 Instructor: Rob Nash  Readings: Chapter 3, P&D

 We have a limited number of hosts so far  Also, a limited geographical distance ◦ As broadcast can only take us so far  We can connect two distant nodes (or networks) via point-to-point connections ◦ But we don’t service any nodes in between  We’d like to build a global network, so we must consider hosts that aren’t directly connected.

 “Nature seems […] to reach many of her ends by long circuitous routes.” – Rudolph Lotze  “Packets are able to reach many different ends by (sometimes) long circuitous routes” ◦ But imagine this dilemma for a second: ◦ How are packets able to navigate an unknown topology?  Ether is simple: send to everybody, but again doesn’t scale

 Your phone isn’t directly connected to all other phone users  Rather, you’re connected to a switch  An operator will provide the “directly connected” illusion by configuring a (temporary) link for use in the call  In the same vein, computer networks have packet switches ◦ For use in forwarding/switching packets  Routing is the process of building a forwarding table (4)

 Very broadly defined here as either:  Connection-oriented: Like a telephone call, with temporary state stored at each switch ◦ X.25 ◦ ATM  Connectionless: Like the postal service, with even less recourse for problems (no RTS, etc.) ◦ IP, UDP  Also, we’ll focus on two specific examples of switching ◦ Ethernet & ATM

 Forwarding is a table lookup ◦ Given the input port and ID, what is the output port and outgoing ID?  Routing is the algorithm that builds the table ◦ A distributed algorithm by nature of the domain ◦ Should be fair ◦ Consider offering a QoS ◦ This has evolved over the history of networks  LAN Switching is an evolution of Ethernet Bridging with performance augmentations

CSS432: Switching and Fowarding 7  Switch Function: ◦ Connects two or more network segments ◦ Forwards packets from input port to output port ◦ Selects a port based on address in packet header Input ports T3 STS-1 T3 STS-1 Switch Output ports

 Covers a large geographic area (> 2500m in Ethernet)  Support large numbers of hosts (>1024 hosts in Ethernet)  Maintain performance (>two packets through a switch) ◦ And for n input ports each with buffer b, we can provide n x b queuing simultaneously  Contrast this to Ethernet, where two hosts will compete for the line

 Point-to-Point  Ethernet MAC  Rings  A switch adds the star topology to our set ◦ Also, the ability to interconnect any of the above networking technologies  As switches may be connected to hosts, or other switches

 Switched networks are more scalable than shared- media networks ◦ Directly due to their ability to support many hosts at full speed (limited to memory capacity)  And, we can use a switch to combine two disparate networks ◦ A SONET STS-3 link with and a few T3s ◦ Each port runs the appropriate link layer protocol  Switching (or forwarding): receiving incoming packets on an input port and selecting the appropriate output port on which to forward the data

 How does a switch make its decision? ◦ This depends on the approach {connectionless, etc} ◦ In general, look at the header of the packet for an identifier (could be a local id, could be an IP addr)  Use this to make your decision by looking up the ID in a table, and forward accordingly  We’ll start simply with the datagram approach

 We can provide unique identifiers to each host on the network (e.g., an address)  We also will be interested in providing identifiers to label each input and output port in a switch

 Each packet contains enough information to enable any switch to forward it  How? Just including the complete destination address in every packet.  Each switch will use the destination address as the key in the lookup  No connection state (thus no setup)  All packets are forwarded independently  Node failure and reroute is possible

CSS432: Switching and Fowarding Switch 3 Host B Switch 2 Host A Switch 1 Host C Host D Host E Host F Host G Host H DestPort A3 B0 C3 D3 E2 F1 G0 H0 Table at Switch 2

 In a simple and static environment, one network operator may know the topology ◦ And, manually install this in switches in the network  In a distributed and dynamic environment, no one operator knows the complete topology ◦ Multiple pathways, failing nodes, etc. ◦ This harder problem is routing (Section 4.2) ◦ For now: routing is an assumed background process, and forwarding is a simple lookup

