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 other header fields are decompressed using this decompressed SN curr1. Generated when several successive packets have failed to be decompressed correctly. If a profile is indicated in the context, the logic of that profile determines what, if any, feedback is to be sent. Whatever the mode is, both the compressor and the decompressor work in one of their three states. Padding or Add-CID octet Justification for the specific reference:.
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 this, the algorithm of section 5.
Any information which initializes the context of the decompressor should be protected by the CRC. 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.
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. Alternatives do, however.
In general, FO state compresses all static fields and most dynamic fields. This section describes some of this state information in an abstract way. If the first bit is 1, the CID uses ietg octets. In summary, ROHC segmentation should be used with a relatively low frequency in the packet flow.
Thus the type needs to be reserved at the framework level. Decompression logic U-mode Decompression in Unidirectional mode is carried out following three steps which are described in subsequent sections. The Bidirectional Optimistic mode is rvc 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.
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Context information is conceptually kept in a table. Thus, the feedback information must contain CID information if the associated compressor can use more than one context. The ROHC algorithm is similar to video compression, in that a base frame and then several difference frames are sent irtf represent an IP packet flow. Contexts are identified by a context identifier, CID, which is sent along with compressed headers and feedback information.
A mismatch in the CRC can be caused by one or more of: In First-Order FO state, the compressor has detected and stored the static fields such as IP addresses and port numbers on both sides of the connection.
Robust Header Compression
Instead, the Optimistic mode makes use of ieyf from decompressor to compressor for transitions in the backward direction and for OPTIONAL improved forward transition. This part of the IR packet is defined by individual profiles. However, it can redefine what profile is associated with a context, see for example 5. The next packets contain variable information, e. 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 rrfc subsequent packets.
The compressor is also sending dynamic packet field differences in FO state. In the Full Context state: Other for any supplementary information:.
Details of the transitions between states and decompression logic are given subsequent to the figure. The RFC defines a generic compression mechanism.
The compressor normally obtains its confidence about decompressor status by sending several packets with the same information according to the lower compression state.
RFC – part 3 of 7
Current information, if any, about IPR issues: 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. First, attempt to determine whether SN LSB wraparound case 3 is likely, and if so, attempt a correction.
Articles needing additional references from October All articles needing additional references Pages using RFC magic links. The contents of this part of the IR packet are defined by the individual profiles.
Once a stream of packets is classified, it is compressed according to the compression profile that fits best. 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 IR-DYN header typically also initializes or refreshes parts of a context, typically the dynamic part. Any explicit references within that referenced document should also be listed: When the compressor is in the IR state, it will stay there until it assumes that the decompressor has correctly received the static context information.
ITU-T work programme
These fields are transmitted in a compressed form to save more bits. In the Unidirectional mode of operation, packets are only sent in one direction: This can lead to an erroneous iegf SN being used in W-LSB decoding, as the reference SN is updated for each successfully decompressed header of certain types. However, in Optimistic mode there are no timeouts. Set of nonnegative integers, each integer indicating a profile supported by the decompressor. Note that, as a result of this protocol, there is no way in ROHC to make any use of a segment that has residual bit errors.
Unsourced material may be challenged and removed. The following principal kinds of feedback are supported. If set, it indicates that this is the last segment of a reconstructed unit. There are two possible reasons eitf the CRC failure: 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.