Download Protocol Stack

Today’s Lecture
• Network applications
18-345: Introduction to
Telecommunication Networks
Lectures 2: Protocol Stack
• Requirements
• Latency and bandwidth
• Internet architecture
• Protocols
• A layered design
• Life of a packet
Peter Steenkiste
Spring 2015
www.cs.cmu.edu/~prs/nets-ece
• Network utilities
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Applications and Application Protocols
Client-Server Paradigm
• Application: communicating,
distributed processes
Typical network app has two pieces: client and server
Client:
• Running in network hosts in
“user space”
• Exchange messages to
implement app
• e.g., email, file transfer, the
Web
application
transport
network
data link
physical
• Initiates contact with server
(“speaks first”)
• Typically requests service from
server,
• For Web, client is implemented in
browser; for e-mail, in mail
reader
• Application protocols
• One “piece” of an app
• Define messages exchanged
by apps and actions taken
• User services provided by
lower layer protocols
Server:
application
transport
network
data link
physical
application
transport
network
data link
physical
• Provides a service to client
• e.g., Web server sends
requested Web page, mail server
delivers e-mail
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application
transport
network
data link
physical
request
reply
application
transport
network
data link
physical
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Page 1
Yesterday’s Applications
FTP: The File Transfer Protocol
• FTP: transfer files to a host
• No distributed file systems!
• Mostly replaced by “the web” – http
FTP
user
interface
FTP
client
user
at host
• Telnet: use a computer remotely
• Similar to ssh today (minus the security)
file transfer
FTP
server
remote file
system
local file
system
• Transfer file to/from remote host
• Client/server model that allows a host (client) to transfer a file to/from
another host (server)
• Application is a minimal wrapper - a command line “user interface”
• Mail: exchange electronic e-mail
• Similar today (kind of)
• Initially host-to-host: [email protected]
• All the heavy lifting is done in the protocol implementation
• Project: will learn about HTTP
• Already very useful!
• Same idea but much richer functionality
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Ftp Commands, Responses
FTP: Separate Control, Data
Connections
Sample Commands:
• Ftp client contacts ftp server at
port 21, specifying TCP as
transport protocol
• Two parallel TCP connections
opened:
•
TCP control connection
port 21
Control: exchange commands,
responses between client, server.
“out of band control”
•
Data: file data to/from server
• sent as ASCII text over control
channel
• USER username
• PASS password
FTP
client
TCP data connection
port 20
• LIST return list of files in
current directory
FTP
server
• RETR filename retrieves
(gets) file
• Ftp server maintains “state”:
current directory, earlier
authentication
Sample Return Codes
• status code and phrase
• 331 Username OK,
password required
• 125 data connection
already open; transfer
starting
• 425 Can’t open data
connection
• 452 Error writing file
• STOR filename stores (puts)
file onto remote host
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Page 2
Today’s Applications
Requirements
• Amazon, Facebook, etc.
• Performance: latency and throughput
• Network reliability
• What matters most?
• 2009 quote: “Amazon found every 100ms of latency
cost them 1% in sales”
• Network service must always be available
• Security: for users and the network itself
• Video streaming
• Privacy, authentication, deal with various attacks, …
• Attacks on the network, versus enabled by the network
• Accounts for very high percentage of bandwidth
• Interactive versus broadcast versus playback
• What matters most?
• Scalability.
• Scale to large numbers of users, traffic flows, …
• Skype audio and video conferencing
• Manageability: monitoring, enforcing policies,
billing, …
• Traditional telephone app
• What matters most?
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Transport Service Requirements
of Common Apps
What Service Does an
Application Need?
Data loss
Timing
• Some apps (e.g., audio) can
tolerate some loss
• Other apps (e.g., file transfer,
telnet) require 100% reliable
data transfer
• Some apps (e.g., Internet
telephony, interactive
games) require low delay to
be “effective”
• Most apps are flexible
Application
file transfer
e-mail
web documents
real-time audio/
video
stored audio/video
interactive games
financial apps
Bandwidth
• Some apps (e.g., multimedia) require minimum amount of
bandwidth to be “effective”
• Other apps (“elastic apps”) make use of whatever bandwidth
they can get
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Data loss
Bandwidth
Time Sensitive
no loss
no loss
no loss
loss-tolerant
elastic
elastic
elastic
audio: 5Kb-1Mb
video:10Kb-5Mb
same as above
few Kbps
elastic
no
no
no
yes, 100’s msec
loss-tolerant
loss-tolerant
no loss
yes, few secs
yes, 100’s msec
yes and no
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Page 3
A Closer Look at Packet Delay
Cut-through
• Sum of a number of different delay components.
