Subject: RE: Preconfigured EXP<-->PHB mapping for E-LSPs

From: Jayaram Beladakere <[email-masked]>
Date: Sat, 7 Apr 2001 12:35:14 -0700

Shahram, Thanks for this clarification.  I have a follow-up question on this
topic.

Suppose that the preconfigured EXP<-->PHB table has not been configured by
the network administrator; thus, all EXP values map to the Default PHB.  If
an E-LSP tunnel is now set up with no Diffserv Context, how should the LSR
encode the outgoing PHBs?  Obviously, the E-LSP tunnel would only support
Default PHB.  But, there are eight EXP field values that map to that PHB.
Which value shall be used for outgoing EXP field?

Otherwise, is it mandatory to specify a Diffserv Context for an E-LSP
tunnel?

Regards, Jayaram

-----Original Message-----
From: Shahram Davari [mailto:[email-masked]]
Sent: Wednesday, March 28, 2001 8:21 AM
To: 'Jayaram Beladakere'; [email-masked]'
Subject: RE: Preconfigured EXP<-->PHB mapping for E-LSPs

Hi Jayaram,

The intention of configured E-LSPs is to have the MPLS+Diffserv network
behave the same as a Diffserv-only network. Since RFC-2474 section 3 says:

"
   Packets received with an unrecognized codepoint SHOULD be forwarded
   as if they were marked for the Default behavior (see Sec. 4), and
   their codepoints should not be changed.  Such packets MUST NOT cause
   the network node to malfunction. "

So the answer to you question is that the EXP should be left unchanged in
the normal swap operation. 

It is however, possible to push another label and have either
non-configured, configured or even signaled E-LSP for the new LSP (which we
call tunnel-LSP). In that case depending on the type of the tunnel (Uniform
or pipe), the top label could have its EXP set to the same EXP, or to the
Default EXP or something else.

In case the tunnel-LSP is a uniform tunnel that is a non-configured (and
non-signaled) E-LSP, then the same behavior as explained in the above quote
from RFC-2474 is the correct behavior (i.e., setting the EXP of the pushed
label to the same EXP as the incoming label). 

Yours,
-Shahram

> -----Original Message-----
> From: Jayaram Beladakere [mailto:[email-masked]]
> Sent: Tuesday, March 27, 2001 2:06 PM
> To: [email-masked]'
> Subject: Preconfigured EXP<-->PHB mapping for E-LSPs
> 
> 
> The following is from "MPLS Support of Differentiated Services"
> (draft-ietf-mpls-diff-ext-08.txt).
> 
> 3.2.1 Preconfigured EXP<-->PHB mapping
> ....
> In case, the preconfigured EXP<-->PHB mapping has not actually been
> configured by the Network Administrator, the LSR should use a
> default preconfigured EXP<-->PHB mapping which maps all EXP values
> to the Default PHB.
> 
> In the above case, the LSR provides Default PHB treatment to 
> all labeled
> packets (belonging to LSPs with preconfigured EXP<-->PHB 
> mapping).  What
> shall be the value of the outgoing EXP field for such labeled 
> packets, both
> with label-swap and label-push operations at the LSR?  In 
> particular, shall
> the outgoing EXP match incoming EXP, or shall it be '000'?  Is there a
> difference if the label operation is swap rather than push?
> 
> Regards,
> Jayaram Beladakere
> Mahi Networks, Inc.
>

---

From: Shahram Davari <[email-masked]>
Date: Mon, 9 Apr 2001 11:52:33 -0700

Hi,

Since this is very similar to what I quoted from RFC-2474, my understanding is that the outgoing EXP should be the same as the incoming EXP. Because from this node's point of view all EXPs mean DF-PHB, so leaving the EXP field unchanged still means DF-PHB to this node. Additionally leaving the EXP field unchanged causes the downstream nodes that have their EXP<=>PHB configured, to treat the packet properly (prevents malfunction).

Yours,
-Shahram

> Sent: Saturday, April 07, 2001 3:35 PM
> To: Shahram Davari; [email-masked]'

---

From: Jayaram Beladakere <[email-masked]>
Date: Mon, 9 Apr 2001 18:36:26 -0700

The question, as I asked previously, may not have been completely clear.  I
will attempt to elaborate.

I understand that many-to-one mapping of many EXP field values to one PHB is
possible with the preconfigured EXP<-->PHB mapping.  The following are two
examples.  i) When the preconfigured EXP<-->PHB mapping has not actually
been configured by the Network Administrator, the LSR uses a default
preconfigured EXP<-->PHB mapping which maps all EXP values to the Default
PHB.  ii) A network administrator may map 000, 001 and 010 to Default PHB
and 110 and 111 to EF PHB.

Consider an E-LSP that is tunneled in another E-LSP using the PIPE MODEL.
Assume that neither LSP is specified with a Diffserv Context, so that both
use the preconfigured EXP<-->PHB mapping.  While the incoming PHB
determination is clear (section 3.3 of draft 0.8), what is not clear is the
encoding of outgoing EXP field (section 3.4.1).  Specifically, I have the
following question:

With many-to-one mapping of EXP-to-PHB in the preconfigured EXP<-->PHB
mapping, how shall the outgoing EXP be encoded for a PHB?  There are many
EXP field values that each correspond to the same PHB.

I am not sure that using the same value as incoming EXP field is the
intention with Pipe model.  Is it mandatory to specify a Diffserv Context
for the tunnel LSP (that maps a PHB to only one outgoing EXP field value)?

Jayaram

-----Original Message-----
From: Shahram Davari [mailto:[email-masked]]
Sent: Monday, April 09, 2001 11:53 AM
To: 'Jayaram Beladakere'; [email-masked]'
Subject: RE: Preconfigured EXP<-->PHB mapping for E-LSPs

Hi,

Since this is very similar to what I quoted from RFC-2474, my understanding
is that the outgoing EXP should be the same as the incoming EXP. Because
from this node's point of view all EXPs mean DF-PHB, so leaving the EXP
field unchanged still means DF-PHB to this node. Additionally leaving the
EXP field unchanged causes the downstream nodes that have their EXP<=>PHB
configured, to treat the packet properly (prevents malfunction).

Yours,
-Shahram

---

From: Shahram Davari <[email-masked]>
Date: Tue, 10 Apr 2001 08:19:00 -0700

Hi,

I think what you are asking is not an MPLS-Diffserv question, rather it is a Diffserv specific question. Because it is equally applicable to DSCP<=>PHB mapping.

So I suggest that you ask this from the Diffserv WG and then the answer will be equally applicable to MPLS-Diffserv. 

But here is my own opinion:

If multiple DSCP/EXP map to the same PHB, the choice of outgoing DSCP/EXP is a local policy matter and does not need standardization.

It all depends on the reasons for having multiple DSCP/EXP mappings to the same PHB. Is it because they refer to multiple instances of the same PHB or is it just for reducing the configuration time. In the former case the choice may depend on specific QoS requirements such as delay requirements, while in the latter case probably it should make no difference which DSCP/EXP is used for a particular outgoing PHB, the choice could be as simple as "the one that involves less packet processing operation", which may translate to: if the outgoing PHB is going to be the same as the incoming PHB, then it probably makes sense to use the same DSCP/EXP as the incoming packet.

Yours,
-Shahram

> Sent: Monday, April 09, 2001 9:36 PM