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RFC 6458

 
 
 

Sockets API Extensions for the Stream Control Transmission Protocol (SCTP)

Part 2 of 4, p. 25 to 56
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5.  Data Structures

   This section discusses important data structures that 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.

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   The msghdr and the related cmsghdr structures are defined and
   discussed in detail in [RFC3542].  They are defined as

   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 */
     int 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 */
   };

   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 3.1.4 and 4.1.8).

   The scatter/gather buffers, or I/O vectors (pointed to by the msg_iov
   field) are treated by SCTP as a single user message for both
   sendmsg() and recvmsg().

   The SCTP stack uses the ancillary data (msg_control field) to
   communicate the attributes, such as SCTP_RCVINFO, of the message
   stored in msg_iov to the socket endpoint.  The different ancillary
   data types are described in Section 5.3.

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

   If a notification has arrived, recvmsg() will return the notification
   in the msg_iov field and set the MSG_NOTIFICATION flag in msg_flags.
   If the MSG_NOTIFICATION flag is not set, recvmsg() will return data.
   See Section 6 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.  Ancillary Data Considerations and Semantics

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

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5.2.1.  Multiple Items and Ordering

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

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

   SCTP_SNDRCV/SCTP_SNDINFO/SCTP_RCVINFO type ancillary data always
   corresponds to the data in the msghdr's msg_iov member.  There can be
   only one such type of ancillary data for each sendmsg() or recvmsg()
   call.

5.2.2.  Accessing and Manipulating Ancillary Data

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

   Implementations may have different padding requirements for ancillary
   data, so portable applications should make use of the macros
   CMSG_FIRSTHDR, CMSG_NXTHDR, CMSG_DATA, CMSG_SPACE, and CMSG_LEN.  See
   [RFC3542] and the SCTP implementation's documentation for more
   information.  The following is an example, from [RFC3542],
   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_len == 0) {
        /* Error handling */
        break;
     }
     if (cmsgptr->cmsg_level == ... && cmsgptr->cmsg_type == ... ) {
       u_char  *ptr;

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

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5.2.3.  Control Message Buffer Sizing

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

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

   Thus, it is essential that applications be able to deterministically
   calculate the maximum required buffer size to pass to recvmsg().  One
   constraint imposed on this specification that makes this possible is
   that all ancillary data definitions are of a fixed length.  One way
   to calculate the maximum required buffer size might be to take the
   sum of the sizes of all enabled ancillary data item structures, as
   calculated by CMSG_SPACE.  For example, if we enabled
   SCTP_SNDRCV_INFO and IPV6_RECVPKTINFO [RFC3542], we would calculate
   and allocate the buffer size as follows:

   size_t total;
   void *buf;

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

   buf = malloc(total);

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

5.3.  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
   msghdr structure's msg_control member used in sendmsg() and
   recvmsg().  Fine-grained control over initialization and sending
   parameters are handled with ancillary data.

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

   By default, on either style of socket, SCTP will pass no ancillary
   data.  Specific ancillary data items can be enabled with socket
   options defined for SCTP; see Section 6.2.

   Note that all ancillary types are of fixed length; see Section 5.2
   for further discussion on this.  These data structures use struct
   sockaddr_storage (defined in [RFC3493]) as a portable, fixed-length
   address format.

   Other protocols may also provide ancillary data to the socket layer
   consumer.  These ancillary data items from other protocols may
   intermingle with SCTP data.  For example, the IPv6 sockets API
   definitions ([RFC3542] and [RFC3493]) define a number of ancillary
   data items.  If a sockets API consumer enables delivery of both SCTP
   and IPv6 ancillary data, they both may appear in the same msg_control
   buffer in any order.  An application may thus need to handle other
   types of ancillary data besides those passed by SCTP.

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

5.3.1.  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 |
            +--------------+-----------+---------------------+

   The sctp_initmsg structure is defined below:

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

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

   sinit_max_instreams:  This value represents the maximum number of
      inbound streams the application is prepared to support.  This
      value is bounded by the actual implementation.  In other words,
      the user may be able to support more streams than the operating
      system.  In such a case, the operating-system limit overrides the
      value requested by the user.  The default value of 0 indicates the
      use of the endpoint's default value.

