Chapter 1: Introduction

Chapter 1: Introduction
What is a Network? What is Internet? Compared with postal service & telephone system “Nuts and Bolts” description Services provided Packet Switching vs. Circuit Switching Fundamental Issues in Computer Networking Protocol and Layered Architecture Internet Protocols, Architecture & History Readings: Chapter 1, Lecture Notes CSci4211: Introduction

Goal and Motivating Questions
What is internet? What’s so special about it? What’s a protocol? How do I build a network? How do I deal with the complexity? What does real Internet look like now? Why I download slowly? Our goal: get “feel” and terminology more depth, detail later in course approach: use Internet as example CSci4211: Introduction 2

Internet is the network!
It’s big! It’s diverse! It’s complex! It’s everywhere (almost)! … and it keeps growing and changing! CSci4211: Introduction 3

Inter-networking Internet: networks of networks
A network can be defined recursively as... two or more networks connected by two or more nodes two or more nodes connected by a link, or Internet: networks of networks started as ARPAnet with only 4 nodes 4 CSci4211: Introduction

Map of Internet

Internet Usage Statistics
source: csci Introduction 6

csci Introduction 7

More gadgets are plugged in … New Era of Internet of Things (IoT)
servers, desktops, laptops, … smart mobile phones, iPads, e-readers, … now TVs, lightbulbs, thermostats, cars, etc., soon fridges, … everything Wireless technologies revolutionizing Internet! WiFi, bluetooth, NFC, Zigbee, 3/4G (soon 5G) cellular networks High-tier Low-tier High Mobility Low Mobility Wide Area Local Area mobile computing location services IoT & Smart Cities 8 CSci4211: Introduction

Internet: a huge transformative & disruptive force!
What has become of the Internet: Information Service and E-Commerce Platform deliver all kinds of information, news, music, video, shopping web, spotify, iTune, youtube, Netflix, Hulu, … Global Information Repository store and search for all kinds of information google, flickr, dropbox, icloud, … Cyberspace and Virtual Communities keep in touch with friends and strangers , facebook, twitter, … Enormous Super-Computer mobile, cloud computing and services We’re increasingly depending on it ! 9 CSci4211: Introduction

So what’s so special about the Internet?
But first, what is a Network? CSci4211: Introduction

What is a Network? There are many types of networks!
Key Features of Networks Providing certain services transport goods, mail, information or data Shared resources used by many users, often concurrently Basic building blocks nodes (active entities): process and transfer goods/data links (passive medium): passive “carrier” of goods/data Typically distributed & “multi-hop”: two “end points” cannot directly reach each other need other nodes/entities to relay CSci4211: Introduction

What is a Network … Compare Internet with
Postal Service and Telephone System Services Provided Various Key Pieces and Their Functions How the pieces work together to provide services CSci4211: Introduction

What’s the Internet: “nuts and bolts” view
mobile network global ISP regional ISP home network institutional Internet: “network of networks” Interconnected ISPs protocols control sending, receiving of messages e.g., TCP, IP, HTTP, Skype, Internet standards RFC: Request for comments IETF: Internet Engineering Task Force CSci4211: Introduction

What’s the Internet: a service view
infrastructure that provides services to applications: Web, VoIP, , games, e-commerce, social nets, … provides programming interface to apps hooks that allow sending and receiving app programs to “connect” to Internet provides service options, analogous to postal service mobile network global ISP regional ISP home network institutional CSci4211: Introduction

Nuts and Bolts Description
Network is fundamentally distributed in nature: a collection of distinct entities: “nodes” and “links” Postal: Mailboxes Local/Branch Postal Offices, Regional, Central Postal Offices Mail Sorting Machines Postmen, Delivery Trucks/Trains/Planes, Roads, … Telephone: Phones Local Switching Office, Central Switching Offices, … Telephone Switches Wires Internet ? CSci4211: Introduction

Internet: Building Blocks
Nodes: PCs, special-purpose hardware, … Hosts (or end systems): servers, PCs, laptops, mobile devices, smart meters, …… Switches: routers, switches, … Links: coax cable, optical fiber, wireless, … point-to-point multiple access … CSci4211: Introduction

Inter-networking A network can be defined recursively as...
two or more nodes connected by a link, or two or more networks connected by two or more nodes Internet: networks of networks CSci4211: Introduction

Service Perspective Basic Services Provided
Postal: deliver mail/package from people to people First class, express mail, bulk rate, certified, registered, … Telephone: connect people for talking You may get a busy dial tone Once connected, consistently good quality, unless using cell phones Internet: transfer information between people/machines Reliable connection-oriented or unreliably connectionless services! You never get a busy dial tone, but things can be very slow! You can’t ask for express delivery (not at the moment at least!) CSci4211: Introduction

Fundamental Issues in Networking
Network is a shared resource Provide services for many people at same time Carry bits/information for many people at same time Switching and Multiplexing How to share resources among multiple users, and transfer data from one node to another node Naming and Addressing How to find name/address of the party (or parties) you would like to communicate with Address: byte-string that identifies a node unicast, multicast and broadcast addresses Routing and (end-to-end) Forwarding: Routing: process of determining how to send packets towards the destination based on its address find out neighbors, build “maps” (routing tables), … transfer data from source to destination “hop-by-hop” CSci4211: Introduction

What’s so special about the Internet?
Internet is based on the notion of “packet switching” enables statistical multiplexing better utilization of network resources for transfer of “bursty” data traffic CSci4211: Introduction

Switching & Multiplexing
Network is a shared resource Provide services for many people at same time Carry bits/information for many people at same time How do we do it? Switching: how to deliver information from point A to point B? Multiplexing: how to share resources among many users Think about postal service and telephone system! Switching and multiplexing are closely related! CSci4211: Introduction

Switching Strategies Circuit switching Packet switching Pros and Cons?
set up a dedicated route (“circuit”) first carry all bits of a “conversation” on one circuit original telephone network Analogy: railroads and trains/subways Packet switching divide information into small chunks (“packets”) each packet delivered independently “store-and-forward” packets Internet (also Postal Service, but they don’t tear your mail into pieces first!) Analogy: highways and cars Pros and Cons? - think taking subways vs. driving cars, during off-peak vs. rush hours! CSci4211: Introduction

