Before examining the IPv6 header, you might find it helpful, for contrasting purposes, to review the IPv4 header shown in Figure 4-2.
Figure 4-2 The structure of the IPv4 header.
Following is a list of the fields in the IPv4 header:
Version The Version field indicates the version of IP and is set to 4. The size of this field is 4 bits.
Internet Header Length The Internet Header Length (IHL) field indicates the number of 4-byte blocks in the IPv4 header. The size of this field is 4 bits. Because an IPv4 header is a minimum of 20 bytes in size, the smallest value of the IHL field is 5. IPv4 options can extend the minimum IPv4 header size in increments of 4 bytes. If an IPv4 option is not an integral multiple of 4 bytes in length, the remaining bytes are padded with padding options, making the entire IPv4 header an integral multiple of 4 bytes. With a maximum IHL value of 0xF, the maximum size of the IPv4 header, including options, is 60 bytes (15 x 4).
Type of Service The Type of Service field indicates the desired service expected by this packet for delivery through routers across the IPv4 internetwork. The size of this field is 8 bits, including bits originally defined in RFC 791 for precedence, delay, throughput, reliability, and cost characteristics. RFC 2474 provides the modern definition as the Differentiated Services (DS) field. The high-order 6 bits of the DS field comprise the DS Code Point (DSCP) field. The DSCP field allows devices in a network to mark, unmark, and classify packets for forwarding. This is usually done based on the needs of an application. For example, Voice over IP and other real-time packets take precedence over e-mail in congested areas of the network. This is commonly referred to as Quality of Service (QoS). The low-order 2 bits of the Type of Service field are used for Explicit Congestion Notification (ECN), as defined in RFC 3168.
Total Length The Total Length field indicates the total length of the IPv4 packet (IPv4 header + IPv4 payload) and does not include link-layer framing. The size of this field is 16 bits, which can indicate an IPv4 packet that is up to 65,535 bytes long.
Identification The Identification field identifies this specific IPv4 packet. The size of this field is 16 bits. The Identification field is selected by the source node of the IPv4 packet. If the IPv4 packet is fragmented, all the fragments retain the Identification field value so that the destination node can group the fragments for reassembly.
Flags The Flags field identifies flags for the fragmentation process. The size of this field is 3 bits; however, only 2 bits are defined for current use. There are two flags—one to indicate whether the IPv4 packet can be fragmented and another to indicate whether more fragments follow the current fragment.
Fragment Offset The Fragment Offset field indicates the position of the fragment relative to the beginning of the original IPv4 payload. The size of this field is 13 bits.
Time-to-Live The Time-to-Live (TTL) field indicates the maximum number of links on which an IPv4 packet can travel before being discarded. The size of this field is 8 bits. The TTL field was originally defined as a time count for the number of seconds the packet could exist on the network. An IPv4 router determined the length of time required (in seconds) to forward the IPv4 packet and decremented the TTL accordingly. Modern routers almost always forward an IPv4 packet in less than a second, and they are required by RFC 791 to decrement the TTL by at least one. Therefore, the TTL becomes a maximum link count with the value set by the sending node. When the TTL equals 0, an ICMPv4 Time Exceeded-Time to Live Exceeded in Transit message is sent to the source of the packet and the packet is discarded.
Protocol The Protocol field identifies the upper-layer protocol. The size of this field is 8 bits. For example, a value of 6 in this field identifies TCP as the upper-layer protocol, a decimal value of 17 identifies UDP, and a value of 1 identifies ICMPv4. The Protocol field is used to identify the upper-layer protocol that is to receive the IPv4 packet payload.
Header Checksum The Header Checksum field provides a checksum on the IPv4 header only. The size of this field is 16 bits. The IPv4 payload is not included in the checksum calculation because the IPv4 payload usually contains its own checksum. Each IPv4 node that receives IPv4 packets verifies the IPv4 header checksum and silently discards the IPv4 packet if checksum verification fails. When a router forwards an IPv4 packet, it must decrement the TTL. Therefore, the Header Checksum value is recomputed at each hop between source and destination.
Source Address The Source Address field stores the IPv4 address of the originating host. The size of this field is 32 bits.
Destination Address The Destination Address field stores the IPv4 address of an intermediate destination (in the case of source routing) or the destination host. The size of this field is 32 bits.
Options The Options field stores one or more IPv4 options. The size of this field is a multiple of 32 bits (4 bytes). If an IPv4 option does not use all 32 bits, padding options must be added so that the IPv4 header is an integral number of 4-byte blocks that can be indicated by the IHL field.