draft-ietf-mmusic-media-loopback-17.txt   draft-ietf-mmusic-media-loopback-18.txt 
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N. Venna N. Venna
Saperix Saperix
P. Jones P. Jones
Cisco Systems, Inc. Cisco Systems, Inc.
A. Roychowdhury A. Roychowdhury
Hughes Systique Corp. Hughes Systique Corp.
C. SivaChelvan C. SivaChelvan
Cisco Systems, Inc. Cisco Systems, Inc.
N. Stratton N. Stratton
BlinkMind, Inc. BlinkMind, Inc.
March 10, 2012 March 26, 2012
An Extension to the Session Description Protocol (SDP) for Media An Extension to the Session Description Protocol (SDP)
Loopback and Real-time Transport Protocol (RTP) for Media Loopback
draft-ietf-mmusic-media-loopback-17 draft-ietf-mmusic-media-loopback-18
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with This Internet-Draft is submitted to IETF in full conformance with
the provisions of BCP 78 and BCP 79. the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. The list of current Internet-Drafts is at Drafts.
http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other months and may be updated, replaced, or obsoleted by other
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as reference material or to cite them other than as "work in as reference material or to cite them other than as "work in
progress." progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
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carefully, as they describe your rights and restrictions with carefully, as they describe your rights and restrictions with
respect to this document. Code Components extracted from this respect to this document. Code Components extracted from this
document must include Simplified BSD License text as described in document must include Simplified BSD License text as described in
Section 4.e of the Trust Legal Provisions and are provided without Section 4.e of the Trust Legal Provisions and are provided without
warranty as described in the Simplified BSD License. warranty as described in the Simplified BSD License.
Abstract Abstract
The wide deployment of Voice over IP (VoIP), Text and Video over IP The wide deployment of Voice over IP (VoIP), Text and Video over IP
services has introduced new challenges in managing and maintaining services has introduced new challenges in managing and maintaining
real-time voice/real-time Text/video quality, reliability, and real-time voice/text/video quality, reliability, and overall
overall performance. In particular, media delivery is an area that performance. In particular, media delivery is an area that needs
needs attention. One method of meeting these challenges is attention. One method of meeting these challenges is monitoring
monitoring the media delivery performance by looping media back to the media delivery performance by looping media back to the
the transmitter. This is typically referred to as "active transmitter. This is typically referred to as "active monitoring"
monitoring" of services. Media loopback is especially popular in of services. Media loopback is especially popular in ensuring the
ensuring the quality of transport to the edge of a given VoIP, quality of transport to the edge of a given VoIP, Real-time Text or
Real-time Text or Video over IP service. Today in networks that Video over IP service. Today in networks that deliver real-time
deliver real-time media, short of running 'ping' and 'traceroute' media, short of running 'ping' and 'traceroute' to the edge,
to the edge, service providers are left without the necessary tools administrators are left without the necessary tools to actively
to actively monitor, manage, and diagnose quality issues with their monitor, manage, and diagnose quality issues with their service.
service. The extension defined herein adds new SDP media The extension defined herein adds new SDP media attributes, which
attributes which enables establishment of media sessions where the enable establishment of media sessions where the media is looped
media is looped back to the transmitter. Such media sessions will back to the transmitter. Such media sessions will serve as
serve as monitoring and troubleshooting tools by providing the monitoring and troubleshooting tools by providing the means for
means for measurement of more advanced VoIP, Real-time Text and measurement of more advanced VoIP, Real-time Text and Video over IP
Video over IP performance metrics. performance metrics.
Table of Contents Table of Contents
1. Introduction..................................................3 1. Introduction..................................................3
1.1 Use Cases Supported.......................................4 1.1 Use Cases Supported.......................................4
2. Terminology...................................................5 2. Terminology...................................................6
3. Offering Entity Behavior......................................6 3. Overview of Operation.........................................6
4. Answering Entity Behavior.....................................6 3.1 SDP Offerer Behavior......................................6
5. SDP Constructs Syntax..............Error! Bookmark not defined. 3.2 SDP Answerer Behavior.....................................6
5.1 Loopback Type Attribute...................................7 4. New SDP Attributes............................................7
5.2 Loopback Mode Attribute...................................7 4.1 Loopback Type Attribute...................................7
5.3 Generating the Offer for Loopback Session.................8 4.2 Loopback Role Attributes: loopback-source and loopback-
5.4 Generating the Answer for Loopback Session................9 mirror........................................................8
5.5 Offerer Processing of the Answer.........................11 5. Rules for Generating the SDP offer/answer.....................9
5.6 Modifying the Session....................................11 5.1 Generating the SDP Offer for Loopback Session.............9
5.7 Establishing Sessions Between Entities Behind NAT........12 5.2 Generating the SDP Answer for Loopback Session...........10
5.3 Offerer Processing of the SDP Answer.....................11
5.4 Modifying the Session....................................12
5.5 Establishing Sessions Between Entities Behind NAT........12
6. RTP Requirements.............................................12 6. RTP Requirements.............................................12
7. Payload formats for Packet loopback..........................12 7. Payload formats for Packet loopback..........................13
7.1 Encapsulated Payload format..............................13 7.1 Encapsulated Payload format..............................13
7.2 Direct loopback RTP payload format.......................15 7.2 Direct loopback RTP payload format.......................16
8. RTCP Requirements............................................16 8. RTCP Requirements............................................17
9. Congestion Control...........................................17 9. Congestion Control...........................................17
10. Examples....................................................17 10. Examples....................................................18
10.1 Offer for specific media loopback type..................17 10.1 Offer for specific media loopback type..................18
10.2 Offer for choice of media loopback type.................18 10.2 Offer for choice of media loopback type.................18
10.3 Response to INVITE request rejecting loopback media.....19 10.3 Answerer rejecting loopback media.......................19
11. Security Considerations.....................................19 11. Security Considerations.....................................20
12. Implementation Considerations...............................20 12. Implementation Considerations...............................21
13. IANA Considerations.........................................20 13. IANA Considerations.........................................21
13.1 SDP Attributes..........................................20 13.1 SDP Attributes..........................................21
13.2 MIME Types..............................................21 13.2 MIME Types..............................................22
14. Normative References........................................30 14. Acknowledgements............................................31
15. Normative References........................................31
16. Informative References......................................32
1. Introduction 1. Introduction
The overall quality, reliability, and performance of VoIP, The overall quality, reliability, and performance of VoIP,
Real-time Text and Video over IP services rely on the performance Real-time Text and Video over IP services rely on the performance
and quality of the media path. In order to assure the quality of and quality of the media path. In order to assure the quality of
the delivered media there is a need to monitor the performance of the delivered media there is a need to monitor the performance of
the media transport. One method of monitoring and managing the the media transport. One method of monitoring and managing the
overall quality of real-time VoIP, Text and Video over IP Services overall quality of real-time VoIP, Text and Video over IP Services
is through monitoring the quality of the media in an active is through monitoring the quality of the media in an active
session. This type of "active monitoring" of services is a method session. This type of "active monitoring" of services is a method
of proactively managing the performance and quality of VoIP based of proactively managing the performance and quality of VoIP based
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VoIP, Text or Video over IP session. A way to achieve this goal is 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 request an endpoint to loop media back to the other endpoint and
to provide media statistics (e.g., RTCP and RTCP XR information). to provide media statistics (e.g., RTCP and RTCP XR information).
Another method involves deployment of special endpoints that always Another method involves deployment of special endpoints that always
loop incoming media back for sessions. Although the latter method loop incoming media back for sessions. Although the latter method
has been used and is functional, it does not scale to support large has been used and is functional, it does not scale to support large
networks and introduces new network management challenges. networks and introduces new network management challenges.
Further, it does not offer the granularity of testing a specific Further, it does not offer the granularity of testing a specific
endpoint that may be exhibiting problems. endpoint that may be exhibiting problems.
