RTCP XR

Abstract

RTCP XR

Authors

Walter Fan

Status

WIP as draft

Updated

2024-08-21

Overview

RFC3611 RTCP Extended Reports (RTCP XR)

packet-by-packet block types:

  • Loss RLE Report Block: Run length encoding of reports concerning the losses and receipts of RTP packets.

  • Duplicate RLE Report Block: Run length encoding of reports concerning duplicates of received RTP packets.

  • Packet Receipt Times Report Block: A list of reception timestamps of RTP packets.

reference time related block types:

  • Receiver Reference Time Report Block: Receiver-end wallclock timestamps. Together with the DLRR Report Block mentioned next, these allow non-senders to calculate round-trip times.

  • DLRR Report Block: The delay since the last Receiver Reference Time Report Block was received.

Two summary metric block types: * Statistics Summary Report Block: Statistics on RTP packet sequence numbers, losses, duplicates, jitter, and TTL or Hop Limit values. * VoIP Metrics Report Block: Metrics for monitoring Voice over IP (VoIP) calls

XR Packet 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|reserved |   PT=XR=207   |             length            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                              SSRC                             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
:                         report blocks                         :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
version (V): 2 bits

Identifies the version of RTP. This specification applies to RTP version two.

padding (P): 1 bit

If the padding bit is set, this XR packet contains some additional padding octets at the end. The semantics of this field are identical to the semantics of the padding field in the SR packet, as defined by 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.

packet type (PT): 8 bits

Contains the constant 207 to identify this as an RTCP XR packet. This value is registered with the Internet Assigned Numbers Authority (IANA), as described in Section 6.1.

length: 16 bits

As described for the RTCP Sender Report (SR) packet (see Section 6.4.1 of the RTP specification [9]). Briefly, the length of this XR packet in 32-bit words minus one, including the header and any padding.

SSRC: 32 bits

The synchronization source identifier for the originator of this XR packet.

report blocks: variable length.

Zero or more extended report blocks. In keeping with the extended report block framework defined below, each block MUST consist of one or more 32-bit words.

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 |         block length          |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
:             type-specific block contents                      :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  • block type (BT): 8 bits

Identifies the block format. Seven block types are defined in Section 4. Additional block types may be defined in future specifications. This field’s name space is managed by the Internet Assigned Numbers Authority (IANA), as described in Section 6.2.

  • type-specific: 8 bits

The use of these bits is determined by the block type definition.

  • block length: 16 bits

The length of this report block, including the header, in 32- bit words minus one. If the block type definition permits, zero is an acceptable value, signifying a block that consists of only the BT, type-specific, and block length fields, with a null type-specific block contents field.

  • type-specific block contents: variable length

The use of this field is defined by the particular block type, subject to the constraint that it MUST be a multiple of 32 bits long. If the block type definition permits, It MAY be zero bits long.

“RTCP Extended Report Block”

类型

名称

内容

1

Loss RLE Report Block

Run Length Chunk, Bit Vector Chunk, Terminating Null Chunk

2

Duplicate RLE Report Block

3

Packet Receipt Times Report Block

4

Receiver Reference Time Report Block

5

DLRR Report Block

6

Statistics Summary Report Block

7

VoIP Metrics Report Block

Packet Loss and Discard Metrics, Burst Metrics, Delay Metrics, Signal Related Metrics, Call Quality or Transmission Quality Metrics, Configuration Parameters, Jitter Buffer Parameters