Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 1 Review of Networking and Design Concepts (I): Brief Version Based in part upon slides of Prof. Raj Jain (OSU), S. Keshav (Cornell), L. Peterson (Princeton), J. Kurose (U Mass) Or GOOGLE: “Shiv RPI” Shivkumar Kalyanaraman Rensselaer Polytechnic Institute

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 2 q Connectivity: q direct (pt-pt, N-users), q indirect (switched, inter-networked) q Concepts: Topologies, Framing, Multiplexing, Flow/Error Control, Reliability, Multiple-access, Circuit/Packet- switching, Addressing/routing, Congestion control q Data link/MAC layer: q SLIP, PPP, LAN technologies … q Interconnection Devices q Chapter 1,2,11 in Doug Comer book q Reading: Saltzer, Reed, Clark: "End-to-End arguments in System Design""End-to-End arguments in System Design" q Reading: Clark: "The Design Philosophy of the DARPA Internet Protocols":"The Design Philosophy of the DARPA Internet Protocols": q Reading: RFC 2775: Internet Transparency: In HTMLIn HTML Overview

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 3 Connectivity... q Building Blocks q links: coax cable, optical fiber... q nodes: general-purpose workstations... q Direct connectivity: q point-to-point q multiple access

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 4 Connectivity… (Continued) q Indirect Connectivity q switched networks => switches q inter-networks => routers

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 5 What is “Connectivity” ? q Direct or indirect access to every other node in the network q Connectivity is the magic needed to communicate if you do not have a link. q Tradeoff: Performance characteristics worse!

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 6 Connectivity … q Internet: q Best-effort (no performance guarantees) q Packet-by-packet q A pt-pt link: q Always-connected q Fixed bandwidth q Fixed delay q Zero-jitter

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 7 Point-to-Point Connectivity Issues q Physical layer: coding, modulation etc q Link layer needed if the link is shared bet’n apps; is unreliable; and is used sporadically q No need for protocol concepts like addressing, names, routers, hubs, forwarding, filtering … AB

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 8 Link Layer: Serial IP (SLIP) q Simple: only framing = Flags + byte-stuffing q Compressed headers (CSLIP) for efficiency on low speed links for interactive traffic. q Problems: q Need other end’s IP address a priori (can’t dynamically assign IP addresses) q No “type” field => no multi-protocol encapsulation q No checksum => all errors detected/corrected by higher layer. q RFCs: 1055, 1144

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 9 Link Layer: PPP q Point-to-point protocol: solves problems with SLIP q Frame format similar to HDLC q Multi-protocol encapsulation, CRC, dynamic address allocation possible q key fields: flags, protocol, CRC q Asynchronous and synchronous communications possible q Link and Network Control Protocols (LCP, NCP) for flexible control & peer-peer negotiation q Can be mapped onto low speed (9.6Kbps) and high speed channels (SONET) q RFCs: 1548, 1332

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 10 Reliability Mechanisms q Mechanisms: q Checksum: detects corruption in pkts & acks q ACK: “packet correctly received” q Duplicate ACK: “packet incorrectly received” q Sequence number: identifies packet or ack q 1-bit sequence number used both in forward & reverse channel q Timeout only at sender q Reliability capabilities achieved: q An error-free channel q A forward & reverse channel with bit-errors q Detects duplicates of packets/acks q NAKs eliminated q A forward & reverse channel with packet-errors (loss)

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 11 Stop and Wait Flow Control Data Ack Data t frame t prop  = t prop t frame = Distance/Speed of Signal Frame size /Bit rate = Distance  Bit rate Frame size  Speed of Signal = 1 2  + 1 U= 2t prop +t frame t frame U  Light in vacuum = 300 m/  s Light in fiber = 200 m/  s Electricity = 250 m/  s

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 12 Sliding Window Protocols Data Ack t frame t prop U= Nt frame 2t prop +t frame = N 2  +1 1 if N>2  +1

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 13 Connecting N users: Directly… q Pt-pt: connects only two users directly… q How to connect N users directly ? q What are the costs of each option? q Does this method of connectivity scale ? AB... Full mesh Bus

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 14 Multiplexing vs Have it all q Multiplexing = sharing q Allows system to achieve “economies of scale” q Cost: waiting time (delay), buffer space & loss q Gain: Money ($$) => Overall system costs less Full MeshBus

