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Transcript
Virtual-Circuit Switching: ATM (Asynchronous Transmission Mode) and MPLS (Multiprotocol Label Switching) 2007. 10 Virtual Circuit (VC) Switching Hybrid of packets and circuits Circuits: establish and teardown along end-toend path Packets: divide the data into packets with identifiers Packets carry a virtual-circuit identifier Associates each packet with the virtual circuit Determines the next link along the path Intermediate nodes maintain state VC Forwarding table entry Allocated resources Timing of Virtual-Circuit Packet Switching Host 1 Node 1 Host 2 Node 2 propagation delay between Host 1 and Node 1 VC establishment Packet 1 Packet 2 Packet 1 Data transfer Packet 3 Packet 2 Packet 3 Packet 1 Packet 2 Packet 3 VC termination Establishing the Circuit Signaling Creating the entries in the forwarding tables Reserving resources for the virtual circuit, if needed Two main approaches to signaling Network administrator configures each node Source sends set-up message along the path Set-up latency Time for the set-up message to traverse the path … and return back to the source Routing End-to-end path is selected during circuit set-up Virtual Circuit Identifier (VC ID) Virtual Circuit Identifier (VC ID) Source set-up: establish path for the VC Switch: mapping VC ID to an outgoing link Packet: fixed length label in the header 1 2 1: 7 2: 7 link 7 1: 14 2: 8 link 14 link 8 Swapping the Label at Each Hop Problem: using VC ID along the whole path Each virtual circuit consumes a unique ID Starts to use up all of the ID space in the network Label swapping Map the VC ID to a new value at each hop Table has old ID, and next link and new ID 1 2 1: 7, 20 20: 14, 78 link 7 2: 7, 53 53: 8, 42 link 14 link 8 Virtual Circuits Similar to IP Datagrams Data divided in to packets Sender divides the data into packets Packet has address (e.g., IP address or VC ID) Store-and-forward transmission Multiple packets may arrive at once Need buffer space for temporary storage Multiplexing on a link No reservations: statistical multiplexing • Packets are interleaved without a fixed pattern Reservations: resources for group of packets • Guarantees to get a certain number of “slots” Virtual Circuits Differ from IP Datagrams Forwarding look-up Virtual circuits: fixed-length connection id IP datagrams: destination IP address Initiating data transmission Virtual circuits: must signal along the path IP datagrams: just start sending packets Router state Virtual circuits: routers know about connections IP datagrams: no state, easier failure recovery Quality of service Virtual circuits: resources and scheduling per VC IP datagrams: difficult to provide QoS Asynchronous Transfer Mode: ATM 1990’s/00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architecture Goal: integrated, end-end transport of carry voice, video, data meeting timing/QoS requirements of voice, video (versus Internet best-effort model) “next generation” telephony: technical roots in telephone world packet-switching (fixed length packets, called “cells”) using virtual circuits ATM reference model ATM architecture adaptation layer: only at edge of ATM network data segmentation/reassembly roughly analagous to Internet transport layer ATM layer: “network” layer cell switching, routing physical layer ATM Physical Layer Physical Medium Dependent (PMD) sublayer SONET/SDH: transmission frame structure (like a container carrying bits); bit synchronization; bandwidth partitions (TDM); several speeds: OC3 = 155.52 Mbps; OC12 = 622.08 Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps TI/T3: transmission frame structure (old telephone hierarchy): 1.5 Mbps/ 45 Mbps unstructured: just cells (busy/idle) ATM Physical Layer (more) Two pieces (sublayers) of physical layer: Transmission Convergence Sublayer (TCS): adapts ATM layer above to PMD sublayer below Physical Medium Dependent (PMD) : depends on physical medium being used TCS Functions: Header checksum generation: 8 bits CRC Cell delineation With “unstructured” PMD sublayer, transmission of idle cells when no data cells to send ATM Layer: Virtual Circuits analogous to IP network layer very different services than IP network layer VC transport: cells carried on VC from source to dest 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. ATM VCs Advantages of ATM VC approach: QoS performance guarantee for connection mapped to VC (bandwidth, delay, delay jitter) Drawbacks of ATM VC approach: Inefficient support of datagram traffic VC introduces call setup latency, processing overhead for short lived connections ATM Layer: ATM cell 5-byte ATM cell header 48-byte payload Why?: small payload -> short cell-creation delay for digitized voice halfway between 32 and 64 (compromise!) Cell header Cell format ATM cell header VCI: virtual channel ID will change from link to link thru net PT: Payload type (e.g. RM cell versus data cell) CLP: Cell Loss Priority bit CLP = 1 implies low priority cell, can be discarded if congestion HEC: Header Error Checksum cyclic redundancy