RTP packet generated by a loopback-mirror. Each received packet
MUST be encapsulated in a different packet, the encapsulated packet MUST be encapsulated in a different packet, the encapsulated packet
MAY be fragmented only if required (for example: due to MTU MUST be fragmented only if required (for example: due to MTU
limitations). limitations).
7.1.1 Usage of RTP Header fields 7.1.1 Usage of RTP Header fields
Payload Type (PT): The assignment of an RTP payload type for this Payload Type (PT): The assignment of an RTP payload type for this
packet format is outside the scope of this document; it is either packet format is outside the scope of this document; it is either
specified by the RTP profile under which this payload format is specified by the RTP profile under which this payload format is
used or more likely signaled dynamically out-of-band (e.g., using used or more likely signaled dynamically out-of-band (e.g., using
SDP; section 7.1.3 defines the name binding). SDP; section 7.1.3 defines the name binding).
skipping to change at page 14, line 5 skipping to change at page 16, line 18
set to 10; otherwise the packet that contains the first fragments set to 10; otherwise the packet that contains the first fragments
sets this field to 00, the packet that contains the last fragment sets this field to 00, the packet that contains the last fragment
sets this field to 01, all other packets set this field to 11. sets this field to 01, all other packets set this field to 11.
Reserved: 2 bits Reserved: 2 bits
This field is reserved for future definition. In the absence of This field is reserved for future definition. In the absence of
such a definition, the bits in this field MUST be set to zero and such a definition, the bits in this field MUST be set to zero and
MUST be ignored by the receiver. MUST be ignored by the receiver.
Any padding octets in the original packet MUST not be included in Any padding octets in the original packet MUST NOT be included in
the loopback packet generated by a loopback-mirror. The the loopback packet generated by a loopback-mirror. The
loopback-mirror MAY add padding octets if required. loopback-mirror MAY add padding octets if required.
7.1.3 Usage of SDP 7.1.3 Usage of SDP
The payload type number for the encapsulated stream can be The payload type number for the encapsulated stream can be
negotiated using a mechanism like SDP. There is no static payload negotiated using a mechanism like SDP. There is no static payload
type assignment for the encapsulated stream, so dynamic payload type assignment for the encapsulated stream, so dynamic payload
type numbers MUST be used. The binding to the name is indicated by type numbers MUST be used. The binding to the name is indicated by
an rtpmap attribute. The name used in this binding is "encaprtp". an rtpmap attribute. The name used in this binding is "encaprtp".
The following is an example SDP fragment for encapsulated RTP. The following is an example SDP fragment for encapsulated RTP.
m=audio 41352 RTP/AVP 112 m=audio 41352 RTP/AVP 112
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
7.2 Direct loopback RTP payload format 7.2 Direct loopback RTP payload format
The direct loopback RTP payload format can be used in scenarios The direct loopback RTP payload format can be used in scenarios
where the 16 byte overhead of the encapsulated payload format is where the 16 byte overhead of the encapsulated payload format is
significant. This payload format MUST not be used in cases where significant. This payload format MUST NOT be used in cases where
the MTU on the loopback path will cause fragmentation of looped the MTU on the loopback path will cause fragmentation of looped
back RTP packets. When using this payload format, the receiver back RTP packets. When using this payload format, the receiver
MUST loop back each received packet in a separate RTP packet. MUST loop back each received packet in a separate RTP packet.
7.2.1 Usage of RTP Header fields 7.2.1 Usage of RTP Header fields
Payload Type (PT): The assignment of an RTP payload type for this Payload Type (PT): The assignment of an RTP payload type for this
packet format is outside the scope of this document; it is either packet format is outside the scope of this document; it is either
specified by the RTP profile under which this payload format is specified by the RTP profile under which this payload format is
used or more likely signaled dynamically out-of-band (e.g., using used or more likely signaled dynamically out-of-band (e.g., using
SDP; section 7.2.3 defines the name binding). SDP; section 7.2.3 defines the name binding).
Marker (M) bit: Set to the value in the received packet. Marker (M) bit: Set to the value in the received packet.