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

   sinit_max_init_timeo:  This value represents the largest timeout or
      retransmission timeout (RTO) value (in milliseconds) to use in
      attempting an INIT.  Normally, the 'RTO.Max' is used to limit the
      doubling of the RTO upon timeout.  For the INIT message, this
      value may override 'RTO.Max'.  This value must not influence
      'RTO.Max' during data transmission and is only used to bound the
      initial setup time.  A default value of 0 indicates the use of the
      endpoint's default value.  This is normally set to the system's
      'RTO.Max' value (60 seconds).

5.3.2.  SCTP Header Information Structure (SCTP_SNDRCV) - DEPRECATED

   This cmsghdr structure specifies SCTP options for sendmsg() and
   describes SCTP header information about a received message through
   recvmsg().  This structure mixes the send and receive path.
   SCTP_SNDINFO (described in Section 5.3.4) and SCTP_RCVINFO (described
   in Section 5.3.5) split this information.  These structures should be
   used, when possible, since SCTP_SNDRCV is deprecated.

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

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   The sctp_sndrcvinfo structure is defined below:

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

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

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

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

      recvmsg() flags:

         SCTP_UNORDERED:  This flag is present when the message was sent
            unordered.

      sendmsg() flags:

         SCTP_UNORDERED:  This flag requests the unordered delivery of
            the message.  If this flag is clear, the datagram is
            considered an ordered send.

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

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

         SCTP_EOF:  Setting this flag invokes the SCTP graceful shutdown
            procedure on the specified association.  Graceful shutdown
            assures that all data queued by both endpoints is
            successfully transmitted before closing the association.

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

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

   sinfo_context:  This value is an opaque 32-bit context datum that is
      used in the sendmsg() function.  This value is passed back to the
      upper layer if an error occurs on the send of a message and is
      retrieved with each undelivered message.

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

   sinfo_tsn:  For the receiving side, this field holds a Transmission
      Sequence Number (TSN) that was assigned to one of the SCTP DATA
      chunks.  For the sending side, it is ignored.

   sinfo_cumtsn:  This field will hold the current cumulative TSN as
      known by the underlying SCTP layer.  Note that this field is
      ignored when sending.

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

   An sctp_sndrcvinfo item always corresponds to the data in msg_iov.

5.3.3.  Extended SCTP Header Information Structure (SCTP_EXTRCV) -
        DEPRECATED

   This cmsghdr structure specifies SCTP options for SCTP header
   information about a received message via recvmsg().  Note that this
   structure is an extended version of SCTP_SNDRCV (see Section 5.3.2)
   and will only be received if the user has set the socket option
   SCTP_USE_EXT_RCVINFO (see Section 8.1.22) to true in addition to any
   event subscription needed to receive ancillary data.  Note that data
   in the next message is not valid unless the current message is
   completely read, i.e., unless the MSG_EOR is set; in other words, if
   the application has more data to read from the current message, then
   no next-message information will be available.

   SCTP_NXTINFO (described in Section 5.3.6) should be used when
   possible, since SCTP_EXTRCV is considered deprecated.

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

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   The sctp_extrcvinfo structure is defined below:

   struct sctp_extrcvinfo {
     uint16_t sinfo_stream;
     uint16_t sinfo_ssn;
     uint16_t sinfo_flags;
     uint32_t sinfo_ppid;
     uint32_t sinfo_context;
     uint32_t sinfo_pr_value;
     uint32_t sinfo_tsn;
     uint32_t sinfo_cumtsn;
     uint16_t serinfo_next_flags;
     uint16_t serinfo_next_stream;
     uint32_t serinfo_next_aid;
     uint32_t serinfo_next_length;
     uint32_t serinfo_next_ppid;
     sctp_assoc_t sinfo_assoc_id;
   };

   sinfo_*:  Please see Section 5.3.2 for details for these fields.

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

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

      SCTP_NEXT_MSG_ISCOMPLETE:  This bit, when set, indicates that the
         next message is completely in the receive buffer.  The
         next_length field thus contains the entire message size.  If
         this flag is set to 0, then the next_length field only contains
         part of the message size, since the message is still being
         received (it is being partially delivered).

