gen_sctp(3erl)

hiii bestie 💖 welcome to the gen_sctp(3erl) manual page — the Erlang Module Definition one — where we make SCTP sockets feel like main character energy.

🌸 NAME 🌸

gen_sctp — functions for communicating with sockets using the SCTP protocol. That's the whole vibe of this section, and honestly? Iconic. ✨

🦋 DESCRIPTION 🦋

This module gives you functions for talking to sockets with the SCTP protocol. The implementation assumes your OS kernel supports SCTP (RFC 2960) through the user-level Sockets API Extensions. No cap.

During development, this implementation was tested on:

• Linux Fedora Core 5.0 (kernel 2.6.15-2054 or later is needed)
• Solaris 10, 11

During OTP adaptation it was tested on:

• SUSE Linux Enterprise Server 10 (x86_64) kernel 2.6.16.27-0.6-smp, with lksctp-tools-1.0.6
• Briefly on Solaris 10
• SUSE Linux Enterprise Server 10 Service Pack 1 (x86_64) kernel 2.6.16.54-0.2.3-smp with lksctp-tools-1.0.7
• FreeBSD 8.2

This module was written for one-to-many style sockets (type seqpacket). With the addition of peeloff/2, one-to-one style sockets (type stream) were introduced. We love an upgrade era. 💅

Record definitions for this module can be found using:

-include_lib("kernel/include/inet_sctp.hrl").

These record definitions use the "new" spelling 'adaptation', not the deprecated 'adaption', regardless of which spelling the underlying C API uses. Spelling matters, bestie. 🎀

💫 DATA TYPES 💫

🦋 Exported data types 🦋

🌟 assoc_id()

An opaque term returned in, for example, #sctp_paddr_change{}, which identifies an association for an SCTP socket. The term is opaque except for the special value 0, which has a meaning such as "the whole endpoint" or "all future associations". Lowkey mysterious, highkey useful. 🔮

🌟 option()

option() = elementary_option() | record_option()

One of the SCTP Socket Options used to set an option. 💖

🌟 option_name()

option_name() =
    elementary_option_name() | record_option() | ro_option()

An option name or one of the SCTP Socket Options used to get an option. Slay. ✨

🌟 option_value()

option_value() =
    elementary_option() | record_option() | ro_option()

One of the SCTP Socket Options as returned when getting an option. 🫶

🌟 sctp_socket()

Socket identifier returned from open/*. Your little socket handle bestie. 💌

💫 DATA TYPES 💫

🔮 Internal data types 🔮

🔥 elementary_option()

elementary_option() =
    {active, true | false | once | -32768..32767} |
    {buffer, integer() >= 0} |
    {debug, boolean()} |
    {dontroute, boolean()} |
    {high_msgq_watermark, integer() >= 1} |
    {linger, {boolean(), integer() >= 0}} |
    {low_msgq_watermark, integer() >= 1} |
    {mode, list | binary} |
    list | binary |
    {priority, integer() >= 0} |
    {recbuf, integer() >= 0} |
    {reuseaddr, boolean()} |
    {ipv6_v6only, boolean()} |
    {sndbuf, integer() >= 0} |
    {sctp_autoclose, integer() >= 0} |
    {sctp_disable_fragments, boolean()} |
    {sctp_i_want_mapped_v4_addr, boolean()} |
    {sctp_maxseg, integer() >= 0} |
    {sctp_nodelay, boolean()} |
    {tos, integer() >= 0} |
    {tclass, integer() >= 0} |
    {ttl, integer() >= 0} |
    {recvtos, boolean()} |
    {recvtclass, boolean()} |
    {recvttl, boolean()}

🔥 elementary_option_name()

elementary_option_name() =
    active | buffer | debug | dontroute | high_msgq_watermark |
    linger | low_msgq_watermark | mode | priority | recbuf |
    reuseaddr | ipv6_v6only | sctp_autoclose |
    sctp_disable_fragments | sctp_i_want_mapped_v4_addr |
    sctp_maxseg | sctp_nodelay | sndbuf | tos | tclass | ttl |
    recvtos | recvtclass | recvttl

🔥 record_option()

record_option() =
    {sctp_adaptation_layer, #sctp_setadaptation{}} |
    {sctp_associnfo, #sctp_assocparams{}} |
    {sctp_default_send_param, #sctp_sndrcvinfo{}} |
    {sctp_delayed_ack_time, #sctp_assoc_value{}} |
    {sctp_events, #sctp_event_subscribe{}} |
    {sctp_initmsg, #sctp_initmsg{}} |
    {sctp_peer_addr_params, #sctp_paddrparams{}} |
    {sctp_primary_addr, #sctp_prim{}} |
    {sctp_rtoinfo, #sctp_rtoinfo{}} |
    {sctp_set_peer_primary_addr, #sctp_setpeerprim{}}

🔥 ro_option()

ro_option() =
    {sctp_get_peer_addr_info, #sctp_paddrinfo{}} |
    {sctp_status, #sctp_status{}}

👑 EXPORTS 👑

💖 abort(Socket, Assoc) -> ok | {error, inet:posix()}

abort(Socket, Assoc) -> ok | {error, inet:posix()}

Types:

Socket = sctp_socket()
Assoc = #sctp_assoc_change{}

Abnormally terminates the association specified by Assoc, without flushing unsent data. The socket itself remains open. Other associations opened on this socket are still valid, and the socket can be used in new associations. So yeah, it's giving "this association? not invited anymore" but the socket stays booked and busy. 💅