The extension defined in this memo introduces new SDP media The extension defined in this document introduces new SDP media
attributes that enable establishment of media sessions where the attributes that enable establishment of media sessions where the
media is looped back to the transmitter. The offer/answer model media is looped back to the transmitter. The SDP offer/answer
[RFC3264] is used to establish a loopback connection. Furthermore, model [RFC3264] is used to establish a loopback connection.
this extension provides guidelines on handling RTP [RFC3550], as Furthermore, this extension provides guidelines on handling RTP
well as usage of RTCP [RFC3550] and RTCP XR [RFC3611] for reporting [RFC3550], as well as usage of RTCP [RFC3550] and RTCP XR [RFC3611]
media related measurements. for reporting media related measurements.
1.1 Use Cases Supported 1.1 Use Cases Supported
As a matter of terminology in this document, packets flow from one As a matter of terminology in this document, packets flow from one
peer acting as a "loopback source", to the other peer acting as a peer acting as a "loopback source", to the other peer acting as a
"loopback mirror", which in turn returns packets to the loopback "loopback mirror", which in turn returns packets to the loopback
source. In advance of the session, the peers negotiate to determine source. In advance of the session, the peers negotiate to determine
which one acts in which role. The negotiation also includes details which one acts in which role, using the SDP offer/answer exchange.
such as the type of loopback to be used. The negotiation also includes details such as the type of loopback
to be used.
This specification supports three use cases: "encapsulated packet This specification supports three use cases: "encapsulated packet
loopback", "direct loopback", and "media loopback". These are loopback", "direct loopback", and "media loopback". These are
distinguished by the treatment of incoming RTP packets at the distinguished by the treatment of incoming RTP packets at the
loopback mirror. loopback mirror.
1.1.1 Encapsulated Packet Loopback 1.1.1 Encapsulated Packet Loopback
In the encapsulated packet loopback case, the entire incoming RTP In the encapsulated packet loopback case, the entire incoming RTP
packet is encapsulated as payload within an outer payload type that packet is encapsulated as payload within an outer RTP packet that
is specific to this use case and specified below (Section 7.1). is specific to this use case and specified in Section 7.1. The
The encapsulated packet is returned to the loopback source. The encapsulated packet is returned to the loopback source. The
loopback source can generate statistics for one-way path loopback source can generate statistics for one-way path
performance up to the RTP level for each direction of travel by performance up to the RTP level for each direction of travel by
examining sequence numbers and timestamps in the outer header and examining sequence numbers and timestamps in the encapsulating
the encapsulated RTP packet payload. The loopback source can also outer RTP header and the encapsulated RTP packet payload. The
play back the returned media content for evaluation. loopback source can also play back the returned media content for
evaluation.
Because the encapsulating payload extends the packet size, it could Because the encapsulating RTP packet header extends the packet
encounter difficulties in an environment where the original RTP size, it could encounter difficulties in an environment where the
packet size is close to the path MTU size. The encapsulating original RTP packet size is close to the path MTU size. The
payload type therefore offers the possibility of RTP-level encapsulating payload format therefore offers the possibility of
fragmentation of the returned packets. The use of this facility RTP-level fragmentation of the returned packets. The use of this
could affect statistics derived for the return path. In addition, facility could affect statistics derived for the return path. In
the increased bit rate required in the return direction may affect addition, the increased bit rate required in the return direction
these statistics more directly in a restricted-bandwidth situation. may affect these statistics more directly in a restricted-bandwidth
situation.
1.1.2 Direct Loopback 1.1.2 Direct Loopback
In the direct loopback case, the loopback mirror copies the payload In the direct loopback case, the loopback mirror copies the payload
of the incoming RTP packet into a new packet, the payload type of of the incoming RTP packet into a new RTP packet, using a payload
which is again specific to this use case and specified below format specific to this use case and specified in Section 7.2. The
(Section 7.2). The loopback mirror returns the new packet to the loopback mirror returns the new packet to the packet source. There
packet source. There is no provision in this case for RTP-level is no provision in this case for RTP-level fragmentation.
fragmentation.
This use case has the advantage of keeping the packet size the same This use case has the advantage of keeping the packet size the same
in both directions. The packet source can compute only two-way in both directions. The packet source can compute only two-way
path statistics from the direct loopback packet header, but can path statistics from the direct loopback packet header, but can
play back the returned media content. play back the returned media content.
It has been suggested that the loopback source, knowing that the It has been suggested that the loopback source, knowing that the
incoming packet will never be passed to a decoder, can store a incoming packet will never be passed to a decoder, can store a
timestamp and sequence number inside the payload of the packet it timestamp and sequence number inside the payload of the packet it
sends to the mirror, then extract that information from the sends to the mirror, then extract that information from the
returned direct loopback packet and compute one-way path statistics returned direct loopback packet and compute one-way path statistics
as in the previous case. Obviously, playout of returned content is as in the previous case. Obviously, playout of returned content is
no longer possible if this is done. no longer possible if this is done.
1.1.3 Media Loopback 1.1.3 Media Loopback
In the media loopback case, the loopback mirror submits the In the media loopback case, the loopback mirror submits the
incoming packet to a decoder appropriate to the incoming payload incoming packet to a decoder appropriate to the incoming payload
type. The packet is taken as close as possible to the analog level, type. The packet is taken as close as possible to the analog level,
then reencoded according to an outgoing format determined by then re-encoded according to an outgoing format determined by SDP
negotiation. The reencoded content is returned to the loopback negotiation. The reencoded content is returned to the loopback
source as an RTP packet with payload type corresponding to the source as an RTP packet with payload type corresponding to the
reencoding format. reencoding format.
This usage allows trouble-shooting at the codec level. The This usage allows trouble-shooting at the codec level. The
capability for path statistics is limited to what is available from capability for path statistics is limited to what is available from
RTCP reports. RTCP reports.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
this document are to be interpreted as described in RFC 2119. this document are to be interpreted as described in RFC 2119.
SDP: Session Description Protocol, as defined in [RFC4566]. This SDP: Session Description Protocol, as defined in [RFC4566]. This
document assumes the SDP offer/answer model is followed, per document assumes the SDP offer/answer model is followed, per
[RFC3264], but does not assume any specific protocol for carrying [RFC3264], but does not assume any specific signaling protocol for
the SDP. carrying the SDP.
The following terms are borrowed from [RFC3264] definitions: offer, The following terms are borrowed from [RFC3264] definitions: offer,
offerer, answer, answerer, and agent. offerer, answer, answerer, and agent.
3. SDP Offerer Behavior 3. Overview of Operation
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.
3.1 SDP Offerer Behavior
An SDP offerer compliant to this memo and attempting to establish a An SDP offerer compliant to this memo and attempting to establish a
media session with media loopback MUST include "loopback" media media session with media loopback MUST include "loopback" media
attributes for each individual media description in the offer attributes for each individual media description in the offer
message. The offerer MUST look for the "loopback" media attributes message. The offerer MUST look for the "loopback" media attributes
in the media description(s) of the response from the answer for in the media description(s) of the response from the answer for
confirmation that the request is accepted. confirmation that the request is accepted.
4. SDP Answerer Behavior 3.2 SDP Answerer Behavior
An SDP answerer compliant to this specification and receiving an An SDP answerer compliant to this specification and receiving an
offer containing media descriptions with the "loopback" media offer containing media descriptions with the "loopback" media
attributes MUST acknowledge the request by including the received attributes MUST acknowledge the request by including the received
"loopback" media attributes for each media description in its "loopback" media attributes for each media description in its
asnwer if it agrees to do the loopback. If the answerer does not asnwer if it agrees to do the loopback. If the answerer does not
want to do loopback or wants to reject the "loopback" request for want to do loopback or wants to reject the "loopback" request for
specific media types, it MAY do so as defined in section Error! specific media types, it MAY do so as defined in this section.
Reference source not found. of this specification.
An answerer MAY reject an offered stream (either with loopback- An answerer MAY reject an offered stream (either with loopback-
source or loopback-mirror) if the loopback-type is not specified, source or loopback-mirror) if the loopback-type is not specified,
the specified loopback-type is not supported, or the endpoint the specified loopback-type is not supported, or the endpoint
cannot honor the offer for any other reason. The loopback request cannot honor the offer for any other reason. The loopback request
MUST be rejected by setting the stream's media port number to zero MUST be rejected by setting the stream's media port number to zero
in the answer as defined in RFC 3264 [RFC3264], or by rejecting the in the answer as defined in RFC 3264 [RFC3264], or by rejecting the
entire offer (e.g., by rejecting the session request entirely). entire offer (i.e., by rejecting the session request entirely).