      SCTP_NEXT_MSG_IS_UNORDERED:  This bit, when set, indicates that
         the next message to be received was sent by the peer as
         unordered.  If this bit is not set (i.e., the bit is 0) the
         next message to be read is an ordered message in the stream
         specified.

      SCTP_NEXT_MSG_IS_NOTIFICATION:  This bit, when set, indicates that
         the next message to be received is not a message from the peer,
         but instead is a MSG_NOTIFICATION from the local SCTP stack.

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

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

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

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

5.3.4.  SCTP Send Information Structure (SCTP_SNDINFO)

   This cmsghdr structure specifies SCTP options for sendmsg().

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

   The sctp_sndinfo structure is defined below:

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

   snd_sid:  This value holds the stream number to which the application
      wishes to send this message.  If a sender specifies an invalid
      stream number, an error indication is returned and the call fails.

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

      SCTP_UNORDERED:  This flag requests the unordered delivery of the
         message.  If this flag is clear, the datagram is considered an
         ordered send.

      SCTP_ADDR_OVER:  This flag, for a one-to-many style socket,
         requests that the SCTP stack override the primary destination
         address with the address found with the sendto()/sendmsg call.

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

      SCTP_EOF:  Setting this flag invokes the SCTP graceful shutdown
         procedures on the specified association.  Graceful shutdown
         assures that all data queued by both endpoints is successfully
         transmitted before closing the association.

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

   snd_ppid:  This value in sendmsg() is an unsigned integer that is
      passed to the remote end in each user message.  Please note that
      the SCTP stack performs no byte order modification of this field.
      For example, if the DATA chunk has to contain a given value in
      network byte order, the SCTP user has to perform the htonl()
      computation.

   snd_context:  This value is an opaque 32-bit context datum that is
      used in the sendmsg() function.  This value is passed back to the
      upper layer if an error occurs on the send of a message and is
      retrieved with each undelivered message.

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

   An sctp_sndinfo item always corresponds to the data in msg_iov.

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5.3.5.  SCTP Receive Information Structure (SCTP_RCVINFO)

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

   To enable the delivery of this information, an application must use
   the SCTP_RECVRCVINFO socket option (see Section 8.1.29).

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

   The sctp_rcvinfo structure is defined below:

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

   rcv_sid:  The SCTP stack places the message's stream number in this
      value.

   rcv_ssn:  This value contains the stream sequence number that the
      remote endpoint placed in the DATA chunk.  For fragmented
      messages, this is the same number for all deliveries of the
      message (if more than one recvmsg() is needed to read the
      message).

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

      SCTP_UNORDERED:  This flag is present when the message was sent
         unordered.

   rcv_ppid:  This value is the same information that was passed by the
      upper layer in the peer application.  Please note that the SCTP
      stack performs no byte order modification of this field.  For
      example, if the DATA chunk has to contain a given value in network
      byte order, the SCTP user has to perform the ntohl() computation.

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   rcv_tsn:  This field holds a TSN that was assigned to one of the SCTP
      DATA chunks.

   rcv_cumtsn:  This field will hold the current cumulative TSN as known
      by the underlying SCTP layer.

   rcv_context:  This value is an opaque 32-bit context datum that was
      set by the user with the SCTP_CONTEXT socket option.  This value
      is passed back to the upper layer if an error occurs on the send
      of a message and is retrieved with each undelivered message.

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

   An sctp_rcvinfo item always corresponds to the data in msg_iov.

5.3.6.  SCTP Next Receive Information Structure (SCTP_NXTINFO)

   This cmsghdr structure describes SCTP receive information of the next
   message that will be delivered through recvmsg() if this information
   is already available when delivering the current message.

   To enable the delivery of this information, an application must use
   the SCTP_RECVNXTINFO socket option (see Section 8.1.30).

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

   The sctp_nxtinfo structure is defined below:

   struct sctp_nxtinfo {
     uint16_t nxt_sid;
     uint16_t nxt_flags;
     uint32_t nxt_ppid;
     uint32_t nxt_length;
     sctp_assoc_t nxt_assoc_id;
   };

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   nxt_sid:  The SCTP stack places the next message's stream number in
      this value.

