Go to main content

man pages section 4: Device and Network Interfaces

Exit Print View

Updated: Wednesday, July 27, 2022



sdp - Sockets Direct Protocol driver


#include <socket.h>
#include <netinet/in.h>


The Sockets Direct Protocol (SDP) is a transport protocol layered over the Infiniband Transport Framework (IBTF). SDP is a standard implementation based on Annex 4 of the Infiniband Architecture Specification Vol 1 and provides reliable byte-stream, flow controlled two-way data transmission that closely mimics the Transmission Control Protocol (TCP).

SDP supports a sockets-based SOCK_STREAM interface to application programs. It also supports graceful close (including half-closed sockets), IP addressing (IPv4 or IPv6), the connecting/accepting connect model, out-of-band (OOB) data and common socket options. The SDP protocol also supports kernel bypass data transfers and data transfers from send-upper-layer-protocol (ULP) buffers to receive ULP buffers. A SDP message includes a BSDH header followed by data. (A BSDH header advertises the amount of available buffers on the local side).

SDP networking functionality is broken into the sdp driver and a function call-based sockfs implementation. A new protocol family of PROTO_SDP is introduced to use the SDP transport provided by the driver.

Sockets utilizing SDP are either active or passive. Active sockets initiate connections to passive sockets. Both active and passive sockets must have their local IP or IPv6 address and SDP port number bound with bind(3C) after the socket is created. By default, SDP sockets are active. A passive socket is created by calling listen(3C) after binding the socket with bind(). This process establishes a queueing parameter for the passive socket. Connections to the passive socket can be received with accept(3C). Active sockets use the connect(3C) call after binding to initiate connections.

In most cases, SDP sends data when it is presented. When outstanding data is not yet acknowledged, SDP gathers small amounts of output to be sent in a single packet once an acknowledgment is received. For a small number of clients this packetization may cause significant delays. To circumvent this problem, SDP provided by the driver supplies SDP_NODELAY, a socket-level boolean option. This is similar to the TCP_NODELAY option.

SDP provides an urgent data mechanism that can be invoked using the out-of-band provisions of send(3C). The out-of-band delivery behavior is identical to TCP. The caller may mark one byte as urgent with the MSG_OOB flag to send(3C). This sets an urgent pointer pointing to the byte in the SDP stream. The receiver of the stream is notified of the urgent data by a SIGURG signal. The SIOCATMARK ioctl(2) request returns a value indicating whether the stream is at the urgent mark. Because the system never returns data across the urgent mark in a single read(2) call, it is possible to advance to the urgent data in a simple loop which reads data, testing the socket with the SIOCATMARK ioctl() request until it reaches the mark.


SDP uses IP/IPv6 addresses to refer to local and remote devices and opens a reliable connected IB connection between two end points. The sdp driver supports a point-to-point connection, however broadcasting and multicasting are not supported.


SDP supports setsockopt() and getsockopt() to set and read socket options. Very few socket options affect SDP protocol operations. Other common socket options are processed but do not affect SDP protocol operation. All socket options are checked for validity. A getsockopt() call returns the values set or toggled by setsockopt(). Socket options that affect protocol operations are SO_LINGER, SO_DEBUG, SO_REUSEADDR, and SO_OOBINLINE.



A connect() operation was attempted on a socket on which a connect() operation had already been performed.


The remote peer forced the connection to be closed. This usually occurs when the remote machine loses state information about the connection due to a crash.


The remote peer actively refused connection establishment. This usually occurs because no process is listening to the port.


A bind() operation was attempted on a socket with a network address/port pair that has already been bound to another socket.


A bind() operation was attempted on a socket with a network address for which no network interface exists.


A bind() operation was attempted with a reserved port number and the effective user ID of the process was not the privileged user.


The system ran out of memory for internal data structures.



64–bit ELF kernel module (x86).


64–bit ELF kernel module (SPARC).


See attributes(7) for descriptions of the following attribute:

x86, SPARC

See Also

read(2), accept(3C), bind(3C), connect(3C), getsockopt(3C), send(3C), socket.h(3HEAD), attributes(7), standards(7)

Infiniband Architecture Specification Vol 1– Annex 4 — November, 2002