Telematics. 7th Tutorial - ATM, Bridging, Spanning Tree, VLAN

Transcription

1 Telematics 7th Tutorial - ATM, Bridging, Spanning Tree, VLAN Bastian Blywis Department of Mathematics and Computer Science Institute of Computer Science 09. December, 2010 Institute of Computer Science Telematics Tutorial 09. December,

2 Outline 1. Frame Size 2. Packets and Cells 3. FDDI Performance 4. ATM 5. Bridge Classification 6. IEEE 802.1D 7. Broken Links 8. Virtual LANs 9. IEEE 802.1q and IEEE Features of Layer 2 Protocols Institute of Computer Science Telematics Tutorial 09. December,

3 Frame Size Consider a 10 MBit/s CSMA/CD LAN with a bus topology of 50 m. The signal propagates with m/s in the medium. 1. Calculate the upper bound of the collision detection time. 2. Calculate the minimum frame length that is required to detect a collision.??? Institute of Computer Science Telematics Tutorial 09. December,

4 Frame Size Maximum time depends on maximum distance: A B 50m 5*10^-7s Worst case: station A senses a free medium and starts to transmit Station B senses a free medium and starts to transmit just at the time the signal from A arrives at B Signal from B has to travel through the whole network before A can detect the collision Maximum time until collision detection is twice the signal propagation time t = 2 bus length signal speed = 2 50m m s = s (1) Institute of Computer Science Telematics Tutorial 09. December,

5 Frame Size To ensure that the stations are able to detect a collision, the frame be at least t = seconds on the medium: frame length min bit rate > t (2) frame length min > 10 Mbps s (3) frame length min > bps s (4) frame length min > 5 bit (5) (6) Institute of Computer Science Telematics Tutorial 09. December,

6 Packets and Cells 1. Discuss the difference between packet and cell switching. 2. What are virtual switched circuits and why they are required in some scenarios???? Institute of Computer Science Telematics Tutorial 09. December,

7 Packets and Cells Packets: Variable length Overhead depending on header/data ratio Bursty traffic and idle periods Packets are usually not empty More complex to handle More complex scheduling Intended for data communication (and initially non-live media content) Routing implemented in software and/or hardware Cells: Fixed size reduced jitter High overhead: small payload, large header Constant stream of cells Cells can be empty Easier to handle Simplified scheduling/multiplexing Intended for data, voice, television telex,... Cell switching implemented in hardware Institute of Computer Science Telematics Tutorial 09. December,

8 Packets and Cells Virtual circuits are like physical circuits Virtual circuits can be temporary or permanent Virtual circuits are established by source and ATM switches determine route Route gets connection identifier Cell switching is then based on connection identifier and not ATM addresses Cells can be lost but arrive in order at destination Virtual circuits of ATM have particular service class depending on application Criterion Service Class A B C D Data Rate Negotiated Maximum Dynamic Take what maximum and average rate adjustment you can get cell rate cell rate to free resources Synchronization yes no Bit rate constant variable Connection mode Connection-oriented Connectionless Institute of Computer Science Telematics Tutorial 09. December,

9 FDDI Performance 100 stations are connected to a large FDDI ring. The token rotation time is 40 ms and the token holding time is set to 10 ms. Calculate the maximum efficiency of the ring.??? Institute of Computer Science Telematics Tutorial 09. December,

10 FDDI Performance Assumption: Every station has unlimited data to send Assumption: Data is send during the whole token holding time One token circulation takes ms + 40ms = 1040 ms Efficiency = time used for data transmission total time (7) Efficiency = 1000 ms 1040 ms (8) Efficiency 0.96 (9) Institute of Computer Science Telematics Tutorial 09. December,

11 ATM 1. The size of an ATM cell is a compromise between the requirements of voice and data communication. Assume that ATM has cells of 1500 bytes. Calculate the time between two successive cells, if data is generated at a rate of 64 kbit/s. How large is the time for the normal ATM cell size? 2. The ATM checksum protects only the cell header. How many bit errors can be corrected with this checksum and why???? Institute of Computer Science Telematics Tutorial 09. December,

