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(I don't know if anyone is interested in this, but let's give it a try. It started as a thought experiment, when I looked for 'lighter' protocols I can implement on the Pico W. I see Spartan mentioned here and there, and I wonder if there's any interest in going even more ... spartan. If you find this interesting, useful or fun, and have ideas how to improve this protocol, I'd love to hear from you at my-first-name@dimakrasner.com!)
Guppy is a simple unencrypted client-to-server protocol, for download of text and text-based interfaces that require upload of short input. It uses UDP and inspired by TFTP, DNS and Spartan. The goal is to design a simple, text-based protocol that is easy to implement and can be used to host a "guplog" even on a microcontroller (like a Raspberry Pi Pico W, ESP32 or ESP8266) and serve multiple requests using a single UDP socket.
Requests are always sent as a single packet, while responses can be chunked and each chunk must be acknowledged by the client. The protocol is designed for short-lived sessions that transfer small text files, therefore it doesn't allow failed downloads to be resumed, and doesn't allow upload of big chunks of data.
Implementers can choose their preferred complexity vs. speed ratio. Out-of-order transmission of chunked responses should allow extremely fast transfer of small textual documents, especially if the network is reliable. However, this requires extra code complexity, memory and bandwidth in both clients and servers. Simple implementations can achieve slow but reliable TFTP-like transfers with minimal amounts of code. Out-of-order transmission doesn't matter much if the server is a blog containing small posts (that fit in one or two chunks) and the client is smart enough to display the beginning of the response while receiving the next chunks.
v0.3.1:
(Response to feedback from tjp)
v0.3:
v0.2:
(Response to feedback from slondr)
v0.1:
If the URL is guppy://localhost/a and the response is "# Title 1\n":
> guppy://localhost/a\r\n (request) < 566837578 text/gemini\r\n# Title 1\n (response) > 566837578\r\n (acknowledgment) < 566837579\r\n (end-of-file) > 566837579\r\n (acknowledgment)
If the URL is guppy://localhost/a and input is "b c":
> guppy://localhost/a?b%20c\r\n < 566837578 text/gemini\r\n# Title 1\n > 566837578\r\n < 566837579\r\n > 566837579\r\n
If the URL is guppy://localhost/a and the response is "# Title 1\nParagraph 1\n":
> guppy://localhost/a\r\n < 566837578 text/gemini\r\n# Title 1\n > 566837578\r\n < 566837579\r\nParagraph 1 > 566837579\r\n < 566837580\r\n\n > 566837580\r\n < 566837581\r\n > 566837581\r\n
If the URL is guppy://localhost/a and the response is "# Title 1\nParagraph 1\n":
> guppy://localhost/a\r\n < 566837578 text/gemini\r\n# Title 1\n < 566837579\r\nParagraph 1 > 566837578\r\n < 566837579\r\nParagraph 1 > 566837579\r\n < 566837579\r\nParagraph 1 < 566837580\r\n\n > 566837580\r\n < 566837581\r\n > 566837581\r\n
If the URL is guppy://localhost/a and the response is "# Title 1\nParagraph 1\n":
> guppy://localhost/a\r\n < 566837578 text/gemini\r\n# Title 1\n < 566837579\r\nParagraph 1 (server sends packet 566837579 without waiting for the client to acknowledge 566837578) > 566837578\r\n < 566837579\r\nParagraph 1 (server sends packet 566837579 again because the client didn't acknowledge it) < 566837580\r\n\n (server sends packet 566837580 without waiting for the client to acknowledge 566837579) > 566837579\r\n < 566837580\r\n\n (server sends packet 566837580 again because the client didn't acknowledge it) < 566837581\r\n (server sends packet 566837581 without waiting for the client to acknowledge 566837580) > 566837580\r\n < 566837581\r\n (server sends packet 566837581 again because the client didn't acknowledge it) > 566837581\r\n
If the URL is guppy://localhost/a and the response is "# Title 1\nParagraph 1\n":
> guppy://localhost/a\r\n < 566837578 text/gemini\r\n# Title 1\n > 566837578\r\n < 566837578 text/gemini\r\n# Title 1\n (acknowledgement arrived after the server re-transmitted the success packet) < 566837579\r\nParagraph 1 < 566837579\r\nParagraph 1 (first continuation packet was lost) > 566837579\r\n < 566837580\r\n\n > 566837580\r\n > 566837580\r\n (first acknowledgement packet was lost and the client re-transmitted it while waiting for a continuation or EOF packet) < 566837581\r\n (server sends EOF after receiving the re-transmitted acknowledgement packet) < 566837581\r\n (first EOF packet was lost while server waits for client to acknowledge EOF) > 566837581\r\n
> guppy://localhost/a\r\n < 0 guppy://localhost/b\r\n
> guppy://localhost/a\r\n < 0 /b\r\n
> guppy://localhost/search\r\n < 1 No search keywords specified\r\n
Sample server in Go, with out-of-order transsmission of up to 8 response chunks, 512b each
Sample client in C, with buffering of 8 response chunks, up to 4K each
git clone -b guppy --recursive https://github.com/dimkr/gplaces cd gplaces make PREFIX=/tmp/guppy CONFDIR=/tmp/guppy/etc install /tmp/guppy/bin/gplaces guppy://hd.206267.xyz
"Must" means a strict requirement, a rule all conformant Guppy client or server must obey.
