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Network Working Group                                    R. R. Stewart
INTERNET-DRAFT                                                   Cisco
                                                                Q. Xie
                                                             L Yarroll
                                                              Motorola
                                                               J. Wood
                                                               K. Poon
                                                      Sun Microsystems
                                                             K. Fujita
                                                                   NEC

expires in six months                                    July 19, 2001

                   Sockets API Extensions for SCTP
                 <draft-ietf-tsvwg-sctpsocket-01.txt>

    Status of This Memo

    This document is an Internet-Draft and is in full conformance with
    all provisions of Section 10 of [RFC2026].  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.

    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.

    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.

    Table of Contents

    1. Introduction............................................ 3
    2. Conventions............................................. 4
     2.1 Data Types............................................ 4
    3. UDP-style Interface..................................... 4
     3.1 Basic Operation....................................... 4
        3.1.1 socket() - UDP Style Syntax...................... 5
        3.1.2 bind() - UDP Style Syntax........................ 5
        3.1.3 listen() - UDP Style Syntax...................... 6
        3.1.4 sendmsg() and recvmsg() - UDP Style Syntax....... 7
        3.1.5 close() - UDP Style Syntax....................... 8
     3.2 Implicit Association Setup............................ 8
     3.3 Non-blocking mode..................................... 9
    4. TCP-style Interface.....................................10
     4.1 Basic Operation.......................................10
      4.1.1 socket() - TCP Style Syntax........................11
      4.1.2 bind() - TCP Style Syntax..........................11

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      4.1.3 listen() - TCP Style Syntax........................12
      4.1.4 accept() - TCP Style Syntax........................12
      4.1.5 connect() - TCP Style Syntax.......................12
      4.1.6 close() - TCP Style Syntax.........................13
      4.1.7 shutdown() - TCP Style Syntax......................13
      4.1.8 sendmsg() and recvmsg() - TCP Style Syntax.........14
      4.1.9 getsockname() .....................................15
      4.1.10 getpeername() ....................................15
   5. Data Structures..........................................16
    5.1 The msghdr and cmsghdr Structures......................16
    5.2 SCTP msg_control Structures............................17
     5.2.1 SCTP Initiation Structure (SCTP_INIT)...............18
     5.2.2 SCTP Header Information Structure (SCTP_SNDRCV).....19
    5.3 SCTP Events and Notifications..........................20
     5.3.1 SCTP Notification Structure.........................21
      5.3.1.1 SCTP_ASSOC_CHANGE................................22
      5.3.1.2 SCTP_PEER_ADDR_CHANGE............................23
      5.3.1.3 SCTP_REMOTE_ERROR................................25
      5.3.1.4 SCTP_SEND_FAILE..................................26
      5.3.1.5 SCTP_SHUTDOWN_EVENT..............................27
     5.4 Ancillary Data Considerations and Semantics...........27
      5.4.1 Multiple Items and Ordering........................27
      5.4.2 Accessing and Manipulating Ancillary Data..........28
      5.4.3 Control Message Buffer Sizing......................28
    6. Common Operations for Both Styles.......................29
     6.1 send(), recv(), sendto(), recvfrom()..................29
     6.2 setsockopt(), getsockopt()............................30
     6.3 read() and write()....................................30
    7. Socket Options..........................................30
     7.1 Read / Write Options..................................31
      7.1.1 Retransmission Timeout Parameters (SCTP_RTOINFO)...31
      7.1.2 Association Retransmission Parameter
            (SCTP_ASSOCRTXINFO)................................32
      7.1.3 Initialization Parameters (SCTP_INITMSG)...........32
      7.1.4 SO_LINGER..........................................33
      7.1.5 SO_NODELAY.........................................33
      7.1.6 SO_RCVBUF..........................................33
      7.1.7 SO_SNDBUF..........................................33
      7.1.8 Automatic Close of associations (SCTP_AUTOCLOSE)...33
      7.1.9 SCTP_SET_PRIMARY_ADDR..............................34
      7.1.10 SCTP_SET_PEER_PRIMARY_ADDR........................34
     7.2 Read-Only Options.....................................34
      7.2.1 Association Status (SCTP_STATUS)...................34
     7.3. Ancillary Data and Notification Interest Options.....35
    8. New Interfaces..........................................36
     8.1 sctp_bindx()..........................................36
     8.2 Branched-off Association, sctp_peeloff()..............37
     8.3 sctp_getpaddrs()......................................38
     8.4 sctp_freepaddrs().....................................38
     8.5 sctp_getladdrs()......................................38
     8.6 sctp_freeladdrs().....................................39
     8.7 sctp_opt_info().......................................39
      8.7.1 Peer Address Parameters............................39
      8.7.2 Peer Address Information...........................40

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    9. Security Considerations.................................41
    10.  Acknowledgements......................................41
    11.  Authors' Addresses....................................41
    12.  References............................................42
    Appendix A: TCP-style Code Example.........................42
    Appendix B: UDP-style Code Example.........................47

1. Introduction

    The sockets API has provided a standard mapping of the Internet
    Protocol suite to many operating systems. Both TCP [RFC793] and UDP
    [RFC768] 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 UDP-style interface

    This set of semantics is similar to that defined for conntionless
    protocols, such as UDP. It is more efficient than a TCP-like
    connection-oriented interface in terms of exploring the new features
    of SCTP.

    Note that SCTP is connection-oriented in nature, and it does not
    support broadcast or multicast communications, as UDP does.

    3) Support a TCP-style interface

    This interface supports the same basic semantics as sockets for
    connection-oriented protocols, such as TCP.

    The purpose of defining this interface is to allow existing
    applications built on connnection-oriented protocols be ported to
    use SCTP with very little effort, and developers familiar with those
    semantics can easily adapt to SCTP.

