OT: mtus

Bruce Dubbs bdubbs at swbell.net
Sat Jul 31 20:42:25 PDT 2004

Kevin P. Fleming wrote:

> The max MTU for Ethernet is 1514 including the frame header, under 
> normal circumstances.
> However, a significant number of Ethernet adapters support "jumbo 
> frames", especially Gigabit adapters. Using 9000-byte frames on my 
> Gigabit link dramatically improves file transfer speeds, so I do that :-)
> Also, there are other network interfaces besides Ethernet that may 
> benefit from having their MTU set to a non-default size.

There are pros and cons here.  If transferring between two systems that 
both understand jumbo frames, the speed can be significantly faster due 
to reduced overhead.  On the other hand, if the source and destination 
systems use differnt MTUs, it can increase overhead due to IP packet 
fragmentation and opens up a class of network attacks such as a teardrop 
attack.  I normally drop all fragmented packets. 

One way to avoid packet fragmentation is to set the df (don't fragment) 
bit in an IP header.  If a connection requires fragmentation any place 
in the route to the destination, an ICMP packet is returned to the host 
which, in turn, retransmits with a lower packet size.  This process 
would add overhead to basically every connection from a network using 
jumbo frames to a network without.  In this case, the most efficient 
transfer method is when the frame size matches the packet size.  To the 
best of my knowledge, most wideband connections support 1500 byte frames.

The bottom line is that YMMV depending on the predominent usage of your 
network.  The issue appears to be beyond the scope of LFS/BLFS, but is 
interesting to discuss.

  -- Bruce

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