Univ. of TehranIntroduction to Computer Network1 Introduction to Computer Networks University of Tehran Dept. of EE and Computer Engineering By: Dr. Nasser Yazdani Lecture 6: Packet Switching

Univ. of TehranIntroduction to Computer Network2 outline Store-and-Forward Switches Bridges and Extended LANs Packet switching Cell Switching Segmentation and Reassembly

Univ. of TehranIntroduction to Computer Network3 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 m in Ethernet. Solution: This is like telephone network. Then, use Switches.

Univ. of TehranIntroduction to Computer Network4 Switches forwards packets from input port to output port 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. Needs some kinds of scheduling for packet delivery. If the buffer overflow, it will be a congestion. Input ports T3 STS-1 T3 STS-1 Switch Output ports

Univ. of TehranIntroduction to Computer Network5 An Ethernet Network Router Outside world Problem:  Shared network limits throughput.  Lots of collisions reduces efficiency.

Univ. of TehranIntroduction to Computer Network6 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 TehranIntroduction to Computer Network7 Ethernet Switching Router Outside world Benefits:  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. Ethernet Switch

Univ. of TehranIntroduction to Computer Network8 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 TehranIntroduction to Computer Network9 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) Ethernet Switch is called Bridge traditionally. A Bridge BC XY Z Port 1 Port 2

Univ. of TehranIntroduction to Computer Network10 One Ethernet Switch in “Packard”

Univ. of TehranIntroduction to Computer Network11 Output ports? Learning Bridges Maintain a forwarding table for hosts. How to populate table. Manually by system admin. (not good) Learn table entries based on source address (flexible). PortHost 1A 1B 1C 2X 2Y 2X A Bridge BC XY Z Port 1 Port 2

Univ. of TehranIntroduction to Computer Network12 Learning Bridges, (Forwarding) 1. If the frame destination address is in the routing table, forward the frame to the corresponding port. 2. Otherwise, broadcast the frame. 3. 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 TehranIntroduction to Computer Network13 Ethernet Switching Learning addresses Router Outside world Ethernet Switch Ethernet Switch Ethernet Switch Ethernet Switch A BCD E FGH I JKL M NOP Q J F Switch learns that ‘F’ is reachable through port 5

Univ. of TehranIntroduction to Computer Network14 Ethernet Switching Learning addresses Router Outside world Ethernet Switch Ethernet Switch Ethernet Switch Ethernet Switch A BCD E FGH I JKL M NOP Q Ethernet addressPort A1 B2 C3 D4 E, F, G, H, Q5 I, J, K, L, M, N, O, P6 Q: How do we prevent loops?

Univ. of TehranIntroduction to Computer Network15 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 TehranIntroduction to Computer Network16 Spanning Tree Algorithm Spanning tree A distributed spanning tree algorithm select which bridges actively forward developed by Radia Perlman now IEEE specification A C E D B K F H J G I B3 B2 B7 B4 B1 B6 B5

Univ. of TehranIntroduction to Computer Network17 Example Spanning Tree B3 B5 B7 B2 B1 B6 B4 Protocol operation : 1.Picks a root 2.For each LAN, picks a designated bridge that is closest to the root. 3.All bridges on a LAN send packets towards the root via the designated bridge. B8

Univ. of TehranIntroduction to Computer Network18 Example Spanning Tree B3 B5 B7 B2 B1 B6 B4 Root B8 B2B4B5B7 B8 B1 Spanning Tree:

Univ. of TehranIntroduction to Computer Network19 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) B3 A C E D B2 B5 B B7 K F H B4 J B1 B6 G I Each bridge forwards frames over each LAN for which it is the designated bridge

Univ. of TehranIntroduction to Computer Network20 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 TehranIntroduction to Computer Network21 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 config messages claiming to be the root

Univ. of TehranIntroduction to Computer Network22 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 TehranIntroduction to Computer Network23 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 TehranIntroduction to Computer Network24 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 TehranIntroduction to Computer Network25 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 TehranIntroduction to Computer Network26 Source Routing List entire path in packet Driving directions (north 3 hops, east, etc..) Router processing, one option 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 TehranIntroduction to Computer Network27 Source Routing Example Receiver Packet 3,4,3 Sender R1 R2 R1 4,33 All routing information is provided by the source.

Univ. of TehranIntroduction to Computer Network28 Source Routing Advantages Switches can be very simple and fast Disadvantages Variable (unbounded) header size Sources must know or discover topology (e.g., failures) Typical use Debugging and management

Univ. of TehranIntroduction to Computer Network29 Virtual Circuits/Tag Switching Use the telephone model virtual circuits Each flow is identified by a Virtual Circuits Identifier (VCI). Connection setup phase, Signaling 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 Each packet carries connection ID Sent from source with 1 st 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 TehranIntroduction to Computer Network30 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 VC1 VC2 VC3 VC2 In, VC Out, VC 1, 1 4, 1 1, 2 4, 3 2, 1 4, 2 ….

Univ. of TehranIntroduction to Computer Network31 Virtual Circuits Examples Receiver Packet 1,5  3,7 Sender ,7  4, ,2  3,6 R2 R3 R In-portIn-VCIOut-portOut-VCI Lookup table for Switch R1 Subsequence packets follow the same circuit

Univ. of TehranIntroduction to Computer Network32 Virtual Circuits Advantages 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. Disadvantages 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 TehranIntroduction to Computer Network33 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 resources.

Univ. of TehranIntroduction to Computer Network34 Datagram Switching No connection setup phase since it is costly. Each packet is forwarded independently Sometimes called connectionless model Analogy: postal system Each switch maintains a forwarding (routing) table Receiver Packet R Sender R1 R2 R1 R R R  3 R  4 R  3 R

Univ. of TehranIntroduction to Computer Network35 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 TehranIntroduction to Computer Network36 Cell Switching 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 TehranIntroduction to Computer Network37 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 TehranIntroduction to Computer Network38 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 = us high priority packet may sit in the queue us in contrast, 53 x 8/100 = 4.24us for ATM near cut-through behavior two 4KB packets arrive at same time link idle for us while both arrive at end of us, still have 8KB to transmit in contrast, can transmit first cell after 4.24us at end of us, just over 4KB left in queue

Univ. of TehranIntroduction to Computer Network39 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 TehranIntroduction to Computer Network40 Cell Format User-Network Interface (UNI) 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 GFC becomes part of VPI field GFCHEC (CRC-8) VPIVCICLPTypePayload 384 (48 bytes)8

Univ. of TehranIntroduction to Computer Network41 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 ATM AAL ATM ……

Univ. of TehranIntroduction to Computer Network42 AAL 3/4 Convergence Sublayer Protocol Data Unit (CS- PDU) 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 TehranIntroduction to Computer Network43 Cell Format Type 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 TehranIntroduction to Computer Network44 AAL5 CS-PDU Format 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 TehranIntroduction to Computer Network45 ATM Layers Higher Layers AAL ATM PHY Aplication TCP IP DLL PHY TCP/IP ATM PHY in ATM is usually SONET. 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.