TCP IP Illustrated

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Unformatted text preview: e total length of the IP datagram in bytes. Using this field and the header length field, we know where the data portion of the IP datagram starts, and its length. Since this is a 16-bit field, the maximum size of an IP datagram is 65535 bytes. (Recall from Figure 2.5 that a Hyperchannel has an MTU of 65535. This means there really isn't an MTU-it uses the largest IP datagram possible.) This field also changes when a datagram is fragmented, which we describe in Section 11.5. Although it's possible to send a 65535-byte IP datagram, most link layers will fragment this. Furthermore, a host is not required to receive a datagram larger than 576 bytes. TCP divides the user's data into pieces, so this limit normally doesn't affect TCP. With UDP we'll encounter numerous applications in later chapters (RIP, TFTP, BOOTP, the DNS, and SNMP) that limit themselves to 512 bytes of user data, to stay below this 576-byte limit. Realistically, however, most implementations today (especially those that support the Network File System, NFS) allow for just over 8192-byte IP datagrams. The total length field is required in the IP header since some data links (e.g., Ethernet) pad small frames to be a minimum length. Even though the minimum Ethernet frame size is 46 bytes (Figure 2.1), an IP datagram can be smaller. If the total length field wasn't provided, the IP layer wouldn't know how much of a 46-byte Ethernet frame was really an IP datagram. The identification field uniquely identifies each datagram sent by a host. It normally increments by one each time a datagram is sent. We return to this field when we look at fragmentation and reassembly in Section 11.5. Similarly, we'll also look at the flags field and the fragmentation offset field when we talk about fragmentation. RFC 791 [Postel 1981a] says that the identification field should be chosen by the upper layer that is having IP send the datagram. This implies that two consecutive IP datagrams, one generated by TCP and one generated by UDP, can have the same identification field. While this i...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.

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