Subject: IPv6 certification - IPv6 Router Advertisement Lifetime 0 and Reachable time 10 seconds

From: Isaac <[email-masked]>
Date: Sun, 24 Jan 2021 16:32:01 +0530

IPv6 IETF team,

We have a query with regard to an enforcement by an IPv6 certification body
(University of New Hampshire Intereroperability Lab,
http://www.ipv6forum.org, http://www/ipv6ready.org ). In the following IPv6
router core protocol specification, the certification body mandates a
vendor product support of a combination: Router Advertisement (RA) lifetime
0 & Router advertisement reachable time 10 seconds (Link
https://www.ipv6ready.org/docs/Core_Conformance_5_0_1.pdf,  page 174,
section 2.2.15 Part B).

For us, it appeared that this combination of RA lifetime 0 and reachable
time of 10 seconds may not be practical. Do you see any use case for the
above mentioned combination? Are you aware of any vendor product
configuration/scenario at which the above combination of values may be
used? Do you think that the combination of RA lifetime of 0 and reachable
time of 10 seconds should be mandated by certification body or this can be
an optional support?

At the moment, we are unable to find a scenario (real world usecase) to
support RA lifetime of 0 and RA reachable time of 10 seconds. Please review
and respond.

Thanks in advance!!!
Isaac.

---

From: Nick Hilliard <[email-masked]>
Date: Sun, 24 Jan 2021 12:08:40 +0000

Isaac wrote on 24/01/2021 11:02:
> support RA lifetime of 0 and RA reachable time of 10 seconds. Please 
> review and respond.

Isaac,

you're referring to an IPv6 Forum document, so they might be more 
qualified to give an answer to your question.

As a potential pointer, rfc4861 documents the following case in section 
6.2.3:

>    A router might want to send Router Advertisements without advertising
>    itself as a default router.  For instance, a router might advertise
>    prefixes for stateless address autoconfiguration while not wishing to
>    forward packets.  Such a router sets the Router Lifetime field in
>    outgoing advertisements to zero.

Nick

---

From: Alexandre Petrescu <[email-masked]>
Date: Sun, 24 Jan 2021 13:26:09 +0100

Le 24/01/2021 à 12:02, Isaac a écrit :
> IPv6 IETF team,
> 
> We have a query with regard to an enforcement by an IPv6
> certification body (University of New Hampshire Intereroperability
> Lab, http://www.ipv6forum.org <http://www.ipv6forum.org>, 
> http://www/ipv6ready.org <http://www/ipv6ready.org> ). In the
> following IPv6 router core protocol specification, the certification
> body mandates a vendor product support of a combination: Router
> Advertisement (RA) lifetime 0 & Router advertisement reachable time
> 10 seconds (Link 
> https://www.ipv6ready.org/docs/Core_Conformance_5_0_1.pdf 
> <https://www.ipv6ready.org/docs/Core_Conformance_5_0_1.pdf>,  page
> 174, section 2.2.15 Part B).

Yes, but which RFC is that?

I am asking because I can acceses RFCs securely on the ietf.org website
from my IPv6-only client; but when I try to access that ipv6ready.org
URL from that IPv6-only client the firefox browser displays an
exclamation mark on the lock near https (that exclamation mark does not
appear when I use IPv4 client).

> For us, it appeared that this combination of RA lifetime 0 and
> reachable time of 10 seconds may not be practical. Do you see any use
> case for the above mentioned combination? Are you aware of any vendor
> product configuration/scenario at which the above combination of
> values may be used? Do you think that the combination of RA lifetime
> of 0 and reachable time of 10 seconds should be mandated by
> certification body or this can be an optional support?
> 
> At the moment, we are unable to find a scenario (real world usecase)
> to support RA lifetime of 0 and RA reachable time of 10 seconds.
> Please review and respond.

If the other router's (the default router, which has a lifetime
non-zero, and must be present, but you dont mention) reachable time is
10 seconds it might make sense that this router's lifetime (this is
presumably a backup router, to take the place of the default router) to
be also 10 seconds.

Alex

> 
> Thanks in advance!!! Isaac.
> 
> -------------------------------------------------------------------- 
> IETF IPv6 working group mailing list [email-masked] Administrative
> Requests: https://www.ietf.org/mailman/listinfo/ipv6 
> --------------------------------------------------------------------
>

---

From: Isaac <[email-masked]>
Date: Sun, 24 Jan 2021 21:37:17 +0530

Thanks Nick for the timely response!!!

