Manual Pages  — SCTP_SEND

If no space is available at the socket to hold the message to be transmitted, then sctp_send() normally blocks, unless the socket has been placed in non-blocking I/O mode. The select(2) system call may be used to determine when it is possible to send more data on one-to-one type (SOCK_STREAM) sockets.

The sinfo structure is used to control various SCTP features and has the following format:

struct sctp_sndrcvinfo {
        uint16_t sinfo_stream;  /* Stream sending to */
        uint16_t sinfo_ssn;     /* valid for recv only */
        uint16_t sinfo_flags;   /* flags to control sending */
        uint32_t sinfo_ppid;    /* ppid field */
        uint32_t sinfo_context; /* context field */
        uint32_t sinfo_timetolive; /* timetolive for PR-SCTP */
        uint32_t sinfo_tsn;        /* valid for recv only */
        uint32_t sinfo_cumtsn;     /* valid for recv only */
        sctp_assoc_t sinfo_assoc_id; /* The association id */
};

The sinfo->sinfo_ppid argument is an opaque 32 bit value that is passed transparently through the stack to the peer endpoint. It will be available on reception of a message (see sctp_recvmsg(3)). Note that the stack passes this value without regard to byte order.

The sinfo->sinfo_flags argument may include one or more of the following:

#define SCTP_EOF          0x0100        /* Start a shutdown procedures */
#define SCTP_ABORT        0x0200        /* Send an ABORT to peer */
#define SCTP_UNORDERED    0x0400        /* Message is un-ordered */
#define SCTP_ADDR_OVER    0x0800        /* Override the primary-address */
#define SCTP_SENDALL      0x1000        /* Send this on all associations */
                                        /* for the endpoint */
/* The lower byte is an enumeration of PR-SCTP policies */
#define SCTP_PR_SCTP_TTL  0x0001        /* Time based PR-SCTP */
#define SCTP_PR_SCTP_BUF  0x0002        /* Buffer based PR-SCTP */
#define SCTP_PR_SCTP_RTX  0x0003        /* Number of retransmissions based PR-SCTP */

The flag SCTP_EOF is used to instruct the SCTP stack to queue this message and then start a graceful shutdown of the association. All remaining data in queue will be sent after which the association will be shut down.

SCTP_ABORT is used to immediately terminate an association. An abort is sent to the peer and the local TCB is destroyed.

SCTP_UNORDERED is used to specify that the message being sent has no specific order and should be delivered to the peer application as soon as possible. When this flag is absent messages are delivered in order within the stream they are sent, but without respect to order to peer streams.

The flag SCTP_ADDR_OVER is used to specify that a specific address should be used. Normally SCTP will use only one of a multi-homed peers addresses as the primary address to send to. By default, no matter what the to argument is, this primary address is used to send data. By specifying this flag, the user is asking the stack to ignore the primary address and instead use the specified address not only as a lookup mechanism to find the association but also as the actual address to send to.

For a one-to-many type (SOCK_SEQPACKET) socket the flag SCTP_SENDALL can be used as a convenient way to make one send call and have all associations that are under the socket get a copy of the message. Note that this mechanism is quite efficient and makes only one actual copy of the data which is shared by all the associations for sending.

The remaining flags are used for the partial reliability extension (RFC3758) and will only be effective if the peer endpoint supports this extension. This option specifies what local policy the local endpoint should use in skipping data. If none of these options are set, then data is never skipped over.

SCTP_PR_SCTP_TTL is used to indicate that a time based lifetime is being applied to the data. The sinfo->sinfo_timetolive argument is then a number of milliseconds for which the data is attempted to be transmitted. If that many milliseconds elapse and the peer has not acknowledged the data, the data will be skipped and no longer transmitted. Note that this policy does not even assure that the data will ever be sent. In times of a congestion with large amounts of data being queued, the sinfo->sinfo_timetolive may expire before the first transmission is ever made.

The SCTP_PR_SCTP_BUF based policy transforms the sinfo->sinfo_timetolive field into a total number of bytes allowed on the outbound send queue. If that number or more bytes are in queue, then other buffer-based sends are looked to be removed and skipped. Note that this policy may also result in the data never being sent if no buffer based sends are in queue and the maximum specified by timetolive bytes is in queue.

The SCTP_PR_SCTP_RTX policy transforms the sinfo->sinfo_timetolive into a number of retransmissions to allow. This policy always assures that at a minimum one send attempt is made of the data. After which no more than sinfo->sinfo_timetolive retransmissions will be made before the data is skipped.

sinfo->sinfo_stream is the SCTP stream that you wish to send the message on. Streams in SCTP are reliable (or partially reliable) flows of ordered messages.

The sinfo->sinfo_assoc_id field is used to select the association to send to on a one-to-many socket. For a one-to-one socket, this field is ignored.

The sinfo->sinfo_context field is used only in the event the message cannot be sent. This is an opaque value that the stack retains and will give to the user when a failed send is given if that notification is enabled (see sctp(4)). Normally a user process can use this value to index some application specific data structure when a send cannot be fulfilled.

The flags argument holds the same meaning and values as those found in sendmsg(2) but is generally ignored by SCTP.

The fields sinfo->sinfo_ssn, sinfo->sinfo_tsn, and sinfo->sinfo_cumtsn are used only when receiving messages and are thus ignored by sctp_send(). The function sctp_sendx() has the same properties as sctp_send() with the additional arguments of an array of sockaddr structures passed in. With the addrs argument being given as an array of addresses to be sent to and the addrcnt argument indicating how many socket addresses are in the passed in array. Note that all of the addresses will only be used when an implicit association is being set up. This allows the user the equivalent behavior as doing a sctp_connectx() followed by a sctp_send() to the association. Note that if the sinfo->sinfo_assoc_id field is 0, then the first address will be used to look up the association in place of the association id. If both an address and an association id are specified, the association id has priority.