draft-ietf-mmusic-media-loopback-04.txt   draft-ietf-mmusic-media-loopback-05.txt 
K. Hedayat K. Hedayat
Internet Draft Brix Networks Internet Draft Brix Networks
Expires: February 2007 P. Jones Expires: April 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 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-04 draft-ietf-mmusic-media-loopback-05
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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1. Introduction..................................................3 1. Introduction..................................................3
2. Terminology...................................................4 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.................7 5.3 Generating the Offer for Loopback Session.................7
5.4 Generating the Answer for Loopback Session................8 5.4 Generating the Answer for Loopback Session................8
5.5 Offerer Processing of the Answer..........................9 5.5 Offerer Processing of the Answer..........................9
5.6 Modifying the Session.....................................9 5.6 Modifying the Session....................................10
6. RTP Requirements.............................................10 6. RTP Requirements.............................................10
7. Payload format for encapsulated RTP Streams..................10 7. Payload formats for Packet loopback..........................10
7.1 Usage of RTP Header fields...............................10 7.1 Encapsulated Payload format..............................11
7.2 RTP Payload Structure....................................11 7.2 Direct loopback RTP payload format.......................13
7.3 Usage of SDP.............................................12 8. RTCP Requirements............................................14
8. RTCP Requirements............................................13 9. Congestion Control...........................................15
9. Congestion Control...........................................13 10. Examples....................................................15
10. Examples....................................................13 10.1 Offer for specific media loopback type..................15
10.1 Offer for specific media loopback type..................13 10.2 Offer for choice of media loopback type.................16
10.2 Offer for choice of media loopback type.................14
10.3 Offer for choice of media loopback type with 10.3 Offer for choice of media loopback type with
rtp-start-loopback...........................................15 rtp-start-loopback...........................................17
10.4 Response to INVITE request rejecting loopback media.....16 10.4 Response to INVITE request rejecting loopback media.....18
10.5 Response to INVITE request rejecting loopback media with 10.5 Response to INVITE request rejecting loopback media with
rtp-start-loopback...........................................16 rtp-start-loopback...........................................18
11. Security Considerations.....................................17 11. Security Considerations.....................................19
12. IANA Considerations.........................................18 12. IANA Considerations.........................................20
13. Acknowledgements............................................18 13. Acknowledgements............................................20
14. References..................................................18 14. References..................................................20
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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The extension defined in this memo introduces new SDP media The extension defined in this memo introduces new SDP media
attributes that enable establishment of media sessions where the attributes that enable establishment of media sessions where the
media is looped back to the transmitter. The offer/answer model media is looped back to the transmitter. The offer/answer model
[RFC3264] is used to establish a loopback connection. Furthermore, [RFC3264] is used to establish a loopback connection. Furthermore,
this extension provides guidelines on handling RTP [RFC3550], as this extension provides guidelines on handling RTP [RFC3550], as
well as usage of RTCP [RFC3550] and RTCP XR [RFC3611] for reporting well as usage of RTCP [RFC3550] and RTCP XR [RFC3611] for reporting
media related measurements. media related measurements.
2. Terminology 2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
and "OPTIONAL" are to be interpreted as described in RFC 2119 and this document are to be interpreted as described in RFC 2119.
indicate requirement levels for compliant 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 = loopback-type-1 | loopback-type-2 loopback-type = loopback-type-1 | loopback-type-2
loopback-type-1 = loopback-type-choice-1 [space loopback-type- loopback-type-1 = loopback-type-choice-1 [space
choice-1] loopback-type-choice-1]
loopback-type-choice-1 = “rtp-pkt-loopback” | “rtp-media-loopback” loopback-type-choice-1 = "rtp-pkt-loopback" | "rtp-media-loopback"
loopback-type-2 = loopback-type-choice-2 loopback-type-2 = loopback-type-choice-2
loopback-type-choice-2 = “rtp-start-loopback” 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, such the reverse direction. Any type of encoding related functions,
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. Section 7 describes the encapsulated payload format loopback path. In this mode the RTP packets are looped back with a
that MUST be used for this type of loopback. new payload type and format. Section 7 describes the payload
formats that MUST be used for this type of loopback.
rtp-media-loopback: This loopback is activated as close as possible rtp-media-loopback: This loopback is activated as close as possible
to the analog interface and after the decoder so that the RTP to the analog interface and after the decoder so that the RTP
packets are subsequently re-encoded prior to transmission back to packets are subsequently re-encoded prior to transmission back to
the sender. the sender.
