ROHC having three modes which is based on availability of feedback channel, error probabilities and header size variation.
U-Mode- Packets are only sent in one direction: from compressor to decompressor, this is suitable where return path is not available.
O-mode/Bi-directional Optimistic- There is feedback channel is used to send error recovery NACK & ACK.
R-Mode/Bidirectional Reliable- In this, feedback channel is used frequently to avoid packet loss due to context invalidation.The periodic context updating packets reduce compression efficiency compared to O-mode.
The ROHC compressor starts in U- mode & depends on availability of the feedback channel, error on link the modes changes also its a system implementation specific.