Arhitecturi și Protocoale de Comunicații (APC) Rețele locale (I)

Transcription

1 Arhitecturi și Protocoale de Comunicații (APC) Rețele locale (I)

2 Outline Shared media LANs Classes of (MAC) protocols, standards. CSMA/CD (Ethernet). Bridged/Switched LANs Types of bridges, standards. Transparent bridges: Learning and Forwarding. Spanning Tree Protocol. LAN switches. LAN infrastructure (annex) 2 nd Part: Wireless LANs Introduction to IEEE WLANs. Octavian Catrina 2

3 Data Link layer (review) LANs: DL connectivity using shared media and LAN switching WANs: DL connectivity using pointto-point links and WAN switching Hub Switches 6 Router Data Link layer (DL) to control the data transfer on physical links: framing, addressing, error control,... Medium Access Control (MAC) to coordinate the access to shared physical transmission media. Octavian Catrina 3... DL/MAC PHY Medium... DL/MAC PHY

4 Data link layer: main functions DL/MAC addresses Identify stations attached to same link. Flat address space. Framing Define a PDU (frame) format that: Allows receivers to determine frame boundaries in the received bit stream. Incorporates control information needed by the data link layer protocol (addresses, error detection code, etc). Error control Detect/correct frames altered during transmission. Medium access control Coordinate the access to shared physical transmission media. Multiplexing and switching Various technologies for LANs (e.g., switched Ethernet) and WANs (e.g., Frame Relay, ATM). Octavian Catrina 4

5 Shared Media LANs MAC protocols for wired LANs

6 Classes of MAC protocols Non-deterministic access Network devices contend for shared medium access. MAC protocol provides contention rules. Collisions can occur during contention. Access time is not predictable/scheduled. Access delay/efficiency: Some time is lost with device contention (collisions). Simple, robust, cheap. Deterministic access Network devices take turns for shared medium access. MAC protocol provides coordination rules. Collisions do not occur. Access times can be scheduled/predictable. Access delay/efficiency: Some time is lost for device coordination. More complex, expensive. Octavian Catrina 6

7 MAC protocols IEEE 802 standards (examples) 802.3: CSMA/CD ( Ethernet) Carrier Sense Multiple Access with Collision Detection. Non-deterministic access, contention MAC protocol : Token Ring; 802.4: Token Bus (historical) Deterministic access, token rotation MAC protocol : Wireless LANs Non-deterministic access using CSMA/CA - CSMA with Collision Avoidance. Deterministic access using polling : 100 VG-AnyLAN (historical) Deterministic access, round-robin scheduling protocol. ANSI/ISO standard (ISO 9314) (historical) FDDI (Fiber Distributed Digital Interface) Deterministic access, timed token rotation protocol. Octavian Catrina 7

8 OSI Physical OSI Data Link CSMA-CD: IEEE Token Ring: IEEE Wireless LAN: IEEE and many others Physical MAC sublayer LLC sublayer Scope IEEE LAN standards For each LAN type: MAC sublayer + PHY layer. Common Logical Link Control (LLC) sublayer: LAN interconnection (bridges/switches): MAC sublayer Framing, addressing, error detection, MAC protocols. Connectionless unreliable service. LLC sublayer Often empty in IP networks... Provides a common DL service interface for all MAC types and protocols for more service types: best effort, reliable connection oriented and connectionless. Logical Link Control LLC: IEEE Octavian Catrina 8

9 ALOHA Non-deterministic access Transmit at will. Towards CSMA/CD (Ethernet) Very low efficiency (up to 18% channel utilization). CSMA: Carrier Sense Multiple Access Transmit only when sensing the medium idle. Collisions still possible due to propagation delay. Efficiency is still quite low. CSMA/CD: CSMA with Collision Detection Adding collision detection allows: (1) interruption of failed transmission and (2) immediate recovery by retransmission. Most widespread technology for wired shared media LANs. Octavian Catrina 9

10 CSMA/CD principle Transmit only when sensing medium idle (CSMA). Detect collision (CD), abort transmission, retry later. A backoff 12: A looses! t 0 t B d=c t 0-2 t 0+ backoff 02: B wins! c = signal propagation speed. d = distance (cable length). = signal propagation delay. t Time elapsed from transmission start until collision detection: 2. Maximum duration of a transmission in case of collision 2. After a collision, the stations must defer the attempts to transmit for a random time interval (backoff) to avoid systematic collisions. Octavian Catrina 10

