Lectures notes for 2010 - KTH

State Cookie
Use of the COOKIE prevents a SYN flood like attack - since resources are not
allocated until the COOKIE ECHO chuck is received.
However, state has to be saved from the initial INIT chunk - therefore it is placed
in the cookie in a way that only the server can access it (hence the cookie is sealed
with an HMAC {aka digest} after being created {aka “baked”}). This requires that
the server has a secret key which it uses to compute this digest.
If the sender of the INIT is an attacker located on another machine, they won’t be
able to receive the cookie if they faked the source address in the INIT - since the
INIT ACK is sent to the address and contains the cookie!
• Without a cookie ⇒ no association is created and no resources (such as TCB) are tied up!
Maguire INIT ACK Chunk 6: 11 of 44
maguire@kth.se 2010.03.21 Internetworking/Internetteknik