💖 close(Socket) -> ok | {error, inet:posix()}

close(Socket) -> ok | {error, inet:posix()}

Types:

Socket = sctp_socket()

Closes the socket and all associations on it. The unsent data is flushed as in eof/2. The close/1 call is blocking or otherwise depending on the value of the linger socket option. If close does not linger or the linger time-out expires, the call returns and the data is flushed in the background. Basically: sometimes it waits, sometimes it ghosts the flush into the background. 😭

🌈 connect(Socket, SockAddr, Opts) -> {ok, #sctp_assoc_change{state = comm_up}} | {error, #sctp_assoc_change{state = cant_assoc}} | {error, inet:posix()}

connect(Socket, SockAddr, Opts) ->
            {ok, #sctp_assoc_change{state = comm_up}} |
            {error, #sctp_assoc_change{state = cant_assoc}} |
            {error, inet:posix()}

Types:

Socket = sctp_socket()
SockAddr = socket:sockaddr_in() | socket:sockaddr_in6()
Opts = [Opt :: option()]

Same as connect(Socket, SockAddr, Opts, infinity). ✨

🌈 connect(Socket, SockAddr, Opts, Timeout) -> {ok, #sctp_assoc_change{state = comm_up}} | {error, #sctp_assoc_change{state = cant_assoc}} | {error, inet:posix()}

connect(Socket, SockAddr, Opts, Timeout) ->
            {ok, #sctp_assoc_change{state = comm_up}} |
            {error, #sctp_assoc_change{state = cant_assoc}} |
            {error, inet:posix()}

Types:

Socket = sctp_socket()
SockAddr = socket:sockaddr_in() | socket:sockaddr_in6()
Opts = [Opt :: option()]
Timeout = timeout()

This is conceptually the same as connect/5, only with the difference that we use a socket address, socket:sockaddr_in() or socket:sockaddr_in6(), instead of an address (inet:ip_address() or inet:hostname()) and port-number. Fancy address object moment. 🎀

🌈 connect(Socket, Addr, Port, Opts) -> {ok, #sctp_assoc_change{state = comm_up}} | {error, #sctp_assoc_change{state = cant_assoc}} | {error, inet:posix()}

connect(Socket, Addr, Port, Opts) ->
            {ok, #sctp_assoc_change{state = comm_up}} |
            {error, #sctp_assoc_change{state = cant_assoc}} |
            {error, inet:posix()}

Types:

Socket = sctp_socket()
Addr = inet:ip_address() | inet:hostname()
Port = inet:port_number()
Opts = [Opt :: option()]

Same as connect(Socket, Addr, Port, Opts, infinity). 💫

🌈 connect(Socket, Addr, Port, Opts, Timeout) -> {ok, #sctp_assoc_change{state = comm_up}} | {error, #sctp_assoc_change{state = cant_assoc}} | {error, inet:posix()}

connect(Socket, Addr, Port, Opts, Timeout) ->
            {ok, #sctp_assoc_change{state = comm_up}} |
            {error, #sctp_assoc_change{state = cant_assoc}} |
            {error, inet:posix()}

Types:

Socket = sctp_socket()
Addr = inet:ip_address() | inet:hostname()
Port = inet:port_number()
Opts = [Opt :: option()]
Timeout = timeout()

Establishes a new association for socket Socket, with the peer (SCTP server socket) specified by Addr and Port. Timeout is expressed in milliseconds. A socket can be associated with multiple peers. So yes, your socket can have multiple connections and we support that. 🫶

⚠️ Warning: Using a value of Timeout less than the maximum time taken by the OS to establish an association (around 4.5 minutes if the default values from RFC 4960 are used), can result in inconsistent or incorrect return values. This is especially relevant for associations sharing the same Socket (that is, source address and port), as the controlling process blocks until connect/* returns. connect_init/* provides an alternative without this limitation. Bestie, do not set that timeout too low and then act surprised. 😭

The result of connect/* is an #sctp_assoc_change{} event that contains, in particular, the new Association ID:

#sctp_assoc_change{
      state             = atom(),
      error             = integer(),
      outbound_streams  = integer(),
      inbound_streams   = integer(),
      assoc_id          = assoc_id()
}

The number of outbound and inbound streams can be set by giving an sctp_initmsg option to connect as in:

connect(Socket, Ip, Port>,
      [{sctp_initmsg,#sctp_initmsg{num_ostreams=OutStreams,
                                   max_instreams=MaxInStreams}}])

All options Opt are set on the socket before the association is attempted. If an option record has undefined field values, the options record is first read from the socket for those values. In effect, Opt option records only define field values to change before connecting. So it's like a little pre-connect makeover. 💅

The returned outbound_streams and inbound_streams are the stream numbers on the socket. These can be different from the requested values (OutStreams and MaxInStreams, respectively) if the peer requires lower values. Negotiation, but make it network. ✨

state can have the following values:

• comm_up: — Association is successfully established. This indicates a successful completion of connect.
• cant_assoc: — The association cannot be established (connect/* failure).

