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Application layered TCP Introduction
APIs » "raw" APIs

## Topics

| Application layered TCP Functions

## Detailed Description

## Overview

altcp (application layered TCP connection API; to be used from TCPIP thread) is an abstraction layer that prevents applications linking hard against the tcp.h functions while providing the same functionality. It is used to e.g. add SSL/TLS (see LWIP_ALTCP_TLS) or proxy-connect support to an application written for the tcp callback API without that application knowing the protocol details.

- This interface mimics the tcp callback API to the application while preventing direct linking (much like virtual functions).
- This way, an application can make use of other application layer protocols on top of TCP without knowing the details (e.g. TLS, proxy connection).
- This is achieved by simply including "lwip/altcp.h" instead of "lwip/tcp.h", replacing "struct tcp_pcb" with "struct altcp_pcb" and prefixing all functions with "altcp_" instead of "tcp_".

With altcp support disabled (LWIP_ALTCP==0), applications written against the altcp API can still be compiled but are directly linked against the tcp.h callback API and then cannot use layered protocols. To minimize code changes in this case, the use of altcp_allocators is strongly suggested.

## Usage

To make use of this API from an existing tcp raw API application:

- Include "lwip/altcp.h" instead of "lwip/tcp.h"
- Replace "struct tcp_pcb" with "struct altcp_pcb"
- Prefix all called tcp API functions with "altcp_" instead of "tcp_" to link against the altcp functions
- altcp_new (and altcp_new_ip_type / altcp_new_ip6) take an altcp_allocator_t as an argument, whereas the original tcp API functions take no arguments.
- An altcp_allocator_t allocator is an object that holds a pointer to an allocator object and a corresponding state (e.g. for TLS, the corresponding state may hold certificates or keys). This way, the application does not even need to know if it uses TLS or pure TCP, this is handled at runtime by passing a specific allocator.
- An application can alternatively bind hard to the altcp_tls API by calling altcp_tls_new or altcp_tls_wrap.
- The TLS layer is not directly implemented by lwIP, but a port to mbedTLS is provided.
- Another altcp layer is proxy-connect to use TLS behind a HTTP proxy (see altcp_proxyconnect.h)

## altcp_allocator_t

An altcp allocator is created by the application by combining an allocator callback function and a corresponding state, e.g.:

static const unsigned char cert[] = {0x2D, ... (see mbedTLS doc for how to create this)};
struct altcp_tls_config * conf = altcp_tls_create_config_client(cert, sizeof(cert));
altcp_allocator_t tls_allocator = {
altcp_tls_alloc, conf
};
altcp_tls_create_config_client
struct altcp_tls_config * altcp_tls_create_config_client(const u8_t *cert, size_t cert_len)
altcp_tls_alloc
struct altcp_pcb * altcp_tls_alloc(void *arg, u8_t ip_type)
Definition altcp_alloc.c:80
altcp_allocator_t
struct altcp_allocator_s altcp_allocator_t

## struct altcp_tls_config

The struct altcp_tls_config holds state that is needed to create new TLS client or server connections (e.g. certificates and private keys).

It is not defined by lwIP itself but by the TLS port (e.g. altcp_tls to mbedTLS adaption). However, the parameters used to create it are defined in altcp_tls.h (see altcp_tls_create_config_server_privkey_cert for servers and altcp_tls_create_config_client / altcp_tls_create_config_client_2wayauth for clients).

For mbedTLS, ensure that certificates can be parsed by 'mbedtls_x509_crt_parse()' and private keys can be parsed by 'mbedtls_pk_parse_key()'.