11 More insight Ensuring collision detection Minimum frame transmission duration: T min > 2. Transmit brief jamming signal after detecting collision. Solving contention Defer retransmission after collision with random delay: backoff. Backoff delay is random multiple N of 2 time slots. Ensures the transmitter with shortest backoff delay wins. Range of N for K consecutive collisions is [0, 2 min(k, 10) -1]. Binary exponential backoff. Adapts backoff delay to load, ensures that contention ends at high load. Reduces collision probability when many senders are active by spreading retransmissions over longer intervals. Limit the number of retries: K max = 16. CSMA/CD efficiency Efficient operation when contention duration (multiple of 2) is much shorter than transmission duration (T): /T << 1. Efficiency decreases as delay (distance) and/or data rate increase. Octavian Catrina 11

12 Ethernet frame format (IEEE 802.3) octets: PA SFD DA SA L/T Data Pad FCS octets covered by FCS PA: Preamble - synchronization ( ) SFD: Start of Frame Delimiter ( ) DA: Destination Address SA: Source Address L/T: Length of data field (octets) if Else, payload type. Pad: Extra bits added when the frame has less than 64 octets. FCS: Frame Check Sequence (CRC-32.) Baseband transmission. MAC addresses: bits: I/G U/L 46-bit address I/G = 0/1 - individual/group (multicast). U/L = 0/1 - local/global administered. 0xFFFFFFFF = broadcast address. Assumed maximum round-trip propagation delay: 2 = 51.2 s Octavian Catrina 12

13 Examples of Ethernet frames Note: Padding is at the end of the frame MAC Length + Padding + LLC + Payload MAC Type + Padding + Payload MAC Type + Payload Octavian Catrina 13

14 CSMA/CD protocol overview Wait for data to send. Format frame Transmission Reception Y Carrier sense ON? N Wait inter-frame gap. Start frame transmission Collision detected? Y Abort frame transmission. Transmit jam signal (32 bits). Increment collision counter K N Complete transmission Status: Done N Incoming signal detected? Y Set carrier sense signal ON. Bit synchronization. Wait for SFD. Receive frame. FCS and frame size OK? Y N K > 16? N Y Backoff: wait N slots, N = random (0, 2 min (K, 10) -1) Discard frame. Status: Attempt limit exceeded Matching dest. address? Y Deliver frame to higher sub-layer N Discard frame. Octavian Catrina 14

15 Ethernet: A success story First Ethernet Xerox (1970's), joined later by DEC and Intel. 1-2 Mbps. Deployed since the beginning of the 1980s. 10 Mbps - Ethernet (IEEE 802.3, 1985) 10BASE5/10BASE2: bus topology, thick/thin coaxial. 10BASE-T/FX: star topology, UTP/optical fiber. Switched Ethernet - early 1990s. 100 Mbps - Fast Ethernet (1996) 100BASE-TX/FX: star, UTP(STP)/optical fiber Mbps - Gigabit Ethernet (1999) 1000BASE-FX/CX/T: star, optical fiber/copper(stp/utp). Normally used in switched Ethernet LANs/MANs. Still supports CSMA/CD. 10 Gbps Ethernet in Switched Ethernet LANs/MANs. 100 Gbps Ethernet in 2010 Octavian Catrina 15

16 Token Ring overview (historical) Outline Logical ring topology with star wiring. One-way links, active interfaces. Copper (shielded twisted pair) or optical fiber cable. 4/16/100 Mbps, < 260 stations, < 100 m between stations. MAC protocol Token rotation (token passing) protocol. Real-time traffic support: - Limited access delay by limited token holding time. - Priorities. Interface Token Concentrator Octavian Catrina 16

17 Token rotation protocol Token Ring interface Repeat mode: Transmit mode: Rx Rx Tx Tx Octavian Catrina 17

18 Shared media: Performance issues Frame transfer delay Medium access delay + Transmission duration + Propagation delay Specific component: medium access delay. Examples: CSMA/CD contention, Token rotation. Main problem: fast increase with traffic load. Throughput Time sharing of the medium Average throughput for each station is a fraction of the shared medium bandwidth.... DL/MAC PHY PHY Repeater PHY... DL/MAC PHY Shared media domain Shared media domain Extended shared media domain Frames delivered by flooding Octavian Catrina 18

