It eliminates fields that are unneeded or rarely used and adds fields that provide better support for real-time traffic. Figure 17 The structure of an IPv6 packet. The IPv6 header is always present and is a fixed size of 40 bytes. The fields in the IPv6 header are described in detail later in this paper. Zero or more extension headers can be present and are of varying lengths. A Next Header field in the IPv6 header indicates the next extension header.
Within each extension header is a Next Header field that indicates the next extension header. The new extension header format allows IPv6 to be augmented to support future needs and capabilities. Unlike options in the IPv4 header, IPv6 extension headers have no maximum size and can expand to accommodate all the extension data needed for IPv6 communication. Normally, it can be up to 65, bytes long. Payloads greater than 65, bytes in length can be sent using the Jumbo Payload option in the Hop-by-Hop Options extension header.
Figure 19 The IPv6 header. Version — 4 bits are used to indicate the version of IP and is set to 6. Traffic Class — Indicates the class or priority of the IPv6 packet. The size of this field is 8 bits.
However, an IPv6 implementation is required to provide a means for an application layer protocol to specify the value of the Traffic Class field for experimentation. Flow Label — Indicates that this packet belongs to a specific sequence of packets between a source and destination, requiring special handling by intermediate IPv6 routers. The size of this field is 20 bits. The Flow Label is used for non-default quality of service connections, such as those needed by real-time data voice and video.
For default router handling, the Flow Label is set to 0. There can be multiple flows between a source and destination, as distinguished by separate non-zero Flow Labels. Payload Length — Indicates the length of the IPv6 payload. The size of this field is 16 bits.
With 16 bits, an IPv6 payload of up to 65, bytes can be indicated. For payload lengths greater than 65, bytes, the Payload Length field is set to 0 and the Jumbo Payload option is used in the Hop-by-Hop Options extension header.
When indicating an upper layer protocol above the Internet layer, the same values used in the IPv4 Protocol field are used here. Hop Limit — Indicates the maximum number of links over which the IPv6 packet can travel before being discarded. The Hop Limit is similar to the IPv4 TTL field except that there is no historical relation to the amount of time in seconds that the packet is queued at the router. Source Address —Stores the IPv6 address of the originating host.
The size of this field is bits. Destination Address — Stores the IPv6 address of the current destination host. In most cases the Destination Address is set to the final destination address.
However, if a Routing extension header is present, the Destination Address might be set to the next router interface in the source route list. Table 5 Values of the Next Header Field.
Products Customers Support Blog. Menu Why us? How we help. Who we help. About us. Request a Live demo. Get a Price Quote. Menu Products Micetro. More products. Menu Customers. The 8-bit field in the IPv6 header can assume different values to enable the source node to differentiate between the packets generated by it by associating different delivery priorities to them.
This field is subsequently used by the originating node and the routers to identify the data packets that belong to the same traffic class and distinguish between packets with different priorities.
The bit flow label field in the IPv6 header can be used by a source to label a set of packets belonging to the same flow. A flow is uniquely identified by the combination of the source address and of a non-zero Flow label. The IPv6 routers must handle the packets belonging to the same flow in a similar fashion. The information on handling of IPv6 data packets belonging to a given flow may be specified within the data packets themselves or it may be conveyed by a control protocol such as the RSVP Resource reSerVation Protocol.
Edited as per inputs from Brian E. When routers receive the first packet of a new flow, they can process the information carried by the IPv6 header, Routing header, and Hop-by-Hop extension headers, and store the result e. The bit Payload length field puts an upper limit on the maximum packet payload to 64 kilobytes.
In case a higher packet payload is required, a Jumbo payload extension header is provided in the IPv6 protocol.
A Jumbo payload Jumbogram is indicated by the value zero in the Payload Length field. Jumbograms are frequently used in supercomputer communication using the IPv6 protocol to transmit heavy data payload. The 8-bit Next Header field identifies the type of header immediately following the IPv6 header and located at the beginning of the data field payload of the IPv6 packet. The 8-bit Hop Limit field is decremented by one, by each node typically a router that forwards a packet.
Length of the Routing header in 8-octet units, not including the first 8 octets. Routing Type 8-bit identifier of a particular Routing header variant.
