Introduction to Networking
Announcements
• Homework 4 due today, Thursday, October 30th
• Prelim II will be Thursday, November 20th, in class
• Nazrul will teach next two Tuesday’s, November 4th and 11th
– Make sure to attend class and to vote Nov 4th
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Goals for today
• Introduction to Networking
– Motivated by distributed systems
• Overview
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Layered Architecture
ISO and Internet Protocols
Addressing
Routing
Circuit vs Packet Switching
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Centralized vs Distributed Systems
Server
Client/Server Model
Peer-to-Peer Model
• Centralized System: System in which major functions are
performed by a single physical computer
– Originally, everything on single computer
– Later: client/server model
• Distributed System: physically separate computers working
together on some task
– Early model: multiple servers working together
• Probably in the same room or building
• Often called a “cluster”
– Later models: peer-to-peer/wide-spread collaboration
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Distributed Systems
Definition:
Loosely coupled processors interconnected by network
• Distributed system is a piece of software that ensures:
– Independent computers appear as a single coherent system
• Lamport: “A distributed system is a system where I can’t
get my work done because a computer that I’ve never
heard of has failed”
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Why use distributed systems?
• These are now a requirement:
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–
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Economics dictate that we buy small computers
Cheap way to provide reliability
We all need to communicate
It is much easier to share resources
Allows a whole set of distributed applications
A whole set of future problems need machine communication
• Collaboration: Much easier for users to collaborate through network
resources (such as network file systems)
– …
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Distributed Systems: Issues
• The promise of distributed systems:
– Higher availability: one machine goes down, use another
– Better durability: store data in multiple locations
– More security: each piece easier to make secure
• Reality has been disappointing
– Worse availability: depend on every machine being up
• Lamport: “a distributed system is one where I can’t do work because some
machine I’ve never heard of isn’t working!”
– Worse reliability: can lose data if any machine crashes
– Worse security: anyone in world can break into system
• Coordination is more difficult
– Must coordinate multiple copies of shared state information (using only a
network)
– What would be easy in a centralized system becomes a lot more difficult
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Distributed Systems Goals
• Connecting resources and users
• Transparency: the ability of the system to mask its complexity
behind a simple interface
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Location: Can’t tell where resources are located
Migration: Resources may move without the user knowing
Replication: Can’t tell how many copies of resource exist
Concurrency: Can’t tell how many users there are
Parallelism: System may speed up large jobs by splitting them into
smaller pieces
– Fault Tolerance: System may hide various things that go wrong in the
system
• Openness: portability, interoperability
• Scalability: size, geography, administrative
• Transparency and collaboration require some way for
different processors to communicate with one another
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Software Concepts
System
Description
Main Goal
Distributed OS
Tightly coupled OS for
multiprocessors and
homogeneous m/cs
Hide and manage
hardware resources
Networked OS
Loosely coupled OS for
heterogeneous computers,
LAN/WAN
Offer local services to
remote clients
Middleware
Additional layer atop NOS
implementing general-purpose
services
Provide distribution
transparency
Machine C
Machine B
Machine A
Distributed Applications
Middleware
Local OS
Local OS
Local OS
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Network
Some Applications
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•
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•
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•
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Air traffic control
Banking, stock markets
Military applications
Health care, hospital automation
Telecommunications infrastructure
E-commerce, e-cash
…
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Few Challenges
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No shared clocks
– How to order events
•
No shared memory
– Inconsistent system state
•
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Scalability
Fault tolerance
– Availability, recoverability
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•
•
Consensus
Self management
Security
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Networking
• Middleware gives guarantees not provided by networking
• How do you connect computers?