Extension (X) bit: Defined by the RTP Profile used. Extension (X) bit: Defined by the RTP Profile used.
skipping to change at page 15, line 33 skipping to change at page 18, line 5
type assignment for the stream, so dynamic payload type numbers type assignment for the stream, so dynamic payload type numbers
MUST be used. The binding to the name is indicated by an rtpmap MUST be used. The binding to the name is indicated by an rtpmap
attribute. The name used in this binding is "rtploopback". attribute. The name used in this binding is "rtploopback".
The following is an example SDP fragment for direct loopback RTP The following is an example SDP fragment for direct loopback RTP
format. format.
m=audio 41352 RTP/AVP 112 m=audio 41352 RTP/AVP 112
a=rtpmap:112 rtploopback/8000 a=rtpmap:112 rtploopback/8000
8. Payload format for Priming the Loopback Pump 8. Payload format for Priming the Loopback Pump
The sole purpose of the payload format described in this section is The sole purpose of the payload format described in this section is
to prime the loopback pump in cases where the loopback process to prime the loopback pump in cases where the loopback process
cannot start because of intermediate devices in the network as cannot start because of intermediate devices in the network as
described in Section 5.7. described in Section 5.7. The loopback-mirror MAY send payload data
of any length and any content as it desires and the loopback-source
MUST NOT interpret the payload data. This payload format MUST NOT
be used for any purpose other than priming the loopback pump.
8.1 Usage of RTP Header fields 8.1 Usage of RTP Header fields
Payload Type (PT): The assignment of an RTP payload type for this Payload Type (PT): The assignment of an RTP payload type for this
packet format is outside the scope of this document; it is either packet format is outside the scope of this document; it is either
specified by the RTP profile under which this payload format is specified by the RTP profile under which this payload format is
used or more likely signaled dynamically out-of-band (e.g., using used or more likely signaled dynamically out-of-band (e.g., using
SDP; section 8.2 defines the name binding). SDP; section 8.2 defines the name binding).
All other fields are set as described in RFC 3550 [RFC3550]. All other fields are set as described in RFC 3550 [RFC3550].
8.2 Usage of SDP 8.2 Usage of SDP
The payload type number for the loopback primer stream can be The payload type number for the loopback primer stream can be
negotiated using a mechanism like SDP. There is no static payload negotiated using a mechanism like SDP. There is no static payload
type assignment for the loopback primer stream, so dynamic payload type assignment for the loopback primer stream, so dynamic payload
type numbers MUST be used. The binding to the name is indicated by type numbers MUST be used. The binding to the name is indicated by
an rtpmap attribute. The name used in this binding is an rtpmap attribute. The name used in this binding is
"loopbkprimer". "loopbkprimer".
The following is an example SDP fragment for loopback primer RTP The following is an example SDP fragment for loopback primer RTP
stream. stream.
m=audio 41352 RTP/AVP 112 m=audio 41352 RTP/AVP 112
a=rtpmap:112 loopbkprimer/8000 a=rtpmap:112 loopbkprimer/8000
9. RTCP Requirements 9. RTCP Requirements
The use of the loopback attribute is intended for monitoring of The use of the loopback attribute is intended for monitoring of
media quality of the session. Consequently the media performance media quality of the session. Consequently the media performance
information should be exchanged between the offering and the information should be exchanged between the offering and the
answering entities. An offering or answering entity that is answering entities. An offering or answering entity that is
compliant to this specification SHOULD support RTCP per [RFC3550] compliant to this specification SHOULD support RTCP per [RFC3550]
and RTCP-XR per RFC 3611 [RFC3611]. Furthermore, if the client or and RTCP-XR per RFC 3611 [RFC3611]. Furthermore, if the client or
the server choose to support RTCP-XR, they SHOULD support RTCP-XR the server choose to support RTCP-XR, they SHOULD support RTCP-XR
Loss RLE report block, Duplicate RLE report block, Statistics Loss RLE report block, Duplicate RLE report block, Statistics
Summary report block, and VoIP Metric Reports Block per sections Summary report block, and VoIP Metric Reports Block per sections
4.1, 4.2, 4.6, and 4.7 of RFC 3611 [RFC3611]. The client and the 4.1, 4.2, 4.6, and 4.7 of RFC 3611 [RFC3611]. The client and the
server MAY support other RTCP-XR reporting blocks as defined by RFC server MAY support other RTCP-XR reporting blocks as defined by RFC
3611 [RFC3611]. 3611 [RFC3611].
