nanog mailing list archives
RE: PMTU-D: remember, your load balancer is broken
From: rdobbins () netmore net
Date: Wed, 14 Jun 2000 09:29:48 -0700
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 <rdobbins () netmore net> // 818.535.5024 voice -----Original Message----- From: Roeland Meyer (E-mail) [mailto:rmeyer () mhsc com] Sent: Wednesday, June 14, 2000 9:07 AM To: Valdis.Kletnieks () vt edu; 'Marc Slemko' Cc: nanog () merit edu Subject: RE: PMTU-D: remember, your load balancer is broken
Valdis.Kletnieks () vt edu: 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).
Current thread:
- Re: PMTU-D: remember, your load balancer is broken, (continued)
- Re: PMTU-D: remember, your load balancer is broken Ryan O`Connell (Jun 14)
- Re: PMTU-D: remember, your load balancer is broken Steven M. Bellovin (Jun 13)
- Re: PMTU-D: remember, your load balancer is broken Valdis . Kletnieks (Jun 13)
- Re: PMTU-D: remember, your load balancer is broken Brett Frankenberger (Jun 14)
- Re: PMTU-D: remember, your load balancer is broken Greg A. Woods (Jun 15)
- Re: PMTU-D: remember, your load balancer is broken Greg A. Woods (Jun 15)
- Re: PMTU-D: remember, your load balancer is broken Marc Slemko (Jun 15)
- Re: PMTU-D: remember, your load balancer is broken Greg A. Woods (Jun 15)
- Re: PMTU-D: remember, your load balancer is broken Valdis . Kletnieks (Jun 13)
- Re: PMTU-D: remember, your load balancer is broken Stephen Sprunk (Jun 14)
- Re: PMTU-D: remember, your load balancer is broken Andrew Brown (Jun 14)
- Re: PMTU-D: remember, your load balancer is broken Valdis . Kletnieks (Jun 15)
- OT: PMTU-D and Chickens Rural CNE (Jun 15)
- RE: PMTU-D: remember, your load balancer is broken Deepak Jain (Jun 14)
- RE: PMTU-D: remember, your load balancer is broken John Fraizer (Jun 14)
- RE: PMTU-D: remember, your load balancer is broken Deepak Jain (Jun 14)