MPLS Architecture Overview V1.1

Course Objectives MPLS overview MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths Loops and TTL LDP overview Day in the Life of a Packet

MPLS Concepts MPLS is a layer 2+ switching Developed to integrate IP and ATM MPLS forwarding is done in the same way as in ATM switches Packet forwarding is done based on Labels

MPLS Origin MPLS is developed by integrating IP switching protocols of different manufactures IP Switching (Ipsilon) Tag Switching (Cisco) Aggregate RouteBased IP Switching (IBM) Cell Switch Router (Toshiba) IP Navigator (Ascend)

MPLS Origin MPLS is developed by the combination of ATM and IP at 1990s. In 1996, Ipsilon proposed IP switching protocol which solve the problem of better supporting IP on ATM switch, this makes an ATM switch a router and at the same time having the high performance of an ATM switch ， breaking the performance bottleneck on the router (can not achieve wire speed on POS3,POS12)

MPLS History When OC48,OC192 interface achieves wire speed, MPLS research switched to these application ： TE (Traffic engineering), VPN. MPLS TE becomes an important method to manage network traffic 、 reduce congestion and ensure QOS on IP network. VPN is regarded as an important method to provide value-added service and explore new service by carriers. Disadvantages ： Change the current network topology, increase the network complexity, VPN security factors.

MPLS Overview Multi-protocol label switch Label is at the layer 2.5, between link layer and network layer. This technology have the advantage of layer 2 switching and layer 3 routing. The successful point of MPLS technology is that it introduced connection-oriented mechanism into IP network; Core idea of MPLS is ： routing on the edge and switching on the core part Establishing of LSP （ label switch path ）： LSP is established by topology not by data. The establishing of LSP can be done by LDP ， CR-LDP ， RSVP protocols.

MPLS Developing Motion Current core network is made up of router and ATM switch Core network based on MPLS is superior than core network based on router and ATM This is the developing motion of MPLS.

Comparison between MPLS and routing MPLS simplifies the process of packet forwarding MPLS supports valid explicit routing MPLS can implement TE best MPLS can support QoS routing Mapping from IP packet to FEC MPLS support dividing network by function MPLS implements single forwarding rule no matter how many levels of Qos are there

Advantage of MPLS Comparison of label switching and hop-by-hop forwarding based on destination IP address ： MPLS is implemented on switch, thus taking full advantage of high speed of switch. Traditional switch can search label and replace them, but can not analyze network header or at least not fast enough to analyze it, so it is difficult to implement forwarding based on network header information on traditional switch When packet comes into MPLS network, ingress router will decide which FEC it belongs to, it can use any information about the packet not only the network layer information. （ for example the port information ）

Advantage of MPLS (Continued.) MPLS can assign different labels to packet coming from different routers, thus making it easy to implement routing based on ingress routers. Because ingress router id can not be sent with the packet, so traditional routing patterns do not support this function. Sometimes it is reasonable to designate a predefined route for the packets than hop-by-hop routing.

Course Objectives MPLS overview MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths Loops and TTL LDP overview Day in the Life of a Packet

MPLS Concepts Unlike IP, classification/label can be based on: Destination Unicast address Traffic Engineering VPN QoS FEC: Forwarding Equivalence Class A FEC can represent a: Destination address prefix, VPN, Traffic Engineering tunnel, Class of Service.

Key words in MPLS Label FEC (Forwarding equivalence class) LSR (Label switching router) LER (Label edge router) LSP (Label switching path) LDP (Label distribution path)

Label Label is a key point in MPLS, it is a short fixed-length and locally used identifier which is used to identify FEC Label processing is done by high speed ASIC chip thus making the delay of packet processing and queuing decrease to a great extent Why fixed-length label? Balancing between forwarding efficiency and switching performance. Although fixed-length making the forwarding efficiency low but it can highly improve switching performance. Label is locally used identifier. It is only meaningful between the sending out port of upstream router and receiving port of downstream router.

Label Format MPLS label is an integer ranged between 0 to ,it is used to identify specific FEC. This label is encapsulated in layer 2 header.

Label stack Two or more MPLS labels, encapsulated after link layer header and before network header. Top label in the stack appear earliest. Network layer header follow the last label in the stack

Label stack Forwarding of packets is based on the top label in the stack ， when LSR receives a packet ， checks the top label and decides the next hop The operation about the label stack ： replace ： Use a new label to replace the top label in the stack Pop (delete) ： pop the top label in the label stack and delete it push (add) ： replace the top label and add a new label into the stack

FEC Forwarding Equivalence Class (FEC) ： A group of packets that have common attributes. These packets will be forwarded in the same way by LSR in the MPLS network ， just because they are forwarded in the same way they are equivalence.

