INTERNET-DRAFT 3 November 2002 Internet Engineering Task Force Expires: 3 May 2003 Audio/Video Transport Working Group Timur Friedman, Paris 6 Ramon Caceres, ShieldIP Kevin Almeroth, UCSB Kamil Sarac, UCSB Alan Clark, Telchemy Robert Cole, AT&T Kaynam Hedayat, Brix Networks RTCP Reporting Extensions draft-ietf-avt-rtcp-report-extns-01.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 Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet- Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Copyright Notice Copyright (C) The Internet Society (2002). All Rights Reserved. Abstract This document defines the XR (extended report) RTCP packet type and seven XR block types. The purpose of the extended reporting format is to convey information that supplements the six statistics that are contained in the report blocks used by SR (sender report) and RR Friedman et al. [Page 1] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 (receiver report) packets. Some applications, such as MINC (multicast inference of network characteristics) or VoIP (voice over IP) monitoring, require other and more detailed statistics. In addition to the block types defined here, additional block types may be defined in the future by adhereing to the simple framework that this document provides. 1. Introduction This document defines the XR (extended report) RTCP packet type for RTCP, the control portion of RTP [8]. 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. XR report blocks carry information that is not appropriately carried in SR or RR profile-specific extensions because it is of use across profiles. Information that is useful to network management falls into this category, for instance. Seven report block formats are defined by this document: - Loss RLE Report Block (Section 4.1): Run-length encoding of RTP packet loss reports. - Duplicate RLE Report Block (Section 4.2): Run-length encoding of reports of RTP packet duplicates. - Timestamp Report Block (Section 4.3): A list of timestamps of received RTP packets. - Statistics Summary Report Block (Section 4.4): Statistics on RTP packet sequence numbers, losses, duplicates, jitter, and TTL values. - Receiver Timestamp Report Block (Section 4.5): Receiver-end timestamps that complement the sender-end timestamps already defined for RTCP. - DLRR Report Block (Section 4.6): The delay since the last receiver timestamp report block was received, allowing non-senders to calculate round-trip times. - VoIP Metrics Report Block (Section 4.7): Metrics for monitoring Voice over IP (VoIP) calls. Friedman et al. [Page 2] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 These blocks are defined within a minimal framework: a type field and a length field are common to all XR blocks. The purpose is to maintain flexibility and to keep overhead low. 0ther block formats, beyond the seven defined here, may be defined within this framework 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 [2] and indicate requirement levels for compliance with this specification. 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. 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. In the absence of such definition, the bits in this field MUST be set to zero and MUST be ignored by the receiver. Friedman et al. [Page 3] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 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 (IANA) [7]. 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. Each block MUST be a multiple of 32 bits long. A block MAY be zero bits long. 3. Extended Report Block Framework Extended report blocks are stacked, one after the other, at the end of an XR packet. An individual block's length is a multiple of 4 octets. The XR header's length field describes 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. Friedman et al. [Page 4] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 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 field MUST be a multiple of 32 bits long. It MAY be zero bits long. 4. Specific Extended Report Blocks This section defines seven extended report blocks: block types for losses, duplicates, packet reception timestamps, detailed reception statistics, receiver timestamps, receiver inter-report delays, and VoIP metrics. An implementation MAY ignore incoming blocks with types either not relevant or unknown to it. Additional block types MUST be registered with the Internet Assigned Numbers Authority (IANA) [7], as described in Section 5. 4.1 Loss RLE Report Block This block type permits detailed reporting upon individual packet receipt and loss events. Such reports could be used, for example, for MINC inference [1] of the topology of the multicast tree used for distributing a source's RTP packets, and of the loss rates along links within that tree. Since a Boolean trace of lost and received RTP packets is potentially lengthy, this block type permits the trace to be compressed through run length encoding. 