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Printable Version: RFC3095.PDF
RFC 3095 Robust Header Compression July 2001
Then W-LSB decoding is used to decompress Offset_m, using the
received LSBs in packet m and Offset_ref. Note that m may contain
zero LSBs for Offset_m, in which case Offset_m = Offset_ref.
Finally, the IP-ID for packet m is regenerated as
IP-ID for m = decompressed SN of packet m + Offset_m
Network byte order:
Some IPv4 stacks do use a counter to generate IP ID values as
described, but do not transmit the contents of this counter in
network byte order, but instead send the two octets reversed. In
this case, the compressor can compress the IP-ID field after
swapping the bytes. Consequently, the decompressor also swaps the
bytes of the IP-ID after decompression to regenerate the original
IP-ID. This requires that the compressor and the decompressor
synchronize on the byte order of the IP-ID field using the NBO or
NBO2 flag (see section 5.7).
Random IP Identifier:
Some IPv4 stacks generate the IP Identifier values using a
pseudo-random number generator. While this may provide some
security benefits, it makes it pointless to attempt compressing
the field. Therefore, the compressor should detect such random
behavior of the field. After detection and synchronization with
the decompressor using the RND or RND2 flag, the field is sent
as-is in its entirety as additional octets after the compressed
header.
4.5.6. Self-describing variable-length values
The values of TS_STRIDE and a few other compression parameters can
vary widely. TS_STRIDE can be 160 for voice and 90 000 for 1 f/s
video. To optimize the transfer of such values, a variable number of
octets is used to encode them. The number of octets used is
determined by the first few bits of the first octet:
First bit is 0: 1 octet.
7 bits transferred.
Up to 127 decimal.
Encoded octets in hexadecimal: 00 to 7F
First bits are 10: 2 octets.
14 bits transferred.
Up to 16 383 decimal.
Encoded octets in hexadecimal: 80 00 to BF FF
Bormann, et al. Standards Track [Page 35]