4.7. Routing Cache

4.7. Routing Cache

The routing cache is also known as the forwarding information base (FIB). This term may be familiar to users of other routing systems.

The routing cache stores recently used routing entries in a fast and convenient hash lookup table, and is consulted before the routing tables. If the kernel finds a matching entry during route cache lookup, it will forward the packet immediately and stop traversing the routing tables.

Because the routing cache is maintained by the kernel separately from the routing tables, manipulating the routing tables may not have an immediate effect on the kernel's choice of path for a given packet. To avoid a non-deterministic lag between the time that a new route is entered into the kernel routing tables and the time that a new lookup in those route tables is performed, use ip route flush cache. Once the route cache has been emptied, new route lookups (if not by a packet, then manually with ip route get) will result in a new lookup to the kernel routing tables.

The following is a listing of the hash lookup keys in the routing cache and a description of each key. Compare this list with the elements identified in Table 4.1, “Keys used for hash table lookups during route selection”.

dst, Destination Address

The destination IP address of the packet. This is the destination address on the packet at the time of the route lookup. The address is a host address. All 32 bits are significant during this lookup.

src, Source Address

The source IP address of the packet. This is the source address on the packet at the time of the route lookup. The address is a host address. All 32 bits are significant during this lookup.

tos, Type of Service

The ToS marking on the packet. If there is no ToS marking on the packet (tos == 0), this lookup key is unused. If there is a ToS marking, the kernel will search for a match with this ToS value. If no matching (dst, src, tos) is found, the kernel will continue the search for a route by traversing the RPDB.

fwmark

The mark on a packet added administratively by the packet filtering engine (ipchains or iptables). This mark is not part of the physical IP packet, and only exists as part of the data structure held in memory on the routing device to represent the IP packet. If there is no fwmark on the packet, this lookup key is unused. When present, the kernel will search for a matching (dst, src, tos?, fwmark) entry. If no matching entry is found, the kernel will continue the search for a route by traversing the RPDB.

iif, inbound interface

The name of the interface on which the packet arrived.

 

The following attributes may be stored for each entry in the routing cache.

cwnd, FIXME Window

FIXME. A) I don't know what it is. B) I don't know how to describe it.

advmss, Advertised Maximum Segment Size

 

src, (Preferred Local) Source Address

 

mtu, Maximum Transmission Unit

 

rtt, Round Trip Time

 

rttvar, Round Trip Time Variation

FIXME. Gotta find some references to this, too.

age

 

users

 

used

 

Collectively the hash keys uniquely identify routes in the forwarding information base (routing cache) and each entry provides attributes of the route.