Conference 7.286::fddi

1104.0. "A note to those who implement new protocols on LANs" by KONING::KONING (Paul Koning, B-16504) Tue Oct 05 1993 15:55

To all of you who may be implementing new LAN-based protocols at some point:

If you are designing some application that uses a new protocol directly layered
on top of a LAN datalink (as opposed to using, say, DECnet or IP), you will
need to use an appropriate protocol identifier.  In many cases, such protocols
also need some sort of multicast.

Since not everyone who might need to know actually does know the relevant
procedures, let me mention them.  Feel free to distribute this information
widely (within Digital).

The protocol in a LAN protocol message can be identified in one of three
ways:

1. For Ethernet format packets, by the protocol type (2 bytes).
2. For 802 format packets:
	a. by the SAP address (1 byte), or
	b. by the SNAP Protocol ID (5 bytes) 

Case 2a applies only to formally standardized protocols; it is not an option
for custom protocols.

If you think you may want your protocol to be in a product (now or later) you
MUST obtain a registered protocol identifier.  This ensures that every
protocol has a distinct identifier.  I'm the registrar for this; get in
touch with me (early on if at all possible!) for the details.

Something you should consider right away: Digital does not have any Ethernet
protocol types available for assignment.  However, we have LOTS of 802 SNAP
Protocol Identifiers left.  So if you are designing a new LAN protocol, be
sure to use SNAP packet format and NOT ETHERNET FORMAT.  Otherwise you'll
find yourself in a nasty hassle trying to scrounge a protocol type somehow.
This tends to annoy all parties involved and creates a hassle you'd probably
rather do without as you head for fieldtest.

So, once again: if you're designing a LAN protocol, use SNAP format packets
ONLY.  Do NOT use Ethernet format packets because you won't be able to get
an Ethernet protocol type assignment.

	paul

(cross-posted: ETHERNET, FDDI, TOKEN-RING, SYSNOTES, DECNETVAX)