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SCTP(4)			 BSD Kernel Interfaces Manual		       SCTP(4)

     sctp — Internet Stream Control Transmission Protocol

     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netinet/sctp.h>



     The SCTP protocol provides reliable, flow-controlled, two-way transmis‐
     sion of data.  It is a message oriented protocol and can support the
     SOCK_STREAM and SOCK_SEQPACKET abstractions.  SCTP uses the standard
     Internet address format and, in addition, provides a per-host collection
     of “port addresses”.  Thus, each address is composed of an Internet
     address specifying the host and network, with a specific SCTP port on the
     host identifying the peer entity.

     There are two models of programming in SCTP.  The first uses the
     SOCK_STREAM abstraction.  In this abstraction sockets utilizing the SCTP
     protocol are either “active” or “passive”.	 Active sockets initiate con‐
     nections to passive sockets.  By default, SCTP sockets are created
     active; to create a passive socket, the listen(2) system call must be
     used after binding the socket with the bind(2) or sctp_bindx(3) system
     calls.  Only passive sockets may use the accept(2) call to accept incom‐
     ing connections.  Only active sockets may use the connect(2) call to ini‐
     tiate connections.

     The other abstraction SOCK_SEQPACKET provides a “connectionless” mode of
     operation in that the user may send to an address (using any of the valid
     send calls that carry a socket address) and an association will be setup
     implicitly by the underlying SCTP transport stack.	 This abstraction is
     the only one capable of sending data on the third leg of the four-way
     handshake.	 A user must still call listen(2) to allow the socket to
     accept connections.  Calling listen(2) however does not restrict the user
     from still initiating implicit connections to other peers.

     The SCTP protocol directly supports multi-homing.	So when binding a
     socket with the “wildcard” address INADDR_ANY, the SCTP stack will inform
     the peer about all of the local addresses that are deemed in scope of the
     peer.  The peer will then possibly have multiple paths to reach the local

     The SCTP transport protocol is also multi-streamed.  Multi-streaming
     refers to the ability to send sub-ordered flows of messages.  A user per‐
     forms this by specifying a specific stream in one of the extended send
     calls such as the sctp_send(3) function call.  Sending messages on dif‐
     ferent streams will allow parallel delivery of data i.e., a message loss
     in stream 1 will not block the delivery of messages sent in stream 2.

     The SCTP transport protocol also provides a unordered service as well.
     The unordered service allows a message to be sent and delivered with no
     regard to the ordering of any other message.

     The FreeBSD implementation of SCTP also supports the following exten‐

     sctp partial reliability This extension allows one to have message be
	     skipped and not delivered based on some user specified parame‐

     sctp dynamic addressing This extension allows addresses to be added and
	     deleted dynamically from an existing association.

     sctp authentication This extension allows the user to authenticate spe‐
	     cific peer chunks (including data) to validate that the peer who
	     sent the message is in fact the peer who setup the association.
	     A shared key option is also provided for so that two stacks can
	     pre-share keys.

     packet drop Some routers support a special satellite protocol that will
	     report losses due to corruption.  This allows retransmissions
	     without subsequent loss in bandwidth utilization.

     stream reset This extension allows a user on either side to reset the
	     stream sequence numbers used by any or all streams.

     SCTP supports a number of socket options which can be set with
     setsockopt(2) and tested with getsockopt(2) or sctp_opt_info(3):

     SCTP_NODELAY		 Under most circumstances, SCTP sends data
				 when it is presented; when outstanding data
				 has not yet been acknowledged, it gathers
				 small amounts of output to be sent in a sin‐
				 gle packet once an acknowledgement is
				 received.  For some clients, such as window
				 systems that send a stream of mouse events
				 which receive no replies, this packetization
				 may cause significant delays.	The boolean
				 option SCTP_NODELAY defeats this algorithm.

     SCTP_RTOINFO		 This option returns specific information
				 about an associations “Retransmission Time
				 Out”.	It can also be used to change the
				 default values.

     SCTP_ASSOCINFO		 This option returns specific information
				 about the requested association.

