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Abstract protocols and transports

Overview

Until now protocol implementations in lws have been done directly to the network-related apis inside lws.

In an effort to separate out completely network implementation details from protocol specification, lws now supports "abstract protocols" and "abstract transports".

The concept is that the implementation is split into two separate chunks of code hidden behind "ops" structs... the "abstract protocol" implementation is responsible for the logical protocol operation and reads and writes only memory buffers.

The "abstract transport" implementation is responsible for sending and receiving buffers on some kind of transport, and again is hidden behind a standardized ops struct.

In the system, both the abstract protocols and transports are found by their name.

An actual "connection" is created by calling a generic api lws_abs_bind_and_create_instance() to instantiate the combination of a protocol and a transport.

This makes it possible to confidently offer the same protocol on completely different transports, eg, like serial, or to wire up the protocol implementation to a test jig sending canned test vectors and confirming the response at buffer level, without any network. The abstract protocol itself has no relationship to the transport at all and is completely unchanged by changes to the transport.

In addition, generic tokens to control settings in both the protocol and the transport are passed in at instantiation-time, eg, controlling the IP address targeted by the transport.

lws SMTP client support has been rewritten to use the new scheme, and lws provides a raw socket transport built-in.

Public API

The public api for defining abstract protocols and transports is found at

`lws_abs_t`

The main structure that defines the abstraction is lws_abs_t, this is a name and then pointers to the protocol and transport, optional tokens to control both the protocol and transport, and pointers to private allocations for both the protocol and transport when instantiated.

The transport is selected using

LWS_VISIBLE LWS_EXTERN const lws_abs_transport_t *
lws_abs_transport_get_by_name(const char *name);

and similarly the protocol by

LWS_VISIBLE LWS_EXTERN const lws_abs_protocol_t *
lws_abs_protocol_get_by_name(const char *name);

At the moment only "raw-skt" is defined as an lws built-in, athough you can also create your own mock transport the same way for creating test jigs.

transport op	meaning
`tx()`	transmit a buffer
`client_conn()`	start a connection to a peer
`close()`	request to close the connection to a peer
`ask_for_writeable()`	request a `writeable()` callback when tx can be used
`set_timeout()`	set a timeout that will close the connection if reached
`state()`	check if the connection is established and can carry traffic

These are called by the protocol to get things done and make queries through the abstract transport.

protocol op	meaning
`accept()`	The peer has accepted the transport connection
`rx()`	The peer has sent us some payload
`writeable()`	The connection to the peer can take more tx
`closed()`	The connection to the peer has closed
`heartbeat()`	Called periodically even when no network events

These are called by the transport to inform the protocol of events and traffic.

Instantiation

The user fills an lws_abs_t and passes a pointer to it to lws_abs_bind_and_create_instance() to create an instantiation of the protocol + transport.

`lws_token_map_t`

The abstract protocol has no idea about a network or network addresses or ports or whatever... it may not even be hooked up to one.

If the transport it is bound to wants things like that, they are passed in using an array of lws_token_map_t at instantiation time.

For example this is passed to the raw socket protocol in the smtp client minimal example to control where it would connect to:

static const lws_token_map_t smtp_abs_tokens[] = {
{
	.u = { .value = "127.0.0.1" },
	.name_index = LTMI_PEER_DNS_ADDRESS,
}, {
	.u = { .lvalue = 25l },
	.name_index = LTMI_PEER_PORT,
}};

Steps for adding new abstract protocols

add the public header in ./include/libwebsockets/abstract/protocols/
add a directory under ./lib/abstract/protocols/
add your protocol sources in the new directory
in CMakeLists.txt:
- add an LWS_WITH_xxx for your protocol
- search for "using any abstract protocol" and add your LWS_WITH_xxx to the if so it also sets LWS_WITH_ABSTRACT if any set
- add a clause to append your source to SOURCES if LWS_WITH_xxx enabled
add your lws_abs_protocol to the list available_abs_protocols in ./lib/abstract/abstract.c

Steps for adding new abstract transports

add the public header in ./include/libwebsockets/abstract/transports/
add your transport sources under ./lib/abstract/transports/
in CMakeLists.txt append your transport sources to SOURCES if LWS_WITH_ABSTRACT and any other cmake conditionals
add an extern for your transport lws_protocols in ./lib/core-net/private.h
add your transport lws_protocols to available_abstract_protocols in ./lib/core-net/vhost.c
add your lws_abs_transport to the list available_abs_transports in ./lib/abstract/abstract.c

Protocol testing

unit tests

lws features an abstract transport designed to facilitate unit testing. This contains an lws_sequencer that performs the steps of tests involving sending the protocol test vector buffers and confirming the response of the protocol matches the test vectors.

test-sequencer

test-sequencer is a helper that sequences running an array of unit tests and collects the statistics and gives a PASS / FAIL result.

See the SMTP client api test for an example of how to use.

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