Re: draft-bonica-6man-frag-deprecate

[Ole Troan <[e-mail]>]

>>   This memo deprecates IPv6 fragmentation and the IPv6 fragment
>> header.
>>   Application and transport layer protocols SHOULD support effective
>>   PMTU discovery [RFC4821], since ICMP-based PMTU discovery [RFC1981]
>>   is unreliable. Any application or transport layer protocol that
>>   cannot support effective PMTU discovery MUST NOT in any
>> circumstances
>>   send IPv6 packets that exceed the IPv6 minimum MTU of 1280 bytes.
> 
> But, if those 1280 packets go into a tunnel they become (1280 + HLEN)
> and are vulnerable to loss. This is even before we start talking about
> tunnels-within-tunnels. It should therefore be well established by now
> that tunnels over IPv6 MUST be capable of some form of frag/reass.

in my understanding the 1280 minimum MTU for IPv6 was already chosen to accommodate nested tunnels.
the expectation made, I believe, was that native links support at least an MTU of 1500 bytes,
allowing 220 bytes for tunnel encap (5 IPv6 headers).

you want to avoid reassembly at tunnel endpoints, if you care about performance that is.

cheers,
Ole