 Hosts can send packets at any time (and to anywhere) ◦ No setup or teardown ◦ All switches can immediately forward this packet, assuming a correct routing table  Hosts don’t know (or care) about the health of the intermediary network or destination node ◦ You could send a packet to a machine that just lost power ◦ Or, you could send a packet through a network whose switches just lost power  Failures may not catastrophically effect communications if alternate routes exists around failed nodes (and the network updates its tables)

 A connection-oriented approach ◦ With a setup, communicate, and teardown phase ◦ This may seem like TCP over IP, but we’ll see this is implemented on top of the connectionless approach  Setup: establishing connection state and path through the network ◦ Each subsequent packet will follow this path  Forwarding tables use VCIs – Virtual Circuit Identifiers – that help uniquely identify connections at a local switch

CSS432: Switching and Fowarding 18 Each switch maintains a VC table The Input Port & VCI uniquely determine a connection Switch 3 Host B Switch 2 Host A Switch 1 VCI = 5 VCI = 11 VCI = 7 VCI = 4 Port (in)VCIPort (out)VCI Port (in)VCIPort (out)VCI Port (in)VCIPort (out)VCI 0734 Switch 1 Switch 2 Switch 3

 PVCs – “permanent Virtual Circuits”, which are long-lived (or network operator configured) table entries  Signaled: a host may set up or delete a VC dynamically and autonomously

 Oracle: How do switches to know what outgoing VCI they should use? ◦ This data is literally downstream of the current switch!  Answer: We fill this data in “in reverse”, after we’ve built a path from A to B. ◦ Then, a setup/connection packet from B to A is sent informing each upstream hop of the VCI it should use  We signal to set up (reserve a VCI entry) and signal to reclaim these resources when done

 At least one RTT delay before any payload is communicated… ◦ Why?  Setup packets differ from payload packets ◦ Since setup contains the full GuID for the destination ◦ So, per-packet overhead is reduced relative to the datagram approach  When we do get to send data, much network topology is known in advance ◦ There is a receiver and route to that receiver, and the receiver is ready to accept data

 Resources are reserved in advance to avoid contention  SWP is used in between node pairs along the circuit  Flow control is used to prevent congestion, and new circuits are declined if not enough resources at a switch

 Popular with telephony companies in the 80s  Physical medium : POTS links or ISDN ◦ ISDN integrates speech and data on the same line ◦ Pre-DSL  From Wiki:  “X.25 is today to a large extent replaced by less complex protocols, especially the Internet protocol (IP) “ Internet protocol

 We see the datagram approach is minimal and doesn’t reserve resources in advance ◦ But, it also cannot make the same guarantee that X.25 can  We can implement a QoS concept using the connection model, as we set the service level per connection ◦ QoS here: a performance or resource guarantee  My packets shouldn’t be delayed (queued) too long  My packets will always be accommodated at each switch

 Frame Relay is used for VPN construction (4.1.8)  ATM is used to link telephony systems across wide areas in a point-to-point configuration

 Consider a pair of Ethernets you’d like to connect  We could just place a “repeater” terminal that collects all packets on one net and broadcasts them to the other ◦ Shout louder! ◦ This forms an extended LAN ◦ The simplest version does no optimization  Note that a “bridge” here could be a host, but it meets our definition of a switch.

