Re: IPv6 Link Local Addresses

On 12-Jun-19 06:31, Erik Kline wrote:
> 
> On Tue, 11 Jun 2019 at 10:42, Kerry Lynn <kerlyn@ieee.org <mailto:kerlyn@ieee.org>> wrote:
> 
>     On Tue, Jun 11, 2019 at 9:05 AM Alexandre Petrescu <alexandre.petrescu@gmail.com <mailto:alexandre.petrescu@gmail.com>> wrote:
> 
> 
> 
>         Le 10/06/2019 à 18:44, Bob Hinden a écrit :
>         > Hi,
>         >
>         > RFC4291 defines Link Local Address as:
>         >
>         >   2.5.6.  Link-Local IPv6 Unicast Addresses
>         >
>         >     Link-Local addresses are for use on a single link.  Link-Local
>         >     addresses have the following format:
>         >
>         >     |   10     |
>         >     |  bits    |         54 bits         |          64 bits           |
>         >     +----------+-------------------------+----------------------------+
>         >     |1111111010|           0             |       interface ID         |
>         >     +----------+-------------------------+----------------------------+
>         >
>         >     Link-Local addresses are designed to be used for addressing on a
>         >     single link for purposes such as automatic address configuration,
>         >     neighbor discovery, or when no routers are present.
>         >
>         >     Routers must not forward any packets with Link-Local source or
>         >     destination addresses to other links.
>         >
>         > This means that link local addresses have a 10 bit prefix (1111111010) followed by 54 bits of zeros.  That is it, nothing more.   Address with different prefixes or with a 1111111010 prefix followed by non-zero 54 bits are not link local addresses.
>         >
>         > This is not ambiguous.
> 
>         Bob,
> 
>         The RFC4291 link local addresses have a 10 bit prefix and 54bits of
>         zero.  That is fe80::/64.
> 
>         The IANA allocation of IPv6 link-local addresses is fe80::/10.
> 
>          > fe80::/10    Link-Scoped Unicast     [RFC3513][RFC4291]     
> 
>         In that sense, the IANA definition and the RFC4291 differ with respect
>         to the definition of link-local addresses.
> 
>         fe80::/10 and fe80::/64 are not the same thing.
> 
>         Alex
> 
> 
>     Bob et al.
> 
>     Would a correct router implementation base its "must not forward" decision
>     on the most-significant 10 bits or most significant 64 bits?
> 
> 
> IMHO, /10.

Correct, IMHO, because clearly the whole of that prefix is defined as LL.

However, it might also be appropriate to log an error if the following
54 bits are non-zero, since they are specified to be zero.

     Brian

> 
> The well-known prefix on a link is the /64.
> 
> The extra empty space between the two does have its uses (https://www.freebsd.org/doc/en/books/developers-handbook/ipv6.html#ipv6-scope-index), even if not officially sanctioned by a document.
> 
>     Kerry
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