[e2e] TCP Option Negotiation

John Wroclawski jtw at lcs.mit.edu
Thu May 17 09:25:31 PDT 2001

At 11:57 AM -0400 5/17/01, Hari Balakrishnan wrote:
>>  Alex,
>>  This seems to be another manifestation of the standard  problem of old
>>  duplicate packet.   Your scenario is a violation of TCP's "quiet time"
>>  requirement upon host crash and restart (it's the same host if it has
>>  the same IP address).  Quiet time is a vital part of TCP's machinery to
>>  protect against old duplicates.
>Not quite.
>Unfortunately the statement: "it's the same host if it has the same 
>IP address"
>is increasingly untrue because of dynamic IP address assignment (e.g., via
>DHCP).  This may well be a theoretical problem, but I've observed (in my home,
>from my FreeBSD DHCP server), turning off a laptop and turning another one on,
>and having the latter receive the former's IP!


It might be arguable that RFC793 actually covers this. Crash/restart 
is used as the motivating example, but the words are more general - 
"in the absence of knowledge about the sequence numbers used on a 
particular connection, the TCP specification recommends that the 
source delay for MSL seconds before emitting segments on that 
connection, to allow time for segments from an earlier connection 
incarnation to drain from the system".

An implementation that truly followed this recommendation would 
enforce a quiet time after any assignment of a dynamic address on the 
grounds that it had no idea where that address had been before, and 
thus no knowledge about previous SN's..


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