SCTP(7P) Protocols SCTP(7P)


sctp, SCTP - Stream Control Transmission Protocol


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






SCTP is a transport protocol layered above the Internet Protocol (IP), or
the Internet Protocol Version 6 (IPv6). SCTP provides a reliable, session
oriented, flow-controlled, two-way transmission of data. It is a message-
oriented protocol and supports framing of individual messages boundaries.
An SCTP association is created between two endpoints for data transfer
which is maintained during the lifetime of the transfer. An SCTP
association is setup between two endpoints using a four-way handshake
mechanism with the use of a cookie to guard against some types of denial
of service (DoS) attacks. These endpoints may be represented by multiple
IP addresses.

An SCTP message includes a common SCTP header followed by one or more
chunks. Included in the common header is a 32-bit field which contains
the checksum (computed using CRC-32c polynomial) of the entire SCTP

SCTP transfers data payloads in the form of DATA chunks. Each DATA chunk
contains a Transmission Sequence Number (TSN), which governs the
transmission of messages and detection of loss. DATA chunk exchanges
follow the Transmission Control Protocol's (TCP) Selective ACK (SACK)
mechanism. The receiver acknowledges data by sending SACK chunks, which
not only indicate the cumulative TSN range received, but also non-
cumulative TSNs received, implying gaps in the received TSN sequence.
SACKs are sent using the delayed acknowledgment method similar to TCP,
that is, one SCTP per every other received packet with an upper bound on
the delay (when there are gaps detected the frequency is increased to one
every received packet). Flow and congestion control follow TCP
algorithms: Slow Start, Congestion Avoidance, Fast Recovery and Fast
retransmit. But unlike TCP, SCTP does not support half-close connection
and "urgent" data.

SCTP is designed to support a number of functions that are critical for
telephony signalling transport, including multi-streaming. SCTP allows
data to be partitioned into multiple streams that have the property of
independent sequenced delivery so that message loss in any one stream
only affects delivery within that stream. In many applications
(particularly telephony signalling), it is only necessary to maintain
sequencing of messages that affect some resource. Other messages may be
delivered without having to maintain overall sequence integrity. A DATA
chunk on an SCTP association contains the Stream Id/Stream Sequence
Number pair, in addition to the TSN, which is used for sequenced delivery
within a stream.

SCTP uses IP's host level addressing and adds its own per-host collection
of port addresses. The endpoints of an SCTP association are identified by
the combination of IP address(es) and an SCTP port number. By providing
the ability for an endpoint to have multiple IP addresses, SCTP supports
multi-homing, which makes an SCTP association more resilient in the
presence of network failures (assuming the network is constructed to
provided redundancy). For a multi-homed SCTP association, a single
address is used as the primary address, which is used as the destination
address for normal DATA chunk transfers. Retransmitted DATA chunks are
sent over alternate address(es) to increase the probability of reaching
the remote endpoint. Continued failure to send DATA chunks over the
primary address results in selecting an alternate address as the primary
address. Additionally, SCTP monitors the accessibility of all alternate
addresses by sending periodic "heartbeats" chunks. An SCTP association
supports multi-homing by exchanging the available list of addresses
during association setup (as part of its four-way handshake mechanism).
An SCTP endpoint is associated with a local address using the
bind(3SOCKET) call. Subsequently, the endpoint can be associated with
additional addresses using sctp_bindx(3SOCKET). By using a special value
of INADDR_ANY with IP or the unspecified address (all zeros) with IPv6 in
the bind() or sctp_bindx() calls, an endpoint can be bound to all
available IP or IPv6 addresses on the system.

SCTP uses a three-way mechanism to allow graceful shutdown, where each
endpoint has confirmation of the DATA chunks received by the remote
endpoint prior to completion of the shutdown. An Abort is provided for
error cases when an immediate shutdown is needed.

Applications can access SCTP using the socket interface as a SOCK_STREAM
(one-to-one style) or SOCK_SEQPACKET (one-to-many style) socket type.

One-to-one style socket interface supports similar semantics as sockets
for connection oriented protocols, such as TCP. Thus, a passive socket is
created by calling the listen(3SOCKET) function after binding the socket
using bind(). Associations to this passive socket can be received using
accept(3SOCKET) function. Active sockets use the connect(3SOCKET)
function after binding to initiate an association. If an active socket
is not explicitly bound, an implicit binding is performed. If an
application wants to exchange data during the association setup phase, it
should not call connect(), but use sendto(3SOCKET)/sendmsg(3SOCKET) to
implicitly initiate an association. Once an association has been
established, read(2) and write(2) can used to exchange data.
Additionally, send(3SOCKET), recv(3SOCKET), sendto(), recvfrom(3SOCKET),
sendmsg(), and recvmsg(3SOCKET) can be used.

One-to-many socket interface supports similar semantics as sockets for
connection less protocols, such as UDP (however, unlike UDP, it does not
support broadcast or multicast communications). A passive socket is
created using the listen() function after binding the socket using
bind(). An accept() call is not needed to receive associations to this
passive socket (in fact, an accept() on a one-to-many socket will fail).
Associations are accepted automatically and notifications of new
associations are delivered in recvmsg() provided notifications are
enabled. Active sockets after binding (implicitly or explicitly) need not
call connect() to establish an association, implicit associations can be
created using sendmsg()/recvmsg() or sendto()/recvfrom() calls. Such
implicit associations cannot be created using send() and recv() calls. On
an SCTP socket (one-to-one or one-to-many), an association may be
established using sendmsg(). However, if an association already exists
for the destination address specified in the msg_name member of the msg
parameter, sendmsg() must include the association id in msg_iov member of
the msg parameter (using sctp_sndrcvinfo structure) for a one-to-many
SCTP socket. If the association id is not provided, sendmsg() fails with
EADDRINUSE. On a one-to-one socket the destination information in the msg
parameter is ignored for an established association.