– Local area network (LAN)
– Wide area network (WAN)
• Let us consider the example of the Internet
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Internet: Example
• Click -> get page
• specifies
- protocol (http)
- location
(www.cnn.com)
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Internet: Locating Resource
• www.cnn.com
– name of a computer
– Implicitly also a file (index.html)
• Map name to internet protocol (IP) address
– Domain name system (DNS)
cnn.com?
cnn.com?
host
com
local
a.b.c.d
a.b.c.d
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Internet: Connection
• Http (hyper-text transport protocol) sets up a connection
– TCP connection (transmission control protocol)
– between the host and cnn.com to transfer the page
• The connection transfers page as a byte stream
– without errors: flow control + error control
Host
www.cnn.com
Page; close
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Internet: End-to-end
• Byte stream flows end to end across many links/switches:
– routing (+ addressing)
• That stream is regulated and controlled by both ends:
– retransmission of erroneous or missing bytes; flow control
end-to-end pacing and
error control
CNN.COM
routing
HOST
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Internet: Packets
• The network transports bytes grouped into packets
• Packets are “self-contained”; routers handle them 1 by 1
• The end hosts worry about errors and pacing
– Destination sends ACKs; Source checks losses
A | B | # , CRC | bytes
CNN.COM: A
HOST: B
C
B: to
C
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Internet: Bits
• Equipment in each node sends packets as string of bits
• That equipment is not aware of the meaning of the bits
• Frames (packetizing) vs. streams
01011...011...110
01011...011...110
Transmitter
Physical Medium
Receiver
Optical
Copper
Wireless
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Internet: Points to remember
• Separation of tasks
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send bits on a link: transmitter/receiver [clock, modulation,…]
send packet on each hop [framing, error detection,…]
send packet end to end [addressing, routing]
pace transmissions [detect congestion]
retransmit erroneous or missing packets [acks, timeout]
find destination address from name [DNS]
• Scalability
– routers don’t know full path
– names and addresses are hierarchical
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Internet : Challenges
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•
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Addressing ?
Routing ?
Reliable transmission ?
Interoperability ?
Resource management ?
Quality of service ?
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Concepts at heart of the Internet
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•
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•
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Protocol
Layered Architecture
Packet Switching
Distributed Control
Open System
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Protocol
• Two communicating entities must agree on:
– Expected order and meaning of messages they exchange
– The action to perform on sending/receiving a message
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Layered Architectures
• How computers manage complex protocol processing?
– Break-up design problem into smaller problems
• More manageable
• Decompose complicated jobs into layers
– each has a well defined task
– Specify well defined protocols to enact.
• Modular design:
– easy to extend/modify.
• Difficult to implement
– careful with interaction of layers for efficiency
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Layered Architecture
network
users
Applications
Web, e-mail, file transfer, ...
Middleware
Reliable/ordered transmission, QOS,
security, compression, ...
Routing
Physical Links
End-to-end transmission,
resource allocation, routing, ...
Point-to-point links,
LANs, radios, ...
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The OSI Model
• Open Systems Interconnect (OSI)
– standard way of understanding conceptual layers of network comm.
– This is a model, nobody builds systems like this.
• Each level
– provides certain functions and guarantees
– communicates with the same level on remote notes.
• A message
– generated at the highest level
– is passed down the levels, encapsulated by lower levels
– until it is sent over the wire.
• On the destination
– Encapsulated message makes its way up the layers
– until the high-level message reaches its high-level destination.
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OSI Levels
Node A Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
Network
Node B
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OSI Levels
• Physical Layer
– electrical details of bits on the wire
• Data Link Layer
– sending “frames” of bits and error detection
• Network Layer
– routing packets to the destination
• Transport Layer
– reliable transmission of messages, disassembly/assembly, ordering,
retransmission of lost packets
• Session Layer
– really part of transport, typ. Not impl.
• Presentation Layer
– data representation in the message
• Application
– high-level protocols (mail, ftp, etc.)
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The ISO Network Message
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The Internet Protocol Layers
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Internet protocol stack
network
users
Application
HTTP, SMTP, FTP, TELNET, DNS, …
Transport
TCP, UDP.
Network
IP
Physical
Point-to-point links,
LANs, radios, ...
