[Oberon] MTU and Ethernet NIC drivers.

PETER EASTHOPE peasthope at shaw.ca
Thu May 7 19:42:33 MEST 2009


Whereas two NO systems connected to the Internet 
via Debian Linux receive files by FTP and receive 
messages by POP3, a sizeable file or message 
can not be sent.  I am considering whether this 
can be solved by reducing the MTU in NO.

Net3Com509.Mod has MaxPkt = 1514.

Net3Com90x.Mod has EthernetFrameSize = 1514.

NetNe2000.Mod has MaxPacketSize = 1516; 
(* 1514 + 2 byte padding to 32 bit boundary *).

NetBase.Mod has MaxDataLen* = 1514.

(1) Are these four constants synonyms?  Presumeably 
they include the MAC header and the CRC checksum.

Length of Mac header = 14.
Length of checksum = 4.
So, length of payload = MTU = 1514 - 14 - 4 = 1496.

(2) Assuming the answer to (1) is "yes" and given that 
RFC 1191 specifies MTU = 1500 for Ethernet, why was 
the value of 1514 rather than 1518 used in two drivers?  

(3) Is NetBase.MaxDataLen = Net3Com509.MaxPkt 
imperative?  If so, definition of MaxPkt rather than 
use of NetBase.MaxDataLen is bad practice.  Correct?

(4) Any thoughts on the observed problem in sending 
non-trivial messages and files?

Thanks for any response,            ... Peter E.

http://carnot.yi.org/ = http://carnot.pathology.ubc.ca/

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