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Transcript
Introduction to Computer Networks Lecture 6: Packet Switching University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani Univ. of Tehran Introduction to Computer Network 1 outline Store-and-Forward Switches Bridges and Extended LANs Packet switching Cell Switching Segmentation and Reassembly Univ. of Tehran Introduction to Computer Network 2 Scalable Networks Limitation of directly connected networks. Limit on the number of hosts; For example, Ethernet is 1024 hosts. Limit on the geographical area of LANs. 2500 m in Ethernet. Solution: This is like telephone network. Then, use Switches. Univ. of Tehran Introduction to Computer Network 3 Switches forwards packets from input port to output port T3 T3 STS-1 Input ports port selected based on address in packet header If two packets are destined to the same output, one must be buffered (queued). This is called contention. Switch T3 T3 STS-1 Output ports Needs some kinds of scheduling for packet delivery. If the buffer overflow, it will be a congestion. Univ. of Tehran Introduction to Computer Network 4 An Ethernet Network Router Problem: Outside world Shared network limits throughput. Lots of collisions reduces efficiency. Univ. of Tehran Introduction to Computer Network 5 Ethernet Switch Ethernet Switch/Bridge Router • If only one computer per port, no collisions can take place (each cable is now a self-contained point-to-point Ethernet link). Problem? How to determine the output port? Univ. of Tehran Introduction to Computer Network 6 Ethernet Switching Ethernet Switch Benefits: Router Outside world Number of collisions is reduced. If only one computer per port, no collisions can take place (each cable is now a self-contained point-to-point Ethernet link). Capacity is increased: the switch can forward multiple frames to different computers at the same time. Univ. of Tehran Introduction to Computer Network 7 Interconnecting LANs Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router. By partitioning a large LAN into multiple smaller networks, there are fewer collisions, and more parallel communications. It is now common for the port of an Ethernet switch to connect to just one (or a small number of) hosts. Univ. of Tehran Introduction to Computer Network 8 Bridges and Extended LANs LANs have physical limitations (e.g., 2500m) Connect two or more LANs with a switch accept and forward strategy level 2 connection (does not add packet header) A B C Port 1 Bridge Port 2 X Y Z Ethernet Switch is called Bridge traditionally. Univ. of Tehran Introduction to Computer Network 9 One Ethernet Switch in “Packard” Univ. of Tehran Introduction to Computer Network 10 Output ports? Learning Bridges Maintain a forwarding table for hosts. Port Host 1 A 1 B 1 C 2 X 2 Y 2 X A B C Port 1 Bridge Port 2 X Y Z How to populate table. Manually by system admin. (not good) Learn table entries based on source address (flexible). Univ. of Tehran Introduction to Computer Network 11 Learning Bridges, (Forwarding) 1. 2. 3. If the frame destination address is in the routing table, forward the frame to the corresponding port. Otherwise, broadcast the frame. Update the table if the source address is not in the table. The table is learned by examining the Ethernet SA of arriving packets. Table is an optimization; need not be complete Always forward broadcast frames Univ. of Tehran Introduction to Computer Network 12 Ethernet Switching Learning addresses B A C 2 3 1 Ethernet Switch 6 I J Ethernet Switch Univ. of Tehran 4 5 K D J F E F G Ethernet Switch Switch learns that ‘F’ is reachable through port 5 N L M O H Outside Q Router world P Ethernet Switch Introduction to Computer Network 13 Ethernet Switching Ethernet address Port A 1 Learning addresses B A C 2 3 1 Ethernet Switch 6 I J B 2 C 3 D 4 E, F, G, H, Q 5 I, J, K, L, M, N, O, P 6 D E F 4 5 K G Ethernet Switch L M N H Outside Q Router world O P Ethernet Switch Ethernet Switch Q: How do we prevent loops? Univ. of Tehran Introduction to Computer Network 14 Preventing loops Spanning Tree Protocol The network of bridges is a graph. The Spanning Tree Protocol finds a subgraph that spans all the vertices without loops. Spanning => all bridges are included. Tree => the topology has no loops. The distributed protocol runs: To determine which bridge is the root of the tree, and Each bridge turns off ports that are not part of the tree. Univ. of Tehran Introduction to Computer Network 15 Spanning Tree Algorithm A Spanning tree B B3 C B5 D B2 B7 E K F B1 G H B6 B4 I A distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman Univ. now IEEE 802.1 specification of Tehran Introduction to Computer Network J 16 Example Spanning Tree B8 B3 Protocol operation: B5 B7 B2 3. B1 B6 Univ. of Tehran 1. 2. B4 Introduction to Computer Network Picks a root For each LAN, picks a designated bridge that is closest to the root. All bridges on a LAN send packets towards the root via the designated bridge. 