title
	ContNet Protocol specification, version 0.3 (2017-08-18)

content
	section Overview
		text
			CNP is a request-response application protocol meant to facilitate hypertext content requests and delivery over the Internet. It is designed to be an alternative to a part of HTTP; specifically, the subset dealing with static content (text, images, videos and similar). The protocol is designed to be simple, easy to implement and unambiguous, as well as strict in order to avoid having to support multiple incorrect implementations.

			It is a binary protocol. While all of the defined keywords are also valid ASCII text (lowercase letters, numbers, symbols and whitespace), CNP does not use any character encoding. Rather, these keywords should be treated as specific bytestrings. Every mention of CNP strings in the specification refers to bytestrings whose content is the provided ASCII-encoded text. This is done to avoid problems with encoding and case insensitivity and simplify parsing, since each keyword has exactly one valid bytestring representation.

			For now, the protocol implements only a limited amount of simple optimizations. While it would be possible to make it slightly faster by including more, that would also increase its complexity and increase the likelihood of implementations supporting a very small or even incorrect subset of the specification. Additional simple optimizations may be added in future versions of CNP.


	section CNP message
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			CNP messages are composed of a header and an optional body. The header is delimited by a newline. Both request and response share the same syntax, with the differences being the semantic meaning of values.


		section Header
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				The header consists of the CNP version, message intent and an arbitrary number of parameters, using a space as the delimiter between fields.

				The header field delimiter is a single space byte (``0x20``). Multiple successive spaces in the header are a syntax error. A space cannot appear as itself in any header value; it must be escaped into a specific byte sequence. This makes splitting a valid header by spaces safe and correct.

				The CNP version part of the header is the string ``"cnp/"`` followed by a tuple of the major and minor version numbers without leading zeroes on non-zero versions, separated by a period (e.g. ``"cnp/0.3"``). Even if the specification and implementation use a patch version, it is not provided in the message header and must have no impact on protocol compatibility. CNP implementations within the same major version should be generally compatible with other minor versions for basic requests, excluding some noncritical feature discrepancy; unknown parameters are ignored. An exception to this are all versions in the major version ``0``; since it's the development version, the protocol may be completely rewritten between two minor versions.

				The intent is a string that defines the type of the message. Valid values depend on whether the message is request or response.

				Parameters are key-value pairs separated by an equals sign (``0x3d``). There may be any number of parameters in a message. The key and value can be arbitrary bytestrings, including empty strings. The order of parameters can be arbitrary and implementations must not depend on it nor alter their behavior based on the order; sorting the parameters or shuffling their order produces an identical request. Each parameter key may appear at most once in a message; duplicate parameter key is an error. The equals sign must always be present for a parameter, even if the key or value is blank. A missing parameter is considered to have the blank value, so there is no difference between a parameter provided with a blank value and one not present in the header.

				Header ends with a line feed byte (``0x0a``). Note that if there is a carriage return before it, it will be considered to be a part of the last token (parameter value or intent).

				The message body is a blob of arbitrary binary data. There is no body delimiter or terminator; if the body ends with a newline, it is considered a part of the body data.


		section Escaping
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				The NUL (``0x00``), line feed (``0x0a``), space (``0x20``), equals sign (``0x3d``) and backslash (``0x5c``) bytes must be escaped by using a backslash and a specific character (see table below) in all parts of the message header, except when used to delimit header fields or parameter key and value. Using any of these characters when not otherwise specified by the syntax (such as space between intent parameters, equals sign between parameter key and value and line feed at the end of the header) is a syntax error. Other characters, such as tab and carriage return, do not have to be escaped and stand for themselves.

				Each raw value can be escaped into exactly one escaped value and each escaped value maps 1:1 to raw values. Because of this, escaped values do not have to be unescaped or normalized before being handled, as long as they are compared with escaped versions of expected values.

				The message body contains no escaping and is plain binary data (unless specified otherwise by a parameter).

			raw
				(NUL)   ->  \0
				(LF)    ->  \n
				(space) ->  \_
				=       ->  \-
				\       ->  \\


		section Request
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				A CNP request is sent by a client that wants to request a resource from a server (or send some data to it).


			section Intent
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					In a request, the intent part of the header contains the hostname of the server concatenated with the path of the requested resource.

