Scribe Notes For Lecture 32 (CS625)
by Amit Srivastava (Y0038)
amits@iitk.ac.in
Date: Nov 11 2003


Multi Protocol Label Switching (MPLS)

Circuit Switching Vs Packet Switching: In case of a circuit switched network a fixed amount of bandwidth is reserved as well as a route is chosen before a connection is established and only this route is used for the complete transaction on the connection. While in case of a packet switched network all the information is packaged into small groups called packets, frames, blocks, or cells. Circuit switching provides better Quality of Service (QOS) while packet switching utilizes the bandwidth more efficiently. Also, information about each and every connection needs to be stored thereby putting a limit on scalibilty.

MPLS tries to combine the benefit of both the above switching technologies. It uses circuit switching within an ISP while packet switching is used within ISP's.

Motivation for MPLS: Initially MPLS was developed to combine the benefits of internetworking and routing in layer 3 and layer 2 i.e. Network Layer and Data Link Layer. But, its major technological significance lies in implementing Traffice Engineering.

Some Components of MPLS:
  • Label: Labels are short fixed length identifiers, assigned to every packet.
  • Label-Switched Path (LSP): A unidirectional path to transport packets within MPLS domain. The path is setup before the data tranmission similar to circuit switching.
  • Forwarding Equivalence Classes (FEC): A group of packets that share the same requirement for transport e.g. packets with same destination IP address prefix
  • Label Switching Router (LSR): High speed routers which switch data traffice within MPLS domain
  • Label Distribution Protocol (LDP)
    • Label request from upstream to downstream
    • Label mapping from downstream to upstream

MPLS Basics:
  • MPLS uses Label Switching
  • A label is assigned for each IP flow
  • A LSP is created between ingress and egress
  • Packet forwarding at each router by table lookup (based on label)
When an ingress recieves a packet it encapsulates it inside a MPLS packet at layer 2 and passed on to egress which then puts of the MPLS headers.

Fig 1

MPLS Packet Format
  
Label
(20 bits)
 Class of Service (CoS)
(3 bits)
 Stack bit (S)
 TTL
(8 bits)


L2
Header
 MPLS
Header
 L3
Header
 L3
Data


L2
Header		 MPLS
Header		 MPLS
Header		 L3
Header		 L3
Data


Fault Tolerance and Path Recovery

Various path recovery mechanisms are employed if some link goes down as shown in Fig 2

                                           Fig 2

Local Recovery (blue broken line): A new path is chosen just between the two routers, the link between whom has gone down.
Global Recovery (green broken line): A new path is chosen between the Ingress and Egress.

Resources:
  1. The MPLS Resource Center (http://www.mplsrc.com)
  2. Multiprotocol Label Switching (MPLS) Traffic Engineering