     SCTP_INITMSG		 This option allows you to get or set the
				 default sending parameters when an associa‐
				 tion is implicitly setup.  It allows you to
				 change such things as the maxium number of
				 streams allowed inbound and the number of
				 streams requested of the peer.

     SCTP_AUTOCLOSE		 For the one-to-many model (SOCK_SEQPACKET)
				 associations are setup implicitly.  This
				 option allows the user to specify a default
				 number of idle seconds to allow the associa‐
				 tion be maintained.  After the idle timer
				 (where no user message have been sent or have
				 been received from the peer) the association
				 will be gracefully closed.  The default for
				 this value is 0, or unlimited (i.e., no auto‐
				 matic close).

     SCTP_SET_PEER_PRIMARY_ADDR	 The dynamic address extension allows a peer
				 to also request a particular address of its
				 be made into the primary address.  This
				 option allows the caller to make such a
				 request to a peer.  Note that if the peer
				 does not also support the dynamic address
				 extension, this call will fail.  Note the
				 caller must provide a valid local address
				 that the peer has been told about during
				 association setup or dynamically.

     SCTP_PRIMARY_ADDR		 This option allows the setting of the primary
				 address that the caller wishes to send to.
				 The caller provides the address of a peer
				 that is to be made primary.

     SCTP_ADAPTATION_LAYER	 The dynamic address extension also allows a
				 user to pass a 32 bit opaque value upon asso‐
				 ciation setup.	 This option allows a user to
				 set or get this value.

     SCTP_DISABLE_FRAGMENTS	 By default SCTP will fragment user messages
				 into multiple pieces that will fit on the
				 network and then later, upon reception,
				 reassemble the pieces into a single user mes‐
				 sage.	If this option is enabled instead, any
				 send that exceeds the path maximum transfer
				 unit (P-MTU) will fail and the message will
				 NOT be sent.

     SCTP_PEER_ADDR_PARAMS	 This option will allow a user to set or get
				 specific peer address parameters.

     SCTP_DEFAULT_SEND_PARAM	 When a user does not use one of the extended
				 send calls (e.g., sctp_sendmsg(3)) a set of
				 default values apply to each send.  These
				 values include things like the stream number
				 to send to as well as the per-protocol id.
				 This option lets a caller both get and set
				 these values.	If the user changes these
				 default values, then these new values will be
				 used as the default whenever no information
				 is provided by the sender (i.e., the non-
				 extended API is used).

     SCTP_EVENTS		 SCTP has non-data events that it can communi‐
				 cate to its application.  By default these
				 are all disabled since they arrive in the
				 data path with a special flag
				 MSG_NOTIFICATION set upon the received mes‐
				 sage.	This option lets a caller both get
				 what events are current being received as
				 well as set different events that they may be
				 interested in receiving.

     SCTP_I_WANT_MAPPED_V4_ADDR	 SCTP supports both IPV4 and IPV6.  An associ‐
				 ation may span both IPV4 and IPV6 addresses
				 since SCTP is multi-homed.  By default, when
				 opening an IPV6 socket, when data arrives on
				 the socket from a peer's V4 address the V4
				 address  will be presented with an address
				 family of AF_INET.  If this is undesireable,
				 then this option can be enabled which will
				 then convert all V4 addresses into mapped V6

     SCTP_MAXSEG		 By default SCTP chooses its message fragmen‐
				 tation point based upon the smallest P-MTU of
				 the peer.  This option lets the caller set it
				 to a smaller value.  Note that while the user
				 can change this value, if the P-MTU is
				 smaller than the value set by the user, then
				 the P-MTU value will override any user set‐

     SCTP_DELAYED_ACK_TIME	 This option lets the user both set and get
				 the delayed ack time (in milliseconds) that
				 SCTP is using.	 The default is 200 millisec‐

				 SCTP at times may need to start delivery of a
				 very large message before the entire message
				 has arrived.  By default SCTP waits until the
				 incoming message is larger than one fourth of
				 the receive buffer.  This option allows the
				 stacks value to be overridden with a smaller

     SCTP_FRAGMENT_INTERLEAVE	 SCTP at times will start partial delivery (as
				 mentioned above).  In the normal case succes‐
				 sive reads will continue to return the rest
				 of the message, blocking if needed, until all
				 of that message is read.  However this means
				 other messages may have arrived and be ready
				 for delivery and be blocked behind the mes‐
				 sage being partially delivered.  If this
				 option is enabled, when a partial delivery
				 message has no more data to be received, then
				 a subsequent read may return a different mes‐
				 sage that is ready for delivery.  By default
				 this option is off since the user must be
				 using the extended API's to be able to tell
				 the difference between messages (via the
				 stream and stream sequence number).

