Future Internet Technologies

Transcription

1 Future Internet Technologies Internet Backbone Dr. Dennis Pfisterer Institut für Telematik, Universität zu Lübeck

2 Contents Routing Protocols Classification Examples: OSPF and BGP Backbone Technologies Frame Relay ATM MPLS #2

3 Routing Protocols #3

4 Classification of Routing Protocols By algorithm class Distance Vector Link State Path Vector By scope Within an AS Interior Gateway Protocol (IGP) Between AS Exterior Gateway Protocol (EGP) AS #1 AS #2 #4

5 Distance Vector Signpost principle No global view of the network s topology Only direction known (next hop and path metric) Procedure Startup: Routers only aware of direct neighbors (e.g., due to L2 connectivity) Neighbors periodically exchange routing tables Local routing tables are updated with received information On update, the routing table is forwarded Destination Next Hop Metric / / Examples RIP (v1 and v2), IGRP #5

6 Example Startup Initially, only neighbors are known/discovered A B C 5 D A B C D Dst. Next Metric Dst. Next Metric Dst. Next Metric Dst. Next Metric A A 2 A A 20 B B B B 1 C C 20 C C C C 5 D D #6

7 Example Transmit data to neighbors Update local routing table Change existing entries if a better metric is available Re-send routing table if entries were updated A C B D A B C D Dst. Next Metric Dst. Next Metric Dst. Next Metric Dst. Next Metric A A 2 A A 20 B B B B 1 C C 20 C C C C 5 D D #7

8 Example Example for router A A 2 B B reaches C with metric 1. This means I can reach C via B with metric which is better than 20 update C reaches B with metric 1. This means I can reach B via C with metric which is worse than 2 don t update C D A B C D Dst. Next Metric Dst. Next Metric Dst. Next Metric Dst. Next Metric A A 2 A A 20 B B B B 1 C CB 203 C C C C 5 D C 25 D D #8

9 Example Final stable state A 2 B 20 1 C 5 D A B C D Dst. Next Metric Dst. Next Metric Dst. Next Metric Dst. Next Metric A A 2 A B 3 A C 7 B B B B 1 B C 6 C B 3 C C C C 5 D B 8 D C 6 D D #9

10 Distance Vector: Issues 2 A B Count-To-Infinity Example for router A Metric CD deteriorates from 5 to C 5 30 D A B C D Dst. Next Metric Dst. Next Metric Dst. Next Metric Dst. Next Metric A A 2 A B 3 A C 7 B B B B 1 B C 6 C B 3 C C C C 30 D B 8 D C 6 D D #10

11 Count-To-Infinity (Router A) C propagates new (worse) route C D to A with metric 30 A keeps best route to D via B (8 < 30+20) B updates its route to D because it learns a (wrong) route via A (which still uses outdated data from B) Cost to D: 30 A C 20 I m keeping the route to D via B (cost 8) B Cost to D: 30 D New better route to D via A (cost 10) Route BAB(old cost)cd #11

12 Count-To-Infinity (Router A) B propagates the new (wrong) data to A A is not aware that this path is via him A updates its routing tables because B announces a deteriorated metric New cost to D: 12 A 20 Cost to D: B Cost to D: 10 New better route to D via A (cost 10) New better route to D via B (cost 11) C 5 30 D #12

13 Count-To-Infinity (Router A) A propagates updated path to D B now learns that D is reachable via A to the (worse) cost of 12 instead of 10 Slowly converging to 30 + link metric Cost to D: 12 New route to D via A (cost 12 was 10) Real problem when 30 is E.g., a link goes down A 2 B Solutions Slit Horizon & Poison Reverse Don t use Distance Vector Cost to D: 12 C D #13

14 Path Vector Extension to path vector protocols to avoid count-to-infinity problems Path to destinations is included in messages Each system appends its ID to the path Loops are easily detected Typically, only the shortest path is kept Example: BGP Destination Path (last entry: next hop) Metric / / #14

15 Link State Every node knows the network s topology Goal: Build a graph of the network Each router calculates shortest paths Uses Dijkstra's algorithm to create a tree of shortest paths with the current router as root Roadmap vs. Signpost principle Updates flooded in the network Fast convergence times No routing loops / Alternative paths available Often high CPU and memory requirements Examples: OSPF and IS-IS Figure OpenStreetMap contributors, CC BY-SA #15

