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Network Working Group                                         R. Stewart
Internet-Draft                                       Cisco Systems, Inc.
Expires: December 11, 2006                                        Q. Xie
                                                          Motorola, Inc.
                                                              L. Yarroll
                                                            TimeSys Corp
                                                                 K. Poon
                                                  Sun Microsystems, Inc.
                                                               M. Tuexen
                                      Univ. of Applied Sciences Muenster
                                                            June 9, 2006


 Sockets API Extensions for Stream Control Transmission Protocol (SCTP)
                   draft-ietf-tsvwg-sctpsocket-13.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   http://www.ietf.org/ietf/1id-abstracts.txt.

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   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on December 11, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document describes a mapping of the Stream Control Transmission



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   Protocol SCTP RFC2960 [RFC2960] into a sockets API.  The benefits of
   this mapping include compatibility for TCP applications, access to
   new SCTP features and a consolidated error and event notification
   scheme.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Conventions  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Data Types . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  one-to-many style Interface  . . . . . . . . . . . . . . . . .  6
     3.1.  Basic Operation  . . . . . . . . . . . . . . . . . . . . .  6
       3.1.1.  socket() - one-to-many style socket  . . . . . . . . .  7
       3.1.2.  bind() - one-to-many style socket  . . . . . . . . . .  8
       3.1.3.  listen() - One-to-many style socket  . . . . . . . . .  9
       3.1.4.  sendmsg() and recvmsg() - one-to-many style socket . .  9
       3.1.5.  close() - one-to-many style socket . . . . . . . . . . 11
       3.1.6.  connect() - one-to-many style socket . . . . . . . . . 11
     3.2.  Implicit Association Setup . . . . . . . . . . . . . . . . 12
     3.3.  Non-blocking mode  . . . . . . . . . . . . . . . . . . . . 13
     3.4.  Special considerations . . . . . . . . . . . . . . . . . . 13
   4.  one-to-one style Interface . . . . . . . . . . . . . . . . . . 15
     4.1.  Basic Operation  . . . . . . . . . . . . . . . . . . . . . 15
       4.1.1.  socket() - one-to-one style socket . . . . . . . . . . 16
       4.1.2.  bind() - one-to-one style socket . . . . . . . . . . . 16
       4.1.3.  listen() - one-to-one style socket . . . . . . . . . . 17
       4.1.4.  accept() - one-to-one style socket . . . . . . . . . . 18
       4.1.5.  connect() - one-to-one style socket  . . . . . . . . . 18
       4.1.6.  close() - one-to-one style socket  . . . . . . . . . . 19
       4.1.7.  shutdown() - one-to-one style socket . . . . . . . . . 19
       4.1.8.  sendmsg() and recvmsg() - one-to-one style socket  . . 20
       4.1.9.  getpeername()  . . . . . . . . . . . . . . . . . . . . 20
   5.  Data Structures  . . . . . . . . . . . . . . . . . . . . . . . 21
     5.1.  The msghdr and cmsghdr Structures  . . . . . . . . . . . . 21
     5.2.  SCTP msg_control Structures  . . . . . . . . . . . . . . . 22
       5.2.1.  SCTP Initiation Structure (SCTP_INIT)  . . . . . . . . 23
       5.2.2.  SCTP Header Information Structure (SCTP_SNDRCV)  . . . 24
       5.2.3.  Extended SCTP Header Information Structure
               (SCTP_EXTRCV)  . . . . . . . . . . . . . . . . . . . . 27
     5.3.  SCTP Events and Notifications  . . . . . . . . . . . . . . 31
       5.3.1.  SCTP Notification Structure  . . . . . . . . . . . . . 31
     5.4.  Ancillary Data Considerations and Semantics  . . . . . . . 41
       5.4.1.  Multiple Items and Ordering  . . . . . . . . . . . . . 41
       5.4.2.  Accessing and Manipulating Ancillary Data  . . . . . . 42
       5.4.3.  Control Message Buffer Sizing  . . . . . . . . . . . . 42
   6.  Common Operations for Both Styles  . . . . . . . . . . . . . . 43
     6.1.  send(), recv(), sendto(), recvfrom() . . . . . . . . . . . 43



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     6.2.  setsockopt(), getsockopt() . . . . . . . . . . . . . . . . 44
     6.3.  read() and write() . . . . . . . . . . . . . . . . . . . . 45
     6.4.  getsockname()  . . . . . . . . . . . . . . . . . . . . . . 45
   7.  Socket Options . . . . . . . . . . . . . . . . . . . . . . . . 46
     7.1.  Read / Write Options . . . . . . . . . . . . . . . . . . . 48
       7.1.1.  Retransmission Timeout Parameters (SCTP_RTOINFO) . . . 48
       7.1.2.  Association Parameters (SCTP_ASSOCINFO)  . . . . . . . 48
       7.1.3.  Initialization Parameters (SCTP_INITMSG) . . . . . . . 50
       7.1.4.  SO_LINGER  . . . . . . . . . . . . . . . . . . . . . . 50
       7.1.5.  SCTP_NODELAY . . . . . . . . . . . . . . . . . . . . . 50
       7.1.6.  SO_RCVBUF  . . . . . . . . . . . . . . . . . . . . . . 51
       7.1.7.  SO_SNDBUF  . . . . . . . . . . . . . . . . . . . . . . 51
       7.1.8.  Automatic Close of associations (SCTP_AUTOCLOSE) . . . 51
       7.1.9.  Set Peer Primary Address
               (SCTP_SET_PEER_PRIMARY_ADDR) . . . . . . . . . . . . . 51
       7.1.10. Set Primary Address (SCTP_PRIMARY_ADDR)  . . . . . . . 52
       7.1.11. Set Adaptation Layer Indicator
               (SCTP_ADAPTATION_LAYER)  . . . . . . . . . . . . . . . 52
       7.1.12. Enable/Disable message fragmentation
               (SCTP_DISABLE_FRAGMENTS) . . . . . . . . . . . . . . . 52
       7.1.13. Peer Address Parameters (SCTP_PEER_ADDR_PARAMS)  . . . 53
       7.1.14. Set default send parameters
               (SCTP_DEFAULT_SEND_PARAM)  . . . . . . . . . . . . . . 55
       7.1.15. Set notification and ancillary events (SCTP_EVENTS)  . 56
       7.1.16. Set/clear IPv4 mapped addresses
               (SCTP_I_WANT_MAPPED_V4_ADDR) . . . . . . . . . . . . . 56
       7.1.17. Set the maximum fragmentation size (SCTP_MAXSEG) . . . 56
       7.1.18. Add a chunk that must be authenticated
               (SCTP_AUTH_CHUNK)  . . . . . . . . . . . . . . . . . . 56
       7.1.19. Get or set the list of supported HMAC Identifiers
               (SCTP_HMAC_IDENT)  . . . . . . . . . . . . . . . . . . 57
       7.1.20. Set a shared key (SCTP_AUTH_KEY) . . . . . . . . . . . 57
       7.1.21. Get or set the active shared key
               (SCTP_AUTH_ACTIVE_KEY) . . . . . . . . . . . . . . . . 58
       7.1.22. Delete a shared key (SCTP_AUTH_DELETE_KEY) . . . . . . 58
       7.1.23. Get or set delayed ack timer
               (SCTP_DELAYED_ACK_TIME)  . . . . . . . . . . . . . . . 59
       7.1.24. Get or set fragmented interleave
               (SCTP_FRAGMENT_INTERLEAVE) . . . . . . . . . . . . . . 59
       7.1.25. Set or Get the sctp partial delivery point
               (SCTP_PARTIAL_DELIVERY_POINT)  . . . . . . . . . . . . 60
       7.1.26. Set or Get the use of extended receive info
               (SCTP_USE_EXT_RCVINFO) . . . . . . . . . . . . . . . . 60
       7.1.27. Set or Get the auto asconf flag (SCTP_AUTO_ASCONF) . . 60
       7.1.28. Set or Get the maximum burst (SCTP_MAX_BURST)  . . . . 61
       7.1.29. Set or Get the default context (SCTP_CONTEXT)  . . . . 61
     7.2.  Read-Only Options  . . . . . . . . . . . . . . . . . . . . 61
       7.2.1.  Association Status (SCTP_STATUS) . . . . . . . . . . . 61



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       7.2.2.  Peer Address Information (SCTP_GET_PEER_ADDR_INFO) . . 62
       7.2.3.  Get the list of chunks the peer requires to be
               authenticated (SCTP_PEER_AUTH_CHUNKS)  . . . . . . . . 63
       7.2.4.  Get the list of chunks the local endpoint requires
               to be authenticated (SCTP_LOCAL_AUTH_CHUNKS) . . . . . 64
       7.2.5.  Get the list of current associations
               (SCTP_GET_ASOC_ID_LIST)  . . . . . . . . . . . . . . . 64
     7.3.  Ancillary Data and Notification Interest Options . . . . . 65
   8.  New Interfaces . . . . . . . . . . . . . . . . . . . . . . . . 67
     8.1.  sctp_bindx() . . . . . . . . . . . . . . . . . . . . . . . 67
     8.2.  Branched-off Association . . . . . . . . . . . . . . . . . 69
     8.3.  sctp_getpaddrs() . . . . . . . . . . . . . . . . . . . . . 69
     8.4.  sctp_freepaddrs()  . . . . . . . . . . . . . . . . . . . . 70
     8.5.  sctp_getladdrs() . . . . . . . . . . . . . . . . . . . . . 70
     8.6.  sctp_freeladdrs()  . . . . . . . . . . . . . . . . . . . . 71
     8.7.  sctp_sendmsg() . . . . . . . . . . . . . . . . . . . . . . 71
     8.8.  sctp_recvmsg() . . . . . . . . . . . . . . . . . . . . . . 71
     8.9.  sctp_connectx()  . . . . . . . . . . . . . . . . . . . . . 72
     8.10. sctp_send()  . . . . . . . . . . . . . . . . . . . . . . . 73
     8.11. sctp_sendx() . . . . . . . . . . . . . . . . . . . . . . . 73
     8.12. sctp_getaddrlen  . . . . . . . . . . . . . . . . . . . . . 74
   9.  Preprocessor Constants . . . . . . . . . . . . . . . . . . . . 75
   10. IANA considerations  . . . . . . . . . . . . . . . . . . . . . 75
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 75
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 76
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 76
     13.1. Normative references . . . . . . . . . . . . . . . . . . . 76
     13.2. Informational References . . . . . . . . . . . . . . . . . 77
   Appendix A.  one-to-one style Code Example . . . . . . . . . . . . 77
   Appendix B.  one-to-many style Code Example  . . . . . . . . . . . 82
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 84
   Intellectual Property and Copyright Statements . . . . . . . . . . 86



















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1.  Introduction

   The sockets API has provided a standard mapping of the Internet
   Protocol suite to many operating systems.  Both TCP RFC793 [RFC0793]
   and UDP RFC768 [RFC0768] have benefited from this standard
   representation and access method across many diverse platforms.  SCTP
   is a new protocol that provides many of the characteristics of TCP
   but also incorporates semantics more akin to UDP.  This document
   defines a method to map the existing sockets API for use with SCTP,
   providing both a base for access to new features and compatibility so
   that most existing TCP applications can be migrated to SCTP with few
   (if any) changes.

   There are three basic design objectives:

   1)  Maintain consistency with existing sockets APIs:
      We define a sockets mapping for SCTP that is consistent with other
      sockets API protocol mappings (for instance, UDP, TCP, IPv4, and
      IPv6).
   2)  Support a one-to-many style interface
      This set of semantics is similar to that defined for connection-
      less protocols, such as UDP.  A one-to-many style SCTP socket
      should be able to control multiple SCTP associations.  This is
      similar to an UDP socket, which can communicate with many peer end
      points.  Each of these associations is assigned an association ID
      so that an applications can use the ID to differentiate them.
      Note that SCTP is connection-oriented in nature, and it does not
      support broadcast or multicast communications, as UDP does.
   3)  Support a one-to-one style interface
      This interface supports a similar semantics as sockets for
      connection-oriented protocols, such as TCP.  A one-to-one style
      SCTP socket should only control one SCTP association.
      One purpose of defining this interface is to allow existing
      applications built on other connection-oriented protocols be
      ported to use SCTP with very little effort.  And developers
      familiar with those semantics can easily adapt to SCTP.  Another
      purpose is to make sure that existing mechanisms in most OSes to
      deal with socket, such as select(), should continue to work with
      this style of socket.
      Extensions are added to this mapping to provide mechanisms to
      exploit new features of SCTP.

   Goals 2 and 3 are not compatible, so in this document we define two
   modes of mapping, namely the one-to-many style mapping and the one-
   to-one style mapping.  These two modes share some common data
   structures and operations, but will require the use of two different
   application programming styles.  Note that all new SCTP features can
   be used with both styles of socket.  The decision on which one to use



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   depends mainly on the nature of applications.

   A mechanism is defined to extract a one-to-many style SCTP
   association into a one-to-one style socket.

   Some of the SCTP mechanisms cannot be adequately mapped to existing
   socket interface.  In some cases, it is more desirable to have new
   interface instead of using existing socket calls.  Section 8 of this
   document describes those new interface.


2.  Conventions

2.1.  Data Types

   Whenever possible, data types from Draft 6.6 (March 1997) of POSIX
   1003.1g are used: uintN_t means an unsigned integer of exactly N bits
   (e.g., uint16_t).  We also assume the argument data types from
   1003.1g when possible (e.g., the final argument to setsockopt() is a
   size_t value).  Whenever buffer sizes are specified, the POSIX 1003.1
   size_t data type is used.


3.  one-to-many style Interface

   The one-to-many style interface has the following characteristics:

   A) Outbound association setup is implicit.

   B) Messages are delivered in complete messages (with one notable
   exception).

   C) There is a 1 to MANY relationship between socket and association.

3.1.  Basic Operation

   A typical server in this style uses the following socket calls in
   sequence to prepare an endpoint for servicing requests:

   1.  socket()
   2.  bind()
   3.  listen()
   4.  recvmsg()
   5.  sendmsg()
   6.  close()

   A typical client uses the following calls in sequence to setup an
   association with a server to request services:



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   1.  socket()
   2.  sendmsg()
   3.  recvmsg()
   4.  close()

   In this style, by default, all the associations connected to the
   endpoint are represented with a single socket.  Each associations is
   assigned an association ID (type is sctp_assoc_t) so that an
   application can use it to differentiate between them.  In some
   implementations, the peer endpoints addresses can also be used for
   this purpose.  But this is not required for performance reasons.  If
   an implementation does not support using addresses to differentiate
   between different associations, the sendto() call can only be used to
   setup an association implicitly.  It cannot be used to send data to
   an established association as the association ID cannot be specified.

   Once as association ID is assigned to an SCTP association, that ID
   will not be reused until the application explicitly terminates the
   association.  The resources belonged to that association will not be
   freed until that happens.  This is similar to the close() operation
   on a normal socket.  The only exception is when the SCTP_AUTOCLOSE
   option (section 7.1.8) is set.  In this case, after the association
   is terminated gracefully automatically, the association ID assigned
   to it can be reused.  All applications using this option should be
   aware of this to avoid the possible problem of sending data to an
   incorrect peer end point.

   If the server or client wishes to branch an existing association off
   to a separate socket, it is required to call sctp_peeloff() and in
   the parameter specifies the association identification.  The
   sctp_peeloff() call will return a new socket which can then be used
   with recv() and send() functions for message passing.  See
   Section 8.2 for more on branched-off associations.

   Once an association is branched off to a separate socket, it becomes
   completely separated from the original socket.  All subsequent
   control and data operations to that association must be done through
   the new socket.  For example, the close operation on the original
   socket will not terminate any associations that have been branched
   off to a different socket.

   We will discuss the one-to-many style socket calls in more details in
   the following subsections.

3.1.1.  socket() - one-to-many style socket

   Applications use socket() to create a socket descriptor to represent
   an SCTP endpoint.



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   The syntax is,

   sd = socket(PF_INET, SOCK_SEQPACKET, IPPROTO_SCTP);

   or,

   sd = socket(PF_INET6, SOCK_SEQPACKET, IPPROTO_SCTP);

   Here, SOCK_SEQPACKET indicates the creation of a one-to-many style
   socket.

   The first form creates an endpoint which can use only IPv4 addresses,
   while, the second form creates an endpoint which can use both IPv6
   and IPv4 addresses.

3.1.2.  bind() - one-to-many style socket

   Applications use bind() to specify which local address the SCTP
   endpoint should associate itself with.

   An SCTP endpoint can be associated with multiple addresses.  To do
   this, sctp_bindx() is introduced in section Section 8.1 to help
   applications do the job of associating multiple addresses.

   These addresses associated with a socket are the eligible transport
   addresses for the endpoint to send and receive data.  The endpoint
   will also present these addresses to its peers during the association
   initialization process, see RFC2960 [RFC2960].

   After calling bind(), if the endpoint wishes to accept new
   associations on the socket, it must call listen() (see section
   Section 3.1.3).

   The syntax of bind() is,

   ret = bind(int sd, struct sockaddr *addr, socklen_t addrlen);

   sd - the socket descriptor returned by socket().
   addr - the address structure (struct sockaddr_in or struct
      sockaddr_in6 RFC2553 [RFC2553]).
   addrlen - the size of the address structure.

   If sd is an IPv4 socket, the address passed must be an IPv4 address.
   If the sd is an IPv6 socket, the address passed can either be an IPv4
   or an IPv6 address.

   Applications cannot call bind() multiple times to associate multiple
   addresses to an endpoint.  After the first call to bind(), all



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   subsequent calls will return an error.

   If addr is specified as a wildcard (INADDR_ANY for an IPv4 address,
   or as IN6ADDR_ANY_INIT or in6addr_any for an IPv6 address), the
   operating system will associate the endpoint with an optimal address
   set of the available interfaces.

   If a bind() is not called prior to a sendmsg() call that initiates a
   new association, the system picks an ephemeral port and will choose
   an address set equivalent to binding with a wildcard address.  One of
   those addresses will be the primary address for the association.
   This automatically enables the multi-homing capability of SCTP.

3.1.3.  listen() - One-to-many style socket

   By default, new associations are not accepted for one-to-many style
   sockets.  An application uses listen() to mark a socket as being able
   to accept new associations.  The syntax is,


      int listen(int sd, int backlog);


   sd - the socket descriptor of the endpoint.
   backlog - if backlog is non-zero, enable listening else disable
      listening.

