----------------------------- Project Gemini "Speculative specification" v0.11.0, March 1st 2020 ----------------------------- This is an increasingly less rough sketch of an actual spec for Project Gemini. Although not finalised yet, further changes to the specification are likely to be relatively small. You can write code to this pseudo-specification and be confident that it probably won't become totally non-functional due to massive changes next week, but you are still urged to keep an eye on ongoing development of the protocol and make changes as required. This is provided mostly so that people can quickly get up to speed on what I'm thinking without having to read lots and lots of old phlog posts and keep notes. Feedback on any part of this is extremely welcome, please email solderpunk@sdf.org. ----------------------------- 1. Overview Gemini is a client-server protocol featuring request-response transactions, broadly similar to gopher or HTTP. Connections are closed at the end of a single transaction and cannot be reused. When Gemini is served over TCP/IP, servers should listen on port 1965 (the first manned Gemini mission, Gemini 3, flew in March '65). This is an unprivileged port, so it's very easy to run a server as a "nobody" user, even if e.g. the server is written in Go and so can't drop privileges in the traditional fashion. 1.1 Gemini transactions There is one kind of Gemini transaction, roughly equivalent to a gopher request or a HTTP "GET" request. Transactions happen as follows: C: Opens connection S: Accepts connection C/S: Complete TLS handshake (see 1.4) C: Validates server certificate (see 1.4.2) C: Sends request (one CRLF terminated line) (see 1.2) S: Sends response header (one CRFL terminated line), closes connection under non-success conditions (see 1.3.1, 1.3.2) S: Sends response body (text or binary data) (see 1.3.3) S: Closes connection C: Handles response (see 1.3.4) 1.2 Gemini requests Gemini requests are a single CRLF-terminated line with the following structure: is a UTF-8 encoded absolute URL, of maximum length 1024 bytes. If the scheme of the URL is not specified, a scheme of gemini:// is implied. Sending an absolute URL instead of only a path or selector is effectively equivalent to building in a HTTP "Host" header. It permits virtual hosting of multiple Gemini domains on the same IP address. It also allows servers to optionally act as proxies. Including schemes other than gemini:// in requests allows servers to optionally act as protocol-translating gateways to e.g. fetch gopher resources over Gemini. Proxying is optional and the vast majority of servers are expected to only respond to requests for resources at their own domain(s). 1.3 Responses Gemini response consist of a single CRLF-terminated header line, optionally followed by a response body. 1.3.1 Response headers Gemini response headers look like this: is a two-digit numeric status code, as described below in 1.3.2 and in Appendix 1. is any non-zero number of consecutive spaces or tabs. is a UTF-8 encoded string of maximum length 1024, whose meaning is dependent. If does not belong to the "SUCCESS" range of codes, then the server MUST close the connection after sending the header and MUST NOT send a response body. If a server sends a which is not a two-digit number or a which exceeds 1024, the client SHOULD close the connection and disregard the response header, informing the user of an error. 1.3.2 Status codes Gemini uses two-digit numeric status codes. Related status codes share the same first digit. Importantly, the first digit of Gemini status codes do not group codes into vague categories like "client error" and "server error" as per HTTP. Instead, the first digit alone provides enough information for a client to determine how to handle the response. By design, it is possible to write a simple but feature complete client which only looks at the first digit. The second digit provides more fine-grained information, for unambiguous server logging, to allow writing comfier interactive clients which provide a slightly more streamlined user interface, and to allow writing more robust and intelligent automated clients like content aggregators, search engine crawlers, etc. The first digit of a response code unambiguously places the response into one of six categories, which define the semantics of the line. 1 INPUT The requested resource accepts a line of textual user input. The line is a prompt which should be displayed to the user. The same resource should then be requested again with the user's input included as a query component. Queries are included in requests as per the usual generic URL definition in RFC3986, i.e. separated from the path by a ?. There is no response body. 2 SUCCESS The request was handled successfully and a response body will follow the response header. The line is a MIME media type which applies to the response body. 3 REDIRECT The server is redirecting the client to a new location for the requested resource. There is no response body. The header text is a new URL for the requested resource. The URL may be absolute or relative. The redirect should be considered temporary, i.e. clients should continue to request the resource at the original address and should not performance convenience actions like automatically updating bookmarks. There is no response body. 4 TEMPORARY FAILURE The request has failed. There is no response body. The nature of the failure is temporary, i.e. an identical request MAY succeed in the future. The contents of may provide additional information on the failure, and should be displayed to human users. 