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

      SCTP_UNORDERED:  This flag is present when the next message was
         sent unordered.

      SCTP_COMPLETE:  This flag indicates that the entire message has
         been received and is in the socket buffer.  Note that this has
         special implications with respect to the nxt_length field; see
         the description for nxt_length below.

      SCTP_NOTIFICATION:  This flag is present when the next message is
         not a user message but instead is a notification.

   nxt_ppid:  This value is the same information that was passed by the
      upper layer in the peer application for the next message.  Please
      note that the SCTP stack performs no byte order modification of
      this field.  For example, if the DATA chunk has to contain a given
      value in network byte order, the SCTP user has to perform the
      ntohl() computation.

   nxt_length:  This value is the length of the message currently within
      the socket buffer.  This might NOT be the entire length of the
      message, since a partial delivery may be in progress.  Only if the
      flag SCTP_COMPLETE is set in the nxt_flags field does this field
      represent the size of the entire next message.

   nxt_assoc_id:  The association handle field of the next message,
      nxt_assoc_id, holds the identifier for the association announced
      in the SCTP_COMM_UP notification.  All notifications for a given
      association have the same identifier.  This field is ignored for
      one-to-one style sockets.

5.3.7.  SCTP PR-SCTP Information Structure (SCTP_PRINFO)

   This cmsghdr structure specifies SCTP options for sendmsg().

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

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   The sctp_prinfo structure is defined below:

   struct sctp_prinfo {
     uint16_t pr_policy;
     uint32_t pr_value;
   };

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

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

   An sctp_prinfo item always corresponds to the data in msg_iov.

5.3.8.  SCTP AUTH Information Structure (SCTP_AUTHINFO)

   This cmsghdr structure specifies SCTP options for sendmsg().

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

   The sctp_authinfo structure is defined below:

   struct sctp_authinfo {
     uint16_t auth_keynumber;
   };

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

   An sctp_authinfo item always corresponds to the data in msg_iov.
   Please note that the SCTP implementation must not bundle user
   messages that need to be authenticated using different shared key
   identifiers.

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5.3.9.  SCTP Destination IPv4 Address Structure (SCTP_DSTADDRV4)

   This cmsghdr structure specifies SCTP options for sendmsg().

            +--------------+----------------+----------------+
            | cmsg_level   | cmsg_type      | cmsg_data[]    |
            +--------------+----------------+----------------+
            | IPPROTO_SCTP | SCTP_DSTADDRV4 | struct in_addr |
            +--------------+----------------+----------------+

   This ancillary data can be used to provide more than one destination
   address to sendmsg().  It can be used to implement sctp_sendv() using
   sendmsg().

5.3.10.  SCTP Destination IPv6 Address Structure (SCTP_DSTADDRV6)

   This cmsghdr structure specifies SCTP options for sendmsg().

            +--------------+----------------+-----------------+
            | cmsg_level   | cmsg_type      | cmsg_data[]     |
            +--------------+----------------+-----------------+
            | IPPROTO_SCTP | SCTP_DSTADDRV6 | struct in6_addr |
            +--------------+----------------+-----------------+

   This ancillary data can be used to provide more than one destination
   address to sendmsg().  It can be used to implement sctp_sendv() using
   sendmsg().

6.  SCTP Events and Notifications

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

   When an SCTP application layer does a recvmsg(), the message read is
   normally a data message from a peer endpoint.  If the application
   wishes to have the SCTP stack deliver notifications of non-data
   events, it sets the appropriate socket option for the notifications
   it wants.  See Section 6.2 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 that provide general
   information.

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

6.1.  SCTP Notification Structure

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

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

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

      SCTP_ASSOC_CHANGE:  This tag indicates that an association has
         either been opened or closed.  Refer to Section 6.1.1 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 an endpoint via a specific transport address).  Please see
         Section 6.1.2 for data structure details.

      SCTP_REMOTE_ERROR:  The attached error message is an Operation
         Error message received from the remote peer.  It includes the
         complete TLV sent by the remote endpoint.  See Section 6.1.3
         for the detailed format.

