5 Data Link Layer Data Link Layer Self-learning, Interconnecting - PDF document

Data Link Layer Data Link Layer Hubs Switch physical- layer (“dumb”) repeaters:  bits coming in one link go out all other links at same  link-layer device: smarter than hubs, take rate active role  all nodes connected to hub can collide with one  store, forward Ethernet frames another  examine incoming frame’s MAC address,  no frame buffering selectively forward frame to one-or-more  no CSMA/CD at hub: host NICs detect collisions outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment  transparent  hosts are unaware of presence of switches twisted pair  plug-and-play, self-learning hub  switches do not need to be configured COMNETS @CSE COMNETS @CSE 25 26 Data Link Layer Data Link Layer Switch: allows multiple simultaneous Switch Table transmissions A A  Q: how does switch know that  hosts have dedicated, C’ A’ reachable via interface 4, C’ B B direct connection to switch B’ reachable via interface 5?  switches buffer packets  A: each switch has a switch 1 2 3 1 2 3  Ethernet protocol used on 6 6 table, each entry: each incoming link, but no 4 4 5 5  (MAC address of host, interface collisions; full duplex to reach host, time stamp) C C  each link is its own collision  looks like a routing table! domain B’ B’  switching: A-to- A’ and B - A’ A’  Q: how are entries created, to- B’ simultaneously, maintained in switch table? switch with six interfaces switch with six interfaces without collisions  something like a routing (1,2,3,4,5,6) (1,2,3,4,5,6)  not possible with dumb hub protocol? COMNETS @CSE COMNETS @CSE 27 28 Data Link Layer Data Link Layer Switch: frame filtering/forwarding Switch: self-learning Source: A Dest: A’ When frame received: A A’ A  switch learns which hosts can be reached through C’ 1. record link associated with sending host B which interfaces 2. index switch table using MAC dest address  when frame received, 1 2 3 3. if entry found for destination 6 switch “learns” location of then { 4 sender: incoming LAN 5 segment if dest on segment from which frame arrived C  records sender/location then drop the frame pair in switch table B’ else forward the frame on interface indicated A’ } else flood Switch table forward on all but the interface MAC addr interface TTL (initially empty) on which the frame arrived A 1 60 COMNETS @CSE COMNETS @CSE 29 30 5

Data Link Layer Data Link Layer Self-learning, Interconnecting switches Source: A Dest: A’ forwarding: A A’ A switches can be connected together example C’ B S 4  frame destination unknown: flood 1 2 3 S 1 S 3 A A’ A A’ A A’ A A’ A A’ 6 S 2 A  destination A 4 5 F I D location known: B C C H G E A’ A selective send B’  Q: sending from A to G - how does S 1 know to A’ forward frame destined to F via S 4 and S 3 ? MAC addr interface TTL  A: self learning! (works exactly the same as in Switch table A 1 60 single-switch case!) A’ 4 60 (initially empty) COMNETS @CSE COMNETS @CSE 31 32 Data Link Layer Data Link Layer Self-learning multi-switch example Institutional network Suppose C sends frame to I, I responds to C mail server S 4 to external 1 network S 1 2 web server S 3 router S 2 A F I D B C IP subnet H G E  Q: show switch tables and packet forwarding in S 1 , S 2 , S 3 , S 4 COMNETS @CSE COMNETS @CSE 33 34 Data Link Layer Data Link Layer Switches vs. Routers Switches vs. Routers (2)  both store-and-forward devices  routers: network layer devices (examine network layer headers) Switch Router  switches are link layer devices Pro Con Pro Con  routers maintain routing tables , implement routing algorithms • Plug-and- • Broadcast • No • Not plug-  switches maintain switch tables , implement play storms broadcast and-play filtering, learning algorithms • Topology storms restricted • More to general spanning topologies tree COMNETS @CSE COMNETS @CSE 35 36 6