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 RTP packet sequence numbers for the trace are specified in the block, the ending sequence number being the last sequence number in the trace plus one. The last sequence number in the trace MAY differ from the sequence number reported on in any accompanying SR or RR packet. The ending sequence number MAY be less than the beginning sequence number. This happens when the sequence numbers that are being reported upon have wrapped around. However, a Loss RLE Block MUST NOT be used to report upon a range of 65,534 or greater in the sequence number space, as there is no means to identify multiple Friedman et al. [Page 5] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 wrap-arounds. The encoding itself consists of a series of 16 bit chunks that describe packet receipts or losses. Each chunk either specifies a run length or a bit vector, or is a null chunk. A run length describes between 1 and 16,383 events that are all the same (either all receipts or all losses). A bit vector describes 15 events that may be mixed receipts and losses. A null chunk describes no events, and is used to 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 necessarily unique. For example, the fol- lowing trace covers 45 packets, of which the 22nd and 24th have been lost and the others received: 1111 1111 1111 1111 1111 1010 1111 1111 1111 1111 1111 1 One way to encode this would be: bit vector 1111 1111 1111 111 bit vector 1111 1101 0111 111 bit vector 1111 1111 1111 111 null chunk Another way to encode this would be: run of 21 receipts bit vector 0101 1111 1111 111 run of 9 receipts null chunk The choice of encoding is left to the application. As part of this freedom of choice, applications MAY terminate a series of run length and bit vector chunks with a bit vector chunk that runs beyond the sequence number space described by the report block. For example, if the 44th packet in the same sequence were lost: 1111 1111 1111 1111 1111 1010 1111 1111 1111 1111 1110 1 This could be encoded as: Friedman et al. [Page 6] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 run of 21 receipts bit vector 0101 1111 1111 111 bit vector 1111 1110 1000 000 null chunk In this example, the last five bits of the second bit vector describe a part of the sequence number space that extends beyond the last sequence number in the trace. These bits have been set to zero. All bits in a bit vector chunk that describe a part of the sequence number space that extends beyond the last sequence number in the trace MUST be set to zero and MUST be ignored by the receiver. A null packet MUST appear at the end of a Loss RLE Block if the num- ber of run length plus bit vector chunks is odd. The null chunk MUST NOT appear in any other context. Caution should be used in sending Loss RLE Blocks because, even with the compression provided by run-length encoding, they can easily con- sume bandwidth out of proportion with normal RTCP packets. The block type includes a mechanism, called thinning, that allows an applica- tion to limit report sizes. A thinning value, T, selects a subset of packets within the sequence number space: those with sequence numbers that are multiples of 2^T. Packet reception and loss reports apply only to those packets. T can vary between 0 and 15. If T is zero then every packet in the sequence number space is reported upon. If T is fifteen then one in every 32,768 packets is reported upon. Suppose that the trace just described begins at sequence number 13,821. The last sequence number in the trace is 13,865. If the trace were to be thinned with a thinning value of T=2, then the fol- lowing sequence numbers would be reported upon: 13,824, 13,828, 13,832, 13,836, 13,840, 13,844, 13,848, 13,852, 13,856, 13,860, 13,864. The thinned trace would be as follows: 1 1 1 1 1 0 1 1 1 1 0 This could be encoded as follows: Friedman et al. [Page 7] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 bit vector 1111 1011 1100 000 null chunk The last four bits in the bit vector, representing sequence numbers 13,868, 13,872, 13,876, and 13,880, extend beyond the trace and are thus set to zero and are ignored by the receiver. With thinning, the loss of the 22nd packet goes unreported because its sequence number, 13,842, is not a multiple of four. Packet receipts for all sequence numbers that are not multiples of four also go unreported. However, in this example thinning has permitted the Loss RLE Block to be shortened by one 32 bit word. Choice of the thinning value is left to the application. The Loss RLE 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=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. rsvd.: 4 bits This field is reserved for future definition. In the absence of such definition, the bits in this field MUST be set to zero and receivers MUST ignore this field. thinning (T): 4 bits The amount of thinning performed on the sequence number 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 Friedman et al. [Page 8] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 32,768 (amounting to two packets within each 16-bit sequence space). length: 16 bits Defined in Section 3. 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. 4.1.1 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 termi- nating 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. 4.1.2 Bit Vector Chunk Friedman et al. [Page 9] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 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 The vector is read from left to right, in order of increasing sequence number (with the appropriate allowance for wrap around). A zero indicates a packet loss and a one indicates a received packet. 