draft-ietf-mmusic-media-loopback-03.txt   draft-ietf-mmusic-media-loopback-04.txt 
K. Hedayat K. Hedayat
Internet Draft Brix Networks Internet Draft Brix Networks
Expires: December 2006 P. Jones Expires: February 2007 P. Jones
Cisco Systems, Inc. Cisco Systems, Inc.
A. Roychowdhury A. Roychowdhury
Hughes Hughes
C. SivaChelvan C. SivaChelvan
Cisco Systems, Inc. Cisco Systems, Inc.
N. Stratton N. Stratton
August 2006
An Extension to the Session Description Protocol (SDP) for Media An Extension to the Session Description Protocol (SDP) for Media
Loopback Loopback
draft-ietf-mmusic-media-loopback-03 draft-ietf-mmusic-media-loopback-04
Status of this Memo Status of this Memo
By submitting this Internet-Draft, each author represents that any By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79. aware will be disclosed, in accordance with Section 6 of 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
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service. The extension defined herein adds new SDP media service. The extension defined herein adds new SDP media
attributes which enables establishment of media sessions where the attributes which enables establishment of media sessions where the
media is looped back to the transmitter. Such media sessions will media is looped back to the transmitter. Such media sessions will
serve as monitoring and troubleshooting tools by providing the serve as monitoring and troubleshooting tools by providing the
means for measurement of more advanced VoIP, Real-time Text and means for measurement of more advanced VoIP, Real-time Text and
Video Over IP performance metrics. Video Over IP performance metrics.
Table of Contents Table of Contents
1. Introduction..................................................3 1. Introduction..................................................3
2. Terminology...................................................3 2. Terminology...................................................4
3. Offering Entity Behavior......................................4 3. Offering Entity Behavior......................................4
4. Answering Entity Behavior.....................................4 4. Answering Entity Behavior.....................................4
5. SDP Constructs Syntax.........................................4 5. SDP Constructs Syntax.........................................4
5.1 Loopback Type Attribute...................................4 5.1 Loopback Type Attribute...................................4
5.2 Loopback Mode Attribute...................................6 5.2 Loopback Mode Attribute...................................6
5.3 Generating the Offer for Loopback Session.................6 5.3 Generating the Offer for Loopback Session.................7
5.4 Generating the Answer for Loopback Session................7 5.4 Generating the Answer for Loopback Session................8
5.5 Offerer Processing of the Answer..........................8 5.5 Offerer Processing of the Answer..........................9
5.6 Modifying the Session.....................................8 5.6 Modifying the Session.....................................9
6. RTP Requirements..............................................8 6. RTP Requirements.............................................10
7. RTCP Requirements.............................................9 7. Payload format for encapsulated RTP Streams..................10
8. Examples......................................................9 7.1 Usage of RTP Header fields...............................10
8.1 Offer for specific media loopback type....................9 7.2 RTP Payload Structure....................................11
8.2 Offer for choice of media loopback type..................10 7.3 Usage of SDP.............................................12
8.3 Offer for choice of media loopback type with 8. RTCP Requirements............................................13
rtp-start-loopback...........................................11 9. Congestion Control...........................................13
8.4 Response to INVITE request rejecting loopback media......12 10. Examples....................................................13
8.5 Response to INVITE request rejecting loopback media with 10.1 Offer for specific media loopback type..................13
rtp-start-loopback...........................................13 10.2 Offer for choice of media loopback type.................14
9. Security Considerations......................................14 10.3 Offer for choice of media loopback type with
10. IANA Considerations.........................................14 rtp-start-loopback...........................................15
11. Acknowledgements............................................14 10.4 Response to INVITE request rejecting loopback media.....16
12. References..................................................14 10.5 Response to INVITE request rejecting loopback media with
rtp-start-loopback...........................................16
11. Security Considerations.....................................17
12. IANA Considerations.........................................18
13. Acknowledgements............................................18
14. References..................................................18
1. Introduction 1. Introduction
The overall quality, reliability, and performance of VoIP, The overall quality, reliability, and performance of VoIP,
Real-time Text and Video over IP services rely on the performance Real-time Text and Video over IP services rely on the performance
and quality of the media path. In order to assure the quality of and quality of the media path. In order to assure the quality of
the delivered media there is a need to monitor the performance of the delivered media there is a need to monitor the performance of
the media transport. One method of monitoring and managing the the media transport. One method of monitoring and managing the
overall quality of VoIP, Real-time Text and Video over IP Services overall quality of VoIP, Real-time Text and Video over IP Services
is through monitoring the quality of the media in an active is through monitoring the quality of the media in an active
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media is looped back to the transmitter. The offer/answer model media is looped back to the transmitter. The offer/answer model
[RFC3264] is used to establish a loopback connection. Furthermore, [RFC3264] is used to establish a loopback connection. Furthermore,
this extension provides guidelines on handling RTP [RFC3550], as this extension provides guidelines on handling RTP [RFC3550], as
well as usage of RTCP [RFC3550] and RTCP XR [RFC3611] for reporting well as usage of RTCP [RFC3550] and RTCP XR [RFC3611] for reporting
media related measurements. media related measurements.
