1 ATM and MPLS ECS 152A

2 Virtualization of networks Virtualization of resources: a powerful abstraction in systems engineering: r computing examples: virtual memory, virtual devices m Virtual machines: e.g., java m IBM VM os from 1960’s/70’s r layering of abstractions: don’t sweat the details of the lower layer, only deal with lower layers abstractly

3 The Internet: virtualizing networks 1974: multiple unconnected nets m ARPAnet m data-over-cable networks m packet satellite network (Aloha) m packet radio network … differing in: m addressing conventions m packet formats m error recovery m routing ARPAnet satellite net "A Protocol for Packet Network Intercommunication", V. Cerf, R. Kahn, IEEE Transactions on Communications, May, 1974, pp

4 The Internet: virtualizing networks ARPAnet satellite net gateway Internetwork layer (IP): r addressing: internetwork appears as a single, uniform entity, despite underlying local network heterogeneity r network of networks Gateway: r “embed internetwork packets in local packet format or extract them” r route (at internetwork level) to next gateway

5 Cerf & Kahn’s Internetwork Architecture What is virtualized? r two layers of addressing: internetwork and local network r new layer (IP) makes everything homogeneous at internetwork layer r underlying local network technology m cable m satellite m 56K telephone modem m today: ATM, MPLS … “invisible” at internetwork layer. Looks like a link layer technology to IP!

6 ATM and MPLS r ATM, MPLS separate networks in their own right m different service models, addressing, routing from Internet r viewed by Internet as logical link connecting IP routers m just like dialup link is really part of separate network (telephone network) r ATM, MPSL: packet-switched, virtual circuit networks

7 Virtual circuits r call setup, teardown for each call before data can flow r each packet carries VC identifier (not destination host address) r every router on source-dest path maintains “state” for each passing connection r link, router resources (bandwidth, buffers) may be allocated to VC “source-to-dest path behaves much like telephone circuit” m performance-wise m network actions along source-to-dest path

8 VC implementation A VC consists of: 1. Path from source to destination 2. VC numbers, one number for each link along path 3. Entries in forwarding tables in routers along path r Packet belonging to VC carries a VC number. r VC number must be changed on each link. m New VC number comes from forwarding table

9 Forwarding table VC number interface number Incoming interface Incoming VC # Outgoing interface Outgoing VC # … … Forwarding table in northwest router: Routers maintain connection state information!

10 Virtual circuits: signaling protocols r used to setup, maintain teardown VC r used in ATM, frame-relay, X.25 r not used in today’s Internet application transport network data link physical application transport network data link physical 1. Initiate call 2. incoming call 3. Accept call 4. Call connected 5. Data flow begins 6. Receive data

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

12 ATM architecture r ATM adaptation layer: only at edge of ATM network m data segmentation/reassembly m roughly analogous to Internet transport layer r ATM layer: “network” layer m cell switching, routing r physical layer

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

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

15 ATM Adaptation Layer (AAL) [more] Different versions of AAL layers, depending on ATM service class: r AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation r AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video r AAL5: for data (eg, IP datagrams) AAL PDU ATM cell User data

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

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

18 ATM VCs r Advantages of ATM VC approach: m QoS performance guarantee for connection mapped to VC (bandwidth, delay, delay jitter) r Drawbacks of ATM VC approach: m Inefficient support of datagram traffic m one PVC between each source/dest pair) does not scale (N*(N-1) connections needed) m SVC introduces call setup latency, processing overhead for short lived connections

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

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

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

22 ATM Physical Layer Physical Medium Dependent (PMD) sublayer r SONET/SDH: transmission frame structure (like a container carrying bits); m bit synchronization; m bandwidth partitions (TDM); m several speeds: OC3 = Mbps; OC12 = Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps r TI/T3: transmission frame structure (old telephone hierarchy): 1.5 Mbps/ 45 Mbps r unstructured: just cells (busy/idle)

23 IP-Over-ATM Classic IP only r 3 “networks” (e.g., LAN segments) r MAC (802.3) and IP addresses IP over ATM r replace “network” (e.g., LAN segment) with ATM network r ATM addresses, IP addresses ATM network Ethernet LANs Ethernet LANs

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

25 Datagram Journey in IP-over-ATM Network r at Source Host: m IP layer maps between IP, ATM dest address (using ARP) m passes datagram to AAL5 m AAL5 encapsulates data, segments cells, passes to ATM layer r ATM network: moves cell along VC to destination r at Destination Host: m AAL5 reassembles cells into original datagram m if CRC OK, datagram is passed to IP

26 IP-Over-ATM Issues: r IP datagrams into ATM AAL5 PDUs r from IP addresses to ATM addresses m just like IP addresses to MAC addresses! ATM network Ethernet LANs

27 Multiprotocol label switching (MPLS) r initial goal: speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding m borrowing ideas from Virtual Circuit (VC) approach m but IP datagram still keeps IP address! PPP or Ethernet header IP header remainder of link-layer frame MPLS header label Exp S TTL

28 MPLS capable routers r a.k.a. label-switched router r forwards packets to outgoing interface based only on label value (don’t inspect IP address) m MPLS forwarding table distinct from IP forwarding tables r signaling protocol needed to set up forwarding m RSVP-TE m forwarding possible along paths that IP alone would not allow (e.g., source-specific routing) !! m use MPLS for traffic engineering r must co-exist with IP-only routers

29 R1 R2 D R3 R4 R A R6 in out out label label dest interface 6 - A 0 in out out label label dest interface 10 6 A D 0 in out out label label dest interface 10 A 0 12 D 0 1 in out out label label dest interface 8 6 A A 1 MPLS forwarding tables

30 Chapter 5: Summary r principles behind data link layer services: m error detection, correction m sharing a broadcast channel: multiple access m link layer addressing r instantiation and implementation of various link layer technologies m Ethernet m switched LANS m PPP m virtualized networks as a link layer: ATM, MPLS