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 details of each state, state transitions, and compression logic are given subsequent to the figure. Rf is part of the normal decompression procedure prior to any CRC test failures.
Maximum reconstructed reception unit. Details of the transitions between states and compression logic are given subsequent to the figure. Either of Feedback or Header must be present. Acknowledges successful decompression of a packet, which means that the context is up-to-date with a high probability.
ROHC feedback format Feedback sent on a ROHC channel consists of one or more concatenated feedback elements, where each feedback element has the following format: It has the following general format: In this state, the 8-bit ROHC header contains three fields:.
Compressor states and logic O-mode Below is the state machine for the compressor in Bidirectional Optimistic mode. Clear description of the referenced document: The decompressor might be able to detect this situation and avoid context damage by using a local clock.
This mode therefore makes ROHC usable over links where a return path from decompressor to compressor is unavailable or undesirable. It has the following general format. If this cannot be ensured, segmentation should be performed at lower layers. The decompressor notes the arrival time, a iof each incoming packet i.
ITU-T A.5 reference justification
The profile to be associated with the CID. If a profile is indicated in the context, the logic of that profile determines what, if any, feedback is to be sent. Actions upon CRC failure This section is written so that it is applicable to all modes. The ROHC algorithm is similar to video compression, in that a base frame and then several difference frames are sent to represent an IP packet flow. The feedback data received by the compressor has the following structure feedback sent on a dedicated feedback channel MAY also use this format: References within the referenced RFC are listed under item 8.
Iettf do not place any particular requirements on the ROHC packet type scheme. If the first bit is 0, the CID uses one octet. This is abbreviated in the same way as with IR packets. Unsourced material may be challenged and removed.
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. Contexts and context identifiers Associated with each compressed flow is a context, which is the state compressor and decompressor maintain in order to correctly compress or decompress the headers of the packet stream.
Robust Header Compression
In First-Order FO state, the compressor has detected and rfv the static fields such as IP addresses and port numbers on both sides of the connection. The following rules are to be used: When the decompressor has determined the size of the feedback data field, it removes the feedback type octet and the Size field if present iettf hands the rest to the same-side associated compressor.
This classification takes advantage of inter-packet redundancy. The reconstructed unit has the format: The degree of stability or maturity of the document: This section describes some of this state information in an abstract way.
These fields are transmitted in a compressed form to save more bits. If the first bit s of that octet are not 0 or 10, the packet Rrfc be discarded without further action. In addition to the kind of feedback, other information may be included in profile- specific feedback information. Padding or Add-CID octet However, in Optimistic mode there are no timeouts. Feedback is described in section 5. The CID space is distinct for each channel, i.
The purpose of this algorithm is to repair the context. Timeouts, downward transition When the optimistic approach is taken gfc described above, there will always be a possibility of failure since the decompressor may not have received sufficient information for correct decompression.
Header either starts with a packet type indication or has a packet type indication immediately following an Add-CID Octet.
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. Please help improve this article by adding citations to reliable sources.
If the decompressed header generated in d. In order to handle potential decompression errors, the compressor sends periodic refreshes of the stream context to the decompressor. A ROHC packet has the following general format in the diagram, colons “: Further, when in the Static Context state, only packets carrying a 7- or 8-bit CRC can be decompressed i.
From Wikipedia, the free encyclopedia. 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.
Thus, FO state is essentially static and pseudo-dynamic compression. A large CID is represented using the encoding scheme of section 4.
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.
Reconstruct and verify the header When reconstructing the header, the decompressor takes the header information already stored in the context and updates it with the information received in the current header.