   Note that one-to-many style socket consumers do not need to call
   accept to retrieve new associations.  Calling accept() on a one-to-
   many style socket should return EOPNOTSUPP.  Rather, new associations
   are accepted automatically, and notifications of the new associations
   are delivered via recvmsg() with the SCTP_ASSOC_CHANGE event (if
   these notifications are enabled).  Clients will typically not call
   listen(), so that they can be assured that the only associations on
   the socket will be ones they actively initiated.  Server or peer-to-
   peer sockets, on the other hand, will always accept new associations,
   so a well-written application using server one-to-many style sockets
   must be prepared to handle new associations from unwanted peers.

   Also note that the SCTP_ASSOC_CHANGE event provides the association
   ID for a new association, so if applications wish to use the
   association ID as input to other socket calls, they should ensure
   that the SCTP_ASSOC_CHANGE event is enabled.

3.1.4.  sendmsg() and recvmsg() - one-to-many style socket

   An application uses sendmsg() and recvmsg() call to transmit data to
   and receive data from its peer.



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   ssize_t sendmsg(int sd, const struct msghdr *message, int flags);

   ssize_t recvmsg(int sd, struct msghdr *message, int flags);

   sd - the socket descriptor of the endpoint.
   message: pointer to the msghdr structure which contains a single user
      message and possibly some ancillary data.  See Section 5 for
      complete description of the data structures.
   flags - No new flags are defined for SCTP at this level.  See Section
      5 for SCTP-specific flags used in the msghdr structure.

   As we will see in Section 5, along with the user data, the ancillary
   data field is used to carry the sctp_sndrcvinfo and/or the
   sctp_initmsg structures to perform various SCTP functions including
   specifying options for sending each user message.  Those options,
   depending on whether sending or receiving, include stream number,
   stream sequence number, various flags, context and payload protocol
   Id, etc.

   When sending user data with sendmsg(), the msg_name field in msghdr
   structure will be filled with one of the transport addresses of the
   intended receiver.  If there is no association existing between the
   sender and the intended receiver, the sender's SCTP stack will set up
   a new association and then send the user data (see Section 3.2 for
   more on implicit association setup).

   If a peer sends a SHUTDOWN, a SCTP_SHUTDOWN_EVENT notification will
   be delivered if that notification has been enabled, and no more data
   can be sent to that association.  Any attempt to send more data will
   cause sendmsg() to return with an ESHUTDOWN error.  Note that the
   socket is still open for reading at this point so it is possible to
   retrieve notifications.

   When receiving a user message with recvmsg(), the msg_name field in
   msghdr structure will be populated with the source transport address
   of the user data.  The caller of recvmsg() can use this address
   information to determine to which association the received user
   message belongs.  Note that if SCTP_ASSOC_CHANGE events are disabled,
   applications must use the peer transport address provided in the
   msg_name field by recvmsg() to perform correlation to an association,
   since they will not have the association ID.

   If all data in a single message has been delivered, MSG_EOR will be
   set in the msg_flags field of the msghdr structure (see section
   Section 5.1).

   If the application does not provide enough buffer space to completely
   receive a data message, MSG_EOR will not be set in msg_flags.



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   Successive reads will consume more of the same message until the
   entire message has been delivered, and MSG_EOR will be set.

   If the SCTP stack is running low on buffers, it may partially deliver
   a message.  In this case, MSG_EOR will not be set, and more calls to
   recvmsg() will be necessary to completely consume the message.  Only
   one message at a time per stream can be partially delivered.

   Note, if the socket is a branched-off socket that only represents one
   association (see Section 3.1), the msg_name field can be used to
   override the primary address when sending data.

3.1.5.  close() - one-to-many style socket

   Applications use close() to perform graceful shutdown (as described
   in Section 10.1 of RFC2960 [RFC2960]) on ALL the associations
   currently represented by a one-to-many style socket.

   The syntax is:


      ret = close(int sd);


   sd - the socket descriptor of the associations to be closed.

   To gracefully shutdown a specific association represented by the one-
   to-many style socket, an application should use the sendmsg() call,
   and including the SCTP_EOF flag.  A user may optionally terminate an
   association non-gracefully by sending with the SCTP_ABORT flag and
   possibly passing a user specified abort code in the data field.  Both
   flags SCTP_EOF and SCTP_ABORT are passed with ancillary data (see
   Section 5.2.2) in the sendmsg call.

   If sd in the close() call is a branched-off socket representing only
   one association, the shutdown is performed on that association only.

3.1.6.  connect() - one-to-many style socket

   An application may use the connect() call in the one-to-many style to
   initiate an association without sending data.

   The syntax is:

   ret = connect(int sd, const struct sockaddr *nam, socklen_t len);






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   sd - the socket descriptor to have a new association added to.
   nam - the address structure (either struct sockaddr_in or struct
      sockaddr_in6 defined in RFC2553 [RFC2553]).
   len - the size of the address.
      Multiple connect() calls can be made on the same socket to create
      multiple associations.  This is different from the semantics of
      connect() on a UDP socket.

3.2.  Implicit Association Setup

   Once the bind() call is complete on a one-to-many style socket, the
   application can begin sending and receiving data using the sendmsg()/
   recvmsg() or sendto()/recvfrom() calls, without going through any
   explicit association setup procedures (i.e., no connect() calls
   required).

   Whenever sendmsg() or sendto() is called and the SCTP stack at the
   sender finds that there is no association existing between the sender
   and the intended receiver (identified by the address passed either in
   the msg_name field of msghdr structure in the sendmsg() call or the
   dest_addr field in the sendto() call), the SCTP stack will
   automatically setup an association to the intended receiver.

   Upon the successful association setup a SCTP_COMM_UP notification
   will be dispatched to the socket at both the sender and receiver
   side.  This notification can be read by the recvmsg() system call
   (see Section 3.1.3).

   Note, if the SCTP stack at the sender side supports bundling, the
   first user message may be bundled with the COOKIE ECHO message
   RFC2960 [RFC2960].

   When the SCTP stack sets up a new association implicitly, it first
   consults the sctp_initmsg structure, which is passed along within the
   ancillary data in the sendmsg() call (see Section 5.2.1 for details
   of the data structures), for any special options to be used on the
   new association.

   If this information is not present in the sendmsg() call, or if the
   implicit association setup is triggered by a sendto() call, the
   default association initialization parameters will be used.  These
   default association parameters may be set with respective
   setsockopt() calls or be left to the system defaults.

   Implicit association setup cannot be initiated by send()/recv()
   calls.





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3.3.  Non-blocking mode

   Some SCTP users might want to avoid blocking when they call socket
   interface function.

   Once all bind() calls are complete on a one-to-many style socket, the
   application must set the non-blocking option by a fcntl() (such as
   O_NONBLOCK).  After which the sendmsg() function returns immediately,
   and the success or failure of the data message (and possible
   SCTP_INITMSG parameters) will be signaled by the SCTP_ASSOC_CHANGE
   event with SCTP_COMM_UP or CANT_START_ASSOC.  If user data could not
   be sent (due to a CANT_START_ASSOC), the sender will also receive a
   SCTP_SEND_FAILED event.  Those event(s) can be received by the user
   calling of recvmsg().  A server (having called listen()) is also
   notified of an association up event by the reception of a
   SCTP_ASSOC_CHANGE with SCTP_COMM_UP via the calling of recvmsg() and
   possibly the reception of the first data message.

   In order to shutdown the association gracefully, the user must call
   sendmsg() with no data and with the SCTP_EOF flag set.  The function
   returns immediately, and completion of the graceful shutdown is
   indicated by an SCTP_ASSOC_CHANGE notification of type
   SHUTDOWN_COMPLETE (see Section 5.3.1.1).  Note that this can also be
   done using the sctp_send() call described in Section 8.10.

   An application is recommended to use caution when using select() (or
   poll()) for writing on a one-to-many style socket.  The reason being
   that interpretation of select on write is implementation specific.
   Generally a positive return on a select on write would only indicate
   that one of the associations represented by the one-to-many socket is
   writable.  An application that writes after the select return may
   still block since the association that was writeable is not the
   destination association of the write call.  Likewise select (or
   poll()) for reading from a one-to-many socket will only return an
   indication that one of the associations represented by the socket has
   data to be read.

   An application that wishes to know that a particular association is
   ready for reading or writing should either use the one-to-one style
   or use the sctp_peeloff() (see Section 8.2) function to separate the
   association of interest from the one-to-many socket.

3.4.  Special considerations

   The fact that a one-to-many style socket can provide access to many
   SCTP associations through a single socket descriptor has important
   implications for both application programmers and system programmers
   implementing this API.  A key issue is how buffer space inside the



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   sockets layer is managed.  Because this implementation detail
   directly affects how application programmers must write their code to
   ensure correct operation and portability, this section provides some
   guidance to both implementors and application programmers.

   An important feature that SCTP shares with TCP is flow control:
   specifically, a sender may not send data faster than the receiver can
   consume it.

   For TCP, flow control is typically provided for in the sockets API as
   follows.  If the reader stops reading, the sender queues messages in
   the socket layer until it uses all of its socket buffer space
   allocation creating a "stalled connection".  Further attempts to
   write to the socket will block or return the error EAGAIN or
   EWOULDBLOCK for a non-blocking socket.  At some point, either the
   connection is closed, or the receiver begins to read again freeing
   space in the output queue.

   For one-to-one style SCTP sockets (this includes sockets descriptors
   that were separated from a one-to-many style socket with
   sctp_peeloff()) the behavior is identical.  For one-to-many style
   SCTP sockets, the fact that we have multiple associations on a single
   socket makes the situation more complicated.  If the implementation
   uses a single buffer space allocation shared by all associations, a
   single stalled association can prevent the further sending of data on
   all associations active on a particular one-to-many style socket.

   For a blocking socket, it should be clear that a single stalled
   association can block the entire socket.  For this reason,
   application programmers may want to use non-blocking one-to-many
   style sockets.  The application should at least be able to send
   messages to the non-stalled associations.

   But a non-blocking socket is not sufficient if the API implementor
   has chosen a single shared buffer allocation for the socket.  A
   single stalled association would eventually cause the shared
   allocation to fill, and it would become impossible to send even to
   non-stalled associations.

   The API implementor can solve this problem by providing each
   association with its own allocation of outbound buffer space.  Each
   association should conceptually have as much buffer space as it would
   have if it had its own socket.  As a bonus, this simplifies the
   implementation of sctp_peeloff().

   To ensure that a given stalled association will not prevent other
   non-stalled associations from being writable, application programmers
   should either:



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   (a) demand that the underlying implementation dedicates independent
      buffer space allotments to each association (as suggested above),
      or
   (b) verify that their application layer protocol does not permit
      large amounts of unread data at the receiver (this is true of some
      request-response protocols, for example), or
   (c) use one-to-one style sockets for association which may
      potentially stall (either from the beginning, or by using
      sctp_peeloff before sending large amounts of data that may cause a
      stalled condition).
      An implementation which dedicates independent buffer space for
      each association should define HAVE_SCTP_MULTIBUF to 1.


4.  one-to-one style Interface

   The goal of this style is to follow as closely as possible the
   current practice of using the sockets interface for a connection
   oriented protocol, such as TCP.  This style enables existing
   applications using connection oriented protocols to be ported to SCTP
   with very little effort.

   Note that some new SCTP features and some new SCTP socket options can
   only be utilized through the use of sendmsg() and recvmsg() calls,
   see Section 4.1.8.  Also note that some socket interfaces may not be
   able to provide data on the third leg of the association set up with
   this interface style.

4.1.  Basic Operation

   A typical server in one-to-one style uses the following system call
   sequence to prepare an SCTP endpoint for servicing requests:

   1. socket()

   2. bind()

   3. listen()

   4. accept()

   The accept() call blocks until a new association is set up.  It
   returns with a new socket descriptor.  The server then uses the new
   socket descriptor to communicate with the client, using recv() and
   send() calls to get requests and send back responses.

   Then it calls




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   5. close()

   to terminate the association.

   A typical client uses the following system call sequence to setup an
   association with a server to request services:

   1. socket()

   2. connect()

   After returning from connect(), the client uses send() and recv()
   calls to send out requests and receive responses from the server.

   The client calls

   3. close()

   to terminate this association when done.

4.1.1.  socket() - one-to-one style socket

   Applications calls socket() to create a socket descriptor to
   represent an SCTP endpoint.

   The syntax is:

     int socket(PF_INET, SOCK_STREAM, IPPROTO_SCTP);

      or,

     int socket(PF_INET6, SOCK_STREAM, IPPROTO_SCTP);

   Here, SOCK_STREAM indicates the creation of a one-to-one style
   socket.

   The first form creates an endpoint which can use only IPv4 addresses,
   while the second form creates an endpoint which can use both IPv6 and
   IPv4 addresses.

4.1.2.  bind() - one-to-one style socket

   Applications use bind() to pass an address to be associated with an
   SCTP endpoint to the system. bind() allows only either a single
   address or a IPv4 or IPv6 wildcard address to be bound.  An SCTP
   endpoint can be associated with multiple addresses.  To do this,
   sctp_bindx() is introduced in Section 8.1 to help applications do the
   job of associating multiple addresses.



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   These addresses associated with a socket are the eligible transport
   addresses for the endpoint to send and receive data.  The endpoint
   will also present these addresses to its peers during the association
   initialization process, see RFC2960 [RFC2960].

   The syntax is:

   int bind(int sd, struct sockaddr *addr, socklen_t addrlen);

   sd: the socket descriptor returned by socket() call.
   addr: the address structure (either struct sockaddr_in or struct
      sockaddr_in6 defined in RFC2553 [RFC2553]).
   addrlen: the size of the address structure.

   If sd is an IPv4 socket, the address passed must be an IPv4 address.
   Otherwise, i.e., the sd is an IPv6 socket, the address passed can
   either be an IPv4 or an IPv6 address.

   Applications cannot call bind() multiple times to associate multiple
   addresses to the endpoint.  After the first call to bind(), all
   subsequent calls will return an error.

   If addr is specified as a wildcard (INADDR_ANY for an IPv4 address,
   or as IN6ADDR_ANY_INIT or in6addr_any for an IPv6 address), the
   operating system will associate the endpoint with an optimal address
   set of the available interfaces.

   If a bind() is not called prior to the connect() call, the system
   picks an ephemeral port and will choose an address set equivalent to
   binding with a wildcard address.  One of those addresses will be the
   primary address for the association.  This automatically enables the
   multi-homing capability of SCTP.

   The completion of this bind() process does not ready the SCTP
   endpoint to accept inbound SCTP association requests.  Until a
   listen() system call, described below, is performed on the socket,
   the SCTP endpoint will promptly reject an inbound SCTP INIT request
   with an SCTP ABORT.

4.1.3.  listen() - one-to-one style socket

   Applications use listen() to ready the SCTP endpoint for accepting
   inbound associations.

   The syntax is:

   int listen(int sd, int backlog);




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   sd: the socket descriptor of the SCTP endpoint.
   backlog: this specifies the max number of outstanding associations
      allowed in the socket's accept queue.  These are the associations
      that have finished the four-way initiation handshake (see Section
      5 of RFC2960 [RFC2960]) and are in the ESTABLISHED state.  Note, a
      backlog of '0' indicates that the caller no longer wishes to
      receive new associations.

4.1.4.  accept() - one-to-one style socket

   Applications use accept() call to remove an established SCTP
   association from the accept queue of the endpoint.  A new socket
   descriptor will be returned from accept() to represent the newly
   formed association.

   The syntax is:

   new_sd = accept(int sd, struct sockaddr *addr, socklen_t *addrlen);

   new_sd - the socket descriptor for the newly formed association.
   sd - the listening socket descriptor.
   addr - on return, will contain the primary address of the peer
      endpoint.
   addrlen - on return, will contain the size of addr.

4.1.5.  connect() - one-to-one style socket

   Applications use connect() to initiate an association to a peer.

   The syntax is:

   int connect(int sd, const struct sockaddr *addr, socklen_t addrlen);

   sd - the socket descriptor of the endpoint.
   addr - the peer's address.
   addrlen - the size of the address.

   This operation corresponds to the ASSOCIATE primitive described in
   section 10.1 of RFC2960 [RFC2960].

   By default, the new association created has only one outbound stream.
   The SCTP_INITMSG option described in Section 7.1.3 should be used
   before connecting to change the number of outbound streams.

   If a bind() is not called prior to the connect() call, the system
   picks an ephemeral port and will choose an address set equivalent to
   binding with INADDR_ANY and IN6ADDR_ANY for IPv4 and IPv6 socket
   respectively.  One of those addresses will be the primary address for



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   the association.  This automatically enables the multi-homing
   capability of SCTP.

   Note that SCTP allows data exchange, similar to T/TCP RFC1644
   [RFC1644], during the association set up phase.  If an application
   wants to do this, it cannot use connect() call.  Instead, it should
   use sendto() or sendmsg() to initiate an association.  If it uses
   sendto() and it wants to change initialization behavior, it needs to
   use the SCTP_INITMSG socket option before calling sendto().  Or it
   can use SCTP_INIT type sendmsg() to initiate an association without
   doing the setsockopt().  Note that some sockets implementations may
   not support the sending of data to initiate an association with the
   one-to-one style (implementations that do not support T/TCP normally
   have this restriction).  Implementations which allow sending of data
   to initiate an association without calling connect() define the
   preprocessor constant HAVE_SCTP_NOCONNECT to 1.

   SCTP does not support half close semantics.  This means that unlike
   T/TCP, MSG_EOF should not be set in the flags parameter when calling
   sendto() or sendmsg() when the call is used to initiate a connection.
   MSG_EOF is not an acceptable flag with SCTP socket.

4.1.6.  close() - one-to-one style socket

   Applications use close() to gracefully close down an association.

   The syntax is:

      int close(int sd);

        sd      - the socket descriptor of the association to be closed.

   After an application calls close() on a socket descriptor, no further
   socket operations will succeed on that descriptor.