17 Example Spanning Tree B8 Spanning Tree: B3 B5 B1 B7 B2 B2 B4 B5 B7 B1 Root B6 Univ. of Tehran B8 B4 Introduction to Computer Network 18 Algorithm Overview Each bridge has unique id (e.g., B1, B2, B3) Select bridge with smallest id as root Select bridge on each LAN closest to root as designated bridge (use id to break ties) Each bridge forwards frames over each LAN for which it is the designated bridge A B B3 C B5 D B2 B7 E K F B1 G H B6 B4 I J Univ. of Tehran Introduction to Computer Network 19 Algorithm Details Bridges exchange configuration messages id for bridge sending the message id for what the sending bridge believes to be root bridge distance (hops) from sending bridge to root bridge Each bridge records current best configuration message for each port Initially, each bridge believes it is the root Univ. of Tehran Introduction to Computer Network 20 Algorithm Detail (cont) When learn not root, stop generating config messages in steady state, only root generates configuration messages When learn not designated bridge, stop forwarding config messages in steady state, only designated bridges forward config messages Root continues to periodically send config messages If any bridge does not receive config message after a period of time, it starts generating Univ. of Tehranmessages Introduction to Computer 21 config claiming toNetwork be the root Spanning Tree Protocol 1. Each bridge periodically sends a configuration message out of every port. A message contains: (ID of sender, ID of root, distance from sender to root). 2. Initially, every bridge claims to be root and sends a distance field of 0. 3. A bridge keeps sending the same message (periodically) until it hears a “better” message. “Better” means: • • • A root with a smaller ID A root with equal ID, but with shorter distance The root ID and distance are the same as we already have, but the sending bridge has a smaller ID. 4. Once a bridge hears a better configuration message, it stops generating its own messages, and just forwards ones that it receives (adding 1 to the distance). 5. If the bridge realizes that it is not the designated bridge for a LAN, it stops sending configuration messages to that LAN. Eventually: • • Only the root bridge generates configuration messages, Other bridges send configuration messages to LANs for which they are the designated bridge. Univ. of Tehran Introduction to Computer Network 22 Broadcast and Multicast Forward all broadcast/multicast frames current practice Learn when no group members downstream Accomplished by having each member of group G send a frame to bridge multicast address with G in source field Univ. of Tehran Introduction to Computer Network 23 Limitations of Bridges Do not scale spanning tree algorithm does not scale broadcast does not scale Do not accommodate heterogeneity Caution: beware of transparency Univ. of Tehran Introduction to Computer Network 24 Packet Switching Schemes Learning bridges are a special case. We need a general form for packet switching or specifying the output in the packet. They are three format for specifying the output in the a packet Source routing: List entire path in the packet Virtual circuits: like telephone circuit, identify packets with ids. Datagram: Put only destination address in packets’ headers. Univ. of Tehran Introduction to Computer Network 25 Source Routing List entire path in packet Router processing, one option Driving directions (north 3 hops, east, etc..) Examine first step in directions Strip first step from packet Forward to step just stripped off Or the address can be implemented by a linked list in the packet header. Univ. of Tehran Introduction to Computer Network 26 Source Routing Example Packet 4,3 3,4,3 2 Sender 1 2 R1 4 3 1 R1 3 4 3 2 1 R2 4 3 Receiver All routing information is provided by the source. Univ. of Tehran Introduction to Computer Network 27 Source Routing Advantages Disadvantages Switches can be very simple and fast Variable (unbounded) header size Sources must know or discover topology (e.g., failures) Typical use Debugging and management Univ. of Tehran Introduction to Computer Network 28 Virtual Circuits/Tag Switching Use the telephone model virtual circuits Each flow is identified by a Virtual Circuits Identifier (VCI). Connection setup phase, Signaling Each packet carries connection ID Use other means to route setup request Each router allocates flow ID on local link Creates mapping of inbound flow ID/port to outbound flow ID/port Sent from source with 1st hop connection ID Router processing Lookup flow ID – simple table lookup Replace flow ID with outgoing flow ID Forward to output port Swap labels Univ. of Tehran Introduction to Computer Network 29 Virtual Circuit Connection setup establishes a path through switches A virtual circuit ID (VCI) identifies path Uses packet switching, with packets containing VCI VCIs are often indices into per-switch connection tables; change at each hop VC1 VC2 1 2 VC1 In, VC Out, VC 1, 1 4, 1 1, 2 4, 3 Univ.2, of 1 Tehran 4, 2 4 VC1 VC2 3 4 1 VC3 2 VC1 3 VC2 VC1 …. Introduction to Computer Network 30 Virtual Circuits Examples In-port Lookup table for Switch R1 Packet Sender In-VCI Out-port Out-VCI 1 5 3 7 4 11 0 8 5 7 1 2 2 R1 R2 4 3 1 4 1,7 4,2 3 2 1,5 3,7 2 1 R3 4 3 Receiver 6 2,2 3,6 Subsequence packets follow the same circuit Univ. of Tehran Introduction to Computer Network 31 Virtual Circuits Advantages Disadvantages More efficient lookup (simple table lookup) More flexible (different path for each flow) Can reserve