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Air travel
Passenger Origin
Passenger Destination
Ticket (purchase)
Ticket (complain)
Baggage (check)
Baggage (claim)
Gates (load)
Gates (unload)
Runway (take off)
Runway (landing)
Airplane routing
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Protocol stack
user X
English
user Y
e-mail client
SMTP
e-mail server
TCP server
TCP
TCP server
IP server
ethernet
driver/card
IP
IEEE 802.3 standard
electric signals
IP server
ethernet
driver/card
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Protocol interfaces
user X
user Y
e-mail client
TCP server
e-mail server
s = open_socket();
socket_write(s, buffer);
…
TCP server
IP server
IP server
ethernet
driver/card
ethernet
driver/card
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Socket
• A communication end-point unique to a machine
• An Internet socket is composed of the following:
– Protocol (TCP, UDP, etc)
– Local IP address
• Address of local machine
– Local port
• Identifier for local process on local machine
– Remote IP address
• Address of remote machine
– Remote port
• Identifier for remote process on remote machine
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Addressing
• Each network interface has a hardware MAC address
– Multiple interfaces  multiple addresses
• Each application communicates via a port
– Port is a logical connection endpoint
– Allows multiple local applications to use network resources
– Up to 65,535
• < 1024 : used by privileged applications
• 1024 ≤ available for use ≤ 49151
• 49152 ≤ Dynamic ports/private ports ≤ 65535
– http ports 80 and 8080
– ssh 20, telnet 23, ftp 21, etc
• Think of a telephone network …
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Addressing and Packet Format
• The ``Data'' segment contains higher
level protocol information.
Start (7 bytes)
– Which protocol is this packet destined for?
Destination (6)
– Which process is the packet destined for?
Source (6)
– Which packet is this in a sequence of
packets?
Length (2)
– What kind of packet is this?
• This is the stuff of the OSI reference
model.
Msg Data (1500)
Checksum (4)
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Ethernet packet dispatching
• An incoming packet comes into the Ethernet controller.
• The Ethernet controller reads it off the network into a
buffer.
• It interrupts the CPU.
• A network interrupt handler reads the packet out of the
controller into memory.
• A dispatch routine looks at the Data part and hands it to a
higher level protocol
• The higher level protocol copies it out into user space.
• A program manipulates the data.
• The output path is similar.
• Consider what happens when you send mail.
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Hi Dad.
Example: Mail
To: Dad
Hi Dad.
SrcAddr: 128.95.1.2
DestAddr: 128.95.1.3
SrcPort: 110,
DestPort: 110Bytes: 1-20
Hi Dad.
To: Dad
Mail Composition And Display
User
Mail Transport Layer
Kernel
To: Dad
Hi Dad.
SrcAddr: 128.95.1.2
DestAddr: 128.95.1.3
SrcPort: 110,
DestPort: 110Bytes: 1-20
To: Dad
Network Transport Layer
Hi Dad.
SrcEther: 0xdeadbeef
DestEther: 0xfeedface
SrcAddr: 128.95.1.2
DestAddr: 128.95.1.3
SrcPort: 100
DestPort: 200Bytes: 1-20
Hi Dad.
Link Layer
To: Dad
Hi Dad.
SrcEther: 0xdeadbeef
DestEther: 0xfeedface
SrcAddr: 128.95.1.2
DestAddr: 128.95.1.3
SrcPort: 100
DestPort: 200Bytes: 1-20
To: Dad
Network
Hi Dad.
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Protocol encapsulation
user X
“Hello”
user Y
e-mail client
“Hello”
e-mail server
TCP server
“Hello”
TCP server
IP server
“Hello”
IP server
“Hello”
ethernet
driver/card
ethernet
driver/card
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End-to-End Argument
• What function to implement in each layer?
• Saltzer, Reed, Clarke 1984
– A function can be correctly and completely implemented only with
the knowledge and help of applications standing at the
communication endpoints
– Argues for moving function upward in a layered architecture
• Should the network guarantee packet delivery ?
