Michiel De WitteandClaude Opus 5
5e86d78e6d
A request handler that leaves the response code unset asks for no response to be sent. That is honoured for a piggybacked ACK (the token gets stripped, so an empty ACK goes out) and for a non-confirmable or reliable response (dropped in no_response()), but not for a separate response set up with coap_register_async(). There the response is turned into a CON, and a 0.00 CON was put on the wire. An Empty message is a ping (RFC 7252 4.2), not a response, and the PDU still carries the request's token, which RFC 7252 4.1 does not allow. Being confirmable, it was then retransmitted up to MAX_RETRANSMIT times, usually at a client port that had long gone away. Drop it like the other two cases, delete it without logging it as a dropped response since the application asked for this, and document in coap_handler(3) that a separate response is not sent either. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
libcoap: A C implementation of the Constrained Application Protocol (RFC 7252)
Copyright (C) 2010—2026 by Olaf Bergmann bergmann@tzi.org and others
ABOUT LIBCOAP
libcoap is a C implementation of a lightweight application-protocol for devices that are constrained their resources such as computing power, RF range, memory, bandwidth, or network packet sizes. This protocol, CoAP, is standardized by the IETF as RFC 7252. For further information related to CoAP, see https://coap.space or CoAP Wiki.
You might want to check out libcoap-minimal for usage examples.
DOCUMENTATION
Documentation and further information can be found at https://libcoap.net.
PACKAGE CONTENTS
This package contains a protocol parser and basic networking functions for platforms with support for malloc() and BSD-style sockets. In addition, there is support for Contiki-NG, Espressif/ESP-IDF, LwIP and RIOT-OS hosted environments.
The following RFCs are supported
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RFC7390: Group Communication for the Constrained Application Protocol (CoAP)
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RFC7641: Observing Resources in the Constrained Application Protocol (CoAP)
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RFC7959: Block-Wise Transfers in the Constrained Application Protocol (CoAP)
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RFC7967: Constrained Application Protocol (CoAP) Option for No Server Response
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RFC8132: PATCH and FETCH Methods for the Constrained Application Protocol (CoAP)
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RFC8323: CoAP (Constrained Application Protocol) over TCP, TLS, and WebSockets
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RFC8516: "Too Many Requests" Response Code for the Constrained Application Protocol
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RFC8613: Object Security for Constrained RESTful Environments (OSCORE)
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RFC8768: Constrained Application Protocol (CoAP) Hop-Limit Option
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RFC8974: Extended Tokens and Stateless Clients in the Constrained Application Protocol (CoAP)
The following Internet Draft is supported
There is (D)TLS support for the following libraries
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OpenSSL (Minimum version 1.1.0) [PKI, PSK and PKCS11]
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GnuTLS (Minimum version 3.3.0) [PKI, PSK, RPK(3.6.6+) and PKCS11]
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Mbed TLS (Minimum version 2.7.10) [PKI and PSK]
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wolfSSL (Minimum version 5.2.0) [PKI, PSK, RPK(5.6.4+) and DTLS1.3(5.4.0+)]
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TinyDTLS [PSK and RPK] [DTLS Only]
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openHiTLS (Minimum version 0.4.0) [PKI and PSK]
The examples directory contain a CoAP client, CoAP Resource Directory server and a CoAP server to demonstrate the use of this library.
BUILDING
Further information can be found at https://libcoap.net/install.html and BUILDING.
LICENSE INFORMATION
This library is published as open-source software without any warranty of any kind. Use is permitted under the terms of the simplified BSD license. It includes public domain software. libcoap binaries may also include open-source software with their respective licensing terms. Please refer to LICENSE for further details.