 Host Configuration Protocol (DHCP) [55]. Additionally, the client can simply attempt to send PCP requests to the default IPv4 and IPv6 routers that the client has configured for itself, or perhaps utilize anycast addresses as recently proposed [96]. 6.4.4 Protocol Messages The PCP protocol is based on the idea that PCP clients indicate which kind of services they would like, and PCP servers inform clients what the clients get. The control of what is allowed is fully on the server side [53]. In the basic operation the PCP client is sending requests for itself, but the PCP also supports requests sent by third parties when the PCP is used in fully trusted networks. PCP messages are transported using UDP with a maximum message size of 1100 bytes, which is small enough to allow some tunneling headers and still fit into the IPv6's minimum Maximum Transmission Unit (MTU) of 1280 bytes. The IP address in PCP messages is always transported in 128-bit fields for the sake of simplicity. When the fields include an IPv4 address, the IPv4-mapped IPv6 address format is used - see Section 3.1.8. In the following sections we look at the overview of the PCP. Full protocol specification features and details are available in the base specification [53]. PCP Requests All PCP requests include the request type, 32-bit requested lifetime, PCP client's own IP address, and then possible optional information elements. The following two requests are defined in the base specification [53]: MAP: The MAP request can be used for creating explicit endpoint independent mappings between internal and external addresses, protocols, and ports, before a transport layer connection is established. In the MAP request the client provides information about a random nonce, an upper layer protocol, and an internal port, and suggest an external IP address and port. PEER: The PEER request can be used to create a new endpoint dependent mapping, or control the lifetime of an existing mapping created by already established transport level connection, to a remote