Chapter 5 MPLS

Labels There are many examples of label substitution protocols already in existence. ATM - label is called VPI/VCI and travels with cell. Frame Relay - label is called a DLCI and travels with frame. TDM - label is called a timeslot its implied, like a lane. X25 - a label is an LCN Proprietary PORS, TAG etc.. One day perhaps Frequency substitution where label is a light frequency?

Hop-by-hop or source (explicit) routing to establish labels Uses label native to the media Multilevel label substitution transport What is MPLS?

 Virtual circuit layer underneath IP  Virtual circuit = virtual wire = label switched path IP Network (ATM) IP Network (Voice) IP Network (Data) MPLS (Virtual Point-to-Point Circuits) Physical Infrastructure (Point-to-Point Circuits)

What is MPLS?  Offer service above IP  Converged network  Realtime voice  Best-effort data  High priority transactions (ATM, control …)  On the same physical infrastructure

What is MPLS?  MPLS Characteristics  Mechanisms to manage traffic flows  Is independent of Layer-2 and Layer-3 protocols  Maps IP-addresses to fixed length labels  Interfaces to existing routing protocols (RSVP, OSPF)  Supports ATM, Frame-Relay and Ethernet

7 Why MPLS?  Leverage existing ATM hardware  Ultra fast forwarding  IP Traffic Engineering  Constraint-based Routing  Virtual Private Networks  Controllable tunneling mechanism  Voice/Video on IP  Delay variation + QoS constraints

ROUTE AT EDGE, SWITCH IN CORE IP Forwarding LABEL SWITCHING IP Forwarding IP #L1IP#L2IP#L3 IP

MPLS Terminology  LDP: Label Distribution Protocol  LSP: Label Switched Path  FEC: Forwarding Equivalence Class  LSR: Label Switching Router  LER: Label Edge Router

WHAT IS A LABEL?  “labels” called a label stack.  A label is a short, four-byte, fixed-length, locally-significant identifier which is used to identify a Forwarding Equivalence Class (FEC).  The label which is put on a particular packet represents the FEC to which that packet is assigned.  Each label stack entry contains four fields: Label - a 20-bit label value. Exp - a 3-bit Traffic Class field for QoS (quality of service) priority (experimental) and ECN (Explicit Congestion Notification). S - a 1-bit bottom of stack flag. If this is set, it signifies that the current label is the last in the stack. TTL - an 8-bit TTL (time to live) field.

FORWARDING EQUIVALENCE CLASS (FEC)  Any subset of packets are treated the same way by a router  Forwarded out the same interface with the same next hop and label  Given the same class of service, output on same queue, given same drop preference, and any other option available to the network operator.  When a packet enters the MPLS network at the ingress node, the packet is mapped into an FEC. The mapping can also be done on a wide variety of parameters (as specified by network manager); address prefix (or host), source/destination IP address pair, port numbers, IP protocol ID or ingress interface. This greater flexibility adds functionality to MPLS that is not available in traditional IP routing.  FECs also allow for greater scalability in MPLS. The aggregation of flows into FECs of variable granularity provides scalability that meets the demands of the public Internet as well as enterprise applications.  In the current LDP specification, only 3 types of FECs are specified:  IP Address Prefix  Router ID  Flow (port, dest-addr, src-addr etc.)  The spec. states that new elements can be added as required.

How Does MPLS Work?  Packets are tagged and routed based on tags.  All traffic with the same label treated the same LER IP Routing Layer IP Routing Layer LSR Payload 13 Payload 5 13Payload13 Payload 5 5

Other Features of MPLS  Label forwarding distinct from IP forwarding  May make non-shortest paths  Label routing linked to IP routing IP Forwarding LER (Perform Labeling) LSR Cloud (Forward by label) IP Forwarding LER (Remove Label) LSR Cloud (Forward by label)

MPLS BUILT ON STANDARD IP Destination based forwarding tables as built by OSPF, IS-IS, RIP, etc.

IP FORWARDING USED BY HOP- BY-HOP CONTROL IP

MPLS Label Distribution Mapping: 0.40 Request: 47.1 Mapping: 0.50 Request: 47.1

Label Switched Path (LSP) IP

LABEL EDGE ROUTER (LER)  Can be an ATM switch or a router  Ingress LER performs the following:  Receives the packet  Adds label  Forwards the packet into the MPLS domain  Egress LER removes the label and delivers the packet

LABEL EDGE ROUTER (LER)

LABEL SWITCHING ROUTER (LSR)  A router/switch that supports MPLS  Can be a router  Can be an ATM switch + label switch controller  Label swapping  Each LSR examines the label on top of the stack  Uses the Label Information Base (LIB) to decide the outgoing path and the outgoing label  Removes the old label and attaches the new label  Forwards the packet on the predetermined path