    Extensions will be 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 UDP-style mapping and the TCP-style
    mapping. These two modes share some common data structures and
    operations, but will require the use of two different programming

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    models.

    A mechanism is defined to convert a UDP-style SCTP socket into a
    TCP-style socket.

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

2. Conventions

2.1 Data Types

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

3. UDP-style Interface

    The UDP-style interface has the following characteristics:

    A) Outbound association setup is implicit.

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

3.1 Basic Operation

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

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

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

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

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

    If the server or client wishes to branch an existing association off

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    to a separate socket, it is required to call sctp_peeloff() and in
    the parameter specifies one of the transport addresses of the
    association. The sctp_peeloff() call will return a new socket which
    can then be used with recv() and send() functions for message
    passing. See Section 8.2 for more on branched-off associations.

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

    We will discuss the UDP-style socket calls in more details in the
    following subsections.

3.1.1 socket() - UDP Style Syntax

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

    The syntax is,

     sd = socket(PF_INET, SOCK_SEQPACKET, IPPROTO_SCTP);

    or,

     sd = socket(PF_INET6, SOCK_SEQPACKET, IPPROTO_SCTP);

    Here, SOCK_SEQPACKET indicates the creation of a UDP-style socket.

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

3.1.2 bind() - UDP Style Syntax

    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 [SCTP].

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

    The syntax of bind() is,

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

     sd      - the socket descriptor returned by socket().
     addr    - the address structure (struct sockaddr_in or struct
               sockaddr_in6 [RFC 2553]),
     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.

    If a bind() or sctp_bindx() 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 multihoming capability
    of SCTP.

3.1.3 listen() - UDP Style Syntax

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

     int listen(int socket, int backlog);

      socket  - the socket descriptor of the endpoint.
      backlog - ignored for UDP-style sockets.

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

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    that the SCTP_ASSOC_CHANGE event is enabled (it is enabled by
    default).

3.1.4 sendmsg() and recvmsg() - UDP Style Syntax

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

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

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

      socket  - 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, TOS, various flags, context and payload
    protocol Id, etc.

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

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

    When receiving a user message with recvmsg(), the msg_name field in
    msghdr structure will be populated with the source transport address
    of the user data. The caller of recvmsg() can use this address
    information to determine to which association the received user
    message belongs. Note that if SCTP_ASSOC_CHANGE events are disabled,
    applications must use the peer transport address provided in the
    msg_name field by recvmsg() to perform correlation to an

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    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
    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.

    Note, if the socket is a branched-off socket that only represents
    one association (see Section 3.1), the msg_name field is not used
    when sending data (i.e., ignored by the SCTP stack).

3.1.5 close() - UDP Style Syntax

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

    The syntax is

      ret = close(int sd);

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

    To gracefully shutdown a specific association represented by the
    UDP-style socket, an application should use the sendmsg() call,
    passing no user data, but including the MSG_EOF flag in the
    ancillary data (see Section 5.2.2).

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

3.2 Implicit Association Setup

    Once all bind() calls are complete on a UDP-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

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    will automatically setup an association to the intended receiver.

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

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

    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.

    Whenever the user which want to avoid blocking must call select()
    before calling sendmsg()/sendto() and recvmsg()/recvfrom(), and
    check the socket status is writable or readable. If the socket
    status isn't writeable or readable, the user should not call
    sendmsg()/sendto() and recvmsg()/recvfrom().

    Once all bind() calls are complete on a UDP-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 signalled by the
    SCTP_ASSOC_CHANGE event with COMMUNICATION_UP or
    CANT_START_ASSOC. If user data could not be sent (due to a
    CANT_START_ASSOC), the sender will also receive a SCTP_SEND_FAILED
    event. Those event(s) can be received by the user calling of
    recvmsg(). A server (having called listen()) is also notified of an
    association up event by the reception of a SCTP_ASSOC_CHANGE with
    COMMUNICATION_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 MSG_EOF flag set. The function
    returns immediately, and completion of the graceful shutdown is

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    indicated by an SCTP_ASSOC_CHANGE notification of type
    SHUTDOWN_COMPLETE (see section 5.3.1.1).

4. TCP-style Interface

    The goal of this model is to follow as closely as possible the
    current practice of using the sockets interface for a connection
    oriented protocol, such as TCP. This model 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.

4.1 Basic Operation

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

    1. socket()
    2. bind()
    3. listen()
    4. accept()

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

    Then it calls

    5. close()

    to terminate the association.

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

    1. socket()
    2. connect()

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

    The client calls

    3. close()

    to terminate this association when done.

4.1.1 socket() - TCP Style Syntax

    Applications calls socket() to create a socket descriptor to

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    represent an SCTP endpoint.

    The syntax is:

     int socket(PF_INET, SOCK_STREAM, IPPROTO_SCTP);

    or,

     int socket(PF_INET6, SOCK_STREAM, IPPROTO_SCTP);

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

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

4.1.2 bind() - TCP Style Syntax

    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 [SCTP].

    The syntax is:

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

      sd      - the socket descriptor returned by socket() call.
      addr    - the address structure (either struct sockaddr_in or struct
                sockaddr_in6 defined in [RFC 2553]).
      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() or sctp_bindx() is not called prior to the connect()
    call, the system picks an ephemeral port and will choose an address

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    set equivalant to binding with a wildcard address. One of those
    addresses will be the primary address for the association. This
    automatically enables the multihoming capability of SCTP.

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

4.1.3 listen() - TCP Style Syntax

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

    The syntax is:

     int listen(int sd, int backlog);

      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 [SCTP]) and are in the
                ESTABLISHED state.

