Scott Bennett bennett at
Wed Jun 3 14:07:49 UTC 2009

     This may seem to some like beating a dead horse, but SCTP really is
coming to the Internet.  It just looks too useful to die like OSI did.  The
more I find out about it, the more it looks like a really good match for
tor.  In fact, it looks like it might be something from a network-
application-developer heaven.  It has been available in FreeBSD since
7.0-RELEASE and was officially announced as being a fully supported part
of FreeBSD when 7.2-RELEASE was made available about a month ago.  More
features will be supported in 8.0-RELEASE whenever that comes out, too.
     There must be similar progress in LINUX kernel development, but I
don't follow that very much, so I'd be grateful if a few of the LINUX users
on this list were to describe the current support status of SCTP in the
various distributions of LINUX.  Sooner or later, even Micro$lop will have
to release it for its miserable excuses for operating systems, too.
     For those on this list who know even less about it so far than I do,
please see the sctp(4) man page from 7.2 below.  It's in serious need of
proofreading, but I'm sure those who are interested can struggle through
it if they can handle what often appears on this list. :)  If this stuff
isn't your bag, just skip the rest of the message.

                                  Scott Bennett, Comm. ASMELG, CFIAG
* Internet:       bennett at                              *
* "A well regulated and disciplined militia, is at all times a good  *
* objection to the introduction of that bane of all free governments *
* -- a standing army."                                               *
*    -- Gov. John Hancock, New York Journal, 28 January 1790         *

SCTP(4)                FreeBSD 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
     connections 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 abstrac-
     tion 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 host.

     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 dynammi-
             cally 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(2):

     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)

FreeBSD 7.2                    December 15, 2006                   FreeBSD 7.2

More information about the tor-talk mailing list