CPS 356: Introduction to Computer Networks Lecture 2: Network Architectures Xiaowei Yang Reference: Chapter 1 of [PD]

Overview Updated course administrative stuff – Grading policy, office hours, piazza Design requirements of the original Internet Concepts of Network Architectures An Example of how the Internet works

Updated Grading Policy Old – Class participation and pop quizzes: 20% – Assignments: 50% In a group assignment, both students get the same grade for the assignment – Exams: 30% New – Class participation and pop quizzes: 10% – Assignments: 60% In a group assignment, both students get the same grade for the assignment – Exams: 30%

Office hours Instructor – Fridays: 3-5pm TA – Tuesdays 7-9pm

Discussion Forum Piazza sign up link – piazza.com/duke/spring2014/compsci356

Overview Updated course administrative stuff – Grading policy, office hours, piazza Design requirements of the original Internet Concepts of Network Architectures An Example of how the Internet works

1 st Mission of this course Understand the concepts and design principles that make the Internet work Design paradigm – Identify requirements, brainstorm design choices/mechanisms, make design decisions – What requirements make sense to you? Scalable connectivity Cost-effective resource sharing Support for different types of services Manageability … – It remains an open challenge how to incorporate other requirements such as security into the Internet design

Features of computer networks Generality Carry many different types of data Support an unlimited range of applications

What’s the Internet? The Internet is a large-scale general-purpose computer network. – Run more than one applications The Internet transfers data between computers. The Internet is a network of networks.

Design requirements and techniques to meet them 1.Scalable connectivity 2.Cost-effective resource sharing 3.Support for common services 4.Manageability

1. Scalable Connectivity A network must provide connectivity among a set of computers – Open vs close: to connect all computers or a subset of them? – Internet is an open network Scalability: A system is designed to grow to an arbitrary large size is said to scale – How to connect an arbitrary large number of computers on a network?

Connectivity recursively occurs at different levels Link-level: connect two or more computers via a physical medium Computers are referred to as nodes The physical medium is referred to as a link Point-to-Point Multiple-Access

Switching Switching is a mechanism to achieve connectivity Nodes that are attached to at least two links forward data from one link to another link They are called switches Computers outside the cloud are called hosts A question: switch vs router, what can become a switch?

Circuit switching – Sets up a circuit before nodes can communicate – Switches connect circuits on different links Packet switching – Data are split into blocks of data called packets – Store and forward – Nodes send packets and switches forward them

An internetwork of networks – Each cloud is a network/a multiple-access link – A node that is connected to two or more networks is commonly called a router Speaks different protocols than switches – An internet can be viewed as a “cloud.” We can recursively build larger clouds by connecting smaller ones – Autonomous system (AS) Internetworking: Another way to achieve connectivity

Addressing and routing Physical connectivity != connectivity Addressing and routing are mechanisms to achieve connectivity Nodes are assigned addresses Routers compute how to reach them by running routing protocols – intra-AS: OSPF, RIP, IS-IS – Inter-AS: BGP

2. Cost-effective resource sharing Question: how do all the hosts share the network when they want to communicate with each other? – Use at the same time – Fair Multiplexing: a system resource is shared among multiple users – Analogy: CPU sharing Mechanisms to multiplexing – Time-division multiplexing (TDM) – Frequency-division multiplexing (FDM) – Statistical multiplexing

MultiplexDemultiplex

TDM and FDM TDM frequency time 4 users Example: FDM frequency time

Problems with FDM and TDM What if a user does not have data to send all the time? – Consider web browsing –  Inefficient use of resources Max # of flows is fixed and known ahead of time – Not practical to change the size of quantum or add additional quanta for TDM – Nor add more frequencies in FDM

Statistical Multiplexing The physical link is shared over time (like TDM) But does not have fixed pattern  statistical multiplexing – Sequence of A & B packets are sent on demand, not predetermined slots A B C 10 Mb/s Ethernet 1.5 Mb/s D E statistical multiplexing queue of packets waiting for output link

Pros and Cons Assumption: traffic is largely bursty Pros: Resources are not wasted when hosts are idle Cons: No guarantee flows would have their turns to transmit Some possible fixes: – Limit maximum packet size – Scheduling which packets got transmitted, e.g., fair queuing

Maximum Packet Size Divide an application message into blocks of data  packets – Segments, frames Maximum packet size limit – Flows send on demand – Must give each flow its turn to send – Solution: defines an upper bound on the size of the block of data

