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Started an introductory section too.

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+# PAIR protocol
+
+The {{i:*PAIR* protocol}}{{hi:*PAIR*}} implements a peer-to-peer pattern, where
+relationships between peers are one-to-one.
+
+### Socket Operations
+
+The [`nng_pair_open`][nng_pair_open] functions create a _PAIR_ socket.
+
+Normally, this pattern will block when attempting to send a message if
+no peer is able to receive the message.
+
+> [!NOTE]
+> Even though this mode may appear to be reliable, because back-pressure
+> prevents discarding messages most of the time, there are topologies involving
+> where messages may be discarded.
+> Applications that require reliable delivery semantics should consider using
+> [_REQ_][req] sockets, or implement their own acknowledgment layer on top of _PAIR_ sockets.
+
+### 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.
+
+> [!TIP]
+> Use version 0 if you need to communicate with other implementations,
+> including the legacy [libnanomsg][nanomsg] library or
+> [mangos][mangos].
+
+Version 1 of the protocol offers improved protection against loops when
+used with [devices][device].
+
+### Polyamorous Mode
+
+> [!NOTE]
+> Polyamorous mode is deprecated, and support for it will likely
+> be removed in a future release, when a suitable mesh protocol is
+> available.
+> In the meantime, applications are encouraged to look to other patterns.
+
+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.
+
+_Polyamorous_{{hi:polyamorous mode}} changes this, to allow a socket to communicate with
+multiple directly-connected peers.
+This mode is enabled by opening a socket using [`nng_pair1_open_poly`][nng_pair_open].
+
+> [!TIP]
+> Polyamorous mode is only available when using pair version 1.
+
+In polyamorous mode 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`][pipe] to use for the outgoing message using
+[`nng_msg_set_pipe`][nng_msg_pipe].
+
+If no remote peer is specified by the sender, then the protocol will select
+any available connected peer.
+
+Most often the value of the outgoing pipe will be obtained from an incoming
+message using [`nng_msg_get_pipe`][nng_msg_pipe],
+such as when replying to an incoming message.
+
+> [!NOTE]
+> Directed send _only_ works with directly connected peers.
+> It will not function across [device][device] proxies.
+
+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_MAXTTL`][NNG_OPT_MAXTTL]:
+  (`int`, version 1 only). Maximum time-to-live.
+
+- `NNG_OPT_PAIR1_POLY`:
+  (`bool`, version 1 only) This option is no longer supported.
+  Formerly it was used to configure _polyamorous_ mode, but that mode
+  is now established by using the `nng_pair1_open_poly` function.
+
+### 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`][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_pair_open]: TODO.md
+[NNG_OPT_MAXTTL]: TODO.md
+[device]: TODO.md
+[nanomsg]: TODO.md
+[mangos]: TODO.md
+[pipe]: TODO.md
+[nng_msg_pipe]: ../api/msg/nng_msg_pipe.md
+[req]: ./req.md