16 Multiple Routes Often, more than a single route to a destination exists Routers must decide, which to prefer Using metrics, a routing protocol selects the best route for a destination Metrics: Hop count, cost, trust, load, reliability, RTT, MTU,...? Routers may run multiple routing protocols E.g., IGP and EGP May again lead to multiple routes to a destination Typically, the one with the lowest administrative distance is chosen see next slide #16

17 Administrative Distance Protocol Measure used by Cisco routers Used to select best path if multiple routes to a destination from multiple routing protocols exist A lower value is preferred/better 255: route not used Directly connected route 0 Static route out an interface 1 Static route to next-hop address 1 EIGRP summary route 5 External BGP 20 Internal EIGRP 90 IGRP 100 OSPF 110 IS-IS 115 RIP 120 EGP 140 ODR 160 External EIGRP 170 Internal BGP 200 DHCP-learned 254 Unknown 255 Adm. Distance #17

18 Open Shortest Path First (OSPF) #18

19 Open Shortest Path First (OSPF) Link state routing algorithm (Better) alternative to RIP OSPF v2 (IPv4, RFC 2328, 1998) OSPF v3 (+IPv6, RFC 5340, 2008) Autonomous System Interior gateway protocol Within a single autonomous system (AS) Messages encapsulated in IP datagrams with protocol number 89 Multicast addressing for route flooding on broadcast network links OSPF messages are never forwarded by routers #19

20 OSPF Neighbors Routers in the same broadcast domain are neighbors after detection Periodic broadcasting of HELLO packets Established a bidirectional binding Routers on the same broadcast medium E.g., same Ethernet Select designated router (DR) and backup designated router (BDR) Act as a hub to reduce traffic between routers List of neighbors: OSPF adjacency database DR BDR #20

21 Open Shortest Path First (OSPF) Changes in link state are flooded to all routers Link State Advertisements (LSA) Link state database (LSDB) and routing table Incremental updates of the link state database are exchanged Applying shortest path algorithm on LSDB creates local routing table Destination Next Hop Metric / / #21

22 OSPF Metrics OSPF s link metric is a unit-less number Could be link speed, round-trip time, throughput of a link, link availability and reliability Cisco s default metric 10 8 /bandwidth (in Bits/second) 100Mbit/s link cost of 1 10Mbit/s a cost of 10 #22

23 Border Gateway Protocol (BGP) #23

24 BGP Autonomous Systems (IGP vs. EGP) Between ASs: EGP (in the Internet: BGP v4) Inside an AS: Arbitrary IGPs Each AS has a unique 16 Bit AS number Assigned by Internet Registries Autonomous System #123 (e.g., using RIP) Autonomous System #345 (e.g., using IS-IS) Autonomous System #789 (e.g., using OSPF) Autonomous System #9876 (e.g., using OSPF) Autonomous System #845 (e.g., using OSPF) Autonomous System #1725 (e.g., using OSPF) #24

25 Border Gateway Protocol (BGP) Current Version: BGP v4 (RFC 4271) Not bound to a specific protocol Supports IPv4, IPv6, MPLS label distribution,... BGP is an exterior gateway protocol IGPs not applicable between AS IGPs use technical, not business metrics (contracts, money, services, SLA) BGP is the core routing protocol on the Internet Path Vector Protocol Not based on the same types of metric as IGP #25

26 Routing between ASs Final routes often determined by policies Rather than purely technical properties Policies define Which routes are accepted Which are advertised (with certain metrics) Which routes are actually chosen #26

27 Types of AS (RFC 1930) Single-homed (stub) AS Use of BGP not required Default route, static routes or an IGP could be used AS #1 Stub- AS Internet Multi-homed non-transit AS For instance multi-homed networks of big companies E.g. with provider-independent address space AS #1 Internet Multi-homed transit AS Connections with multiple other ASs Forwards BGP data Multi Transit ISP Internet #27

28 BGP Messages and State Machine Neighboring BGP routers (BPG peers) Manually configured Establishes a TCP session on port 179 Connections are maintained by periodically sending keep-alive messages (every 30sec) Four types of messages OPEN UPDATE NOTIFICATION KEEPALIVE Figure source: #28

