[e2e] Routers accessing TCP header

Jon Crowcroft Jon.Crowcroft at cl.cam.ac.uk
Mon Feb 21 03:53:19 PST 2005

in principle, 
	anything is possible given enough money...

in practice,
depends on the router 
	deep packet inspection takes lotsa cycles or usesup valuble logic
	in any custom processor stuff on line cards, or 
	means your packet goes to the control procxessor ("slow path") but
	also messes up any router that has seperate paths for the headers 
	or any router with a switch that turns packets into cells etc etc etc
	will all make life very hard

	if its a zebra/xorp/gated or any other unix variant, 
	then its probably  about as easy as changing ns...and about as reliable:)

depends on the packet
	if its crypted, you lose

Hack proposal: copy the TCP sequence number into the IP Id field then its in the normal IP header
(yes i know the TCP sequence number is 32 but, but it  usually varies by a MSS size, and you could just put the
deltas in the IP field - surely noone depends on them going ++ (they'd stil be  _monotocally_ increaseing,
just less monotonously:)

and the ip frag stuff would still all work right?

In missive <64196. at>, "Arjuna Sathiaseelan" typed:

 >>Dear all,
 >>  I have implemented a mechanism which requires routers to access the TCP
 >>header from the IP packet mainly the TCP sequence numbers. Implementing
 >>this on ns-2 is not a problem. But in real world, is this a possibility?
 >>Can the routers access the TCP header mainly the TCP sequence numbers? I
 >>would be very much obliged if someone could help me with this.



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