T-MPLS Techno logy and App lication on Carrier Ethernet

QU Hua1, Zhao JIhong2,Mao Hongbao2

（1. X i'an Jiaotong University, X i'an 710049, P. R. China;

2. Xi'an University of Posts and Telecommunications, X i'an 710061, P. R. China）

Abstract:As a connec tion-oriented packet transport technology,Transport Multi-Protocol Labe l Sw itching（T-MPLS）is an im portant bearer technology for Carrier Ethernet（CE）.This artic le starts w ith the technolog ical characteristics o f T-MPLS,then goes on to the latest T-MPLS standard ization p rog ress and cha llenges, fo llowed by the descrip tion of T-MPLS based CE architecture. Next, the artic le covers severa l key technologies related with T-MPLS, inc lud ing the data forward ing p lane,control p lane,network survivab ility,and Operation,Adm inistration and Maintenance（OAM）.Fina lly,the artic le analyzes the app lications o f T-MPLS based CE.

B eing a carrier-c lass transport technology,Transport Multi-Protocol LabelSw itching（T-MPLS）［1］elim inates the unnecessary connectionless IP-based p rocessing capabilities ofMPLS,and uses a unified data transportp lane.Essentially,itenab les

connection-oriented packet transport,and can p rovide carrier-c lass services.Itboasts almost the same end-to-end OAM,Quality ofService（QoS）and less than 50m s p rotection sw itching capability as the trad itionaltransport technologies,and has the hierarchical structure sim ilar to Synchronous Digital Hierarchy（SDH）and Op ticalTransport Network（OTN）w ithmatched network managementand intelligentcontrol capabilities.All thementioned features guarantee its w ide app lication.

Carrier Ethernet（CE）［2］does not refer to a specific network technology,buta generalconcep tion;generally speaking,allthe standard ized,scalab le and highly-reliab le networking technologies thatp rovide Ethernetinterface,QoS guarantee and servicemanagement capabilities,and thatbearmultip le services,can be called CE technologies.Defined by the Metro EthernetForum（MEF）,CE involves five aspec ts:standard ized services,scalability,reliability,QoS,and carrier-c lass networkmanagement.So farmost potentialCE technologies are T-MPLS,VirtualPrivate LAN Service（VPLS）and Provider Backbone Transport（PBT）.

1 Technological Characteristics and Standardization Progress of T-MPLS

1.1 Technological Characteristics of T-MPLS

T-MPLS,as defined by ITU-TSG15,is a MPLS-based connection-oriented packet transport technology.Itis deemed as the naturalextension of MPLS from the core network to the Metropolitan Area Network（MAN）and Access Network（AN）;itcan be simp ly described as the follow ing equation:T-MPLS=MPLS-the comp lexity of L3+Operation,Adm inistration and Maintenance（OAM）+Traffic Engineering（TE）.For carrier-c lass app lications,T-MPLShas the following features:

（1）Itd iscards the com p licated controlp rotocolstack defined by IETF for MPLS,sim p lifies the data p lane,and deletes unnecessary forwarding actions（e.g.,Penultimate Hop Popp ing（PHP）,labelcombination ofMPLS,and Equal CostMultipath（ECMP）routing）.

（2）Itboasts perfectOAM mechanism.

（3）Itallows for performance monitoring.

（4）Itensures the end-to-end less than 50m s p rotection.

（5）For the sake of TEand end-to-end transport,itdoes not support the connectionless transport mode.Therefore,allconnections are exp licitly end-to-end,whichmakes T-MPLS a pure transport-oriented,operation-based transportnetwork technology.

（6）Itsupports Differentiated Service（DiffServ）QoS,multicast,and g lobal labelspace,and p rovides bi-directional LabelSwitching Path（LSP）.

（7）The transportp lane and control p lane of T-MPLS are independent,which is d ifferent from MPLS thatdepends on the controlp lane.This feature better ensures expandability and flexibility.

These featuresmake T-MPLS qualified as the CE bearer technology.Besides,T-MPLS supports the Ethernet VirtualConnection（EVC）service,and meets the abovementioned five requirements ofCE.