4.1.7.  shutdown() - one-to-one style socket

   SCTP differs from TCP in that it does not have half closed semantics.
   Hence the shutdown() call for SCTP is an approximation of the TCP
   shutdown() call, and solves some different problems.  Full TCP-
   compatibility is not provided, so developers porting TCP applications
   to SCTP may need to recode sections that use shutdown().  (Note that
   it is possible to achieve the same results as half close in SCTP
   using SCTP streams.)

   The syntax is:

      int shutdown(int sd, int how);



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   sd -  the socket descriptor of the association to be closed.
   how - Specifies the type of shutdown.  The values are as follows:
      SHUT_RD - Disables further receive operations.  No SCTP protocol
         action is taken.
      SHUT_WR - Disables further send operations, and initiates the SCTP
         shutdown sequence.
      SHUT_RDWR - Disables further send and receive operations and
         initiates the SCTP shutdown sequence.

   The major difference between SCTP and TCP shutdown() is that SCTP
   SHUT_WR initiates immediate and full protocol shutdown, whereas TCP
   SHUT_WR causes TCP to go into the half closed state.  SHUT_RD behaves
   the same for SCTP as TCP.  The purpose of SCTP SHUT_WR is to close
   the SCTP association while still leaving the socket descriptor open,
   so that the caller can receive back any data SCTP was unable to
   deliver (see Section 5.3.1.4 for more information).

   To perform the ABORT operation described in RFC2960 [RFC2960] section
   10.1, an application can use the socket option SO_LINGER.  It is
   described in Section 7.1.4.

4.1.8.  sendmsg() and recvmsg() - one-to-one style socket

   With a one-to-one style socket, the application can also use
   sendmsg() and recvmsg() to transmit data to and receive data from its
   peer.  The semantics is similar to those used in the one-to-many
   style (section Section 3.1.3), with the following differences:

   1) When sending, the msg_name field in the msghdr is not used to
   specify the intended receiver, rather it is used to indicate a
   preferred peer address if the sender wishes to discourage the stack
   from sending the message to the primary address of the receiver.  If
   the transport address given is not part of the current association,
   the data will not be sent and a SCTP_SEND_FAILED event will be
   delivered to the application if send failure events are enabled.

4.1.9.  getpeername()

   Applications use getpeername() to retrieve the primary socket address
   of the peer.  This call is for TCP compatibility, and is not multi-
   homed.  It does not work with one-to-many style sockets.  See
   Section 8.3 for a multi-homed/one-to-many style version of the call.

   The syntax is:

      int getpeername(int sd, struct sockaddr *address,
                      socklen_t *len);




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   sd - the socket descriptor to be queried.
   address - On return, the peer primary address is stored in this
      buffer.  If the socket is an IPv4 socket, the address will be
      IPv4.  If the socket is an IPv6 socket, the address will be either
      an IPv6 or IPv4 address.
   len - The caller should set the length of address here.  On return,
      this is set to the length of the returned address.

   If the actual length of the address is greater than the length of the
   supplied sockaddr structure, the stored address will be truncated.


5.  Data Structures

   We discuss in this section important data structures which are
   specific to SCTP and are used with sendmsg() and recvmsg() calls to
   control SCTP endpoint operations and to access ancillary information
   and notifications.

5.1.  The msghdr and cmsghdr Structures

   The msghdr structure used in the sendmsg() and recvmsg() calls, as
   well as the ancillary data carried in the structure, is the key for
   the application to set and get various control information from the
   SCTP endpoint.

   The msghdr and the related cmsghdr structures are defined and
   discussed in details in RFC2292 [RFC2292].  Here we will cite their
   definitions from RFC2292 [RFC2292].

   The msghdr structure:

   struct msghdr {
     void      *msg_name;        /* ptr to socket address structure */
     socklen_t  msg_namelen;     /* size of socket address structure */
     struct iovec  *msg_iov;     /* scatter/gather array */
     size_t     msg_iovlen;      /* # elements in msg_iov */
     void      *msg_control;     /* ancillary data */
     socklen_t  msg_controllen;  /* ancillary data buffer length */
     int        msg_flags;       /* flags on received message */
   };

   The cmsghdr structure:








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   struct cmsghdr {
     socklen_t  cmsg_len;   /* #bytes, including this header */
     int        cmsg_level; /* originating protocol */
     int        cmsg_type;  /* protocol-specific type */
                /* followed by unsigned char cmsg_data[]; */
     };

   In the msghdr structure, the usage of msg_name has been discussed in
   previous sections (see Section 3.1.3 and Section 4.1.8).

   The scatter/gather buffers, or I/O vectors (pointed to by the msg_iov
   field) are treated as a single SCTP data chunk, rather than multiple
   chunks, for both sendmsg() and recvmsg().

   The msg_flags are not used when sending a message with sendmsg().

   If a notification has arrived, recvmsg() will return the notification
   with the MSG_NOTIFICATION flag set in msg_flags.  If the
   MSG_NOTIFICATION flag is not set, recvmsg() will return data.  See
   Section 5.3 for more information about notifications.

   If all portions of a data frame or notification have been read,
   recvmsg() will return with MSG_EOR set in msg_flags.

5.2.  SCTP msg_control Structures

   A key element of all SCTP-specific socket extensions is the use of
   ancillary data to specify and access SCTP-specific data via the
   struct msghdr's msg_control member used in sendmsg() and recvmsg().
   Fine-grained control over initialization and sending parameters are
   handled with ancillary data.

   Each ancillary data item is proceeded by a struct cmsghdr (see
   Section 5.1), which defines the function and purpose of the data
   contained in in the cmsg_data[] member.

   There are two kinds of ancillary data used by SCTP: initialization
   data, and, header information (SNDRCV).  Initialization data (one-to-
   many style only) sets protocol parameters for new associations.
   Section 5.2.1 provides more details.  Header information can set or
   report parameters on individual messages in a stream.  See
   Section 5.2.2 for how to use SNDRCV ancillary data.

   By default on a one-to-one style socket, SCTP will pass no ancillary
   data; on a one-to-many style socket, SCTP will only pass SCTP_SNDRCV
   and SCTP_ASSOC_CHANGE information.  Specific ancillary data items can
   be enabled with socket options defined for SCTP; see Section 7.3.




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   Note that all ancillary types are fixed length; see Section 5.4 for
   further discussion on this.  These data structures use struct
   sockaddr_storage (defined in RFC2553 [RFC2553]) as a portable, fixed
   length address format.

   Other protocols may also provide ancillary data to the socket layer
   consumer.  These ancillary data items from other protocols may
   intermingle with SCTP data.  For example, the IPv6 socket API
   definitions (RFC2292 [RFC2292] and RFC2553 [RFC2553]) define a number
   of ancillary data items.  If a socket API consumer enables delivery
   of both SCTP and IPv6 ancillary data, they both may appear in the
   same msg_control buffer in any order.  An application may thus need
   to handle other types of ancillary data besides that passed by SCTP.

   The sockets application must provide a buffer large enough to
   accommodate all ancillary data provided via recvmsg().  If the buffer
   is not large enough, the ancillary data will be truncated and the
   msghdr's msg_flags will include MSG_CTRUNC.

5.2.1.  SCTP Initiation Structure (SCTP_INIT)

   This cmsghdr structure provides information for initializing new SCTP
   associations with sendmsg().  The SCTP_INITMSG socket option uses
   this same data structure.  This structure is not used for recvmsg().

   cmsg_level    cmsg_type      cmsg_data[]
   ------------  ------------   ----------------------
   IPPROTO_SCTP  SCTP_INIT      struct sctp_initmsg

   Here is the definition of the sctp_initmsg structure:

   struct sctp_initmsg {
      uint16_t sinit_num_ostreams;
      uint16_t sinit_max_instreams;
      uint16_t sinit_max_attempts;
      uint16_t sinit_max_init_timeo;
   };

   sinit_num_ostreams: 16 bits (unsigned integer)

   This is an integer number representing the number of streams that the
   application wishes to be able to send to.  This number is confirmed
   in the SCTP_COMM_UP notification and must be verified since it is a
   negotiated number with the remote endpoint.  The default value of 0
   indicates to use the endpoint default value.

   sinit_max_instreams: 16 bits (unsigned integer)




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   This value represents the maximum number of inbound streams the
   application is prepared to support.  This value is bounded by the
   actual implementation.  In other words the user MAY be able to
   support more streams than the Operating System.  In such a case, the
   Operating System limit overrides the value requested by the user.
   The default value of 0 indicates to use the endpoints default value.

   sinit_max_attempts: 16 bits (unsigned integer)

   This integer specifies how many attempts the SCTP endpoint should
   make at resending the INIT.  This value overrides the system SCTP
   'Max.Init.Retransmits' value.  The default value of 0 indicates to
   use the endpoints default value.  This is normally set to the
   system's default 'Max.Init.Retransmit' value.

   sinit_max_init_timeo: 16 bits (unsigned integer)

   This value represents the largest Time-Out or RTO value (in
   milliseconds) to use in attempting an INIT.  Normally the 'RTO.Max'
   is used to limit the doubling of the RTO upon timeout.  For the INIT
   message this value MAY override 'RTO.Max'.  This value MUST NOT
   influence 'RTO.Max' during data transmission and is only used to
   bound the initial setup time.  A default value of 0 indicates to use
   the endpoints default value.  This is normally set to the system's
   'RTO.Max' value (60 seconds).

5.2.2.  SCTP Header Information Structure (SCTP_SNDRCV)

   This cmsghdr structure specifies SCTP options for sendmsg() and
   describes SCTP header information about a received message through
   recvmsg().

   cmsg_level    cmsg_type      cmsg_data[]
   ------------  ------------   ----------------------
   IPPROTO_SCTP  SCTP_SNDRCV    struct sctp_sndrcvinfo

   Here is the definition of sctp_sndrcvinfo:

   struct sctp_sndrcvinfo {
      uint16_t sinfo_stream;
      uint16_t sinfo_ssn;
      uint16_t sinfo_flags;
      uint32_t sinfo_ppid;
      uint32_t sinfo_context;
      uint32_t sinfo_timetolive;
      uint32_t sinfo_tsn;
      uint32_t sinfo_cumtsn;
      sctp_assoc_t sinfo_assoc_id;



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   };


   sinfo_stream: 16 bits (unsigned integer)

   For recvmsg() the SCTP stack places the message's stream number in
   this value.  For sendmsg() this value holds the stream number that
   the application wishes to send this message to.  If a sender
   specifies an invalid stream number an error indication is returned
   and the call fails.

   sinfo_ssn: 16 bits (unsigned integer)

   For recvmsg() this value contains the stream sequence number that the
   remote endpoint placed in the DATA chunk.  For fragmented messages
   this is the same number for all deliveries of the message (if more
   than one recvmsg() is needed to read the message).  The sendmsg()
   call will ignore this parameter.

   sinfo_ppid: 32 bits (unsigned integer)

   This value in sendmsg() is an unsigned integer that is passed to the
   remote end in each user message.  In recvmsg() this value is the same
   information that was passed by the upper layer in the peer
   application.  Please note that the SCTP stack performs no byte order
   modification of this field.  For example, if the DATA chunk has to
   contain a given value in network byte order, the SCTP user has to
   perform the htonl() computation.

   sinfo_context: 32 bits (unsigned integer)

   This value is an opaque 32 bit context datum that is used in the
   sendmsg() function.  This value is passed back to the upper layer if
   a error occurs on the send of a message and is retrieved with each
   undelivered message (Note: if a endpoint has done multiple sends, all
   of which fail, multiple different sinfo_context values will be
   returned.  One with each user data message).

   sinfo_flags: 16 bits (unsigned integer)

   This field may contain any of the following flags and is composed of
   a bitwise OR of these values.

   recvmsg() flags:







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   SCTP_UNORDERED - This flag is present when the message was sent non-
      ordered.

   sendmsg() flags:
   SCTP_UNORDERED - This flag requests the un-ordered delivery of the
      message.  If this flag is clear the datagram is considered an
      ordered send.

   SCTP_ADDR_OVER - This flag, in the one-to-many style, requests the
      SCTP stack to override the primary destination address with the
      address found with the sendto/sendmsg call.

   SCTP_ABORT - Setting this flag causes the specified association to
      abort by sending an ABORT message to the peer (one-to-many style
      only).  The ABORT chunk will contain an error cause 'User
      Initiated Abort' with cause code 12.  The cause specific
      information of this error cause is provided in msg_iov.

   SCTP_EOF - Setting this flag invokes the SCTP graceful shutdown
      procedures on the specified association.  Graceful shutdown
      assures that all data enqueued by both endpoints is successfully
      transmitted before closing the association (one-to-many style
      only).

   SCTP_SENDALL - This flag, if set, will cause a one-to-many model
      socket to send the message to all associations that are currently
      established on this socket.  For the one-to-one socket, this flag
      has no effect.

   sinfo_timetolive: 32 bit (unsigned integer)

   For the sending side, this field contains the message time to live in
   milliseconds.  The sending side will expire the message within the
   specified time period if the message as not been sent to the peer
   within this time period.  This value will override any default value
   set using any socket option.  Also note that the value of 0 is
   special in that it indicates no timeout should occur on this message.

   sinfo_tsn: 32 bit (unsigned integer)

   For the receiving side, this field holds a TSN that was assigned to
   one of the SCTP Data Chunks.

   sinfo_cumtsn: 32 bit (unsigned integer)

   This field will hold the current cumulative TSN as known by the
   underlying SCTP layer.  Note this field is ignored when sending and
   only valid for a receive operation when sinfo_flags are set to



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   SCTP_UNORDERED.

   sinfo_assoc_id: sizeof (sctp_assoc_t)

   The association handle field, sinfo_assoc_id, holds the identifier
   for the association announced in the SCTP_COMM_UP notification.  All
   notifications for a given association have the same identifier.
   Ignored for one-to-one style sockets.

   A sctp_sndrcvinfo item always corresponds to the data in msg_iov.

5.2.3.  Extended SCTP Header Information Structure (SCTP_EXTRCV)

   This cmsghdr structure specifies SCTP options for SCTP header
   information about a received message via recvmsg().  Note that this
   structure is an extended version of SCTP_SNDRCV and will only be
   received if the user has set the socket option SCTP_USE_EXT_RCVINFO
   to true in addition to any event subscription needed to receive
   ancillary data..

   cmsg_level    cmsg_type      cmsg_data[]
   ------------  ------------   ----------------------
   IPPROTO_SCTP  SCTP_SNDRCV    struct sctp_sndrcvinfo

   Here is the definition of sctp_sndrcvinfo:

   struct sctp_extrcvinfo {
      uint16_t sinfo_stream;
      uint16_t sinfo_ssn;
      uint16_t sinfo_flags;
      uint32_t sinfo_ppid;
      uint32_t sinfo_context;
      uint32_t sinfo_timetolive;
      uint32_t sinfo_tsn;
      uint32_t sinfo_cumtsn;
      sctp_assoc_t sinfo_assoc_id;
      uint16_t next_flags;
      uint16_t next_stream;
      uint32_t next_asocid;
      uint32_t next_length;
      uint32_t next_ppid;
   };


   sinfo_stream: 16 bits (unsigned integer)

   For recvmsg() the SCTP stack places the message's stream number in
   this value.  For sendmsg() this value holds the stream number that



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   the application wishes to send this message to.  If a sender
   specifies an invalid stream number an error indication is returned
   and the call fails.

   sinfo_ssn: 16 bits (unsigned integer)

   For recvmsg() this value contains the stream sequence number that the
   remote endpoint placed in the DATA chunk.  For fragmented messages
   this is the same number for all deliveries of the message (if more
   than one recvmsg() is needed to read the message).  The sendmsg()
   call will ignore this parameter.

   sinfo_ppid: 32 bits (unsigned integer)

   This value in sendmsg() is an unsigned integer that is passed to the
   remote end in each user message.  In recvmsg() this value is the same
   information that was passed by the upper layer in the peer
   application.  Please note that the SCTP stack performs no byte order
   modification of this field.  For example, if the DATA chunk has to
   contain a given value in network byte order, the SCTP user has to
   perform the htonl() computation.

   sinfo_context: 32 bits (unsigned integer)

   This value is an opaque 32 bit context datum that is used in the
   sendmsg() function.  This value is passed back to the upper layer if
   a error occurs on the send of a message and is retrieved with each
   undelivered message (Note: if a endpoint has done multiple sends, all
   of which fail, multiple different sinfo_context values will be
   returned.  One with each user data message).

   sinfo_flags: 16 bits (unsigned integer)

   This field may contain any of the following flags and is composed of
   a bitwise OR of these values.

   recvmsg() flags:
   SCTP_UNORDERED - This flag is present when the message was sent non-
      ordered.

   sendmsg() flags:
   SCTP_UNORDERED - This flag requests the un-ordered delivery of the
      message.  If this flag is clear the datagram is considered an
      ordered send.







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   SCTP_ADDR_OVER - This flag, in the one-to-many style, requests the
      SCTP stack to override the primary destination address with the
      address found with the sendto/sendmsg call.

   SCTP_ABORT - Setting this flag causes the specified association to
      abort by sending an ABORT message to the peer (one-to-many style
      only).  The ABORT chunk will contain an error cause 'User
      Initiated Abort' with cause code 12.  The cause specific
      information of this error cause is provided in msg_iov.

   SCTP_EOF - Setting this flag invokes the SCTP graceful shutdown
      procedures on the specified association.  Graceful shutdown
      assures that all data enqueued by both endpoints is successfully
      transmitted before closing the association (one-to-many style
      only).

   SCTP_SENDALL - This flag, if set, will cause a one-to-many model
      socket to send the message to all associations that are currently
      established on this socket.  For the one-to-one socket, this flag
      has no effect.

   sinfo_timetolive: 32 bit (unsigned integer)

   For the sending side, this field contains the message time to live in
   milliseconds.  The sending side will expire the message within the
   specified time period if the message as not been sent to the peer
   within this time period.  This value will override any default value
   set using any socket option.  Also note that the value of 0 is
   special in that it indicates no timeout should occur on this message.

   sinfo_tsn: 32 bit (unsigned integer)

   For the receiving side, this field holds a TSN that was assigned to
   one of the SCTP Data Chunks.

   sinfo_cumtsn: 32 bit (unsigned integer)

   This field will hold the current cumulative TSN as known by the
   underlying SCTP layer.  Note this field is ignored when sending and
   only valid for a receive operation when sinfo_flags are set to
   SCTP_UNORDERED.

   sinfo_assoc_id: sizeof (sctp_assoc_t)

   The association handle field, sinfo_assoc_id, holds the identifier
   for the association announced in the SCTP_COMM_UP notification.  All
   notifications for a given association have the same identifier.
   Ignored for one-to-one style sockets.