4.1.4 accept() - TCP Style Syntax

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

    The syntax is:

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

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

4.1.5 connect() - TCP Style Syntax

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

    The syntax is

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

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

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

    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() or sctp_bindx() is not called prior to the connect()
    call, the system picks an ephemeral port and will choose an address
    set equivalent to binding with INADDR_ANY and IN6ADDR_ANY for IPv4
    and IPv6 socket respectively. One of those addresses will be the
    primary address for the association.  This automatically enables the
    multihoming 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 connect() call.  Instead, it should use sendto()
    or sendmsg() to initiate an association.  If it uses sendto() and it
    wants to change initialization behavior, it needs to use the
    SCTP_INITMSG socket option before calling sendto().  Or it can use
    SCTP_INIT type sendmsg() to initiate an association without doing
    the setsockopt().

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

4.1.6 close() - TCP Style Syntax

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

    The syntax is:

     int close(int sd);

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

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

4.1.7 shutdown() - TCP Style Syntax

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

    The syntax is:


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     int shutdown(int socket, int how);

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

      how     - Specifies the type of shutdown.  The  values  are
                as follows:

                SHUT_RD
                      Disables further receive operations. No SCTP
                      protocol action is taken.

                SHUT_WR
                      Disables further send operations, and initiates
                      the SCTP shutdown sequence.

                SHUT_RDWR
                      Disables further send  and  receive  operations
                      and initiates the SCTP shutdown sequence.

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

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

4.1.8 sendmsg() and recvmsg() - TCP Style Syntax

    With a TCP-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 UDP-style model (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
    different peer address if the sender does not want to send the
    message over the primary address of the receiver. If the transport
    address given is not part of the current association, the data will
    not be sent and a SCTP_SEND_FAILED event will be delivered to the
    application if send failure events are enabled.

    When receiving, if a message is not received from the primary
    address, the SCTP stack will fill in the msg_name field on return so
    that the application can retrieve the source address information of
    the received message.

    2) An application must use close() to gracefully shutdown an
    association, or use SO_LINGER option with close() to abort an
    association.  It must not use the MSG_ABORT or MSG_EOF flag in

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    sendmsg().  The system returns an error if an application tries to
    do so.

4.1.9 getsockname()

    Applications use getsockname() to retrieve the locally-bound socket
    address of the specified socket. The is especially useful if the
    caller let SCTP chose a local port. This call is for TCP
    compatibility, and is not multihomed. It does not work with
    UDP-style sockets. See section 8.5 for a multihomed/UDP-sockets
    version of the call.

    The syntax is:

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

      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 mapped IPv4 address.

      len     - The caller should set the length of address here.
                On return, this is set to the length of the returned
                address.

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

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

4.1.10 getpeername()

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

    The syntax is:

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

      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 mapped IPv4

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                address.

      len     - The caller should set the length of address here.
                On return, this is set to the length of the returned
                address.

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

5. Data Structures

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

5.1 The msghdr and cmsghdr Structures

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

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

    The msghdr structure:

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

   The cmsghdr structure:

    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 Sections 3.1.3 and 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

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    multiple chunks, for both sendmsg() and recvmsg().

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

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

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

5.2 SCTP msg_control Structures

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

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

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

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

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

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

    The sockets application must provide a buffer large enough to
    accomodate 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.

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5.2.1 SCTP Initiation Structure (SCTP_INIT)

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

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

    Here is the definition of the sctp_initmsg structure:

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

    sinit_num_ostreams: 16 bits (unsigned integer)

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

    sinit_max_instreams: 16 bits (unsigned integer)

    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 endpoint's default
    value.

    sinit_max_attempts: 16 bits (unsigned integer)

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

    sinit_max_init_timeo: 16 bits (unsigned integer)

    This value represents the largest Time-Out or RTO value to use in
    attempting a 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

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    time.  A default value of 0 indicates to use the endpoint's default
    value.  This is normally set to the system's 'RTO.Max' value (60
    seconds).

5.2.2 SCTP Header Information Structure (SCTP_SNDRCV)

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

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

    Here is the defintion 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;
       sctp_assoc_t sinfo_assoc_id;
    };

    sinfo_stream: 16 bits (unsigned integer)

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

    sinfo_ssn: 16 bits (unsigned integer)

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

    sinfo_ppid:32 bits (unsigned integer)

    This value in sendmsg() is an opaque unsigned value 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 byte order issues are NOT accounted
    for and this information is passed opaquely by the SCTP stack from
    one end to the other.

    sinfo_context:32 bits (unsigned integer)

    This value is an opaque 32 bit context datum that is used in the
    sendmsg() function.  This value is passed back to the upper layer if

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    a error occurs on the send of a message and is retrieved with each
    undelivered message (Note: if a endpoint has done multple sends, all
    of which fail, multiple different sinfo_context values will be
    returned.  One with each user data message).

    sinfo_flags: 16 bits (unsigned integer)

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

    recvmsg() flags:

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

    sendmsg() flags:

      MSG_UNORDERED - This flag requests the un-ordered delivery of the
                      message.  If this flag is clear the datagram is
                      considered an ordered send.

      MSG_ADDR_OVER - This flag, in the UDP model, requests the SCTP
                      stack to override the primary destination address
                      with the address found with the sendto/sendmsg
                      call.

      MSG_ABORT     - Setting this flag causes the specified association
                      to abort by sending an ABORT message to the peer
                      (UDP-style only).

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

    sinfo_assoc_id: sizeof (sctp_assoc_t)

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

    A sctp_sndrcvinfo 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

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

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

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

5.3.1 SCTP Notification Structure

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

    union sctp_notification {
        uint16_t sn_type;             /* Notification type. */
        struct sctp_assoc_change;
        struct sctp_paddr_change;
        struct sctp_remote_error;
        struct sctp_shutdown_event;
    };

    sn_type: sizeof (uint16_t)

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

    sn_type                      Description
    ---------              ---------------------------

    SCTP_ASSOC_CHANGE      This tag indicates that an
                           association has either been
                           opened or closed.  Refer to
                           5.3.1.1 for details.

