Internet Protocol: Routing IP Datagrams Chapter 8

Introduction All internet services use an underlying, connectionless packet delivery system and the basic unit of transfer in TCP/IP is a datagram This chapter shows how routers forward datagrams and deliver them to the final destinations

Routing in an Internet Routing refers to choosing a path over which to send packets A router is the computer making the choice of that path Ideally, to find the best path, routing software would look at such factors as: –network load, datagram length, or type of service Most routing software is not that sophisticated and looks at “shortest path”

Routing in an Internet Routers have direct connections to two or more networks Both hosts and routers participate in routing a datagram to its destination The source host makes an initial routing decision as to which router it should forward its datagram Sometimes a machine could function as a host or a router, but distinctions must be made

Figure 8.1

Direct and Indirect Delivery Direct delivery –transmission of a datagram across a single physical network directly to another –both machines attach directly to the same underlying physical transmission system Indirect delivery –the destination is not on a directly attached network –the sender passes the datagram to a router for delivery

Datagram Delivery over a Single Network To transfer an IP datagram, the sender –encapsulates the datagram in a frame –maps the destination IP address into a physical address (eg. using ARP) –uses the network hardware to deliver it How does the sender know if the destination lies on a directly connected network (this network)? –by comparing the network portion of the with its own

Table-Driven IP Routing There is an IP routing table on each machine: router or host This table stores information about possible destinations and how to reach them What information is stored in a routing table? –(N, R) pairs –where N is the of a destination network –and R is the of the next router on the path to that network

Next-Hop Routing The routing table indicates only the next step along the path from R to N Each entry in the routing table points to a router that can be reached across a single network All routers (R) listed in a machine’s routing table must be directly connected to this machine See Figure 8.2 (next slide) The size of a routing table depends on the number of networks in the internet

Figure 8.2

Next-Hop Routing To hide information, routing tables should be kept small IP routing software only keeps information about destination network addresses, not individual hosts Consequences: –all traffic destined for a network takes same path –don’t know if destination host is even operational

Default Routes If no route appears in the routing table for a particular network, the datagram could be sent to a default router This is good when a site has a small set of local addresses and only one connection to the rest of the internet Two tests are done in this case: –one for hosts on the local network –a default that points to the only router

Host-Specific Routes Most IP routing software allows per-host routes to be specified The ability to specify a special route to one machine may be useful

The IP Routing Algorithm Extract destination IP address D from the datagram Compute the network prefix N If N matches any directly connected network address Get D’s hardware address Encapsulate the datagram in a frame Send the frame to D Else If the table contains a host-specific route for D Build a frame and send it to the next-hop specified Else If the table contains a route for network N Build and send a frame to the next-hop specified Else If the table contains a default route, send it Else declare a routing error

Routing with IP Addresses The original datagram is not altered during routing Specifically, the source and destination are unchanged When IP executes the routing algorithm, it selects a new IP address, the of the machine to which the datagram is to be sent on its next hop The network interface software binds the next to a forms a frame, places the datagram in the data portion of the frame and sends it

Routing with IP Addresses Why doesn’t the IP software use hardware addresses when storing and computing routes? –routes are easier to examine and change –to hide the details of the underlying networks - this allows the underlying network architecture to be changed without having to update tables See the address boundary shown in Figure 8.4 –a conceptual division between low-level software that understands hardware addresses and internet software that understands high-level addresses

Figure 8.4

Handling Incoming Datagrams When an IP datagram arrives at a host: –the network interface software delivers it to the IP module for processing –if the datagram’s matches the host’s IP software on the host accepts the datagram, and passes it to a higher level protocol software for further processing –if no match, the datagram is discarded

Handling Incoming Datagrams When an IP datagram arrives at a router: –the network interface software delivers it to the IP module for processing –if the datagram’s matches the router’s IP software on the host accepts the datagram, and passes it to a higher level protocol software for further processing - usually for testing and router management –if not a match, the TTL is decremented, the checksum is recomputed and the datagram is forwarded, using the routing table and the routing algorithm Hosts should not perform router functions

Summary Direct delivery is possible if the destination machine is on a network to which the sending machine attaches Otherwise, the sender forwards the datagram to a router Datagrams travel from router to router until they reach their final destination. Or? Transmission of a datagram from one machine to the next requires encapsulating it in a frame

For Next Time Read Chapter 9 Make the equivalent of about 15 slides outlining Chapters 8 and 9