10. Congestion Control 10. Congestion Control
All the participants in a loopback session SHOULD implement All the participants in a loopback session SHOULD implement
congestion control mechanisms as defined by the RTP profile under congestion control mechanisms as defined by the RTP profile under
which the loopback mechanism is implemented. For audio video which the loopback mechanism is implemented. For audio video
profiles, implementations SHOULD conform to the mechanism defined profiles, implementations SHOULD conform to the mechanism defined
in Section 2 of RFC 3551. in Section 2 of RFC 3551.
11. Examples 11. Examples
This section provides examples for media descriptions using SDP for This section provides examples for media descriptions using SDP for
different scenarios. The examples are given for SIP-based different scenarios. The examples are given for SIP-based
transactions and are abbreviated and do not show the complete transactions and are abbreviated and do not show the complete
signaling for convenience. signaling for convenience.
11.1 Offer for specific media loopback type 11.1 Offer for specific media loopback type
A client sends an INVITE request with SDP which looks like: A client sends an INVITE request with offer SDP which looks like:
v=0 v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=Example s=Example
i=An example session i=An example session
e=alice@example.com e=alice@example.com
c=IN IP4 host.atlanta.example.com c=IN IP4 host.atlanta.example.com
t=0 0 t=0 0
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-source:0 a=loopback-source:0
a=rtpmap:0 pcmu/8000 a=rtpmap:0 pcmu/8000
The client is offering to source the media and expects the server The client is offering to source the media and expects the server
to mirror the RTP stream per rtp-media-loopback loopback type. to mirror the RTP stream per rtp-media-loopback loopback type.
A server sends a response with SDP which looks like: A server sends a response with answer SDP which looks like:
v=0 v=0
o=bob 2890844526 2890842807 IN IP4 host.biloxi.example.com o=bob 2890844526 2890842807 IN IP4 host.biloxi.example.com
s=Example s=Example
i=An example session i=An example session
e=bob@example.com e=bob@example.com
c=IN IP4 host.biloxi.example.com c=IN IP4 host.biloxi.example.com
t=0 0 t=0 0
m=audio 49270 RTP/AVP 0 m=audio 49270 RTP/AVP 0
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-mirror:0 a=loopback-mirror:0
a=rtpmap:0 pcmu/8000 a=rtpmap:0 pcmu/8000
The server is accepting to mirror the media from the client at the The server is accepting to mirror the media from the client at the
media level. media level.
11.2 Offer for choice of media loopback type 11.2 Offer for choice of media loopback type
A client sends an INVITE request with SDP which looks like:
A client sends an INVITE request with offer SDP which looks like:
v=0 v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=Example s=Example
i=An example session i=An example session
e=alice@example.com e=alice@example.com
c=IN IP4 host.atlanta.example.com c=IN IP4 host.atlanta.example.com
t=0 0 t=0 0
m=audio 49170 RTP/AVP 0 112 113 m=audio 49170 RTP/AVP 0 112 113
a=loopback:rtp-media-loopback rtp-pkt-loopback a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source:0 a=loopback-source:0
a=rtpmap:0 pcum/8000 a=rtpmap:0 pcmu/8000
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
a=rtpmap:113 rtploopback/8000 a=rtpmap:113 rtploopback/8000
The client is offering to source the media and expects the server The client is offering to source the media and expects the server
to mirror the RTP stream at either the media or rtp level. to mirror the RTP stream at either the media or rtp level.
A server sends a response with SDP which looks like: A server sends a response with answer SDP which looks like:
v=0 v=0
o=box 2890844526 2890842807 IN IP4 host.biloxi.example.com o=box 2890844526 2890842807 IN IP4 host.biloxi.example.com
s=Example s=Example
i=An example session i=An example session
e=bob@example.com e=bob@example.com
c=IN IP4 host.biloxi.example.com c=IN IP4 host.biloxi.example.com
t=0 0 t=0 0
m=audio 49270 RTP/AVP 112 m=audio 49270 RTP/AVP 0 112
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-mirror:0 a=loopback-mirror:0
a=rtpmap:0 pcmu/8000 a=rtpmap:0 pcmu/8000
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
The server is accepting to mirror the media from the client at the The server is accepting to mirror the media from the client at the
packet level using the encapsulated RTP payload format. packet level using the encapsulated RTP payload format.
