= nng_options(5)
//
// Copyright 2018 Staysail Systems, Inc. <info@staysail.tech>
// Copyright 2018 Capitar IT Group BV <info@capitar.com>
//
// This document is supplied under the terms of the MIT License, a
// copy of which should be located in the distribution where this
// file was obtained (LICENSE.txt).  A copy of the license may also be
// found online at https://opensource.org/licenses/MIT.
//

== NAME

nng_options - socket, dialer, listener, and pipe options

== SYNOPSIS

[source, c]
----
#include <nng/nng.h>

#define NNG_OPT_SOCKNAME      "socket-name"
#define NNG_OPT_RAW           "raw"
#define NNG_OPT_PROTO         "protocol"
#define NNG_OPT_PROTONAME     "protocol-name"
#define NNG_OPT_PEER          "peer"
#define NNG_OPT_PEERNAME      "peer-name"
#define NNG_OPT_RECVBUF       "recv-buffer"
#define NNG_OPT_SENDBUF       "send-buffer"
#define NNG_OPT_RECVFD        "recv-fd"
#define NNG_OPT_SENDFD        "send-fd"
#define NNG_OPT_RECVTIMEO     "recv-timeout"
#define NNG_OPT_SENDTIMEO     "send-timeout"
#define NNG_OPT_LOCADDR       "local-address"
#define NNG_OPT_REMADDR       "remote-address"
#define NNG_OPT_URL           "url"
#define NNG_OPT_MAXTTL        "ttl-max"
#define NNG_OPT_RECVMAXSZ     "recv-size-max"
#define NNG_OPT_RECONNMINT    "reconnect-time-min"
#define NNG_OPT_RECONNMAXT    "reconnect-time-max"
#define NNG_OPT_TCP_NODELAY   "tcp-nodelay"
#define NNG_OPT_TCP_KEEPALIVE "tcp-nodelay"
----

== DESCRIPTION

This page documents the various standard options that can be set or
retrieved on objects in the _nng_ library.

Sockets (<<nng_socket.5#,`nng_socket`>> objects) use the functions
<<nng_getopt.3#,`nng_getopt()`>>
and <<nng_setopt.3#,`nng_setopt()`>> to set and retrieve option values.

Dialers (<<nng_dialer.5#,`nng_dialer`>> objects) use the functions
<<nng_dialer_getopt.3#,`nng_dialer_getopt()`>> and
<<nng_dialer_setopt.3#,`nng_dialer_setopt()`>> to set and retrieve option
values.

Listeners (<<nng_listener.5#,`nng_listener`>> objects) use the functions
<<nng_listener_getopt.3#,`nng_listener_getopt()`>>
and <<nng_listener_setopt.3#,`nng_listener_setopt()`>> to set and
retrieve option values.

Pipes (<<nng_pipe.5#,`nng_pipe`>> objects) can only retrieve option values using
the <<nng_pipe_getopt.3#,`nng_pipe_getopt()`>> function.

In addition to the options listed here, transports and protocols will generally
have some of their own options, which will be documented with the transport
or protocol.

=== Options

In the following list of options, the name of the option is supplied,
along with the data type of the underlying value.
Some options are only meaningful or supported in certain contexts; for
example there is no single meaningful address for a socket, since sockets
can have multiple dialers and endpoints associated with them.
An attempt has been made to include details about such restrictions in the
description of the option.

[[NNG_OPT_LOCADDR]]
((`NNG_OPT_LOCADDR`))::
(`<<nng_sockaddr.5#,nng_sockaddr>>`)
This read-only option may be used on listeners, dialers and connected pipes, and
represents the local address used for communication.
Not all transports support this option, and some transports may support it
listeners but not dialers.

[[NNG_OPT_RAW]]
((`NNG_OPT_RAW`))::
(((raw mode)))
(((cooked mode)))
(`bool`)
This read-only option indicates whether the socket is in "`raw`" mode.
If `true`, the socket is in "`raw`" mode, and if `false` the socket is
in "`cooked`" mode.
Raw mode sockets generally do not have any protocol-specific semantics applied
to them; instead the application is expected to perform such semantics itself.
(For example, in "`cooked`" mode a <<nng_rep.7#,_rep_>> socket would
automatically copy message headers from a received message to the corresponding
reply, whereas in "`raw`" mode this is not done.)
See <<nng.7#raw_mode,Raw Mode>> for more details.

[[NNG_OPT_RECONNMINT]]
((`NNG_OPT_RECONNMINT`))::
(((reconnect time, minimum)))
(`<<nng_duration.5#,nng_duration>>`)
This is the minimum amount of time (milliseconds) to wait before attempting
to establish a connection after a previous attempt has failed.
This can be set on a socket, but it can also be overridden on an individual
dialer.
The option is irrelevant for listeners.