– Think about a file transfer program
– Read file from disk, send it, the receiver reads packets and writes
them to the disk
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End-to-End Argument
• If the network guaranteed packet delivery
– one might think that the applications would be simpler
• No need to worry about retransmits
– But need to check that file was written to the remote disk intact
• A check is necessary if nodes can fail
– Consequently, applications need to perform their retransmits
• No need to burden the internals of the network with
properties that can, and must, be implemented at the
periphery
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End-to-End Argument
• An Occam’s razor for Internet design
– If there is a problem, the simplest explanation is probably the correct
one
• Application-specific properties are best provided by the
applications, not the network
– Guaranteed, or ordered, packet delivery, duplicate suppression,
security, etc.
• The internet performs the simplest packet routing and
delivery service it can
– Packets are sent on a best-effort basis
– Higher-level applications do the rest
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Two ways to handle networking
• Circuit Switching
– What you get when you make a phone call
– Dedicated circuit per call
• Packet Switching
– What you get when you send a bunch of letters
– Network bandwidth consumed only when sending
– Packets are routed independently
• Message Switching
– It’s just packet switching, but routers perform store-and-forward
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Circuit Switching
• End-to-end resources reserved for “call”
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Link bandwidth, switch capacity
Dedicated resources: no sharing
Circuit-like (guaranteed) performance
Call setup required
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Packet Switching
• Each end-to-end data stream divided into packets
– User’s packets share network resources
• Compared to dedicated allocation
– Each packet uses full link bandwidth
• Compared to dividing bandwidth into pieces
– Resources are used as needed
• Compared to resource reservation
• Resource contention:
– Aggregate demand can exceed amount available
– Congestion: packets queue, wait for link use
– Store and forward: packets move one hop at a time
• Transmit over link
• Wait turn at next link
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Routing
• Goal: move data among routers from source to dest.
• Datagram packet network:
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Destination address determines next hop
Routes may change during session
Analogy: driving, asking directions
No notion of call state
• Circuit-switched network:
– Call allocated time slots of bandwidth at each link
– Fixed path (for call) determined at call setup
– Switches maintain lots of per call state: resource allocation
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Packet vs. Circuit Switching
• Reliability: no congestion, in-order data in circuit-switch
• Packet switching: better bandwidth use
• State, resources: packet switching has less state
– Good: less control plane processing resources along the way
– More data plane (address lookup) processing
• Failure modes (routers/links down)
– Packet switch reconfigures sub-second timescale
– Circuit switching: more complicated
• Involves all switches in the path
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A small Internet
W
w,e5
b,e4
B
V
Scenario:
A wants to send data to B.
R
r3
r1,e1
r2,e2
a,e3
A
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Packet forwarding
Host A
Host B
Router R
Router W
HTTP
HTTP
TCP
TCP
IP
IP
ethernet
eth
IP
link
link
IP
eth
ethernet
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Summary (1/2)
• Network: physical connection that allows two computers to
communicate
– Packet: unit of transfer, sequence of bits carried over the network
• Protocol: Agreement between two parties as to how
information is to be transmitted
• Internet Protocol (IP)
– Used to route messages through routes across globe
– 32-bit addresses, 16-bit ports
• Reliable, Ordered, Arbitrary-sized Messaging:
– Built through protocol layering on top of unreliable,
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Summary(2/2)
• Layering
– building complex services from simpler ones
• End-to-end argument
– Application-specific properties are best provided by the applications,
not the network
• Packet vs Circuit Switching
– Post card (packet) vs phone call (circuit)
– Bandwidth and congestion
• Packet - better bandwidth usage, but potentially congested links
• Circuit - no congenstion, but potentially lower link utilization
– Failures and reconfiguration
• Packet - Failed routed detected and routed around
• Circuit - reconfigure entire path if any router fails
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