Chapter
12
. Appendix B Troubleshooting
Solwise SAR715PV ADSL Ethernet Router User’s Guide
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12.6.2 So what is it?
A maximum transmission unit (MTU) is the largest size packet or
frame, specified in octets (eight-bit bytes), that can be sent in a
packet- or frame-based network such as the Internet. The Internet's
Transmission Control Protocol uses the MTU to determine the
maximum size of each packet in any transmission. Too large an
MTU size may mean retransmissions if the packet encounters a
router that can't handle that large a packet. Too small an MTU size
means relatively more header overhead and more
acknowledgements that have to be sent and handled. Most
computer operating systems provide a default MTU value that is
suitable for most users.
12.6.3 Why is it important?
The main reason one would be concerned with changing a default
MTU value is the problem of IP fragmentation. IP fragmentation
occurs when you pass through a router with an MTU less than the
MTU you are using. If your TCP/IP stack is set to a MTU of 1500
and you hit a router with a MTU set to 576. The router will fragment
your packets down to 576. When you finally reach your destination,
those datagrams will have to be re-assembled and this will slow
down your throughput, e.g., download time.
You can check on fragmentation and other errors by using the
netstat -s (use netstat -s | more to stop the scroll and netstat -x to
see the options available to you) command and checking the IP
statistics. For example, note the Datagram is the MTU.
IP Statistics
Packets Received = 152113
Received Header Errors = 0
Received Address Errors = 0
Datagrams Forwarded = 0
Unknown Protocols Received = 0
Received Packets Discarded = 0
Received Packets Delivered = 152113
Output Requests = 122510
Routing Discards = 0
Discarded Output Packets = 0
Output Packet No Route = 0
Reassembly Required = 0
Reassembly Successful = 0
Reassembly Failures = 0
Datagrams Successfully Fragmented = 0
Datagrams Failing Fragmentation = 0
Fragments Created = 0
Another problem to check out is whether or not you are having
excessive retransmissions. The lower your retransmit rate
(Segments Sent/Segments Retransmitted) the better. If you have a
retransmit rate in Windows, check Microsoft there are some
patches can fix this type of problem. In this example (using netstat -
s), the rate is about 1.5%. Note the Segment is the MSS.
TCP Statistics