Data Link Layer Data Link Layer Link Layer Virtualization of networks  Virtualization of resources : powerful  1 Introduction and  5 Ethernet abstraction in systems engineering services  6 Link-layer switches  Computing examples: virtual memory,  2 Error detection  7 Link virtualization: virtual devices and correction ATM  Virtual machines: e.g., java  3 Multiple access  IBM VM os from 1960’s/70’s protocols  Layering of abstractions : don’t sweat the  4 Link-layer details of the lower layer, only deal with Addressing lower layers abstractly COMNETS @CSE COMNETS @CSE 37 38 Data Link Layer Data Link Layer Connecting different networks is a The idea of the Internet challenge 1974: multiple differing in: unconnected nets  addressing conventions  ARPAnet  packet formats gateway  data-over-cable networks  error recovery  packet satellite network  routing (Aloha)  packet radio network ARPAnet satellite net (1) Add a Gateway : (2) Add Internetwork layer (IP):  “embed internetwork packets in local packet format or extract  addressing: internetwork them” appears as single, uniform  route (at internetwork level) to entity, despite underlying next gateway local network heterogeneity ARPAnet satellite net  network of networks "A Protocol for Packet Network Intercommunication", V. Cerf, R. Kahn, IEEE Transactions on Communications, COMNETS @CSE COMNETS @CSE 39 40 May, 1974, pp. 637-648. Data Link Layer Data Link Layer Ethernet Central to the Internet: Virtualizing To IP, no difference! The new layer (IP) makes everything homogeneous at internetwork layer underlying local network technology => to a homogeneous link layer service that delivers an IP datagram from one host to another host Switch • cable Switch • satellite • 56K telephone modem • today: ATM PSTN • “ invisible ” at internetwork layer. • Looks like a link layer technology to IP! COMNETS @CSE COMNETS @CSE 41 42 7

Data Link Layer Data Link Layer ATM Asynchronous Transfer Mode: ATM  1990’s/00 standard for high -speed (155Mbps to  ATM separate networks in their own right 622 Mbps and higher) Broadband Integrated Service Digital Network architecture  different service models, addressing, routing from Internet  Goal: integrated, end-end transport of carry voice, video, data  viewed by Internet as logical link connecting  meeting timing/QoS requirements of voice, video IP routers (versus Internet best-effort model)  just like dialup link is really part of separate  “next generation” telephony: technical roots in network (telephone network) telephone world  ATM: of technical interest in their own right  packet-switching using virtual circuits (fixed length packets, called “cells”) COMNETS @CSE COMNETS @CSE 43 44 Data Link Layer Data Link Layer ATM architecture ATM Adaptation Layer (AAL)  ATM Adaptation Layer (AAL): “adapts” upper AAL AAL layers (IP or native ATM applications) to ATM ATM ATM ATM ATM layer below  AAL present only in end systems , not in switches physical physical physical physical  AAL layer segment (header/trailer fields, data) switch end system end system switch fragmented across multiple ATM cells  adaptation layer : only at edge of ATM network  analogy: TCP segment in many IP packets  data segmentation/reassembly  roughly analogous to Internet transport layer AAL AAL  ATM layer: “network” layer ATM ATM ATM ATM  cell switching, routing physical physical physical physical  physical layer switch switch end system end system COMNETS @CSE COMNETS @CSE 45 46 Data Link Layer Data Link Layer IP-Over-ATM ATM: network or link layer? IP over ATM Classic IP only  replace “network” (e.g., LAN segment)  3 “networks” (e.g., Vision: end-to-end with ATM network LAN segments) IP transport: “ATM from network  ATM addresses, IP desktop to desktop”  MAC (802.3) and IP ATM addresses addresses  ATM is a network network technology ATM Reality: used to connect network IP backbone routers  “IP over ATM”  ATM as switched link layer, connecting IP routers Ethernet Ethernet LANs LANs COMNETS @CSE COMNETS @CSE 47 48 8

Data Link Layer Data Link Layer IP-Over-ATM Homework #4 app transport app transport IP IP Page 528 IP AAL AAL Eth ATM Eth ATM phy phy phy phy ATM phy 2, 7, 8, 14, 15 ATM phy No submission required. COMNETS @CSE COMNETS @CSE 49 50 9