2.4 Architectural Assumptions

The current Internet architecture is based on a set of assumptions about the communication system. The assumptions most relevant to routers are as follows:

• The Internet is a network of networks.

  Each host is directly connected to some particular network(s); its connection to the Internet is only conceptual. Two hosts on the same network communicate with each other using the same set of protocols that they would use to communicate with hosts on distant networks.

• Routers do not keep connection state information.

  To improve the robustness of the communication system, routers are designed to be stateless, forwarding each IP packet independently of other packets. As a result, redundant paths can be exploited to provide robust service in spite of failures of intervening routers and networks.

  All state information required for end-to-end flow control and reliability is implemented in the hosts, in the transport layer or in application programs. All connection control information is thus co-located with the end points of the communication, so it will be lost only if an end point fails. Routers control message flow only indirectly, by dropping packets or increasing network delay.

  Note that future protocol developments may well end up putting some more state into routers. This is especially likely for multicast routing, resource reservation, and flow based forwarding.

• Routing complexity should be in the routers.

  Routing is a complex and difficult problem, and ought to be performed by the routers, not the hosts. An important objective is to insulate host software from changes caused by the inevitable evolution of the Internet routing architecture.

• The system must tolerate wide network variation.

  A basic objective of the Internet design is to tolerate a wide range of network characteristics - e.g., bandwidth, delay, packet loss, packet reordering, and maximum packet size. Another objective is robustness against failure of individual networks, routers, and hosts, using whatever bandwidth is still available. Finally, the goal is full open system interconnection: an Internet router must be able to interoperate robustly and effectively with any other router or Internet host, across diverse Internet paths.

  Sometimes implementors have designed for less ambitious goals. For example, the LAN environment is typically much more benign than the Internet as a whole; LANs have low packet loss and delay and do not reorder packets. Some vendors have fielded implementations that are adequate for a simple LAN environment, but work badly for general interoperation. The vendor justifies such a product as being economical within the restricted LAN market. However, isolated LANs seldom stay isolated for long. They are soon connected to each other, to organization-wide internets, and eventually to the global Internet system. In the end, neither the customer nor the vendor is served by incomplete or substandard routers.

  The requirements in this document are designed for a full-function router. It is intended that fully compliant routers will be usable in almost any part of the Internet.