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Brief Tutorial on Networks and Communications. CS-4513 Distributed Systems

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brief tutorial on networks and communications

Brief Tutorial onNetworks and Communications

CS-4513 Distributed Systems

(Slides include materials from Operating System Concepts, 7th ed., by Silbershatz, Galvin, & Gagne, Distributed Systems: Principles & Paradigms, 2nd ed. By Tanenbaum and Van Steen, and Distributed Systems: Concepts and Design, 4th ed., by Coulouris, et. al.)

Networks and Communications

context
Context
  • Networking was formerly regarded as “just another form of I/O”
  • Today, focus is Distributed Computing
      • Shared files and other resources among physically separated systems on networks
        • NFS, remote printing, etc.
      • Integrated computations across network
        • Airline reservations, ATMs, etc.
      • Interactive games and multimedia
  • Note: this topic overlaps with CS-3516
      • Also with CS-513/ECE-506

Networks and Communications

topics
Topics
  • Fundamentals
      • Socket interface
      • Protocol Stack
      • Kinds of network connections
  • Kinds of Communication
      • Remote Procedure Call
      • Message-oriented communication
      • Stream-oriented communication
  • Naming
      • Names, addresses, routes

Networks and Communications

slide4

Computer A

Computer B

Computer C

The Network

Process k

Process i

Process j

Networks and Communications

network goal
Network Goal
  • Allow activities on multiple computer systems to communicate with each other
      • Shared memory, files, or data
      • Message passing
      • Remote Procedure Call
      • Integrated applications — distributed across physical space
  • Create abstractions that make these (relatively) transparent

Networks and Communications

principal abstraction socket
Principal Abstraction – Socket
  • Originally created in BSD Unix
  • Subsequently, part of most operating systems
  • Allows opening a connection between two processes across network
  • Connection:
    • a serial conversation between two end points
      • e.g., processes, threads, tasks on different machines
    • organized as a sequence of messages or datagrams
    • distinct from all other connections

Networks and Communications

slide7

Thread r

Computer A

Computer B

Computer C

The Network

Process p

Process k

Process i

Process j

Task q

Networks and Communications

definition protocol
Definition — Protocol
  • Formal set of rules that govern the formats, contents, and meanings of messages from computer to computer, process to process, etc.
  • Must be agreed to by all parties to a communication
  • May be defined in terms of other protocols

Networks and Communications

there are many many protocols
There are many, many protocols
  • TCP, UDP, IP, NCP, SMTP, SNNP, NNTP, FTP, TFTP, POP, IMAP, HTTP, VMRL, …
  • Appletalk, Netware, …
  • Remote Procedure Call, NFS, …
  • CORBA, GLOBE, JINI, …
  • Network Streaming, …

How to make sense out of all of them?

Networks and Communications

network stack
Network Stack
  • 1983 – Open System Interconnection (OSI) 7 layer Reference Model
    • Working group of the International Standards Organization (ISO)
    • Defines seven layers
      • Describe how applications communicate with each other
        • Via network-aware devices
    • Most day-to-day protocols
      • work on a slightly modified layer system
      • E.g. TCP/ IP uses a 6-rather than a 7-layer model

Networks and Communications

osi 7 layer model
OSI 7-layer model
  • Primarily a software and protocol architecture
  • Layers of model correspond to layers of abstraction
  • Each layer has a well-defined function
  • Layers chosen so that …
    • international standards can be defined
  • Boundaries between layers chosen to …
    • minimize information flow across interfaces
  • Number of layers:–
    • Large enough
      • Distinct functions need not be thrown together
    • Small enough
      • Architecture does not become unwieldy

Networks and Communications

the osi 7 layer model in a nutshell

Application

Layer

Presentation

Layer

Session

Layer

Transport

Layer

Network

Layer

Data Link

Layer

Physical

Layer

The OSI 7-layer model(in a nutshell)

Silbershatz, §§16.6-16.7Coulouris et al, §3.3.4

Networks and Communications

annotated osi 7 layer stack
Annotated OSI 7-Layer Stack

Silbershatz, page 630

Networks and Communications

the osi 7 layer model continued
The OSI 7-layer model (continued)
  • Layer 2 – Data Link Layer
    • Take the raw transmission facility and transform it into an abstract link that appears free of errors to layer 3.
      • Error correcting coding (e.g. FEC)
      • Rate Control (Slow device not overrun by high speed device)
      • Defines Packet abstraction
  • Layer 1 – Physical Layer
    • Defines the physical and electrical characteristics of the network.
      • Transmitting of raw bits over the communication channel
      • Defines Bit abstraction