Analogy: railroad and train
CSci4211: Introduction 23

Analogy: Highway and cars
CSci4211: Introduction

Circuit Switching network resources (e.g., bandwidth) divided into “pieces” pieces allocated to calls resource piece idle if not used by owning call (no sharing) dividing link bandwidth into “pieces” frequency division time division code division Trivia Q: You must have heard of the term “CDMA” (think the company Qualcom, for which it is most associated with), what does “CD” in CDMA stands for? CSci4211: Introduction

Circuit Switching: FDM and TDM
4 users Example: FDM frequency time TDM frequency time Two simple multiple access control techniques. Each mobile’s share of the bandwidth is divided into portions for the uplink and the downlink. Also, possibly, out of band signaling. As we will see, used in AMPS, GSM, IS-54/136 CSci4211: Introduction

Numerical example How long does it take to send a file of 640,000 bits from host A to host B over a circuit-switched network? All links are Mbps Each link uses TDM with 24 slots/sec 500 msec to establish end-to-end circuit Let’s work it out! 10.5 seconds CSci4211: Introduction

Networks with Circuit Switching e. g
Networks with Circuit Switching e.g., conventional (fixed-line) telephone networks End-end resources reserved for “call” link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required CSci4211: Introduction

Circuit Switched Networks
All resources (e.g. communication links) needed by a call dedicated to that call for its duration Example: telephone network Call blocking when all resources are used Under circuit switching, all the resources needed by a call are dedicated to that call for the duration of the call. This is used in telephone networks. Good thing about it is its guaranteed service. You are assured of the required resources for the entire duration of the call. The bad thing is that resources are not utilized efficiently. There may be silent periods during the talk but the dedicated network resources cannot be used by other calls. Whether you are shouting or silent you consume the same amount of network resources. What happens when the resource demands exceed the resources available. For example, there is only one circuit/link. Lets say is A is currently occupying the circuits between C-D and D-E. Now suppose B wants to call E. Then B is blocked. Once call is admitted, you are guaranteed to have good quality of service. CSci4211: Introduction

Bandwidth division into “pieces”
Packet Switching Each end-end “data stream” divided into packets users A, B packets share network resources each packet uses full link bandwidth resources used as needed resource contention: aggregate resource demand can exceed amount available congestion: packets queue, wait for link use store and forward: packets move one hop at a time Node receives complete packet before forwarding Packets may suffer delay or losses! Bandwidth division into “pieces” Dedicated allocation Resource reservation CSci4211: Introduction

Statistical Multiplexing
Time division, but on demand rather than fixed Reschedule link on a per-packet basis Packets from different sources interleaved on the link Buffer packets that are contending for the link Buffer buildup is called congestion This is packet switching, used in computer networks CSci4211: Introduction

Packet Switching: Statistical Multiplexing
100 Mb/s Ethernet C A statistical multiplexing 1.5 Mb/s B queue of packets waiting for output link D E Sequence of A & B packets does not have fixed pattern, shared on demand  statistical multiplexing. TDM: each host gets same slot in revolving TDM frame. CSci4211: Introduction

Packet-switching: store-and-forward
L R R R Takes L/R seconds to transmit (push out) packet of L bits on to link or R bps Entire packet must arrive at router before it can be transmitted on next link: store and forward delay = 3L/R (assuming zero propagation delay) Example: L = 7.5 Mbits R = 1.5 Mbps delay = ? 15 sec more on delay later … CSci4211: Introduction

Packet switching versus circuit switching
Packet switching allows more users to use network! 1 Mb/s link each user: 100 kb/s when “active” active 10% of time circuit-switching: 10 users packet switching: with 35 users, probability > 10 active less than .0004 N users 1 Mbps link Q: how did we get value ? CSci4211: Introduction

Circuit Switching vs Packet Switching
Item Circuit-switched Packet-switched Dedicated “copper” path Yes No Bandwidth available Fixed Dynamic Potentially wasted bandwidth No (not really!) Store-and-forward transmission Each packet/bit always follows the same route Not necessarily Call setup Required Not Needed When can congestion occur At setup time On every packet Effect of congestion Call blocking Queuing delay Is packet switching is always preferable? Ideally we want circuit switching type service with the efficiency of packet switching. Computer networks use packet switching. CSci4211: Introduction

Packet switching vs. circuit switching
Is packet switching a “slam dunk winner?” Great for bursty data resource sharing simpler, no call setup Excessive congestion: packet delay and loss protocols needed for reliable data transfer, congestion control Q: How to provide circuit-like behavior? bandwidth guarantees needed for audio/video apps still an unsolved problem (chapter 7) Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)? CSci4211: Introduction

What’s so special about the Internet?
Internet is based on the notion of “packet switching” enables statistical multiplexing better utilization of network resources for transfer of “bursty” data traffic Internet’s key organizational/architectural principle: “smart” end systems + “dumb” networks architecture: functional division & function placement hourglass Internet architecture: enables diverse applications and accommodates evolving technologies “dumb” network (core): simple packet-switched, store-forward, connectionless “datagram” service, with core functions: global addressing, routing & forwarding “smart” end systems/edges: servers, PCs, mobile devices, …; diverse and ever-emerging new applications! CSci4211: Introduction

Internet Hourglass Architecture
enabling diverse applications & new types of end devices p2p file sharing, skype, YouTube, Netflix, Cloud Computing bitTorrent, DHT, SIP, DASH, …. accommodating evolving & new technologies network edge/end hosts network core WiFi, Bluetooth, Docsis, gMPLS, DWDM/fiber, …, 3G/4G cellular, …. CSci4211: Introduction

“Dumb” Networks & “Smart” End Systems
Five Layer Architecture: Lower three layers are implemented everywhere Top two layers are implemented only at hosts Application Transport Host A Application Transport Host B Router Network Network Network Datalink Datalink Datalink Physical Physical Physical Physical medium CSci4211: Introduction

An Overview of Network Structure: a “horizontal view”
network edge: applications and hosts network core: routers network of networks access networks, physical media: communication links CSci4211: Introduction

What’s the Internet: “nuts and bolts” view
millions of connected computing devices: hosts = end systems running network apps communication links fiber, copper, radio, satellite transmission rate = bandwidth routers: forward packets (chunks of data) local ISP company network regional ISP router workstation server mobile CSci4211: Introduction 41

