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What is the difference of MRU versus MTU??

Posted on 2002-03-04
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Last Modified: 2006-11-17

hi

Can anybody explain to me what is Maximum Receive Unit (MRU) and its differences / usages when compared to MTU?
Couldn't find a satisfactory answer to that on the Net.

Thanks!

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Question by:thiamwah
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by:stevenlewis
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MTU = maximum transmission unit
one is the recieve side, and one is the transmission side, ideally they should match
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by:thiamwah
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how does MTU interplay with MRU..??
How does MRU affect the size of the largest segment sent when say: MTU > MRU and  when conversely MRU < MTU..?
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SteveJ earned 50 total points
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I've never seen a router with an MRU setting. So now that I've confessed ignorance here's some verbiage that you can ignore.

If I transmit a packet that's 1500 bytes (because the MTU on that particular interface of my router is set to 1500) through a bunch of routers on a network where the MTU is set at 512 bytes my packet will be fragmented OR if any of the intervening routers have a "NO Fragment" flag set, then my 1500 byte packet will be dropped at that router.

Some routers can be configured to send an ICMP MTU discovery packet which will determine the MTU along the path that your packet would take, report the value to the router OS and the router will dynamically set the MTU. However, there's security implications to this so some people disable "allow MTU discovery" on their routers.

There's an unwritten, general rule of thumb which says that you should set the MTU at 512 (ON THE SERIAL INTERFACE) bytes as a sort of minimum common denominator within the internet to keep from fragmenting your packets (which of course adds overhead thereby making the transmission a little less efficient, not to mention the time added to the transmission to fragment and then reassemble the packet.)

There's another unwritten, general rule of thumb that says for every router tech that's any good there's half a dozen who have no clue and will set the MTU at 256 or 1600 bytes.

Good luck.
Steve
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