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      SCTP_SEND_FAILED_EVENT:  The attached datagram could not be sent
         to the remote endpoint.  This structure includes the original
         SCTP_SNDINFO that was used in sending this message; i.e., this
         structure uses the sctp_sndinfo per Section 6.1.11.

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

      SCTP_ADAPTATION_INDICATION:  This notification holds the peer's
         indicated adaptation layer.  Please see Section 6.1.6.

      SCTP_PARTIAL_DELIVERY_EVENT:  This notification is used to tell a
         receiver that the partial delivery has been aborted.  This may
         indicate that the association is about to be aborted.  Please
         see Section 6.1.7.

      SCTP_AUTHENTICATION_EVENT:  This notification is used to tell a
         receiver that either an error occurred on authentication, or a
         new key was made active.  See Section 6.1.8.

      SCTP_SENDER_DRY_EVENT:  This notification is used to inform the
         application that the sender has no more user data queued for
         transmission or retransmission.  See Section 6.1.9.

   sn_flags:  These are notification-specific flags.

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

6.1.1.  SCTP_ASSOC_CHANGE

   Communication notifications inform the application that an SCTP
   association has either begun or ended.  The identifier for a new
   association is provided by this notification.  The notification
   information has the following format:

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

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   sac_type:  This field should be set to SCTP_ASSOC_CHANGE.

   sac_flags:  This field is currently unused.

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

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

      SCTP_COMM_UP:  A new association is now ready, and data may be
         exchanged with this peer.  When an association has been
         established successfully, this notification should be the
         first one.

      SCTP_COMM_LOST:  The association has failed.  The association is
         now in the closed state.  If SEND_FAILED notifications are
         turned on, an SCTP_COMM_LOST is accompanied by a series of
         SCTP_SEND_FAILED_EVENT events, one for each outstanding
         message.

      SCTP_RESTART:  SCTP has detected that the peer has restarted.

      SCTP_SHUTDOWN_COMP:  The association has gracefully closed.

      SCTP_CANT_STR_ASSOC:  The association setup failed.  If
         non-blocking mode is set and data was sent (on a one-to-many
         style socket), an SCTP_CANT_STR_ASSOC is accompanied by a
         series of SCTP_SEND_FAILED_EVENT events, one for each
         outstanding message.

   sac_error:  If the state was reached due to an error condition (e.g.,
      SCTP_COMM_LOST), any relevant error information is available in
      this field.  This corresponds to the protocol error codes defined
      in [RFC4960].

   sac_outbound_streams and sac_inbound_streams:  The maximum number of
      streams allowed in each direction is available in
      sac_outbound_streams and sac_inbound streams.

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

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   sac_info:  If sac_state is SCTP_COMM_LOST and an ABORT chunk was
      received for this association, sac_info[] contains the complete
      ABORT chunk as defined in Section 3.3.7 of the SCTP specification
      [RFC4960].  If sac_state is SCTP_COMM_UP or SCTP_RESTART, sac_info
      may contain an array of uint8_t describing the features that the
      current association supports.  Features may include

      SCTP_ASSOC_SUPPORTS_PR:  Both endpoints support the protocol
         extension described in [RFC3758].

      SCTP_ASSOC_SUPPORTS_AUTH:  Both endpoints support the protocol
         extension described in [RFC4895].

      SCTP_ASSOC_SUPPORTS_ASCONF:  Both endpoints support the protocol
         extension described in [RFC5061].

      SCTP_ASSOC_SUPPORTS_MULTIBUF:  For a one-to-many style socket, the
         local endpoints use separate send and/or receive buffers for
         each SCTP association.

6.1.2.  SCTP_PEER_ADDR_CHANGE

   When a destination address of a multi-homed peer encounters a state
   change, a peer address change event is sent.  The notification has
   the following format:

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

   spc_type:  This field should be set to SCTP_PEER_ADDR_CHANGE.

   spc_flags:  This field is currently unused.

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

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

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

      SCTP_ADDR_AVAILABLE:  This address is now reachable.  This
         notification is provided whenever an address becomes reachable.