2. Terminology 2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED", In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 and "OPTIONAL" are to be interpreted as described in RFC 2119 and
[RFC3264] and indicate requirement levels for compliant indicate requirement levels for compliant implementations.
implementations.
3. Offering Entity Behavior 3. Offering Entity Behavior
An offering entity compliant to this memo and attempting to An offering entity compliant to this memo and attempting to
establish a media session with media loopback MUST include establish a media session with media loopback MUST include
"loopback" media attributes for each individual media description "loopback" media attributes for each individual media description
in the offer message. The offering entity MUST look for the in the offer message. The offering entity MUST look for the
"loopback" media attributes in the media description(s) of the "loopback" media attributes in the media description(s) of the
response from the answering entity for confirmation that the response from the answering entity for confirmation that the
request is accepted. request is accepted.
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which receives an offer with the "loopback" media attributes MAY which receives an offer with the "loopback" media attributes MAY
ignore the attribute and treat the incoming offer as a normal ignore the attribute and treat the incoming offer as a normal
request. request.
5. SDP Constructs Syntax 5. SDP Constructs Syntax
Two new media attributes are defined: one indicates the type of Two new media attributes are defined: one indicates the type of
loopback and one indicates the mode of the loopback. loopback and one indicates the mode of the loopback.
5.1 Loopback Type Attribute 5.1 Loopback Type Attribute
The loopback type is a property media attribute with the following The loopback type is a property media attribute with the following
syntax: syntax:
a=loopback:<loopback-type> a=loopback:<loopback-type>
Following is the Augmented BNF [RFC2234] for loopback-type: Following is the Augmented BNF [RFC2234] for loopback-type:
loopback-type = 1*2(loopback-type-choice) [ space "rtp-start- loopback-type = loopback-type-1 | loopback-type-2
loopback" ] loopback-type-1 = loopback-type-choice-1 [space loopback-type-
loopback-type-choice = "rtp-pkt-loopback" | "rtp-media-loopback” | choice-1]
“rtp-start-loopback" loopback-type-choice-1 = “rtp-pkt-loopback” | “rtp-media-loopback”
loopback-type-2 = loopback-type-choice-2
loopback-type-choice-2 = “rtp-start-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, rtp-media-loopback, and loopback-type values are rtp-pkt-loopback, rtp-media-loopback, and
rtp-start-loopback. rtp-start-loopback.
rtp-pkt-loopback: In this mode, the RTP packets are looped back to rtp-pkt-loopback: In this mode, the RTP packets are looped back to
the sender at a point before the encoder/decoder function in the the sender at a point before the encoder/decoder function in the
receive direction to a point after the encoder/decoder function in receive direction to a point after the encoder/decoder function in
the send direction. This effectively re-encapsulates the RTP the send direction. This effectively re-encapsulates the RTP
payload with the RTP/UDP/IP overheads appropriate for sending it in payload with the RTP/UDP/IP overheads appropriate for sending it in
the reverse direction. Any type of encoding related functions, the reverse direction. Any type of encoding related functions, such
such as packet loss concealment, MUST NOT be part of this type of as packet loss concealment, MUST NOT be part of this type of
loopback path. loopback path. Section 7 describes the encapsulated payload format
that MUST be used for this type of loopback.
rtp-media-loopback: This loopback is activated as close as possible rtp-media-loopback: This loopback is activated as close as possible
to the analog interface and after the decoder so that the RTP to the analog interface and after the decoder so that the RTP
packets are subsequently re-encoded prior to transmission back to packets are subsequently re-encoded prior to transmission back to
the sender. the sender.
rtp-start-loopback: In certain scenarios it is possible that the rtp-start-loopback: In certain scenarios it is possible that the
media transmitted by the offering entity is blocked by a network media transmitted by the loopback-source is blocked by a network
element until the answering entity starts transmitting packets. element until the loopback-mirror starts transmitting packets.
