draft-ietf-mmusic-media-loopback-27.txt   rfc6849.txt 
MMUSIC Working Group H. Kaplan (ed.)
Internet-Draft Acme Packet
Intended status: Proposed Standard K. Hedayat
Expires: July 14, 2013 EXFO
N. Venna
Saperix
P. Jones
Cisco Systems, Inc.
N. Stratton
BlinkMind, Inc.
January 14, 2013
An Extension to the Session Description Protocol (SDP)
and Real-time Transport Protocol (RTP) for Media Loopback
draft-ietf-mmusic-media-loopback-27
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with
the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-Drafts
as reference material or to cite them other than as "work in
progress".
The list of current Internet-Drafts can be accessed at Internet Engineering Task Force (IETF) H. Kaplan, Ed.
http://www.ietf.org/1id-abstracts.html Request for Comments: 6849 Acme Packet
Category: Standards Track K. Hedayat
The list of Internet-Draft Shadow Directories can be accessed at ISSN: 2070-1721 EXFO
http://www.ietf.org/shadow.html. N. Venna
Saperix
P. Jones
Cisco Systems, Inc.
N. Stratton
BlinkMind, Inc.
February 2013
This Internet-Draft will expire on July 14, 2013. An Extension to the Session Description Protocol (SDP)
and Real-time Transport Protocol (RTP) for Media Loopback
Copyright Notice Abstract
Copyright (c) 2012 IETF Trust and the persons identified as the The wide deployment of Voice over IP (VoIP), real-time text, and
document authors. All rights reserved. Video over IP services has introduced new challenges in managing and
maintaining real-time voice/text/video quality, reliability, and
overall performance. In particular, media delivery is an area that
needs attention. One method of meeting these challenges is
monitoring the media delivery performance by looping media back to
the transmitter. This is typically referred to as "active
monitoring" of services. Media loopback is especially popular in
ensuring the quality of transport to the edge of a given VoIP, real-
time text, or Video over IP service. Today, in networks that deliver
real-time media, short of running 'ping' and 'traceroute' to the
edge, administrators are left without the necessary tools to actively
monitor, manage, and diagnose quality issues with their service. The
extension defined herein adds new Session Description Protocol (SDP)
media types and attributes that enable establishment of media
sessions where the media is looped back to the transmitter. Such
media sessions will serve as monitoring and troubleshooting tools by
providing the means for measurement of more advanced VoIP, real-time
text, and Video over IP performance metrics.
This document is subject to BCP 78 and the IETF Trust's Legal Status of This Memo
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with
respect to this document. Code Components extracted from this
document must include Simplified BSD License text as described in
Section 4.e of the Trust Legal Provisions and are provided without
warranty as described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF This is an Internet Standards Track document.
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s)
controlling the copyright in such materials, this document may not
be modified outside the IETF Standards Process, and derivative
works of it may not be created outside the IETF Standards Process,
except to format it for publication as an RFC or to translate it
into languages other than English.
Abstract This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
The wide deployment of Voice over IP (VoIP), Text and Video over IP Information about the current status of this document, any errata,
services has introduced new challenges in managing and maintaining and how to provide feedback on it may be obtained at
real-time voice/text/video quality, reliability, and overall http://www.rfc-editor.org/info/rfc6849.
performance. In particular, media delivery is an area that needs
attention. One method of meeting these challenges is monitoring
the media delivery performance by looping media back to the
transmitter. This is typically referred to as "active monitoring"
of services. Media loopback is especially popular in ensuring the
quality of transport to the edge of a given VoIP, Real-time Text or
Video over IP service. Today in networks that deliver real-time
media, short of running 'ping' and 'traceroute' to the edge,
administrators are left without the necessary tools to actively
monitor, manage, and diagnose quality issues with their service.
The extension defined herein adds new SDP media types and
attributes, which enable establishment of media sessions where the
media is looped back to the transmitter. Such media sessions will
serve as monitoring and troubleshooting tools by providing the
means for measurement of more advanced VoIP, Real-time Text and
Video over IP performance metrics.
Table of Contents Copyright Notice
1. Introduction..................................................3 Copyright (c) 2013 IETF Trust and the persons identified as the
1.1 Use Cases Supported.......................................4 document authors. All rights reserved.
2. Terminology...................................................6
3. Overview of Operation.........................................6
3.1 SDP Offerer Behavior......................................6
3.2 SDP Answerer Behavior.....................................7
4. New SDP Attributes............................................7
4.1 Loopback Type Attribute...................................7
4.2 Loopback Role Attributes: loopback-source and loopback-
mirror........................................................8
5. Rules for Generating the SDP offer/answer.....................9
5.1 Generating the SDP Offer for Loopback Session.............9
5.2 Generating the SDP Answer for Loopback Session...........10
5.3 Offerer Processing of the SDP Answer.....................12
5.4 Modifying the Session....................................12
5.5 Establishing Sessions Between Entities Behind NAT........12
6. RTP Requirements.............................................13
7. Payload formats for Packet loopback..........................13
7.1 Encapsulated Payload format..............................14
7.2 Direct loopback RTP payload format.......................16
8. SRTP Behavior................................................17
9. RTCP Requirements............................................18
10. Congestion Control..........................................18
11. Examples....................................................18
11.1 Offer for specific media loopback type..................19
11.2 Offer for choice of media loopback type.................19
11.3 Answerer rejecting loopback media.......................20
12. Security Considerations.....................................21
13. Implementation Considerations...............................22
14. IANA Considerations.........................................22
14.1 SDP Attributes..........................................22
14.2 Media Types.............................................23
15. Acknowledgements............................................31
16. Normative References........................................31
17. Informative References......................................32
1. Introduction This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
The overall quality, reliability, and performance of VoIP, This document may contain material from IETF Documents or IETF
Real-time Text and Video over IP services rely on the performance Contributions published or made publicly available before November
and quality of the media path. In order to assure the quality of 10, 2008. The person(s) controlling the copyright in some of this
the delivered media there is a need to monitor the performance of material may not have granted the IETF Trust the right to allow
the media transport. One method of monitoring and managing the modifications of such material outside the IETF Standards Process.
overall quality of real-time VoIP, Text and Video over IP Services Without obtaining an adequate license from the person(s) controlling
is through monitoring the quality of the media in an active the copyright in such materials, this document may not be modified
session. This type of "active monitoring" of services is a method outside the IETF Standards Process, and derivative works of it may
of proactively managing the performance and quality of VoIP based not be created outside the IETF Standards Process, except to format
services. it for publication as an RFC or to translate it into languages other
than English.
The goal of active monitoring is to measure the media quality of a Table of Contents
VoIP, Text or Video over IP session. A way to achieve this goal is
to request an endpoint to loop media back to the other endpoint and
to provide media statistics (e.g., RTCP and RTCP-XR information).
Another method involves deployment of special endpoints that always
loop incoming media back for all sessions. Although the latter
method has been used and is functional, it does not scale to
support large networks and introduces new network management
challenges. Further, it does not offer the granularity of testing
a specific endpoint that may be exhibiting problems.
The extension defined in this document introduces new SDP media 1. Introduction ....................................................3
types and attributes that enable establishment of media sessions 1.1. Use Cases Supported ........................................4
where the media is looped back to the transmitter. The SDP 2. Terminology .....................................................6
offer/answer model [RFC3264] is used to establish a loopback 3. Overview of Operation ...........................................6
connection. Furthermore, this extension provides guidelines on 3.1. SDP Offerer Behavior .......................................6
handling RTP [RFC3550], as well as usage of RTP Control Protocol 3.2. SDP Answerer Behavior ......................................7
(RTCP) [RFC3550] and RTCP Extended Reports (RTCP-XR) [RFC3611] for 4. New SDP Attributes ..............................................7
reporting media related measurements. 4.1. Loopback-Type Attribute ....................................7
4.2. Loopback-Role Attributes: loopback-source and
loopback-mirror ............................................8
5. Rules for Generating the SDP Offer/Answer .......................9
5.1. Generating the SDP Offer for Loopback Session ..............9
5.2. Generating the SDP Answer for Loopback Session ............10
5.3. Offerer Processing of the SDP Answer ......................12
5.4. Modifying the Session .....................................12
5.5. Establishing Sessions between Entities behind NATs ........12
6. RTP Requirements ...............................................13
7. Payload Formats for Packet Loopback ............................13
7.1. Encapsulated Payload Format ...............................14
7.2. Direct Loopback RTP Payload Format ........................16
8. SRTP Behavior ..................................................17
9. RTCP Requirements ..............................................18
10. Congestion Control ............................................18
11. Examples ......................................................18
11.1. Offer for Specific Media Loopback Type ...................19
11.2. Offer for Choice of Media Loopback Type ..................19
11.3. Answerer Rejecting Loopback Media ........................20
12. Security Considerations .......................................21
13. Implementation Considerations .................................22
14. IANA Considerations ...........................................22
14.1. SDP Attributes ...........................................22
14.2. Media Types ..............................................23
15. Acknowledgements ..............................................31
16. References ....................................................31
16.1. Normative References .....................................31
16.2. Informative References ...................................32
1.1 Use Cases Supported 1. Introduction
As a matter of terminology in this document, packets flow from one The overall quality, reliability, and performance of VoIP, real-time
peer acting as a "loopback source", to the other peer acting as a text, and Video over IP services rely on the performance and quality
"loopback mirror", which in turn returns packets to the loopback of the media path. In order to assure the quality of the delivered
source. In advance of the session, the peers negotiate to determine media, there is a need to monitor the performance of the media
which one acts in which role, using the SDP offer/answer exchange. transport. One method of monitoring and managing the overall quality
The negotiation also includes details such as the type of loopback of real-time VoIP, real-time text, and Video over IP services is
to be used. through monitoring the quality of the media in an active session.
This type of "active monitoring" of services is a method of
proactively managing the performance and quality of VoIP-based
services.
This specification supports three use cases: "encapsulated packet The goal of active monitoring is to measure the media quality of a
loopback", "direct loopback", and "media loopback". These are VoIP, real-time text, or Video over IP session. A way to achieve
distinguished by the treatment of incoming RTP packets at the this goal is to request an endpoint to loop media back to the other
loopback mirror. endpoint and to provide media statistics (e.g., RTP Control Protocol
(RTCP) [RFC3550] and RTCP Extended Reports (RTCP-XR) [RFC3611]
information). Another method involves deployment of special
endpoints that always loop incoming media back for all sessions.
Although the latter method has been used and is functional, it does
not scale to support large networks and introduces new network
management challenges. Further, it does not offer the granularity of
testing a specific endpoint that may be exhibiting problems.
1.1.1 Encapsulated Packet Loopback The extension defined in this document introduces new SDP media types
and attributes that enable establishment of media sessions where the
media is looped back to the transmitter. The SDP offer/answer model
[RFC3264] is used to establish a loopback connection. Furthermore,
this extension provides guidelines on handling RTP [RFC3550], as well
as usage of RTCP [RFC3550] and RTCP-XR [RFC3611] for reporting media-
related measurements.
In the encapsulated packet loopback case, the entire incoming RTP 1.1. Use Cases Supported
packet is encapsulated as payload within an outer RTP packet that
is specific to this use case and specified in 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 encapsulating
outer RTP header and the encapsulated RTP packet payload. The
loopback source can also play back the returned media content for
evaluation.