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   A sctp_sndrcvinfo item always corresponds to the data in msg_iov.

   next_flags: 16 bit (unsigned integer)

   This bitmask will hold one or more of the following values:

   SCTP_NEXT_MSG_AVAIL - This bit, when set to 1, indicates that next
      message information is available i.e.: next_stream, next_asocid,
      next_length and next_ppid fields all have valid values.  If this
      bit is set to 0, then these fields are not valid and should be
      ignored.

   SCTP_NEXT_MSG_ISCOMPLETE - This bit, when set, indicates that the
      next message is completely in the receive buffer.  The next_length
      field thus contains the entire message size.  If this flag is set
      to 0, then the next_length field only contains part of the message
      size since the message is still being received (it is being
      partially delivered).
   SCTP_NEXT_MSG_IS_UNORDERED - This bit, when set, indicates that the
      next message to be received was sent by the peer as unordered.  If
      this bit is not set (i.e the bit is 0) the next message to be read
      is an ordered message in the stream specified.

   next_stream: 16 bit (unsigned integer)

   This value, when valid (see next_flags), contains the next stream
   number that will be received on a subsequent call to one of the
   receive message functions.

   next_asocid: 32 bit (unsigned integer)

   This value, when valid (see next_flags), contains the next
   association identification that will be received on a subsequent call
   to one of the receive message functions.

   next_length: 32 bit (unsigned integer)

   This value, when valid (see next_flags), contains the length of the
   next message that will be received on a subsequent call to one of the
   receive message functions.  Note that this length may be a partial
   length depending on the settings of next_flags.

   next_ppid: 32 bit (unsigned integer)

   This value, when valid (see next_flags), contains the ppid of the
   next message that will be received on a subsequent call to one of the
   receive message functions.




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5.3.  SCTP Events and Notifications

   An SCTP application may need to understand and process events and
   errors that happen on the SCTP stack.  These events include network
   status changes, association startups, remote operational errors and
   undeliverable messages.  All of these can be essential for the
   application.

   When an SCTP application layer does a recvmsg() the message read is
   normally a data message from a peer endpoint.  If the application
   wishes to have the SCTP stack deliver notifications of non-data
   events, it sets the appropriate socket option for the notifications
   it wants.  See Section 7.3 for these socket options.  When a
   notification arrives, recvmsg() returns the notification in the
   application-supplied data buffer via msg_iov, and sets
   MSG_NOTIFICATION in msg_flags.

   This section details the notification structures.  Every notification
   structure carries some common fields which provides general
   information.

   A recvmsg() call will return only one notification at a time.  Just
   as when reading normal data, it may return part of a notification if
   the msg_iov buffer is not large enough.  If a single read is not
   sufficient, msg_flags will have MSG_EOR clear.  The user MUST finish
   reading the notification before subsequent data can arrive.

5.3.1.  SCTP Notification Structure

   The notification structure is defined as the union of all
   notification types.

    union sctp_notification {
     struct {
       uint16_t sn_type;             /* Notification type. */
       uint16_t sn_flags;
       uint32_t sn_length;
       } sn_header;
       struct sctp_assoc_change     sn_assoc_change;
       struct sctp_paddr_change     sn_paddr_change;
       struct sctp_remote_error     sn_remote_error;
       struct sctp_send_failed      sn_send_failed;
       struct sctp_shutdown_event   sn_shutdown_event;
       struct sctp_adaptation_event sn_adaptation_event;
       struct sctp_pdapi_event      sn_pdapi_event;
       struct sctp_authkey_event    sn_auth_event;
    };




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   sn_type: 16 bits (unsigned integer)

   The following list describes the SCTP notification and event types
   for the field sn_type.

   SCTP_ASSOC_CHANGE: This tag indicates that an association has either
      been opened or closed.  Refer to Section 5.3.1.1 for details.
   SCTP_PEER_ADDR_CHANGE: This tag indicates that an address that is
      part of an existing association has experienced a change of state
      (e.g. a failure or return to service of the reachability of a
      endpoint via a specific transport address).  Please see
      Section 5.3.1.2 for data structure details.
   SCTP_REMOTE_ERROR: The attached error message is an Operational Error
      received from the remote peer.  It includes the complete TLV sent
      by the remote endpoint.  See Section 5.3.1.3 for the detailed
      format.
   SCTP_SEND_FAILED: The attached datagram could not be sent to the
      remote endpoint.  This structure includes the original
      SCTP_SNDRCVINFO that was used in sending this message i.e. this
      structure uses the sctp_sndrecvinfo per Section 5.3.1.4.
   SCTP_SHUTDOWN_EVENT: The peer has sent a SHUTDOWN.  No further data
      should be sent on this socket.
   SCTP_ADAPTATION_INDICATION: This notification holds the peers
      indicated adaptation layer.  Please see Section 5.3.1.6.
   SCTP_PARTIAL_DELIVERY_EVENT: This notification is used to tell a
      receiver that the partial delivery has been aborted.  This may
      indicate the association is about to be aborted.  Please see
      Section 5.3.1.7
   SCTP_AUTHENTICATION_EVENT: This notification is used to tell a
      receiver that either an error occurred on authentication, or a new
      key was made active.  Section 5.3.1.8

   All standard values for sn_type are greater than 2^15.  Values from
   2^15 and down are reserved.

   sn_flags: 16 bits (unsigned integer)

   These are notification-specific flags.

   sn_length: 32 bits (unsigned integer)

   This is the length of the whole sctp_notification structure including
   the sn_type, sn_flags, and sn_length fields.

5.3.1.1.  SCTP_ASSOC_CHANGE

   Communication notifications inform the ULP that an SCTP association
   has either begun or ended.  The identifier for a new association is



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   provided by this notification.  The notification information has the
   following format:

     struct sctp_assoc_change {
        uint16_t sac_type;
        uint16_t sac_flags;
        uint32_t sac_length;
        uint16_t sac_state;
        uint16_t sac_error;
        uint16_t sac_outbound_streams;
        uint16_t sac_inbound_streams;
        sctp_assoc_t sac_assoc_id;
        uint8_t  sac_info[0];
     };

   sac_type:

   It should be SCTP_ASSOC_CHANGE.

   sac_flags: 16 bits (unsigned integer)

   Currently unused.

   sac_length: 32 bits (unsigned integer)

   This field is the total length of the notification data, including
   the notification header.

   sac_state: 16 bits (signed integer)

   This field holds one of a number of values that communicate the event
   that happened to the association.  They include:

   Event Name Description

   ---------------- ---------------
   SCTP_COMM_UP - A new association is now ready and data may be
      exchanged with this peer.
   SCTP_COMM_LOST - The association has failed.  The association is now
      in the closed state.  If SEND FAILED notifications are turned on,
      a SCTP_COMM_LOST is followed by a series of SCTP_SEND_FAILED
      events, one for each outstanding message.
   SCTP_RESTART - SCTP has detected that the peer has restarted.
   SCTP_SHUTDOWN_COMP - The association has gracefully closed.







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   SCTP_CANT_STR_ASSOC - The association failed to setup.  If non
      blocking mode is set and data was sent (in the udp mode), a
      SCTP_CANT_STR_ASSOC is followed by a series of SCTP_SEND_FAILED
      events, one for each outstanding message.

   sac_error: 16 bits (signed integer)

   If the state was reached due to a error condition (e.g.
   SCTP_COMM_LOST) any relevant error information is available in this
   field.  This corresponds to the protocol error codes defined in
   RFC2960 [RFC2960].

   sac_outbound_streams: 16 bits (unsigned integer)

   sac_inbound_streams: 16 bits (unsigned integer)

   The maximum number of streams allowed in each direction are available
   in sac_outbound_streams and sac_inbound streams.

   sac_assoc_id: sizeof (sctp_assoc_t)

   The association id field, holds the identifier for the association.
   All notifications for a given association have the same association
   identifier.  For one-to-one style socket, this field is ignored.

   sac_info: variable

   If the sac_state is SCTP_COMM_LOST and an ABORT chunk was received
   for this association, sac_info[] contains the complete ABORT chunk as
   defined in the SCTP specification RFC2960 [RFC2960] section 3.3.7.

5.3.1.2.  SCTP_PEER_ADDR_CHANGE

   When a destination address on a multi-homed peer encounters a change
   an interface details event is sent.  The information has the
   following structure:


       struct sctp_paddr_change {
          uint16_t spc_type;
          uint16_t spc_flags;
          uint32_t spc_length;
          struct sockaddr_storage spc_aaddr;
          uint32_t spc_state;
          uint32_t spc_error;
          sctp_assoc_t spc_assoc_id;
        }




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   spc_type:

   It should be SCTP_PEER_ADDR_CHANGE.

   spc_flags: 16 bits (unsigned integer)

   Currently unused.

   spc_length: 32 bits (unsigned integer)

   This field is the total length of the notification data, including
   the notification header.

   spc_aaddr: sizeof (struct sockaddr_storage)

   The affected address field, holds the remote peer's address that is
   encountering the change of state.

   spc_state: 32 bits (signed integer)

   This field holds one of a number of values that communicate the event
   that happened to the address.  They include:

   Event Name Description

   ---------------- ---------------
   SCTP_ADDR_AVAILABLE - This address is now reachable.
   SCTP_ADDR_UNREACHABLE - The address specified can no longer be
      reached.  Any data sent to this address is rerouted to an
      alternate until this address becomes reachable.
   SCTP_ADDR_REMOVED - The address is no longer part of the association.
   SCTP_ADDR_ADDED - The address is now part of the association.
   SCTP_ADDR_MADE_PRIM - This address has now been made to be the
      primary destination address.
   SCTP_ADDR_CONFIRMED - This address has now been confirmed as a valid
      address.

   spc_error: 32 bits (signed integer)

   If the state was reached due to any error condition (e.g.
   SCTP_ADDR_UNREACHABLE) any relevant error information is available in
   this field.

   spc_assoc_id: sizeof (sctp_assoc_t)

   The association id field, holds the identifier for the association.
   All notifications for a given association have the same association
   identifier.  For one-to-one style socket, this field is ignored.



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5.3.1.3.  SCTP_REMOTE_ERROR

   A remote peer may send an Operational Error message to its peer.
   This message indicates a variety of error conditions on an
   association.  The entire ERROR chunk as it appears on the wire is
   included in a SCTP_REMOTE_ERROR event.  Please refer to the SCTP
   specification RFC2960 [RFC2960] and any extensions for a list of
   possible error formats.  SCTP error notifications have the format:


   struct sctp_remote_error {
       uint16_t sre_type;
       uint16_t sre_flags;
       uint32_t sre_length;
       uint16_t sre_error;
       sctp_assoc_t sre_assoc_id;
       uint8_t sre_data[0];
   };


   sre_type:

   It should be SCTP_REMOTE_ERROR.

   sre_flags: 16 bits (unsigned integer)

   Currently unused.

   sre_length: 32 bits (unsigned integer)

   This field is the total length of the notification data, including
   the notification header and the contents of sre_data.

   sre_error: 16 bits (unsigned integer)

   This value represents one of the Operational Error causes defined in
   the SCTP specification, in network byte order.

   sre_assoc_id: sizeof (sctp_assoc_t)

   The association id field, holds the identifier for the association.
   All notifications for a given association have the same association
   identifier.  For one-to-one style socket, this field is ignored.

   sre_data: variable

   This contains the ERROR chunk as defined in the SCTP specification
   RFC2960 [RFC2960] section 3.3.10.



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5.3.1.4.  SCTP_SEND_FAILED

   If SCTP cannot deliver a message it may return the message as a
   notification.


   struct sctp_send_failed {
      uint16_t ssf_type;
      uint16_t ssf_flags;
      uint32_t ssf_length;
      uint32_t ssf_error;
      struct sctp_sndrcvinfo ssf_info;
      sctp_assoc_t ssf_assoc_id;
      uint8_t ssf_data[0];
   };


   ssf_type:

   It should be SCTP_SEND_FAILED.

   The flag value will take one of the following values:

   SCTP_DATA_UNSENT - Indicates that the data was never put on the wire.
   SCTP_DATA_SENT - Indicates that the data was put on the wire.  Note
      that this does not necessarily mean that the data was (or was not)
      successfully delivered.

   ssf_length: 32 bits (unsigned integer)

   This field is the total length of the notification data, including
   the notification header and the payload in ssf_data.

   ssf_error: 16 bits (unsigned integer)

   This value represents the reason why the send failed, and if set,
   will be a SCTP protocol error code as defined in RFC2960 [RFC2960]
   section 3.3.10.

   ssf_info: sizeof (struct sctp_sndrcvinfo)

   The original send information associated with the undelivered
   message.

   ssf_assoc_id: sizeof (sctp_assoc_t)

   The association id field, sf_assoc_id, holds the identifier for the
   association.  All notifications for a given association have the same



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   association identifier.  For one-to-one style socket, this field is
   ignored.

   ssf_data: variable length

   The undelivered message, exactly as delivered by the caller to the
   original send*() call.

5.3.1.5.  SCTP_SHUTDOWN_EVENT

   When a peer sends a SHUTDOWN, SCTP delivers this notification to
   inform the application that it should cease sending data.

       struct sctp_shutdown_event {
           uint16_t sse_type;
           uint16_t sse_flags;
           uint32_t sse_length;
           sctp_assoc_t sse_assoc_id;
       };

   sse_type

   It should be SCTP_SHUTDOWN_EVENT

   sse_flags: 16 bits (unsigned integer)

   Currently unused.

   sse_length: 32 bits (unsigned integer)

   This field is the total length of the notification data, including
   the notification header.  It will generally be sizeof (struct
   sctp_shutdown_event).

   sse_flags: 16 bits (unsigned integer)

   Currently unused.

   sse_assoc_id: sizeof (sctp_assoc_t)

   The association id field, holds the identifier for the association.
   All notifications for a given association have the same association
   identifier.  For one-to-one style socket, this field is ignored.

5.3.1.6.  SCTP_ADAPTATION_INDICATION

   When a peer sends a Adaptation Layer Indication parameter , SCTP
   delivers this notification to inform the application that of the



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   peers requested adaptation layer.

       struct sctp_adaptation_event {
           uint16_t sai_type;
           uint16_t sai_flags;
           uint32_t sai_length;
           uint32_t sai_adaptation_ind;
           sctp_assoc_t sai_assoc_id;
       };

   sai_type

   It should be SCTP_ADAPTATION_INDICATION

   sai_flags: 16 bits (unsigned integer)

   Currently unused.

   sai_length: 32 bits (unsigned integer)

   This field is the total length of the notification data, including
   the notification header.  It will generally be sizeof (struct
   sctp_adaptation_event).

   sai_adaptation_ind: 32 bits (unsigned integer)

   This field holds the bit array sent by the peer in the adaptation
   layer indication parameter.  The bits are in network byte order.

   sai_assoc_id: sizeof (sctp_assoc_t)

   The association id field, holds the identifier for the association.
   All notifications for a given association have the same association
   identifier.  For one-to-one style socket, this field is ignored.

5.3.1.7.  SCTP_PARTIAL_DELIVERY_EVENT

   When a receiver is engaged in a partial delivery of a message this
   notification will be used to indicate various events.

       struct sctp_pdapi_event {
           uint16_t pdapi_type;
           uint16_t pdapi_flags;
           uint32_t pdapi_length;
           uint32_t pdapi_indication;
           sctp_assoc_t pdapi_assoc_id;
       };




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   pdapi_type

   It should be SCTP_PARTIAL_DELIVERY_EVENT

   pdapi_flags: 16 bits (unsigned integer)

   Currently unused.

   pdapi_length: 32 bits (unsigned integer)

   This field is the total length of the notification data, including
   the notification header.  It will generally be sizeof (struct
   sctp_pdapi_event).

   pdapi_indication: 32 bits (unsigned integer)

   This field holds the indication being sent to the application
   possible values include:

   SCTP_PARTIAL_DELIVERY_ABORTED

   pdapi_assoc_id: sizeof (sctp_assoc_t)

   The association id field, holds the identifier for the association.
   All notifications for a given association have the same association
   identifier.  For one-to-one style socket, this field is ignored.

5.3.1.8.  SCTP_AUTHENTICATION_EVENT

   When a receiver is using authentication this message will provide
   notifications regarding new keys being made active as well as errors.

       struct sctp_authkey_event {
           uint16_t auth_type;
           uint16_t auth_flags;
           uint32_t auth_length;
           uint32_t auth_keynumber;
           uint32_t auth_altkeynumber;
           uint32_t auth_indication;
           sctp_assoc_t auth_assoc_id;
       };

   auth_type

   It should be SCTP_AUTHENTICATION_EVENT

   auth_flags: 16 bits (unsigned integer)




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   Currently unused.

   auth_length: 32 bits (unsigned integer)

   This field is the total length of the notification data, including
   the notification header.  It will generally be sizeof (struct
   sctp_authkey_event).

   auth_keynumber: 32 bits (unsigned integer)

   This field holds the keynumber set by the user for the effected key.
   If more than one key is involved, this will contain one of the keys
   involved in the notification.

   auth_altkeynumber: 32 bits (unsigned integer)

   This field holds an alternate keynumber which is used by some
   notifications.

   auth_indication: 32 bits (unsigned integer)

   This field hold the error or indication being reported.  The
   following values are currently defined:
   SCTP_AUTH_NEWKEY - this report indicates that a new key has been made
      active (used for the first time by the peer) and is now the active
      key.  The auth_keynumber field holds the user specified key
      number.

   auth_assoc_id: sizeof (sctp_assoc_t)

   The association id field, holds the identifier for the association.
   All notifications for a given association have the same association
   identifier.

5.4.  Ancillary Data Considerations and Semantics

   Programming with ancillary socket data contains some subtleties and
   pitfalls, which are discussed below.

5.4.1.  Multiple Items and Ordering

   Multiple ancillary data items may be included in any call to
   sendmsg() or recvmsg(); these may include multiple SCTP or non-SCTP
   items, or both.

   The ordering of ancillary data items (either by SCTP or another
   protocol) is not significant and is implementation-dependent, so
   applications must not depend on any ordering.



