[to our little ad hoc group AND internet area list] On 2-aug-2007, at 1:38, Templin, Fred L wrote:
Instead, I have proposed a BCP approach on the int-area mailing list. Here's what I said there:
+ Focusing only on the ULP, can we say that 1500 bytes is + the nominal MTU of today's Internet, and therefore all + attached devices SHOULD configure a minMRU of 1500?
We can say that, but is such a statement helpful? I can't remember ever seeing someone reduce their MTU below 1500 when they are physically and logically capable of receiving 1500 byte packets.
Then, + can we also say that applications that send packets larger + than 1500 bytes (with DF=1) are RECOMMENDED to use RFC4821?
RFC 4821 should be recommended whenever path MTU discovery is used, which is pretty much in all implementations. PMTUD black holes are often the result of MTUs lower than 1500 in the path so limiting this recommendation to 1500+ interfaces makes little sense.
+ Finally, could we also say that tunnel decapsulators SHOULD + configure a minMRU of 2048 to account for encapsulations + that might extend a 1500 byte ULP out to the IEEE 802.as + maximum frame size?
Not sure what you mean here. Obviously people should be prepared to receive the largest possible packets supported by the hardware configuration in use. That means 1500 bytes for ethernet, or whatever jumboframe size is in effect. Trying to receive more than the hardware will do is an exercise in futility...
However, we CAN recommend that people make their tunnel MRUs equal to the size of the largest physical interface MRU - encapsulation size, even if the tunnel MTU is smaller than that. But I would recommend making the tunnel MTU equal to that MRU rather than use the IPv6-in- IPv4 tunneling practice of a 1280 byte MTU to accommodate multiple levels of encapsulation.
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