Because the encapsulating RTP packet header extends the packet As a matter of terminology in this document, packets flow from one
size, it could encounter difficulties in an environment where the peer acting as a "loopback source", to the other peer acting as a
original RTP packet size is close to the path Maximum Transmission "loopback mirror", which in turn returns packets to the loopback
Unit (MTU) size. The encapsulating payload format therefore offers source. In advance of the session, the peers negotiate to determine
the possibility of RTP-level fragmentation of the returned packets. which one acts in which role, using the SDP offer/answer exchange.
The use of this facility could affect statistics derived for the The negotiation also includes details such as the type of loopback to
return path. In addition, the increased bit rate required in the be used.
return direction may affect these statistics more directly in a
restricted-bandwidth situation.
1.1.2 Direct Loopback 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.
In the direct loopback case, the loopback mirror copies the payload 1.1.1. Encapsulated Packet Loopback
of the incoming RTP packet into a new RTP packet, using a payload
format specific to this use case and specified in 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 the encapsulated packet loopback case, the entire incoming RTP
in both directions. The packet source can compute only two-way packet is encapsulated as payload within an outer RTP packet that is
path statistics from the direct loopback packet header, but can specific to this use case and specified in Section 7.1. The
play back the returned media content. 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 encapsulating outer RTP header and the
encapsulated RTP packet payload. The loopback source can also play
back the returned media content for evaluation.
It has been suggested that the loopback source, knowing that the Because the encapsulating RTP packet header extends the packet size,
incoming packet will never be passed to a decoder, can store a it could encounter difficulties in an environment where the original
timestamp and sequence number inside the payload of the packet it RTP packet size is close to the path Maximum Transmission Unit (MTU)
sends to the mirror, then extract that information from the size. The encapsulating payload format therefore offers the
returned direct loopback packet and compute one-way path statistics possibility of RTP-level fragmentation of the returned packets. The
as in the previous case. Obviously, playout of returned content is use of this facility could affect statistics derived for the return
no longer possible if this is done. path. In addition, the increased bit rate required in the return
direction may affect these statistics more directly in a restricted-
bandwidth situation.
1.1.3 Media Loopback 1.1.2. Direct Loopback
In the media loopback case, the loopback mirror submits the In the direct loopback case, the loopback mirror copies the payload
incoming packet to a decoder appropriate to the incoming payload of the incoming RTP packet into a new RTP packet, using a payload
type. The packet is taken as close as possible to the analog level, format specific to this use case and specified in Section 7.2. The
then re-encoded according to an outgoing format determined by SDP loopback mirror returns the new packet to the packet source. There
negotiation. The reencoded content is returned to the loopback is no provision in this case for RTP-level fragmentation.
source as an RTP packet with payload type corresponding to the
reencoding format.
This usage allows trouble-shooting at the codec level. The This use case has the advantage of keeping the packet size the same
capability for path statistics is limited to what is available from in both directions. The packet source can compute only two-way path
RTCP reports. statistics from the direct loopback packet header but can play back
the returned media content.
2. Terminology 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.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 1.1.3. Media Loopback
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119
[RFC2119].
SDP: Session Description Protocol, as defined in [RFC4566]. This In the media loopback case, the loopback mirror submits the incoming
document assumes the SDP offer/answer model is followed, per packet to a decoder appropriate to the incoming payload type. The
[RFC3264], but does not assume any specific signaling protocol for packet is taken as close as possible to the analog level, then
carrying the SDP. re-encoded according to an outgoing format determined by SDP
negotiation. The re-encoded content is returned to the loopback
source as an RTP packet with payload type corresponding to the
re-encoding format.
The following terms are borrowed from [RFC3264] definitions: offer, This usage allows troubleshooting at the codec level. The capability
offerer, answer, answerer, and agent. for path statistics is limited to what is available from RTCP
reports.
3. Overview of Operation 2. Terminology
This document defines two loopback 'types', two 'roles', and two The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
encoding formats for loopback. For any given SDP offerer or "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
answerer pair, one side is the source of RTP packets, while the document are to be interpreted as described in RFC 2119 [RFC2119].
other is the mirror looping packets/media back. Those define the
two loopback roles. As the mirror, two 'types' of loopback can be
performed: packet-level or media-level. When media-level is used,
there is no further choice of encoding format - there is only one
format: whatever is indicated for normal media, since the "looping"
is performed at the codec level. When packet-level looping is
performed, however, the mirror can either send back RTP in an
encapsulated format or direct-loopback format. The rest of this
document describes these loopback types, roles, and encoding
formats, and the SDP offer/answer rules for indicating them.
3.1 SDP Offerer Behavior SDP: Session Description Protocol, as defined in [RFC4566]. This
document assumes that the SDP offer/answer model is followed,
per [RFC3264], but does not assume any specific signaling
protocol for carrying the SDP.
An SDP offerer compliant to this specification and attempting to The following terms are borrowed from [RFC3264] definitions: offer,
establish a media session with media loopback will include offerer, answer, answerer, and agent.
"loopback" media attributes for each individual media description
in the offer message that it wishes to have looped back. Note that
the offerer may choose to only request loop back for some media
descriptions/streams but not others. For example it might wish to
request loopback for a video stream but not audio, or vice-versa.
The offerer will look for the "loopback" media attributes in the 3. Overview of Operation
media description(s) of the response from the SDP answer for
confirmation that the request is accepted.
3.2 SDP Answerer Behavior This document defines two loopback 'types', two 'roles', and two
encoding formats for loopback. For any given SDP offerer or answerer
pair, one side is the source of RTP packets, while the other is the
mirror looping packets/media back. Those define the two loopback
roles. As the mirror, two 'types' of loopback can be performed:
packet-level or media-level. When media-level is used, there is no
further choice of encoding format -- there is only one format:
whatever is indicated for normal media, since the "looping" is
performed at the codec level. When packet-level looping is
performed, however, the mirror can either send back RTP in an
encapsulated format or direct loopback format. The rest of this
document describes these loopback types, roles, and encoding formats,
and the SDP offer/answer rules for indicating them.
In order to accept a loopback offer (that is, an offer containing 3.1. SDP Offerer Behavior
"loopback" in the media description), an SDP answerer includes the
"loopback" media attribute in each media description for which it
desires loopback.
An answerer can reject an offered stream (either with loopback- An SDP offerer compliant to this specification and attempting to
source or loopback-mirror) if the loopback-type is not specified, establish a media session with media loopback will include "loopback"
the specified loopback-type is not supported, or the endpoint media attributes for each individual media description in the offer
cannot honor the offer for any other reason. The loopback request message that it wishes to have looped back. Note that the offerer
is rejected by setting the stream's media port number to zero in may choose to only request loopback for some media
the answer as defined in RFC 3264 [RFC3264], or by rejecting the descriptions/streams but not others. For example, it might wish to
entire offer (i.e., by rejecting the session request entirely). request loopback for a video stream but not audio, or vice versa.
Note that an answerer that is not compliant to this specification The offerer will look for the "loopback" media attributes in the
and which receives an offer with the "loopback" media attributes media description(s) of the response from the SDP answer for
would ignore the attributes and treat the incoming offer as a confirmation that the request is accepted.
normal request. If the offerer does not wish to establish a
"normal" RTP session, it would need to terminate the session upon
receiving such an answer.
4. New SDP Attributes 3.2. SDP Answerer Behavior
Three new SDP media-level attributes are defined: one indicates the In order to accept a loopback offer (that is, an offer containing
type of loopback, and the other two define the role of the agent. "loopback" in the media description), an SDP answerer includes the
"loopback" media attribute in each media description for which it
desires loopback.
4.1 Loopback Type Attribute An answerer can reject an offered stream (either with loopback-source
or loopback-mirror) if the loopback-type is not specified, the
specified loopback-type is not supported, or the endpoint cannot
honor the offer for any other reason. The loopback request is
rejected by setting the stream's media port number to zero in the
answer as defined in RFC 3264 [RFC3264] or by rejecting the entire
offer (i.e., by rejecting the session request entirely).
This specification defines a new "loopback" attribute, which Note that an answerer that is not compliant to this specification and
indicates that the agent wishes to perform loopback, and the type that receives an offer with the "loopback" media attributes would
of loopack that the agent is able to do. The loopback-type is a ignore the attributes and treat the incoming offer as a normal
value media attribute [RFC4566] with the following syntax: request. If the offerer does not wish to establish a "normal" RTP
session, it would need to terminate the session upon receiving such
an answer.
a=loopback:<loopback-type> 4. New SDP Attributes
Following is the Augmented BNF [RFC5234] for loopback-type: Three new SDP media-level attributes are defined: one indicates the
type of loopback, and the other two define the role of the agent.
attribute /= loopback-attr 4.1. Loopback-Type Attribute
; attribute defined in RFC 4566
loopback-attr = "loopback:" SP loopback-type This specification defines a new "loopback" attribute, which
loopback-type = loopback-choice [1*SP loopback-choice] indicates that the agent wishes to perform loopback, and the type of
loopback-choice = loopback-type-pkt / loopback-type-media loopback that the agent is able to do. The loopback-type is a value
loopback-type-pkt = "rtp-pkt-loopback" media attribute [RFC4566] with the following syntax:
loopback-type-media = "rtp-media-loopback"
The loopback-type is used to indicate the type of loopback. The a=loopback:<loopback-type>
loopback-type values are rtp-pkt-loopback, and rtp-media-loopback.
rtp-pkt-loopback: In this mode, the RTP packets are looped back to Following is the Augmented BNF [RFC5234] for loopback-type:
the sender at a point before the encoder/decoder function in the
receive direction to a point after the encoder/decoder function in
the send direction. This effectively re-encapsulates the RTP
payload with the RTP/UDP/IP headers appropriate for sending it in
the reverse direction. Any type of encoding related functions,
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
new payload type and format. Section 7 describes the payload
formats that are to be used for this type of loopback. This type
of loopback applies to the encapsulated and direct loopback use-
cases described in Section 1.1.
rtp-media-loopback: This loopback is activated as close as possible attribute =/ loopback-attr
to the analog interface and after the decoder so that the RTP ; attribute defined in RFC 4566
packets are subsequently re-encoded prior to transmission back to
the sender. This type of loopback applies to the media loopback
use-case described in Section 1.1.3.
4.2 Loopback Role Attributes: loopback-source and loopback-mirror loopback-attr = "loopback:" SP loopback-type
loopback-type = loopback-choice [1*SP loopback-choice]
loopback-choice = loopback-type-pkt / loopback-type-media
loopback-type-pkt = "rtp-pkt-loopback"
loopback-type-media = "rtp-media-loopback"
The loopback-type is used to indicate the type of loopback. The
loopback-type values are rtp-pkt-loopback and rtp-media-loopback.