Note that an answerer that is not compliant to this specification Note that an answerer that is not compliant to this specification
and which receives an offer with the "loopback" media attributes and which receives an offer with the "loopback" media attributes
would ignore the attribute and treat the incoming offer as a normal would ignore the attributes and treat the incoming offer as a
request. If the offerer does not wish to establish a "normal" RTP normal request. If the offerer does not wish to establish a
session, it would need to terminate the session upon receiving such "normal" RTP session, it would need to terminate the session upon
an answer. receiving such an answer.
5. New SDP Attributes 4. New SDP Attributes
Three new SDP media-level attributes are defined: one indicates the Three new SDP media-level attributes are defined: one indicates the
type of loopback, and the other two define the mode of the type of loopback, and the other two define the role of the agent.
loopback.
5.1 Loopback Type Attribute 4.1 Loopback Type Attribute
This specification defines a new 'loopback' attribute, which This specification defines a new 'loopback' attribute, which
indicates the type of loopack that the agent is able to do. The indicates that the agent wishes to perform loopback, and the type
loopback type is a property media attribute with the following of loopack that the agent is able to do. The loopback-type is a
syntax: property media attribute with the following syntax:
a=loopback:<loopback-type> a=loopback:<loopback-type>
Following is the Augmented BNF [RFC5234] for loopback-type: Following is the Augmented BNF [RFC5234] for loopback-type:
Loopback-attr = "a=loopback:" Loopback-attr = "a=loopback:" SP loopback-type
loopback-type = loopback-choice [1*SP loopback-choice] loopback-type = loopback-choice [1*SP loopback-choice]
loopback-choice = loopback-type-pkt / loopback-type-media loopback-choice = loopback-type-pkt / loopback-type-media
loopback-type-pkt = "rtp-pkt-loopback" loopback-type-pkt = "rtp-pkt-loopback"
loopback-type-media = "rtp-media-loopback" loopback-type-media = "rtp-media-loopback"
The loopback type is used to indicate the type of loopback. The The loopback-type is used to indicate the type of loopback. The
loopback-type values are rtp-pkt-loopback, and rtp-media-loopback. loopback-type values are rtp-pkt-loopback, and rtp-media-loopback.
rtp-pkt-loopback: In this mode, the RTP packets are looped back to rtp-pkt-loopback: In this mode, the RTP packets are looped back to
the sender at a point before the encoder/decoder function in the the sender at a point before the encoder/decoder function in the
receive direction to a point after the encoder/decoder function in receive direction to a point after the encoder/decoder function in
the send direction. This effectively re-encapsulates the RTP the send direction. This effectively re-encapsulates the RTP
payload with the RTP/UDP/IP headers appropriate for sending it in payload with the RTP/UDP/IP headers appropriate for sending it in
the reverse direction. Any type of encoding related functions, the reverse direction. Any type of encoding related functions,
such as packet loss concealment, MUST NOT be part of this type of such as packet loss concealment, MUST NOT be part of this type of
loopback path. In this mode the RTP packets are looped back with a loopback path. In this mode the RTP packets are looped back with a
new payload type and format. Section 7 describes the payload new payload type and format. Section 7 describes the payload
formats that MUST be used for this type of loopback. formats that MUST be used for this type of loopback. 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 rtp-media-loopback: This loopback is activated as close as possible
to the analog interface and after the decoder so that the RTP to the analog interface and after the decoder so that the RTP
packets are subsequently re-encoded prior to transmission back to packets are subsequently re-encoded prior to transmission back to
the sender. the sender. This type of loopback applies to the media loopback
use-case described in Section 1.1.3.
5.2 Loopback Mode Attribute 4.2 Loopback Role Attributes: loopback-source and loopback-mirror
The loopback mode defines two value media attributes that are used The loopback role defines two value media attributes that are used
to indicate the mode of the loopback. These attributes are to indicate the role of the agent generating the SDP offer or
additional mode attributes like sendonly, recvonly, etc. The answer. The syntax of the two loopback role media attributes are as
syntax of the loopback mode media attributes are based on the follows:
following:
a=<loopback-mode>:<fmt>... a=loopback-source
The loopback-mode values are 'loopback-source' and 'loopback- and
mirror'.
a=loopback-mirror
loopback-source: This attribute specifies that the entity that loopback-source: This attribute specifies that the entity that
generated the SDP is the media source and expects the receiver of generated the SDP is the media source and expects the receiver of
the SDP message to act as a loopback-mirror. the SDP message to act as a loopback-mirror.
loopback-mirror: This attribute specifies that the entity that loopback-mirror: This attribute specifies that the entity that
generated the SDP will mirror (echo) all received media back to the generated the SDP will mirror (echo) all received media back to the
sender of the RTP stream. No media is generated locally by the sender of the RTP stream. No media is generated locally by the
looping back entity for transmission in the mirrored stream. looping back entity for transmission in the mirrored stream.
<fmt> is a media format description. The format description has the The "m=" line in the SDP MUST include all the payload types that
semantics as defined in section 5.14 of RFC 4566[RFC4566]. When will be used during the loopback session. The complete payload
loopback-mode is specified as loopback-source, the media format space for the session is specified in the "m=" line and the rtpmap
corresponds to the RTP payload types the entity that generated the attribute is used to map from the payload type number to an
SDP is willing to send. When loopback-mode is specified as encoding name denoting the payload format to be used.
loopback-mirror, the media format corresponds to the RTP payload
types the mirror is willing to receive. The "m=" line in the SDP
MUST include all the payload types that will be used during the
loopback session including those specified in the loopback-mode
attribute line. The complete payload space for the call 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.
5.3 Generating the Offer for Loopback Session 5. Rules for Generating the SDP offer/answer
5.1 Generating the SDP Offer for Loopback Session
If an offerer wishes to make a loopback request, it MUST include If an offerer wishes to make a loopback request, it MUST include
both the loopback-type and loopback-mode attributes in a valid SDP both the loopback-type and loopback-role attributes in a valid SDP
offer: offer:
Example: m=audio 41352 RTP/AVP 0 8 100 Example: m=audio 41352 RTP/AVP 0 8 100
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-source:0 8 100 a=loopback-source
a=rtpmap:0 pcmu/8000 a=rtpmap:0 pcmu/8000
a=rtpmap:8 pcma/8000 a=rtpmap:8 pcma/8000
a=rtpmap:100 G7221/16000/1 a=rtpmap:100 G7221/16000/1
A loopback offer in a given media description MUST NOT contain the Since media loopback requires bidirectional RTP, its normal
standard mode attributes sendonly, recvonly, sendrecv, or inactive. direction mode is "sendrecv"; the "sendrecv" direction attribute
MAY be encoded in SDP or not, as per section 5.1 of [RFC3264],
The loopback-mode attributes (loopback-source and loopback-mirror) since it is implied by default. If either the loopback source or
replace the standard attributes. 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).
The offerer may offer more than one loopback-type in the SDP offer. The offerer may offer more than one loopback-type in the SDP offer.
The port number and the address in the offer (m/c= lines) indicate The port number and the address in the offer (m/c= lines) indicate
where the offerer would like to send and receive the media stream. where the offerer would like to receive the media stream(s). The
The payload type numbers indicate the value of the payload the payload type numbers indicate the value of the payload the offerer
offerer expects to send and receive. If the offerer is the expects to receive. However, the answer might indicate a subset of
loopback-source, the subset of payload types indicated in the payload type numbers from those given in the offer. In that case,
a=loopback-source line are the payload types for the codecs the the offerer MUST only send the payload types received in the
offerer is willing to send. However, the answer might indicate a answer, per normal SDP offer/answer rules.
different payload type number for the same codec in the loopback-
mirror line. In that case, the offerer MUST send the payload type If the offer indicates rtp-pkt-loopback support, the offer MUST
received in the answer. If the offerer is the loopback-mirror, the also contain either an encapsulated or direct loopback encoding
subset of payload types indicated in the a=loopback-mirror line are format encoding names, or both, as defined in later sections of
the payload types for the codecs the offerer is willing to receive. this 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.
