Transcript ICS 156: Advanced Computer Networks

Introduction & Lab I
Lab Equipment & Organization
Shlomo Hershkop
Columbia University
Fall 2006
Announcements

Introducing the TA




Lab Setup
Online Poll


Archana Rao
maybe one more (depending on enrollment numbers)
please take the office hours poll so we can accommodate
everyone’s preferences
Lab times



we need to confirm all lab groups (3/4 members)
might switch around some people for spreading out experience
need to confirm specific lab times
Internet Lab Equipment

4 Cisco 2600 Routers

4 Linux PCs
(Intel Celeron 400MHz, 256MB Ram, 40GB disk, cdrom, floppy)

4 Ethernet hubs
2x 5-port Hub 3Com OfficeConnect Dual Speed (10/100)
2x 8-port Hub NETGEAR DS108

Dual monitor, keyboard, mouse

1 KVM switch


can accommodate two users at once
Cables

pretty colors
Internet Lab Equipment
Linux PCs

PCs are labeled as:
RackPC1, RackPC2, etc.

PCs run Linux (Redhat)

Each PC has:





a floppy drive,
a cdrom drive,
2 usb ports
a serial port,
5x 10/100 Mbps Ethernet
interface cards (NICs) named eth0 – eth4.
Linux PC
Cisco Routers



Routers are labeled: Router1, Router2, Router3, Router4.
Routers run Cisco IOS 12.0 or a later version
Each router has:
 a console port
 an auxiliary port
 two 10/100 Mbps Fast Ethernet interfaces
Ethernet Hubs

Each hub has 4 or more RJ-45 ports

Ports can operate at 10 Mbps or 100 Mbps
Lab Sequence
Core Labs:
Lab 1 Introduction to
the Internet Lab
Lab 2 - Single
Segment IP
Networks
Lab 7 - NAT
and DHCP
Lab 3 - Static
Routing
Lab 4 Dynamic
Routing
Protocols
Lab 5 Transport
Protocols:
UDP and TCP
Lab 8 - Domain
Name System
Lab 9 - SNMP
Lab 10 - IP
Multicast
Advanced Labs:
Lab 6 - LAN
switching
Core Labs

Lab 1 – Introduction to the Internet Lab
Overview of the Internet Lab equipment; introduction to
ethereal and tcpdump.

Lab 2 – Single Segment IP Networks
Configuring a network interface for IP networking;
address resolution with ARP;
security problems of common Internet applications.
Core Labs (cont.)

Lab 3 – Static routing
IP forwarding and routing between IP networks; setup a Linux PC
and a Cisco router as an IP router; manual configuration of routing
tables.

Lab 4 – Dynamic Routing Protocols
Routing protocols RIP, OSPF and BGP.

Lab 5 – Transport Protocols: UDP and TCP
Data transmissions with TCP and UDP; TCP connection management; TCP
flow control; retransmissions in TCP; TCP congestion control.
Advanced Labs
 Lab 6 - LAN switching
LAN switching in Ethernet networks; forwarding of Ethernet frames between LAN
switches/bridges; spanning tree protocol for loop free routing between interconnected
LANs.
 Lab 7 - NAT and DHCP
Setup of a private network; dynamic assignment of IP addresses with DHCP.
 Lab 8 – Domain Name System
Domain name resolution with DNS; name server hierarchy; setup of a DNS root
server.
 Lab Extra Credit
Maybe something to do with wireless
Structure of the Labs

Each lab has three phases:
 Pre-laboratory Assignment (Prelab)
 Lab Session
 Lab Reports
Structure of the Labs (cont.)

Pre-laboratory Assignment (Pre-lab)




Exercises to be completed in advance of the associated lab
session.
The pre-labs ask you to acquire background knowledge that
is needed during the lab exercises.
Each pre-lab has a question sheet that must be completed
before the corresponding lab session.
The answers to the prelab questions are graded.
Structure of the Labs (cont.)

Lab Session.