I understand your comment regarding the prerogative of IPv6 forum in this
regard. Meanwhile, we need a technical answer/analysis of the combination
of RA lifetime 0 and Reachable time 10s whether that makes sense or whether
it was clearly envisioned in the original IPv6 design. We know that RFC
puts forth a set of 'may', 'might' conditions which are deemed optional in
certian corner cases (possibly). We are already having discussions with the
certification body but we need to go with a clear cut technical response of
whether RA lifetime 0 and reachable time 10 seconds makes sense or not.
Same way, section 6.2.3 in RFC4861 puts forth a 'might' condition. RA with
a lifetime 0 and with advertised prefixes might mean that there may be a
second router in the LAN segment which advertises a positive lifetime. And
this itself is a corner scenario we believe and common scenario would be a
single router in a LAN segment who always advertises with a positive
lifetime until he decides to cease to be default gatewway for clients
(probably he is ging down as well). But the combination of RA lifetime 0
and reachable time 10 seconds doesn't make sense to us and we are clueless
as to how that can be supported. We do not want to deisgn some throw away
logic just for certfication purpose and we do think thats neither the
purpose of certification bodies nor the end customers. We need a solid
technical answer from the IETF IPv6 official body in this regard. Please
review and respond.

Thanks,
Isaac.

---

From: Ole Troan <[email-masked]>
Date: Sun, 24 Jan 2021 17:50:47 +0100

Isaac,

The two variables are independent. 
The example you cited is perfectly fine.

the RA lifetime says: “don’t use me as a default router” and the reachable time configures hosts on the link to consider a neighbor entry in the ND cache reachable for 10s (for NUD).

Best regards,
Ole, 6man co-chair

> On 24 Jan 2021, at 17:07, Isaac <[email-masked]> wrote:
> 
> 
> I understand your comment regarding the prerogative of IPv6 forum in this regard. Meanwhile, we need a technical answer/analysis of the combination of RA lifetime 0 and Reachable time 10s whether that makes sense or whether it was clearly envisioned in the original IPv6 design. We know that RFC puts forth a set of 'may', 'might' conditions which are deemed optional in certian corner cases (possibly). We are already having discussions with the certification body but we need to go with a clear cut technical response of whether RA lifetime 0 and reachable time 10 seconds makes sense or not. Same way, section 6.2.3 in RFC4861 puts forth a 'might' condition. RA with a lifetime 0 and with advertised prefixes might mean that there may be a second router in the LAN segment which advertises a positive lifetime. And this itself is a corner scenario we believe and common scenario would be a single router in a LAN segment who always advertises with a positive lifetime until he decides to cease to be default gatewway for clients (probably he is ging down as well). But the combination of RA lifetime 0 and reachable time 10 seconds doesn't make sense to us and we are clueless as to how that can be supported. We do not want to deisgn some throw away logic just for certfication purpose and we do think thats neither the purpose of certification bodies nor the end customers. We need a solid technical answer from the IETF IPv6 official body in this regard. Please review and respond.
> 
> Thanks,
> Isaac.
> 
> IETF IPv6 working group mailing list
> [email-masked]
> Administrative Requests: https://www.ietf.org/mailman/listinfo/ipv6

---

From: Nick Hilliard <[email-masked]>
Date: Sun, 24 Jan 2021 17:55:40 +0000

Isaac wrote on 24/01/2021 16:07:
> Meanwhile, we need a technical answer/analysis of the combination of
> RA lifetime 0 and Reachable time 10s whether that makes sense or
> whether it was clearly envisioned in the original IPv6 design.

RA lifetime = 0 means that the route specified in the RA should not be 
used as a default route.

RA reachable time is the time in milliseconds that the issuing router 
should be considered reachable from the point of view of NUD.

The tuple { ra_lifetime = 0, ra_reachable = 10000 } is valid.  As 
described in the RFC, it might be emitted from a device which was 
providing prefix information for the l2 domain, and nothing else; for 
example, specifying a non-routed ULA prefix for the local l2 domain.

> We do not want to deisgn some throw away logic just for certfication
> purpose and we do think thats neither the purpose of certification
> bodies nor the end customers. We need a solid technical answer from
> the IETF IPv6 official body in this regard. Please review and
> respond.