19 Shared media: benefits and limitations Benefits Cost effective LAN connectivity. High data rate at end systems and low transfer delay for the lowest cost. Limitations Does not scale up for larger LANs and higher medium bandwidth. More stations Less bandwidth per station. Larger network and/or higher bandwidth decrease of medium bandwidth utilization: larger time fraction spent with transmission coordination by MAC protocol. Higher traffic Rapid increase of medium access delay. Octavian Catrina 19

20 Scalable LAN connectivity Meeting bandwidth requirements Increase medium bandwidth. Limited by data transmission technology, cost, MAC protocol efficiency. Improve bandwidth utilization. The only scalable solution. Rely on Data Link or/and Network layer functions for scalable, bandwidth-efficient connectivity. Towards scalable solutions in LANs Limit shared media domains. Segmentation with bridges, switches or routers. Other issues: Limit MAC broadcasts, use full-duplex instead of half-duplex, optimize paths, add redundant paths with load sharing, etc. Octavian Catrina 20

21 Switched LANs Bridges, Ethernet switches

22 LAN connectivity using bridges Bridges were introduced in the late 1980s to interconnect shared media LAN segments, at MAC (sub)layer. A bridge distinguishes inter-segment frames from intra-segment (local) frames, based on MAC addresses. Forwards inter-segment frames toward the destination. Filters (does not forward) intra-segment (local) frames. Inter-segment Forward Intra-segment No forward... Bridge DL/MAC PHY MAC PHY MAC PHY DL/MAC PHY Octavian Catrina 22

23 Transparent vs. Source routing bridges Transparent bridge Design goal: transparent interconnection. Stations ignore the presence of bridges. No HW/SW changes in stations: plug and play. Bridges discover attached stations and only forward inter-segment traffic. Standard: IEEE 802.1D-2004 (supersedes 802.1D-1990, 1998). Source routing bridge Design goal: optimal paths and load sharing. Source stations discover paths to destinations. Frame headers carry a path description. Bridges forward frames according to the path in the header. Historical: IEEE (Token Ring), ISO (FDDI). Transparent bridges were widely deployed in Ethernet LANs. They rapidly won the competition with source routing bridges, which were used only in Token Ring LANs. Octavian Catrina 23

24 Transparent bridges The bridge learns the location of the stations relative to its ports from the source MAC address of the received frames. This information is stored in a forwarding table and used to decide what frames to forward and where. Forward Destination at another port. Filter Destination at the same port. Flood Destination unknown or broadcast frame or multicast frame Forward Flood Filter Octavian Catrina 24

25 Learning and forwarding (1) Octavian Catrina 25

26 Learning and forwarding (2) Adaptation to topology changes What happens if station 1 is moved to the rightmost segment? It becomes unreachable (until it sends some frame)! The bridge must be able to automatically update the forwarding table to network topology changes. A timestamp is stored whenever a table entry is created or updated. Entries older than a preset max age are discarded. Octavian Catrina 26

27 Learning and forwarding (3) Bridge Architecture Receive & store frame on port P Frame buffer memory MAC Learning Logic Forwarding Table (FT) MAC Address Port Age Frame buffer memory MAC Learn SA Learn: Add FT entry: SA, port P, age No Read Source Address (SA) Is SA in FT? Yes Read Destination Address (DA) PHY Forwarding Logic PHY No Is DA broadcast? Yes Frame Frame Is DA in FT? Yes Read port for DA from FT = Q No Flood frame No Is P = Q? Yes Forward frame to port Q Filter (discard) frame Flood: Forward on all ports except P Octavian Catrina 27

28 Loops in bridged LANs Loops: How do they occur? Intended interconnections: LAN topology with alternate paths, able to maintain connectivity in case of failure (fault tolerance). Involuntary/accidental interconnections. Transparent bridges and loops In this example, the frame is flooded because its destination address is not in the table. Similar behavior for broadcast destination address. Frame from 1 to 2 12 The frame loops and proliferates! up 1 up DA Port 1 down DA Port 1 down Learn station 1. Flood. 12 Learn station Flood. Double delivery Learn station 1. Flood Learn station 1. Flood. Octavian Catrina 28