Other states do not normally occur in the output from connect/*. Rather, they can occur in #sctp_assoc_change{} events received instead of data in recv/* calls. All of them indicate losing the association because of various error conditions, and are listed here for the sake of completeness:

• comm_lost:
• restart:
• shutdown_comp:

Field error can provide more detailed diagnostics. The error field value can be converted into a string using error_string/1. Diagnostics queen. 🔍

💜 connect_init(Socket, SockAddr, Opts) -> ok | {error, inet:posix()}

connect_init(Socket, SockAddr, Opts) -> ok | {error, inet:posix()}

Types:

Socket = sctp_socket()
SockAddr = socket:sockaddr_in() | socket:sockaddr_in6()
Opts = [option()]

Same as connect_init(Socket, SockAddr, Opts, infinity). 🌸

💜 connect_init(Socket, SockAddr, Opts, Timeout) -> ok | {error, inet:posix()}

connect_init(Socket, SockAddr, Opts, Timeout) ->
                 ok | {error, inet:posix()}

Types:

Socket = sctp_socket()
SockAddr = socket:sockaddr_in() | socket:sockaddr_in6()
Opts = [option()]
Timeout = timeout()

This is conceptually the same as connect_init/5, only with the difference that we use a socket address, socket:sockaddr_in() or socket:sockaddr_in6(), instead of an address (inet:ip_address() or inet:hostname()) and port-number. Same energy, different address wrapper. 🦋

💜 connect_init(Socket, Addr, Port, Opts) -> ok | {error, inet:posix()}

connect_init(Socket, Addr, Port, Opts) ->
                 ok | {error, inet:posix()}

Types:

Socket = sctp_socket()
Addr = inet:ip_address() | inet:hostname()
Port = inet:port_number()
Opts = [option()]

Same as connect_init(Socket, Addr, Port, Opts, infinity). ⭐

connect_init(Socket, Addr, Port, Opts, Timeout)

connect_init(Socket, Addr, Port, Opts, Timeout) ->
                ok | {error, inet:posix()}

Types:

   Socket = sctp_socket()
   Addr = inet:ip_address() | inet:hostname()
   Port = inet:port_number()
   Opts = [option()]
   Timeout = timeout()

okay so this one initiates a new association for socket Socket with the peer (that's the SCTP server socket) you specify with Addr and Port. slay. The major difference between this API and connect/* is that the return value is literally the return value of the underlying OS connect(2) system call. If you get ok, the actual result of the association establishment comes to the calling process as an #sctp_assoc_change{} event. So the calling process better be ready to receive that, or poll for it using recv/*, depending on what the active option is set to. no cap. The parameters are the same as described in connect/*, except for the Timeout value. Also, the timer associated with Timeout only supervises the IP resolution of Addr. periodt.

---

connectx_init(Socket, SockAddrs, Opts)

connectx_init(Socket, SockAddrs, Opts) ->
                 {ok, assoc_id()} | {error, inet:posix()}

Types:

   Socket = sctp_socket()
   SockAddrs =
       [{inet:ip_address(), inet:port_number()} |
        inet:family_address() |
        socket:sockaddr_in() |
        socket:sockaddr_in6()]
   Opts = [option()]

Similar to connectx_init/5 except it uses socket addresses and has no Timeout. Since the addresses don't need lookup and the connect is non-blocking, this call returns immediately. iconic. The value of each socket address port must be the same or zero. At least one socket address must have a non-zero port. fr fr.

---

connectx_init(Socket, Addrs, Port, Opts)

connectx_init(Socket, Addrs, Port, Opts) ->
                 {ok, assoc_id()} | {error, inet:posix()}

Types:

   Socket = sctp_socket()
   Addrs = [inet:ip_address() | inet:hostname()]
   Port = inet:port_number() | atom()
   Opts = [option()]

Same as connectx_init(Socket, Addrs, Port, Opts, infinity). literally just calls that with infinity. 💅

---

connectx_init(Socket, Addrs, Port, Opts, Timeout)

connectx_init(Socket, Addrs, Port, Opts, Timeout) ->
                 {ok, assoc_id()} | {error, inet:posix()}

Types:

   Socket = sctp_socket()
   Addrs = [inet:ip_address() | inet:hostname()]
   Port = inet:port_number() | atom()
   Opts = [option()]
   Timeout = timeout()

Initiates a new association for socket Socket with the peer (SCTP server socket) specified by Addrs and Port. This API is similar to connect_init/* except the underlying OS sctp_connectx(3erl) system call is used. If successful, the association ID is returned, which will be received in a subsequent #sctp_assoc_change{} event. The parameters are as described in connect_init/5.

NOTE: This API allows the OS to use all Addrs when establishing an association, but does not guarantee it will. Therefore, if the connection fails, the user may want to rotate the order of addresses for a subsequent call. iykyk.

---

controlling_process(Socket, Pid)

controlling_process(Socket, Pid) -> ok | {error, Reason}

Types:

   Socket = sctp_socket()
   Pid = pid()
   Reason = closed | not_owner | badarg | inet:posix()

Assigns a new controlling process Pid to Socket. Same implementation as gen_udp:controlling_process/2. no cap. 🫶

---

eof(Socket, Assoc)

eof(Socket, Assoc) -> ok | {error, Reason}

Types:

   Socket = sctp_socket()
   Assoc = #sctp_assoc_change{}
   Reason = term()

Gracefully terminates the association specified by Assoc, with flushing of all unsent data. The socket itself remains open. Other associations opened on this socket are still valid. The socket can be used in new associations. periodt. 💖

---

error_string(ErrorNumber)

error_string(ErrorNumber) -> ok | string() | unknown_error

Types:

   ErrorNumber = integer()

Translates an SCTP error number from, for example, #sctp_remote_error{} or #sctp_send_failed{} into an explanatory string, or one of the atoms ok for no error or undefined for an unrecognized error. ngl that's handy. ✨