• Propagation delay on each link.
Prop + xmit
2*(Prop + xmit)
2*prop + xmit
Store & Forward
Packet Delay Components
• Proportional to the length of the link
• Transmission delay on each link.
• Proportional to the packet size and 1/link speed
• Processing delay on each router.
• Depends on the speed of the router
When does cut-through matter?
• Queuing delay on each router.
• Depends on the traffic load and queue size
Next: Routers have finite speed (processing delay)
D B C B A A
Routers may buffer packets (queueing delay)
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A Word about Units
Application-level Delay
• What do “Kilo” and “Mega” mean?
• Depends on context
• Storage works in powers of two.
• 1 Byte = 8 bits
• 1 KByte = 1024 Bytes
• 1 MByte = 1024 Kbytes
Delay of
one packet
• Networks work in decimal units.
Average
sustained
throughput
• Network hardware sends bits, not Bytes
• 1 Kbps = 1000 bits per second
• To avoid confusion, use 1 Kbit/second
• Why? Historical: CS versus ECE.
Size
Delay* +
Throughput
Units: seconds +
bits/(bits/seconds)
* For minimum sized packet
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Page 4
Some Examples
A Closer Look at Throughput
• How long does it take to send a 100 Kbit file?
•
• When streaming packets, the network works
like a pipeline.
Assume a perfect world
• Is the transfer latency or throughput limited?
• What about a 10 Kbit file?
Throughput
Latency
• All links forward different packets in parallel
• Throughput is determined by the slowest
stage.
100 Kbit/s
1 Mbit/s
100 Mbit/s
500 sec
1.0005
0.1005
0.0105
0.1005
0.0006
0.0015
10 msec
1.01
0.11
0.02
0.11
0.0101
0.011
100 msec
0.2
1.1
0.11
0.2
0.1001
0.101
• Called the bottleneck link
• Does not matter why the link is slow!
• Low link bandwidth
• Many users sharing the link bandwidth
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30
104
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17 267
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Bandwidth Sharing
Fair Sharing of Bandwidth
• Bandwidth received on the
bottleneck link determines
BW
end-to-end throughput.
• Router before the bottleneck 100
link decides how much
bandwidth each user gets.
•
• All else being equal, fair
means that users get equal
treatment.
•
Users that try to send at a
higher rate will see packet loss
• User bandwidth can fluctuate
quickly as flows are added or
end, or as flows change their
transmit rate.
Sounds fair
• When things are not equal,
we need a policy that
determines who gets how
much bandwidth.
•
•
Time
•
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Users who pay more get more
bandwidth
Users with a higher “rank” get
more bandwidth
Certain classes of applications
get priority
BW
100
Time
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Page 5
But It is Not that Simple
Today’s Lecture
• Network applications
• Requirements
• Latency and bandwidth
• Internet architecture
• Protocols
• A layered design
• Life of a packet
Bottleneck
• Network utilities
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Lots of Protocols (and Acronyms!)
•
•
•
•
•
•
•
•
•
•
•
What are Protocols?
IP: Internet protocol
UDP: User datagram protocol
TCP: Transmission control protocol
FTP: File transfer protocol
SMTP: Simple mail transfer protocol
HTTP: Hypertext transfer protocol
ARP: Address resolution protocol
BGP: Border gateway protocol
ICMP: Internet control message protocol
DHCP: Dynamic host configuration protocol
And many more …
• An agreement between parties on
how communication should take
place
Friendly greeting
• Module in layered structure
Muttered reply
• Protocols define: Interface to peer
(syntax & semantics)
•
•
•
Actions taken on receipt of a messages
Format and order of messages
Error handling, termination, ordering of
requests, etc.
