1. Introduction to Networking

2. 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

3. Goals for today Introduction to Networking Overview
Motivated by distributed systems
Overview
Layered Architecture
ISO and Internet Protocols
Addressing
Routing
Circuit vs Packet Switching

4. Centralized vs Distributed Systems
Peer-to-Peer Model
Server
Client/Server 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

5. Loosely coupled processors interconnected by network
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”

6. Why use distributed systems?
These are now a requirement:
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) …

7. 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

8. Distributed Systems Goals
Connecting resources and users
Transparency: the ability of the system to mask its complexity behind a simple interface
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

9. Software Concepts Machine A Machine B Machine C
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
Local OS
Middleware
Distributed Applications
Network
Machine A
Machine B
Machine C

10. Some Applications Air traffic control Banking, stock markets
Military applications
Health care, hospital automation
Telecommunications infrastructure
E-commerce, e-cash …

11. Few Challenges No shared clocks No shared memory Scalability
How to order events
No shared memory
Inconsistent system state
Scalability
Fault tolerance
Availability, recoverability
Consensus
Self management
Security

12. 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

13. Internet: Example Click -> get page
specifies - protocol (http) - location (
Great brainstorm here too – what all kinds of traffic do you think will happen as a result of
This activity

14. Internet: Locating Resource
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
local
com
a.b.c.d
a.b.c.d

15. 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
Connect OK
Get page
Page; close

16. 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

17. 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

18. 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

19. Internet: Points to remember
Separation of tasks
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

20. Internet : Challenges Addressing ? Routing ? Reliable transmission ?
Interoperability ?
Resource management ?
Quality of service ?

21. Concepts at heart of the Internet
Protocol
Layered Architecture
Packet Switching
Distributed Control
Open System

22. Protocol Two communicating entities must agree on:
Expected order and meaning of messages they exchange
The action to perform on sending/receiving a message
Asking the time

23. 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

24. Layered Architecture users network Applications
Web, , file transfer, ...
Middleware
Reliable/ordered transmission, QOS,
security, compression, ...
Routing
End-to-end transmission,
resource allocation, routing, ...
Physical Links
Point-to-point links,
LANs, radios, ...

25. The OSI Model Open Systems Interconnect (OSI) Each level A message
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.

26. OSI Levels Node A Node B Application Application Presentation
Session
Session
Transport
Transport
Network
Network
Data Link
Data Link
Physical
Physical
Network

27. OSI Levels Physical Layer Data Link Layer Network 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.)

28. The ISO Network Message

29. The Internet Protocol Layers

30. Internet protocol stack
users
network
Application
HTTP, SMTP, FTP, TELNET, DNS, …
Transport
TCP, UDP.
Network IP
Physical
Point-to-point links,
LANs, radios, ...

31. 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

32. Protocol stack user X English user Y e-mail client e-mail server SMTP
TCP server
TCP server
TCP
Peers exchange units meaningful to each end; communicate
Uses services of lower layer to avoid complexity
IP server IP
IP server
IEEE standard
ethernet
driver/card
ethernet
driver/card
electric signals

33. Protocol interfaces user X user Y e-mail client e-mail server
TCP server
s = open_socket();
socket_write(s, buffer); …
TCP server
IP server
IP server
ethernet
driver/card
ethernet
driver/card

34. 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

35. 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 …

36. Addressing and Packet Format
The ``Data'' segment contains higher level protocol information.
Which protocol is this packet destined for?
Which process is the packet destined for?
Which packet is this in a sequence of packets?
What kind of packet is this?
This is the stuff of the OSI reference model.
Start (7 bytes)
Destination (6)
Source (6)
Length (2)
Msg Data (1500)
Checksum (4)

37. 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.

38. Mail Composition And Display Network Transport Layer
Example: Mail
Hi Dad.
Hi Dad.
Hi Dad.
To: Dad
Hi Dad.
To: Dad
Mail Composition And Display
SrcAddr:
DestAddr:
SrcPort: 110,
DestPort: 110Bytes: 1-20
SrcAddr:
DestAddr:
SrcPort: 110,
DestPort: 110Bytes: 1-20
Mail Transport Layer
User
Kernel
Hi Dad.
To: Dad
Hi Dad.
To: Dad
Network Transport Layer
SrcEther: 0xdeadbeef
DestEther: 0xfeedface
SrcEther: 0xdeadbeef
DestEther: 0xfeedface
Link Layer
SrcAddr:
DestAddr:
SrcPort: 100
DestPort: 200Bytes: 1-20
SrcAddr:
DestAddr:
SrcPort: 100
DestPort: 200Bytes: 1-20
Hi Dad.
To: Dad
Hi Dad.
To: Dad
Network

39. Protocol encapsulation
user X
user Y
“Hello”
client
server
“Hello”
TCP server
TCP server
“Hello”
IP server
IP server
“Hello”
ethernet
driver/card
ethernet
driver/card
“Hello”

40. 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

41. 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

42. 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

43. 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

44. Circuit Switching End-to-end resources reserved for “call”
Link bandwidth, switch capacity
Dedicated resources: no sharing
Circuit-like (guaranteed) performance
Call setup required

45. 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

46. Routing Goal: move data among routers from source to dest.
Datagram packet network:
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

47. 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

48. A small Internet W b,e4 w,e5 B V Scenario: A wants to send data to B.
Do Neotrace
r2,e2
r1,e1
a,e3 A

49. Packet forwarding Host A Host B Router R Router W HTTP HTTP TCP TCP IP
eth
link
link
eth
ethernet
ethernet

50. 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,

51. Summary(2/2) Layering End-to-end argument Packet vs Circuit Switching
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