Subject: Multiple BFD sessions over one path

From: Mach Chen <[email-masked]>
Date: Sat, 19 Sep 2009 11:27:45 +0800

Hi,

After reading the two drafts (draft-ietf-bfd-generic-05 and 
draft-ietf-bfd-v4v6-1hop-09), I have some confusion about the session 
limitation over a specific network-layer path.

Section 2 of draft-ietf-bfd-v4v6-1hop-09 says:
"Each BFD session between a pair of systems MUST traverse a separate 
network-layer path in both directions..."

But section 6 of draft-ietf-bfd-generic-05 says:
"...if multiple differentiated services [DIFFSERV] are being operated over 
IPv4, an independent BFD session may be run for each service level...", this 
implicates that there may be mulitiple BFD sessions over one 
network-layer(ipv4) path, but it seems that this is not allowed in 
draft-ietf-bfd-v4v6-1hop-09. How to understand this conflict?  or do I miss 
something?

Best regards,
Mach Chen

---

From: Dave Katz <[email-masked]>
Date: Sat, 19 Sep 2009 00:39:58 -0600

The point of the text in 1hop is that there isn't any use in having  
more than one session over a "path", and in any case it can't be done  
because there would be no way to demultiplex the initial packet of  
multiple sessions on the same "path" (because the first packet carries  
discriminator 0).

But diffserv is essentially a tunnel of sorts;  each service level has  
potentially different forwarding characteristics, so it may be useful  
to run BFD over more than one level (and the sessions can be  
demultiplexed because they are contextualized by the diffserv level).

Essentially, each diffserv service level is considered to be a  
separate path, not unlike GRE or IPsec or what have you.

Implementors are free to do as they please;  the point of the text is  
to not preclude creative use of BFD where it can be useful.

--Dave

On Sep 18, 2009, at 9:27 PM, Mach Chen wrote:

> Hi,
>
> After reading the two drafts (draft-ietf-bfd-generic-05 and draft- 
> ietf-bfd-v4v6-1hop-09), I have some confusion about the session  
> "...if multiple differentiated services [DIFFSERV] are being  
> operated over IPv4, an independent BFD session may be run for each  
> service level...", this implicates that there may be mulitiple BFD  
> sessions over one network-layer(ipv4) path, but it seems that this  
> is not allowed in draft-ietf-bfd-v4v6-1hop-09. How to understand  
> this conflict?  or do I miss something?
>
> Best regards,
> Mach Chen
>
>
>
>

---

From: Mach Chen <[email-masked]>
Date: Sat, 19 Sep 2009 16:07:16 +0800

Hi Dave,

Thanks for your response!

Based on your explanation, the point is whether there is a way to 
demultiplex the initial patcket, if there is, means no need to limit the BFD 
sesion. So, IMHO, it's better to clarify this in 1hop(since the draft is 
still in IESG review state, seems not too late to do it:-).

Have a good weekend!

Best regards,
Mach

--------------------------------------------------
From: "Dave Katz" <[email-masked]>
Sent: Saturday, September 19, 2009 2:39 PM
To: "Mach Chen" <[email-masked]>
Cc: <[email-masked]>
Subject: Re: Multiple BFD sessions over one path

> The point of the text in 1hop is that there isn't any use in having  more 
> than one session over a "path", and in any case it can't be done  because 
> there would be no way to demultiplex the initial packet of  multiple 
> sessions on the same "path" (because the first packet carries 
> discriminator 0).
>
> potentially different forwarding characteristics, so it may be useful  to 
> run BFD over more than one level (and the sessions can be  demultiplexed 
> because they are contextualized by the diffserv level).
>
> Essentially, each diffserv service level is considered to be a  separate 
> path, not unlike GRE or IPsec or what have you.
>
> Implementors are free to do as they please;  the point of the text is  to 
> not preclude creative use of BFD where it can be useful.
>
> --Dave
>
>
>
>> "...if multiple differentiated services [DIFFSERV] are being  operated 
>> over IPv4, an independent BFD session may be run for each  service 
>> level...", this implicates that there may be mulitiple BFD  sessions over 
>> one network-layer(ipv4) path, but it seems that this  is not allowed in 
>> draft-ietf-bfd-v4v6-1hop-09. How to understand  this conflict?  or do I 
>> miss something?
>>
>> Best regards,
>> Mach Chen
>>
>>
>>
>>
>

---

From: Dave Katz <[email-masked]>
Date: Sat, 19 Sep 2009 03:09:02 -0600

But the point is that, the way 1hop is specified, there is *no* way to  
demultiplex the initial packet--thus the requirement.

The only mechanism for multiple sessions is to have multiple paths,  
with one session per path.  A "path" ends up with the somewhat  
reflexive definition of "something you can demultiplex by."

As such, I believe that the language in the two drafts is proper.

--Dave

On Sep 19, 2009, at 2:07 AM, Mach Chen wrote:

> demultiplex the initial patcket, if there is, means no need to limit  
> the BFD sesion. So, IMHO, it's better to clarify this in 1hop(since  
> the draft is still in IESG review state, seems not too late to do  
> it:-).
>
> Have a good weekend!
>
> Best regards,
> Mach
>
>
>> The point of the text in 1hop is that there isn't any use in  
>> having  more than one session over a "path", and in any case it  
>> can't be done  because there would be no way to demultiplex the  
>> initial packet of  multiple sessions on the same "path" (because  
>> the first packet carries discriminator 0).
>>
>> But diffserv is essentially a tunnel of sorts;  each service level  
>> has potentially different forwarding characteristics, so it may be  
>> useful  to run BFD over more than one level (and the sessions can  
>> be  demultiplexed because they are contextualized by the diffserv  
>> level).
>>
>> Implementors are free to do as they please;  the point of the text  
>> is  to not preclude creative use of BFD where it can be useful.
>>
>> --Dave
>>

---

From: Vishwas Manral <[email-masked]>
Date: Sat, 19 Sep 2009 13:40:57 -0700

Hi Dave,

> demultiplex the initial packet--thus the requirement.
Like in BFD for MPLS LSP's, if we have negotiated the session
discriminator offline or have configured it statically we can
demultiplex the first packets too.

Thanks,
Vishwas

---

From: Dave Katz <[email-masked]>
Date: Sat, 19 Sep 2009 14:54:20 -0600

The 1hop spec explicitly disallows this.

This isn't to say that other applications are possible, but the point  
of the 1hop spec is to provide an interoperable baseline for a  
particular type of deployment.

--Dave