29 Loop-free full connectivity Spanning tree Sub-graph providing loop-free full connectivity. Spanning Tree Protocol (STP) STP allows transparent bridges to: Discover a spanning tree in the physical topology. Reduce the physical topology to a spanning tree by disabling some bridge ports. Monitor connectivity and enable/disable ports to adapt to network changes or failures. STP implements a distributed algorithm for determining and maintaining a spanning tree. Bridges communicate using control messages called BPDU (Bridge Protocol Data Units). Example of BPDU in slide 13. Octavian Catrina 29 L3 L4 1 L3 L2 3 5 L1 L L6 L4 6 L5 L6 Physical topology Root L1 L5 Spanning tree: Connects the root bridge to all LANs and all bridges on shortest paths. Provides loop-free, symmetric (two-way) connectivity. 6 Spanning tree

30 STP: Algorithm overview Bridge ID: Priority + MAC address. Cost assigned to each port: What "costs" to send a frame through that port (inv. of link bandwidth). Elect the root bridge: The bridge with the lowest bridge ID. Find the root path cost (RPC) of each port: The lowest path cost to reach the root by that port. Select the root port (R) of each bridge: Port with the lowest RPC. Select the designated bridge for each LAN segment: Bridge with the lowest RPC. The port of that bridge connected to the LAN is marked as designated port (D). Block () all the other ports (i.e., which are not R or D). Octavian Catrina 30 L4 R L1 D 1 RPC=0 R 1 RPC= D 1 Root 2 3 L5 2 2 D 3 RPC=0 3 RPC=3 L2 R 1 RPC=2 1 R 1 RPC=1 R 2 RPC= D RPC=3 L6 2 2 D 1 RPC=3 L3 1 RPC= D 2 RPC=2 Bridge identifiers (1, 2,...) Port identifiers (1, 2,...)

31 STP: Protocol overview BPDU: Contains the ID of the root assumed by the sender, the RPC, the sender's ID,... BPDU flow: BPDUs are multicast to all bridges on a LAN segment. The root bridge sends BPDUs periodically. Designated bridges relay BPDUs on their D ports. At power on: Initially, any bridge issues BPDUs claiming to be root. Eventually, only the elected root still issues BPDUs and the designated bridges relay them. Upon failure (worst case): The periodic BPDUs do not reach the bridge any more. After a preset time it restarts the spanning tree algorithm. Octavian Catrina 31 R L1 D 1 RPC=0 R 1 RPC=1 1 (1,0,1) (1,1,2) 1 2 D 1 Root 2 3 L5 2 2 D 3 RPC=0 3 RPC=3 L2 R 1 RPC=2 1 R 1 RPC=1 R 2 RPC= (1,1,3) (1,2,4) 1 D RPC=3 L6 2 2 D 1 RPC=3 L3 1 RPC=2 1 1 (1,2,5) 5 2 L4 D 2 RPC=2 BPDUs transmitted on D ports after convergence. Main info: Root Bridge ID, My RPC, My Bridge ID.

32 High performance multiport bridge Layer 2 (LAN) switches Parallel architecture, hardware implementation Multiple frames can be forwarded at the same time. Very low frame forwarding latency by early table lookup (next slide). Typically one station per switch port No more shared transmission media full duplex operation is possible. Asymmetric switching Ports can have different bit rates (fast ports for servers and inter-switch links). Octavian Catrina 33

33 Switching modes Store-and-forward switching The switch starts forwarding after receiving the complete frame. Frame forwarding delay (if no queuing): T = L/R. Delay is long and varies with frame length. All errors filtered. Preamble SFD DA SA Len Data FCS Cut-through Fragment-free Store-and-Forward Cut-through switching A switch can start forwarding after receiving the frame's DA. Usually: Start forwarding after receiving F = 64 octets. "Fragment-free": filters frames < 64 octets (collision fragments). Frame forwarding delay (if no queuing): T = F/R. Delay is very short and constant. Bit errors not filtered. Forwarding starts before receiving FCS. Octavian Catrina 34

