Re: [v6ops] IPv6 link-local traffic questions

Makes sense.

Thanks

Gyan

On Sat, Mar 14, 2020 at 4:12 AM Karl Auer <kauer@biplane.com.au> wrote:

> On Sat, 2020-03-14 at 02:07 -0400, Gyan Mishra wrote:
> > I always wondered why RFC 4862 states that ND & RD packets have TTL
> > 255.
>
> >From RFC4861:
>
>    "By setting the Hop Limit to 255, Neighbor Discovery
>     is immune to off-link senders that accidentally or
>     intentionally send ND messages.  In IPv4, off-link
>     senders can send both ICMP Redirects and Router
>     Advertisement messages."
>
> RFC4861 also requires RA and NA packets with any other TTL value to be
> silently discarded by any receiving node.
>
> I can't find my original source, but my notes in a training course I
> developed a while ago have this to say about why:
>
>    "Why does 255 mean “local source”? Because routers
>     decrement the hop limit on packets passing through
>     them, and discard packets where the hop count
>     decrements to zero. It is impossible to decrement
>     a one-byte counter to 255 without passing through
>     zero, because 255 is the maximum value such a counter
>     can have. So the value 255 means that the packet must
>     have been generated on the local link."
>
> Probably would be better expressed as "must have originated on the
> local link".
>
> Regards, K.
>
> --
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-- 

Gyan  Mishra

Network Engineering & Technology

Verizon

Silver Spring, MD 20904

Phone: 301 502-1347

Email: gyan.s.mishra@verizon.com