• Example: Buying airline ticket
Destination?
Pittsburgh
Thank you
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Page 6
Solution #1
How to Design a Network?
• Has many users
• Offers diverse services
• Mixes very diverse
technologies
Operating System
Application
Links
• Components built by
many companies
• Diverse ownership
• Can evolve over time
TCP
Router Software
Operating System
(many protocols) Application
Tw. Pair
FTP
Wireless
HTTP
UDP
The Network
Coax
DNS
Computer
Network Interface
Router Hardware
Computer
Bridge HW/SW
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Solution #2?
Web
Optical
Telnet
Web
Protocol Software
Voice
FTP
Solution #3
Telnet
Voice
Web
Video
FTP
Telnet
Voice
Intermediate Layer
Tw. Pair
Coax
Optical
Wireless
Tw. Pair
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Coax
Optical
Wireless
Protocol and
Service Levels
Types of Protocols
host
host
host
Application
host
host
host
host
Application
Channel
• Core network: responsible for transferring data
between a sending and receiving host.
• End-to-end protocols: present a network service to
applications and users.
Host to
Host
• May add value to the core network protocols
Hardware
• Driven by differences in constraints: scalability, power,
management, speed, etc.
Layering: modular approach to network functionality
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Layering Characteristics
A Layer Network Model
• Each layer relies on services from layer
below and exports services to layer above
• Interface defines interaction with peer on
other hosts – called protocols
• Modules hide implementation - layers can
change without disturbing other layers
(black box)
The Open Systems Interconnection (OSI) Model
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Application
Application
6
Presentation
Presentation
5
Session
Session
4
Transport
Transport
3
Network
Network
Network
Network
2
Data link
Data link
Data link
Data link
1
Physical
Physical
Physical
Physical
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OSI Model: 7 Protocol Layers
Different Sources of Components
•
•
•
•
•
•
•
• Application: web
server/browser, mail,
distributed game,..
• Presentation/session
Physical: how to transmit bits
Data link: how to transmit frames
Network: how to route packets
Transport: how to send packets end2end
Session: how to tie flows together
Presentation: byte ordering, security
Application: everything else
• Often part of application
• Transport/network
• Typically part of the operating
system
• Datalink
• Often written by vendor of the
network interface hardware
• TCP/IP has been amazingly successful, and it
is not based on a rigid OSI model. The OSI
model has been very successful at shaping
thought
• Physical
• Hardware: card and link
Application
Presentation
Session
Transport
Network
Data link
Physical
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The Internet Engineering
Task Force
Application & Upper Layers
• Standardization is key to network interoperability
• Application Layer: Provides
services that are frequently
required by applications: DNS,
web acess, file transfer, email…
• Presentation Layer: machineindependent representation of
data…
• Session Layer: dialog
management, recovery from
Mostly
errors, …
incorporated into
Application Layer
• The hardware/software of communicating parties are often not built
by the same vendor  yet they can communicate because they
use the same protocol
Application
Application
Application
Application
Layer
Layer
Presentation
library
Layer
sockets
Session
Transport
Layer
Layer
Transport
Layer
• Internet Engineering Task Force
• Based on working groups that focus on specific issues
• Request for Comments
• Document that provides information or defines standard
• Requests feedback from the community
• Can be “promoted” to standard under certain conditions
• consensus in the committee
• interoperating implementations
• Project 1 will look at the Internet Relay Chat (IRC) RFC
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Page 9
Life of Packet
Layer Encapsulation
User A
Application
User B
Presentation
Get index.html
Session
Transport
Connection ID
Network
Data Link
Source/Destination
Physical
Link Address
Host
Bridge/Switch
Router/Gateway
Host
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Multiplexing and Demultiplexing
• There may be multiple
implementations of each
layer.
• Multiple choices at each layer
TCP
TCP
IP
IP
• How does the receiver know
what version of a layer to
use?
• Each header includes a
demultiplexing field that is
used to identify the next
layer.
• Filled in by the sender
• Used by the receiver
• Multiplexing occurs at
multiple layers. E.g., IP,
TCP, …
Protocol Demultiplexing
FTP
HTTP
NV
TCP
V/HL
TOS
ID
TTL
UDP
Length
IPX
Flags/Offset
Prot.