Application

Layer

Presentation

Layer

Session

Layer

Transport

Layer

Network

Layer

Data Link

Layer

Physical

Layer

Networks and Communications

the osi 7 layer model continued1

Application

Layer

Presentation

Layer

Session

Layer

Transport

Layer

Network

Layer

Data Link

Layer

Physical

Layer

The OSI 7-layer model (continued)
  • Layer 2 – Data Link Layer
    • Take the raw transmission facility and transform it into an abstract link that appears free of errors to layer 3.
      • Error correcting coding (e.g. FEC)
      • Rate Control (Slow device not overrun by high speed device)
      • Defines Packet abstraction
  • Layer 1 – Physical Layer
    • Defines the physical and electrical characteristics of the network.
      • Transmitting of raw bits over the communication channel
      • Defines Bit abstraction

Networks and Communications

the osi 7 layer model continued2

Application

Layer

Presentation

Layer

Session

Layer

Transport

Layer

Network

Layer

Data Link

Layer

Physical

Layer

The OSI 7-layer model (continued)
  • Layer 3 – Network Layer
    • Controlling the operation of the subnet
    • How packets are routed
    • Congestion Control
    • Accounting function (billing)
      • Network Statistics
    • Example - IP layer (IPv4, IPv6)
      • Differences between v4, v6 source/destination addressing
        • V4 – 32 bit addressing
        • V6 – 128 bit addressing
    • Defines Internet abstraction – i.e., packets that can be sent from anywhere to anywhere

Networks and Communications

the osi 7 layer model continued3

Application

Layer

Presentation

Layer

Session

Layer

Transport

Layer

Network

Layer

Data Link

Layer

Physical

Layer

The OSI 7-layer model (continued)
  • Layer 4 – Transport Layer
    • Accept data from higher layers
      • Split it up into smaller units if need be
      • Passes these to the network layer
      • Ensures that the packets all arrive correctly at the destination in the right order
      • Isolates higher layers from changes in the underlying hardware
    • Two types of service to provide
      • Reliable or unreliable delivery
    • True end-to-end layer
    • Example:– TCP or UDP
    • Defines Connection abstraction – i.e., data to destination

Networks and Communications

the osi 7 layer model continued4

Application

Layer

Presentation

Layer

Session

Layer

Transport

Layer

Network

Layer

Data Link

Layer

Physical

Layer

The OSI 7-layer model (continued)
  • Layer 7 – Application Layer
    • User layer protocol, multiple protocols required
    • Example – http, ftp, smtp
  • Layer 6 – Presentation Layer
    • Performs certain functions that are requested sufficiently often to warrant finding a general solution for them rather than letting each user solve the problem
    • Example – encoding data
  • Layer 5 - Session Layer
    • Allows users on different machines to establish sessions between them
    • Example SSL, RPC

Networks and Communications

summary osi 7 layer model

Sending

Process

Receiving

Process

Data

Application

Layer

Application

Layer

AH

Data

Presentation

Layer

Presentation

Layer

PH

Data

Session

Layer

Session

Layer

SH

Data

Transport

Layer

Transport

Layer

TH

Data

Network

Layer

Network

Layer

NH

Data

Data Link

Layer

Data Link

Layer

DH

Data

DT

Physical

Layer

Physical

Layer

Bits

Summary — OSI 7-layer model

Networks and Communications

layered protocols
Layered Protocols
  • OSI 7-layer model was intended to be a foundation of a family of international standard protocols
  • Those protocols never gained much acceptance
  • Role of Session and Presentation layers is murky, at best.
  • Internet protocols (TCP/IP, etc.) are dominant

Networks and Communications

the tcp ip protocol layers

TCP/IP

The TCP/IP Protocol Layers

Networks and Communications

the tcp ip protocol layers1

TCP/IP

The TCP/IP Protocol Layers

Subsumed by middleware

Defined by manufacturers, industry sub-groups, and separate standards bodies

Networks and Communications

modified layers
Modified Layers

Networks and Communications

examples of middleware
Examples of Middleware
  • Authentication protocols
  • Commit protocols for atomic transactions
  • Multimedia protocols
  • Remote Procedure Call protocols (RPC)

Networks and Communications

styles of communication
Styles of Communication
  • Message-oriented
  • Remote Procedure Call
  • Streaming

Networks and Communications

some terms
Some Terms
  • Packet:
    • A unit of communication at Data Link layer
    • aka datagram
  • IP Address:
    • A four-part “number” used by Network Layer to route a packet from one computer to another
  • Port:
    • A 16-bit number used within one computer to identify who/where to send packet to
  • Well-known port:
    • A port with number < 1024, used by agreement for standard services – e.g.,
      • telnet (23), ftp (21), smtp (25), pop (110)

Networks and Communications

more terms
More Terms
  • Socket:
    • End point of a communication
    • Usually used in pairs, one for each direction
    • Comprises [IP Address: Port #]
  • Connection:
    • A logical linkage between pairs of sockets at two endpoints for purposes of a particular communication between those endpoints
    • i.e., a serial conversation between endpoints
      • Usually two-way

Networks and Communications

connection
Connection
  • The backbone of most message-oriented communication protocols
  • Each party retains knowledge of the other
  • Each party retains information about state of the other (vis a vis the protocol itself)
  • Each party “knows” if connection is broken
  • Note: some popular protocols are “connection-less”
    • one side retains no state information about other side

Networks and Communications

establishing a connection
Establishing a Connection
  • Process a on machine m creates a socket
      • OS assigns a new port number q to that socket
  • Process a attempts to open a connection to machine n:p
      • p is a well-known port
  • Process b on machine n is listening on p
      • Receives request from m:q
  • Process b forks a process or spawns a thread c to talk with m:q, then resumes listening on p
  • Thread/process c
      • Creates a new socket r for this connection
      • Replies to m:q with return address n:r
  • a and c continue to communicate over this pair of sockets until they are finished.