The loopback role defines two property media attributes [RFC4566] rtp-pkt-loopback: In this mode, the RTP packets are looped back to
that are used to indicate the role of the agent generating the SDP the sender at a point before the encoder/decoder function in the
offer or answer. The syntax of the two loopback role media receive direction to a point after the encoder/decoder function in
attributes are as follows: the send direction. This effectively re-encapsulates the RTP
payload with the RTP/UDP/IP headers appropriate for sending it in
the reverse direction. Any type of encoding-related functions,
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 new payload type and format. Section 7 describes the payload
formats that are to be used for this type of loopback. This type
of loopback applies to the encapsulated and direct loopback use
cases described in Section 1.1.
a=loopback-source rtp-media-loopback: This loopback is activated as close as possible
to the analog interface and after the decoder so that the RTP
packets are subsequently re-encoded prior to transmission back to
the sender. This type of loopback applies to the media loopback
use case described in Section 1.1.3.
and 4.2. Loopback-Role Attributes: loopback-source and loopback-mirror
a=loopback-mirror The loopback role defines two property media attributes [RFC4566]
that are used to indicate the role of the agent generating the SDP
offer or answer. The syntax of the two loopback-role media
attributes is as follows:
Following is the Augmented BNF [RFC5234] for loopback-type: a=loopback-source
attribute /= loopback-source / loopback-mirror and
; attribute defined in RFC 4566
loopback-source = "loopback-source"
loopback-mirror = "loopback-mirror"
loopback-source: This attribute specifies that the entity that 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 entity that Following is the Augmented BNF [RFC5234] for loopback-source and
generated the SDP will mirror (echo) all received media back to the loopback-mirror:
sender of the RTP stream. No media is generated locally by the
looping back entity for transmission in the mirrored stream.
The "m=" line in the SDP includes all the payload types that will attribute =/ loopback-source / loopback-mirror
be used during the loopback session. The complete payload space for ; attribute defined in RFC 4566
the session is specified in the "m=" line and the rtpmap attribute loopback-source = "loopback-source"
is used to map from the payload type number to an encoding name loopback-mirror = "loopback-mirror"
denoting the payload format to be used.
5. Rules for Generating the SDP offer/answer loopback-source: This attribute specifies that the entity that
generated the SDP is the media source and expects the receiver of
the SDP message to act as a loopback mirror.
5.1 Generating the SDP Offer for Loopback Session loopback-mirror: This attribute specifies that the entity that
generated the SDP will mirror (echo) all received media back to
the sender of the RTP stream. No media is generated locally by
the looping-back entity for transmission in the mirrored stream.
If an offerer wishes to make a loopback request, it includes both The "m=" line in the SDP includes all the payload types that will be
the loopback-type and loopback-role attributes in a valid SDP used during the loopback session. The complete payload space for the
offer: session is specified in the "m=" line, and the rtpmap attribute is
used to map from the payload type number to an encoding name denoting
the payload format to be used.
Example: m=audio 41352 RTP/AVP 0 8 100 5. Rules for Generating the SDP Offer/Answer
a=loopback:rtp-media-loopback
a=loopback-source
a=rtpmap:0 pcmu/8000
a=rtpmap:8 pcma/8000
a=rtpmap:100 G7221/16000/1
Since media loopback requires bidirectional RTP, its normal 5.1. Generating the SDP Offer for Loopback Session
direction mode is "sendrecv"; the "sendrecv" direction attribute
MAY be encoded in SDP or not, as per Section 5.1 of [RFC3264],
since it is implied by default. If either the loopback source or
mirror wish to disable loopback use during a session, the direction
mode attribute "inactive" MUST be used as per [RFC3264]. The
direction mode attributes "recvonly" and "sendonly" are
incompatible with the loopback mechanism and MUST NOT be indicated
when generating an SDP Offer or Answer. When receiving an SDP
Offer or Answer, if "recvonly" or "sendonly" is indicated for
loopback, the SDP-receiving agent SHOULD treat it as a protocol
failure of the loopback negotiation and terminate the session
through its normal means (e.g., by sending a SIP BYE if SIP is
used), or reject the offending media stream.
The offerer may offer more than one loopback-type in the SDP offer. If an offerer wishes to make a loopback request, it includes both the
The port number and the address in the offer (m/c= lines) indicate loopback-type and loopback-role attributes in a valid SDP offer:
where the offerer would like to receive the media stream(s). The
payload type numbers indicate the value of the payload the offerer
expects to receive. However, the answer might indicate a subset of
payload type numbers from those given in the offer. In that case,
the offerer MUST only send the payload types received in the
answer, per normal SDP offer/answer rules.
If the offer indicates rtp-pkt-loopback support, the offer MUST Example: m=audio 41352 RTP/AVP 0 8 100
also contain either an encapsulated or direct loopback encoding a=loopback:rtp-media-loopback
format encoding name, or both, as defined in Sections 7.1 and 7.2 a=loopback-source
of this document. If the offer only indicates rtp-media-loopback a=rtpmap:0 pcmu/8000
support, then neither encapsulated nor direct loopback encoding a=rtpmap:8 pcma/8000
formats apply and they MUST NOT be in the offer. a=rtpmap:100 G7221/16000/1
If loopback-type is rtp-pkt-loopback, the loopback-mirror MUST send Since media loopback requires bidirectional RTP, its normal direction
and the loopback-source MUST receive the looped back packets mode is "sendrecv"; the "sendrecv" direction attribute MAY be encoded
encoded in one of the two payload formats (encapsulated RTP or in SDP or not, as per Section 5.1 of [RFC3264], since it is implied
direct loopback) as defined in Section 7. by default. If either the loopback source or mirror wishes to
disable loopback use during a session, the direction mode attribute
"inactive" MUST be used as per [RFC3264]. The direction mode
attributes "recvonly" and "sendonly" are incompatible with the
loopback mechanism and MUST NOT be indicated when generating an SDP
offer or answer. When receiving an SDP offer or answer, if
"recvonly" or "sendonly" is indicated for loopback, the SDP-receiving
agent SHOULD treat it as a protocol failure of the loopback
negotiation and terminate the session through its normal means (e.g.,
by sending a SIP BYE if SIP is used) or reject the offending media
stream.
Example: m=audio 41352 RTP/AVP 0 8 112 The offerer may offer more than one loopback-type in the SDP offer.
a=loopback:rtp-pkt-loopback The port number and the address in the offer (m/c= lines) indicate
a=loopback-source where the offerer would like to receive the media stream(s). The
a=rtpmap:112 encaprtp/8000 payload type numbers indicate the value of the payload the offerer
expects to receive. However, the answer might indicate a subset of
payload type numbers from those given in the offer. In that case,
the offerer MUST only send the payload types received in the answer,
per normal SDP offer/answer rules.
Example: m=audio 41352 RTP/AVP 0 8 112 If the offer indicates rtp-pkt-loopback support, the offer MUST also
a=loopback:rtp-pkt-loopback contain either an encapsulated or direct loopback encoding format
a=loopback-source encoding name, or both, as defined in Sections 7.1 and 7.2 of this
a=rtpmap:112 rtploopback/8000 document. If the offer only indicates rtp-media-loopback support,
then neither encapsulated nor direct loopback encoding formats apply
and they MUST NOT be in the offer.
5.2 Generating the SDP Answer for Loopback Session If loopback-type is rtp-pkt-loopback, the loopback mirror MUST send,
and the loopback source MUST receive, the looped-back packets encoded
in one of the two payload formats (encapsulated RTP or direct
loopback) as defined in Section 7.
As with the offer, an SDP answer for loopback follows SDP Example: m=audio 41352 RTP/AVP 0 8 112
offer/answer rules for the direction attribute, but directions of a=loopback:rtp-pkt-loopback
"sendonly" or "recvonly" do not apply for loopback operation. a=loopback-source
a=rtpmap:112 encaprtp/8000
The port number and the address in the answer (m/c= lines) indicate Example: m=audio 41352 RTP/AVP 0 8 112
where the answerer would like to receive the media stream. The a=loopback:rtp-pkt-loopback
payload type numbers indicate the value of the payload types the a=loopback-source
answerer expects to send and receive. a=rtpmap:112 rtploopback/8000
An answerer includes both the loopback role and loopback type 5.2. Generating the SDP Answer for Loopback Session
attributes in the answer to indicate that it will accept the
loopback request. When a stream is offered with the loopback-source
attribute, the corresponding stream in the response will be
loopback-mirror and vice versa, provided the answerer is capable of
supporting the requested loopback-type.
For example, if the offer contains the loopback-source attribute: As with the offer, an SDP answer for loopback follows SDP
offer/answer rules for the direction attribute, but directions of
"sendonly" or "recvonly" do not apply for loopback operation.
m=audio 41352 RTP/AVP 0 8 The port number and the address in the answer (m/c= lines) indicate
a=loopback:rtp-media-loopback where the answerer would like to receive the media stream. The
a=loopback-source payload type numbers indicate the value of the payload types the
answerer expects to send and receive.
The answer that is capable of supporting the offer must contain the An answerer includes both the loopback-role and loopback-type
loopback-mirror attribute: attributes in the answer to indicate that it will accept the loopback
request. When a stream is offered with the loopback-source
attribute, the corresponding stream in the response will be
loopback-mirror and vice versa, provided the answerer is capable of
supporting the requested loopback-type.
m=audio 12345 RTP/AVP 0 8 For example, if the offer contains the loopback-source attribute:
a=loopback:rtp-media-loopback
a=loopback-mirror
If a stream is offered with multiple loopback type attributes, the m=audio 41352 RTP/AVP 0 8
answer MUST include only one of the loopback types that are a=loopback:rtp-media-loopback
accepted by the answerer. The answerer SHOULD give preference to a=loopback-source
the first loopback-type in the SDP offer.
For example, if the offer contains: The answer that is capable of supporting the offer must contain the
loopback-mirror attribute:
m=audio 41352 RTP/AVP 0 8 112 m=audio 12345 RTP/AVP 0 8
a=loopback:rtp-media-loopback rtp-pkt-loopback a=loopback:rtp-media-loopback
a=loopback-source a=loopback-mirror
a=rtpmap:112 encaprtp/8000
The answer that is capable of supporting the offer and chooses to If a stream is offered with multiple loopback-type attributes, the
loopback the media using the rtp-media-loopback type must contain: answer MUST include only one of the loopback types that are accepted
by the answerer. The answerer SHOULD give preference to the first
loopback-type in the SDP offer.
m=audio 12345 RTP/AVP 0 8 For example, if the offer contains:
a=loopback:rtp-media-loopback
a=loopback-mirror
As specified in Section 7, if the loopback-type is m=audio 41352 RTP/AVP 0 8 112
rtp-pkt-loopback, either the encapsulated RTP payload format or a=loopback:rtp-media-loopback rtp-pkt-loopback
direct loopback RTP payload format MUST be used for looped back a=loopback-source
packets. a=rtpmap:112 encaprtp/8000
For example, if the offer contains: The answer that is capable of supporting the offer and chooses to
loopback the media using the rtp-media-loopback type must contain:
m=audio 41352 RTP/AVP 0 8 112 113 m=audio 12345 RTP/AVP 0 8
a=loopback:rtp-pkt-loopback a=loopback:rtp-media-loopback
a=loopback-source a=loopback-mirror
a=rtpmap:112 encaprtp/8000
a=rtpmap:113 rtploopback/8000
The answer that is capable of supporting the offer must contain one As specified in Section 7, if the loopback-type is rtp-pkt-loopback,
of the following: either the encapsulated RTP payload format or direct loopback RTP
payload format MUST be used for looped-back packets.
m=audio 12345 RTP/AVP 0 8 112 For example, if the offer contains:
a=loopback:rtp-pkt-loopback
a=loopback-mirror
a=rtpmap:112 encaprtp/8000
m=audio 12345 RTP/AVP 0 8 113 m=audio 41352 RTP/AVP 0 8 112 113
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-mirror a=loopback-source
a=rtpmap:113 rtploopback/8000 a=rtpmap:112 encaprtp/8000
a=rtpmap:113 rtploopback/8000
The previous examples used the 'encaprtp' and 'rtploopback' The answer that is capable of supporting the offer must contain one
encoding names, which will be defined in Sections 7.1.3 and 7.2.3. of the following:
5.3 Offerer Processing of the SDP Answer m=audio 12345 RTP/AVP 0 8 112
a=loopback:rtp-pkt-loopback
a=loopback-mirror
a=rtpmap:112 encaprtp/8000
If the received SDP answer does not contain an a=loopback-mirror or m=audio 12345 RTP/AVP 0 8 113
a=loopback-source attribute, it is assumed that the loopback a=loopback:rtp-pkt-loopback
extensions are not supported by the remote agent. This is not a a=loopback-mirror
protocol failure, and instead merely completes the SDP offer/answer a=rtpmap:113 rtploopback/8000
exchange with whatever normal rules apply; the offerer MAY decide
to end the established RTP session (if any) through normal means of
the upper-layer signaling protocol (e.g., by sending a SIP BYE).