Segments Left 8-bit unsigned integer. Number of route segments remaining, i. If, after processing a Routing header of a received packet, an intermediate node determines that the packet is to be forwarded onto a link whose link MTU is less than the size of the packet, the node must discard the packet and send an ICMP Packet Too Big message to the packet's Source Address.
For the Type 0 Routing header, Hdr Ext Len is equal to two times the number of addresses in the header. Routing Type 0. Reserved bit reserved field. Initialized to zero for transmission; ignored on reception. Multicast addresses must not appear in a Routing header of Type 0, or in the IPv6 Destination Address field of a packet carrying a Routing header of Type 0. A Routing header is not examined or processed until it reaches the node identified in the Destination Address field of the IPv6 header.
Note: unlike IPv4, fragmentation in IPv6 is performed only by source nodes, not by routers along a packet's delivery path -- see section 5. Identifies the initial header type of the Fragmentable Part of the original packet defined below.
Reserved 8-bit reserved field. Fragment Offset bit unsigned integer. The offset, in 8-octet units, of the data following this header, relative to the start of the Fragmentable Part of the original packet. Res 2-bit reserved field.
Identification 32 bits. See description below. In order to send a packet that is too large to fit in the MTU of the path to its destination, a source node may divide the packet into fragments and send each fragment as a separate packet, to be reassembled at the receiver.
For every packet that is to be fragmented, the source node generates an Identification value. If a Routing header is present, the Destination Address of concern is that of the final destination.
However, it is not required that a source node know the maximum packet lifetime. Rather, it is assumed that the requirement can be met by maintaining the Identification value as a simple, bit, "wrap-around" counter, incremented each time a packet must be fragmented. It is an implementation choice whether to maintain a single counter for the node or multiple counters, e. The Fragmentable Part consists of the rest of the packet, that is, any extension headers that need be processed only by the final destination node s , plus the upper-layer header and data.
The Fragmentable Part of the original packet is divided into fragments, each, except possibly the last "rightmost" one, being an integer multiple of 8 octets long.
A Fragment Offset containing the offset of the fragment, in 8-octet units, relative to the start of the Fragmentable Part of the original packet. The Fragment Offset of the first "leftmost" fragment is 0.
An M flag value of 0 if the fragment is the last "rightmost" one, else an M flag value of 1. The Identification value generated for the original packet. The lengths of the fragments must be chosen such that the resulting fragment packets fit within the MTU of the path to the packets' destination s. The Unfragmentable Part of the reassembled packet consists of all headers up to, but not including, the Fragment header of the first fragment packet that is, the packet whose Fragment Offset is zero , with the following two changes: The Next Header field of the last header of the Unfragmentable Part is obtained from the Next Header field of the first fragment's Fragment header.
The Payload Length of the reassembled packet is computed from the length of the Unfragmentable Part and the length and offset of the last fragment. For example, a formula for computing the Payload Length of the reassembled original packet is: PL.
The Fragmentable Part of the reassembled packet is constructed from the fragments following the Fragment headers in each of the fragment packets. The Fragment header is not present in the final, reassembled packet.
The following error conditions may arise when reassembling fragmented packets: If insufficient fragments are received to complete reassembly of a packet within 60 seconds of the reception of the first-arriving fragment of that packet, reassembly of that packet must be abandoned and all the fragments that have been received for that packet must be discarded.
If the first fragment i. If the length of a fragment, as derived from the fragment packet's Payload Length field, is not a multiple of 8 octets and the M flag of that fragment is 1, then that fragment must be discarded and an ICMP Parameter Problem, Code 0, message should be sent to the source of the fragment, pointing to the Payload Length field of the fragment packet. If the length and offset of a fragment are such that the Payload Length of the packet reassembled from that fragment would exceed 65, octets, then that fragment must be discarded and an ICMP Parameter Problem, Code 0, message should be sent to the source of the fragment, pointing to the Fragment Offset field of the fragment packet.
The following conditions are not expected to occur, but are not considered errors if they do: The number and content of the headers preceding the Fragment header of different fragments of the same original packet may differ.
Whatever headers are present, preceding the Fragment header in each fragment packet, are processed when the packets arrive, prior to queueing the fragments for reassembly. Only those headers in the Offset zero fragment packet are retained in the reassembled packet. The Next Header values in the Fragment headers of different fragments of the same original packet may differ.
Only the value from the Offset zero fragment packet is used for reassembly. Identifies the type of header immediately following the Destination Options header.
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