4.1.3 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| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 4.2 Duplicate RLE Report Block This block type permits per-sequence-number reports on duplicates in a source's RTP packet stream. Such information can be used for net- work diagnosis, and provide an alternative to packet losses as a basis for multicast tree topology inference. The Duplicate RLE Block format is identical to the Loss RLE Block format. Only the interpretation is different, in that the informa- tion concerns packet duplicates rather than packet losses. The trace to be encoded in this case also consists of zeros and ones, but a zero here indicates the presence of duplicate packets for a given sequence number, whereas a one indicates that no duplicates were received. The existence of a duplicate for a given sequence number is deter- mined over the entire reporting period. For example, if packet num- ber 12,593 arrives, followed by other packets with other sequence numbers, the arrival later in the reporting period of another packet Friedman et al. [Page 10] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 numbered 12,593 counts as a duplicate for that sequence number. The duplicate does not need to follow immediately upon the first packet of that number. Care must be taken that a report does not cover a range of 65,534 or greater in the sequence number space. No distinction is made between the existance of a single duplicate packet and multiple duplicate packets for a given sequence number. Note also that since there is no duplicate for a lost packet, a loss is encoded as a one in a Duplicate RLE Block. The Duplicate RLE 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=33 | rsvd. | T | 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. rsvd.: 4 bits This field is reserved for future definition. In the absence of such definition, the bits in this field MUST be set to zero and receivers MUST ignore this field. thinning (T): 4 bits The amount of thinning performed on the sequence number space. length: 16 bits Defined in Section 3. begin_seq: 32 bits The first sequence number that this block reports on. Friedman et al. [Page 11] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 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. 4.3 Timestamp Report Block This block type permits per-sequence-number reports on packet receipt timestamps for a given source's RTP packet stream. Such information can be used for MINC inference of the topology of the multicast tree used to distribute the source's RTP packets, and of the delays along the links within that tree. It can also be used to measure partial path characteristics and to model distributions for packet jitter. Timestamps consume more bits than loss or duplicate information, and do not lend themselves to run length encoding. The use of thinning is encouraged to limit the size of Timestamp Report Blocks. The Timestamp 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=48 | rsvd. | T | length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | SSRC of source | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | begin_seq | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | end_seq | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP timestamp 1 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP timestamp 2 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RTP timestamp n | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Friedman et al. [Page 12] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 block type (BT): 8 bits A Timestamp Report Block is identified by the constant 48. rsvd.: 4 bits This field is reserved for future definition. In the absence of such definition, the bits in this field MUST be set to zero and receivers MUST ignore this field. thinning (T): 4 bits The amount of thinning performed on the sequence number space. length: 16 bits Defined in Section 3. 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 i: 32 bits The timestamp reflects the packet arrival time at the receiver. It is preferable for the timestamp to be established at the link layer interface, or in any case as close as possible to the wire arrival time. Units and format are the same as for the timestamp in RTP data packets. As opposed to RTP data packet timestamps, in which nominal values may be used instead of system clock values in order to convey information useful for periodic playout, the receiver timestamps should reflect the actual time as closely as possible. The initial value of the timestamp is random, and subsequent timestamps are off- set from this value. 4.4 Statistics Summary Report Block This block reports statistics beyond the information carried in the standard RTCP packet format, but not as fine grained as that carried in the report blocks previously described. Information is recorded about lost packets, duplicate packets, jitter measurements, and TTL values (TTL values being taken from the TTL field of IPv4 packets, if the data packets are carried over IPv4). Such information can be useful for network management. 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. Friedman et al. [Page 13] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 The Statistics Summary 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=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. 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. In the absence of such definition, all bits in this field MUST be set to zero, and receivers MUST ignore this field. length: 16 bits Defined in Section 3. begin_seq: 32 bits The first sequence number that this block reports on. Friedman et al. [Page 14] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 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. 