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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 RFC 6458

Network Working Group                                         R. Stewart
Internet-Draft                                                    Huawei
Intended status: Informational                                   K. Poon
Expires: August 5, 2010                           Sun Microsystems, Inc.
                                                               M. Tuexen
                                      Muenster Univ. of Applied Sciences
                                                             V. Yasevich
                                                                      HP
                                                                  P. Lei
                                                     Cisco Systems, Inc.
                                                        February 1, 2010


 Sockets API Extensions for Stream Control Transmission Protocol (SCTP)
                   draft-ietf-tsvwg-sctpsocket-21.txt

Abstract

   This document describes a mapping of the Stream Control Transmission
   Protocol SCTP 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.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on August 5, 2010.

Copyright Notice




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   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   Contributions published or made publicly available before November
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   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.



























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  6
   2.  Data Types . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  One-to-Many Style Interface  . . . . . . . . . . . . . . . . .  7
     3.1.  Basic Operation  . . . . . . . . . . . . . . . . . . . . .  7
       3.1.1.  socket() . . . . . . . . . . . . . . . . . . . . . . .  8
       3.1.2.  bind() . . . . . . . . . . . . . . . . . . . . . . . .  9
       3.1.3.  listen() . . . . . . . . . . . . . . . . . . . . . . . 10
       3.1.4.  sendmsg() and recvmsg()  . . . . . . . . . . . . . . . 10
       3.1.5.  close()  . . . . . . . . . . . . . . . . . . . . . . . 12
       3.1.6.  connect()  . . . . . . . . . . . . . . . . . . . . . . 13
     3.2.  Implicit Association Setup . . . . . . . . . . . . . . . . 13
     3.3.  Non-blocking mode  . . . . . . . . . . . . . . . . . . . . 14
     3.4.  Special considerations . . . . . . . . . . . . . . . . . . 15
   4.  One-to-One Style Interface . . . . . . . . . . . . . . . . . . 16
     4.1.  Basic Operation  . . . . . . . . . . . . . . . . . . . . . 17
       4.1.1.  socket() . . . . . . . . . . . . . . . . . . . . . . . 17
       4.1.2.  bind() . . . . . . . . . . . . . . . . . . . . . . . . 18
       4.1.3.  listen() . . . . . . . . . . . . . . . . . . . . . . . 19
       4.1.4.  accept() . . . . . . . . . . . . . . . . . . . . . . . 19
       4.1.5.  connect()  . . . . . . . . . . . . . . . . . . . . . . 20
       4.1.6.  close()  . . . . . . . . . . . . . . . . . . . . . . . 21
       4.1.7.  shutdown() . . . . . . . . . . . . . . . . . . . . . . 21
       4.1.8.  sendmsg() and recvmsg()  . . . . . . . . . . . . . . . 22
       4.1.9.  getpeername()  . . . . . . . . . . . . . . . . . . . . 22
   5.  Data Structures  . . . . . . . . . . . . . . . . . . . . . . . 23
     5.1.  The msghdr and cmsghdr Structures  . . . . . . . . . . . . 23
     5.2.  SCTP msg_control Structures  . . . . . . . . . . . . . . . 24
       5.2.1.  SCTP Initiation Structure (SCTP_INIT)  . . . . . . . . 24
       5.2.2.  SCTP Header Information Structure (SCTP_SNDRCV)  . . . 25
       5.2.3.  Extended SCTP Header Information Structure
               (SCTP_EXTRCV)  . . . . . . . . . . . . . . . . . . . . 28
       5.2.4.  SCTP Send Information Structure (SCTP_SNDINFO) . . . . 29
       5.2.5.  SCTP Receive Information Structure (SCTP_RCVINFO)  . . 31
       5.2.6.  SCTP Next Receive Information Structure
               (SCTP_NXTINFO) . . . . . . . . . . . . . . . . . . . . 32
       5.2.7.  SCTP PR-SCTP Information Structure (SCTP_PRINFO) . . . 32
       5.2.8.  SCTP AUTH Information Structure (SCTP_AUTHINFO)  . . . 33
     5.3.  SCTP Events and Notifications  . . . . . . . . . . . . . . 33
       5.3.1.  SCTP Notification Structure  . . . . . . . . . . . . . 34
       5.3.2.  SCTP_ASSOC_CHANGE  . . . . . . . . . . . . . . . . . . 35
       5.3.3.  SCTP_PEER_ADDR_CHANGE  . . . . . . . . . . . . . . . . 36
       5.3.4.  SCTP_REMOTE_ERROR  . . . . . . . . . . . . . . . . . . 37
       5.3.5.  SCTP_SEND_FAILED . . . . . . . . . . . . . . . . . . . 38
       5.3.6.  SCTP_SHUTDOWN_EVENT  . . . . . . . . . . . . . . . . . 39
       5.3.7.  SCTP_ADAPTATION_INDICATION . . . . . . . . . . . . . . 40
       5.3.8.  SCTP_PARTIAL_DELIVERY_EVENT  . . . . . . . . . . . . . 40



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       5.3.9.  SCTP_AUTHENTICATION_EVENT  . . . . . . . . . . . . . . 41
       5.3.10. SCTP_SENDER_DRY_EVENT  . . . . . . . . . . . . . . . . 42
       5.3.11. SCTP_NOTIFICATIONS_STOPPED_EVENT . . . . . . . . . . . 43
     5.4.  Ancillary Data Considerations and Semantics  . . . . . . . 43
       5.4.1.  Multiple Items and Ordering  . . . . . . . . . . . . . 43
       5.4.2.  Accessing and Manipulating Ancillary Data  . . . . . . 43
       5.4.3.  Control Message Buffer Sizing  . . . . . . . . . . . . 44
   6.  Common Operations for Both Styles  . . . . . . . . . . . . . . 45
     6.1.  send(), recv(), sendto(), and recvfrom() . . . . . . . . . 45
     6.2.  setsockopt() and getsockopt()  . . . . . . . . . . . . . . 47
     6.3.  read() and write() . . . . . . . . . . . . . . . . . . . . 48
     6.4.  getsockname()  . . . . . . . . . . . . . . . . . . . . . . 48
   7.  Socket Options . . . . . . . . . . . . . . . . . . . . . . . . 48
     7.1.  Read / Write Options . . . . . . . . . . . . . . . . . . . 50
       7.1.1.  Retransmission Timeout Parameters (SCTP_RTOINFO) . . . 50
       7.1.2.  Association Parameters (SCTP_ASSOCINFO)  . . . . . . . 51
       7.1.3.  Initialization Parameters (SCTP_INITMSG) . . . . . . . 52
       7.1.4.  SO_LINGER  . . . . . . . . . . . . . . . . . . . . . . 53
       7.1.5.  SCTP_NODELAY . . . . . . . . . . . . . . . . . . . . . 53
       7.1.6.  SO_RCVBUF  . . . . . . . . . . . . . . . . . . . . . . 53
       7.1.7.  SO_SNDBUF  . . . . . . . . . . . . . . . . . . . . . . 54
       7.1.8.  Automatic Close of Associations (SCTP_AUTOCLOSE) . . . 54
       7.1.9.  Set Primary Address (SCTP_PRIMARY_ADDR)  . . . . . . . 54
       7.1.10. Set Adaptation Layer Indicator
               (SCTP_ADAPTATION_LAYER)  . . . . . . . . . . . . . . . 55
       7.1.11. Enable/Disable Message Fragmentation
               (SCTP_DISABLE_FRAGMENTS) . . . . . . . . . . . . . . . 55
       7.1.12. Peer Address Parameters (SCTP_PEER_ADDR_PARAMS)  . . . 55
       7.1.13. Set Default Send Parameters
               (SCTP_DEFAULT_SEND_PARAM)  . . . . . . . . . . . . . . 58
       7.1.14. Set Notification and Ancillary Events (SCTP_EVENTS)  . 58
       7.1.15. Set/Clear IPv4 Mapped Addresses
               (SCTP_I_WANT_MAPPED_V4_ADDR) . . . . . . . . . . . . . 58
       7.1.16. Get or Set the Maximum Fragmentation Size
               (SCTP_MAXSEG)  . . . . . . . . . . . . . . . . . . . . 58
       7.1.17. Get or Set the List of Supported HMAC Identifiers
               (SCTP_HMAC_IDENT)  . . . . . . . . . . . . . . . . . . 59
       7.1.18. Get or Set the Active Shared Key
               (SCTP_AUTH_ACTIVE_KEY) . . . . . . . . . . . . . . . . 60
       7.1.19. Get or Set Delayed SACK Timer (SCTP_DELAYED_SACK)  . . 60
       7.1.20. Get or Set Fragmented Interleave
               (SCTP_FRAGMENT_INTERLEAVE) . . . . . . . . . . . . . . 61
       7.1.21. Set or Get the SCTP Partial Delivery Point
               (SCTP_PARTIAL_DELIVERY_POINT)  . . . . . . . . . . . . 62
       7.1.22. Set or Get the Use of Extended Receive Info
               (SCTP_USE_EXT_RCVINFO) . . . . . . . . . . . . . . . . 63
       7.1.23. Set or Get the Auto ASCONF Flag (SCTP_AUTO_ASCONF) . . 63
       7.1.24. Set or Get the Maximum Burst (SCTP_MAX_BURST)  . . . . 63



