| 单词 | Figure I-2 |
| 释义 | Figure I-2 As indicated in Figure I-2, the IPv6 header is 40 octets, compared to the IPv4 header of 20 octets, and can be extended as necessary through optional headers associated with higher layer protocols such as Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). In a departure from IPv4, the IP header and any extension headers associated with the datagram are in addition to the payload length.The IPv6 datagram contains the following fields: • VERS (VERSion): 4 bits identifying the IP version number.The version number is 6. • Class: 8 bits used by originating nodes and/or forwarding routers to identify and distinguish between different packet classes or priorities, as set by upper-layer protocols.This field replaces the IPv4 Type of Service (ToS) field. • Flow Label: 20 bits used by a source host indicating any special handling requested for a flow, or sequence, of datagrams. Real-time voice, audio, or video communications exemplify applications that require special flow treatment. • Payload Length: 16 bits describing the total length of the datagram.This field is much like the Total Length field in IPv4, although the Payload Length field does not include the IP header. IPv6 supports jumbo payloads larger than the traditional 65,535 octets. At a minimum, all network links must support a maximum transmission unit (MTU) of at least 1280 octets.The IPv6 payload length includes any IPv6 header extensions,TCP or UDP headers, and any Application Layer headers that might be associated with the datagram. 74570c09.qxd 9/11/07 12:22 PM Page 256 |
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