ATM and MultiProtocol Label Switching MPLS By Behzad

ATM and Multi-Protocol Label Switching (MPLS) By Behzad Akbari Spring 2011 These slides are based in parts on the slides of J. Kurose (UMASS) and Shivkumar (RPI) 1

Outline n n n ATM basics IP over ATM MPLS basics MPLS VPN MPLS traffic engineering 2

Asynchronous Transfer Mode: ATM n n 1990’s/00 standard for high-speed (155 Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architecture Goal: integrated, end-end transport of carry voice, video, data q meeting timing/Qo. S requirements of voice, video (versus Internet best-effort model) q “next generation” telephony: technical roots in telephone world q packet-switching (fixed length packets, called “cells”) using virtual circuits 3

ATM architecture AAL n n n ATM ATM physical end system switch end system adaptation layer: only at edge of ATM network q data segmentation/reassembly q roughly analagous to Internet transport layer ATM layer: “network” layer q cell switching, routing physical layer 4

ATM: network or link layer? Vision: end-to-end transport: “ATM from desktop to desktop” q ATM is a network technology Reality: used to connect IP backbone routers q “IP over ATM” q ATM as switched link layer, connecting IP routers IP network ATM network 5

ATM Adaptation Layer (AAL) n n n ATM Adaptation Layer (AAL): “adapts” upper layers (IP or native ATM applications) to ATM layer below AAL present only in end systems, not in switches AAL layer segment (header/trailer fields, data) fragmented across multiple ATM cells q analogy: TCP segment in many IP packets AAL ATM ATM physical end system switch end system 6

ATM Adaptation Layer (AAL) [more] Different versions of AAL layers, depending on ATM service class: n n n AAL 1: for CBR (Constant Bit Rate) services, e. g. circuit emulation AAL 2: for VBR (Variable Bit Rate) services, e. g. , MPEG video AAL 5: for data (eg, IP datagrams) User data AAL PDU ATM cell 7

ATM Layer Service: transport cells across ATM network n analogous to IP network layer n very different services than IP network layer Network Architecture Internet Service Model Guarantees ? Congestion Bandwidth Loss Order Timing feedback best effort none ATM CBR ATM VBR ATM ABR ATM UBR constant rate guaranteed minimum none no no no yes yes yes no no (inferred via loss) no congestion yes no no 8

ATM Layer: Virtual Circuits n VC transport: cells carried on VC from source to dest q q n n call setup, teardown for each call before data can flow each packet carries VC identifier (not destination ID) every switch on source-dest path maintain “state” for each passing connection link, switch resources (bandwidth, buffers) may be allocated to VC: to get circuit-like perf. Permanent VCs (PVCs) q long lasting connections q typically: “permanent” route between to IP routers Switched VCs (SVC): q dynamically set up on per-call basis 9

ATM VCs n n Advantages of ATM VC approach: q Qo. S performance guarantee for connection mapped to VC (bandwidth, delay jitter) Drawbacks of ATM VC approach: q Inefficient support of datagram traffic q one PVC between each source/dest pair) does not scale (N*2 connections needed) q SVC introduces call setup latency, processing overhead for short lived connections 10

ATM Layer: ATM cell n n 5 -byte ATM cell header 48 -byte payload q Why? : small payload -> short cell-creation delay for digitized voice q halfway between 32 and 64 (compromise!) Cell header Cell format 11

ATM cell header n n VCI: virtual channel ID q will change from link to link thru net PT: Payload type (e. g. RM cell versus data cell) CLP: Cell Loss Priority bit q CLP = 1 implies low priority cell, can be discarded if congestion HEC: Header Error Checksum q cyclic redundancy check 12

ATM Physical Layer (more) Two pieces (sublayers) of physical layer: n Transmission Convergence Sublayer (TCS): adapts ATM layer above to PMD sublayer below n Physical Medium Dependent: depends on physical medium being used TCS Functions: q Header checksum generation: 8 bits CRC q Cell delineation q With “unstructured” PMD sublayer, transmission of idle cells when no data cells to send 13

ATM Physical Layer Physical Medium Dependent (PMD) sublayer n SONET/SDH: transmission frame structure (like a container carrying bits); q bit synchronization; q bandwidth partitions (TDM); q several speeds: OC 3 = 155. 52 Mbps; OC 12 = 622. 08 Mbps; OC 48 = 2. 45 Gbps, OC 192 = 9. 6 Gbps n n TI/T 3: transmission frame structure (old telephone hierarchy): 1. 5 Mbps/ 45 Mbps unstructured: just cells (busy/idle) 14

IP-Over-ATM Classic IP only n 3 “networks” (e. g. , LAN segments) n MAC (802. 3) and IP addresses IP over ATM n replace “network” (e. g. , LAN segment) with ATM network n ATM addresses, IP addresses ATM network Ethernet LANs 15

