TCP sequences bytes. A lot of people assume that TCP sequences packets or
segments, but that's not true.
The sequence number in a TCP header is the sequence number of the first
byte in the payload. It's not a segment number. The ACK is the number of
the next byte of payload expected. It's not a segment number. The sliding
window keeps track of how many bytes have been sent and acknowledged.
The 3-way handshake kind of breaks this rule, which is probably why people
get confused. They never go past the 3-way handshake. With the 3-way
handshake, there are no payload bytes. The recipient's ACK number is
nonetheless one more than the other side's SEQ number.
Follow the sequence number and ACKs in the following trace, after you get
past the 3-way handshake in packets 1-3:
1 MyPC Server TCP HTTP S= 2821020, L= 0, A= 0
2 Server MyPC TCP HTTP S=2134278484, L= 0, A= 2821021
3 MyPC Server TCP HTTP S= 2821021, L= 0, A=2134278485
4 MyPC Server TCP HTTP S= 2821021, L= 384, A=2134278485
5 Server MyPC TCP HTTP S=2134278485, L= 0, A= 2821405
6 Server MyPC TCP HTTP S=2134278485, L= 156, A= 2821405
7 MyPC Server TCP HTTP S= 2821405, L= 0, A=2134278641
8 MyPC Server TCP HTTP S= 2821405, L= 304, A=2134278641
9 Server MyPC TCP HTTP S=2134278641, L= 0, A= 2821709
10 Server MyPC TCP HTTP S=2134278641, L= 156, A= 2821709
11 MyPC Server TCP HTTP S= 2821709, L= 0, A=2134278797
Priscilla
At 02:10 PM 6/2/01, Chuck Larrieu wrote:
>I am reading Doug Comer's excellent book Internetworking with TCP/IP vol 1.
>Highly recommended, and I wish I had followed the good advice of several
>other people on this list, and read the book a year ago.
>
>I have a question on sliding windows as Comer describes it: "The TCP sliding
>window mechanism operates at the octet level, not at the segment or packet
>level" Comer goes on to describe the operation of the mechanism, and
>indicates that acknowledgements occur octet for octet.
>
>This strikes me as highly inefficient, and something that would render TCP
>unusable in networks of any size. Because acknowledgements are based on
>sequence numbers, there would have to be a TCP header for every octet. Not
>good at all.
>
>I am no doubt missing something fundamental here. Perhaps TCP stack
>implementations are written in such a way that the "octets" being sent and
>acknowledge via the sliding window mechanism are really segments / packets?
>I.e. hundreds of octets at a time?
>
>Can someone enlighten me?
>
>Chuck
>
>One IOS to forward them all.
>One IOS to find them.
>One IOS to summarize them all
>And in the routing table bind them.
>
>-JRR Chambers-
________________________
Priscilla Oppenheimer
http://www.priscilla.com
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=6904&t=6899
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