20.1 Introduction to Connectionless Communication

Connectionless communication is an abstraction based on transmission of single messages or datagrams between sender and receiver. A datagram is a unit of data transferred from one endpoint to another. Connectionless communication makes no association between the endpoints, and a process can use a single connectionless endpoint to send messages to or receive messages from many other endpoints.

Figure 20.1 illustrates connectionless communication among four processes running on different hosts . Process A receives messages from several different sources on the same communication endpoint. Process A uses this communication endpoint both to reply to the message from C and to send a message to D. Process C uses its connectionless communication endpoint to send messages to both A and D. Since each message includes the sender's return address, the receiver knows where to send the response.

Figure 20.1. Four processes with connectionless communication endpoints.

This chapter develops a model for connectionless communication based on UDP, the User Datagram Protocol. UDP is used in many common Internet applications and protocols, including DNS ( name service), NFS (distributed file system), NTP (time protocol), RTP (realtime transfer protocol) and SNMP (network management).

A UDP communication endpoint is identified by host IP address and port number. The receiver can extract the address of the sender's communication endpoint and use the information as a return address in replying to the message. Because no connection is involved, connectionless communication might not follow the client-server model. However, the client-server communication pattern holds for many applications, with clients sending request messages to servers on well-known ports (e.g., NFS servers use port 2049).

While the connection-oriented TCP protocol provides an error-free byte stream, UDP is unreliable. A UDP datagram might not arrive at its destination, or it might arrive before a message that was sent earlier. The sender has no information about the success or failure of the transmission. Even if the datagram arrives at the destination host, the network subsystem might drop the message before delivering it to the application because the endpoint buffers are full. Thus, while UDP has very low overhead, the application must handle considerably more complex errors than with TCP.

UDP datagrams are transmitted atomically, that is, a given datagram either arrives in its entirety at the destination endpoint or it does not arrive at all. To achieve this, modern network subsystems assemble UDP datagrams and verify UDP checksums. If a checksum is not correct, the subsystem discards the packet. Unfortunately, the computation of UDP checksums in IPv4 is optional, and some older systems disable checking by default. The UDP checksum guards against transmission errors, but not against malicious attackers . Such an attacker could modify both the data and the checksum in a consistent way. UDP does not authenticate what was sent and so has no way of detecting that an attack has occurred. Authentication must take place in a higher-level layer or in the application itself.

As with connection-oriented protocols, we introduce a simplified interface for connectionless communication, based on a socket implementation with UDP. Section 20.2 describes the UICI UDP interface. Sections 20.3 and 20.4 use this interface to implement the simple-request and the request-reply protocols, respectively. Section 20.5 adds timeouts and retries to the request-reply protocol. Section 20.6 outlines the implementation of request-reply- acknowledge protocols. Section 20.7 describes the implementation of each function in the UICI UDP interface in terms of sockets and UDP. Section 20.8 compares the UDP and TCP protocols. Section 20.9 discusses multicast communication and adds two functions to UICI UDP to support multicast communication.