IP tunnel MTU

Mon Oct 29 16:39:19 UTC 2012

Hi Ray,

MSS rewriting has been well known and broadly applied for a long
time now, but only applies to TCP. The subject of MSS rewriting
comes up all the time in the IETF wg discussions, but has failed
to reach consensus as a long-term alternative.

Plus, MSS rewriting does no good for tunnels-within-tunnels. If
the innermost tunnel rewrites MSS to a value that *it* thinks is
safe there is no guarantee that the packets will fit within any
outer tunnels that occur further down the line.

What I want to get to is an indefinite tunnel MTU; i.e., admit
any packet into the tunnel regardless of its size then make any
necessary adaptations from within the tunnel. That is exactly
what SEAL does:

 https://datatracker.ietf.org/doc/draft-templin-intarea-seal/

Thanks - Fred
fred.l.templin at boeing.com

> -----Original Message-----
> From: Ray Soucy [mailto:rps at maine.edu]
> Sent: Monday, October 29, 2012 7:55 AM
> To: Templin, Fred L
> Cc: Dobbins, Roland; NANOG list
> Subject: Re: IP tunnel MTU
> 
> The core issue here is TCP MSS. PMTUD is a dynamic process for
> adjusting MSS, but requires that ICMP be permitted to negotiate the
> connection.  The realistic alternative, in a world that filters all
> ICMP traffic, is to manually rewrite the MSS.  In IOS this can be
> achieved via "ip tcp adjust-mss" and on Linux-based systems, netfilter
> can be used to adjust MSS for example.
> 
> Keep in mind that the MSS will be smaller than your MTU.
> Consider the following example:
> 
>  ip mtu 1480
>  ip tcp adjust-mss 1440
>  tunnel mode ipip
> 
> IP packets have 20 bytes of overhead, leaving 1480 bytes for data.  So
> for an IP-in-IP tunnel, you'd set your MTU of your tunnel interface to
> 1480.  Subtract another 20 bytes for the tunneled IP header and 20
> bytes (typical) for your TCP header and you're left with 1440 bytes
> for data in a TCP connection.  So in this case we write the MSS as
> 1440.
> 
> I use IP-in-IP as an example because it's simple.  GRE tunnels can be
> a little more complex.  While the GRE header is typically 4 bytes, it
> can grow up to 16 bytes depending on options used.
> 
> So for a typical GRE tunnel (4 byte header), you would subtract 20
> bytes for the IP header and 4 bytes for the GRE header from your base
> MTU of 1500.  This would mean an MTU of 1476, and a TCP MMS of 1436.
> 
> Keep in mind that a TCP header can be up to 60 bytes in length, so you
> may want to go higher than the typical 20 bytes for your MSS if you're
> seeing problems.
> 
> 
> 
> 
> On Tue, Oct 23, 2012 at 10:07 AM, Templin, Fred L
> <Fred.L.Templin at boeing.com> wrote:
> > Hi Roland,
> >
> >> -----Original Message-----
> >> From: Dobbins, Roland [mailto:rdobbins at arbor.net]
> >> Sent: Monday, October 22, 2012 6:49 PM
> >> To: NANOG list
> >> Subject: Re: IP tunnel MTU
> >>
> >>
> >> On Oct 23, 2012, at 5:24 AM, Templin, Fred L wrote:
> >>
> >> > Since tunnels always reduce the effective MTU seen by data packets
> due
> >> to the encapsulation overhead, the only two ways to accommodate
> >> > the tunnel MTU is either through the use of path MTU discovery or
> >> through fragmentation and reassembly.
> >>
> >> Actually, you can set your tunnel MTU manually.
> >>
> >> For example, the typical MTU folks set for a GRE tunnel is 1476.
> >
> > Yes; I was aware of this. But, what I want to get to is
> > setting the tunnel MTU to infinity.
> >
> >> This isn't a new issue; it's been around ever since tunneling
> technologies
> >> have been around, and tons have been written on this topic.  Look at
> your
> >> various router/switch vendor Web sites, archives of this list and
> others,
> >> etc.
> >
> > Sure. I've written a fair amount about it too over the span
> > of the last ten years. What is new is that there is now a
> > solution near at hand.
> >
> >> So, it's been known about, dealt with, and documented for a long time.
> In
> >> terms of doing something about it, the answer there is a) to allow the
> >> requisite ICMP for PMTU-D to work to/through any networks within your
> span
> >> of administrative control and b)
> >
> > That does you no good if there is some other network further
> > beyond your span of administrative control that does not allow
> > the ICMP PTBs through. And, studies have shown this to be the
> > case in a non-trivial number of instances.
> >
> >> b) adjusting your own tunnel MTUs to
> >> appropriate values based upon experimentation.
> >
> > Adjust it down to what? 1280? Then, if your tunnel with the
> > adjusted MTU enters another tunnel with its own adjusted MTU
> > there is an MTU underflow that might not get reported if the
> > ICMP PTB messages are lost. An alternative is to use IP
> > fragmentation, but recent studies have shown that more and
> > more operators are unconditionally dropping IPv6 fragments
> > and IPv4 fragmentation is not an option due to wrapping IDs
> > at high data rates.
> >
> > Nested tunnels-within-tunnels occur in operational scenarios
> > more and more, and adjusting the MTU for only one tunnel in
> > the nesting does you no good if there are other tunnels that
> > adjust their own MTUs.
> >
> >> Enterprise endpoint networks are notorious for blocking *all* ICMP (as
> >> well as TCP/53 DNS) at their edges due to 'security' misinformation
> >> propagated by Confused Information Systems Security Professionals and
> >> their ilk.  Be sure that your own network policies aren't part of the
> >> problem affecting your userbase, as well as anyone else with a need to
> >> communicate with properties on your network via tunnels.
> >
> > Again, all an operator can control is that which is within their
> > own administrative domain. That does no good for ICMPs that are
> > lost beyond their administrative domain.
> >
> > Thanks - Fred
> > fred.l.templin at boeing.com
> >
> >> -----------------------------------------------------------------------
> >> Roland Dobbins <rdobbins at arbor.net> // <http://www.arbornetworks.com>
> >>
> >>         Luck is the residue of opportunity and design.
> >>
> >>                      -- John Milton
> >>
> >
> >
> 
> 
> 
> --
> Ray Patrick Soucy
> Network Engineer
> University of Maine System
> 
> T: 207-561-3526
> F: 207-561-3531
> 
> MaineREN, Maine's Research and Education Network
> www.maineren.net