 Consider a shared-media example  Consider the star topology offered by switching ◦ Note that each host has its own dedicated link  In the MAC example, link contention is an issue ◦ In the switching example, I can send as much as the switch can forward (or buffer) on my own link

CSS432: Switching and Fowarding 28  Connecting two or more LANs ◦ Repeater  L1 – Physical Layer  Limitations: <= 2500m and <= 1024 nodes ◦ Bridge (or LAN switch)  L2 – link layer  No physical limitations  Fowarding frames using MAC address  Static configuration + partial dynamic configuration (Spanning Tree Protocol) ◦ Router  L3 – Network Layer  Routing IP packets using IP address  Dynamic configuration A Bridge BC XY Z Port 1 Port 2

CSS432: Switching and Fowarding29 Learning Bridges Do not forward when unnecessary  Ex. A frame sent from A to B Maintain forwarding table HostPort A1 B1 C1 X2 Y2 Z2 Learn table entries based on source address  Ex. An entry for A is registered upon receiving a frame from A  Ex. When receiving a frame from B, don’t forward to Port 2 Table is an optimization; need not be complete Entries are expired after a specific period of time A Bridge BC XY Z Port 1 Port 2 Based on datagram switching

 How could a network come to have cycles in it? ◦ Perhaps it’s a multi-site distributed net where no one administrator knows the complete topology ◦ Introduced by accident? ◦ More likely: introduced for redundancy!  However, Learning Bridges can fail if a cycle exists, so we need a strategy to address graph cycles.

 Algorithm deactivates ports to remove cycles ◦ The spanning tree determines which bridges to use, and which bridges should “sit out”  Note that a bridge may forward on some ports, but not others  Formalized in the IEEE Specification ◦ Bridges adopt this distributed algorithm (as we’ll see)  Concept: remove edges from your graph until no cycles exist (the tree is a subset of the graph) ◦ Oddity: vertices in this graph are both hosts and switches

 When the network has settled, certain bridges will be designated to forward packets over their IO ports based on their distance to the root (or ID number if a tie)  Other bridges or ports will simply be disabled  Each bridge decides the ports over which it will and will not forward frames

 Elect the smallest ID as the root ◦ Roots always forward over all ports  Each bridge computes the distance between it and the root ◦ Usually a per-hop count  Trades this information with its neighbors, keeping track of “best” paths ◦ Ie, shortest hop count in this context ◦ Bridges that offer the best paths become designated  Finally all bridges determine the root feeder, which is the only bridge that forwards to the root ◦ Chosen so it is closest to the root

CSS432: Switching and Fowarding34 STP Overview Each bridge has unique id (e.g., B1, B2, B3) Select a bridge with smallest id as root Select a 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 if it is a designated bridge root 1 hop B5 < B7 1 hop B4 < B6 1 hop 2 hops

CSS432: Switching and Fowarding35 STP Details (use p. 191) Initially, each bridge believes it is the root When learn not the root, stop generating configuration messages  in steady state, only the root generates messages When learn not a designated bridge, stop forwarding configuration messages  in steady state, only designated bridges forward configuration messages If any bridge does not receive configuration message after a period of time, it starts generating configuration messages claiming to be the root. B3 A C E D B2 B5 B B7 K F H B4 J B1 B6 G I Bridges exchange configuration messages (Y, d, X)  Y: the id of root to be  d: #hops from X to Y  X: the sending bridge id (1, 0, 1) (1, 1, 2) (1, 1, 5) (1, 0, 1)

36  STP: ◦ It won’t forward frames over alternative paths for the sake of:  Routing around a congested bridge  Routing along a shorter path like one from a node on B to another node on K ◦ Scales linearly, and uses broadcast mechanism  Bridges in general: ◦ Not scalable (“tens of”)  STP  Broadcast (forwarding all broadcast/multicast frames in the current practice) ◦ Homogenous networks only (uses network’s frame header)  Ethernet to Ethernet  Token ring to Token ring  ATM to ATM  Idea: Partition LANS using coloring/tiling to limit the number  Of network segments that will broadcast

 “It is never safe to design network software under the assumption that it will run over a single Ethernet segment.”  “Bridges happen.” ◦ Drop frames if congested (rare on Ethernet alone) ◦ Frames could be reordered in an extended LAN  Not in a singular Ethernet segment