Lab exercises that are performed on the equipment of the
Internet lab. All lab exercises can be completed without
supervision. The time to complete a lab session should
be three hours on the average, but may vary. Complete
the laboratory activities to the extent that you can. The
activities during the lab session are not graded, however,
data collected during the lab session are needed to
complete a lab report.
Floppy disk symbol in the lab manual indicates when you
have to collect data.
Floppy disk
symbol
Structure of the Labs (cont.)

Lab Reports.

After each lab session, you prepare a lab report that
summarizes and analyzes the findings from the lab session.
A notepad symbol indicates an assignment for the lab
report. The lab reports should be submitted as a typewritten
document.

The lab report is generally due 1 week after the lab session.
The lab report is graded.

Note:

Lab reports should not include irrelevant data
Notepad
symbol
In the Lab:
1.
2.
3.
4.
5.
I am trying to get USB keys, in the meantime, either
have one of the group members dig one up (no
questions asked, or use provided floppies (please
don’t offload them on ebay)
Reboot Linux PCs
Complete exercises as described in the lab manual
Take measurements as instructed
Save data to floppy disk
Additional notes

The equipment of the Internet Lab is not connected to the
Internet.

you can bring in a laptop (best) and connect wired or wirelessly

Each lab has an anonymous feedback sheet. The feedback is
used to improve the setup and organization of the labs.

Since you have administrative (root) privileges on the Internet
Lab equipment, exercise caution when modifying the
configuration of the Internet Lab equipment.
TCP/IP Networking
An Example
Introductory material.
This module illustrates the interactions of the protocols of the TCP/IP
protocol suite with the help of an example. The example intents to
motivate the study of the TCP/IP protocols.
A simple TCP/IP Example

A user on host argon.netlab.edu (“Argon”) makes web access
to URL http://neon.netlab.edu/index.html.

What actually happens in the network?
HTTP Request and HTTP response
Web server runs an HTTP server program
 HTTP client Web browser runs an HTTP client program
 sends an HTTP request to HTTP server
 HTTP server responds with HTTP response

HTTP Request
GET /example.html HTTP/1.1
Accept: image/gif, */*
Accept-Language: en-us
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/4.0
Host: 192.168.123.144
Connection: Keep-Alive
HTTP Response
HTTP/1.1 200 OK
Date: Sat, 25 May 2002 21:10:32 GMT
Server: Apache/1.3.19 (Unix)
Last-Modified: Sat, 25 May 2002 20:51:33 GMT
ETag: "56497-51-3ceff955"
Accept-Ranges: bytes
Content-Length: 81
Keep-Alive: timeout=15, max=100
Connection: Keep-Alive
Content-Type: text/html
<HTML>
<BODY>
<H1>Internet Lab</H1>
Click <a href="http://www.netlab.net/index.html">here</a> for the
Internet Lab webpage.
</BODY>
</HTML>
• How does the HTTP request get from Argon to Neon ?
From HTTP to TCP
Argon
Neon
HTTP client
HTTP request / HTTP response
HTTP server
TCP client
TCP connection
TCP server

To send request, HTTP client program establishes an TCP
connection to the HTTP server Neon.

The HTTP server at Neon has a TCP server running
Resolving hostnames and port numbers

Since TCP does not work with hostnames and also would not know how
to find the HTTP server program at Neon, two things must happen:
1. The name “neon.netlab.edu” must be translated into a
32-bit IP address.
2. The HTTP server at Neon must be identified by a 16-bit
port number.
Translating a hostname into an IP address

The translation of the hostname neon.netlab.edu into an IP address
is done via a database lookup

The distributed database used is called the Domain Name System
(DNS)

All machines on the Internet have an IP address:
argon.netlab.edu
128.143.137.144
neon.netlab.edu
128.143.71.21
Finding the port number

Note: Most services on the Internet are reachable via well-known ports.

E.g. HTTP servers on the Internet can be reached at port number “80”.

