4.5. DLRR Report Block

4.5. DLRR Report Block

This block extends RTCP's delay since the last Sender Report (DLSR) mechanism [9, Sec. 6.3.1] so that non-senders may also calculate round trip times, as proposed in [18]. It is termed DLRR for delay since the last Receiver Report, and may be sent in response to a Receiver Timestamp Report Block (see previous section) from a receiver to allow that receiver to calculate its round trip time to the respondent. 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=5      |   reserved    |         block 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 Report Block is identified by the constant 5.

reserved: 8 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.

block 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 Reference Time Report Block from participant SSRC_n. If no such block has been received from SSRC_n, the LRR 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 Reference Time Report Block and sending this receiver report block. If no Receiver Reference Time Report Block has been received from SSRC_n, the DLRR value is set to zero.

SSRC_i: 32 bits The SSRC of the originator of the Receiver Reference Time Report Block to which this sub-block is responding.