| 单词 | Figure I-1 |
| 释义 | Figure I-1 The total size of the IPv4 datagram, including the header (shown in Figure I-1), can be up to 65,535 octets in length.At a minimum, all networks must support a packet of at least 576 octets.As illustrated in Figure I-1, the minimum size of the IP header is 20 octets.The IPv4 datagram contains the following fields: • VERS (VERSion): 4 bits identifying the IP version number.The version number is 4. • IHL (Internet Header Length): 4 bits indicating the length of the header, which has a minimum value of five 32-bit words, or 20 octets.The IHL also provides a measurement of where the TCP header, or other higher-layer, header begins. • Type of Service (ToS): 8 bits indicating the quality of service (QoS) requested for the datagram. Although TCP/IP networks currently do not provide guaranteed QoS, the networks will attempt to honor QoS requests in terms of parameters that include packet precedence (i.e., priority), low delay, high throughput, and high reliability. • Total Length: 16 bits describing the total length of the datagram, including the IP header.The maximum size is 65,535 octets (216–1, with 0 not considered as it has no value). All network hosts must be able to handle a datagram of at least 576 octets. • Identification: 16 bits that are used in fragmentation control. In the event that the receiving network cannot accommodate a datagram of the specified total length, that datagram must be frag- mented. Each fragment must contain a copy of the identification field and certain other fields in the IP header so they can be reassociated and the datagram can be reconstituted. • Flags: 3 bits that define the manner in which the fragmentation occurs.The first bit always is set at 0.The second bit defines whether fragmentation is permitted.The third bit is used to identify the last fragment in a series of fragments. • Fragment Offset: 13 bits that identify where the fragment fits in the complete set of fragments that comprise the original datagram.This field is used to sequence the fragments correctly, as they may arrive at the destination device out of sequence. • Time To Live (TTL): 8 bits that specify the length of time in seconds that the datagram can live in the Internet system.The maximum length of time is 255 seconds (28–1, with 0 not considered, as it is the official time of death), or 4.25 minutes. From the instant the IP datagram enters the Internet, each gateway and host that acts on the datagram decrements the TTL by at least one second.When the TTL reaches 0, the datagram is declared dead and is discarded.The TTL mechanism prevents packets from wandering the Internet for eternity, at which point they would have no value, and would only contribute to overall network congestion. Over time, the TTL field has been redefined to indicate, as an option, the number of hops, that is the number of routers through which the packet travels.The default TTL is 64. 74570c09.qxd 9/11/07 12:22 PM Page 254 |
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