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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 4 (IP, IPv4)                      9  IP Datagram Size, Maximum Transmission Unit (MTU), Fragmentation and Reassembly

IP Datagram Size, the Maximum Transmission Unit (MTU), and Fragmentation Overview 1 2 3 4 IP Message Reassembly Process

The IP Fragmentation Process: An Example

The device performing the fragmentation follows a specific algorithm to divide the message into fragments for transmission. The exact implementation of the fragmentation process depends on the device. Let's take the same example from the previous topic, an IP message 12,000 bytes wide (including the 20-byte IP header) that needs to be sent over a link with an MTU of 3,300. Here's a typical method by which this fragmentation might be performed (you may find the illustration in Figure 90 helpful):

1. Create First Fragment: The first fragment is created by taking the first 3,300 bytes of the 12,000-byte IP datagram. This includes the original header, which becomes the IP header of the first fragment (with certain fields changed as described below). So, 3,280 bytes of data are in the first fragment. This leaves 8,700 bytes to encapsulate (11,980 minus 3,280).
   
   
2. Create Second Fragment: The next 3,280 bytes of data are taken from the 8,700 bytes that remain after the first fragment was built, and paired with a new header to create fragment #2. This leaves 5,420 bytes.
   
   
3. Create Third Fragment: The third fragment is created from the next 3,280 bytes of data, with a 20-byte header. This leaves 2,140 bytes of data.
   
   
4. Create Fourth Fragment: The remaining 2,140 bytes are placed into the fourth fragment, with a 20-byte header of course.

I want to emphasize two important points here. First, IP fragmentation does not work by fully encapsulating the original IP message into the Data fields of the fragments. If this were done, the first 20 bytes of the Data field of the first fragment would contain the original IP header. This technique is used by some other protocols, such as the PPP Multilink Protocol, but not by IP. The original IP header is “transformed” into the IP header of the first fragment.

Second, note that the total number of bytes transmitted increases: we are sending 12,060 bytes (3,300 times three plus 2,160) instead of 12,000. The extra 60 bytes are from the additional headers in the second, third and fourth fragments. (The increase in size could theoretically be even larger if the headers contain options.)

IP Datagram Size, the Maximum Transmission Unit (MTU), and Fragmentation Overview 1 2 3 4 IP Message Reassembly Process