One example of this scenario is the presence of an RTP relay in the Loopback-source and loopback-mirror are loopback modes defined in
path of the media. RTP relays exist in VoIP networks for purpose section 5.2. One example of this scenario is the presence of an RTP
of NAT and Firewall traversal. If an RTP relay is present the relay in the path of the media. RTP relays exist in VoIP networks
offering entity’s packets are dropped by the RTP relay until the for purpose of NAT and Firewall traversal. If an RTP relay is
answering entity has started transmitting media and the media state present, the loopback-source’s packets are dropped by the RTP relay
within the RTP relay is established. This loopback attribute is until the loopback-mirror has started transmitting media and the
used to specify the media type for transmitting media packets by media state within the RTP relay is established. This loopback
the answering entity prior to the loopback process for the purpose attribute is used to specify the media type for transmitting media
of setting media state within the network. In the presence of this packets by the loopback-mirror prior to the loopback process for
loopback attribute the answering entity will transmit media, the purpose of setting media state within the network. In the
according to the description that contains this attribute, until it presence of this loopback attribute the loopback-mirror will
receives media from the offering entity. The answering entity MAY transmit media, according to the description that contains this
include this attribute in the answer if it is not present in the attribute, until it receives media from the loopback-source. The
offer. This may be necessary if the answering entity is aware of loopback-mirror MAY include this attribute in the answer if it is
NAT’s, firewalls, or RTP relays on the path of the call. In this not present in the offer. This may be necessary if the loopback-
case the offering entity MUST accept media according to mirror is aware of NAT’s, firewalls, or RTP relays on the path of
rtp-start-loopback attribute. After the first media packet is the call. In this case the loopback-source MUST accept media
received from the offering entity, the answering entity MUST according to rtp-start-loopback attribute. After the first media
terminate the transmission of rtp-start-loopback media and MUST packet is received from the loopback-source, the loopback-mirror
start looping back media as defined by the other loopback MUST terminate the transmission of rtp-start-loopback media and
MUST start looping back media as defined by the other loopback
attributes present in the offer. If an offer includes the attributes present in the offer. If an offer includes the
rtp-start-loopback attribute it MUST also include at least one rtp-start-loopback attribute it MUST also include at least one
other attribute as defined in this section. The offering entity is other attribute as defined in this section. The loopback-source is
able to filter rtp-start-loopback packets from other types of able to filter rtp-start-loopback packets from other types of
loopback with the payload type of the packet. The media port number loopback with the payload type of the packet. The media port number
for rtp-start-loopback MUST be the same as the corresponding for rtp-start-loopback MUST be the same as the corresponding
loopback attribute that will take over after the reception of first loopback attribute that will take over after the reception of first
media packet from the offering entity. media packet from the offering entity.
It is recommended that an offering entity specifying media with It is recommended that an offering entity specifying media with
either rtp-pkt-loopback or rtp-media-loopback attribute also either rtp-pkt-loopback or rtp-media-loopback attribute also
specify the rtp-start-loopback attribute unless the offering entity specify the rtp-start-loopback attribute unless the offering entity
is certain that its media will not be blocked by a network entity is certain that its media will not be blocked by a network entity
as explained above. as explained above.
5.2 Loopback Mode Attribute 5.2 Loopback Mode Attribute
The loopback mode is a value media attribute that is used to The loopback mode is a value media attribute that is used to
indicate the mode of the loopback. These attributes can be viewed indicate the mode of the loopback. These attributes are additional
as additional mode attributes similar to sendonly, recvonly, etc. mode attributes like sendonly, recvonly, etc. The syntax of the
The syntax of the loopback mode media attribute is: loopback mode media attribute is:
a=<loopback-mode> a=<loopback-mode>:<fmt>...
The loopback-mode values are loopback-source and loopback-mirror. The loopback-mode values are loopback-source and loopback-mirror.
loopback-source: This attribute specifies that the sender is the loopback-source: This attribute specifies that the sender is the
media source and expects the receiver to act as a loopback-mirror. media source and expects the receiver to act as a loopback-mirror.
loopback-mirror: This attribute specifies that the receiver will loopback-mirror: This attribute specifies that the receiver will
mirror (echo) all received media back to the sender of the RTP mirror (echo) all received media back to the sender of the RTP
stream. No media is generated locally by the receiver for stream. No media is generated locally by the receiver for
transmission in the mirrored stream unless rtp-start-loopback is transmission in the mirrored stream unless rtp-start-loopback is
requested requested.
<fmt> is a media format description. The format descrption has the
semantics as defined in section 5.14 of RFC 4566 [RFC2234]. When
loopback-mode is specified as loopback-source, the media format
corresponds to the RTP payload types the source is willing to send.
When loopback-mode is specified as loopback-mirror, the media
format corresponds to the RTP payload types the mirror is willing
to receive. The payload types specified in m= line for a loopback-
source specify the payloads the source is willing to receive.
