The ROHC framework, along with a set of compression profiles, was initially defined in RFC To improve and simplify the ROHC specifications, this. Canonical URL: ; File formats: Plain Text PDF Discuss this RFC: Send questions or comments to [email protected] Robust Header Compression (ROHC) is a standardized method to compress the IP, UDP, The ROHC compression scheme differs from other compression schemes, such as IETF RFC and RFC , by the fact that it performs According to RFC , the ROHC scheme has three modes of operation, as follows.
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If the local repair attempt fails, it must move to a lower state to avoid. This mode therefore makes ROHC usable over links where a return path from decompressor to compressor is unavailable or undesirable. Feedback elements carry internal CID information. A mismatch in the CRC can be caused by one or more ietv The link layer may, however, be limited in the size of packets it ffc offer in this efficient mode, or it may be desirable to request only a limited largest size.
The Bidirectional Optimistic mode is similar to the Unidirectional mode, except that a feedback channel is used to send error recovery requests and optionally acknowledgments of significant context updates from the decompressor to compressor.
Robust Header Compression
The compressor can use the following logic to parse the feedback field. The compressor normally obtains its confidence about decompressor status by sending several packets with the same information according to the lower compression state.
These constraints have led to a design which includes – optional padding, – a feedback packet type, – an optional Add-CID octet which provides 4 bits of CID, and – a simple segmentation and reassembly mechanism. In this state, the 8-bit ROHC header contains three fields:. Per-context parameters, profiles Per-context parameters are established with IR headers see section 5.
Relationship with other existing or emerging documents: When receiving an ACK Uthe compressor should reduce the frequency of IR packets since the static information has been correctly received, but it is not required to stop sending IR packets. This has the advantage of allowing ROHC to survive many packet losses in its highest compression state, as long as the base frames are not lost.
ROHC segmentation should only be used for occasional packets with sizes larger than what is efficient to accommodate, e.
RFC – RObust Header Compression (ROHC): Corrections and Clarifications to RFC
The O-mode aims to maximize tfc efficiency and aims for sparse usage of the feedback channel. Initially, all contexts are in no context state, i. It has the following general format. The profile to be associated with the CID.
The compressor converts the large overhead to only a few bytes, while the decompressor does the opposite. This means that the compressor transits to a higher compression state when it is fairly confident that the decompressor has received enough information to correctly decompress packets sent according to the higher compression state. The exact details of how feedback is realized is to be specified in a “ROHC over X” document, for each lower layer X in question.
Feedback in Unidirectional mode The Unidirectional mode of operation is designed to operate over links where a feedback channel is not available.
SO state is compressing all dynamic fields predictively using a sequence number and checksum. The CID space is distinct for each channel, i. This section describes some of this state information in an abstract way.
RFC – The RObust Header Compression (ROHC) Framework
If the decompressed header generated in b. The number of consecutive packets to send for confidence is not defined in this document.
This classification takes advantage of inter-packet redundancy. The following mechanism allows the decompressor to judge if the context was updated incorrectly by an earlier packet and, if so, to attempt a repair. This explicit length information allows piggybacking and also sending more than one feedback element in a packet.
Other for any supplementary information: Oetf material may be challenged and removed.
ITU-T work programme
The decompressor does not attempt to decompress headers at all in the No Context and Static Context states unless sufficient information is included in the packet itself. If set, it indicates that this is the last segment of a reconstructed unit. They are basically finite state machines.
The decompressor notes the arrival time, a iof each incoming packet i. The following algorithm MAY be used: Justification for the specific reference:. It may be extended by defining new compression profiles dedicated to specific protocol headers. The IR-DYN header typically also initializes or refreshes parts of a context, typically the dynamic part. The degree of stability or maturity of the document: The reconstructed unit has the format: There are two possible reasons for the CRC failure: Data structures The ROHC protocol is based on a number of parameters that form part of the negotiated channel state and the per-context state.