IP tunnel MTU

Shahab Vahabzadeh sh.vahabzadeh at gmail.com
Mon Oct 29 19:32:51 UTC 2012


Hi there,
I have the same problem in my network, I have GRE tunnel for transfering
users real internet traffic, they have problems with browsing websites like
yahoo.com or microsoft.com.
I had to set ip mtu 1500 to solve it, and it occurs fragmantation...
Thanks

On Mon, Oct 29, 2012 at 10:47 PM, Ray Soucy <rps at maine.edu> wrote:

> Sorry, glanced at this and thought it was someone having problems with
> tunnel MTU without adjusting TCP MSS.
>
> Nice work, though my preference is to avoid tunnels at all costs :-)
>
>
>
>
> On Mon, Oct 29, 2012 at 12:39 PM, Templin, Fred L
> <Fred.L.Templin at boeing.com> wrote:
> > 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
>
>
>
> --
> 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
>
>


-- 
Regards,
Shahab Vahabzadeh, Network Engineer and System Administrator

Cell Phone: +1 (415) 871 0742
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