Basic Idea MPLS is a hybrid model adopted by IETF to incorporate best properties in both packet routing & circuit 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) MPLS supports a range of access technologies, including T1/E1, ATM, Frame Relay, and DSL. Multiprotocol Label Switching (MPLS) is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table. IP Router MPLS ATM Switch Control: Control: Control: IP Router IP Router ATM Forum Software Software Software Forwarding: Forwarding: Forwarding: Longest-match Label Swapping Label Swapping Lookup
History In Mid-90s, many ISPs migrated from router based cores to IP-over- ATM, this provided: Greater Bandwidth Deterministic forwarding performance Traffic engineering support No specific Internet backbone networking equipment available for ISPs. However, Continued Internet growth increased stress on ATM networks: Bandwidth limitations 20 percent ―cell tax‖ Designed for different tasks (IP—connectionless, ATM—connection- oriented) Standard being developed by IETF (Internet Engineering Task Force) since 1997
History (Cont.) Packets labeled and sent through network on paths rather than hop-to-hop as in IP data-grams Each multilayer switch ran standard IP routing software (OSPF, BGP-4) Different label binding approaches Data-driven model Label bindings created when data packets arrive. Labels created either when first packet in a flow or after a number of packets in a flow have arrived. IP Switching and CSR used this technique. Control-driven model Label bindings created when control information arrives. Assigned in response to processing of protocol traffic, control traffic (such as RSVP), or static configuration. --Control-driven model used in MPLS!Note:OSPF-Open Shortest-Path First BGP-Border Gateway Protocol RSVP-Resource Reservation Protocol
Terminology/Components LSR (Label Switched Router) High speed routers which switch data traffic within MPLS domain Swaps labels on packets in core of network. LSP (Label Switch Path) A unidirectional path to transport packets within MPLS domain. The path is setup before the data transmission similar to circuit switching Path through network based on a FEC (simplex in nature). LER (Label Edge Router) Attach Labels to packets based on a FEC. Operates at the edge of the access network & MPLS network Responsible for assignment and removal of labels Supports Multiple Protocols connected to dissimilar networks (such as frame relay, ATM and Ethernet)
Terminology/Components LIB (Label Information Base) Table maintained by the Routers MPLS equivalent to IP routing table, contains FEC-to-Label bindings. FEC (Forwarding Equivalence Class) Group of packets sharing the same type of transport. A path is a representation of a FEC Label Distribution Protocol (LDP) IETF defined protocol for explicit signaling and management
MPLS Operation 1a. Routing protocols (e.g. OSPF-TE, IS-IS-TE) exchange reach ability to destination networks 4. LER at egress1b. Label Distribution Protocol (LDP) removes label andestablishes label mappings to destination delivers packetnetwork IP IP 2. Ingress LER receives packet and “labels packets 3. LSR forwards packets using label swapping
LSRs and LERs The devices used for MPLS can be classified into label edge routers (LERs) and label switching routers (LSRs). A LSR is a high-speed router device in the core of an MPLS network. Participates in the establishment of LSPs, using the appropriate label signaling protocol Does high-speed switching of the data traffic based on the established paths. A LER is a device that operates at the edge of the access network and MPLS network. Supports multiple ports connected to dissimilar networks (such as frame relay, ATM, and Ethernet) Forwards this traffic on to the MPLS network after establishing LSPs, using the label signaling protocol at the ingress and distributing the traffic back to the access networks at the egress. Plays important role in the assignment and removal of labels, as traffic enters or exits an MPLS network.
Labels The MPLS forwarding component is based on the label- swapping algorithm. Label encapsulated in MPLS header, which is in between the Layer 2 and IP header. If Layer 2 technology supports labels (ATM VPI/VCI, Frame Relay DLCI), MPLS label and header encapsulated in the Layer 2 label field.
Why Label Swap? Label swapping provides a significant number of operational benefits when compared to conventional hop-by-hop network layer routing. Gives an ISP flexibility in the way that it assigns packets to FECs. Destination address (like conventional IP routing) Source address. Application type. Point of entry/exit to/from the label-swapping network. CoS conveyed in the packet header. Any combination of the above. ISPs can construct customized LSPs that support specific application requirements (for instance, VPNs). LSPs can be designed to: minimize the number of hops bandwidth requirements bypass points of congestion Offer ISPs precise control over the flow of traffic in their networks.
MPLS header Label field- Actual MPLS label (20bits). CoS field- ―Class of Service‖ can effect queuing and discard algorithms applied to packets (3 bits). S (Stack) field- supports a hierarchical label stack (1 bit). TTL field- ―Time-to-live‖ provides conventional IP TTL functionality (8 bits).
Label Creation Topology-based method uses normal processing of routing protocols (such as OSPF and BGP) Request-based method uses processing of request-based control traffic (such as RSVP)Note:OSPF-Open shortest-path first BGP- Border Gateway Protocol RSVP-Resource Reservation Protocol
Label Spaces Labels used by an LSR for FEC-label bindings are split into 2 categories: Per platform-label values are unique across an entire LSR. Per interface-label values are associated w/ interfaces. Label values provided on different interfaces could be the same.
Label Distribution No single method of signaling required Enhancements of existing routing protocols (to allow piggybacking of label information) include: Border Gateway Protocol (BGP) Resource Reservation Protocol (RSVP) LDP (Label Distribution Protocol)- Defined by IETF for signaling and management of label space. --Extensions have been defined to support explicit routing based on QoS and CoS requirements.
Label Distribution schemes LDP—maps unicast IP destinations into labels RSVP, CR–LDP—used for traffic engineering and resource reservation BGP—external labels (VPN)
MPLS features and security Traffic Engineering MPLS networks provide Efficient Link Utilization separation of address and Class of Service (CoS) traffic Packets from one VPN do not Differentiated types of service inadvertently go to another across an MPLS network. VPN Virtual Private Networks (VPNs) Malicious spoofing is impossible A VPN is a private connection over an shared network
Summary Improves packet-forwarding performance in the network MPLS enhances and simplifies packet forwarding through routers using Layer-2 switching paradigms. MPLS is simple, which allows for easy implementation. MPLS increases network performance because it enables routing by switching at wireline speeds. Supports QoS and CoS for service differentiation MPLS uses traffic-engineered path setup and helps achieve service-level guarantees. MPLS incorporates provisions for constraint-based and explicit path setup. Supports network scalability MPLS can be used to avoid the N2 overlay problem associated with meshed IP–ATM networks. Integrates IP and ATM in the network MPLS provides a bridge between access IP and core ATM. MPLS can reuse existing router/ATM switch hardware, effectively joining the two disparate networks. Builds interoperable networks MPLS is a standards-based solution that achieves synergy between IP and ATM networks. MPLS facilitates IP–over-synchronous optical network (SONET) integration in optical switching. MPLS helps build scalable VPNs with traffic-engineering capability.
…However Some Internet Purists complain that MPLS breaks some critical Internet architectural principles: MPLS supports tunneling, which breaks the transparency paradigm. MPLS supports sessions, it breaks the datagram model. But MPLS provides great value to ISPs, such as lower operating costs and ability to provide QoS to businesses.
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