Connected: An Internet Encyclopedia
3.2.4 Decompressor processing
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3.2.4 Decompressor processing
3.2.4 Decompressor processing
Because of the simplex communication model, processing at the
decompressor is much simpler than at the compressor --- all the
decisions have been made and the decompressor simply does what the
compressor has told it to do.
The decompressor is called with the incoming packet,/22/ the length and
type of the packet and the compression state structure for the incoming
serial line. A (possibly re-constructed) IP packet will be returned.
The decompressor can receive four types of packet: the three generated
by the compressor and a TYPE_ERROR pseudo-packet generated when the
receive framer detects an error./23/ The first step is a `switch' on
the packet type:
- If the packet is TYPE_ERROR or an unrecognized type, a `toss' flag
is set in the state to force COMPRESSED_TCP packets to be discarded
until one with the C bit set or an UNCOMPRESSED_TCP packet arrives.
Nothing (a null packet) is returned.
- If the packet is TYPE_IP, an unmodified copy of it is returned and
the state is not modified.
- If the packet is UNCOMPRESSED_TCP, the state index from the IP
protocol field is checked./24/ If it's illegal, the toss flag is
set and nothing is returned. Otherwise, the toss flag is cleared,
the index is copied to the state's last connection received field, a
copy of the input packet is made,/25/ the TCP protocol number is
restored to the IP protocol field, the packet header is copied to
the indicated state slot, then the packet copy is returned.
If the packet was not handled above, it is COMPRESSED_TCP and a new
TCP/IP header has to be synthesized from information in the packet plus
the last packet's header in the state slot. First, the explicit or
implicit connection number is used to locate the state slot:
- If the C bit is set in the change mask, the state index is checked.
If it's illegal, the toss flag is set and nothing is returned.
Otherwise, last connection received is set to the packet's state
index and the toss flag is cleared.
- If the C bit is clear and the toss flag is set, the packet is
ignored and nothing is returned.
At this point, last connection received is the index of the appropriate
state slot and the first byte(s) of the compressed packet (the change
mask and, possibly, connection index) have been consumed. Since the
TCP/IP header in the state slot must end up reflecting the newly arrived
packet, it's simplest to apply the changes from the packet to that
header then construct the output packet from that header concatenated
with the data from the input packet. (In the following description,
`saved header' is used as an abbreviation for `the TCP/IP header saved
in the state slot'.)
- The next two bytes in the incoming packet are the TCP checksum.
They are copied to the saved header.
- If the P bit is set in the change mask, the TCP PUSH bit is set in
the saved header. Otherwise the PUSH bit is cleared.
- If the low order four bits (S, A, W and U) of the change mask are
all set (the `unidirectional data' special case), the amount of user
data in the last packet is calculated by subtracting the TCP and IP
header lengths from the IP total length in the saved header. That
amount is then added to the TCP sequence number in the saved header.
- If S, W and U are set and A is clear (the `terminal traffic' special
case), the amount of user data in the last packet is calculated and
added to both the TCP sequence number and ack fields in the saved
header.
- Otherwise, the change mask bits are interpreted individually in the
order that the compressor set them:
- If the U bit is set, the TCP URG bit is set in the saved header
and the next byte(s) of the incoming packet are decoded and
stuffed into the TCP Urgent Pointer. If the U bit is clear, the
TCP URG bit is cleared.
- If the W bit is set, the next byte(s) of the incoming packet are
decoded and added to the TCP window field of the saved header.
- If the A bit is set, the next byte(s) of the incoming packet are
decoded and added to the TCP ack field of the saved header.
- If the S bit is set, the next byte(s) of the incoming packet are
decoded and added to the TCP sequence number field of the saved
header.
- If the I bit is set in the change mask, the next byte(s) of the
incoming packet are decoded and added to the IP ID field of the
saved packet. Otherwise, one is added to the IP ID.
At this point, all the header information from the incoming packet has
been consumed and only data remains. The length of the remaining data
is added to the length of the saved IP and TCP headers and the result is
put into the saved IP total length field. The saved IP header is now up
to date so its checksum is recalculated and stored in the IP checksum
field. Finally, an output datagram consisting of the saved header
concatenated with the remaining incoming data is constructed and
returned.
Next: 4 Error handling
Connected: An Internet Encyclopedia
3.2.4 Decompressor processing