Introduction to Stream Control Transmission Protocol

The SCTP Web site (www.sctp.org) has a list of implementations of SCTP. There are implementations for BSD and Windows, and since 2001, there has been a Linux kernel project at sourceforge.net/projects/lksctp. At present, SCTP is not in any Microsoft release, so applications running on Windows need to install one of the available stacks.

SCTP is included in the Linux kernel as an experimental network protocol. SCTP is normally built as a module. It may be necessary to load the module using modprobe sctp. To build user applications, you may need to install the SCTP tools—in Fedora Core 6, these are in the RPM packages lksctp-tools-1.0.6-1.fc6.i386.rpm and lksctp-tools-devel-1.0.6-1.fc6.i386.rpm. On Fedora Core 6, I also had to add a symbolic link from /usr/lib/libsctp.so to /usr/lib/libsctp.so.1.

The lksctp-tools package contains the libraries to run SCTP applications. It also contains a program called checksctp, which tells you if your kernel has support for SCTP. When you run this program, it prints either “SCTP supported” or an error message.

The devel package contains the sctp.h header file, so you can compile and build your own applications, and man pages for the SCTP function calls.

Most firewalls can be configured to deal with SCTP packets, but the documentation for each firewall may not mention SCTP explicitly. For example, the man page for iptables says, “The specified protocol [in a rule] can be one of tcp, udp, icmp, or all...”. But, it then goes on to say, “A protocol name from /etc/protocols is also allowed”, and in that file, we find that protocol 132 is sctp. So, rules for SCTP can be added to iptables in the same way as TCP and UDP rules.

For example, an iptables rule to accept SCTP connections to port 13 would be:

-A INPUT -p sctp -m sctp -i eth0 --dport 13 -j ACCEPT

Webmin is a popular administration tool for managing things like iptables rules. Unfortunately, as of version 1.340, it could not accept this rule, because it is hard-wired to accept port numbers only for TCP and UDP, not realising that SCTP also uses port numbers. Such a rule would need to be entered by hand into the iptables configuration file /etc/sysconfig/iptables. This will be fixed in later versions of Webmin after I logged a bug report, but similar problems may occur in other tools.

As with TCP and UDP, SCTP provides a socket API for applications. A server creates a socket bound to a port and then uses this to accept a connection from a client. A client also creates a socket and then connects to a server. Both then use the socket file descriptor to read and write messages. SCTP is not a superset of TCP. Nevertheless, when restricted to a similar style of connection as TCP, there are sufficient similarities that an SCTP socket often can be used as a drop-in replacement for a TCP socket. When used in this way, SCTP sockets are called one-to-one sockets, as they simply connect one host to a single other host.

To create a TCP socket, use the system call:

sockfd = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)

This creates an IPv4 socket. To create an IPv6 socket, replace the first parameter with AF_INET6. The last parameter often is given as zero, meaning “use the only protocol value in the family”. It is better to use IPPROTO_TCP explicitly, because SCTP introduces another possible value.

To create an SCTP one-to-one socket, simply replace IPPROTO_TCP with IPPROTO_SCTP:

sockfd = socket(AF_INET, SOCK_STREAM, IPPROTO_SCTP)

and that (in many cases) is it! The client or server is now talking the SCTP protocol instead of TCP.

To see this in action, Listings 1 (echo_client.c) and 2 (echo_server.c) give a simple echo-client and server, where the server returns a string sent to it when a client connects to it. Only the line above needs to change in both the client and the server (with also an extra include file, sctp.h).