11.3 Offer for choice of media loopback type with loopback primer 11.3 Offer for choice of media loopback type with loopback primer
A client sends an INVITE request with SDP which looks like: A client sends an INVITE request with offer SDP which looks like:
v=0 v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=Example s=Example
i=An example session i=An example session
e=alice@example.com e=alice@example.com
c=IN IP4 host.atlanta.example.com c=IN IP4 host.atlanta.example.com
t=0 0 t=0 0
m=audio 49170 RTP/AVP 0 112 113 114 m=audio 49170 RTP/AVP 0 112 113 114
a=loopback:rtp-media-loopback rtp-pkt-loopback a=loopback:rtp-media-loopback rtp-pkt-loopback
skipping to change at page 19, line 30 skipping to change at page 21, line 46
A server sends a response with SDP which looks like: A server sends a response with SDP which looks like:
v=0 v=0
o=bob 2890844526 2890842807 IN IP4 host.biloxi.example.com o=bob 2890844526 2890842807 IN IP4 host.biloxi.example.com
s=Example s=Example
i=An example session i=An example session
e=user@example.com e=user@example.com
c=IN IP4 host.biloxi.example.com c=IN IP4 host.biloxi.example.com
t=0 0 t=0 0
m=audio 49270 RTP/AVP 113 m=audio 49270 RTP/AVP 0 113 114
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-mirror:0 114 a=loopback-mirror:0 114
a=rtpmap:0 pcmu/8000 a=rtpmap:0 pcmu/8000
a=rtpmap:113 rtploopback/8000 a=rtpmap:113 rtploopback/8000
a=rtmpa:114 loopbkprimer/8000 a=rtmap:114 loopbkprimer/8000
The server is accepting to mirror the media from the client at the The server is accepting to mirror the media from the client at the
packet level using the direct loopback RTP payload format. The packet level using the direct loopback RTP payload format. The
server is also accepting to source media until it receives media server is also accepting to source media until it receives media
packets from the client. packets from the client.
11.4 Response to INVITE request rejecting loopback media 11.4 Response to INVITE request rejecting loopback media
A client sends an INVITE request with SDP which looks like: A client sends an INVITE request with offer SDP which looks like:
v=0 v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=Example s=Example
i=An example session i=An example session
e=user@example.com e=user@example.com
c=IN IP4 host.atlanta.example.com c=IN IP4 host.atlanta.example.com
t=0 0 t=0 0
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-source:0 a=loopback-source:0
a=rtpmap:0 pcmu/8000 a=rtpmap:0 pcmu/8000
The client is offering to source the media and expects the server The client is offering to source the media and expects the server
to mirror the RTP stream at the media level. to mirror the RTP stream at the media level.
A server sends a response with SDP which looks like: A server sends a response with answer SDP which looks like:
v=0 v=0
o=bob 2890844526 2890842807 IN IP4 host.biloxi.example.com o=bob 2890844526 2890842807 IN IP4 host.biloxi.example.com
s=Example s=Example
i=An example session i=An example session
e=user@example.com e=user@example.com
c=IN IP4 host.biloxi.example.com c=IN IP4 host.biloxi.example.com
t=0 0 t=0 0
m=audio 0 RTP/AVP 0 m=audio 0 RTP/AVP 0
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-mirror:0 a=loopback-mirror:0
a=rtpmap:0 pcmu/8000 a=rtpmap:0 pcmu/8000
NOTE: Loopback request may be rejected by either not including the NOTE: Loopback request may be rejected by either not including the
loopback mode attribute (for backward compatibility) or setting the loopback mode attribute (for backward compatibility) or setting the
media port number to zero, or both, in the response. media port number to zero, or both, in the response.
