Dropped connections with tcp_tw_recycle=1

Sven Ulland sveniu at opera.com
Tue Sep 22 09:51:59 CEST 2009

Nils Goroll wrote:
>> The outer conditional verifies that the incoming SYN has
>> a timestamp, that tcp_tw_recycle is enabled, and that the origin
>> exists in our peer cache. Note that it only checks the IP of the
>> origin. Doesn't it make sense to also match on port?
> My understanding is that the fact that the connection is in
> TIME_WAIT implies that the source port should not be reused at this
> time.

Right, you're saying that the srcaddr+srcport pair of a connection in
TIME_WAIT should not be reused under this scheme (i.e. the SYN can be
dropped), and I agree. Then I don't understand why a new connection
originating from a *different* source port (although from the same
source IP) is also considered a dupe and dropped. SYN retries don't
change/increase the source port afterall. Is this a mistake in the
TCP code, or maybe in my understanding of the issue?


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