80 Networking and Online Games: Understanding and Engineering
80 Networking and Online Games: Understanding and Engineering Multiplayer Internet Games reveal links that include significant propagation delays. Traceroute can assess paths using different sized probe packets, which can be used to reveal serialisation delay sensitivities along a path. (As of writing, Windows XP s tracert did not allow configuration of the probe packet s size.) Both traceroute and tracert can resolve the IP addresses into domain names of routers seen along a path. If the ISPs have used human-readable names, you can sometimes infer things about the geographical path being followed (e.g. if city names are encoded in the domain names of the router interfaces seen by traceroute). There are many web sites around the Internet that offer traceroute facilities from their location. You can find a list of pointers to such sites at http://www.traceroute.org For example, Figure 5.4 shows one of the listed sites (located at Telstra in Canberra, Australia) revealing the effect of propagation delay when performing a traceroute to www.lucent.com (192.11.226.2, based in New Jersey, USA). The traceroute output shows about 5 ms between first and seventh hop, then ~145 ms between seventh and eighth hops (from Sydney to Los Angeles) followed by another jump between Los Angeles and New York of ~70 ms (essentially next door to New Jersey). Note that the ~145-ms RTT between Sydney and Los Angeles is substantially higher than the ~80 ms estimate in Section 5.2.1. This can be attributed to the lower propagation speed of signals in optical fibre, queuing delays over multiple hops and the fibre s indirect physical path between Sydney and Los Angeles being much longer than 12,000 km. Another approach is to use information gathered by a modified game server. Figure 5.5 shows one published experiment where RTT samples from an active Quake III Arena server were used to plot the average jitter (per map played) against the average latency (per map played) [ARM2004]. One of the paper s conclusions was that jitter typically never exceeded 20 % of the average latency. Another observation from Figure 5.5 is that there are two broad clusters one where jitter is low regardless of latency (out to 300 ms latency) and another where jitter is roughly proportional to latency. The former is attributed to paths where most latency is propagation delay. The latter is attributed to 1 FastEthernet6-0.civ-service1.Canberra.telstra.net (203.50.1.65) 0.225 ms 0.193 ms 0.268 ms [..] 7 i-7-0.syd-core02.net.reach.com (202.84.221.90) 5.457 ms 5.636 ms 5.349 ms 8 i-0-0.wil-core02.net.reach.com (202.84.144.101) 153.923 ms 153.935 ms 154.057 ms [..] 11 0.so-3-0-0.CL1.LAX15.ALTER.NET (152.63.117.90) 153.84 ms 154.67 ms 154.303 ms 12 0.so-5-0-0.XL1.NYC9.ALTER.NET (152.63.0.174) 227.725 ms 265.947 ms 227.653 ms [..] 17 192.11.226.2 (192.11.226.2) 228.585 ms 229.29 ms 227.779 ms Figure 5.4 Traceroute from Australia to the USA showing long-haul propagation delays
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