Subject: Question: Hop-by-Hop Header and Router Alert

From: [email-masked]
Date: Mon, 26 May 2008 15:32:39 +0200

Hi all, 

I’m contacting you as I’ve a question regarding the Hop-by-Hop header option ‘Router Alert’ and its exact use. I hope I don't disturb you...

Currently I’m playing with some new ideas regarding mobile IP. These ideas include new messages and so on. 
The new messages are included within IPv6 packages and send toward a certain destination. While sending these messages toward the final destination it is necessary for them to be examined by routers on the paths. Hence, the Hop-by-Hop header with the ‘Router Alert’ option SHOULD be used indicating that the contents of the datagram may be interesting to the router. To give the router a hint about the datagram contents the router alert’s value field SHOULD be used therefore and inform the router about the contents of the payload or about the following IPv6 header, e. g. ICMPv6 message.

First question: 

Would it be okay to use the router alert option and its value field to indicate the router that it should not only process the hop-by-hop header before forwarding the message to the next hop,  but also to process the next header within the IPv6 packet, e.g. a ICMPv6 message?

Second question:

What will happen if a router doesn’t recognize the value in the router alert’s value field? Will it continue parsing the datagram and then forward the packet to the next hop (two MSB in the Hop-by-Hop option type field set to ‘00’), or will it immediately stop parsing and forward the received IPv6 packet to the next hop (two MSB in the Hop-by-Hop option type field set to ‘00’)?

I hope you can provide me with some information. Nevertheless I want to thank you in advance!

Best regards

Bernd

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From: Suresh Krishnan <[email-masked]>
Date: Mon, 26 May 2008 17:12:01 -0400

Hi Bernd,

[email-masked] wrote:
> Hi all, 
> 
> I’m contacting you as I’ve a question regarding the Hop-by-Hop header 
> option ‘Router Alert’ and its exact use. I hope I don't disturb you...

I am generally against any new proposal that uses hop by hop options 
(including router alerts) since they could increase the processing loads 
on intermediate routers and could be used as a DoS vector,

> 
> Currently I’m playing with some new ideas regarding mobile IP.
> These ideas include new messages and so on. The new messages are
> included within IPv6 packages and send toward a certain destination.
> While sending these messages toward the final destination it is
> necessary for them to be examined by routers on the paths. Hence,
> the Hop-by-Hop header with the ‘Router Alert’ option SHOULD be used
> indicating that the contents of the datagram may be interesting to
> the router. To give the router a hint about the datagram contents
> the router alert’s value field SHOULD be used therefore and inform
> the router about the contents of the payload or about the following
> IPv6 header, e. g. ICMPv6 message.
> 
> First question: 
> 
> Would it be okay to use the router alert option and its value field
> to indicate the router that it should not only process the hop-by-hop
> header before forwarding the message to the next hop,  but also to
> process the next header within the IPv6 packet, e.g. a ICMPv6
> message?

The router alert option does not have any processing rules associated 
with it. It just says "hey router, you need to take a closer look at the 
packet". This usually results (at least in our implementation) to take 
out the packet from the forwarding plane (fast path) and send it off to 
the control plane (slow path) for further processing. Here it will be 
examined in detail based on the applications that have registered to 
receive such packets (RSVP,MLD etc.). So your application will receive a 
copy of this packet and after that it can do whatever it wants to do.

> 
> Second question:
> 
> What will happen if a router doesn’t recognize the value in the
> router alert’s value field? Will it continue parsing the datagram
> and then forward the packet to the next hop (two MSB in the 
> Hop-by-Hop option type field set to ‘00’), or will it immediately
> stop parsing and forward the received IPv6 packet to the next 
> hop (two MSB in the Hop-by-Hop option type field set to ‘00’)?

The router code will not see this as an error. It will forward the 
packet through as if nothing happened. This is required for incremental 
deployment(e.g. non RSVP routers among RSVP routers)

Cheers
Suresh

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From: "Christopher Morrow" <[email-masked]>
Date: Mon, 26 May 2008 19:09:47 -0400

On Mon, May 26, 2008 at 5:12 PM, Suresh Krishnan
<[email-masked]> wrote:
> Hi Bernd,
>
> [email-masked] wrote:
>> Hi all,
>>
>> I'm contacting you as I've a question regarding the Hop-by-Hop header
>> option 'Router Alert' and its exact use. I hope I don't disturb you...
>

most larger ip backbones today don't pay attention (drop/ignore)
Router-Alert ip-options in ipv4, I don't expect that trend to change
in ipv6... If you are planning on using hop-by-hop or router-alert in
ipv6 you probably are planning on a failed solution.

-chris

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From: Suresh Krishnan <[email-masked]>
Date: Mon, 26 May 2008 19:11:17 -0400

> ipv6 you probably are planning on a failed solution.

Amen to that. I think the IETF needs to advise people to not use these 
to design new protocols. I did volunteer to write up a document stating 
just that, but there was sufficient interest in keeping these options alive.

Cheers
Suresh

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From: "Christopher Morrow" <[email-masked]>
Date: Mon, 26 May 2008 21:53:23 -0400

On Mon, May 26, 2008 at 7:11 PM, Suresh Krishnan
<[email-masked]> wrote:
> Amen to that. I think the IETF needs to advise people to not use these to
> design new protocols. I did volunteer to write up a document stating just
> that, but there was sufficient interest in keeping these options alive.

what's confusing to me is that these survive while RH0 was
deprecated... they pose essentially the same sets of problems.

-Chris

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From: Pekka Savola <[email-masked]>
Date: Tue, 27 May 2008 07:55:16 +0300 (EEST)

On Mon, 26 May 2008, Suresh Krishnan wrote:
>> Second question:
>>
>> What will happen if a router doesn?t recognize the value in the
>> router alert?s value field? Will it continue parsing the datagram
>> Hop-by-Hop option type field set to ?00?), or will it immediately
>> hop (two MSB in the Hop-by-Hop option type field set to ?00?)?
>

FWIW, I share Suresh's sentiments about Hop-by-Hop and Router Alert 
options :-)

FYI, here is discussion on what Linux does (which is a bit different 
given that there is no strict fast/slow path separation).  An 
application running on the router needs to set IPV6_ROUTER_ALERT 
sockopt with value of the desired RA option it wants to see.  Other 
messages are not delivered to the application and are forwarded as 
normal.  If some application gets the packet, it is responsible for 
re-injecting a new packet:

http://www.mail-archive.com/[email-masked]/msg50634.html

-- 
Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
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From: [email-masked]
Date: Tue, 27 May 2008 09:56:00 +0200

Hi Suresh,

thanks for your helpful response!

>(including router alerts) since they could increase the processing loads >on intermediate routers and could be used as a DoS vector,

Okay, if you say that you're against any new proposal that uses hop by hop options combined with router alert, then it could be better to define a new hop-by-hop option, which is only recognized by certain router (regarding mobile ip these routers could be home agents). or you're generally against the using of the hop-by-hop header, regardless of the router alert is used or not?

Thanks a lot

Bernd
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