The network edge: end systems (hosts): client/server model
run application programs e.g. Web, at “edge of network” client/server model client host requests, receives service from always-on server e.g. Web browser/server; client/server peer-peer model: minimal (or no) use of dedicated servers e.g. Skype, BitTorrent, KaZaA CSci4211: Introduction

The network edge: end systems (hosts): client/server model
run application programs e.g. Web, at “edge of network” client/server model client host requests, receives service from always-on server e.g. Web browser/server; client/server Cloud & Mobile Computing peer-peer model: minimal (or no) use of dedicated servers e.g. Skype, BitTorrent, KaZaA cloud computing CSci4211: Introduction

Network edge: connection-oriented service
Goal: data transfer between end systems handshaking: setup (prepare for) data transfer ahead of time Hello, hello back human protocol set up “state” in two communicating hosts TCP - Transmission Control Protocol Internet’s connection-oriented service TCP service [RFC 793] reliable, in-order byte-stream data transfer loss: acknowledgements and retransmissions flow control: sender won’t overwhelm receiver congestion control: senders “slow down sending rate” when network congested CSci4211: Introduction

Network edge: connectionless service
Goal: data transfer between end systems same as before! UDP - User Datagram Protocol [RFC 768]: connectionless unreliable data transfer no flow control no congestion control App’s using TCP: HTTP (Web), FTP (file transfer), Telnet (remote login), SMTP ( ), Flash videos, DASH stream videos App’s using UDP: streaming media, teleconferencing, DNS, Internet telephony CSci4211: Introduction

The Network Core mesh of interconnected routers shared by many users
the fundamental questions: how network is shared how to find the other party (person, website, …) you want how is data transferred through net? CSci4211: Introduction

On the Internet Edge … Internet Large # of (mobile & stationary) users
Large # of “dumb” or smart devices & appliances Some “always-on,” high-speed connection Others intermittent connectivity with varying bandwidth Diverse applications and services Heterogeneous technologies social networks music streaming games web video streaming & IPTV others Internet smart pads & e-readers home users sensors & smart home surveillance & security VoIP banking & e-commerce dumb & smart phones POTS CSci4211: Introduction

Within the Internet “Cloud”
Network Core: big ISPs (& cellular providers) with large geographical span As well as medium & smaller ISPs And the “other end/edge”: big content providers with huge data centers High bandwidth, dense and rich topology Enormous computing & storage capacities to support cloud, mobile computing/services CSci4211: Introduction

Motivating Questions 3-5
Well, Internet is too complex for me to learn. How can they even build it? And what’s a protocol & why do we need protocols? Motivating Questions 3-5 CSci4211: Introduction

Network Architecture (or organizational principles)
Networks are complex! many “pieces”: hosts routers links of various media hardware, software applications protocols ….. Question: Is there any hope of organizing structure or principle of network? Or at least our discussion of networks? Network architecture: “blue prints” (or principles) regarding functional division and function placement CSci4211: Introduction

Organization of air travel
ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing a series of steps CSci4211: Introduction

Layering of airline functionality
ticket (purchase) baggage (check) gates (load) runway (takeoff) airplane routing departure airport arrival intermediate air-traffic control centers ticket (complain) baggage (claim gates (unload) runway (land) ticket baggage gate takeoff/landing Layers: each layer implements a service via its own internal-layer actions relying on services provided by layer below CSci4211: Introduction

Why Layering? Dealing with complex systems:
explicit structure allows identification, relationship of complex system’s pieces layered reference model for discussion modularization eases maintenance, updating of system change of implementation of layer’s service transparent to rest of system e.g., change in gate procedure doesn’t affect rest of system CSci4211: Introduction

Internet Protocol Stack
application: supporting network applications FTP, SMTP, HTTP, DASH, … transport: process-process data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements PPP, Ethernet physical: bits “on the wire” application transport network link physical CSci4211: Introduction

Layered Architecture Layering simplifies the architecture of complex system Layer N relies on services from layer N-1 to provide a service to layer N+1 Interfaces define the services offered Service required from a lower layer is independent of its implementation Layer N change doesn’t affect other layers Information/complexity hiding Similar to object oriented methodology Network software is designed using layered approach to make the complex system manageable. Layer N uses the services provided by the lower layers to offer a service to upper layers. Interfaces define the services offered by a layer. By keeping the interface same, the implementation of a layer can be changed without affecting the rest of the system. This type of information and complexity hiding is similar to object oriented approach. CSci4211: Introduction

Protocols and Services
Protocols are used to implement services Peering entities in layer N provide service by communicating with each other using the service provided by layer N-1 Logical vs physical communication Lets look at the relation between services, protocols and interfaces. The interface of a layer defines the service provided by that layer and protocols are used to implement the service according to the interface specification. The protocol is between peer entities and they communicate with each other using the service provided by the lower layer. Here is an example of communication between peer layers. It is important to understand the difference between logical vs physical communication. For example, layer 4’s communicate with each other using a layer 4 protocol. They exchange messages as if they can talk to each other directly. But in reality, messages go thru the router. Layer 4 uses the services offered by layer 3 to send the message to layer 4 on the other side. The dotted line shows the logical communication path and the solid line, physical communication path. This approach allows design, implementation and testing of a layer software independent of other layers. CSci4211: Introduction

What’s a protocol? human protocols: “what’s the time?”
“I have a question” introductions network protocols: machines rather than humans all communication activity in Internet governed by protocols (why this concept is so important!!!) CSci4211: Introduction

Bob can understand English
Human protocol Alice protocols define: Format. Order of msgs sent and received among network entities (two or more) Actions taken on msg transmission, receipt Bob Hi Hi Got the time? 2:00pm Q: What are the purposes of first hi-hi exchange Bob is awake and can hear Bob can understand English Bob can speak English Bob is willing to talk 1. Make sure Bob is awake Bob can understand English 2 Bob can speak English 3 4 Bob is willing to talk CSci4211: Introduction

What’s a protocol? a human protocol and a computer network protocol:
Hi TCP connection request Hi TCP connection response Got the time? Get 2:00 <file> time Q: Other human protocols? (e.g., in-class interaction) CSci4211: Introduction