---

listen(Socket, IsServer) and listen(Socket, Backlog)

listen(Socket, IsServer) -> ok | {error, Reason}

listen(Socket, Backlog) -> ok | {error, Reason}

Types:

   Socket = sctp_socket()
   Backlog = integer()
   Reason = term()

Sets up a socket to listen on the IP address and port number it is bound to. For type seqpacket sockets (the default), IsServer must be true or false. In contrast to TCP, there is no listening queue length in SCTP. If IsServer is true, the socket accepts new associations, that is, it becomes an SCTP server socket. For type stream sockets, Backlog defines the backlog queue length just like in TCP. slay. 💅

---

open() / open(Port) / open(Opts) / open(Port, Opts)

open() -> {ok, Socket} | {error, inet:posix()}

open(Port) -> {ok, Socket} | {error, inet:posix()}

open(Opts) -> {ok, Socket} | {error, inet:posix()}

open(Port, Opts) -> {ok, Socket} | {error, inet:posix()}

Types:

   Opts = [Opt]
   Opt =
       {ifaddr, IP | SockAddr} |
       {ip, IP} |
       {port, Port} |
       inet:address_family() |
       {type, SockType} |
       {netns, file:filename_all()} |
       {bind_to_device, binary()} |
       option()
   IP = inet:ip_address() | any | loopback
   SockAddr = socket:sockaddr_in() | socket:sockaddr_in6()
   Port = inet:port_number()
   SockType = seqpacket | stream
   Socket = sctp_socket()

Creates an SCTP socket and binds it to the local addresses specified by all {ip,IP} (or synonymously {ifaddr,IP}) options. This feature is called SCTP multi-homing. The default IP and Port are any and 0, meaning bind to all local addresses on any free port. It is also possible to use {ifaddr, SockAddr}, in which case it takes precedence over the ip and port options. These options can however be used to update the address and port of ifaddr (if they occur after ifaddr in the options list), although this is not recommended. Other options: inet6 sets up the socket for IPv6. inet sets up the socket for IPv4. This is the default. A default set of socket options is used. In particular, the socket is opened in binary and passive mode, with SockType seqpacket, and with reasonably large kernel and driver buffers. If the socket is in passive mode, data can be received through the recv/1,2 calls. If the socket is in active mode, received data is delivered to the controlling process as messages: {sctp, Socket, FromIP, FromPort, {AncData, Data}}. See recv/1,2 for a description of the message fields.

Note: This message format unfortunately differs slightly from the gen_udp message format with ancillary data, and from the recv/1,2 return tuple format. ngl that's a bit annoying but you'll deal. 😭

---

peeloff(Socket, Assoc)

peeloff(Socket, Assoc) -> {ok, NewSocket} | {error, Reason}

Types:

   Socket = sctp_socket()
   Assoc = #sctp_assoc_change{} | assoc_id()
   NewSocket = sctp_socket()
   Reason = term()

Branches off an existing association Assoc in a socket Socket of type seqpacket (one-to-many style) into a new socket NewSocket of type stream (one-to-one style). The existing association argument Assoc can be either a #sctp_assoc_change{} record as returned from, for example, recv/*, connect/*, or from a listening socket in active mode. It can also be just the field assoc_id integer from such a record. iconic. 👑

---

recv(Socket) / recv(Socket, Timeout)

recv(Socket) ->
        {ok, {FromIP, FromPort, AncData, Data}} | {error, Reason}

recv(Socket, Timeout) ->
        {ok, {FromIP, FromPort, AncData, Data}} | {error, Reason}

Types:

   Socket = sctp_socket()
   Timeout = timeout()
   FromIP = inet:ip_address()
   FromPort = inet:port_number()
   AncData = [#sctp_sndrcvinfo{} | inet:ancillary_data()]
   Data =
       binary() |
       string() |
       #sctp_sndrcvinfo{} |
       #sctp_assoc_change{} |
       #sctp_paddr_change{} |
       #sctp_adaptation_event{}
   Reason =
       inet:posix() |
       #sctp_send_failed{} |
       #sctp_paddr_change{} |
       #sctp_pdapi_event{} |
       #sctp_remote_error{} |
       #sctp_shutdown_event{}

Receives the Data message from any association of the socket. If the receive times out, {error,timeout} is returned. The default time-out is infinity. FromIP and FromPort indicate the address of the sender. AncData is a list of ancillary data items that can be received along with the main Data. This list can be empty, or contain a single #sctp_sndrcvinfo{} record if receiving of such ancillary data is enabled (see option sctp_events). It is enabled by default, as such ancillary data provides an easy way of determining the association and stream over which the message is received. (An alternative way is to get the association ID from FromIP and FromPort using socket option sctp_get_peer_addr_info, but this does still not produce the stream number). AncData may also contain ancillary data from the socket options recvtos, recvtclass or recvttl, if that is supported by the platform for the socket. The Data received can be a binary() or a list() of bytes (integers in the range 0 through 255) depending on the socket mode, or an SCTP event. Possible SCTP events:

• #sctp_sndrcvinfo{}
• #sctp_assoc_change{}
• #sctp_paddr_change{} — here's the record:

#sctp_paddr_change{
      addr      = {ip_address(),port()},
      state     = atom(),
      error     = integer(),
      assoc_id  = assoc_id()
}

Indicates change of the status of the IP address of the peer specified by addr within association assoc_id. Possible values of state (mostly self-explanatory) include:

• addr_unreachable
• addr_available
• addr_removed
• addr_added
• addr_made_prim
• addr_confirmed — bestie, if you get an error here (something like addr_unreachable, for example), the error field is where the extra diagnostics live. In those cases, the #sctp_paddr_change{} event gets automatically converted into an error term that recv hands back to you. And you can turn that error field value into a string with error_string/1. 🫶
• #sctp_send_failed{} — the "my send did NOT eat" event 💀

#sctp_send_failed{
      flags     = true | false,
      error     = integer(),
      info      = #sctp_sndrcvinfo{},
      assoc_id  = assoc_id()
      data      = binary()
}

The sender gets this event when a send operation fails, no cap.

