Qustion about minimum Ethertype value.

[Gorry Fairhurst <gorry@erg.abdn.ac.uk>]

A question was raised regarding the Type value that should separate the 
ULE Type 1 and Type 2 headers.  The current rev -00 of the ULE Spec is 
incorrect (bearing in mind tagged LLC frames that may be longer than 
1500B). The current text says:

>>>
"Ethertypes were originally specified by Xerox under the DIX
framework for Ethernet. After specification of IEEE 802.3 [LLC], the
set of Ethertypes less than or equal to 1500 (0x05FC), assumed the
role of a length indicator. Ethernet receivers use this feature to
discriminate LLC format frames. Hence any IEEE Ethertype <= 1500
indicates an LLC frame, and the actual value indicates the length of
the LLC frame."
<<<

Based on the IEEE specification (see below), I propose that the text in 
the ULE Spec should now be ammended to:

>>>
"Ethertypes were originally specified by Xerox under the DIX framework 
for Ethernet. After specification of IEEE 802.3 [LLC], the set of 
Ethertypes less than 1536 (0x600 in hexadecimal), assumed the role of a 
length indicator. Ethernet receivers use this feature to discriminate 
LLC format frames. Hence any IEEE Ethertype < 1536 indicates an LLC 
frame, and the actual value indicates the length of the LLC frame."
<<<

This implies changes to the protocol text in other places also where 
1500/1501 are referenced in the text.

Do other people agree with this change?

Gorry Fairhurst
(ULE Author)



------

The IEEE allocation is described at:

http://standards.ieee.org/regauth/ethertype/type-tut.html

It says:

"The IEEE 802.3, 1998 Length/Type Field, originally known as EtherType, 
is a two-octet field which takes one of two meanings, depending on its 
numeric value. For numeric evaluation, the first octet is the most 
significant octet of this field. This document only deals with the type 
interpretation of the Length/Type Field.

See standards IEEE 802.3, 1998 Clause 3.2.6 Length/Type Field 
specifications and IEEE 802.1H-1995 for use of the Type Field with other 
media access methods.

When the value of this field is greater than or equal to 1536 decimal 
(equal to 0600 hexadecimal) the Type Field indicates the nature of the 
MAC client protocol (Type interpretation). The length and type 
interpretations of this field are mutually exclusive.

Therefore when the value of the two octet field is equal to or greater 
than 0600 hex then it is a Type Field and the value of the Type Field is 
obtained from the IEEE Type Field Registrar. New values obtained from 
the IEEE Type Field Registrar will not interfere with the existing Type 
Field assignments from Xerox or the IEEE. Former assignments are still 
valid."

The IANA view is:

Assignments:

Ethernet          Exp. Ethernet    Description          References
-------------     -------------   -----------           ----------
decimal  Hex      decimal  octal
  0000   0000-05DC   -       -    IEEE802.3 Length Field   [XEROX]
  0257   0101-01FF   -       -    Experimental             [XEROX]
  0512   0200        512   1000   XEROX PUP (see 0A00)   [8,XEROX]
  0513   0201        -      -     PUP Addr Trans (see 0A01)[XEROX]
         0400                     Nixdorf                  [XEROX]
  1536   0600       1536   3000   XEROX NS IDP         [133,XEROX]


The corresponding IEEE Registry is at:

http://standards.ieee.org/regauth/ethertype/type-pub.html