سوي يچينگ و مسيريابي در شبكه

Transcription

1 سوي يچينگ و مسيريابي در شبكه دكتر فرهاد فغاني استاديار دانشكده مهندسي برق قسمت ششم : Multi-Protocol Label Switching (MPLS) 1

2 One of the many ways of getting from A to B: BROADCAST: Go everywhere, stop when you get to B, never ask for directions. HOP BY HOP ROUTING: Continually ask who s closer to B go there, repeat stop when you get to B. Going to B? You d better go to X, its on the way. SOURCE ROUTING: Ask for a list (that you carry with you) of places to go that eventually lead you to B. Going to B? Go straight 5 blocks, take the next left, 6 more blocks and take a right at the lights. 2

3 Disadvantages of IP Routing: Connectionless e.g. no QoS Each router has to make independent forwarding decisions based on the IP-address Large IP Header At least 20 bytes Routing in Network Layer Slower than Switching Usually designed to obtain shortest path Do not take into account additional metrics 3

4 Disadvantages of ATM: Complex Expensive Not widely adopted 4

5 Idea: Combine the forwarding algorithm used in ATM with IP. Use a fixed length label in the packet header to decide packet forwarding 5

6 MPLS: MPLS is arranged between Layer 2 and Layer 3 6

7 Why MPLS? Leverage existing ATM hardware Ultra fast forwarding IP Traffic Engineering Constraint-based Routing Virtual Private Networks Controllable tunneling mechanism Voice/Video on IP Delay variation + QoS constraints 7

8 MPLS Characteristics Mechanisms to manage traffic flows of various granularities (Flow Management) such as flows among different hardware, machines, or even flows among different applications. Is independent of Layer-2 and Layer-3 protocols Maps IP-addresses to fixed length labels Interfaces to existing routing protocols (such as RSVP, OSPF) supports the IP, ATM, and frame-relay Layer 2 protocols. 8

9 MPLS Basics LSR - Routers that support MPLS are called Label Switch Router LER - LSR at the edge of the network is called Label Edge Router (a.k.a Edge LSR) Ingress LER is responsible for adding labels to unlabeled IP packets. Egress LER is responsible for removing the labels. Label Switch Path (LSP) the path defined by the labels through LSRs between two LERs. Label Forwarding Information Base (LFIB) a forwarding table (mapping) between labels to outgoing interfaces. Forward Equivalent Class (FEC) All IP packets follow the same path on the MPLS network and receive the same treatment at each node. 9

10 MPLS Basics Resides at the edge of an MPLS network and assigns and removes the labels from the packets. Support multiple ports connected to dissimilar networks (such as frame relay, ATM, and Ethernet). 10

11 Label1 Edge Router - LER Resides at the edge of an MPLS network and assigns and removes the labels from the packets. Support multiple ports connected to dissimilar networks (such as frame relay, ATM, and Ethernet). 11

12 Label1 Switching Router - LSR Is a high speed router in the core on an MPLS network. ATM switches can be used as LSRs without changing their hardware. Label switching is equivalent to VP/VC switching. 12

13 How does it work? Add label at the ingress LER remove label at the egress LER LER LSR LSR LER IP IP #L1 IP #L2 IP #L3 IP IP Routing Label Switching Label Switching IP Routing 13

14 Position of LERs and LSRs 14

15 Label 1 A label is a short, fixed-length value carried in the packet's header to identify a forwarding equivalence class (FEC). A label is analogous to a connection identifier, such as an ATM VPI/VCI or a Frame Relay DLCI, because it has only link-local significance, does not encode information from the network layer header, and maps traffic to a specific FEC. 15

16 Label 1 Generic label format 16

17 Label Creation Topology-based method uses normal processing of routing protocols (such as OSPF and BGP) Request-based method uses processing of request-based control traffic (such as RSVP) Traffic-based method uses the reception of a packet to trigger the assignment and distribution of a label The topology- and request-based methods are examples of control-driven label bindings, while the traffic-based method is an example of data-driven binding 17

18 Label distribution MPLS does not specify a single method for label distribution BGP has been enhanced to piggyback the label information within the contents of the protocol RSVP has also been extended to support piggybacked exchange of labels. IETF has also defined a new protocol known as the label distribution protocol (LDP) for explicit signaling and management 18

19 MPLS Operation Label Path: R1 => R2 => R3 => R4 19

20 Label Forwarding Information Base (LFIB) Router Incoming Label Incoming Interface R1 --- E0 Destination Network (FEC) Outgoing Interface Outgoing Label S1 6 R2 6 S S2 11 R3 11 S S3 7 R4 7 S E

21 MPLS Process Label Switch Path Routing Protocol FEC FEC FEC Classification Label assignment LFIB Label Swapping LFIB LFIB Label removal Layer 2 Layer 2 Layer 2 Layer 1 Layer 1 Layer 1 21 Ingress Node Core Node Egress Node

