Re: confusion in mipv6 I-D?

the only restriction is that the source address on the BU MUST be a
topologically correct address (otherwise packet is dropped bu ingress
filtering).

> I still wonder what happens if both the Alternate Care-of Address and 
> the Home Address option are present.  In that case can the Source 
> Address field be either of those addresses, 

typically CoA when in visited link and home address when the MN is
on the home link.

> or perhaps neither?

yes. for example, a MN could have two Care-of addresses, CoA1 and
CoA2. in the Binding Update, it could set the source address to CoA1,
include CoA2 in alt CoA option and home address in the Home Address
option. when the HA processes this Binding Update, the home address is
bound to CoA2.

Vijay