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       7.1.25. Set or Get the Default Context (SCTP_CONTEXT)  . . . . 64
       7.1.26. Enable or Disable Explicit EOR Marking
               (SCTP_EXPLICIT_EOR)  . . . . . . . . . . . . . . . . . 64
       7.1.27. Enable SCTP Port Reusage (SCTP_REUSE_PORT) . . . . . . 64
       7.1.28. Set Notification Event (SCTP_EVENT)  . . . . . . . . . 65
     7.2.  Read-Only Options  . . . . . . . . . . . . . . . . . . . . 65
       7.2.1.  Association Status (SCTP_STATUS) . . . . . . . . . . . 65
       7.2.2.  Peer Address Information (SCTP_GET_PEER_ADDR_INFO) . . 66
       7.2.3.  Get the List of Chunks the Peer Requires to be
               Authenticated (SCTP_PEER_AUTH_CHUNKS)  . . . . . . . . 67
       7.2.4.  Get the List of Chunks the Local Endpoint Requires
               to be Authenticated (SCTP_LOCAL_AUTH_CHUNKS) . . . . . 68
       7.2.5.  Get the Current Number of Associations
               (SCTP_GET_ASSOC_NUMBER)  . . . . . . . . . . . . . . . 68
       7.2.6.  Get the Current Identifiers of Associations
               (SCTP_GET_ASSOC_ID_LIST) . . . . . . . . . . . . . . . 68
     7.3.  Write-Only Options . . . . . . . . . . . . . . . . . . . . 69
       7.3.1.  Set Peer Primary Address
               (SCTP_SET_PEER_PRIMARY_ADDR) . . . . . . . . . . . . . 69
       7.3.2.  Add a Chunk That Must Be Authenticated
               (SCTP_AUTH_CHUNK)  . . . . . . . . . . . . . . . . . . 69
       7.3.3.  Set a Shared Key (SCTP_AUTH_KEY) . . . . . . . . . . . 70
       7.3.4.  Deactivate a Shared Key (SCTP_AUTH_DEACTIVATE_KEY) . . 70
       7.3.5.  Delete a Shared Key (SCTP_AUTH_DELETE_KEY) . . . . . . 71
     7.4.  Ancillary Data and Notification Interest Options . . . . . 72
   8.  New Functions  . . . . . . . . . . . . . . . . . . . . . . . . 75
     8.1.  sctp_bindx() . . . . . . . . . . . . . . . . . . . . . . . 75
     8.2.  sctp_peeloff() . . . . . . . . . . . . . . . . . . . . . . 76
     8.3.  sctp_getpaddrs() . . . . . . . . . . . . . . . . . . . . . 77
     8.4.  sctp_freepaddrs()  . . . . . . . . . . . . . . . . . . . . 77
     8.5.  sctp_getladdrs() . . . . . . . . . . . . . . . . . . . . . 78
     8.6.  sctp_freeladdrs()  . . . . . . . . . . . . . . . . . . . . 78
     8.7.  sctp_sendmsg() . . . . . . . . . . . . . . . . . . . . . . 79
     8.8.  sctp_recvmsg() . . . . . . . . . . . . . . . . . . . . . . 79
     8.9.  sctp_connectx()  . . . . . . . . . . . . . . . . . . . . . 80
     8.10. sctp_send()  . . . . . . . . . . . . . . . . . . . . . . . 81
     8.11. sctp_sendx() . . . . . . . . . . . . . . . . . . . . . . . 82
     8.12. sctp_getaddrlen()  . . . . . . . . . . . . . . . . . . . . 83
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 83
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 83
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 84
   12. Normative References . . . . . . . . . . . . . . . . . . . . . 84
   Appendix A.  One-to-One Style Code Example . . . . . . . . . . . . 85
   Appendix B.  One-to-Many Style Code Example  . . . . . . . . . . . 90
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 91






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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 [RFC0793] and UDP
   [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 a UDP socket,
       which can communicate with many peer endpoints.  Each of these
       associations is assigned an association ID so that an application
       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 operating systems 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
   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.



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


2.  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:
   o  Outbound association setup is implicit.
   o  Messages are delivered in complete messages (with one notable
      exception).
   o  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:
   o  socket()
   o  bind()
   o  listen()
   o  recvmsg()
   o  sendmsg()
   o  close()

   A typical client uses the following calls in sequence to setup an
   association with a server to request services:
   o  socket()
   o  sendmsg()
   o  recvmsg()
   o  close()

   In this style, by default, all the associations connected to the
   endpoint are represented with a single socket.  Each association 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



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   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 belonging 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 and 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 endpoint.

   If the server or client wishes to branch an existing association off
   to a separate socket, it is required to call sctp_peeloff() and to
   specify the association identifier.  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.  The returned socket is a one-to-one style socket.

   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 detail in
   the following subsections.

3.1.1.  socket()

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

   The function prototype is

   int socket(int domain,
              int type,
              int protocol);

   and one uses PF_INET or PF_INET6 as the domain, SOCK_SEQPACKET as the
   type and IPPROTO_SCTP as the protocol.

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



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   socket.

   Using the PF_INET domain indicates the creation of an endpoint which
   can use only IPv4 addresses, while PF_INET6 creates an endpoint which
   can use both IPv6 and IPv4 addresses.

3.1.2.  bind()

   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 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 [RFC4960].

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

   The function prototype of bind() is

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

   and the arguments are
   sd:  The socket descriptor returned by socket().
   addr:  The address structure (struct sockaddr_in or struct
      sockaddr_in6, see [RFC3493]).
   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
   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.



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   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()

   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 function prototype is

   int listen(int sd,
              int backlog);

   and the arguments are
   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 the ones those 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()

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

   The function prototypes are





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

   and

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

   using the arguments:
   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 the
   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 an SCTP_INIT
   cmsg structure is used with NULL data, an association will be
   established using the parameters from the struct sctp_initmsg
   structure.  If no SCTP_INIT cmsg structure is used in combination
   with NULL data, an association is established using the default
   parameters.  If NULL data is used, no association exists and the
   SCTP_ABORT or SCTP_EOF flags are present, then -1 must be returned
   and an errno should be set to something like EDONOTBESTUPID.  Sending
   a message using sendmsg() is atomic unless explicit EOR marking is
   enabled on the socket specified by sd.

   If a peer sends a SHUTDOWN, an 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.



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   When receiving a user message with recvmsg(), the msg_name field in
   the 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.
   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 can be partially delivered in any stream.  The
   socket option SCTP_FRAGMENT_INTERLEAVE controls various aspects of
   what interlacing of messages occurs for both the one-to-one and the
   one-to-many model sockets.  Please consult Section 7.1.20 for further
   details on message delivery options.

   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()

   Applications use close() to perform graceful shutdown (as described
   in Section 10.1 of [RFC4960]) on ALL the associations currently
   represented by a one-to-many style socket.

   The function prototype is

   int close(int sd);

   and the argument is
   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 include the SCTP_EOF flag.  A user may optionally terminate an
   association non-gracefully by sending with the SCTP_ABORT flag and



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   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()

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

   The function prototype is

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

   and the arguments are
   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 [RFC3493]).
   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

   Implicit association setup applies only to one-to-many style sockets.
   For one-to-one style sockets implicit association setup must not be
   used.

   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 an SCTP_COMM_UP notification



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   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
   [RFC4960].

   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.

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 an
   SCTP_SEND_FAILED event.  Events can be received by the user calling
   recvmsg().  A server (having called listen()) is also notified of an
   association up event by the reception of an 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 5.3.2).  Note that this can also be
   done using the sctp_send() call described in Section 8.10.




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   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 the 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() returns 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
   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 implementers 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



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   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 implementer
   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 implementer 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:
   o  demand that the underlying implementation dedicates independent
      buffer space allotments to each association (as suggested above),
      or
   o  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
   o  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).


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 bundle DATA chunks with the COOKIE chunk when using this
   interface style.




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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:
   o  socket()
   o  bind()
   o  listen()
   o  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
   o  close()
   to terminate the association.

   A typical client uses the following system call sequence to setup an
   association with a server to request services:
   o  socket()
   o  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
   o  close()
   to terminate this association when done.

4.1.1.  socket()

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

   The function prototype is

   int socket(int domain,
              int type,
              int protocol);

   and one uses PF_INET or PF_INET6 as the domain, SOCK_STREAM as the
   type and IPPROTO_SCTP as the protocol.

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

   Using the PF_INET domain indicates the creation of an endpoint which



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   can use only IPv4 addresses, while PF_INET6 creates an endpoint which
   can use both IPv6 and IPv4 addresses.

4.1.2.  bind()

   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.

   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 [RFC4960].

   The function prototype of bind() is

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

   and the arguments are
   sd:  The socket descriptor returned by socket().
   addr:  The address structure (struct sockaddr_in or struct
      sockaddr_in6, see [RFC3493]).
   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.




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   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()

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

   The function prototype is

   int listen(int sd,
              int backlog);

   and the arguments are
   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 [RFC4960]) 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()

   Applications use the 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 function prototype is

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

   and the arguments are
   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.
   The functions returns the socket descriptor for the newly formed
   association.






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4.1.5.  connect()

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

   The function prototype is

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

   and the arguments are
   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 [RFC4960].

   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_INIT for IPv4 and IPv6 socket
   respectively.  One of those addresses will be the primary address for
   the association.  This automatically enables the multi-homing
   capability of SCTP.

