[/
    Copyright (c) 2013-2017 Vinnie Falco (vinnie dot falco at gmail dot com)

    Distributed under the Boost Software License, Version 1.0. (See accompanying
    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
]

[section:client Handshaking (Clients)]

A WebSocket session begins when a client sends the HTTP
[@https://tools.ietf.org/html/rfc7230#section-6.7 Upgrade]
request for
[@https://tools.ietf.org/html/rfc6455#section-1.3 websocket],
and the server sends an appropriate HTTP response indicating that
the request was accepted and that the connection has been upgraded.
The HTTP Upgrade request must include the
[@https://tools.ietf.org/html/rfc7230#section-5.4 Host]
field, and the
[@https://tools.ietf.org/html/rfc7230#section-5.3 target]
of the resource to request. The stream member functions
[link beast.ref.websocket__stream.handshake.overload1 `handshake`] and
[link beast.ref.websocket__stream.async_handshake.overload1 `async_handshake`]
are used to send the request with the required host and target strings.
```
    ...
    ws.set_option(websocket::keep_alive(true));
    ws.handshake("localhost", "/");
```

The implementation will create and send an HTTP request that typically
looks like this:

[table WebSocket Upgrade HTTP Request
[[Serialized Octets][Description]]
[[
```
    GET / HTTP/1.1
    Host: localhost
    Upgrade: websocket
    Connection: upgrade
    Sec-WebSocket-Key: 2pGeTR0DsE4dfZs2pH+8MA==
    Sec-WebSocket-Version: 13
    User-Agent: Beast
```
][
    The host and target parameters become part of the Host field
    and request-target in the resulting HTTP request. The key is
    generated by the implementation. Callers may add fields or
    modify fields by providing a ['decorator], described below.
]]]

[heading Decorators]

If the caller wishes to add or modify fields, the member functions
[link beast.ref.websocket__stream.handshake_ex `handshake_ex`] and
[link beast.ref.websocket__stream.async_handshake_ex `async_handshake_ex`]
are provided which allow an additional function object, called a
['decorator], to be passed. The decorator is invoked to modify
the HTTP Upgrade request as needed. This example sets a subprotocol
on the request:
```
    void decorate(websocket::request_type& req)
    {
        req.fields.insert("Sec-WebSocket-Protocol", "xmpp;ws-chat");
    }
    ...
    ws.handshake_ex("localhost", "/", &decorate);

```

The HTTP Upgrade request produced by the previous call will look thusly:

[table Decorated WebSocket Upgrade HTTP Request
[[Serialized Octets][Description]]
[[
    ```
    GET / HTTP/1.1
    Host: localhost
    Upgrade: websocket
    Connection: upgrade
    Sec-WebSocket-Key: 2pGeTR0DsE4dfZs2pH+8MA==
    Sec-WebSocket-Version: 13
    Sec-WebSocket-Protocol: xmpp;ws-chat
    User-Agent: Beast
    ```
][
    Undefined behavior results if the decorator modifies the fields
    specific to perform the WebSocket Upgrade , such as the Upgrade
    and Connection fields.
]]]

[heading Filtering]

When a client receives an HTTP Upgrade response from the server indicating
a successful upgrade, the caller may wish to perform additional validation
on the received HTTP response message. For example, to check that the
response to a basic authentication challenge is valid. To achieve this,
overloads of the handshake member function allow the caller to store the
received HTTP message in an output reference argument as
[link beast.ref.websocket__response_type `response_type`]
as follows:
```
    websocket::response_type res;
    ws.handshake(res, "localhost", "/");
    if(! res.fields.exists("Sec-WebSocket-Protocol"))
        throw std::invalid_argument("missing subprotocols");
```

[endsect]