29 UPDATE Message Announcement and withdrawal of routes Receivers decide whether to update their routing table May trigger new UPDATE messages Contents Field AS_PATH Multi-Exit Discriminator (MED) IGP Metric Local Preference Weight Communities Next Hop Origin Description Loop-free path (comprised of AS ids) to a destination (e.g., a CIDR prefix). An AS id may occur multiple times (e.g., to make a path unattractive) For multiple paths to a neighboring AS, the lowest value determines, which one to prefer. Internal AS-cost to traverse the AS for this path. For multiple paths within an AS, the highest value selects the preferred path. Local weight of a path (not forwarded). Group of destinations sharing a single policy. Last BGP router to this destination (not necessarily the next physical hop). BGP router from which this route was learned. #29

30 Path Selection Router may learn multiple paths to a single destination AS operators can influence the chosen path by adapting BGP parameters and rules of routers Process of path selection is known as BGP Path Selection Process #30

31 BGP Path Selection Process 1. Highest weight 2. Highest local preference 3. Prefer locally originated routes 4. Prefer shortest AS-path 5. Lowest origin code 6. Lowest Multi-Exit Discriminator 7. Prefer EBGP over IBGP path 8. Lowest IGP metric to next hop 9. Oldest route for EBGP paths #31

32 Backbone Technologies #32

33 Interconnection problem Companies, institutions, and state facilities often need data connections to remote places Typical: Use of leased lines Leased lines Always available, no dial-up Set up by a telecommunications common carrier Monthly fee depending on distance and data rate Carrier North America Japan Europe Level zero 64 kbit/s (DS0) 64 kbit/s 64 kbit/s First level Mbit/s (T1) Mbit/s Mbit/s (E1) Second level Mbit/s (T2) or Mbit/s Mbit/s (E2) Third level Mbit/s (T3) Mbit/s Mbit/s (E3) Fourth level Mbit/s (T4) Mbit/s Mbit/s (E4) Fifth level Mbit/s (T5) Mbit/s Mbit/s (E5) 33

34 Interconnection problem Leased line are exclusively used No resource sharing possible, usage often <100% They are expensive Especially if a company has many distributed locations Companies looked for a way to lower costs Physical Connections Logical View Private Network #1 Network Private Network #2 34

35 Frame Relay

36 Frame Relay Packet switched network Specifies physical and data link layer Often used to interconnect LANs over WANs Application Transport Internet Link Layer Provides virtual circuits Point-to-point or point-to-multipoint Speeds vary between 64kbit/s, 45MBits/s and higher FR Switch FR Switch Frame Relay Network Local Area Network (LAN) FR Switch 36

37 Frame Relay: Properties Users have a leased line to connect to a FR cloud No mesh network required multiple virtual circuits on one leased line Low cost due to effective resource sharing Customers buy a certain service level CIR: Committed Information Rate (constant rate) EIR: Excess Information Rate (short bursts) Providers can sell more than 100% of the available bandwidth Often bad quality of service due to over-commiting of resources FR is better suited for constant than for bursty traffic E.g., file transfer, etc. Still, it is used for LAN interconnection (bursty traffic) due to low cost 37

38 Frame Relay: Properties Best-effort service No error-correction, no frame acknowledgments, no retransmission No sequence numbering No flow control (discards frames on overload or errors) Terminology Data Terminal Equipment (DTE) User Equipment Data Communications Equipment (DCE) Provider DTE DCE DTE DCE Frame Relay Network DCE DTE 38

39 Frame Relay: Ports, Circuits, Protocols

40 Frame Relay: Frame Format Frame Format Flag (1 Byte) Address (2, 3, 4 Byte) Payload (variable) FCS (2 Byte) Flag (1 Byte) DLCI C/R EA DLCI FECN BECN DE E/A high (1 Bit) (1 Bit) low (1 Bit) (1 Bit) (1 Bit) (6 Bit) (4 Bit) Fields Flag (0x7E): Frame Delimiter (Begin/End) Byte stuffing DLCI (highest 6 bits): Data Link Connection Identifier EA (Extended Address Bit): 0 more to follow DLCI (lowest 4 bits) FECN (Forward Explicit Congestion Notification) bit BECN (Backward Explicit Congestion Notification) bit DE (Discard Eligibility) bit 2 nd EA: 0 more to follow 40

