Changes in version 8 w.r.t. Shim6 messages length

[marcelo bagnulo braun <marcelo@it.uc3m.es>]

Hi,

in response to Iljitsch comment, I have added the following text in the 
Common Shim6 message format section, based on Iljitsch suggestion:

Shim6 signalling packets may not be larger than 1280 bytes, including
the IPv6 header and any intermediate headers between the IPv6 header and
the Shim6 header. One way to meet this requirement is to omit part of 
the
locator address information if with this information included,
the packet would become larger than 1280 bytes.Another option is to 
perform
option engineering, dividing into different Shim6 messages the 
information to
be transmitted. An implementation may
impose administrative restrictions to avoid excessively large Shim6
packets, such as a limitation on the number of locators to be used.

Let me know if you want additional changes w.r.t. this issue

Thanks, marcelo