1.2 T-MPLS Architecture and Networking Model

T-MPLS LSP has two sub layers:the path sub layerand tunnelsub layer,as shown in Figure 1.The c lient layersupported by T-MPLS canmake any Layer2（L2）technologies adop t themapp ing technology defined by IETF for the Pseudow ire Emulation Edge-to-Edge（PWE3）PWmodel.The circuitsub layer corresponds to the VirtualCircuit（VC）LSP connec tion.Moreover,itadap ts the c lient layerservice to the data forward ing domain of T-MPLS,and in the adap tation p rocess,italso adap ts the OAM information used for VC LSP management.The tunnelsub layer corresponds to the tunnelLSP connection,supports agg regation of T-MPLSVC LSP,which can be either the nesting mode or themultip lexing of T-MPLS and VC LSP,and also adap ts the OAM information for tunnel LSP management.

The CEarchitecture is setup w ithin the framework of the hierarchical network.It requires that the operations of the hierarchicalnetwork are independent of the customer layerand control layer so that the user service can be transported in CE over T-MPLS in amore transparent,safe and intactway.G.8110.1［3］（T-MPLSArchitecture）defines the adap tation and p rocessing of messages between the ETH/T-MPLS hierarchicalnetworks.If T-MPLS supports CE,the Ethernetservice can be carried over T-MPLS to form the structure of Ethernetover T-MPLS.In the CE over T-MPLS architec ture,shown in Figure 2,the T-MPLS technology can be em p loyed to support the MEF-defined EVC service,and the EVC service can in turn be used to transfer the Ethernet frame structure.

1.3 T-MPLS Standardization

In the p rogress ofstandard ization,

T-MPLS is the p ioneeras com pared to other CE technologies.

ITU-TSG15 embarked on p reparing G.8110.1 in 2005 and since then,ITU-T has started formulating a series ofG.81XX recommendations related w ith T-MPLS,as shown in Figure 3.The p rimary purpose of ITU-T to start the T-MPLS-related tasks was to define some secondary func tions ofMPLS for the transportnetwork,such as the connection-oriented attributes.However,to fullymeet the requirements of the transportnetwork,itis necessary tomake some expansions for the MPLS functions defined by IETF,forexam p le,OAM and p rotec tion sw itching.T-MPLSOAM was originally designed to be compatib le w ith EthernetOAM,however,IETFpointed out that these expansions were incom patib le with IETFMPLS.

In July 2007,IETFsubm itted an officialreport to ITU-Ton the compatibility issues between MPLSand T-MPLS,noting the possib le network p rob lem s in the eventof T-MPLSand MPLS interconnection.Moreover,it suggested a jointdevelopmentofMPLS expansion,and assured that the MPLS and T-MPLS networks are independent ofeach otherand theremustbe no denom ination confusions.

In Sep tember2007,ITU-TSG15Q12 reached an ag reementw ith IETFon the standards of T-MPLS struc ture.Moreover,the T-MPLSOAM standardization work ofSG13 Q5 w illbe forwarded to SG15 in the study period 2009-2013.In Ap ril2007,the recommendations aboutOAM（G.8113 and G.8114）were notapp roved because IETFpointed out61 p rob lem s to be solved.G.8113 w illbe pub lished as Y.Supp le.4.

In Feb ruary 2008,ITU-Tand IETFset up the JointWorking Team（JWT）for the study of T-MPLS technology.The JWT w illexam ine the app roved oragreed ITU-Tp roposals.In March 2008,the IETFMPLS Interoperability Design Group discussed the requirements of ITU-T OAM and data forward ing.In the JWT workmeeting held in March 2008,IETF MPLSwas defined as Multi-Protocol LabelSw itching TransportProfile（MPLS-TP）to adap t to the expansion of transport requirements,and IETFwould be responsib le for the task;ITU-Twould inc lude MPLS-TP into its new recommendations.Itwas expec ted that the T-MPLS standard serieswould be comp leted after2008.Atp resent,the JWT is focusing on the data forwarding p lane,controlp lane technology,network elementmanagementand interface/equipment,network survivability,and OAM mechanism.

In China,China Communications Standard Association（CCSA）has finished＂MSTPTechnicalRequirements:Nesting MPLS＂.It is now working on the technical reportnamed＂T-MPLS TechnicalRequirements＂.

