Subject: Is IP TOS field changed by LSR?

From: Sung-eok Jeon <[email-masked]>
Date: Thu, 19 Oct 2000 16:47:22 -0400 (EDT)

Dear Concerns,

How are you doing?

I and my friend are using the linux-mpls-ldp.pre7-0.200 

to setup a MPLS network supporting serveral service classes.

While we are trying to do setup our own network, 

we come upon a weird phenomenon. 

We think we observed that the TOS field of IP header is changed

from LSR to LSR. 

Is it really true that the TOS field is modified from LSR to LSR? 

If the above is true? what is the reason?  

(I guess the front part of IP header might be used as a Label field. 

 Is it right? )
 
-------------------------------------------------------------------------

The followings are what we are now trying to do :

1) Setup LSP( Source-LSR1-LSR2-Destination)

2) marking the TOS of IP header at LSR1.

   Because we want to support multiple services, we want to

   use the TOS field of the IP header.

3) Ping from source to destination and observe the packet content

   with Smartbit (between LSR1 and LSR2/ between LSR2 and destination).

==> Different TOS values according to different observing position.

-------------------------------------------------------------------------   

I hope some of you tell me if there is any mechanism modifying 

the TOS field of IP header in the program.

If ture, Is there any other new version which does not change the TOS field?  

Thanks in advance.

Sung-eok Jeon

---

From: Eyad Saheb <[email-masked]>
Date: Fri, 20 Oct 2000 12:03:43 -0400

>> I'm not familiar with the LDP implementation on Linux, but MPLS should
not
>> be modifying anything above/behind the label (shim or not); that would
>> defeat the Multi-Protocol aspect.
>
> You are right.  In the MPLS domain, the modification should only on
>shim label. 
>
> 
>> ps. Why not use the EXP bits to differentiate service classes ?  Using
the
>> IP TOS bits means you're going through the IP stack at each hop.  Perhaps
>> your problem is there.
>
>In fact, EXP is already suggested to be used for differentiate service
>classes. 

	I know, I wasn't inventing it.  Just thought perhaps you were
unaware.

>However, the IP header has six bits in TOS (eliminte 2 bits not
>used by differential service). So it is possible to use different labels
>for different service classes.
 
	In my understanding, DiffServ over MPLS can be done two ways:
L-LSPs,  E-LSPs. L-LSPs seems to be what you're doing (using the label to
infer service type).  If this is the case, then you don't need to look at
the IP header (you shouldn't be looking there anyways).  Therefore, your
network should provide different levels of service even if the IP header
gets modified along the way (that seems to be a different story).

Eyad

---

From: Shen Gangxiang <[email-masked]>
Date: Sat, 21 Oct 2000 11:05:19 +0800

I think it is impossible as MPLS shim header is independent from IP header
except TTL field. I guess you may have something wrong during your
implementation.

Gangxiang

-----Original Message-----
From: Sung-eok Jeon [mailto:[email-masked]]
Sent: Friday, October 20, 2000 4:47 AM
To: [email-masked]
Subject: Is IP TOS field changed by LSR? 

Dear Concerns,

How are you doing?

from LSR to LSR. 

 Is it right? )

Thanks in advance.

Sung-eok Jeon