    SCTP_PEER_ADDR_CHANGE  This tag indicates that an
                           address that is part of an existing
                           association has experienced a
                           change of state (e.g. a failure
                           or return to service of the
                           reachability of a endpoint
                           via a specific transport
                           address).  Please see 5.3.1.2
                           for data structure details.


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    SCTP_REMOTE_ERROR      The attached error message
                           is an Operational Error received from
                           the remote peer.  It includes the complete
                           TLV sent by the remote endpoint.
                           See section 5.3.1.3 for the detailed format.

    SCTP_SEND_FAILED       The attached datagram
                           could not be sent to the remote endpoint.
                           This structure includes the
                           original SCTP_SNDRCVINFO
                           that was used in sending this
                           message i.e. this structure
                           uses the sctp_sndrecvinfo per
                           section 5.3.1.4.

    SCTP_SHUTDOWN_EVENT    The peer has sent a SHUTDOWN. No further
                           data should be sent on this socket.

5.3.1.1 SCTP_ASSOC_CHANGE

    Communication notifications inform the ULP that an SCTP association
    has either begun or ended. The identifier for a new association is
    provided by this notificaion. 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;
    };

    sac_type:

    It should be SCTP_ASSOC_CHANGE.

    sac_flags: 16 bits (unsigned integer)

    Currently unused.

    sac_length: sizeof (uint32_t)

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

    sac_state: 32 bits (signed integer)

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


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    Event Name           Description
    ----------------     ---------------
    COMMUNICATION_UP     A new association is now ready
                         and data may be exchanged with this
                         peer.

    COMMUNICATION_LOST   The association has failed.  The association
                         is now in the closed state.  If SEND FAILED
                         notifications are turned on, a COMMUNICATION_LOST
                         is followed by a series of SCTP_SEND_FAILED
                         events, one for each outstanding message.

    RESTART              SCTP has detected that the peer has restarted.

    SHUTDOWN_COMPLETE    The association has gracefully closed.

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

    sac_error:  32 bits (signed integer)

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

    sac_outbound_streams:  16 bits (unsigned integer)
    sac_inbound_streams:  16 bits (unsigned integer)

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

    sac_assoc_id: sizeof (sctp_assoc_t)

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

5.3.1.2 SCTP_PEER_ADDR_CHANGE

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

    struct sctp_paddr_change{
       uint16_t spc_type;
       uint16_t spc_flags;
       uint32_t spc_length;
       struct sockaddr_storage spc_aaddr;
       int spc_state;
       int spc_error;

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       sctp_assoc_t spc_assoc_id;
     }

    spc_type:

    It should be SCTP_PEER_ADDR_CHANGE.

    spc_flags: 16 bits (unsigned integer)

    Currently unused.

    spc_length: sizeof (uint32_t)

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

    spc_aaddr: sizeof (struct sockaddr_storage)

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

    spc_state:  32 bits (signed integer)

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

    Event Name           Description
    ----------------     ---------------
    ADDRESS_AVAILABLE    This address is now reachable.

    ADDRESS_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.

    ADDRESS_REMOVED      The address is no longer part of
                         the association.

    ADDRESS_ADDED        The address is now part of the
                         association.

    ADDRESS_MADE_PRIM    This address has now been made
                         to be the primary destination address.

    spc_error:  32 bits (signed integer)

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

    spc_assoc_id: sizeof (sctp_assoc_t)

    The association id field, holds the identifier for the association.

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    All notifications for a given association have the same association
    identifier.  For TCP style socket, this field is ignored.

5.3.1.3 SCTP_REMOTE_ERROR

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

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

    sre_type:

    It should be SCTP_REMOTE_ERROR.

    sre_flags: 16 bits (unsigned integer)

    Currently unused.

    sre_length: sizeof (uint32_t)

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

    sre_error: 16 bits (unsigned integer)

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

    sre_len: 16 bits (unsigned integer)

    This value represents the length of the operational error payload in
    plus the size of sre_error and sre_len in network byte order.

    sre_assoc_id: sizeof (sctp_assoc_t)

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

    sre_data: variable

    This contains the payload of the operational error as defined in the

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    SCTP specification [SCTP] section 3.3.10.

5.3.1.4 SCTP_SEND_FAILED

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

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

    ssf_type:

    It should be SCTP_SEND_FAILED.

    ssf_flags: 16 bits (unsigned integer)

    The flag value will take one of the following values

    SCTP_DATA_UNSENT  - Indicates that the data was never put on
                        the wire.

    SCTP_DATA_SENT    - Indicates that the data was put on the wire.
                        Note that this does not necessarily mean that the
                        data was (or was not) successfully delivered.


    ssf_length: sizeof (uint32_t)

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

    ssf_error: 16 bits (unsigned integer)

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

    ssf_info: sizeof (struct sctp_sndrcvinfo)

    The original send information associated with the undelivered
    message.

    ssf_assoc_id: sizeof (sctp_assoc_t)

    The association id field, sf_assoc_id, holds the identifier for the
    association.  All notifications for a given association have the
    same association identifier.  For TCP style socket, this field is

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    ignored.

    ssf_data: variable length

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

5.3.1.5 SCTP_SHUTDOWN_EVENT

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

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

    sse_type

    It should be SCTP_SHUTDOWN_EVENT

    sse_flags: 16 bits (unsigned integer)

    Currently unused.

    sse_length: sizeof (uint32_t)

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

    sse_assoc_id: sizeof (sctp_assoc_t)

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

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-dependant, so
    applications must not depend on any ordering.