12 ATM The time between two cells results from the time until enough data for transmission, i.e., a new cell is available. Payload of 1500 byte Payload of 48 byte bit bps = ms (10) 48 8 bit = 6 ms (11) bps Speech transmission requires short delays! Institute of Computer Science Telematics Tutorial 09. December,

13 ATM ATM header 4 bytes for VPI, VCI, PTI, and CLP fields 1 byte Header Error Control (HEC) To correct 1-bit errors the following equation must hold: (m + r + 1) 2 r (12) ( ) 2 8 (13) (14) 1-bit errors can be corrected Institute of Computer Science Telematics Tutorial 09. December,

14 ATM 2-bit errors can be detected but not corrected. For n = m + r there are ( n 2) codewords which are differing in 2 bits from a valid codeword Each of the valid 2 m codewords thus has be surrounded by ( n) n(n 1) 2 = 2 (invalid) codewords to correct all 2-bit errors Thus the following equation has to hold: 2-bit errors cannot be corrected n(n 1) 2 m 2 2 n (15) n(n 1) 2 m 2 2 m+r (16) n(n 1) 2 m 2 2 m 2 r (17) n(n 1) 2 2 r (18) (19) Institute of Computer Science Telematics Tutorial 09. December,

15 Bridge Classification 1. Bridges can be classified as local and remote bridges. Discuss the differences. 2. What is the difference between transparent and source bridging???? Institute of Computer Science Telematics Tutorial 09. December,

16 Local bridges Bridge Classification Provide a direct connection between multiple LAN segments in the same area Remote bridges Connect multiple LAN segments in different areas Bridges connected over telecommunication lines, by PPP, or L2TP Transparent bridging Bridge in promiscuous mode Forwarding of frames based on forwarding database Flooding to all segments if port to reach destination is unknown Bridge learns location of stations (backwards learning) Source (route) bridging Used in Token Ring networks Source specifies the bridges a frame has to traverse Discover procedure 1. Route is found by flooding a particular packet from the source to the destination 2. Each bridge the packet traverses attaches its bridge number and network segment (also for loop detection) 3. Destination receives the packet (multiple times over different routes) and replies by unicast (modified route request packet) 4. Replies traverse to the source based on the route in the header STP not required Institute of Computer Science Telematics Tutorial 09. December,

17 Bridge Classification A X Ethernet HDLC Internet PPP Y Ethernet B Payload L3 L2 Payload L3 L2 Payload L3 L2 Payload L3 L2 L3 L2 L3 L2 Institute of Computer Science Telematics Tutorial 09. December,

18 IEEE 802.1D 1. What is the task of the spanning tree protocol and how does it work? 2. What are the disadvantages of the original Spanning Tree Protocol (STP)? 3. Capture and/or create an STP packet. In which types of networks can STP be used???? Institute of Computer Science Telematics Tutorial 09. December,

19 IEEE 802.1D Spanning Tree Protocol: STP creates loop-free topology for bridged LANs Data Link Layer protocol Based on IEEE LLC Documented in IEEE 802.1d Distributed algorithm Institute of Computer Science Telematics Tutorial 09. December,

20 IEEE 802.1D Root bridge selection based on bridge id: configured priority number and MAC address Bridges initially assume to be the root and broadcast BPDU packet to address 01:80:c2:00:00:00 (bridge group address, multicast) Bridges compare their bridge id with received BPDUs Cost and root ID are updated (minimum) Eventually one bridge wins All other bridges determine lowest cost path to root bridge via root port All other root paths are blocked LAN segments select designated ports to designated bridges Protocol ID Protocol Version ID BPDU Type Flags Root ID Root Path Cost Bridge ID Port ID Message Age Max Age Hello Time Forward Delay Version 1 Length Institute of Computer Science Telematics Tutorial 09. December,

