As the RTP control protocol, TCP plays an important role in audio and video quality and session control under weak networks.
1. RTCP Header
V: The version number of RTCP, which must be equal to 2;
P: If set, the padding bit indicates that the packet contains an additional padding octet at the end, which is not part of the control information, but is included in the length field; FMT: Feedback message type,
and The meaning varies depending on the type of PT. For example, when PT=200, it indicates the number of ReportBlock; when PT=205, it indicates the type of feedback packet, 1: NACK lost packet retransmission, 15: TWCC congestion control PT
: load type, please refer to the following table
length for details: The length of this packet (in 32-bit bytes minus 1), including headers and any padding.
2、RTCP Payload
RTCP mainly involves 3 RFC documents
3. RTCP sender report (SR)
Official document: RFC 3550 - RTP: A Transport Protocol for Real-Time Applications
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
header |V=2|P| FMT | PT=SR=200 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
sender | NTP timestamp, most significant word |
info +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NTP timestamp, least significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RTP timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sender packet count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sender octet count |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
report | SSRC_1 (SSRC of first source) |
block +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1 | fraction lost | cumulative number of packets lost |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extended highest sequence number received |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| interarrival jitter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| last SR (LSR) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| delay since last SR (DLSR) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+- FMT: In SR, FMT indicates the number of report blocks that follow, which can be 0 (only streams are sent), or multiple.
- SSRC of sender: Synchronization source identifier, the number of the initiator of the SR packet
- NTP timestamp: 64-bit unsigned integer, indicating the system NTP time when the SR message is sent;
- RTP timestamp: 32-bit unsigned integer, indicating the RTP time at the moment when the SR message is sent. The initial value of the RTP timestamp is random and is a relative time. After adding to the maximum value, it will flip and start from 0;
- packet count: the total number of packets sent by the sender;
- packet octet: the total number of bytes sent by the sender;
- The relevant parameters of the report block are consistent with those of RR, which will be introduced later.
3. RTCP Receiver Report (RR)
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
header |V=2|P| FMT | PT=RR=201 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
report | SSRC_1 (SSRC of first source) |
block +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
1 | fraction lost | cumulative number of packets lost |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| extended highest sequence number received |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| interarrival jitter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| last SR (LSR) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| delay since last SR (DLSR) |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+- FMT: In RR, FMT also indicates the number of report blocks that follow
- SSRC of sender: Synchronization source identifier, the number of the initiator of the RR packet
- SSRC_1: SSRC identifier of the source to which the information in the receive report block belongs
- fraction lost: Packet loss rate, indicating the lost part of the RTP packet from the source SSRC_n after the last SR or RR packet was sent, expressed as a fixed-point number, and the binary point is located on the left edge of the field.
- cumulative number of packets lost: The cumulative number of lost packets, indicating the total number of lost RTP packets from source SSRC_n since the start of reception.
- extended highest sequence number received: The extended highest sequence number received, the lower 16 bits indicate the maximum sequence number of the received RTP packet, and the upper 16 bits indicate the number of times the RTP packet sequence number has been flipped.
- Interarrival jitter: Interarrival jitter, which refers to the estimated statistical variance of the interarrival time of RTP data packets.
- Calculated as follows:
- Define the arrival interval jitter value: J, define the deviation value: D,
- If Ri is the arrival time of packet i in units of RTP timestamp and Si is the RTP timestamp of packet i, then for two packets i and j, D can be expressed as
- D(i,j) = (Rj - Ri) - (Sj - Si) = (Rj - Sj) - (Ri - Si)
- J(i) = J(i-1) + (|D(i-1,i)| - J(i-1))/16
- A gain parameter of 1/16 provides a good noise reduction ratio while maintaining a reasonable convergence rate
- LSR: Only the middle 32 bits of the NTP timestamp in the last received SR packet are intercepted. If no SR has been received, this field is set to 0.
- DLSR: Delay in 1/65536 second since last received SR packet to send RR packet
4.RTCP RTP Feedback
As one of the WebRTC RTCP messages, RTP Feedback contains a lot of content, so it is introduced here separately.
In the RTCP Header , the payload type of this type of message is Payload Type=205, and the feedback message type FMT is as follows:
| FMT | Name | Long Name | Description | Reference |
| 1 | NACK | Generic negative acknowledgement | packet loss retransmission request | RFC4585 |
| 3 | TMMBR | Temporary Maximum Media Stream Bit Rate Request | Temporary maximum stream bitrate request | RFC5104 |
| 4 | TMMBN | Temporary Maximum Media Stream Bit Rate Notification | Temporary maximum stream bitrate notification | RFC5104 |
| 7 | TLLEI | Transport-Layer Third-Party Loss Early Indication | Transport Layer Third Party Loss Early Indication | RFC6642 |
| 8 | ECN | Explicit Congestion Notification | explicit congestion notification | RFC6679 |
| 15 | TWCC | Transport Wide Congestion Control | Transport Wide Congestion Control | draft-holmer-rmcat-transport-wide-cc-extensions-01 |
4.1. Packet loss retransmission request (NACK)
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| FMT=1 | PT=205 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PID | BLP |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- PID: Packet ID, which refers to the sequence number of the lost RTP packet.
