You are watching: Ipv4 and ipv6 use the same packet format.
4.7.1 IPv6 Packet FormatThe format of the IPv6 packet is shown in figure 4.7-1. The many importantchanges introduced in IPv6 are evident in the packet format:A centralized 40 byte header. As discussed below, a numberof IPv4 fields have ben to reduce or made optional. The result 40-bytefixed-length header enables for much faster processing of the IP packet.A new encoding the options permits for much more flexible alternatives processing.Figure 4.7-1: IPv6 packet formatThe IPv6 packet layout is presented in number 4.7-1. As detailed above,a to compare of number 4.7-1 with figure 4.4-8 discover the simpler, morestreamlined framework of the IPv6 packet. The adhering to packet fieldsare characterized in IPv6:version. This 4 bit ar identifies the IP variation number.Not surprisingly, IPv6 dead a worth of "6" in this field. Notethat placing a "4" in this ar does not develop a precious IPv4 packet (ifit did, life would certainly be a lot easier -- check out the discussion below regardingthe change from IPv4 come IPv6.flow label. As debated above, this ar is offered to identifya "flow" that packets.payload length. This 16-bit value is treated together an unsigned integergiven the number of bytes in the IPv6 packet complying with the resolved length,40 byte packet header.next header. This ar identifies the protocol to whjonathanlewisforcongress.comh thecontents (data field) that this packet will certainly be yielded (e.g., to TCP orUDP). The field uses the very same values as the Protocol field in theIPv4 header.hop limit. The components of this field are decremented by oneby every router the forward the packet. If the hop limit count reacheszero, the packet is discarded.source and also destination address. An IP v6 resolve has the followingstructure:data. This is the payload portion of the IPv6 packet.When the packet reaches its destination, the payload will be eliminated fromthe IP packet and passed on to the protocol stated in the nex headerfield.The discussion above identified the objective of the fields that areincluded in the IPv6 packet. Comparing the IPv6 packet format inFigure 4.7-1 v the IPv4 packet format that we saw previously in Figure4.4-8, us notjonathanlewisforcongress.come that several fields showing up in the IPv4 packet space nolonger present in the IPv6 packet:Fragmentation/Reassembly. IPv6 walk not administer for fragmentationand reassembly. If an IPv6 packet received by a router is also largeto be forwarded over the outgoing link, the router simply drops the packetand sends a "Packet also Big" jonathanlewisforcongress.comMP error post (see below)back come the sender. The sender deserve to then resend the data, utilizing a smallerIP packet size. Fragmentation and reassembly is a time-consumingoperating; remove this functionality from the routers and placing itsquarely in the end systems considerably accelerates IP forwardingwithin the network.Checksum. since the transport layer (e.g, TCP and UDP) and datalink (e.g., Ethernet) protocols in the net layers perform checksumming,the developers of IP most likely felt that this usability was suffjonathanlewisforcongress.comientlyredundant in the network layer the it can be removed. Once again,fast processing of IP packets to be a main concern. Recall fromour discussion of IPv4 in section 4.4.1, that due to the fact that the IPv4 header containsa TTL ar (similar come the hop limit field in IPv6), the IPv4 header checksumneeded to be recomputed in ~ every router. As with fragmentation andreassembly, this also was a costly operation in IPv4.Options. An choices field is no much longer a part of the traditional IPheader. However, it has actually not unable to do away. Instead, the options fieldis one of the possible "next headers" pointed come from within the IPv6 header.That is, just as TCP or UDP protocol headers can be the following header withinan IP packet, for this reason too can an options field. The removed of the alternatives filedresults in a addressed length, 40 byte IP header.
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