Subject: Packet decode example in draft-ietf-ipdvb-ule-01.tx

From: Gorry Fairhurst <[email-masked]>
Date: Mon, 20 Sep 2004 11:48:21 +0100

Someone very kindly pointed a slight "problem" with the worked example
provided in the  Informative Appendix, ANNEXE B, of the draft ULE spec. The
current text reads:

" An example of ULE encapsulation carrying an ICMPv6 packet generated
by ping6. 

 ULE SNDU Length  :            63 decimal
 D-bit value  :                0 (NPA Present)
 ULE Protocol Type :           0x86dd (IPv6)
 Destination ULE NPA Address:  01:02:03:04:05:06 **** MULTICAST ****
 ULE CRC32 :                   0x784679a5

 Source IPv6:                  2001:660:3008:1789::5
 Destination IPv6:             2001:660:3008:1789::6

 SNDU contents (including CRC-32):

 0000:  00 3f 86 dd 01 02 03 04 05 06 60 00 00 00 00 0d
 0010:  3a 40 20 01 06 60 30 08 17 89 00 00 00 00 00 00
 0020:  00 05 20 01 06 60 30 08 17 89 00 00 00 00 00 00
 0030:  00 06 80 00 9d 8c 06 38 00 04 00 00 00 00 00 78
 0040:  46 79 a5 "

Although the CRC-32 is correct, there's a slight irregularity in the MAC
address cited in the example.

The cited mac address is multicast, whereas the IPV6 address is Unicast...
Looking at section 4.5 of ULE: " The ****least significant bit**** of the
first byte of the address is set to 1 for multicast frames, and the
remaining bytes specify the link layer multicast address." Note my
highlighting with "***".

Although this does not invalidate the example - from the point of view of
validating the CRC32 calculation - it is not a good example of expected use.

Could someone perhaps please compute the CRC32 again over the same packet
using a unicast NPA address instead?

Gorry