INTERNET-DRAFT 28 February 2002
Internet Engineering Task Force Expires: 28 August 2002
Audio/Video Transport Working Group
Timur Friedman, LiP6
Ramon Caceres, ShieldIP
Kevin Almeroth, UCSB
Kamil Sarac, UCSB
RTCP Reporting Extensions
draft-friedman-avt-rtcp-report-extns-02.txt
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document defines the XR (extended report) RTCP packet type, for
carrying information beyond that which is contained in the SR (sender
report) and RR (receiver report) packets that are defined in the RTP
specification. Within their "reception report blocks", SR and RR
packets are limited to reporting six specified statistics on any
given data source. This document describes how other information can
be reported in "extended report blocks" that are contained within an
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XR packet. Some specific block formats are provided here. For other
formats that may be defined as the need arises, this document
specifies a simple framework that they must adhere to.
1. Introduction
This document defines the XR (extended report) RTCP packet type for
RTCP, the control portion of RTP [7]. The definition consists of
three parts. First, Section 2 of this document defines a general
packet framework capable of including a number of different "extended
report blocks." Second, Section 3 defines the general format for
such blocks. Third, Section 4 defines a number of such blocks.
The extended report blocks convey information beyond that which is
already contained in the reception report blocks of RTCP's SR or RR
packets. For example, while a reception report block contains an
average loss rate field, an application might opt to use an extended
report block that details exactly which packets were received and
which were lost.
The framework for these blocks is minimal: only a type field and a
length field are required. The purpose is to maintain flexibility
and to keep overhead low. While some specific block formats are
provided here, others may be defined as the need arises.
1.1 Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [3] and
indicate requirement levels for compliant RTP implementations.
2. XR Packet Format
The XR packet consists of a header of two 32-bit words, followed by a
number, possibly zero, of extended report blocks. This packet format
has been deployed, as described in [4] and [1], as an RTCP APP
(application-specific) packet. The XR packet header is identical to
that of the APP packet, with the name field removed and the subtype
field cleared.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|reserved | PT=XP=205 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC/CSRC |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: report blocks :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
version (V): 2 bits
Identifies the version of RTP. This specification applies to RTP ver¡
sion two (2).
padding (P): 1 bit
If the padding bit is set, this individual RTCP packet contains some
additional padding octets at the end that are not part of the control
information but are included in the length field. The last octet of
the padding is a count of how many padding octets should be ignored,
including itself (it will be a multiple of four). A full description
of padding in RTCP packets may be found in the RTP specification.
reserved: 5 bits
This field is reserved for future definition. The bits in this field
MUST be set to zero unless otherwise defined.
packet type (PT): 8 bits
Contains the constant 205 to identify this as an RTCP XR packet.
This is a proposed value, pending assignment of a number by the
Internet Assigned Numbers Authority [6].
length: 16 bits
The length of this RTCP packet in 32-bit words minus one, including
the header and any padding. (The offset of one makes zero a valid
length and avoids a possible infinite loop in scanning a compound
RTCP packet, while counting 32-bit words avoids a validity check for
a multiple of 4.)
SSRC: 32 bits
The synchronization source identifier for the originator of this XR
packet.
report blocks: variable length.
Zero or more extended report blocks. The blocks MUST be a multiple
of 32 bits long. They MAY be zero bits long.
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3. Extended Report Block Framework
Extended report blocks MUST be stacked, one after the other, at the
end of an XR packet. An individual block's length MUST be a multiple
of 4 octets. The XR header's length field MUST describe the total
length of the packet, including these extended report blocks.
Each block has block type and length fields that facilitate parsing.
A receiving application can demultiplex the blocks based upon their
type, and can use the length information to locate each successive
block, even in the presence of block types it does not recognize.
An extended report block has the following format:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT | type-specific | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: type-specific data :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
block type (BT): 8 bits
Identifies the specific block format.
type-specific: 8 bits
The use of these bits is defined by the particular block type.
length: 16 bits
The length of this report block in 32-bit words minus one, including
the header.
type-specific data: variable length
This MUST be a multiple of 32 bits long. It MAY be zero bits long.
