On Tue, 19 Apr 2011, Bob Hinden wrote:
Dan,
On the other hand, the difference between 1500 and 1280 is so small, I
wonder if breaking things just because you want to send packets
at 1500 bytes makes a lot of sense.
One other thing, if this makes the IPv6 experience worse than industry
standard for IPv4, then maybe it is also not a good idea.
The IPv6 headers are 20 bytes bigger than IPv4 headers, so a fairer
comparison is 1500 against 1260 (1260=1280-20). That is, with a 1500
byte MTU with IPv4, the effective data payload is 1480 bytes (assuming
no IP options, which is a reasonable assumption with IPv4); with a
1280 byte MTU with IPv6, the effective data payload is 1240 bytes
(assuming no IPv6 extensions). That's a 16.2% reduction in data
payload size from IPv4 to IPv6, with a commensurate increase in
number of packets to send the same data (assuming MTU-sized packets).
I am a little confused by the comparisons being made in this thread.
There is no guarantee that an 1500 IPv4 packet won't be fragmented, so a path
that drops ICMPv4 packet too big messages will cause PMTU to fail. The 1280
number is the size of an MTU that IPv6 traffic can go on with out a need to be
fragmented (that is, no PMTU issue).
If a path can deliver 1500 byte IPv4 packets, it can also deliver 1500 byte
IPv6 packets. The resulting payloads will be 20 byte less for IPv6, but that
less than a 2% difference in payload size.
I doubt middle boxes are going to let ICMPv4 packet too big messages through,
and drop ICMPv6 packet too big messages.
Then middleboxes does not follow RFC 4890:
Recommendations for Filtering ICMPv6 Messages in Firewalls
Best Regards,
Janos Mohacsi
Am I missing something here?
Bob
This isn't quite "packets per second will increase by 16.2%", though,
as of course not all packets are 'full'. But there will be a pps
increase.
-d
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