Support for dhcp clients is implemented by files in

  /usr/lib/network/dhcp/

The file name determines the name of the client for the profile, so
support for a client named dhcpcd is provided by the file:

  /usr/lib/network/connections/dhcpcd

Files that implement support for a connection type should NOT be
executable. Such files should contain valid Bash code, among which two
functions, namely <client_name>_start and <client_name>_stop. For
the client named dhcpcd these would be:

  dhcpcd_start
  dhcpcd_stop

These functions are responsible for starting and stopping the dhcp
client. When the functions are called, three bash files are already
sourced, so all functions and variables in those files are available.
The readily sourced files are:

  /usr/lib/network/network
  /usr/lib/network/globals
  /etc/netctl/<profile>

Here, <profile> is the profile file specifying the desired network
configuration.

When called, the start and stop functions get as their first argument
the version of the IP protocol that the dhcp client is expected to use.
In the case of starting the client for IPv6, an additional second
argument 'noaddr' may be supplied, which indicates that the dhcp client
should configure everything but an IP address.