4. Specific Extended Report Blocks
This section defines five extended report blocks: an experimental
block type and block types for losses, duplicates, timestamps, and
detailed statistics. Other block types MAY be defined in the future.
Any such definition MUST include block type numbers assigned by the
Internet Assigned Numbers Authority [6].
4.1 Experimental Block
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This type MUST be used for extended report block types that have not
been standardized. In addition to the standard type and length
fields, it includes a 32 bit name field that serves to distinguish
one experimental block type from another.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=0 | app-specific | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| name (ASCII) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: application-specific data :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
block type (BT): 8 bits
Block type 0 identifies this as an experimental block.
app-specific: 8 bits
The use of these bits is defined by the application that uses this
block.
length: 16 bits
The length of this report block in 32-bit words minus one, including
the header.
name: 4 octets
A name chosen by the person definining the experimental block to be
unique with respect to other experimental blocks the application
might receive.
application-specific data: variable length.
This MUST be a multiple of 32 bits long. It MAY be zero bits long.
4.2 Loss RLE Block
With this block type, a boolean trace of lost and received packets
can be conveyed in compressed form using run length encoding. This
block type has been deployed on the internet, as part of an RTCP APP
(application-specific) packet, as described in [4] and [1].
Caution SHOULD be used in sending such blocks because, even with com¡
pression, they can easily consume bandwidth out of proportion with
normal RTCP packets.
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Each block reports on a single source, identified by its SSRC. The
receiver that is supplying the report is identified in the header of
the RTCP packet.
The beginning and ending sequence numbers for the trace are specified
in the block, the ending sequence number being the last sequence num¡
ber in the trace plus one. The last sequence number in the trace MAY
or may not be the sequence number reported on accompanying SR or RR
packets, depending on the needs of the application.
The encoding itself consists of a series of 16 bit chunks. Each
chunk either specifies a run length or a bit vector, or, if the trace
otherwise encodes into an odd number of chunks, MUST be a terminating
null chunk used to round out the block to a 32 bit word boundary.
The mapping from a sequence of lost and received packets into a
sequence of chunks is not unique and is left to the application. A
run length chunk can describe runs of between 1 and 16,383 packet
losses or receipts whereas a bit vector chunk can describe a sequence
of 15 packet losses and receipts. It is RECOMMENDED that the
description of run lengths of 14 or shorter be subsumed into bit vec¡
tor chunks, for purposes of brevity.
A bit vector chunk MAY purport to contain information on packets at
or beyond the ending sequence number. Any such purported information
MUST be ignored.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=17 | rsvd. | T | block length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| begin_seq | end_seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| chunk 1 | chunk 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| chunk n-1 | chunk n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
block type (BT): 8 bits
A Loss RLE block is identified by the constant 17 = 0x11.
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rsvd.: 4 bits
This field is reserved for future definition. The bits in this field
MUST be set to zero unless otherwise defined.
thinning (T): 4 bits
The amount of thinning performed on the sequence space. Only those
packets with sequence numbers 0 mod 2^T are reported on by this
block. A value of 0 indicates that there is no thinning, and all
packets are reported on. The maximum thinning is one packet in every
32,768 (amounting to two packets within each 16-bit sequence space).
length: 16 bits
The length of this report block in 32-bit words minus one, including
the header.
begin_seq: 16 bits
The first sequence number that this block reports on.
end_seq: 16 bits
The last sequence number that this block reports on plus one.
chunk i: 16 bits
There are three chunk types: run length, bit vector, and terminating
null. If the chunk is all zeroes then it is a terminating null
chunk. Otherwise, the leftmost bit of the chunk determines its type:
0 for run length and 1 for bit vector.
4.2.1 Run-Length Chunk
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|R| run length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
chunk type (C): 1 bit
A zero identifies this as a runlength chunk.
run type (R): 1 bit
Zero indicates a run of losses. One indicates a run of received
packets.
run length: 14 bits
A value between 1 and 16,383. The value MUST not be zero (zeroes in
both the run type and run length fields would make the chunk a
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terminating null chunk). Run lengths of 15 or less MAY be described
with a run length chunk despite the fact that they could also be
described as part of a bit vector chunk.