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 loopback-source is blocked by a network media transmitted by the loopback-source is blocked by a network
element until the loopback-mirror starts transmitting packets. element until the loopback-mirror starts transmitting packets.
Loopback-source and loopback-mirror are loopback modes defined in Loopback-source and loopback-mirror are loopback modes defined in
section 5.2. One example of this scenario is the presence of an RTP section 5.2. One example of this scenario is the presence of an
relay in the path of the media. RTP relays exist in VoIP networks RTP relay in the path of the media. RTP relays exist in VoIP
for purpose of NAT and Firewall traversal. If an RTP relay is networks for purpose of NAT and Firewall traversal. If an RTP
present, the loopback-source’s packets are dropped by the RTP relay relay is present, the loopback-source's packets are dropped by the
until the loopback-mirror has started transmitting media and the RTP relay until the loopback-mirror has started transmitting media
media state within the RTP relay is established. This loopback and the media state within the RTP relay is established. This
attribute is used to specify the media type for transmitting media loopback attribute is used to specify the media type for
packets by the loopback-mirror prior to the loopback process for transmitting media packets by the loopback-mirror prior to the
the purpose of setting media state within the network. In the loopback process for the purpose of setting media state within the
presence of this loopback attribute the loopback-mirror will network. In the presence of this loopback attribute the loopback-
transmit media, according to the description that contains this mirror will transmit media, according to the description that
attribute, until it receives media from the loopback-source. The contains this attribute, until it receives media from the loopback-
loopback-mirror MAY include this attribute in the answer if it is source. The loopback-mirror MAY include this attribute in the
not present in the offer. This may be necessary if the loopback- answer if it is not present in the offer. This may be necessary if
mirror is aware of NAT’s, firewalls, or RTP relays on the path of the loopback-mirror is aware of NAT's, firewalls, or RTP relays on
the call. In this case the loopback-source MUST accept media the path of the call. In this case the loopback-source MUST accept
according to rtp-start-loopback attribute. After the first media media according to rtp-start-loopback attribute. After the first
packet is received from the loopback-source, the loopback-mirror media packet is received from the loopback-source, the loopback-
MUST terminate the transmission of rtp-start-loopback media and mirror MUST terminate the transmission of rtp-start-loopback media
MUST start looping back media as defined by the other loopback 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 loopback-source 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
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like to receive the media stream. The payload type numbers like to receive the media stream. The payload type numbers
indicate the value of the payload the offerer expects to receive. indicate the value of the payload the offerer expects to receive.
The RTP payload types indicated in the a=loopback-source line are The RTP payload types indicated in the a=loopback-source line are
the payload types for the codecs the offerer is willing to send. the payload types for the codecs the offerer is willing to send.
However, the answer might indicate a different payload type number However, the answer might indicate a different payload type number
for the same codec. In that case, the offerer MUST send the for the same codec. In that case, the offerer MUST send the
payload type received in the answer. payload type received in the answer.
If loopback-type is rtp-pkt-loopback, the loopback-mirror MUST send If loopback-type is rtp-pkt-loopback, the loopback-mirror MUST send
and the loopback-source MUST receive the looped back packets and the loopback-source MUST receive the looped back packets
encoded in an encapsulated RTP payload as defined in section 7. encoded in one of the two payload formats (encapsulated RTP or
payload loopback) as defined in section 7.
Example: m=audio 41352 RTP/AVP 112 Example: m=audio 41352 RTP/AVP 112
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-source:0 8 a=loopback-source:0 8
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
Example: m=audio 41352 RTP/AVP 112
a=loopback:rtp-pkt-loopback
a=loopback-source:0 8
a=rtpmap:112 rtploopback/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
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a=loopback-source:0 8 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 m=audio 41352 RTP/AVP 0 8
a=loopback:rtp-media-loopback a=loopback:rtp-media-loopback
a=loopback-mirror:0 8 a=loopback-mirror:0 8
As specified in section 7, if the loopback-type is As specified in section 7, if the loopback-type is
rtp-pkt-loopback, the encapsulated RTP payload format MUST be used rtp-pkt-loopback, either the encapsulated RTP payload format or
for looped back packets. direct loopback RTP payload format MUST be used for looped back
packets.
