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 Message Fragmentation Process 1 2 IP Datagram Delivery and Routing

The Reassembly Process

As we saw in looking at how fragmentation works, it involves a fair bit of complexity. Several IP header fields are filled in when a message is fragmented to give the receiving device the information it requires to properly reassemble the fragments. The receiving device follows a procedure to keep track of the fragments as they are received and build up its copy of the total received message from the source device. Most of its efforts are geared around dealing with the potential difficulties associated with IP being an unreliable protocol.

The details of implementation of the reassembly process are specific to each device, but generally include the following functions:

• Fragment Recognition and Fragmented Message Identification: The recipient knows it has received a message fragment the first time it sees a datagram with the More Fragments bit set to one or the Fragment Offset a value other than zero. It identifies the message based on: the source and destination IP addresses; the protocol specified in the header; and the Identification field generated by the sender.
  
  
• Buffer Initialization: The receiving device initializes a buffer where it can store the fragments of the message as they are received. It keeps track of which portions of this buffer have been filled with received fragments, perhaps using a special table. By doing this, it knows when the buffer is partially filled with received fragments and when it is completely full.
  
  
• Timer Initialization: The receiving device sets up a timer for reassembly of the message. Since it is possible that some fragments may never show up, this timer ensures that the device will not wait “forever” trying to reassemble the message.
  
  
• Fragment Receipt and Processing: Whenever a fragment of this message arrives (as indicated by it having the same source and destination addresses, protocol and Identification as the first fragment), the fragment is processed. It is inserted into the message buffer in the location indicated by its Fragment Offset field. The device also makes note of the fact that this portion of the message has been received.

Reassembly is finished when the entire buffer has been filled and the fragment with the More Fragments bit set to zero is received, indicating that it is the last fragment of the datagram. The reassembled datagram is then processed like a normal, unfragmented datagram would be. On the other hand, if the timer for the reassembly expires with any of the fragments missing, the message cannot be reconstructed. The fragments are discarded, and an ICMP Time Exceeded message generated. Since IP is unreliable, it relies on higher layer protocols such as TCP to determine that the message was not properly received and then retransmit it.

IP Message Fragmentation Process 1 2 IP Datagram Delivery and Routing