CARD: Modification in resolution to issues #49 and #51

It turns out there are some constraints imposed on the resolution of issues
#49 and #51 by the fact that CARD messages are sent as options on Neighbor
Discovery in IPv6, or Mobile IPv4 ICMP messages in IPv4. These constraints
require modification to the recommended resolutions as follows:

issue-#49: Length indicator for Router Certificate sub-option:

The recommended resolution was to change all field lengths to 16 bits with
units of octets, rather than 8 bits with units of 8 octets. It turns out
that the length field is constrained by the ND options TLD format to be 8
bits with units of octets, allowing a maximum of about 2K octets. This is
not long enough for a general certificate chain. Most X.509 certs are rather
long, typically on the order of about 1K, so more than on cert is unlikely
to fit into a message.

The following is a modification of the resolution to meet these constraints:

- Change the Router Certificate suboption length units to 8 octets.

- Drop the Context ID and require only a Address Option and L2-ID option in
a message with a certificate.

- If more than one certificate is required, the router sends multiple CARD
Reply Messages.

- Add a flag indicating if the certificate is the last one to be sent. The
host should be able to deduce this by whether the chain is complete.

In addtion, the following modification is advisable, since X.500 names used
for certificates can be long:

- Modify the units for the Trusted Anchor option from octets to 8 octets.

issue-#51: Version number needed.

The TLV format of the ND options won't allow the version number to be put
into the lower bits of the option type, since the type is assigned by IANA.

The following is a modification of the resolution to meet these constraints;

- Put the 3 bit version number immediately after the T and L fields, which
are specified by RFC 2461.

Comments?

            jak