TCP/IP Tutorial and Technical Overview

Table of Contents Routing Protocols
TCP/IP Tutorial and Technical Overview

3.1 Basic IP Routing

The fundamental function for routers is present in all IP implementations:

An incoming IP datagram that specifies a ``destination IP address'' other than the local host's IP address(es), is treated as a normal outgoing IP datagram.

This outgoing IP datagram is subject to the IP routing algorithm of the local host, which selects the next hop for the datagram (the next host to send it to). This new destination can be located on any of the physical networks to which the intermediate host is attached. If it is a physical network other than the one on which the host originally received the datagram, then the net result is that the intermediate host has forwarded the IP datagram from one physical network to another.

Figure: Router Operation of IP - Can be performed by all IP implementations.

The normal IP routing table contains information about the locally attached networks and the IP addresses of other routers located on these networks, plus the networks they attach to. It can be extended with information on IP networks that are farther away, and can also contain a default route, but it still remains a table with limited information; that is, it represents only a part of the whole internet. That is why this kind of router is called a router with partial routing information.

Some considerations apply to these routers with partial information:

They do not know about all internet networks.
They allow local sites autonomy in establishing and modifying routes.
A routing entry error in one of the routers may introduce inconsistencies, thereby making part of the network unreachable.

Some error reporting should be implemented by routers with partial information via the Internet Control Message Protocol (ICMP) described in Internet Control Message Protocol (ICMP). They should be able to report the following errors back to the source host:

Unknown IP destination network by an ICMP Destination Unreachable message.
Redirection of traffic to more suitable routers by sending ICMP Redirect messages.
Congestion problems (too many incoming datagrams for the available buffer space) by an ICMP Source Quench message.
The ``Time-to-Live'' field of an IP datagram has reached zero. This is reported with an ICMP Time Exceeded message.
Also, the following base ICMP operations and messages should be supported:
- Parameter problem
- Address mask
- Time stamp
- Information request/reply
- Echo request/reply

A more intelligent router is required if:

The router has to know routes to all possible IP networks, as was the case for the ARPANET core gateways.
The router has to have dynamic routing tables, which are kept up-to-date with minimal or no manual intervention.
The router has to be able to advertise local changes to other routers.

These more advanced forms of routers use additional protocols to communicate with each other. A number of protocols of this kind exist, and descriptions of the important ones will be given in the following sections. The reasons for this multiplicity of different protocols are basically fourfold:

Using Internet terminology, there is a concept of a group of networks, called an Autonomous System (AS), which is administered as a unit. The AS concept arose because the TCP/IP protocols were developed with the ARPANET already in place.
Routing within an AS and routing outside an AS are treated as different issues and are addressed by different protocols.
Over two decades several routing protocols were tested in the Internet. Some of them performed well, others had to be abandoned.
The emergence of Autonomous Systems of different sizes called for different routing solutions. For small to medium sized ASs a group of routing protocols based upon Distance Vector (RIP, for example) became very popular. However, such protocols do not perform well for large interconnected networks. Link State protocols like OSPF are much better suited for such networks.
To exchange routing information between ASs border gateway protocols were developed.

Before discussing the various routing protocols, we will review the routing architectures used in the early Internet, since this will help in understanding the role played by the differing routing protocols. This overview will also show the difference between Interior and Exterior routing. We will then discuss the various protocols used for the two types of routing.

3.1.1 Routing Daemons

The routing protocols are often implemented using one of two daemons: (8)

routed: Pronounced ``route D''. This is a basic routing daemon for interior routing supplied with the majority of TCP/IP implementations. It uses the RIP protocol (see Routing Information Protocol (RIP)).
gated: Pronounced ``gate D''. This is a more sophisticated daemon on UNIX-based systems for interior and exterior routing. It can employ a number of additional protocols such as OSPF (see Open Shortest Path First Protocol (OSPF) Version 2) and BGP (see Border Gateway Protocol (BGP)).
See IP Routing Protocols in IBM TCP/IP Products for details of routing daemon implementations in IBM products.

Table of Contents Historical Perspective