Path MTU Discovery is an algorithm implemented in TCP/IP
which should normally prevent IP fragmentation of TCP frames from ever
occurring on the network, and is enabled by default in all Microsoft Windows
operating systems. This algorithm was implemented because the process of
fragmenting and re-assembling IP frames is time consuming and
inefficient.
However, fragmentation may still occur on the network
under certain circumstances. The way that Path MTU Discovery works is that a
host will set a bit flag in the IP header of all TCP frames it transmits which
informs routers that fragmentation is not allowed. This is known as the �Don�t
Fragment� or DF bit. When a router receives a frame that is too large to be
transmitted onto the next network, it will check to see if the DF bit is set.
If it is not, then the frame is fragmented and forward on to the destination.
If the DF bit is set, then the router should discard the frame and return an
ICMP message to the sender indicating that fragmentation was required but the
DF bit was set. This message will also inform the sender of the Maximum
Transmission Unit (MTU) for the network in question. The sender will then limit
all subsequent frames for this TCP session to the specified MTU size,thereby
eliminating the need for fragmentation.
This process will fail if a
router between the source and destination needs to fragment the frame and
either fails to return the ICMP message to the sender, or the message gets
blocked due to packet filtering. This is known as a �black hole router�. In
this case, the frame will be discarded silently and the sender will retransmit
the frame several times until the TCP session terminates. Another possibility
is that a router may be configured to ignore the DF bit and will fragment the
frame anyway. Lastly, other older operating systems or network devices may not
support Path MTU Discovery, and will not set the DF bit, which willresult in
fragmentation on the network. Differences in MTU values typically occur when
data must cross different types of network media (VPN connections, Token Ring,
Ethernet, etc.). To resolve this issue, identify the router which is preventing
Path MTU Discovery from working properly and correct its configuration.
For additional information
about how to locate such a router and to see some alternative, but less desirable solutions, click the following article number to view the article in the Microsoft Knowledge Base:
314825�
How to troubleshoot black hole router issues