Subject: Interoperability among Label Encoding.

From: Helpdesk MIS <[email-masked]>
Date: Wed, 14 Jun 2000 11:51:49 -0400

From:  Yongjun Im

Dear Sir/Madam,

I have some questions about interoperability among 
MPLS label encoding.

The following paragraphs are cited from "draft-ietf-mpls-arch-06.txt".

"3.25.3. Interoperability among Encoding Techniques
   
   Unfortunately, ATM switches have no capability for translating from
   one encoding technique to another.  The MPLS architecture therefore
   requires that whenever it is possible for two ATM switches to be
   successive LSRs along a level m LSP for some packet, that those two
   ATM switches use the same encoding technique.

   Naturally there will be MPLS networks which contain a combination of
   ATM switches operating as LSRs, and other LSRs which operate using an
   MPLS shim header. In such networks there may be some LSRs which have
   ATM interfaces as well as "MPLS Shim" interfaces. This is one example
   of an LSR with different label stack encodings on different hops.
   Such an LSR may swap off an ATM encoded label stack on an incoming
   interface and replace it with an MPLS shim header encoded label stack
   on the outgoing interface."

The following figure shows an example of interoperability among
shim header(e.g, POS, GbE etc) and ATM header(vpi/vci).
        
        R1------R2------R3                 R6-----R7-----R8.
                                        \              /   
                                        R4------R5

         Shim Header                ATM          Shim Header .
            (POS)                                             (GbE)
        
In the example, LSP consists of LSR R1,R2,..,R7,R8.
R1 and R2 have only POS interfaces.
R4 and R5 have only ATM interfaces.
R7 and R8 have only Gigabit Ethernet interfaces.
R3 have POS and ATM interfaces.
R6 have ATM and GbE interfaces.

Q1: 
When LSR R3 receives a shim-labled packet from R2,
does it swaps off a shim header(POS) on an incoming interface,
encapsulate an IP packet(or fragment) with AAL5 (including ATM shim header),
and forward the segmented cells including ATM label(vpi/vci) to R4 ?

Q2: 
When LSR R5 receives a ATM-labled packet from R5,
does it swaps off a ATM label on an incoming interface,
reassemble AAL5 packet (including shim header),
swap off a AAL5 shim header,
encapsulate an IP packet with MAC frame(including new shim header),
and forwards the MAC frame to R7 ?

Any response will be appreciated.

Thanks in advance.

Best regards.

Yongjun 
   
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Yong-Jun Im (in Korean: 임용준)
Research Engineer
Digital Switching System Lab., LGIC R&D Center
533, Hogye-dong, Dongan-gu, Anyang-shi, Kyonggi-do, 431-080, KOREA
E-mail) [email-masked]  Office) +82-343-450-7750
Fax) +82-343-450-7009   Mobile) +82-11-201-8248
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