So: Argon simply knows the port number of the HTTP server at a
remote machine.

On most Unix systems, the well-known ports are listed in a file with
name /etc/services. The well-known port numbers of some of the most
popular services are:
ftp21
finger
79
telnet
23
http
80
smtp
25
nntp
119
Requesting a TCP Connection

The HTTP client at argon.netlab.edu requests the TCP client to establish a
connection to port 80 of the machine with address 128.141.71.21
Invoking the IP Protocol



The TCP client at Argon sends a request to establish a connection to port 80 at
Neon
This is done by asking its local IP module to send an IP datagram to
128.143.71.21
(The data portion of the IP datagram contains the request to open a connection)
Sending the IP datagram to an IP router

Argon (128.143.137.144) can deliver the IP datagram directly to Neon
(128.143.71.21), only if it is on the same local network (“subnet”)

But Argon and Neon are not on the same local network
(Q: How does Argon know this?)

So, Argon sends the IP datagram to its default gateway

The default gateway is an IP router

The default gateway for Argon is Router137.netlab.edu (128.143.137.1).
The route from Argon to Neon

Note that the gateway has a different name for each of its interfaces.
Finding the MAC address of the gateway

To send an IP datagram to Router137, Argon puts the IP datagram in an
Ethernet frame, and transmits the frame.

However, Ethernet uses different addresses, so-called Media Access
Control (MAC) addresses (also called: physical address, hardware
address).

Therefore, Argon must first translate the IP address 128.143.137.1 into a
MAC address.

The translation of addressed is performed via the Address Resolution
Protocol (ARP)
Address resolution with ARP
Invoking the device driver

The IP module at Argon, tells its Ethernet device driver to
send an Ethernet frame to address 00:e0:f9:23:a8:20
Sending an Ethernet frame

The Ethernet device driver of Argon sends the Ethernet frame to
the Ethernet network interface card (NIC)

The NIC sends the frame onto the wire
Forwarding the IP datagram


The IP router receives the Ethernet frame at interface 128.143.137.1, recovers
the IP datagram and determines that the IP datagram should be forwarded to the
interface with name 128.143.71.1
The IP router determines that it can deliver the IP datagram directly
Another lookup of a MAC address

The router needs to find the MAC address of Neon.

Again, ARP is invoked, to translate the IP address of Neon
(128.143.71.21) into the MAC address of neon (00:20:af:03:98:28).
Invoking the Device Driver at the Router

The IP protocol at Router71, tells its Ethernet device driver to send an
Ethernet frame to address 00:20:af:03:98:28
Sending another Ethernet frame

The Ethernet device driver of Router71 sends the Ethernet
frame to the Ethernet NIC, which transmits the frame
onto the wire.
Data has arrived at Neon

Neon receives the Ethernet frame

The payload of the Ethernet frame is an IP
datagram which is passed to the IP
protocol.

The payload of the IP datagram is a TCP
segment, which is passed to the TCP
server
Wrapping up the example

Data traverses a sequence of layers

Each layer has protocols to handle the
packets

Next Lecture (Lab 2)


Layered architecture of the Internet
Protocols at each layer
Review of Important
Networking Concepts
Introductory material using Prof. Liebeherr on-line notes
Review of important networking concepts: protocol architecture, protocol
layers, encapsulation, demultiplexing, network abstractions.
Networking Concepts