check ATM Service 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 no constant yes rate guaranteed yes rate guaranteed no minimum none no no no yes yes yes yes yes no no (inferred via loss) no congestion no congestion yes yes no no ATM Adaptation Layer (AAL) 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 analogy: TCP segment in many IP packets ATM Adaptation Layer (AAL) [more] Different versions of AAL layers, depending on ATM service class: AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation (phone) AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video AAL5: for data (eg, IP datagrams) User data AAL PDU ATM cell IP-Over-ATM app transport IP Eth phy IP AAL Eth ATM phy phy ATM phy ATM phy app transport IP AAL ATM phy How far along are we? Standardization bodies - ATM Forum, ITU-T We may never see end-to-end ATM (1997) Backbone: - 1995 vBNS (ATM) - 1998 Abilene (SONET) - 2000 IP over DWDM ATM - too complex - too expansive <IP> Internet technology + ATM philosophy but ATM ideas continue to powerfully influence design of next-generation Internet ex: MPLS, admission ctl., resource reservation, …... Multiprotocol label switching (MPLS) initial goal: speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) but IP datagram still keeps IP address! PPP or Ethernet MPLS header header label 20 IP header Exp S TTL 3 1 5 remainder of link-layer frame Label Substitution Have a friend go to B ahead of you using one of the previous two techniques. At every road they reserve a lane just for you. At every intersection they post a big sign that says for a given lane which way to turn and what new lane to take. Label Encapsulation MPLS Encapsulation is specified over various media types. Top labels may use existing format, lower label(s) use a new “shim” label format. MPLS Link Layers MPLS -- run over multiple link layers Following link layers currently exist: • ATM: label -- in VCI/VPI field of ATM header • Frame Relay: label -- in DLCI field in FR header • PPP/LAN: uses ‘shim’ header inserted between L2 and L3 headers Translation between link layers types must be supported MPLS is between L2 and L3 It intended to be “multi-protocol” below and above MPLS capable routers a.k.a. label-switched router forwards packets to outgoing interface based only on label value (don’t inspect IP address) MPLS forwarding table distinct from IP forwarding tables signaling protocol needed to set up forwarding Hop-by-hop or source routing to establish labels forwarding possible along paths that IP alone would not allow (e.g., source-specific routing) !! use MPLS for traffic engineering RSVP-TE must co-exist with IP-only routers MPLS forwarding tables in label out label out interface dest 10 A 0 12 8 D A 0 1 in label out label 10 6 A 12 9 D dest out interface 1 0 R6 0 0 D 1 1 R3 R4 R5 0 0 R2 in label out label dest 8 6 A out interface 0 in label 6 out R1 label dest - A A out interface 0 Best of Both Worlds MPLS + IP form a middle ground that combines the best of IP and the best of virtual circuit switching technologies ATM and Frame Relay cannot easily come to the middle so IP has! Multi-Protocol Label Switching Key ideas of MPLS Label-switched path spans group of routers Explicit path set-up, including backup paths Flexible mapping of data traffic to paths Motivating applications Small routing tables and fast look-ups Virtual Private Networks Traffic engineering Path protection and fast reroute Status of MPLS Deployed in practice Small control and data plane overhead in core Virtual Private Networks Traffic engineering and fast reroute Challenges Protocol complexity Configuration complexity Difficulty of collecting measurement data Continuing evolution Standards Operational practices and tools Optical Networks 1 st Generation: optical fibers substitute copper as physical layer Submarine Systems SONET (synchronous optical) in TDM FDDI for LAN, Gbit Ethernet etc. 2 nd Generation: optical switching and multiplexing/ WDM broadcast-and-select networks WDM rings wavelength routing networks 3 th Generation: optical packet switching??? Optical Switch 1-input 2-outoput illustration with four wavelengths Input & Output fiber array Wavelength Dispersive Element 1-D MEMS Micro-mirror Array Input Fiber Output Fiber 1 1011 Digital Mirror Control Electronics Output Fiber 2 1-D MEMS (micro-electromechanical system) with dispersive optics Dispersive element separates the ’s from inputs MEMS independently switches each Dispersive element recombines the switched ’s into outputs All-Optical Switching Optical Cross-Connects (OXC) Wavelength Routing Switches (WRS) route a channel from any I/P port to any O/P port Natively switch s while they are still multiplexed Eliminate redundant optical-electronic-optical conversions DWDM Demux DWDM Fibers in DWDM Mux DWDM Fibers out All-optical DWDM Demux OXC DWDM Mux MPS MPS = Multi-Protocol Lambda Switching MPLS + OXC Combining MPLS traffic eng control with OXC All packets with one label are sent on one wavelength Next Hop Forwarding Label Entry (NHFLE) <Input port, > to <output port, > mapping