[OpenAFS-devel] .35 sec rx delay bug?

[Buhrmaster, Gary]

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> this isnt the first time i have heard of a router refusing to fragment
> udp.  

There is theory, and there is practice.  In practice, the following
are true (in no particular order)

* Some OS's (or NAT devices, or firewalls, or proxy agents,
  or load balancers, or ..) will tag all packets with 
  "don't fragment" as part of their function.
* Some NAT/routers/firewalls/proxies will not pass
  return ICMP messages.
* More and more providers are now using MPLS (or
  IPSEC tunnels, or GRE tunnels, or ...) to enable 
  new functionality (i.e. marketing opportunities), 
  which on some platforms reduces the effective 
  MTU to less than the ethernet MTU of 1500 somewhere 
  along some paths (usually near the end user, since 
  the backbones can usually handle the additional 
  bytes of the label).
* Almost all mainstream routers now do forwarding in
  hardware, but exceptional conditions (including
  fragmentation) are done in alternative (software)
  paths.
* Almost all mainstream routers now have something in
  the hardware that limits control plane traffic "to 
  protect the router".  This includes generating 
  "unreachables" and/or fragmenting packets.  These
  limits tend to be (somewhat) low, since the control
  engine tends to be rather slow by current processor
  standards.  This can substantially limit the amount
  of traffic that can get fragmented (either by
  control plane limiting, or simply by the time it
  takes to do the fragmenting).
* Best Practices for most mainstream providers will
  not send (or severely limit) ICMP unreachables to
  protect the routing infrastructure (either with
  use of the control plane limits, or via other
  global values).
* Some percentage of intermediate routers will use
  RFC1918 addressing, or unadvertised internal
  routing IP addresses.
* Most mainstream providers use some form of IP address
  space validation, blocking traffic from invalid 
  addresses (this is also considered an anti-SPAM
  measure).  Non-advertised addresses (including the
  RFC1918 ones) are often considered invalid, even 
  if the packet is an ICMP unreachable, which will
  drop the packet.

One can talk about how some of these practices are
"bad", or "wrong", or "stupid".  That does not change
the fact that they are, and that they are more likely
to expand than be eliminated (have you not heard that
the Internet is "The Web"?)

And what this all says is that one should not depend on
UDP fragmentation nor icmp unreachables to determine
a paths current (or future, after rerouting) effective
MTU.  The common heuristics require occasional sending of
packets (of various sizes) to determine the useful
path MTU without (path) fragmentation, but one needs
to consider that a low rate, the control plane
limiting may still allow the router to fragment
or send ICMP unreachables.  It is only at the higher 
rates that the packets get dropped.


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