22 Label Encapsulation Label information can be carried in a packet in a variety of ways: A small, shim label header inserted between the Layer 2 and network layer headers. As part of the Layer 2 header, if the Layer 2 header provides adequate semantics (such as ATM). As part of the network layer header (future, such as IPv6). In general, MPLS can be implemented over any media type, including point-to-point, Ethernet, Frame Relay, and ATM links. The label-forwarding component is independent of the network layer protocol. 22

23 Label Encapsulation L2 Label ATM FR Ethernet PPP VPI/VCI DLCI Shim Label L2 Header Label IP Header Datagram MPLS Encapsulation is specified over various media types. Labels may use existing format (e.g., VPI/VCI) or use a new shim label format. 23

24 Shim Header The Label (Shim Header) is represented as a sequence of Label Stack Entry Each Label Stack Entry is 4 bytes (32 bits) 20 Bits is reserved for the Label Identifier (also named Label) Label (20 bits) Exp (3 bits) S (1 bit) TTL (8bits) Label : Label value (0 to 15 are reserved for special use) Exp : Experimental Use S : Bottom of Stack (set to 1 for the last entry in the label) TTL : Time To Live 24

25 Forward Equivalent Class (FEC) Classification A packet can be mapped to a particular FEC based on the following criteria: destination IP address, source IP address, TCP/UDP port, class of service (CoS) or type of service (ToS), application used, any combination of the previous criteria. Ingress Label FEC Egress Label /

26 Forwarding Equivalence Classes (FEC) LER LSR LER LSR IP1 IP1 #L1 IP1 #L2 IP1 #L3 IP1 IP2 IP2 #L1 IP2 #L2 IP2 #L3 IP2 IP3 IP3 #L4 IP3 #L5 IP3 #L6 IP3 IP4 IP4 #L4 IP4 #L5 IP4 #L6 IP4 26 FEC = A group of packets that are treated the same way by a router. The concept of FECs provides for flexibility, scalability, and traffic engineering. In legacy routing, the ToS field is used to determine FEC at each hop. In MPLS it is only done once at the network ingress.

27 MPLS Applications Traffic Engineering Virtual Private Network Quality of Service (QoS) 27

28 Traffic Engineering Traffic engineering allows a network administrator to make the path deterministic and bypass the normal routed hop-by-hop paths. An administrator may elect to explicitly define the path between stations to ensure QoS or have the traffic follow a specified path to reduce traffic loading across certain hops. The network administrator can reduce congestion by forcing the frame to travel around the overloaded segments. Traffic engineering, then, enables an administrator to define a policy for forwarding frames rather than depending upon dynamic routing protocols. Traffic engineering is similar to source-routing in that an explicit path is defined for the frame to travel. However, unlike source-routing, the hop-by-hop definition is not carried with every frame. Rather, the hops are configured in the LSRs ahead of time along with the appropriate label values. 28

29 MPLS Traffic Engineering Overload!! IP LER 1 Overload!! LER 4 IP IP L IP L Forward to LSR 2 LSR 3 LSR 4 LSR X IP L LSR 2 LSR 3 End-to-End forwarding decision determined by ingress node. Enables Traffic Engineering 29

30 MPLS-based VPN One of most popular MPLS applications is the implementation of VPN. The basic concept is the same as ATM transparent LAN. Using label (instead of IP address) to interconnect multiple sites over a carrier s network. Each site has its own private IP address space. Different VPNs may use the same IP address space. Same as Frame Relay separation of different user traffic but more fashionable to use word VPN today. 30

31 MPLS VPN Connection Model VPN_A MPLS Edge MPLS Core MPLS Edge VPN_A VPN_B VPN_A VPN_A VPN_B VPN_B VPN_A: /24, /24, /24 VPN_B: /24, /24, /24 31

32 MPLS VPN - Example E1 E1 E2 E3 E1 E2 E3 E E1 10 E3 10 E1 30 E2 30 E3 -- E E2 20 E3 20 E1 40 E2 40 E3 -- E2 LSP 70 E3 -- E1 80 E3 -- E1 50 E2 70 E1 60 E2 80 E1 -- E1 50 E3 -- E2 60 E3 LSP 32

33 MPLS and QoS An important proposed MPLS capability is quality of service (QoS) support. QoS mechanisms: Pre-configuration based on physical interface Classification of incoming packets into different classes Classification based on network characteristics (such as congestion, throughput, delay, and loss) A label corresponding to the resultant class is applied to the packet. Labeled packets are handled by LSRs in their path without needing to be reclassified. MPLS enables simple logic to find the state that identifies how the packet should be scheduled. The exact use of MPLS for QoS purposes depends a great deal on how QoS is deployed. Support various QoS protocols, such as IntServ, DiffServ, and RSVP. 33

34 FEC QoS Classification LER LSR MPLS label based on 1. physical interface 2. Source IP address 3. Destination IP address 4. Type of Service (ToS) 5. Protocol information 6. etc. A priority scheme for different label switch path (LSP) 34