A one-to-one style association can be created from a one-to-many
association by branching it off using the sctp_peeloff(3SOCKET) call;
send() and recv() can be used on such peeled off associations. Calling
close(2) on a one-to-many socket will gracefully shutdown all the
associations represented by that one-to-many socket.

The sctp_sendmsg(3SOCKET) and sctp_recvmsg(3SOCKET) functions can be used
to access advanced features provided by SCTP.

SCTP provides the following socket options which are set using
setsockopt(3SOCKET) and read using getsockopt(3SOCKET). The option level
is the protocol number for SCTP, available from getprotobyname(3SOCKET).


Turn on/off any Nagle-like algorithm (similar to TCP_NODELAY).


Set the receive buffer.


Set the send buffer.


For one-to-many style socket, automatically close any association
that has been idle for more than the specified number of seconds. A
value of '0' indicates that no associations should be closed


Specify various notifications and ancillary data the user wants to


Retrieve current status information about an SCTP association.


Gather and reset per endpoint association statistics.

Example Usage:

#include <netinet/sctp.h>

struct sctp_assoc_stats stat;
int rc;

int32_t len = sizeof (stat);

* Per endpoint stats use the socket descriptor for sctp association.

/* Gather per endpoint association statistics */
rc = getsockopt(sd, IPPROTO_SCTP, SCTP_GET_ASSOC_STATS, &stat, &len);

Extract from the modified header file:


* SCTP socket option used to read per endpoint association statistics.

* A socket user request reads local per endpoint association stats.
* All stats are counts except sas_maxrto, which is the max value
* since the last user request for stats on this endpoint.
typedef struct sctp_assoc_stats {
uint64_t sas_rtxchunks; /* Retransmitted Chunks */
uint64_t sas_gapcnt; /* Gap Acknowledgements Received */
uint64_t sas_maxrto; /* Maximum Observed RTO this period */
uint64_t sas_outseqtsns; /* TSN received > next expected */
uint64_t sas_osacks; /* SACKs sent */
uint64_t sas_isacks; /* SACKs received */
uint64_t sas_octrlchunks; /* Control chunks sent - no dups */
uint64_t sas_ictrlchunks; /* Control chunks received - no dups */
uint64_t sas_oodchunks; /* Ordered data chunks sent */
uint64_t sas_iodchunks; /* Ordered data chunks received */
uint64_t sas_ouodchunks; /* Unordered data chunks sent */
uint64_t sas_iuodchunks; /* Unordered data chunks received */
uint64_t sas_idupchunks; /* Dups received (ordered+unordered) */
} sctp_assoc_stats_t;


The ability of SCTP to use multiple addresses in an association can
create issues with some network utilities. This requires a system
administrator to be careful in setting up the system.

For example, the tcpd allows an administrator to use a simple form of
address/hostname access control. While tcpd can work with SCTP, the
access control part can have some problems. The tcpd access control is
only based on one of the addresses at association setup time. Once as
association is allowed, no more checking is performed. This means that
during the life time of the association, SCTP packets from different
addresses of the peer host can be received in the system. This may not be
what the system administrator wants as some of the peer's addresses are
supposed to be blocked.

Another example is the use of IP Filter, which provides several functions
such as IP packet filtering (ipf(1M)) and NAT ipnat(1M)). For packet
filtering, one issue is that a filter policy can block packets from some
of the addresses of an association while allowing packets from other
addresses to go through. This can degrade SCTP's performance when failure
occurs. There is a more serious issue with IP address rewrite by NAT. At
association setup time, SCTP endpoints exchange IP addresses. But IP
Filter is not aware of this. So when NAT is done on a packet, it may
change the address to an unacceptable one. Thus the SCTP association
setup may succeed but packets cannot go through afterwards when a
different IP address is used for the association.


ipf(1M), ipnat(1M), ndd(1M), ioctl(2), close(2), read(2), write(2),
accept(3SOCKET), bind(3SOCKET), connect(3SOCKET),
getprotobyname(3SOCKET), getsockopt(3SOCKET), libsctp(3LIB),
listen(3SOCKET), recv(3SOCKET), recvfrom(3SOCKET), recvmsg(3SOCKET),
sctp_bindx(3SOCKET), sctp_getladdrs(3SOCKET), sctp_getpaddrs(3SOCKET),
sctp_freepaddrs(3SOCKET), sctp_opt_info(3SOCKET), sctp_peeloff(3SOCKET),
sctp_recvmsg(3SOCKET), sctp_sendmsg(3SOCKET), send(3SOCKET),
sendmsg(3SOCKET), sendto(3SOCKET), socket(3SOCKET), ipfilter(5), tcp(7P),
udp(7P), inet(7P), inet6(7P), ip(7P), ip6(7P)

R. Stewart, Q. Xie, K. Morneault, C. Sharp, H. Schwarzbauer, T. Taylor,
I. Rytina, M. Kalla, L. Zang, V. Paxson, RFC 2960, Stream Control
Transmission Protocol, October 2000

L. Ong, J. Yoakum, RFC 3286, An Introduction to Stream Control
Transmission Protocol (SCTP), May 2002

J. Stone, R. Stewart, D. Otis, RFC 3309, Stream Control Transmission
Protocol (SCTP) Checksum Change, September 2002.


A socket operation may fail if:

The socket type is other than SOCK_STREAM and

An association was dropped due to excessive

The remote peer refused establishing an association.

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 an
invalid network address.

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

July 30, 2009 SCTP(7P)