• flags — a Boolean saying whether the data actually made it out over the wire or not. 🛰️
• error — extended diagnostics, hit it with error_string/1.
• info — the original #sctp_sndrcvinfo{} record that was used in the failed send/*.
• data — literally the whole original data chunk that was attempted. The entire thing. 😭

Heads up: in the current implementation of the Erlang/SCTP binding, this event is internally converted into an error term returned by recv/*. So you don't see the record, you see the error.

• #sctp_adaptation_event{} — the adaptation-layer gossip 🦋

#sctp_adaptation_event{
      adaptation_ind = integer(),
      assoc_id       = assoc_id()
}

Delivered when a peer sends an adaptation layer indication parameter (which you set up through the option sctp_adaptation_layer). Important caveat though: with the current implementation of the Erlang/SCTP binding, this event is disabled by default. 🙈

• #sctp_pdapi_event{} — partial delivery said nope 🎀

#sctp_pdapi_event{
      indication = sctp_partial_delivery_aborted,
      assoc_id   = assoc_id()
}

A partial delivery failure. In the current implementation of the Erlang/SCTP binding, this event is internally converted into an error term returned by recv/*.

---

💅 send(Socket, SndRcvInfo, Data) -> ok | {error, Reason}

Types:

Socket = sctp_socket()
SndRcvInfo = #sctp_sndrcvinfo{}
Data = binary() | iolist()
Reason = term()

Sends the Data message with all the sending parameters from a #sctp_sndrcvinfo{} record. So this is the deluxe, full-control version: you get to specify the PPID (passed to the remote end) and the context (passed to the local SCTP layer), which you can use for stuff like error identification. ✨

But ngl, that level of fine-grained user control is rarely required. send/4 is enough for most applications. Don't overthink it, bestie.

🌸 send(Socket, Assoc, Stream, Data) -> ok | {error, Reason}

Types:

Socket = sctp_socket()
Assoc = #sctp_assoc_change{} | assoc_id()
Stream = integer()
Data = binary() | iolist()
Reason = term()

Sends a Data message over an existing association and the specified stream. Simple, iconic, does exactly what it says. 👑

---

🔥 SCTP SOCKET OPTIONS 🔥

Okay so here's the vibe: the set of admissible SCTP socket options is by construction orthogonal to the sets of TCP, UDP, and generic inet options. Meaning — only the options listed right here are allowed for SCTP sockets. No sneaking in random ones. 🚫

You can set options on the socket using open/1,2 or inet:setopts/2, and retrieve them using inet:getopts/2. You can also change options when calling connect/4,5. Easy.

• {mode, list|binary} (or just list or binary) — determines the type of data returned from recv/1,2. 🎀
• {active, true|false|once|N} — the whole activity situation:
  • If false (passive mode, and this is the default), the caller has to do an explicit recv call to go get the available data from the socket. You snooze, you lose.
  • If true|once|N (active modes), received data or events get sent to the owning process. Peep open/0..2 for the message format.
  • If true (full active mode) there is no flow control. 😳

Note that this can cause the message queue to overflow, causing for example the virtual machine to run out of memory and crash. Bestie. Please. Be careful. 💀

• If once, only one message is automatically placed in the message queue, and after that the mode is automatically reset to passive. This gives you flow control plus the ability for the receiver to listen for its incoming SCTP data interleaved with other inter-process messages. Very demure, very mindful. 🧚
• If active is specified as an integer N in the range -32768 to 32767 (inclusive), that number is added to the socket's counting of data messages to be delivered to the controlling process. If the result of the addition is negative, the count is set to 0. Once the count reaches 0 — either through the delivery of messages or by being explicitly set with inet:setopts/2 — the socket mode is automatically reset to passive ({active, false}). And when a socket in this active mode transitions to passive mode, the message {sctp_passive, Socket} is sent to the controlling process to notify it that if it wants more data messages from the socket, it must call inet:setopts/2 to set the socket back into an active mode. So thoughtful of it. 🫶

• {tos, integer()} — sets the Type-Of-Service field on the IP datagrams that are sent, to the specified value. This effectively determines a prioritization policy for the outbound packets. The acceptable values are system-dependent, so iykyk on your box.
• {priority, integer()} — a protocol-independent equivalent of tos above. Setting priority implies setting tos as well. Two for one. ✨
• {dontroute, true|false} — defaults to false. If true, the kernel does not send packets through any gateway, only sends them to directly connected hosts. Strictly local, no detours.
• {reuseaddr, true|false} — defaults to false. If true, the local binding address {IP,Port} of the socket can be reused immediately. No waiting in state CLOSE_WAIT is performed (which can be required for high-throughput servers). Huge for servers that refuse to wait. 🔥
• {sndbuf, integer()} — the size, in bytes, of the OS kernel send buffer for this socket. Sending errors would occur for datagrams larger than val(sndbuf). Setting this option also adjusts the size of the driver buffer (see buffer above).
• {recbuf, integer()} — the size, in bytes, of the OS kernel receive buffer for this socket. Sending errors would occur for datagrams larger than val(recbuf). Setting this option also adjusts the size of the driver buffer (see buffer above).
• {sctp_module, module()} — overrides which callback module is used. Defaults to inet_sctp for IPv4 and inet6_sctp for IPv6. Swappable, iconique.
• {sctp_rtoinfo, #sctp_rtoinfo{}} — the retransmission time-out situation:

