62 Networking and Online Games: Understanding and Engineering
64 Networking and Online Games: Understanding and Engineering Multiplayer Internet Games manually configured to map 128.80.6.200:28000 to 192.168.0.13:27960. The master server sees the Quake III Arena server at 128.80.6.200:28000. However, without special configuration the NAT/NAPT router will not allow new players to actually connect through 128.80.6.200:28000 to the game server itself. We discuss this again in Chapter 12.) NAT/NAPT has its admirers and detractors. Nevertheless, it does serve a purpose for private networks that cannot afford lots of public IP addresses or wish to avoid renumbering of their internal networks on a regular basis. Consumer home routers/gateways invariably support some form of NAT/NAPT functionality. Demand is driven by the deployment of broadband IP access over Asymmetric Digital Subscriber Line (ADSL) or cable modem services, and the fact that many homes have multiple computers. Typically the home router has one Ethernet port to the ADSL modem or cable modem, and one or more Ethernet ports for the internal, home network. (To assist in address management of a small home network, many home routers also support the dynamic host configuration protocol described in the following section.) 4.3.3 Dynamic Host Configuration Protocol The DHCP [RFC2131] automates the configuration of various fundamental parameters hosts need to know before they can become functional members of an IP network. For example, every host minimally needs to know the following: The host s own IP address The subnet mask for the subnet on which it sits The IP address of at least one router to be used as the default route for all traffic destined outside the local subnet. Without these pieces of information, a host cannot properly set the source IP address of its outbound packets, cannot know if it is the destination of inbound unicast packets, and cannot build a basic forwarding table that differentiates between on-link and off-link next hops. DHCP allows hosts to automatically establish the preceding information, and provides two key benefits: The need for manual intervention is minimised when installing and turning on new hosts. IP addresses can be leased for configurable periods of time to temporary hosts. Minimising administrative burdens clearly saves money and time, and increases overall convenience. The benefits of dynamic address leasing become apparent in networks where not all hosts are attached and operational at the same time. 4.3.3.1 Configuring a Host DHCP is a client server protocol. Each host has a DHCP client embedded in it, and the local network has one or more nodes running DHCP servers. DHCP runs on top of UDP, which at first glance suggests a Catch-22 situation with the unconfigured IP
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