   Note that SCTP allows data exchange, similar to T/TCP [RFC1644],
   during the association set up phase.  If an application wants to do
   this, it cannot use the connect() call.  Instead, it should use
   sendto() or sendmsg() to initiate an association.  If it uses
   sendto() and it wants to change the 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).

   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 an SCTP socket.






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4.1.6.  close()

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

   The function prototype is

   int close(int sd);

   and the argument is
   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()

   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 function prototype is

   int shutdown(int sd,
                int how);

   and the arguments are
   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 5.3.5 for more information).

   To perform the ABORT operation described in [RFC4960] section 10.1,



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   an application can use the socket option SO_LINGER.  It is described
   in Section 7.1.4.

4.1.8.  sendmsg() and recvmsg()

   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 socket is connected and the transport address
       given is not part of the current association, the data will not
       be sent and an SCTP_SEND_FAILED event will be delivered to the
       application if send failure events are enabled.
   2.  Using sendmsg() on a non-connected one-to-one style socket for
       implicit connection setup may or may not work depending on the
       SCTP implementation.

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 function prototype is

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

   and the arguments are:
   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.






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5.  Data Structures

   In this section we discuss 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 detail in [RFC3542].  Here we will cite their
   definitions from [RFC3542].

   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 */
   };

   and the cmsghdr structure:

   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().




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   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 the cmsg_data[] member.

   By default on either style socket, SCTP will pass no ancillary data;
   Specific ancillary data items can be enabled with socket options
   defined for SCTP; see Section 7.4.

   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 [RFC3493]) 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 ([RFC3542] and [RFC3493]) 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 those 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().



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            +--------------+-----------+---------------------+
            | 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:  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:  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:  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:  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().  This structure mixes the send and receive path.



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   SCTP_SNDINFO described in Section 5.2.4 and SCTP_RCVINFO described in
   Section 5.2.5 split this information.  These structures should be
   used, when possible, since SCTP_SNDRCV might be deprecated in the
   future.

          +--------------+-------------+------------------------+
          | 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_pr_value;
     uint32_t sinfo_tsn;
     uint32_t sinfo_cumtsn;
     sctp_assoc_t sinfo_assoc_id;
   };

   sinfo_stream:  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:  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_flags:  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
            procedure on the specified association.  Graceful shutdown
            assures that all data queued 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_ppid:  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:  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 an error occurs on the send of a message and is
      retrieved with each undelivered message (Note: if an endpoint has
      done multiple sends, all of which fail, multiple different
      sinfo_context values will be returned.  One with each user data
      message).
   sinfo_pr_value:  The meaning of this field depends on the PR-SCTP
      policy specified by the sinfo_pr_policy field.  It is ignored when
      SCTP_PR_SCTP_NONE is specified.  In case of SCTP_PR_SCTP_TTL the
      lifetime is specified.
   sinfo_tsn:  For the receiving side, this field holds a TSN that was
      assigned to one of the SCTP Data Chunks.
   sinfo_cumtsn:  This field will hold the current cumulative TSN as
      known by the underlying SCTP layer.  Note this field is ignored
      when sending.
   sinfo_assoc_id:  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.

   An sctp_sndrcvinfo item always corresponds to the data in msg_iov.




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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 (see Section 5.2.2)
   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.  Note that next message data is not
   valid unless the current message is completely read, i.e. the MSG_EOR
   is set, in other words if you have more data to read from the current
   message then no next message information will be available.

   SCTP_NXTINFO described in Section 5.2.6 should be used when possible,
   since SCTP_EXTRCV is considered deprecated.

          +--------------+-------------+------------------------+
          | cmsg_level   | cmsg_type   | cmsg_data[]            |
          +--------------+-------------+------------------------+
          | IPPROTO_SCTP | SCTP_EXTRCV | struct sctp_extrcvinfo |
          +--------------+-------------+------------------------+

   Here is the definition of sctp_extrcvinfo structure:

   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_pr_value;
     uint32_t sinfo_tsn;
     uint32_t sinfo_cumtsn;
     uint16_t serinfo_next_flags;
     uint16_t serinfo_next_stream;
     uint32_t serinfo_next_aid;
     uint32_t serinfo_next_length;
     uint32_t serinfo_next_ppid;
     sctp_assoc_t sinfo_assoc_id;
   };

   sinfo_*:  Please see Section 5.2.2 for the details for these fields.
   serinfo_next_flags:  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.



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      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.
      SCTP_NEXT_MSG_IS_NOTIFICATION:  This bit, when set, indicates that
         the next message to be received is not a message from the peer,
         but instead is a MSG_NOTIFICATION from the local SCTP stack.
   serinfo_next_stream:  This value, when valid (see
      serinfo_next_flags), contains the next stream number that will be
      received on a subsequent call to one of the receive message
      functions.
   serinfo_next_aid:  This value, when valid (see serinfo_next_flags),
      contains the next association identification that will be received
      on a subsequent call to one of the receive message functions.
   serinfo_next_length:  This value, when valid (see
      serinfo_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.
   serinfo_next_ppid:  This value, when valid (see serinfo_next_flags),
      contains the ppid of the next message that will be received on a
      subsequent call to one of the receive message functions.

5.2.4.  SCTP Send Information Structure (SCTP_SNDINFO)

   This cmsghdr structure specifies SCTP options for sendmsg().

           +--------------+--------------+---------------------+
           | cmsg_level   | cmsg_type    | cmsg_data[]         |
           +--------------+--------------+---------------------+
           | IPPROTO_SCTP | SCTP_SNDINFO | struct sctp_sndinfo |
           +--------------+--------------+---------------------+

   Here is the definition of the sctp_sndinfo structure:

   struct sctp_sndinfo {
     uint16_t snd_sid;
     uint16_t snd_flags;
     uint32_t snd_ppid;
     uint32_t snd_context;
     sctp_assoc_t snd_assoc_id;



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   };

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

      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 queued 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.
   snd_ppid:  This value in sendmsg() is an unsigned integer that is
      passed to the remote end in each user message.  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.
   snd_context:  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 an error occurs on the send of a message and is
      retrieved with each undelivered message (Note: if an endpoint has
      done multiple sends, all of which fail, multiple different
      sinfo_context values will be returned.  One with each user data
      message).
   snd_assoc_id:  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.

   An sctp_sndinfo item always corresponds to the data in msg_iov.




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5.2.5.  SCTP Receive Information Structure (SCTP_RCVINFO)

   This cmsghdr structure describes SCTP header information about a
   received message through recvmsg().

   To receive this information you must subscribe to the SCTP_RCV_EVENT
   using the SCTP_EVENT option.

           +--------------+--------------+---------------------+
           | cmsg_level   | cmsg_type    | cmsg_data[]         |
           +--------------+--------------+---------------------+
           | IPPROTO_SCTP | SCTP_RCVINFO | struct sctp_rcvinfo |
           +--------------+--------------+---------------------+

   Here is the definition of the sctp_rcvinfo structure:

   struct sctp_rcvinfo {
     uint16_t rcv_sid;
     uint16_t rcv_ssn;
     uint16_t rcv_flags;
     uint32_t rcv_ppid;
     uint32_t rcv_tsn;
     uint32_t rcv_cumtsn;
     sctp_assoc_t rcv_assoc_id;
   };

   rcv_sid:  The SCTP stack places the message's stream number in this
      value.
   rcv_ssn:  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).
   rcv_flags:  This field may contain any of the following flags and is
      composed of a bitwise OR of these values.

      SCTP_UNORDERED:  This flag is present when the message was sent
         non-ordered.
   rcv_ppid:  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.
   rcv_tsn:  This field holds a TSN that was assigned to one of the SCTP
      Data Chunks.







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   rcv_cumtsn:  This field will hold the current cumulative TSN as known
      by the underlying SCTP layer.
   rcv_assoc_id:  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_rcvinfo item always corresponds to the data in msg_iov.

5.2.6.  SCTP Next Receive Information Structure (SCTP_NXTINFO)

   This cmsghdr structure describes SCTP receive information of the next
   message which will be delivered through recvmsg() if this information
   is available.  It uses the same structure as the SCTP Receive
   Information Structure.

   To receive this information you must subscribe to the SCTP_NXT_EVENT
   using the SCTP_EVENT option.

           +--------------+--------------+---------------------+
           | cmsg_level   | cmsg_type    | cmsg_data[]         |
           +--------------+--------------+---------------------+
           | IPPROTO_SCTP | SCTP_NXTINFO | struct sctp_rcvinfo |
           +--------------+--------------+---------------------+

5.2.7.  SCTP PR-SCTP Information Structure (SCTP_PRINFO)

   This cmsghdr structure specifies SCTP options for sendmsg().

            +--------------+-------------+--------------------+
            | cmsg_level   | cmsg_type   | cmsg_data[]        |
            +--------------+-------------+--------------------+
            | IPPROTO_SCTP | SCTP_PRINFO | struct sctp_prinfo |
            +--------------+-------------+--------------------+

   Here is the definition of the sctp_prinfo structure:

   struct sctp_prinfo {
     uint16_t pr_policy;
     uint32_t pr_value;
   };

   pr_policy:  This specifies which PR-SCTP policy is used.  Using
      SCTP_PR_SCTP_NONE results in a reliable transmission.  When
      SCTP_PR_SCTP_TTL is used, the PR-SCTP policy "timed reliability"
      defined in [RFC3758] is used.  In this case, the lifetime is
      provided in pr_value.