If loopback-type is rtp-pkt-loopback, the loopback-mirror MUST send If loopback-type is rtp-pkt-loopback, the loopback-mirror MUST send
and the loopback-source MUST receive the looped back packets and the loopback-source MUST receive the looped back packets
encoded in one of the two payload formats (encapsulated RTP or encoded in one of the two payload formats (encapsulated RTP or
direct loopback) as defined in section 7. direct loopback) as defined in section 7.
Example: m=audio 41352 RTP/AVP 0 8 112 Example: m=audio 41352 RTP/AVP 0 8 112
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-source:0 8 a=loopback-source
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
Example: m=audio 41352 RTP/AVP 0 8 112 Example: m=audio 41352 RTP/AVP 0 8 112
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-source:0 8 a=loopback-source
a=rtpmap:112 rtploopback/8000 a=rtpmap:112 rtploopback/8000
5.4 Generating the Answer for Loopback Session 5.2 Generating the SDP Answer for Loopback Session
As with the offer, an SDP answer for loopback MUST follow SDP
offer/answer rules for the direction attribute, but directions of
"sendonly" or "recvonly" do not apply for loopback operation. \
As with the offer, an SDP answer for loopback MUST NOT contain the
standard mode attributes sendonly, recvonly, sendrecv, or inactive.
The port number and the address in the answer (m/c= lines) indicate The port number and the address in the answer (m/c= lines) indicate
where the answerer would like to receive the media stream. The where the answerer would like to receive the media stream. The
payload type numbers indicate the value of the payload types the payload type numbers indicate the value of the payload types the
answerer expects to send and receive. The loopback-mode attributes answerer expects to send and receive.
(a=loopback-source or a=loopback-miror) MUST contain at least one
codec the answerer is willing to send or receive depending on
whether it is the loopback-source or the loopback-mirror. In
addition, the "m=" line MUST contain at least one codec that the
answerer is willing to send or receive depending on whether it is
the loopback-mirror or the loopback-source.
If the offerer is the loopback-source, the answerer MUST be a
loopback-mirror and the subset of payload types indicated in the
a=loopback-mirror line are the payload types for the codecs the
answerer is willing to receive. Similarly, if the offerer is the
loopback-mirror, the answerer MUST be aloopback-source and the
subset of payload types indicated in the a=loopback-source line are
the payload types for the codecs the answerer is willing to send.
If an answerer wishes to accept the loopback request it MUST If an answerer wishes to accept the loopback request it MUST
include both the loopback mode and loopback type attributes in the include both the loopback role and loopback type attributes in the
answer. When a stream is offered with the loopback-source answer. When a stream is offered with the loopback-source
attribute, the corresponding stream in the response MUST be attribute, the corresponding stream in the response MUST be
loopback-mirror and vice versa, provided that answerer is capable loopback-mirror and vice versa, provided that answerer is capable
of supporting the requested loopback-type. of supporting the requested loopback-type.
For example, if the offer contains the loopback-source attribute: For example, if the offer contains the loopback-source attribute:
m=audio 41352 RTP/AVP 0 8 m=audio 41352 RTP/AVP 0 8
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-source:0 8 a=loopback-source
The answer that is capable of supporting the offer MUST contain the The answer that is capable of supporting the offer MUST contain the
loopback-mirror attribute: loopback-mirror attribute:
m=audio 41352 RTP/AVP 0 8 m=audio 12345 RTP/AVP 0 8
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-mirror:0 8 a=loopback-mirror
If a stream is offered with multiple loopback type attributes, the If a stream is offered with multiple loopback type attributes, the
answer MUST include only one of the loopback types that are answer MUST include only one of the loopback types that are
accepted by the answerer. The answerer SHOULD give preference to accepted by the answerer. The answerer SHOULD give preference to
the first loopback-type in the SDP offer. the first loopback-type in the SDP offer.
For example, if the offer contains: For example, if the offer contains:
m=audio 41352 RTP/AVP 0 8 112 m=audio 41352 RTP/AVP 0 8 112
a=loopback:rtp-media-loopback rtp-pkt-loopback a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source:0 8 a=loopback-source
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
The answer that is capable of supporting the offer and chooses to The answer that is capable of supporting the offer and chooses to
loopback the media using the rtp-media-loopback type MUST contain: loopback the media using the rtp-media-loopback type MUST contain:
m=audio 41352 RTP/AVP 0 8 m=audio 12345 RTP/AVP 0 8
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-mirror:0 8 a=loopback-mirror
As specified in section 7, if the loopback-type is As specified in section 7, if the loopback-type is
rtp-pkt-loopback, either the encapsulated RTP payload format or rtp-pkt-loopback, either the encapsulated RTP payload format or
direct loopback RTP payload format MUST be used for looped back direct loopback RTP payload format MUST be used for looped back
packets. packets.
For example, if the offer contains: For example, if the offer contains:
m=audio 41352 RTP/AVP 0 8 112 113 m=audio 41352 RTP/AVP 0 8 112 113
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-source:0 8 a=loopback-source
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
a=rtpmap:113 rtploopback/8000 a=rtpmap:113 rtploopback/8000
The answer that is capable of supporting the offer must contain one The answer that is capable of supporting the offer must contain one
of the following: of the following:
m=audio 41352 RTP/AVP 0 8 112 m=audio 12345 RTP/AVP 0 8 112
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-mirror:0 8 a=loopback-mirror
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
m=audio 41352 RTP/AVP 0 8 113 m=audio 12345 RTP/AVP 0 8 113
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-mirror:0 8 a=loopback-mirror
a=rtpmap:113 rtploopback/8000 a=rtpmap:113 rtploopback/8000
The previous examples used the 'encaprtp' and 'rtploopback' The previous examples used the 'encaprtp' and 'rtploopback'
encoding names, which will be defined in sections 7.1.3 and 7.2.3. encoding names, which will be defined in sections 7.1.3 and 7.2.3.
5.5 Offerer Processing of the Answer 5.3 Offerer Processing of the SDP Answer
If the received answer does not contain a=loopback-mirror or If the received SDP answer does not contain an a=loopback-mirror or
a=loopback-source, it is assumed that the loopback extensions are a=loopback-source attribute, it is assumed that the loopback
not supported by the remote agent. This is not a protocol failure, extensions are not supported by the remote agent. This is not a
and instead merely completes the SDP offer/answer exchange with protocol failure, and instead merely completes the SDP offer/answer
whatever normal rules apply; the offerer MAY decide to end the exchange with whatever normal rules apply; the offerer MAY decide
established RTP session (if any) through normal means of the upper- to end the established RTP session (if any) through normal means of
layer signaling protocol (e.g., by sending a SIP BYE). the upper-layer signaling protocol (e.g., by sending a SIP BYE).
5.6 Modifying the Session 5.4 Modifying the Session
At any point during the loopback session, either participant MAY At any point during the loopback session, either participant MAY
issue a new offer to modify the characteristics of the previous issue a new offer to modify the characteristics of the previous
session, as defined in section 8 of RFC 3264 [RFC3264]. This also session, as defined in section 8 of RFC 3264 [RFC3264]. This also
includes transitioning from a normal media processing mode to includes transitioning from a normal media processing mode to
loopback mode, and vice a versa. loopback mode, and vice a versa.
5.7 Establishing Sessions Between Entities Behind NAT 5.5 Establishing Sessions Between Entities Behind NAT
ICE/STUN/TURN provide a general solution to establishing media ICE/STUN/TURN provide a general solution to establishing media
sessions between entities that are behind NATs. Loopback sessions sessions between entities that are behind NATs, as defined in
that involve one or more end points behind NATs SHOULD use these [RFC5245]. Loopback sessions that involve one or more end points
general solutions wherever possible. behind NATs SHOULD use these general solutions wherever possible.
