3.3.1.2 Gateway Selection

To efficiently route a series of datagrams to the same destination, the source host MUST keep a "route cache" of mappings to next-hop gateways. A host uses the following basic algorithm on this cache to route a datagram; this algorithm is designed to put the primary routing burden on the gateways [IP:11].

1. If the route cache contains no information for a particular destination, the host chooses a "default" gateway and sends the datagram to it. It also builds a corresponding Route Cache entry.

2. If that gateway is not the best next hop to the destination, the gateway will forward the datagram to the best next-hop gateway and return an ICMP Redirect message to the source host.

3. When it receives a Redirect, the host updates the next-hop gateway in the appropriate route cache entry, so later datagrams to the same destination will go directly to the best gateway.

Since the subnet mask appropriate to the destination address is generally not known, a Network Redirect message SHOULD be treated identically to a Host Redirect message; i.e., the cache entry for the destination host (only) would be updated (or created, if an entry for that host did not exist) for the new gateway.

DISCUSSION:

This recommendation is to protect against gateways that erroneously send Network Redirects for a subnetted network, in violation of the gateway requirements [INTRO:2].

When there is no route cache entry for the destination host address (and the destination is not on the connected network), the IP layer MUST pick a gateway from its list of "default" gateways. The IP layer MUST support multiple default gateways.

As an extra feature, a host IP layer MAY implement a table of "static routes". Each such static route MAY include a flag specifying whether it may be overridden by ICMP Redirects.

DISCUSSION:

A host generally needs to know at least one default gateway to get started. This information can be obtained from a configuration file or else from the host startup sequence, e.g., the BOOTP protocol (see [INTRO:1]).

It has been suggested that a host can augment its list of default gateways by recording any new gateways it learns about. For example, it can record every gateway to which it is ever redirected. Such a feature, while possibly useful in some circumstances, may cause problems in other cases (e.g., gateways are not all equal), and it is not recommended.

A static route is typically a particular preset mapping from destination host or network into a particular next-hop gateway; it might also depend on the Type-of- Service (see next section). Static routes would be set up by system administrators to override the normal automatic routing mechanism, to handle exceptional situations. However, any static routing information is a potential source of failure as configurations change or equipment fails.