See section 3.2.1.8c of RFC 1122:
If host receives a datagram containing a completed
source route (i.e., the pointer points beyond the last
field), the datagram has reached its final destination;
the option as received (the recorded route) MUST be
passed up to the transport layer (or to ICMP message
processing). This recorded route will be reversed and
used to form a return source route for reply datagrams
(see discussion of IP Options in Section 4). When a
return source route is built, it MUST be correctly
formed even if the recorded route included the source
host (see case (B) in the discussion below).
--Steve Bellovin, http://www.research.att.com/~smb (me)
http://www.wilyhacker.com ("Firewalls" book)