TCP IP Illustrated

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Unformatted text preview: riable useloopback and initializes it to 1. If this variable is set to 0, however, the Ethernet driver sends local packets onto the network instead of sending them to the loopback driver. This may or may not work, depending on your Ethernet interface card and device driver. 2.8 MTU As we can see from Figure 2.1, there is a limit on the size of the frame for both Ethernet encapsulation and 802.3 encapsulation. This limits the number of bytes of data to 1500 and 1492, respectively. This characteristic of the link layer is called the MTU, its maximum transmission unit. Most types of networks have an upper limit. If IP has a datagram to send, and the datagram is larger than the link layer's MTU, IP performs fragmentation, breaking the datagram up into smaller pieces (fragments), so that each fragment is smaller than the MTU. We discuss IP fragmentation in Section 11.5. Figure 2.5 lists some typical MTU values, taken from RFC 1191 [Mogul and Deering 1990]. The listed MTU for a point-to-point link (e.g., SLIP or PPP) is not a physical characteristic of the network media. Instead it is a logical limit to provide adequate response time for interactive use. In the Section 2.10 we'll see where this limit comes from. In Section 3.9 we'll use the netstat command to print the MTU of an interface. Network Hyperchannel 16 Mbits/sec token ring (IBM) 4 Mbits/sec token ring (IEEE 802.5) FDDI Ethernet IEEE 802.3/802.2 X.25 Point-to-Point (low delay) MTU (bytes) 65535 17914 4464 4352 1500 1492 576 296 Figure 2.5 Typical maximum transmission units (MTUs). 2.9 Path MTU When two hosts on the same network are communicating with each other, it is the MTU of the network that is important. But when two hosts are communicating across multiple networks, each link can have a different MTU. The important numbers are not the MTUs of the two networks to which the two hosts connect, but rather the smallest MTU of any data link that packets traverse between the two hosts. This is called the path MTU. The path MTU between any two hosts need not be constant....
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