4.2.2 Bit Vector Chunk
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C| bit vector |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
chunk type (C): 1 bit
A one identifies this as a bit vector chunk.
bit vector: 15 bits
In the bit vector, as in the run length chunk, a zero indicates a
loss and a one indicates a received packet.
4.2.3 Terminating Null Chunk
This chunk is all zeroes.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.3 Duplicate RLE Block
This block is identical in format to the Loss RLE Block type but car¡
ries information about individual or runs of duplicate packets. A
zero indicates the presence of duplicate packets for a given sequence
number, whereas a one indicates that no duplicates were received.
Note that a packet loss is encoded as a one in this case.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=33 | reserved | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| begin_seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| end_seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| chunk 1 | chunk 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| chunk n-1 | chunk n |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
block type (BT): 8 bits
A Duplicate RLE block is identified by the constant 33 = 0x21.
reserved: 8 bits
This field is reserved for future definition All of the bits in this
field MUST be set to zero unless otherwise defined.
length: 16 bits
The length of this report block in 32-bit words minus one, including
the header.
begin_seq: 32 bits
The first sequence number that this block reports on.
end_seq: 32 bits
The last sequence number that this block reports on plus one.
chunk i: 16 bits
There are three chunk types: run length, bit vector, and terminating
null. All zeroes indicates a terminating null. Otherwise, the left¡
most bit of the chunk determines its type: 0 for run length and 1 for
bit vector. See the descriptions of these block types in the section
on the Loss RLE Block, above, for details.
4.4 Timestamp Report Block
This block carries RTCP-style timestamps for each packet in the range
of packet sequence numbers. A similar caution, but more emphatic, is
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made for timestamp report blocks as was made for Loss RLE Block pack¡
ets. For each packet in the sequence number range, a 32 bit value
MUST be recorded and sent. This could easily consume significant
bandwidth. Care SHOULD be taken in the size of the sequence space
over which to monitor timestamps.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=48 | reserved | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| begin_seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| end_seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP timestamp (pkt n) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
block type (BT): 8 bits
A Timestamp block is identified by the constant 48 = 0x30.
reserved: 8 bits
This field is reserved for future definition. All bits in this field
MUST be set to zero unless otherwise defined.
length: 16 bits
The length of this report block in 32-bit words minus one, including
the header.
begin_seq: 32 bits
The first sequence number that this block reports on.
end_seq: 32 bits
The last sequence number that this block reports on plus one.
RTP timestamp: 32 bits
Corresponds to the same units as the RTP timestamp in RTP data pack¡
ets. The timestamp is established upon packet arrival. It can be
used to measure partial path characteristics and to model distribu¡
tions for packet jitter.
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4.4.1 Statistics Summary Block
This block reports detailed statistics above and beyond the informa¡
tion carried in the standard RTCP packet format. Information is
recorded about lost packets, duplicate packets, jitter measurements,
and TTL values. The packet contents are dependent upon a bit vector
carried in the first part of the header. Not all values need to be
carried in each packet. Header fields for values not carried are not
included in the packet.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=1 |L|D|J|T|resvd. | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| begin_seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| end_seq |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| lost_packets |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| dup_packets |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| min_jitter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| max_jitter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| avg_jitter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| dev_jitter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| min_ttl | max_ttl | avg_ttl | dev_ttl |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
block type (BT): 8 bits
A Statistics Summary block is identified by the constant 1 = 0x01.
content bits (L,D,J,T): 4 bits
Bit set to 1 if packet contains (L)oss, (D)uplicate, (J)itter, and/or
(T)TL report.
resvd.: 4 bits
This field is reserved for future definition. All bits in this field
MUST be set to zero unless otherwise defined.