[[NNG_OPT_RECONNMAXT]]
((`NNG_OPT_RECONNMAXT`))::
(((`NNG_OPT_RECONNMAXT`)))
(((reconnect time, maximum)))
(`<<nng_duration.5#,nng_duration>>`)
This is the maximum amount of time
(milliseconds) to wait before attempting to establish a connection after
a previous attempt has failed.
If this is non-zero, then the time between successive connection attempts
will start at the value of `NNG_OPT_RECONNMINT`,
and grow exponentially, until it reaches this value.
If this value is zero, then no exponential
backoff between connection attempts is done, and each attempt will wait
the time specified by `NNG_OPT_RECONNMINT`.
This can be set on a socket, but it can also be overridden on an individual
dialer.
The option is irrelevant for listeners.

[[NNG_OPT_RECVBUF]]
((`NNG_OPT_RECVBUF`))::
(((buffer, receive)))
(((receive, buffer)))
(`int`)
This is the depth of the socket's receive buffer as a number of messages.
Messages received by a transport may be buffered until the application
has accepted them for delivery.

[[NNG_OPT_RECVFD]]
((`NNG_OPT_RECVFD`))::
(((poll)))
(((select)))
(((receive, polling)))
(`int`)
This read-only option is used to obtain an integer file descriptor suitable
for use with
http://pubs.opengroup.org/onlinepubs/7908799/xsh/poll.html[`poll()`],
http://pubs.opengroup.org/onlinepubs/7908799/xsh/select.html[`select()`],
(or on Windows systems
https://msdn.microsoft.com/en-us/library/windows/desktop/ms741669(v=vs.85).aspx[`WSAPoll()`])
and similar functions.
This descriptor will be *readable* when a message is available for receiving
on the socket.
When no message is ready for receiving, then this file descriptor will *not*
be readable.

IMPORTANT: Appplications should never attempt to read or write to the
returned file descriptor.
Furthermore, applications should not attempt to use the actual socket (of
type <<nng_socket.5#,`nng_socket`>>) with polling functions,
since it is merely an internal
identifier and will not necessarily referency any operting system object or
handle.

TIP: While this option may help applications integrate into existing polling
loops, it is more efficient, and often easier, to use the asynchronous I/O
objects instead.  See <<nng_aio_alloc.3#,`nng_aio_alloc()`>>.

[[NNG_OPT_RECVMAXSZ]]
((`NNG_OPT_RECVMAXSZ`))::
(((receive, maximum size)))
(`size_t`)
This is the maximum message size that the will be accepted from a remote peer.
If a peer attempts to send a message larger than this, then the message
will be discarded.
If the value of this is zero, then no limit on message sizes is enforced.
This option exists to prevent certain kinds of denial-of-service attacks,
where a malicious agent can claim to want to send an extraordinarily
large message, without sending any data.
This option can be set for the socket, but may be overridden for on a
per-dialer or per-listener basis.

NOTE: Some transports may have further message size restrictions!

[[NNG_OPT_RECVTIMEO]]
((`NNG_OPT_RECVTIMEO`))::
(((receive, timeout)))
(((timeout, receive)))
(`<<nng_duration.5#,nng_duration>>`)
This is the socket receive timeout in milliseconds.
When no message is available for receiving at the socket for this period of
time, receive operations will fail with a return value of `NNG_ETIMEDOUT`.

[[NNG_OPT_REMADDR]]
((`NNG_OPT_REMADDR`))::
(`<<nng_sockaddr.5#,nng_sockaddr>>`)
This read-only option may be used on dialers and connected pipes, and
represents the address of a remote peer.
Not all transports support this option.

[[NNG_OPT_SENDBUF]]
((`NNG_OPT_SENDBUF`))::
(((send, buffer)))
(((buffer, send)))
(`int`)
This is the depth of the socket send buffer as a number of messages.
Messages sent by an application may be buffered by the socket until a
transport is ready to accept them for delivery.
This value must be an integer between 0 and 8192, inclusive.

NOTE: Not all protocols support buffering sent messages;
generally multicast protocols like <<nng_pub.7#,_pub_>> will
simply discard messages when they cannot be delivered immediately.

[[NNG_OPT_SENDFD]]
((`NNG_OPT_SENDFD`))::
(((poll)))
(((select)))
(((send, polling)))
(`int`)
This read-only option is used to obtain an integer file descriptor suitable
for use with
http://pubs.opengroup.org/onlinepubs/7908799/xsh/poll.html[`poll()`],
http://pubs.opengroup.org/onlinepubs/7908799/xsh/select.html[`select()`],
(or on Windows systems
https://msdn.microsoft.com/en-us/library/windows/desktop/ms741669(v=vs.85).aspx[`WSAPoll()`])
and similar functions.
This descriptor will be *readable* when the socket is able to accept a
message for sending without blocking.
When the socket is no longer able to accept such messages without blocking,
the descriptor will *not* be readable.

