Multiprotocol Label Switching The future of IP Backbone

Multiprotocol Label Switching The future of IP Backbone Technology Ravikumar Pragada & Girish Srinivasan 1

Overview • • • Need for MPLS Basics Benefits Label Switched Path Label Distribution Protocol Hierarchy in MPLS Explicit Routing Loop Detection Traffic Engineering Constraint Based Routing Tag Switching IP Switching 2

Conventional IP Networks & Routing • Client networks are connected to backbone via edge routers – LAN, PSTN, ADSL • Data packets are routed based on IP address and other information in the header • Functional components – Forwarding • responsible for actual forwarding across a router • consists of set of procedures to make forwarding decisions – Control • responsible for construction and maintenance of the forwarding table • consists of routing protocols such as OSPF, BGP and PIM

Need for Multiprotocol Label Switching (MPLS) • Forwarding function of a conventional router – a capacity demanding procedure – constitutes a bottle neck with increase in line speed • MPLS simplifies forwarding function by taking a totally different approach by introducing a connection oriented mechanism inside the connectionless IP networks 4

Label Switching • Decomposition of network layer routing into control and forwarding components applicable • Label switching forwarding component algorithm uses – forwarding table – label carried in the packet • What is a Label ? – Short fixed length entity 5

MPLS Basics • A Label Switched Path (LSP) is set up for each route • A LSP for a particular packet P is a sequence of routers, <R 1, R 2………. . Rn> for all i, 1< i < n: Ri transmits P to R[i+1] by means of a label • Edge routers – analyze the IP header to decide which LSP to use – add a corresponding local Label Switched Path Identifier, in the form of a label 6 – forward the packet to the next hop

MPLS Basics contd. . • Subsequent nodes – just forward the packet along the LSP – simplify the forwarding function greatly – increase performance and scalability dramatically • New advanced functionality for Qo. S, differentiated services can be introduced in the edge routers • Backbone can focus on capacity and performance • Routing information obtained using a common intra domain routing protocol such as OSPF 7

Basic Model for MPLS Network Internet LER IP LER LSR MPLS LSR LER LSR = Label Switched Router LER = Label Edge Router IP 8

MPLS Benefits Comparing MPLS with existing IP core and IP/ATM technologies, MPLS has many advantages and benefits: • The performance characteristics of layer 2 networks • The connectivity and network services of layer 3 networks • Improves the price/performance of network layer routing • Improved scalability 9

MPLS Benefits contd. . • Improves the possibilities for traffic engineering • Supports the delivery of services with Qo. S guarantees • Avoids need for coordination of IP and ATM address allocation and routing information 10

Necessity of L 3 Forwarding • For security – To allow packet filtering at firewalls – Requires examination of packet contents, including the IP header • For forwarding at the initial router - used when hosts don’t do MPLS • For Scaling – Forward on a finer granularity than the labels can provide 11

Carrying a Label • Certain link layer technologies can carry label as a part of their link layer header – e. g ATM & Frame Relay • Link layers that do not support labels in their header carry them in a “shim” label header 12

Establishing Label Switched Path • LSPs are generated and maintained in a distributed fashion • Each LSR negotiates a label for each Forwarding Equivalence Class (FEC) with its upstream and downstream neighbors using a distribution method • Label Information Base (LIB) - Result of negotiation 13

LDP - Terminology • 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” 14

LDP Message Exchange • Discovery messages - used to announce and maintain the presence of an LSR • Session messages - used to establish, maintain and terminate sessions between LDP peers • Advertisement messages - used to create, change, and delete label mappings • Notification messages - used to provide advisory information and to signal error information 15

LDP Message Format 0 1 2 3 0123456789012345678901 16

LDP Protocol Data Units (PDUs) • LDP message exchanges are accomplished by sending LDP PDUs • Each LDP PDU is an LDP header followed by LDP message • The LDP header is: 0 1 2 3 0123456789012345678901 17

Forwarding Equivalence Class (FEC) • Introduced in MPLS standards to denote packet forwarding classes • Comprises traffic – to a particular destination – to destination with distinct service requirements • Why FEC? – To precisely specify which IP packets are mapped to each LSP – Done by providing a FEC specification for each LSP 18