6. RTP Requirements Furthermore, 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] SHOULD 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
accepting a media with rtp-pkt-loopback loopback-type MUST loopback RTP/RTCP MUST be used. In other words both agents MUST send
the incoming RTP packets using either the encapsulated RTP payload packets from the same source address and port they receive packets
on.
6. RTP Requirements
A looback source MUST NOT send multiple source streams on the same
5-tuple, since there is no means for the mirror to indicate which
is which in its mirrored RTP packets.
A loopback mirror that is compliant to this specification and
accepts media with rtp-pkt-loopback loopback-type MUST loopback the
incoming RTP packets using either the encapsulated RTP payload
format or the direct loopback RTP payload format as defined in format or the direct loopback RTP payload format as defined in
section 7 of this specification. section 7 of this specification.
An answering entity that is compliant to this specification and A device that is compliant to this specification and performing the
accepting a media with the loopback type rtp-media-loopback MUST mirroring using the loopback type rtp-media-loopback MUST transmit
transmit all received media back to the sender, unless congestion all received media back to the sender, unless congestion feedback
feedback or other lower-layer constraints prevent it from doing so. or other lower-layer constraints prevent it from doing so. The
The incoming media MUST be treated as if it were to be played (e.g. incoming media MUST be treated as if it were to be played (e.g. the
the media stream MAY receive treatment from PLC algorithms). The media stream MAY receive treatment from PLC algorithms). The
answering entity MUST re-generate all the RTP header fields as it mirroring entity MUST re-generate all the RTP header fields as it
would when transmitting media. The answering entity MAY choose to would when transmitting media. The mirroring entity MAY choose to
encode the loopback media according to any of the media encode the loopback media according to any of the media
descriptions supported by the offering entity. Furthermore, in descriptions supported by the offering entity. Furthermore, in
cases where the same media type is looped back, the answering cases where the same media type is looped back, the mirroring
entity MAY choose to preserve number of frames/packet and bitrate entity MAY choose to preserve number of frames/packet and bitrate
of the encoded media according to the received media. of the encoded media according to the received media.
7. Payload formats for Packet loopback 7. Payload formats for Packet loopback
The payload formats described in this section MUST be used by a The payload formats described in this section MUST be used by a
loopback-mirror when rtp-pkt-loopback is the specified loopback-mirror when 'rtp-pkt-loopback' is the specified
loopback-type. Two different formats are specified here - an loopback-type. Two different formats are specified here - an
encapsulated RTP payload format and a direct loopback RTP payload encapsulated RTP payload format and a direct loopback RTP payload
format. The encapsulated RTP payload format should be used when format. The encapsulated RTP payload format should be used when
the incoming RTP header information needs to be preserved during the incoming RTP header information needs to be preserved during
the loopback operation. This is useful in cases where loopback the loopback operation. This is useful in cases where loopback
source needs to measure performance metrics in both directions. source needs to measure performance metrics in both directions.
However, this comes at the expense of increased packet size as However, this comes at the expense of increased packet size as
described in section 7.1. The direct loopback RTP payload format described in section 7.1. The direct loopback RTP payload format
should be used when bandwidth requirement prevents the use of should be used when bandwidth requirement prevents the use of
encapsulated RTP payload format. encapsulated RTP payload format.
skipping to change at page 13, line 18 skipping to change at page 13, line 47
loopback-mirror. As described in RFC 3550 [RFC3550], sequence loopback-mirror. As described in RFC 3550 [RFC3550], sequence
numbers and timestamps in the RTP header are generated with initial numbers and timestamps in the RTP header are generated with initial
random values for security reasons. If this were not mandated and random values for security reasons. If this were not mandated and
the source payload is sequence number aware, the loopback-mirror the source payload is sequence number aware, the loopback-mirror
will be required to understand that payload format to generate will be required to understand that payload format to generate
looped back packets that do not violate RFC 3550 [RFC3550]. looped back packets that do not violate RFC 3550 [RFC3550].
Requiring looped back packets to be in one of the two formats means Requiring looped back packets to be in one of the two formats means
loopback-mirror does not have to look into the actual payload loopback-mirror does not have to look into the actual payload
received before generating the loopback packets. received before generating the loopback packets.
7.1 Encapsulated Payload format 7.1 Encapsulated Payload format
A received RTP packet is encapsulated in the payload section of the A received RTP packet is encapsulated in the payload section of the
RTP packet generated by a loopback-mirror. Each received packet RTP packet generated by a loopback-mirror. Each received packet
MUST be encapsulated in a different packet, the encapsulated packet MUST be encapsulated in a separate encapsulating RTP packet; the
MUST be fragmented only if required (for example: due to MTU encapsulated packet MUST be fragmented only if required (for
limitations). example: due to MTU limitations).
7.1.1 Usage of RTP Header fields 7.1.1Usage of RTP Header fields
Payload Type (PT): The assignment of an RTP payload type for this Payload Type (PT): The assignment of an RTP payload type for this
packet format is outside the scope of this document; it is either packet format is outside the scope of this document; it is either
specified by the RTP profile under which this payload format is specified by the RTP profile under which this payload format is
used or more likely signaled dynamically out-of-band (e.g., using used or more likely signaled dynamically out-of-band (e.g., using
SDP; section 7.1.3 defines the name binding). SDP; section 7.1.3 defines the name binding).
Marker (M) bit: If the received RTP packet is looped back in Marker (M) bit: If the received RTP packet is looped back in
multiple RTP packets, the M bit is set to 1 in the last packet, multiple encapsulating RTP packets, the M bit is set to 1 in every
otherwise it is set to 0. fragment except the last packet, otherwise it is set to 0.
Extension (X) bit: Defined by the RTP Profile used. Extension (X) bit: Defined by the RTP Profile used.
Sequence Number: The RTP sequence number SHOULD be generated by the Sequence Number: The RTP sequence number SHOULD be generated by the
loopback-mirror in the usual manner with a constant random offset loopback-mirror in the usual manner with a constant random offset
as described in RFC 3550 [RFC3550]. as described in RFC 3550 [RFC3550].
Timestamp: The RTP timestamp denotes the sampling instant for when Timestamp: The RTP timestamp denotes the sampling instant for when
the loopback-mirror is transmitting this packet to the loopback- the loopback-mirror is transmitting this packet to the loopback-
source. The RTP timestamp MUST use the same clock rate used by the source. The RTP timestamp MUST use the same clock rate as that of
loopback-source. The initial value of the timestamp SHOULD be the encapsulated packet. The initial value of the timestamp SHOULD
random for security reasons (see Section 5.1 of RFC 3550 be random for security reasons (see Section 5.1 of RFC 3550
[RFC3550]). [RFC3550]).
SSRC: set as described in RFC 3550 [RFC3550]. SSRC: set as described in RFC 3550 [RFC3550].
CC and CSRC fields are used as described in RFC 3550 [RFC3550]. CC and CSRC fields are used as described in RFC 3550 [RFC3550].
7.1.2 RTP Payload Structure 7.1.2RTP Payload Structure
The RTP header in the encapsulated packet MUST be followed by the The outer RTP header of the encapsulating packet MUST be followed
payload header defined in this section. If the received RTP packet by the payload header defined in this section. If the received RTP
has to be looped back in multiple packets due to fragmentation, the packet has to be looped back in multiple encapsulating packets due
RTP header in each packet MUST be followed by the payload header to fragmentation, the encapsulating RTP header in each packet MUST
defined in this section. The header is devised so that the be followed by the payload header defined in this section. The
loopback-source can decode looped back packets in the presence of header is devised so that the loopback-source can decode looped
moderate packet loss [RFC3550]. back packets in the presence of moderate packet loss [RFC3550].
