56 Networking and Online Games: Understanding and Engineering
58 Networking and Online Games: Understanding and Engineering Multiplayer Internet Games For the curious reader: If you are running Windows XP or similar, entering arp -a in a console window will show the current local ARP cache entries. Under many versions of Unix (for example, FreeBSD or Linux) the command arp -an will show the current local ARP cache entries. 4.2.3.3 Time to Live Transient errors in router forwarding tables can sometimes create loops in the IP network, known as routing loops. Routing loops tend to occur shortly after topology changes, while the network s routing protocol converges on a new set of shortest-path trees. Routing loops often act like black holes in the network packets head into the region of the routing loop, and then get stuck, consuming bandwidth as they circulate. Under extreme circumstances, the routing loop can disrupt the routing protocol itself, by saturating links carrying routing protocol update messages. To prevent endless looping, IP packets carry an 8-bit Time to Live (TTL) field (see Figure 4.5). In practice, the TTL represents hops to live a limit on the maximum number of router hops a packet can traverse before it expires in transit. A packet s TTL field is set to a nonzero value by the source, and is decremented by one every time the packet passes through a router. The packet is discarded when its TTL field is decremented to zero (whether or not it has reached its final destination). If the source sets the TTL too low, some distant regions of the Internet may become unreachable. (Indeed, there were examples of this occurring with a popular PC operating system s default TTL in the early 1990s as the Internet became more topologically convoluted.) Setting the TTL too high increases the potential disruption a source s packets can cause during routing loops (by increasing the length of time the packets stay in transit around the loop). Many operating systems today set their initial TTL to 64 or some multiple of 32 above that (more by historical quirk than any particular mandatory requirement). 4.2.3.4 Maximum Transmission Units and IP Fragmentation Although in principle, IP packets may be as large as 64 K bytes, most link layer technologies impose a substantially smaller limit on link level frame size. For example, Ethernet imposes a limit of 1500 bytes on the size of IP packets that can be carried in an Ethernet frame. The underlying link layer s frame size limit is reflected at the IP layer by a parameter known as the Maximum Transmission Unit (MTU). When forwarding an IP packet larger than the link s MTU, IP interfaces must perform IP fragmentation chopping (fragmenting) the IP packet up into a sequence of smaller IP packets that all fit under the MTU limit. IP fragmentation occurs underneath the TCP or UDP layer, which allows the source UDP-or TCP-based applications to be unaware of the actual MTUs of links along the path to the destination. The ultimate destination is responsible for reassembling the fragments into the original packet, and then treating the reassembled IP packet as though it had arrived in one piece. IP fragmentation tends to occur when a packet s path originates on a link with a large MTU, and then at some point along the route passes across a link with a smaller MTU. It is not considered a good thing, as it creates less efficient data transfer along the path [RFC1191][RFC1981].
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