1sctp(7P) Protocols sctp(7P)
2
6 sctp, SCTP - Stream Control Transmission Protocol
7
9 #include <sys/socket.h>
10 #include <netinet/in.h>
11 s = socket(AF_INET, SOCK_STREAM, IPPROTO_SCTP);
13 s = socket(AF_INET, SOCK_SEQPACKET, IPPROTO_SCTP);
16 s = socket(AF_INET6, SOCK_STREAM, IPPROTO_SCTP);
19 s = socket(AF_INET6, SOCK_SEQPACKET, IPPROTO_SCTP);
22
25 SCTP is a transport protocol layered above the Internet Protocol (IP),
26 or the Internet Protocol Version 6 (IPv6). SCTP provides a reliable,
27 session oriented, flow-controlled, two-way transmission of data. It is
28 a message- oriented protocol and supports framing of individual mes‐
29 sages boundaries. An SCTP association is created between two endpoints
30 for data transfer which is maintained during the lifetime of the trans‐
31 fer. An SCTP association is setup between two endpoints using a four-
32 way handshake mechanism with the use of a cookie to guard against some
33 types of denial of service (DoS) attacks. These endpoints may be repre‐
34 sented by multiple IP addresses.
35 An SCTP message includes a common SCTP header followed by one or more
38 chunks. Included in the common header is a 32-bit field which contains
39 the checksum (computed using CRC-32c polynomial) of the entire SCTP
40 packet.
41 SCTP transfers data payloads in the form of DATA chunks. Each DATA
44 chunk contains a Transmission Sequence Number (TSN), which governs the
45 transmission of messages and detection of loss. DATA chunk exchanges
46 follow the Transmission Control Protocol's (TCP) Selective ACK (SACK)
47 mechanism. The receiver acknowledges data by sending SACK chunks, which
48 not only indicate the cumulative TSN range received, but also non-cumu‐
49 lative TSNs received, implying gaps in the received TSN sequence. SACKs
50 are sent using the delayed acknowledgment method similar to TCP, that
51 is, one SCTP per every other received packet with an upper bound on the
52 delay (when there are gaps detected the frequency is increased to one
53 every received packet). Flow and congestion control follow TCP algo‐
54 rithms: Slow Start, Congestion Avoidance, Fast Recovery and Fast
55 retransmit. But unlike TCP, SCTP does not support half-close connection
56 and "urgent" data.
57 SCTP is designed to support a number of functions that are critical for
60 telephony signalling transport, including multi-streaming. SCTP allows
61 data to be partitioned into multiple streams that have the property of
62 independent sequenced delivery so that message loss in any one stream
63 only affects delivery within that stream. In many applications (partic‐
64 ularly telephony signalling), it is only necessary to maintain sequenc‐
65 ing of messages that affect some resource. Other messages may be deliv‐
66 ered without having to maintain overall sequence integrity. A DATA
67 chunk on an SCTP association contains the Stream Id/Stream Sequence
68 Number pair, in addition to the TSN, which is used for sequenced deliv‐
69 ery within a stream.
70 SCTP uses IP's host level addressing and adds its own per-host collec‐
73 tion of port addresses. The endpoints of an SCTP association are iden‐
74 tified by the combination of IP address(es) and an SCTP port number. By
75 providing the ability for an endpoint to have multiple IP addresses,
76 SCTP supports multi-homing, which makes an SCTP association more
77 resilient in the presence of network failures (assuming the network is
78 constructed to provided redundancy). For a multi-homed SCTP associa‐
79 tion, a single address is used as the primary address, which is used as
80 the destination address for normal DATA chunk transfers. Retransmitted
81 DATA chunks are sent over alternate address(es) to increase the proba‐
82 bility of reaching the remote endpoint. Continued failure to send DATA
83 chunks over the primary address results in selecting an alternate
84 address as the primary address. Additionally, SCTP monitors the acces‐
85 sibility of all alternate addresses by sending periodic "heartbeats"
86 chunks. An SCTP association supports multi-homing by exchanging the
87 available list of addresses during association setup (as part of its
88 four-way handshake mechanism). An SCTP endpoint is associated with a
89 local address using the bind(3SOCKET) call. Subsequently, the endpoint
90 can be associated with additional addresses using sctp_bindx(3SOCKET).
91 By using a special value of INADDR_ANY with IP or the unspecified
92 address (all zeros) with IPv6 in the bind() or sctp_bindx() calls, an
93 endpoint can be bound to all available IP or IPv6 addresses on the sys‐
94 tem.
95 SCTP uses a three-way mechanism to allow graceful shutdown, where each
98 endpoint has confirmation of the DATA chunks received by the remote
99 endpoint prior to completion of the shutdown. An Abort is provided for
100 error cases when an immediate shutdown is needed.
