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Week 3 Virtual LANs, Wireless LANs, PPP, ATM

Week 3 Virtual LANs, Wireless LANs, PPP, ATM. Virtual LANs. It is the territory over which a broadcast or multicast packet is delievered (also known as a broadcast domain) The difference in a VLAN and a LAN, if there is any, is in packaging

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Week 3 Virtual LANs, Wireless LANs, PPP, ATM

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  1. Week 3Virtual LANs, Wireless LANs, PPP, ATM Week 3

  2. Virtual LANs • It is the territory over which a broadcast or multicast packet is delievered (also known as a broadcast domain) • The difference in a VLAN and a LAN, if there is any, is in packaging • Virtual LANs allow you to have separate LANs among ports on the same switch • For example, a switch might be told that ports 1-32 are in VLAN A and ports 33-64 are in VLAN B Week 3

  3. VLAN A Q Q V V J J A A R M M K K D D B B VLAN B Virtual and Physical LANs Week 3

  4. Why VLANs • IP requires that all nodes on a LAN share the same IP address prefix; therefore a node that moves to a different LAN must change its address • Changing IP addresses manually is annoying • IP broadcasts traffic within a LAN, something that can cause congestion in a large LAN • Routing IP (rather than bridging) was slow • It might be tempting to bridge everything making your whole topology one giant LAN from the perspective of IP and use layer 2 switches Week 3

  5. Disadvantages of one single LAN • Broadcast traffic (such as ARP) grows in proportion to the number of stations • Users can snoop on the traffic of other users on the same LAN, so it might be safer to isolate groups of users onto different LANs • Some protocols are overly chatty or they get into modes such as broadcast storms. • So it seems desirable for users that need to talk to each other a lot to be in the same LAN but keep other groups of users in separate LANs • A VLAN makes us broadcast domain as large as we want it Week 3

  6. Mapping ports to VLANs • The switch has ports 1 to k in one VLAN and has ports k+1 to 2k in another LAN • The switch can be configured with a port/VLAN mapping • The switch can be configured with a table of VLAN/MAC address mappings. It then dynamically determines the VLAN/port mapping based on the learned MAC address of the station attached to the port. • The switch can be configured with a table of VLAN/IP prefix mappings. It then dynamically determines the VLAN/port mapping based on the source IP address from the station attached to the port. • The switch can be configured with a table of VLAN/protocol mappings. It then dynamically determines the VLAN/port mappings based on the protocol type of the stations attached to the port. Week 3

  7. VLAN forwarding with separate router a.b.c.H f.g.k.Q h q 2 9 d 3 x 13 11 a.b.c.D 7 f.g.k.X j f a.b.c.R1 f.g.k.R2 Router R Router connects VLANs Week 3

  8. VLAN forwarding with switch as router • Router does not use up ports • The switch must know that R’s mac address on VLAN A is f and on VLAN B is j. a..b.c.H f.g.k.Q 9 2 Switch/ Router R VLAN A VLAN B 3 12 a..b.c.D f.g.k.X Week 3

  9. X.B Z.C X.A Z.D a b Q.F c Q.D Q.E Q.F Dynamic binding of links to VLANs The switch now learns that there are two VLANs on port a If enough stations move around, advantage disappears Week 3

  10. VLAN Tagging VLAN 2 VLAN 1 VLAN 2 VLAN 2 VLAN1 VLAN 1 Interswitch port; Packets can belong in either VLAN1 or VLAN2 IEEE standardized a scheme for VLAN tagging VLAN 1 VLAN 1 VLAN 2 Week 3

  11. 802.11b 2.4-5 GHz unlicensed radio spectrum up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code widely deployed, using base stations 802.11a 5-6 GHz range up to 54 Mbps 802.11g 2.4-5 GHz range up to 54 Mbps All use CSMA/CA for multiple access All have base-station and ad-hoc network versions IEEE 802.11 Wireless LAN Week 3

  12. AP AP Internet 802.11 LAN architecture • wireless host communicates with base station • base station = access point (AP) • Basic Service Set (BSS) (aka “cell”) in infrastructure mode contains: • wireless hosts • access point (AP): base station • ad hoc mode: hosts only hub, switch or router BSS 1 BSS 2 Week 3

  13. 802.11: Channels, association • 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies • AP admin chooses frequency for AP • interference possible: channel can be same as that chosen by neighboring AP! • host: must associate with an AP • scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address • selects AP to associate with • may perform authentication • will typically run DHCP to get IP address in AP’s subnet Week 3