41 Frame Relay: Virtual Circuits Frames are of variable length (up to 8192 bytes) Virtual Circuits Permanent Virtual Circuits (setup by the service provider) Identified by Data Link Connection Identifier (DLCI) numbers Have only local meaning (at a DCE) Example DCE 77 DCE 1 DCE 77 2 DCE 41

42 Frame Relay: Summary Resulting network consists of a number of virtual circuits May be a full meshed network between all stations or only a partially interconnected network Frames do not carry any higher-layer protocol information Higher-layer protocol may be a property of the circuit Alternative: First payload byte indicates higher layer protocol Administered by ISO/ITU, contains values protocols including IP Issue of routing/bridging of frames RFC Multiprotocol Interconnect over Frame Relay RFC PPP in Frame Relay RFC Multiprotocol Interconnect over Frame Relay Logical View Private Network #1 Private Network #2 42

43 Asynchronous Transfer Mode (ATM)

44 Web Coverage: Frame Relay vs. ATM Frame Relay Asynchronous Transfer Mode Source: Google 44

45 Situation in the 1980s Two major networks types at that time Telephony Services Data Networks Telephony Services Circuit-switched telephony service POTS (Plain Old Telephone Service) with high QoS Data Networking (mostly Frame Relay) Packet switched networking technology Best-effort service, no QoS guarantees Interconnection of distributed office buildings, warehouses, retail stores, etc. Customers not really satisfied with QoS Network providers had to maintain separate networks High costs 45

46 Asynchronous Transfer Mode (ATM) ATM as a unifying networking technology Operators wanted to maintain only one backbone Multiplex all kinds of services over one network E.g., POTS, Frame Relay, Videoconferencing Goal: Transport all kinds of traffic with different requirements Speech: Fixed bandwidth, low latency, loss, and jitter Data: Bursty traffic, varying bandwidth requirements 46

47 Asynchronous Transfer Mode (ATM) Consequences Long data frames must be split into several small packets Otherwise: Results in jitter for concurrently transmitted phone calls A A A A A A D A A D A A A A 47

48 Data vs. Telephony Data Communications Telecommunicatio ns ATM Traffic support Data Voice Data, voice, video Transmission unit Packet Frame Cell Transmission length Variable Fixed Fixed Switching type Packet Circuit Cell Connection type Connection-less or Connectionoriented Connectionoriented Connectionoriented Time sensitivity None to some All Adaptive Delivery Best effort Guaranteed Defined class or guaranteed Media access Shared or dedicated Dedicated Dedicated 48

49 Use of ATM Although ATM provides a full network stack, it was never widely used on desktop PCs ATM has become mainly a backbone technology Today s ATM usage Internet- and Telephony backbones GSM infrastructure ADSL Backbone HYBNET ( Hybrides Breitbandnetz ) of the German ARD television 49

50 Asynchronous Transfer Mode (ATM) ATM architecture Connection-oriented, digital transmission service ATM relays individual fixed-size cells Prior to cell transmission, a virtual circuit is established What s asynchronous about ATM? Communicating entities (e.g., client and server) may transmit at (very) different rates ATM Network ATM Edge Switch ATM Endpoint ATM Switch 50

51 Asynchronous Transfer Mode (ATM) ATM transmits cells of fixed size ATM Cell (53 Bytes): Compromise between US (64) / EU (32) 5 Bytes header and exactly 48 Bytes payload Larger packets are split into multiple cells Packet ATM Cells Header (5 Bytes) Payload (48 Bytes) 51

52 Asynchronous Transfer Mode (ATM) Architecture and Interfaces Mesh network of ATM routers Two Interfaces User-Network-Interface (UNI) Network-Network-Interface (NNI) UNI NNI 52

53 ATM Cell Header (UNI and NNI) GFC field undefined Typically NNI is used exclusively User-Network Interface (UNI) Network-Network Interface (NNI) GFC VPI VPI VPI VCI VPI VCI VCI VCI PT CLP HEC VCI VCI PT CLP HEC Payload and padding if necessary (48 bytes) Payload and padding if necessary (48 bytes) GFC: Generic Flow Control, VPI: Virtual Path Identifier, VCI: Virtual channel identifier, PT: Payload Type, CLP: Cell Loss Priority, HEC: Header Error Control 53