Protocols Protocol: rules by which network elements communicate
Protocols define the agreement between peering entities The format and the meaning of messages exchanged Protocols in everyday life Examples: traffic control, open round-table discussion etc Protocol is a common term you hear while talking about networks. Protocol is simply the rules by which two peer entities communicate with each other. Protocol defines the syntax of a message, semantics of messages, and actions to be taken upon receipt of a message. In other words, protocols define the format and meaning of the messages exchanged. Protocols are common in everyday life. For example, traffic control. We know what the meaning of a signal and what action to take when you see a signal. Similarly round table discussion, question and answer session in the class etc. Traffic control also helps explain why there are so many protocols. You have signal lights, two-way stop, four-way stop, yield. Each of them are ideal for some traffic conditions. When there is lot of traffic, its better to have signals each way getting a time slice. Under light traffic, its better to have four-way stop type coordination. CSci4211: Introduction

Protocol Packets Protocol data units (PDUs):
packets exchanged between peer entities Service data units (SDUs): packets handed to a layer by an upper layer Data at one layer is encapsulated in packet at a lower layer Envelope within envelope: PDU = SDU + (optional) header or trailer Some terminology here. The packets exchanged between peer entities are called PDUs. Packets handed to a layer by an upper layer is called SDU. Data at one layer is encapsulated in a PDU packet at the next layer. This is like putting an envelope within another envelope. A protocol at a layer doesn’t interpret the data handed by the upper layer. For example, application layer data such as HTTP request message is encapsulated in TCP packet, which in turn in IP and then in Ethernet packet. Each of the layers treat the upper layer’s packet as simply payload. CSci4211: Introduction

Encapsulation source destination application transport network link
message M application transport network link physical segment Ht M Ht datagram Ht Hn M Hn frame Ht Hn Hl M link physical switch destination network link physical Ht Hn M Ht Hn Hl M M application transport network link physical Ht Hn M Ht M Ht Hn M router Ht Hn Hl M CSci4211: Introduction

Internet and ISO/OSI Reference Models
The layered approach is fine but which layer should provide what functionality. International Standards Organization (ISO) defined a reference model specifying the tasks of each layer. It is a seven layer model. Though its protocols are not very popular, it is still considered a good reference model. The most popular internet protocol stack has only 5 layers. It doesn’t have presentation and session layers. A bit of presentation layer job is done by the application layer. Other layers also don’t exactly correspond to OSI layers but roughly similar. We first talk about ISO reference model and then discuss internet protocols. CSci4211: Introduction

ISO/OSI Reference Model
Application layer Examples: smtp, http, ftp, dash, etc process-to-process communication all layers exist to support this layer Presentation layer (OSI only) conversion of data to common format Example: “little endian” vs. “big endian” byte orders multimedia streaming presentation (e.g., mpeg-dash) Session layer (OSI only) session setup (and authentication) recovery from failure (broken session) Internet applications perform presentation/session layer functions, e.g., “little” & “big” endian conversions All the layers exist to support the application layer because this where the end user applications reside. Examples of application layer protocols are smtp for , http for web, and ftp for file transfer. Presentation layer deals with representation of data. Different brands of computers use different internal representations for integers, characters etc. So there is a need for conversion of data to common format. For example, some machines like Intel PCs use little endian representation. That means low order byte is stored at lowest address. Other machines like Mac, SUN use big endian. So when you communicate between these two machines you need to communicate using a common format. Session layer protocols deal with session setup and authentication. They also deal with recovery from a failure during a session. This layer is present only in ISO reference model and such functionality is not provided in Internet protocols. CSci4211: Introduction

ISO/OSI Reference Model (cont’d)
Transport layer: end-to-end data delivery, e.g., connection-oriented (TCP) or connection-less (UDP) services error control, flow/congestion control, … Network layer: examples: IP, X.25 (global) naming and addressing, routing (build routing tables) forwarding packets hop-by-hop across networks avoidance of congested/failed links, traffic engineering, … Data link layer: data transfer between “neighboring” elements Examples: Ethernet, WiFi, PPP framing and error/flow control media access control Physical layer (EE stuff) encoding/decoding information (bits) into physical media modulating & transmitting raw bits (0/1) over wire Transport layer is responsible for end to end delivery of packets. It is an end to end layer and thus a transport protocol is between peer entities in the end-systems. Some of the functions of transport layer are reliable in-order delivery and flow control. This is one of the most complex layers. TCP and UDP are the transports protocols in the Internet Data link layer’s main task is to organize the data into frames and transmit them without errors between neighboring elements. Data link layer protocols are responsible for ensuring that speed matching sender and receiver. When the underlying medium is shared, a sublayer known media access control (MAC) regulates the access to the medium. Ethernet and PPP are some of the data link layer protocols. Physical layer is concerned with transmitting raw bits over the wire. How 1 and 0 are coded etc. CSci4211: Introduction

Comments on Layering Layering simplifies the architecture of complex system Advantages modularization eases maintenance and updating hide lower layer complexity/implementation details from higher layers Layering considered harmful? Q: which layer should implement what functionality? e.g., reliability, hop-by-hop basis or end-to-end basis? Possible Drawbacks? possible duplication of functionality between layers error recovery at link layer and transport layer Other possible drawbacks? What are the advantages of layering approach. As we said earlier, it makes it easy to maintain. We can update a layer’s implementation without affecting the rest of the system. Any drawbacks? A key question is which layer should implement what functionality? You might have noticed that there is overlap of duties among different layers. This could result in duplication of functionality between layers making the implementation wasteful and inefficient. A classic argument is whether to provide control on hop-by-hop basis or end-to-end basis. For example, should error recovery be done at each hop by data link layer or end to end by transport layer. Lets look at the slide 9. Suppose a packet gets corrupted during the transmission at the second hop. It is quicker to recover from the error locally instead of waiting for the sender to timeout and retransmit after a while. But on the other hand, if this is rare event, the complexity introduced at the data link layer for this purpose is not justified. In that case it is better to let the end to end transport layer deal with error recovery. In general, layering principle is followed but occasionally violated for efficiency. CSci4211: Introduction

