Converted protocols to mdbook.

Started an introductory section too.

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+# REP Protocol
+
+The {{i:*REP* protocol}}{{hi:*REP*}} is one half of a {{i:request/reply pattern}}.
+In this pattern, a requester sends a message to one replier, who
+is expected to reply.
+The request is resent if no reply arrives,
+until a reply is received or the request times out.
+
+> [!TIP]
+> This protocol is useful in setting up RPC-like services.
+> It is also reliable, in that a requester will keep retrying until
+> a reply is received.
+
+The _REP_ protocol is the replier side, and the
+[_REP_][req] protocol is the requester side.
+
+## Socket Operations
+
+The [`nng_rep0_open`][nng_rep_open] functions create a replier socket.
+This socket may be used to receive messages (requests), and then to send
+replies.
+
+Generally a reply can only be sent after receiving a request.
+
+Send operations will result in `NNG_ESTATE` if no corresponding request
+was previously received.
+
+Likewise, only one receive operation may be pending at a time.
+Any additional concurrent receive operations will result in `NNG_ESTATE`.
+
+[Raw mode][raw] sockets ignore all these restrictions.
+
+## Context Operations
+
+This protocol supports the creation of [contexts][context] for concurrent
+use cases using [`nng_ctx_open`][nng_ctx_open].
+
+Each context may have at most one outstanding request, and operates
+independently of the others.
+The restrictions for order of operations with sockets apply equally
+well for contexts, except that each context will be treated as if it were
+a separate socket.
+
+## Protocol Versions
+
+Only version 0 of this protocol is supported.
+(At the time of writing, no other versions of this protocol have been defined.)
+
+## Protocol Options
+
+The _REP_ protocol has no protocol-specific options.
+
+## Protocol Headers
+
+The _REP_ protocol uses a {{ii:backtrace}} in the header.
+This is more fully documented in the [_REQ_][req] chapter.
+
+[nng_rep_open]: TODO.md
+[nng_ctx_open]: TODO.md
+[raw]: TODO.md
+[context]: TODO.md
+[req]: ./req.md