54 Virtual Circuit (VC) Before data transmission A virtual circuit between communicating entities is established E.g., for a data transmission or a phone call Virtual Circuit Fixed path through the ATM network (like IP source routing) Signaled to the corresponding routers along the path Associated with QoS requirements (i.e., resources are reserved) 54

55 Virtual Circuit (VC) Each VC uniquely identified by two identifiers Virtual Path Identifier (VPI) Virtual Circuit Identifier (VCI) VC ids only valid per link 9/4 4/7 VPI/VCI 55

56 ATM Cell Forwarding Clients use (different) ids for the same VC Circuits are bidirectional 9/4 4/7 VPI/VCI #56

57 Label Switching Inside the ATM network, the same principle is applied VPI/VCI labels are switched on each link 9/4 4/8 1/9 6/7 4/7 VPI/VCI #57

58 ATM Cell Forwarding Switches can have a number of physical interfaces Connects to other ATM switches or end users Exchanges ATM cells over these connections Task of an ATM switch Determine on which interface to forward a specific cell Forwarding decision depends on VPI/VCI #58

59 Why VPI/VCI? VCs going in the same general direction are logically bundled together into a virtual path Virtual Path Identifier (VPI) Fewer paths than channels Switches may forward cells only based on the VPI Quick route lookup in short list Virtual Channel Identifier (VCI) Close to the destination, switches forward also based on the VCI Virtual Path Virtual Circuit #59

60 Forwarding Equivalent Class (FEC) Packets towards the same destination and/or with similar traffic characteristics form a FEC ATM: same VPI (and maybe VCI) Characteristics determining the FEC depend on router configuration Includes at least destination IP address May include other parameters such as source IP, contract of the customer, Virtual Path Virtual Circuit #60

61 ATM Cell Forwarding Incoming Outgoing ATM Switches have a routing table Outgoing interface determined based on incoming VPI/VCI of cells Interface #1 Interface #2 Interface #3 Interface #1 Interface #2 Interface #3 Before forwarding, the VPI/VCI ids are changed Important concept of label switching Incoming Outgoing VPI VCI VPI VCI Interface #61

62 Virtual Circuits: Types and Properties Permanent Virtual Circuits (PVCs) Long-term association, set up by the ATM provider Switched Virtual Circuits (SVCs) Set up on a per-case basis, disconnected when finished Sporadic or short-lived data transmission ISDN-like signaling for connection setup/teardown Properties Despite label switching at each router, a virtual end-to-end connection is established Cells always travel the same route through the network (unlike IP) Major advantage: QoS parameters can be assigned to each channel 62

63 ATM Adaptation Layer (AAL) ATM Adaptation Layer defines segmentation and reassembly of higherlayer packets into ATM cells E.g., IP, Frame Relay, Ethernet, ISDN, 5 different AALs are specified AAL used is not encoded in the cells Must be configured at/signaled between endpoints Application Application AAL AAL ATM ATM ATM ATM PHY PHY PHY PHY 63

64 ATM Adaptation Layer (AAL) 5 different AALs standardized AAL1: Constant Bit Rate (CBR) Synchronous, connection-oriented traffic (e.g., T1, E1, n*64 kbit/s) AAL2: Variable Bit Rate (VBR-RT) Connection-oriented, synchronous traffic E.g., voice data from GSM connections AAL3/4: VBR data traffic Connection-oriented, asynchronous traffic (e.g. X.25 data) Connectionless packet data (e.g., Frame Relay) AAL5 (simplified version of AAL3/4) Like AAL 3/4 but without error correction Examples: IP over ATM, Ethernet Over ATM, and LAN Emulation (LANE) 64

65 ATM Adaptation Layer 5 (AAL5) Transmit stochastic, packet-switched data over ATM Simple AAL w/o any extra header per packet low overhead Segmentation and Reassembly Packets are split up into multiple ATM cells Last cell is padded (P) with 0-40 bytes 8 byte trailer (T) at the end of related ATM cells IP header TCP header Payload ATM Cell P T #65