    SCTP_SNDRCV items must always correspond to the data in the msghdr's

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    msg_iov member.  There can be only a single SCTP_SNDRCV info for
    each sendmsg() or recvmsg() call.

5.4.2 Accessing and Manipulating Ancillary Data

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

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

       struct msghdr   msg;
       struct cmsghdr  *cmsgptr;

       /* fill in msg */

       /* call recvmsg() */

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

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

5.4.3 Control Message Buffer Sizing

    The information conveyed via SCTP_SNDRCV events will often be
    fundamental to the correct and sane operation of the sockets
    application. This is particularly true of the UDP semantics, but
    also of the TCP 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

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    sum the sizes of all enabled ancillary data item structures, as
    calculated by CMSG_SPACE. For example, if we enabled
    SCTP_SNDRCV_INFO and IPV6_RECVPKTINFO [RFC2292], we would calculate
    and allocate the buffer size as follows:

    size_t total;
    void *buf;

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

    buf = malloc(total);

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

6. Common Operations for Both Styles

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

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

    The syntax is:

      ssize_t send(int sd, connst void *msg, size_t len, int flags);
      ssize_t sendto(int sd, const void *msg, size_t len, int flags,
                     const struct sockaddr *to, int 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, int *fromlen);

       sd      - the socket descriptor of an SCTP endpoint.
       msg     - the message to be sent.
       len     - the size of the message or the size of buffer.
       to      - one of the peer addresses of the association to be
                 used to send the message.
       tolen   - the size of the address.
       buf     - the buffer to store a received message.
       from    - the buffer to store the peer address used to send the
                 received message.
       fromlen - the size of the from address
       flags   - (described below).

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

    SCTP has the concept of multiple streams in one association.  The
    above calls do not allow the caller to specify on which stream a
    message should be sent. The system uses stream 0 as the default

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    stream for send() and sendto(). recv() and recvfrom() return data
    from any stream, but the caller can not distinguish the different
    streams. This may result in data seeming to arrive out of
    order. Similarly, if a data chunk is sent unordered, recv() and
    recvfrom() provide no indication.

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

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

    Note, the send and recv calls, when used in the UDP-style model, may
    only be used with branched off socket descriptors (see Section 8.2).

6.2 setsockopt(), getsockopt()

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

    The syntax is:

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

      sd      - the socket descript.
      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).

6.3 read() and write()

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

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

7. Socket Options

    The following sub-section describes various SCTP level socket
    options that are common to both models.  SCTP associations can be

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    multihomed.  Therefore, certain option parameters include a
    sockaddr_storage structure to select which peer address the option
    should be applied to.

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

    For the TCP-style sockets and branched off UDP-style sockets (see
    section 8.2) this association ID parameter is ignored.  In the cases
    noted below where the parameter is ignored, an application can pass
    to the system a corresponding option structure similar to those
    described below but without the association ID parameter, which
    should be the last field of the option structure.  This can make the
    option setting/getting operation more efficient.  If an application
    does this, it should also specify an appropriate optlen value
    (i.e. sizeof (option parameter) - sizeof (struct sctp_assoc_t)).

    Note that socket or IP level options is set or retrieved per socket.
    This means that for UDP-style sockets, those options will be applied
    to all associations belonging to the socket.  And for TCP-style
    model, 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.

7.1 Read / Write Options

7.1.1 Retransmission Timeout Parameters (SCTP_RTOINFO)

    The protocol parameters used to initialize and bound retransmission
    timeout (RTO) are tunable.  See [SCTP] for more information on how
    these parameters are used in RTO calculation.  The peer address
    parameter is ignored for TCP style socket.

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

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

    srto_initial    - This contains the initial RTO value.
    srto_max and srto_min - These contain the maximum and minumum bounds
                            for all RTOs.
    srto_assoc_id   - (UDP style socket) This is filled in the application,
                      and identifies the association for this query.

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


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

7.1.2 Association Retransmission Parameter (SCTP_ASSOCRTXINFO)

    The protocol parameter used to set the number of retransmissions
    sent before an association is considered unreachable.
    See [SCTP] for more information on how this parameter is used.  The
    peer address parameter is ignored for TCP style socket.

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

    struct sctp_assocparams {
        uint16_t        sasoc_asocmaxrxt;
        sctp_assoc_t    sasoc_assoc_id;
    };

    sasoc_asocmaxrxt - This contains the maximum retransmission attempts
                       to make for the association.
    sasoc_assoc_id   - (UDP style socket) This is filled in the application,
                       and identifies the association for this query.

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

    The maximum number of retransmissions before an address is
    considered unreachable is also tunable, but is address-specific, so
    it is covered in a seperate 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() shall return an error.  The reason for this, from
    [SCTP] 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 intialization.  The structure used to access and modify
    these parameters is defined in section 5.2.1.  The option name
    argument to setsockopt() and getsockopt() is SCTP_INITMSG.

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

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7.1.4 SO_LINGER

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

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

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

7.1.5 SCTP_NODELAY

    Turn 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.

7.1.6 SO_RCVBUF

    Sets receive buffer size. For SCTP TCP-style sockets, this controls
    the receiver window size. For UDP-style sockets, this controls the
    receiver window size for all associations bound to the socket
    descriptor used in the setsockopt() or getsockopt() call. The option
    applies to each association's window size seperately. Expects an
    integer.

7.1.7 SO_SNDBUF

    Sets send buffer size. For SCTP TCP-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 UDP-style sockets, the effect is
    the same, except that it applies to all associations bound to the
    socket descriptor used in the setsockopt() or getsockopt() call. The
    option applies to each association's window size seperately. Expects
    an integer.