"Should" means a recommendation, something minimal clients or servers should do.
"May" means a soft recommendation, something good clients or servers should do.
If no port is specified in a guppy:// URL, clients and servers must fall back to 6775 ('gu').
Interactive clients must be able to display text/plain documents.
Interactive clients must be able to parse text/gemini (without the Spartan := type) documents and allow users to follow links.
If encoding is unspecified via the charset parameter of the MIME type field, the client must assume it's UTF-8. Clients which support ASCII but do not support UTF-8 may render documents with replacement characters.
In Guppy, all URLs can be (theoretically) accompanied by user-provided input. The client must provide the user with means for sending a request with user-provided input, to any link line.
Server authors should inform users when input is required and describe what kind of input, using the link's user-friendly description.
If input is expected but not provided by the user, the server must respond with an error packet.
The protocol is unencrypted, and these concerns are beyond the scope of this document.
Clients and servers may restrict packet size, to allow slower but more reliable transfer. However, servers must transmit responses larger than 512 bytes in chunks of at least 512 bytes. If the response is less than 512 bytes, servers must send it as one piece, without continuation packets.
Requests (the URL plus 2 bytes for the trailing \r\n) must fit in 2048 bytes.
Client and servers should handle timeouts gracefully and must distinguish between timeout and proper termination of the session.
Servers should transmit multiple packets at once, instead of waiting for the client to acknolwedge a packet before sending the next one.
Servers may limit the number of packets awaiting acknowledgement from the client, and wait with sending of the next continuation packets until the client acknowledges some or even all unacknowledged packets.
The server must not assume that lost continuation packet n does not need to be retransmitted, if packet n+1 is acknowledged by the client.
Clients should re-transmit request and acknowledgement packets after a while, if nothing is received from the server. Servers must ignore duplicate acknowledgement packets and additional request packets arriving from the each source address and source port combination, until the previous session has ended.
Trivial clients may ignore out-of-order packets and wait for the next packet to be retransmitted if received but ignored, at the cost of slow transfer speeds.
Clients that receive continuation or end-of-file packets before the success packet should cache and acknowledge the packets, to prevent the server from sending them again and reduce overall transfer time.
Clients may limit the number of buffered packets and keep up to x chunks of the response in memory, when the server transmits many out-of-order packets. However, clients that save a limited number of out-of-order packets must leave room for the first response packet instead of failing when many continuation packets exhaust the buffer.
Clients should start displaying the response as soon as the first chunk is received.
There are 7 packet types:
All packets begin with a "header", followed by \r\n.
TL;DR -
host/path input\r\n
The query part specifies user-provided input, percent-encoded.
The server must respond with a success, redirect or error packet.
seq type\r\n data
The sequence number is an arbitrary number between 2 and 2147483647 (maximum value of a signed 32-bit integer), followed by a space character (0x20 byte). Clients must not assume that the sequence number cannot begin with 1 and confuse success packets with sequence number 10 or 123 with error packets. Servers must pick a low enough sequence number, so the sequence number of the end-of-file packet does not exceed 2147483647.