5 PERMANENT FAILURE The request has failed. There is no response body. The nature of the failure is permanent, i.e. identical future requests will reliably fail for the same reason. The contents of may provide additional information on the failure, and should be displayed to human users. Automatic clients such as aggregators or indexing crawlers should should not repeat this request. 6 CLIENT CERTIFICATE REQUIRED The requested resource requires client-certificate authentication to access. If the request was made without a certificate, it should be repeated with one. If the request was made with a certificate, the server did not accept it and the request should be repeated with a different certificate. The contents of may provide additional information on certificate requirements or the reason a certificate was rejected. Note that for basic interactive clients for human use, errors 4 and 5 may be effectively handled identically, by simply displaying the contents of under a heading of "ERROR". The temporary/permanent error distinction is primarily relevant to well-behaving automated clients. Basic clients may also choose not to support client-certificate authentication, in which case only four distinct status handling routines are required (for statuses beginning with 1, 2, 3 or a combined 4-or-5). The full two-digit system is detailed in Appendix 1. Note that for each of the six valid first digits, a code with a second digit of zero corresponds is a generic status of that kind with no special semantics. This means that basic servers without any advanced functionality need only be able to return codes of 10, 20, 30, 40 or 50. The Gemini status code system has been carefully designed so that the increased power (and correspondingly increased complexity) of the second digits is entirely "opt-in" on the part of both servers and clients. 1.3.3 Response bodies Response bodies are just raw content, text or binary, ala gopher. There is no support for compression, chunking or any other kind of content or transfer encoding. The server closes the connection after the final byte, there is no "end of response" signal like gopher's lonely dot. Response bodies only accompany responses whose header indicates a SUCCESS status (i.e. a status code whose first digit is 2). For such responses, is a MIME media type as defined in RFC 2046. If a MIME type begins with "text/" and no charset is explicitly given, the charset should be assumed to be UTF-8. Compliant clients MUST support UTF-8-encoded text/* responses. Clients MAY optionally support other encodings. Clients receiving a response in a charset they cannot decode SHOULD gracefully inform the user what happened instead of displaying garbage. If is an empty string, the MIME type MUST default to "text/gemini; charset=utf-8". 1.3.4 Response body handling Response handling by clients should be informed by the provided MIME type information. Gemini defines one MIME type of its own (text/gemini) whose handling is discussed below in 1.3.5. In all other cases, clients should do "something sensible" based on the MIME type. Minimalistic clients might adopt a strategy of printing all other text/* responses to the screen without formatting and saving all non-text responses to the disk. Clients for unix systems may consult /etc/mailcap to find installed programs for handling non-text types. 1.3.5 text/gemini responses 1.3.5.1 Overview In the same sense that HTML is the "native" response format of HTTP and plain text is the native response format of gopher, Gemini defines its own native response format - though of course, thanks to the inclusion of a MIME type in the response header Gemini can be used to serve plain text, rich text, HTML, Markdown, LaTeX, etc. Response bodies of type "text/gemini" are a kind of lightweight hypertext format, which takes inspiration from gophermaps and from Markdown. The format permits richer typographic possibilities than the plain text of Gopher, but remains extremely easy to parse. The format is line-oriented, and a satisfactory rendering can be achieved with a single pass of a document, processing each line independently. As per gopher, links can only be displayed one per line, encouraging neat, list-like structure. Similar to how the two-digit Gemini status codes were designed so that simple clients can function correctly while ignoring the second digit, the text/gemini format has been designed so that simple clients can ignore the more advanced features and still remain very usable. 