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 15 Virtualization q The multiplexed shared resource with a level of indirection will seem like a unshared virtual resource! q I.e. Multiplexing + indirection = virtualization q “Refer” to the virtual resource as if it were the physical resource. q Eg: virtual memory, virtual circuits… q Connectivity: a virtualization created by the Internet! q Indirection requires binding and unbinding…... Physical Bus = AB AB Virtual Pt-Pt Link

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 16 Statistical Multiplexing q Reduce resource requirements (eg: bus capacity) by exploiting statistical knowledge of the system. q Eg: average rate <= service rate <= peak rate q For a stable multiplexed system: q Gain = peak rate/service rate. q Cost: buffering, queuing delays, losses. q Statistical Multiplexing useful only if peak rate differs significantly from average rate.

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 17 Stability of a Multiplexed System Average Input Rate > Average Output Rate => system is unstable! How to ensure stability ? 1.Reserve enough capacity so that demand is less than reserved capacity 2.Dynamically detect overload and adapt either the demand or capacity to resolve overload

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 18 What’s a performance tradeoff ? q R=link bandwidth (bps) q L=packet length (bits) q a=average packet arrival rate Traffic intensity = La/R You cannot get something for nothing! a.k.a zero-sum game.

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 19 What’s a performance tradeoff ? q La/R ~ 0: average queuing delay small q La/R -> 1: delays become large q La/R > 1: average delay infinite (service degrades unboundedly => instability)! Summary: Multiplexing using bus topologies has both direct resource costs and intangible costs like potential instability, buffer/queuing delay.

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 20 How to build Scalable Networks? q Direct connectivity does not scale! q Inefficiency limits scaling … q Direct connectivity is inefficient & hence does not scale q Mesh: inefficient in terms of # of links q Bus architecture: 1 expensive link, N cheap links. Inefficient in bandwidth use

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 21 Filtering, forwarding … q Filtering: choose a subset of elements from a set q Don’t let information go where its not supposed to… q Filtering => More efficient => more scalable Filtering is the key to efficiency & scaling q Forwarding: actually sending packets to a filtered subset of link/node(s) q Packet sent to one link/node => efficient q Solution: Build nodes which focus on filtering/forwarding and achieve indirect connectivity “switches” & “routers”

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 22 Connecting N users: Indirectly q Star: One-hop path to any node, reliability, forwarding function q “Switch” S can filter and forward! q Switch may forward multiple pkts in parallel for additional efficiency! Star S

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 23 Ring Star S Tree Topologies: Indirect Connectivity

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 24 MAC Protocols: a taxonomy Three broad classes: q Channel Partitioning q divide channel into smaller “pieces” (time slots, frequency) q allocate piece to node for exclusive use q Random Access q allow collisions q “recover” from collisions q “Taking turns”: Token-based q tightly coordinate shared access to avoid collisions Goal: efficient, fair, simple, decentralized

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 25 Review: Multiple Access Protocols q Aloha at University of Hawaii: Transmit whenever you like Worst case utilization = 1/(2e) =18% q CSMA: Carrier Sense Multiple Access Listen before you transmit q CSMA/CD: CSMA with Collision Detection Listen while transmitting. Stop if you hear someone else. q Ethernet uses CSMA/CD. Standardized by IEEE committee.

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 26 10Base5 Ethernet Cabling Rules q Thick coax q Length of the cable is limited to 2.5 km, no more than 4 repeaters between stations  No more than 500 m per segment  “10Base5” 2.5m 500 m Repeater Terminator Transceiver

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 27 10Base5 Cabling Rules (Continued) q No more than 2.5 m between stations q Transceiver cable limited to 50 m 2.5m 500 m Repeater Terminator Transceiver

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 28 Inter-connection Devices q Repeater: Layer 1 (PHY) device that restores data and collision signals: a digital amplifier q Hub: Multi-port repeater + fault detection q Note: broadcast at layer 1 q Bridge: Layer 2 (Data link) device connecting two or more collision domains. q Key: a bridge attempts to filter packets and forward them from one collision domain to the other. q It snoops on passing packets and learns the interface where different hosts are situated, and builds a L2 forwarding table q MAC multicasts propagated throughout “extended LAN.” q Note: Limited filtering intelligence and forwarding capabilities at layer 2

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 29 Interconnection Devices (Continued) q Router: Network layer device. IP, IPX, AppleTalk. Interconnects broadcast domains. q Does not propagate MAC multicasts. q Switch: q Key: has a switch fabric that allows parallel forwarding paths q Layer 2 switch: Multi-port bridge w/ fabric q Layer 3 switch: Router w/ fabric and per-port ASICs These are functions. Packaging varies.