Packet scheduling Scheduling: which packet to send First come first serve (FIFQ) Weighted fair queuing

Switching vs multiplexing TDM and FDM are used in circuit switching – Require a setup as max # of flows is fixed SM is used in packet switching

Congestion Aggregate incoming rate > outgoing rate An open question A large buffer can help temporary congestion

Packet switching versus circuit switching 1 Mb/s link each user: – 100 kb/s when “active” – active 10% of time circuit-switching: fixed capacity – 10 users packet switching: – with 35 users, probability > 10 active less than.0004 Packet switching allows more users to use network! N users 1 Mbps link

3. Support for common services Application developers want a network to provide services that make application programs communicate with each other, not just sending packets – E.g. reliably delivering an message from a sender to a receiver Many complicated things need to happen – Can you name a few? Design choices – Application developers build all functions they need – Network provides common services  a layered network architecture Build it once, and shared many times

Interactive request/reply Streaming of data Bulk data transfer … Key challenges: what services/channels to provide that can satisfy most applications at lowest costs? Approach: identify common patterns, then decide – What functions to implement – Where to implement those functions We will discuss end-to-end arguments in future class

Ex: how to provide reliability as a common service Failures may occur at different scopes – Bit transmission errors – Packet loss – Component failures: link, node Design choices – Link layer – Every hop in the router – End systems In future classes, we will discuss how to cope with these failures

4. Manageability Manage the network as it grows and when things go wrong An open research challenge – Datacenter networks – Backbones – Home networks IP cameras, printers, network attached storage

Overview Updated course administrative stuff – Grading policy, office hours, piazza Design requirements of the original Internet Concepts of Network Architectures An Example of how the Internet works

Network Architectures Many ways to build a network Use network architectures to characterize different ways of building a network The general blueprints that guide the design and implementation of networks are referred to as network architectures

Central concepts Layering Protocols

Layering An abstraction to handle complexity – A unifying model that capture important aspect of a system – Encapsulate the model in an object that has an interface for others to interact with – Hide the details from the users of the object Not so strict

Advantages of layering Simplify the design tasks – Each layer implements simpler functions Modular design – Can provide new services by modifying one layer

Protocols The abstract objects that make up the layers of a network system are called protocols Each protocol defines two different interfaces – Service interface – Peer interface

A protocol graph Peer-to-peer communication is indirect – Except at the hardware level Potentially multiple protocols at each level Show the suite of protocols that make up a network system with a protocol graph

A sample protocol graph

Protocol standardization Standard bodies such as IETF govern procedures for introducing, validating, and approving protocols – The Internet protocol suite uses open standard Set of rules governing the form and content of a protocol graph are called a network architecture

We reject kings, presidents, and voting. We believe in rough consensus and running code - David Clark

Encapsulation Upper layer sends a message using the service interface A header, a small data structure, to add information for peer-to-peer communication, is attached to the front message – Sometimes a trailer is added to the end Message is called payload or data This process is called encapsulation

Multiplexing & Demultiplexing Same ideas apply up and down the protocol graph

Examples of Network Architectures

The protocol graph of Internet No strict layering. One can do cross-layer design Hourglass shaped: IP defines a common method for exchanging packets among different networks To propose a new protocol, one must produce both a spec and one/two implementations Link layer Network layer Transport layer Applicatoin layer

Functions of the Layers Link Layer: – Service: Reliable transfer of frames over a link Media Access Control on a LAN – Functions: Framing, media access control, error checking Network Layer: – Service: Move packets from source host to destination host – Functions: Routing, addressing Transport Layer: – Service: Delivery of data between hosts – Functions: Connection establishment/termination, error control, flow control Application Layer: – Service: Application specific (delivery of , retrieval of HTML documents, reliable transfer of file) – Functions: Application specific

The Open Systems Interconnection (OSI) architecture Seven-layer

International Telecommunications Union (ITU) publishes protocol specs based on the OSI reference model – X dot series Physical layer: handles raw bits Data link layer: aggregate bits to frames. Network adaptors implement it Network layer: handles host-to-host packet delivery. Data units are called packets Transport: implements process channel. Data units are called messages Session layer: handles multiple transport streams belong to the same applications Presentation layer: data format, e.g., integer format, ASCII string or not Application layer: application specific protocols