1.3.5.2 Line-orientation As mentioned, the text/gemini format is line-oriented. Each line of a text/gemini document has a single "line type". It is possible to unambiguously determine a line's type purely by inspecting its first three characters. A line's type determines the manner in which it should be presented to the user. Any details of presentation or rendering associated with a particular line type are strictly limited in scope to that individual line. There are 6 different line types in total. However, a fully functional and specification compliant Gemini client need only recognise and handle 4 of them - these are the "core line types", (see 1.3.5.3). Advanced clients can also handle the additional "advanced line types" (see 1.3.5.4). Simple clients can treat all advanced line types as one of the core line types and still offer an adequate user experience. 1.3.5.3 Core line types The four core line types are: 1.3.5.3.1 Text lines Text lines are the most fundamenal line type - any line which does not match the definition of another line type defined below defaults to being a text line. The majority of lines in a typical text/gemini document will be text lines. Text lines should be presented to the user, after being wrapped to the appropriate width for the client's viewport (see below). Text lines may be presented to the user in a visually pleasing manner for general reading, the precise meaning of which is at the client's discretion. For example, variable width fonts may be used, spacing may be normalised, with spaces between sentences being made wider than spacing between words, and other such typographical niceties may be applied. Clients may permit users to customise the appearance of text lines by altering the font, font size, text and background colour, etc. Authors should not expect to exercise any control over the precise rendering of their text lines, only of their actual textual content. Content such as ASCII art, computer source code, etc. which may appear incorrectly when treated as such should be enclosed beween preformatting toggle lines (see 1.3.5.3.3). Blank lines are instances of text lines and have no special meaning. They should be rendered individually as vertical blank space each time they occur. In this way they are analogous to
tags in HTML. Consecutive blank lines should NOT be collapsed into a fewer blank lines. Note also that consecutive non-blank text lines do not form any kind of coherent unit or block such as a "paragraph": all text lines are independent entities. Text lines which are longer than can fit on a client's display device SHOULD be "wrapped" to fit, i.e. long lines should be split (ideally at whitespace or at hyphens) into multiple consecutive lines of a device-appropriate width. This wrapping is applied to each line of text independently. Multiple consecutive lines which are shorter than the client's display device MUST NOT be combined into fewer, longer lines. In order to take full advantage of this method of text formatting, authors of text/gemini content SHOULD avoid hard-wrapping to a pecific fixed width, in contrast to the convention in Gopherspace where text is typically wrapped at 80 characters or fewer. Instead, text which should be displayed as a contiguous block should be written as a single long line. Most text editors can be configured to "soft-wrap", i.e. to write this kind of file while displaying the long lines wrapped to fit the author's display device. Authors who insist on hard-wrapping their content MUST be aware that the content will display neatly on clients whose display device is as wide as the hard-wrapped length or wider, but will appear with irregular line widths on narrower clients. 1.3.5.3.2 Link lines Lines beginning with the two characters "=>" are link lines, which have the following syntax: =>[][] where: * is any non-zero number of consecutive spaces or tabs * Square brackets indicate that the enclosed content is optional. * is a URL, which may be absolute or relative. If the URL does not include a scheme, a scheme of gemini:// is implied. All the following examples are valid link lines: => gemini://example.org/ => gemini://example.org/ An example link => gemini://example.org/foo Another example link at the same host =>gemini://example.org/bar Yet another example link at the same host => foo/bar/baz.txt A relative link => gopher://example.org:70/1 A gopher link Note that link URLs may have schemes other than gemini://. This means that Gemini documents can simply and elegantly link to documents hosted via other protocols, unlike gophermaps which can only link to non-gopher content via a non-standard adaptation of the `h` item-type. Clients can present links to users in whatever fashion the client author wishes. 1.3.5.3.3 Preformatting toggle lines Any line whose first three characters are "```" (i.e. three consecutive back ticks with no leading whitespace) are preformatted toggle lines. These lines should NOT be included in the rendered output shown to the user. Instead, these lines toggle the parser between preformatted mode being "on" or "off". Preformatted mode should be "off" at the beginning of a document. The current status of preformatted mode is the only internal state a parser is required to maintain. When preformatted mode is "on", the usual rules for identifying line types are suspended, and all lines should be identified as preformatted text lines (see 1.3.5.3.4). Preformatting toggle lines can be thought of as analogous to 
 and