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   SCTP_SNDRCV items must always correspond to the data in the msghdr's
   msg_iov member.  There can be only a single SCTP_SNDRCV info for each
   sendmsg() or recvmsg() call.

5.4.2.  Accessing and Manipulating Ancillary Data

   Applications can infer the presence of data or ancillary data by
   examining the msg_iovlen and msg_controllen msghdr members,
   respectively.

   Implementations may have different padding requirements for ancillary
   data, so portable applications should make use of the macros
   CMSG_FIRSTHDR, CMSG_NXTHDR, CMSG_DATA, CMSG_SPACE, and CMSG_LEN.  See
   RFC2292 [RFC2292] and your SCTP implementation's documentation for
   more information.  Following is an example, from RFC2292 [RFC2292],
   demonstrating the use of these macros to access ancillary data:

   struct msghdr   msg;
   struct cmsghdr  *cmsgptr;

   /* fill in msg */

   /* call recvmsg() */

   for (cmsgptr = CMSG_FIRSTHDR(&msg); cmsgptr != NULL;
        cmsgptr = CMSG_NXTHDR(&msg, cmsgptr)) {
        if (cmsgptr->cmsg_level == ... && cmsgptr->cmsg_type == ... ) {
           u_char  *ptr;

           ptr = CMSG_DATA(cmsgptr);
           /* process data pointed to by ptr */
        }
   }

5.4.3.  Control Message Buffer Sizing

   The information conveyed via SCTP_SNDRCV events will often be
   fundamental to the correct and sane operation of the sockets
   application.  This is particularly true of the one-to-many semantics,
   but also of the one-ton-one semantics.  For example, if an
   application needs to send and receive data on different SCTP streams,
   SCTP_SNDRCV events are indispensable.

   Given that some ancillary data is critical, and that multiple
   ancillary data items may appear in any order, applications should be
   carefully written to always provide a large enough buffer to contain
   all possible ancillary data that can be presented by recvmsg().  If
   the buffer is too small, and crucial data is truncated, it may pose a



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   fatal error condition.

   Thus it is essential that applications be able to deterministically
   calculate the maximum required buffer size to pass to recvmsg().  One
   constraint imposed on this specification that makes this possible is
   that all ancillary data definitions are of a fixed length.  One way
   to calculate the maximum required buffer size might be to take the
   sum the sizes of all enabled ancillary data item structures, as
   calculated by CMSG_SPACE.  For example, if we enabled
   SCTP_SNDRCV_INFO and IPV6_RECVPKTINFO RFC2292 [RFC2292], we would
   calculate and allocate the buffer size as follows:

   size_t total;
   void *buf;

   total = CMSG_SPACE(sizeof (struct sctp_sndrcvinfo)) +
           CMSG_SPACE(sizeof (struct in6_pktinfo));

   buf = malloc(total);

   We could then use this buffer for msg_control on each call to
   recvmsg() and be assured that we would not lose any ancillary data to
   truncation.


6.  Common Operations for Both Styles

6.1.  send(), recv(), sendto(), recvfrom()

   Applications can use send() and sendto() to transmit data to the peer
   of an SCTP endpoint. recv() and recvfrom() can be used to receive
   data from the peer.

   The syntax is:

   ssize_t send(int sd, const void *msg, size_t len, int flags);
   ssize_t sendto(int sd, const void *msg, size_t len, int flags,
                  const struct sockaddr *to, socklen_t tolen);
   ssize_t recv(int sd, void *buf, size_t len, int flags);
   ssize_t recvfrom(int sd, void *buf, size_t len, int flags,
                    struct sockaddr *from, socklen_t *fromlen);

   sd - the socket descriptor of an SCTP endpoint.
   msg - the message to be sent.







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   len - the size of the message or the size of buffer.
   to - one of the peer addresses of the association to be used to send
      the message.
   tolen - the size of the address.
   buf - the buffer to store a received message.
   from - the buffer to store the peer address used to send the received
      message.
   fromlen - the size of the from address
   flags - (described below).

   These calls give access to only basic SCTP protocol features.  If
   either peer in the association uses multiple streams, or sends
   unordered data these calls will usually be inadequate, and may
   deliver the data in unpredictable ways.

   SCTP has the concept of multiple streams in one association.  The
   above calls do not allow the caller to specify on which stream a
   message should be sent.  The system uses stream 0 as the default
   stream for send() and sendto(). recv() and recvfrom() return data
   from any stream, but the caller can not distinguish the different
   streams.  This may result in data seeming to arrive out of order.
   Similarly, if a data chunk is sent unordered, recv() and recvfrom()
   provide no indication.

   SCTP is message based.  The msg buffer above in send() and sendto()
   is considered to be a single message.  This means that if the caller
   wants to send a message which is composed by several buffers, the
   caller needs to combine them before calling send() or sendto().
   Alternately, the caller can use sendmsg() to do that without
   combining them. recv() and recvfrom() cannot distinguish message
   boundaries.

   In receiving, if the buffer supplied is not large enough to hold a
   complete message, the receive call acts like a stream socket and
   returns as much data as will fit in the buffer.

   Note, the send() and recv() calls may not be used for a one-to-many
   style socket.

   Note, if an application calls a send function with no user data and
   no ancillary data the SCTP implementation should reject the request
   with an appropriate error message.  An implementation is NOT allowed
   to send a Data chunk with no user data RFC2960 [RFC2960].

6.2.  setsockopt(), getsockopt()

   Applications use setsockopt() and getsockopt() to set or retrieve
   socket options.  Socket options are used to change the default



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   behavior of sockets calls.  They are described in Section 7

   The syntax is:

   ret = getsockopt(int sd, int level, int optname, void *optval,
                    socklen_t *optlen);
   ret = setsockopt(int sd, int level, int optname, const void *optval,
                    socklen_t optlen);


   sd - the socket descriptor.
   level - set to IPPROTO_SCTP for all SCTP options.
   optname - the option name.
   optval - the buffer to store the value of the option.
   optlen - the size of the buffer (or the length of the option
      returned).

   All socket options set on a 1-to-1 listening sockets also apply all
   accepted sockets.  All socket options set on a 1-to-many socket using
   the assoc_id 0 applies for all future assocs on the socket.

6.3.  read() and write()

   Applications can use read() and write() to send and receive data to
   and from peer.  They have the same semantics as send() and recv()
   except that the flags parameter cannot be used.

   Note, these calls, when used in the one-to-many style, may only be
   used with branched off socket descriptors (see Section 8.2).

6.4.  getsockname()

   Applications use getsockname() to retrieve the locally-bound socket
   address of the specified socket.  This is especially useful if the
   caller let SCTP chose a local port.  This call is for where the
   endpoint is not multi-homed.  It does not work well with multi-homed
   sockets.  See Section 8.5 for a multi-homed version of the call.

   The syntax is:

   int getsockname(int sd, struct sockaddr *address,
                   socklen_t *len);

   sd - the socket descriptor to be queried.







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   address - On return, one locally bound address (chosen by the SCTP
      stack) is stored in this buffer.  If the socket is an IPv4 socket,
      the address will be IPv4.  If the socket is an IPv6 socket, the
      address will be either an IPv6 or IPv4 address.
   len - The caller should set the length of address here.  On return,
      this is set to the length of the returned address.

   If the actual length of the address is greater than the length of the
   supplied sockaddr structure, the stored address will be truncated.

   If the socket has not been bound to a local name, the value stored in
   the object pointed to by address is unspecified.


7.  Socket Options

   The following sub-section describes various SCTP level socket options
   that are common to both styles.  SCTP associations can be multi-
   homed.  Therefore, certain option parameters include a
   sockaddr_storage structure to select which peer address the option
   should be applied to.

   For the one-to-many style sockets, an sctp_assoc_t structure
   (association ID) is used to identify the the association instance
   that the operation affects.  So it must be set when using this style.

   For the one-to-one style sockets and branched off one-to-many style
   sockets (see Section 8.2) this association ID parameter is ignored.

   Note that socket or IP level options are set or retrieved per socket.
   This means that for one-to-many style sockets, those options will be
   applied to all associations belonging to the socket.  And for one-to-
   one style, those options will be applied to all peer addresses of the
   association controlled by the socket.  Applications should be very
   careful in setting those options.

   For some IP stacks getsockopt() is read-only; so a new interface will
   be needed when information must be passed both in to and out of the
   SCTP stack.  The syntax for sctp_opt_info() is,

   int sctp_opt_info(int sd,
                     sctp_assoc_t id,
                     int opt,
                     void *arg,
                     socklen_t *size);

   The sctp_opt_info() call is a replacement for getsockopt() only and
   will not set any options associated with the specified socket.  A



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   setsockopt() must be used to set any writeable option.

   For one-to-many style sockets, id specifies the association to query.
   For one-to-one style sockets, id is ignored.

   opt specifies which SCTP socket option to get.  It can get any socket
   option currently supported that requests information (either read/
   write options or read only) such as:
   SCTP_RTOINFO
   SCTP_ASSOCINFO
   SCTP_DEFAULT_SEND_PARAM
   SCTP_GET_PEER_ADDR_INFO
   SCTP_PRIMARY_ADDR
   SCTP_PEER_ADDR_PARAMS
   SCTP_STATUS
   SCTP_AUTH_ACTIVE_KEY
   SCTP_PEER_AUTH_CHUNKS
   SCTP_LOCAL_AUTH_CHUNKS

   arg is an option-specific structure buffer provided by the caller.
   See Section 8.5) subsections for more information on these options
   and option-specific structures.

   sctp_opt_info() returns 0 on success, or on failure returns -1 and
   sets errno to the appropriate error code.

   All options that support specific settings on an association by
   filling in either an association id variable or a sockaddr_storage
   SHOULD also support setting of the same value for the entire endpoint
   (i.e. future associations).  To accomplish this the following logic
   is used when setting one of these options:

   a) If an address is specified via a sockaddr_storage that is included
      in the structure, the address is used to lookup the association
      and the settings are applied to the specific address (if
      appropriate) or to the entire association.
   b) If an association identification is filled in but not a
      sockaddr_storage (if present), the association is found using the
      association identification and the settings should be applied to
      the entire association (since a specific address is not
      specified).  Note this also applies to options that hold an
      association identification in their structure but do not have a
      sockaddr_storage field.
   c) If neither the sockaddr_storage or association identification is
      set, i.e. the sockaddr_storage is set to all 0's (INADDR_ANY) and
      the association identification is 0, the settings are a default
      and to be applied to the endpoint (all future associations).




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7.1.  Read / Write Options

7.1.1.  Retransmission Timeout Parameters (SCTP_RTOINFO)

   The protocol parameters used to initialize and bound retransmission
   timeout (RTO) are tunable.  See RFC2960 [RFC2960] for more
   information on how these parameters are used in RTO calculation.

   The following structure is used to access and modify these
   parameters:

   struct sctp_rtoinfo {
       sctp_assoc_t          srto_assoc_id;
       uint32_t              srto_initial;
       uint32_t              srto_max;
       uint32_t              srto_min;
   };

   srto_initial - This contains the initial RTO value.
   srto_max and srto_min - These contain the maximum and minimum bounds
      for all RTOs.
   srto_assoc_id - (one-to-many style socket) This is filled in the
      application, and identifies the association for this query.  If
      this parameter is '0' (on a one-to-many style socket), then the
      change effects the entire endpoint.

   All parameters are time values, in milliseconds.  A value of 0, when
   modifying the parameters, indicates that the current value should not
   be changed.

   To access or modify these parameters, the application should call
   getsockopt or setsockopt() respectively with the option name
   SCTP_RTOINFO.

7.1.2.  Association Parameters (SCTP_ASSOCINFO)

   This option is used to both examine and set various association and
   endpoint parameters.

   See RFC2960 [RFC2960] for more information on how this parameter is
   used.  The peer address parameter is ignored for one-to-one style
   socket.

   The following structure is used to access and modify this parameters:







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   struct sctp_assocparams {
      sctp_assoc_t    sasoc_assoc_id;
      uint16_t        sasoc_asocmaxrxt;
      uint16_t        sasoc_number_peer_destinations;
      uint32_t        sasoc_peer_rwnd;
      uint32_t        sasoc_local_rwnd;
      uint32_t        sasoc_cookie_life;
   };

   sasoc_asocmaxrxt - This contains the maximum retransmission attempts
      to make for the association.
   sasoc_number_peer_destinations - This is the number of destination
      addresses that the peer has.
   sasoc_peer_rwnd - This holds the current value of the peers rwnd
      (reported in the last SACK) minus any outstanding data (i.e. data
      inflight).
   sasoc_local_rwnd - This holds the last reported rwnd that was sent to
      the peer.
   sasoc_cookie_life - This is the associations cookie life value used
      when issuing cookies.
   sasoc_assoc_id - This is filled in the application, and identifies
      the association for this query.

   This information may be examined for either the endpoint or a
   specific association.  To examine a endpoints default parameters the
   association id (sasoc_assoc_id) should must be set to the value '0'.
   The values of the sasoc_peer_rwnd is meaningless when examining
   endpoint information.

   All parameters are time values, in milliseconds.  A value of 0, when
   modifying the parameters, indicates that the current value should not
   be changed.

   The values of the sasoc_asocmaxrxt and sasoc_cookie_life may be set
   on either an endpoint or association basis.  The rwnd and destination
   counts (sasoc_number_peer_destinations,
   sasoc_peer_rwnd,sasoc_local_rwnd) are NOT settable and any value
   placed in these is ignored.

   To access or modify these parameters, the application should call
   getsockopt or setsockopt() respectively with the option name
   SCTP_ASSOCINFO.

   The maximum number of retransmissions before an address is considered
   unreachable is also tunable, but is address-specific, so it is
   covered in a separate option.  If an application attempts to set the
   value of the association maximum retransmission parameter to more
   than the sum of all maximum retransmission parameters, setsockopt()



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   shall return an error.  The reason for this, from RFC2960 [RFC2960]
   section 8.2:

   Note: When configuring the SCTP endpoint, the user should avoid
   having the value of 'Association.Max.Retrans' larger than the
   summation of the 'Path.Max.Retrans' of all the destination addresses
   for the remote endpoint.  Otherwise, all the destination addresses
   may become inactive while the endpoint still considers the peer
   endpoint reachable.

7.1.3.  Initialization Parameters (SCTP_INITMSG)

   Applications can specify protocol parameters for the default
   association initialization.  The structure used to access and modify
   these parameters is defined in Section 5.2.1).  The option name
   argument to setsockopt() and getsockopt() is SCTP_INITMSG.

   Setting initialization parameters is effective only on an unconnected
   socket (for one-to-many style sockets only future associations are
   effected by the change).  With one-to-one style sockets, this option
   is inherited by sockets derived from a listener socket.

7.1.4.  SO_LINGER

   An application using the one-to-one style socket can use this option
   to perform the SCTP ABORT primitive.  The linger option structure is:

   struct  linger {
       int     l_onoff;                /* option on/off */
       int     l_linger;               /* linger time */
   };

   To enable the option, set l_onoff to 1.  If the l_linger value is set
   to 0, calling close() is the same as the ABORT primitive.  If the
   value is set to a negative value, the setsockopt() call will return
   an error.  If the value is set to a positive value linger_time, the
   close() can be blocked for at most linger_time ms.  If the graceful
   shutdown phase does not finish during this period, close() will
   return but the graceful shutdown phase continues in the system.

   Note, this is a socket level option NOT an SCTP level option.  So
   when setting SO_LINGER you must specify a level of SOL_SOCKET in the
   setsockopt() call.

7.1.5.  SCTP_NODELAY

   Turn on/off any Nagle-like algorithm.  This means that packets are
   generally sent as soon as possible and no unnecessary delays are



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   introduced, at the cost of more packets in the network.  Expects an
   integer boolean flag.

7.1.6.  SO_RCVBUF

   Sets receive buffer size in octets.  For SCTP one-to-one style
   sockets, this controls the receiver window size.  For one-to-many
   style sockets the meaning depends on the constant HAVE_SCTP_MULTIBUF
   (see Section 3.4).  If the implementation defines HAVE_SCTP_MULTIBUF
   as 1, this controls the receiver window size for each association
   bound to the socket descriptor.  If the implementation defines
   HAVE_SCTP_MULTIBUF as 0, this controls the size of the single receive
   buffer for the whole socket.  The call expects an integer.

7.1.7.  SO_SNDBUF

   Sets send buffer size.  For SCTP one-to-one style sockets, this
   controls the amount of data SCTP may have waiting in internal buffers
   to be sent.  This option therefore bounds the maximum size of data
   that can be sent in a single send call.  For one-to-many style
   sockets, the effect is the same, except that it applies to one or all
   associations (see Section 3.4) bound to the socket descriptor used in
   the setsockopt() or getsockopt() call.  The option applies to each
   association's window size separately.  The call expects an integer.

7.1.8.  Automatic Close of associations (SCTP_AUTOCLOSE)

   This socket option is applicable to the one-to-many style socket
   only.  When set it will cause associations that are idle for more
   than the specified number of seconds to automatically close using the
   graceful shutdown procedure.  An association being idle is defined as
   an association that has NOT sent or received user data.  The special
   value of '0' indicates that no automatic close of any associations
   should be performed, this is the default value.  The option expects
   an integer defining the number of seconds of idle time before an
   association is closed.

   An application using this option should enable receiving the
   association change notification.  This is the only mechanism an
   application is informed about the closing of an association.  After
   an association is closed, the association ID assigned to it can be
   reused.  An application should be aware of this to avoid the possible
   problem of sending data to an incorrect peer end point.

7.1.9.  Set Peer Primary Address (SCTP_SET_PEER_PRIMARY_ADDR)

   Requests that the peer mark the enclosed address as the association
   primary.  The enclosed address must be one of the association's



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   locally bound addresses.  The following structure is used to make a
   set primary request:

   struct sctp_setpeerprim {
       sctp_assoc_t            sspp_assoc_id;
       struct sockaddr_storage sspp_addr;
   };

   sspp_addr - The address to set as primary
   sspp_assoc_id - This is filled in by the application, and identifies
      the association for this request.

   This functionality is optional.  Implementations that do not support
   this functionality should return EOPNOTSUPP.

