Multiprotocol Label Switching The future of IP Backbone Technology

1
Multiprotocol Label SwitchingThe future of IP
Backbone Technology

• Ravikumar Pragada
• Girish Srinivasan

2
Overview

• Need for MPLS
• 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

3
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

4
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

5
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

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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,
• ltR1,R2..Rngtfor all i, 1lt i lt n Ri
  transmits P to Ri1 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
• forward the packet to the next hop

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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 QoS,
  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

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Basic Model for MPLS Network
MPLS
LSR Label Switched Router LER Label Edge
Router
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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

10
MPLS Benefits contd..

• Improves the possibilities for traffic
  engineering
• Supports the delivery of services with QoS
  guarantees
• Avoids need for coordination of IP and ATM
  address allocation and routing information

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Necessity of L3 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 dont do MPLS
• For Scaling
• Forward on a finer granularity than the labels
  can provide

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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

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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

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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

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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

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LDP Message Format
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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

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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

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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
  packets IP destination address

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Rules for Mapping packet to a LSP

• If exactly one LSPs Host Address FEC element
  packets 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

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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

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LDP Identifiers

• A six octet quantity
• used to identify specific label space within an
  LSR
• First four octets encode LSRs IP address
• Last two octets identify specific label space
• Representation ltIP addressgt ltlabel space idgt
• e.g., 171.32.27.280, 192.0.3.52
• Last two octets for platform-wide label spaces
  are always both zero

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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

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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 the targeted Hello
• LDP Link Hello sent by an LSR
• carries the LDP identifier for the label space
  the LSR intends to use for the interface

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Session establishment

• Exchange of LDP discovery Hellos triggers session
  establishment
• Two step process
• Transport connection establishment
• If LSR1 does not already have a LDP session for
  the exchange of label spaces LSR1a and LSR2b,
  it attempts to open a TCP connection with LSR2
• LSR1 determines the transport addresses at its
  end (A1) and LSR2s end (A2) of the TCP
  connection
• If A1gtA2, LSR1 plays the active role otherwise
  it is passive
• Session initialization
• Negotiate session parameters by exchanging LDP
  initialization messages

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Session Initialization State Transition Diagram
Rx - Receive Tx - Transmit
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Session Initialization State Transition Table
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Session Initialization State Transition Table
(cont.)
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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
  distribution method used by its peer

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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

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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

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Label Retention Mode

• Conservative Label Retention Mode
• Advertised label mappings are retained only if
  they are used for 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

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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

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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

Domain 2
C
D
1
Domain 3
Domain 1
6
2
3
4
5
B
F
E
A
Note Internal routers in domains 1 and 3 not
shown
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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
  lower level label

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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

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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

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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 L3 path
• Third approach
• Allows single stream to be split into multiple
  streams, by using L3 forwarding
• e.g. might use a hash function on source and
  destination IP addresses
• Conserves paths at the cost of switching
  performance

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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
• MPLS makes explicit routing practical

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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 link
  state routing protocol)

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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

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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

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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 L3 forwarding

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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

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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

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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 QoS
  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

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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

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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

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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

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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

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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

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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

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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

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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
• run a shortest path algorithm on the residual
  graph

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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

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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

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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

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

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Tag Switching
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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

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Destination Based forwarding model of Tag
Switching
192.16/16
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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

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Initial TFIB Entries
66
TFIB Entries after Tag Distribution
67
Behavior during routing change
Link Down
68
Updated TFIB
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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

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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

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Hierarchy of Routing knowledge model
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TFIB Entries in Routing Domain A
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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
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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

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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

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Multicast model in Tag Switching
77
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 flow
  is set up using the RESV message

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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

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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

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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

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Removing ATM Control Plane
IP IFMP
ATM hardware
(a)
(b)

• (a) IP over Standard ATM
• (b) IP Switching

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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 doesnt yet have a label

83
IP Switch Architecture
84
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 ltsource address, destination
  addressgt
• can also refer to a flow at finer granularity,
  e.g., different applications between same pair of
  machines, identified by lt source address, source
  port, destination address, destination portgt

85
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

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Flow Redirection
87
Ipsilon Flow Management Protocol (IFMP)

• Designed to communicate flow to label binding
  information
• IFMP is a soft state protocol
• IFMPs Adjacency Protocol
• Used to communicate and discover information
  about neighbors
• Adjacency message sent as limited broadcast
• IFMPs Redirection Protocol
• used to send appropriate messages for flow-label
  bindings

88
IFMPs 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

89
IFMP Redirect Protocol Message Format
IFMP REDIRECT message body
90
Encapsulation of Redirected Flows
91
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, such as link failure, happens on the
  switch

92
GSMP contd..

• GSMP packets are LLC/SNAP encapsulated and sent
  over ATM link using AAL5
• 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 QoS using a priority field

93
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
• contributed GSMP