LSP - FEC Mapping • FEC specified as a set of two elements (currently) 1. IP Address Prefix - any length from 0 - 32 2. Host Address - 32 bit IP address • A given packet matches a particular LSP if and only if IP Address Prefix FEC element matches packet’s IP destination address 19

Rules for Mapping packet to a LSP • If exactly one LSP’s Host Address FEC element ~ packet’s IP destination address, packet is mapped to that LSP • If there are multiple LSPs satisfying the above condition, then the packet is mapped to one of those LSPs† • If a packet matches exactly one LSP, packet is mapped to that LSP • If packet matches multiple LSPs, mapped to one with the longest prefix match † Which LSP to be chosen - outside the scope of this presentation 20

Label Spaces • Useful for assignment and distribution of labels • Two types of label spaces – Per interface label space: Interface-specific labels used for interfaces that use interface resources for labels – Per platform label space: Platform-wide incoming labels used for interfaces that can share the same label space 21

LDP Identifiers • A six octet quantity • used to identify specific label space within an LSR • First four octets encode LSR’s IP address • Last two octets identify specific label space • Representation <IP address> : <label space id> – e. g. , 171. 32. 27. 28: 0, 192. 0. 3. 5: 2 • Last two octets for platform-wide label spaces are always both zero 22

LDP Discovery • A mechanism that enables an LSR to discover potential LDP peers • Avoids unnecessary explicit configuration of LSR label switching peers • Two variants of the discovery mechanism – basic discovery mechanism: used to discover LSR neighbors that are directly connected at the link level – extended discovery mechanism: used to locate LSRs that are not directly connected at the link level 23

LDP Discovery contd. . • Basic discovery mechanism – To engage - send LDP Hellos periodically – LDP Hellos sent as UDP packets for all routers on that subnet • Extended discovery mechanism – To engage - send LDP targeted Hellos periodically – Targeted Hellos are sent to a specific address – Targeted LSR decides whether to respond or to ignore th targeted Hello • LDP Link Hello sent by an LSR – carries the LDP identifier for the label space the LSR 24 intends to use for the interface

Session establishment • Exchange of LDP discovery Hellos triggers session establishment • Two step process – Transport connection establishment • If LSR 1 does not already have a LDP session for the exchange of label spaces LSR 1: a and LSR 2: b, it attempts to open a TCP connection with LSR 2 • LSR 1 determines the transport addresses at it’s end (A 1) and LSR 2’s end (A 2) of the TCP connection • If A 1>A 2, LSR 1 plays the active role; otherwise it is passive – Session initialization • Negotiate session parameters by exchanging LDP initialization 25 messages

Session Initialization State Transition Diagram Rx Any other msg or Timeout Tx NAK msg NON EXISTENT Session connection Rx Any LDP msg except established Init msg or Timeout (Passive Role) Rx Acceptable Init msg/ Tx Init msg & Keep. Alive msg INITIALIZED OPENSENT OPENREC Rx Keep. Alive msg (Active Role) Tx Init msg Rx Acceptable Init msg Tx Keep. Alive msg OPERATIONAL All other LDP msgs Rx - Receive Tx - Transmit Rx Any other msg or Timeout Tx NAK msg Rx Shutdown msg or Timeout Tx Shutdown msg

Session Initialization State Transition Table 27

Session Initialization State Transition Table (cont. )

Label Distribution and Management • Two label distribution techniques – Downstream on demand label distribution: An LSR can distribute a FEC label binding in response to an explicit request – Downstream Unsolicited label distribution: Allows an LSR to distribute label bindings to LSRs that have not explicitly requested them • Both can be used in the same network at the same time; however, each LSR must be aware of the 29 distribution method used by its peer

Label Distribution Control Mode • Independent Label Distribution Control – Each LSR may advertise label mappings to its neighbors at any time – In independent Downstream on Demand mode LSR answers without waiting for a label mapping from next hop – In independent Downstream Unsolicited mode LSR advertises label mapping for a FEC whenever it is prepared – Consequence: upstream label can be advertised before a downstream label is received 30