7.1.10.  Set Primary Address (SCTP_PRIMARY_ADDR)

   Requests that the local SCTP stack use the enclosed peer address as
   the association primary.  The enclosed address must be one of the
   association peer's addresses.  The following structure is used to
   make a set peer primary request:

   struct sctp_setprim {
       sctp_assoc_t            ssp_assoc_id;
       struct sockaddr_storage ssp_addr;
   };

   ssp_addr - The address to set as primary
   ssp_assoc_id - This is filled in by the application, and identifies
      the association for this request.

7.1.11.  Set Adaptation Layer Indicator (SCTP_ADAPTATION_LAYER)

   Requests that the local endpoint set the specified Adaptation Layer
   Indication parameter for all future INIT and INIT-ACK exchanges.


       struct sctp_setadaptation {
           uint32_t   ssb_adaptation_ind;
       };


   ssb_adaptation_ind - The adaptation layer indicator that will be
      included in any outgoing Adaptation Layer Indication parameter

7.1.12.  Enable/Disable message fragmentation (SCTP_DISABLE_FRAGMENTS)

   This option is a on/off flag and is passed an integer where a non-



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   zero is on and a zero is off.  If enabled no SCTP message
   fragmentation will be performed.  Instead if a message being sent
   exceeds the current PMTU size, the message will NOT be sent and
   instead a error will be indicated to the user.

7.1.13.  Peer Address Parameters (SCTP_PEER_ADDR_PARAMS)

   Applications can enable or disable heartbeats for any peer address of
   an association, modify an address's heartbeat interval, force a
   heartbeat to be sent immediately, and adjust the address's maximum
   number of retransmissions sent before an address is considered
   unreachable.  The following structure is used to access and modify an
   address's parameters:

   struct sctp_paddrparams {
       sctp_assoc_t            spp_assoc_id;
       struct sockaddr_storage spp_address;
       uint32_t                spp_hbinterval;
       uint16_t                spp_pathmaxrxt;
       uint32_t                spp_pathmtu;
       uint32_t                spp_sackdelay;
       uint32_t                spp_flags;
       uint32_t                spp_ipv6_flowlabel;
       uint8_t                 spp_ipv4_tos;
   };

   spp_assoc_id -  (one-to-many style socket) This is filled in the
      application, and identifies the association for this query.
   spp_address - This specifies which address is of interest.
   spp_hbinterval - This contains the value of the heartbeat interval,
      in milliseconds.  Note that unless the spp_flag is set to
      SPP_HB_ENABLE the value of this field is ignored.  Note also that
      a value of zero indicates the current setting should be left
      unchanged.  To set an actual value of zero the use of the flag
      SPP_HB_TIME_IS_ZERO should be used.
   spp_pathmaxrxt - This contains the maximum number of retransmissions
      before this address shall be considered unreachable.  Note that
      unless the spp_flag is set to SPP_PMTUD_ENABLE the value of this
      field is ignored.  Note also that a value of zero indicates the
      current setting should be left unchanged.
   spp_pathmtu - When Path MTU discovery is disabled the value specified
      here will be the "fixed" path mtu (i.e. the value of the spp_flags
      field must include the flag SPP_PMTUD_DISABLE for this field to
      have any effect).  Note that if the spp_address field is empty
      then all associations on this address will have this fixed path
      mtu set upon them.  If an address is specified, then only that
      address will be effected.




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   spp_sackdelay - When delayed sack is enabled, this value specifies
      the number of milliseconds that sacks will be delayed for.  This
      value will apply to all addresses of an association if the
      spp_address field is empty.  Note that unless the spp_flag is set
      to SPP_SACKDELAY_ENABLE the value of this field is ignored.  Note
      also that a value of zero indicates the current setting should be
      left unchanged.
   spp_ipv6_flowlabel- This field is used in conjunction with the
      SPP_IPV4_FLOWLABEL flag.
   spp_ipv4_tos- This field is used in conjunction with the SPP_IPV4_TOS
      flag.
   spp_flags- These flags are used to control various features on an
      association.  The flag field is a bit mask which may contain zero
      or more of the following options:

      SPP_HB_ENABLE - Enable heartbeats on the specified address.  Note
         that if the address field is empty all addresses for the
         association have heartbeats enabled upon them.
      SPP_HB_DISABLE - Disable heartbeats on the specified address.
         Note that if the address field is empty all addresses for the
         association will have their heartbeats disabled.  Note also
         that SPP_HB_ENABLE and SPP_HB_DISABLE are mutually exclusive,
         only one of these two should be specified.  Enabling both
         fields will have undetermined results.
      SPP_HB_DEMAND - Request a user initiated heartbeat to be made
         immediately.
      SPP_HB_TIME_IS_ZERO - Specify's that the time for heartbeat delay
         is to be set to the value of 0 milliseconds.
      SPP_PMTUD_ENABLE - This field will enable PMTU discovery upon the
         specified address.  Note that if the address field is empty
         then all addresses on the association are effected.
      SPP_PMTUD_DISABLE - This field will disable PMTU discovery upon
         the specified address.  Note that if the address field is empty
         then all addresses on the association are effected.  Not also
         that SPP_PMTUD_ENABLE and SPP_PMTUD_DISABLE are mutually
         exclusive.  Enabling both will have undetermined results.
      SPP_SACKDELAY_ENABLE - Setting this flag turns on delayed sack.
         The time specified in spp_sackdelay is used to specify the sack
         delay for this address.  Note that if spp_address is empty then
         all addresses will enable delayed sack and take on the sack
         delay value specified in spp_sackdelay.
      SPP_SACKDELAY_DISABLE - Setting this flag turns off delayed sack.
         If the spp_address field is blank then delayed sack is disabled
         for the entire association.  Note also that this field is
         mutually exclusive to SPP_SACKDELAY_ENABLE, setting both will
         have undefined results.





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      SPP_IPV6_FLOWLABEL - Setting this flag enables setting of the IPV6
         flowlabel value associated with either the association or the
         specific address.  If the address field is filled in, then the
         specific destination address has this value set upon it.  If
         the association is specified, but not the address, then the
         flowlabel value is set for any future destination addresses
         that may be added.  The value is obtained in the
         spp_ipv6_flowlabel field.

         Upon retrieval, this flag will be set to indicate that the
         spp_ipv6_flowlabel field has a valid value returned.  If a
         specific destination addresses is set (in the spp_address
         field) when called then the value returned is that of the
         address.  If just an association is specified (and no address)
         then the association default flowlabel is returned.  If neither
         an association nor an destination is specified, then the
         sockets default flowlabel is returned.  For non IPv6 sockets,
         then this flag will be left cleared.
      SPP_IPV4_TOS - Setting this flag enables setting of the IPV4 tos
         value associated with either the association or specific
         address.  If the address field is filled in, then the specific
         destination address has this value set upon it.  If the
         association is specified, but not the address, then the tos
         value is set for any future destination addresses that may be
         added.  The value is obtained in the spp_ipv4_tos field.

         Upon retrieval, this flag will be set to indicate that the
         spp_ipv4_tos field has a valid value returned.  If a specific
         destination addresses is set when called (in the spp_address
         field) then that specific destination addresses tos value is
         returned.  If just an association is specified then the
         association default tos is returned.  If neither an association
         nor an destination is specified, then the sockets default tos
         is returned.  For non IPv4 sockets, then this flag will be left
         cleared.

   To read or modify these parameters, the application should call
   sctp_opt_info() with the SCTP_PEER_ADDR_PARAMS option.

7.1.14.  Set default send parameters (SCTP_DEFAULT_SEND_PARAM)

   Applications that wish to use the sendto() system call may wish to
   specify a default set of parameters that would normally be supplied
   through the inclusion of ancillary data.  This socket option allows
   such an application to set the default sctp_sndrcvinfo structure.
   The application that wishes to use this socket option simply passes
   in to this call the sctp_sndrcvinfo structure defined in
   Section 5.2.2) The input parameters accepted by this call include



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   sinfo_stream, sinfo_flags, sinfo_ppid, sinfo_context,
   sinfo_timetolive.  The sinfo_assoc_id field specifies the default
   association used for send operations on one-to-many style sockets.
   It is ignored on the one-to-one style.  Note that setting the
   sinfo_assoc_id field to zero indicates that the users wishes to set
   the endpoint default send parameters for all future associations.

7.1.15.  Set notification and ancillary events (SCTP_EVENTS)

   This socket option is used to specify various notifications and
   ancillary data the user wishes to receive.  Please see Section 7.3)
   for a full description of this option and its usage.

7.1.16.  Set/clear IPv4 mapped addresses (SCTP_I_WANT_MAPPED_V4_ADDR)

   This socket option is a boolean flag which turns on or off mapped V4
   addresses.  If this option is turned on and the socket is type
   PF_INET6, then IPv4 addresses will be mapped to V6 representation.
   If this option is turned off, then no mapping will be done of V4
   addresses and a user will receive both PF_INET6 and PF_INET type
   addresses on the socket.

   By default this option is turned on and expects an integer to be
   passed where non-zero turns on the option and zero turns off the
   option.

7.1.17.  Set the maximum fragmentation size (SCTP_MAXSEG)

   This socket option specifies the maximum size to put in any outgoing
   SCTP DATA chunk.  If a message is larger than this size it will be
   fragmented by SCTP into the specified size.  Note that the underlying
   SCTP implementation may fragment into smaller sized chunks when the
   PMTU of the underlying association is smaller than the value set by
   the user.  The option expects an integer.

   The default value for this option is '0' which indicates the user is
   NOT limiting fragmentation and only the PMTU will effect SCTP's
   choice of DATA chunk size.

7.1.18.  Add a chunk that must be authenticated (SCTP_AUTH_CHUNK)

   This option adds a chunk type that the user is requesting to be
   received only in an authenticated way.  Changes to the list of chunks
   will only effect future associations on the socket.







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   struct sctp_authchunk {
       uint8_t                 sauth_chunk;
   };

     sauth_chunks    - This parameter contains a chunk type
                       that the user is requesting to be authenticated.


   The chunk types for INIT, INIT-ACK, COOKIE-ECHO, COOKIE-ACK,
   SHUTDOWN-COMPLETE, and AUTH chunks MUST not be used.  If they are
   used an error MUST be returned.  The usage of this option enables
   SCTP-AUTH in cases where it is not required by other means (for
   example the use of ADD-IP).

7.1.19.  Get or set the list of supported HMAC Identifiers
         (SCTP_HMAC_IDENT)

   This option gets or sets the list of HMAC algorithms that the local
   endpoint requires the peer to use.


   struct sctp_hmacalgo {
       uint32_t                shmac_idents[];
   };


   shmac_idents - This parameter contains an array of HMAC Identifiers
      that the local endpoint is requesting the peer to use, in priority
      order.

7.1.20.  Set a shared key (SCTP_AUTH_KEY)

   This option will set a shared secret key which is used to build an
   association shared key.

   struct sctp_authkey {
       sctp_assoc_t            sca_assoc_id;
       uint32_t                sca_keynumber;
       uint8_t                 sca_key[];
   };


   sca_assoc_id - This parameter, if non-zero, indicates what
      association that the shared key is being set upon.  Note that if
      this element contains zero, then the shared key is set upon the
      endpoint and all future associations will use this key (if not
      changed by subsequent calls to SCTP_AUTH_KEY).  For one-to-one
      sockets, this parameter is ignored.  Note, however, that this



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      option will set a key on the association if the socket is
      connected, otherwise this will set a key on the endpoint.
   sca_keynumber - this parameter is the shared key identifier by which
      the application will refer to this key.  If a key of the specified
      index already exists, then this new key will replace the old
      existing key.  Note that shared key identifier '0' defaults to a
      null key.
   sca_key - This parameter contains an array of bytes that is to be
      used by the endpoint (or association) as the shared secret key.
      Note, if the length of this field is zero, a null key is set.

7.1.21.  Get or set the active shared key (SCTP_AUTH_ACTIVE_KEY)

   This option will get or set the active shared key to be used to build
   the association shared key.


   struct sctp_authkeyid {
       sctp_assoc_t            scact_assoc_id;
       uint32_t                scact_keynumber;
   };


   scact_assoc_id - This parameter, if non-zero, indicates what
      association that the shared key identifier is being set active
      upon.  Note that if this element contains zero, then the
      activation applies to the endpoint and all future associations
      will use the specified shared key identifier.  For one-to-one
      sockets, this parameter is ignored.  Note, however, that this
      option will set the active key on the association if the socket is
      connected, otherwise this will set the default active key for the
      endpoint.
   scact_keynumber - this parameter is the shared key identifier which
      the application is requesting to become the active shared key to
      be used for sending authenticated chunks.  The key identifier MUST
      correspond to an existing shared key.  Note that shared key
      identifier '0' defaults to a null key.

7.1.22.  Delete a shared key (SCTP_AUTH_DELETE_KEY)

   This option will delete a shared secret key from use.


   struct sctp_authkeyid {
       sctp_assoc_t            scact_assoc_id;
       uint32_t                scact_keynumber;
   };




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   scact_assoc_id - This parameter, if non-zero, indicates what
      association that the shared key identifier is being deleted from.
      Note that if this element contains zero, then the shared key is
      deleted from the endpoint and all associations will no longer use
      the specified shared key identifier (unless otherwise set on the
      association using SCTP_AUTH_KEY).  For one-to-one sockets, this
      parameter is ignored.  Note, however, that this option will delete
      the key from the association if the socket is connected, otherwise
      this will delete the key from the endpoint.
   scact_keynumber - this parameter is the shared key identifier which
      the application is requesting to be deleted.  The key identifier
      MUST correspond to an existing shared key and MUST NOT be the
      current active key.  Note if this parameter is zero, use of the
      null key identifier '0' is disabled on the endpoint and/or
      association.

7.1.23.  Get or set delayed ack timer  (SCTP_DELAYED_ACK_TIME)

   This options will get or set the delayed ack timer.  The time is set
   in milliseconds.  If the assoc_id is 0, then this sets or gets the
   endpoints default delayed ack timer value.  If the assoc_id field is
   non-zero, then the set or get effects the specified association.


   struct sctp_assoc_value {
       sctp_assoc_t            assoc_id;
       uint32_t                assoc_value;
   };


   assoc_id - This parameter, indicates which association the user is
      performing an action upon.  Note that if this field's value is
      zero then the endpoints default value is changed (effecting future
      associations only).
   assoc_value - This parameter contains the number of milliseconds that
      the user is requesting the delayed ACK timer be set to.  Note that
      this value is defined in the standard to be between 200 and 500
      milliseconds.

7.1.24.  Get or set fragmented interleave (SCTP_FRAGMENT_INTERLEAVE)

   This options will at a minimum specify if the implementation is doing
   fragmented interleave.  Fragmented interleave, for a one to many
   socket, is when subsequent calls to receive a message may return
   parts of messages from different associations.  Some implementations
   may allow you to turn this value on or off.  If so, when turned off,
   no fragment interleave will occur (which will cause a head of line
   blocking amongst multiple associations sharing the same one to many



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   socket).  When this option is turned on, then each receive call may
   come from a different association (thus the user must receive data
   with the extended calls (e.g. sctp_recvmsg) to keep track of which
   association each receive belongs to.

   This option takes a boolean value.  A non-zero value indicates that
   fragmented interleave is on.  A value of zero indicates that
   fragmented interleave is off.

   Note that it is important that an implementation that allows this
   option to be turned on, have it off by default.  Otherwise an unaware
   application using the one to many model may become confused and act
   incorrectly.

7.1.25.  Set or Get the sctp partial delivery point
         (SCTP_PARTIAL_DELIVERY_POINT)

   This option will set or get the SCTP partial delivery point.  This
   point is the size of a message where the partial delivery API will be
   invoked to help free up rwnd space for the peer.  Setting this to a
   lower value will cause partial delivery's to happen more often.  The
   calls argument is an integer that sets or gets the partial delivery
   point.

7.1.26.  Set or Get the use of extended receive info
         (SCTP_USE_EXT_RCVINFO)

   This option will enable or disable the use of the extended version of
   the sctp_sndrcvinfo structure.  If this option is disabled, then the
   normal sctp_sndrcvinfo structure is returned in all receive message
   calls.  If this option is enabled then the sctp_extrcvinfo structure
   is returned in all receive message calls.

   Note that the sctp_extrcvinfo structure is never used in any send
   call.

7.1.27.  Set or Get the auto asconf flag (SCTP_AUTO_ASCONF)

   This option will enable or disable the use of the automatic
   generation of ASCONF chunks to add and delete addresses to an
   existing association.  Note that this option has two caveats namely:
   a) it only effects sockets that are bound to all addresses on the
   machine, and b) the system administrator may have an overriding
   control that turns the asconf feature off no matter what setting the
   socket option may have.






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7.1.28.  Set or Get the maximum burst (SCTP_MAX_BURST)

   This option will allow a user to change the maximum burst of packets
   that can be emitted by this association.  Note that the default value
   is 4, and some implementations may restrict this setting so that it
   can only be lowered.

7.1.29.  Set or Get the default context (SCTP_CONTEXT)

   The context field in the sctp_sndrcvinfo structure is normally only
   used when a failed message is retrieved holding the value that was
   sent down on the actual send call.  This option allows the setting of
   a default context on an association basis that will be received on
   reading messages from the peer.  This is especially helpful in the
   one-2-many model for an application to keep some reference to an
   internal state machine that is processing messages on the
   association.  Note that the setting of this value only effects
   received messages from the peer and does not effect the value that is
   saved with outbound messages.

   To set or get this option the user fills in the following structure:


   struct sctp_assoc_value {
       sctp_assoc_t            assoc_id;
       uint32_t                assoc_value;
   };


   assoc_id - This parameter, indicates which association the user is
      performing an action upon.  Note that if this field's value is
      zero then the endpoints default value is changed (effecting future
      associations only).
   assoc_value - This parameter contains the context.

