BTL Implementation Guideline

This is a document written by BTL to assist developers comply with off-specification, common practice issues.

Discussion around Address Lengths

                       2.15 Be prepared to handle MAC addresses of any size less than or equal to 6 bytes
                           MAC addresses are not limited to 1, 2, or 6 bytes as might be thought from Table 6-2 of ASHRAE 135-
                           2012. Any size MAC address should be allowed which is less than or equal to 6 bytes. If the device is a
                           router, then any size MAC address with a DLEN of 7 bytes or less shall be supported.
                           As an example, gateways to proprietary systems are allowed to provide MAC addresses that are not
                           included in the standard table.
                           
                       2.17 Addresses are expected to be no more than 6 bytes
                           Before Addendum 135-2008q-2 which defined the VMAC layer, the SSPC was considering the use of
                           MAC addresses up to 18 bytes. With the adoption of 135-2008q-2 and the VMAC layer approach,
                           addresses are expected to be no more than 6 bytes. So there is no longer a need for implementations to
                           expect longer addresses. The support for LON addresses of length 7 is so restrictive--only usable as a
                           DLEN and never as an SLEN--that only routers and devices that themselves will utilize the form of LON
                           addresses of length 7, are required to support that.

                       So where does that leave us?
                           Nobody on the planet (that I am aware of) uses 7 byte addressing.
                           A DLEN of 7 is unidirectional (a length of 7 is never allowed in SLEN, so the response does not include the 7 byte SLEN).
                           LonTalk is deprecated
                           This is a router test only
                           A SLEN of 7 is incorrectly decoded by wireshark.
                           We recomment simply stick to 6 as your maximum length, but be prepared to decode 7 when receiving messages.