5.4 Modifying the Session The previous examples used the 'encaprtp' and 'rtploopback' encoding
names, which will be defined in Sections 7.1.3 and 7.2.3.
At any point during the loopback session, either participant MAY 5.3. Offerer Processing of the SDP Answer
issue a new offer to modify the characteristics of the previous
session, as defined in Section 8 of RFC 3264 [RFC3264]. This also
includes transitioning from a normal media processing mode to
loopback mode, and vice versa.
5.5 Establishing Sessions Between Entities Behind NAT If the received SDP answer does not contain an a=loopback-mirror or
a=loopback-source attribute, it is assumed that the loopback
extensions are not supported by the remote agent. This is not a
protocol failure and instead merely completes the SDP offer/answer
exchange with whatever normal rules apply; the offerer MAY decide to
end the established RTP session (if any) through normal means of the
upper-layer signaling protocol (e.g., by sending a SIP BYE).
Interactive Connectivity Establishment (ICE) [RFC5245], Traversal 5.4. Modifying the Session
Using Relays around NAT (TURN) [RFC5766], and Session Traversal
Utilities for NAT (STUN) [RFC5389] provide a general solution to
establishing media sessions between entities that are behind
Network Address Translators (NATs). Loopback sessions that involve
one or more endpoints behind NATs can also use these general
solutions wherever possible.
If ICE is not supported, then in the case of loopback, the At any point during the loopback session, either participant MAY
mirroring entity will not send RTP packets, and therefore will not issue a new offer to modify the characteristics of the previous
automatically create the NAT pinhole in the way that other SIP session, as defined in Section 8 of RFC 3264 [RFC3264]. This also
sessions do. Therefore, if the mirroring entity is behind a NAT, includes transitioning from a normal media processing mode to
it MUST send some packets to the identified address/port(s) of the loopback mode, and vice versa.
peer, in order to open the NAT pinhole. Using ICE, this would be
accomplished with the STUN connectivity check process, or through a
TURN server connection. If ICE is not supported, either [RFC6263]
or Section 10 of ICE [RFC5245] can be followed to open the pinhole
and keep the NAT binding alive/refreshed.
Note that for any form of NAT traversal to function, symmetric 5.5. Establishing Sessions between Entities behind NATs
RTP/RTCP [RFC4961] MUST be used, unless the mirror can control the
NAT(s) in its path to create explicit pinholes. In other words
both agents MUST send packets from the source address and port they
receive packets on, unless some mechanism is used to avoid that
need (e.g., by using Port Control Protocol).
6. RTP Requirements Interactive Connectivity Establishment (ICE) [RFC5245], Traversal
Using Relays around NAT (TURN) [RFC5766], and Session Traversal
Utilities for NAT (STUN) [RFC5389] provide a general solution to
establishing media sessions between entities that are behind Network
Address Translators (NATs). Loopback sessions that involve one or
more endpoints behind NATs can also use these general solutions
wherever possible.
A loopback source MUST NOT send multiple source streams on the same If ICE is not supported, then in the case of loopback, the mirroring
5-tuple, since there is no means for the mirror to indicate which entity will not send RTP packets and therefore will not automatically
is which in its mirrored RTP packets. create the NAT pinhole in the way that other SIP sessions do.
Therefore, if the mirroring entity is behind a NAT, it MUST send some
packets to the identified address/port(s) of the peer, in order to
open the NAT pinhole. Using ICE, this would be accomplished with the
STUN connectivity check process or through a TURN server connection.
If ICE is not supported, either [RFC6263] or Section 10 of ICE
[RFC5245] can be followed to open the pinhole and keep the NAT
binding alive/refreshed.
A loopback mirror that is compliant to this specification and Note that for any form of NAT traversal to function, symmetric
accepts media with rtp-pkt-loopback loopback type loops back the RTP/RTCP [RFC4961] MUST be used, unless the mirror can control the
incoming RTP packets using either the encapsulated RTP payload NAT(s) in its path to create explicit pinholes. In other words, both
format or the direct loopback RTP payload format as defined in agents MUST send packets from the source address and port they
Section 7 of this specification. receive packets on, unless some mechanism is used to avoid that need
(e.g., by using the Port Control Protocol).
A device that is compliant to this specification and performing the 6. RTP Requirements
mirroring using the loopback type rtp-media-loopback MUST transmit
all received media back to the sender, unless congestion feedback
or other lower-layer constraints prevent it from doing so. The
incoming media is treated as if it were to be played; for example,
the media stream may receive treatment from Packet Loss Concealment
(PLC) algorithms. The mirroring entity re-generates all the RTP
header fields as it would when transmitting media. The mirroring
entity MAY choose to encode the loopback media according to any of
the media descriptions supported by the offering entity.
Furthermore, in cases where the same media type is looped back, the
mirroring entity can choose to preserve number of frames/packet and
bitrate of the encoded media according to the received media.
7. Payload formats for Packet loopback A loopback source MUST NOT send multiple source streams on the same
The payload formats described in this section MUST be used by a 5-tuple, since there is no means for the mirror to indicate which is
loopback-mirror when 'rtp-pkt-loopback' is the specified which in its mirrored RTP packets.
loopback-type. Two different formats are specified here - an
encapsulated RTP payload format and a direct loopback RTP payload
format. The encapsulated RTP payload format should be used when
the incoming RTP header information needs to be preserved during
the loopback operation. This is useful in cases where loopback
source needs to measure performance metrics in both directions.
However, this comes at the expense of increased packet size as
described in Section 7.1. The direct loopback RTP payload format
should be used when bandwidth requirements prevent the use of
encapsulated RTP payload format.
7.1 Encapsulated Payload format A loopback mirror that is compliant to this specification and accepts
media with the loopback type rtp-pkt-loopback loops back the incoming
RTP packets using either the encapsulated RTP payload format or the
direct loopback RTP payload format as defined in Section 7 of this
specification.
A received RTP packet is encapsulated in the payload section of the A device that is compliant to this specification and performing the
RTP packet generated by a loopback-mirror. Each received packet is mirroring using the loopback type rtp-media-loopback MUST transmit
encapsulated in a separate encapsulating RTP packet; the all received media back to the sender, unless congestion feedback or
encapsulated packet would be fragmented only if required (for other lower-layer constraints prevent it from doing so. The incoming
example: due to MTU limitations). media is treated as if it were to be played; for example, the media
stream may receive treatment from Packet Loss Concealment (PLC)
algorithms. The mirroring entity re-generates all the RTP header
fields as it would when transmitting media. The mirroring entity MAY
choose to encode the loopback media according to any of the media
descriptions supported by the offering entity. Furthermore, in cases
where the same media type is looped back, the mirroring entity can
choose to preserve the number of frames/packets and the bit rate of
the encoded media according to the received media.
7.1.1 Usage of RTP Header fields 7. Payload Formats for Packet Loopback
Payload Type (PT): The assignment of an RTP payload type for this The payload formats described in this section MUST be used by a
packet format is outside the scope of this document; it is either loopback mirror when 'rtp-pkt-loopback' is the specified
specified by the RTP profile under which this payload format is loopback-type. Two different formats are specified here -- an
used or more likely signaled dynamically out-of-band (e.g., using encapsulated RTP payload format and a direct loopback RTP payload
SDP; Section 7.1.3 defines the name binding). format. The encapsulated RTP payload format should be used when the
incoming RTP header information needs to be preserved during the
loopback operation. This is useful in cases where the loopback
source needs to measure performance metrics in both directions.
However, this comes at the expense of increased packet size as
described in Section 7.1. The direct loopback RTP payload format
should be used when bandwidth requirements prevent the use of the
encapsulated RTP payload format.
Marker (M) bit: If the received RTP packet is looped back in 7.1. Encapsulated Payload Format
multiple encapsulating RTP packets, the M bit is set to 1 in every
fragment except the last packet, otherwise it is set to 0.
Extension (X) bit: Defined by the RTP Profile used. A received RTP packet is encapsulated in the payload section of the
RTP packet generated by a loopback mirror. Each received packet is
encapsulated in a separate encapsulating RTP packet; the encapsulated
packet would be fragmented only if required (for example, due to MTU
limitations).
Sequence Number: The RTP sequence number SHOULD be generated by the 7.1.1. Usage of RTP Header Fields
loopback-mirror in the usual manner with a constant random offset
as described in RFC 3550 [RFC3550].
Timestamp: The RTP timestamp denotes the sampling instant for when Payload Type (PT): The assignment of an RTP payload type for this
the loopback-mirror is transmitting this packet to the loopback- packet format is outside the scope of this document; it is either
source. The RTP timestamp MUST use the same clock rate as that of specified by the RTP profile under which this payload format is
the encapsulated packet. The initial value of the timestamp SHOULD used or more likely signaled dynamically out-of-band (e.g., using
be random for security reasons (see Section 5.1 of RFC 3550 SDP; Section 7.1.3 defines the name binding).
[RFC3550]).
SSRC: set as described in RFC 3550 [RFC3550]. Marker (M) bit: If the received RTP packet is looped back in multiple
encapsulating RTP packets, the M bit is set to 1 in every fragment
except the last packet; otherwise, it is set to 0.
CC and CSRC fields are used as described in RFC 3550 [RFC3550]. Extension (X) bit: This bit is defined by the RTP profile used.
7.1.2 RTP Payload Structure Sequence Number: The RTP sequence number SHOULD be generated by the
loopback mirror in the usual manner with a constant random offset
as described in RFC 3550 [RFC3550].
The outer RTP header of the encapsulating packet is followed by the Timestamp: The RTP timestamp denotes the sampling instant for when
payload header defined in this section, after any header the loopback mirror is transmitting this packet to the loopback
extension(s). If the received RTP packet has to be looped back in source. The RTP timestamp MUST use the same clock rate as that of
multiple encapsulating packets due to fragmentation, the the encapsulated packet. The initial value of the timestamp
encapsulating RTP header in each packet is followed by the payload SHOULD be random for security reasons (see Section 5.1 of RFC 3550
header defined in this section. The header is devised so that the [RFC3550]).
loopback-source can decode looped back packets in the presence of
moderate packet loss [RFC3550]. The RTP payload of the
encapsulating RTP packet starts with the payload header defined in
this section.