While supporting CE,T-MPLSalso faces the follow ing challenges:

（1）Resourcemanagement:The logic labelhas to bemapped to the p roper bandw id th;the connec ted bandw id th tends to be dynam ic;Connection Accep tance Control（CAC）is needed;more connections are needed as compared w ith thatofa circuit-sw itched network;and the bandw id th managementis comp lex.

（2）Bi-d irec tionalnon-symmetric LSP:The bi-d irectional LSP is overlapp ing and non-symmetric;signaling has to be expanded;and resource allocation and management have difficulties.

（3）Nesting LSP:The nesting LSP imp roves expandability,but ithas d ifficulties in p rotection and restoration.

（4）Link discovery and fault management:With the func tion of link d iscovery,the link connection between nodes can be d iscovered automatically,hence lessmanualconfiguration is needed and more unexpected configuration errors can be avoided.

（5）Link ManagementProtocol（LMP）is an op tion in T-MPLS link d iscovery and management.The discovery mechanism can also be imp lemented by OAM on the data transportp lane.

（6）T-MPLS survivability:The p rotec tion technology inc ludes 1:1,1+1 and ring network p rotection;the restoration technology inc ludes hard/soft routing,p re-computation/dynam ic restoration.The p rob lem s to be solved in this aspec tinc lude themode of starting the restoration action,segmental p rotection/restoration and end-to-end p rotec tion/restoration,and the survivability based on Grade of Service（GoS）.

（7）Service supported:Point-to-Point（P2P）LSP（E-Line）,Point-to-Multipoint（P2MP）LSP（E-Tree）and multip le P2P LSP（E-LAN）.

（8）Multi-levelinterconnection:Signaling interoperability,flood ing and extraction of TE link,Path Com putation Element（PCE）,multi-levelsurvivability,and peeroroverlay.

2 Data Forwarding Plane Technology of T-MPLS

T-MPLSuses the dual-labelswitching and forward ing mode,thatis,T-MPLS allocates two types of label to the c lient while itp rovides the c lientw ith packet transport.The two labeltypes are the virtualchannel/PW label,and the transportsw itching path/tunnel label.The channel labelconnects the c lients atboth ends together,and is used for the term inalequipment to d ifferentiate c lient data.The tunnel labelis used fordata sw itching and forwarding in the T-MPLS packetpath.

The T-MPLS data forwarding technology is based on the MPLS data forward ing,labeloperation and washer label formatting specified in IETF recommendation RFC3031/3032［4］.Ithas the follow ing features:

（1）Uni-d irec tionaland bi-d irectional T-MPLS LSP:Two uni-d irectionalLSPs on the data p lane transported in the same physicalpath form a bi-d irectional LSP.

（2）Differentiated services:Itsupports expansion-based LSP or label LSP,and enab les data forward ing ofdifferentiated services,based on Pipe Modeland Short Pipe Modelspecified in RFC3270［5］.The Uniform Modelin RFC 3270 is not considered in T-MPLS because itdoes notsuit the L2VPN data forward ing mode.

（3）Itsupports Time-to-Live（TTL）p rocessing and coordinates w ith tunnel LSP,w ith considerations of Pipe-Model and Short-Pipe LSP data forward ing modes specified in RFC3443［6］.

▲Figure 4. Delivering RACF in the architecture of CE over T-MPLS.

P2MPmulticastis an im portant research d irection of T-MPLS.Itinc ludes next-hop label forwarding access,packetcopying,data p lane（tree or point-to-point）,and labelallocation and layering.Since the JWTwas setup not long ago,there are no otherdocuments covering details of T-MPLSmulticast excep tG8110.1 thatmentions the supportof T-MPLS formulticast.The MPLSand Frame Relay Alliance（MFA）conference,held in Chicago in March 2007,believed that the details of uni-d irectionalP2MPmulticastof T-MPLS in G.8110.1 can be im p roved based on the IETFRFC aboutMPLS P2MPmulticast.The latestp roposal G.8110.1 Amendment1［7］about T-MPLS architecture has given the schematic diag ram of T-MPLSuni-d irec tional P2MP connection architecture.

3 Control Plane Technology of T-MPLS

CE's g reateststreng th is its separated and independentcontrol function and transport function.The transportp lane is responsib le for data forwarding and transportonly,while the controlp lane delivers all functions related w ith network resource controland management.

