Maximum Sequence Number

[Rami88]

TCP determines if a data segment is "old" or "new" by testing whether its sequence number is within 2**31 bytes of the left edge of the window, and if it is not, discarding the data as "old". To insure that new data is never mistakenly considered old and vice-versa, the left edge of the sender's window has to be at most 2**31 away from the right edge of the receiver's window.
Similarly with the sender's right edge and receiver's left edge. Since the right and left edges of either the sender's or receiver's window differ by the window size, and since the sender and receiver windows can be out of phase by at most the window size, the above constraints imply that 2 * the max window size must be less than 2**31, or

max window < 2**30

from RFC 1323.

My question is why 2**31 not 2**32 because the sequence number is 32-bit not 31-bit long?

 

[mmerlinn]

If the sequence number is a signed integer, one bit is required for the sign (+ or -).

mmerlinn

http://mmerlinn.com

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[Rami88]

mmerlinn thank you for your response but this is an RFC and not a program. It is definitely unsigned and starts at 0.

 

[allworxguy]

I'm just guessing... 5 binary bits gives you 0 - 31. 00000 - 11111. 32 would would require a 6th bit 100000.

Marv ccna

http://www.telemarv.ca