FEC FEC – packets forwarded in the same way Same destination prefix unicast packet Same destination address multicast packet Same Qos packets MPLS Domain LER a LSR y LER b LER c LER d LER e LER f LSR x LSR z LSP Ingress Egress FEC p FEC q FEC p+q

MPLS key points LSR –responsible for establishing LSP for FEC LER –responsible for FEC classification, TE ， begin the process of establishing LSP, IP packet forwarding LSP –used for IP packet forwarding LDP –responsible for assigning labels MPLS Domain LER a LSR y LER b LER c LER d LER e LER f LSR x LSR z LSP Ingress Egress FEC p FEC q FEC p+q

MPLS key points Label switch router (LSR) ： one node in MPLS network, it is located in the middle of MPLS network, it runs MPLS control protocol and layer 3 routing protocol, its responsibility lies in: exchanging routing information with other LSR to form route table, implementing the mapping from FEC to IP packets ， redistributing label binding information, establishing label forwarding table and maintaining it.

MPLS key points Label edge router (LER) ： Responsible for connecting MPLS domain and non-MPLS domain. Implementing the function of classifying service 、 assigning labels 、 taking off labels etc. LER is the key device in implementing MPLS.

MPLS Key Points Label switch path (LSP) ： The forwarding path made up of the LSRs along the way to forward packets which belong to the same FEC. Label distributing protocol (LDP) ： Responsible for controlling the label binding information exchanging process between LSRs ， LSR establish and maintain the LIB (Label information base) according to the binding information between label and FEC.

MPLS Key points Upstream router Downstream router

Course Objectives MPLS overview MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths Loops and TTL LDP overview Day in the Life of a Packet

LSRs and Labels LSR: Label Switch Router Edge-LSR: LSRs that do label imposition and disposition ATM-LSR: An ATM switch with Label Switch Controller

LSRs and Labels An IP routing protocol is used within the routing domain (e.g.:OSPF, i- ISIS) A label distribution protocol is used to distribute address/label mappings between adjacent neighbors The ingress LSR receives IP packets, performs packet classification, assign a label, and forward the labelled packet into the MPLS network Core LSRs switch packets/cells based on the label value The egress LSR removes the label before forwarding the IP packet outside the MPLS network IGP domain with a label distribution protocol

LSRs and Labels PPP Header Layer 3 Header Shim Header PPP Header(Packet over SONET/SDH) Ethernet Hdr Layer 3 Header Shim Header Ethernet FR Hdr Layer 3 Header Shim Header Frame Relay ATM Cell Header HEC DATA CLP PTI VCI GFC VPI Label HEC DATA CLP PTI VCI GFC VPI Label Subsequent cells

Course Objectives MPLS overview MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths Loops and TTL LDP overview Day in the Life of a Packet

Label Assignment and Distribution Labels have link-local significance Each LSR binds his own label mappings Each LSR assign labels to his FECs Labels are assigned and exchanged between adjacent neighboring LSR Applications may require non-adjacent neighbors

Label Assignment and Distribution Rtr-C is the downstream neighbor of Rtr-B for destination /24 Rtr-B is the downstream neighbor of Rtr-A for destination /24 LSRs know their downstream neighbors through the IP routing protocol Next-hop address is the downstream neighbor /24 Rtr-BRtr-ARtr-C /24 Upstream and Downstream LSRs

Label Assignment and Distribution LSRs distribute labels to the upstream neighbors /24 Rtr-B Rtr-A Rtr-C /24 Next-Hop InLab -... AddressPrefix OutI/F 1... OutLab InI/F 0... Next-Hop InLab AddressPrefix OutI/F 1... OutLab InI/F 0... Next-Hop InLab AddressPrefix OutI/F 1... OutLab -... InI/F 0... Use label 40 for destination /24 Use label 30 for destination /24 IGP derived routes Unsolicited Downstream Distribution

Label Assignment and Distribution Upstream LSRs request labels to downstream neighbors Downstream LSRs distribute labels upon request /24 Rtr-B Rtr-A Rtr-C /24 Use label 30 for destination /24 Use label 40 for destination /24 Request label for destination /24 On-Demand Downstream Distribution

Label Assignment and Distribution Label Retention Modes Liberal retention mode LSR retains labels from all neighbors –Improve convergence time, when next-hop is again available after IP convergence –Require more memory and label space Conservative retention mode LSR retains labels only from next-hops neighbors –LSR discards all labels for FECs without next-hop –Free memory and label space

Label Assignment and Distribution Label Distribution Modes Independent LSP control LSR binds a Label to a FEC independently, whether or not the LSR has received a Label the next-hop for the FEC The LSR then advertises the Label to its neighbor Ordered LSP control LSR only binds and advertise a label for a particular FEC if: –it is the egress LSR for that FEC or –it has already received a label binding from its next-hop

Label Assignment and Distribution Several protocols for label exchange LDP Maps unicast IP destinations into labels RSVP, CR-LDP Used in traffic engineering BGP External labels (VPN) PIM For multicast states label mapping

Course Objectives MPLS overview MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths Loops and TTL LDP overview Day in the Life of a Packet

Label Switch Path (LSP) LSPs are derived from IGP routing information LSPs may diverge from IGP shortest path LSP tunnels (explicit routing) with TE LSPs are unidirectional Return traffic takes another LSP LSP follows IGP shortest pathLSP diverges from IGP shortest path IGP domain with a label distribution protocol

LSP establishing process The establishing of LSP in MPLS network including these three steps: Every node run routing protocols such as BGP 、 OSPF 、 IS-IS to form its own route table According to the route table, every node establish label information base under the control of LDP From the ingress LSR 、 middle LSR and egress LSR, the ingress lable and outgress lable together form a LSP.