The type field specifies the response MIME type and must not be empty.
The server may send a chunked response, by sending one or more continuation packets.
seq\r\n data
The sequence number must increase by 1 in every continuation packet.
If the client begins to display a textual response before the entire response has been received, it must not assume that the response is split on a line boundary: a long line may be sent in multiple response packets.
In addition, the client must not assume that response packets arrive in the correct order: it may receive a continuation packet before the success packet containng the first chunk of the response.
seq\r\n
The server must mark the end of the transmission by sending a continuation packet without any data, even if the response fits in a single packet.
Clients must wait for the "end of file" packet, to differentiate between timeout, a partially received response and a successfully received response.
seq\r\n
The client must acknowledge every success, continuation or EOF packet by echoing its sequence number back to the server. If the server keeps sending the same continuation packet, the acknowledgement packets for it were probably lost and the client must retry acknowledgement to avoid additional waste of bandwidth.
The server may attempt re-transmission of a success, continuation or EOF packet after a while, if not acknowledgement by the client.
0 url\r\n
The URL may be relative.
The client must inform the user of the redirection.
The client may remember redirected URLs. The server must not assume clients don't do this.
The client should limit the number of redirects during the handling of a single request.
The client may forbid a series of redirects, and may prompt the user to confirm each redirect.
1 error\r\n
Clients must display the error to the user.
Using an error packet to tell the user they need to request a URL with input mixes telling the user there was an error (e.g. something broke) with an instruction to the user
This is intentional: errors are for the user, not for the client. They should be human-readable.
This also means that for any URL that should accept user input, the author would need to configure the Guppy server to return an error, which is kind of onerous.
Gemini servers respond with status code 1x when they expect input but none is provided. This is a similar mechanism, but without introducing a special status code requiring clients to implement the "retry the request after showing a prompt" logic.
Using an Error Packet to signify user input:
* user downloads gemtext
...
There's a missing first step here: user follows a link that says "enter search keywords" or "new post", then decides to attach input to the request.
It seems like this probably was changed but the acknowledgement section wasn't updated.
[...]
This contradicts just about everything said elsewhere about out-of-order packet handling so it probably just wasn't updated in some prior iteration.
True.
Note also that with the spec-provided 512 byte minimum chunk size, storing the sequence number in an unsigned 16-bit number caps the
guaranteed download size at 16MB.
True. Although we're talking about small text files here, I increased the sequence number range to allow larger transfers.
One possible (but not guaranteed) ack failure indication would be receiving a re-transmission of an already-acked packet, but this is something the spec elsewhere
suggests clients ignore, and is a pretty awkward heuristic to code.
Clients must re-acknowledge the packet if received again, so the server can stop sending it and continue if it's waiting for it to be acknowledged before it sends the next ones.
The server won't be able to distinguish re-transmission of the "same" request packet from a legitimate re-request [...]. These are indistinguishable because request packets don't have a sequence number.
The server can use the source address and port combination to ignore additional requests in the same "session", hence no application layer request ID is needed. That's one thing that UDP does provide :)
It's a classic mistake to look at complicated machinery like TCP and assume it's bloated.
I'm not assuming it's bloated, and Guppy is not a reaction to the so-called "bloat" of TCP. It's an experiment in designing a protocol simpler than Gopher and Spartan, which provides a similar feature set but with faster transfer speeds (for small documents) and using a much simpler software stack (i.e. including the implementation of TCP/IP, instead of judging simplicity by the application layer protocol alone).
Even though TCP contains a more complicated and convoluted solution to the problems of re-ordering and re-transmission, its use would be a massive simplification both for this spec and especially for implementors.
Implementors can implement a TFTP-style client, one that sends a request, waits for a single packet, acknowledges it, waits for the next packet and so on. If the client displays the first chunk of the response while waiting for the next one, and the document fits in 3-4 response packets, such a client should be good enough for most content and most users. Clients are compatible with servers that don't understand out-of-order transmission, and vice versa, so it is possible to implement a super simple but still useful Guppy client.