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   pr_value:  The meaning of this field depends on the PR-SCTP policy
      specified by the sinfo_pr_policy field.  It is ignored when
      SCTP_PR_SCTP_NONE is specified.  In case of SCTP_PR_SCTP_TTL the
      lifetime in milliseconds is specified.

   An sctp_prinfo item always corresponds to the data in msg_iov.

5.2.8.  SCTP AUTH Information Structure (SCTP_AUTHINFO)

   This cmsghdr structure specifies SCTP options for sendmsg().

          +--------------+---------------+----------------------+
          | cmsg_level   | cmsg_type     | cmsg_data[]          |
          +--------------+---------------+----------------------+
          | IPPROTO_SCTP | SCTP_AUTHINFO | struct sctp_authinfo |
          +--------------+---------------+----------------------+

   Here is the definition of the sctp_authinfo structure:

   struct sctp_authinfo {
     uint16_t auth_keyid;
   };

   auth_keyid:  This specifies the shared key identifier used for
      sending the user message.

   An sctp_authinfo item always corresponds to the data in msg_iov.

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 7.4 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 provide general
   information.



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   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 sctp_tlv {
       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;
     struct sctp_sender_dry_event sn_sender_dry_event;
   };

   sn_type:  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 5.3.2 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 an endpoint via a specific transport address).  Please see
         Section 5.3.3 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 5.3.4 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_sndrcvinfo per Section 5.3.5.





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      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 peer's
         indicated adaptation layer.  Please see Section 5.3.7.
      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 5.3.8.
      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.  See Section 5.3.9.
      SCTP_SENDER_DRY_EVENT:  This notification is used to inform the
         application that the sender has no user data queued anymore,
         neither for transmission nor retransmission.  See
         Section 5.3.10.
   sn_flags:  These are notification-specific flags.
   sn_length:  This is the length of the whole sctp_notification
      structure including the sn_type, sn_flags, and sn_length fields.

5.3.2.  SCTP_ASSOC_CHANGE

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

   sac_type:  It should be SCTP_ASSOC_CHANGE.
   sac_flags:  Currently unused.
   sac_length:  This field is the total length of the notification data,
      including the notification header.
   sac_state:  This field holds one of a number of values that
      communicate the event that happened to the association.  They
      include:






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      SCTP_COMM_UP:  A new association is now ready and data may be
         exchanged with this peer.  When an association has been
         established successfully, this notification should be the first
         one.
      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 accompanied 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.
      SCTP_CANT_STR_ASSOC:  The association failed to setup.  If non
         blocking mode is set and data was sent (on a one-to-many style
         socket), a SCTP_CANT_STR_ASSOC is accompanied by a series of
         SCTP_SEND_FAILED events, one for each outstanding message.
   sac_error:  If the state was reached due to an 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 [RFC4960].
   sac_outbound_streams:
   sac_inbound_streams:  The maximum number of streams allowed in each
      direction are available in sac_outbound_streams and sac_inbound
      streams.
   sac_assoc_id:  The association id field holds the identifier for the
      association.  All notifications for a given association have the
      same association identifier.  For a one-to-one style socket, this
      field is ignored.
   sac_info:  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 [RFC4960] section
      3.3.7.  If the sac_state is SCTP_COMM_UP or SCTP_RESTART, sac_info
      may contain an array of features that the current association
      supports.  Features may include
      SCTP_PR:  Both endpoints support the protocol extension described
         in [RFC3758].
      SCTP_AUTH:  Both endpoints support the protocol extension
         described in [RFC4895].
      SCTP_ASCONF:  Both endpoints support the protocol extension
         described in [RFC5061].
      SCTP_MULTIBUF:  For a one-to-many style socket, the local
         endpoints use separate send and/or receive buffers for each
         SCTP association.

5.3.3.  SCTP_PEER_ADDR_CHANGE

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




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   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;
   }

   spc_type:  It should be SCTP_PEER_ADDR_CHANGE.
   spc_flags:  Currently unused.
   spc_length:  This field is the total length of the notification data,
      including the notification header.
   spc_aaddr:  The affected address field holds the remote peer's
      address that is encountering the change of state.
   spc_state:  This field holds one of a number of values that
      communicate the event that happened to the address.  They include:
      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:  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:  The association id field holds the identifier for the
      association.  All notifications for a given association have the
      same association identifier.  For a one-to-one style socket, this
      field is ignored.

5.3.4.  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 an SCTP_REMOTE_ERROR event.  Please refer to the SCTP
   specification [RFC4960] and any extensions for a list of possible
   error formats.  SCTP error notifications have the format:






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   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[];
   };

   sre_type:  It should be SCTP_REMOTE_ERROR.
   sre_flags:  Currently unused.
   sre_length:  This field is the total length of the notification data,
      including the notification header and the contents of sre_data.
   sre_error:  This value represents one of the Operational Error causes
      defined in the SCTP specification, in network byte order.
   sre_assoc_id:  The association id field holds the identifier for the
      association.  All notifications for a given association have the
      same association identifier.  For a one-to-one style socket, this
      field is ignored.
   sre_data:  This contains the ERROR chunk as defined in the SCTP
      specification [RFC4960] section 3.3.10.

5.3.5.  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[];
   };

   ssf_type:  It should be SCTP_SEND_FAILED.
   ssf_flags:  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.







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   ssf_length:  This field is the total length of the notification data,
      including the notification header and the payload in ssf_data.
   ssf_error:  This value represents the reason why the send failed, and
      if set, will be an SCTP protocol error code as defined in
      [RFC4960] section 3.3.10.
   ssf_info:  The send information associated with the undelivered
      message.  The ssf_info.sinfo_flags field will also contain an
      indication if the beginning of the message and/or end of the
      message is present.  In cases where no data has been sent on the
      wire, this field will have or'ed in the value SCTP_DATA_NOT_FRAG,
      which is a composition of both a "BEGIN" and "END" fragmentation
      bit.  In cases where only part of the data has been sent, this
      field will have or'ed in the value SCTP_DATA_LAST_FRAG, which
      corresponds to the "END" bit.  Note that the message itself may be
      more than one chunk.  If the ssf_info.sinfo_flags field holds
      neither of these two values then a piece that has been fragmented
      and sent but not acknowledged is present.  This piece is from an
      unspecified position in the message and the application can make
      no assumptions about the data itself.  Applications wanting to
      examine a recovered message should look for the
      SCTP_DATA_NOT_FRAG.  Without this flag the application should
      assume part of the message arrived and take appropriate steps to
      audit and recover any lost or missing data.
   ssf_assoc_id:  The association id field, ssf_assoc_id, holds the
      identifier for the association.  All notifications for a given
      association have the same association identifier.  For a one-to-
      one style socket, this field is ignored.
   ssf_data:  The undelivered message or part of the undelivered message
      will be present in the ssf_data field.  Note that the
      ssf_info.sinfo_flags field as noted above should be used to
      determine if a complete message is present or just a piece of the
      message.  Note that only user data is present in this field, any
      chunk headers or SCTP common headers must be removed by the SCTP
      stack.

5.3.6.  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;
       };





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   sse_type:  It should be SCTP_SHUTDOWN_EVENT.
   sse_flags:  Currently unused.
   sse_length:  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:  Currently unused.
   sse_assoc_id:  The association id field holds the identifier for the
      association.  All notifications for a given association have the
      same association identifier.  For a one-to-one style socket, this
      field is ignored.

5.3.7.  SCTP_ADAPTATION_INDICATION

   When a peer sends an Adaptation Layer Indication parameter as
   described in [RFC5061], SCTP delivers this notification to inform the
   application about the peer's adaptation layer indication.

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

5.3.8.  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.









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

   pdapi_type:  It should be SCTP_PARTIAL_DELIVERY_EVENT.
   pdapi_flags:  Currently unused.
   pdapi_length:  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:  This field holds the indication being sent to the
      application.  Possible values include:
      SCTP_PARTIAL_DELIVERY_ABORTED:  This notification indicates that
         the partial delivery of a user message has been aborted.
   pdapi_stream:  This field holds the stream on which the partial
      delivery event happened.
   pdapi_seq:  This field holds the stream sequence number which was
      partially delivered.
   pdapi_assoc_id:  The association id field holds the identifier for
      the association.  All notifications for a given association have
      the same association identifier.  For a one-to-one style socket
      this field is ignored.

5.3.9.  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;
     uint16_t auth_keynumber;
     uint16_t auth_altkeynumber;
     uint32_t auth_indication;
     sctp_assoc_t auth_assoc_id;
   };

   auth_type:  It should be SCTP_AUTHENTICATION_EVENT.







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   auth_flags:  Currently unused.
   auth_length:  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:  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:  This field holds an alternate keynumber which is
      used by some notifications.
   auth_indication:  This field holds 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.
      SCTP_AUTH_NO_AUTH:  This report indicates that the peer does not
         support SCTP-AUTH.
      SCTP_AUTH_FREE_KEY:  This report indicates that the SCTP
         implementation will not use the key identifier specified in
         auth_keynumber anymore.
   auth_assoc_id:  The association id field holds the identifier for the
      association.  All notifications for a given association have the
      same association identifier.  For a one-to-one style socket this
      field is ignored.

