Shim6, was: Re: filtering /48 is going to be necessary

Thu Mar 15 04:48:37 UTC 2012

William Herrin wrote:

> I've been an IRTF RRG participant and in my day job I build backend
> systems for mobile messaging devices used in some very challenging and
> very global IP and non-IP environments.

I know non-IP mobile environment is heavily encumbered. So, I
can understand why you insist on using DNS for mobility only
to make IP mobility as encumbered as non-IP ones.

> Au contraire. Triangle elimination is a problem because the IP address
> can't change with session survivability. But that's because TCP and
> UDP require it. If A follows from B and B follows from C then A
> follows from C: TCP is at fault.

If a correspondent host CH send packets to a mobile host MH,
it may be tunneled by a home agent HA or, with triangle
elimination, tunneled by CH itself, in both of which cases,
IP address of internal packets within tunnels are that of
CH and MH's home address, which is handled by TCP just
normally.

>> Ignoring that DNS does not work so fast, TCP becomes "it wasn't
>> sure what addresses it should be talking to" only after a long
>> timeout.
> 
> Says who? Our hypothetical TCP can become "unsure" as soon as the
> first retransmission if we want it to. It can even become unsure when
> handed a packet to send after a long delay with no traffic. There's
> little delay kicking off the recheck either way.

That may be a encumbered way of doing things in non-IP, or
bell headed, mobile systems, where 0.05 second of voice
loss is acceptable but 0.2 second of voice loss is
significant.

However, on the Internet, 0.05 second of packet losses can
be significant and things work end to end.

In this case, your peer, a mobile host, is the proper
end, because it is sure when it has lost or are
losing a link.

Then, the end establishes a new link with a new IP and
initiate update messages for triangle elimination at
proper timing without unnecessary checking.

According to the end to end argument of Saltzer et. al:

      The function in question can completely and correctly be
      implemented only with the knowledge and help of the application
      standing at the end points of the communication system.

the mobility module of the mobile host has "the knowledge"
for proper timing to update triangle elimination "the function
in question".

					Masataka Ohta