I'm not sure who "we" refers to here.

Obviously, if you're writing an ipv6 stack, it's your choice as to what 
goes into your implementation.  There doesn't seem to be any ambiguity 
in the rfc, however.  From a more general point of view, the IETF's 
position on communication protocols is to be liberal in what you expect 
and conservative in what you send, so if you decline to support 
something like this, it would open up questions as to whether your 
implementation conformed with the rfc.

Whether or not it should be part of the IPv6 Forum's certification spec 
is an issue for them, not the ietf.

Nick

---

From: Isaac <[email-masked]>
Date: Mon, 25 Jan 2021 00:40:33 +0530

Thanks Ole. Do hosts rely on RA packets for updating neighbor cache
post determining that the router is no longer the default gateway? They can
use NS/NA packets also right?

BTW, what do you think is the configuration/scenario on a router which
sends a RA packet with lifetime 0 and reachable time 10 seconds? Are you
aware of any particular vendor's configuration which generates such packets?

Thanks,
Isaac.

On Sun, Jan 24, 2021 at 10:20 PM Ole Troan <[email-masked]> wrote:

> the RA lifetime says: “don’t use me as a default router” and the reachable
> time configures hosts on the link to consider a neighbor entry in the ND
> cache reachable for 10s (for NUD).
>
> Best regards,
> Ole, 6man co-chair
>

---

From: Ole Troan <[email-masked]>
Date: Sun, 24 Jan 2021 20:32:47 +0100

Isaac,

> On 24 Jan 2021, at 20:10, Isaac <[email-masked]> wrote:
> 
> 
> Thanks Ole. Do hosts rely on RA packets for updating neighbor cache post determining that the router is no longer the default gateway? They can use NS/NA packets also right?

The router lifetime is used to determine if this router is used as the default router. The hosts receive configuration information from the router, independently of that. 

> BTW, what do you think is the configuration/scenario on a router which sends a RA packet with lifetime 0 and reachable time 10 seconds? Are you aware of any particular vendor's configuration which generates such packets?

Yes, certainly cisco’s can do that. Presumably all others too. Setting RA lifetime = 0 and reachable time to 10 does not affect behavior on the router much. It will likely  adjust its own perception of the reachable time on the link, but it might also have a separate configuration knob for that. 

Cheers 
Ole

---

From: Timothy Winters <[email-masked]>
Date: Sun, 24 Jan 2021 14:53:12 -0500

Issac,

All the current IPv6 Ready Logo routers can set this value in this
configuration.   There are about 1000 routers on the list.

Regards,
Tim
IPv6 Ready Logo Chair

On Sun, Jan 24, 2021 at 2:33 PM Ole Troan <[email-masked]> wrote:

> The router lifetime is used to determine if this router is used as the
> default router. The hosts receive configuration information from the
> router, independently of that.
>
> Yes, certainly cisco’s can do that. Presumably all others too. Setting RA
> lifetime = 0 and reachable time to 10 does not affect behavior on the
> router much. It will likely  adjust its own perception of the reachable
> time on the link, but it might also have a separate configuration knob for
> that.
>
> Cheers
> Ole
>
>
>
>> the RA lifetime says: “don’t use me as a default router” and the
>> reachable time configures hosts on the link to consider a neighbor entry in
>> the ND cache reachable for 10s (for NUD).
>>
>> Best regards,
>> Ole, 6man co-chair
>>
>>> to

---

From: Isaac <[email-masked]>
Date: Mon, 25 Jan 2021 01:25:48 +0530

Thanks Ole.

"Yes, certainly cisco’s can do that. Presumably all others too. Setting RA
lifetime = 0 and reachable time to 10 does not affect behavior on the
router much. It will likely  adjust its own perception of the reachable
time on the link, but it might also have a separate configuration knob for
that. "

Need a favor, (if you have it handy) could you please point to a
configuration by Cisco router (any of the product type would do) which
sends out the RA lifetime 0 and reachable time 10 seconds and how commonly
(practically) that is used? (Especially in the context of modern global
networks where customers want things done autonmously/automatically with
the least number of knobs possible)

Thanks,
Isaac.

