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NIST and SP800-119



On 2011-02-16, at 02:44, Douglas Otis wrote:

> Routers indicate local MTUs, but minimum MTUs are not assured to have 1280 octets when IPv4 translation is involved.
> See Section 5 in rfc2460.

I've heard that interpretation of 2460 before from Bill Manning, but I still don't see it myself. The text seems fairly clear that 1280 is the minimum MTU for any interface, regardless of the type of interface (tunnel, PPP, whatever). In particular,

   Links that have a configurable MTU (for example, PPP links [RFC-
   1661]) must be configured to have an MTU of at least 1280 octets; it
   is recommended that they be configured with an MTU of 1500 octets or
   greater, to accommodate possible encapsulations (i.e., tunneling)
   without incurring IPv6-layer fragmentation.

That same section indicates that pMTUd is strongly recommended in IPv6 rather than mandatory, but in the context of embedded devices that can avoid implementing pMTUd by never sending a packet larger than the minimum MTU. Such devices would break if there was an interface (of any kind) in the path with a sub-1280 MTU.


Joe