LABEL SWITCHING ROUTER (LSR)  Upstream Router (Ru) – router that sends packets  Downstream Router (Rd) – router that receives packets  Need not be an end router  Rd for one link can be the Ru for the other Ru Rd Ru Rd

LABEL SWITCHING ROUTER (LSR)

POSITIONS OF LERs & LSRs

LABEL SWITCHED PATH (LSP)  LSP defines the path through LSRs from ingress to egress router  FEC is determined at the LER-ingress  LSPs are unidirectional

LABEL SWITCHED PATH (LSP) LSP

ROUTE SELECTION  Refers to the method of selecting an LSP for a particular FEC  Done by LDP  Set of procedures and messages  Messages exchanged between LSRs to establish an LSP  LSRs associate an FEC with each LSP created  Two types of LDP  Hop by hop routing  Explicit routing

ROUTE SELECTION Hop-by-Hop Routing Explicit Routing Source routing of control traffic Builds a path from source to destination Requires manual provisioning, or automated creation mechanisms. LSPs can be ranked so some reroute very quickly and/or backup paths may be pre-provisioned for rapid restoration Operator has routing flexibility (policy- based, QoS-based, Adapts well to traffic engineering Distributes routing of control traffic Builds a set of trees either fragment by fragment like a random fill, or backwards, or forwards in organized manner. Reroute on failure impacted by convergence time of routing protocol Existing routing protocols are destination prefix based Difficult to perform traffic engineering, QoS-based routing Explicit routing shows great promise for traffic engineering

Explicit Routing - MPLS vs. Traditional Routing Connectionless nature of IP implies that routing is based on information in each packet header Source routing is possible, but path must be contained in each IP header Lengthy paths increase size of IP header, make it variable size, increase overhead Some gigabit routers require ‘slow path’ option-based routing of IP packets Source routing has not been widely adopted in IP and is seen as impractical Some network operators may filter source routed packets for security reasons MPLS’s enables the use of source routing by its connection-oriented capabilities - paths can be explicitly set up through the network - the ‘label’ can now represent the explicitly routed path Loose and strict source routing can be supported MPLS makes the use of source routing in the Internet practical

Label Distribution Protocol (LDP)  Label Distribution Protocol (LDP)  set of procedures by which LSRs establish LSPs  mapping between network-layer routing information directly to data-link layer switched paths  LDP peers:  two LSRs which use LDP to exchange label/stream mapping  information exchange known as “LDP Session”

Label Distribution Protocol (LDP) - Purpose Label distribution ensures that adjacent routers have a common view of FEC label bindings Routing Table: Addr-prefix Next Hop /8 LSR2 Routing Table: Addr-prefix Next Hop /8 LSR2 LSR1 LSR2 LSR3 IP Packet Routing Table: Addr-prefix Next Hop /8 LSR3 Routing Table: Addr-prefix Next Hop /8 LSR3 For /8 use label ‘17’ Label Information Base: Label-In FEC Label-Out /8 XX Label Information Base: Label-In FEC Label-Out /8 XX Label Information Base: Label-In FEC Label-Out XX /8 17 Label Information Base: Label-In FEC Label-Out XX /8 17 Step 1: LSR creates binding between FEC and label value Step 2: LSR communicates binding to adjacent LSR Step 3: LSR inserts label value into forwarding base Common understanding of which FEC the label is referring to! Label distribution can either piggyback on top of an existing routing protocol, or a dedicated label distribution protocol (LDP) can be created Label distribution can either piggyback on top of an existing routing protocol, or a dedicated label distribution protocol (LDP) can be created

Label Distribution - Methods LSR1 LSR2 Label Distribution can take place using one of two possible methods Downstream Label Distribution Label-FEC Binding LSR2 and LSR1 are said to have an “LDP adjacency” (LSR2 being the downstream LSR) LSR2 discovers a ‘next hop’ for a particular FEC LSR2 generates a label for the FEC and communicates the binding to LSR1 LSR1 inserts the binding into its forwarding tables If LSR2 is the next hop for the FEC, LSR1 can use that label knowing that its meaning is understood LSR1 LSR2 Downstream-on-Demand Label Distribution Label-FEC Binding LSR1 recognizes LSR2 as its next-hop for an FEC A request is made to LSR2 for a binding between the FEC and a label If LSR2 recognizes the FEC and has a next hop for it, it creates a binding and replies to LSR1 Both LSRs then have a common understanding Request for Binding Both methods are supported, even in the same network at the same time For any single adjacency, LDP negotiation must agree on a common method