Label Distribution Control Mode contd. . • Ordered Label Distribution Control – Initiates transmission of label mapping for a FEC only if it has next FEC next hop or is the egress – If not, the LSR waits till it gets a label from downstream LSR – LSR acts as an egress for a particular FEC, if • next hop router for FEC is outside of label switching network • FEC elements are reachable by crossing a domain boundary 31

Label Retention Mode • Conservative Label Retention Mode – Advertised label mappings are retained only if they are used forwarding packets – Downstream on Demand Mode typically used with Conservative Label Retention Mode – Advantage: only labels required are maintained – Disadvantage: a change in routing causes delay • Liberal Retention Mode – All label mappings are retained regardless of whether LSR is next hop or not – reaction to routing changes will be quick 32

Label Information Base • LSR maintains learned labels in Label Information Base (LIB) • Each entry of LIB associates an FEC with an (LDP Identifier, label) pair • When next hop changes for a FEC, LSR will retrieve the label for the new next hop from the LIB 33

Hierarchical Operation in MPLS Example: • External Routers A, B, C, D, E, F - Talk BGP • Internal Routers 1, 2, 3, 4, 5, 6 - Talk OSPF C Domain #2 1 A B Domain #1 2 3 4 5 D 6 F E Domain #3 Note: Internal routers in domains 1 and 3 not shown 34

Hierarchical Operation contd. . • When IP packet traverses domain #2, it will contain two labels, encoded as a “label stack” • Higher level label used between routers C and D, which is encapsulated inside a lower level label used within Domain #2 • Operation at C – C needs to swap BGP label to put label that D expects – C also needs to add an OSPF label that 1 expects – C therefore pushes down the BGP label and adds a 35 lower level label

Label Stack • Multiple labels are carried in data packets – e. g. data packet carried across Domain #2 • Concept of stacking – provides a mechanism to segregate streams within a switched path – one useful application of this technique is in Virtual Private Networks • Advantage of Hierarchical MPLS is that the internal routers need not know about higher level (BGP) routing 36

Multipath • Many IP routing protocols support the notion of equal-cost multipath routes • Few possible approaches for handling multipath within MPLS • First approach: – separate switched path from each ingress node to the merge point – preserves switching performance, but at the cost of proliferating the number of switched paths 37

Multipath contd. . • Second approach – Only one switched path from one ingress node to a destination – Conserves switched paths but cannot balance loads across downstream links as well as other approaches – LSP may be different from the normal L 3 path • Third approach: – Allows single stream to be split into multiple streams, by using L 3 forwarding – e. g. might use a hash function on source and destination IP addresses – Conserves paths at the cost of switching performance

Explicit Routing in MPLS • Two options for route selection: – Hop by hop routing – Explicit routing • Explicit Routing (aka Source Routing) is a very powerful technique – With pure datagram routing overhead of carrying complete explicit route is prohibitive – MPLS allows explicit route to be carried only at the time the LSP is setup, and not with each packet 39 – MPLS makes explicit routing practical

Explicit Routing in MPLS contd. . • In an explicitly routed LSP – the LSP next hop is not chosen by the local node – selected by a single node, usually the ingress • The sequence of LSRs may be chosen by – configuration (e. g. , by an operator or by a centralized server) – an algorithm (e. g. , the ingress node may make use of topological information learned from a 40 link state routing protocol)

Loops and Loop Handling • Routing protocols used in conjunction with MPLS are based on distributed computation which may contain loops • Loops handling - 3 categories – Loop Survival – Loop Detection – Loop Prevention 41

Loop Survival • Minimizes the impact of loops by limiting the amount of resources consumed by the loop • Method – based on use of TTL field which is decrement at each hop – Use of dynamic routing protocol converging rapidly to non-looping paths – Use of fair queuing 42

Loop Detection • Loops may be setup but they are subsequently detected • The detected loop is then broken by dropping label relationship • Broken loops now necessitates packets to be forwarded using L 3 forwarding 43