Similarly, for the loopback-mirror these payload types specify the
payloads it is willing to send.
The loopback mode attribute does not apply to rtp-start-loopback The loopback mode attribute does not apply to rtp-start-loopback
attribute and MUST be ignored if received by the answering entity. attribute and MUST be ignored if received by the answering entity.
5.3 Generating the Offer for Loopback Session 5.3 Generating the Offer for Loopback Session
If an offerer wishes to make a loopback request, it MUST include If an offerer wishes to make a loopback request, it MUST include
both the loopback-type and loopback-mode attribute in a valid SDP both the loopback-type and loopback-mode attribute in a valid SDP
offer: offer:
Example: a=loopback:rtp-media-loopback Example: m=audio 41352 RTP/AVP 0 8
a=loopback-source a=loopback:rtp-media-loopback
a=loopback-source:0 8
Note: A loopback offer in a given media description MUST NOT Note: A loopback offer in a given media description MUST NOT
contain the standard mode attributes sendonly, recvonly, sendrecv contain the standard mode attributes sendonly, recvonly, sendrecv
or inactive. or inactive. The loopback-mode attributes (loopback-source and
loopback-mirror) replace the standard attributes.
The offerer may offer more than one loopback-type in the SDP offer. The offerer may offer more than one loopback-type in the SDP offer.
In this case the answer MUST include only one of the loopback types In this case the answer MUST include only one of the loopback types
that are accepted by the answerer. The answerer SHOULD give that are accepted by the answerer. The answerer SHOULD give
preference to the first loopback-type in the SDP offer. preference to the first loopback-type in the SDP offer.
For loopback-source media (e.g. audio) streams, the port number and For loopback-source media (e.g. audio) streams, the port number and
the address in the offer indicate where the offerer would like to the address in the offer (m= line) indicate where the offerer would
receive the media stream. The payload type numbers indicate the like to receive the media stream. The payload type numbers
value of the payload the offerer expects to receive, and would indicate the value of the payload the offerer expects to receive.
prefer to send. However, the answer might indicate a different The RTP payload types indicated in the a=loopback-source line are
payload type number for the same codec. In that case, the offerer the payload types for the codecs the offerer is willing to send.
MUST send the payload type received in the answer. However, the answer might indicate a different payload type number
for the same codec. In that case, the offerer MUST send the
payload type received in the answer.
If loopback-type is rtp-pkt-loopback, the loopback-mirror MUST send
and the loopback-source MUST receive the looped back packets
encoded in an encapsulated RTP payload as defined in section 7.
Example: m=audio 41352 RTP/AVP 112
a=loopback:rtp-pkt-loopback
a=loopback-source:0 8
a=rtpmap:112 encaprtp/8000
Note: NAT devices may change the actual port number that is used Note: NAT devices may change the actual port number that is used
for transmission and the expected receive port. for transmission and the expected receive port.
5.4 Generating the Answer for Loopback Session 5.4 Generating the Answer for Loopback Session
If an answerer wishes to accept the loopback request it MUST If an answerer wishes to accept the loopback request it MUST
include both the loopback mode and loopback type attribute in the include both the loopback mode and loopback type attribute in the
answer. If a stream is offered with loopback-source or answer. If a stream is offered with loopback-source or
loopback-mirror attributes, the corresponding stream MUST be loopback-mirror attributes, the corresponding stream MUST be
loopback-mirror or loopback-source respectively, provided that loopback-mirror or loopback-source respectively, provided that
answerer is capable of supporting the requested loopback-type. answerer is capable of supporting the requested loopback-type.
For example, if the offer contains: For example, if the offer contains:
m=audio 41352 RTP/AVP 0 8
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-source a=loopback-source:0 8
The answer that is capable of supporting the offer MUST contain: The answer that is capable of supporting the offer MUST contain:
m=audio 41352 RTP/AVP 0 8
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-mirror a=loopback-mirror:0 8
As previously stated if a stream is offered with multiple loopback As previously stated if a stream is offered with multiple loopback
type attributes, the corresponding stream MUST contain only one type attributes, the corresponding stream MUST contain only one
loopback type attribute selected by the answerer. loopback type attribute selected by the answerer.
For example, if the offer contains: For example, if the offer contains:
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 a=loopback-source:0 8
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
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-mirror a=loopback-mirror:0 8
As specified in section 7, if the loopback-type is
rtp-pkt-loopback, the encapsulated RTP payload format MUST be used
for looped back packets.