21 IEEE 802.1D Problems of STP: Slow convergence (30-50s) Excessive loss of BPDUs Duplex mismatch Unidirectional links Root bridge spoofing or misconfiguration Location of root bridge determines delay of frames Alternatives and Extensions: Rapid Spanning Tree Protocol (RSTP) Per-VLAN Spanning Tree (PVST) Multiple Spanning Tree Protocol (MSTP) Rapid Per-VLAN Spanning Tree (R-PVST) Institute of Computer Science Telematics Tutorial 09. December,

22 IEEE 802.1D Figure: Captured STP Packet Institute of Computer Science Telematics Tutorial 09. December,

23 Broken Links How can a LAN automatically resolve broken links or switches???? Institute of Computer Science Telematics Tutorial 09. December,

24 Broken Links LANs should contain loops to be able to create alternative routes STP will create loop-free topology BPDUs are sent out periodically and are used to detect topology changes Topology Change Notification (TCN) Topology Change Notification Acknowledgment (TCA) Institute of Computer Science Telematics Tutorial 09. December,

25 Virtual LANs 1. What is the purpose of a virtual LAN? 2. What information can be used to automatically assign hosts to a particular VLAN???? Institute of Computer Science Telematics Tutorial 09. December,

26 Virtual LANs Virtual LAN Creates logical network topology on physical topology Data link layer protocol Limits broadcast domain, broadcast storms Increases security Can be used for load balancing / scheduling Assignment based on: Port MAC address Layer 3 protocol Time Institute of Computer Science Telematics Tutorial 09. December,

27 IEEE q Virtual LANs Additional header with VLAN-tag Unique VLAN ID for each virtual LAN Priority field not related to VLAN but to distinguish different types of data traffic, e.g., Voice over Ethernet Canonical Format Indicator (CFI) Originally: marks endianess of MAC addresses Actual use: indicates IEEE frame in payload QinQ for multiple VLAN tags Figure: Ethernet Frame with IEEE 802.1q VLAN Header Institute of Computer Science Telematics Tutorial 09. December,

28 Virtual LANs Figure: Captured Frame with IEEE 802.1q Institute of Computer Science Telematics Tutorial 09. December,

29 IEEE 802.1q and IEEE Create, inject, and capture an Ethernet frame that contains an IEEE 802.1q header with the following values: Priority = Excellent Effort Canonical Format Indicator set to 1 VLAN ID = 0xA0 Type set to the value of ARP 2. Add a Logical Link Control an frame. Use the following values: Unnumbered Frame Type DSAP = Novell NetWare SSAP = Xerox Network Systems I/G = 0 C/R = 0 Choose any value for the command field 3. Add a SNAP header to the frame. Use the following values: OUI = 0xABCD Type = 0x88CE 4. What layer 3 PDU is stored as data in the frame? 5. How large is the frame without the data???? Institute of Computer Science Telematics Tutorial 09. December,

30 IEEE 802.1q and IEEE >>> eth = Ether(src="fe:fe:fe:fe:fe:fe", dst="ab:ab:ab:ab:ab:ab") >>> vlan = Dot1Q(vlan=0xA0, prio=2, type=0x0806) >>> pkg = eth/vlan >>> pkg.show() ###[ Ethernet ]### dst= ab:ab:ab:ab:ab:ab src= fe:fe:fe:fe:fe:fe type= 0x8100 ###[ 802.1Q ]### prio= 2 id= 0 vlan= 160 type= 0x806 >>> sendp(pkg) Institute of Computer Science Telematics Tutorial 09. December,

31 IEEE 802.1q and IEEE Figure: Ethernet Frame with IEEE 802.1q Header and missing ARP Institute of Computer Science Telematics Tutorial 09. December,

