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This documentation is for version v1.1.0 of nng, but the latest released version is v1.3.2. see the documentation for v1.3.2 for the most up-to-date information.


Version 0
#include <nng/protocol/pair0/pair.h>
Version 1
#include <nng/protocol/pair1/pair.h>


The pair protocol implements a peer-to-peer pattern, where relationships between peers are one-to-one.

Version 1 of this protocol supports an optional polyamorous mode where a peer can maintain multiple partnerships. Using this mode requires some additional sophistication in the application.

Socket Operations

The nng_pair_open() functions create pair socket.

Normally, this pattern will block when attempting to send a message if no peer is able to receive the message.

Even though this mode may appear to be "reliable", because back-pressure prevents discarding messages most of the time, there are topologies involving devices (see nng_device()) or raw mode sockets (see NNG_OPT_RAW) where messages may be discarded. Applications that require reliable delivery semantics should consider using req sockets, or implement their own acknowledgment layer on top of pair sockets.

In order to avoid head-of-line blocking conditions, polyamorous mode pair sockets (version 1 only) discard messages if they are unable to deliver them to a peer.

Protocol Versions

Version 0 is the legacy version of this protocol. It lacks any header information, and is suitable when building simple one-to-one topologies.

Use version 0 if you need to communicate with other implementations, including the legacy nanomsg library or mangos.

Version 1 of the protocol offers improved protection against loops when used with nng_device(). It also offers polyamorous mode for forming multiple partnerships on a single socket.

Version 1 of this protocol is considered experimental at this time.

Polyamorous Mode

Normally pair sockets are for one-to-one communication, and a given peer will reject new connections if it already has an active connection to another peer.

In polyamorous mode, which is only available with version 1, a socket can support many one-to-one connections. In this mode, the application must choose the remote peer to receive an outgoing message by setting the nng_pipe to use for the outgoing message with the nng_msg_set_pipe() function.

Most often the value of the outgoing pipe will be obtained from an incoming message using the nng_msg_get_pipe() function, such as when replying to an incoming message.

In order to prevent head-of-line blocking, if the peer on the given pipe is not able to receive (or the pipe is no longer available, such as if the peer has disconnected), then the message will be discarded with no notification to the sender.

Protocol Options

The following protocol-specific options are available.


(bool, version 1 only) This option enables the use of polyamorous mode. The value is read-write, and takes an integer Boolean value. The default false value (0) indicates that legacy monogamous mode should be used.


(int, version 1 only). Maximum time-to-live.

Protocol Headers

Version 0 of the pair protocol has no protocol-specific headers.

Version 1 of the pair protocol uses a single 32-bit unsigned value. The low-order (big-endian) byte of this value contains a "hop" count, and is used in conjunction with the NNG_OPT_MAXTTL option to guard against device forwarding loops. This value is initialized to 1, and incremented each time the message is received by a new node.

NNG Reference Manual vv1.1.0 © 2019 Staysail Systems, Inc, © 2018 Capitar IT Group BV
This document is supplied under the MIT License.
nanomsg™ and nng™ are trademarks of Garrett D'Amore.