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 IP @

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 IP @ of a destination network– and R is the IP @ 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 IP @s are unchanged

• When IP executes the routing algorithm, it selects a new IP address, the IP @ of the machine to which the datagram is to be sent on its next hop

• The network interface software binds the next hop @ to a hardware @, 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 destination @ matches the host’s IP @, 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 destination @ matches the router’s IP @, 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