Re: Is Fragmentation at IP layer even needed ?

["Joel M. Halpern" <[e-mail]>]

I would note that tunnel mechanisms either need a very good path "size" 
reporting mechanism or a way to fragment.  As the tunnel entry and exit 
are end-points, and as path "size" discovery has many problems, many 
tunnel mechanisms do us IP (v4 or v6) fragmentation.
Admittedly, none of them like it, and most recommend that if you can do 
path MTU discovery you should, but we want things to work.

Yours,
Joel

On 2/8/16 1:23 PM, Warren Kumari wrote:
>
>
> On Mon, Feb 8, 2016 at 9:05 AM David Borman <[e-mail]
> <mailto:[e-mail]>> wrote:
>
>
>      > On Feb 8, 2016, at 10:08 AM, Alexey Eromenko <[e-mail]
>     <mailto:[e-mail]>> wrote:
>      >
>      > On Mon, Feb 8, 2016 at 5:52 PM, John Levine <[e-mail]
>     <mailto:[e-mail]>> wrote:
>      > >2. What kind of UDP applications use such big packets, over 1280
>     bytes ?
>      >
>      > >Um, the DNS with EDNS0, and particularly with DNSSEC?  Is this a
>     trick question?
>      >
>      >
>      > Yes.
>      > The trick lies right there in IPv6 specification:
>      >
>      > "A node must be able to accept a fragmented packet that, after
>      >    reassembly, is as large as 1500 octets."
>      > ... which is not much greater than 1280 bytes defined as minimum
>     MTU for IPv6.
>      >
>      > Basically, Fragmentation, as specified in RFC-2460 (IPv6), solves
>     the problem ONLY for packet-sized between 1280 and 1500 bytes. Which
>     is why I doubt if it is useful at all.
>
>     That’s the minimum required implementation, which guarantees that
>     you can send at least a basic ethernet sized packet to any host.
>     What is omitted in the above qoute is next 2 sentences:
>         "A node is permitted to
>         accept fragmented packets that reassemble to more than 1500 octets.
>         An upper-layer protocol or application that depends on IPv6
>         fragmentation to send packets larger than the MTU of a path should
>         not send packets larger than 1500 octets unless it has assurance
>     that
>         the destination is capable of reassembling packets of that larger
>         size.”
>     So if you are writing an application that needs >1500 octets, use an
>     IPv6 implementation that supports >1500 octet fragmentation and
>     reassembly.
>
>
> ... but as an application writer (or, basically anyone else), I have no
> control over the "IPv6 implementation". Even if I'm in an environment
> where I do control the OS / model of all devices, and I know they
> support >1500 octet, it seems like a bad idea to *rely* on that.
> Sometime I'm going to want to change OS / add some other device, be able
> to interact with some other system. This sounds like vendor lock at its
> worst...
>
> W
>
>
>
>                      -David Borman
>
>      >
>      > (I also understand, that IPv4 fragmentation is much more
>     flexible, yes. It can go as small as 68 byte packets.)
>