Documents
Resources
Learning Center
Upload
Plans & pricing Sign in
Sign Out

Label

VIEWS: 0 PAGES: 40

									Multiprotocol Label Switching




                 Engineering Internet QoS   1
Overview


L3 Switching vs Routing
What is MPLS?
MPLS on ATM: Issues
Traffic Engineering Using MPLS
MPLS Simulation Results




                  Engineering Internet QoS   2
Introduction

MPLS stands for “Multiprotocol” label switching
  • techniques applicable to any L3 protocol
A router supporting MPLS is know as Label
 Switched Router (LSR)
IP routing/control software is combined with
 label swapping
  • similar to ATM VCI/VPI swapping


                  Engineering Internet QoS         3
Proprietary Protocols

 The Cell Switching Router (CSR)
   • ftp://ftp.wide.toshiba.co.jp/pub/csr/
 Ipsilon’s IP Switching
   • www.ipsilon.com
 Cisco’s Tag Switching
   • www.cisco.com
 IBM’s Aggregate Route-Based IP Switching (ARIS)
   • www.networking.ibm.com/isr/ip/
 IETF MPLS working group
   • www.ietf.org/html.charters/mpls-charter.html




                         Engineering Internet QoS   4
Why label switching

Scalablity of L3 routing and simplicity of L2
 forwarding
  • high performance
  • use of ATM hardware
  • faster than IP lookup
  • integration of IP and ATM
new Services such as VPNs
hierarchy of Routing

                   Engineering Internet QoS      5
Terminologies

  flow :   a single instance of an application to application flow of
   data
  forwarding equivalence class:           a group of IP packets
   which are forwarded along same path (or same treatment)
  label:   a short fixed length physically contiguous identifier
   which is used to identify a FEC (local significance)
  label Switched Router:              an MPLS node which is capable
   of forwarding L3 packets




                          Engineering Internet QoS                       6
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com




MPLS Domain




     LSR: label switching router
      LER: label edge router
                                                  Engineering Internet QoS                                                         7
Conventional IP Routing

packet travels from one router to the next
  • an independent forwarding decision made at each
    hop
At each hop packets assigned to a Forwarding
 Equivalence Class (FEC)
  • packets considered to be in same FEC if the routing
    table contains some prefix X such that X is the
    longest match for each packet’s destination address



                    Engineering Internet QoS              8
MPLS Approach


 assignments of packets to FEC done once
   • as the packet enters the network
 FEC to which packet is assigned is encoded
  with a label
 packet is forwarded to next hop with label
   • no further analysis of packet header at
     next hop

                 Engineering Internet QoS      9
Label encoding

 MPLS encapsulation should contain
  • the label stack field
  • time-to-live (TTL) field
  • a 3 bit Experimental field (earlier COS)
 MPLS uses 32 bits for label encoding
                 label                  exp s             ttl
     Label: Label Value, 20 bits (0-16 reserved)
     Exp.:     Experimental, 3 bits (earlier Class of Service)
     S:        Bottom of Stack, 1 bit (1 = last entry in label stack)
     TTL:      8 bit Time to Live



                              Engineering Internet QoS                  10
Reserved Labels

 Multiprotocol = IP, IPX, etc.
  Protocol is inferred from the last label in the stack.
 0 = IPv4 Explicit Null Label
   Pop this last label. Use IPv4 header.
 1 = Router Alert Label
    Looked by the router software.
  Packet forwarded based on the next label in stack
  Similar to “Router Alert Option” in IP Packets
 2 = IPv5 Explicit Null Label
 3 = Implicit Null Label
  Used only for assignment and distribution
  Should not appear in any label stack in the packets

                       Engineering Internet QoS            11
TTL Handling

At the beginning TTL = TTL from IP header
At every hop: TTL = TTL -1
Drop the packet if TTL=0
At the exit: TTL in IP Header = TTL from label
In some cases, entire MPLS domain may be
 considered one hop, e.g., ATM




                  Engineering Internet QoS        12
MPLS Encapsulation

  For ATM VPI/VCI field and for Frame Relay DLCI
   field used
  a “shim” layer between link layer and network layer
   headers for non-ATM/Frame Relay network
    • protocol independent
    • shim layer may consist of sequence of label stack
       entries
    • If PPP used, protocol field may identify frames
       that carry labels