Networks and Communications

typical client server connection
Typical Client-Server Connection
  • Create socket
      • On server side
  • Bind
      • I.e., connect socket to port # (usually well-known port)
  • Listen
      • Sit and wait for a communication to come in
  • Accept
      • Create new socket for purpose of responding to this caller

Networks and Communications

notes
Notes
  • Responder to request for connection does not have to be the original server machine
      • Delegate workload to other server systems
  • Systems often include a connection ID as part of request to open connection
      • Unique or randomly chosen
      • Reduces spoofing of server responses
  • Unix/Linux will not re-use a socket # within 30 seconds
      • To avoid confusion between old connection and new

Networks and Communications

reliable connections
Reliable Connections
  • Transport layer partitions messages into packets
      • TCP – Transmission Control Protocol
      • Sequence number of current packet
      • Sequence number of last packet received correctly
  • Receiver keeps track of seq. # of packets
      • Reassembles in right order
      • Notify sender of missing, broken packets
  • Sender keeps copy of each packet until receipt acknowledged
      • Retransmits packets if no acknowledgement
  • Window defines how many packet buffers to maintain for efficient transmission
      • Allows many packets in “flight”

Networks and Communications

reliable connections continued
Reliable Connections (continued)

Packet i

Packet i+1

Packet i+2

Packet i+3

rec’d i

Packet i+k

time

Networks and Communications

reliable connections continued1
Reliable Connections (continued)

Packet i

Packet i+1

Packet i+2

Packet i+3

rec’d i

rec’d i

Packet i+k

time

Networks and Communications

reliable connections continued2
Reliable Connections (continued)

Packet i

Packet i+1

Packet i+2

Packet i+3

rec’d i

rec’d i

Packet i+k

rec’d i+2

time

Networks and Communications

reliable connections continued3
Reliable Connections (continued)

Packet i

Packet i+1

Packet i+2

Packet i+3

rec’d i

lost

rec’d i

Packet i+k

rec’d i+2

rec’d i+2

time

Networks and Communications

reliable connections continued4
Reliable Connections (continued)
  • If acknowledgement received for packet i
      • Delete from buffer all packets  i
  • If no acknowledgement received within a reasonable time for packet k
      • Retransmit from buffer all packets  k
  • Result
      • Recovers from loss of packets
      • Recovers from loss of acknowledgements
      • Works well for reasonably reliable internet
      • Doesn’t work so well for noisy, unreliable networks

Networks and Communications

reminder
Reminder
  • How do we know if a packet is received correctly?
  • Cyclic Redundancy Check (CRC)
    • Polynomial computed from packet header and body
        • Usually 16 or 32 bits, computed by hardware
    • Appended to message
    • Recomputed on reception, compared with transmitted CRC
        • Equal  packet received correctly

Networks and Communications

examples of connection based protocols
Examples of Connection-based Protocols
  • Telnet(virtual terminal)
    • 2-way communication by character stream
    • Line-by-line organization
  • SMTP(Simple Mail Transport Protocol)
    • For sending mail
    • Layered on top of telnet protocol
  • POP (Post Office Protocol)
    • For receiving your mail
    • Layered on top of telnet protocol
  • FTP (File Transfer Protocol)
    • For transmitting ASCII or binary files
    • Binary data transmission not layered on telnet protocol

Networks and Communications

connection less communication
Connection-less communication
  • Some communication protocols don’t need the overhead of reliable connections
    • When some number of errors can be tolerated
    • Where recovery from those errors is easy
  • UDP – User Datagram Protocol
    • The internet connection-less protocol (layer 4)
    • Breaks messages into packets
    • Messages delivered atomically or not at all
    • Does not send acknowledgement of correct receipt

Networks and Communications

examples
Examples
  • HTTP (HyperText Transport Protocol)
    • Web server responds directly to requests
    • If client does not get response, retries request
  • NFS (Network File System)
    • For access to files on servers as if they are local
    • If client does not get response, retries request
  • RPC (Remote Procedure Call)
    • Next topic

Networks and Communications

summary
Summary
  • Socket, connection
  • Network stack, 7-layer model
  • Establishing a connection
  • Reliable transmission
  • Reading assignment
    • Coulouris, Chapter 3

Networks and Communications