1= nng_pair(7) 2// 3// Copyright 2020 Staysail Systems, Inc. <info@staysail.tech> 4// Copyright 2018 Capitar IT Group BV <info@capitar.com> 5// 6// This document is supplied under the terms of the MIT License, a 7// copy of which should be located in the distribution where this 8// file was obtained (LICENSE.txt). A copy of the license may also be 9// found online at https://opensource.org/licenses/MIT. 10// 11 12== NAME 13 14nng_pair - pair protocol 15 16== SYNOPSIS 17 18.Version 0 19[source,c] 20---- 21#include <nng/protocol/pair0/pair.h> 22---- 23 24.Version 1 25[source,c] 26---- 27#include <nng/protocol/pair1/pair.h> 28---- 29 30== DESCRIPTION 31 32(((protocol, _pair_))) 33The ((_pair_ protocol)) implements a peer-to-peer pattern, where 34relationships between peers are one-to-one. 35 36=== Socket Operations 37 38The xref:nng_pair_open.3.adoc[`nng_pair_open()`] functions create a _pair_ socket. 39 40Normally, this pattern will block when attempting to send a message if 41no peer is able to receive the message. 42 43NOTE: Even though this mode may appear to be reliable, because back-pressure 44prevents discarding messages most of the time, there are topologies involving 45_devices_ (see xref:nng_device.3.adoc[`nng_device()`]) or raw mode sockets 46(see xref:nng_options.5.adoc#NNG_OPT_RAW[`NNG_OPT_RAW`]) where 47messages may be discarded. 48Applications that require reliable delivery semantics should consider using 49xref:nng_req.7.adoc[_req_] sockets, or 50implement their own acknowledgment layer on top of _pair_ sockets. 51 52=== Protocol Versions 53 54Version 0 is the legacy version of this protocol. 55It lacks any header 56information, and is suitable when building simple one-to-one topologies. 57 58TIP: Use version 0 if you need to communicate with other implementations, 59including the legacy https://github.com/nanomsg/nanomsg[nanomsg] library or 60https://github.com/go-mangos/mangos[mangos]. 61 62Version 1 of the protocol offers improved protection against loops when 63used with xref:nng_device.3.adoc[`nng_device()`]. 64 65=== Polyamorous Mode 66 67WARNING: Polyamorous mode is deprecated, and support for it will likely 68be removed in a future release. 69Applications are strongly discouraged from making further use of it. 70 71Normally pair sockets are for one-to-one communication, and a given peer 72will reject new connections if it already has an active connection to another 73peer. 74 75((_Polyamorous_ mode)) changes this, to allow a socket to communicate with 76multiple directly-connected peers. 77This mode is enabled by opening a socket with the 78xref:nng_pair_open.3.adoc[`nng_pair1_open_poly()`] 79function 80 81NOTE: Polyamorous mode is only available when using pair version 1. 82 83In polyamorous mode a socket can support many one-to-one connections. 84In this mode, the application must 85choose the remote peer to receive an outgoing message by setting the 86xref:nng_pipe.5.adoc[`nng_pipe`] to use for the outgoing message with 87the xref:nng_msg_set_pipe.3.adoc[`nng_msg_set_pipe()`] function. 88 89If no remote peer is specified by the sender, then the protocol will select 90any available connected peer. 91 92Most often the value of the outgoing pipe will be obtained from an incoming 93message using the xref:nng_msg_get_pipe.3.adoc[`nng_msg_get_pipe()`] function, 94such as when replying to an incoming message. 95 96NOTE: Directed send _only_ works with directly connected peers. 97It will not function across xref:nng_device.3.adoc[device] proxies. 98 99In order to prevent head-of-line blocking, if the peer on the given pipe 100is not able to receive (or the pipe is no longer available, such as if the 101peer has disconnected), then the message will be discarded with no notification 102to the sender. 103 104=== Protocol Options 105 106The following protocol-specific options are available. 107 108xref:nng_options.5.adoc#NNG_OPT_MAXTTL[`NNG_OPT_MAXTTL`]:: 109 110 (`int`, version 1 only). Maximum time-to-live. 111 112((`NNG_OPT_PAIR1_POLY`)):: 113 114 (`bool`, version 1 only) This option is no longer supported. 115 Formerly it was used to configure _polyamorous_ mode, but that mode 116 is now established by using the `nng_pair1_open_poly()` function. 117 118=== Protocol Headers 119 120Version 0 of the pair protocol has no protocol-specific headers. 121 122Version 1 of the pair protocol uses a single 32-bit unsigned value. The 123low-order (big-endian) byte of this value contains a "hop" count, and is 124used in conjunction with the 125xref:nng_options.5.adoc#NNG_OPT_MAXTTL[`NNG_OPT_MAXTTL`] option to guard against 126device forwarding loops. 127This value is initialized to 1, and incremented each time the message is 128received by a new node. 129 130== SEE ALSO 131 132[.text-left] 133xref:nng_pair_open.3.adoc[nng_pair_open(3)], 134xref:nng_options.5.adoc[nng_options(5)], 135xref:nng.7.adoc[nng(7)] 136