5.3.10.  SCTP_SENDER_DRY_EVENT

   When the SCTP implementation has no user data anymore to send or
   retransmit, this notification is given to the user.  If the user
   subscribes to this event and SCTP has at this point of time no user
   data to send or retransmit, this notification is also given to the
   user.

   struct sctp_sender_dry_event {
     uint16_t sender_dry_type;
     uint16_t sender_dry_flags;
     uint32_t sender_dry_length;
     sctp_assoc_t sender_dry_assoc_id;
   };

   sender_dry_type:  It should be SCTP_SENDER_DRY_EVENT.
   sender_dry_flags:  Currently unused.
   sender_dry_length:  This field is the total length of the
      notification data, including the notification header.  It will
      generally be sizeof(struct sctp_sender_dry_event).






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5.3.11.  SCTP_NOTIFICATIONS_STOPPED_EVENT

   Notifications, when subscribed to, are reliable.  They are always
   delivered as long as there is space in the socket receive buffer.
   However, if an implementation experiences a notification storm, it
   may run out of socket buffer space.  When this occurs it may wish to
   disable notifications.  If the implementation chooses to do this, it
   will append a final notification SCTP_NOTIFICATIONS_STOPPED_EVENT.
   This notification is an empty sctp_tlv (see the union above), that
   merely has this type in the sn_type field, the sn_length field set to
   the sizeof an sctp_tlv structure and the sn_flags set to 0.  If an
   application receives this notification, it will need to resubscribe
   to any notifications of interest to it.

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.

   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
   [RFC3542] and your SCTP implementation's documentation for more
   information.  The following is an example, from [RFC3542],
   demonstrating the use of these macros to access ancillary data:






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   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-to-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
   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 [RFC3542], we would calculate
   and allocate the buffer size as follows:









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   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 (buf) 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(), and 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 function prototypes are

   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,



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                    socklen_t *fromlen);

   and the arguments are
   sd:  The socket descriptor of an SCTP endpoint.
   msg:  The message to be sent.
   len:  the size of the message or the size of the 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.  Sending a message using send() or sendto() is atomic
   unless explicit EOR marking is enabled on the socket specified by sd.
   Using sendto() on a non-connected one-to-one style socket for
   implicit connection setup may or may not work depending on the SCTP
   implementation. 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.




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   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 [RFC4960].

6.2.  setsockopt() and getsockopt()

   Applications use setsockopt() and getsockopt() to set or retrieve
   socket options.  Socket options are used to change the default
   behavior of socket calls.  They are described in Section 7.

   The function prototypes are

   int getsockopt(int sd,
                  int level,
                  int optname,
                  void *optval,
                  socklen_t *optlen);

   and

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

   and the arguments are
   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 one-to-one style listening socket also
   apply to all accepted sockets.  For one-to-many style sockets often a
   socket option will pass a structure that includes an assoc_id field.
   This field can be filled with the association id of a particular
   association and unless otherwise specified can be filled with one of
   the following constants:
   SCTP_FUTURE_ASSOC:  Specifies that only future associations created
      after this socket option will be effected by this call.
   SCTP_CURRENT_ASSOC:  Specifies that only currently existing
      associations will be effected by this call, future associations
      will still receive the previous default value.





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   SCTP_ALL_ASSOC:  Specifies that all current and future associations
      will be effected by this call.

6.3.  read() and write()

   Applications can use read() and write() to send and receive data to
   and from a 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, should 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 single homed
   endpoints.  It does not work well with multi-homed endpoints.  See
   Section 8.5 for a multi-homed version of the call.

   The function prototype is

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

   and the arguments are
   sd:  The socket descriptor to be queried.
   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 the 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



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   should be applied to.

   For the one-to-many style sockets, an sctp_assoc_t structure
   (association ID) is used to identify 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 into 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
   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.

   The field 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_CONTEXT







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   SCTP_AUTH_ACTIVE_KEY
   SCTP_PEER_AUTH_CHUNKS
   SCTP_LOCAL_AUTH_CHUNKS

   The arg field 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 the 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:
   o  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.
   o  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.
   o  If neither the sockaddr_storage nor association identification is
      set, i.e. the sockaddr_storage is set to all 0 (INADDR_ANY) and
      the association identification is SCTP_FUTURE_ASSOC, the settings
      are a default and to be applied to the endpoint.

7.1.  Read / Write Options

7.1.1.  Retransmission Timeout Parameters (SCTP_RTOINFO)

   The protocol parameters used to initialize and limit the
   retransmission timeout (RTO) are tunable.  See [RFC4960] 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;



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   };

   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:  This parameter is ignored for one-to-one style
      sockets.  For one-to-many style sockets the application may fill
      in an association identification or one of the predefined
      constants.

   All times are given 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 [RFC4960] for more information on how this
   parameter is used.

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

   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_assoc_id:  This parameter is ignored for one-to-one style
      sockets.  For one-to-many style sockets the application may fill
      in an association identification or one of the predefined
      constants.
   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.







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   sasoc_peer_rwnd:  This holds the current value of the peers rwnd
      (reported in the last SACK) minus any outstanding data (i.e. data
      in flight).
   sasoc_local_rwnd:  This holds the last reported rwnd that was sent to
      the peer.
   sasoc_cookie_life:  This is the association's cookie life value used
      when issuing cookies.

   The values of the sasoc_peer_rwnd is meaningless when examining
   endpoint information.

   All time values are given 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()
   may return an error.  The reason for this, from [RFC4960] 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



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   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 listening socket.

7.1.4.  SO_LINGER

   An application can use this option to perform the SCTP ABORT
   primitive.  This option affects all associations related to the
   socket.

   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 will continue 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
   introduced, at the cost of more packets in the network.  Expects an
   integer boolean flag.  Turning this option on disables any Nagle-like
   algorithm.

7.1.6.  SO_RCVBUF

   Sets the 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 is implementation dependent.  It might
   control the receive buffer for each association bound to the socket
   descriptor or it might control the receive buffer for the whole
   socket.  The call expects an integer.






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7.1.7.  SO_SNDBUF

   Sets the 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 association
   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 endpoint.

7.1.9.  Set Primary Address (SCTP_PRIMARY_ADDR)

   Requests that the local SCTP stack uses the enclosed peer address as
   the association's 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.







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   ssp_assoc_id:  This parameter is ignored for one-to-one style
      sockets.  For one-to-many style sockets it identifies the
      association for this request.  Note that the predefined constants
      are NOT allowed.

7.1.10.  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.

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

   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.

7.1.11.  Enable/Disable Message Fragmentation (SCTP_DISABLE_FRAGMENTS)

   This option is a on/off flag and is passed as an integer where a non-
   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 an error will be indicated to the user.

7.1.12.  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_flags;
     uint32_t spp_ipv6_flowlabel;
     uint8_t spp_ipv4_tos;
   };



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   spp_assoc_id:  This parameter is ignored for one-to-one style
      sockets.  For one-to-many style sockets it identifies the
      association for this query.  Note that the predefined constants
      are NOT allowed.
   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 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).  Note that if the
      spp_address field is empty then all destinations for this
      association will have this fixed path MTU set upon them.  If an
      address is specified, then only that address will be effected.
      Note also that this option cannot be set on the endpoint, but must
      be set on each individual association.  Also, when disabling PMTU
      discovery, the implementation may disallow this behavior if the
      "fixed" path MTU is below the constant value SCTP_SMALLEST_PMTU.
   spp_ipv6_flowlabel:  This field is used in conjunction with the
      SPP_IPV6_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:  Specifies that the time for heartbeat delay
         is to be set to the value of 0 milliseconds.






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      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.  Note also
         that SPP_PMTUD_ENABLE and SPP_PMTUD_DISABLE are mutually
         exclusive.  Enabling both will have undetermined results.
      SPP_IPV6_FLOWLABEL:  Setting this flag enables the 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 address 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's default flowlabel is returned.  If neither an
         association nor a destination is specified, then the socket's
         default flowlabel is returned.  For non IPv6 sockets, this flag
         will be left cleared.
      SPP_IPV4_TOS:  Setting this flag enables the setting of the IPV4
         TOS value associated with either the association or a 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 address is set when called (in the spp_address
         field) then that specific destination address' 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, 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.





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7.1.13.  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
   the sctp_sndrcvinfo structure defined in Section 5.2.2 to this call.
   The input parameters accepted by this call include sinfo_stream,
   sinfo_flags, sinfo_ppid, sinfo_context, sinfo_pr_policy and
   sinfo_pr_value.  The sinfo_flags is composed of a bitwise OR of
   SCTP_UNORDERED, SCTP_EOF, and SCTP_SENDALL.  The sinfo_assoc_id field
   specifies the association to apply the parameters to.  In a one-to-
   many style sockets any of the predefined constants are also allowed
   in this field.  The field is ignored on the one-to-one style.

7.1.14.  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 7.4
   for a full description of this option and its usage.  Note that this
   option is considered deprecated and present for backward
   compatibility.  New applications should use the SCTP_SET_EVENT
   option.  See Section 7.4 for a full description of that option as
   well.

7.1.15.  Set/Clear IPv4 Mapped Addresses (SCTP_I_WANT_MAPPED_V4_ADDR)

   This socket option is a boolean flag which turns on or off the
   mapping of IPv4 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 off and expects an integer to be
   passed where non-zero turns on the option and zero turns off the
   option.

7.1.16.  Get or Set the Maximum Fragmentation Size (SCTP_MAXSEG)

   This option will get or set 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 default value for this option is '0' which indicates
   the user is NOT limiting fragmentation and only the PMTU will effect



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   SCTP's choice of DATA chunk size.  Note also that values set larger
   than the maximum size of an IP datagram will effectively let SCTP
   control fragmentation (i.e. the same as setting this option to 0).