ITU-T,IETFand Op tical Internetworking Forum（OIF）are responsib le for the standard ization of the T-MPLS controlp lane.ITU-T focuses on the requirements,architecture and functions of the p lane.IETFmainly p repares the design and expansion of the controlp rotocols,forexamp le,signaling,routing and LMP p rotocol;IETF also c larifies and add resses specific issues related w ith the controlp lane.OIF puts its effortsmainly on User Network Interface（UNI）and External Network Network Interface（E-NNI）.

T-MPLS dep loys the GeneralMultip le ProtocolSw itching/Automatic Sw itching Op ticalNetwork（GMPLS/ASON）as its unified controlp lane to perform label d istribution and T-MPLS path estab lishment.This is a critical b reakthrough and reform in the history of transportnetwork technology.In the architecture ofCEover T-MPLS,the controlp lane is desc ribed vivid ly as the network Resource and Accep tance ControlFunction（RACF）［8-9］and the Network AttachmentControlFunction（NACF）.Figure 4 shows the RACF scheme in the architecture of CEover T-MPLS.

RACF ismade up ofPolicy Decision FunctionalEntity（PD-FE）and Transport Resource ControlFunctionalEntity（TRC-FE）,responsib le for technology-unrelated and technology-related resource accep tance and control functions respectively.TRC-FE is a functionalentity custom ized especially for the T-MPLS-based transportnetwork.Tomeet the specific requirements ofCE,TRC-FE has to deliversuch functions as real-time monitoring and information collection of transportnetwork resource（topology and link capacity）,CAC,and technology-related QoS requirements and attributes.

4 Survivability Technology of T-MPLS

One of themajor characteristics of T-MPLS is its perfectsurvivability technology.The survivability of the T-MPLS network is very im portant to guarantee its network performances,inc luding the transportp lane-based p rotec tion sw itching and control p lane-based restoration.The p rotec tion sw itching based on the transportp lane inc ludes linear p rotection switching and shared p rotec tion ring.The restoration based on the controlp lanemeans that,aftera faultoccurs and w ith the coord ination of the controlp lane,the working path is com puted again for the service or the p rotection path is p re-com puted.

So faronly the linearp rotec tion sw itching for T-MPLS survivability has been standardized.The T-MPLS shared ring p rotec tion has been d rafted.

（1）Linear Protec tion Sw itching The linearp rotection sw itching for T-MPLS survivability has been standardized in ITU-TG.8131/Y.1382［10］,which specifies both the T-MPLS end-to-end path p rotection structure and T-MPLS subnetconnec tion p rotection structure,as wellas defines the uni-d irec tional/bi-directional sw itching types and the return/non-return operation types.Based on APS coordination,T-MPLS can perform p rotection sw itching on the section layer,path layerand circuit layer.The p rotection sw itching can be initiated by themanagementp lane or initiated by the fault ind ication signalafteran equipment faultis detec ted.

（2）Shared Protection Ring

ITU-TG.8132［11］specified the T-MPLSShared Protection Ring（TM-SPRing）standards,which has two p rotectionmechanisms,w rapp ing and steering,to supportP2Pand P2MP T-MPLS connection.

▲Figure 5. TM-SPRing working principle and signaling procedure.

TM-SPRing is a bi-directional dual-ring topology w ith each section of op ticalpath along the ring working at the same rate.The transportd irection of the outer loop is c lockw ise while thatof the inner loop is anti-c lockw ise.The p rotection requires no ded icated bandw id th backup and two of the loops can be used to transportdata.By broadcasting switching control messages on the ring,allnodes on the ring w illbe notified ofa fault route or node.TM-SPRing p rovides reliab le mechanism of less than 50ms p rotection sw itching forallp rotected services.

Figure 5 dep ic ts the TM-SPRing signaling p rocedure of T-MPLS w rapp ing and steering p rotec tion mechanism s.In Figure 5（a）,the service traffic Q is in normalnetwork status,and the outer loop is taken.Aftera link fault occurs,w rapping p rotection,as shown in Figure 5（b）,is emp loyed to perform p rotection sw itching operation on the upstream Node B of the neighboring faulty link,w ith a new path of A-B-A-D-E-F-C-F,while steering p rotection,as shown in Figure 5（c）,is emp loyed and the service traffic from Node A to Node D is rerouted to the inner loop to get to the destination node,w ith a new path ofA-D-E-F.