---

From: Ole Troan <[email-masked]>
Date: Sun, 24 Jan 2021 22:00:06 +0100

Isaac,

> Need a favor, (if you have it handy) could you please point to a configuration by Cisco router (any of the product type would do) which sends out the RA lifetime 0 and reachable time 10 seconds and how commonly (practically) that is used? (Especially in the context of modern global networks where customers want things done autonmously/automatically with the least number of knobs possible)

A quick search would have found that information for you.
Basically:
interface Ethernet0
 ipv6 nd ra lifetime 0
 ipv6 nd reachable-time 10000

https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipv6/command/ipv6-cr-book/ipv6-i3.html

Best regards,
Ole

---

From: Isaac <[email-masked]>
Date: Mon, 25 Jan 2021 10:25:33 +0530

Ole/Tim Winters/IETF team,

Yes, we understand these knobs but we wanted to understand more on
the scenario/topology. More importantly we wanted to understand the real
world scenario when this combination of RA lifetime 0 and reachable time 10
seconds is used and the technical merit of it for which we did not get
clear response (especially in the modern global IPv6 networks context).
It's surprising that the certification bodies haven't clearly mandated only
common/practical (although IETF has mentioned that these paramers need to
be configurable but never said explicitly that all permutation/combination
of values need to be supported. Vendors (definitely want) comply to RFCs
but do not want allow impractical values) use cases but have listed even
the corner scenario which may never be used. We understand that there are
thousand vendors who have implemented this combination. But we fear that
these are extra burden for vendors considering that vendors go ahead for
certification without questioning the certification body itself
believing that the certification body does its job of validating the modern
technical relevance. Ideally, we expect the certification body (if not
IETF) to re visit all the tests periodically to understand the relevancy as
time passes and modify if required (which is the purpose of the
certification body we believe). Sorry to have spilled certain discussions
pertaining to certification body in this forum. But we do not have much
option as we want technical answer from the IETF group. Let's not stop with
the high statements in RFC. The reason we approcahed IETF is to go one
level deep (especially in the context of modern day global networks) to
undertand the relevance of RA lifetime 0 and reachable time 10 seconds
whether it makes sense to support. These are our 2 cents contribution to
the community (if there is someone to listen!)

Thanks,
Isaac.

---

From: "Manfredi (US), Albert E" <[email-masked]>
Date: Mon, 25 Jan 2021 05:15:34 +0000

From: ipv6 <[email-masked]> On Behalf Of Isaac

> Yes, we understand these knobs but we wanted to understand more on the scenario/topology. More importantly we wanted to understand the real world scenario when this combination of RA lifetime 0 and reachable time 10 seconds is used and the technical merit of it for which we did not get clear response (especially in the modern global IPv6 networks context). It's surprising that the certification bodies haven't clearly mandated only common/practical (although IETF has mentioned that these paramers need to be configurable but never said explicitly that all permutation/combination of values need to be supported. Vendors (definitely want) comply to RFCs but do not want allow impractical values) use cases but have listed even the corner scenario which may never be used. We understand that there are thousand vendors who have implemented this combination. But we fear that these are extra burden for vendors considering that vendors go ahead for certification without questioning the certification body itself believing that the certification body does its job of validating the modern technical relevance. Ideally, we expect the certification body (if not IETF) to re visit all the tests periodically to understand the relevancy as time passes and modify if required (which is the purpose of the certification body we believe). Sorry to have spilled certain discussions pertaining to certification body in this forum. But we do not have much option as we want technical answer from the IETF group. Let's not stop with the high statements in RFC. The reason we approcahed IETF is to go one level deep (especially in the context of modern day global networks) to undertand the relevance of RA lifetime 0 and reachable time 10 seconds whether it makes sense to support. These are our 2 cents contribution to the community (if there is someone to listen!)

Isaac, I'm trying to understand your point. At least one scenario in which the RA lifetime is set to 0, and reachable time is set to 10 seconds, was explained a couple of times. It can be used for a router to provide the IPv6 prefix, for example for SLAAC, but for that same router NOT to be used as the default router, for the subnet in question.

Is it that this scenario seems unrealistic and unnecessary, to you?

Thanks,
Bert

---

From: Isaac <[email-masked]>
Date: Mon, 25 Jan 2021 11:10:20 +0530

Bert,

In a way, yes. Do you think this combination of RA lifetime 0 and reachable
time 10 seconds need to be mandated for all vendors?

