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.temp...@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.temp...@boeing.com > >> -----Original Message----- >> From: Ray Soucy [mailto:r...@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.temp...@boeing.com> wrote: >> > Hi Roland, >> > >> >> -----Original Message----- >> >> From: Dobbins, Roland [mailto:rdobb...@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.temp...@boeing.com >> > >> >> ----------------------------------------------------------------------- >> >> Roland Dobbins <rdobb...@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
