Introduction to Stream Control Transmission Protocol

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The first in a series on SCTP and how to migrate TCP applications.

The usual C compile command can be used to create object modules and executables. If the program uses SCTP-specific functions (the programs in Listings 1 and 2 don't), you also need to link in the SCTP library:

cc -o echo_client echo_client.c -lsctp

Is it worthwhile to take an application that runs over TCP and move it to SCTP? The disadvantages are that SCTP is not as well supported as TCP, the tools are sometimes not aware of SCTP and the API is still evolving. On the other hand, it benefits from the experience of 20 years of seeing TCP and UDP applications in practice. For example, SCTP is secure from SYN attacks by design, and the protocol has no known security holes. SCTP also will take advantage of multihoming when needed automatically. If packets are getting lost, due to, say, congestion, SCTP will use different interfaces to try to avoid the losses, and this could result in faster throughput.

The withsctp Tool

In the previous section, I discussed how to alter the source code of a client or server to use SCTP instead of TCP. The sctp-tools package contains a program called withsctp, which essentially does the same to binary code. This program acts as a wrapper around a TCP application to turn it into an SCTP application. It first saves the address of the “real” socket() function call, and then inserts its own version of socket() into the load library path. This new version of socket() simply gets the parameters of the function call, changes the third parameter from IPPROTO_TCP to IPPROTO_SCTP and calls the “real” socket() function.

For example, the xinetd dæmon can run a group of TCP and UDP services. The services are those listed in the directory /etc/xinetd.d, which have enable = yes or disable = no. The TCP services all can be run over SCTP by:

withsctp xinetd

One of the simplest services that is run by xinetd is daytime. The service accepts a connection and returns an ASCII string for the current date. A quick Google search turns up source code for many clients, but the simplest way is to run Telnet:


telnet <host-name> 13

If you have daytime running as an SCTP service rather than a TCP service, use withsctp to connect to it:


withsctp telnet <host-name> 13

This is a quick way of testing whether a TCP service can be converted to SCTP.

Message Orientation

TCP is a byte-oriented protocol—that is, you write bytes and read bytes. The UNIX system calls read() and write() typically are used for this. TCP also has send()/recv(), which have an extra flags parameter, but these do not change the byte-transfer model.

SCTP, on the other hand, is message-oriented, more like UDP. Most Internet applications have a message structure to their communications rather than merely a sequence of bytes. For example, a single HTTP request has a header and body section, and even the header section is composed of an arbitrary number of lines. The sender has to compose the parts into the single request, and the receiver of such a message has to parse it back into its component messages. A few protocols are only byte-oriented (for example, the file transfer mode of FTP), but these are the minority.

SCTP makes it easy to use a message-based structure—within limits. A write() call writes a complete message. The corresponding read() reads this complete message. So, to send an HTTP header over SCTP, you could do a write of each line, followed by a write of an empty line. The receiver would read each line as a separate message, stopping after reading an empty line. There would be no need to parse the received bytes into a set of lines before processing each one. Note that if the original TCP application already used a series of writes followed by a single read, expecting TCP to concatenate all the messages, the application would need to be modified to match each write to a corresponding read statement.

The caveats are with big messages. Applications that want to take advantage of these messaging capabilities must be careful when sending big messages (say 32KB or more). To send a message, you aren't merely passing a pointer to data on the stack, you're actually moving that data across the network. That means putting it into buffers on the sender side, passing it through buffers in intermediate nodes and, finally, delivering it to a buffer in the reading application. All of these buffers have limits that cannot be exceeded.

For example, say a sender uses a buffer with a size set by the socket option SO_SNDBUF. An attempt to write a message larger than that will fail and return -1. The size of this is generous, typically about 64KB. It can be changed by using setsockopt(sockfd, SOL_SOCKET, SO_SNDBUF, &val, &val_len), where val is an integer variable containing the length to which you want to set the buffer. But, then other limits may come into play. Each host along the route from sender to receiver will have a maximum packet size that it will pass along. The Path Maximum Transmission Unit (PMTU) is the minimum of all of these. If the message (plus any IP and SCTP headers) is larger than the PMTU, it will be fragmented and delivered in pieces. The sender can guard against this by setting the SCTP option SCTP_DISABLE_FRAGMENTS so that a message is delivered as a single entity or not at all, but this typically will only decrease the maximum possible message size.

The receiver of a message also has a receiving buffer size, which is controlled by the socket option SO_RCVBUF. It will not receive messages larger than this—fragmenting them if necessary. The major problem from the receiving side is how to deal with fragmented messages. The system calls read() and recv() do not contain any information about message boundaries, as they are byte-oriented. Fortunately, SCTP has a new system call, sctp_recvmsg(), which returns status information about the read in an integer parameter. In particular, if the MSG_EOR bit (message end-of-record) is set, read of a message has been completed. If it is not set, the message has been fragmented and more of the message needs to be read. This can be used by the reader to build up a complete message before processing it.

Listing 3 shows how the sctp_recvmsg() call can be used to receive fragmented messages and build them up into a complete message. It does so by reading each part of a message as it comes in and adding it to the parts already received. When a part arrives with the MSG_EOR bit set in the flags, the message is complete and can be returned to the reading application.

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Excellent!

Johan H's picture

An excellent article concerning introduction to SCTP.
Very good!

/Best regards
J

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