Design MPLS | Multiprotocol Label Switching | Virtual Private Network

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Design concepts of MPLS
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  EE6364 DCW MPLS-1  Multi-Protocol Label Switching (MPLS) ã  MPLS Terminology ã  MPLS Forwarding Component ã  MPLS Control Components   RSVP-TE   LDP ã  MPLS Fast Re-Route ã  Pseudo-Wire Emulation  EE6364 DCW MPLS-2  Introduction 1   In the connection-oriented protocol, users and the network negotiate the QoS during the call setup process. For example, in the ATM connection admission control (CAC) process, the users include the traffic descriptor and QoS requests in the SETUP messages sent to the network. Based on the traffic descriptor and the QoS requested, the network can either accept or reject the connection request. If the connection request is accepted, a virtual circuit is established and a corresponding VPI/VCI is assigned to the virtual circuit. The network reserves resources (e.g., bandwidth and buffer) for as long as this virtual circuit exists. For each ATM cell received, the switch examines the VPI/VCI to identify the virtual circuit and the QoS requested, and handles the cell accordingly. Each switch switches the ATM cell received by comparing the VPI/VCI with the entries in the switch routing table. The switching process is relatively simple. IP traffic is connectionless. Routers route the IP packets based on the destination IP addresses. The forwarding decision is made on a hop-by-hop basis. No connection set up is needed. Thus, the routers can not reserve bandwidth for a session. In addition, the TOS field in the IP packet header is not supported by many routers. Thus, there is no QoS support in the IP networks. Each router routes the packet received by matching the destination IP address with the entries in the routing table using the longest prefix matching algorithm. This switching process is more complicated, and could cause longer delay. Multiprotocol label switching (MPLS) introduces the connection-oriented paradigm into the IP traffic flow. With MPLS, IP packets are routed based on a fixed-length short label, similar to the ATM’s VPI/VCI and frame relay’s DLCI. 1 E. Rosen, A. Viswanathan, R. Callon, “Multiprotocol Label Switching Architecture,” RFC 3031, January, 2001. E. Rosen, et. al., “MPLS Label Stack Encoding,” RFC 3032, January, 2001.  EE6364 DCW MPLS-3 MPLS Terminologies: 1. MPLS Node:  A node which is running MPLS. It is aware of MPLS control protocols and is capable of forwarding packets based on labels. 2. MPLS Domain:  A contiguous set of nodes which operate MPLS routing and forwarding, and which are under one administrative domain. 3. Forward Equivalence Class (FEC):  A group of IP packets that are forwarded in the same manner (e.g., forwarded to the same next hop, treated with the same priority). 4. Label:  A short fixed length identifier which is carried by a packet and is used to identify a FEC, usually of local significance. 5. Shim:  A space in a packet between the layer2 and layer 3 headers. A label is encoded in the shim. 6. Labeled Packet  A packet into which a label has been encoded. 7. Label Switch Router (LSR):  An MPLS node which is capable of forwarding layer 3 packets. A LSR can be at the ingress or egress of the MPLS domain, or at the core of the domain. If a LSR is in the core of the MPLS domain. It routes the packet based on the label and swap the label before the packet is sent to the output port of the node. If a LSR is at the ingress or egress of the MPLS domain. It generates a label in the ingress, and removes the label at the egress. 8. Label Swapping: The operation consisting of looking up an incoming label to determine the outing label, encapsulation, output port, and other data handling information. 9. Label Switched Path (LSP):  A sequence of LSRs that is to be followed by a packet. 10. Label Stack:  An order set of labels. 11. Label Binding: The process that two MPLS nodes agree on the assignment of a label to a FEC.  EE6364 DCW MPLS-4 MPLS Forwarding Components MPLS involves (1) control component, and (2) packet forwarding component. The control component includes label binding between MPLS nodes, and the establishment of a forwarding table. The packet forwarding component includes table look-up, label swapping and forwarding of the packet from the input port to the output port. Label MPLS is layer 2 independent. The link layer can be ATM, frame relay, Ethernet MAC layer, or PPP. If ATM is used as the data link layer, the label is the ATM VPI/VCI. If frame relay is used, the label is the DLCI. In these two data link layers, the label is located in the link layer header. If other data link layer is used, the label will be place in a shim, which is located between the layer2 and layer 3 headers. The MPLS label is four octets long. The position and the format of the label are as follows: In the above figure, Label Value: Values 0 to 15 are reserved. Exp: Experimental use (3 bits); can be used for Classes of Service. S: Bottom of stack. This bit is set to one for the last entry in the label stack. TTL: Time to live. It is used to prevent looping.  A packet with a label is called a labeled packet. A normal IP packet without MPLS label is called an un-labeled packet. If a labeled packet is encapsulated by the PPP header, the protocol number in the PPP header is 0X0281 for unicast and 0X0283 for multicast MPLS. If a Label ValueExpSTTL 20318BitsL2 HeaderShimL3 HeaderData
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