IP-Over-ATM app transport IP Eth phy IP AAL Eth ATM phy app transport IP AAL ATM phy 16

Datagram Journey in IP-over-ATM Network n at Source Host: q q q n n IP layer maps between IP, ATM dest address (using ARP) passes datagram to AAL 5 encapsulates data, segments cells, passes to ATM layer ATM network: moves cell along VC to destination at Destination Host: q AAL 5 reassembles cells into original datagram q if CRC OK, datagram is passed to IP 17

IP-Over-ATM Issues: n IP datagrams into ATM AAL 5 PDUs n from IP addresses to ATM addresses q just like IP addresses to 802. 3 MAC addresses! ATM network Ethernet LANs 18

Re-examining Basics: Routing vs Switching 19

IP Routing vs IP Switching 20

MPLS: Best of Both Worlds PACKET ROUTING IP HYBRID MPLS +IP CIRCUIT SWITCHING ATM TDM Caveat: one cares about combining the best of both worlds only for large ISP networks that need both features! Note: the “hybrid” also happens to be a solution that bypasses IP-over-ATM mapping woes! 21

History: Ipsilon’s IP Switching: Concept Hybrid: IP routing (control plane) + ATM switching (data plane) 22

Ipsilon’s IP Switching ATM VCs setup when new IP “flows” seen, I. e. , “data-driven” VC setup 23

Issues with Ipsilon’s IP switching 24

Tag Switching Key difference: tags can be setup in the background using IP routing protocols (I. e. control-driven VC setup) 25

Multi-Protocol Label Switching (MPLS) 26

Background n It was meant to improve routing performance on the Internet q q n MPLS is similar to virtual circuits q n Routing is difficult using CIDR (longest prefix matching) Using the label-swapping paradigm to optimize network performance Only a fixed-sized label is used (like a VCID) with local scope It is very datagram oriented though q It uses IP addressing and IP routing protocols 27

Goals of MPLS n n To enable IP capability on devices that cannot handle IP traffic q Making cell switches behave as routers Increased performance q Using the label-swapping paradigm to optimize network performance Forward packets along “explicit routes” (pre-calculated routes not used in “regular” routing) q MPLS also permits explicit backbone routing, which specifies in advance the hops that a packet will take across the network. q This should allow more deterministic, or predictable, performance that can be used to guarantee Qo. S To support certain virtual private network services 28

IP Regular Destination Based Forwarding Address Prefix I/F 128. 89 1 128. 89 0 171. 69 1 … … 0 128. 89 0 1 128. 89. 25. 4 Data 0 128. 89. 25. 4 Data Packets Forwarded Based on IP Address 171. 69 29

MPLS Example: Routing Information Out In Address Out Label I’face Label Prefix 128. 89 1 128. 89 0 171. 69 1 … … Out In Address Out Label I’face Label Prefix 128. 89 0 … … 0 0 1 You Can Reach 128. 89 and 171. 69 Thru Me Routing Updates (OSPF, EIGRP, …) 128. 89 You Can Reach 128. 89 Thru Me 1 You Can Reach 171. 69 Thru Me 171. 69 30

Labels for Destination-Based Forwarding n A label is allocated for each prefix in its table q q n The label is chosen locally Think of them as indices into the routing table Router advertises this to its neighbors q “label distribution protocol” (LDP) n Packets addressed to the prefix should, for efficiency, be tagged with the label. n The label of an incoming packet is “swapped” before being forwarded to the next router. 31

MPLS Example: Assigning Labels Out In Address Out Label I’face Label Prefix - 128. 89 1 4 4 128. 89 0 9 - 171. 69 1 5 5 171. 69 1 7 … … … … Out In Address Out Label I’face Label Prefix 9 128. 89 0 - … … 0 128. 89 0 1 Use Label 9 for 128. 89 Use Label 4 for 128. 89 and Use Label 5 for 171. 69 Label Distribution Protocol (LDP) 1 Use Label 7 for 171. 69 (downstream allocation) 32

MPLS Example: Forwarding Packets Out In Address Out Label I’face Label Prefix - 128. 89 1 4 4 128. 89 0 9 - 171. 69 1 5 5 171. 69 1 7 … … … … Out In Address Out Label I’face Label Prefix 9 128. 89 0 - … … 0 128. 89 0 1 128. 89. 25. 4 9 128. 89. 25. 4 Data 1 128. 89. 25. 4 Data 4 128. 89. 25. 4 Data Label Switch Forwards Based on Label 33

