From: David Arnold (arnold_at_dstc.edu.au)
Date: Tue Oct 21 2003 - 02:07:33 PDT
-->"John" == John Skinner <john_at_johnskinner.net> writes:
John> EtherTalk IS NOT AppleTalk encapsulated over TCP/IP
right. in fact, it's AppleTalk over Ethernet.
in summary, ethernet frames have a field which allows different
higher-level protocols encapsulated in the frame to be distinguished.
it's possible for an Ethernet switch to drop frames depending upon the
value of this field.
just when i was about to post i remembered that there was more to it
than that. so i pulled out some references, and since i looked it up,
i though i might as well post the full details of how this works.
if anyone else cares about the bit below, you get my Geek of the Week
award :-)
there are multiple versions of Ethernet, and they use different
mechanisms to make this distinction.
DIX-style ethernet (aka Ethernet II) can be used for IP (see RFC-894).
it uses a 16 bit field following the destination and source MAC
addresses to distinguish payload types. the value for IPv4 is 0x0800;
ARP is 0x0806.
to my knowledge, EtherTalk cannot use DIX ethernet frames (at least
Inside AppleTalk, 2nd ed doesn't mention it).
IEEE 802-style ethernet uses a two-part frame header. both IPv4 and
EtherTalk use the Type 1 802.2 packets (IPv4 over IEEE-802 networks is
defined in RFC-1042).
the first part, the 802.2 LLC header, is 3 bytes.
for both EtherTalk and IPv4, these are 0xAA, 0xAA and 0x3.
the second part distinguishes all protocols using the 0xAA SAP, and is
called the SNAP header. it is 5 bytes long
for IPv4, these bytes are 0x00, 0x00, 0x00, 0x08, 0x00
for ARP, these bytes are 0x00, 0x00, 0x00, 0x08, 0x06
for AppleTalk data, these bytes are 0x08, 0x00, 0x07, 0x80, 0x9b
for AARP, these bytes are 0x00, 0x00, 0x00, 0x80, 0xf3
i fired up NetNames on my ump2k to verify this with ethereal, but i'm
getting a -61001 error from the chooser, so i'm sorry to say i can't
confirm any of this theory :-)
d
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