0 1 2 3 Synchronization source (SSRC): This field is set as described in
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 RFC 3550 [RFC3550].
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| receive timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F | R | CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| transmit timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Encapsulating RTP Packet Payload Header
The 12 octets after the receive timestamp are identical to the The CSRC count (CC) and contributing source (CSRC) fields are used as
encapsulated RTP header of the received packet except for the first described in RFC 3550 [RFC3550].
2 bits of the first octet. In effect, the received RTP packet is
encapsulated by creating a new outer RTP header followed by 4 new
bytes of a receive timestamp, followed by the original received RTP
header and payload, except that the first two bits of the received
RTP header are overwritten as defined here.
Receive Timestamp: 32 bits 7.1.2. RTP Payload Structure
The Receive timestamp denotes the sampling instant for when the The outer RTP header of the encapsulating packet is followed by the
last octet of the received media packet that is being encapsulated payload header defined in this section, after any header
by the loopback-mirror is received from the loopback-source. The extension(s). If the received RTP packet has to be looped back in
same clock rate MUST be used by the loopback-source. The initial multiple encapsulating packets due to fragmentation, the
value of the timestamp SHOULD be random for security reasons (see encapsulating RTP header in each packet is followed by the payload
Section 5.1 of RFC 3550 [RFC3550]). header defined in this section. The header is devised so that the
loopback source can decode looped-back packets in the presence of
moderate packet loss [RFC3550]. The RTP payload of the encapsulating
RTP packet starts with the payload header defined in this section.
Fragmentation (F): 2 bits 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| receive timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F | R | CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| transmit timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
First Fragment (00) /Last Fragment (01) /No Fragmentation(10)/ Figure 1. Encapsulating RTP Packet Payload Header
Intermediate Fragment (11). This field identifies how much of the
received packet is encapsulated in this packet by the loopback-
mirror. If the received packet is not fragmented, this field is
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 01, all other packets set this field to 11.
7.1.3 Usage of SDP The 12 octets after the receive timestamp are identical to the
encapsulated RTP header of the received packet except for the first 2
bits of the first octet. In effect, the received RTP packet is
encapsulated by creating a new outer RTP header followed by 4 new
bytes of a receive timestamp, followed by the original received RTP
header and payload, except that the first two bits of the received
RTP header are overwritten as defined here.
The payload type number for the encapsulated stream can be Receive timestamp: 32 bits
negotiated using SDP. There is no static payload type assignment
for the encapsulating stream, so dynamic payload type numbers MUST
be used. The binding to the name is indicated by an rtpmap
attribute. The name used in this binding is "encaprtp".
The following is an example SDP fragment for encapsulated RTP. The receive timestamp denotes the sampling instant for when the last
octet of the received media packet that is being encapsulated by the
loopback mirror is received from the loopback source. The same clock
rate MUST be used by the loopback source. The initial value of the
timestamp SHOULD be random for security reasons (see Section 5.1 of
RFC 3550 [RFC3550]).
m=audio 41352 RTP/AVP 112 Fragmentation (F): 2 bits
a=rtpmap:112 encaprtp/8000
7.2 Direct loopback RTP payload format Possible values are First Fragment (00), Last Fragment (01),
No Fragmentation (10), or Intermediate Fragment (11). This field
identifies how much of the received packet is encapsulated in this
packet by the loopback mirror. If the received packet is not
fragmented, this field is 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 01, and all other
packets set this field to 11.
The direct loopback RTP payload format can be used in scenarios 7.1.3. Usage of SDP
where the 16 byte overhead of the encapsulated payload format is of
concern, or simply due to local policy. When using this payload
format, the receiver loops back each received RTP packet payload
(not header) in a separate RTP packet.
Because a direct loopback format does not retain the original RTP The payload type number for the encapsulated stream can be negotiated
headers, there will be no indication of the original payload-type using SDP. There is no static payload type assignment for the
sent to the mirror, in looped-back packets. Therefore, the encapsulating stream, so dynamic payload type numbers MUST be used.
loopback source SHOULD only send one payload type per loopback RTP The binding to the name is indicated by an rtpmap attribute. The
session, if direct mode is used. name used in this binding is "encaprtp".
7.2.1 Usage of RTP Header fields The following is an example SDP fragment for encapsulated RTP.
Payload Type (PT): The assignment of an RTP payload type for the m=audio 41352 RTP/AVP 112
encapsulating packet format is outside the scope of this document; a=rtpmap:112 encaprtp/8000
it is either specified by the RTP profile under which this payload
format is used or more likely signaled dynamically out-of-band
(e.g., using SDP; Section 7.2.3 defines the name binding).
Marker (M) bit: Set to the value in the received packet. 7.2. Direct Loopback RTP Payload Format
Extension (X) bit: Defined by the RTP Profile used. The direct loopback RTP payload format can be used in scenarios where
the 16-byte overhead of the encapsulated payload format is of
concern, or simply due to local policy. When using this payload
format, the receiver loops back each received RTP packet payload (not
header) in a separate RTP packet.
Sequence Number: The RTP sequence number SHOULD be generated by the Because a direct loopback format does not retain the original RTP
loopback-mirror in the usual manner with a constant random offset, headers, there will be no indication of the original payload-type
as per [RFC3550]. sent to the mirror, in looped-back packets. Therefore, the loopback
source SHOULD only send one payload type per loopback RTP session if
direct mode is used.
Timestamp: The RTP timestamp denotes the sampling instant for when 7.2.1. Usage of RTP Header Fields
the loopback-mirror is transmitting this packet to the
loopback-source. The same clock rate MUST be used as that of the
received RTP packet. The initial value of the timestamp SHOULD be
random for security reasons (see Section 5.1 of RFC 3550
[RFC3550]).
SSRC: set as described in RFC 3550 [RFC3550]. Payload Type (PT): The assignment of an RTP payload type for the
encapsulating packet format is outside the scope of this document;
it is either specified by the RTP profile under which this payload
format is used or more likely signaled dynamically out-of-band
(e.g., using SDP; Section 7.2.3 defines the name binding).
CC and CSRC fields are used as described in RFC 3550 [RFC3550]. Marker (M) bit: This bit is set to the value in the received packet.
7.2.2 RTP Payload Structure Extension (X) bit: This bit is defined by the RTP profile used.
This payload format does not define any payload specific headers. Sequence Number: The RTP sequence number SHOULD be generated by the
The loopback-mirror simply copies the RTP payload data from the loopback mirror in the usual manner with a constant random offset,
payload portion of the RTP packet received from the loopback- as per [RFC3550].
source.
7.2.3 Usage of SDP Timestamp: The RTP timestamp denotes the sampling instant for when
the loopback mirror is transmitting this packet to the loopback
source. The same clock rate MUST be used as that of the received
RTP packet. The initial value of the timestamp SHOULD be random
for security reasons (see Section 5.1 of RFC 3550 [RFC3550]).
The payload type number for the payload loopback stream can be SSRC: This field is set as described in RFC 3550 [RFC3550].
negotiated using a mechanism like SDP. There is no static payload
type assignment for the stream, so dynamic payload type numbers
MUST be used. The binding to the name is indicated by an rtpmap
attribute. The name used in this binding is "rtploopback".
The following is an example SDP fragment for direct loopback RTP The CC and CSRC fields are used as described in RFC 3550 [RFC3550].
format.
m=audio 41352 RTP/AVP 112 7.2.2. RTP Payload Structure
a=rtpmap:112 rtploopback/8000
8. SRTP Behavior This payload format does not define any payload-specific headers.
The loopback mirror simply copies the RTP payload data from the
payload portion of the RTP packet received from the loopback source.
Secure RTP [RFC3711] MAY be used for loopback sessions. SRTP 7.2.3. Usage of SDP
operates at a lower logical layer than RTP, and thus if both sides
negotiate to use SRTP, each side uses its own key, performs
encryption/decryption, authentication, etc. Therefore the loopback
function on the mirror occurs after the SRTP packet has been
decrypted and authenticated, as a normal cleartext RTP packet
without an MKI or authentication tag; once the cleartext RTP packet
or payload is mirrored - either at the media-layer, direct packet-
layer, or encapsulated packet-layer - it is encrypted by the mirror
using its own key.
In order to provide the same level of protection to both forward The payload type number for the payload loopback stream can be
and reverse media flows (media to and from the mirror), if SRTP is negotiated using a mechanism like SDP. There is no static payload
used it MUST be used in both directions with the same properties. type assignment for the stream, so dynamic payload type numbers MUST
be used. The binding to the name is indicated by an rtpmap
attribute. The name used in this binding is "rtploopback".
9. RTCP Requirements The following is an example SDP fragment for the direct loopback RTP
format.
The use of the loopback attribute is intended for monitoring of m=audio 41352 RTP/AVP 112
media quality of the session. Consequently the media performance a=rtpmap:112 rtploopback/8000
information should be exchanged between the offering and the
answering entities. An offering or answering agent that is
compliant to this specification SHOULD support RTCP per [RFC3550]
and RTCP-XR per RFC 3611 [RFC3611]. Furthermore, if the offerer or
answerer choose to support RTCP-XR, they SHOULD support RTCP-XR
Loss Run Length Encoding (RLE) report block, Duplicate RLE report
block, Statistics Summary report block, and VoIP Metric Reports
Block per Sections 4.1, 4.2, 4.6, and 4.7 of RFC 3611 [RFC3611].
The offerer and the answerer MAY support other RTCP-XR reporting
blocks as defined by RFC 3611 [RFC3611].
10. Congestion Control 8. SRTP Behavior
All the participants in a media-level loopback session SHOULD Secure RTP (SRTP) [RFC3711] MAY be used for loopback sessions. SRTP
implement congestion control mechanisms as defined by the RTP operates at a lower logical layer than RTP, and thus if both sides
profile under which the loopback mechanism is implemented. For negotiate to use SRTP, each side uses its own key and performs
audio video profiles, implementations SHOULD conform to the encryption/decryption, authentication, etc. Therefore, the loopback
mechanism defined in Section 2 of RFC 3551 [RFC3551]. function on the mirror occurs after the SRTP packet has been
decrypted and authenticated, as a normal cleartext RTP packet without
a Master Key Identifier (MKI) or authentication tag; once the
cleartext RTP packet or payload is mirrored -- either at the media-
layer, direct packet-layer, or encapsulated packet-layer -- it is
encrypted by the mirror using its own key.
For packet-level loopback types, the loopback source SHOULD In order to provide the same level of protection to both forward and
implement congestion control. The mirror will simply reflect back reverse media flows (media to and from the mirror), if SRTP is used
the RTP packets it receives (either in encapsulated or direct it MUST be used in both directions with the same properties.
modes), therefore the source needs to control the congestion of
both forward and reverse paths by reducing its sending rate to the
mirror. This keeps the loopback mirror implementation simpler, and
provides more flexibility for the source performing a loopback
test.
