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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It is ffc to know for certain which of these is the actual cause. The compressor converts the large overhead to only a few bytes, while the decompressor does the opposite. The compressor can use the following logic to parse the feedback field.
ITU-T work programme
For transition from the FO to the SO state, the compressor should be confident that the decompressor has all parameters needed to decompress according to a fixed pattern.
Need for updates, downward transition In addition to the downward state transitions carried out due to periodic timeouts, the compressor must also ketf transit back to the FO state when rcc header to be compressed does not conform to the established pattern. Other for any supplementary information: Header either starts with a packet type indication or has a packet type indication immediately following an Add-CID Octet.
The link layer may, however, be limited in the size of packets it can offer in this efficient mode, or it may be desirable to request only a limited largest size.
RFC – RObust Header Compression (ROHC): Corrections and Clarifications to RFC
This explicit length information allows piggybacking and also sending more than one feedback element in a packet. Optional reference to a channel in the reverse direction.
When CRC checks fail only occasionally, assume residual errors in the current header and simply discard the packet. In summary, ROHC segmentation should be used with a relatively low frequency in the packet flow. When receiving a compressed header the SN SN curr1 is decompressed using ref 0 as the reference. The segmentation scheme was designed to reduce packet size variations that may occur due to outliers in the header size distribution.
The sequence 0395 is reconstructed by replacing the sequence number LSBs in the context with those received in the header. Set of nonnegative integers, each integer indicating a profile supported by the decompressor.
For better performance, the packets are classified into streams before being compressed. Optional octet indicating the size of the feedback data field in octets. Relationship with other existing or emerging documents: Any explicit references within that referenced document should also be listed:. The most important differences are a more intensive usage of the feedback channel, and a stricter logic at both the compressor and the decompressor that prevents loss of context synchronization between compressor and decompressor, except for very high residual bit error rates.
The following steps is an outline of initial decompressor processing which upon reception of a ROHC packet can determine its contents. It is anticipated that feedback to the compressor can be realized in many ways, depending on the properties of the particular lower layer. When a CRC mismatch is caused by residual bit errors in the current header case 1 abovethe decompressor should stay in its current state to avoid unnecessary loss of subsequent packets.
An IR header contains a profile identifier, which determines how the rest of the header is to be interpreted. The purpose of this algorithm is to repair the context.
In O-mode and R-mode, feedback is sent according to sections 5. Unsourced material may be challenged and removed. Every state refers to a defined behaviour and compression level.
Setting the mode parameter in the ACK packet to U indicates that the compressor is to stay in Unidirectional mode. Feedback elements carry internal CID information. A mismatch in the CRC can be caused by one or more of: Data compression Internet Standards. The procedure for finding the size of the feedback data is as follows: It is not intended to replace link layer segmentation functions; these SHOULD be used whenever available and efficient for the task at hand.
Note that N can be negative.
Robust Header Compression
Other useful information describing the “Quality” of the document: Alternatives 1 and 2 apply only to compressed headers in channels where the CID space is small. Contexts are identified by a context identifier, CID, which is sent along with compressed headers and feedback information. Therefore, the compressor MUST periodically transit to lower compression states. Feedback logic O-mode The feedback logic defines what feedback to send due to different events when operating in the various states.
Compressor states and logic U-mode Below is the state machine for the compressor in Unidirectional mode. Repair of incorrect SN updates The CRC can fail to detect residual errors in the compressed header because of its limited length, i.
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. 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.
In Second-Order SO state, the compressor is suppressing all dynamic fields such as RTP sequence numbers, and sending only a logical sequence number and partial checksum to cause the other side to predictively generate and verify the headers of the next expected packet. From Wikipedia, the free encyclopedia.
This, the minimal, packet type is used when parameters of all SN- functions are known by the decompressor, and the header to be compressed adheres to these functions. 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.