Unix Network Programming, Volume 1: The Sockets Networking API (3rd Edition)

We will now use some of the elementary functions from Chapters 4 and Chapter 9 to write a complete one-to-many SCTP client/server example. Our simple example is similar to the echo server presented in Chapter 5, and performs the following steps:

1. A client reads a line of text from standard input and sends the line to the server. The line follows the form [#] text , where the number in brackets is the SCTP stream number on which the text message should be sent.

2. The server receives the text message from the network, increases the stream number on which the message arrived by one, and sends the text message back to the client on this new stream number.

3. The client reads the echoed line and prints it on its standard output, displaying the stream number, stream sequence number, and text string.

Figure 10.1 depicts this simple client/server along with the functions used for input and output.

We show two arrows between the client and server depicting two unidirectional streams being used, even though the overall association is full-duplex . The fgets and fputs functions are from the standard I/O library. We do not use the writen and readline functions defined in Section 3.9 since they are unnecessary. Instead, we use the sctp_sendmsg and sctp_recvmsg functions defined in Sections 9.9 and Sections 9.10, respectively.

For this example, we use a one-to-many-style server. We make this choice for one important reason. The examples in Chapter 5 can be modified to run over SCTP with one minor change: modify the socket function call to specify IPPROTO_SCTP instead of IPPROTO_TCP as the third argument. Simply making this change, however, would not take advantage of any of the additional features provided by SCTP except multihoming. Using the one-to-many style allows us to exercise all of SCTP's features.