MPLS Operation 1 a. Existing routing protocols (e. g. OSPF, IS-IS) establish reachability to destination networks. 1 b. Label Distribution Protocol (LDP) establishes label to destination network mappings. 2. Ingress Edge LSR receives packet, performs Layer 3 value-added services, and labels(PUSH) packets. 4. Edge LSR at egress removes(POP) label and delivers packet. 3. LSR switches packets using label swapping(SWAP). 34

Remarks n Rather than longest prefix-matching we use label matching q n Regular IP routing is still used q q n Labels can be very efficient, simply an index into the routing table E. g. , we could use OSPF to determine the routes Then we use labels for efficiency in per-hop routing Note that a “Setup” phase (like in VC’s) is not used 35

Placement of “labels” For Ethernet, the “protocol number used” is 0 x 8847 for MPLS I. e. , the “protocol number” of IP is not used. Thus, IP never sees the message! 36

Label Header 0 1 2 3 4 5 6 7 8 9 0 1 Label = 20 bits S = Bottom of Stack, 1 bit • • • EXP S TTL EXP = Class of Service, 3 bits TTL = Time to Live, 8 bits Header= 4 bytes, Label = 20 bits. Can be used over Ethernet, 802. 3, or PPP links Contains everything needed at forwarding time 37

Some Definitions n Forwarding Equivalence Class (FEC): a group of IP packets which are forwarded in the same manner (e. g. , over the same path, with the same forwarding treatment) n Labeled Switched Router (LSR): A router capable of supporting MPLS labels. n Labeled Switched Path: a sequence of LSR’s so that data can traverse the entire path using labels. 38

Traffic Aggregates: Forwarding Equivalence Classes LSR LER LSP IP 1 IP 2 IP 1 #L 1 IP 1 #L 2 IP 1 #L 3 IP 2 #L 1 IP 2 #L 2 IP 2 #L 3 IP 2 Packets are destined for different address prefixes, but can be mapped to common path • FEC = “A subset of packets that are all treated the same way by a router” • The concept of FECs provides for a great deal of flexibility and scalability • In conventional routing, a packet is assigned to a FEC at each hop (i. e. L 3 look-up), in MPLS it is only done once at the network ingress 39

Label Switched Path (LSP) IP 47. 1. 1. 1 1 47. 1 3 3 1 1 47. 3 3 2 2 47. 2 2 IP 47. 1. 1. 1 40

Label Merging n When multiple input streams corresponding to the same FEC exit using the same MPLS label. In. Label Next. Hop Label 10 Port 3 30 25 Port 3 30 Dest Next. Hop Label D Port 1 10 R 1 R 2 Netw D R 4 Port 3 Port 1 Port 5 Dest Next. Hop D Port 5 Label 25 R 3 41

Non-Label Merging n Each source-destination pair has its own label at each LSR router. In. Label Next. Hop Label 10 Port 3 5 25 Port 3 8 Dest Next. Hop Label D Port 1 10 R 1 R 2 Netw D R 4 Port 3 Port 1 Port 5 Dest Next. Hop D Port 5 Label 25 R 3 42

Pushing-Requesting Labels n n n R 2 can “push” a label to R 1, indicating which label to use to reach D R 1 can “request” a label from R 2 to be used to reach D. If using non-merging, usually R 1 requests a label from R 2 Netw D R 2 R 4 R 1 43

ATM n Most importantly, we can use ATM switches for IP n We can turn “ATM Cell switches” into “label switching routers” usually only by changing the software and not the hardware of the switch. 44

IP over ATM (Before MPLS) n We had every router with a VC over an ATM network to every other router q n Known as an “overlay” network Whole ATM network looked like a single “subnet” to the IP Routers q ATM switches are not aware that the payload is an IP packet 45

IP disassembly into ATM cells n n IP becomes an “application” to the ATM layer. IP packets have to be broken into small 48 -byte pieces, and placed into ATM Cells are sent over the ATM circuit (e. g. from R 1 to R 6), the switches only see ATM Cells, not IP packet At R 6, the cells are regrouped and the IP packet restored 46

ATM switches as LSRs (using MPLS) ATM switches are now “peers” of MPLS routers n n No longer viewed as a single subnet, each link is now a subnet 47

Advantages of MPLS vs overlay n Each MPLS router has fewer “adjacencies” (i. e. neighbors) q q n This reduces the OSPF traffic to the router significantly In OSPF you receive the topology of the entire network via each of your neighbors. Each router now has a view of the entire topology q q Not possible in overlay networks (ATM network “black box”) Routers have better control of paths in case of link failures n n In overlay networks, the ATM switches would do the rerouting ATM switches may still support native ATM if desired. 48