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length: 16 bits
The length of this report block in 32-bit words minus one, including
the header.
begin_seq: 32 bits
The first sequence number that this block reports on.
end_seq: 32 bits
The last sequence number that this block reports on plus one.
lost_packets: 32 bits
Number of lost packets in the above sequence number interval.
dup_packets: 32 bits
Number of duplicate packets in the above sequence number interval.
min_jitter: 32 bits
The minimum relative transit time between two packets in the above
sequence number interval. All jitter values are measured as the dif¡
ference between a packet's RTP timestamp and the reporter's clock at
the time of arrival, measured in the same units.
max_jitter: 32 bits
The maximum relative transit time between two packets in the above
sequence number interval.
avg_jitter: 32 bits
The average relative transit time between each two packet series in
the above sequence number interval.
dev_jitter: 32 bits
The standard deviation of the relative transit time between each two
packet series in the above sequence number interval.
min_ttl: 8 bits
The minimum TTL value of data packets in sequence number range.
max_ttl: 8 bits
The maximum TTL value of data packets in sequence number range.
avg_ttl: 8 bits
The average TTL value of data packets in sequence number range.
dev_ttl: 8 bits
The standard deviation of TTL values of data packets in sequence num¡
ber range.
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4.4.2 Receiver Timestamp Report Block
This block extends RTCP's timestamp reporting so that non-senders may
also send timestamps. It recapitulates the NTP timestamp fields from
the RTCP Sender Report [7, Sec. 6.3.1]. A non-sender may estimate
its RTT to other participants, as proposed in [8], by sending this
report block and receiving DLRR report blocks (see next section) in
reply.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=2 | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NTP timestamp, most significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NTP timestamp, least significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
block type (BT): 8 bits
A Receiver Timestamp block is identified by the constant 2 = 0x02.
reserved: 24 bits
This field is reserved for future definition. The bits in this field
MUST be set to zero unless otherwise defined.
NTP timestamp: 64 bits
Indicates the wallclock time when this block was sent so that it may
be used in combination with timestamps returned in DLRR report blocks
from other receivers to measure round-trip propagation to those
receivers. Receivers should expect that the measurement accuracy of
the timestamp may be limited to far less than the resolution of the
NTP timestamp. The measurement uncertainty of the timestamp is not
indicated as it may not be known. A report block sender that can keep
track of elapsed time but has no notion of wallclock time may use the
elapsed time since joining the session instead. This is assumed to be
less than 68 years, so the high bit will be zero. It is permissible
to use the sampling clock to estimate elapsed wallclock time. A
report sender that has no notion of wallclock or elapsed time may set
the NTP timestamp to zero.
4.4.3 DLRR Report Block
This block extends RTCP's DLSR mechanism [7, Sec. 6.3.1] so that non-
senders may also calculate round trip times, as proposed in [8]. It
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is termed DLRR for Delay since Last Receiver Report, and may be sent
in response to a Receiver Timestamp report block (see previous sec¡
tion) from a receiver to allow that receiver to calculate its round
trip time to the respondant. The report consists of one or more 3
word sub-blocks: one sub-block per receiver report.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=3 | reserved | length |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC_1 (SSRC of first receiver) | sub-
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block
| last RR (LRR) | 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| delay since last RR (DLRR) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| SSRC_2 (SSRC of second receiver) | sub-
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block
: ... : 2
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
block type (BT): 8 bits
A DLRR block is identified by the constant 3 = 0x03.
reserved: 8 bits
This field is reserved for future definition. All bits in this field
MUST be set to zero unless otherwise defined.
length: 16 bits
The length of this report block in 32-bit words minus one, including
the header. The number of sub-blocks is length divided by three (3).
last RR timestamp (LRR): 32 bits
The middle 32 bits out of 64 in the NTP timestamp (as explained in
the previous section) received as part of a Receiver Timestamp report
block from participant SSRC_n. If no such block has been received,
the field is set to zero.
delay since last RR (DLRR): 32 bits
The delay, expressed in units of 1/65536 seconds, between receiving
the last Receiver Timestamp report block from participant SSRC_n and
sending this DLRR report block. If no Receiver Timestamp report
block has been received yet from SSRC_n, the DLRR field is set to
zero (or the DLRR is omitted entirely). Let SSRC_r denote the
receiver issuing this DLRR report block. Participant SSRC_n can
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compute the round-trip propagation delay to SSRC_r by recording the
time A when this Receiver Timestamp report block is received. It
calculates the total round-trip time A-LSR using the last SR times¡
tamp (LSR) field, and then subtracting this field to leave the round-
trip propagation delay as (A- LSR - DLSR). This is illustrated in [7,
Fig. 2].