bandwidth at connection setup Easier for hardware implementations Small per-packet header overhead. Still need to route connection setup request More complex failure recovery – must recreate connection state Typical uses ATM – combined with fix sized cells MPLS – tag switching for IP networks Univ. of Tehran Introduction to Computer Network 32 Virtual Circuit Model Typically wait full RTT for connection setup before sending first data packet. While the connection request contains the full address for destination, each data packet contains only a small identifier, making the per-packet header overhead small. If a switch or a link in a connection fails, the connection is broken and a new one needs to be established. Connection setup provides an opportunity to reserve Univ. of Tehran resources. Introduction to Computer Network 33 Datagram Switching No connection setup phase since it is costly. Each packet is forwarded independently Sometimes called connectionless model Analogy: postal system Packet Each switch maintains a forwarding (routing) table Univ. of Tehran Sender R R 1 2 R1 4 3 1 2 R1 4 R4 3 R3 R 2 1 R2 3 4 Receiver R R3 Introduction to Computer Network 34 Datagram Model There is no round trip time delay waiting for connection setup; a host can send data as soon as it is ready. Source host has no way of knowing if the network is capable of delivering a packet or if the destination host is even up. Since packets are treated independently, it is possible to route around link and node failures. Since every packet must carry the full address of the destination, the overhead per packet is higher. Univ. of Tehran Introduction to Computer Network 35 Cell Switching (ATM) Connection-oriented packet-switched network Used in both WAN and LAN settings Signaling (connection setup) Protocol: Q.2931 Specified by ATM forum Packets are called cells 5-byte header + 48-byte payload Commonly transmitted over SONET other physical layers possible Univ. of Tehran Introduction to Computer Network 36 Variable vs Fixed-Length Packets No Optimal Length if small: high header-to-data overhead if large: low utilization for small messages Fixed-Length Easier to Switch in Hardware simpler enables parallelism Univ. of Tehran Introduction to Computer Network 37 Big vs Small Packets Small Improves Queue behavior finer-grained pre-emption point for scheduling link maximum packet = 4KB link speed = 100Mbps transmission time = 4096 x 8/100 = 327.68us high priority packet may sit in the queue 327.68us in contrast, 53 x 8/100 = 4.24us for ATM near cut-through behavior two 4KB packets arrive at same time link idle for 327.68us while both arrive at end of 327.68us, still have 8KB to transmit in contrast, can transmit first cell after 4.24us at end of 327.68us, just over 4KB left in queue Univ. of Tehran Introduction to Computer Network 38 Big vs Small (cont) Small Improves Latency (for voice) voice digitally encoded at 64Kbps (8-bit samples at 8KHz) need full cell’s worth of samples before sending cell example: 1000-byte cells implies 125ms per cell (too long) smaller latency implies no need for echo cancellors ATM Compromise: 48 bytes = (32+64)/2 Univ. of Tehran Introduction to Computer Network 39 Cell Format User-Network Interface (UNI) 4 8 16 3 1 8 384 (48 bytes) GFC VPI VCI Type CLP HEC (CRC-8) Payload host-to-switch format GFC: Generic Flow Control (still being defined) VCI: Virtual Circuit Identifier VPI: Virtual Path Identifier Type: management, congestion control, AAL5 (later) CLP: Cell Loss Priority HEC: Header Error Check (CRC-8) Network-Network Interface (NNI) switch-to-switch format Univ. Tehranbecomes part Introduction to Computer ofGFC of VPI fieldNetwork 40 Segmentation and Reassembly ATM Adaptation Layer (AAL) AAL 1 and 2 designed for applications that need guaranteed rate (e.g., voice, video) AAL 3/4 designed for packet data AAL 5 is an alternative standard for packet data AAL AAL … … ATM Univ. of Tehran ATM Introduction to Computer Network 41 AAL 3/4 Convergence Sublayer Protocol Data Unit (CSPDU) 8 8 16 < 64 KB CPI Btag BASize User data 0– 24 8 8 16 Pad 0 Etag Len CPI: common part indicator (version field) Btag/Etag:beginning and ending tag BAsize: hint on amount of buffer space to allocate Length: size of whole PDU Univ. of Tehran Introduction to Computer Network 42 Cell Format 40 ATM header 4 10 Type SEQ MID 352 (44 bytes) Payload 6 10 Length CRC-10 Type 2 BOM: beginning of message COM: continuation of message EOM end of message SEQ: sequence of number MID: message id Length: number of bytes of PDU in this cell It uses 4 extra bytes for SAR, not good! Univ. of Tehran Introduction to Computer Network 43 AAL5 CS-PDU Format < 64 KB 0– 47 bytes 16 16 32 Data Pad Reserved Len CRC-32 pad so trailer always falls at end of ATM cell Length: size of PDU (data only) CRC-32 (detects missing or misordered cells) Cell Format end-of-PDU bit in Type field of ATM header Univ. of Tehran Introduction to Computer Network 44 ATM Layers Aplication PHY in ATM is usually SONET. TCP Higher Layers IP AAL DLL ATM PHY PHY TCP/IP ATM • Sending IP over ATM is done by address translation. • ATM is also used in LAN. Then, It tries to emulate LAN. • The Technology is called LANE. Univ. of Tehran Introduction to Computer Network 45