tags in HTML. 1.3.5.3.4 Preformatted text lines Preformatted text lines should be presented to the user in a "neutral", monowidth font without any alteration to whitespace or stylistic enhancements. Graphical clients should use scrolling mechanisms to present preformatted text lines which are longer than the client viewport, in preference to wrapping. In displaying preformatted text lines, clients should keep in mind applications like ASCII art and computer source code: in particular, source code in langugaes with significant whitespace (e.g. Python) should be able to be copied and pasted from the client into a file and interpreted/compiled without any problems arising from the client's manner of displaying them. 1.3.5.4 Advanced line types The following advanced line types MAY be recognised by advanced clients. Simple clients may treat them all as text lines as per 1.3.5.3.1 without any loss of essential function. 1.3.5.4.1 Heading lines Lines beginning with "#" are heading lines. Heading lines consist of one, two or three consecutive "#" characters, followed by optional whitespace, followed by heading text. The number of # characters indicates the "level" of header; #, ## and ### can be thought of as analogous to

,

and

in HTML. Heading text should be presented to the user, and clients MAY use special formatting, e.g. a larger or bold font, to indicate its status as a header (simple clients may simply print the line, including its leading #s, without any styling at all). However, the main motivation for the definition of heading lines is not stylistic but to provide a machine-readable representation of the internal structure of the document. Advanced clients can use this information to, e.g. display an automatically generated and hierarchically formatted "table of contents" for a long document in a side-pane, allowing users to easily jump to specific sections without excessive scrolling. CMS-style tools automatically generating menus or Atom/RSS feeds for a directory of text/gemini files can use first heading in the file as a human-friendly title. 1.3.5.4.2 Unordered list items Lines beginning with a * are unordered list items. This line type exists purely for stylistic reasons. The * may be replaced in advanced clients by a bullet symbol. Any text after the * character should be presented to the user as if it were a text line, i.e. wrapped to fit the viewport and formatted "nicely". Advanced clients can take the space of the bullet symbol into account when wrapping long list items to ensure that all lines of text corresponding to the item are offset an equal distance from the left of the screen. 1.4 TLS 1.4.1 Version requirements Use of TLS for Gemini transactions is mandatory. Servers MUST use TLS version 1.2 or higher and SHOULD use TLS version 1.3 or higher. Clients MAY refuse to connect to servers using TLS version 1.2 or lower. 1.4.2 Server certificate validation Clients can validate TLS connections however they like (including not at all) but the strongly RECOMMENDED approach is to implement a lightweight "TOFU" certificate-pinning system which treats self-signed certificates as first- class citizens. This greatly reduces TLS overhead on the network (only one cert needs to be sent, not a whole chain) and lowers the barrier to entry for setting up a Gemini site (no need to pay a CA or setup a Let's Encrypt cron job, just make a cert and go). TOFU stands for "Trust On First Use" and is public-key security model similar to that used by OpenSSH. The first time a Gemini client connects to a server, it accepts whatever certificate it is presented. That certificate's fingerprint and expiry date are saved in a persistent database (like the .known_hosts file for SSH), associated with the server's hostname. On all subsequent connections to that hostname, the received certificate's fingerprint is computed and compared to the one in the database. If the certificate is not the one previously received, but the previous certificate's expiry date has not passed, the user is shown a warning, analogous to the one web browser users are shown when receiving a certificate without a signature chain leading to a trusted CA. This model is by no means perfect, but it is not awful and is vastly superior to just accepting self-signed certificates unconditionally. 1.4.3 Transient client certificate sessions Self-signed client certificates can optionally be used by Gemini clients to permit servers to recognise subsequent requests from the same client as belonging to a single "session". This facilitates maintaining state in server-side applications. The functionality is very similar to HTTP cookies, but with important differences. Whereas HTTP cookies are originally created by a webserver and given to a client via a response header, client certificates are created by the client and given to the server as part of the TLS handshake: Client certificates are fundamentally a client-centric means of identification. Further, whereas HTTP cookies can be "resurrected" by webservers after a client deletes them if the server recognises the client by means of browser finger-printing or some other tracking technology (leading to unkillable "super cookies"), if a client deletes a client certificate and also the accompanying private key (which the server has never seen), then the session ID can never be recreated. Thus clients not only need to opt in to a certificate session, but