SYNOPSIS
#include <nng/protocol/pair0/pair.h>
int nng_pair0_open(nng_socket *s);#include <nng/protocol/pair1/pair.h>
int nng_pair1_open(nng_socket *s);DESCRIPTION
The nng_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() call creates a 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(3)) or raw mode sockets where messages may be discarded. Applications that require reliable delivery semantics should consider using nng_req(7) sockets, or implement their own acknowledgement 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(3). 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 ougoing message by setting the value of the pipe ID on the outgoing message using the nng_msg_set_pipe(3) function.
Most often the value of the outgoing pipe ID will be obtained from an incoming message using the nng_msg_get_pipe(3) 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.
NNG_OPT_PAIR1_POLY-
(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.
NNG_OPT_MAXTTL-
(Version 1 only). Maximum time-to-live. This option is an integer value between 0 and 255, inclusive, and is the maximum number of "hops" that a message may pass through until it is discarded. The default value is 8. A value of 0 may be used to disable the loop protection, allowing an infinite number of hops.
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.
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 conjuction 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.