TFTP
H. Checksum
NET1
Source IP address
Network
IP
Type
Field
Protocol
Field
TCP/UDP
IP
NET2
…
NETn
Port
Number
Destination IP address
Options..
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Server and Client
The Internet Protocol Suite
Server and Client exchange messages over
the network through a common Socket API
FTP
Clients
Server
user
space
ports
HTTP
NV
TCP
Socket API
TCP/UDP
IP
IP
Ethernet Adapter
Ethernet Adapter
UDP TCP
UDP
Narrow Waist
IP
TCP/UDP
Applications
TFTP
Data Link
kernel
space
NET1
NET2
…
NETn
Physical
The Hourglass Model
The waist facilitates interoperability
… but evolution is hard
hardware
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IP based on a Minimalist Approach
Today’s Lecture
• Dumb network
• Network applications
• IP provide minimal functionalities to support connectivity
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• Requirements
• Latency and bandwidth
• Addressing, forwarding, routing
• Smart end system
• Transport layer or application performs more sophisticated
functionalities
• Internet architecture
• Flow control, error control, congestion control
• A layered design
• Protocols
• Life of a packet
• Advantages
• Accommodate heterogeneous technologies (Ethernet,
modem, satellite, wireless)
• Support diverse applications (telnet, ftp, Web, X windows)
• Decentralized network administration
• Network utilities
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Page 11
Network tools
•
•
•
•
•
ping
• Application to determine if host is reachable
• Based on Internet Control Message Protocol
• ICMP informs source host about errors
encountered in IP packet processing by routers or
by destination host
• ICMP Echo message requests reply from
destination host
• PING sends echo message & sequence #
• Determines reachability & round-trip delay
• Sometimes disabled for security reasons
ping
traceroute
ipconfig
tcpdump
…
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ipconfig
traceroute
• Find route from local host to a remote host
• Time-to-Live (TTL)
• Utility in Microsoft Windows to display
TCP/IP information about a host
• Many options
• IP packets have TTL field that specifies maximum #
hops traversed before packet discarded
• Each router decrements TTL by 1
• When TTL reaches 0 packet is discarded
• Simplest: IP address, subnet mask, default
gateway for the host
• Information about each IP interface of a host
• Traceroute
•
•
•
•
Send UDP to remote host with TTL=1
First router will reply ICMP Time Exceeded Message
Send UDP to remote host with TTL=2, …
Each step reveals next router in path to remote host
• DNS hostname, IP addresses of DNS servers,
physical address of network card, IP address, …
• Renew IP address from DHCP server
• tracert (windows), tracepath (linux)
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tcpdump and Network Protocol
Analyzers
netstat
• Queries a host about TCP/IP network
status
• Status of network drivers & their
interface cards
• tcpdump program captures IP packets on a network
interface (usually Ethernet NIC)
• Filtering used to select packets of interest
• Packets & higher-layer messages can be displayed and
analyzed
• tcpdump basis for many network protocol analyzers for
troubleshooting networks
• We use the open source Ethereal analyzer to generate
examples (or wireshark, etc.)
• www.ethereal.com
• #packets in, #packets out, errored packets,
…
• State of routing table in host
• TCP/IP active server processes
• TCP active connections
How the layers work together:
Network Analyzer Example
Top Pane shows
frame/packet
Ethereal
windows
sequence
Middle Pane shows
encapsulation for a
given frame
Internet



User clicks on http://www.nytimes.com/
Ethereal network analyzer captures all frames observed
by its Ethernet NIC (or Wireshark)
Sequence of frames and contents of frame can be
examined in detail down to individual bytes
Bottom Pane shows hex & text
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Top pane: frameTCP
sequence
Connection
DNS Query
Setup
Middle pane: Encapsulation
HTTP
Request &
Response
Ethernet Frame
Ethernet
Destination and
Source Addresses
Protocol Type
Middle pane: Encapsulation
MiddleAnd
pane:
a lot ofEncapsulation
other stuff!
TCP Segment
IP Packet
Source and
Destination Port
Numbers
IP Source and
Destination
Addresses
GET
HTTP
Request
Protocol Type
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