libwebsockets/lib/abstract

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 abstract protocol implementation only operates on callback events and reads and writes to buffers... separately when it is instantiated, it can be bound to an "abstract transport" which handles all the details of sending and receiving on whatever the transport is.

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.

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 transports.h

lws_abstract_t

The main structure that defines the abstraction is lws_abstract_t, this is a name and then about a dozen function pointers for various events and operations.

The transport defines about half of these and exports this lws_abstract_t * via its name, it can be retreived using

LWS_VISIBLE LWS_EXTERN const lws_abstract_t *
lws_abstract_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.

When you instantiate an abstract protocol, it defines the other half of the lws_abstract_t operations and is combined with the transport lws_abstract_t * to get the full set of operations necessary for the protocol to operate on the 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.

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,
}};