Internet Protocol “Zoo”
….. Flash ….. DASH SMTP telnet, ssh NFS/RPC FTP, SCP DNS HTTP RealAudio RealVideo SOAP IPTV application VoIP P2P Now lets look at the protocols used in the Internet. There are tons of protocols and not all the protocols are listed here. Why so many? Each protocol serves a specific purpose. There so many link layer protocols and application layer protocols. There are two transport protocols namely TCP and UDP. But there is only one network layer protocol called Internet Protocol (IP). This protocol in the middle is the glue that holds the whole thing together. That’s why Internet protocol stack is said to resemble an hour glass. This architecture makes it easy to interface with new technologies and support new applications. They only need to interact with IP. This Internet protocol stack is also referred to as TCP/IP protocol stack as these two are most popular protocols. We will briefly look at some of these protocols and cover them in more detail later in the semester. ICMP, OSPF, RIP, BGP, … MPLS/gMPLS PPP 2.5G/3G/4G (GPRS,UMTS, WiMAX, LTE, …) Cellular Radio Networks WiFi DWDM DSL or DOCSIS CSci4211: Introduction

What real Internet looks like now?
CSci4211: Introduction

access via WiFi hotspots
Internet Structure Internet: “networks of networks”! LANs International lines Regional or local ISP local ISPs company university National or tier-1 ISP IXPs or private peering Regional ISPs access via WiFi hotspots Internet eXcange Points Home users Home users CSci4211: Introduction

Internet structure: network of networks
Roughly hierarchical At center: “tier-1” ISPs (e.g., Verizon, Sprint, AT&T, L3, Cable and Wireless), national/international coverage treat each other as equals IXP Tier-1 providers also interconnect at Internet Exchange Point Tier 1 ISP Tier-1 providers interconnect (peer) privately Tier 1 ISP Tier 1 ISP CSci4211: Introduction

Tier-1 ISP: e.g., Sprint … …. to/from backbone peering
to/from customers peering to/from backbone …. POP: point-of-presence CSci4211: Introduction

“Tier-2” ISPs: smaller (often regional) ISPs Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs Tier-2 ISPs also peer privately with each other, interconnect at IXP Tier-2 ISP Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet tier-2 ISP is customer of tier-1 provider Tier 1 ISP IXP Tier 1 ISP Tier 1 ISP CSci4211: Introduction

“Tier-3” ISPs and local ISPs last hop (“access”) network (closest to end systems) local ISP Tier 3 Local and tier- 3 ISPs are customers of higher tier ISPs connecting them to rest of Internet Tier-2 ISP Tier 1 ISP IXP Tier 1 ISP Tier 1 ISP CSci4211: Introduction

a packet passes through many networks! traceroute A local ISP Tier 3 ISP local ISP local ISP local ISP Tier-2 ISP Routing & forwarding: how do packets go from A to B? Tier 1 ISP IXP Tier 1 ISP Tier 1 ISP local ISP B local ISP local ISP local ISP CSci4211: Introduction

Map of Internet

Why it takes so long to download my friends’ pictures from web
Why it takes so long to download my friends’ pictures from web? Or why can’t I access the Internet now? Motivating Question 6 CSci4211: Introduction

Fundamental Problems in Networking …
Or what can go wrong? Bit-level errors: due to electrical interferences “Frame-level” errors: media access delay or frame collision due to contention/collision/interference Packet-level errors: packet delay or loss due to network congestion/buffer overflow Out of order delivery: packets may takes different paths Link/node failures: cable is cut or system crash What are the fundamental problems in networking. There are many things that can go wrong. Due to noise and interference, it is possible that a bit transmitted as 0 is interpreted as 1 at the receiver. As we discussed earlier, in packet switching it is possible that buffers at a congested link may overflow. This results in packet loss. We will see later that it is not necessary that all the packets to a destination follow the same path. Each packet is routed in isolation and so it possible that two packets take two different paths, experience different delays and reach the destination out of order. How do we deal with link and node failures. The cable may get cut. This is not as unusual as it seems. The systems may crash. So what can be done. Lets look some potential solutions. One way to deal with bit level errors is to add redundancy in the packet so that we can detect such bit errors and discard the packet. Or we can add enough redundancy such that we know how to correct/repair the error. We can use selective retransmission with timeout to recover the lost packets. If each packet received is acknowledged, sender can retransmit a packet if the acknowledgement doesn’t reach within the timeout period. To deal with out of order delivery, we can assign each packet a sequence number and buffer the packets reached out of order at the receiver and reorder them at the receiver. A router can send are you alive packets to its neighbors periodically to confirm that the link and the node are up. If it doesn’t get a response, it can declare them down and reset the routing tables such that failed links and nodes are avoided. These are not the only problems. This is just a small representative list of problems. The basic goal of networking software is to fill the gap between expectations of applications and the capabilities of the underlying technology. For example, if applications expect a reliable transmission and underlying channel is noisy, it’s the job of networking software to add error correction bits or error detection bits with retransmission. The networking software that addresses these problems is generally designed in a modular way to make the complexity manageable. CSci4211: Introduction

Four sources of packet delay
1. nodal processing: check bit errors determine output link 2. queueing time waiting at output link for transmission depends on congestion level of router A B propagation transmission nodal processing queueing CSci4211: Introduction

Delay in packet-switched networks
3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R 4. Propagation delay: d = length of physical link s = propagation speed in medium (~2x108 m/sec) propagation delay = d/s A B propagation transmission nodal processing queueing Note: s and R are very different quantitites! CSci4211: Introduction

Nodal delay dproc = processing delay dqueue = queuing delay
typically a few microsecs or less dqueue = queuing delay depends on congestion dtrans = transmission delay = L/R, significant for low-speed links dprop = propagation delay a few microsecs to hundreds of msecs CSci4211: Introduction

Statistical Multiplexing and Queueing
B C 10 Mbs Ethernet 1.5 Mbs 45 Mbs D E statistical multiplexing queue of packets waiting for output link CSci4211: Introduction

Queueing delay (revisited)
R=link bandwidth (bps) L=packet length (bits) a=average packet arrival rate traffic intensity = La/R La/R ~ 0: average queueing delay small La/R -> 1: delays become large La/R > 1: more “work” arriving than can be serviced, average delay infinite! CSci4211: Introduction

Queueing delay and Packet loss
Queue (aka buffer) preceding link in buffer has finite capacity When packet arrives to full queue, packet is dropped (aka lost) lost packet may be retransmitted by previous node, by source end system, or not retransmitted at all CSci4211: Introduction

“Real” Internet delays and routes
What do “real” Internet delay & loss look like? Traceroute program: provides delay measurement from source to router along end-end Internet path towards destination. For all i: sends three packets that will reach router i on path towards destination router i will return packets to sender sender times interval between transmission and reply. 3 probes 3 probes 3 probes CSci4211: Introduction