For example, if the offer contains: For example, if the offer contains:
m=audio 41352 RTP/AVP 112 m=audio 41352 RTP/AVP 112 113
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-source:0 8 a=loopback-source:0 8
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
a=rtpmap:113 rtploopback/8000
The answer that is capable of supporting the offer MUST contain: The answer that is capable of supporting the offer MUST contain one
of the following:
m=audio 41352 RTP/AVP 112 m=audio 41352 RTP/AVP 112
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-mirror:0 8 a=loopback-mirror:0 8
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
m=audio 41352 RTP/AVP 113
a=loopback:rtp-pkt-loopback
a=loopback-mirror:0 8
a=rtpmap:113 rtploopback/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
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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 using the encapsulated RTP payload format the incoming RTP packets using either the encapsulated RTP payload
as defined in section 7 of this specification. format or the direct loopback RTP payload format as defined in
section 7 of this specification.
An answering entity that is compliant to this specification and An answering entity that is compliant to this specification and
accepting a media with 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. Payload format for encapsulated RTP Streams 7. Payload formats for Packet loopback
The payload format 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. A received RTP packet is encapsulated in the payload loopback-type. Two different formats are specified here - an
section of the RTP packet generated by a loopback-mirror.Each encapsulated RTP payload format and a direct loopback RTP payload
received packet MUST be encapsulated in a different packet, the format. The encapsulated RTP payload format should be used when
encapsulated packet MAY be fragmented only if required (for the incoming RTP header information needs to be preserved during
example: due to MTU limitations). the loopback operation. This is useful in cases where loopback
source needs to measure performance metrics in both directions.
However, this comes at the expense of increased packet size as
described in section 7.1. The direct loopback RTP payload format
should be used when bandwidth requirement prevent the use of
encapsulated RTP payload format.
7.1 Usage of RTP Header fields 7.1 Encapsulated Payload format
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).
7.1.1 Usage of RTP Header fields
Payload Type (PT): The assignment of an RTP payload type for this Payload Type (PT): The assignment of an RTP payload type for this
packet format is outside the scope of this document; it is either packet format is outside the scope of this document; it is either
specified by the RTP profile under which this payload format is specified by the RTP profile under which this payload format is
used or more likely signaled dynamically out-of-band (e.g., using used or more likely signaled dynamically out-of-band (e.g., using
SDP; section 7.3 defines the name binding). SDP; section 7.3 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 RTP packets, the M bit is set to 1 in the last packet,
otherwise it is set to 0. otherwise it is set to 0.
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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 based on the same clock used by the source. The RTP timestamp MUST 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]).
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 RTP Payload Structure 7.1.2 RTP Payload Structure
The RTP header in the encapsulated packet MUST be followed by the The RTP header in the encapsulated packet MUST be followed by the
payload header defined in this section. If the received RTP packet payload header defined in this section. If the received RTP packet
has to be looped back in multiple packets due to fragmentation, the has to be looped back in multiple packets due to fragmentation, the
RTP header in each packet MUST be followed by the payload header RTP header in each packet MUST be followed by the payload header
defined in this section. The header is devised so that the defined in this section. The header is devised so that the
loopback-source can usefully decode looped back packets in the loopback-source can usefully decode looped back packets in the
presence of moderate packet loss [RFC3550]. presence of moderate packet loss [RFC3550].
0 1 2 3 0 1 2 3
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loopback-mirror is received from the loopback-source. The Receive loopback-mirror is received from the loopback-source. The Receive
timestamp MUST be based on the same clock used by the loopback- timestamp MUST be based on the same clock used by the loopback-
source. The initial value of the timestamp SHOULD be random for source. The initial value of the timestamp SHOULD be random for
security reasons (see Section 5.1 of RFC 3550 [RFC3550]). security reasons (see Section 5.1 of RFC 3550 [RFC3550]).
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 set mirror. If the received packet is not fragmented, this field is
to 10; otherwise the packet that contains the first fragments sets set to 10; otherwise the packet that contains the first fragments
this field to 00, the packet that contains the last fragment sets sets this field to 00, the packet that contains the last fragment
this field to 01, all other packets set this field to 11. sets this field to 01, all other packets set this field to 11.
Reserved: 2 bits Reserved: 2 bits
This field is reserved for future definition. In the absence of This field is reserved for future definition. In the absence of
such a definition, the bits in this field MUST be set to zero and such a definition, the bits in this field MUST be set to zero and
MUST be ignored by the receiver. MUST be ignored by the receiver.