For example, if the offer contains:
m=audio 41352 RTP/AVP 112
a=loopback:rtp-pkt-loopback
a=loopback-source:0 8
a=rtpmap:112 encaprtp/8000
The answer that is capable of supporting the offer MUST contain:
m=audio 41352 RTP/AVP 112
a=loopback:rtp-pkt-loopback
a=loopback-mirror:0 8
a=rtpmap:112 encaprtp/8000
5.4.1 Rejecting the Loopback Offer 5.4.1 Rejecting the Loopback Offer
An offered stream with loopback-source MAY be rejected if the An offered stream with loopback-source MAY be rejected if the
loopback-type is not specified, the specified loopback-type is not loopback-type is not specified, the specified loopback-type is not
supported, or the endpoint cannot honor the offer for any other supported, or the endpoint cannot honor the offer for any other
reason. The Loopback request may be rejected by setting the media reason. The Loopback request may be rejected by setting the media
port number to zero in the answer as per RFC 3264 [RFC3264]. port number to zero in the answer as per RFC 3264 [RFC3264].
5.5 Offerer Processing of the Answer 5.5 Offerer Processing of the Answer
The answer to a loopback-source MUST be loopback-mirror. The The answer to a loopback-source MUST be loopback-mirror. The
answer to a loopback-mirror MUST be loopback-source. In addition, answer to a loopback-mirror MUST be loopback-source. The loopback-
the "m=" line MUST contain at least one codec that the answerer is mode line MUST contain at least one codec the answerer is willing
willing to both send and receive. 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 answer does not contain a=loopback-mirror or If the answer does not contain a=loopback-mirror or
a=loopback-source or contains any other standard mode attributes, a=loopback-source or contains any other standard mode attributes,
it is assumed that the loopback extensions are not supported by the it is assumed that the loopback extensions are not supported by the
target UA. target UA.
5.6 Modifying the Session 5.6 Modifying the Session
At any point during the loopback session, either participant may At any point during the loopback session, either participant may
issue a new offer to modify the characteristics of the previous issue a new offer to modify the characteristics of the previous
session. In case of SIP this is defined in section 8 of RFC 3264 session. In case of SIP this is defined in section 8 of RFC 3264
[RFC3264]. This also includes transitioning from a normal media [RFC3264]. This also includes transitioning from a normal media
processing mode to loopback mode, and vice a versa. processing mode to loopback mode, and vice a versa.
6. RTP Requirements 6. RTP Requirements
An answering entity that is compliant to this specification and An answering entity that is compliant to this specification and
accepting a media with rtp-pkt-loopback loopback-type MUST loopback accepting a media with rtp-pkt-loopback loopback-type MUST loopback
the incoming RTP packets while re-generating only the SSRC field of the incoming RTP packets using the encapsulated RTP payload format
the RTP header. Note that during the rtp-pkt-loopback mode the as defined in section 7 of this specification.
answering entity does not have control over the encoding of the
media and cannot perform certain functions including congestion
control on the looped back media. However, since the purpose of the
loopback is to characterize the round-trip path at the RTP level,
this limitation is acceptable.
An answering entity that is compliant to this specification and An answering entity that is compliant to this specification and
accepting a media with rtp-media-loopback loopback-type MUST accepting a media with rtp-media-loopback loopback-type MUST
transmit all received media back to the sender. The incoming media transmit all received media back to the sender. The incoming media
MUST be treated as if it were to be played (e.g. the media stream MUST be treated as if it were to be played (e.g. the media stream
MAY receive treatment from PLC algorithms). The answering entity MAY receive treatment from PLC algorithms). The answering entity
MUST re-generate all the RTP header fields as it would when MUST re-generate all the RTP header fields as it would when
transmitting media. The answering entity MAY choose to encode the transmitting media. The answering entity MAY choose to encode the
loopback media according to any of the media descriptions supported loopback media according to any of the media descriptions supported
by the offering entity. Furthermore, in cases where the same media by the offering entity. Furthermore, in cases where the same media
type is looped back, the answering entity MAY choose to preserve type is looped back, the answering entity MAY choose to preserve
number of frames/packet and bitrate of the encoded media according number of frames/packet and bitrate of the encoded media according
to the received media. to the received media.
7. RTCP Requirements 7. Payload format for encapsulated RTP Streams
The payload format described in this section MUST be used by a
loopback-mirror when rtp-pkt-loopback is the specified
loopback-type. A received RTP packet is encapsulated in the payload
section of the RTP packet generated by a loopback-mirror.Each
received packet MUST be encapsulated in a different packet, the
encapsulated packet MAY be fragmented only if required (for
example: due to MTU limitations).
7.1 Usage of RTP Header fields
Payload Type (PT): The assignment of an RTP payload type for this
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 7.3 defines the name binding).
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,
otherwise it is set to 0.
Extension (X) bit: Defined by the RTP Profile used.