34 Final example Physical topology. All links have the same cost. STP Bridge IDs are B1, B2, B8 and B1 < B2 < < B8. Root B B B B S Active topology created by STP and delivery of the frame X S. Designated Port: D Root Port: R Blocked Port Root B1 4 D R 1 2 B3 D D D D 1 1 D R B D 3 4 D D D D R 1 2 B4 3 D D D D S B1 Adr. Port S 1 X SA=X, DA=S B5 B6 B7 B We assume the following initial contents of the forwarding tables: B2 Adr. Port S 5 B3 Adr. Port Y 4 B4 Adr. Port S 1 SA=X, DA=S Bridge actions for the delivery of the frame X S and final contents of the forwarding tables: B1 Learn Fwd Adr. Port S 1 X 4 R R R R B5 B6 B7 B D D D D D D D D X B2 Learn Fwd Adr. Port S 5 X 1 B3 Learn Flood Adr. Port Y 4 X 6 B4 Nothing Adr. Port S 1 B5 Adr. Port X 4 B6 Adr. Port B7 Adr. Port Y 4 B8 Adr. Port Z 4 B5 Flood Adr. Port X 4 B6 Learn Flood Adr. Port X 1 B7 Learn Flood Adr. Port Y 4 X 1 B8 Learn Flood Adr. Port Z 4 X 1 Octavian Catrina 35

35 Bridges: benefits and limitations Benefits Cost effective solution for LAN connectivity with isolation of shared media segments. Minimal installation and maintenance effort. LAN connectivity independent of Network layer protocol. Limitations No broadcast control: single MAC broadcast domain. Imperfect filtering: flooding if destination unknown. No optimal paths, no load sharing (for transparent bridges). Transfer delay increases (for store and forward operation). Does not scale up for large inter-networks. 1/10 Gigabit Ethernet High performance and cost effective solutions for campus network backbone. Also, attractive solution for MANs. Octavian Catrina 36

36 Evolution of bridged LAN standards IEEE 802.1D MAC Bridges First standard for transparent bridges was 802.1D Various amendments have been incorporated in 2 revisions of the 802.1D standard, issued in 1998 and Amendment 802.1w-2001 specified Rapid STP (RSTP). RSTP was introduced to replace the original STP and provide much faster convergence after topology changes (from seconds to a few seconds in the worst case). Current version of the MAC Bridges standard is 802.1D IEEE 802.1Q VLAN Bridges Support for Virtual LANs was introduced in 802.1Q Development continued with a long series of amendments and 2 revisions of the standard, in 2005 and On-going work. Current version of the VLAN Bridges standard is 802.1Q Octavian Catrina 37

37 Towards more efficient connectivity Inefficient interconnection in traditional bridged LANs Traditional bridges can handle redundant paths in the physical topology, to provide fault tolerance. However, the spanning tree active topology cannot provide optimal paths and load balancing on alternate paths. Several recent developments aim at providing more efficient connectivity in bridged LANs (at MAC layer), using techniques borrowed from IP routing protocols. Shortest Path Bridging (SPB) Specified in 2012 by the IEEE, in amendment 802.1aq Transparent Interconnection of Lots of Links (TRILL) Proposed by Radia Perlman, specified in by the IETF, in RFC 5556, 6325, 6326, 6327, etc. Competing solutions supported by different vendors Octavian Catrina 38

38 Annex LAN Infrastructure

39 Star topology: Catchment area Transmission range Depends on physical medium characteristics, data rate, transmission technology, etc. Standardized (IEEE 802, EIA/TIA-568). Catchment area Area around the hub within transmission range. Example Catchment area for Ethernet 10BASE-T and 100Base-TX LANs (UTP). Octavian Catrina 40

40 Star topology: EIA/TIA 568/569 cabling < 3m < 6m < 90m Patch panel Wiring closet Specially designated room for wiring a data (or voice) network. Center of star topology. Horizontal cabling Cabling extending from a wiring closet to workstations. Octavian Catrina 41

41 Extended star topology: Multi-story building MDF: Main Distribution Facility Primary communications room (wiring closet) for a building or campus. Central point (root) of an extended star topology. IDF: Intermediate Distribution Facility Secondary communications room, lower hierarchy level in an extended star topology. POP: Point-Of-Presence Communications room serving as point of interconnection with local telecom network. Octavian Catrina 42

42 Extended star: Multi-building campus Octavian Catrina 43

43 Extended star: EIA/TIA 568/569 cabling Wiring closet HCC: Horizontal Cross Connect (patch panels). VCC: Vertical Cross Connect (patch panels). Wiring closet Octavian Catrina 44