I have a question to the IETF group here, when RA lifetime is 0 (which
means the the router is no longer to be used as gateway by hosts) and
prefixes are provided (prefixes have their own lifetime and flags and in
this rare scenario there is going to be a second router acting as gateway),
why do we need reachable time of 10 seconds for the 1st router?

Thanks,
Isaac.

On Mon, Jan 25, 2021 at 10:45 AM Manfredi (US), Albert E <
[email-masked]> wrote:

> Isaac, I'm trying to understand your point. At least one scenario in which
> the RA lifetime is set to 0, and reachable time is set to 10 seconds, was
> explained a couple of times. It can be used for a router to provide the
> IPv6 prefix, for example for SLAAC, but for that same router NOT to be used
> as the default router, for the subnet in question.
>

---

From: Mark Smith <[email-masked]>
Date: Mon, 25 Jan 2021 17:05:56 +1100

On Mon, 25 Jan 2021, 16:40 Isaac, <[email-masked]> wrote:

> Bert,
>
> In a way, yes. Do you think this combination of RA lifetime 0 and
> reachable time 10 seconds need to be mandated for all vendors?
>

Yes.

Because they're separate and unrelated parameters.

One parameter is about a router being a default router.

The other parameter is to do with the Neighbour Discovery protocol.

The only thing these parameters have in common is they're both carried in
an RA.

If you're reading a non-IETF spec that says they're tightly coupled
together parameters, where as the IETF RFCs and people in this IETF working
group say they aren't, which is wrong?

>> Isaac, I'm trying to understand your point. At least one scenario in
>> which the RA lifetime is set to 0, and reachable time is set to 10 seconds,
>> was explained a couple of times. It can be used for a router to provide the

---

From: Alexandre Petrescu <[email-masked]>
Date: Mon, 25 Jan 2021 10:24:30 +0100

Le 25/01/2021 à 05:55, Isaac a écrit :
> the scenario/topology. More importantly we wanted to understand the
> real world scenario when this combination of RA lifetime 0 and
> reachable time 10 seconds is used and the technical merit of it

The technical merit of it is this: when the default router has a
reachable time of 10seconds the backup router has to do the same.

The merit is in that the Host receiving the RA continues as before when
the backup becomes the normal.

This is an explanation I am trying, but I never tried it in practice.
It might be wrong.

Alex

  for which we did not
> get clear response (especially in the modern global IPv6 networks 
> context). It's surprising that the certification bodies haven't
> clearly mandated only common/practical (although IETF has mentioned
> that these paramers need to be configurable but never said explicitly
> that all permutation/combination of values need to be supported.
> Vendors (definitely want) comply to RFCs but do not want allow
> impractical values) use cases but have listed even the corner
> scenario which may never be used. We understand that there are
> thousand vendors who have implemented this combination. But we fear
> that these are extra burden for vendors considering that vendors go
> ahead for certification without questioning the certification body
> itself believing that the certification body does its job of
> validating the modern technical relevance. Ideally, we expect the
> certification body (if not IETF) to re visit all the tests
> periodically to understand the relevancy as time passes and modify if
> required (which is the purpose of the certification body we believe).
> Sorry to have spilled certain discussions pertaining to certification
> body in this forum. But we do not have much option as we want
> technical answer from the IETF group. Let's not stop with the high
> statements in RFC. The reason we approcahed IETF is to go one level 
> deep (especially in the context of modern day global networks) to 
> undertand the relevance of RA lifetime 0 and reachable time 10
> seconds whether it makes sense to support. These are our 2 cents
> contribution to the community (if there is someone to listen!)
> 
> Thanks, Isaac.
> 
> On Mon, Jan 25, 2021 at 2:30 AM Ole Troan <[email-masked] 
> <mailto:[email-masked]>> wrote:
> 
> Isaac,
> 
>> configuration by Cisco router (any of the product type would do) 
>> which sends out the RA lifetime 0 and reachable time 10 seconds and
>> how commonly (practically) that is used? (Especially in the context
>> of modern global networks where customers want things done 
>> autonmously/automatically with the least number of knobs possible)
> 
> A quick search would have found that information for you. Basically: 
> interface Ethernet0 ipv6 nd ra lifetime 0 ipv6 nd reachable-time
> 10000
> 
<https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipv6/command/ipv6-cr-book/ipv6-i3.html>
> Best regards, Ole

---

From: Nick Hilliard <[email-masked]>
Date: Mon, 25 Jan 2021 12:11:36 +0000

> It might be wrong.

it is wrong.  RA lifetime = 0 means: don't install this router as a 
default router.