Summary of New Terms Layering is an abstraction that captures important aspects of the system, provides service interfaces, and hides implementation details Protocols are abstract objects that make up the layers of a network system are A protocol graph represents protocols that make up a system – Nodes are protocols – Links are depend-on relations Set of rules governing the form and content of a protocol graph are called a network architecture Attaching a header/trailer to an upper layer data unit is referred to as encapsulation

An Example

A user on host argon.tcpip-lab.edu (“Argon”) makes web access to URL tcpip-lab.edu/index.html. What actually happens in the network? A simple TCP/IP Example

HTTP Request and HTTP response Web server runs an HTTP server program HTTP client Web browser runs an HTTP client program sends an HTTP request to HTTP server HTTP server responds with HTTP response

HTTP Request GET /example.html HTTP/1.1 Accept: image/gif, */* Accept-Language: en-us Accept-Encoding: gzip, deflate User-Agent: Mozilla/4.0 Host: Connection: Keep-Alive

HTTP Response HTTP/ OK Date: Sat, 25 May :10:32 GMT Server: Apache/ (Unix) Last-Modified: Sat, 25 May :51:33 GMT ETag: " ceff955" Accept-Ranges: bytes Content-Length: 81 Keep-Alive: timeout=15, max=100 Connection: Keep-Alive Content-Type: text/html Internet Lab Click here for the Internet Lab webpage. How does the HTTP request get from Argon to Neon ?

From HTTP to TCP To send request, HTTP client program establishes an TCP connection to the HTTP server Neon. The HTTP server at Neon has a TCP server running

Resolving hostnames and port numbers Since TCP does not work with hostnames and also would not know how to find the HTTP server program at Neon, two things must happen: 1. The name “neon.tcpip-lab.edu” must be translated into a 32-bit IP address. 2. The HTTP server at Neon must be identified by a 16-bit port number.

Translating a hostname into an IP address The translation of the hostname neon.tcpip-lab.edu into an IP address is done via a database lookup – gethostbyname(host) The distributed database used is called the Domain Name System (DNS) All machines on the Internet have an IP address: argon.tcpip-lab.edu neon.tcpip-lab.edu

Finding the port number Note: Most services on the Internet are reachable via well-known ports. E.g. All HTTP servers on the Internet can be reached at port number “80”. So: Argon simply knows the port number of the HTTP server at a remote machine. On most Unix systems, the well-known ports are listed in a file with name /etc/services. The well-known port numbers of some of the most popular services are: ftp21finger79 telnet23http80 smtp 25nntp 119

Requesting a TCP Connection The HTTP client at argon.tcpip-lab.edu requests the TCP client to establish a connection to port 80 of the machine with address connect(s, (struct sockaddr*)&sin, sizeof(sin))

Invoking the IP Protocol The TCP client at Argon sends a request to establish a connection to port 80 at Neon This is done by asking its local IP module to send an IP datagram to (The data portion of the IP datagram contains the request to open a connection)

Sending the IP datagram to the default router Argon sends the IP datagram to its default router The default gateway is an IP router The default gateway for Argon is Router137.tcpip-lab.edu ( ).

Invoking the device driver The IP module at Argon, tells its Ethernet device driver to send an Ethernet frame to address 00:e0:f9:23:a8:20 Ethernet address of the default router is found out via ARP

The route from Argon to Neon Note that the router has a different name for each of its interfaces.

Sending an Ethernet frame The Ethernet device driver of Argon sends the Ethernet frame to the Ethernet network interface card (NIC) The NIC sends the frame onto the wire

Forwarding the IP datagram The IP router receives the Ethernet frame at interface recovers the IP datagram 2. determines that the IP datagram should be forwarded to the interface with name The IP router determines that it can deliver the IP datagram directly

The IP protocol at Router71, tells its Ethernet device driver to send an Ethernet frame to address 00:20:af:03:98:28 Invoking the Device Driver at the Router

Sending another Ethernet frame The Ethernet device driver of Router71 sends the Ethernet frame to the Ethernet NIC, which transmits the frame onto the wire.

Data has arrived at Neon Neon receives the Ethernet frame The payload of the Ethernet frame is an IP datagram which is passed to the IP protocol. The payload of the IP datagram is a TCP segment, which is passed to the TCP server

Summary Updated course administrative stuff – Grading policy, office hours, piazza Design requirements of the original Internet Concepts of Network Architectures An Example of how the Internet works