#sctp_rtoinfo{
      assoc_id = assoc_id(),
      initial  = integer(),
      max      = integer(),
      min      = integer()
}

Determines retransmission time-out parameters, in milliseconds, for the association(s) specified by assoc_id.

assoc_id = 0 (the default) indicates the whole endpoint. See RFC 2960 and Sockets API Extensions for SCTP for the exact semantics of the field values. 📚

• {sctp_associnfo, #sctp_assocparams{}} — association params, the deep lore:

#sctp_assocparams{
      assoc_id                 = assoc_id(),
      asocmaxrxt               = integer(),
      number_peer_destinations = integer(),
      peer_rwnd                = integer(),
      local_rwnd               = integer(),
      cookie_life              = integer()
}

Determines association parameters for the association(s) specified by assoc_id.

assoc_id = 0 (the default) indicates the whole endpoint. See Sockets API Extensions for SCTP for the discussion of their semantics. Rarely used — like, lowkey nobody touches this. 🙈

• {sctp_initmsg, #sctp_initmsg{}} — the "hi nice to meet you" handshake settings:

#sctp_initmsg{
     num_ostreams   = integer(),
     max_instreams  = integer(),
     max_attempts   = integer(),
     max_init_timeo = integer()
}

Determines the default parameters that this socket tries to negotiate with its peer while establishing an association with it. It's to be set after open/* but before the first connect/*. Also, #sctp_initmsg{} can be used as ancillary data with the first call of send/* to a new peer (when a new association is created). Timing matters, bestie. ⏰

• num_ostreams — number of outbound streams
• max_instreams — maximum number of inbound streams
• max_attempts — maximum retransmissions while establishing an association
• max_init_timeo — time-out, in milliseconds, for establishing an association

• {sctp_autoclose, integer() >= 0} — determines the time, in seconds, after which an idle association is automatically closed. 0 means the association is never automatically closed. Set it and forget it. 🎀
• {sctp_nodelay, true|false} — turns on|off the Nagle algorithm for merging small packets into larger ones. This improves throughput at the expense of latency. Classic tradeoff, choose your fighter. ⚖️
• {sctp_disable_fragments, true|false} — if true, induces an error on an attempt to send a message larger than the current PMTU size (which would require fragmentation/reassembling). Notice that message fragmentation does not affect the logical atomicity of its delivery; this option is provided for performance reasons only. So don't panic about correctness. 💅
• {sctp_i_want_mapped_v4_addr, true|false} — turns on|off automatic mapping of IPv4 addresses into IPv6 ones (if the socket address family is AF_INET6).
• {sctp_maxseg, integer()} — determines the maximum chunk size if message fragmentation is used. If 0, the chunk size is limited by the Path MTU only.
• {sctp_primary_addr, #sctp_prim{}} — picking the main character address 👑

#sctp_prim{
      assoc_id = assoc_id(),
      addr     = {IP, Port}
}
 IP = ip_address()
 Port = port_number()

For the association specified by assoc_id, {IP,Port} must be one of the peer addresses. This option determines that the specified address is treated by the local SCTP stack as the primary address of the peer.

• {sctp_set_peer_primary_addr, #sctp_setpeerprim{}} — telling them who's primary:

#sctp_setpeerprim{
      assoc_id = assoc_id(),
      addr     = {IP, Port}
}
 IP = ip_address()
 Port = port_number()

When set, informs the peer to use {IP, Port} as the primary address of the local endpoint for the association specified by assoc_id. Basically a polite FYI to the other side. 🫶

• {sctp_adaptation_layer, #sctp_setadaptation{}} — the adaptation-layer flex:

#sctp_setadaptation{
      adaptation_ind = integer()
}

When set, requests that the local endpoint uses the value specified by adaptation_ind as the Adaptation Indication parameter for establishing new associations. For details, see RFC 2960 and Sockets API Extensions for SCTP. Go read the lore. 📖

okay bestie, next up in the socket-option lore is the per-address parameter situation 💅

{sctp_peer_addr_params, #sctp_paddrparams{}}

#sctp_paddrparams{
      assoc_id   = assoc_id(),
      address    = {IP, Port},
      hbinterval = integer(),
      pathmaxrxt = integer(),
      pathmtu    = integer(),
      sackdelay  = integer(),
      flags      = list()
}
IP = ip_address()
Port = port_number()

This one determines various per-address parameters for the association specified by assoc_id and the peer address address. And yes, because the SCTP protocol supports multi-homing, more than one address can correspond to a specified association — it's giving "main character with backup besties" energy 🌈

