36.4 SCTP Applications

SCTP supports legacy ns applications, but they obviously cannot completely exploit all SCTP's features. For these applications, the TCL-bound SCTP configuration parameters can be used to set reliability and ordering options. However, these options cannot be controlled per message using these parameters. Only SCTP-aware application can be written to do so. ns applications wishing to become SCTP-aware can use the sendmsg API as follows (see ~ns/apps/sctp_app1.{cc, h} as an example).

1. Create and fill an instance of the AppData_S structure (see ~ns/sctp/sctp.h). The AppData_S structure has the following fields:

   usNumStreams is the number of outgoing streams to setup during negotiation. Although this field is passed with every sendmsg call, it is only used during association setup. Once the association is established, this field is ignored.

   usNumUnreliable is the number of outgoing streams which are unreliable (now called partially reliable). The sender simply sets the lowest outgoing stream to unreliable/partially-reliable; the remaining ones are reliable. This field is also only used during association establishment.

   usStreamId is the stream ID of a message.

   usReliability is the reliability level (k-rtx value) of a message. This field is ignored if the message is sent on a reliable stream.

   eUnordered is the unordered boolean flag for a message.

   uiNumBytes is the number of bytes in a message.

2. Pass this object as the second parameter in SCTP's sendmsg:

   sctpAgent-sendmsg(numBytes, (char *)appData);