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7. Distributed Systems > Chapter 2: Communication Basics > Flashcards

Chapter 2: Communication Basics Flashcards

1
Q

ISO‘s OSI model NN

A
  • Open Systems Interconnection model
  • Basis for standards development on systems interconnection.
  • Reference model for networking standards development
  • ISO/IEC7498-1
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2
Q

OSI model

A
  • Application
    • HTTP
  • Presentation
  • Session
  • Transport
    • TCP / UDP
  • Network
    • IP and Routing
  • Data Link
  • Physical
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3
Q

IP

A
  • 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
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4
Q

IP Routing

A
  • 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
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5
Q

TCP

A
  • 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, …
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6
Q

UDP

A
  • 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, …
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7
Q

Ports

A
  • 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
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8
Q

Application layer protocols

A
  • Example: A browser wants to retreive a web page
    • Typically HTTP 1.1 over TCP is used
  • Presentedprotocols
    • HTTP
  • Other protocols
    • FTP
    • SMTP
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9
Q

Protocols

A
  • 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, …
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10
Q

Common application layer protocols

A
  • 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)

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11
Q

HTTP/1.1

A

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
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12
Q

HTTP/1.1 methods

A
  • 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
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13
Q

HTTP/2.0

A
  • 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
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14
Q

The Hourglass Evolution of the Internet Protocol Stack

A
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15
Q

Socket: Problem description

A
  • 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)
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16
Q

A Client/Server world

A
  • 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
17
Q

Protocol design considerations

A
  • 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?
18
Q

Simple protocol design

A
  • 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
19
Q

K/V-store server API NN

A
  • 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
20
Q

Using the “network” / Abstracting the network NN

A

Sockets are network programming abstractions.

  • User interface Business Layer
  • Internal API
  • Serialization/Protocol
  • Connection and error handling
  • Networking API
  • Protocols (TCP, …)
21
Q

Communication end-points & sockets

A
  • 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!
22
Q

Sockets are file descriptors

A
  • 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
23
Q

Socket types

A
  • Internetsockets…
  • Types of Sockets
    • SOCK_STREAM
      • TCP based
      • Ordering guaranteed
      • Error-free
    • SOCK_DGRAM
      • UDP based
  • IPv4 & IPv6
24
Q

SOCK_STREAM Sockets

A
  • 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)
25
Q

Stream sockets in practice

A
  • 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?
26
Q

Two end-points determine a connection

A
  • End-point is determined by
    • IP address (host address); e.g., 192.168.100.100
    • Port number, e.g., 8888
27
Q

“Error binding socket”

A
  • 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
28
Q

Byte Order

A
  • 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
29
Q

Setting up & using sockets !!

A
30
Q

Java NIO

A
  • 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
31
Q

Asynchronous vs. Synchronous

A
  • 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
32
Q

Synchronous I/O

A
33
Q

Asyncronous I/O

A
34
Q

The “Latency is Zero” fallacy

A
  • 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

7. Distributed Systems (21 decks)

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