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

   struct sctp_assoc_value {
     sctp_assoc_t assoc_id;
     uint32_t assoc_value;
   };

   assoc_id:  This parameter is ignored for one-to-one style sockets.
      For one-to-many style sockets this parameter indicates which
      association the user is performing an action upon.  Note that any
      of the predefined constants are also allowed in this field.
   assoc_value:  This parameter specifies the maximum size in bytes.

7.1.17.  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.

   The following structure is used to get or set these identifiers:

   struct sctp_hmacalgo {
     uint32_t shmac_number_of_idents;
     uint16_t shmac_idents[];
   };

   shmac_number_of_idents:  This field gives the number of elements
      present in the array 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.  The following identifiers are valid:
      *  SCTP_AUTH_HMAC_ID_SHA1
      *  SCTP_AUTH_HMAC_ID_SHA256

   Note that the list supplied must include SCTP_AUTH_HMAC_ID_SHA1 and
   may include any of the other values in its preferred order (lowest
   list position has the highest preference in algorithm selection).
   Note also that the lack of SCTP_AUTH_HMAC_ID_SHA1, or the inclusion
   of an unknown HMAC identifier (including optional identifiers unknown
   to the implementation) will cause the set option to fail and return
   an error.






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7.1.18.  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.

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

   struct sctp_authkeyid {
     sctp_assoc_t scact_assoc_id;
     uint16_t scact_keynumber;
   };

   scact_assoc_id:  This parameter, if non-zero, indicates the
      association that the shared key identifier is 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.

   When used with setsockopt() the SCTP implementation must use the
   indicated shared key identifier for all messages being given to an
   SCTP implementation via a send call after the setsockopt() call until
   changed again.  Therefore, the SCTP implementation must not bundle
   user messages which should be authenticated using different shared
   key identifiers.

   Initially the key with key identifier 0 is the active key.

7.1.19.  Get or Set Delayed SACK Timer (SCTP_DELAYED_SACK)

   This option will effect the way delayed acks are performed.  This
   option allows you to get or set the delayed ack time, in
   milliseconds.  It also allows changing the delayed ack frequency.
   Changing the frequency to 1 disables the delayed sack algorithm.  If
   the sack_assoc_id is 0, then this sets or gets the endpoints default
   values.  If the sack_assoc_id field is non-zero, then the set or get
   effects the specified association for the one-to-many model (the
   assoc_id field is ignored by the one-to-one model).  Note that if
   sack_delay or sack_freq are 0 when setting this option, the current
   values will remain unchanged.



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

   struct sctp_sack_info {
     sctp_assoc_t sack_assoc_id;
     uint32_t sack_delay;
     uint32_t sack_freq;
   };

   sack_assoc_id:  This parameter is ignored for one-to-one style
      sockets.  For one-to-many style sockets this parameter indicates
      which association the user is performing an action upon.  Note
      that any of the predefined constants may also be used for one-to-
      many style sockets.
   sack_delay:  This parameter contains the number of milliseconds that
      the user is requesting the delayed ACK timer to be set to.  Note
      that this value is defined in the standard to be between 200 and
      500 milliseconds.
   sack_freq:  This parameter contains the number of packets that must
      be received before a sack is sent without waiting for the delay
      timer to expire.  The default value is 2, setting this value to 1
      will disable the delayed sack algorithm.

7.1.20.  Get or Set Fragmented Interleave (SCTP_FRAGMENT_INTERLEAVE)

   Fragmented interleave controls how the presentation of messages
   occurs for the message receiver.  There are three levels of fragment
   interleave defined.  Two of the levels effect the one-to-one model,
   while the one-to-many model is effected by all three levels.

   This option takes an integer value.  It can be set to a value of 0, 1
   or 2.  Attempting to set this level to other values will return an
   error.

   Setting the three levels provides the following receiver
   interactions:

   level 0:  Prevents the interleaving of any messages.  This means that
      when a partial delivery begins, no other messages will be received
      except the message being partially delivered.  If another message
      arrives on a different stream (or association) that could be
      delivered, it will be blocked waiting for the user to read all of
      the partially delivered message.
   level 1:  Allows interleaving of messages that are from different
      associations.  For the one-to-one model, level 0 and level 1 thus
      have the same meaning since a one-to-one socket always receives
      messages from the same association.  Note that setting the one-to-
      many model to this level may cause multiple partial deliveries



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      from different associations but for any given association, only
      one message will be delivered until all parts of a message have
      been delivered.  This means that one large message, being read
      with an association identification of "X", will block other
      messages from association "X" from being delivered.
   level 2:  Allows complete interleaving of messages.  This level
      requires that the sender carefully observes not only the peer
      association identification (or address) but must also pay careful
      attention to the stream number.  With this option enabled a
      partially delivered message may begin being delivered for
      association "X" stream "Y" and the next subsequent receive may
      return a message from association "X" stream "Z".  Note that no
      other messages would be delivered for association "X" stream "Y"
      until all of stream "Y"'s partially delivered message was read.
      Note that this option also effects the one-to-one model.  Also
      note that for the one-to-many model not only may another streams
      message from the same association be delivered from the next
      receive, some other associations message may be delivered upon the
      next receive.

   An implementation should default the one-to-many model to level 1.
   The reason for this is that otherwise it is possible that a peer
   could begin sending a partial message and thus block all other peers
   from sending data.  However a setting of level 2 requires the
   application to not only be aware of the association (via the
   association id or peer's address) but also the stream number.  The
   stream number is NOT present unless the user has subscribed to the
   sctp_data_io_events (see Section 7.4).  This is also why we recommend
   that the one-to-one model be defaulted to level 0 (level 1 for the
   one-to-one model has no effect).  Note that an implementation should
   return an error if an application attempts to set the level to 2 and
   has NOT subscribed to the sctp_data_io_events.

   For applications that have subscribed to events those events appear
   in the normal socket buffer data stream.  This means that unless the
   user has set the fragmentation interleave level to 0, notifications
   may also be interleaved with partially delivered messages.

7.1.21.  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 deliveries to happen more often.  The
   call's argument is an integer that sets or gets the partial delivery
   point.  Note also that the call will fail if the user attempts to set
   this value larger than the socket receive buffer size.



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   Note that any single message having a length smaller than or equal to
   the SCTP partial delivery point will be delivered in one single read
   call as long as the user provided buffer is large enough to hold the
   message.

7.1.22.  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.  This option is present for
   compatibility with older applications and is deprecated.  Future
   applications should use SCTP_NXTINFO to retrieve this same
   information via ancillary data.

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

7.1.23.  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.

7.1.24.  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.

   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;
   };








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   assoc_id:  This parameter is ignored for one-to-one style sockets.
      For one-to-many style sockets this parameter indicates which
      association the user is performing an action upon.  Note that any
      of the predefined constants may be used for one-to-many style
      sockets.
   assoc_value:  This parameter contains the maximum burst.

7.1.25.  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-to-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 is ignored for one-to-one style sockets.
      For one-to-many style sockets this parameter indicates which
      association the user is performing an action upon.  Note that any
      of the predefined constants may be used for one-to-many style
      sockets.
   assoc_value:  This parameter contains the context.

7.1.26.  Enable or Disable Explicit EOR Marking (SCTP_EXPLICIT_EOR)

   This boolean flag is used to enable or disable explicit end of record
   (EOR) marking.  When this option is enabled, a user may make multiple
   send system calls to send a record and must indicate that they are
   finished sending a particular record by including the SCTP_EOR flag.
   If this boolean flag is disabled then each individual send system
   call is considered to have an SCTP_EOR indicator set on it implicitly
   without the user having to explicitly add this flag.

7.1.27.  Enable SCTP Port Reusage (SCTP_REUSE_PORT)

   This option only supports one-to-one style SCTP sockets.  If used on
   a one-to-many style SCTP socket an error is indicated.



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   This setsockopt() call must not be used after calling bind() or
   sctp_bindx() for a one-to-one style SCTP socket.  If using bind() or
   sctp_bindx() on a socket with the SCTP_REUSE_PORT option, all other
   SCTP sockets bound to the same port must have set the
   SCTP_REUSE_PORT.  Calling bind() or sctp_bindx() for a socket without
   having set the SCTP_REUSE_PORT option will fail if there are other
   sockets bound to the same port.  At most one socket being bound to
   the same port may be listening.

   It should be noted that the behavior of the socket level socket
   option to reuse ports and/or addresses for SCTP sockets is
   unspecified.

7.1.28.  Set Notification Event (SCTP_EVENT)

   This socket option is used to set a specific notification or
   ancillary data option.  Please see Section 7.4 for a full description
   of this option and its usage.

7.2.  Read-Only Options

   The options defined in this subsection are read-only.  Using this
   option in a setsockopt() call will result in an error indicating
   EOPNOTSUPP.

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:

   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;
   };






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   sstat_assoc_id:  This parameter is ignored for one-to-one style
      sockets.  For one-to-many style sockets it holds the identifier
      for the association.  All notifications for a given association
      have the same association identifier.  Note that the one-to-many
      predefined constants may not be used with this option.
   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_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.

