Unix Network Programming Episode 71

SCTP_ASSOC_CHANGE

This notification informs an application that a change has occurred to an association; either a new association has begun or an existing association has ended. The information provided with this event is defined as follows:

struct sctp_assoc_change {
	u_int16_t sac_type;
	u_int16_t sac_flags;
	u_int32_t sac_length;
	u_int16_t sac_state;
	u_int16_t sac_error;
	u_int16_t sac_outbound_streams;
	u_int16_t sac_inbound_streams;
	sctp_assoc_t sac_assoc_id;
	uint8_t sac_info[];
};

The sac_state describes the type of event that has occurred on the association, and will take one of the following values:

SCTP_COMM_UP

This state indicates that a new association has just been started. The inbound and outbound streams fields indicate how many streams are available in each direction. The association identification is filled with a unique value that can be used to communicate with the local SCTP stack regarding this association.

SCTP_COMM_LOST

This state indicates that the association specified by the association identification has closed due to either an unreachability threshold being triggered (i.e., the SCTP endpoint timed out multiple times and hit its threshold, which indicates the peer is no longer reachable), or the peer performed an abortive close (usually with the SO_LINGER option or by using sendmsg with a MSG_ABORT flag) of the association. Any user-specific information will be found in the sac_info field of the notification.

SCTP_RESTART

This state indicates that the peer has restarted. The most likely cause of this notification is a peer crash and restart. The application should verify the number of streams in each direction, since these values may change during a restart.

SCTP_SHUTDOWN_COMP

This state indicates that a shutdown initiated by the local endpoint (via either a shutdown call or a sendmsg with a MSG_EOF flag) has completed. For the one-to-one style, after receiving this notification, the socket descriptor can be used again to connect to a different peer.

SCTP_CANT_STR_ASSOC

This state indicates that a peer did not respond to an association setup attempt (i.e., the INIT message).

SCTP_PEER_ADDR_CHANGE

This notification indicates that one of the peer’s addresses has experienced a change of state. This change may either be a failure, such as the destination is not responding when sent to, or a recovery, such as a destination that was in a failed state has recovered. The structure that accompanies an address change is as follows:

struct sctp_paddr_change {
	u_int16_t spc_type;
	u_int16_t spc_flags;
	u_int32_t spc_length;
	struct sockaddr_storage spc_aaddr;
	u_int32_t spc_state;
	u_int32_t spc_error;
	sctp_assoc_t spc_assoc_id;
};

SCTP_REMOTE_ERROR

A remote peer may send an operational error message to the local endpoint. These messages can indicate a variety of error conditions for the association. The entire error chunk will be passed to the application in wire format when this notification is enabled. The format of the message will be as follows:

struct sctp_remote_error {
	u_int16_t sre_type;
	u_int16_t sre_flags;
	u_int32_t sre_length;
	u_int16_t sre_error;
	sctp_assoc_t sre_assoc_id;
	u_int8_t sre_data[];
};

SCTP_SEND_FAILED

When a message cannot be delivered to a peer, the message is sent back to the user through this notification. This notification is usually soon followed by an association failure notification. In most cases, the only way a message will not be delivered is if the association has failed. The only time a message failure will occur without an association failure is when the partial reliability extension of SCTP is being used.

When an error notification is sent, the following format will be read by the application:

struct sctp_send_failed {
	u_int16_t ssf_type;
	u_int16_t ssf_flags;
	u_int32_t ssf_length;
	u_int32_t ssf_error;
	struct sctp_sndrcvinfo ssf_info;
	sctp_assoc_t ssf_assoc_id;
	u_int8_t ssf_data[];
};