IP Header Identification
Data travels through a network in packets. An email message, streaming video or other file sent over the Internet is comprised of many such packets, which are reassembled into their original form by a computer on the receiving end. Every packet has a header that contains the data needed to route the data to its intended destination. Sometimes a node may need to split a packet into smaller units called fragments. The header provides identification for each fragment, allowing the destination node to reassemble the fragments in their proper order.
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Source and Destination
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Internet Protocol versions 4 and 6 have different header fields reserved for packet identification, but both contain the IP address of the packet's source, the device that sent the original data, and the packet's destination in their headers. IPv4 packet headers reserve 32 bits for the IP "Source Address" and "Destination Address" fields, allowing IPv4 to provide addresses for approximately 4.3 billion devices connected to the Internet. The Internet Assigned Numbers Authority distributed the last available IPv4 addresses on February 3, 2011. IPv6 solves this problem by providing 128 bits for its address fields, multiplying the number of available addresses exponentially.
Fragmentation Handling in IPv4
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An IPv4 packet header's "Identification," "Flags" and "Fragment Offset" fields provide the necessary instructions for rebuilding the original packet. The Identification field contains a numerical value assigned to all of a packet's fragments, ensuring that the destination device knows which fragments belong to the packets it receives. The Flags field contains three bits, each of which can have a value of “1” or “0.” The third bit in the Flags field is the "More Fragments" flag, which has a value of “1” if it's the last fragment of a packet, letting the destination node know that it has all the fragments needed to rebuild the packet. The Fragment Offset field's value is unique to every fragment, indicating the fragment's sequential order in the original packet and allowing the receiving device to reassemble the fragments in the correct order.
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Fragmentation Handling in IPv6
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Standard IPv6 packet headers don't contain fields for storing fragmentation information. Instead, if a packet needs to be fragmented, devices using the IPv6 protocol will append an extension header onto the packet containing the information that the receiving device needs to reconstruct the packet. The device transmitting the data adds the extension header, because under IPv6, only the nodes transmitting packets can fragment them. The extension header contains an "Identification" field that serves the same function as the "Identification" field in IPv4 packets. Once the packet reaches its destination, the device uses the source and destination information, along with the extension header's data, to rebuild the original packet.
Identifying Packets Requiring Priority Delivery
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IPv4's "Precedence" field allows the transmitting node to assign a priority to a packet's delivery, from routine passage through the network to instructing the network to place greater priority on the packet's delivery than on all other packets with lower IP precedence field values. The IPv6 packet header provides a "Traffic Class" field, which assigns delivery priority for the packet and a "Flow Label" field that tells the network to assign the same priority to every packet within a specific data flow.
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References
- TCP/IP - The Ultimate Protocol Guide: Complete 2 Volume Set; Philip M. Miller
- Cisco: General Networking: Internet Addressing and Routing First Step: IP Header Format
- Sams Teach Yourself TCP/IP in 24 Hours; Joe Casad
- Understanding Voice Over IP Technology; Nicholas Wittenberg
- Ars Technica; River of IPv4 Addresses Officially Runs Dry; Iljitsch can Beijnum
- IPv6.com: IPv6 Header Deconstructed
Resources
- Photo Credit Siri Stafford/Digital Vision/Getty Images