Any padding octets in the original packet MUST not be included in Any padding octets in the original packet MUST not be included in
the loopback packet generated by a loopback-mirror. The loopback- the loopback packet generated by a loopback-mirror. The
mirror MAY add padding octets if required. loopback-mirror MAY add padding octets if required.
7.3 Usage of SDP 7.1.3Usage of SDP
The payload type number for the encapsulated stream can be The payload type number for the encapsulated stream can be
negotiated using a mechanism like SDP. There is no static payload negotiated using a mechanism like SDP. There is no static payload
type assignment for the encapsulate stream, so dynamic payload type type assignment for the encapsulated stream, so dynamic payload
numbers MUST be used. The binding to the name is indicated by an type numbers MUST be used. The binding to the name is indicated by
rtpmap attribute. The name used in this binding is “encaprtp”. an rtpmap attribute. The name used in this binding is "encaprtp".
The following is an example SDP fragment for encapsulated RTP. The following is an example SDP fragment for encapsulated RTP.
m=audio 41352 RTP/AVP 112 m=audio 41352 RTP/AVP 112
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
7.2 Direct loopback RTP payload format
The direct loopback RTP payload format can be used in scenarios
where the 16 byte overhead of the encapsulated payload format is
significant. It MUST not be used in cases where the MTU on the
loopback path is less than the MTU on the transmit path. When
using this payload format, the receiver MUST loop back each
received packet in a separate RTP packet.
7.2.1 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: Set to the value in the received packet.
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].
7.2.2 RTP Payload Structure
This payload format does not define any payload specific headers.
The loopback-mirror simply copies the payload data from the payload
portion of the packet received from the loopback-source.
7.2.3 Usage of SDP
The payload type number for the payload loopback stream can be
negotiated using a mechanism like SDP. There is no static payload
type assignment for the stream, so dynamic payload type numbers
MUST be used. The binding to the name is indicated by an rtpmap
attribute. The name used in this binding is "rtploopback".
The following is an example SDP fragment for encapsulated RTP.
m=audio 41352 RTP/AVP 112
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 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
skipping to change at page 14, line 36 skipping to change at page 16, line 26
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 0 112 m=audio 49170 RTP/AVP 0 112 113
a=loopback:rtp-media-loopback rtp-pkt-loopback a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source:0 a=loopback-source:0
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
a=rtpmap:113 rtploopback/8000
The client is offering to source the media and expects the server The client is offering to source the media and expects the server
to mirror the RTP stream at either the media or rtp level. to mirror the RTP stream at either the media or rtp level.
A server sends a response with SDP which looks like: A server sends a response with 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
skipping to change at page 15, line 10 skipping to change at page 17, line 4
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 112 m=audio 49170 RTP/AVP 112
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-mirror:0 a=loopback-mirror:0
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
The server is accepting to mirror the media from the client at the The server is accepting to mirror the media from the client at the
packet level. packet level using the encapsulated RTP payload format.
10.3 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 0 112 m=audio 49170 RTP/AVP 0 112 113
a=loopback:rtp-media-loopback rtp-pkt-loopback a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source:0 a=loopback-source:0
a=rtpmap:112 encaprtp/8000 a=rtpmap:112 encaprtp/8000
a=rtpmap:113 rtploopback/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 112 m=audio 49170 RTP/AVP 113
a=loopback:rtp-pkt-loopback a=loopback:rtp-pkt-loopback
a=loopback-mirror:0 a=loopback-mirror:0
a=rtpmap:112 encaprtp/8000 a=rtpmap:113 rtploopback/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 using the direct loopback RTP payload format. The
it receives media packets from the client. server is also accepting to source media until it receives media
packets from the client.
10.4 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
skipping to change at page 18, line 40 skipping to change at page 20, line 37
[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.,
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, 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 2234, November 1997. Specification: ABNF", RFC 2234, November 1997.
[RFC2119] Bradner, S.,"Key words for use in RFCs to Indicate [RFC2119] Bradner, S.,"Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2736] Handley, M., Perkins, C., "Guidelines for Writers of [RFC2736] Handley, M., Perkins, C., "Guidelines for Writers of
RTP Payload Format Specifications, RFC 2736, BCP RTP Payload Format Specifications", RFC 2736, BCP
0036, December 1999. 0036, December 1999.
[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.
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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