5. Acknowledgements
We thank the following people: Colin Perkins, Steve Casner, and Hen¡
ning Schulzrinne for their considered guidance; Nick Duffield for
extensive ongoing contributions; Sue Moon for helping foster collabo¡
ration between the authors of this document; and Mounir Benzaid for
drawing our attention to the reporting needs of MLDA.
6. Intellectual Property
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to per¡
tain to the implementation or use of the technology described in this
document or the extent to which any license under such rights might
or might not be available; neither does it represent that it has made
any effort to identify any such rights. Information on the IETF's
procedures with respect to rights in standards-track and standards-
related documentation can be found in BCP 11 [5]. Copies of claims
of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such propri¡
etary rights by implementors or users of this specification can be
obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
7. References
[1] A. Adams, T. Bu, R. Cßceres, N.G. Duffield, T. Friedman, J.
Horowitz, F. Lo Presti, S.B. Moon, V. Paxson, and D. Towsley, "The
Use of End-to-End Multicast Measurements for Characterizing Internal
Network Behavior," IEEE Communications Magazine, May 2000.
[2] S. Bradner, "The Internet Standards Process -- Revision 3," BCP
Friedman, Caceres, Almeroth, and Sarac [Page 15]
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9, RFC 2026, IETF, October 1996.
[3] S. Bradner, "Key words for use in RFCs to indicate requirement
levels," BCP 14, RFC 2119, IETF, March 1997.
[4] R. Caceres, N.G. Duffield, and T. Friedman, "Impromptu measure¡
ment infrastructures using RTP," Proc. IEEE Infocom 2002, New York,
23-27 June 2002, to appear.
[5] R. Hovey and S. Bradner, "The Organizations Involved in the IETF
Standards Process," BCP 11, RFC 2028, IETF, October 1996.
[6] J. Reynolds and J. Postel, "Assigned Numbers," STD 2, RFC 1700,
IETF, October 1994.
[7] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: A
transport protocol for real-time applications," RFC 1889, IETF,
February 1996.
[8] D. Sisalem and A. Wolisz, "MLDA: A TCP-friendly Congestion Con¡
trol Framework for Heterogeneous Multicast Environments", Eighth
International Workshop on Quality of Service (IWQoS 2000), Pitts¡
burgh, 5-7 June 2000.
8. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this doc¡
ument itself may not be modified in any way, such as by removing the
copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of develop¡
ing Internet standards in which case the procedures for copyrights
defined in the Internet Standards process must be followed, or as
required to translate it into languages other than English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
Friedman, Caceres, Almeroth, and Sarac [Page 16]
draft-friedman-avt-rtcp-report-extns-02.txt 28 February 2002
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MER¡
CHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
9. Authors' Addresses
Timur Friedman <timur.friedman@lip6.fr>
Laboratoire d'informatique de Paris 6
8, rue du Capitaine Scott
75015 PARIS, FRANCE
Ramon Caceres <ramon@shieldip.com>
ShieldIP, Inc.
11 West 42nd Street, 31st Floor
New York, NY 10036, USA
Kevin Almeroth <almeroth@cs.ucsb.edu>
Department of Computer Science
University of California
Santa Barbara, CA 93106, USA
Kamil Sarac <ksarac@cs.uscb.edu>
Department of Computer Science
University of California
Santa Barbara, CA 93106, USA
10. Expiry
This draft expires 28 August 2002.
Friedman, Caceres, Almeroth, and Sarac [Page 17]