11. Examples 9. RTCP Requirements
This section provides examples for media descriptions using SDP for The use of the loopback attribute is intended for the monitoring of
different scenarios. The examples are given for SIP-based media quality of the session. Consequently, the media performance
transactions and are abbreviated and do not show the complete information should be exchanged between the offering and the
signaling for convenience. answering entities. An offering or answering agent that is compliant
to this specification SHOULD support RTCP per [RFC3550] and RTCP-XR
per RFC 3611 [RFC3611]. Furthermore, if the offerer or answerer
chooses to support RTCP-XR, they SHOULD support the RTCP-XR Loss Run
Length Encoding (RLE) Report Block, Duplicate RLE Report Block,
Statistics Summary Report Block, and VoIP Metrics Report Block per
Sections 4.1, 4.2, 4.6, and 4.7 of RFC 3611 [RFC3611]. The offerer
and the answerer MAY support other RTCP-XR reporting blocks as
defined by RFC 3611 [RFC3611].
11.1 Offer for specific media loopback type 10. Congestion Control
An agent sends an SDP offer which looks like: All the participants in a media-level loopback session SHOULD
implement congestion control mechanisms as defined by the RTP profile
under which the loopback mechanism is implemented. For audio/video
profiles, implementations SHOULD conform to the mechanism defined in
Section 2 of RFC 3551 [RFC3551].
v=0 For packet-level loopback types, the loopback source SHOULD implement
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com congestion control. The mirror will simply reflect back the RTP
s=- packets it receives (either in encapsulated or direct modes);
c=IN IP4 host.atlanta.example.com therefore, the source needs to control the congestion of both forward
t=0 0 and reverse paths by reducing its sending rate to the mirror. This
m=audio 49170 RTP/AVP 0 keeps the loopback mirror implementation simpler and provides more
a=loopback:rtp-media-loopback flexibility for the source performing a loopback test.
a=loopback-source
a=rtpmap:0 pcmu/8000
The agent is offering to source the media and expects the answering 11. Examples
agent to mirror the RTP stream per rtp-media-loopback loopback
type.
An answering agent sends an SDP answer which looks like: This section provides examples for media descriptions using SDP for
different scenarios. The examples are given for SIP-based
transactions; for convenience, they are abbreviated and do not show
the complete signaling.
v=0 11.1. Offer for Specific Media Loopback Type
o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
s=-
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49270 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-mirror
a=rtpmap:0 pcmu/8000
The answerer is accepting to mirror the media from the offerer at An agent sends an SDP offer that looks like:
the media level.
11.2 Offer for choice of media loopback type v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=-
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-source
a=rtpmap:0 pcmu/8000
An agent sends an SDP offer which looks like: The agent is offering to source the media and expects the answering
agent to mirror the RTP stream per the loopback type
rtp-media-loopback.
v=0 An answering agent sends an SDP answer that looks like:
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=-
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0 112 113
a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source
a=rtpmap:0 pcmu/8000
a=rtpmap:112 encaprtp/8000
a=rtpmap:113 rtploopback/8000
The offerer is offering to source the media and expects the v=0
answerer to mirror the RTP stream at either the media or rtp level. o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
s=-
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49270 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-mirror
a=rtpmap:0 pcmu/8000
An answering agent sends an SDP answer which looks like: The answerer agrees to mirror the media from the offerer at the media
level.
v=0 11.2. Offer for Choice of Media Loopback Type
o=box 1234567890 1122334455 IN IP4 host.biloxi.example.com
s=-
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49270 RTP/AVP 0 112
a=loopback:rtp-pkt-loopback
a=loopback-mirror
a=rtpmap:0 pcmu/8000
a=rtpmap:112 encaprtp/8000
The answerer is accepting to mirror the media from the offerer at An agent sends an SDP offer that looks like:
the packet level using the encapsulated RTP payload format.
11.3 Answerer rejecting loopback media v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=-
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0 112 113
a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source
a=rtpmap:0 pcmu/8000
a=rtpmap:112 encaprtp/8000
a=rtpmap:113 rtploopback/8000
The offerer is offering to source the media and expects the answerer
to mirror the RTP stream at either the media or RTP level.
An agent sends an SDP offer which looks like: An answering agent sends an SDP answer that looks like:
v=0 v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
s=- s=-
c=IN IP4 host.atlanta.example.com c=IN IP4 host.biloxi.example.com
t=0 0 t=0 0
m=audio 49170 RTP/AVP 0 m=audio 49270 RTP/AVP 0 112
a=loopback:rtp-media-loopback a=loopback:rtp-pkt-loopback
a=loopback-source a=loopback-mirror
a=rtpmap:0 pcmu/8000 a=rtpmap:0 pcmu/8000
a=rtpmap:112 encaprtp/8000
The offerer is offering to source the media and expects the The answerer agrees to mirror the media from the offerer at the
answerer to mirror the RTP stream at the media level. packet level using the encapsulated RTP payload format.
An answering agent sends an SDP answer which looks like: 11.3. Answerer Rejecting Loopback Media
v=0 An agent sends an SDP offer that looks like:
o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
s=-
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 0 RTP/AVP 0
a=rtpmap:0 pcmu/8000
Note in this case the answerer did not indicate loopback support, v=0
although it could have and still used a port number of 0 to o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
indicate it does not wish to accept that media session. s=-
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-source
a=rtpmap:0 pcmu/8000
Alternatively, the answering agent could have simply rejected the The offerer is offering to source the media and expects the answerer
entire SDP offer through some higher-layer signaling protocol means to mirror the RTP stream at the media level.
(e.g., by rejecting the SIP INVITE request if the SDP offer was in
the INVITE).
12. Security Considerations An answering agent sends an SDP answer that looks like:
The security considerations of [RFC3264] and [RFC3550] apply. v=0
o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
s=-
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 0 RTP/AVP 0
a=rtpmap:0 pcmu/8000
Note in this case that the answerer did not indicate loopback
support, although it could have and still used a port number of 0 to
indicate that it does not wish to accept that media session.
Given that media loopback may be automated without the end user's Alternatively, the answering agent could have simply rejected the
knowledge, the answerer of the media loopback should be aware of entire SDP offer through some higher-layer signaling protocol means
denial of service attacks. It is RECOMMENDED that session requests (e.g., by rejecting the SIP INVITE request if the SDP offer was in
for media loopback be authenticated and the frequency of such the INVITE).
sessions limited by the answerer.
If the higher-layer signaling protocol were not authenticated, a 12. Security Considerations
malicious attacker could create a session between two parties the
attacker wishes to target, with each party acting as the loopback-
mirror to the other, of rtp-pkt-loopback type. A few RTP packets
sent to either party would then infinitely loop among the two, as
fast as they could process them, consuming their resources and
network bandwidth.
Furthermore, media-loopback provides a means of attack indirection, The security considerations of [RFC3264] and [RFC3550] apply.
whereby a malicious attacker creates a loopback session as the
loopback-source, and uses the mirror to reflect the attacker's
packets against a target - perhaps a target the attacker could not
reach directly, such as one behind a firewall for example. Or the
attacker could initiate the session as the loopback-mirror, in the
hopes of making the peer generate media against another target.
If end-user devices such as mobile phones answer loopback requests Given that media loopback may be automated without the end user's
without authentication and without notifying the end-user, then an knowledge, the answerer of the media loopback should be aware of
attacker could cause the battery to drain, and possibly deny the denial-of-service attacks. It is RECOMMENDED that session requests
end-user normal phone service or cause network data usage fees. for media loopback be authenticated and the frequency of such
This could even occur naturally if a legitimate loopback session sessions limited by the answerer.
does not terminate properly and the end device does not have a
timeout mechanism for such.
For the reasons noted above, end user devices SHOULD provide a If the higher-layer signaling protocol were not authenticated, a
means of indicating to the human user that the device is in a malicious attacker could create a session between two parties the
loopback session, even if it is an authenticated session. Devices attacker wishes to target, with each party acting as the loopback
that answer or generate loopback sessions SHOULD either perform mirror to the other, of the rtp-pkt-loopback type. A few RTP packets
keepalive/refresh tests of the session state through some means, or sent to either party would then infinitely loop among the two, as
time out the session automatically. fast as they could process them, consuming their resources and
network bandwidth.
13. Implementation Considerations Furthermore, media loopback provides a means of attack indirection,
whereby a malicious attacker creates a loopback session as the
loopback source and uses the mirror to reflect the attacker's packets
against a target -- perhaps a target the attacker could not reach
directly, such as one behind a firewall, for example. Or, the
attacker could initiate the session as the loopback mirror, in the
hopes of making the peer generate media against another target.
The media loopback approach described in this document is a If end-user devices such as mobile phones answer loopback requests
complete solution that would work under all scenarios. However, it without authentication and without notifying the end user, then an
is possible that the solution may not be light-weight enough for attacker could cause the battery to drain, and possibly deny the end
some implementations. In light of this concern, this section user normal phone service or cause network data usage fees. This
clarifies which features of the loopback proposal MUST be could even occur naturally if a legitimate loopback session does not
implemented for all implementations and which features MAY be terminate properly and the end device does not have a timeout
deferred if the complete solution is not desired. mechanism for such.
All implementations MUST at least support the rtp-pkt-loopback mode For the reasons noted above, end-user devices SHOULD provide a means
for loopback-type, with direct media loopback payload encoding. In of indicating to the human user that the device is in a loopback
addition, for the loopback role, all implementations of an SDP session, even if it is an authenticated session. Devices that answer
offerer MUST at least be able to act as a loopback-source. These or generate loopback sessions SHOULD either perform keepalive/refresh
requirements are intended to provide a minimal level of tests of the session state through some means or time out the session
interoperability between different implementations. automatically.
14. IANA Considerations 13. Implementation Considerations
[Note to RFC Editor: Please replace "XXXX" with the appropriate RFC The media loopback approach described in this document is a complete
number on publication] solution that would work under all scenarios. However, it is
possible that the solution may not be lightweight enough for some
implementations. In light of this concern, this section clarifies
which features of the loopback proposal MUST be implemented for all
implementations and which features MAY be deferred if the complete
solution is not desired.
14.1 SDP Attributes All implementations MUST at least support the rtp-pkt-loopback mode
for loopback-type, with direct media loopback payload encoding. In
addition, for the loopback role, all implementations of an SDP
offerer MUST at least be able to act as a loopback source. These
requirements are intended to provide a minimal level of
interoperability between different implementations.
This document defines three new media-level SDP attributes. IANA 14. IANA Considerations
has registered the following attributes:
Contact name: Kaynam Hedayat 14.1. SDP Attributes
Email address: kaynam.hedayat@exfo.com
Telephone number: +1-978-367-5611
Attribute name: loopback
Type of attribute: Media level.
Subject to charset: No.
Purpose of attribute: The 'loopback' attribute is used to
indicate the type of media loopback.
Allowed attribute values: The parameters to 'loopback' may be
one or more of "rtp-pkt-loopback" and
"rtp-media-loopback". See Section 5
of RFC XXXX for syntax.
Contact name: Kaynam Hedayat This document defines three new media-level SDP attributes. IANA has
Email address: kaynam.hedayat@exfo.com registered the following attributes.
Telephone number: +1-978-367-5611
Attribute name: loopback-source
Type of attribute: Media level.
Subject to charset: No.
Purpose of attribute: The 'loopback-source' attribute
specifies that the sender is the media
source and expects the receiver to act
as a loopback-mirror.