1st step ： form of route table Dynamic routing protocols help each router form route table

Traditional ‘hop by hop’ forwarding IP

2nd step ： form of LIB Mapping: 0.40 Request: 47.1 Mapping: 0.50 Request: 47.1

3rd step ： form of LSP IP

Routing method in MPLS hop-by-hop routing This method allows each node to select the next hop for each FEC independently This kind of routing method is commonly used in IP network

Routing method in MPLS Explicit Routing In this kind of routing method, each LSR can not select next hop independently ， on the contrary ， path selection is done under the network management policy, for example, the ingress or egress LSR define the way of the LSP. When the ingress of egress LSR define every hop along the LSP, we call it “strict explicit routing” ， if it only define part of the nodes along the way ， we call it “loose explicit routing” “strict explicit routing” is also called “source routing” in IP network ， but compared to IP source routing, strict explicit routing has higher efficiency.

IP hop-by-hop routing

#216 #14 #462 #972 #14 #972 A B C Route= {A,B,C} Explicit Routing

IP ER-LSP

Label Switch Path (LSP) Penultimate Hop Popping The label at the top of the stack is removed (popped) by the upstream neighbor of the egress LSR The egress LSR requests the “popping” through the label distribution protocol Egress LSR advertises implicit-null label The egress LSR will not have to do a lookup and remove itself the label One lookup is saved in the egress LSR

Label Switch Path (LSP) Penultimate Hop Popping Egress LSR needs to do an IP lookup for finding more specific route Egress LSR need NOT receive a labelled packet 0 1 Summary route for / /24 Next-Hop InLabAddressPrefixOutI/FOutLabInI/F /162pop Next-Hop InLabAddressPrefixOutI/FOutLabInI/F / Egress LSR summarises more specific routes and advertises a label for the new FEC Summary route is propagate through the IGP and label is assigned by each LSR Use label “implicit-null” for FEC /16 Summary route for /16 Use label 4 for FEC / /24 Address Prefix and mask /24 Next-Hop InterfaceSerial / Serial /16...Null

Course Objectives MPLS overview MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths Loops and TTL LDP overview Day in the Life of a Packet

Loops and TTL In IP networks TTL is used to prevent packets to travel indefinitely in the network MPLS may use same mechanism as IP, but not on all encapsulations TTL is present in the label header for PPP and LAN headers (shim headers) ATM cell header does not have TTL

Loops and TTL LSRs using ATM do not have TTL capability Some suggested options: hop-count object in LDP Path Vector object in LDP

Loops and TTL TTL is decremented prior to enter the non-TTL capable LSP If TTL is 0 the packet is discarded at the ingress point TTL is examined at the LSP exit IGP domain with a label distribution protocol LSR-1 LSR-2 LSR-4 LSR-5 LSR- 3 LSR-6 Egress IP packet TTL = 6 Label = 25 IP packet TTL = 6 IP packet TTL = 10 LSR-6 --> 25 Hops=4 IP packet TTL = 6 Label = 39 IP packet TTL = 6 Label = 21

Course Objectives MPLS overview MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths Loops and TTL LDP overview Day in the Life of a Packet

LDP Concepts Label Distribution Protocol Labels map to FECs for Unicast Destination Prefix LDP works between adjacent/non-adjacent peers LDP sessions are established between peers

LDP

LDP Messages Discovery messages Used to discover and maintain the presence of new peers Hello packets (UDP) sent to all-routers multicast address Once neighbor is discovered, the LDP session is established over TCP

LDP Messages Session messages Establish, maintain and terminate LDP sessions Advertisement messages Create, modify, delete label mappings Notification messages Error signalling

Course Objectives MPLS overview MPLS Concepts LSRs and labels Label assignment and distribution Label Switch Paths Loops and TTL LDP overview Day in the Life of a Packet

Day in the life of a Packet Egress LSR needs to do an IP lookup for finding more specific route 0 Summary route for / /24 Next-Hop InLabAddressPrefixOutI/FOutLabInI/F /162pop Next-Hop InLabAddressPrefixOutI/FOutLabInI/F / Egress LSR summarises more specific routes and advertises a label for the new FEC Summary route is propagate through the IGP and label is assigned by each LSR Use label “implicit-null” for FEC /16 Summary route for /16 Use label 4 for FEC / /24 Address Prefix and mask /24 Next-Hop InterfaceSerial / Serial /16...Null Use label 7 for FEC /16 Next-Hop InLabAddressPrefixOutI/FOutLabInI/F / CE PE PP

Day in the life of a Packet Basic Layout

Day in the life of a Packet Database Layout

Summary MPLS basic conception MPLS working process LSP FEC label distribution LDP