      SCTP_ADDR_UNREACHABLE:  The address specified can no longer be
         reached.  Any data sent to this address is rerouted to an
         alternate until this address becomes reachable.  This
         notification is provided whenever an address becomes
         unreachable.

      SCTP_ADDR_REMOVED:  The address is no longer part of the
         association.

      SCTP_ADDR_ADDED:  The address is now part of the association.

      SCTP_ADDR_MADE_PRIM:  This address has now been made the primary
         destination address.  This notification is provided whenever an
         address is made primary.

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

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

6.1.3.  SCTP_REMOTE_ERROR

   A remote peer may send an Operation Error message to its peer.  This
   message indicates a variety of error conditions on an association.
   The entire ERROR chunk as it appears on the wire is included in an
   SCTP_REMOTE_ERROR event.  Please refer to the SCTP specification
   [RFC4960] and any extensions for a list of possible error formats.
   An SCTP error notification has the following format:

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

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   sre_type:  This field should be set to SCTP_REMOTE_ERROR.

   sre_flags:  This field is currently unused.

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

   sre_error:  This value represents one of the Operation Error causes
      defined in the SCTP specification [RFC4960], in network byte
      order.

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

   sre_data:  This contains the ERROR chunk as defined in Section 3.3.10
      of the SCTP specification [RFC4960].

6.1.4.  SCTP_SEND_FAILED - DEPRECATED

   Please note that this notification is deprecated.  Use
   SCTP_SEND_FAILED_EVENT instead.

   If SCTP cannot deliver a message, it can return back the message as a
   notification if the SCTP_SEND_FAILED event is enabled.  The
   notification has the following format:

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

   ssf_type:  This field should be set to SCTP_SEND_FAILED.

   ssf_flags:  The flag value will take one of the following values:

      SCTP_DATA_UNSENT:  This value indicates that the data was never
         put on the wire.

      SCTP_DATA_SENT:  This value indicates that the data was put on the
         wire.  Note that this does not necessarily mean that the data
         was (or was not) successfully delivered.

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

   ssf_error:  This value represents the reason why the send failed, and
      if set, will be an SCTP protocol error code as defined in
      Section 3.3.10 of [RFC4960].

   ssf_info:  This field includes the ancillary data (struct
      sctp_sndrcvinfo) used to send the undelivered message.  Regardless
      of whether ancillary data is used or not, the ssf_info.sinfo_flags
      field indicates whether the complete message or only part of the
      message is returned in ssf_data.  If only part of the message is
      returned, it means that the part that is not present has been sent
      successfully to the peer.

      If the complete message cannot be sent, the SCTP_DATA_NOT_FRAG
      flag is set in ssf_info.sinfo_flags.  If the first part of the
      message is sent successfully, SCTP_DATA_LAST_FRAG is set.  This
      means that the tail end of the message is returned in ssf_data.

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

   ssf_data:  The undelivered message or part of the undelivered message
      will be present in the ssf_data field.  Note that the
      ssf_info.sinfo_flags field as noted above should be used to
      determine whether a complete message or just a piece of the
      message is present.  Note that only user data is present in this
      field; any chunk headers or SCTP common headers must be removed by
      the SCTP stack.

6.1.5.  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:  This field should be set to SCTP_SHUTDOWN_EVENT.

   sse_flags:  This field is currently unused.

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

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

6.1.6.  SCTP_ADAPTATION_INDICATION

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

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

   sai_type:  This field should be set to SCTP_ADAPTATION_INDICATION.

   sai_flags:  This field is currently unused.

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

   sai_adaptation_ind:  This field holds the bit array sent by the peer
      in the Adaptation Layer Indication parameter.

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

6.1.7.  SCTP_PARTIAL_DELIVERY_EVENT

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

   struct sctp_pdapi_event {
     uint16_t pdapi_type;
     uint16_t pdapi_flags;
     uint32_t pdapi_length;

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     uint32_t pdapi_indication;
     uint32_t pdapi_stream;
     uint32_t pdapi_seq;
     sctp_assoc_t pdapi_assoc_id;
   };

   pdapi_type:  This field should be set to SCTP_PARTIAL_DELIVERY_EVENT.

   pdapi_flags:  This field is currently unused.