Allowed attribute values: None.
Contact name: Kaynam Hedayat Contact name: Kaynam Hedayat
Email address: kaynam.hedayat@exfo.com Email address: kh274@cornell.edu
Telephone number: +1-978-367-5611 Telephone number: +1-617-899-3279
Attribute name: loopback-mirror 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-mirror' attribute Purpose of attribute: The 'loopback' attribute is used to
specifies that the receiver will indicate the type of media loopback.
mirror (echo) all received media back Allowed attribute values: The parameters for 'loopback' may be
to the sender of the RTP stream. one or more of "rtp-pkt-loopback" and
Allowed attribute values: None. "rtp-media-loopback". See Section 4
of RFC 6849 for syntax.
14.2 Media Types Contact name: Kaynam Hedayat
Email address: kh274@cornell.edu
Telephone number: +1-617-899-3279
Attribute name: loopback-source
Type of attribute: Media level.
Subject to charset: No.
Purpose of attribute: The 'loopback-source' attribute
specifies that the sender is the media
source and expects the receiver to act
as a loopback mirror.
Allowed attribute values: N/A
The IANA has registered the following media types: Contact name: Kaynam Hedayat
Email address: kh274@cornell.edu
Telephone number: +1-617-899-3279
Attribute name: loopback-mirror
Type of attribute: Media level.
Subject to charset: No.
Purpose of attribute: The 'loopback-mirror' attribute
specifies that the receiver will
mirror (echo) all received media back
to the sender of the RTP stream.
Allowed attribute values: N/A
14.2.1 audio/encaprtp 14.2. Media Types
To: ietf-types@iana.org The IANA has registered the following media types.
Subject: Registration of media type audio/encaprtp 14.2.1. audio/encaprtp
Type name: audio To: ietf-types@iana.org
Subtype name: encaprtp Subject: Registration of media type audio/encaprtp
Required parameters: Type name: audio
rate: RTP timestamp clock rate, which is equal to the Subtype name: encaprtp
sampling rate. The typical rate is 8000; other rates
may be specified. This is specified by the loop back
source, and reflected by the mirror.
Optional parameters: none Required parameters:
Encoding considerations: This media type is framed. rate: RTP timestamp clock rate, which is equal to the sampling
rate. This is specified by the loopback source and reflected by
the mirror.
Security considerations: See Section 12 of RFC XXXX. Optional parameters: N/A
Interoperability considerations: none Encoding considerations: This media type is framed.
Published specification: RFC XXXX. Security considerations: See Section 12 of RFC 6849.
Applications which use this media type: Applications wishing Interoperability considerations: N/A
to monitor and ensure the quality of transport to the
edge of a given VoIP Service.
Additional information: none Published specification: RFC 6849.
Contact: the authors of RFC XXXX. Applications that use this media type: Applications wishing to
monitor and ensure the quality of transport to the edge of a given
VoIP service.
Intended usage: LIMITED USE Additional information: N/A
Restrictions on usage: This media type depends on RTP Contact: the authors of RFC 6849.
framing, and hence is only defined for transfer via
RTP. Transfer within other framing protocols is not
defined at this time.
Author: Intended usage: LIMITED USE
Kaynam Hedayat.
Change controller: IETF PAYLOAD working Restrictions on usage: This media type depends on RTP framing and
group delegated from the IESG. hence is only defined for transfer via RTP. Transfer within other
framing protocols is not defined at this time.
14.2.2 video/encaprtp Author: Kaynam Hedayat.
To: ietf-types@iana.org Change controller: IETF PAYLOAD working group delegated from
the IESG.
Subject: Registration of media type video/encaprtp 14.2.2. video/encaprtp
Type name: video To: ietf-types@iana.org
Subtype name: encaprtp Subject: Registration of media type video/encaprtp
Required parameters: Type name: video
rate: RTP timestamp clock rate, which is equal to the Subtype name: encaprtp
sampling rate. This is specified by the loop back
source, and reflected by the mirror.
Optional parameters: none Required parameters:
Encoding considerations: This media type is framed. rate: RTP timestamp clock rate, which is equal to the sampling
rate. This is specified by the loopback source and reflected by
the mirror.
Security considerations: See Section 12 of RFC XXXX. Optional parameters: N/A
Interoperability considerations: none Encoding considerations: This media type is framed.
Published specification: RFC XXXX. Security considerations: See Section 12 of RFC 6849.
Applications which use this media type: Applications wishing Interoperability considerations: N/A
to monitor and ensure the quality of transport to the Published specification: RFC 6849.
edge of a given Video Over IP Service.
Additional information: none Applications that use this media type: Applications wishing to
monitor and ensure the quality of transport to the edge of a given
Video Over IP service.
Contact: the authors of RFC XXXX. Additional information: N/A
Intended usage: LIMITED USE Contact: the authors of RFC 6849.
Restrictions on usage: This media type depends on RTP Intended usage: LIMITED USE
framing, and hence is only defined for transfer via
RTP. Transfer within other framing protocols is not
defined at this time.
Author: Restrictions on usage: This media type depends on RTP framing and
Kaynam Hedayat. hence is only defined for transfer via RTP. Transfer within other
framing protocols is not defined at this time.
Change controller: IETF PAYLOAD working Author: Kaynam Hedayat.
group delegated from the IESG.
14.2.3 text/encaprtp Change controller: IETF PAYLOAD working group delegated from
the IESG.
To: ietf-types@iana.org 14.2.3. text/encaprtp
Subject: Registration of media type text/encaprtp To: ietf-types@iana.org
Type name: text Subject: Registration of media type text/encaprtp
Subtype name: encaprtp Type name: text
Required parameters: Subtype name: encaprtp
rate: RTP timestamp clock rate, which is equal to the Required parameters:
sampling rate. This is specified by the loop back
source, and reflected by the mirror.
Optional parameters: none rate: RTP timestamp clock rate, which is equal to the sampling
rate. This is specified by the loopback source and reflected by
the mirror.
Encoding considerations: This media type is framed. Optional parameters: N/A
Security considerations: See Section 12 of RFC XXXX. Encoding considerations: This media type is framed.
Interoperability considerations: none Security considerations: See Section 12 of RFC 6849.
Published specification: RFC XXXX. Interoperability considerations: N/A
Applications which use this media type: Applications wishing Published specification: RFC 6849.
to monitor and ensure the quality of transport to the
edge of a given Real-Time Text Service.
Additional information: none Applications that use this media type: Applications wishing to
monitor and ensure the quality of transport to the edge of a given
real-time text service.
Contact: the authors of RFC XXXX. Additional information: N/A
Intended usage: LIMITED USE Contact: the authors of RFC 6849.
Restrictions on usage: This media type depends on RTP Intended usage: LIMITED USE
framing, and hence is only defined for transfer via
RTP. Transfer within other framing protocols is not
defined at this time.
Author: Restrictions on usage: This media type depends on RTP framing and
Kaynam Hedayat. hence is only defined for transfer via RTP. Transfer within other
framing protocols is not defined at this time.
Change controller: IETF PAYLOAD working Author: Kaynam Hedayat.
group delegated from the IESG.
14.2.4 application/encaprtp Change controller: IETF PAYLOAD working group delegated from
the IESG.
To: ietf-types@iana.org 14.2.4. application/encaprtp
Subject: Registration of media type To: ietf-types@iana.org
application/encaprtp
Type name: application Subject: Registration of media type application/encaprtp
Subtype name: encaprtp Type name: application
Required parameters: Subtype name: encaprtp
rate: RTP timestamp clock rate, which is equal to the Required parameters:
sampling rate. This is specified by the loop back
source, and reflected by the mirror.
Optional parameters: none rate: RTP timestamp clock rate, which is equal to the sampling
rate. This is specified by the loopback source and reflected by
the mirror.
Encoding considerations: This media type is framed. Optional parameters: N/A
Security considerations: See Section 12 of RFC XXXX. Encoding considerations: This media type is framed.
Interoperability considerations: none Security considerations: See Section 12 of RFC 6849.
Published specification: RFC XXXX. Interoperability considerations: N/A
Applications which use this media type: Applications wishing Published specification: RFC 6849.
to monitor and ensure the quality of transport to the
edge of a given Real-Time Application Service.
Additional information: none Applications that use this media type: Applications wishing to
monitor and ensure the quality of transport to the edge of a given
real-time application service.
Contact: the authors of RFC XXXX. Additional information: N/A
Intended usage: LIMITED USE Contact: the authors of RFC 6849.
Restrictions on usage: This media type depends on RTP Intended usage: LIMITED USE
framing, and hence is only defined for transfer via
RTP. Transfer within other framing protocols is not
defined at this time.
Author: Restrictions on usage: This media type depends on RTP framing and
Kaynam Hedayat. hence is only defined for transfer via RTP. Transfer within other
framing protocols is not defined at this time.
Change controller: IETF PAYLOAD working Author: Kaynam Hedayat.
group delegated from the IESG.
14.2.5 audio/rtploopback Change controller: IETF PAYLOAD working group delegated from
the IESG.
To: ietf-types@iana.org 14.2.5. audio/rtploopback
Subject: Registration of media type audio/rtploopback To: ietf-types@iana.org
Type name: audio Subject: Registration of media type audio/rtploopback
Subtype name: rtploopback Type name: audio
Required parameters: Subtype name: rtploopback
rate:RTP timestamp clock rate, which is equal to the Required parameters:
sampling rate. The typical rate is 8000; other rates
may be specified. This is specified by the loop back
source, and reflected by the mirror.
Optional parameters: none rate: RTP timestamp clock rate, which is equal to the sampling
rate. This is specified by the loopback source and reflected by
the mirror.
Encoding considerations: This media type is framed. Optional parameters: N/A
Security considerations: See Section 12 of RFC XXXX. Encoding considerations: This media type is framed.
Interoperability considerations: none Security considerations: See Section 12 of RFC 6849.
Published specification: RFC XXXX. Interoperability considerations: N/A
Applications which use this media type: Applications wishing Published specification: RFC 6849.
to monitor and ensure the quality of transport to the
edge of a given VoIP Service.
Additional information: none Applications that use this media type: Applications wishing to
monitor and ensure the quality of transport to the edge of a given
VoIP service.
Contact: the authors of RFC XXXX. Additional information: N/A
Intended usage: LIMITED USE Contact: the authors of RFC 6849.
Restrictions on usage: This media type depends on RTP Intended usage: LIMITED USE
framing, and hence is only defined for transfer via
RTP. Transfer within other framing protocols is not
defined at this time.
Author: Restrictions on usage: This media type depends on RTP framing and
Kaynam Hedayat. hence is only defined for transfer via RTP. Transfer within other
framing protocols is not defined at this time.
Change controller: IETF PAYLOAD working Author: Kaynam Hedayat.
group delegated from the IESG.
14.2.6 video/rtploopback Change controller: IETF PAYLOAD working group delegated from
the IESG.
To: ietf-types@iana.org 14.2.6. video/rtploopback
Subject: Registration of media type video/rtploopback To: ietf-types@iana.org
Type name: video Subject: Registration of media type video/rtploopback
Subtype name: rtploopback Type name: video
Required parameters: Subtype name: rtploopback
rate:RTP timestamp clock rate, which is equal to the Required parameters:
sampling rate. This is specified by the loop back
source, and reflected by the mirror.
