Stream Control Transmission Protocol (SCTP) Associations
An SCTP association is a generalisation of a TCP connection. Usually a TCP connection is one-to-one between two network interfaces, one on a server and the other on a client. In contrast, an SCTP association is many-to-many in two ways:
Multiple network interfaces on a server can be associated with multiple interfaces on a client. For example, suppose the server and client both have an Ethernet card and a Wi-Fi card connected to the Internet. Then, data can flow over a single association in up to four possible ways: Ethernet to Ethernet, Ethernet to Wi-Fi, Wi-Fi to Ethernet or Wi-Fi to Wi-Fi.
An association also can carry multiple logical streams. These are numbered from zero upward. So, for example, stream zero could carry control instructions, while stream one could carry small pieces of data (such as small files), and stream two could carry larger pieces of data (such as an MPEG movie). The three streams are logically independent, so that delays on one stream do not cause delays on any other stream.
Note that a single socket can have multiple associations—that is, one socket can be used to talk to multiple other hosts. In general, these different associations are distinguished by having an association ID. The socket API for SCTP distinguishes between situations where exactly one association can exist (a one-to-one socket) or where a socket can manage many associations (a one-to-many socket). The first case corresponds to the TCP-like case that I discussed in the first article on SCTP [LJ, September 2007]. The second case will be covered in the next article. In this article, I look only at a single association, which is applicable to both the one-to-one and the one-to-many sockets.
TCP and UDP use a single network interface on an endpoint, by specifying its IP address in a socketaddr structure. If you specify the wildcard address INADDR_ANY, a server will listen on all interfaces while a client will choose only one. In any case, communication takes place only between a single interface on each endpoint. As an aside, if you want to know what all the interfaces on your machine are, use the call ioctl() with parameter SIOCGIFCONF. How to do this is described in WR Stevens et al., Unix Network Programming, vol 1, section 17.5.
Using only one interface reduces reliability when more than one is available. A network cable may have poor connections, or you may be too far from a wireless access point for a reliable signal. On the other hand, using all of the interfaces may not always be desirable. For example, in Australia, the download charges for 3-G or WiMAX connections are ridiculously expensive, so you would use that interface only if no others were available. Or, a bridge would expose the internal and external interfaces separately to different groups of users.
SCTP allows an application to choose a subset of interfaces on either the source or destination side of an association. Some implementations also will allow interfaces to be added or removed dynamically, so the application can adjust to different states of the network connections. By registering for association-change events (which will be discussed in the next article), one endpoint can track changes in the interfaces at the other end.
The normal socket call bind() just takes a single sockaddr parameter to bind the socket to a single IP address (or to the wildcard address). SCTP extends this by introducing a new call, sctp_bindx(), which takes an array of sockaddrs to bind the socket to all of these addresses. The socket is bound only to a single port though; all of the port numbers in the array of sockaddrs must have the same port number. And, if addresses are added or removed later, they must have the same bound port value. Otherwise, the call fails.
There is another wrinkle to sctp_bindx() regarding IPv4 and IPv6 addresses. The socket can be passed a set of only IPv4 sockaddrs, a set of only IPv6 sockaddrs or a mixture of both. The two types of socket address structures, sockaddr_in and sockaddr_in6, have different sizes, so mixing these in the same array can cause alignment issues. SCTP packs the structures together with no wasted space between them. So, you can't just use an index into the array, you have to copy the right number of bytes for each structure and then move up by that amount.
The call to bind a set of addresses is:
int sctp_bindx(int sd, struct sockaddr *addrs, ↪int addrcount, int flags)