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//! A minimal CoAP server implementation built on [embedded_nal]. //! //! Usage and operation //! ------------------- //! //! Until the project has matured further, see [the example] for usage. The general steps are: //! //! [the example]: https://gitlab.com/chrysn/coap-message-demos/-/blob/master/examples/std_embedded_nal_minicoapserver.rs //! //! * Get a network stack with a UDP server socket that implments [embedded_nal::UdpServer] //! //! * Create a CoAP handler that implements [coap_handler::Handler]; the //! [coap_handler::implementations] module contains some building blocks //! (including some to combine handlers for individual resources into a handler that picks //! sub-handlers from the URI path). //! //! * Whenever there is indication that a request might have come in, call [poll] with the stack, //! the socket and the handler. This will accept the data from the socket, decode the CoAP //! message, pass it to the handler, and send back the response. //! //! It returns successful if a message was processed (or something came in that could be //! ignored), propagates out errors from the socket, and returns [WouldBlock](embedded_nal::nb::Error::WouldBlock) if it //! turms out the socket was *not* ready. //! //! By applying the belowmentioned constraints and exercising some of the liberties designed into //! CoAP, the server does not need to hold any state of its own. //! //! Caveats //! ------- //! //! * The server does not perform any amplification mitigation (and the handler can't for lack of //! remote information); use this only in environments where this is acceptable (e.g. in closed //! networks). //! //! * The server does not perform any message deduplication. All handler functions must therefore //! be idempotent. //! //! * The response logic is implemented using [nb](embedded_nal::nb) and does not attempt to store responses for //! later invocations. If a request comes in and the response can't be sent right away, it is //! discarded. //! //! * Being based on embedded-nal, it binds to the any-address but leaves the network stack to //! choose the sending address; this leads to subtle bugs when runnign on a system with multiple //! IP addresses. //! //! * Messages are created with as little copying as [embedded_nal] permits. For writable messages, //! that means that they need to be written to in ascending CoAP option number. This is in //! accordance with the implemented [coap_message::MinimalWritableMessage] and //! [coap_message::MutableWritableMessage] traits. //! //! That restriction enables this crate to not only be `no_std`, but to not require `alloc` //! either. //! //! Roadmap //! ------- //! //! The goal of this server is to become a component that can be used easily to bring CoAP //! connectivity to embedded devices at the bare minimum, while still being practically usable. //! //! This means that the amplification mitigation topic will need to be addressed, and that security //! backing must be catered for (probably by referring to an OSCORE/EDHOC mix-in). //! //! Other than that, this implementation's plan is to stay simple and utilize the optimizations //! CoAP offers, even if this means limiting the application (eg. to immediate responses, and to //! idempotent handlers). #![no_std] mod message; use num_derive::{FromPrimitive, ToPrimitive}; use num_traits::{FromPrimitive, ToPrimitive}; use embedded_nal::nb::Result; const COAP_VERSION: u8 = 1; /// Maximum size of a CoAP message we need to expect /// /// Also used in creating an output buffer as it's allocated the same way anyway. const MAX_SIZE: usize = 1152; #[derive(FromPrimitive, ToPrimitive, PartialEq)] enum Type { CON = 0, NON = 1, ACK = 2, RST = 3, } /// Attempt to process one message out of the given `socket` on a UDP `stack`. /// /// Any CoAP requests are dispatched to the handler. A response is built immediately and sent. /// /// Failure to perform any action immediately makes the function return `WouldBlock`, and it should /// be called again whenever there is indication that the network device is ready again. Any errors /// from the stack are propagated out. Errors in message processing (eg. invalid CoAP messages) are /// treated as per the protocol and are not indicated separately; they cause a successful return. /// /// Note that the caveats in the module description apply. pub fn poll<ST>( stack: &ST, socket: &mut ST::UdpSocket, handler: &mut impl coap_handler::Handler, ) -> Result<(), ST::Error> where ST: embedded_nal::UdpServer + ?Sized, { // Receive step let (extracted, addr, t_in, msgid, token) = { // too bad it needs to be initialized -- but see // https://github.com/rust-embedded-community/embedded-nal/issues/12 & co let mut buf: [u8; MAX_SIZE] = [0; MAX_SIZE]; let (len, addr) = stack.receive(socket, &mut buf)?; let buf = &buf[..len]; // All the UDP-specific format parsing if len < 4 { // Ignoring too short a request return Ok(()); } let ver = buf[0] >> 6; if ver != COAP_VERSION { // Mismatching version: MUST be silently ignored return Ok(()); } let t_in = Type::from_u8((buf[0] >> 4) & 0x03) .expect("Success guaranteed by numberof variants and input size"); if t_in == Type::ACK || t_in == Type::RST { // We're not a client and always respond piggy-backed, thus never expect ACKs or RSTs return Ok(()); } let tkl = buf[0] & 0x0f; let tkl: usize = tkl.into(); let code = buf[1]; let msgid = u16::from_be_bytes([buf[2], buf[3]]); if len < 4 + tkl { // Another form of too short a message return Ok(()); } let token = match heapless::Vec::<_, heapless::consts::U8>::from_slice(&buf[4..4 + tkl]) { Ok(t) => t, // MUST be processed as a message format error -- silently ignoring them. _ => return Ok(()), }; let tail = &buf[4 + tkl..]; let msg = coap_message_utils::inmemory::Message::new(code, tail); (handler.extract_request_data(&msg), addr, t_in, msgid, token) }; // Send step { // Could make this smaller based on asking the handler let mut buf: [u8; MAX_SIZE] = [0; MAX_SIZE]; let t_out = if t_in == Type::CON { Type::ACK } else { Type::NON }; buf[0] = (COAP_VERSION << 6) | ((t_out.to_u8().unwrap()) << 4) | (token.len() as u8); buf[2..4].copy_from_slice(&msgid.to_be_bytes()); buf[4..4 + token.len()].copy_from_slice(&token); let (header, tokentail) = buf.split_at_mut(4); let code = &mut header[1]; let tail = &mut tokentail[token.len()..]; let mut message = message::Message::new(code, tail); handler.build_response(&mut message, extracted); let written = 4 + token.len() + message.finish(); // Note that this `?` not only propagates errors but also WouldBlock -- and by doing so // discards the incoming request. stack.send_to(socket, addr, &mut buf[..written])?; }; Ok(()) }