                     Engineering Internet QoS             13
Label Processing

  label used as index into a table which specifies next
   hop and a new label
    • longest match calculation eliminated at
      subsequent hops
  old label replaced with new label and packet
   forwarded to next hop
  label may also contain class of service (COS)
    • for scheduling/ discarding packets etc
    • again saves header processing


                      Engineering Internet QoS             14
Label Distribution

 Who assigns labels for communication
  between A and B?
   • A, B, or someone else?
   • Downstream, upstream, ...
 Where is the control for the entire path?
  A, B, ingress or egress LSR?
 Separate protocol or existing route distribution
  mechanisms?
   • IETF’s Label Distribution Protocol (LDP)
       based on Cisco’s TDP and IBM’s ARIS protocol


                         Engineering Internet QoS      15
 Sample Network Topology




Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com

                                                     Engineering Internet QoS                                                       16
  Routing Tables

                                      Table at R2
                                                                                                Table at R4

                          17      162.25.8               1                                   162.25.8              1
        Index               18 192.35.10                 0
        of table                                                                            192.35.10              1
                            19




Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com


                                                      Engineering Internet QoS                                                        17
 Label Binding Example




Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com

                                                     Engineering Internet QoS                                                       18
 Label allocation

                    Table at R4                                                       Table at R2
          In label out label     Addr prefix     Interface                    In label out label     Addr prefix     Interface


              ?        17 162.25.8                   1                          17        19 162.25.8                   1
              ?        18 192.35.10                  1                          18        16 192.35.10                  0




Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com

                                                     Engineering Internet QoS                                                       19
 Label Switching




Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com

                                                     Engineering Internet QoS                                                       20
 Hierarchy Routing




                                                                BGP: Border Gateway Protocol (inter-domain)
                                                                IGP: Interior Gateway Protocol (intra-domain)

               IGP Routers (R3, R4) in transit domain A maintain all the routes provided
               by Interdomain routing
                         - necessary to forward transit traffic to/from domains B & C
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com

                                                     Engineering Internet QoS                                                       21
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com




  Hierarchy Routing




                     BGP entries                                                  IGP entries
                       in            out           next                                    in            out            next
                       label         label         hop                                     label         label          hop

             R1          ….          21             ….                          R2              ?          52           R3

             R2            21         31          R5                             R3           52           62           R4

             R5           31          41          R6                             R4           62           72           R5

             R6           41         ….             ….                           R5           72            ?           R5

                                                       Engineering Internet QoS                                                       22
Stack of labels

  Packets carry several labels organised as a label
   stack
  packet forwarded from one domain to other contains
   one label
  packet forwarded through a transit domain contains
   two labels
  LSRs use label from top of stack
    • labels pushed (ingress) and poped (egress) at
      domain boundaries



                   Engineering Internet QoS             23
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com




 push/pop on stack
                               swap label 21 to 31                                     next hop R5 (self)
                               R5 not directly connected                               pop off the top label of the stack
                               Find direct conn. and label                             swap this label 31 to 41 (BGP)
                               push label52 and send to R3




                    BGP entries                                                 IGP entries
                     in            out            next                                    in            out            next
                     label         label          hop                                     label         label          hop

            R1         ….           21             ….
                                                             push              R2             ?           52           R3

            R2           21         31           R5                            R3            52           62           R4

            R5           31         41          R6                             R4            62           72           R5          pop
            R6           41        ….             ….                           R5            72            ?           R5

                                                      Engineering Internet QoS                                                      24
MPLS over ATM


  ATM switches performing label switching are called
   ATM-LSRs
  labels need to be encoded in VCI/VPI fields
  MPLS forwarding similar to “label swapping” in ATM
    • same ATM user plane may be used
    • control plane to be changed (MPLS)
        protocols BGP, OSPF .. replaces UNI, PNNI




                      Engineering Internet QoS          25
 MPLS over ATM

Packets have a shim with Label=0.
 Actual label is encoded in VPI/VCI.
No TTL decrement
n-to-1 merge or n-to-n multipoint not supported
VCI = 0 through 32 should not be used in labels
If LSRs are connected via SVCs, VCI/VPIs cannot be
 used as labels. LSRs exchange VCIDs.