 Many ways to build economy & high-end switches ◦ More advanced fabrics are implemented in high-end (core) switches  The high level concepts overlap, however  One idea: Get a box and a few NICs (DMA) ◦ Not a bad experimentation setup for new protocols ◦ Or cross-protocol examination  Not so hot for performance  Another idea: Custom Hardware ◦ A shared-memory switch  memory with dual ports  Crossbar switch  Switches that attempt to self-route ( , Batcher & Banyan)

CSS432: Switching and Fowarding 39  Advantage: flexible because a workstation has a CPU.  Example ◦ 33MHz 32bit I/O bus  1Gbps for one way from NIC to main memory  500Mbps for a round trip between NIC and main memory  Enough to support five 100Mbps Ethenet ◦ What if a packet is very small like 64byes  The workstation has 500,000 packets per second (pps).  Throughput: 500,000 x 64 x 8 = 256Mbps NIC I/O ctlr CPU Main memory I/O Bus LAN A LAN B LAN C Workstation

CSS432: Switching and Fowarding 40  A simple design  Shared bus or memory becomes a bottleneck. (Max. 16 bus masters) Output Port Input Port Shared memory Shared bus Control processor DMA from port to port

CSS432: Switching and Fowarding 41  Without a collision, all inputs delivered to each output  All inputs may go to the same output which causes a collision in the output buffer.

 Connection-oriented packet switching ◦ Uses signaling (Protocol Q.2931)  WAN, but more recently LANs  Runs on various physical mediums ◦ SONET ◦ Shared Media such as Wireless ◦ Shared-Media like Ethernet (with LANE)  Packets are called cells, which are fixed length ( Bytes)

 LAN packets V.S. ATM cells ◦ Consider also CISC v.s. RISC  In this light, certain features of ATM shine  Observations for a short and simple approach: ◦ Its easier to build HW to do simple (short) jobs ◦ The processing of data is simpler when fixed format  RISC ISA commonly has only a few instruction formats  Off topic: & Dec.Intel.Xerox Ethernet standard  Meaning: Compatibility can be simpler with a common format ◦ Simple and short data {frames, instructions} can often be “trained” or “pipelined”

 Observation: homogenous packet length lends to homogenous switching structures ◦ Short and uniform structures can make the task of exploiting parallelism easier  Either at the hardware level  See simultaneous multithreading  Or along protocol stack (simultaneous packet processing, self-organizing streams, etc.) ◦ Uniformity at higher levels tends to promote uniform hardware designs  Since this is not custom, often cheaper to build this fast, scalable hardware

 Fixed length instructions help to align, fetch, prefetch, optimize, synchronize, reorder etc. ◦ See the original 360 and Robert Tomasulo  Variable length instructions are more complex by design, ◦ possibly requiring multiple cycles to fetch a longer instruction  And/or more trips across the bus to and from memory  All said and done, Ethernet LANs are just as convincing due to their speed, cost, success & adoption rate

 Error detection is implemented at endpoints ◦ End-to-end but not at each switch (i.e., at data link layer)  Congestion control ◦ Admission control  If switches are completely reserved, decline connections

 Fixed-size cells can make this easier  One Approach: use some SONET overhead to point to the start of the cell in the payload  Another Approach: CRC every 5 bytes ◦ If you see no error, you’re likely at an ATM header  Repeat this approach looking for the same results every 53 bytes  See p.199 for the frame format

 Not exaustive  ATM offers Qos features  ATM offers flow control, LANs are “best effort”  ATMS are conservative resource-wise ◦ Connectionless protocols are minimalist  ATM can guarantee resources ahead of time ◦ Useful esp. for voice-grade guarantees  Fixed length V.S. variable length packets  No broadcast (natively) V.S. only broadcast

 Layers were built ontop of ATM to support other styles of networks and services ◦ AAL 1-2 is for voice grade guaranteed bit rates ◦ AAL 3-4 is for packet data over ATM  This requires S&R, since MTU for Ethernet >> 53B

 When packets are being discarded frequently due to lack of resources ◦ arrivalRate > sendRate + bufferSpace for some t