Chapter 2: Communication Basics

Question 1

ISO‘s OSI model NN

Answer 1

• Open Systems Interconnection model
• Basis for standards development on systems interconnection.
• Reference model for networking standards development
• ISO/IEC7498-1

Question 2

OSI model

Answer 2

• Application
  
  • HTTP
• Presentation
• Session
• Transport
  
  • TCP / UDP
• Network
  
  • IP and Routing
• Data Link
• Physical

Question 3

IP

Answer 3

• Two versions
  
  • IPv4, e.g., 131.159.52.25
  • IPv6, e.g., fe80::3e97:eff:fe41:2d
• Characteristics
  
  • Relays datagrams accross networks
  • Routing enables internetworking
  • Deliver packet from source to host address
  • Foundation for TCP/UDP

Question 4

IP Routing

Answer 4

• Example: We want to reach the chairs website from home.
  
  • Which route should the packets go?
• Requires routing protocol
  
  • Many (open, proprietary) routing protocols available,
  • Protocol used in the internet is BGP
• BGP routing
  
  • Border Gateway Protocol
  • Routing between AS (Autonomous systems)
  • AS is typical a provider or bigger organization
  • Exchanges routing and reachability information between AS (Autonomous System

Question 5

TCP

Answer 5

• Transmission control protocol
• Characteristics
  
  • Connection oriented protocol, session is initiated
  • Provides reliability through ACK
  • Provides ordering, sequencing
  • Flow control, sender connot overflow receiver
• Slow compared to UDP
  
  • But provides reliability
• Usage
  
  • HTTP, RPC, …

Question 6

UDP

Answer 6

• User Datagram Protocol
• Characteristics
  
  • Connectionless, no session
  • Best-effort
  • No guaranteed delivery
  • No ordering guarantees
  • Point-to-point and point-to-multipoint
• Faster than TCP
  
  • But application is responsible to fullfill guarantees
• Usages
  
  • DNS, Video, Voice, …

Question 7

Ports

Answer 7

• Example: Computer with a given IP address handles Mail, IM, Torrent, and Web browsing
  
  • How should the OS differentiate among these networked applications, if packets arrive?
• A port is a 16 bit number to locally (per host) identify the connection
  
  • Separate for UDP and TCP
• Numbers (vary by OS)
  
  • 0-1023 “reserved”, must be root to use
  • 1024 – 65535 are available to regular user
• Well-known, reserved services
  • http80/tcp
  • ftp 21/tcp
  • ssh22/tcp
  • Others: telnet 23/tcp; finger 79/tcp; snmp 161/udp

Question 8

Application layer protocols

Answer 8

• Example: A browser wants to retreive a web page
  
  • Typically HTTP 1.1 over TCP is used
• Presentedprotocols
  
  • HTTP
• Other protocols
  
  • FTP
  • SMTP

Question 9

Protocols

Answer 9

• Set of rules that specifies data transfer between computing end-points, often including
  
  • Connection establishment & tear-down
  • Communication
  • Data representation
• Often based on standards, de facto standards, open specifications, …

Question 10

Common application layer protocols

Answer 10

• DHCP (Dynamic Host Configuration Protocol)

HTTP (Hypertext Transfer Protocol)

FTP (File Transfer Protocol)

Telnet (Telnet Remote Protocol)

SSH (Secure Shell Remote Protocol)

SIP(SessionInitiationProtocol)

POP3(PostOfficeProtocol3)

SMTP(SimpleMailTransferProtocol)

IMAP (Internet Message Access Protocol)

Question 11

HTTP/1.1

Answer 11

Definition

• HyperText Transfer Protocol
• Request/Response protocol for client-server communication
• Normally used on top of TCP

Usagescenarios – Browser

• RESTFUL API‘s
• SOAP over HTTP
• NoSQL databases, e.g., CouchDB

Question 12

HTTP/1.1 methods

Answer 12

• Get – retrieve information
• Head – similar to get but must not return body message
• Post – post form
• Put – update
• delete –
• trace – diagnostics
• connect – initialize secure connection

Question 13

HTTP/2.0

Answer 13

• Standard Proposed in May 2015 as RFC7540
• Supported already by all major browser
• Main difference to HTTP/1.1
  
  • Better utilization of available network capacity
  • Headers are also compressed
  • On a single connection requests and response are interleaved
  • Prioritization of requests

Question 14

The Hourglass Evolution of the Internet Protocol Stack

Answer 14

Question 15

Socket: Problem description

Answer 15

• Move data from Application I on Host A to Application K on Host B
• Send a message from Application I on Host A to Application K on Host B
• Invoke an operation/service from Application I on Host A on Application K on Host B and return a result (or failure)

Question 16

A Client/Server world

Answer 16

• Client
  
  • Issues requests to server(e.g.,send&receive)
• Server
  
  • Starts up and listens for connections, requests, and sends/receives
• Client/Server examples
  
  • telnet/telnetd
  • ftp/ftpd(sftp/sftpd)
  • Firefox/Apache
• Client and server are roles, our following discussion also applies in peer-to-peer contexts
  
  • Same application may play both roles

Question 17

Protocol design considerations

Answer 17

• How are API calls (e.g., get / set) sent from the client to the server?
  
