Link Layer and LANs

Link Layer and LANs

Chapter Link Layer and LANs A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers) They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs They obviously represent a lot of work on our part In return for use, we only ask the following:  If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)  If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material Thanks and enjoy! JFK/KWR Computer Networking: A Top Down Approach Featuring the Internet, 3rd edition Jim Kurose, Keith Ross Addison-Wesley, July 2004 All material copyright 1996-2006 J.F Kurose and K.W Ross, All Rights Reserved 5: DataLink Layer 5-1 Chapter 5: The Data Link Layer Our goals: Ì understand principles behind data link layer services: r r r r error detection, correction sharing a broadcast channel: multiple access link layer addressing reliable data transfer, flow control: done! Ì instantiation and implementation of various link layer technologies 5: DataLink Layer 5-2 Link Layer Ì 5.1 Introduction and Ì Ì Ì Ì services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet Ì 5.6 Hubs and switches Ì 5.7 PPP Ì 5.8 Link Virtualization: ATM and MPLS 5: DataLink Layer 5-3 Link Layer: Introduction Some terminology: “link” Ì hosts and routers are nodes Ì communication channels that connect adjacent nodes along communication path are links r r r wired links wireless links LANs Ì layer-2 packet is a frame, encapsulates datagram data-link layer has responsibility of transferring datagram from one node to adjacent node over a link 5: DataLink Layer 5-4 Link layer: context Ì Datagram transferred by different link protocols over different links: r e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last link Ì Each link protocol provides different services r e.g., may or may not provide rdt over link transportation analogy Ì trip from Princeton to Lausanne r limo: Princeton to JFK r plane: JFK to Geneva r train: Geneva to Lausanne Ì tourist = datagram Ì transport segment = communication link Ì transportation mode = link layer protocol Ì travel agent = routing algorithm 5: DataLink Layer 5-5 Link Layer Services Ì Framing, link access: r r r encapsulate datagram into frame, adding header, trailer channel access if shared medium “MAC” addresses used in frame headers to identify source, dest • different from IP address! Ì Reliable delivery between adjacent nodes r we learned how to this already (chapter 3)! r seldom used on low bit error link (fiber, some twisted pair) r wireless links: high error rates • Q: why both link-level and end-end reliability? 5: DataLink Layer 5-6 Link Layer Services (more) Ì Flow Control: r Ì pacing between adjacent sending and receiving nodes Error Detection: r r errors caused by signal attenuation, noise receiver detects presence of errors: • signals sender for retransmission or drops frame Ì Error Correction: r receiver identifies and corrects bit error(s) without resorting to retransmission Ì Half-duplex and full-duplex r with half duplex, nodes at both ends of link can transmit, but not at same time 5: DataLink Layer 5-7 Adaptors Communicating datagram sending node rcving node link layer protocol frame frame adapter adapter Ì link layer implemented in Ì receiving side r looks for errors, rdt, flow “adaptor” (aka NIC) control, etc r Ethernet card, PCMCI r extracts datagram, passes card, 802.11 card to rcving node Ì sending side: r r encapsulates datagram in a Ì frame adds error checking bits, Ì rdt, flow control, etc adapter is semiautonomous link & physical layers 5: DataLink Layer 5-8 Link Layer Ì 5.1 Introduction and Ì Ì Ì Ì services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet Ì 5.6 Hubs and switches Ì 5.7 PPP Ì 5.8 Link Virtualization: ATM 5: DataLink Layer 5-9 Error Detection EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields • Error detection not 100% reliable! • protocol may miss some errors, but rarely • larger EDC field yields better detection and correction 5: DataLink Layer 5-10 ATM Layer Service: transport cells across ATM network Ì analogous to IP network layer Ì 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 rate guaranteed minimum none no no no yes yes yes yes yes yes no yes no no (inferred via loss) no congestion no congestion yes no yes no no 5: DataLink Layer 5-95 ATM Layer: Virtual Circuits Ì VC transport: cells carried on VC from source to dest r call setup, teardown for each call before data can flow r each packet carries VC identifier (not destination ID) r every switch on source-dest path maintain “state” for each passing connection r link,switch resources (bandwidth, buffers) may be allocated to VC: to get circuit-like perf Ì Permanent VCs (PVCs) long lasting connections r typically: “permanent” route between to IP routers Ì