How to route IP packets? n Can we send IP messages to our neighbors? q q q We can use a special VCID (say 0) to send IP messages to our neighbor. Each node has a VCID 0 with each of its neighbors (a “single hop” VCID Thus, to send an IP message to a neighbor n n Disassemble the IP packet into ATM Cells Send them on VCID 0 of the link of the desired neighbor The neighbor reassembles the IP packet Since we can send an IP message to any neighbor q This implies ATM LSR’s can execute ANY Internet protocol based on IP (e. g. , OSPF, RIP, etc) and forward IP datagrams 49

End-to-end VC’s n Disassembly/reassembly at each hop is wasteful q n It is better to establish an e-2 -e VC for each source/destination pair, e. g. , from R 1 to R 6 From OSPF (or other mechanism), each router knows which other router is ATM or regular router q q q R 1 “requests” a label from LSR 1 for destination R 6 LSR 1 requests a label from LSR 3 for destination R 6 LSR 3 requests a label from R 6 50

Explicit Routing n n Similar to “source routing” but done by a router “Fish” network due to its shape R 1 -> R 7 : R 1 R 3 R 6 R 7 R 2 -> R 7 : R 2 R 3 R 4 R 5 R 7 q n Perhaps we want to balance the load somehow Cannot be done with regular IP q IP routing does not look at the source of the message 51

Explicitly Routed (ER-) LSP Route= {A, B, C} #14 #216 #972 B A #14 #972 C #462 ER-LSP follows route that source chooses. In other words, the control message to establish the LSP (label request) is source routed. 52

Explicitly Routed (ER-) LSP Contd IP 47. 1. 1. 1 1 47. 1 3 1 1 47. 3 3 3 2 2 47. 2 2 IP 47. 1. 1. 1 53

Explicit Route Advantages n Traffic Engineering q q n You can control how much traffic travels through some point in the network This is done by controlling the paths taken by traffic Fast-rerouting q You can bypass broken links quickly with explicit routing. n q No need to wait for a routing protocol (OSPF) to react. How? n n Keep track of two paths, regular path and backup path If the regular path fails use the backup 54

Virtual Private Networks n n We can do VPN’s with MPLS. Virtual Private Network q q q A group of connected networks Connections may be over multiple networks not belonging to the group (e. g. over the Internet) E. g. , joining the networks of several branches of a company into a “private internetwork” 55

Virtual Private Networks C A B K L M C K L A B M 56

Tunneling n IP Tunnel q Virtual point-to-point link between an arbitrarily connected pair of nodes Network 1 R 1 Internetwork Network 2 R 2 IP Tunnel 10. 0. 0. 1 IP Dest = 2. x IP Payload 57

Tunneling n Advantages of tunneling q q n Transparent transmission of packets over heterogeneous networks n The data carried may not even be IP messages! n n Multicast Local Addresses Only need to change relevant routers (end points) Coupled with encryption, gives you a secure private internetwork. End-points of tunnels my have features not available in other Internet routers. Disadvantages q q q Increases packet size Processing time needed to encapsulate and decapsulate packets Management at tunnel-aware routers 58

Virtual Private Networks with MPLS n n n We can do similarly with MPLS We can connect different sites with an MPLS tunnel We can send regular IP traffic through the tunnel, or any other type of traffic. 59

“Layer 2” tunnel n Use MPLS to provide a tunnel between two q q n LANs (Ethernet, etc) ATM points Any data can be “wrapped” with a label q q It need not be IP datagrams LSR does not look “beyond” the label 60

Demultiplexing Label n What to do with the packet once it reaches the other side of the tunnel? q A “demultiplexing” label needs to be added to inform the end-point router what to do with the packet. 61

E. g. , Emulate a VC n ATM cells with a specific VCID come in at the entrance of the tunnel n ATM cells at the end of the tunnel should have the appropriate VCID for the next switch after the router. 62

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Emulate a VC (steps) 1. 2. 3. 4. 5. 6. An ATM cell arrives to the input LSR with VCID 101 The head router attaches the demultiplexing label and identifies the emulated circuit The head router attaches the tunnel label (to reach the tail router) Routers in the middle forward as usual The tail router removes the tunnel label, finds the demultiplexing label, and identifies the VC The tail router modifies the VCID to the next ATM switch value (202) and sends it to the ATM switch. 64

Label Stacks n The previous example has a stack of two labels n You can have larger stacks of labels in the header. n In the example q q It enables to have a tunnel And many types of traffic within the tunnel 65

“Layer 3” VPN’s n The packet being carried is an IP packet q n Hence the name “layer 3” VPNs Service provider (see picture next. . ) q q Has many customers Each customer has many sites n q These sites are linked with tunnels to appear to be one large Internetwork Each customer can only reach its own sites n The customer is isolated from the rest of the Internet and from other customers 66

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