Nick

---

From: Alexandre Petrescu <[email-masked]>
Date: Mon, 25 Jan 2021 15:44:23 +0100

> it is wrong.  RA lifetime = 0 means: don't install this router as a 

YEs, it means to not install rhis router now as a default router, but 
maybe do it later.

It is a matter of local configuration, but an Ethernet subnet might have 
several routers with lifetime==0 and one router with lifetime non-zero 
(the default router).  All these routers would rather have the same 
reachable time 10seconds.  The reachable time would depend on the kind 
of link layer technology, which is only one.

   Host1   Host2  ... Hostn
    |       |          |
   ------------------------
      |        |         |
      R1       R2        R3
      |
     Internet

and later on, when R1 fails, the situation would become:

   Host1   Host2  ... Hostn
    |       |          |
   ------------------------
              |         |
              R2        R3
              |
            Internet

Alex

---

From: Nick Hilliard <[email-masked]>
Date: Mon, 25 Jan 2021 15:09:31 +0000

it means:

>     A Lifetime of 0 indicates that the router is not a
>     default router and SHOULD NOT appear on the default
>     router list.

+ other stuff like:

>    Note that system management may disable a router's IP forwarding
>    capability (i.e., changing the system from being a router to being a
>    host), a step that does not necessarily imply that the router's
>    interfaces stop being advertising interfaces.  In such cases,
>    subsequent Router Advertisements MUST set the Router Lifetime field
>    to zero.

Can you point the bit where RA Lifetime = 0 means "maybe do it later"?

Nick

---

From: Alexandre Petrescu <[email-masked]>
Date: Mon, 25 Jan 2021 16:39:57 +0100

Le 25/01/2021 à 16:09, Nick Hilliard a écrit :
>>     router list. >

In RFC 4861 it is there:> "AdvDefaultLifetime
>                      The value to be placed in the Router Lifetime field
>                      of Router Advertisements sent from the interface,
>                      in seconds.These limits may be overridden
>                      by specific documents that describe how IPv6
>                      operates over different link layers."

In RFC 6275 "Mobile IPv6" it is something about multiple Home Agents
(routers), using Router Lifetime fields in Router Advertisements, that
might dynamically take a main role there:

>       In addition, the sending home agent MAY
>       dynamically set the Home Agent Preference value, for example,
>       basing it on the number of mobile nodes it is currently serving or
>       on its remaining resources for serving additional mobile nodes;
>       such dynamic settings are beyond the scope of this document.

In RFC 5798 "VRRP" there is something about Master and Backup VRRP 
routers there:
> 8.2.3.  Router Advertisements
> 
>    When a Backup VRRP router has become Master for a virtual router, it
>    is responsible for sending Router Advertisements for the virtual
>    router as specified in Section 6.4.3.  The Backup routers must be
>    configured to send the same Router Advertisement options as the
>    address owner.
Alex

---

From: Bob Hinden <[email-masked]>
Date: Mon, 25 Jan 2021 10:14:22 -0800

Isaac,

> On Jan 24, 2021, at 9:40 PM, Isaac <[email-masked]> wrote:
> 
> Bert,
> 
> In a way, yes. Do you think this combination of RA lifetime 0 and reachable time 10 seconds need to be mandated for all vendors?

As many people have said, they are separate parameters.   Support for them needs to be implemented that way.

> 
> I have a question to the IETF group here, when RA lifetime is 0 (which means the the router is no longer to be used as gateway by hosts) and prefixes are provided

The key word is not being used as a "default router".  It can still be used as a router for a specific prefix.

> (prefixes have their own lifetime and flags and in this rare scenario there is going to be a second router acting as gateway), why do we need reachable time of 10 seconds for the 1st router?

It’s not that rare.

I think you have gotten the answer to your question(s).

