2.2.12.1 Picture Loss Indication (PLI)
The Picture Loss Indication (PLI) feedback message is the only payload-specific message this protocol supports. It is identified by PT=PSFB and FMT=1, specified in [RFC4585] section 6.3.1.
A PLI feedback message MAY be used to request sync frames or instantaneous decoder refresh (IDR) on one or more H264 streams in a simulcast stream.
This protocol supports two types of PLI messages: standard PLI and extended PLI. The type of PLI to use is configured by the method specified in [MS-SDPEXT] section 3.1.5.30.
The standard PLI message format is specified in [RFC4585] section 6.3.1.
The extended PLI defined in this protocol differs from the standard PLI that it contains one Feedback Control Information (FCI) field. All other fields in the common packet format are same as defined in [RFC4585] section 6.1.
The FCI field is defined as follows:
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Request Id |
Reserved |
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Request Id (2 bytes): A 16-bit unsigned number that uniquely identifies the PLI. A PLI can be sent multiple times to improve redundancy. All the retransmitted PLIs MUST carry the same Request Id. A different PLI MUST carry a different Request Id.
Reserved (2 bytes): Reserved field. The sender SHOULD set it to 0. The receiver MUST ignore it.
SFR0 (1 byte): Sync frame request byte #0. Each bit in a SFR byte corresponds to one Priority Id which identifies one H264 stream. The bit value of 1 indicates a sync frame is requested on the H264 stream identified by the corresponding Priority Id. The bit value of 0 indicates a sync frame is not requested for the H264 stream identified by the corresponding Priority Id. The more significant bit corresponds to a higher Priority Id value. The most significant bit in SFR0 corresponds to Priority Id 7. The least significant bit in SFR0 corresponds to Priority Id 0.
SFR1 (1 byte): Sync frame request byte #1. This SFR byte defines the sync frame request for Priority ID 8~15.
SFR2 (1 byte): Sync frame request byte #2. This SFR byte defines the sync frame request for Priority ID 16~23.
SFR3 (1 byte): Sync frame request byte #3. This SFR byte defines the sync frame request for Priority ID 24~31.
SFR4 (1 byte): Sync frame request byte #4. This SFR byte defines the sync frame request for Priority ID 32~39.
SFR5 (1 byte): Sync frame request byte #5. This SFR byte defines the sync frame request for Priority ID 40~47.
SFR6 (1 byte): Sync frame request byte #6. This SFR byte defines the sync frame request for Priority ID 48~55.
SFR7 (1 byte): Sync frame request byte #7. This SFR byte defines the sync frame request for Priority ID 56~63.