7.1.8 Automatic Close of associations (SCTP_AUTOCLOSE)

    This socket option is applicable to the UDP-style socket only. When
    set it will cause associations that are idle for more than the
    specified number of seconds to automatically close. An association
    being idle is defined an association that has NOT sent or received
    user data. The special value of '0' indicates that no automatic

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    close of any associations should be performed. The option expects
    an integer defining the number of seconds of idle time before
    an association is closed.

7.1.9 Set Primary Address (SCTP_SET_PRIMARY_ADDR)

    Requests that the peer mark 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 primary request:

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

    ssp_addr            The address to set as primary
    ssp_assoc_id        (UDP style socket) This is filled in by the
                        application, and identifies the association
                        for this request.

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

7.1.10 Set Peer Primary Address (SCTP_SET_PEER_PRIMARY_ADDR)

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

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

    sspp_addr           The address to set as primary
    sspp_assoc_id       (UDP style socket) This is filled in by the
                        application, and identifies the association
                        for this request.

7.2 Read-Only Options

7.2.1 Association Status (SCTP_STATUS)

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

    struct sctp_status {
        int32_t         sstat_state;
        uint32_t        sstat_rwnd;

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        uint16_t        sstat_unackdata;
        uint16_t        sstat_penddata;
        struct sctp_paddrinfo sstat_primary;
        sctp_assoc_t    sstat_assoc_id;
    };

    sstat_state    - This contains the association's current state one
                     of the following values:

                     SCTP_CLOSED
                     SCTP_BOUND
                     SCTP_LISTEN
                     SCTP_COOKIE_WAIT
                     SCTP_COOKIE_ECHOED
                     SCTP_ESTABLISHED
                     SCTP_SHUTDOWN_PENDING
                     SCTP_SHUTDOWN_SENT
                     SCTP_SHUTDOWN_RECEIVED
                     SCTP_SHUTDOWN_ACK_SENT


    sstat_rwnd     - This contains the association  peer's current
                     receiver window size.
    sstat_unackdata - This is the number of unacked data chunks.
    sstat_penddata  - This is the number of data chunks pending receipt.
    sstat_primary   - This is information on the current primary peer
                      address.
    sstat_assoc_id  - (UDP style socket) This holds the an identifier for the
                      association.  All notifications for a given association
                      have the same association identifier.

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

7.3.  Ancillary Data and Notification Interest Options

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

    The following optional information is available to the application:

    1.  SCTP_RECVDATAIOEVNT: Per-message information (i.e. stream number,
        TSN, SSN, etc. described in section 5.2.2)
    2.  SCTP_RECVASSOCEVNT: (described in section 5.3.1.1)
    3.  SCTP_RECVPADDREVNT: (described in section 5.3.1.2)
    4.  SCTP_RECVPEERERR: (described in section 5.3.1.3)
    5.  SCTP_RECVSENDFAILEVNT: (described in section 5.3.1.4)
    6.  SCTP_RECVSHUTDOWNEVNT: (described in section 5.3.1.5);

    To receive any ancillary data or notifications, first the
    application registers it's interest by calling setsockopt() to turn
    on the corresponding flag:


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    int on = 1;

    setsockopt(fd, IPPROTO_SCTP, SCTP_RECVDATAIOEVNT,   &on, sizeof(on));
    setsockopt(fd, IPPROTO_SCTP, SCTP_RECVASSOCEVNT,    &on, sizeof(on));
    setsockopt(fd, IPPROTO_SCTP, SCTP_RECVPADDREVNT,    &on, sizeof(on));
    setsockopt(fd, IPPROTO_SCTP, SCTP_RECVSENDFAILEVNT, &on, sizeof(on));
    setsockopt(fd, IPPROTO_SCTP, SCTP_RECVPEERERR,      &on, sizeof(on));
    setsockopt(fd, IPPROTO_SCTP, SCTP_RECVSHUTDOWNEVNT, &on, sizeof(on));

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

    By default a TCP-style socket has all options off.

    By default a UDP-style socket has SCTP_RECVDATAIOEVNT and
    SCTP_RECVASSOCEVNT on and all other options off.

8. New Interfaces

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

8.1 sctp_bindx()

    The syntax of sctp_bindx() is,

     int sctp_bindx(int sd, struct sockaddr_storage *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 a struct sockaddr_storage, so each
    address is a fixed length. The caller specifies the number of
    addresses in the array with addrcnt.

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

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

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

    SCTP_BINDX_ADD_ADDR

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    SCTP_BINDX_REM_ADDR

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

    An application can use sctp_bindx(SCTP_BINDX_ADD_ADDR) to associate
    additional addresses with an endpoint after calling bind().  Or use
    sctp_bindx(SCTP_BINDX_REM_ADDR) to remove some addresses a listening
    socket is associated with so that no new association accepted will
    be associated with those addresses.

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

8.2 Branched-off Association

    After an association is established on a UDP-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 UDP-style socket but branch off those
    associations carrying high volume data traffic into their own
    separate socket descriptors.

    The application uses sctp_peeloff() call to branch off an
    association into a separate socket (Note the semantics are somewhat
    changed from the traditional TCP-style accept() call).

    The syntax is:

     new_sd = sctp_peeloff(int sd, sctp_assoc_t *assoc_id, int *addrlen)

     new_sd  - the new socket descriptor representing the branched-off
               association.

     sd      - the original UDP-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 TCP-style accept() call, this would be an out
                parameter, but for the UDP-style call, this is an in
                parameter).

      addrlen - an integer pointer to the size of the sockaddr structure
                addr (in a traditional TCP-style call, this would be a out
                parameter, but for the UDP-style call this is an in

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                parameter).