- BLP: The bitmask of lost data packets, indicating whether the 16 RTP packets after the PID are lost. If the i-th bit is 1, it means that the RTP packet with the sequence number PID+i is lost, and if no packet is lost, this bit is 0.
4.2 Temporary Maximum Media Stream Bitrate Request (TMMBR)
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| FMT=3 | PT=205 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MxTBR Exp | MxTBR Mantissa |Measured Overhead|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- MxTBR Exp: Index
- MxTBR Mantissa: Coefficients
- Measured Overhead: Measured average packet overhead value in bytes
- Explanation : TMMBR is used for flow control. The receiver requests the sender to transmit the data stream according to the specified maximum bit rate. It is usually used to ensure the fluency of VOIP communication (temporarily reduce the quality) under the condition of network jitter. Maximum bitrate = coefficient * 2^exponent.
4.3 Temporary Maximum Media Stream Bitrate Notification (TMMBN)
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| FMT=4 | PT=205 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MxTBR Exp | MxTBR Mantissa |Measured Overhead|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Description: This feedback message is used to notify the sender of the TMMBR message that one or more TMMBR messages have been received
4.4 Transport Layer Third Party Loss Early Indication (TLLEI)
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| FMT=7 | PT=205 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PID | BLP |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- PID: Packet ID, which refers to the sequence number of the lost RTP packet.
- BPL: The bitmask of lost data packets, indicating whether the 16 RTP packets after the PID are lost. If the i-th bit is 1, it means that the RTP packet with the sequence number PID+i is lost, and if no packet is lost, this bit is 0.
4.5. Explicit Congestion Notification (ECN)
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| FMT=8 | PT=205 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extended Highest Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ECT (0) Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ECT (1) Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ECN-CE Counter | not-ECT Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Lost Packets Counter | Duplication Counter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- Extended Highest Sequence Number: Extended Highest Sequence Number, indicating the maximum RTP sequence number related to this report.
- ECT (0) Counter ：
- ECT (1) Counter：
- ECN-CE Counter：
- not-ECT Counter：
- Lost Packets Counter：
- Duplication Counter：
4.6 Transport Wide Congestion Control (TWCC)
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| FMT=15 | PT=205 | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of packet sender |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SSRC of media source |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| base sequence number | packet status count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| reference time | fb pkt. count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| packet chunk | packet chunk |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| packet chunk | recv delta | recv delta |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| recv delta | recv delta | zero padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- base sequence number: the sequence number of the first RTP packet in this feedback
- packet status count: the total number of RTP packets included in this feedback
- reference time: Indicates the reference time, in units of 64ms
- feedback packet count: Feedback packet count, used by the receiver to detect whether the feedback packet is lost
- packet chunk: data packet block, records the arrival status of RTP packets, and the sequence numbers of these recorded RTP packets are calculated by base sequence number
- recv delta: For the status of each "received data packet", that is, the received RTP packet, add the corresponding arrival time interval information in the recv delta list to record the arrival time information of the RTP packet. Through the previous reference time and recv delta information, we can get the arrival time of the RTP packet.
5WebRTC RTCP XR
RTCP Extended Report (XR) is used to supplement the six statistics in the report block of RTCP's Sender Report (SR) and Receiver Report (RR) packets.
| Value | Name | Name | Description | Reference |
| 1 | RRT | Receiver Reference Time | receiver reference time | rfc3611#section-4.4 |
| 2 | DLSR | Delay since the last Sender Report | Delay since last sender report | rfc3611#section-4.5 |
5.1 Receiver Reference Time (RRT)
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| BT=4 | reserved | block length = 2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NTP timestamp, most significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NTP timestamp, least significant word |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- block type (BT): block type, the report block is a constant 4
- block length: block length, 2*32bit
- NTP timestamp: 64bit NTP system time
5.2 Delay since last sender report (DLSR)
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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): block type, the report block is constant 5
- block length: block length, 3*32bit
- last RR (LRR): The NTP timestamp in the last received RRT packet, only the middle 32bit is intercepted. This field is set to 0 if no RRT has been received.
- delay since last RR (DLRR): The delay between the last RRT packet received and the DLSR sent, in units of 1/65536 seconds
After the receiver sends RTCP-XR RRT, the sender carries RTCP-XR DLSR in the next RTCP, which enables the receiver to estimate the RTT parameters of the network.