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Re: PMTU-D: remember, your load balancer is broken

  • From: Paul Vixie
  • Date: Mon Jun 19 16:40:39 2000

[email protected] ("Bora Akyol") writes:
> FYI, there are core routers out there that can support line rate
> HW-based fragmentation. I believe this is necessary in any router
> with heterogeneous interfaces.

OK, somebody check me on this.  "Line rate" means back to back, minimum
spaced packets.  Fragmentation means turning single packets into multiple
packets.  Each such packet will have its own headers, thus, fragmentation
means turning a single header into multiple headers.  Headers take space.

Therefore if there's a 100Mb/s FDDI (MTU 4400) on the input side of a router,
and a 100Mb/s FastEthernet (MTU 1500) on the output side of that router, then
there WILL be lossage due to fragmentation, since "line rate" on the output
side has more IP headers octets per IP payload octet than "line rate" on the
input side did.

This is a contrived example but there are more complex and more subtle flows
in real life that would excite this problem.  Fragmentation is like classful
addressing -- an interesting early architectural error that shows how much
experimentation was going on while IP was being designed.
Paul Vixie <[email protected]>
SVP for Internet Services, MFNX

M.I.B.H. Inc. is a subsidiary of Metromedia Fiber Network, Inc.