Sequence Number: The RTP sequence number SHOULD be generated by the
loopback-mirror in the usual manner with a constant random offset.
Timestamp: The RTP timestamp denotes the sampling instant for when
the loopback-mirror is transmitting this packet to the loopback-
source. The RTP timestamp MUST based on the same clock 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]).
SSRC: set as described in RFC 3550 [RFC3550].
CC and CSRC fields are used as described in RFC 3550 [RFC3550].
7.2 RTP Payload Structure
The RTP header in the encapsulated packet MUST be followed by the
payload header defined in this section. If the received RTP packet
has to be looped back in multiple packets due to fragmentation, the
RTP header in each packet MUST be followed by the payload header
defined in this section. The header is devised so that the
loopback-source can usefully decode looped back packets in the
presence of moderate packet loss [RFC3550].
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 |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The 12 octets after the receive timestamp are identical to the RTP
header in the received packet except for the first 4 bits of the
first octet.
Receive Timestamp: 32 bits
The Receieve timestamp denotes the sampling instant for when the
last octet of the media packet that is being encapsulated by the
loopback-mirror is received from the loopback-source. The Receive
timestamp MUST be based on the same clock 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]).
Fragmentation (F): 2 bits
First Fragment (00) /Last Fragment (01) /No Fragmentation(10)/
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.
Reserved: 2 bits
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.
7.3 Usage of SDP
The payload type number for the encapsulated stream can be
negotiated using a mechanism like SDP. There is no static payload
type assignment for the encapsulate 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.
m=audio 41352 RTP/AVP 112
a=rtpmap:112 encaprtp/8000
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 entity that is
compliant to this specification SHOULD support RTCP per [RFC3550] compliant to this specification SHOULD support RTCP per [RFC3550]
and RTCP-XR per RFC 3611 [RFC3611]. Furthermore, if the client or and RTCP-XR per RFC 3611 [RFC3611]. Furthermore, if the client or
the server choose to support RTCP-XR, they SHOULD support RTCP-XR the server choose to support RTCP-XR, they SHOULD support RTCP-XR
Loss RLE report block, Duplicate RLE report block, Statistics Loss RLE report block, Duplicate RLE report block, Statistics
Summary report block, and VoIP Metric Reports Block per sections Summary report block, and VoIP Metric Reports Block per sections
4.1, 4.2, 4.6, and 4.7 of RFC 3611 [RFC3611]. The client and the 4.1, 4.2, 4.6, and 4.7 of RFC 3611 [RFC3611]. The client and the
server MAY support other RTCP-XR reporting blocks as defined by RFC server MAY support other RTCP-XR reporting blocks as defined by RFC
3611 [RFC3611]. 3611 [RFC3611].
8. Examples 9. Congestion Control
All the participants in a 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.
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.
8.1 Offer for specific media loopback type 10.1 Offer for specific media loopback type
A client sends an INVITE request with SDP which looks like: A client sends an INVITE request with SDP which looks like:
v=0 v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4 o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example s=Example
i=An example session i=An example session
e=user@example.com e=user@example.com
c=IN IP4 192.168.0.12/127 c=IN IP4 192.168.0.12/127
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 a=loopback-source:0
The client is offering to source the media and expects the server The client is offering to source the media and expects the server
to mirror the RTP stream per rtp-media-loopback loopback type. to mirror the RTP stream per rtp-media-loopback loopback type.
A server sends a response with SDP which looks like: A server sends a response with SDP which looks like:
v=0 v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4 o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example s=Example
i=An example session i=An example session
e=user@example.com e=user@example.com
c=IN IP4 192.168.0.12/127 c=IN IP4 192.168.0.12/127
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-mirror a=loopback-mirror:0
The server is accepting to mirror the media from the client at the The server is accepting to mirror the media from the client at the
media level. media level.
8.2 Offer for choice of media loopback type 10.2 Offer for choice of media loopback type
A client sends an INVITE request with SDP which looks like: A client sends an INVITE request with SDP which looks like:
v=0 v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4 o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example s=Example
i=An example session i=An example session
e=user@example.com e=user@example.com
c=IN IP4 192.168.0.12/127 c=IN IP4 192.168.0.12/127
t=0 0 t=0 0
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 0 112
a=loopback:rtp-media-loopback rtp-pkt-loopback a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source a=loopback-source:0
a=rtpmap:112 encaprtp/8000
The client is offering to source the media and expects the server The client is offering to source the media and expects the server
to mirror the RTP stream at either the media or rtp level. to mirror the RTP stream at either the media or rtp level.