7.2.  Read-Only Options

7.2.1.  Association Status (SCTP_STATUS)

   Applications can retrieve current status information about an
   association, including association state, peer receiver window size,
   number of unacked data chunks, and number of data chunks pending
   receipt.  This information is read-only.  The following structure is
   used to access this information:







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   struct sctp_status {
       sctp_assoc_t    sstat_assoc_id;
       int32_t         sstat_state;
       uint32_t        sstat_rwnd;
       uint16_t        sstat_unackdata;
       uint16_t        sstat_penddata;
       uint16_t        sstat_instrms;
       uint16_t        sstat_outstrms;
       uint32_t        sstat_fragmentation_point;
       struct sctp_paddrinfo sstat_primary;
   };


   sstat_state - This contains the association's current state one of
      the following values:
      SCTP_CLOSED
      SCTP_BOUND
      SCTP_LISTEN
      SCTP_COOKIE_WAIT
      SCTP_COOKIE_ECHOED
      SCTP_ESTABLISHED
      SCTP_SHUTDOWN_PENDING
      SCTP_SHUTDOWN_SENT
      SCTP_SHUTDOWN_RECEIVED
      SCTP_SHUTDOWN_ACK_SENT
   sstat_rwnd - This contains the association peer's current receiver
      window size.
   sstat_unackdata - This is the number of unacked data chunks.
   sstat_penddata - This is the number of data chunks pending receipt.
   sstat_primary - This is information on the current primary peer
      address.
   sstat_assoc_id - (one-to-many style socket) This holds the an
      identifier for the association.  All notifications for a given
      association have the same association identifier.
   sstat_instrms - The number of streams that the peer will be using
      inbound.
   sstat_outstrms - The number of streams that the endpoint is allowed
      to use outbound.
   sstat_fragmentation_point - The size at which SCTP fragmentation will
      occur.

   To access these status values, the application calls getsockopt()
   with the option name SCTP_STATUS.  The sstat_assoc_id parameter is
   ignored for one-to-one style socket.

7.2.2.  Peer Address Information (SCTP_GET_PEER_ADDR_INFO)

   Applications can retrieve information about a specific peer address



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   of an association, including its reachability state, congestion
   window, and retransmission timer values.  This information is read-
   only.  The following structure is used to access this information:


   struct sctp_paddrinfo {
       sctp_assoc_t    spinfo_assoc_id;
       struct sockaddr_storage spinfo_address;
       int32_t         spinfo_state;
       uint32_t        spinfo_cwnd;
       uint32_t        spinfo_srtt;
       uint32_t        spinfo_rto;
       uint32_t        spinfo_mtu;
   };


   spinfo_address - This is filled in the application, and contains the
      peer address of interest.

   On return from getsockopt():
   spinfo_state - This contains the peer addresses's state (either
      SCTP_ACTIVE or SCTP_INACTIVE and possibly the modifier
      SCTP_UNCONFIRMED)
   spinfo_cwnd - This contains the peer addresses's current congestion
      window.
   spinfo_srtt - This contains the peer addresses's current smoothed
      round-trip time calculation in milliseconds.
   spinfo_rto - This contains the peer addresses's current
      retransmission timeout value in milliseconds.
   spinfo_mtu - The current P-MTU of this address.
   spinfo_assoc_id - This is field may be filled in by the application,
      if so, this field will have priority in looking up the association
      over the address specified in spinfo_address.  Note that if the
      address does not belong to the association specified then this
      call will fail.  If the application does NOT fill in the
      spinfo_assoc_id, then the address will be used to lookup the
      association and on return this field will have the valid
      association id.  In other words, this call can be used to
      translate a address into an association id.

   To retrieve this information, use sctp_opt_info() with the
   SCTP_GET_PEER_ADDR_INFO options.

7.2.3.  Get the list of chunks the peer requires to be authenticated
        (SCTP_PEER_AUTH_CHUNKS)

   This option gets a list of chunks for a specified association that
   the peer requires to be received authenticated only.



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   struct sctp_authchunks {
       sctp_assoc_t            gauth_assoc_id;
       uint8_t                 gauth_chunks[];
   };


   gauth_assoc_id - This parameter, indicates which association the user
      is requesting the list of peer authenticated chunks.  For one-to-
      one sockets, this parameter is ignored.
   gauth_chunks - This parameter contains an array of chunks that the
      peer is requesting to be authenticated.

7.2.4.  Get the list of chunks the local endpoint requires to be
        authenticated (SCTP_LOCAL_AUTH_CHUNKS)

   This option gets a list of chunks for a specified association that
   the local endpoint requires to be received authenticated only.


   struct sctp_authchunks {
       sctp_assoc_t            gauth_assoc_id;
       uint8_t                 gauth_chunks[];
   };


   gauth_assoc_id - This parameter, indicates which association the user
      is requesting the list of local authenticated chunks.  For one-to-
      one sockets, this parameter is ignored.
   gauth_chunks - This parameter contains an array of chunks that the
      local endpoint is requesting to be authenticated.

7.2.5.  Get the list of current associations (SCTP_GET_ASOC_ID_LIST)

   This option gets a list of current associations that are associated
   with a socket.


   struct sctp_assoc_ids {
           uint16_t asls_assoc_start;      /* array of index's start at 0 */
           uint8_t asls_numb_present;
           uint8_t asls_more_to_get;
           sctp_assoc_t asls_assoc_id[MAX_ASOC_IDS_RET];
   };








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   asls_assoc_start - This parameter gives an initial starting place to
      begin collecting association information.  Normally a user sets
      this initially to 0.  Subsequent calls that need to get more
      association identifications (in cases where the socket has more
      than MAX_ASOC_IDS_RET established i.e asls_more_to_get returns set
      to false) are made by incrementing the asls_assoc_start by the
      number of previous association id's seen.  This cycle is repeated
      until the asls_more_to_get field becomes true, indicating that all
      association id's have been retrieved.
   asls_numb_present - This field holds the total number of associations
      that have been copied into the asls_assoc_id array.  This value
      will NOT be larger than MAX_ASOC_IDS_RET.
   asls_more_to_get - This field holds a boolean flag indicating to the
      caller if more association id's are available then asked for.
      Subsequent calls to this option will be needed to gather these
      association id's.
   asls_assoc_id - an array of association id's that are currently
      active under this socket.
   MAX_ASOC_IDS_RET This value is a constant defined to limit the number
      of association identifications returned with each call.  This
      value can be no larger than 255..

7.3.  Ancillary Data and Notification Interest Options

   Applications can receive per-message ancillary information and
   notifications of certain SCTP events with recvmsg().

   The following optional information is available to the application:

   1.  SCTP_SNDRCV (sctp_data_io_event): Per-message information (i.e.
       stream number, TSN, SSN, etc. described in Section 5.2.2)
   2.  SCTP_ASSOC_CHANGE (sctp_association_event): (described in
       Section 5.3.1.1)
   3.  SCTP_PEER_ADDR_CHANGE (sctp_address_event): (described in
       Section 5.3.1.2)
   4.  SCTP_SEND_FAILED (sctp_send_failure_event): (described in
       Section 5.3.1.4)
   5.  SCTP_REMOTE_ERROR (sctp_peer_error_event): (described in
       Section 5.3.1.3)
   6.  SCTP_SHUTDOWN_EVENT (sctp_shtudown_event): (described in
       Section 5.3.1.5)
   7.  SCTP_PARTIAL_DELIVERY_EVENT (sctp_partial_delivery_event):
       (described in Section 5.3.1.7)
   8.  SCTP_ADAPTATION_INDICATION (sctp_adaptation_layer_event):
       (described in Section 5.3.1.6)
   9.  SCTP_AUTHENTICATION_INDICATION (sctp_authentication_event):
       (described in Section 5.3.1.8)




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   To receive any ancillary data or notifications, first the application
   registers it's interest by calling the SCTP_EVENTS setsockopt() with
   the following structure.

   struct sctp_event_subscribe{
       uint8_t sctp_data_io_event;
       uint8_t sctp_association_event;
       uint8_t sctp_address_event;
       uint8_t sctp_send_failure_event;
       uint8_t sctp_peer_error_event;
       uint8_t sctp_shutdown_event;
       uint8_t sctp_partial_delivery_event;
       uint8_t sctp_adaptation_layer_event;
       uint8_t sctp_authentication_event;
   };

   sctp_data_io_event - Setting this flag to 1 will cause the reception
   of SCTP_SNDRCV information on a per message basis.  The application
   will need to use the recvmsg() interface so that it can receive the
   event information contained in the msg_control field.  Please see
   Section 5.2 for further details.  Setting the flag to 0 will disable
   reception of the message control information.

   sctp_association_event - Setting this flag to 1 will enable the
   reception of association event notifications.  Setting the flag to 0
   will disable association event notifications.  For more information
   on event notifications please see Section 5.3.

   sctp_address_event - Setting this flag to 1 will enable the reception
   of address event notifications.  Setting the flag to 0 will disable
   address event notifications.  For more information on event
   notifications please see Section 5.3.

   sctp_send_failure_event - Setting this flag to 1 will enable the
   reception of send failure event notifications.  Setting the flag to 0
   will disable send failure event notifications.  For more information
   on event notifications please see Section 5.3.

   sctp_peer_error_event - Setting this flag to 1 will enable the
   reception of peer error event notifications.  Setting the flag to 0
   will disable peer error event notifications.  For more information on
   event notifications please see Section 5.3.

   sctp_shutdown_event - Setting this flag to 1 will enable the
   reception of shutdown event notifications.  Setting the flag to 0
   will disable shutdown event notifications.  For more information on
   event notifications please see Section 5.3.




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   sctp_partial_delivery_event - Setting this flag to 1 will enable the
   reception of partial delivery notifications.  Setting the flag to 0
   will disable partial delivery event notifications.  For more
   information on event notifications please see Section 5.3.

   sctp_adaptation_layer_event - Setting this flag to 1 will enable the
   reception of adaptation layer notifications.  Setting the flag to 0
   will disable adaptation layer event notifications.  For more
   information on event notifications please see Section 5.3.

   sctp_authentication_event - Setting this flag to 1 will enable the
   reception of authentication layer notifications.  Setting the flag to
   0 will disable authentication layer event notifications.  For More
   information please see Section 5.3.

   An example where an application would like to receive data io events
   and association events but no others would be as follows:

   {
      struct sctp_event_subscribe event;

      memset(&event,0,sizeof(event));

      event.sctp_data_io_event = 1;
      event.sctp_association_event = 1;

      setsockopt(fd, IPPROTO_SCTP, SCTP_EVENTS, &event, sizeof(event));
   }

   Note that for one-to-many style SCTP sockets, the caller of recvmsg()
   receives ancillary data and notifications for ALL associations bound
   to the file descriptor.  For one-to-one style SCTP sockets, the
   caller receives ancillary data and notifications for only the single
   association bound to the file descriptor.

   By default both the one-to-one style and one-to-many style socket has
   all options off.


8.  New Interfaces

   Depending on the system, the following interface can be implemented
   as a system call or library function.

8.1.  sctp_bindx()

   The syntax of sctp_bindx() is,




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   int sctp_bindx(int sd, struct sockaddr *addrs, int addrcnt,
                  int flags);

   If sd is an IPv4 socket, the addresses passed must be IPv4 addresses.
   If the sd is an IPv6 socket, the addresses passed can either be IPv4
   or IPv6 addresses.

   A single address may be specified as INADDR_ANY or IN6ADDR_ANY, see
   Section 3.1.2 for this usage.

   addrs is a pointer to an array of one or more socket addresses.  Each
   address is contained in its appropriate structure.  For an IPv6
   socket, an array of sockaddr_in6 would be returned.  For a IPv4
   socket, an array of sockaddr_in would be returned.  The caller
   specifies the number of addresses in the array with addrcnt.  Note
   that the wildcard addresses cannot be used with this function, doing
   so will result in an error.

   On success, sctp_bindx() returns 0.  On failure, sctp_bindx() returns
   -1, and sets errno to the appropriate error code.

   For SCTP, the port given in each socket address must be the same, or
   sctp_bindx() will fail, setting errno to EINVAL.

   The flags parameter is formed from the bitwise OR of zero or more of
   the following currently defined flags:

   SCTP_BINDX_ADD_ADDR

   SCTP_BINDX_REM_ADDR

   SCTP_BINDX_ADD_ADDR directs SCTP to add the given addresses to the
   association, and SCTP_BINDX_REM_ADDR directs SCTP to remove the given
   addresses from the association.  The two flags are mutually
   exclusive; if both are given, sctp_bindx() will fail with EINVAL.  A
   caller may not remove all addresses from an association; sctp_bindx()
   will reject such an attempt with EINVAL.

   An application can use sctp_bindx(SCTP_BINDX_ADD_ADDR) to associate
   additional addresses with an endpoint after calling bind().  Or use
   sctp_bindx(SCTP_BINDX_REM_ADDR) to remove some addresses a listening
   socket is associated with so that no new association accepted will be
   associated with those addresses.  If the endpoint supports dynamic
   address a SCTP_BINDX_REM_ADDR or SCTP_BINDX_ADD_ADDR may cause a
   endpoint to send the appropriate message to the peer to change the
   peers address lists.

   Adding and removing addresses from a connected association is



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   optional functionality.  Implementations that do not support this
   functionality should return EOPNOTSUPP.

8.2.  Branched-off Association

   After an association is established on a one-to-many style socket,
   the application may wish to branch off the association into a
   separate socket/file descriptor.

   This is particularly desirable when, for instance, the application
   wishes to have a number of sporadic message senders/receivers remain
   under the original one-to-many style socket but branch off those
   associations carrying high volume data traffic into their own
   separate socket descriptors.

   The application uses sctp_peeloff() call to branch off an association
   into a separate socket (Note the semantics are somewhat changed from
   the traditional one-to-one style accept() call).  Note that the new
   socket is a one-to-one style socket.  Thus it will be confined to
   operations allowed for a one-to-one style socket.

   The syntax is:

   new_sd = sctp_peeloff(int sd, sctp_assoc_t assoc_id);

      the new socket descriptor representing the branched-off
      association.
      the original one-to-many style socket descriptor returned from the
      socket() system call (see Section 3.1.1).
      the specified identifier of the association that is to be branched
      off to a separate file descriptor (Note, in a traditional one-to-
      one style accept() call, this would be an out parameter, but for
      the one-to-many style call, this is an in parameter).

8.3.  sctp_getpaddrs()

   sctp_getpaddrs() returns all peer addresses in an association.  The
   syntax is,

     int sctp_getpaddrs(int sd, sctp_assoc_t id,
                        struct sockaddr **addrs);


   On return, addrs will point to an array dynamically allocated
   sockaddr structures of the appropriate type for the socket type.  The
   caller should use sctp_freepaddrs() to free the memory.  Note that
   the in/out parameter addrs must not be NULL.




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   If sd is an IPv4 socket, the addresses returned will be all IPv4
   addresses.  If sd is an IPv6 socket, the addresses returned can be a
   mix of IPv4 or IPv6 addresses.

   For one-to-many style sockets, id specifies the association to query.
   For one-to-one style sockets, id is ignored.

   On success, sctp_getpaddrs() returns the number of peer addresses in
   the association.  If there is no association on this socket,
   sctp_getpaddrs() returns 0, and the value of *addrs is undefined.  If
   an error occurs, sctp_getpaddrs() returns -1, and the value of *addrs
   is undefined.

8.4.  sctp_freepaddrs()



    sctp_freepaddrs() frees all resources allocated by
    sctp_getpaddrs(). Its syntax is,

    void sctp_freepaddrs(struct sockaddr *addrs);
    addrs is the array of peer addresses returned by sctp_getpaddrs().


8.5.  sctp_getladdrs()

   sctp_getladdrs() returns all locally bound address(es) on a socket.
   The syntax is,

       int sctp_getladdrs(int sd, sctp_assoc_t id,
                          struct sockaddr **ss);


   On return, addrs will point to a dynamically allocated array of
   sockaddr structures of the appropriate type for the socket type.  The
   caller should use sctp_freeladdrs() to free the memory.  Note that
   the in/out parameter addrs must not be NULL.

   If sd is an IPv4 socket, the addresses returned will be all IPv4
   addresses.  If sd is an IPv6 socket, the addresses returned can be a
   mix of IPv4 or IPv6 addresses.

   For one-to-many style sockets, id specifies the association to query.
   For one-to-one style sockets, id is ignored.

   If the id field is set to the value '0' then the locally bound
   addresses are returned without regard to any particular association.




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   On success, sctp_getladdrs() returns the number of local addresses
   bound to the socket.  If the socket is unbound, sctp_getladdrs()
   returns 0, and the value of *addrs is undefined.  If an error occurs,
   sctp_getladdrs() returns -1, and the value of *addrs is undefined.

8.6.  sctp_freeladdrs()

     sctp_freeladdrs() frees all resources allocated by
     sctp_getladdrs(). Its syntax is,

     void sctp_freeladdrs(struct sockaddr *addrs);

     addrs is the array of peer addresses returned by sctp_getladdrs().

8.7.  sctp_sendmsg()

   An implementation may provide a library function (or possibly system
   call) to assist the user with the advanced features of SCTP.


       sctp_sendmsg(). Its syntax is,

       ssize_t sctp_sendmsg(int sd,
                         const void *msg,
                         size_t len,
                         const struct sockaddr *to,
                         socklen_t tolen,
                         uint32_t ppid,
                         uint32_t flags,
                         uint16_t stream_no,
                         uint32_t timetolive,
                         uint32_t context)


   sd - is the socket descriptor
   msg - is the message to be sent.
   len - is the length of the message.
   to - is the destination address of the message.
   tolen - is the length of the destination address.
   ppid - is the same as sinfo_ppid (see section 5.2.2)
   flags - is the same as sinfo_flags (see section 5.2.2)
   stream_no - is the same as sinfo_stream (see section 5.2.2)
   timetolive - is the same as sinfo_timetolive (see section 5.2.2)
   context - is the same as sinfo_context (see section 5.2.2)

8.8.  sctp_recvmsg()

   An implementation may provide a library function (or possibly system



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   call) to assist the user with the advanced features of SCTP.  Note
   that in order for the sctp_sndrcvinfo structure to be filled in by
   sctp_recvmsg() the caller must enable the sctp_data_io_events with
   the SCTP_EVENTS option.  Note that the setting of the
   SCTP_USE_EXT_RCVINFO will effect this function as well, causing the
   sctp_sndrcvinfo information to be extended.


       sctp_recvmsg(). Its syntax is,

       ssize_t sctp_recvmsg(int sd,
                        void *msg,
                        size_t len,
                        struct sockaddr *from,
                        socklen_t *fromlen
                        struct sctp_sndrcvinfo *sinfo
                        int *msg_flags)


   sd - is the socket descriptor
   msg - is a message buffer to be filled.
   len - is the length of the message buffer.
   from - is a pointer to a address to be filled with the sender of this
      messages address.
   fromlen - is the from length.
   sinfo - A pointer to a sctp_sndrcvinfo structure to be filled upon
      receipt of the message.
   msg_flags - A pointer to a integer to be filled with any message
      flags (e.g.  MSG_NOTIFICATION).

