Connected: An Internet Encyclopedia
5.2.4.2 Local/Remote Decision
Up:
Connected: An Internet Encyclopedia
Up:
Requests For Comments
Up:
RFC 1812
Up:
5. INTERNET LAYER - FORWARDING
Up:
5.2 FORWARDING WALK-THROUGH
Up:
5.2.4 Determining the Next Hop Address
Prev: 5.2.4.1 IP Destination Address
Next: 5.2.4.3 Next Hop Address
5.2.4.2 Local/Remote Decision
5.2.4.2 Local/Remote Decision
After it has been determined that the IP packet needs to be forwarded
according to the rules specified in Section [5.2.3], the following
algorithm MUST be used to determine if the Immediate Destination is
directly accessible (see [INTERNET:2]).
- For each network interface that has not been assigned any IP
address (the unnumbered lines as described in Section [2.2.7]),
compare the router-id of the other end of the line to the IP
Destination Address. If they are exactly equal, the packet can
be transmitted through this interface.
- DISCUSSION
-
In other words, the router or host at the remote end of the line
is the destination of the packet or is the next step in the source
route of a source routed packet.
- If no network interface has been selected in the first step, for
each IP address assigned to the router:
- isolate the network prefix used by the interface.
- IMPLEMENTATION
-
The result of this operation will usually have been computed and
saved during initialization.
- Isolate the corresponding set of bits from the IP Destination
Address of the packet.
- Compare the resulting network prefixes. If they are equal to
each other, the packet can be transmitted through the
corresponding network interface.
- If the destination was neither the router-id of a neighbor on an
unnumbered interface nor a member of a directly connected network
prefix, the IP Destination is accessible only through some other
router. The selection of the router and the next hop IP address
is described in Section [5.2.4.3]. In the case of a host that is
not also a router, this may be the configured default router.
Ongoing work in the IETF [ARCH:9, NRHP] considers some cases such as
when multiple IP (sub)networks are overlaid on the same link layer
network. Barring policy restrictions, hosts and routers using a
common link layer network can directly communicate even if they are
not in the same IP (sub)network, if there is adequate information
present. The Next Hop Routing Protocol (NHRP) enables IP entities to
determine the "optimal" link layer address to be used to traverse
such a link layer network towards a remote destination.
- If the selected "next hop" is reachable through an interface
configured to use NHRP, then the following additional steps apply:
- Compare the IP Destination Address to the destination addresses
in the NHRP cache. If the address is in the cache, then send
the datagram to the corresponding cached link layer address.
- If the address is not in the cache, then construct an NHRP
request packet containing the IP Destination Address. This
message is sent to the NHRP server configured for that
interface. This may be a logically separate process or entity
in the router itself.
- The NHRP server will respond with the proper link layer address
to use to transmit the datagram and subsequent datagrams to the
same destination. The system MAY transmit the datagram(s) to
the traditional "next hop" router while awaiting the NHRP reply.
Next: 5.2.4.3 Next Hop Address
Connected: An Internet Encyclopedia
5.2.4.2 Local/Remote Decision