					The hostname may be a domain name or an IP address, optionally with a port number, and it may not contain any slash (``0x2f``) bytes. In general, it should be the address that the client sends the request to. The format of the hostname is the same as in URLs.

					The path is a Unix-style absolute filepath with an optional trailing slash. It may (and probably should) be cleaned up by CNP implementations before being processed by collapsing multiple consecutive slashes into single ones, removing path entries that consist of a single dot and resolving double-dot parent directories, while leaving a trailing slash if one was present (for example, the path ``"/../../.\/\/\/foo/bar/.."`` becomes ``"/foo"`` and ``"\/\/foo/bar/../"`` becomes ``"/foo/"``). If possible, implementations should avoid sending filepaths that need to be cleaned. The minimal path is ``"/"``; blank path is an error.

					The hostname is case-insensitive, while the path is case-sensitive. Hostname may be normalized by clients and servers, but a specific path (after cleanup) represents one specific resource.

					To separate the hostname from the filepath, split the intent before the first slash.


			section Parameters
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					The following request parameters are defined (default value is assumed if the parameter value is blank; any absent parameter must be treated as if it had the default value):


				section length
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						length={number}

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						The length of the request body data in bytes.

						This field is required if any data will be sent to the server.

						Default value: ``"0"`` (empty body)


				section name
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						name={identifier}

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						The name of the content being sent in the request body. Must not contain slashes (``0x2f``) or NUL bytes (``0x00``), including escaped NUL bytes (``\0``).

						Can be used to provide filename metadata. If the original filename was a Unicode filename instead of a bytestring, UTF-8 should be used to encode it.

						Default value: ``""`` (no name provided)


				section type
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						type={identifier}/{identifier}

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						MIME type of the content being sent in the request body.

						Default value: ``"application/octet-stream"``


				section if_modified:
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						if_modified={timestamp}

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						Only send the resource in the response if it has been modified since the RFC 3339 UTC timestamp provided, according to the server's time. Otherwise, reply with a ``"not_modified"`` response and no body data.

						The RFC 3339 timestamp format should be the following, as a strftime format string format: ``%Y-%m-%dT%H:%M:%SZ``, for example: ``1970-12-31T23:59:30Z``.

						The timestamp should usually be either the server's ``"modified"`` parameter value from the time the cached resource was requested or the ``"time"`` parameter value from that request if the former was not provided. If neither was present in the previous response for the current request's intent, the client should not use an ``if_modified`` parameter.

						Default value: ``""`` (do not perform this check, just send the resource)


		section Response
			text
				CNP requests are answered by a CNP response by the server. Most of the time, the response will probably contain the requested resource.


			section Intent
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					The intent part of the response header is one of the defined response types. It signifies whether the response contains the resource requested, a redirect, report of an error or other results.


				section ok
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						The request was successfully resolved and the response body contains the requested resource.


				section not_modified
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						The resource has not been modified since the time in the request's ``"if_modified"`` parameter. The response body is blank and the client should use the cached resource.


				section redirect
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						The client should make a new request to the location provided in the ``"location"`` parameter. The response body may contain a page as if it was an ``"ok"`` response.

						The new request should be blank (contain no body data, as if that path was just entered anew by the user).

						Clients are not required to follow the redirect. Servers should not assume that a client will always follow the redirect immediately or at all. Interactive user agents may prompt the user for confirmation before opening the new page. Otherwise, following redirects up to a certain count is generally the expected behavior.

						User agents do not have to display the provided page and may directly perform the redirect. If the redirect is not performed and a page was provided, it should be displayed instead.


				section error
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						There was an error answering the request. The server must also provide the ``"reason"`` response parameter. The body data may contain a page as if it was an ``"ok"`` response.