IMPORTANT: Appplications should never attempt to read or write to the
returned file descriptor.
Furthermore, applications should not attempt to use the actual socket (of
type <<nng_socket.5#,`nng_socket`>>) with polling functions,
since it is merely an internal
identifier and will not necessarily referency any operting system object or
handle.

TIP: While this option may help applications integrate into existing polling
loops, it is more efficient, and often easier, to use the asynchronous I/O
objects instead.  See <<nng_aio_alloc.3#,`nng_aio_alloc`()>>.

[[NNG_OPT_SENDTIMEO]]
((`NNG_OPT_SENDTIMEO`))::
(((send, timeout)))
(((timeout, send)))
(`<<nng_duration.5#,nng_duration>>`)
This is the socket send timeout in milliseconds.
When a message cannot be queued for delivery by the socket for this period of
time (such as if send buffers are full), the operation will fail with a
return value of `NNG_ETIMEDOUT`.

[[NNG_OPT_SOCKNAME]]
((`NNG_OPT_SOCKNAME`))::
(((name, socket)))
(string)
This the socket name.
By default this is a string corresponding to the value of the socket.
The string must fit within 64-bytes, including the terminating
`NUL` byte, but it can be changed for other application uses.

[[NNG_OPT_MAXTTL]]
((`NNG_OPT_MAXTTL`))::
(`int`)
(((time-to-live)))
This is the maximum number of "`hops`" a message may traverse (see
<<nng_device.3#,`nng_device()`>>).
The intention here is to prevent ((forwarding loops)) in device chains.
When this is supported, it can have a value between 1 and 255, inclusive.

NOTE: Not all protocols support this option.
Those that do generally have a default value of 8.

TIP: Each node along a forwarding path may have it's own value for the
maximum time-to-live, and performs its own checks before forwarding a message.
Therefore it is helpful if all nodes in the topology use the same value for
this option.

[[NNG_OPT_URL]]
((`NNG_OPT_URL`))::
(((URI)))
(((URL)))
(string)
This read-only option is used to obtain the URL with which a listener
or dialer was configured.
Accordingly it can only be used with dialers, listeners, and pipes.

NOTE: Some transports will canonify URLs before returning them to the
application.

[[NNG_OPT_PROTO]]
((`NNG_OPT_PROTO`))::
(`int`)
This read-only option is used to obtain the 16-bit number for the socket's protocol.

[[NNG_OPT_PEER]]
((`NNG_OPT_PEER`))::
(`int`)
This read-only option is used to obtain the 16-bit number of the
peer protocol for the socket.


[[NNG_OPT_PROTONAME]]
((`NNG_OPT_PROTONAME`))::
(string)
This read-only option is used to obtain the name of the socket's protocol.

[[NNG_OPT_PEERNAME]]
((`NNG_OPT_PEERNAME`))::
(string)
This read-only option is used to obtain the name of the peer protocol for
the socket.

[[NNG_OPT_TCP_NODELAY]]
((`NNG_OPT_TCP_NODELAY`))::
(`bool``)
This option is used to disable (or enable) the use of Nagle's algorithm
for TCP connections.
When `true` (the default), messages are sent immediately by the underlying
TCP stream without waiting to gather more data.
When `false`, Nagle's algorithm is enabled, and the TCP stream may
wait briefly in attempt to coalesce messages.
Nagle's algorithm is useful on low-bandwidth connections to reduce overhead,
but it comes at a cost to latency.

NOTE: This setting may apply to transports that are built on top of TCP.
See the transport documentation for each transport for details.

[[NNG_OPT_TCP_KEEPALIVE]]
((`NNG_OPT_TCP_KEEPALIVE`))::
(`bool``)
This option is used to enable the sending of keep-alive messages on
the underlying TCP stream.
This option is `false` by default.
When enabled, if no messages are seen for a period of time, then
a zero length TCP message is sent with the ACK flag set in an attempt
to tickle some traffic from the peer.
If none is still seen (after some platform-specific number of retries and
timeouts), then the remote peer is presumed dead, and the connection is closed.

NOTE: This setting may apply to transports that are built on top of TCP.
See the transport documentation for each transport for details.

TIP: This option has two purposes.
First, it can be used to detect dead peers on an otherwise quiescent network.
Second, it can be used to keep connection table entries in NAT and other
middleware from being expiring due to lack of activity.

== SEE ALSO

<<nng_dialer_getopt.3#,nng_dialer_getopt(3)>>,
<<nng_dialer_setopt.3#,nng_dialer_setopt(3)>>,
<<nng_getopt.3#,nng_getopt(3)>>,
<<nng_listener_getopt.3#,nng_listener_getopt(3)>>,
<<nng_listener_setopt.3#,nng_listener_setopt(3)>>,
<<nng_pipe_getopt.3#,nng_pipe_getopt(3)>>,
<<nng_setopt.3#,nng_setopt(3)>>,
<<nng.7#,nng(7)>>
