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North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical RE: PMTU-D: remember, your load balancer is broken
The 576 value of the MS PPP MTU is merely a default - it can be changed with a registry hack. Yes, fragmentation has indeed become a Great Evil due to the large amounts of data we're pushing, and the time/resources required for fragmentation/defragmentation. Forcing excessive fragmentation/defragmentation is an effective DoS. As far as increasing the MTU size on your LAN links, you need to exercise a lot of care when so doing. I personally have never tried to change the MTU size on an Ethernet segment of any type (Ethernet_II/1500 has worked admirably, and I'm unsure of the result if I tried it); on Token Ring, going up to 4096 has indeed been beneficial in the past when dealing with large database writes, etc. Of course, the protocol I was using at the time supported 4096-byte frame sizes on Token Ring. I thought the frame-size limits for Gigabit Ethernet were 64-1518/1522 bytes? And isn't that the limit on most host IP stacks for Ethernet media? Or am I off in left field, here? Finally, I would say that on any medium, <100% utilization in and of itself isn't grounds for fiddling with the MTU. There are lots of other things to look at, first. --------------------------------------------------------------- Roland Dobbins <[email protected]> // 818.535.5024 voice -----Original Message----- From: Roeland Meyer (E-mail) [mailto:[email protected]] Sent: Wednesday, June 14, 2000 9:07 AM To: [email protected]; 'Marc Slemko' Cc: [email protected] Subject: RE: PMTU-D: remember, your load balancer is broken > [email protected]: Wednesday, June 14, 2000 8:07 AM > > On Tue, 13 Jun 2000 22:36:08 MDT, Marc Slemko said: > > It is also a concern that, in my experience, many of the links with > > MTUs <1500 are also the links with greater packet loss, etc. so > > you really don't want fragmentation on them. > > The worst part here is that I suspect that most of these > links (just on > sheer numbers of shipped product) are the aformentioned Win98 576-MTU. I just set my dial PPP ports to MTU=512+40=552, is this wrong? Where does the MTU=576 number come from? > I seem to remember that the *original* motivation for slow-start and > all that was Van Jacobson's observation that the most common cause of > a TCP retransmit was that an *entire* packet had been silently dropped > due to queueing congestion, and could thus be treated identical to > an ICMP Source Quench. > > Has this changed? Has "fragmentation" become a Great Evil, > rather than > an annoyance that some links have to deal with? I'm having some trouble getting full throughput from a GigE pipe. Even in the 100baseTX/FDX down-stream, I'm not getting full link utilization (everything on switches, Cat6509 and 3512XLs). I'm considering increasing MTU sizes to MTU=4096+40, or even larger. Most of the data transmissions fall into the 5KB-50KB range. The site can be considered a large portal. What would be the effect on my upstream? Would it create problems? The only systems that see the Internet are the web-servers (dual NICs).
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