Switched VCs (SVC): r dynamically set up on per-call basis r 5: DataLink Layer 5-96 ATM VCs Ì Advantages of ATM VC approach: QoS performance guarantee for connection mapped to VC (bandwidth, delay, delay jitter) Ì Drawbacks of ATM VC approach: r Inefficient support of datagram traffic r one PVC between each source/dest pair) does not scale (N*2 connections needed) r SVC introduces call setup latency, processing overhead for short lived connections r 5: DataLink Layer 5-97 ATM Layer: ATM cell Ì 5-byte ATM cell header Ì 48-byte payload Why?: small payload -> short cell-creation delay for digitized voice r halfway between 32 and 64 (compromise!) r Cell header Cell format 5: DataLink Layer 5-98 ATM cell header Ì VCI: virtual channel ID will change from link to link thru net Ì PT: Payload type (e.g RM cell versus data cell) Ì CLP: Cell Loss Priority bit r CLP = implies low priority cell, can be discarded if congestion Ì HEC: Header Error Checksum r cyclic redundancy check r 5: DataLink Layer 5-99 ATM Physical Layer (more) Two pieces (sublayers) of physical layer: Ì Transmission Convergence Sublayer (TCS): adapts ATM layer above to PMD sublayer below Ì Physical Medium Dependent: depends on physical medium being used TCS Functions: r Header checksum generation: bits CRC r Cell delineation r With “unstructured” PMD sublayer, transmission of idle cells when no data cells to send 5: DataLink Layer 5-100 ATM Physical Layer Physical Medium Dependent (PMD) sublayer Ì SONET/SDH: transmission frame structure (like a container carrying bits); r bit synchronization; r bandwidth partitions (TDM); r several speeds: OC3 = 155.52 Mbps; OC12 = 622.08 Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps Ì TI/T3: transmission frame structure (old telephone hierarchy): 1.5 Mbps/ 45 Mbps Ì unstructured: just cells (busy/idle) 5: DataLink Layer 5-101 IP-Over-ATM Classic IP only Ì “networks” (e.g., LAN segments) Ì MAC (802.3) and IP addresses IP over ATM Ì replace “network” (e.g., LAN segment) with ATM network Ì ATM addresses, IP addresses ATM network Ethernet LANs Ethernet LANs 5: DataLink Layer 5-102 IP-Over-ATM app transport IP Eth phy IP AAL Eth ATM phy phy ATM phy ATM phy app transport IP AAL ATM phy 5: DataLink Layer 5-103 Datagram Journey in IP-over-ATM Network Ì at Source Host: r IP layer maps between IP, ATM dest address (using ARP) r passes datagram to AAL5 r AAL5 encapsulates data, segments cells, passes to ATM layer Ì ATM network: moves cell along VC to destination Ì at Destination Host: AAL5 reassembles cells into original datagram r if CRC OK, datagram is passed to IP r 5: DataLink Layer 5-104 IP-Over-ATM Issues: Ì IP datagrams into ATM AAL5 PDUs Ì from IP addresses to ATM addresses r just like IP addresses to 802.3 MAC addresses! ATM network Ethernet LANs 5: DataLink Layer 5-105 Multiprotocol label switching (MPLS) Ì initial goal: speed up IP forwarding by using fixed length label (instead of IP address) to forwarding r r borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address! PPP or Ethernet header MPLS header label 20 IP header remainder of link-layer frame Exp S TTL 5: DataLink Layer 5-106 MPLS capable routers Ì a.k.a label-switched router Ì forwards packets to outgoing interface based only on label value (don’t inspect IP address) r MPLS forwarding table distinct from IP forwarding tables Ì signaling protocol needed to set up forwarding r RSVP-TE r forwarding possible along paths that IP alone would not allow (e.g., source-specific routing) !! r use MPLS for traffic engineering Ì must co-exist with IP-only routers 5: DataLink Layer 5-107 MPLS forwarding tables in label out label dest 10 12 out interface A D A 0 in label out label dest out interface R4 R5 A 12 R6 10 D 0 R3 D A outR1 label dest out interface R2 in label out label dest A out interface in label - A 5: DataLink Layer 5-108 Chapter 5: Summary Ì principles behind data link layer services: r error detection, correction r sharing a broadcast channel: multiple access r link layer addressing Ì instantiation and implementation of various link layer technologies r Ethernet r switched LANS r PPP r virtualized networks as a link layer: ATM, MPLS 5: DataLink Layer 5-109 ... 5.4 Link- Layer Addressing 5.5 Ethernet Ì 5.6 Hubs and switches Ì 5.7 PPP Ì 5.8 Link Virtualization: ATM and MPLS 5: DataLink Layer 5-3 Link Layer: Introduction Some terminology: ? ?link? ?? Ì hosts and. .. semiautonomous link & physical layers 5: DataLink Layer 5-8 Link Layer Ì 5.1 Introduction and Ì Ì Ì Ì services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link- Layer Addressing... over a link 5: DataLink Layer 5-4 Link layer: context Ì Datagram transferred by different link protocols over different links: r e.g., Ethernet on first link, frame relay on intermediate links,