70 Networking and Online Games: Understanding and Engineering
72 Networking and Online Games: Understanding and Engineering Multiplayer Internet Games Ethernet LAN and a nominally 56-Kbps V.90 dial-up connection using Point-to-Point Protocol (PPP) [RFC1661]): On the Ethernet link, a 1500-byte IP packet becomes 1526 bytes long (8 bytes of ethernet preamble, 12 bytes for source and destination MAC address, two bytes ethernet protocol type and 4 bytes trailing Cyclic Redundancy Check [CRC]), or 12,208 bits. At 100 Mbps, it takes 122 microseconds to transmit the frame containing this packet. On a V.90 dial-up link, the uplink is limited to 33.6 Kbps while the downlink rarely exceeds 51 Kbps. If we further assume PPP encapsulation of 8 bytes, the 1500-byte IP packet requires 1508 8 = 12,064 bits to transmit. Thus, a 1500-byte IP packet takes 359 ms to transmit towards the ISP (upstream) and 237 ms towards the client (downstream). Serialisation latency is primarily an issue with low-speed links common in consumer access networks (for example, dial-up modem service or consumer Asymmetric Digital Subscriber Link, ADSL). Using the above numbers, even a small 40-byte IP packet (48 bytes including PPP overhead) takes 11.4 ms on a V.90 upstream and 7.5 ms on a V.90 downstream (contributing 19 ms to any RTT measured using 40-byte packets). A similar situation occurs on high speed links when your ISP imposes temporary rate caps. For example, consider an ISP using ADSL2 + to offer 4 Mbps downstream service and a customer who has exceeded the download limit for the month. The ISP temporarily applies a rate of 64 Kbps until the end of the month, imposed at the IP packet level. Although each packet is still individually transmitted at 4 Mbps, the ISP achieves a 64Kbps long-term rate cap by limiting the number of packets per second that can be sent. The effective serialisation delay is as though the ADSL2 + link was literally running at 64 Kbps. Serialisation latency should only be calculated once (at one end of the link) since the receiving end is pulling bits off the link at the same rate that the transmitting end is sending them. Aside from a slight offset in time due to propagation delay, the transmission and reception processes occur essentially concurrently. A rough rule of thumb for serialisation delay is latency (ms) = 8*(link layer frame length in bytes)/(link speed in Kbps) (Note that for some link technologies, such as Asynchronous Transfer Mode (ATM) and Data over Cable Service Interface Specification (DOCSIS), the relationship between link layer frame length and IP packet length is nonlinear and nontrivial to calculate.) 5.2.3 Queuing Delays One of the core underlying assumptions of the Internet s best effort philosophy is that everyone s traffic is largely bursty and uncorrelated, allowing us to benefit from a concept known as statistical multiplexing. Multiplexing occurs when multiple inbound streams of packet traffic converges on a single outbound link at a particular router or switch. The inbound packets are multiplexed (interleaved in time) onto the outbound link. However, unlike traditional telephone company networks IP routers do not prearrange guaranteed timeslots on the outbound link for the competing inbound packet streams.
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