2. XR Packet Format

An XR packet consists of a header of two 32-bit words, followed by a number, possibly zero, of extended report blocks. This type of packet is laid out in a manner consistent with other RTCP packets, as concerns the essential version, packet type, and length information. XR packets are thus backwards compatible with RTCP receiver implementations that do not recognize them, but that ought to be able to parse past them using the length information. A padding field and an SSRC field are also provided in the same locations that they appear in other RTCP packets, for simplicity. The format is as follows:

 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.

2. XR Packet Format​

2. XR Packet Format