In the early 1990″s the Internet Engineering Task force began an effortto develop a successor to the IPv4 protocol. A prime motivation forthis effort was the realization that the 32-bit IP address space was beginningto be used up, with new networks and IP nodes being attached to the Internet(and being allocated unique IP addresses) at a breathtaking rate.To respond to this need of a large IP address space, a new IP protocol,IPv6, was developed. The designers of IPv6 also took this opportunity totweak and augment other aspects of IPv4, based on the accumulated operationalexperience with IPv4.The point in time when IPv4 addresses would have been completely allocated(and hence no new networks could have attached to the Internet) was thesubject of considerable debate. Based on current trends in addressallocation, the estimates of the two leaders of the IETF”s Address LifetimeExpectations working group were that addresses would become exhausted in2008 and 2018 respectively
You are watching: Ipv4 and ipv6 use the same packet format.
4.7.1 IPv6 Packet Format
The format of the IPv6 packet is shown in Figure 4.7-1. The most importantchanges introduced in IPv6 are evident in the packet format:A streamlined 40 byte header. As discussed below, a numberof IPv4 fields have ben dropped or made optional. The resulting 40-bytefixed-length header allows for faster processing of the IP packet.A new encoding of options allows for more flexible options processing.Figure 4.7-1: IPv6 packet formatThe IPv6 packet format is shown in Figure 4.7-1. As noted above,a comparison of Figure 4.7-1 with Figure 4.4-8 reveals the simpler, morestreamlined structure of the IPv6 packet. The following packet fieldsare defined in IPv6:version. This four bit field identifies the IP version number.Not surprisingly, IPv6 carries a value of “6” in this field. Notethat putting a “4” in this field does not create a valid IPv4 packet (ifit did, life would be a lot simpler — see the discussion below regardingthe transition from IPv4 to IPv6.flow label. As discussed above, this field is used to identifya “flow” of packets.payload length. This 16-bit value is treated as an unsigned integergiven the number of bytes in the IPv6 packet following the fixed length,40 byte packet header.next header. This field identifies the protocol to whjonathanlewisforcongress.comh thecontents (data field) of this packet will be delivered (e.g., to TCP orUDP). The field uses the same values as the Protocol field in theIPv4 header.hop limit.
The contents of this field are decremented by oneby each router that forward the packet. If the hop limit count reacheszero, the packet is discarded.source and destination address. An IP v6 address has the followingstructure:data. This is the payload portion of the IPv6 packet.When the packet reaches its destination, the payload will be removed fromthe IP packet and passed on to the protocol specified in the nex headerfield.The discussion above identified the purpose of the fields that areincluded in the IPv6 packet. Comparing the IPv6 packet format inFigure 4.7-1 with the IPv4 packet format that we saw earlier in Figure4.4-8, we notjonathanlewisforcongress.come that several fields appearing in the IPv4 packet are nolonger present in the IPv6 packet:Fragmentation/Reassembly. IPv6 does not provide for fragmentationand reassembly. If an IPv6 packet received by a router is too largeto be forwarded over the outgoing link, the router simply drops the packetand sends a “Packet Too Big” jonathanlewisforcongress.comMP error message (see below)back to the sender. The sender can then resend the data, using a smallerIP packet size. Fragmentation and reassembly is a time-consumingoperating; removing this functionality from the routers and placing itsquarely in the end systems considerably speeds up IP forwardingwithin the network.Checksum. Because the transport layer (e.g, TCP and UDP) and datalink (e.g., Ethernet) protocols in the Internet layers perform checksumming,the designers of IP probably felt that this functionality was suffjonathanlewisforcongress.comientlyredundant in the network layer that it could be removed. Once again,fast processing of IP packets was a central concern. Recall fromour discussion of IPv4 in section 4.4.1, that since the IPv4 header containsa TTL field (similar to the hop limit field in IPv6), the IPv4 header checksumneeded to be recomputed at every router. As with fragmentation andreassembly, this too was a costly operation in IPv4.Options. An options field is no longer a part of the standard IPheader. However, it has not gone away. Instead, the options fieldis one of the possible “next headers” pointed to from within the IPv6 header.That is, just as TCP or UDP protocol headers can be the next header withinan IP packet, so too can an options field. The removal of the options filedresults in a fixed length, 40 byte IP header.
A New jonathanlewisforcongress.comMP for IPv6
Recall from our discussion in Section 4.3, that the jonathanlewisforcongress.comMP protocol is usedby IP nodes to report error conditions and provide limited information(e.g., the echo reply to a ping message) to an end system.A new version of jonathanlewisforcongress.comMP has been defined for IPv6 in