66 ATM Adaptation Layer 5 (AAL5) AAL5 Trailer 8 Byte at the end of a packet Follows potential padding bytes Contents: length, CRC, 2 unused fields How to identify the end of a packet? No special AAL5 data per cell (only in last one) Not possible to distinguish trailer from data Cross-layer -solution: Use of ATM field PT Lowest bit of PT (payload type) is set Indicates that this is the last cell of a packet GFC VPI VCI VPI VCI VCI PT CLP HEC Payload and padding if necessary (48 bytes) AAL5 Trailer UU CPI Length CRC32 66

67 Traffic Engineering Limit (or guarantee) certain a bandwidth, delay, jitter, packet loss, Defined in service level agreements (SLAs) Allows flexible network planning and pricing ATM switches at the edges enforce traffic characteristics per virtual circuit Enforce so-called Traffic Contracts ATM Network #67

68 Traffic Contracts ATM supports four different types of traffic classes Have many parameters for fine-grained tuning to match a contract with a customer Traffic Contract Constant Bit Rate (CBR) Variable Bit Rate (VBR) Available Bit Rate (ABR) Unspecified Bit Rate (UBR) Description Constant Peak Cell Rate (PCR) specified Average cell rate is specified (may peak at a certain level for a defined interval length) A minimum guaranteed rate is specified Remaining transmission capacity is used 68

69 Enforcing Traffic Contracts Enforcing traffic contracts: Traffic Shaping, Policing Traffic shaping At ATM network s entry point Combination of queuing, cell prioritization, and cell dropping achieved through policing Policing Measures to enforce traffic contracts Simple policing inspects individual cells and drops them if necessary Advanced policing marks packets (set Cell Loss Probability bit in ATM header) 69

70 Traffic Contracts: Policing Disadvantageous for AAL packets spanning multiple cells Dropping one cell of an IP packet makes all other cells of this packet worthless Waste of bandwidth (better: drop all or none) Advanced schemes available Partial Packet Discard (PPD) and Early Packet Discard (EPD) Drops a series of related cells (requires specific AAL knowledge) Example for AAL5 If one cell is dropped, all further ones are dropped until the frame end bit in the ATM header

71 IP and ATM How to set up (ATM) circuits? IP Network ATM Network 71

72 Multi-Protocol Label Switching (MPLS)

73 MPLS: Motivation ATM solved many issues in the 90s Data and phone connections multiplexed over a single infrastructure Connection-less and connection- oriented higher-layer protocols supported ATM Unifying network for all kinds of applications Providers massively invested in ATM backbones Today: new requirements due to massive Internet growth 73

74 MPLS: Motivation IP traffic grown by several orders of magnitude ATM allowed interconnection of IP routers Mostly a fully meshed network BGP to determine IP routes Routing is a major issue in the backbone Routing decisions performed on per-packet basis Backbone routers hold 300,000+ BGP routes Cost of routing slowed down internet connections Consumers Access Network Backbone Simplification of backbone routing required Lead to the definition of MPLS [RFC3031] Successor of Cisco s tag switching protocol 74

75 Routing in IP 1) Inspect IP header 2) Assign packet to a FEC (forwarding equivalent class) 3) Select next hop for FEC 4) Transmit packet to next hop Routing decisions are repeated at every hop Goal: Minimize number of routing decisions in the backbone 75

76 Routing in MPLS Important observations Layer-2 Switches: perform good, are cheap Layer-3 routers: forward IP, expensive, slow, other functionality (filtering) Price/Performance: switches better than routers Goal Separate routing from forwarding Forward IP packets at the cost of a Layer-2 switch 76

77 Routing in MPLS MPLS was designed with the strengths and weaknesses of ATM in mind Goal: integrate ATM-like functionality and IP routing Let IP routers have ATM switch functionality Allows using IP routing data to set up virtual circuit (VC) I.e., BGP routing tables can be used to determine network routes 77

78 MPLS Data packets are assigned labels Packet forwarding only based on labels Packet content not examined Supports arbitrary transport media and protocols Transport media: Frame Relay, ATM, SONET, DSL, Ethernet, Protocols: IPv4, IPv6, Ethernet, Ingress router Egress router 78