0 1 2 3 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 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 | | receive timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F | R | CC |M| PT | sequence number | | F | R | CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| transmit timestamp | | transmit timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier | | synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers | | contributing source (CSRC) identifiers |
| .... | | .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The 12 octets after the receive timestamp are identical to the RTP The 12 octets after the receive timestamp are identical to the
header in the received packet except for the first 4 bits of the encapsulated RTP header of the received packet except for the first
first octet. 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 Receive Timestamp: 32 bits
The Receive timestamp denotes the sampling instant for when the The Receive timestamp denotes the sampling instant for when the
last octet of the received media packet that is being encapsulated last octet of the received media packet that is being encapsulated
by the loopback-mirror is received from the loopback-source. The by the loopback-mirror is received from the loopback-source. The
Receive timestamp MUST be based on the same clock used by the Receive timestamp MUST be based on the same clock used by the
loopback-source. The initial value of the timestamp SHOULD be loopback-source. The initial value of the timestamp SHOULD be
random for security reasons (see Section 5.1 of RFC 3550 random for security reasons (see Section 5.1 of RFC 3550
[RFC3550]). [RFC3550]).
skipping to change at page 15, line 8 skipping to change at page 15, line 48
Fragmentation (F): 2 bits Fragmentation (F): 2 bits
First Fragment (00) /Last Fragment (01) /No Fragmentation(10)/ First Fragment (00) /Last Fragment (01) /No Fragmentation(10)/
Intermediate Fragment (11). This field identifies how much of the Intermediate Fragment (11). This field identifies how much of the
received packet is encapsulated in this packet by the loopback- received packet is encapsulated in this packet by the loopback-
mirror. If the received packet is not fragmented, this field is mirror. If the received packet is not fragmented, this field is
set to 10; otherwise the packet that contains the first fragments set to 10; otherwise the packet that contains the first fragments
sets this field to 00, the packet that contains the last fragment sets this field to 00, the packet that contains the last fragment
sets this field to 01, all other packets set this field to 11. sets this field to 01, all other packets set this field to 11.
Reserved: 2 bits 7.1.3Usage of SDP
This field is reserved for future definition. In the absence of
such a definition, the bits in this field MUST be set to zero and
MUST be ignored by the receiver.
Any padding octets in the original packet MUST NOT be included in
the loopback packet generated by a loopback-mirror. The
loopback-mirror MAY add padding octets if required.
7.1.3 Usage of SDP
The payload type number for the encapsulated stream can be The payload type number for the encapsulated stream can be
negotiated using a mechanism like SDP. There is no static payload negotiated using SDP. There is no static payload type assignment
type assignment for the encapsulated stream, so dynamic payload for the encapsulating stream, so dynamic payload type numbers MUST
type numbers MUST be used. The binding to the name is indicated by be used. The binding to the name is indicated by an rtpmap
an rtpmap attribute. The name used in this binding is "encaprtp". attribute. The name used in this binding is "encaprtp".
The following is an example SDP fragment for encapsulated RTP. The following is an example SDP fragment for encapsulated RTP.
m=audio 41352 RTP/AVP 112 m=audio 41352 RTP/AVP 112
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
7.2 Direct loopback RTP payload format 7.2 Direct loopback RTP payload format
The direct loopback RTP payload format can be used in scenarios The direct loopback RTP payload format can be used in scenarios
where the 16 byte overhead of the encapsulated payload format is where the 16 byte overhead of the encapsulated payload format is of
significant. This payload format MUST NOT be used in cases where concern, or simply due to local policy. When using this payload
the MTU on the loopback path will cause fragmentation of looped format, the receiver MUST loop back each received RTP packet
back RTP packets. When using this payload format, the receiver payload (not header) in a separate RTP packet.
MUST loop back each received packet in a separate RTP packet.
7.2.1 Usage of RTP Header fields Because a direct loopback format does not retain the original RTP
headers, there will be no indication of the original payload-type
sent to the mirror, in looped returning packets. Therefore, the
loopback source SHOULD only send one payload type per loopback RTP
session, if direct mode is used.
Payload Type (PT): The assignment of an RTP payload type for this 7.2.1Usage of RTP Header fields
packet format is outside the scope of this document; it is either
specified by the RTP profile under which this payload format is Payload Type (PT): The assignment of an RTP payload type for the
used or more likely signaled dynamically out-of-band (e.g., using encapsulating packet format is outside the scope of this document;
SDP; section 7.2.3 defines the name binding). 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. Marker (M) bit: Set to the value in the received packet.
Extension (X) bit: Defined by the RTP Profile used. Extension (X) bit: Defined by the RTP Profile used.
Sequence Number: The RTP sequence number SHOULD be generated by the Sequence Number: The RTP sequence number SHOULD be generated by the
loopback-mirror in the usual manner with a constant random offset. loopback-mirror in the usual manner with a constant random offset.
Timestamp: The RTP timestamp denotes the sampling instant for when Timestamp: The RTP timestamp denotes the sampling instant for when
the loopback-mirror is transmitting this packet to the the loopback-mirror is transmitting this packet to the
loopback-source. The RTP timestamp MUST be based on the same clock loopback-source. The RTP timestamp MUST be based on the same clock
used by the loopback-source. The initial value of the timestamp as that of the received RTP packet. The initial value of the
SHOULD be random for security reasons (see Section 5.1 of RFC 3550 timestamp SHOULD be random for security reasons (see Section 5.1 of
[RFC3550]). RFC 3550 [RFC3550]).
SSRC: set as described in RFC 3550 [RFC3550]. SSRC: set as described in RFC 3550 [RFC3550].
CC and CSRC fields are used as described in RFC 3550 [RFC3550]. CC and CSRC fields are used as described in RFC 3550 [RFC3550].
7.2.2 RTP Payload Structure 7.2.2RTP Payload Structure
This payload format does not define any payload specific headers. This payload format does not define any payload specific headers.
The loopback-mirror simply copies the payload data from the payload The loopback-mirror simply copies the RTP payload data from the
portion of the packet received from the loopback-source. payload portion of the RTP packet received from the loopback-
source.
7.2.3 Usage of SDP 7.2.3Usage of SDP
The payload type number for the payload loopback stream can be The payload type number for the payload loopback stream can be
negotiated using a mechanism like SDP. There is no static payload negotiated using a mechanism like SDP. There is no static payload
type assignment for the stream, so dynamic payload type numbers type assignment for the stream, so dynamic payload type numbers
MUST be used. The binding to the name is indicated by an rtpmap MUST be used. The binding to the name is indicated by an rtpmap
attribute. The name used in this binding is "rtploopback". attribute. The name used in this binding is "rtploopback".
The following is an example SDP fragment for direct loopback RTP The following is an example SDP fragment for direct loopback RTP
format. format.
m=audio 41352 RTP/AVP 112 m=audio 41352 RTP/AVP 112
a=rtpmap:112 rtploopback/8000 a=rtpmap:112 rtploopback/8000
8. RTCP Requirements 8. RTCP Requirements
The use of the loopback attribute is intended for monitoring of The use of the loopback attribute is intended for monitoring of
media quality of the session. Consequently the media performance media quality of the session. Consequently the media performance
information should be exchanged between the offering and the information should be exchanged between the offering and the
answering entities. An offering or answering entity that is answering entities. An offering or answering agent that is
compliant to this specification SHOULD support RTCP per [RFC3550] compliant to this specification SHOULD support RTCP per [RFC3550]
and RTCP-XR per RFC 3611 [RFC3611]. Furthermore, if the client or and RTCP-XR per RFC 3611 [RFC3611]. Furthermore, if the offerer or
the server choose to support RTCP-XR, they SHOULD support RTCP-XR answerer choose to support RTCP-XR, they SHOULD support RTCP-XR
Loss RLE report block, Duplicate RLE report block, Statistics Loss RLE report block, Duplicate RLE report block, Statistics
Summary report block, and VoIP Metric Reports Block per sections Summary report block, and VoIP Metric Reports Block per sections
4.1, 4.2, 4.6, and 4.7 of RFC 3611 [RFC3611]. The client and the 4.1, 4.2, 4.6, and 4.7 of RFC 3611 [RFC3611]. The offerer and the
server MAY support other RTCP-XR reporting blocks as defined by RFC answerer MAY support other RTCP-XR reporting blocks as defined by
3611 [RFC3611]. RFC 3611 [RFC3611].
9. Congestion Control 9. Congestion Control
All the participants in a loopback session SHOULD implement All the participants in a loopback session SHOULD implement
congestion control mechanisms as defined by the RTP profile under congestion control mechanisms as defined by the RTP profile under
which the loopback mechanism is implemented. For audio video which the loopback mechanism is implemented. For audio video
profiles, implementations SHOULD conform to the mechanism defined profiles, implementations SHOULD conform to the mechanism defined
in Section 2 of RFC 3551. in Section 2 of RFC 3551.