Layered Architecture to reduce complexity


Encapsulation
Abstractions
Sending a packet from Argon to Neon
neon.netlab.edu
"Neon"
128.143.71.21
argon.netlab.edu
"Argon"
128.143.137.144
router137.netlab.edu
"Router137"
128.143.137.1
router71.netlab.edu
"Router71"
128.143.71.1
Router
Ethernet Network
Ethernet Network
Sending a packet from Argon to Neon
128.143.71.21 is not on my local network.
Therefore, I need to send the packet to my
128.143.71.21
on my local
network.
default
gateway withisaddress
128.143.137.1
DNS:
DNS:
The is
IPisthe
address
address
of
Therefore, I can send the packet directly.
ARP:What
What
theIPMAC
of“neon.netlab.edu
“neon.netlab.edu
””is? of
address
128.143.137.1?
ARP:
TheofMAC
address
128.143.71.21
128.143.137.1 is 00:e0:f9:23:a8:20
argon.netlab.edu
"Argon"
128.143.137.144
ARP: What is the MAC
ARP:
TheofMAC
address of
address
128.143.71.21?
neon.netlab.edu
128.143.137.1 is 00:20:af:03:98:28
"Neon"
128.143.71.21
router137.netlab.edu
"Router137"
128.143.137.1
router71.netlab.edu
"Router71"
128.143.71.1
Router
frame
Ethernet Network
frame
Ethernet Network
What’s a protocol?
human protocols:
 “what’s the time?”
 “I have a question”
 introductions
… specific msgs sent
… specific actions taken
when msgs received, or
other events
network protocols:
 machines rather than
humans
 all communication activity
in Internet governed by
protocols
protocols define format, order of
msgs sent and received among
network entities, and actions
taken on msg transmission,
receipt
What’s a protocol?
a human protocol and a computer network protocol:
Hi
TCP connection
req
Hi
TCP connection
response
Got the
time?
Get http://www.awl.com/kurose-ross
2:00
<file>
time
Q: Other human protocols?
Communications Architecture

The complexity of the communication task is reduced
by using multiple protocol layers:




Each protocol is implemented independently
Each protocol is responsible for a specific subtask
Protocols are grouped in a hierarchy
A structured set of protocols is called a
communications architecture or protocol suite
TCP/IP Protocol Suite

The TCP/IP protocol suite
is the protocol
architecture of the
Internet
Application
User-level programs
Transport
Operating system
Network
Data Link


The TCP/IP suite has four
layers: Application,
Transport, Network, and
Data Link Layer
End systems (hosts)
implement all four layers.
Data Link
Media Access
Control (MAC)
Sublayer in
Local Area
Networks
Functions of the Layers


Data Link Layer:

Service:

Functions:
Network Layer:



Service:
Functions:
Move packets from source host to destination host
Routing, addressing
Transport Layer:



Reliable transfer of frames over a link
Media Access Control on a LAN
Framing, media access control, error checking
Service:
Functions:
Delivery of data between hosts
Connection establishment/termination, error
control, flow control
Application Layer:


Service:
HTML
Functions:
Application specific (delivery of email, retrieval of
documents, reliable transfer of file)
Application specific
TCP/IP Suite and OSI Reference Model
The TCP/IP protocol stack does not define
the lower layers of a complete protocol
stack
Application
Layer
Application
Layer
Transport
Layer
Network
Layer
(Data) Link
Layer
Presentation
Layer
Session
Layer
Transport
Layer
Network
Layer
(Data) Link
Layer
Physical
Layer
TCP/IP Suite
OSI
Reference
Model
Assignment of Protocols to Layers
ping
application
HTTP
Telnet
FTP
TCP
DNS
SNMP
Application
Layer
Transport
Layer
UDP
Routing Protocols
ICMP
RIP
IP
IGMP
PIM
Network
Layer
OSPF
DHCP
ARP
Ethernet
Network
Interface
Data Link
Layer
Layered Communications