101 Applications can access SCTP using the socket interface as a
104 SOCK_STREAM (one-to-one style) or SOCK_SEQPACKET (one-to-many style)
105 socket type.
106 One-to-one style socket interface supports similar semantics as sockets
109 for connection oriented protocols, such as TCP. Thus, a passive socket
110 is created by calling the listen(3SOCKET) function after binding the
111 socket using bind(). Associations to this passive socket can be
112 received using accept(3SOCKET) function. Active sockets use the con‐
113 nect(3SOCKET) function after binding to initiate an association. If an
114 active socket is not explicitly bound, an implicit binding is per‐
115 formed. If an application wants to exchange data during the association
116 setup phase, it should not call connect(), but use
117 sendto(3SOCKET)/sendmsg(3SOCKET) to implicitly initiate an association.
118 Once an association has been established, read(2) and write(2) can used
119 to exchange data. Additionally, send(3SOCKET), recv(3SOCKET), sendto(),
120 recvfrom(3SOCKET), sendmsg(), and recvmsg(3SOCKET) can be used.
121 One-to-many socket interface supports similar semantics as sockets for
124 connection less protocols, such as UDP (however, unlike UDP, it does
125 not support broadcast or multicast communications). A passive socket
126 is created using the listen() function after binding the socket using
127 bind(). An accept() call is not needed to receive associations to this
128 passive socket (in fact, an accept() on a one-to-many socket will
129 fail). Associations are accepted automatically and notifications of new
130 associations are delivered in recvmsg() provided notifications are
131 enabled. Active sockets after binding (implicitly or explicitly) need
132 not call connect() to establish an association, implicit associations
133 can be created using sendmsg()/recvmsg() or sendto()/recvfrom() calls.
134 Such implicit associations cannot be created using send() and recv()
135 calls. On an SCTP socket (one-to-one or one-to-many), an association
136 may be established using sendmsg(). However, if an association already
137 exists for the destination address specified in the msg_name member of
138 the msg parameter, sendmsg() must include the association id in msg_iov
139 member of the msg parameter (using sctp_sndrcvinfo structure) for a
140 one-to-many SCTP socket. If the association id is not provided,
141 sendmsg() fails with EADDRINUSE. On a one-to-one socket the destination
142 information in the msg parameter is ignored for an established associa‐
143 tion.
144 A one-to-one style association can be created from a one-to-many asso‐
147 ciation by branching it off using the sctp_peeloff(3SOCKET) call;
148 send() and recv() can be used on such peeled off associations. Calling
149 close(2) on a one-to-many socket will gracefully shutdown all the asso‐
150 ciations represented by that one-to-many socket.
151 The sctp_sendmsg(3SOCKET) and sctp_recvmsg(3SOCKET) functions can be
154 used to access advanced features provided by SCTP.
155 SCTP provides the following socket options which are set using setsock‐
158 opt(3SOCKET) and read using getsockopt(3SOCKET). The option level is
159 the protocol number for SCTP, available from getprotobyname(3SOCKET).
160 SCTP_NODELAY
162 Turn on/off any Nagle-like algorithm (similar to TCP_NODELAY).
164 SO_RCVBUF
167 Set the receive buffer.
169 SO_SNDBUF
172 Set the send buffer.
174 SCTP_AUTOCLOSE
177 For one-to-many style socket, automatically close any association
179 that has been idle for more than the specified number of seconds. A
180 value of '0' indicates that no associations should be closed auto‐
181 matically.
182 SCTP_EVENTS
185 Specify various notifications and ancillary data the user wants to
187 receive.
188 SCTP_STATUS
191 Retrieve current status information about an SCTP association.
193 SCTP_GET_ASSOC_STATS
196 Gather and reset per endpoint association statistics.
198 Example Usage:
202 #include <netinet/sctp.h>
204 struct sctp_assoc_stats stat;
206 int rc;
207 int32_t len = sizeof (stat);
209 /*
211 * Per endpoint stats use the socket descriptor for sctp association.
212 */
213 /* Gather per endpoint association statistics */
215 rc = getsockopt(sd, IPPROTO_SCTP, SCTP_GET_ASSOC_STATS, &stat, &len);
216 Extract from the modified header file:
220 sctp.h
222 /*
224 * SCTP socket option used to read per endpoint association statistics.
225 */
226 #define SCTP_GET_ASSOC_STATS 24
227 /*
229 * A socket user request reads local per endpoint association stats.
230 * All stats are counts except sas_maxrto, which is the max value
231 * since the last user request for stats on this endpoint.
