The router applies the algorithm in the previous section to determine if the IP Destination Address is adjacent. If so, the next hop address is the same as the IP Destination Address. Otherwise, the packet must be forwarded through another router to reach its Immediate Destination. The selection of this router is the topic of this section.
If the packet contains an SSRR, the router MUST discard the packet and reply with an ICMP Bad Source Route error. Otherwise, the router looks up the IP Destination Address in its routing table to determine an appropriate next hop address.
Per the IP specification, a Strict Source Route must specify a sequence of nodes through which the packet must traverse; the packet must go from one node of the source route to the next, traversing intermediate networks only. Thus, if the router is not adjacent to the next step of the source route, the source route can not be fulfilled. Therefore, the router rejects such with an ICMP Bad Source Route error.
The goal of the next-hop selection process is to examine the entries in the router's Forwarding Information Base (FIB) and select the best route (if there is one) for the packet from those available in the FIB.
Conceptually, any route lookup algorithm starts out with a set of candidate routes that consists of the entire contents of the FIB. The algorithm consists of a series of steps that discard routes from the set. These steps are referred to as Pruning Rules. Normally, when the algorithm terminates there is exactly one route remaining in the set. If the set ever becomes empty, the packet is discarded because the destination is unreachable. It is also possible for the algorithm to terminate when more than one route remains in the set. In this case, the router may arbitrarily discard all but one of them, or may perform "load-splitting" by choosing whichever of the routes has been least recently used.
With the exception of rule 3 (Weak TOS), a router MUST use the following Pruning Rules when selecting a next hop for a packet. If a router does consider TOS when making next-hop decisions, the Rule 3 must be applied in the order indicated below. These rules MUST be (conceptually) applied to the FIB in the order that they are presented. (For some historical perspective, additional pruning rules, and other common algorithms in use, see Appendix E.)
Rule 3 is optional in that Section [5.3.2] says that a router only SHOULD consider TOS when making forwarding decisions.
This rule discards any routes to destinations other than the IP Destination Address of the packet. For example, if a packet's IP Destination Address is 10.144.2.5, this step would discard a route to net 188.8.131.52/16 but would retain any routes to the network prefixes 10.0.0.0/8 and 10.144.0.0/16, and any default routes.
More precisely, we assume that each route has a destination attribute, called route.dest and a corresponding prefix length, called route.length, to specify which bits of route.dest are significant. The IP Destination Address of the packet being forwarded is ip.dest. This rule discards all routes from the set of candidates except those for which the most significant route.length bits of route.dest and ip.dest are equal.
For example, if a packet's IP Destination Address is 10.144.2.5 and there are network prefixes 10.144.1.0/24, 10.144.2.0/24, and 10.144.3.0/24, this rule would keep only 10.144.2.0/24; it is the only route whose prefix has the same value as the corresponding bits in the IP Destination Address of the packet.
Longest Match is a refinement of Basic Match, described above. After performing Basic Match pruning, the algorithm examines the remaining routes to determine which among them have the largest route.length values. All except these are discarded.
For example, if a packet's IP Destination Address is 10.144.2.5 and there are network prefixes 10.144.2.0/24, 10.144.0.0/16, and 10.0.0.0/8, then this rule would keep only the first (10.144.2.0/24) because its prefix length is longest.
Each route has a type of service attribute, called route.tos, whose possible values are assumed to be identical to those used in the TOS field of the IP header. Routing protocols that distribute TOS information fill in route.tos appropriately in routes they add to the FIB; routes from other routing protocols are treated as if they have the default TOS (0000). The TOS field in the IP header of the packet being routed is called ip.tos.
The set of candidate routes is examined to determine if it contains any routes for which route.tos = ip.tos. If so, all routes except those for which route.tos = ip.tos are discarded. If not, all routes except those for which route.tos = 0000 are discarded from the set of candidate routes.
Additional discussion of routing based on Weak TOS may be found in [ROUTE:11].
The effect of this rule is to select only those routes that have a TOS that matches the TOS requested in the packet. If no such routes exist then routes with the default TOS are considered. Routes with a non-default TOS that is not the TOS requested in the packet are never used, even if such routes are the only available routes that go to the packet's destination.
Each route has a metric attribute, called route.metric, and a routing domain identifier, called route.domain. Each member of the set of candidate routes is compared with each other member of the set. If route.domain is equal for the two routes and route.metric is strictly inferior for one when compared with the other, then the one with the inferior metric is discarded from the set. The determination of inferior is usually by a simple arithmetic comparison, though some protocols may have structured metrics requiring more complex comparisons.
Vendor Policy is sort of a catch-all to make up for the fact that the previously listed rules are often inadequate to choose from the possible routes. Vendor Policy pruning rules are extremely vendor-specific. See section [184.108.40.206].
This algorithm has two distinct disadvantages. Presumably, a router implementor might develop techniques to deal with these disadvantages and make them a part of the Vendor Policy pruning rule.
It is also worth noting a deficiency in the way that TOS is supported: routing protocols that support TOS are implicitly preferred when forwarding packets that have non-zero TOS values.
The Basic Match and Longest Match pruning rules generalize the treatment of a number of particular types of routes. These routes are selected in the following, decreasing, order of preference:
If, after application of the pruning rules, the set of routes is empty (i.e., no routes were found), the packet MUST be discarded and an appropriate ICMP error generated (ICMP Bad Source Route if the IP Destination Address came from a source route option; otherwise, whichever of ICMP Destination Host Unreachable or Destination Network Unreachable is appropriate, as described in Section [220.127.116.11]).