4.5 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 [9], by sending this report block and receiving DLRR report blocks (see next section) in Friedman et al. [Page 15] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 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. reserved: 24 bits This field is reserved for future definition. In the absence of such definition, the bits in this field MUST be set to zero, and receivers MUST ignore this field. 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. 4.6 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 [9]. It 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. Friedman et al. [Page 16] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 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. reserved: 8 bits This field is reserved for future definition. In the absence of such definition, all bits in this field MUST be set to zero, and receivers MUST ignore this field. length: 16 bits Defined in Section 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 com- pute the round-trip propagation delay to SSRC_r by recording the time A when this Receiver Timestamp report block is received. It calcu- lates the total round-trip time A-LSR using the last SR timestamp (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]. Friedman et al. [Page 17] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 4.7 VoIP Metrics Report Block 4.7.1 Summary The VoIP Metrics report block provides metrics for monitoring voice over IP (VoIP) calls. These metrics include packet loss and discard metrics, delay metrics, analog metrics, and voice quality metrics. The block reports separately on packets lost on the IP channel, and those that have been received but then discarded by the receiving jitter buffer. It also reports on the combined effect of losses and discards, as both have equal effect on call quality. In order to properly assess the quality of a Voice over IP call it is desirable to consider the degree of burstiness of packet loss [4]. Following a Gilbert-Elliott model [5], an interval, bounded by lost and/or discarded packets, with a high rate of losses and/or discards is a "burst," and an interval between two bursts is a "gap." Bursts correspond to intervals of time during which the packet loss rate is high enough to produce noticeable degradation in audio quality. Gaps correspond to periods of time during which only isolated lost packets may occur, and in general these can be masked by packet loss con- cealment. Delay reports include the transit delay between RTCP end points and the VoIP end system processing delays, both of which con- tribute to the user perceived delay. Additional metrics include sig- nal, echo, noise, and distortion levels. Call quality metrics include R factors (E Model) [5] and MOS scores (Mean Opinion Scores). An implementation that sends these blocks SHOULD send at least one every ten seconds for the duration of a call, and SHOULD send one upon call termination. An implementation MUST supply values for all fields defined here. 4.7.2 VoIP Metrics block structure The block is encoded as seven 32-bit words: Friedman et al. [Page 18] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 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=64 | reserved | length=6 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | loss rate | discard rate | burst duration | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | burst density | gap duration | gap density | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | round trip delay | end system delay | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | signal power | doubletalk | noise level | Gmin | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | R factor | ext. R factor | MOS-LQ | MOS-CQ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | RX Config | JB Nominal | JB Maximum | JB Abs Max | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ block type (BT): 8 bits A VoIP Metrics block is identified by the constant 64. reserved: 8 bits This field is reserved for future definition. In the absence of such definition, all bits in this field MUST be set to zero, and receivers MUST ignore this field. length: 16 bits As defined in Section 3, this is the constant 6 for this block type. 4.7.3 Packet loss and discard metrics It is very useful to distinguish between packets lost by the network and those discarded due to jitter. Both have equal effect on the quality of the voice stream however having separate counts helps identify the source of quality degradation. These fields MUST be pop- ulated. loss rate: 8 bits The fraction of RTP data packets from the source lost since the beginning of reception, expressed as a fixed point number with the binary point at the left edge of the field. This value is calculated by dividing the total number of packets lost (after the effects of applying any error protection such as FEC) by the total number of packets expected, multiplying the result of the division by 256, and taking the integer part. The numbers of duplicated packets and dis- carded packets do not enter into this calculation. Since receivers Friedman et al. [Page 19] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 cannot be required to maintain unlimited buffers, a receiver MAY cat- egorize late-arriving packets as lost. The degree of lateness that triggers a loss SHOULD be significantly greater than that which trig- gers a discard. discard rate: 8 bits The fraction of RTP data packets from the source that have been dis carded since the beginning of reception, due to late or early arrival, under-run or overflow at the receiving jitter buffer. This value is expressed as a fixed point number with