• hbinterval — heartbeat interval, in milliseconds 💓
• pathmaxrxt — maximum number of retransmissions before this address is considered unreachable (and an alternative address is selected)
• pathmtu — fixed Path MTU, if automatic discovery is disabled (see flags below)
• sackdelay — delay, in milliseconds, for SAC messages (if the delay is enabled, see flags below)
• flags — the following flags are available:
  • hb_enable — enables heartbeat
  • hb_disable — disables heartbeat
  • hb_demand — initiates heartbeat immediately
  • pmtud_enable — enables automatic Path MTU discovery
  • pmtud_disable — disables automatic Path MTU discovery
  • sackdelay_enable — enables SAC delay
  • sackdelay_disable — disables SAC delay

---

{sctp_default_send_param, #sctp_sndrcvinfo{}}

#sctp_sndrcvinfo{
      stream     = integer(),
      ssn        = integer(),
      flags      = list(),
      ppid       = integer(),
      context    = integer(),
      timetolive = integer(),
      tsn        = integer(),
      cumtsn     = integer(),
      assoc_id   = assoc_id()
}

okay so #sctp_sndrcvinfo{} is a whole dual-purpose icon: it's used both in this socket option and as ancillary data while sending or receiving SCTP messages. When it's set as an option, it provides default values for subsequent send calls on the association specified by assoc_id ✨

assoc_id = 0 (default) indicates the whole endpoint.

The following fields typically must be specified by the sender:

• sinfo_stream — stream number (0-base) within the association to send the messages through
• sinfo_flags — the following flags are recognised:
  • unordered — the message is to be sent unordered
  • addr_over — the address specified in send overwrites the primary peer address
  • abort — aborts the current association without flushing any unsent data
  • eof — gracefully shuts down the current association, with flushing of unsent data

Other fields are rarely used. For complete information, see RFC 2960 and Sockets API Extensions for SCTP.

---

{sctp_events, #sctp_event_subscribe{}}

#sctp_event_subscribe{
        data_io_event          = true | false,
        association_event      = true | false,
        address_event          = true | false,
        send_failure_event     = true | false,
        peer_error_event       = true | false,
        shutdown_event         = true | false,
        partial_delivery_event = true | false,
        adaptation_layer_event = true | false
}

This option determines which SCTP Events are to be received (through recv/*) along with the data 🦋 The only exception is data_io_event, which enables or disables receiving of #sctp_sndrcvinfo{} ancillary data, not events. By default, all flags except adaptation_layer_event are enabled, although sctp_data_io_event and association_event are used by the driver itself and not exported to the user level.

---

{sctp_delayed_ack_time, #sctp_assoc_value{}}

#sctp_assoc_value{
      assoc_id    = assoc_id(),
      assoc_value = integer()
}

Rarely used. Determines the ACK time (specified by assoc_value, in milliseconds) for the specified association or the whole endpoint if assoc_value = 0 (default).

---

{sctp_status, #sctp_status{}}

#sctp_status{
      assoc_id            = assoc_id(),
      state               = atom(),
      rwnd                = integer(),
      unackdata           = integer(),
      penddata            = integer(),
      instrms             = integer(),
      outstrms            = integer(),
      fragmentation_point = integer(),
      primary             = #sctp_paddrinfo{}
}

Read-only, bestie: this option is read-only. It determines the status of the SCTP association specified by assoc_id 👑

The following are the possible values of state (the state designations are mostly self-explanatory):

• sctp_state_empty — default. Means that no other state is active.
• sctp_state_closed — state name, that's the whole vibe.
• sctp_state_cookie_wait — state name.
• sctp_state_cookie_echoed — state name.
• sctp_state_established — state name.
• sctp_state_shutdown_pending — state name.
• sctp_state_shutdown_sent — state name.
• sctp_state_shutdown_received — state name.
• sctp_state_shutdown_ack_sent — state name.

Semantics of the other fields:

• sstat_rwnd — current receiver window size of the association
• sstat_unackdata — number of unacked data chunks
• sstat_penddata — number of data chunks pending receipt
• sstat_instrms — number of inbound streams
• sstat_outstrms — number of outbound streams
• sstat_fragmentation_point — message size at which SCTP fragmentation occurs
• sstat_primary — information on the current primary peer address (see below for the format of #sctp_paddrinfo{})

---

{sctp_get_peer_addr_info, #sctp_paddrinfo{}}

#sctp_paddrinfo{
      assoc_id  = assoc_id(),
      address   = {IP, Port},
      state     = inactive | active | unconfirmed,
      cwnd      = integer(),
      srtt      = integer(),
      rto       = integer(),
      mtu       = integer()
}
IP = ip_address()
Port = port_number()

Also read-only, bestie: this option is read-only. It determines the parameters specific to the peer address specified by address within the association specified by assoc_id 🔥

Field address fmust be set by the caller; all other fields are filled in on return. If assoc_id = 0 (default), the address is automatically translated into the corresponding association ID. This option is rarely used. For the semantics of all fields, see RFC 2960 and Sockets API Extensions for SCTP.

---

and yeah, there are still more socket options waiting in the queue, so keep scrolling bestie 🫶

✨ SCTP EXAMPLES ✨

okay bestie, real talk — here are some actual Erlang SCTP examples so you can see this API in the wild 💅 first up: an Erlang SCTP server that receives SCTP messages and prints them on the standard output. she's giving listen-and-print realness, no cap.

-module(sctp_server).