A server sends a response with SDP which looks like: A server sends a response with SDP which looks like:
v=0 v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4 o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example s=Example
i=An example session i=An example session
e=user@example.com e=user@example.com
c=IN IP4 192.168.0.12/127 c=IN IP4 192.168.0.12/127
t=0 0 t=0 0
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 112
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-mirror a=loopback-mirror:0
a=rtpmap:112 encaprtp/8000
The server is accepting to mirror the media from the client at the The server is accepting to mirror the media from the client at the
packet level. packet level.
8.3 Offer for choice of media loopback type with rtp-start-loopback 10.3 Offer for choice of media loopback type with rtp-start-loopback
A client sends an INVITE request with SDP which looks like: A client sends an INVITE request with SDP which looks like:
v=0 v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4 o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example s=Example
i=An example session i=An example session
e=user@example.com e=user@example.com
c=IN IP4 192.168.0.12/127 c=IN IP4 192.168.0.12/127
t=0 0 t=0 0
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 0 112
a=loopback:rtp-media-loopback rtp-pkt-loopback a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source a=loopback-source:0
a=rtpmap:112 encaprtp/8000
m=audio 49170 RTP/AVP 100 m=audio 49170 RTP/AVP 100
a=loopback:rtp-start-loopback a=loopback:rtp-start-loopback
The client is offering to source the media and expects the server The client is offering to source the media and expects the server
to mirror the RTP stream at either the media or rtp level. The to mirror the RTP stream at either the media or rtp level. The
client also expects the server to source media until it receives client also expects the server to source media until it receives
packets from the server per media described with the packets from the server per media described with the
rtp-start-loopback attribute. rtp-start-loopback attribute.
A server sends a response with SDP which looks like: A server sends a response with SDP which looks like:
v=0 v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4 o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example s=Example
i=An example session i=An example session
e=user@example.com e=user@example.com
c=IN IP4 192.168.0.12/127 c=IN IP4 192.168.0.12/127
t=0 0 t=0 0
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 112
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-mirror a=loopback-mirror:0
a=rtpmap:112 encaprtp/8000
m=audio 49170 RTP/AVP 100 m=audio 49170 RTP/AVP 100
a=rtpmap:100 pcmu/8000 a=rtpmap:100 pcmu/8000
a=loopback:rtp-start-loopback a=loopback:rtp-start-loopback
The server is accepting to mirror the media from the client at the The server is accepting to mirror the media from the client at the
packet level. The server is also accepting to source media until packet level. The server is also accepting to source media until
it receives media packets from the client. it receives media packets from the client.
8.4 Response to INVITE request rejecting loopback media 10.4 Response to INVITE request rejecting loopback media
A client sends an INVITE request with SDP which looks like: A client sends an INVITE request with SDP which looks like:
v=0 v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4 o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example s=Example
i=An example session i=An example session
e=user@example.com e=user@example.com
c=IN IP4 192.168.0.12/127 c=IN IP4 192.168.0.12/127
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 a=loopback-source:0
The client is offering to source the media and expects the server The client is offering to source the media and expects the server
to mirror the RTP stream at the media level. to mirror the RTP stream at the media level.
A server sends a response with SDP which looks like: A server sends a response with SDP which looks like:
v=0 v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4 o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example s=Example
i=An example session i=An example session
e=user@example.com e=user@example.com
c=IN IP4 192.168.0.12/127 c=IN IP4 192.168.0.12/127
t=0 0 t=0 0
m=audio 0 RTP/AVP 0 m=audio 0 RTP/AVP 0
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-mirror a=loopback-mirror:0
NOTE: Loopback request may be rejected by either not including the NOTE: Loopback request may be rejected by either not including the
loopback mode attribute (for backward compatibility) or setting the loopback mode attribute (for backward compatibility) or setting the
media port number to zero, or both, in the response. media port number to zero, or both, in the response.
8.5 Response to INVITE request rejecting loopback media with 10.5 Response to INVITE request rejecting loopback media with
rtp-start-loopback rtp-start-loopback
A client sends an INVITE request with SDP which looks like: A client sends an INVITE request with SDP which looks like:
v=0 v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4 o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example s=Example
i=An example session i=An example session
e=user@example.com e=user@example.com
c=IN IP4 192.168.0.12/127 c=IN IP4 192.168.0.12/127
t=0 0 t=0 0
m=audio 49170 RTP/AVP 0 m=audio 49170 RTP/AVP 0
skipping to change at page 13, line 22 skipping to change at page 17, line 15
v=0 v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4 o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example s=Example
i=An example session i=An example session
e=user@example.com e=user@example.com
c=IN IP4 192.168.0.12/127 c=IN IP4 192.168.0.12/127
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 a=loopback-source:0
m=audio 49170 RTP/AVP 100 m=audio 49170 RTP/AVP 100
a=loopback:rtp-start-loopback a=loopback:rtp-start-loopback
The client is offering to source the media and expects the server The client is offering to source the media and expects the server
to mirror the RTP stream at the media level. The client also to mirror the RTP stream at the media level. The client also
expects the server to source media until it receives packets from expects the server to source media until it receives packets from
the server per media described with the rtp-start-loopback the server per media described with the rtp-start-loopback
attribute. attribute.