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 30 Interconnection Devices H H B H H Router Extended LAN =Broadcast domain LAN= Collision Domain Network Datalink Physical Transport Router Bridge/Switch Repeater/Hub Gateway Application Network Datalink Physical Transport Application

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 31 Ethernet (IEEE 802) Address Format q 48-bit flat address => no hierarchy to help forwarding q Hierarchy only for administrative/allocation purposes q Assumes that all destinations are (logically) directly connected. q Address structure does not explicitly acknowledge indirect connectivity q => Sophisticated filtering cannot be done! G/L bit (Global/Local) G/I bit (Group/Individual) OUI (Organizationally Unique ID)

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 32 Inter-Networks: Networks of Networks = Internet … … …… Our goal is to design this black box on the right

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 33 Inter-Networks: Networks of Networks q What is it ? q “Connect many disparate physical networks and make them function as a coordinated unit … ” - Douglas Comer q Many => scale q Disparate => heterogeneity q Result: Universal connectivity! q The inter-network looks like one large switch, q User interface is sub-network independent

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 34 Inter-Networks: Networks of Networks q Internetworking involves two fundamental problems: heterogeneity and scale q Concepts: q Translation, overlays, address & name resolution, fragmentation: to handle heterogeneity q Hierarchical addressing, routing, naming, address allocation, congestion control: to handle scaling q Two broad approaches: circuit-switched and packet- switched

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 35 How to design large inter-networks? Circuit-Switching q Divide link bandwidth into “pieces” q Reserve pieces on successive links and tie them together to form a “circuit” q Map traffic into the reserved circuits q Resources wasted if unused: expensive. – Mapping can be done without “headers”. – Everything inferred from timing.

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 36 How to design large inter-networks? Packet-Switching q Chop up data (not links!) into “packets” q Packets: data + meta- data (header) q “Switch” packets at intermediate nodes q Store-and-forward if bandwidth is not immediately available. Bandwidth division into “pieces” Dedicated allocation Resource reservation

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 37 Packet Switching A B C 10 Mbs Ethernet 1.5 Mbs 45 Mbs D E statistical multiplexing queue of packets waiting for output link  Cost: self-descriptive header per-packet, buffering and delays due to statistical multiplexing at switches.  Need to either reserve resources or dynamically detect and adapt to overload for stability

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 38 Spatial vs Temporal Multiplexing q Spatial multiplexing: Chop up resource into chunks. Eg: bandwidth, cake, circuits… q Temporal multiplexing: resource is shared over time, I.e. queue up jobs and provide access to resource over time. Eg: FIFO queueing, packet switching q Packet switching is designed to exploit both spatial & temporal multiplexing gains, provided performance tradeoffs are acceptable to applications. q Packet switching is potentially more efficient => potentially more scalable than circuit switching !

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 39 Scalable Forwarding, Structured Addresses q Address has structure which aids the forwarding process. q Address assignment is done such that nodes which can be reached without resorting to L3 forwarding have the same prefix (network ID) q A simple comparison of network ID of destination and current network (broadcast domain) identifies whether the destination is “directly” connected q I.e. Reachable through L2 forwarding only q Within L3 forwarding, further structure can aid hierarchical organization of routing domains (because routing algorithms have other scalability issues) Network IDHost ID Demarcator

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 40 Flat vs Structured Addresses q Flat addresses: no structure in them to facilitate scalable routing q Eg: IEEE 802 LAN addresses q Hierarchical addresses: q Network part (prefix) and host part q Helps identify direct or indirectly connected nodes

Shivkumar Kalyanaraman Rensselaer Polytechnic Institute 41 Summary: Laundry List of Problems q Basics: Direct/indirect connectivity, topologies q Link layer issues: q Framing, Error control, Flow control q Multiple access & Ethernet: q Cabling, Pkt format, Switching, bridging vs routing q Internetworking problems: Naming, addressing, Resolution, fragmentation, congestion control, traffic management, Reliability, Network Management