Optional parameters: none rate: RTP timestamp clock rate, which is equal to the sampling
rate. This is specified by the loopback source and reflected by
the mirror.
Encoding considerations: This media type is framed. Optional parameters: N/A
Security considerations: See Section 12 of RFC XXXX. Encoding considerations: This media type is framed.
Interoperability considerations: none Security considerations: See Section 12 of RFC 6849.
Published specification: RFC XXXX.
Applications which use this media type: Applications wishing Interoperability considerations: N/A
to monitor and ensure the quality of transport to the
edge of a given Video Over IP Service.
Additional information: none Published specification: RFC 6849.
Contact: the authors of RFC XXXX. Applications that use this media type: Applications wishing to
monitor and ensure the quality of transport to the edge of a given
Video Over IP service.
Intended usage: LIMITED USE Additional information: N/A
Restrictions on usage: This media type depends on RTP Contact: the authors of RFC 6849.
framing, and hence is only defined for transfer via
RTP. Transfer within other framing protocols is not
defined at this time.
Author: Intended usage: LIMITED USE
Kaynam Hedayat. Restrictions on usage: This media type depends on RTP framing and
hence is only defined for transfer via RTP. Transfer within other
framing protocols is not defined at this time.
Change controller: IETF PAYLOAD working Author: Kaynam Hedayat.
group delegated from the IESG.
14.2.7 text/rtploopback Change controller: IETF PAYLOAD working group delegated from
the IESG.
To: ietf-types@iana.org 14.2.7. text/rtploopback
Subject: Registration of media type text/rtploopback To: ietf-types@iana.org
Type name: text Subject: Registration of media type text/rtploopback
Subtype name: rtploopback Type name: text
Required parameters: Subtype name: rtploopback
rate:RTP timestamp clock rate, which is equal to the Required parameters:
sampling rate. This is specified by the loop back
source, and reflected by the mirror.
Optional parameters: none rate: RTP timestamp clock rate, which is equal to the sampling
rate. This is specified by the loopback source and reflected by
the mirror.
Encoding considerations: This media type is framed. Optional parameters: N/A
Security considerations: See Section 12 of RFC XXXX. Encoding considerations: This media type is framed.
Interoperability considerations: none Security considerations: See Section 12 of RFC 6849.
Published specification: RFC XXXX. Interoperability considerations: N/A
Applications which use this media type: Applications wishing Published specification: RFC 6849.
to monitor and ensure the quality of transport to the
edge of a given Real-Time Text Service.
Additional information: none Applications that use this media type: Applications wishing to
monitor and ensure the quality of transport to the edge of a given
real-time text service.
Contact: the authors of RFC XXXX. Additional information: N/A
Intended usage: LIMITED USE Contact: the authors of RFC 6849.
Restrictions on usage: This media type depends on RTP Intended usage: LIMITED USE
framing, and hence is only defined for transfer via
RTP. Transfer within other framing protocols is not
defined at this time.
Author: Restrictions on usage: This media type depends on RTP framing and
Kaynam Hedayat. hence is only defined for transfer via RTP. Transfer within other
framing protocols is not defined at this time.
Change controller: IETF PAYLOAD working Author: Kaynam Hedayat.
group delegated from the IESG.
14.2.8 application/rtploopback Change controller: IETF PAYLOAD working group delegated from
the IESG.
To: ietf-types@iana.org 14.2.8. application/rtploopback
Subject: Registration of media type To: ietf-types@iana.org
application/rtploopback
Type name: application Subject: Registration of media type application/rtploopback
Subtype name: rtploopback Type name: application
Required parameters: Subtype name: rtploopback
rate:RTP timestamp clock rate, which is equal to the Required parameters:
sampling rate. This is specified by the loop back
source, and reflected by the mirror.
Optional parameters: none rate: RTP timestamp clock rate, which is equal to the sampling
rate. This is specified by the loopback source and reflected by
the mirror.
Encoding considerations: This media type is framed. Optional parameters: N/A
Security considerations: See Section 12 of RFC XXXX. Encoding considerations: This media type is framed.
Interoperability considerations: none Security considerations: See Section 12 of RFC 6849.
Published specification: RFC XXXX. Interoperability considerations: N/A
Applications which use this media type: Applications wishing Published specification: RFC 6849.
to monitor and ensure the quality of transport to the
edge of a given Real-Time Application Service.
Additional information: none Applications that use this media type: Applications wishing to
monitor and ensure the quality of transport to the edge of a given
real-time application service.
Contact: the authors of RFC XXXX. Additional information: N/A
Intended usage: LIMITED USE Contact: the authors of RFC 6849.
Restrictions on usage: This media type depends on RTP Intended usage: LIMITED USE
framing, and hence is only defined for transfer via
RTP. Transfer within other framing protocols is not
defined at this time.
Author: Restrictions on usage: This media type depends on RTP framing and
Kaynam Hedayat. hence is only defined for transfer via RTP. Transfer within other
framing protocols is not defined at this time.
Change controller: IETF PAYLOAD working Author: Kaynam Hedayat.
group delegated from the IESG.
15. Acknowledgements Change controller: IETF PAYLOAD working group delegated from
the IESG.
This document's editor would like to thank the original authors of 15. Acknowledgements
the document: Kaynam Hedayat, Nagarjuna Venna, Paul E. Jones, Arjun
Roychowdhury, Chelliah SivaChelvan, and Nathan Stratton. The
editor has made fairly insignificant changes in the end. Also,
we'd like to thank Magnus Westerlund, Miguel Garcia, Muthu Arul
Mozhi Perumal, Jeff Bernstein, Paul Kyzivat, Dave Oran, Flemming
Andreasen, Gunnar Hellstrom, Emil Ivov and Dan Wing for their
feedback, comments and suggestions.
16. Normative References This document's editor would like to thank the original authors of
the document: Kaynam Hedayat, Nagarjuna Venna, Paul E. Jones, Arjun
Roychowdhury, Chelliah SivaChelvan, and Nathan Stratton. The editor
has made fairly insignificant changes in the end. Also, we'd like to
thank Magnus Westerlund, Miguel Garcia, Muthu Arul Mozhi Perumal,
Jeff Bernstein, Paul Kyzivat, Dave Oran, Flemming Andreasen, Gunnar
Hellstrom, Emil Ivov, and Dan Wing for their feedback, comments, and
suggestions.
[RFC2119] Bradner, S.,"Key words for use in RFCs to Indicate 16. References
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer 16.1. Normative References
Model with the Session Description Protocol (SDP)",
RFC 3264, June 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R. and V. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Jacobson, "RTP: A Transport Protocol for Real-Time Requirement Levels", BCP 14, RFC 2119, March 1997.
Applications", STD 64, RFC 3550, July 2003.
[RFC3551] Schulzrinne, H., Casner, S., "RTP Profile for Audio [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
and Video Conferences with Minimial Control", STD 65, with Session Description Protocol (SDP)", RFC 3264,
RFC 3551, July 2003. June 2002.
[RFC3611] Almeroth, K., Caceres, R., Clark, A., Cole, R., [RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Duffield, N., Friedman, T., Hedayat, K., Sarac, K. Jacobson, "RTP: A Transport Protocol for Real-Time
and M. Westerlund, "RTP Control Protocol Extended Applications", STD 64, RFC 3550, July 2003.
Reports (RTCP XR)", RFC 3611, November 2003.
[RFC3711] Baugher, M., et al, "The Secure Real-time Transport [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
Protocol (SRTP)", RFC 3711, March 2004. Video Conferences with Minimal Control", STD 65,
RFC 3551, July 2003.
[RFC4566] Handley, M., Jacobson, V., Perkins, C., "SDP: Session [RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
Description Protocol", RFC 4566, July 2006. "RTP Control Protocol Extended Reports (RTCP XR)",
RFC 3611, November 2003.
[RFC4961] Wing, D., "Symmetric RTP / RTP Control Protocol [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
(RTCP)", RFC 4961, July 2007. Norrman, "The Secure Real-time Transport Protocol
(SRTP)", RFC 3711, March 2004.
[RFC5234] Crocker, P. Overell, "Augmented ABNF for Syntax [RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Specification: ABNF", RFC 5234, October 2005. Description Protocol", RFC 4566, July 2006.
17. Informative References [RFC4961] Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)",
BCP 131, RFC 4961, July 2007.
[RFC5245] Rosenberg, J., "Interactive Connectivity [RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Establishment (ICE): A Protocol for Network Address Syntax Specifications: ABNF", STD 68, RFC 5234,
Translator (NAT) Traversal for Offer/Answer January 2008.
Protocols", RFC 5245, April 2010.
[RFC6263] Marjou, X., Sollaud, A., "Application Mechanism for 16.2. Informative References
Keeping Alive the NAT Mappings Associated with RTP /
RTP Control Protocol (RTCP) Flows", RFC 6263, June
2011.
Authors' Addresses [RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
Hadriel Kaplan [RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
Acme Packet "Session Traversal Utilities for NAT (STUN)", RFC 5389,
100 Crosby Drive October 2008.
Bedford, MA 01730
USA
EMail: hkaplan@acmepacket.com [RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal
URI: http://www.acmepacket.com Using Relays around NAT (TURN): Relay Extensions to
Kaynam Hedayat Session Traversal Utilities for NAT (STUN)", RFC 5766,
EXFO April 2010.
285 Mill Road
Chelmsford, MA 01824
US
EMail: kaynam.hedayat@exfo.com [RFC6263] Marjou, X. and A. Sollaud, "Application Mechanism for
URI: http://www.exfo.com/ Keeping Alive the NAT Mappings Associated with RTP / RTP
Control Protocol (RTCP) Flows", RFC 6263, June 2011.
Nagarjuna Venna Authors' Addresses
Saperix
738 Main Street, #398
Waltham, MA 02451
US
EMail: vnagarjuna@saperix.com Hadriel Kaplan (editor)
URI: http://www.saperix.com/ Acme Packet
100 Crosby Drive
Bedford, MA 01730
US
EMail: hkaplan@acmepacket.com
URI: http://www.acmepacket.com
Paul E. Jones Kaynam Hedayat
Cisco Systems, Inc. EXFO
7025 Kit Creek Rd. 285 Mill Road
Research Triangle Park, NC 27709 Chelmsford, MA 01824
US US
EMail: kh274@cornell.edu
URI: http://www.exfo.com/
EMail: paulej@packetizer.com Nagarjuna Venna
URI: http://www.cisco.com/ Saperix
c/o DogPatch Labs
One Cambridge Center, 6th Floor
Cambridge, MA 02142
US
EMail: vnagarjuna@saperix.com
URI: http://www.saperix.com/
Nathan Stratton Paul E. Jones
BlinkMind, Inc. Cisco Systems, Inc.
2027 Briarchester Dr. 7025 Kit Creek Rd.
Katy, TX 77450 Research Triangle Park, NC 27709
US
EMail: paulej@packetizer.com
URI: http://www.cisco.com/
EMail: nathan@robotics.net Nathan Stratton
URI: http://www.robotics.net/ BlinkMind, Inc.
2027 Briarchester Dr.
Katy, TX 77450
US
EMail: nathan@robotics.net
URI: http://www.robotics.net/
 End of changes. 371 change blocks. 
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