A.4.1. O verview
Configuration detection has been a fundamental discipline for routing in packet-switched networks. Detected objects are network layer entities in packet switches and links between them. The means are provided to detect those objects, such as neighbour acquisition and routing update protocol.
Routing operations include routing control and traffic forwarding. Because of our interest in configuration detection, the latter is relevant. Routing control consists of the following activities [Lai8 8], [IS089a]:
1. routing information collection (e.g. measurement)
2. information distribution
3. route calculation and maintenance
It is the first two above, i.e. routing information collection and information distribution, with which a node in a network acquires the knowledge of the network configuration. From routing information collection, a node can see the available nodes near by, and from information distribution, it is informed o f the reachable destinations.
In the following subsections, we describe these configuration detection aspects o f routing operations. The next subsection presents routing information exchange in the DARPA Internet as an example o f gateway-gateway configuration detection. Subsection A.4.3 and Subsection A .4.4 describe the ISO and DARPA IP protocols respectively for a host to find available gateways.
A.4.2. The Internet
The Internet has developed out o f the original DARPA Internet, but has broadened its scope over the years. Much of the argument in this section is based on the DARPA Internet portion, which was strongly managed. The current growth is m ore democratic in nature. There is now a range o f routing and management procedures, so that there is a serious inter-operability and systems management problem.
The DARPA Internet gateways originally used the Gateway-Gateway Protocol (GGP) for routing information exchange [HiS82], while the gateways were relatively homogeneous in terms of their hardware and software, and their num ber was relatively small. As more networks have joined the Internet, the number o f gateways connecting them has increased. The heterogeneity of their software and hardware has increased as new gateways often have radically different software [SeR84], and they come from a wider range of vendors [PaT87b]. The GGP was neither flexible enough to cope with the size and heterogeneity o f the Internet gateways, nor specified sufficiently tightly enough for multivendor inter-operability.
The Exterior Gateway Protocol was specified to remedy this situation. In this scheme, the Internet System is regarded as being composed o f a num ber o f Autonomous Systems, each of which has a separate administration on routing. In other words, each Autonomous System has its own routing scheme which may differ from that in another Autonomous System. One Autonomous System could make use of others as intermediate hops to the final destination o f the traffic. The system is loosely configured as a federated system [Sol85]; its architecture is influenced strongly by the federal nature o f the funding agencies. Thus the Defense Research portion o f the Internet System is divided in a hierarchical manner, with an Autonomous System o f specific gateways on top; these are called core gateways and managed by BBN exclusively. Other Autonomous Systems are connected to the core gateways as child nodes by means o f stub gateways. Core gateways exchange routing information by GGP internally and now using another protocol; a core gateway and a stub gateway exchange the information by EGP.
The functions of EGP includes the followings [Com8 8]:
1. acquisition of tactically selected neighbours
2. testing neighbour reachability
3. advertising network reachability to neighbours
The protocol does not specify which system is to be configured as neighbour; this needs human intervention. The human decision maker, therefore, is supposed to have a knowl edge o f neighbours. Neighbour reachability is tested by Hello and I-Heard-You messages exchanges. By advertisement o f reachable networks from neighbours, a gateway can obtain the Internet structure outside its Autonomous System. A n Autonomous System can obtain its internal configuration by an appropriate internal routing information exchange protocol. In regard to configuration detection, EGP gives a mechanism to provide a knowledge o f net works and gateways to other Autonomous Systems. Intra-Autonomous System configuration has to be detected by other mechanisms, such as human input or another protocol used in that Autonomous System. Other Autonomous Systems are also connected in with quite different protocol suites. For example the collection of the N SFNET backbone and the Regional Networks, funded partially by the National Science Foundation, have a similar philosophy — but based on a core managed by MERIT Inc., which is a membership consortium o f some universities in the State of Michigan, U.S.A., with support from IBM and MCI. The resulting interoperability problems are currently being resolved.
A.4.3. A subnet of an ISO Connectionless mode Internet
In an ISO connectionless mode internet, intermediate and end systems use the connectionless mode protocol [IS086(DIS 8473)] for internetworking operation. Routing information is exchanged by different protocols. For example, there is one for use between some o f the intermediate systems in the Internet. There is another for use between the end systems and the intermediate systems [ISO 9542] in a subnet, which allows a system to know about the existence o f other systems. In this protocol, an end system multicasts a Hello message to all the intermediate network entities periodically, and so does an intermediate system to all the end systems. A Hello message from an end system carries the information on
available network ports (NSAPs) for each subnet port (SNPA) in the end system. A Hello message from an intermediate system conveys the information on the network entity title of the intermediate system. The current configuration of a subnet can be detected by listening to those Hello messages. A system can listen to those messages by enabling its multicast address ports. This suite is used by some o f the Autonomous Systems in the Internet.
A.4.4. T he G atew ay Discovery P rotocol
Gateway discovery, a mechanism to enable a host to locate at least one operational router for the traffic destined to the outside of the subnet where the host resides, is a problem. Discovery is done by use of the specified pair of ICMP messages, a Gateway Query and a Gateway Report. At boot time, a host multicasts a Gateway Query and routers reply with Gateway Reports. This is a type 4 learning. Moreover, the routers periodically multicast unsolicited Gateway Reports. By listening to those reports, a host can learn the operational gateways. This is a type 3 learning.