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

   pdapi_indication:  This field holds the indication being sent to the
      application.  Currently, there is only one defined value:

      SCTP_PARTIAL_DELIVERY_ABORTED:  This indicates that the partial
         delivery of a user message has been aborted.  This happens, for
         example, if an association is aborted while a partial delivery
         is going on or the user message gets abandoned using PR-SCTP
         while the partial delivery of this message is going on.

   pdapi_stream:  This field holds the stream on which the partial
      delivery event happened.

   pdapi_seq:  This field holds the stream sequence number that was
      being partially delivered.

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

6.1.8.  SCTP_AUTHENTICATION_EVENT

   [RFC4895] defines an extension to authenticate SCTP messages.  The
   following notification is used to report different events relating to
   the use of this extension.

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

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   auth_type:  This field should be set to SCTP_AUTHENTICATION_EVENT.

   auth_flags:  This field is currently unused.

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

   auth_keynumber:  This field holds the key number for the affected key
      indicated in the event (depends on auth_indication).

   auth_indication:  This field holds the error or indication being
      reported.  The following values are currently defined:

      SCTP_AUTH_NEW_KEY:  This report indicates that a new key has been
         made active (used for the first time by the peer) and is now
         the active key.  The auth_keynumber field holds the user-
         specified key number.

      SCTP_AUTH_NO_AUTH:  This report indicates that the peer does not
         support SCTP authentication as defined in [RFC4895].

      SCTP_AUTH_FREE_KEY:  This report indicates that the SCTP
         implementation will no longer use the key identifier specified
         in auth_keynumber.

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

6.1.9.  SCTP_SENDER_DRY_EVENT

   When the SCTP stack has no more user data to send or retransmit, this
   notification is given to the user.  Also, at the time when a user app
   subscribes to this event, if there is no data to be sent or
   retransmit, the stack will immediately send up this notification.

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

   sender_dry_type:  This field should be set to SCTP_SENDER_DRY_EVENT.

   sender_dry_flags:  This field is currently unused.

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

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

6.1.10.  SCTP_NOTIFICATIONS_STOPPED_EVENT

   SCTP notifications, when subscribed to, are reliable.  They are
   always delivered as long as there is space in the socket receive
   buffer.  However, if an implementation experiences a notification
   storm, it may run out of socket buffer space.  When this occurs, it
   may wish to disable notifications.  If the implementation chooses to
   do this, it will append a final notification
   SCTP_NOTIFICATIONS_STOPPED_EVENT.  This notification is a union
   sctp_notification, where only the sctp_tlv structure (see the union
   above) is used.  It only contains this type in the sn_type field, the
   sn_length field set to the size of an sctp_tlv structure, and the
   sn_flags set to 0.  If an application receives this notification, it
   will need to re-subscribe to any notifications of interest to it,
   except for the sctp_data_io_event (note that SCTP_EVENTS is
   deprecated).

   An endpoint is automatically subscribed to this event as soon as it
   is subscribed to any event other than data io events.

6.1.11.  SCTP_SEND_FAILED_EVENT

   If SCTP cannot deliver a message, it can return back the message as a
   notification if the SCTP_SEND_FAILED_EVENT event is enabled.  The
   notification has the following format:

   struct sctp_send_failed_event {
     uint16_t ssfe_type;
     uint16_t ssfe_flags;
     uint32_t ssfe_length;
     uint32_t ssfe_error;
     struct sctp_sndinfo ssfe_info;
     sctp_assoc_t ssfe_assoc_id;
     uint8_t  ssfe_data[];
   };

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   ssfe_type:  This field should be set to SCTP_SEND_FAILED_EVENT.

   ssfe_flags:  The flag value will take one of the following values:

      SCTP_DATA_UNSENT:  This value indicates that the data was never
         put on the wire.

      SCTP_DATA_SENT:  This value 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.