“Real” Internet delays and routes
Let’s Traceroute to CSci4211: Introduction

Throughput throughput: rate (bits/time unit) at which bits transferred between sender/receiver instantaneous: rate at given point in time average: rate over longer period of time pipe that can carry fluid at rate Rc bits/sec) pipe that can carry fluid at rate Rs bits/sec) link capacity Rs bits/sec link capacity Rc bits/sec server sends bits (fluid) into pipe server, with file of F bits to send to client CSci4211: Introduction

Throughput (cont’d) Rs < Rc What is average end-end throughput?
Rc bits/sec Rs bits/sec Rs > Rc What is average end-end throughput? Rs bits/sec Rc bits/sec link on end-end path that constrains end-end throughput bottleneck link CSci4211: Introduction

Throughput: Internet scenario
Rs per-connection end-end throughput: min(Rc,Rs,R/10) in practice: Rc or Rs is often bottleneck Rs Rs R Rc Rc Rc 10 connections (fairly) share backbone bottleneck link R bits/sec CSci4211: Introduction

What’s the Internet: Recap
protocols control sending, receiving of messages e.g., TCP, IP, HTTP, FTP, PPP Internet: “network of networks” loosely hierarchical public Internet versus private intranet Internet standards RFC: Request for comments IETF: Internet Engineering Task Force IEEE router workstation server mobile local ISP regional ISP company network CSci4211: Introduction 89

Fundamental Issues in Networking
Network is a shared resource Provide services for many people at same time Carry bits/information for many people at same time Switching and Multiplexing How to share resources among multiple users, and transfer data from one node to another node Naming and Addressing How to find name/address of the party (or parties) you would like to communicate with Address: byte-string that identifies a node unicast, multicast and broadcast addresses Routing and Switching/Forwarding: process of determining how to send packets towards the destination based on its address: finding out neighbors, building routing tables transferring data from source to destination CSci4211: Introduction

Fundamental Problems in Networking …
Or what can go wrong? Bit-level errors: due to electrical interferences “Frame-level” errors: media access delay or frame collision due to contention/collision/interference Packet-level errors: packet delay or loss due to network congestion/buffer overflow Out of order delivery: packets may takes different paths Link/node failures: cable is cut or system crash What are the fundamental problems in networking. There are many things that can go wrong. Due to noise and interference, it is possible that a bit transmitted as 0 is interpreted as 1 at the receiver. As we discussed earlier, in packet switching it is possible that buffers at a congested link may overflow. This results in packet loss. We will see later that it is not necessary that all the packets to a destination follow the same path. Each packet is routed in isolation and so it possible that two packets take two different paths, experience different delays and reach the destination out of order. How do we deal with link and node failures. The cable may get cut. This is not as unusual as it seems. The systems may crash. So what can be done. Lets look some potential solutions. One way to deal with bit level errors is to add redundancy in the packet so that we can detect such bit errors and discard the packet. Or we can add enough redundancy such that we know how to correct/repair the error. We can use selective retransmission with timeout to recover the lost packets. If each packet received is acknowledged, sender can retransmit a packet if the acknowledgement doesn’t reach within the timeout period. To deal with out of order delivery, we can assign each packet a sequence number and buffer the packets reached out of order at the receiver and reorder them at the receiver. A router can send are you alive packets to its neighbors periodically to confirm that the link and the node are up. If it doesn’t get a response, it can declare them down and reset the routing tables such that failed links and nodes are avoided. These are not the only problems. This is just a small representative list of problems. The basic goal of networking software is to fill the gap between expectations of applications and the capabilities of the underlying technology. For example, if applications expect a reliable transmission and underlying channel is noisy, it’s the job of networking software to add error correction bits or error detection bits with retransmission. The networking software that addresses these problems is generally designed in a modular way to make the complexity manageable. CSci4211: Introduction

Fundamental Problems in Networking
What can be done? Add redundancy to detect and correct erroneous packets Acknowledge received packets and retransmit lost packets Assign sequence numbers and reorder packets at the receiver Sense link/node failures and route around failed links/nodes Goal: to fill the gap between what applications expect and what underlying technology provides What are the fundamental problems in networking. There are many things that can go wrong. Due to noise and interference, it is possible that a bit transmitted as 0 is interpreted as 1 at the receiver. As we discussed earlier, in packet switching it is possible that buffers at a congested link may overflow. This results in packet loss. We will see later that it is not necessary that all the packets to a destination follow the same path. Each packet is routed in isolation and so it possible that two packets take two different paths, experience different delays and reach the destination out of order. How do we deal with link and node failures. The cable may get cut. This is not as unusual as it seems. The systems may crash. So what can be done. Lets look some potential solutions. One way to deal with bit level errors is to add redundancy in the packet so that we can detect such bit errors and discard the packet. Or we can add enough redundancy such that we know how to correct/repair the error. We can use selective retransmission with timeout to recover the lost packets. If each packet received is acknowledged, sender can retransmit a packet if the acknowledgement doesn’t reach within the timeout period. To deal with out of order delivery, we can assign each packet a sequence number and buffer the packets reached out of order at the receiver and reorder them at the receiver. A router can send are you alive packets to its neighbors periodically to confirm that the link and the node are up. If it doesn’t get a response, it can declare them down and reset the routing tables such that failed links and nodes are avoided. These are not the only problems. This is just a small representative list of problems. The basic goal of networking software is to fill the gap between expectations of applications and the capabilities of the underlying technology. For example, if applications expect a reliable transmission and underlying channel is noisy, it’s the job of networking software to add error correction bits or error detection bits with retransmission. The networking software that addresses these problems is generally designed in a modular way to make the complexity manageable. CSci4211: Introduction

The Internet Network layer
Transport layer: TCP, UDP IP protocol addressing conventions packet handling conventions Routing protocols path selection RIP, OSPF, BGP Network layer routing table ICMP protocol error reporting router “signaling” This shows the different components of network layer under Internet protocol stack. IP protocol is concerned with addressing and packet forwarding. The routing table itself is either setup manually or using routing protocols such as Routing Information Protocol (RIP), Open Shortest Path First (OSPF) and Border Gateway Protocol (BGP). These protocols are used to exchange information about the network between routers and update routing tables in response to link and node failures. Internet Control Message Protocol (ICMP) is for reporting errors such as destination not reachable, number of hops exceeded the specified maximum etc. Data Link layer (Ethernet, WiFi, PPP, …) Physical Layer (fiber optics, radio, …) CSci4211: Introduction

