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IP Message Reassembly Process
(Page 2 of 2)
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.
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Version 3.0 - Version Date: September 20, 2005
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