the binary point at the left edge of the field. It is calculated by dividing the total number of packets discarded (excluding duplicate packet discards) by the total number of packets expected, multiplying the result of the division by 256, and taking the integer part. burst metrics: A burst is defined as a longest sequence of packets bounded by lost or discarded packets with the constraint that within a burst the num- ber of successive packets that were received, and not discarded due to delay variation, is less than some value Gmin. A gap is defined as the interval between bursts, and has the property that any lost or discarded packets must be preceded and followed by at least Gmin packets that were received and not discarded. This gives a maximum loss/discard density within a gap of: 1 / (Gmin + 1). burst duration: 16 bits The mean duration, expressed in milliseconds, of the burst intervals that have occurred since the beginning of reception. The duration of each interval is calculated based upon the packets that mark the beginning and end of that interval. It is equal to the timestamp of the end packet, plus the duration of the end packet, minus the times tamp of the beginning packet. If the actual values are not avail able, estimated values MUST be used. If there have been no burst intervals, the burst duration value MUST be zero. burst density: 8 bits The fraction of RTP data packets within burst intervals since the beginning of reception that were either lost or discarded. This value is expressed as a fixed point number with the binary point at the left edge of the field. It is calculated by dividing the total number of packets lost or discarded (excluding duplicate packet dis- cards) within burst intervals by the total number of packets expected within the burst intervals, multiplying the result of the division by 256, and taking the integer part. gap duration: 16 bits The mean duration, expressed in milliseconds, of the gap intervals that have occurred since the beginning of reception. The duration of Friedman et al. [Page 20] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 each interval is calculated based upon the packet that marks the end of the prior burst and the packet that marks the beginning of the subsequent burst. It is equal to the timestamp of the subsequent burst packet, minus the timestamp of the prior burst packet, plus the duration of the prior burst packet. If the actual values are not available, estimated values MUST be used. In the case of a gap that occurs at the beginning of reception, the sum of the timestamp of the prior burst packet and the duration of the prior burst packet are replaced by the reception start time. In the case of a gap that occurs at the end of reception, the timestamp of the subsequent burst packet is replaced by the reception end time. If there have been no gap intervals, the gap duration value MUST be zero. gap density: 8 bits The fraction of RTP data packets within inter-burst gaps since the beginning of reception that were either lost or discarded. The value is expressed as a fixed point number with the binary point at the left edge of the field. It is calculated by dividing the total num- ber of packets lost or discarded (excluding duplicate packet dis cards) within gap intervals by the total number of packets expected within the gap intervals, multiplying the result of the division by 256, and taking the integer part. For example, if the packet spacing is 10mS and a 1 denotes a received packet and 0, a lost, and X, a discarded, packet then the following pattern: 11110111111111111111111X111X1011110111111111111111111X111111111 |--burst---| would have a burst duration of 120mS, a burst density of 0.33, a gap duration of 510mS and a gap density of 0.04, for a GMIN value of 4 or larger. 4.7.4 Delay metrics For the purpose of the following definitions, the RTP interface is the interface between the RTP instance and the voice application (i.e. FEC/de-interleaving/ de-multiplexing, jitter buffer). For example, the time delay due to RTP payload multiplexing would be con- sidered to be part of the voice application or end-system delay whereas delay due to multiplexing RTP frames within a UDP frame would be considered part of the RTP reported delay. This distinction is consistent with the use of RTCP for delay measurements. round trip delay: 16 bits Friedman et al. [Page 21] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 The most recently calculated round trip time between RTP interfaces, expressed in milliseconds. This value is the time of receipt of the most recent RTCP packet from source SSRC, minus the LSR (last SR) time reported in its SR (sender report), minus the DLSR (delay since last SR) reported in its SR. A non-zero LSR value is REQUIRED in order to calculate round trip delay. A value of 0 is permissible dur- ing the first 2-3 RTCP exchanges as insufficient data may be avail- able to determine delay however MUST be populated as soon as a delay estimate is available. end system delay: 16 bits The most recently estimated end system delay, expressed in millisec- onds. End system delay is defined as the total encoding, decoding and jitter buffer delay determined at the reporting endpoint. This is the time required for an RTP frame to be buffered, decoded, con- verted to analog form, looped back at the local analog interface, encoded, and made available for transmission as an RTP frame. The manner in which this value is estimated is implementation dependent. This parameter MUST be provided