Introduction: Summary
Answers to 6 motivating questions What is internet? What so special about it? What internet looks like now? How I deal with the complexity? What’s a protocol? How I build a network? Why do I suffer delays? You now have: context, overview, “feel” of networking more depth, detail to follow! CSci4211: Introduction

Internet Summary Computer networks/Internet use packet switching
Layered architecture for handling complexity & attaining maintainability Key notions: protocols, services and interfaces Internet is based on TCP/IP protocol suite Networks of networks! Shared, distributed and complex system in global scale No centralized authority Fundamental issues in networking addressing/naming routing/forwarding error/flow/congestion control, media access control This is what we learnt so far. We talked about statistical multiplexing and packet switching, why its better to use packet switching instead of circuit switching in computer networks. Also some of the fundamental issues that need to be addressed by the networking software are routing/forwarding and error/flow/congestion control. Next we looked at the layered architecture for addressing these issues using a modular approach. Finally, we briefly discussed TCP/IP protocol stack used in Internet CSci4211: Introduction

Readings for Next Week Read Chapter 1 Review these lecture notes
Read the supplementary notes that follow these one if you have time Read Chapter 2: sections 2.1 –2.6 Learn how web works Learn how works Understand what Domain Name System does for us P2P File Sharing Glance through Chapter 7: sections This is what we learnt so far. We talked about statistical multiplexing and packet switching, why its better to use packet switching instead of circuit switching in computer networks. Also some of the fundamental issues that need to be addressed by the networking software are routing/forwarding and error/flow/congestion control. Next we looked at the layered architecture for addressing these issues using a modular approach. Finally, we briefly discussed TCP/IP protocol stack used in Internet CSci4211: Introduction

Supplementary Readings
Physical Media Access Network Technologies History of Internet Internet “Governing” Bodies Network Security: Cyber Attacks CSci4211: Introduction

Access networks and physical media
Q: How to connect end systems to edge router? residential access nets institutional access networks (school, company) mobile access networks keep in mind: bandwidth (bits per second) of access network? shared or dedicated? CSci4211: Introduction

Physical media bit: propagates between transmitter/receiver pairs
physical link: what lies between transmitter & receiver guided media: signals propagate in solid media: copper, fiber, coax unguided media: signals propagate freely, e.g., radio twisted pair (TP) two insulated copper wires Category 5: 100 Mbps, 1 Gbps Ethernet Category 6: 10Gbps CSci4211: Introduction

Host: sends packets of data
host sending function: takes application message breaks into smaller chunks, known as packets, of length L bits transmits packet into access network at transmission rate R link transmission rate, aka link capacity, aka link bandwidth two packets, L bits each 2 1 R: link transmission rate host packet transmission delay time needed to transmit L-bit packet into link L (bits) R (bits/sec) = = CSci4211: Introduction

Physical media: coax, fiber
coaxial cable: two concentric copper conductors bidirectional broadband: multiple channels on cable HFC fiber optic cable: glass fiber carrying light pulses, each pulse a bit high-speed operation: high-speed point-to-point transmission (e.g., 10’s-100’s Gbps transmission rate) low error rate: repeaters spaced far apart immune to electromagnetic noise CSci4211: Introduction

Physical media: radio Radio link types:
microwave e.g. up to 45 Mbps channels LAN (e.g., waveLAN) 2Mbps, 11Mbps wide-area (e.g., cellular) e.g. CDPD, 10’s Kbps satellite up to 50Mbps channel (or multiple smaller channels) 270 Msec end-end delay geosynchronous versus LEOS signal carried in electromagnetic spectrum no physical “wire” bidirectional propagation environment effects: reflection obstruction by objects interference CSci4211: Introduction

A closer look at network structure:
network edge: hosts: clients and servers servers often in data centers mobile network global ISP regional ISP home network institutional access networks, physical media: wired, wireless communication links network core: interconnected routers network of networks CSci4211: Introduction

Residential access: Dial-up Modem
telephone network Internet home dial-up modem ISP modem (e.g., AOL) home PC central office Uses existing telephony infrastructure Home is connected to central office up to 56Kbps direct access to router (often less) Can’t surf and phone at same time: not “always on” CSci4211: Introduction

Access network: digital subscriber line (DSL)
central office telephone network voice, data transmitted at different frequencies over dedicated line to central office DSL modem splitter DSLAM DSL access multiplexer ISP use existing telephone line to central office DSLAM data over DSL phone line goes to Internet voice over DSL phone line goes to telephone net < 2.5 Mbps upstream transmission rate (typically < 1 Mbps) < 24 Mbps downstream transmission rate (typically < 10 Mbps) CSci4211: Introduction

Access Network: cable modems
CSci4211: Introduction Diagram:

Access network: cable network
cable headend … cable modem splitter Channels V I D E O A T C N R L 1 2 3 4 5 6 7 8 9 frequency division multiplexing: different channels transmitted in different frequency bands CSci4211: Introduction

Access network: cable network
cable headend … data, TV transmitted at different frequencies over shared cable distribution network cable modem splitter cable modem termination system CMTS ISP HFC: hybrid fiber coax asymmetric: up to 30Mbps downstream transmission rate, 2 Mbps upstream transmission rate network of cable, fiber attaches homes to ISP router homes share access network to cable headend unlike DSL, which has dedicated access to central office CSci4211: Introduction

to/from headend or central office
Access network: home network wireless devices to/from headend or central office often combined in single box wireless access point (54 Mbps) router, firewall, NAT cable or DSL modem wired Ethernet (1 Gbps) CSci4211: Introduction

Enterprise access networks (Ethernet)
institutional link to ISP (Internet) institutional router Ethernet switch institutional mail, web servers typically used in companies, universities, etc. 10 Mbps, 100Mbps, 1Gbps, 10Gbps transmission rates today, end systems typically connect into Ethernet switch CSci4211: Introduction