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

   ssfe_error:  This value represents the reason why the send failed,
      and if set, will be an SCTP protocol error code as defined in
      Section 3.3.10 of [RFC4960].

   ssfe_info:  This field includes the ancillary data (struct
      sctp_sndinfo) used to send the undelivered message.  Regardless of
      whether ancillary data is used or not, the ssfe_info.sinfo_flags
      field indicates whether the complete message or only part of the
      message is returned in ssf_data.  If only part of the message is
      returned, it means that the part that is not present has been sent
      successfully to the peer.

      If the complete message cannot be sent, the SCTP_DATA_NOT_FRAG
      flag is set in ssfe_info.sinfo_flags.  If the first part of the
      message is sent successfully, SCTP_DATA_LAST_FRAG is set.  This
      means that the tail end of the message is returned in ssf_data.

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

   ssfe_data:  The undelivered message or part of the undelivered
      message will be present in the ssf_data field.  Note that the
      ssf_info.sinfo_flags field as noted above should be used to
      determine whether a complete message or just a piece of the
      message is present.  Note that only user data is present in this
      field; any chunk headers or SCTP common headers must be removed by
      the SCTP stack.

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6.2.  Notification Interest Options

6.2.1.  SCTP_EVENTS Option - DEPRECATED

   Please note that this option is deprecated.  Use the SCTP_EVENT
   option described in Section 6.2.2 instead.

   To receive SCTP event notifications, an application registers its
   interest by setting the SCTP_EVENTS socket option.  The application
   then uses recvmsg() to retrieve notifications.  A notification is
   stored in the data part (msg_iov) of the msghdr structure.  The
   socket option uses the following structure:

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

   sctp_data_io_event:  Setting this flag to 1 will cause the reception
      of SCTP_SNDRCV information on a per-message basis.  The
      application will need to use the recvmsg() interface so that it
      can receive the event information contained in the msg_control
      field.  Setting the flag to 0 will disable the reception of the
      message control information.  Note that this flag is not really a
      notification and is stored in the ancillary data (msg_control),
      not in the data part (msg_iov).

   sctp_association_event:  Setting this flag to 1 will enable the
      reception of association event notifications.  Setting the flag to
      0 will disable association event notifications.

   sctp_address_event:  Setting this flag to 1 will enable the reception
      of address event notifications.  Setting the flag to 0 will
      disable address event notifications.

   sctp_send_failure_event:  Setting this flag to 1 will enable the
      reception of send failure event notifications.  Setting the flag
      to 0 will disable send failure event notifications.

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   sctp_peer_error_event:  Setting this flag to 1 will enable the
      reception of peer error event notifications.  Setting the flag to
      0 will disable peer error event notifications.

   sctp_shutdown_event:  Setting this flag to 1 will enable the
      reception of shutdown event notifications.  Setting the flag to 0
      will disable shutdown event notifications.

   sctp_partial_delivery_event:  Setting this flag to 1 will enable the
      reception of partial delivery event notifications.  Setting the
      flag to 0 will disable partial delivery event notifications.

   sctp_adaptation_layer_event:  Setting this flag to 1 will enable the
      reception of adaptation layer event notifications.  Setting the
      flag to 0 will disable adaptation layer event notifications.

   sctp_authentication_event:  Setting this flag to 1 will enable the
      reception of authentication layer event notifications.  Setting
      the flag to 0 will disable authentication layer event
      notifications.

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

   An example where an application would like to receive data_io_events
   and association_events but no others would be as follows:

   {
     struct sctp_event_subscribe events;

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

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

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

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

   By default, both the one-to-one style and the one-to-many style
   socket do not subscribe to any notification.

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6.2.2.  SCTP_EVENT Option

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

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

   se_assoc_id:  The se_assoc_id field is ignored for one-to-one style
      sockets.  For one-to-many style sockets, this field can be a
      particular association identifier or SCTP_{FUTURE|CURRENT|
      ALL}_ASSOC.

   se_type:  The se_type field can be filled with any value that would
      show up in the respective sn_type field (in the sctp_tlv structure
      of the notification).

   se_on:  The se_on field is set to 1 to turn on an event and set to 0
      to turn off an event.

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

   {
     struct sctp_event event;

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

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

   By default, both the one-to-one style and the one-to-many style
   socket do not subscribe to any notification.


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