7.2.2.  Peer Address Information (SCTP_GET_PEER_ADDR_INFO)

   Applications can retrieve information about a specific peer address
   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:










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   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_assoc_id:  This parameter is ignored for one-to-one style
      sockets.  For one-to-many style sockets the following applies:
      This field may be filled by the application, if so, this field
      will have priority in looking up the association using 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 an address into an association id.
      Note that the predefined constants are not allowed on this option.
   spinfo_address:  This is filled by the application, and contains the
      peer address of interest.
   spinfo_state:  This contains the peer address' state (either
      SCTP_ACTIVE or SCTP_INACTIVE and possibly the modifier
      SCTP_UNCONFIRMED).
   spinfo_cwnd:  This contains the peer address' current congestion
      window.
   spinfo_srtt:  This contains the peer address' current smoothed round-
      trip time calculation in milliseconds.
   spinfo_rto:  This contains the peer address' current retransmission
      timeout value in milliseconds.
   spinfo_mtu:  The current P-MTU of this address.

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.

   The following structure is used to access these parameters:

   struct sctp_authchunks {
     sctp_assoc_t gauth_assoc_id;
     guint32_t gauth_number_of_chunks
     uint8_t gauth_chunks[];
   };




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   gauth_assoc_id:  This parameter indicates for which association the
      user is requesting the list of peer authenticated chunks.  For
      one-to-one sockets, this parameter is ignored.  Note that the
      predefined constants are not allowed with this option.
   gauth_number_of_chunks:  This parameter gives the number of elements
      in the array gauth_chunks.
   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.

   The following structure is used to access these parameters:

   struct sctp_authchunks {
     sctp_assoc_t gauth_assoc_id;
     uint32_t gauth_number_of_chunks;
     uint8_t gauth_chunks[];
   };

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

7.2.5.  Get the Current Number of Associations (SCTP_GET_ASSOC_NUMBER)

   This option gets the current number of associations that are attached
   to a one-to-many style socket.  The option value is an uint32_t.

7.2.6.  Get the Current Identifiers of Associations
        (SCTP_GET_ASSOC_ID_LIST)

   This option gets the current list of SCTP association identifiers of
   the SCTP associations handled by a one-to-many style socket.

   The option value has the structure

   struct sctp_assoc_ids {
     uint32_t gaids_number_of_ids;
     sctp_assoc_t gaids_assoc_id[];
   };



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   The caller must provide a large enough buffer to hold all association
   identifiers.  If the buffer is too small, an error must be returned.
   The user can use the SCTP_GET_ASSOC_NUMBER socket option to get an
   idea how large the buffer has to be. gaids_number_of_ids gives the
   number of elements in the array gaids_assoc_id.

7.3.  Write-Only Options

   The options defined in this subsection are write-only.  Using this
   option in a getsockopt() or sctp_opt_info() call will result in an
   error indicating EOPNOTSUPP.

7.3.1.  Set Peer Primary Address (SCTP_SET_PEER_PRIMARY_ADDR)

   Requests that the peer marks the enclosed address as the association
   primary.  The enclosed address must be one of the association's
   locally bound addresses.

   The following structure is used to make a set peer 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 parameter is ignored for one-to-one style
      sockets.  For one-to-many style sockets it identifies the
      association for this request.  Note that the predefined constants
      are not allowed on this option.

7.3.2.  Add a Chunk That Must Be Authenticated (SCTP_AUTH_CHUNK)

   This set 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.

   The following structure is used to add a chunk:

   struct sctp_authchunk {
     uint8_t sauth_chunk;
   };

   sauth_chunk:  This parameter contains a chunk type that the user is
      requesting to be authenticated.

   The chunk types for INIT, INIT-ACK, SHUTDOWN-COMPLETE, and AUTH
   chunks must not be used.  If they are used, an error must be



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   returned.  The usage of this option enables SCTP-AUTH in cases where
   it is not required by other means (for example the use of dynamic
   address reconfiguration).

7.3.3.  Set a Shared Key (SCTP_AUTH_KEY)

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

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

   struct sctp_authkey {
     sctp_assoc_t sca_assoc_id;
     uint16_t sca_keynumber;
     uint16_t sca_keylength;
     uint8_t sca_key[];
   };

   sca_assoc_id:  This parameter, if non-zero, indicates what
      association the shared key is being set upon.  Note that any of
      the predefined constants can be used.  For one-to-one sockets,
      this parameter is ignored.  Note, however, that this 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 shared 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_keylength:  This parameter is the length of the array sca_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.

7.3.4.  Deactivate a Shared Key (SCTP_AUTH_DEACTIVATE_KEY)

   This set option indicates that the application will not send user
   messages anymore requiring the usage of the indicated key identifier.

   struct sctp_authkeyid {
     sctp_assoc_t scact_assoc_id;
     uint16_t scact_keynumber;
   };







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   scact_assoc_id:  This parameter, if non-zero, indicates what
      association the shared key identifier is being deactivated for.
      Note that the predefined constants may be used with this option.
      For one-to-one sockets, this parameter is ignored.  Note, however,
      that this option will deactivate the key from the association if
      the socket is connected, otherwise this will deactivate the key
      from the endpoint.
   scact_keynumber:  This parameter is the shared key identifier which
      the application is requesting to be deactivated.  The key
      identifier must correspond to an existing shared key.  Note if
      this parameter is zero, use of the null key identifier '0' is
      deactivated on the endpoint and/or association.

   The currently active key cannot be deactivated.

7.3.5.  Delete a Shared Key (SCTP_AUTH_DELETE_KEY)

   This set option will delete a shared secret key in the SCTP
   implementation.

   struct sctp_authkeyid {
     sctp_assoc_t scact_assoc_id;
     uint16_t scact_keynumber;
   };

   scact_assoc_id:  This parameter, if non-zero, indicates which
      association 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 in use
      for any packet being sent by the SCTP implementation.  This means
      in particular, that it must be deactivated first.  Note if this
      parameter is zero, use of the null key identifier '0' is deleted
      from the endpoint and/or association.

   Only deactivated keys which are no longer used by the kernel can be
   deleted.







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7.4.  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:
   SCTP_SNDRCV (sctp_data_io_event):  Per-message information (i.e.
      stream number, TSN, SSN, etc. described in Section 5.2.2)
   SCTP_ASSOC_CHANGE (sctp_association_event):  described in
      Section 5.3.2
   SCTP_PEER_ADDR_CHANGE (sctp_address_event):  described in
      Section 5.3.3
   SCTP_SEND_FAILED (sctp_send_failure_event):  described in
      Section 5.3.5
   SCTP_REMOTE_ERROR (sctp_peer_error_event):  described in
      Section 5.3.4
   SCTP_SHUTDOWN_EVENT (sctp_shutdown_event):  described in
      Section 5.3.6
   SCTP_PARTIAL_DELIVERY_EVENT (sctp_partial_delivery_event):  described
      in Section 5.3.8
   SCTP_ADAPTATION_INDICATION (sctp_adaptation_layer_event):  described
      in Section 5.3.7
   SCTP_AUTHENTICATION_EVENT (sctp_authentication_event):  described in
      Section 5.3.9)
   SCTP_SENDER_DRY_EVENT (sctp_sender_dry_event):  described in
      Section 5.3.10
   SCTP_NOTIFICATIONS_STOPPED_EVENT (sctp_tlv):  described in
      Section 5.3.11

   To receive any ancillary data or notifications, first the application
   registers its 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;
     uint8_t sctp_sender_dry_event;
   };






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   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.  Setting the flag to 0 will disable the 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.
   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.
   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.
   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.
   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.
   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.
   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.
   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.
   sctp_sender_dry_event:  Setting this flag to 1 will enable the
      reception of sender dry notifications.  Setting the flag to 0 will
      disable sender dry event notifications.

   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 events;

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

     events.sctp_data_io_event = 1;
     events.sctp_association_event = 1;

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




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   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 only for the single
   association bound to the file descriptor.

   The SCTP_EVENTS socket option has one issue for future compatibility.
   As new features are added the structure (sctp_event_subscribe) must
   be expanded.  This can cause an ABI issue unless an implementation
   has added padding at the end of the structure.  To avoid this
   problem, SCTP_EVENTS has been deprecated and a new option SCTP_EVENT
   socket option has taken its place.  The option is used with the
   following structure:

   struct sctp_event {
           sctp_assoc_t se_assoc_id;
           uint16_t     se_type;
           uint8_t      se_on;
   };

   se_assoc_id:  The se_assoc_id field is ignored for one-to-one style
      sockets.  For one-to-many style sockets any this field can be a
      particular association id or one of the defined constants.
   se_type:  The se_type field can be filled with any value that would
      show up in the respective sn_type field (in the sctp_tlv structure
      of the notification).  In addition SCTP_SNDRCV_EVENT,
      SCTP_RCV_EVENT, and SCTP_NXT_EVENT can be used.
   se_on:  The se_on field is set to 1 to turn on an event and set to 0
      to turn off an event.

   To use this option the user fills in this structure and then calls
   the setsockopt to turn on or off an individual event.  The following
   is an example use of this option:

   {
     struct sctp_event event;

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

     event.se_assoc_id = SCTP_FUTURE_ASSOC;
     event.se_type = SCTP_SENDER_DRY_EVENT;
     event.se_on = 1;
     setsockopt(fd, IPPROTO_SCTP, SCTP_EVENT, &event, sizeof(event));
   }

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




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8.  New Functions

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

8.1.  sctp_bindx()

   This function allows the user to bind a specific subset of addresses
   or, if the SCTP extension described in [RFC5061] is supported, add or
   delete specific addresses.