10. Examples 10. Examples
This section provides examples for media descriptions using SDP for This section provides examples for media descriptions using SDP for
different scenarios. The examples are given for SIP-based different scenarios. The examples are given for SIP-based
transactions and are abbreviated and do not show the complete transactions and are abbreviated and do not show the complete
signaling for convenience. signaling for convenience.
10.1 Offer for specific media loopback type 10.1 Offer for specific media loopback type
An agent sends an SDP offer which looks like: An agent sends an SDP offer which looks like:
v=0 v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=Example s=-
i=An example session
e=alice@example.com
c=IN IP4 host.atlanta.example.com c=IN IP4 host.atlanta.example.com
t=0 0 t=0 0
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-source:0 a=loopback-source
a=rtpmap:0 pcmu/8000 a=rtpmap:0 pcmu/8000
The agent is offering to source the media and expects the answering The agent is offering to source the media and expects the answering
agent to mirror the RTP stream per rtp-media-loopback loopback agent to mirror the RTP stream per rtp-media-loopback loopback
type. type.
An answering agent sends an SDP answer which looks like: An answering agent sends an SDP answer which looks like:
v=0 v=0
o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
s=Example s=-
i=An example session
e=bob@example.com
c=IN IP4 host.biloxi.example.com c=IN IP4 host.biloxi.example.com
t=0 0 t=0 0
m=audio 49270 RTP/AVP 0 m=audio 49270 RTP/AVP 0
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-mirror:0 a=loopback-mirror
a=rtpmap:0 pcmu/8000 a=rtpmap:0 pcmu/8000
The answerer is accepting to mirror the media from the offerer at The answerer is accepting to mirror the media from the offerer at
the media level. the media level.
10.2 Offer for choice of media loopback type 10.2 Offer for choice of media loopback type
An agent sends an SDP offer which looks like: An agent sends an SDP offer which looks like:
v=0 v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=Example s=-
i=An example session
e=alice@example.com
c=IN IP4 host.atlanta.example.com c=IN IP4 host.atlanta.example.com
t=0 0 t=0 0
m=audio 49170 RTP/AVP 0 112 113 m=audio 49170 RTP/AVP 0 112 113
a=loopback:rtp-media-loopback rtp-pkt-loopback a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source:0 a=loopback-source
a=rtpmap:0 pcmu/8000 a=rtpmap:0 pcmu/8000
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
a=rtpmap:113 rtploopback/8000 a=rtpmap:113 rtploopback/8000
The offerer is offering to source the media and expects the The offerer is offering to source the media and expects the
answerer to mirror the RTP stream at either the media or rtp level. answerer to mirror the RTP stream at either the media or rtp level.
An answering agent sends an SDP answer which looks like: An answering agent sends an SDP answer which looks like:
v=0 v=0
o=box 1234567890 1122334455 IN IP4 host.biloxi.example.com o=box 1234567890 1122334455 IN IP4 host.biloxi.example.com
s=Example s=-
i=An example session
e=bob@example.com
c=IN IP4 host.biloxi.example.com c=IN IP4 host.biloxi.example.com
t=0 0 t=0 0
m=audio 49270 RTP/AVP 0 112 m=audio 49270 RTP/AVP 0 112
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-mirror:0 a=loopback-mirror
a=rtpmap:0 pcmu/8000 a=rtpmap:0 pcmu/8000
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
The answerer is accepting to mirror the media from the offerer at The answerer is accepting to mirror the media from the offerer at
the packet level using the encapsulated RTP payload format. the packet level using the encapsulated RTP payload format.
10.3 Answerer rejecting loopback media 10.3 Answerer rejecting loopback media
An agent sends an SDP offer which looks like: An agent sends an SDP offer which looks like:
v=0 v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=Example s=-
i=An example session
e=user@example.com
c=IN IP4 host.atlanta.example.com c=IN IP4 host.atlanta.example.com
t=0 0 t=0 0
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-source:0 a=loopback-source
a=rtpmap:0 pcmu/8000 a=rtpmap:0 pcmu/8000
The offerer is offering to source the media and expects the The offerer is offering to source the media and expects the
answerer to mirror the RTP stream at the media level. answerer to mirror the RTP stream at the media level.
An answering agent sends an SDP answer which looks like: An answering agent sends an SDP answer which looks like:
v=0 v=0
o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
s=Example s=-
i=An example session
e=user@example.com
c=IN IP4 host.biloxi.example.com c=IN IP4 host.biloxi.example.com
t=0 0 t=0 0
m=audio 0 RTP/AVP 0 m=audio 0 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-mirror:0
a=rtpmap:0 pcmu/8000 a=rtpmap:0 pcmu/8000
11. Security Considerations Note in this case the answerer did not indicate loopback support,
although it could have and still used a port number of 0 to
indicate it does not wish to accept that media session.
The security considerations of [RFC3264] apply. Furthermore, given Alternatively, the answering agent could have simply rejected the
that media loopback may be automated without the end user's entire SDP offer through some higher-layer signaling protocol means
knowledge, the server of the media loopback should be aware of (e.g., by rejecting the SIP INVITE request if the SDP offer was in
denial of service attacks. It is recommended that sessions with the INVITE).
media loopback are authenticated and the frequency of such sessions
is limited by the server.
12. Implementation Considerations 11. Security Considerations
The security considerations of [RFC3264] and [RFC3550] apply.
Given that media loopback may be automated without the end user's
knowledge, the answerer of the media loopback should be aware of
denial of service attacks. It is recommended that session requests
for media loopback be authenticated and the frequency of such
sessions limited by the answerer.
If the higher-layer signaling protocol were not authenticated, a
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,
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
without authentication and without notifying the end-user, then an
attacker could cause the battery to drain, and possibly deny the
end-user normal phone service or cause network data usage fees.
This could even occur naturally if a legitimate loopback session
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
means of indicating to the human user that the device is in a
loopback session, even if it is an authenticated session. Devices
which answer or generate loopback sessions SHOULD either perform
keepalive/refresh tests of the session state through some means, or
time out the session automatically.
12. Implementation Considerations
The media loopback approach described in this document is a The media loopback approach described in this document is a
complete solution that would work under all scenarios. However, it complete solution that would work under all scenarios. However, it
is believed that the solution may not be light-weight enough for is believed that the solution may not be light-weight enough for
the common case. In light of this concern, this section clarifies the common case. In light of this concern, this section clarifies
which features of the loopback proposal MUST be implemented for all which features of the loopback proposal MUST be implemented for all
implementations and which features MAY be deferred if the complete implementations and which features MAY be deferred if the complete
solution is not desired. solution is not desired.
All implementations MUST support the rtp-pkt-loopback option for All implementations MUST at least support the rtp-pkt-loopback mode
loopback-type attribute. In addition, for the loopback-mode for loopback-type, with direct media loopback payload encoding. In
attribute, all implementations of an offerer MUST at a minimum be addition, for the loopback role, all implementations of an SDP
able to act as a loopback-source. All implementation MUST also at a offerer MUST at least be able to act as a loopback-source.
minimum support the direct media loopback payload type. The rtp-
media-loopback attribute MAY be implemented in complete
implementations of this draft.
13. IANA Considerations 13. IANA Considerations
13.1 SDP Attributes 13.1 SDP Attributes
This document defines three new media-level SDP attributes. IANA This document defines three new media-level SDP attributes. IANA
has registered the following attributes: has registered the following attributes:
Contact name: Kaynam Hedayat Contact name: Kaynam Hedayat
<kaynam.hedayat@exfo.com>. <kaynam.hedayat@exfo.com>.
Attribute name: "loopback". Attribute name: "loopback".
Type of attribute: Media level. Type of attribute: Media level.
Subject to charset: No. Subject to charset: No.