8.3 sctp_getpaddrs()

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

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

    On return, addrs will point to a dynamically allocated array of
    struct sockaddr_storages, one for each peer address. The caller
    should use sctp_freepaddrs() to free the memory. 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 UDP-style sockets, id specifies the association to query. For
    TCP-style sockets, id is ignored.

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

8.4 sctp_freepaddrs()

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

    void sctp_freepaddrs(struct sockaddr_storage *addrs);

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

8.5 sctp_getladdrs()

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

    int sctp_getladdrs(int sock, sctp_assoc_t id,
                       struct sockaddr_storage **ss);

    On return, addrs will point to a dynamically allocated array of
    struct sockaddr_storages, one for each local address. The caller
    should use sctp_freeladdrs() to free the memory. 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.


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    For UDP-style sockets, id specifies the association to query. For
    TCP-style sockets, id is ignored.

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

8.6 sctp_freeladdrs()

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

    void sctp_freeladdrs(struct sockaddr_storage *addrs);

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

8.7 sctp_opt_info()

    getsockopt() is read-only, so a new interface is required when
    information must be passed both in to and out of the SCTP stack. The
    syntax for scpt_opt_info() is,

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

    For UDP-style sockets, id specifies the association to query. For
    TCP-style sockets, id is ignored.

    opt specifies which SCTP option to get or set. It can be one of the
    following:

    SCTP_SET_PEER_ADDR_PARAMS
    SCTP_GET_PEER_ADDR_PARAMS
    SCTP_GET_PEER_ADDR_INFO


    arg is an option-specific structure buffer provided by the caller.
    See 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.

8.7.1 Peer Address Parameters

    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 {
        struct sockaddr_storage spp_address;

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        uint32_t                spp_hbinterval;
        uint16_t                spp_pathmaxrxt;
        sctp_assoc_t            spp_assoc_id;
    };

    spp_address     - This specifies which address is of interest.
    spp_hbinterval  - This contains the value of the heartbeat interval,
                      in milliseconds.  A value of 0, when modifying the
                      parameter, specifies that the heartbeat on this
                      address should be disabled. A value of UINT32_MAX
                      (4294967295), when modifying the parameter,
                      specifies that a heartbeat should be sent
                      immediately to the peer address, and the current
                      interval should remain unchanged.
    spp_pathmaxrxt  - This contains the maximum number of
                      retransmissions before this address shall be
                      considered unreachable.
    spp_assoc_id    - (UDP style socket) This is filled in the application,
                      and identifies the association for this query.

    To modify these parameters, the application should call
    sctp_opt_info() with the SCTP_SET_PEER_ADDR_PARAMS option. To get
    these parameters, the application should use
    SCTP_GET_PEER_ADDR_PARAMS.

8.7.2 Peer Address Information

    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:

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

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

    On return from getsockopt():

     spinfo_state     - This contains the peer addresses's state (either
                        SCTP_ACTIVE or SCTP_INACTIVE).
     spinfo_cwnd      - This contains the peer addresses's current congestion
                        window.
     spinfo_srtt      - This contains the peer addresses's current smoothed
                        round-trip time calculation in milliseconds.
     spinfo_rto       - This contains the peer addresses's current

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                        retransmission timeout value in milliseconds.
     spinfo_assoc_id  - (UDP style socket) This is filled in the application,
                        and identifies the association for this query.

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

9. 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 unprivelged users should not be able to set protocol
    parameters which could result in the congestion control algorithm
    being more agressive than permitted on the public Internet.  These
    paramaters are:

    struct sctp_rtoinfo

    If an unprivileged user inherits a UDP-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.

10. Acknowledgements

    The authors wish to thank Mike Bartlett, Jon Berger, Renee Ravis,
    and many others on the TSVWG mailing list for contributing valuable
    comments.

11.  Authors' Addresses

Randall R. Stewart                      Tel: +1-815-477-2127
Cisco Systems, Inc.                     EMail: rrs@cisco.com
Crystal Lake, IL 60012
USA

Qiaobing Xie                            Tel: +1-847-632-3028
Motorola, Inc.                          EMail: qxie1@email.mot.com
1501 W. Shure Drive, Room 2309
Arlington Heights, IL 60004
USA

La Monte H.P. Yarroll                   NIC Handle: LY
Motorola, Inc.                          EMail: piggy@acm.org
1501 W.  Shure Drive, IL27-2315
Arlington Heights, IL 60004
USA

Jonathan Wood
Sun Microsystems, Inc.                  Email: jonathan.wood@sun.com

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901 San Antonio Road
Palo Alto, CA 94303
USA

Kacheong Poon
Sun Microsystems, Inc.                  Email: kacheong.poon@sun.com
901 San Antonio Road
Palo Alto, CA 94303
USA

Ken Fujita                              Tel: +81-471-82-1131
NEC Corporation                         Email: fken@cd.jp.nec.com
1131, Hinode, Abiko
Chiba, 270-1198
Japan

12.  References


[RFC793]  Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
          September 1981.

[RFC768]  Postel, J. (ed.), "User Datagram Protocol", STD 6, RFC 768,
          August 1980.

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

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

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

[RFC2553] R. Gilligan, S. Thomson, J. Bound, W. Stevens. "Basic Socket
          Interface Extensions for IPv6," RFC 2553, March 1999.