11.5 Response to INVITE request rejecting loopback media with loopback 11.5 Response to INVITE request rejecting loopback media with
primer payload type loopback primer payload type
A client sends an INVITE request with SDP which looks like: A client sends an INVITE request with offer SDP which looks like:
v=0 v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=Example s=Example
i=An example session i=An example session
e=alice@example.com e=alice@example.com
c=IN IP4 host.atlanta.example.com c=IN IP4 host.atlanta.example.com
t=0 0 t=0 0
m=audio 49170 RTP/AVP 0 100 m=audio 49170 RTP/AVP 0 100
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-source:0 a=loopback-source:0
a=rtpmap:0 pcum/8000 a=rtpmap:0 pcmu/8000
a=rtpmap:100 loopbkprimer/8000 a=rtpmap:100 loopbkprimer/8000
The client is offering to source the media and expects the server The client is offering to source the media and expects the server
to mirror the RTP stream at the media level. The client also to mirror the RTP stream at the media level. The client (offerer)
expects the server to source media until it receives packets from also expects the server (answerer) to source media until it
the server using the loopbkprimer payload type. receives packets from the server using the loopbkprimer payload
type.
A server sends a response with SDP which looks like: A server sends a response with answer SDP which looks like:
v=0 v=0
o=bob 2890844526 2890842807 IN IP4 host.biloxi.example.com o=bob 2890844526 2890842807 IN IP4 host.biloxi.example.com
s=Example s=Example
i=An example session i=An example session
e=bob@example.com e=bob@example.com
c=IN IP4 host.biloxi.example.com c=IN IP4 host.biloxi.example.com
t=0 0 t=0 0
m=audio 0 RTP/AVP 0 m=audio 0 RTP/AVP 0
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-mirror:0 a=loopback-mirror:0
NOTE: Loopback request may be rejected by either not including the NOTE: Loopback request may be rejected by either not including the
loopback mode attribute (for backward compatibility) or setting the loopback mode attribute (for backward compatibility) or setting the
media port number to zero, or both, in the response. media port number to zero, or both, in the response.
12. Security Considerations 12. Security Considerations
The security considerations of [RFC3261] apply. Furthermore, given The security considerations of [RFC3261] and [RFC3264] apply.
that media loopback may be automated without the end user's Furthermore, given that media loopback may be automated without the
knowledge, the server of the media loopback should be aware of end user's knowledge, the server of the media loopback should be
denial of service attacks. It is recommended that sessions with aware of denial of service attacks. It is recommended that sessions
media loopback are authenticated and the frequency of such sessions with media loopback are authenticated and the frequency of such
is limited by the server. sessions is limited by the server.
13. Implementation Considerations 13. Implementation Considerations
The media loopback approach described in this document is a The media loopback approach described in this document is a
complete solution that would work under all scenarios. However, it complete solution that would work under all scenarios. However, it
is believed that the solution may not be light-weight enough for is believed that the solution may not be light-weight enough for
the common case. In light of this concern, this section clarifies the common case. In light of this concern, this section clarifies
which features of the loopback proposal MUST be implemented for all which features of the loopback proposal MUST be implemented for all
implementations and which features MAY be deferred if the complete implementations and which features MAY be deferred if the complete
solution is not desired. solution is not desired.
All implementations MUST support the rtp-pkt-loopback option for All implementations MUST support the rtp-pkt-loopback option for
loopback-type attribute. In addition, for the loopback-mode loopback-type attribute. In addition, for the loopback-mode
attribute, all implementations of an offerer MUST at a minimum be attribute, all implementations of an offerer MUST at a minimum be
able to act as a loopback-source. All implementation MUST also at a able to act as a loopback-source. All implementation MUST also at a
minimum support the direct media loopback payload type. The rtp- minimum support the direct media loopback payload type. The rtp-
media-loopback attribute MAY be implemented in complete media-loopback attribute MAY be implemented in complete
implementations of this draft. implementations of this draft.
14. IANA Considerations 14. IANA Considerations
14.1 SDP Attributes 14.1 SDP Attributes
This document defines three new media-level SDP attributes. IANA This document defines three new media-level SDP attributes. IANA
has registered the following attributes: has registered the following attributes:
Contact name: Kaynam Hedayat Contact name: Kaynam Hedayat
<kaynam.hedayat@exfo.com>. <kaynam.hedayat@exfo.com>.
Attribute name: "loopback". Attribute name: "loopback".
Type of attribute: Media level. Type of attribute: Media level.
Subject to charset: No. Subject to charset: No.
Purpose of attribute: The 'loopback' attribute is used to Purpose of attribute: The 'loopback' attribute is used to
skipping to change at page 23, line 4 skipping to change at page 25, line 24
Contact name: Kaynam Hedayat Contact name: Kaynam Hedayat
<kaynam.hedayat@exfo.com>. <kaynam.hedayat@exfo.com>.
