Re: [v4tov6transition] IPv6 VPNs configured over 1280 MTU tunnels

"Templin, Fred L" <Fred.L.Templin@boeing.com> Wed, 13 October 2010 15:04 UTC

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From: "Templin, Fred L" <Fred.L.Templin@boeing.com>
To: Washam Fan <washam.fan@gmail.com>
Date: Wed, 13 Oct 2010 08:05:30 -0700
Thread-Topic: [v4tov6transition] IPv6 VPNs configured over 1280 MTU tunnels
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Cc: Softwires <softwires@ietf.org>, "v4tov6transition@ietf.org" <v4tov6transition@ietf.org>
Subject: Re: [v4tov6transition] IPv6 VPNs configured over 1280 MTU tunnels
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Hi Washam, 

> -----Original Message-----
> From: Washam Fan [mailto:washam.fan@gmail.com] 
> Sent: Monday, October 11, 2010 10:48 PM
> To: Templin, Fred L
> Cc: Rémi Després; Softwires; v4tov6transition@ietf.org
> Subject: Re: [v4tov6transition] IPv6 VPNs configured over 
> 1280 MTU tunnels
> 
> Hi Fred,
> 
> I might see your points.

Good.

> Let H represents Host, N represents NAT, S represents the 6a44
> servers. PMTU(H,S) stands for the PMTU between H and S. HOME_MTU
> represents MTU within home LAN (i.e., unmanaged network), SP_MTU
> represents MTU within SP network(i.e., managed network). We assume
> both SP_MTU and SP_MTU should exceed or equal to 1308.

This is an incomplete characterization. PMTU is not
necessarily symmetric, e.g., even just within the end
user it could be different in the H->N direction than
in the N->H direction. Also, there could be multiple N's
in the path; each imparting their own MTU uncertainties.
 
> if HOME_MTU>=SP_MTU, PMTU(H,S)=SP_MTU, the H could configure SP_MTU-28
> as the IPv6 virtual interface MTU.

A couple of things here. First, you seem to be assuming a
static S instead of a redundantly replicated S with anycast,
where there may be many distinct paths with varing SP_MTUs to
reach the multiple S's. Second, how does H discover SP_MTU;
by engaging in an initial probing by sending large packets
and getting a PTB message back? Again, this won't give a
deterministic SP_MTU value if  S is replicated across paths
with varying PMTUs.

> H->S direction: if S received or trigger any ICMPv6 PTB messages, it
> can forward ICMPv6-in-UDP-in-IPv4 to H.

You keep saying this, and I keep saying that you are right
but that this has never been a matter for concern. PMTUD
*beyond* the tunnel is not in question; it is only PMTUD
*within* the tunnel that bears further discussion.

> S->H direction: S would reject any IPv6 packets exceeding SP_MTU-28
> with ICMPv6 PTB.
> I don't see problems here.

Huh? Is S supposed to cache the SP_MTU to each and every
potential host H? I assume the goal is for a completely
stateless S - right?

> If HOME_MTU<SP_MTU,PMTU(H,S)=HOME_MTU, the H could configure
> HOME_MTU-28 as IPv6 virtual interface MTU.

So again, H could discover this only by sending probes?
And again, any probing would be non-deterministic if S
is an anycast (and not unicast) address.

> H->S direction: if S received or trigger any ICMPv6 PTB messages, it
> can forward ICMPv6-in-UDP-in-IPv4 to H.

Yes, we have confirmed this several times now; not
a point in question.

> I don't expect the size of the encapsulated packet is too much.

Encapsulated PTB could be as big as 1280 + 20 + 8.

> S->H direction: if S forward some IPv6 packets whose size is between
> HOME_MTU-28 and SP_MTU-28, N might trigger a ICMPv4 PTB or filtering
> them. For the former, S might don't have enough info to infer which
> IPv6 original packet it is related,

Right.

> for the latter, black hole might occur. I see problems here.

Right.