Loop Detection (cont. ) • Method is based on transmitting a Loop Detection Control Packet (LDCP) whenever a route changes • LDCP is forwarded towards the destination until – last MPLS node along the path is reached – TTL of the LDCP expires – it returns to the node which originated it 44

Loop Prevention • Ensures that loops are never set up • labels are not used until it is sure to be loop free • Methods – labels are propagated starting at the egress switch – use source routing to set up label bindings from the egress switch to each ingress switch 45

Leaf Detects loop immediately LSR Link removed from tree Change in Link Ingress Node Egress Node 46

Traffic Engineering and Performance Objectives • Traffic Engineering (TE) is concerned with performance optimization of operational networks • The key performance objectives – traffic oriented - aspects that enhance the Qo. S of traffic streams e. g minimization of packet loss – resource oriented - aspects that pertain to the optimization of resource utilization e. g efficient management of bandwidth 47

Performance Objectives (cont. ) • Minimizing congestion is a major traffic and resource oriented performance objective • Congestion manifest under two scenarios – network resources are insufficient or inadequate • can be solved by capacity expansion or classical congestion control techniques – traffic streams are inefficiently mapped onto available resources • can be reduced by adopting load balancing policies 48

Traffic and Resource Control • The traffic engineer acts as the controller in an adaptive feedback control system which includes – a set of interconnected network elements – a network performance monitoring system & – network configuration management tools • The traffic engineer formulates control policies, observes the state of the network, characterizes the traffic and applies the control actions in accordance to the control policy 49

MPLS and Traffic Engineering • Main components used – Traffic Trunk - aggregation of traffic flows of the same class which are placed inside a Label Switched Path – Induced MPLS Graph • analogous to a virtual topology in an overlay model • logically mapped onto the physical network through the selections o LSPs for traffic trunk • comprises a set of LSRs which act as nodes of the graph and a set of LSPs which provide logical point to point connectivity between LSRs and thus act as edges of the graph 50

Augmented Capabilities • Set of attributes associated with traffic trunks which collectively specify their behavioral characteristics • Set of attributes associated with resources which constrain the placement of traffic trunks through them • A “constraint based routing” framework which is used to select paths for traffic trunks subject to constraints imposed 51

Basic operation on traffic trunks • • • Establish - create an instance of a traffic trunk Activate - cause to start passing traffic Deactivate - stop passing traffic Modify Attributes Reroute - administratively or by underlying protocols • Destroy - reclaim all resources such as label space and bandwidth 52

Basic attributes of traffic trunk • Traffic parameter attribute - capture the characteristics of the traffic streams • Generic Path selection and maintenance attributes - defines rules for selecting route taken by traffic trunk and rules of maintaining the paths • Priority attribute • Preemption attribute • Resilience attribute • Policing attribute 53

Resource Attributes • Part of the topology state parameters used to constrain the routing of traffic trunks through specific resources • Main components – Maximum Allocation Multiplier (MAM) administratively configured to determine the proportion of resource available for allocation – Resource Class Attribute - administratively assigned parameters which express some notion of “Class” for resources 54

Constraint Based Routing • Enables a demand driven, resource reservation aware, routing paradigm to co-exist with current topology driven protocols • uses the following inputs – traffic trunk attributes – resource attributes – other topology state information • Basic features – prune the resources that do not meet the requirements of the traffic trunk attribute 55 – run a shortest path algorithm on the residual graph

Constraint Based Routing (cont. ) • Strict & Loose Explicit Routes – Constraint Based LSP (CRLSP) is calculated at one point at the edge of the network based on certain criteria – special char. such as assigning certain bandwidth can be supported – The route is encoded as a series of Explicit routed hops contained in a CR based route TLV 56

Constraint Based Routing (cont. ) • Traffic Characteristics – Described in the Traffic Parameter TLV in terms of peak rate, committed rate and service granularity • Preemption – Setup and Holding priorities are used to rank new and existing paths respectively to determine if new paths can preempt existing paths – Allocation of these priorities is a network policy 57

Constraint Based Routing (cont. ) • Route Pinning – applicable to segments of an LSP that are loosely routed i. e the next hop is an abstract node – used if the LSP need not be changed • Resource Class – While setup , indication must be given as to which class the CRLSP can draw resources from 58