Attribute name: "loopback-mirror". Attribute name: "loopback-mirror".
Type of attribute: Media level. Type of attribute: Media level.
Subject to charset: No. Subject to charset: No.
Purpose of attribute: The 'loopback-mirror' attribute Purpose of attribute: The 'loopback-mirror' attribute
specifies that the receiver will specifies that the receiver will
mirror (echo) all received media back mirror (echo) all received media back
to the sender of the RTP stream. to the sender of the RTP stream.
Allowed attribute values: The parameter to 'loopback-mirror' is Allowed attribute values: The parameter to 'loopback-mirror' is
a media format ("<fmt>") description a media format ("<fmt>") description
as defined in RFC 4566 Section 5.14. as defined in RFC 4566 Section 5.14.
14.2 MIME Types 14.2 MIME Types
The IANA has registered the following MIME types: The IANA has registered the following MIME types:
14.2.1 audio/encaprtp 14.2.1 audio/encaprtp
To: ietf-types@iana.org To: ietf-types@iana.org
Subject: Registration of media type audio/encaprtp Subject: Registration of media type audio/encaprtp
Type name: audio Type name: audio
skipping to change at page 36, line 30 skipping to change at page 39, line 5
framing, and hence is only defined for transfer via framing, and hence is only defined for transfer via
RTP. Transfer within other framing protocols is not RTP. Transfer within other framing protocols is not
defined at this time. defined at this time.
Author: Author:
Kaynam Hedayat. Kaynam Hedayat.
Change controller: IETF Audio/Video Transport working Change controller: IETF Audio/Video Transport working
group delegated from the IESG. group delegated from the IESG.
15. Acknowledgements 15. Additional Authors and Acknowledgements
The authors wish to thank Nagarjuna Venna, Muthu ArulMozhi Perumal, The following people have contributed to the task of authoring this
Flemming Andreasen, Jeff Bernstein, Paul Kyzivat, and Dave Oran for document: Chelliah Sivachelvan (Cisco).
their comments and suggestions.
16. Normative References The authors acknowledge the contributions and comments of , Muthu
ArulMozhi Perumal, Flemming Andreasen, Jeff Bernstein, Paul
Kyzivat, and Dave Oran.
16. Normative References
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G.,
Johnston, A., Peterson, J., Sparks, R., Handley, M. Johnston, A., Peterson, J., Sparks, R., Handley, M.
and E. Schooler, "SIP: Session Initiation Protocol", and E. Schooler, "SIP: Session Initiation Protocol",
RFC 3261, June 2002. RFC 3261, June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer
Model with the Session Description Protocol (SDP)", Model with the Session Description Protocol (SDP)",
RFC 3264, June 2002. RFC 3264, June 2002.
skipping to change at page 38, line 4 skipping to change at page 40, line 26
Kaynam Hedayat Kaynam Hedayat
EXFO EXFO
285 Mill Road 285 Mill Road
Chelmsford, MA 01824 Chelmsford, MA 01824
US US
Phone: +1 978 367 5611 Phone: +1 978 367 5611
EMail: kaynam.hedayat@exfo.com EMail: kaynam.hedayat@exfo.com
URI: http://www.exfo.com/ URI: http://www.exfo.com/
Nagarjuna Venna Nagarjuna Venna
EXFO Saperix
285 Mill Road 738 Main Street, #398
Chelmsford, MA 01824 Waltham, MA 02451
US US
Phone: +1 978 367 5703 Phone: +1 978 367 5703
EMail: nagarjuna.venna@exfo.com EMail: vnagarjuna@saperix.com
URI: http://www.exfo.com/ URI: http://www.saperix.com/
Paul E. Jones Paul E. Jones
Cisco Systems, Inc. Cisco Systems, Inc.
7025 Kit Creek Rd. 7025 Kit Creek Rd.
Research Triangle Park, NC 27709 Research Triangle Park, NC 27709
US US
Phone: +1 919 392 6948 Phone: +1 919 392 6948
EMail: paulej@packetizer.com EMail: paulej@packetizer.com
URI: http://www.cisco.com/ URI: http://www.cisco.com/
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