Distribution Control: Ordered v. Independent Independent LSP Control Ordered LSP Control Next Hop (for FEC) Outgoing Label Incoming Label MPLS path forms as associations are made between FEC next-hops and incoming and outgoing labels Each LSR makes independent decision on when to generate labels and communicate them to upstream peers Communicate label-FEC binding to peers once next-hop has been recognized LSP is formed as incoming and outgoing labels are spliced together Label-FEC binding is communicated to peers if: - LSR is the ‘egress’ LSR to particular FEC - label binding has been received from upstream LSR LSP formation ‘flows’ from egress to ingress Definition Comparison Labels can be exchanged with less delay Does not depend on availability of egress node Granularity may not be consistent across the nodes at the start May require separate loop detection/mitigation method Requires more delay before packets can be forwarded along the LSP Depends on availability of egress node Mechanism for consistent granularity and freedom from loops Used for explicit routing and multicast Both methods are supported in the standard and can be fully interoperable

Label Retention Methods LSR1 LSR2 LSR3 LSR4 LSR5 Binding for LSR5 Binding for LSR5 Binding for LSR5 An LSR may receive label bindings from multiple LSRs Some bindings may come from LSRs that are not the valid next-hop for that FEC Liberal Label RetentionConservative Label Retention LSR1 LSR2 LSR3 LSR4 Label Bindings for LSR5 Valid Next Hop LSR4’s Label LSR3’s Label LSR2’s Label LSR1 LSR2 LSR3 LSR4 Label Bindings for LSR5 Valid Next Hop LSR4’s Label LSR3’s Label LSR2’s Label LSR maintains bindings received from LSRs other than the valid next hop If the next-hop changes, it may begin using these bindings immediately May allow more rapid adaptation to routing changes Requires an LSR to maintain many more labels LSR only maintains bindings received from valid next hop If the next-hop changes, binding must be requested from new next hop Restricts adaptation to changes in routing Fewer labels must be maintained by LSR Label Retention method trades off between label capacity and speed of adaptation to routing changes

MPLS Header  Lightweight  8 bit TTL  20 bit label tag  3 bit QoS tag  1 bit stack  Indicates last LSR tag  Allows heirarchical tagging Payload13 Payload138Payload135 Payload13

Provisioning vs. Signaling  Signaling  Seconds  Provisioning  Minutes to days  Separate control message protocol  Distribute labels and forwarding info  RSVP  Label Distribution Protocol

Comparing MPLS to IP  IP over MPLS vs IP only  Qos  Performance  Tunneling  VPN  Traffic Engineering

MPLS vs IP: QoS  MPLS  Per hop QoS  Using labels to prioritize  20 bit identifier space  IP  Per hop QoS  Use IP&TCP header  104 bit identifier space

MPLS vs IP: Performance  MPLS  Forward on short tags  Not prefix match on address  IP  Routers can forward at gigabit/s

MPLS vs IP: Tunneling (VPN)  MPLS  Lightweight tunnels  32 bit header  No security  IP  Heavyweight tunnels  ~160 (?) bit header  No security  (without IPSEC)

MPLS vs IP: Traffic Engineering  MPLS  Arbitrary (non-shortest) paths  Virtual circuits  MPLS routing linked to IP routing  Flexible aggregation  IP  Route announcement manipulation  Path cost manipulation

MPLS vs IP: Future QoS  MPLS  Propagate QoS between networks  RSVP  IP  Propagate QoS between networks  RSVP

MPLS Advantages & Disadvantages  Advantages:  Improves packet-forwarding performance in the network  Supports QoS and CoS for service differentiation  Supports network scalability  Improves the possibilities for traffic engineering  Integrates IP and ATM in the network  Builds interoperable networks  Disadvantages:  An additional layer is added  The router has to understand MPLS

Summary of Motivations for MPLS Simplified forwarding based on exact match of fixed length label - initial drive for MPLS was based on existance of cheap, fast ATM switches Separation of routing and forwarding in IP networks - facilitates evolution of routing techniques by fixing the forwarding method - new routing functionality can be deployed without changing the forwarding techniques of every router in the Internet Facilitates the integration of ATM and IP - allows carriers to leverage their large investment of ATM equipment - eliminates the adjacency problem of VC-mesh over ATM Enables the use of explicit routing/source routing in IP networks - can be easily used for such things as traffic management, QoS routing Promotes the partitioning of functionality within the network - move granular processing of packets to edge; restrict core to packet forwarding - assists in maintaining scalability of IP protocols in large networks Improved routing scalability through stacking of labels - removes the need for full routing tables from interior routers in transit domain; only routes to border routers are required Applicability to both cell and packet link-layers - can be deployed on both cell (eg. ATM) and packet (eg. FR, Ethernet) media - common management and techniques simplifies engineering Many drivers exist for MPLS above and beyond high speed forwarding