Fred
fred.l.templin@boeing.com
 
> Thanks,
> washam
> 
> 2010/10/12 Templin, Fred L <Fred.L.Templin@boeing.com>om>:
> > Hi Washam,
> >
> >> -----Original Message-----
> >> From: Washam Fan [mailto:washam.fan@gmail.com]
> >> Sent: Monday, October 11, 2010 7:38 PM
> >> To: Templin, Fred L
> >> Cc: Rémi Després; Softwires; v4tov6transition@ietf.org
> >> Subject: Re: [v4tov6transition] IPv6 VPNs configured over
> >> 1280 MTU tunnels
> >>
> >> Hi,
> >>
> >> If 6a44 deployed in a managed ISP network, the 6a44 client could be
> >> configured with IPv4_MTU-28 for its IPv6 MTU.
> >
> > My understanding is that the 6a44 server is in the managed
> > ISP network, and the 6a44 client is in the UNmanaged end
> > user network.
> >
> >> For inbound direction,
> >> 6a44 server would reject any IPv6 packets whose size exceeds
> >> IPv4_MTU-28 with a ICMPv6 PTB message, so no ICMPv6-ICMPv4 
> translation
> >> needed.
> >
> > Huh? How does the server have any idea what MTU the client
> > set on its tunnel interface? The client is behind a NAT in
> > an unmanaged end user network. Are you expecting the server
> > to probe the path MTU to the client and maintain state? (An
> > IRON server might be in position to do something like that,
> > but I was assuming the 6a44 server would be stateless, right?)
> >
> >> For outbound direction, 6a44 server would encapsulate any
> >> ICMPv6 PTB messages it received in UDP in IPv4, and then forward to
> >> the 6a44 client, so no NAT filtering worried.
> >
> > Correct, but no one has suggested an MTU problem for what
> > happens to IPv6 packets *beyond* the tunnel; we are only
> > discussing the case of MTU issues *within* the tunnel.
> >
> > Fred
> > fred.l.templin@boeing.com
> >
> >> Thanks,
> >> washam
> >>
> >> 2010/10/12 Templin, Fred L <Fred.L.Templin@boeing.com>om>:
> >> > Remi,
> >> >
> >> >> -----Original Message-----
> >> >> From: Rémi Després [mailto:remi.despres@free.fr]
> >> >> Sent: Monday, October 11, 2010 10:05 AM
> >> >> To: Templin, Fred L
> >> >> Cc: Washam Fan; Softwires; v4tov6transition@ietf.org
> >> >> Subject: Re: [v4tov6transition] IPv6 VPNs configured over
> >> >> 1280 MTU tunnels
> >> >>
> >> >>
> >> >> Le 11 oct. 2010 à 18:42, Templin, Fred L a écrit :
> >> >>
> >> >> >> ...
> >> >> >> Actually, the 6a44 specification should, instead of 1280,
> >> >> >> require IPv4 MTU - 28 octets, both for hairpinning and
> >> >> >> traversal cases.
> >> >> >
> >> >> > How can you be sure that IPv4 PMTUD will work in
> >> >> > the traversal case?
> >> >>
> >> >> In the to-host direction, because the ISP network is all what
> >> >> is left to traverse before reaching the CPE.
> >> >
> >> > In what you call the to-host direction, any ICMPv4
> >> > returned from the ISP network might not have enough
> >> > information for stateless translation to ICMPv6.
> >> >
> >> >> In the from host direction, one can't be sure, but 
> doesnt' need to.
> >> >> If a smaller PMTU is encountered further downstream, a PTB
> >> >> ICMPv6 error message will be returned from there.
> >> >
> >> > In the from-host direction, any ICMPv4 returned from
> >> > the ISP network might not be delivered to the tunnel
> >> > endpoint due to NAT filtering, and might not have
> >> > enough information for stateless translation to ICMPv6.
> >> >
> >> > Fred
> >> > fred.l.templin@boeing.com
> >> >
> >> >> RD
> >> >
> >>
>