Unix Network Programming Episode 62

• sinfo_pid provides the default value to use when setting the SCTP payload protocol identifier in all data transmissions.
• sinfo_context specifies the default value to place in the sinfo_context field, which is provided as a local tag when messages that could not be sent to a peer are retrieved.
• sinfo_timetolive dictates the default lifetime that will be applied to all message sends. The lifetime field is used by SCTP stacks to know when to discard an outgoing message due to excessive delay (prior to its first transmission). If the two endpoints support the partial reliability option, then the lifetime is also used to specify how long a message is valid after its first transmission.
• sinfo_tsn is ignored when setting the default options. When receiving a message with the recvmsg function or sctp_recvmsg function, this field will hold the value the peer placed in the transport sequence number (TSN) field in the SCTP DATA chunk.
• sinfo_cumtsn is ignored when setting the default options. When receiving a message with the recvmsg function or sctp_recvmsg function, this field will hold the current cumulative TSN the local SCTP stack has associated with its remote peer.
• sinfo_assoc_id specifies the association identification that the requester wishes the default parameters to be set against. For one-to-one sockets, this field is ignored.

SCTP_DISABLE_FRAGMENTS Socket Option

SCTP normally fragments any user message that does not fit in a single SCTP packet into multiple DATA chunks. Setting this option disables this behavior on the sender. When disabled by this option, SCTP will return the error EMSGSIZE and not send the message. The default behavior is for this option to be disabled; SCTP will normally fragment user messages.

SCTP_EVENTS Socket Option

This socket option allows a caller to fetch, enable, or disable various SCTP notifications. An SCTP notification is a message that the SCTP stack will send to the application. The message is read as normal data, with the msg_flags field of the recvmsg function being set to MSG_NOTIFICATION. An application that is not prepared to use either recvmsg or sctp_recvmsg should not enable events. Eight different types of events can be subscribed to by using this option and passing an sctp_event_subscribe structure. Any value of 0 represents a non-subscription and a value of 1 represents a subscription.

The sctp_event_subscribe structure takes the following form:

struct sctp_event_subscribe {
	u_int8_t sctp_data_io_event;
	u_int8_t sctp_association_event;
	u_int8_t sctp_address_event;
	u_int8_t sctp_send_failure_event;
	u_int8_t sctp_peer_error_event;
	u_int8_t sctp_shutdown_event;
	u_int8_t sctp_partial_delivery_event;
	u_int8_t sctp_adaption_layer_event;
};

SCTP_GET_PEER_ADDR_INFO Socket Option

This option retrieves information about a peer address, including the congestion window, smoothed RTT and MTU. This option may only be used to retrieve information about a specific peer address. The caller provides a sctp_paddrinfo structure with the spinfo_address field filled in with the peer address of interest, and should use sctp_opt_info instead of getsockopt for maximum portability. The sctp_paddrinfo structure has the following format:

struct sctp_paddrinfo {
	sctp_assoc_t spinfo_assoc_id;
	struct sockaddr_storage spinfo_address;
	int32_t spinfo_state;
	u_int32_t spinfo_cwnd;
	u_int32_t spinfo_srtt;
	u_int32_t spinfo_rto;
	u_int32_t spinfo_mtu;
}

The data returned to the caller provides the following:

• spinfo_assoc_id contains association identification information, also provided in the “communication up” notification (SCTP_COMM_UP). This unique value can be used as a shorthand method to represent the association for almost all SCTP operations.
• spinfo_address is set by the caller to inform the SCTP socket on which address to return information. On return, its value should be unchanged.
• spinfo_state holds one or more of the values seen in Figure 7.18.
• spinfo_cwnd represents the current congestion window recorded for the peer address. A description of how the the cwnd value is managed can be found on page 177 of [Stewart and Xie 2001].
• spinfo_srtt represents the current estimate of the smoothed RTT for this address.
• spinfo_rto represents the current retransmission timeout in use for this address.
• spinfo_mtu represents the current path MTU as discovered by path MTU discovery.

You can learn more options in this book.