Wireless access networks
shared wireless access network connects end system to router via base station aka “access point” wide-area wireless access provided by telco (cellular) operator, 10’s km between 1 and 10 Mbps 3G, 4G: LTE wireless LANs: within building (100 ft.) 802.11b/g/n (WiFi): 11, 54, 450 Mbps transmission rate to Internet to Internet CSci4211: Introduction

The network core mesh of interconnected routers
packet-switching: hosts break application-layer messages into packets forward packets from one router to the next, across links on path from source to destination each packet transmitted at full link capacity CSci4211: Introduction

Origin of Internet? Three Major Actors:
Started by U.S. research/military organizations: Three Major Actors: DARPA: Defense Advanced Research Projects Agency funds technology with military goals DoD: U.S. Department of Defense early adaptor of Internet technology for production use NSF: National Science Foundation funds university research CSci4211: Introduction

Pre-Internet Modes of Human Telecommunications
The Dark Age before the Internet: before 1960 Non-electrical (source: wikipedia) Prehistoric: Fires, Beacons, Smoke signals, drums, Horns 6th century BCE: (snail) mail (e.g., delivered by human couriers on horse) 5th century BCE: Pigeon post 4th century BCE: Hydraulic semaphores, heliographs (shield signals) 15th century CE: Maritime flag semaphores 1672: First experimental acoustic (mechanical) telephone 1790: Semaphore lines (optical telegraphs) 1867: Signal lamps; 1877: Acoustic phonograph Electrical: 1830: telegraph 1876: circuit-switching (telephone) 1896: radio TV (1940?) , and later cable TV (1970s) CSci4211: Introduction

Internet History 1961-1972: Early packet-switching principles
1961: Kleinrock - queueing theory shows effectiveness of packet-switching 1964: Baran - packet-switching in military nets 1967: ARPAnet conceived by Advanced Research Projects Agency 1969: first ARPAnet node operational 1972: ARPAnet public demonstration NCP (Network Control Protocol) first host-host protocol first program ARPAnet has 15 nodes CSci4211: Introduction

Internet History 1972-1980: Internetworking, new and proprietary nets
1970: ALOHAnet satellite network in Hawaii 1974: Cerf and Kahn - architecture for interconnecting networks 1976: Ethernet at Xerox PARC ate70’s: proprietary architectures: DECnet, SNA, XNA late 70’s: switching fixed length packets (ATM precursor) 1979: ARPAnet has 200 nodes Cerf and Kahn’s internetworking principles: minimalism, autonomy - no internal changes required to interconnect networks best effort service model stateless routers decentralized control define today’s Internet architecture CSci4211: Introduction

Internet History 1980-1990: new protocols, a proliferation of networks
1983: deployment of TCP/IP 1982: smtp protocol defined 1983: DNS defined for name-to-IP-address translation 1985: ftp protocol defined 1988: TCP congestion control new national networks: Csnet, BITnet, NSFnet, Minitel 100,000 hosts connected to confederation of networks CSci4211: Introduction

Internet History 1990, 2000’s: commercialization, the Web, new apps
Late 1990’s – 2000’s: more killer apps: instant messaging, P2P file sharing network security to forefront est. 50 million host, 100 million+ users backbone links running at Gbps Napster, BitTorrent, … Myspace, Facebook, twitter,.. YouTube, Netflix, Hulu, … Now to the future: … (your invention here!) Early 1990’s: ARPAnet decommissioned 1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995) early 1990s: Web hypertext [Bush 1945, Nelson 1960’s] HTML, HTTP: Berners-Lee 1994: Mosaic, later Netscape late 1990’s: commercialization of the Web CSci4211: Introduction

Who Runs the Internet “nobody” really!
standards: Internet Engineering Task Force (IETF) names/numbers: The Internet Corporation for Assigned Names and Numbers (ICANN) DNS root server operators, domain name registrars networks: ISPs (Internet Service Providers), IXPs (Internet Exchange Points), …… fibers: telephone companies (mostly) content: companies, universities, governments, individuals, …; content distribution networks, … CSci4211: Introduction

Internet “Governing” Bodies
Internet Society (ISOC): membership organization raise funds for IAB, IETF& IESG, elect IAB Internet Engineering Task Force (IETF): a body of several thousands or more volunteers organized in working groups (WGs) meet three times a year + Internet Architecture Board architectural oversight, elected by ISOC Steering Group (IESG): approves standards, Internet standards, subset of RFC RFC: “Request For Comments”, since 1969 most are not standards, also experimental, informational and historic(al) CSci4211: Introduction

Internet Names and Addresses
Internet Corporation for Assigned Names and Numbers (ICAAN): coordinate IPv4 & IPv6 address spaces, keep track of numbers (e.g., protocol identifiers), delegates Internet address assignment to regional Internet registries manage top-level domain names & operations of root name servers designate authority for each top-level domain; create new TLDs Regional Internet Registries: AfriNIC, APNIC, ARIN, LACMIC, RIPE NCC: manage the allocation and registration of Internet number resources e.g., hand out blocks of addresses to ISPs; assign AS numbers maintain WHOIS registries …. CSci4211: Introduction

Network security field of network security:
how bad guys can attack computer networks how we can defend networks against attacks how to design architectures that are immune to attacks Internet not originally designed with (much) security in mind original vision: “a group of mutually trusting users attached to a transparent network”  Internet protocol designers playing “catch-up” security considerations in all layers!

Bad guys: put malware into hosts via Internet
malware can get in host from: virus: self-replicating infection by receiving/executing object (e.g., attachment) worm: self-replicating infection by passively receiving object that gets itself executed spyware malware can record keystrokes, web sites visited, upload info to collection site infected host can be enrolled in botnet, used for spam. DDoS attacks

Bad guys: attack server, network infrastructure
Denial of Service (DoS): attackers make resources (server, bandwidth) unavailable to legitimate traffic by overwhelming resource with bogus traffic 1. select target 2. break into hosts around the network (see botnet) target 3. send packets to target from compromised hosts

Bad guys can sniff packets
packet “sniffing”: broadcast media (shared Ethernet, wireless) promiscuous network interface reads/records all packets (e.g., including passwords!) passing by A C src:B dest:A payload B wireshark software used for end-of-chapter labs is a (free) packet-sniffer

Bad guys can use fake addresses
IP spoofing: send packet with false source address A C src:B dest:A payload B … lots more on security (throughout, Chapter 8)

Chapter 1: Introduction

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