8.9.  sctp_connectx()

   An implementation may provide a library function (or possibly system
   call) to assist the user with associating to an endpoint that is
   multi-homed.  Much like sctp_bindx() this call allows a caller to
   specify multiple addresses at which a peer can be reached.  The way
   the SCTP stack uses the list of addresses to set up the association
   is implementation dependent.  This function only specifies that the
   stack will try to make use of all the addresses in the list when
   needed.

   Note that the list of addresses passed in is only used for setting up
   the association.  It does not necessarily equal the set of addresses
   the peer uses for the resulting association.  If the caller wants to
   find out the set of peer addresses, it must use sctp_getpaddrs() to
   retrieve them after the association has been set up.





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       sctp_connectx(). Its syntax is,


       int sctp_connectx(int sd,
                         struct sockaddr *addrs,
                         int addrcnt)


   sd - is the socket descriptor
   addrs - is an array of addresses.
   addrcnt - is the number of addresses in the array.

8.10.  sctp_send()

   An implementation may provide another alternative function or system
   call to assist an application with the sending of data without the
   use of the CMSG header structures.  The function takes the following
   form:

       sctp_send(). Its syntax is,

       int sctp_send(int sd,
                     const void *msg,
                     size_t len,
                     const struct sctp_sndrcvinfo *sinfo,
                     int flags);


   sd - is the socket descriptor
   msg - The message to be sent
   len - The length of the message
   sinfo - A pointer to a sctp_sndrcvinfo structure used as described in
      5.2.2 for a sendmsg call.
   flags - is used in the same format as the sendmsg call flags (e.g.
      MSG_DONTROUTE).

   This function call may also be used to terminate an association using
   an association identification by setting the sinfo.sinfo_flags to
   SCTP_EOF and the sinfo.sinfo_assoc_id to the association that needs
   to be terminated.  In such a case the len of the message would be
   zero.

8.11.  sctp_sendx()

   An implementation may provide another alternative function or system
   call to assist an application with the sending of data without the
   use of the CMSG header structures that also gives a list of
   addresses.  The list of addresses is provided for implicit



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   association setup.  In such a case the list of addresses serves the
   same purpose as the addresses given in sctp_connectx (see
   Section 8.9).


       sctp_sendx(). Its syntax is,

       int sctp_sendx(int sd,
                      const void *msg,
                      size_t len,
                      struct sockaddr *addrs,
                      int addrcnt,
                      struct sctp_sndrcvinfo *sinfo,
                      int flags);

   sd - is the socket descriptor
   msg - The message to be sent
   len - The length of the message
   addrs - is an array of addresses.
   addrcnt - is the number of addresses in the array.
   sinfo - A pointer to a sctp_sndrcvinfo structure used as described in
      5.2.2 for a sendmsg call.
   flags - is used in the same format as the sendmsg call flags (e.g.
      MSG_DONTROUTE).

   Note that on return from this call the sinfo structure will have
   changed in that the sinfo_assoc_id will be filled in with the new
   association id.

   This function call may also be used to terminate an association using
   an association identification by setting the sinfo.sinfo_flags to
   SCTP_EOF and the sinfo.sinfo_assoc_id to the association that needs
   to be terminated.  In such a case the len of the message would be
   zero.

8.12.  sctp_getaddrlen

   For application binary portability it is sometimes desirable to know
   what the kernel thinks is the length of a socket address family.
   This function, when called with a valid family type will return the
   length that the operating system uses in the specified family's
   socket address structure.


      int sctp_getaddrlen(sa_family_t family);






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9.  Preprocessor Constants

   For application portability it is desirable to define pre-processor
   constants for determination if sctp is present and supports various
   features.  The following pre-processor constants should be defined in
   a include file, sctp.h.
   HAVE_SCTP - If this constant is defined to 1, then an implementation
      of SCTP is available.
   HAVE_KERNEL_SCTP - If this constant is defined to 1, then a kernel
      SCTP implementation is available through the sockets interface.
   HAVE_SCTP_PRSCTP - If this constant is defined to 1, then the SCTP
      implementation supports the partial reliability extension to SCTP.
   HAVE_SCTP_ADDIP - If this constant is defined to 1, then the SCTP
      implementation supports the dynamic address extension to SCTP.
   HAVE_SCTP_CANSET_PRIMARY - If this constant is defined to 1, then the
      SCTP implementation supports the ability to request setting of the
      remote primary address.
   HAVE_SCTP_SAT_NETWORK_CAPABILITY - If this constant is defined to 1,
      then the SCTP implementation supports the satellite network
      extension to SCTP.
   HAVE_SCTP_MULTIBUF - If this constant is defined to 1, then the SCTP
      implementation dedicates separate buffer space to each association
      on a one-to-many socket.  If this constant is defined to 0, then
      the implementation provides a single block of shared buffer space
      for a one-to-many socket.
   HAVE_SCTP_NOCONNECT - If this constant is defined to 1, then the SCTP
      implementation supports initiating an association on a one-to-one
      style socket without the use of connect(), as outlined in
      Section 4.1.5.
   HAVE_SCTP_EXT_RCVINFO - If this constant is defined to 1, then the
      SCTP implementation supports the use of the extended style
      sndrecinfo structure, sctp_extrcvinfo.


10.  IANA considerations

   This document contains no IANA considerations.


11.  Security Considerations

   Many TCP and UDP implementations reserve port numbers below 1024 for
   privileged users.  If the target platform supports privileged users,
   the SCTP implementation SHOULD restrict the ability to call bind() or
   sctp_bindx() on these port numbers to privileged users.

   Similarly unprivileged users should not be able to set protocol
   parameters which could result in the congestion control algorithm



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   being more aggressive than permitted on the public Internet.  These
   parameters are:

   struct sctp_rtoinfo

   If an unprivileged user inherits a one-to-many style socket with open
   associations on a privileged port, it MAY be permitted to accept new
   associations, but it SHOULD NOT be permitted to open new
   associations.  This could be relevant for the r* family of protocols.


12.  Acknowledgments

   Special acknowledgment is given to Ken Fujita and Jonathan Woods who
   helped extensively in the early formation of this document.

   The authors also wish to thank Kavitha Baratakke, Mike Bartlett, Jon
   Berger, Mark Butler, Scott Kimble, Renee Revis, and many others on
   the TSVWG mailing list for contributing valuable comments.

   A special thanks to Phillip Conrad, for his suggested text, quick and
   constructive insights, and most of all his persistent fighting to
   keep the interface to SCTP usable for the application programmer.


13.  References

13.1.  Normative references

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

   [RFC1644]  Braden, B., "T/TCP -- TCP Extensions for Transactions
              Functional Specification", RFC 1644, July 1994.

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2292]  Stevens, W. and M. Thomas, "Advanced Sockets API for
              IPv6", RFC 2292, February 1998.

   [RFC2553]  Gilligan, R., Thomson, S., Bound, J., and W. Stevens,



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              "Basic Socket Interface Extensions for IPv6", RFC 2553,
              March 1999.

   [RFC2960]  Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
              Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
              Zhang, L., and V. Paxson, "Stream Control Transmission
              Protocol", RFC 2960, October 2000.

13.2.  Informational References


Appendix A.  one-to-one style Code Example

   The following code is a simple implementation of an echo server over
   SCTP.  The example shows how to use some features of one-to-one style
   IPv4 SCTP sockets, including:

   o  Opening, binding, and listening for new associations on a socket;
   o  Enabling ancillary data
   o  Enabling notifications
   o  Using ancillary data with sendmsg() and recvmsg()
   o  Using MSG_EOR to determine if an entire message has been read
   o  Handling notifications

   #include <stdio.h>
   #include <sys/types.h>
   #include <sys/socket.h>
   #include <netinet/in.h>
   #include <arpa/inet.h>
   #include <stdlib.h>
   #include <unistd.h>
   #include <netinet/sctp.h>
   #include <sys/uio.h>

   #define BUFLEN  100

   static void
   handle_event(void *buf)
   {
       struct sctp_assoc_change *sac;
       struct sctp_send_failed *ssf;
       struct sctp_paddr_change *spc;
       struct sctp_remote_error *sre;
       union sctp_notification *snp;
       char addrbuf[INET6_ADDRSTRLEN];
       const char *ap;
       struct sockaddr_in *sin;
       struct sockaddr_in6 *sin6;



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       snp = buf;

       switch (snp->sn_header.sn_type) {
       case SCTP_ASSOC_CHANGE:
             sac = &snp->sn_assoc_change;
             printf("^^^ assoc_change: state=%hu, error=%hu, instr=%hu "
                 "outstr=%hu\n", sac->sac_state, sac->sac_error,
                 sac->sac_inbound_streams, sac->sac_outbound_streams);
             break;
       case SCTP_SEND_FAILED:
             ssf = &snp->sn_send_failed;
             printf("^^^ sendfailed: len=%hu err=%d\n", ssf->ssf_length,
                 ssf->ssf_error);
             break;

       case SCTP_PEER_ADDR_CHANGE:
             spc = &snp->sn_paddr_change;
             if (spc->spc_aaddr.ss_family == AF_INET) {
               sin = (struct sockaddr_in *)&spc->spc_aaddr;
               ap = inet_ntop(AF_INET, &sin->sin_addr,
                              addrbuf, INET6_ADDRSTRLEN);
             } else {
               sin6 = (struct sockaddr_in6 *)&spc->spc_aaddr;
               ap = inet_ntop(AF_INET6, &sin6->sin6_addr,
                              addrbuf, INET6_ADDRSTRLEN);
             }
             printf("^^^ intf_change: %s state=%d, error=%d\n", ap,
                    spc->spc_state, spc->spc_error);
             break;
       case SCTP_REMOTE_ERROR:
             sre = &snp->sn_remote_error;
             printf("^^^ remote_error: err=%hu len=%hu\n",
                 ntohs(sre->sre_error), ntohs(sre->sre_length));
             break;
       case SCTP_SHUTDOWN_EVENT:
             printf("^^^ shutdown event\n");
             break;
       default:
             printf("unknown type: %hu\n", snp->sn_header.sn_type);
             break;
       };
   }

   static void *
   mysctp_recvmsg(int fd, struct msghdr *msg, void *buf, size_t *buflen,
       ssize_t *nrp, size_t cmsglen)
   {
       ssize_t nr = 0, nnr = 0;



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       struct iovec iov[1];

       *nrp = 0;
       iov->iov_base = buf;
       iov->iov_len = *buflen;
       msg->msg_iov = iov;
       msg->msg_iovlen = 1;


       for (;;) {
   #ifndef MSG_XPG4_2
   #define MSG_XPG4_2 0
   #endif
               msg->msg_flags = MSG_XPG4_2;
               msg->msg_controllen = cmsglen;

               nnr = recvmsg(fd, msg, 0);
               if (nnr <= 0) {
                       /* EOF or error */
                       *nrp = nr;
                       return (NULL);
               }
               nr += nnr;

               if ((msg->msg_flags & MSG_EOR) != 0) {
                       *nrp = nr;
                       return (buf);
               }

               /* Realloc the buffer? */
               if (*buflen == (size_t)nr) {
                       buf = realloc(buf, *buflen * 2);
                       if (buf == 0) {
                               fprintf(stderr, "out of memory\n");
                               exit(1);
                       }
                       *buflen *= 2;
               }
               /* Set the next read offset */
               iov->iov_base = (char *)buf + nr;
               iov->iov_len = *buflen - nr;
        }
   }

   static void
   echo(int fd, int socketModeone_to_many)
   {
       ssize_t nr;



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       struct sctp_sndrcvinfo *sri;
       struct msghdr msg[1];
       struct cmsghdr *cmsg;
       char cbuf[sizeof (*cmsg) + sizeof (*sri)];
       char *buf;
       size_t buflen;
       struct iovec iov[1];
       size_t cmsglen = sizeof (*cmsg) + sizeof (*sri);
       /* Allocate the initial data buffer */
       buflen = BUFLEN;
       if (!(buf = malloc(BUFLEN))) {
               fprintf(stderr, "out of memory\n");
               exit(1);
       }

       /* Set up the msghdr structure for receiving */
       memset(msg, 0, sizeof (*msg));
       msg->msg_control = cbuf;
       msg->msg_controllen = cmsglen;
       msg->msg_flags = 0;
       cmsg = (struct cmsghdr *)cbuf;
       sri = (struct sctp_sndrcvinfo *)(cmsg + 1);

       /* Wait for something to echo */
       while (buf = mysctp_recvmsg(fd, msg,
                buf, &buflen, &nr, cmsglen)) {

               /* Intercept notifications here */
               if (msg->msg_flags & MSG_NOTIFICATION) {
                       handle_event(buf);
                       continue;
               }

               iov->iov_base = buf;
               iov->iov_len = nr;
               msg->msg_iov = iov;
               msg->msg_iovlen = 1;

               printf("got %u bytes on stream %hu:\n", nr,
                   sri->sinfo_stream);
               write(0, buf, nr);

               /* Echo it back */
               msg->msg_flags = MSG_XPG4_2;
               if (sendmsg(fd, msg, 0) < 0) {
                       perror("sendmsg");
                       exit(1);
               }



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       }

       if (nr < 0) {
               perror("recvmsg");
       }
       if(socketModeone_to_many == 0)
            close(fd);
   }

   int main()
   {
       struct sctp_event_subscribe event;
       int lfd, cfd;
       int onoff = 1;
       struct sockaddr_in sin[1];
       if ((lfd = socket(AF_INET, SOCK_STREAM, IPPROTO_SCTP)) == -1) {
               perror("socket");
               exit(1);
       }

       sin->sin_family = AF_INET;
       sin->sin_port = htons(7);
       sin->sin_addr.s_addr = INADDR_ANY;
       if (bind(lfd, (struct sockaddr *)sin, sizeof (*sin)) == -1) {
               perror("bind");
               exit(1);
       }

       if (listen(lfd, 1) == -1) {
               perror("listen");
               exit(1);
       }

       /* Wait for new associations */
       for (;;) {
               if ((cfd = accept(lfd, NULL, 0)) == -1) {
                       perror("accept");
                       exit(1);
               }

               /* Enable all events */
               event.sctp_data_io_event = 1;
               event.sctp_association_event = 1;
               event.sctp_address_event = 1;
               event.sctp_send_failure_event = 1;
               event.sctp_peer_error_event = 1;
               event.sctp_shutdown_event = 1;
               event.sctp_partial_delivery_event = 1;



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               event.sctp_adaptation_layer_event = 1;
               if (setsockopt(cfd, IPPROTO_SCTP,
                   SCTP_EVENTS, &event,
                   sizeof(event)) != 0) {
                   perror("setevent failed");
                   exit(1);
               }
               /* Echo back any and all data */
               echo(cfd,0);
       }
   }


Appendix B.  one-to-many style Code Example

   The following code is a simple implementation of an echo server over
   SCTP.  The example shows how to use some features of one-to-many
   style IPv4 SCTP sockets, including:

   o  Opening and binding of a socket;
   o  Enabling ancillary data
   o  Enabling notifications
   o  Using ancillary data with sendmsg() and recvmsg()
   o  Using MSG_EOR to determine if an entire message has been read
   o  Handling notifications

   Note most functions defined in Appendix A are reused in this example.

   int main()
   {
       int fd;
       int idleTime = 2;
       struct sockaddr_in sin[1];
       struct sctp_event_subscribe event;

       if ((fd = socket(AF_INET, SOCK_SEQPACKET, IPPROTO_SCTP)) == -1) {
         perror("socket");
         exit(1);
       }

       sin->sin_family = AF_INET;
       sin->sin_port = htons(7);
       sin->sin_addr.s_addr = INADDR_ANY;
       if (bind(fd, (struct sockaddr *)sin, sizeof (*sin)) == -1) {
         perror("bind");
         exit(1);
       }




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       /* Enable all notifications and events */
       event.sctp_data_io_event = 1;
       event.sctp_association_event = 1;
       event.sctp_address_event = 1;
       event.sctp_send_failure_event = 1;
       event.sctp_peer_error_event = 1;
       event.sctp_shutdown_event = 1;
       event.sctp_partial_delivery_event = 1;
       event.sctp_adaptation_layer_event = 1;
       if (setsockopt(fd, IPPROTO_SCTP,
           SCTP_EVENTS, &event,
           sizeof(event)) != 0) {
           perror("setevent failed");
           exit(1);
       }
       /* Set associations to auto-close in 2 seconds of
        * inactivity
        */
       if (setsockopt(fd, IPPROTO_SCTP, SCTP_AUTOCLOSE,
                         &idleTime, 4) < 0) {
         perror("setsockopt SCTP_AUTOCLOSE");
         exit(1);
       }

       /* Allow new associations to be accepted */
       if (listen(fd, 1) < 0) {
         perror("listen");
         exit(1);
       }

       /* Wait for new associations */
       while(1){
         /* Echo back any and all data */
         echo(fd,1);
       }
   }















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Authors' Addresses

   Randall R. Stewart
   Cisco Systems, Inc.
   4875 Forest Drive
   Suite 200
   Columbia, SC  29206
   USA

   Phone:
   Email: rrs@cisco.com


   Qiaobing Xie
   Motorola, Inc.
   1501 W. Shure Drive, #2309
   Arlington Heights, IL  60004
   USA

   Phone:
   Email: qxie1@email.mot.com


   La Monte H.P. Yarroll
   TimeSys Corp
   925 Liberty Ave.
   Pittsburgh, PA  15222
   USA

   Phone:
   Email: piggy@acm.org


   Kacheong Poon
   Sun Microsystems, Inc.
   4150 Network Circle
   Santa Clara, CA  95054
   USA

   Phone:
   Email: kacheong.poon@sun.com










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   Michael Tuexen
   Univ. of Applied Sciences Muenster
   Stegerwaldstr. 39
   48565 Steinfurt
   Germany

   Email: tuexen@fh-muenster.de












































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   Copyright (C) The Internet Society (2006).  This document is subject
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   Internet Society.




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