A server sends a response with SDP which looks like: A server sends a response with SDP which looks like:
v=0 v=0
o=user1 2890844526 2890842807 IN IP4 126.16.64.4 o=user1 2890844526 2890842807 IN IP4 126.16.64.4
s=Example s=Example
i=An example session i=An example session
e=user@example.com e=user@example.com
c=IN IP4 192.168.0.12/127 c=IN IP4 192.168.0.12/127
t=0 0 t=0 0
m=audio 0 RTP/AVP 0 m=audio 0 RTP/AVP 0
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-mirror a=loopback-mirror:0
m=audio 0 RTP/AVP 0 m=audio 0 RTP/AVP 0
a=loopback:rtp-start-loopback a=loopback:rtp-start-loopback
NOTE: Loopback request may be rejected by either not including the NOTE: Loopback request may be rejected by either not including the
loopback mode attribute (for backward compatibility) or setting the loopback mode attribute (for backward compatibility) or setting the
media port number to zero, or both, in the response. media port number to zero, or both, in the response.
9. Security Considerations 11. Security Considerations
The security considerations of [RFC3261] apply. Furthermore, given The security considerations of [RFC3261] apply. Furthermore, given
that media loopback may be automated without the end user's that media loopback may be automated without the end user's
knowledge, the server of the media loopback should be aware of knowledge, the server of the media loopback should be aware of
denial of service attacks. It is recommended that sessions with denial of service attacks. It is recommended that sessions with
media loopback are authenticated and the frequency of such sessions media loopback are authenticated and the frequency of such sessions
is limited by the server. is limited by the server.
10. IANA Considerations 12. IANA Considerations
There are no IANA considerations associated with this There are no IANA considerations associated with this
specification. specification.
11. Acknowledgements 13. Acknowledgements
The authors wish to thank Nagarjuna Venna, Flemming Andreasen, Jeff The authors wish to thank Nagarjuna Venna, Flemming Andreasen, Jeff
Bernstein, Paul Kyzivat, and Dave Oran for their comments and Bernstein, Paul Kyzivat, and Dave Oran for their comments and
suggestions. suggestions.
12. References 14. References
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., [RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G.,
Johnston, A., Peterson, J., Sparks, R., Handley, M. Johnston, A., Peterson, J., Sparks, R., Handley, M.
and E. Schooler, "SIP: Session Initiation Protocol", and E. Schooler, "SIP: Session Initiation Protocol",
RFC 3261, STD 1, June 2002. RFC 3261, June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer [RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer
Model with the Session Description Protocol (SDP)", Model with the Session Description Protocol (SDP)",
RFC 3264, STD 1, 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", RFC 3550, STD 1, 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.,
Duffield, N., Friedman, T., Hedayat, K., Sarac, K. Duffield, N., Friedman, T., Hedayat, K., Sarac, K.
and M. Westerlund, "RTP Control Protocol Extended and M. Westerlund, "RTP Control Protocol Extended
Reports (RTCP XR)", RFC 3611, STD 1, November 2003. Reports (RTCP XR)", RFC 3611, November 2003.
[RFC2234] Crocker, P. Overell, "Augmented ABNF for Syntax [RFC2234] Crocker, P. Overell, "Augmented ABNF for Syntax
Specification: ABNF”, RFC 3611, STD 1, November 1997. Specification: ABNF”, RFC 2234, November 1997.
[RFC2119] Bradner, S.,"Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2736] Handley, M., Perkins, C., "Guidelines for Writers of
RTP Payload Format Specifications”, RFC 2736, BCP
0036, December 1999.
[RFC3551] Schulzrinne, H., Casner, S., "RTP Profile for Audio
and Video Conferences with Minimial Control”, STD 65,
RFC 3551, July 2003.
[RFC4566] Handley, M., Jacobson, V., Perkins, C., "SDP: Session
Description Protocol”, RFC 4566, July 2006.
Authors' Addresses Authors' Addresses
Kaynam Hedayat Kaynam Hedayat
Brix Networks Brix Networks
285 Mill Road 285 Mill Road
Chelmsford, MA 01824 Chelmsford, MA 01824
US US
Phone: +1 978 367 5611 Phone: +1 978 367 5611
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