Implementation Consideration Management Interface MPLS Constraint Based Routing Process Resource Attribute Availability Database Conventional IGP Process Link State Database 59

Quality of Service using CRLSP • Delay Sensitive Service – the network commits to deliver with high probability, user datagrams at a rate of PDR with minimum delay and delay requirements – Datagrams in excess of PDR will be discarded • Throughput Sensitive Service – the network commits to deliver at a rate of at least CDR – Datagrams with higher CDR have lower probability of being delivered • Best Effort Service – No expected service is guaranteed 60

Tag Switching 61

Destination Based Routing • A TSR participates in unicast routing protocols to construct its mapping between FECs and next hops • This mapping is used by the Tag Switching Control component for constructing the TFIB which is used for actual packet forwarding 62

Destination Based forwarding model of Tag Switching A B if 0 if 1 C if 2 if 1 if 2 if 0 if 1 if 2 if 1 D E if 0 192. 16/16 if 0 TSR 63

Information for constructing TFIB • A local binding between the FEC and a tag – takes a tag from the pool of free tags and uses it as an index in the TFIB to set the incoming tag entry • A mapping between the FEC and the next hop for that FEC (provided by the routing protocol(s) running on the TSR) • A remote binding between the FEC and a tag that is received from the next hop 64

Initial TFIB Entries 65

TFIB Entries after Tag Distribution 66

Behavior during routing change A B if 0 if 1 if 2 E if 0 Link Down if 0 C if 2 if 1 D if 0 TSR 67

Updated TFIB 68

Hierarchy of Routing Knowledge • All TSRs within a routing domain participate in a common intra-domain routing protocol and construct TFIB corresponding to destinations within the domain • All border TSRs or TERs within a domain and directly connected TERs from other domains also exchange Tag binding information via inter-domain routing protocol 69

Hierarchy of Routing Knowledge (cont. ) • To support forwarding in the presence of hierarchy of routing knowledge, Tag switching allows a packet to carry several tags organized as a tag stack • At the ingress a tag is pushed onto the tag stack, and at the egress a tag is popped off a the stack 70

Hierarchy of Routing knowledge model Routing domain B V Routing domain C Routing domain A T X Y W Z TSR 71

TFIB Entries in Routing Domain A 72

Label Stack During Hierarchical Routing TSR Z distributes label 2 to TSR W and TSR W gives label 5 to TSR T for the purpose of inter-domain routing Top of Stack 10 Top of Stack 2 2 Stack after processing in TSR T Stack after processing in TSR W 73

Multicast in Tag Switching • Selects the distribution tree based only on – tag carried in a packet – interface on which the packet arrives • TSR maintains its TFIB on a per interface basis • TSRs connected to a common sub-network agree among themselves on a common tag associated with a particular multicast tree 74

Multicast in Tag Switching (cont. ) • Procedures are used to partition the set of tags for use with multicast into disjoint subsets and care is taken to avoid overlapping with the help of HELLO packets • TSR connected to a common sub-network and those which are a part of the same distribution tree elect one TSR that will create the tag bindings and distribute them and any TSR can join the group using the JOIN command 75

Multicast model in Tag Switching A B TSR if 0 D if 1 if 2 if 0 E if 0 F 76

RSVP with Tag Switching • RSVP is supported by the help of a RSVP object - the tag Object • The tag object binding information for an RSVP flow is carried in the RSVP “RESV” message • The RESV message carries the tag object containing the tag given by a TSR and also information about the local resources to be used • The reservation state is refreshed once the 77 flow is set up using the RESV message

Explicit Routes • Tag switching supports explicit routes with the help of a RSVP object - the Explicit Route Object • The object is carried in the RSVP “PATH” message • The tag information is carried in the Tag Object by the RSVP “RESV” 78

IP Switching • Introduced by Ipsilon • Already been tested in the field • Significant Innovation: Defined a switch management protocol (GSMP) along with label binding protocol called Ipsilon Flow Management Protocol (IFMP) • General Switch Management Protocol (GSMP) - allows an ATM switch to be controlled by an “IP switch controller” 79