						User agents do not have to display the provided page and may just inform the user of the error reason. If a page is provided, it should be used to inform the user of the details of the error.


			section Parameters
				text
					The following response parameters are defined (default value is assumed if the parameter value is blank; any absent parameter must be treated as if it had the default value):


				section length
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						length={number}

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						The length of the response body data in bytes.

						If this parameter is not provided or is blank, the client should read all data until the connection is closed, end of file is reached or equivalent. Despite that, the length parameter should be sent whenever possible.

						Valid in all response types.

						Default value: ``""`` (read until EOF)


				section name
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						name={identifier}

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						The name of the content being sent in the response body. Must not contain slashes (``0x2f``) or NUL bytes (``0x00``), including escaped NUL bytes (``\0``).

						Can be used to provide filename metadata. If the original filename was a Unicode filename instead of a bytestring, UTF-8 should be used to encode it.

						Valid in response types where the body data can be file (``ok``, ``redirect``, ``error``).

						Default value: ``""`` (no name provided)


				section type
					raw
						type={identifier}/{identifier}

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						MIME type of the content being sent in the response body.

						Valid in response types where the body data can be file (``ok``, ``redirect``, ``error``).

						Default value: ``"application/octet-stream"``


				section time
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						time={timestamp}

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						The current time on the server as an RFC 3339 UTC timestamp.

						May be sent with any response type, but most useful in the ``"ok"`` response, where it may be used by the client in an ``"if_modified"`` request parameter later.

						Default value: ``""`` (no timestamp provided)


				section modified
					raw
						modified={timestamp}

					text fmt
						RFC3339 UTC timestamp representing the time the requested file was last modified.

						Valid in ``"ok"`` and ``"not_modified"`` responses, where it may be used by the client in an ``"if_modified"`` request parameter later.

						Default value: ``""`` (no timestamp provided)


				section location
					raw
						location={identifier}/{identifier}

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						The location to redirect to in the format of a CNP request intent.

						If the host part of the intent in the value is empty, the current host should be reused. If the host is ``"."``, the current host should be reused and the path should be appended to the current path, excluding the last filename after the final slash in the current path (if any). Otherwise, the new request should be sent to the provided host, which has to be resolved to a server address (instead of just sending a request with the new host to the current server).

						For example, if the request was sent to ``"example.com/foo/bar"`` and the redirect location was ``"/baz"``, the new request's intent is ``"example.com/baz"``, but if the location parameter was ``"./baz"``, the new intent is ``"example.com/foo/baz"``.

						Valid in ``"redirect"`` response type.

						Default value: ``""`` (not providing this parameter in a redirect response is an error, no redirect happens)


				section reason
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						reason={identifier}

					text
						Describes the reason for the error.

						Defined values:

					list
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							``syntax``: the request was not a valid CNP message

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							``version``: the request CNP version is not supported by the server

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							``invalid``: the received CNP message is not a valid CNP request (invalid intent format, invalid value of a defined parameter)

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							``not_supported``: a requested feature is not supported by the server, so the request cannot be answered

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							``too_large``: the server does not want to accept so much data (either header or body)

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							``not_found``: the requested path was not found on the server

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							``denied``: server does not allow access to this content (might require authentication first)

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							``rejected``: the request did not match the server's requirements for that path, but was a valid CNP request (e.g. missing parameters the application requires or an API call provided incorrect type of data)

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							``server_error``: internal server error

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						Valid in ``"error"`` response type.

						Default value: ``"server_error"`` (servers should provide this parameter in the ``"reason"`` response if possible)


		section EBNF
			raw ebnf
				raw_character = ? any byte except "\0", "\n", " ", "=", "\\" ? ;
				escaped_character = "\\" ( "0" | "n" | "_" | "-" | "\\" ) ;
				ident_character = raw_character | escaped_character ;
				identifier = ident_character, { ident_character } ;
				number = "0" | ( "1" … "9" ), { "0" … "9" } ;

				version = "cnp/", number, ".", number ;
				intent = identifier ;
				parameter = [ identifier ], "=", [ identifier ] ;

				header = version, " ", intent, { " ", parameter }, "\n" ;
				body = { ? any byte ? } ;

				message = header, body ;