-export([server/0,server/1,server/2]).
-include_lib("kernel/include/inet.hrl").
-include_lib("kernel/include/inet_sctp.hrl").

server() ->
    server(any, 2006).

server([Host,Port]) when is_list(Host), is_list(Port) ->
    {ok, #hostent{h_addr_list = [IP|_]}} = inet:gethostbyname(Host),
    io:format("~w -> ~w~n", [Host, IP]),
    server([IP, list_to_integer(Port)]).

server(IP, Port) when is_tuple(IP) orelse IP == any orelse IP == loopback,
                      is_integer(Port) ->
    {ok,S} = gen_sctp:open(Port, [{recbuf,65536}, {ip,IP}]),
    io:format("Listening on ~w:~w. ~w~n", [IP,Port,S]),
    ok     = gen_sctp:listen(S, true),
    server_loop(S).

server_loop(S) ->
    case gen_sctp:recv(S) of
    {error, Error} ->
        io:format("SCTP RECV ERROR: ~p~n", [Error]);
    Data ->
        io:format("Received: ~p~n", [Data])
    end,
    server_loop(S).

that's the whole server vibe: open, listen, recv, print, loop. iconic. 🎀

---

next, an Erlang SCTP client that interacts with the above server 🫶 and bestie, the stream math is the main character here: in this example the client creates an association with the server with 5 outbound streams. so sending "Test 0" over stream 0 succeeds, but sending "Test 5" over stream 5 fails. the client then aborts the association, which results in that the corresponding event is received on the server side. it's literally cause and effect, and we're obsessed 😭🔥

-module(sctp_client).

-export([client/0, client/1, client/2]).
-include_lib("kernel/include/inet.hrl").
-include_lib("kernel/include/inet_sctp.hrl").

client() ->
    client([localhost]).

client([Host]) ->
    client(Host, 2006);

client([Host, Port]) when is_list(Host), is_list(Port) ->
    client(Host,list_to_integer(Port)),
    init:stop().

client(Host, Port) when is_integer(Port) ->
    {ok,S}     = gen_sctp:open(),
    {ok,Assoc} = gen_sctp:connect
        (S, Host, Port, [{sctp_initmsg,#sctp_initmsg{num_ostreams=5}}]),
    io:format("Connection Successful, Assoc=~p~n", [Assoc]),

    io:write(gen_sctp:send(S, Assoc, 0, <<"Test 0">>)),
    io:nl(),
    timer:sleep(10000),
    io:write(gen_sctp:send(S, Assoc, 5, <<"Test 5">>)),
    io:nl(),
    timer:sleep(10000),
    io:write(gen_sctp:abort(S, Assoc)),
    io:nl(),

    timer:sleep(1000),
    gen_sctp:close(S).

---

and finally, a simple Erlang SCTP client that uses the connect_init API 🌸 she's doing two connections and living her best {active, true} life.

-module(ex3).

-export([client/4]).
-include_lib("kernel/include/inet.hrl").
-include_lib("kernel/include/inet_sctp.hrl").

client(Peer1, Port1, Peer2, Port2)
  when is_tuple(Peer1), is_integer(Port1), is_tuple(Peer2), is_integer(Port2) ->
    {ok,S}     = gen_sctp:open(),
    SctpInitMsgOpt = {sctp_initmsg,#sctp_initmsg{num_ostreams=5}},
    ActiveOpt = {active, true},
    Opts = [SctpInitMsgOpt, ActiveOpt],
    ok = gen_sctp:connect(S, Peer1, Port1, Opts),
    ok = gen_sctp:connect(S, Peer2, Port2, Opts),
    io:format("Connections initiated~n", []),
    client_loop(S, Peer1, Port1, undefined, Peer2, Port2, undefined).

client_loop(S, Peer1, Port1, AssocId1, Peer2, Port2, AssocId2) ->
    receive
        {sctp, S, Peer1, Port1, {_Anc, SAC}}
          when is_record(SAC, sctp_assoc_change), AssocId1 == undefined ->
            io:format("Association 1 connect result: ~p. AssocId: ~p~n",
                      [SAC#sctp_assoc_change.state,
                       SAC#sctp_assoc_change.assoc_id]),
            client_loop(S, Peer1, Port1, SAC#sctp_assoc_change.assoc_id,
                        Peer2, Port2, AssocId2);

        {sctp, S, Peer2, Port2, {_Anc, SAC}}
          when is_record(SAC, sctp_assoc_change), AssocId2 == undefined ->
            io:format("Association 2 connect result: ~p. AssocId: ~p~n",
                      [SAC#sctp_assoc_change.state, SAC#sctp_assoc_change.assoc_id]),
            client_loop(S, Peer1, Port1, AssocId1, Peer2, Port2,
                       SAC#sctp_assoc_change.assoc_id);

        {sctp, S, Peer1, Port1, Data} ->
            io:format("Association 1: received ~p~n", [Data]),
            client_loop(S, Peer1, Port1, AssocId1,
                        Peer2, Port2, AssocId2);

        {sctp, S, Peer2, Port2, Data} ->
            io:format("Association 2: received ~p~n", [Data]),
            client_loop(S, Peer1, Port1, AssocId1,
                        Peer2, Port2, AssocId2);

        Other ->
            io:format("Other ~p~n", [Other]),
            client_loop(S, Peer1, Port1, AssocId1,
                        Peer2, Port2, AssocId2)

    after 5000 ->
            ok
    end.

---

🔗 SEE ALSO 🔗

• gen_tcp(3erl)
• gen_udp(3erl)
• inet(3erl)
• RFC 2960 (Stream Control Transmission Protocol)
• Sockets API Extensions for SCTP

---

and that's the whole page, bestie 💖 shoutout to Ericsson AB for kernel 8.5.4.2 — this is the gen_sctp(3erl) manual page, and we are so done (affectionate) 💅✨