IP Switching Overview • IP over ATM models are complex and inefficient - involve running two control planes – ATM Forum signaling and routing – IP routing and address resolution on top • In contrast IP Switching uses – IP component plus label binding protocol – completely removes ATM control plane • Goal: To integrate ATM switches and IP routing in a simple and efficient way 80

Removing ATM Control Plane ATM IP MARS NHRP ARP PNNI IP Q. 2931 ATM hardware IFMP ATM hardware (a) IP over Standard ATM (b) IP Switching (b) 81

IP Switching Architecture • Switch controller – control processor of the system – uses GSMP to communicate with ATM switch itself – runs IP routing and forwarding code • Default VC – defined to get control traffic before IP Switching is performed – uses well known VCI/VPI value – also used for data that doesn’t yet have a label

IP Switch Architecture Switch controller Default VC Data VC IFMP To upstream switch GSMP Flow Classification and control Routing and forwarding GSMP To downstream switch Default VC Switch Data VC 83

IP Switching Basics • IP Switching relies on IP protocols – to establish routing information – to determine next hop • Flow classification and control module selects flows from incoming traffic • IP flow refers to a sequence of datagrams – from one source to one destination, identified by the ordered pair <source address, destination address> – can also refer to a flow at finer granularity, e. g. , different applications between same pair of machines, identified by < source address, source port, destination address, destination port>

Flow Redirection • Redirection: Process of binding labels to flows and establishing label switched paths • Example: – data is flowing from A via B to C on default VC – B sends a redirect to A specifying flow y and the label (VPI/VCI) on which it expects to receive – If C issues a redirect to B for flow y, B forwards y on the VPI/VCI specified by C – Since same flow y enters B on one VC and leaves on another, B uses GSMP to inform its switching element to set up the appropriate switching path

Flow Redirection Redirect: Flow y VPI/VCI 3/57 A B C Default VC Switch Controller Switch Element 3/57 Switch B issues a REDIRECT message to switch A Redirect: Flow y VPI/VCI 3/57 A Redirect: Flow y VPI/VCI 2/22 B Default VC C Default VC Switch Controller Switch Element 3/57 2/22 Switch B and C redirect the same flow, allowing it to be switched at B

Ipsilon Flow Management Protocol (IFMP) • Designed to communicate flow to label binding information • IFMP is a soft state protocol • IFMP’s Adjacency Protocol: – Used to communicate and discover information about neighbors – Adjacency message sent as limited broadcast • IFMP’s Redirection Protocol – used to send appropriate messages for flowlabel bindings 87

IFMP’s Redirection Protocol • Different message types defined: – REDIRECT: used to bind label to a flow – RECLAIM: enables label to be unbound for subsequent re-use – RECLAIM ACK: Acknowledgement for RECLAIM message – ERROR: Used to deal with various error conditions • Common header format 88

IFMP Redirect Protocol Message Format IFMP REDIRECT message body

Encapsulation of Redirected Flows Encapsulation of IP packet on the default VC Encapsulation of IP packet on the redirected VCs 90

General Switch Management Protocol (GSMP) • GSMP is a master/slave protocol – ATM switch is the slave – Master could be any general purpose computer • The protocol allows the master to – Establish and release VC connections across the switch – Perform port management (Up, Down, Reset, Loopback) – Request Data (configuration information, statistics) – Allows slave to inform master if something interesting, 91 such as link failure, happens on the switch

GSMP contd. . • GSMP packets are LLC/SNAP encapsulated and sent over ATM link using AAL 5 • GSMP Adjacency Protocol – used to gain information about the system at the other end of the link and – to monitor link status • GSMP Connection Management Protocol – used to ensure consistency between the GSMP master and slave – also specifies the Qo. S using a priority field 92

Implementations & Contributions • IP Switching products – available since 1996 – Ipsilon product family uses Intel Pentium-based PC as the switch controller – Also offers a number of ATM switches that are controlled by the switch controller • IP Switching made the following significant contributions to label switching effort: – first to deliver real products and caused activity that resulted in the development of Tag Switching and ultimately the formation of MPLS working group 93 – contributed GSMP