  14. B A C C C’s signal strength A’s signal strength B A space IEEE 802.11: multiple access • avoid collisions: 2+ nodes transmitting at same time • 802.11: CSMA - sense before transmitting • don’t collide with ongoing transmission by other node • 802.11: no collision detection! • difficult to receive (sense collisions) when transmitting due to weak received signals (fading) • can’t sense all collisions in any case: hidden terminal, fading • goal: avoid collisions: CSMA/C(ollision)A(voidance) Week 3

  15. DIFS data SIFS ACK IEEE 802.11 MAC Protocol: CSMA/CA 802.11 sender 1 if INITIALLY sense channel idle for DIFSthen transmit entire frame (no CD) 2 ifsense channel busy then start random backoff time timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval, repeat 2 802.11 receiver - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) sender receiver Week 3

  16. Avoiding collisions (more) idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames • sender first transmits small request-to-send (RTS) packets to BS using CSMA • RTSs may still collide with each other (but they’re short) • BS broadcasts clear-to-send CTS in response to RTS • RTS heard by all nodes • sender transmits data frame • other stations defer transmissions Avoid data frame collisions completely using small reservation packets! Week 3

  17. RTS(B) RTS(A) reservation collision RTS(A) CTS(A) CTS(A) DATA (A) ACK(A) ACK(A) Collision Avoidance: RTS-CTS exchange B A AP defer time Week 3

  18. 6 4 2 2 6 6 6 2 0 - 2312 frame control duration address 1 address 2 address 3 address 4 payload CRC seq control 802.11 frame: addressing Address 4: used only in ad hoc mode Address 1: MAC address of wireless host or AP to receive this frame Address 3: MAC address of router interface to which AP is attached Address 2: MAC address of wireless host or AP transmitting this frame Week 3

  19. router AP Internet R1 MAC addr AP MAC addr source address dest. address 802.3frame AP MAC addr H1 MAC addr R1 MAC addr address 3 address 2 address 1 802.11 frame 802.11 frame: addressing H1 R1 Week 3

  20. 6 4 2 2 6 6 6 2 0 - 2312 frame control duration address 1 address 2 address 3 address 4 payload CRC seq control 2 2 4 1 1 1 1 1 1 1 1 Protocol version Type Subtype To AP From AP More frag Retry Power mgt More data WEP Rsvd 802.11 frame: more frame seq # (for reliable ARQ) duration of reserved transmission time (RTS/CTS) frame type (RTS, CTS, ACK, data) Week 3

  21. H1 remains in same IP subnet: IP address can remain same switch: which AP is associated with H1? self-learning (Ch. 5): switch will see frame from H1 and “remember” which switch port can be used to reach H1 router 802.11: mobility within same subnet hub or switch BBS 1 AP 1 AP 2 H1 BBS 2 Week 3

  22. Point to Point Data Link Control • one sender, one receiver, one link: easier than broadcast link: • no Media Access Control • no need for explicit MAC addressing • e.g., dialup link, ISDN line • popular point-to-point DLC protocols: • PPP (point-to-point protocol) • HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack! Week 3

  23. PPP Design Requirements [RFC 1557] • packet framing: encapsulation of network-layer datagram in data link frame • carry network layer data of any network layer protocol (not just IP) at same time • ability to demultiplex upwards • bit transparency: must carry any bit pattern in the data field • error detection (no correction) • connection liveness: detect, signal link failure to network layer • network layer address negotiation: endpoint can learn/configure each other’s network address Week 3

  24. PPP non-requirements • no error correction/recovery • no flow control • out of order delivery OK • no need to support multipoint links (e.g., polling) Error recovery, flow control, data re-ordering all relegated to higher layers! Week 3

  25. PPP Data Frame • Flag: delimiter (framing) • Address: does nothing (only one option) • Control: does nothing; in the future possible multiple control fields • Protocol: upper layer protocol to which frame delivered (eg, PPP-LCP, IP, IPCP, etc) Week 3

  26. PPP Data Frame • info: upper layer data being carried • check: cyclic redundancy check for error detection Week 3

  27. Byte Stuffing • “data transparency” requirement: data field must be allowed to include flag pattern <01111110> • Q: is received <01111110> data or flag? • Sender: adds (“stuffs”) extra < 01111101> byte before each < 01111110> data byte • Receiver: • 01111101 and 01111110 bytes in a row: discard first byte, continue data reception • single 01111110: flag byte Week 3

  28. Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data Week 3

  29. PPP Data Control Protocol Before exchanging network-layer data, data link peers must • configure PPP link (max. frame length, authentication) • learn/configure network layer information • for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address Week 3

  30. 5.1 Introduction and services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet 5.6 Hubs and switches 5.7 PPP 5.8 Link Virtualization: ATM and MPLS Link Layer Week 3

