[Syncookies]: Add support for TCP options via timestamps.
Allow the use of SACK and window scaling when syncookies are used and the client supports tcp timestamps. Options are encoded into the timestamp sent in the syn-ack and restored from the timestamp echo when the ack is received. Based on earlier work by Glenn Griffin. This patch avoids increasing the size of structs by encoding TCP options into the least significant bits of the timestamp and by not using any 'timestamp offset'. The downside is that the timestamp sent in the packet after the synack will increase by several seconds. changes since v1: don't duplicate timestamp echo decoding function, put it into ipv4/syncookie.c and have ipv6/syncookies.c use it. Feedback from Glenn Griffin: fix line indented with spaces, kill redundant if () Reviewed-by: NHagen Paul Pfeifer <hagen@jauu.net> Signed-off-by: NFlorian Westphal <fw@strlen.de> Signed-off-by: NDavid S. Miller <davem@davemloft.net>
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