  • May they have given us any hints in the code?
• What parts of a call need to be conveyed to the server?
  
  • E.g., int storage_get(*table, *key, *record, *conn);
• What’s the simplest possible way of sending this information?
• What else needs to be represented in the protocol between the client library and the storage server?

Question 18

Simple protocol design

Answer 18

• Simplicity
  
  • Simple encoding of client requests
  • Simple encoding of server response
  • Need to worry about error conditions
  • Don’t worry about saving a bit of bandwidth or computation
  • Consider debugging your communication
• Interoperability
  
  • We can not assume client and server run on the same hardware / OS
• Portability
  
  • We restrict ourselves to IPv4

Question 19

K/V-store server API NN

Answer 19

• Telnet-based
  
  • \r\n as line delimiter
  • A „ „ space as delimiter between arguments • Values should be base64 encode
  • ISO-8859-1 encoding
  • Works with plain telnet or plain sockets
• Example interactions
  
  • PUT tablename key value\r\n
    
    • OK if stored successfull
  • GET tablename key
    
    • Value is returned

Question 20

Using the “network” / Abstracting the network NN

Answer 20

Sockets are network programming abstractions.

• User interface Business Layer
• Internal API
• Serialization/Protocol
• Connection and error handling
• Networking API
• Protocols (TCP, …)

Question 21

Communication end-points & sockets

Answer 21

• Identify the application that sends and receives data and the host the application runs on
  
  • Multiple applications running on the same machine
• End-points are programmatically represented by sockets (#include )
• Create the end-points
• Connect the end-points
• Listen and accept connections on end-points
• Transmit messages via sending and receiving
• Diligently handle errors!

Question 22

Sockets are file descriptors

Answer 22

• Socket is a programming abstraction for communicating among processes (applications) based on (Unix) file descriptors
• A file descriptor is an integer representing an open file managed by the OS
  
  • In Unix any I/O is done by reading/writing from/to file descriptors
  • For sockets, also called socket descriptor

Question 23

Socket types

Answer 23

• Internetsockets…
• Types of Sockets
  
  • SOCK_STREAM
    
    • TCP based
    • Ordering guaranteed
    • Error-free
  • SOCK_DGRAM
    
    • UDP based
• IPv4 & IPv6

Question 24

SOCK_STREAM Sockets

Answer 24

• Are reliable, two-way connected communication streams
  
  • Output two items into the socket in the order“1, 2”, they arrive in the order “1, 2” at the opposite end (“… if they arrive at all” )
  • Transmission is error-free (i.e., messages are not corrupted)

Question 25

Stream sockets in practice

Answer 25

* Telnet uses stream sockets * **Web browsers & HTTP** use stream sockets to fetch pages * Stream sockets are based on the **Transmission Control Protocol (TCP)** * TCP ensures that data arrives **sequentially** and **error-free** (given no failures) * When is this kind of reliability not required? * What kind of reliability for live video streaming? * What kind of socket for e-mail? * For calling your darling over the internet?

Question 26

Two end-points determine a connection

Answer 26

* End-point is determined by * **IP address** (host address); e.g., 192.168.100.100 * **Port** number, e.g., 8888

Question 27

“Error binding socket"

Answer 27

* **Port** is **used** by another process * ps aux | grep server * Kill it, if it is your own * Still an error? Wait a few seconds till the OS cleans it * Use a **different** **port** **number**, if it is someone else’s who might be logged in to the machine via ssh

Question 28

Byte Order

Answer 28

* The order bytes are represented by a computer architecture in memory, on the wire, ... * E.g., 0x **A3F1** as two bytes // 0x HEX number * Stored as **A3** F1 // **Big-Endian** (“Big end first”) * Motorola 6800, 68k, PowerPC (Intel), ... * Network byte order * Stored as **F1** A3 // **Little-Endian** (“Little end first”) * Intel or Intel-compatible processors * x86, 6502, Z80, VAX, PDP-11

Question 29

Setting up & using sockets !!

Answer 29

Question 30

Java NIO

Answer 30

* **Nonblocking** sockets introduced with Java 1.4 * JDK7 comes with NIO.2 API * **Asynchronous** I/O on network and files * **Multicasting** using DatagramChannel * Needs operating system support * Linux:epoll * FreeBSD/MacOS-X:kqueue * Windows/Solaris: IOCP * Requires different kind of application architecture * Keywords: Reactive Programming, **Event Loops** * More about that in the Application Servers Lecture

Question 31

Asynchronous vs. Synchronous

Answer 31

* **Syncronous** * **A thread** enters into action and **waits** until I/O is **completed** * **Limited in scalability**, one thread per I/O connection * Overhead, context switching * **Asyncronous** * **Passes the request immediatly** to the kernel and continous with other tasks

Question 32

Synchronous I/O

Answer 32

Question 33

Asyncronous I/O

Answer 33

Question 34

The “Latency is Zero” fallacy

Answer 34

* In the context of remote calls a fatal misconception * Latency is the time it takes to move data from client to storage server vs amount of data transferred (bandwidth) * Typical **WAN round trip times are 30ms+** * **Transfer as much data with as few calls as possible** vs. many fine grained calls