32 IEEE 802.1q and IEEE Some Service Access Point (SAP) identifiers: 0x04 - IBM SNA 0x80 - Xerox Network Systems 0xAA - SubNetwork Access Protocol (SNAP) 0xE0 - Novell NetWare 0xF4 - Lan Manager FE -CLNS See also: IEEE Logical Link Control (LLC) Public Listing >>> ieee8023 = Dot3(src="fe:fe:fe:fe:fe:fe", dst="ab:ab:ab:ab:ab:ab") >>> llc = LLC(dsap=0xE0, ssap=0x80, ctrl=0) >>> pkg = ieee8023/llc >>> pkg.show() ###[ ]### dst= ab:ab:ab:ab:ab:ab src= fe:fe:fe:fe:fe:fe len= None ###[ LLC ]### dsap= 0xe1 ssap= 0x80 ctrl= 0 >>> sendp(pkg, iface="lo") Institute of Computer Science Telematics Tutorial 09. December,

33 IEEE 802.1q and IEEE Figure: Ethernet Frame with IEEE LLC Header (incomplete) Institute of Computer Science Telematics Tutorial 09. December,

34 IEEE 802.1q and IEEE >>> snap = SNAP(OUI=0xABCD, code=0x88ce) >>> llc = LLC(dsap=0xAA, ssap=0xaa, ctrl=0x03) >>> pkg = ieee8023/llc/snap >>> pkg.show() ###[ ]### dst= ab:ab:ab:ab:ab:ab src= fe:fe:fe:fe:fe:fe len= None ###[ LLC ]### dsap= 0xaa ssap= 0xaa ctrl= 0 ###[ SNAP ]### OUI= 0xabcd code= 0x88ce >>> sendp(pkg, iface="lo") Institute of Computer Science Telematics Tutorial 09. December,

35 IEEE 802.1q and IEEE Figure: Ethernet Frame with IEEE LLC and SNAP Header (incomplete) Institute of Computer Science Telematics Tutorial 09. December,

36 IEEE 802.1q and IEEE Ethertype 0x88CE, Remote Direct Memory Access over Ethernet (RDMAoE): A protocol for low-latency, low-overhead memory-to-memory communications among hosts interconnected by Ethernet. 5. Frame length = 26 bytes IEEE header = 14 bytes IEEE header = 8 bytes FCS = 4 bytes (Padding is required if the network layer PDU is not large enough!) Institute of Computer Science Telematics Tutorial 09. December,

37 Features of Layer 2 Protocols 1. List features and services of layer 2 protocols. 2. Do Ethernet, IEEE 802.2, ATM, and PPP provide these features and services? Are any additional features and services provided???? Institute of Computer Science Telematics Tutorial 09. December,

38 Features of Layer 2 Protocols Example Data Link Layer services: Framing Error detection and correction Flow control Reliability: acknowledgements, retransmissions Medium access Addressing Multiplexing Note: Discuss other services! Institute of Computer Science Telematics Tutorial 09. December,

39 Features of Layer 2 Protocols Ethernet IEEE ATM PPP Framing? Error detection ( ) Error correction ( )? Flow control ( ) ( ) Acknowledgements ( ) ( ) Retransmission ( Medium access Addressing ( ) Multiplexing Table: Features of some Data Link Layer protocols, up for discussion Institute of Computer Science Telematics Tutorial 09. December,

40 Features of Layer 2 Protocols Ethernet flow control Specified in IEEE 802.3x Only for full-duplex connections PAUSE frames to signal full buffer Type field set to 0x8808 (MAC Control) MAC Control opcode = 0x0001 Send to multicast address 01:80:C2:00:00:01 Frame is not forwarded Two byte quanta, length of pause in 512 bit times Figure: Ethernet pause frame Institute of Computer Science Telematics Tutorial 09. December,

41 Features of Layer 2 Protocols Figure: Ethernet pause frame Institute of Computer Science Telematics Tutorial 09. December,

42 Features of Layer 2 Protocols Some additional features: Ethernet Extensible family of protocols, many optional features Power over Ethernet Virtual LANs see IEEE and IEEE standards and further sub-standards IEEE ATM PPP Connection oriented service Acknowledged datagram service Virtual switched circuits Guarantee of quality criteria Services classes Session initiation, testing, termination Compression Institute of Computer Science Telematics Tutorial 09. December,

43 The Last Slide TM Thank you for your attention. Questions? Institute of Computer Science Telematics Tutorial 09. December,