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

[STEVENS] W.R. Stevens,  M. Thomas, E. Nordmark, "Advanced Sockets API for
          IPv6," <draft-ietf-ipngwg-rfc2292bis-01.txt>, December 1999
          (Work in progress)

Appendix A: TCP-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 TCP-style IPv4
SCTP sockets, including:

  o Opening, binding, and listening for new associations on a socket;
  o Enabling ancillary data
  o Enabling notifications

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  o Using ancillary data with sendmsg() and recvmsg()
  o Using MSG_EOR to determine if an entire message has been read
  o Handling notifications




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_type) {
        case SCTP_ASSOC_CHANGE:
                sac = &snp->sn_assoc_change;
                printf("^^^ assoc_change: state=%hu, error=%hu, instr=%hu "
                    "outstr=%hu\n", sac->sac_state, sac->sac_error,
                    sac->sac_inbound_streams, sac->sac_outbound_streams);
                break;
        case SCTP_SEND_FAILED:
                ssf = &snp->sn_send_failed;
                printf("^^^ sendfailed: len=%hu err=%d\n", ssf->ssf_length,
                    ssf->ssf_error);
                break;
        case SCTP_PEER_ADDR_CHANGE:
                spc = &snp->sn_intf_change;
                if (spc->spc_addr.ss_family == AF_INET) {
                        sin = (struct sockaddr_in *)&spc->spc_addr;
                        ap = inet_ntop(AF_INET, &sin->sin_addr, addrbuf,
                            INET6_ADDRSTRLEN);
                } else {
                        sin6 = (struct sockaddr_in6 *)&spc->spc_addr;
                        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_len));
                break;
        case SCTP_SHUTDOWN_EVENT:

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                printf("^^^ shutdown event\n");
                break;
        default:
                printf("unknown type: %hu\n", snp->sn_type);
                break;
        }
}

static void *
sctp_recvmsg(int fd, struct msghdr *msg, void *buf, size_t *buflen,
    ssize_t *nrp, size_t cmsglen)
{
        ssize_t nr = 0;
        struct iovec iov[1];

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

        for (;;) {
                msg->msg_flags = MSG_XPG4_2;
                msg->msg_iov->iov_len = *buflen;
                msg->msg_controllen = cmsglen;

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

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

                /* Realloc the buffer? */
                if (*buflen == 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;

        }
}


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static void
echo(int fd, int socketModeUDP)
{
        ssize_t nr;
        struct sctp_sndrcvinfo *sri;
        struct msghdr msg[1];
        struct cmsghdr *cmsg;
        char cbuf[sizeof (*cmsg) + sizeof (*sri)];
        char *buf;
        size_t buflen;
        struct iovec iov[1];
        size_t cmsglen = sizeof (*cmsg) + sizeof (*sri);

        /* Allocate the initial data buffer */
        buflen = BUFLEN;
        if (!(buf = malloc(BUFLEN))) {
                fprintf(stderr, "out of memory\n");
                exit(1);
        }

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

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

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

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

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

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


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

main()
{
        int lfd, cfd;
        int onoff = 1;
        struct sockaddr_in sin[1];

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

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

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

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

                /* Enable ancillary data */
                if (setsockopt(cfd, IPPROTO_SCTP, SCTP_RECVDATAIOEVNT,
                    &onoff, 4) < 0) {
                        perror("setsockopt RECVDATAIOEVNT");
                        exit(1);
                }
                /* Enable notifications */
                if (setsockopt(cfd, IPPROTO_SCTP, SCTP_RECVASSOCEVNT,
                    &onoff, 4) < 0) {
                        perror("setsockopt SCTP_RECVASSOCEVNT");
                        exit(1);
                }
                if (setsockopt(cfd, IPPROTO_SCTP, SCTP_RECVSENDFAILEVNT,
                    &onoff, 4) < 0) {
                        perror("setsockopt SCTP_RECVASSOCEVNT");
                        exit(1);

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                }
                if (setsockopt(cfd, IPPROTO_SCTP, SCTP_RECVPADDREVNT,
                    &onoff, 4) < 0) {
                        perror("setsockopt SCTP_RECVPADDREVNT");
                        exit(1);
                }
                if (setsockopt(cfd, IPPROTO_SCTP, SCTP_RECVPEERERR,
                    &onoff, 4) < 0) {
                        perror("setsockopt SCTP_RECVPEERERR");
                        exit(1);
                }
                if (setsockopt(cfd, IPPROTO_SCTP, SCTP_RECVSHUTDOWNEVNT,
                    &onoff, 4) < 0) {
                        perror("setsockopt SCTP_RECVSHUTDOWNEVNT");
                        exit(1);
                }

                /* Echo back any and all data */
                echo(cfd,0);
        }
}

Appendix B: UDP-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 UDP-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.



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

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

        sin->sin_family = AF_INET;
        sin->sin_port = htons(7);

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

        /* Enable notifications */

        /* SCTP_RECVASSOCEVNT and SCTP_RECVDATAIOEVNT are on by default */

        /* if a send fails we want to know it */
        if (setsockopt(fd, IPPROTO_SCTP, SCTP_RECVSENDFAILEVNT,
                       &onoff, 4) < 0) {
          perror("setsockopt SCTP_RECVASSOCEVNT");
          exit(1);
        }
        /* if a network address change or event transpires
         * we wish to know it
         */
        if (setsockopt(fd, IPPROTO_SCTP, SCTP_RECVPADDREVNT,
                       &onoff, 4) < 0) {
          perror("setsockopt SCTP_RECVPADDREVNT");
          exit(1);
        }
        /* We would like all error TLV's from the peer */
        if (setsockopt(fd, IPPROTO_SCTP, SCTP_RECVPEERERR,
                       &onoff, 4) < 0) {
          perror("setsockopt SCTP_RECVPEERERR");
          exit(1);
        }
        /* And of course we would like to know about shutdown's */
        if (setsockopt(fd, IPPROTO_SCTP, SCTP_RECVSHUTDOWNEVNT,
                       &onoff, 4) < 0) {
          perror("setsockopt SCTP_RECVSHUTDOWNEVNT");
          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, 0) < 0) {
          perror("listen");
          exit(1);
        }

        /* Wait for new associations */
        while(1){
          /* Echo back any and all data */

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          echo(fd,1);
        }
}




















































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