   The function prototype is

   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 in combination with non
   wildcard addresses on a socket 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:
   o  SCTP_BINDX_ADD_ADDR
   o  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



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   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 reconfiguration an SCTP_BINDX_REM_ADDR or SCTP_BINDX_ADD_ADDR
   may cause an endpoint to send the appropriate message to the peer to
   change the peer's address lists.

   Adding and removing addresses from a connected association is an
   optional functionality.  Implementations that do not support this
   functionality should return EOPNOTSUPP.

   sctp_bindx() can be called on an already bound socket or on an
   unbound socket.  If the socket is unbound and the first port number
   in the addrs is zero, the kernel will choose a port number.  All port
   numbers after the first one being 0 must also be zero.  If the first
   port number is not zero, the following port numbers must be zero or
   have the same value as the first one.  For an already bound socket,
   all port numbers provided must be the bound one or 0.

   sctp_bindx() is an atomic operation.  Therefore, the binding will be
   either successful on all addresses or fail on all addresses.  If
   multiple addresses are provided and the sctp_bindx() call fails there
   is no indication which address is responsible for the failure.  The
   only way to get a specific error indication is to call sctp_bindx()
   with only one address sequentially.

8.2.  sctp_peeloff()

   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 the 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.



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   The function prototype is

   int sctp_peeloff(int sd,
                    sctp_assoc_t assoc_id);

   and the arguments are
   sd:  The original one-to-many style socket descriptor returned from
      the socket() system call (see Section 3.1.1).
   assoc_id:  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).
   The function returns a non-negative file descriptor representing the
   branched-off association, or -1 if an error occurred.  The variable
   errno is then set appropriately.

8.3.  sctp_getpaddrs()

   sctp_getpaddrs() returns all peer addresses in an association.

   The function protoype 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.

   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().




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   The function prototype is

   void sctp_freepaddrs(struct sockaddr *addrs);

   and 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 function prototype 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.

   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().

   The function prototype is

   void sctp_freeladdrs(struct sockaddr *addrs);

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




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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.

   The function prototype 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 pr_value,
                        uint32_t context);

   and the arguments are:
   sd:  The socket descriptor
   msg:  The message to be sent.
   len:  The length of the message.
   to:  The destination address of the message.
   tolen:  The length of the destination address.
   ppid:  The same as sinfo_ppid (see Section 5.2.2)
   flags:  The same as sinfo_flags (see Section 5.2.2)
   stream_no:  The same as sinfo_stream (see Section 5.2.2)
   pr_value:  The same as sinfo_pr_value (see Section 5.2.2).
   context:  The same as sinfo_context (see Section 5.2.2)
   The call returns the number of characters sent, or -1 if an error
   occurred.  The variable errno is then set appropriately.

   Sending a message using sctp_sendmsg() is atomic (unless explicit EOR
   marking is enabled on the socket specified by sd).

   Using sctp_sendmsg() on a non-connected one-to-one style socket for
   implicit connection setup may or may not work depending on the SCTP
   implementation.

8.8.  sctp_recvmsg()

   An implementation may provide a library function (or possibly system
   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.



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   The function prototype 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);

   and the arguments are
   sd:  The socket descriptor.
   msg:  The message buffer to be filled.
   len:  The length of the message buffer.
   from:  A pointer to an address to be filled with the sender of this
      messages address.
   fromlen:  An in/out parameter describing the from length.
   sinfo:  A pointer to an sctp_sndrcvinfo structure to be filled upon
      receipt of the message.
   msg_flags:  A pointer to an integer to be filled with any message
      flags (e.g.  MSG_NOTIFICATION).  Note that this field is an in-out
      field.  Options for the receive may also be passed into the value
      (e.g.  MSG_PEEK).  On return from the call, the msg_flags value
      will be different than what was sent in to the call.  If
      implemented via a recvmsg() call, the msg_flags should only
      contain the value of the flags from the recvmsg() call.
   The call returns the number of bytes received, or -1 if an error
   occurred.  The variable errno is then set appropriately.

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.

   The function prototype is




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

   and the arguments are:
   sd:  The socket descriptor.
   addrs:  An (packed) array of addresses.
   addrcnt:  The number of addresses in the array.
   id:  An output parameter that if passed in as a non-NULL will return
      the association identification for the newly created association
      (if successful).

   The call returns 0 on success or -1 if an error occurred.  The
   variable errno is then set appropriately.

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 prototype is

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

   and the arguments are
   sd:  The socket descriptor.
   msg:  The message to be sent.
   len:  The length of the message.
   sinfo:  A pointer to an sctp_sndrcvinfo structure used as described
      in Section 5.2.2 for a sendmsg call.
   flags:  The same flags as used by the sendmsg call flags (e.g.
      MSG_DONTROUTE).
   The call returns the number of bytes sent, or -1 if an error
   occurred.  The variable errno is then set appropriately.

   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.

   Using sctp_send() on a non-connected one-to-one style socket for



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   implicit connection setup may or may not work depending on the SCTP
   implementation.

   Sending a message using sctp_send() is atomic unless explicit EOR
   marking is enabled on the socket specified by sd.

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
   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).

   The function prototype is

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

   and the arguments are:
   sd:  The socket descriptor.
   msg:  The message to be sent.
   len:  The length of the message.
   addrs:  is an array of addresses.
   addrcnt:  The number of addresses in the array.
   sinfo:  A pointer to a sctp_sndrcvinfo structure used as described in
      Section 5.2.2 for a sendmsg call.
   flags:  The same flags as used by the sendmsg call flags (e.g.
      MSG_DONTROUTE).
   The call returns the number of bytes sent, or -1 if an error
   occurred.  The variable errno is then set appropriately.

   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.



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   Sending a message using sctp_send() is atomic unless explicit EOR
   marking is enabled on the socket specified by sd.

   Using sctp_sendx() on a non-connected one-to-one style socket for
   implicit connection setup may or may not work depending on the SCTP
   implementation.

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.

   The function prototype is:

   int sctp_getaddrlen(sa_family_t family);

   This function, when called with a valid family type returns the
   length that the operating system uses in the specified family's
   socket address structure.  In case of an error, -1 is returned and
   the variable errno is then set appropriately.


9.  IANA Considerations

   This document requires no actions from IANA.


10.  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
   being more aggressive than permitted on the public Internet.  These
   parameters are:
   o  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.

   Applications using the one-to-many style sockets and using the
   interleave level if 0 are subject to denial of service attacks as
   described in Section 7.1.20.



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11.  Acknowledgments

   Special acknowledgment is given to Ken Fujita, Jonathan Woods,
   Qiaobing Xie, and La Monte Yarroll, 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, Andreas Fink,
   Jonathan Leighton, Irene Ruengeler, 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.


12.  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.

   [RFC3493]  Gilligan, R., Thomson, S., Bound, J., McCann, J., and W.
              Stevens, "Basic Socket Interface Extensions for IPv6",
              RFC 3493, February 2003.

   [RFC3542]  Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
              "Advanced Sockets Application Program Interface (API) for
              IPv6", RFC 3542, May 2003.

   [RFC3758]  Stewart, R., Ramalho, M., Xie, Q., Tuexen, M., and P.
              Conrad, "Stream Control Transmission Protocol (SCTP)
              Partial Reliability Extension", RFC 3758, May 2004.

   [RFC4895]  Tuexen, M., Stewart, R., Lei, P., and E. Rescorla,
              "Authenticated Chunks for the Stream Control Transmission
              Protocol (SCTP)", RFC 4895, August 2007.

   [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
              RFC 4960, September 2007.

   [RFC5061]  Stewart, R., Xie, Q., Tuexen, M., Maruyama, S., and M.
              Kozuka, "Stream Control Transmission Protocol (SCTP)



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              Dynamic Address Reconfiguration", RFC 5061,
              September 2007.


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;

       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,



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                 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;
       struct iovec iov;

       *nrp = 0;
       iov.iov_base = buf;
       iov.iov_len = *buflen;
       msg->msg_iov = &#65533;
       msg->msg_iovlen = 1;



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       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;
       struct sctp_sndrcvinfo *sri;
       struct msghdr msg;
       struct cmsghdr *cmsg;
       char cbuf[sizeof (*cmsg) + sizeof (*sri)];
       char *buf;
       size_t buflen;
       struct iovec iov;
       size_t cmsglen = sizeof (*cmsg) + sizeof (*sri);
       /* Allocate the initial data buffer */



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       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 = &#65533;
               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);
               }
       }

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




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   int main()
   {
       struct sctp_event_subscribe event;
       int lfd, cfd;
       int onoff = 1;
       struct sockaddr_in sin;
       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;
               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);



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       }
   }


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;
       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);
       }

       /* 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,



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           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); /* from appendix a */
       }
   }


Authors' Addresses

   Randall R. Stewart
   Huawei
   Chapin, SC  29036
   USA

   Email: rstewart@huawei.com


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

   Email: kacheong.poon@sun.com






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

   Email: tuexen@fh-muenster.de


   Vladislav Yasevich
   HP
   110 Spitrook Rd
   Nashua, NH, 03062
   USA

   Email: vladislav.yasevich@hp.com


   Peter Lei
   Cisco Systems, Inc.
   8735 West Higgins Road
   Suite 300
   Chicago, IL  60631
   USA

   Email: peterlei@cisco.com

























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