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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The following principal kinds of feedback are supported. When a final segment header is received, the decompressor reassembles the segment carried in this packet and any nonfinal segments that immediately preceded it into a single reconstructed unit, in the order they were received. For transition from the FO to the SO state, the compressor should be confident that the decompressor has ffc parameters needed to decompress according to a fixed pattern.

If the decompressed header generated in d. The profile to be associated with the CID.

Robust Header Compression

RoHC has a direct impact on call set up performance. Maximum reconstructed reception unit. Decompression logic, timer-based timestamp decompression In Bidirectional mode or if there is some other way for the compressor to obtain the decompressor’s clock resolution and the link’s jittertimer-based timestamp decompression iftf be used to improve compression efficiency when RTP Timestamp values are proportional to wall-clock time.

Redundant information is transmitted in the first packets only. Thus the type needs to be reserved at rc framework level. If decompression of any of the three headers fails, the decompressor MUST discard that header and the previously generated headers, and act according to rules a through c of section 5.


Upon reception of an ACK for an updating packet, the compressor knows that the decompressor has received the acknowledged packet and the rff to a higher compression state can be carried out immediately. Feedback elements always start with a packet type indication. Feedback logic O-mode The feedback logic defines what feedback to send due to different events when operating in the various states.

The reconstructed unit has the iietf This is part of the normal decompression procedure prior to any CRC test failures.

The total size of the feedback data field is determinable upon reception by the decompressor, by inspection of the Code field and possibly the Size field. If decompression of the third packet using the new context also succeeds, the context repair is deemed successful and this and subsequent decompressed packets are delivered to the upper layers.

The IP Identifier usually increases by the same delta as the sequence number and the timestamp by the same delta times a fixed value.

This section describes some of this state information in an abstract way. What differs is the decompression and feedback logic. NACKs carry the SN of the latest packet successfully decompressed, and this information MAY be used by the compressor to determine what fields need to be updated.

ITU-T work programme

Decompressor states and logic U-mode Below is the state machine for the decompressor in Unidirectional mode. In addition to the kind of feedback, other information may be included in profile- specific feedback information. Comments on RFCs and corresponding changes are accommodated through the existing standardization process. Indicates that the static context of the decompressor is not valid or has not been established.


It is interpreted according to the profile indicated in the Profile field. The context table is indexed using the CID which is sent along with compressed headers and feedback information.

There are two possible reasons for the CRC failure: It can typically also refresh parts of the context.

Robust Header Compression – Wikipedia

Details of the transitions between states and decompression logic are given subsequent to the figure. Clear description of the referenced document: The segmentation scheme should only be used for packet sizes that are larger than the maximum size in the allowed set of sizes from the lower layers.

The length information decouples the decompressor from the compressor in the sense that the decompressor can process the compressed header immediately without waiting for the compressor to hand it back after parsing the feedback information. On the other hand, when the mismatch is caused by a damaged context case 2the decompressor should attempt to repair the context locally.

The 3-bit CRC present in some header formats will eventually detect context damage reliably, since the probability of undetected context damage decreases exponentially with each new header processed. Framework and four profiles: Using this mechanism, the decompressor may be able to repair the context after excessive loss, at the expense of discarding two packets. How a compressor is associated with feedback on a particular channel needs to be defined in a “ROHC over X” document.

To accommodate the occasional packet that is larger than that largest size negotiated, ROHC defines a simple segmentation protocol.