3.2.1.8 Options: RFC-791 Section 3.2 (original) (raw)

Source Route Options

A host MUST support originating a source route and MUST be able to act as the final destination of a source route.

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).

An IP header containing more than one Source Route option MUST NOT be sent; the effect on routing of multiple Source Route options is implementation- specific.

Section 3.3.5 presents the rules for a host acting as an intermediate hop in a source route, i.e., forwarding a source-routed datagram.

DISCUSSION:

If a source-routed datagram is fragmented, each fragment will contain a copy of the source route. Since the processing of IP options (including a source route) must precede reassembly, the original datagram will not be reassembled until the final destination is reached.

Suppose a source routed datagram is to be routed from host S to host D via gateways G1, G2, ... Gn. There was an ambiguity in the specification over whether the source route option in a datagram sent out by S should be (A) or (B):

(A):  {>>G2, G3, ... Gn, D}     <--- CORRECT

(B):  {S, >>G2, G3, ... Gn, D}  <---- WRONG

(where >> represents the pointer). If (A) is sent, the datagram received at D will contain the option: {G1, G2, ... Gn >>}, with S and D as the IP source and destination addresses. If (B) were sent, the datagram received at D would again contain S and D as the same IP source and destination addresses, but the option would be: {S, G1, ...Gn >>}; i.e., the originating host would be the first hop in the route.