79 MPLS MPLS operates at layer 2.5 Between Layer 2 (link layer) and Layer 3 (network layer) No special signaling protocols Much fewer complexity No small cells like ATM Interleaving data and voice has become feasible due to high line speeds 40 Gbit/s: 1500 byte 300ns Application Transport (L4) Network (L3) MPLS (L2.5) Link Layer (L2) Packet Cells 79

80 Label Switched Path (LSP) Path through an MPLS network LSPs are essentially like ATM VCs Main difference: LSPs are unidirectional For bidirectional communication, two LSPs must be set up 1 8 Label 80

81 Label Switching As with ATM VPI/VCI labels, MPLS labels are switched along a path Difference to ATM: Clients are not aware of MPLS They use their protocol as normal (e.g., IP or Ethernet) Label #81

82 MPLS routers: Label Switch Routers (LSRs) Handle IP and MPLS traffic Edge routers also called LERs (Label Edge Routers) Packets enter MPLS networks at an ingress router Ingress routers classify unlabeled IP packets and append labels Packets leave the MPLS network at egress routers Egress routers remove the label and forward the (plain) IP packet Ingress Router Egress Router Label Label 82

83 MPLS Routing (for IP-based networks) Ingress router inserts MPLS shim header (label + x) Backbone routers only use the label to determine next hop No inspection of the payload (i.e., the IP header) necessary Routing decision only happens on the ingress router Labels only have local meaning as in ATM and Frame Relay Labels are switched from router to router Layer 2 IP Header Layer 2Payload Shim Hdr IP Header Payload (1) Inspect Shim Header (Label) (2) Determine next hop and switch label 83

84 MPLS Label Stacking MPLS networks may be nested Labels can be stacked #84

85 Label Stacking Multiple labels can be inserted Labels are pushed on MPLS entry And pop-ed on exit Layer 2 Shim Hdr Layer 3 Layer 2 Shim Hdr Shim Hdr Layer 3 Layer 2 Shim Hdr Shim Hdr Shim Hdr Layer 3 #85

86 MPLS Shim Header Fixed length (32 bit) Label Exp S TTL Four fields: Field Label (20 Bit) Exp (3 Bit) S (1 Bit) TTL (8 Bit) Description Unique LSP identifier Experimental field Corresponds to Cisco s Class of Service field) Stack Bit 0: additional labels 1: end of stack) Time to Live #86

87 MPLS Label Distribution Options BGP extensions RFC 4364: BGP/MPLS IP Virtual Private Networks (VPNs) RFC 3107: Carrying Label Information in BGP-4 RFC 4781: Graceful Restart Mechanism for BGP with MPLS Label Distribution Protocol (LDP, RFC 5036) Uses shortest paths from Interior Gateway Protocol (e.g., OSPF) Constraint-based Routing Label Distribution Protocol (CR-LDP) Discontinued in favor of RSVP-TE Resource Reservation Protocol-Traffic Engineering (RSVP-TE) Defined in RFC 5151 next slide #87

88 MPLS Label Distribution: RSVP-TE Resource Reservation Protocol (RSVP, RFC 2205) Request or deliver specific levels of quality of service (QoS) for application data streams or flows Not widely deployed RSVP-Traffic Engineering (RSVP-TE, RFC 5151) Extension of RSVP for Traffic Engineering Establishment of MPLS label switched paths (LSPs) RSVP Flow MPLS Label Path Parameters such as bandwidth, delay and jitter, and explicit hops #88

89 MPLS Pros Routing decisions happen only once Higher performance in the backbone Routing decisions can factor in information not contained in the IP header Packets arriving on different ports (i.e., customers) Ingress routers may assign different types of traffic different FECs Prioritize voice traffic or traffic of better paying customers Assign low priority to data streams or normal IP traffic MPLS can be used over a variety of networks IP: Layer 2,5 header inserted between Layer-2 and IP ATM: Use VCI/VPI as label Frame Relay: Use DLCI as label 89

90 MPLS Pros and Cons Further Pros MPLS may use standard BGP techniques to setup labels Customers can get private Layer-2 networks Inside these networks, private BGP is used to fill routing tables Routers maintain different forwarding tables per customer Cons Big change to the Internet s architecture Another Layer (2.5) inserted into the layer model Complicates the architecture and debugging Standard tools (traceroute) cannot be used 90

91 Literature [potaroo.net] IPv4 Address Report, #91