     SCTP_AUTH_CHUNK		 By default only the dynamic addressing chunks
				 are authenticated.  This option lets a user
				 request an additional chunk be authenticated
				 as well.  Note that successive calls to this
				 option will work and continue to add more
				 chunks that require authentication.  Note
				 that this option only effects future associa‐
				 tions and not existing ones.

     SCTP_AUTH_KEY		 This option allows a user to specify a shared
				 key that can be later used to authenticate a

     SCTP_HMAC_IDENT		 This option will let you get or set the list
				 of HMAC algorithms used to authenticate
				 peers.	 Note that the HMAC values are in pri‐
				 ority order where the first HMAC identifier
				 is the most prefered and the last is the
				 least prefered.

     SCTP_AUTH_ACTIVE_KEY	 This option allows you to make a key active
				 for the generation of authentication informa‐
				 tion.	Note that the peer must have the same
				 key or else the data will be discarded.

     SCTP_AUTH_DELETE_KEY	 This option allows you to delete an old key.

     SCTP_USE_EXT_RECVINFO	 The sockets api document allows an extended
				 send/receive information structure to be
				 used.	The extended structure includes addi‐
				 tional fields related to the next message to
				 be received (after the current receive com‐
				 pletes) if such information is known.	By
				 default the system will not pass this infor‐
				 mation.  This option allows the user to
				 request this information.

     SCTP_AUTO_ASCONF		 By default when bound to all address and the
				 system administrator has enables automatic
				 dynamic addresses, the SCTP stack will auto‐
				 matically generate address changes into add
				 and delete requests to any peers by setting
				 this option to true.  This option allows an
				 endpoint to disable that behavior.

     SCTP_MAXBURST		 By default SCTP implements micro-burst con‐
				 trol so that as the congestion window opens
				 up no large burst of packets can be gener‐
				 ated.	The default burst limit is four.  This
				 option lets the user change this value.

     SCTP_CONTEXT		 Many sctp extended calls have a context
				 field.	 The context field is a 32 bit opaque
				 value that will be returned in send failures.
				 This option lets the caller set the default
				 context value to use when none is provided by
				 the user.

     SCTP_EXPLICIT_EOR		 By default, a single send is a complete mes‐
				 sage.	SCTP generates an implied record
				 boundary.  If this option is enabled, then
				 all sends are part of the same message until
				 the user indicates an end of record with the
				 special flag SCTP_EOR passed in the
				 sctp_sndrcvinfo flags field.  This effec‐
				 tively makes all sends part of the same mes‐
				 sage until the user specifices differently.
				 This means that a caller must NOT change the
				 stream number until after the SCTP_EOR is
				 passed to SCTP else an error will be

     SCTP_STATUS		 This option is a read only option that
				 returns various status information about the
				 specified association.

     SCTP_GET_PEER_ADDR_INFO	 This read only option returns information
				 about a peer address.

     SCTP_PEER_AUTH_CHUNKS	 This read only option returns a list of the
				 chunks the peer requires to be authenticated.

     SCTP_LOCAL_AUTH_CHUNKS	 This read only option returns a list of the
				 locally required chunks that must be authen‐

     SCTP_RESET_STREAMS		 This socket option is used to cause a stream
				 sequence number or all stream sequence num‐
				 bers to be reset.  Note that the peer SCTP
				 endpoint must also support the stream reset
				 extension as well.

     accept(2), bind(2), connect(2), listen(2), sctp_bindx(3),
     sctp_connectx(3), sctp_opt_info(3), sctp_recvmsg(3), sctp_sendmsg(3)

BSD			       December 15, 2006			   BSD

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