From:Steve Adams
Date:17-Nov-2000 22:40
Subject:   MTU and SDU for SQL*Net

You are right that the ideal SDU (session data unit) size is the MTU (maximum transmission unit) size less the relevant headers, not the MTU itself. However, your numbers are not quite right. The 1500 bytes quoted as the MTU for Ethernet is exclusive of the 14-bytes Ethernet header and 4-byte Ethernet CRC trailer. But there is also an IP header, a TCP header and an SQL*Net/Net8 header which account for 20, 20 and 10 bytes respectively. So the ideal SDU for Ethernet is actually 1450 bytes.

Just for completeness, the equivalent numbers for the other common link layer protocols are 4302 bytes for FDDI, 1442 bytes for IEEE 802.3/802.2, 526 bytes for X.25 and just 246 bytes for PPP. The numbers for IPC are 32718 bytes for NT, 8142 bytes for HP-UX, and 5070 bytes on all other platforms.

I've talked to Oracle about MTU and SDU to tell that their advise to set SDU to o/s MTU is not correct. After I explained why they say it sounds like I am right, but it seems they ignore me further on. They start asking if network is really troubling my site. That wasn't the point, I want clarity in my point!

What is my point? If o/s MTU is 1500 (my hpux mtu's 1500), then a SDU of 1500 is not optimal, because within the o/s MTU there must be space for heading (20 bytes) and footing (4 bytes, I think). So effective data that an IP package can contain will be 1500-(20+4)= 1476 bytes! If SDU is set to 1500, the data must broken down to 1476 and 24 bytes!

Please give my your opinion.