in all VoIP metrics reports. Note that the one way symmetric VoIP segment delay may be calculated from the round trip and end system delays as follows. If the round trip delay is denoted RTD and the end system delays associated with the two endpoints are ESD(A) and ESD(B) then: one way symmetric voice path delay = ( RTD + ESD(A) + ESD(B) ) / 2 4.7.5 Signal related metrics The following metrics are intended to provide real time information related to the non-packet elements of the voice over IP system to assist with the identification of problems affecting call quality. The values identified below must be determined for the received audio signal. The information required to populate these fields may not be available in all systems, although it is strongly recommended that this data SHOULD be provided to support problem diagnosis. signal level: 8 bits The voice signal relative level is defined as the ratio of the signal level to overflow signal level, expressed in decibels as a signed integer in two's complement form. This is measured only for packets containing speech energy. The intent of this metric is not to pro- vide a precise measurement of the signal level but to provide a real time indication that the signal level may be excessively high or low. If the full range (overflow level) of the Vocoder's Digital to Analog conversion function is +/- L and the value of a decoded sample during Friedman et al. [Page 22] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 a talkspurt is V then the signal level is given by Signal level = 10 log10 ( mean( abs(V) / L ) ) A value of 127 indicates that this parameter is unavailable. doubletalk level: 8 bits The doubletalk level is defined as the proportion of voice frame intervals during which speech energy was present in both sending and receiving directions. High levels of doubletalk can provide an indi- cation of delay or echo related problems. The value is expressed as a fixed point number with the binary point at the left edge of the field. It is calculated by dividing the total number of voice frame intervals by the number of voice frame intervals during which energy was present in both sending and receiving directions, multiplying the result of the division by 256, and taking the integer part. A value of 255 indicates that this value is unavailable noise level: 8 bits The noise level is defined as the ratio of the silent period back ground noise level to overflow signal power, expressed in decibels as a signed integer in two's complement form. If the full range (over- flow level) of the Vocoder's Digital to Analog conversion function is +/- L and the value of a decoded sample during a silence period is V then the noise level is given by Noise level = 10 log10 ( mean( abs(V) / L ) ) A value of 127 indicates that this parameter is unavailable. 4.7.6 Call quality/ transmission quality metrics The following metrics are direct measures of the transmission quality or call quality, and incorporate the effects of CODEC type, packet loss, discard, burstiness, delay etc. These metrics may not be available in all systems however SHOULD be provided in order to sup- port problem diagnosis. R factor: 8 bits The R factor is a voice quality metric describing the segment of the call that is carried over this RTP session. It is expressed as an integer in the range 0 to 100, with a value of 94 corresponding to "toll quality" and values of 50 or less regarded as unusable. This Friedman et al. [Page 23] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 metric is defined as including the effects of delay, consistent with ITU-T G.107 [6] and ETSI TS 101 329-5 [5]. A value of 127 indicates that this parameter is unavailable. ext. R factor: 8 bits The external R factor is a voice quality metric describing the seg ment of the call that is carried over a network segment external to the RTP segment, for example a cellular network. Its values are interpreted in the same manner as for the RTP R factor. This metric is defined as including the effects of delay, consistent with ITU-T G.107 [6] and ETSI TS 101 329-5 [5], and relates to the outward voice path from the Voice over IP termination for which this metrics block applies. Note that an overall R factor may be estimated from the RTP segment R factor and the external R factor, as follows: R total = RTP R factor + ext. R factor - 94 A value of 127 indicates that this parameter is unavailable. MOS-LQ: 8 bits The estimated mean opinion score for listening quality (MOS-LQ) is a voice quality metric on a scale from 1 to 5, in which 5 represents excellent and 1 represents unacceptable. This metric is defined as not including the effects of delay and can be compared to MOS scores obtained from listening quality (ACR) tests. It is expressed as an integer in the range 10 to 50, corresponding to MOS x 10. For exam- ple, a value of 35 would correspond to an estimated MOS score of 3.5. A value of 127 indicates that this parameter is unavailable. MOS-CQ: 8 bits The estimated mean opinion score for conversational quality (MOS-CQ) is defined as including the effects of delay and other effects that would affect conversational quality. The metric may be calculated by converting an R factor determined according to ITU-T G.107 [6] or ETSI TS 101 329-5 [5] into an estimated MOS using the equation speci- fied in G.107 A value of 127 indicates that this parameter is unavailable. 