Re: PMTU-D: remember, your load balancer is broken
Valdis.Kletnieks@vt.edu writes:
Has this changed? Has "fragmentation" become a Great Evil, ... ?
Yes. http://research.compaq.com/wrl/techreports/abstracts/87.3.html says: Research Report 87/3, December 1987 87.3 -- Fragmentation Considered Harmful Christopher A. Kent, Jeffrey C. Mogul PostScript Version gzipped PostScript Version Adobe Acrobat PDF Version Internetworks can be built from many different kinds of networks, with varying limits on maximum packet size. Throughput is usually maximized when the largest possible packet is sent; unfortunately, some routes can carry only very small packets. The IP protocol allows a gateway to fragment a packet if it is too large to be transmitted. Fragmentation is at best a necessary evil; it can lead to poor performance or complete communication failure. There are a variety of ways to reduce the likelihood of fragmentation; some can be incorporated into existing IP implementations without changes in protocol specifications. Others require new protocols, or modifications to existing protocols. wrl-techops@pa.dec.com Copyright © 2000 Compaq Computer Corporation I was there, I saw the research that went into this, I know the guys who did the work, and I agree completely with the conclusions thus presented.
On Sat, 17 Jun 2000 11:59:30 PDT, Paul Vixie <vixie@mibh.net> said:
Valdis.Kletnieks@vt.edu writes:
Has this changed? Has "fragmentation" become a Great Evil, ... ?
Yes. http://research.compaq.com/wrl/techreports/abstracts/87.3.html says: (abstract trimmed) Research Report 87/3, December 1987
87.3 -- Fragmentation Considered Harmful Fragmentation is at best a necessary evil; it can lead to poor performance or complete communication failure. There are a variety of
Yeah, I've known about that for a while. What I *meant* was: Has fragmentation been reclassified from "necessary evil that can cause problems" to "Great Evil that must be avoided at all costs"? For instance, we probably all agree that fragging on a core router is Bad Juju and should be avoided if at all possible. On the other hand, how far should we jump through hoops (such as PMTU-D etc) to avoid fragging on a last-hop modem link from a terminal server to a PC? I already spend far too much of my day (even with a lot of tools) sending flame-grams to ISPs who drop us spam, or have open mail relays, or start running NTP or tools that query ports 13/37 and forget to open the firewall, and then complain about my machine probing them.. Enabling PMTU-D (even if it won't buy *my* boxes that much since their local MTU is 1500) and getting people to fix their ICMP configurations for the benefit of those sites that WILL profit is an option, but only if there's general consensus that it's a fight worth fighting... Valdis Kletnieks Operating Systems Analyst Virginia Tech
FYI, there are core routers out there that can support line rate HW-based fragmentation. I believe this is necessary in any router with heterogeneous interfaces. Bora ----- Original Message ----- From: <Valdis.Kletnieks@vt.edu> To: "Paul Vixie" <vixie@mibh.net> Cc: <nanog@merit.edu> Sent: Saturday, June 17, 2000 10:06 PM Subject: Re: PMTU-D: remember, your load balancer is broken
On Sat, 17 Jun 2000 11:59:30 PDT, Paul Vixie <vixie@mibh.net> said:
Valdis.Kletnieks@vt.edu writes:
Has this changed? Has "fragmentation" become a Great Evil, ... ?
Yes. http://research.compaq.com/wrl/techreports/abstracts/87.3.html
says:
(abstract trimmed)
Research Report 87/3, December 1987
87.3 -- Fragmentation Considered Harmful Fragmentation is at best a necessary evil;
it
can lead to poor performance or complete communication failure. There are a variety
of
Yeah, I've known about that for a while. What I *meant* was:
Has fragmentation been reclassified from "necessary evil that can cause problems" to "Great Evil that must be avoided at all costs"?
For instance, we probably all agree that fragging on a core router is Bad Juju and should be avoided if at all possible. On the other hand, how far should we jump through hoops (such as PMTU-D etc) to avoid fragging on a last-hop modem link from a terminal server to a PC?
I already spend far too much of my day (even with a lot of tools) sending flame-grams to ISPs who drop us spam, or have open mail relays, or start running NTP or tools that query ports 13/37 and forget to open the firewall, and then complain about my machine probing them..
Enabling PMTU-D (even if it won't buy *my* boxes that much since their local MTU is 1500) and getting people to fix their ICMP configurations for the benefit of those sites that WILL profit is an option, but only if there's general consensus that it's a fight worth fighting...
Valdis Kletnieks Operating Systems Analyst Virginia Tech
participants (3)
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Bora Akyol -
Paul Vixie -
Valdis.Kletnieks＠vt.edu