From: "Terry Baranski" <tbaranski@mail.com> Date: Tue, 3 Feb 2004 16:42:55 -0600 Sender: owner-nanog@merit.edu

Leo Bicknell wrote:

...

Since most POS is 4470, adding a jumbo frame GigE edge makes this application work much more efficiently, even if it doesn't enable jumbo (9k) frames end to end. The interesting thing here is it means there absolutely is a PMTU issue, a 9K edge with a 4470 core.

This brings up the question of what other MTUs are common on the Internet, as well as which ones are simply defaults (i.e., could easily be increased) and which ones are the result of device/protocol limitations.

And why 4470 for POS? Did everyone borrow a vendor's FDDI-like default or is there a technical reason? PPP seems able to use 64k packets (as can the frame-based version of GFP, incidentally, POS's likely replacement).

4470 was, as you surmised, to allow a full sized FDDI packet to be packed into a single POS packet. At the time FDDI was using larger packets than anything else. Now the recommendation for research and education networks (Abilene, ESnet, NASA, and many Asian and European R&Es) is 9000 and, within that community, is almost universally adopted when the hardware will support it. -- R. Kevin Oberman, Network Engineer Energy Sciences Network (ESnet) Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab) E-mail: oberman@es.net Phone: +1 510 486-8634