Purpose of attribute: The 'loopback' attribute is used to Purpose of attribute: The 'loopback' attribute is used to
skipping to change at page 21, line 6 skipping to change at page 22, line 14
Contact name: Kaynam Hedayat Contact name: Kaynam Hedayat
<kaynam.hedayat@exfo.com>. <kaynam.hedayat@exfo.com>.
Attribute name: "loopback-source". Attribute name: "loopback-source".
Type of attribute: Media level. Type of attribute: Media level.
Subject to charset: No. Subject to charset: No.
Purpose of attribute: The 'loopback-source' attribute Purpose of attribute: The 'loopback-source' attribute
specifies that the sender is the media specifies that the sender is the media
source and expects the receiver to act source and expects the receiver to act
as a loopback-mirror. as a loopback-mirror.
Allowed attribute values: The parameter to 'loopback-source' is Allowed attribute values: None.
a media format ("<fmt>") description
as defined in RFC 4566 Section 5.14.
Contact name: Kaynam Hedayat Contact name: Kaynam Hedayat
<kaynam.hedayat@exfo.com>. <kaynam.hedayat@exfo.com>.
Attribute name: "loopback-mirror". Attribute name: "loopback-mirror".
Type of attribute: Media level. Type of attribute: Media level.
Subject to charset: No. Subject to charset: No.
Purpose of attribute: The 'loopback-mirror' attribute Purpose of attribute: The 'loopback-mirror' attribute
specifies that the receiver will specifies that the receiver will
mirror (echo) all received media back mirror (echo) all received media back
to the sender of the RTP stream. to the sender of the RTP stream.
Allowed attribute values: The parameter to 'loopback-mirror' is Allowed attribute values: None.
a media format ("<fmt>") description
as defined in RFC 4566 Section 5.14.
13.2 MIME Types 13.2 MIME Types
The IANA has registered the following MIME types: The IANA has registered the following MIME types:
13.2.1 audio/encaprtp 13.2.1 audio/encaprtp
To: ietf-types@iana.org To: ietf-types@iana.org
Subject: Registration of media type audio/encaprtp Subject: Registration of media type audio/encaprtp
Type name: audio Type name: audio
skipping to change at page 30, line 21 skipping to change at page 31, line 25
framing, and hence is only defined for transfer via framing, and hence is only defined for transfer via
RTP. Transfer within other framing protocols is not RTP. Transfer within other framing protocols is not
defined at this time. defined at this time.
Author: Author:
Kaynam Hedayat. Kaynam Hedayat.
Change controller: IETF Audio/Video Transport working Change controller: IETF Audio/Video Transport working
group delegated from the IESG. group delegated from the IESG.
14. Normative References 14. Acknowledgements
This document's editor would like to thank the original authors of
the document: Kaynam Hedayat, et al. The editor has made fairly
insignificant changes in the end. Also, we'd like to thank Magnus
Westerlund, Miguel Garcia, Flemming Andreason, Gunnar Hellstrom,
Emil Ivov and Dan Wing for their feedback, comments and
suggestions.
15. Normative References
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer
Model with the Session Description Protocol (SDP)", Model with the Session Description Protocol (SDP)",
RFC 3264, June 2002. RFC 3264, June 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R. and V. [RFC3550] Schulzrinne, H., Casner, S., Frederick, R. and V.
Jacobson, "RTP: A Transport Protocol for Real-Time Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003. Applications", STD 64, RFC 3550, July 2003.
[RFC3611] Almeroth, K., Caceres, R., Clark, A., Cole, R., [RFC3611] Almeroth, K., Caceres, R., Clark, A., Cole, R.,
skipping to change at page 31, line 15 skipping to change at page 32, line 25
[RFC3551] Schulzrinne, H., Casner, S., "RTP Profile for Audio [RFC3551] Schulzrinne, H., Casner, S., "RTP Profile for Audio
and Video Conferences with Minimial Control", STD 65, and Video Conferences with Minimial Control", STD 65,
RFC 3551, July 2003. RFC 3551, July 2003.
[RFC4566] Handley, M., Jacobson, V., Perkins, C., "SDP: Session [RFC4566] Handley, M., Jacobson, V., Perkins, C., "SDP: Session
Description Protocol", RFC 4566, July 2006. Description Protocol", RFC 4566, July 2006.
[RFC4855] Casner, S., "Media Type Registration of RTP Payload [RFC4855] Casner, S., "Media Type Registration of RTP Payload
Formats", RFC 4855, February 2007. Formats", RFC 4855, February 2007.
16. Informative References
[RFC5245] Rosenberg, J., "Interactive Connectivity
Establishment (ICE): A Protocol for Network Address
Translator (NAT) Traversal for Offer/Answer
Protocols", RFC 5245, April 2010.
[RFC6263] Marjou, X., Sollaud, A., "Application Mechanism for
Keeping Alive the NAT Mappings Associated with RTP /
RTP Control Protocol (RTCP) Flows", RFC 6263, June
2011.
Authors' Addresses Authors' Addresses
Hadriel Kaplan Hadriel Kaplan
Acme Packet Acme Packet
100 Crosby Drive 100 Crosby Drive
Bedford, MA 01730 Bedford, MA 01730
USA USA
EMail: hkaplan@acmepacket.com EMail: hkaplan@acmepacket.com
URI: http://www.acmepacket.com URI: http://www.acmepacket.com
skipping to change at page 31, line 25 skipping to change at page 33, line 4
Authors' Addresses Authors' Addresses
Hadriel Kaplan Hadriel Kaplan
Acme Packet Acme Packet
100 Crosby Drive 100 Crosby Drive
Bedford, MA 01730 Bedford, MA 01730
USA USA
EMail: hkaplan@acmepacket.com EMail: hkaplan@acmepacket.com
URI: http://www.acmepacket.com URI: http://www.acmepacket.com
Kaynam Hedayat Kaynam Hedayat
EXFO EXFO
285 Mill Road 285 Mill Road
Chelmsford, MA 01824 Chelmsford, MA 01824
US US
Phone: +1 978 367 5611
EMail: kaynam.hedayat@exfo.com EMail: kaynam.hedayat@exfo.com
URI: http://www.exfo.com/ URI: http://www.exfo.com/
Nagarjuna Venna Nagarjuna Venna
Saperix Saperix
738 Main Street, #398 738 Main Street, #398
Waltham, MA 02451 Waltham, MA 02451
US US
Phone: +1 978 367 5703
EMail: vnagarjuna@saperix.com EMail: vnagarjuna@saperix.com
URI: http://www.saperix.com/ URI: http://www.saperix.com/
Paul E. Jones Paul E. Jones
Cisco Systems, Inc. Cisco Systems, Inc.
7025 Kit Creek Rd. 7025 Kit Creek Rd.
Research Triangle Park, NC 27709 Research Triangle Park, NC 27709
US US
Phone: +1 919 392 6948
EMail: paulej@packetizer.com EMail: paulej@packetizer.com
URI: http://www.cisco.com/ URI: http://www.cisco.com/
Arjun Roychowdhury Arjun Roychowdhury
Hughes Systique Corp. Hughes Systique Corp.
15245 Shady Grove Rd, Ste 330 15245 Shady Grove Rd, Ste 330
Rockville MD 20850 Rockville MD 20850
US US
Phone: +1 301 527 1629
EMail: arjun@hsc.com EMail: arjun@hsc.com
URI: http://www. hsc.com/ URI: http://www. hsc.com/
Chelliah SivaChelvan Chelliah SivaChelvan
Cisco Systems, Inc. Cisco Systems, Inc.
2200 East President George Bush Turnpike 2200 East President George Bush Turnpike
Richardson, TX 75082 Richardson, TX 75082
US US
Phone: +1 972 813 5224
EMail: chelliah@cisco.com EMail: chelliah@cisco.com
URI: http://www.cisco.com/ URI: http://www.cisco.com/
Nathan Stratton Nathan Stratton
BlinkMind, Inc. BlinkMind, Inc.
2027 Briarchester Dr. 2027 Briarchester Dr.
Katy, TX 77450 Katy, TX 77450
Phone: +1 832 330 3810
EMail: nathan@robotics.net EMail: nathan@robotics.net
URI: http://www.robotics.net/ URI: http://www.robotics.net/
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