once they have done so they retain a guaranteed ability to opt out of it at any point and the server cannot defeat this ability. Gemini requests typically will be made without a client certificate being sent to the server. If a requested resource is part of a server-side application which requires persistent state, a Gemini server can return a status code of 61 (see Appendix 1 below) to request that the client repeat the request with a "transient certificate" to initiate a client certificate section. Interactive clients for human users MUST inform users that such a session has been requested and require the user to approve generation of such a certificate. Transient certificates MUST NOT be generated automatically. Transient certificates are limited in scope to a particular domain. Transient certificates MUST NOT be reused across different domains. Transient certificates MUST be permanently deleted when the matching server issues a response with a status code of 21 (see Appendix 1 below). Transient certificates MUST be permanently deleted when the client process terminates. Transient certificates SHOULD be permanently deleted after not having been used for more than 24 hours. Appendix 1. Full two digit status codes 10 INPUT As per definition of single-digit code 1 in 1.3.2. 20 SUCCESS As per definition of single-digit code 2 in 1.3.2. 21 SUCCESS - END OF CLIENT CERTIFICATE SESSION The request was handled successfully and a response body will follow the response header. The line is a MIME media type which applies to the response body. In addition, the server is signalling the end of a transient client certificate session which was previously initiated with a status 61 response. The client should immediately and permanently delete the certificate and accompanying private key which was used in this request. 30 REDIRECT - TEMPORARY As per definition of single-digit code 3 in 1.3.2. 31 REDIRECT - PERMANENT The requested resource should be consistently requested from the new URL provided in future. Tools like search engine indexers or content aggregators should update their configurations to avoid requesting the old URL, and end-user clients may automatically update bookmarks, etc. Note that clients which only pay attention to the initial digit of status codes will treat this as a temporary redirect. They will still end up at the right place, they just won't be able to make use of the knowledge that this redirect is permanent, so they'll pay a small performance penalty by having to follow the redirect each time. 40 TEMPORARY FAILURE As per definition of single-digit code 4 in 1.3.2. 41 SERVER UNAVAILABLE The server is unavailable due to overload or maintenance. (cf HTTP 503) 42 CGI ERROR A CGI process, or similar system for generating dynamic content, died unexpectedly or timed out. 43 PROXY ERROR A proxy request failed because the server was unable to successfully complete a transaction with the remote host. (cf HTTP 502, 504) 44 SLOW DOWN Rate limiting is in effect. is an integer number of seconds which the client must wait before another request is made to this server. (cf HTTP 429) 50 PERMANENT FAILURE As per definition of single-digit code 5 in 1.3.2. 51 NOT FOUND The requested resource could not be found but may be available in the future. (cf HTTP 404) (struggling to remember this important status code? Easy: you can't find things hidden at Area 51!) 52 GONE The resource requested is no longer available and will not be available again. Search engines and similar tools should remove this resource from their indices. Content aggregators should stop requesting the resource and convey to their human users that the subscribed resource is gone. (cf HTTP 410) 53 PROXY REQUEST REFUSED The request was for a resource at a domain not served by the server and the server does not accept proxy requests. 59 BAD REQUEST The server was unable to parse the client's request, presumably due to a malformed request. (cf HTTP 400) 60 CLIENT CERTIFICATE REQUIRED As per definition of single-digit code 6 in 1.3.2. 61 TRANSIENT CERTIFICATE REQUESTED The server is requesting the initiation of a transient client certificate session, as described in 1.4.3. The client should ask the user if they want to accept this and, if so, generate a disposable key/cert pair and re-request the resource using it. The key/cert pair should be destroyed when the client quits, or some reasonable time after it was last used (24 hours? Less?) 62 AUTHORISED CERTIFICATE REQUIRED This resource is protected and a client certificate which the server accepts as valid must be used - a disposable key/cert generated on the fly in response to this status is not appropriate as the server will do something like compare the certificate fingerprint against a white-list of allowed certificates. The client should ask the user if they want to use a pre-existing certificate from a stored "key chain". 63 CERTIFICATE NOT ACCEPTED The supplied client certificate is not valid for accessing the requested resource. 64 FUTURE CERTIFICATE REJECTED The supplied client certificate was not accepted because its validity start date is in the future. 65 EXPIRED CERTIFICATE REJECTED The supplied client certificate was not accepted because its expiry date has passed.