Subject: Recommended value for the Hop Limit?

From: Fernando Gont <[email-masked]>
Date: Wed, 19 Aug 2009 03:26:50 -0300

Hello, folks,

Is there a recommended value for the Hop Limit of IPv6 packets?

The IANA ipv6-parameters file states that the Hop Limit is not a
parameter (?).

FWIW, the recommended value for the IPv4 TTL is 64, and it seems that
some v6 implementations follow the same advice for the IPv6 Hop Limit.
e.g., at least in some Junipers it defaults to 64.
(http://www.juniper.net/techpubs/software/erx/junose700/swcmdref-a-m/html/i-commands285.html)

Thanks!

Kind regards,
-- 
Fernando Gont
e-mail: [email-masked] || [email-masked]
PGP Fingerprint: 7809 84F5 322E 45C7 F1C9 3945 96EE A9EF D076 FFF1

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From: Fred Baker <[email-masked]>
Date: Wed, 19 Aug 2009 00:19:16 -0700

There are a number of RFCs with specific values. You might look at RFC  
2473 (64), RFC 3122 (255), and RFC 3315 (32). Neighbor Discovery wants  
a hop limit of one. each of those specifies a specific case in which  
the value applies.

For IPv6 unicast traffic, I think it's fair to say "wider than the  
network diameter". IPv4 seems to be using 64 pretty consistently for  
that these days. I remember when the magic number was 16 and when it  
was 32.

On Aug 18, 2009, at 11:26 PM, Fernando Gont wrote:

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> IETF IPv6 working group mailing list
> [email-masked]
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From: Fernando Gont <[email-masked]>
Date: Wed, 19 Aug 2009 04:35:12 -0300

Hello, Fred,

> 2473 (64), RFC 3122 (255), and RFC 3315 (32). Neighbor Discovery wants a
> hop limit of one. 

(Side-tracked, but: think it wants 255, for GTSM)

>each of those specifies a specific case in which the
> value applies.

Yes. However, I was wondering about the recommended default value for
"normal" unicast traffic.

> that these days. I remember when the magic number was 16 and when it was
> 32.

I was expecting a recommended value in the IANA ipv6-parameters file.
Not sure why it's not there.

Thanks!

Kind regards,

---

From: Thomas Narten <[email-masked]>
Date: Mon, 24 Aug 2009 12:19:25 -0400

Fred Baker <[email-masked]> writes:

> 2473 (64),

Specifically, the text at issue says:

>           Hop Limit:
> 
>             The tunnel IPv6 header hop limit is set to a pre-configured
>             value - see section 6.3.
> 
>             The default value for hosts is the Neighbor Discovery
>             advertised hop limit [ND-Spec].  The default value for
>             routers is the default IPv6 Hop Limit value from the
>             Assigned Numbers RFC (64 at the time of writing this
>             document).

The intention of that wording was that the value to be used was the
same one as recommended for general use for IPv4, which IANA records.

http://www.iana.org/assignments/ip-parameters says:

> IP TIME TO LIVE PARAMETER
> 
> The current recommended default time to live (TTL) for the Internet
> Protocol (IP) is 64 [RFC791, RFC1122].

Obviously, this could be a bit clearer. IANA should have a similar
entry for Hop Limit, which might well just point to the IPv4 TTL
value.

Thomas

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From: Fernando Gont <[email-masked]>
Date: Mon, 24 Aug 2009 14:37:17 -0300

> same one as recommended for general use for IPv4, which IANA records.

What about nodes that are e.g. statically configured?

> entry for Hop Limit, which might well just point to the IPv4 TTL

Yes, I believe the ipv6-parameters file should specify what the
default value for the Hop Limit is. Any hint on how to get this done?

Thanks!

Kind regards,
Fernando

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From: Thomas Narten <[email-masked]>
Date: Mon, 24 Aug 2009 13:41:37 -0400

> What about nodes that are e.g. statically configured?

The intention of the Hop Limit, I think, is that implementations (not
operations) configure it by default to 64. The value rarely seems to
change, so an operator/user should only need to change it if for some
reason the default is not good enough.

So it really shouldn't in general need to be statically configured.

> default value for the Hop Limit is. Any hint on how to get this done?

A note to IANA might be sufficient, and at some point, when we update
the relevant specs, we should clarify the language. I don't see this
as being a huge problem that needs fixing. It has been years since
anyone asked a question about what the documents say on this point...

Thomas

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From: Fernando Gont <[email-masked]>
Date: Mon, 24 Aug 2009 15:02:30 -0300

> reason the default is not good enough.

I just just arguing that the quoted text wouldn't apply to
static-configuration scenarios (in which parameters are not learn by
means of ICMPv6)

> anyone asked a question about what the documents say on this point...

I don't think ths is a huge point, either. Just that it would be great
if this was clarified.

Thanks!

Kind regards,
Fernando