4.7.7 Configuration parameters: Gmin: 8 bits Friedman et al. [Page 24] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 The gap threshold. This field contains the value used for this report block to determine if a gap exists. The recommended value of 16 (octal 0x10) corresponds to a burst interval having a minimum den- sity of 6.25% of lost or discarded packets, which may cause notice- able degradation in call quality; during gap intervals, if packet loss or dis card occurs, each lost or discarded packet would be pre- ceded by and followed by a sequence of at least 16 received non-dis- carded packets. Note that lost or discarded packets that occur within Gmin packets of a report being generated may be reclassified as being part of a burst or gap in later reports. ETSI TS 101 329-5 [5] defines a computationally efficient algorithm for measuring burst and gap density using a packet loss/discard event driven approach. Gmin MUST not be zero and MUST be provided. Receiver Configuration byte: 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |PLC|JBA|JB rate| +-+-+-+-+-+-+-+-+ PLC - packet loss concealment Standard (11) / enhanced (10) / disabled (01) / unspecified (00). When PLC=11 then a simple replay or interpolation algorithm is being used to fill-in the missing packet - this is typically able to con- ceal isolated lost packets with loss rates under 3%. When PLC=10 then an enhanced interpolation algorithm is being used - this would typically be able to conceal lost packets for loss rates of 10% or more. When PLC=01 then silence is inserted in place of lost packets. When PLC = 00 then no information is available concerning the use of PLC however for some CODECs this may be inferred. JBA - Jitter Buffer Adaptive Adaptive (11) / non-adaptive (10) / reserved (01)/ unknown (00). When Jitter Buffer is adaptive then its size is being dynamically adjusted to deal with varying levels of jitter. When non-adaptive then the Jitter Buffer size is maintained at a fixed level. When either adap- tive or non-adaptive modes are specified then the Jitter Buffer Size parameters below MUST be specified. JB Rate - Jitter Buffer Rate J = adjustment rate (0-15). This represents the implementation spe- cific adjustment rate of a Jitter Buffer in adaptive mode. This parameter is defined in terms of the approximate time taken to fully adjust to a step change in peak to peak jitter from 30mS to 100mS such that: Friedman et al. [Page 25] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 adjustment time = 2* J * frame size (mS) This parameter is intended only to provide a guide to the degree of "aggressiveness" of a an adaptive jitter buffer and may be estimated. A value of 0 indicates that the adjustment time is unknown for this implementation. 4.7.7 Jitter Buffer Parameters Jitter Buffer - nominal size in frames (8 bit) This is the current nominal fill point within the jitter buffer, which corresponds to the nominal jitter buffer delay for packets that arrive exactly on time. This parameter MUST be provided for both fixed and adaptive jitter buffer implementations. Jitter Buffer Maximum - size in frames (8 bit) This is the current maximum jitter buffer level corresponding to the earliest arriving packet that would not be discarded. In simple queue implementations this may correspond to the nominal size. In adaptive jitter buffer implementations this value may dynamically vary up to Jitter Buffer Absolute Maximum. This parameter MUST be provided for both fixed and adaptive jitter buffer implementations. Jitter Buffer Absolute Maximum - size in frames (8 bit) This is the absolute maximum size that the adaptive jitter buffer can reach under worst case jitter conditions. This parameter MUST be provided for adaptive jitter buffer implementations and its value MUST be set to JB Maximum for fixed jitter buffer implementations. Example of burst packet loss calculation. This is an event driven algorithm for measuring burst characteristics and is hence extremely computationally efficient. Given the following definition of states: State 1 = received a packet during a gap State 2 = received a packet during a burst State 3 = lost a packet during a burst State 4 = lost an isolated packet during a gap The "c" variables below correspond to state transition counts, i.e. c14 is the transition from state 1 to state 4. It is possible to infer one of a pair of state transition counts to an accuracy of 1 which is generally sufficient for this application. "pkt" is the Friedman et al. [Page 26] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 count of packets received since the last packet was declared lost or discarded and "lost" is the number of packets lost within the current burst. if ( packet_lost ) loss_count++; if ( packet_discarded ) discard_count++; if (pkt >= gmin) { if (lost == 1) c14++; else c13++; lost = 1; c11 += pkt; } else { lost++; if (pkt == 0) c33++; else { c23++; c22 += (pkt - 1); } } At each reporting interval the burst and gap metrics can be calcu- lated as follows. Friedman et al. [Page 27] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 /* calculate additional transition counts */ c31 = c13; c32 = c23; ctotal = c11 + c14 + c13 + c22 + c23 + c31 + c32 + c33; /* calculate burst and densities */ p32 = c32 / (c31 + c32 + c33); if ((c22 + c23) < 1) p23 = 1; else p23 = 1 - c22/(c22 + c23); burst_density = 256 * p23 / (p23 + p32); gap_density = 256 * c14 / (c11 + c14); /* calculate burst and gap durations in mS */ m = frameDuration_in_mS * framesPerRTPPkt; gap_length = (c11 + c14 + c13) * m / c13; burst_length = ctotal * m / c13 - lgap; /* calculate loss and discard densities */ loss_density = 256 * loss_count / ctotal; discard_density = 256 * discard_count / ctotal; 5. IANA Considerations The extended report block type (BT) field defined by this document is a name space to be managed by the Internet Assigned Numbers Authority (IANA). The field contains eight bits, allowing 256 values, of which seven are defined here: 1 (Statistics Summary Block, see Section 4.4) 2 (Receiver Timestamp Report Block, see Section 4.5) 3 (DLRR Report Block, see Section 4.6) 17 (Loss RLE Block, see Section 4.1) 33 (Duplicate RLE Block, see Section 4.2) 48 (Timestamp Report Block, see Section 4.3) 64 (VoIP Metrics Report Block, see Section 4.7) In addition, the value 0 is reserved for experimental use. No review is necessary by the IANA in order for it to record the assignment of additional numbers from this name space. Such numbers are to be assigned as part of the IETF standards process. Friedman et al. [Page 28] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 6. Security Considerations This document extends the RTCP reporting mechanism, so all security considerations for RTCP reports also apply to the XR packets described here. This section details the additional security consid- erations that apply to the extensions. The extensions introduce heightened confidentiality concerns. Stan- dard RTCP reports contain a limited number of summary statistics. The information contained in XR reports is both more detailed and more extensive (covering a larger number of parameters). The per packet information contained in Loss RLE, Duplicate RLE, and Times- tamp Report Blocks facilitates MINC inference of multicast distribu- tion trees for RTP data packets, and inference of link characteris- tics such as loss and delay. This inference reveals information that might otherwise be considered confidential to autonomous system administrators. The VoIP Metrics Report Block provides information on the quality of ongoing voice calls. Though such information might be carried in application specific format in standard RTP sessions, making it available in a standard format here makes it more available to potential eavesdroppers. No new mechanisms are introduced in this document to ensure confiden- tiality. Already available authentification and encryption proce- dures should be used when confidentiality is a concern to end hosts. Autonomous system administrators concerned about the loss of confi- dentiality regarding their networks can filter traffic to exclude RTCP packets containing the XR report blocks concerned. The extensions also make certain denial of service attacks easier. This is because of the potential to create RTCP packets much larger than average with the per packet reporting capabilities of the Loss RLE, Duplicate RLE, and Timestamp Report Blocks. Because of the automatic bandwidth adjustment mechanisms in RTCP, if some session participants are sending large RTCP packets, all participants will see their RTCP reporting intervals lengthened, meaning they will be able to report less frequently. No new mechanisms are introduced in this document to prevent such denial of service attacks. 7. 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 Friedman et al. [Page 29] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 drawing our attention to the reporting needs of MLDA. 8. 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 [7]. 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. 9. References [1] A. Adams, T. Bu, R. Caceres, 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, "Key words for use in RFCs to indicate requirement levels," BCP 14, RFC 2119, IETF, March 1997. [3] R. Caceres, N.G. Duffield, and T. Friedman, "Impromptu measure- ment infrastructures using RTP," Proc. IEEE Infocom 2002. [4] A. D. Clark, "Modeling the Effects of Burst Packet Loss and Recency on Subjective Voice Quality," Proc. IP Telephony Workshop 2001. [5] ETSI, "Quality of Service (QoS) measurement methodologies," ETSI TS 101 329-5 V1.1.1 (2000-11), November 2000. [6] ITU-T, "The E-Model, a computational model for use in transmis- sion planning," Recommendation G.107 (05/00), May 2000. Friedman et al. [Page 30] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 [7] J. Reynolds and J. Postel, "Assigned Numbers," STD 2, RFC 1700, IETF, October 1994. [8] H. Schulzrinne, S. Casner, R. Frederick, and V. Jacobson, "RTP: A transport protocol for real-time applications," RFC 1889, IETF, February 1996. [9] D. Sisalem and A. Wolisz, "MLDA: A TCP-friendly Congestion Con- trol Framework for Heterogeneous Multicast Environments", Proc. IWQoS 2000. 10. 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 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 University of Paris 6 Laboratoire LiP6-CNRS 8, rue du Capitaine Scott 75015 PARIS, FRANCE Friedman et al. [Page 31] draft-ietf-avt-rtcp-report-extns-01.txt 3 November 2002 Ramon Caceres ShieldIP, Inc. 11 West 42nd Street, 31st Floor New York, NY 10036, USA Kevin Almeroth Department of Computer Science University of California Santa Barbara, CA 93106, USA Kamil Sarac Department of Computer Science University of California Santa Barbara, CA 93106, USA Alan Clark Telchemy Incorporated 3360 Martins Farm Road, Suite 200 Suwanee, GA 30024 Tel: +1 770 614-6944 Fax: +1 770 614-3951 Robert Cole AT&T Labs 330 Saint Johns Street, 2nd Floor Havre de Grace, MD, USA 21078 Tel: +1 410 939-8732 Fax: +1 410 939-8732 Kaynam Hedayat Brix Networks 285 Mill Road Chelmsford, MA 01824 Tel: +1 978 367-5600 Fax: +1 978 367-5700 Friedman et al. [Page 32]