232 */
233 typedef struct sctp_assoc_stats {
234 uint64_t sas_rtxchunks; /* Retransmitted Chunks */
235 uint64_t sas_gapcnt; /* Gap Acknowledgements Received */
236 uint64_t sas_maxrto; /* Maximum Observed RTO this period */
237 uint64_t sas_outseqtsns; /* TSN received > next expected */
238 uint64_t sas_osacks; /* SACKs sent */
239 uint64_t sas_isacks; /* SACKs received */
240 uint64_t sas_octrlchunks; /* Control chunks sent - no dups */
241 uint64_t sas_ictrlchunks; /* Control chunks received - no dups */
242 uint64_t sas_oodchunks; /* Ordered data chunks sent */
243 uint64_t sas_iodchunks; /* Ordered data chunks received */
244 uint64_t sas_ouodchunks; /* Unordered data chunks sent */
245 uint64_t sas_iuodchunks; /* Unordered data chunks received */
246 uint64_t sas_idupchunks; /* Dups received (ordered+unordered) */
247 } sctp_assoc_stats_t;
248
251 The ability of SCTP to use multiple addresses in an association can
252 create issues with some network utilities. This requires a system
253 administrator to be careful in setting up the system.
254 For example, the tcpd allows an administrator to use a simple form of
257 address/hostname access control. While tcpd can work with SCTP, the
258 access control part can have some problems. The tcpd access control is
259 only based on one of the addresses at association setup time. Once as
260 association is allowed, no more checking is performed. This means that
261 during the life time of the association, SCTP packets from different
262 addresses of the peer host can be received in the system. This may not
263 be what the system administrator wants as some of the peer's addresses
264 are supposed to be blocked.
265 Another example is the use of IP Filter, which provides several func‐
268 tions such as IP packet filtering (ipf(1M)) and NAT ipnat(1M)). For
269 packet filtering, one issue is that a filter policy can block packets
270 from some of the addresses of an association while allowing packets
271 from other addresses to go through. This can degrade SCTP's performance
272 when failure occurs. There is a more serious issue with IP address re‐
273 write by NAT. At association setup time, SCTP endpoints exchange IP
274 addresses. But IP Filter is not aware of this. So when NAT is done on a
275 packet, it may change the address to an unacceptable one. Thus the SCTP
276 association setup may succeed but packets cannot go through afterwards
277 when a different IP address is used for the association.
278
280 ipf(1M), ipnat(1M), ndd(1M), ioctl(2), close(2), read(2), write(2),
281 accept(3SOCKET), bind(3SOCKET), connect(3SOCKET), getprotoby‐
282 name(3SOCKET), getsockopt(3SOCKET), libsctp(3LIB), listen(3SOCKET),
283 recv(3SOCKET), recvfrom(3SOCKET), recvmsg(3SOCKET),
284 sctp_bindx(3SOCKET), sctp_getladdrs(3SOCKET), sctp_getpaddrs(3SOCKET),
285 sctp_freepaddrs(3SOCKET), sctp_opt_info(3SOCKET),
286 sctp_peeloff(3SOCKET), sctp_recvmsg(3SOCKET), sctp_sendmsg(3SOCKET),
287 send(3SOCKET), sendmsg(3SOCKET), sendto(3SOCKET), socket(3SOCKET),
288 ipfilter(5), tcp(7P), udp(7P), inet(7P), inet6(7P), ip(7P), ip6(7P)
289 R. Stewart, Q. Xie, K. Morneault, C. Sharp, H. Schwarzbauer, T. Taylor,
292 I. Rytina, M. Kalla, L. Zang, V. Paxson, RFC 2960, Stream Control
293 Transmission Protocol, October 2000
294 L. Ong, J. Yoakum, RFC 3286, An Introduction to Stream Control Trans‐
297 mission Protocol (SCTP), May 2002
298 J. Stone, R. Stewart, D. Otis, RFC 3309, Stream Control Transmission
301 Protocol (SCTP) Checksum Change, September 2002.
302
304 A socket operation may fail if:
305 EPROTONOSUPPORT The socket type is other than SOCK_STREAM and
307 SOCK_SEQPACKET.
308 ETIMEDOUT An association was dropped due to excessive retrans‐
311 missions.
312 ECONNREFUSED The remote peer refused establishing an association.
315 EADDRINUSE A bind() operation was attempted on a socket with a
318 network address/port pair that has already been
319 bound to another socket.
320 EINVAL A bind() operation was attempted on a socket with an
323 invalid network address.
324 EPERM A bind() operation was attempted on a socket with a
327 "reserved" port number and the effective user ID of
328 the process was not the privileged user.
329SunOS 5.11 30 Jul 2009 sctp(7P)