Bob

> 
> Thanks,
> Isaac.
> 
> On Mon, Jan 25, 2021 at 10:45 AM Manfredi (US), Albert E <[email-masked]> wrote:

---

From: Nick Hilliard <[email-masked]>
Date: Mon, 25 Jan 2021 19:30:22 +0000

Alex,

I've read the link you cited, but maybe am missing something here.  Can 
you be clearer about situations where Router Lifetime = 0 means 
something other than "do not install this router address in the routing 
table"?

The limits you note in rfc 4861 seem to refer to this value not being 
equal to zero.

6275 only mentions AdvDefaultLifetime to say that it should be at least 
one second.

The VRRP situation requires that the backup routers use the same RA 
options, which would presumably not be zero, because then no routers 
would be active.

Nick

---

From: Ted Lemon <[email-masked]>
Date: Mon, 25 Jan 2021 14:50:08 -0500

On Jan 25, 2021, at 2:30 PM, Nick Hilliard <[email-masked]> wrote:
> I've read the link you cited, but maybe am missing something here.  Can you be clearer about situations where Router Lifetime = 0 means something other than "do not install this router address in the routing table”?

It doesn’t mean that. It means “do not install a default route pointing to this router.”

An example of where this is used in practice is with stub routers, where we want to publish reachability to a stub network prefix without claiming to be able to route to the internet. So we send an RA with a router lifetime of zero and with a Route Information Option for the prefix in use on the stub network.

Someone else earlier mentioned the example of where you want to provide a prefix for use on the local link, but do not want to claim the default route, because you aren’t presently able to route packets to the internet. This can be useful in a small subnetted network where a routing protocol like babel is operating, but there’s no egress route to the internet. In this case, you still want to advertise reachability, but you don’t want to claim the default route; some other router on the link may in fact have reachability to the internet, and by claiming the default route you would be breaking that connectivity.

---

From: Mark Smith <[email-masked]>
Date: Tue, 26 Jan 2021 09:15:03 +1100

On Tue, 26 Jan 2021, 06:50 Ted Lemon, <[email-masked]> wrote:

> you be clearer about situations where Router Lifetime = 0 means something
> other than "do not install this router address in the routing table”?
>
>

> An example of where this is used in practice is with stub routers, where
> we want to publish reachability to a stub network prefix without claiming
> to be able to route to the internet. So we send an RA with a router
> lifetime of zero and with a Route Information Option for the prefix in use
> on the stub network.
>

I imagine a more general scenario when thinking of  RA RL=0 use case.

One router is the link's default router, likely to the Internet, so RA RL
!= 0.

Another router on the link provides access to a specific subset of
destinations e.g. perhaps over an IPsec VPN.

That router announces RA RL=0, and Route Information Options for those
destinations, so hosts send directly to the subset destination router for
those destinations.

It's not the only way to get this forwarding working, however RA RIOs is
the most efficient for this subset of destinations because hosts of
informed of exactly which router they need to reach them.

> Someone else earlier mentioned the example of where you want to provide a
> prefix for use on the local link, but do not want to claim the default
> route, because you aren’t presently able to route packets to the internet.
> This can be useful in a small subnetted network where a routing protocol
> like babel is operating, but there’s no egress route to the internet. In
> this case, you still want to advertise reachability, but you don’t want to
> claim the default route; some other router on the link may in fact have
> reachability to the internet, and by claiming the default route you would
> be breaking that connectivity.
>

---

From: Ted Lemon <[email-masked]>
Date: Mon, 25 Jan 2021 17:21:26 -0500

On Jan 25, 2021, at 5:15 PM, Mark Smith <[email-masked]> wrote:
> It's not the only way to get this forwarding working, however RA RIOs is the most efficient for this subset of destinations because hosts of informed of exactly which router they need to reach them.

Right. We don’t expect the host to participate in the routing plane, so the route has to be communicated somehow, and RA is the obvious tool to do it. Otherwise I think you’d just have to type it in, which is obviously not scalable.

---

From: Alexandre Petrescu <[email-masked]>
Date: Tue, 26 Jan 2021 16:30:47 +0100

Nick,

> something other than "do not install this router address in the routing 

I agree with you - no I can not be clearer.

When router lifetime == 0 it means that it must not be a default router. 
  It could be used for other things, but certainly it's not a default 
router.

These other things vary.

> options, which would presumably not be zero, because then no routers 

YEs.

Alex

>>>> YEs, it means to not install rhis router now as a default router, 
>>>> but maybe do it later.
>>>
>>> it means:
>>>