December 2005
Intermediate to advanced
1066 pages
33h 38m
English
Content preview from Understanding Linux Network InternalsBecome an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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Packet Reception Versus Packet Transmission
The routing table is used to route both packets that are transmitted and those that are received, because either type may be delivered locally or forwarded. But besides those obvious uses of the routing table, there are others that are less obvious.
Figure 30-9 shows a couple of examples of routing table use. It distinguishes between lookups triggered by the reception of data (left side) and the transmission of data (right side). Note that the routing information required to forward an input packet is collected when the packet is first received, which explains why the Forwarding block does not have an arrow toward the Routing block in Figure 30-9. The figure also includes pointers to those chapters where you can find more details about a given kernel component. Here are some details concerning the activities shown:
Address Resolution Protocol (ARP) packets are not routed, but ARP may need to do a route lookup to enforce some sanity checks. See Chapter 28.
IP-over-IP is a simple tunneling protocol that encapsulates IP packets within larger IP packets. When the IP handler is handed an ingress IP-over-IP packet, it redelivers the payload to the IP layer. The inner IP packet is routed like any other ingress packet, so the routing subsystem needs to make another routing lookup.
Figure 30-9. Ingress and egress traffic routing
Routing a packet normally ...
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