Unix Network Programming Episode 61

SCTP Socket Options

The relatively large number of socket options for SCTP (17 at present writing) reflects the finer grain of control SCTP provides to the application developer. We specify the level as IPPROTO_SCTP.

Several options used to get information about SCTP require that data be passed into the kernel (e.g., association ID and/or peer address).

SCTP_ADAPTION_LAYER Socket Option

During association initialization, either endpoint may specify an adaption layer indication. This indication is a 32-bit unsigned integer that can be used by the two applications to coordinate any local application adaption layer. This option allows the caller to fetch or set the adaption layer indication that this endpoint will provide to peers.

SCTP_ASSOCINFO Socket Option

The SCTP_ASSOCINFO socket option can be used for three purposes: (i) to retrieve information about an existing association, (ii) to change the parameters of an existing association, and/or (iii) to set defaults for future associations. When retrieving information about an existing association, the sctp_opt_info function should be used instead of getsockopt. This option takes as input the sctp_assocparams structure.

struct sctp_assocparams {
sctp_assoc_t sasoc_assoc_id;
u_int16_t sasoc_asocmaxrxt;
u_int16_t sasoc_number_peer_destinations;
u_int32_t sasoc_peer_rwnd;
u_int32_t sasoc_local_rwnd;
u_int32_t sasoc_cookie_life;
};

These fields have the following meaning:

• sasoc_assoc_id holds the identification for the association of interest. If this value is set to 0 when calling the setsockopt function, then sasoc_asocmaxrxt and sasoc_cookie_life represent values that are to be set as defaults on the socket. Calling getsockopt will return association-specific information if the association ID is supplied; otherwise, if this field is 0, the default endpoint settings will be returned.
• sasoc_asocmaxrxt holds the maximum number of retransmissions an association will make without acknowledgment before giving up, reporting the peer unusable and closing the association.
• sasoc_number_peer_destinations holds the number of peer destination addresses. It cannot be set, only retrieved.
• sasoc_peer_rwnd holds the peer’s current calculated receive window. This value represents the total number of data bytes that can yet be sent. This field is dynamic; as the local endpoint sends data, this value decreases. As the remote application reads data that has been received, this value increases. This value cannot be changed by this socket option call.
• sasoc_local_rwnd represents the local receive window the SCTP stack is currently reporting to the peer. This value is dynamic as well and is influenced by the SO_SNDBUF socket option. This value cannot be changed by this socket option call.
• sasoc_cookie_life represents the number of milliseconds for which a cookie, given to a remote peer, is valid. Each state cookie sent to a peer has a lifetime associated with it to prevent replay attacks. The default value of 60,000 milliseconds can be changed by setting this option with a sasoc_assoc_id value of 0.

SCTP_AUTOCLOSE Socket Option

This option allows us to fetch or set the autoclose time for an SCTP endpoint. The autoclose time is the number of seconds an SCTP association will remain open when idle. Idle is defined by the SCTP stack as neither endpoint sending or receiving user data. The default is for the autoclose function to be disabled.

SCTP_DEFAULT_SEND_PARAM Socket Option

SCTP has many optional send parameters that are often passed as ancillary data or used with the sctp_sendmsg function call (which is often implemented as a library call that passes ancillary data for the user). An application that wishes to send a large number of messages, all with the same parameters, can use this option to set up the default parameters and thus avoid using ancillary data or the sctp_sendmsg call. This option takes as input the sctp_sndrcvinfo structure.

struct sctp_sndrcvinfo {
u_int16_t sinfo_stream;
u_int16_t sinfo_ssn;
u_int16_t sinfo_flags;
u_int32_t sinfo_ppid;
u_int32_t sinfo_context;
u_int32_t sinfo_timetolive;
u_int32_t sinfo_tsn;
u_int32_t sinfo_cumtsn;
sctp_assoc_t sinfo_assoc_id;
};

These fields are defined as follows:

• sinfo_stream specifies the new default stream to which all messages will be sent.
• sinfo_ssn 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 stream sequence number (SSN) field in the SCTP DATA chunk.
• sinfo_flags dictates the default flags to apply to all future message sends. Allowable flag values can be found in Figure 7.16.