                   Engineering Internet QoS     26
Encoding labels in ATM header

   SVC Encoding
     • VPI/VCI field to encode the label from top of stack
     • ATM signaling can be used
   SVP Encoding
     • VPI encodes top label
     • VCI encodes 2nd label
   SVP Multipoint Encoding



                     Engineering Internet QoS            27
Cell Interleave Problem




Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com

                                                  Engineering Internet QoS                                                        28
Request Separate labels
                get a label for 162.25




                                                                     request separate labels for
                    R1                                               R1 and R2 from R3                                  162.25
                                                       if2

                                               if0                   ATM                                           R3
 get a label for 162.25
                                                                                            if1


                   R2
                                    in        in             out            addr
                                                                                             interface
                                    interface label          label          prefix


                                         0      21           51            162.25                  1

                                         2      21           41           162.25                   1
                                                                                                         Predefined VPI/VCI
                                                                                                         used for Label binding
                                                      Engineering Internet QoS                                             29
VC - Merge




                                                                                    Delay cells from one frame
                                                                                    (AAL5 EOF marker)




Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com

                                                   Engineering Internet QoS                                                        30
MPLS on ATM: Issues

VCI field is sufficient for one level tagging
 VPI may be used for the 2nd level
LSR switches need to participate in network
 layer routing protocols (OSPF, BGP)
Multiple tags per destination may be used to
 avoid frame merging
VPI/VCI space may be segmented for label
 switching and normal ATM switching


                  Engineering Internet QoS       31
Traffic Engineering Using MPLS

Traffic Engineering
  • Performance Optimization
  • Efficient resource allocation
  • Constrained routing / Load balancing
  • Maximum throughput, Min delay, min loss
     Quality of service
  • Meet policy requirements of operators



               Engineering Internet QoS       32
 Shortest-Path Routing

                                            MPLS Backbone
                        R1                                                           R5
     A
                                           R2


                                                             R4                      R6
                        R3
                                                                                                                          C
                B
                                                                            R2->R5->R6 link 45Mbps
                                                                            R2->R4->R6 link 15Mbps
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com

                                                     Engineering Internet QoS                                                       33
Constrained Routed LSP

Hard to do non-shortest path routing with
 connectionless forwarding
Constrained Routed label-switched paths allows
 selective non-shortest path routing
  • load balancing across path possible
  • Signaling protocols for such path
    establishment being developed by IETF
      M-RSVP is based on Resource Reservation
       Protocol RSVP (soft-state model)
      CR-LDP defines hard-state signaling protocols

                    Engineering Internet QoS           34
 CR-LSP Example

                                                MPLS Backbone
                      R1                                                           R5
     A
                                           R2

                                                                                   R6
                                                             R4
                        R3
                                                                                                                          C
                B



Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com

                                                     Engineering Internet QoS                                                       35
Traffic Trunk

In MPLS networks: “Traffic Trunks” = SVCs
A set of traffic parameters can be specified to
 determine the “Forwarding Equivalence Class
 (FEC)” or set of packets assigned to that trunk
Features of trunks
  • Multiple trunks can be used in parallel to the same
    egress.
  • Traffic trunks are routable entities like VCs
  • Each traffic trunk can have a set of associated
    characteristics, e.g., priority, preemption, policing,
    overbooking
                      Engineering Internet QoS               36
Traffic Trunks Features

Traffic Trunk Features
  • Trunk paths are setup based on policies or specified
    resource availability.
  • A traffic trunk can have alternate sets of paths in
    case of failure of the main path. Trunks can be
    rerouted.
  • Some trunks may preempt other trunks. A trunk can
    be preemptor, non-preemptor, preemptable, or non-
    preemptable.
  • Each trunk can have its own overbooking rate

                    Engineering Internet QoS           37
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com




Flows, Trunks, LSPs, and Links

 Label Switched Path (LSP):
  All packets with the same label
 Trunk: Same Label+Exp
 Flow: Same MPLS+IP+TCP headers
 DL Label Exp S TTL                                           IP         TCP




                                               Engineering Internet QoS                                                        38
MPLS Simulation Results

Total network throughput improves significantly
 with proper traffic engineering
Congestion-unresponsive flows affect
 congestion- responsive flows
  • Separate trunks for different types of flows
Trunks should be end-to-end
  • Trunk + No Trunk = No Trunk


                  Engineering Internet QoS     39
Summary

Simplified forwarding based on exact match of
 fixed length label
Separation of routing and forwarding in IP
 networks
Facilitates the integration of ATM and IP
Enables the use of explicit routing/source routing
 in IP networks
Improved routing scalability (hierarchy) through
 stacking of labels
Traffic Engineering Benefits
                   Engineering Internet QoS      40

								
To top