（3）Restoration Technology

T-MPLS restoration is fulfilled on the controland managementp lanes.The d istributed controlp lane technology based on GMPLS/ASON is dep loyed to perform LSP restoration.The restoration app lies to any topology struc tures,and it can also coordinate w ith other transport network technology layers,such as SDH,OTN,and WDM.

There are stillmany p rob lems to be solved for T-MPLS survivability,inc lud ing the triggermechanism,multi-ring p rotec tion function,service-oriented restoration,multi-domain restoration technology,and dynam ic path restoration function.

5 T-MPLS OAM

▲Figure 6. OAM mechanism of T-MPLS.

Another im portantcharacteristic of T-MPLS is thatitboasts ofpowerful OAM capabilities.ITU-TY.Sup4［12］elaborately defines the T-MPLSOAM mechanism so that the transportentities atall layers,nomatter if they belong to the user,service p rovideror carrier,are ab le to perform faultdetection,fault isolation,and performancemonitoring,as wellas to learn about the comp leteness and path status ofsending and receiving information on theirown layers.This is whatmakes T-MPLS extraord inary as com pared to other PacketTransport Network（PTN）schemes.

The T-MPLSOAM mechanism com p rises three parts:fault management,performance management,and other functions,as shown in Figure 6,where the MPLSOAM functions ofare given in the smallcirc le while the fans outside the smallcirc le are ded icated to the three parts of T-MPLS OAM func tions.

Figure 6 c learly shows thatT-MPLS OAM expands MPLSOAM.According to the com parison,T-MPLS hasmanymore types ofOAM ProtocolData Unit（PDU）than MPLS,and each PDU perform s one function.The functions togetherare enab ling T-MPLSOAM to supporterror managementand performance management,while alsom inim ize service interrup tion,restoration time and operation resource.These T-MPLSOAM func tions are basically equivalent to those of the traditional transportnetwork,hencemaking the T-MPLSOAM capab le ofp rovid ing carrier-c lass services.

▲Figure 7. T-MPLS over OTN.

6 T-MPLS Applications

As an MPLS-based connection-oriented packet transport technology,T-MPLS is supported bymany telecom vendors inc lud ing ZTE,Huawei,Cisco,Juniper and Alcatel-Lucent.These vendors are ab le tomake equipmentw ith sophisticated Internet Protocol/Multi-ProtocolLabelSw itching（IP/MPLS）functions,and theirp roducts account foramajormarketshare.They also support the MPLS-based evolutions,such as T-MPLS.Other telecom vendors who used to support PBTonly are also show ing interests in T-MPLS,by supporting both PBTand T-MPLS in theirp roducts,in a bid to meetcarrier requirements and increase marketshare.ZTE has developed the T-MPLS func tions on itsmulti-service transportp roduc tZXMPS385,which,w ith European standard-com p liant structure design,canmeet requirements ofvarious currentand future services.In 2007,ZTE passed the g lobal interoperability testof T-MPLS in the CE interoperability testhosted by European Advanced Network TestCenter（EANTC）.T-MPLS can be carried over SDH and OTN.With the evolution ofop ticalnetwork,however,OTN w ill ultimately rep lace SDH to form the integ rated packet/Op ticalData Unit（ODU）/wavelength network.Therefore,ITU-T imp roved G.709,the originalOTN interface p rotocol,in November2007,and put forward G.709/Y.1331 Amendment2［13］that expanded the c lient signalofODUw ith T-MPLS added to it［14］.Figure 7 shows the T-MPLS overOTN structure,where T-MPLS is carried over the op ticalchanneleitherbyway ofODU via SDH or directly through ODU.

7 Conclusion

Being the com p letely connection-oriented packet transport technology,T-MPLSboasts of the packetnetwork characteristics,inc lud ing flexibility,high efficiency and the capability to carrymultip le services.It is endowed w ith the advantages of the transportnetwork,forexam p le,good survivability and OAM,and is ab le to supportCE favorab ly.However,T-MPLS is also facing challenges from other packet transport technologies,inc luding VPLS and PBT.As an emerging technology,T-MPLS is yet to getmature w ith the supportofstandardization organizations,network carriers and equipmentvendors.