  31. Virtualization of networks Virtualization of resources: a powerful abstraction in systems engineering: • computing examples: virtual memory, virtual devices • Virtual machines: e.g., java • IBM VM os from 1960’s/70’s • layering of abstractions: don’t sweat the details of the lower layer, only deal with lower layers abstractly Week 3

  32. The Internet: virtualizing networks … differing in: • addressing conventions • packet formats • error recovery • routing 1974: multiple unconnected nets • ARPAnet • data-over-cable networks • packet satellite network (Aloha) • packet radio network satellite net ARPAnet "A Protocol for Packet Network Intercommunication", V. Cerf, R. Kahn, IEEE Transactions on Communications, May, 1974, pp. 637-648. Week 3

  33. Internetwork layer (IP): • addressing: internetwork appears as a single, uniform entity, despite underlying local network heterogeneity • network of networks The Internet: virtualizing networks Gateway: • “embed internetwork packets in local packet format or extract them” • route (at internetwork level) to next gateway gateway satellite net ARPAnet Week 3

  34. Cerf & Kahn’s Internetwork Architecture What is virtualized? • two layers of addressing: internetwork and local network • new layer (IP) makes everything homogeneous at internetwork layer • underlying local network technology • cable • satellite • 56K telephone modem • today: ATM, MPLS … “invisible” at internetwork layer. Looks like a link layer technology to IP! Week 3

  35. Generic connection – oriented network • For A to talk to B, there must be a special call setup packet that travels from A to B, specifying B as the destination. • Each router along the path must make a routing decision based on B’s address • This is the identical problem in IP • In addition to simply forwarding the call setup packet, the goal is to assign the call a small identifier, which we now call the CI (connection identifier) • CIs can be small because they are handed out dynamically and are significant only on a link • They only need to be large enough to distinguish between the total number of calls that might simultaneously be routed on the same link Week 3

  36. A B R5 R2 R1 X R3 R4 A wants to talk to B and use CI 57 22b,79 79c,22 57 c,33 c • Why does the CI have to change hop by hop? • The answer is that it would be very difficult to choose a CI that was unused on all the links along the path 33d,79 33  a,57 b 79a,33 a c c a a b Week 3

  37. ATM and MPLS • ATM, MPLS separate networks in their own right • different service models, addressing, routing from Internet • viewed by Internet as logical link connecting IP routers • just like dialup link is really part of separate network (telephone network) • ATM, MPLS: of technical interest in their own right Week 3

  38. 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 Week 3

  39. 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 Week 3

  40. ATM: network or link layer? Vision: end-to-end transport: “ATM from desktop to desktop” • ATM is a network technology Reality: used to connect IP backbone routers • “IP over ATM” • ATM as switched link layer, connecting IP routers IP network ATM network Week 3

  41. 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 Week 3

  42. 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 • AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video • AAL5: for data (eg, IP datagrams) User data AAL PDU ATM cell Week 3

  43. ATM Layer Service: transport cells across ATM network • analogous to IP network layer • very different services than IP network layer Guarantees ? Network Architecture Internet ATM ATM ATM ATM Service Model best effort CBR VBR ABR UBR Congestion feedback no (inferred via loss) no congestion no congestion yes no Bandwidth none constant rate guaranteed rate guaranteed minimum none Loss no yes yes no no Order no yes yes yes yes Timing no yes yes no no Week 3

  44. ATM Layer: Virtual Circuits • 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. • Permanent VCs (PVCs) • long lasting connections • typically: “permanent” route between to IP routers • Switched VCs (SVC): • dynamically set up on per-call basis Week 3

  45. 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 • one PVC between each source/dest pair) does not scale (N*2 connections needed) • SVC introduces call setup latency, processing overhead for short lived connections Week 3

  46. 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 Week 3

  47. 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 Week 3

  48. 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 • One PVC between each source/dest pair) does not scale (N*2 connections needed) • SVC introduces call setup latency, processing overhead for short lived connections Week 3

  49. Virtual Path Concept • The connection identifier in the ATM cell header has two complexities: • It’s hierarchical and divided into two subfields VPI (Virtual Path Identifier) and VCI (Virtual Circuit Identifier) • VCI is 16 bits • VPI is 12 bits • What’s a VPI? There might be very high speed backbone carrying many millions of calls • The split between VPI and VCI saves the switches in the backbone from requiring that their call mapping database keep track of millions of calls Week 3

  50. Virtual Path Concept • The backbone routers only use the VPIU field then if needed • Outside the backbone, the switches treat the entire VPI:VCI field as one nonhierarchical unit • VP switch looks at only the VPI portion • VC switch looks at both Week 3

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