Subject: RE: Is IP TOS field changed by LSR?

From: Eyad Saheb <[email-masked]>
Date: Fri, 20 Oct 2000 10:54:13 -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.

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.

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On Behalf Of Sung-eok
Jeon
Sent: Thursday, October 19, 2000 4:47 PM
To: [email-masked]
Subject: Is IP TOS field changed by LSR? 

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: Wushao Wen <[email-masked]>
Date: Fri, 20 Oct 2000 08:49:16 -0700 (PDT)

Please read my commments below.
> defeat the Multi-Protocol aspect.
 You are right.  In the MPLS domain, the modification should only on
shim label. 

> IP TOS bits means you're going through the IP stack at each hop.  Perhaps

In fact, EXP is already suggested to be used for differentiate service
classes. 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. 

Wushao