An entity of a particular layer can only communicate with:
1. a peer layer entity using a common protocol (Peer
Protocol)
2. adjacent layers to provide services and to receive
services
N+1 Layer
N+1 Layer
Entity
N+1 Layer Protocol
N+1 Layer
Entity
N Layer
Entity
N Layer Protocol
N Layer
Entity
N-1 Layer
Entity
N-1 Layer Protocol
N-1 Layer
Entity
layer N+1/N
interface
N Layer
layer N/N-1
interface
N-1 Layer
Service Primitives
Communication services are invoked via function
calls. The functions are called service primitives
N+1 Layer
Entity
Request
Delivery
N Layer
Entity
N+1 Layer Peer Protocol
N+1 Layer
Entity
Indicate
Delivery
N Layer
Entity
Service Primitives
Recall: A layer N+1 entity sees the lower layers
only as a service provider
N+1 Layer
Entity
N+1 Layer Peer Protocol
N+1 Layer
Entity
Indicate
Delivery
Request
Delivery
Service Provider
Layers in the Example
HTTP
HTTP protocol
HTTP
TCP
TCP protocol
TCP
IP
Ethernet
IP
IP protocol
Ethernet
argon.netlab.edu
128.143.137.144
Ethernet
IP protocol
Ethernet
Ethernet
router71.netlab.edu router137.netlab.edu
128.143.137.1
128.143.71.1
00:e0:f9:23:a8:20
IP
Ethernet
neon.netlab.edu
128.143.71.21
Layers in the Example
HTTP
TCP
IP
Frame is an IP
datagram
Ethernet
HTTP
Send HTTP Request
to neon
Establish a connection to 128.143.71.21 at
port 80Open TCP connection to
128.143.71.21 port 80
IP datagram is a TCP
segment for port 80
IP data-gram
to
Send a datagram (which
contains
a connection
IPSend
Send IP datagram
to
128.143.71.21
request) to 128.143.71.21
128.143.71.21
Frame is an IP
datagram
Send the datagram to 128.143.137.1
Ethernet
Ethernet
TCP
IP
Send the datagram
Ethernet
to 128.143.7.21
argon.netlab.edu
neon.tcpip-lab.edu
router71.netlab.edu router137.netlab.edu
Send Ethernet frame
Send Ethernet frame
128.143.71.1
128.143.137.144
128.143.71.21
128.143.137.1
to 00:20:af:03:98:28
to 00:e0:f9:23:a8:20
00:e0:f9:23:a8:20
Layers and Services

Service provided by TCP to HTTP:


Service provided by IP to TCP:


unreliable transmission of IP datagrams across an IP
network
Service provided by Ethernet to IP:


reliable transmission of byte streams over a logical
connection
transmission of a frame across an Ethernet segment
Other services:


DNS: translation between domain names and IP addresses
ARP: Translation between IP addresses and MAC addresses
Encapsulation & Demultiplexing

As data is moving down the protocol stack, each
protocol is adding layer-specific control
information
HTTP
User data
HTTP Header
User data
HTTP Header
User data
TCP
TCP Header
IP
TCP segment
IP Header
Ethernet
TCP Header
HTTP Header
User data
IP datagram
Ethernet
Header
IP Header
TCP Header
HTTP Header
Ethernet frame
User data
Ethernet
Trailer
Encapsulation & Demultiplexing in our
Example

Let us look in detail at the Ethernet frame
between Argon and the Router, which contains
the TCP connection request to Neon.

This is the frame in hexadecimal notation.
00e0
4500
8990
0000
05b4
f923
002c
808f
0000
a820
9d08
4715
6002
00a0
4000
065b
2000
2471
8006
0050
598e
e444
8bff
0009
0000
0800
808f
465b
0204
Encapsulation & Demultiplexing
6 bytes
destination address
4 bytes
source address
type
Ethernet Header
CRC
IP Header
TCP Header
Ethernet frame
Application data
Ethernet Trailer
Encapsulation & Demultiplexing: Ethernet
Header
6 bytes
00:e0:f9:23:a8:20
4 bytes
0:a0:24:71:e4:44
0x0800
Ethernet Header
CRC
IP Header
TCP Header
Ethernet frame
Application data
Ethernet Trailer
Encapsulation & Demultiplexing: IP Header
32 bits
version
(4 bits)
header
length
DS
flags
(3 bits)
Identification (16 bits)
TTL Time-to-Live
(8 bits)
Total Length (in bytes)
(16 bits)
ECN
Protocol
(8 bits)
Fragment Offset (13 bits)
Header Checksum (16 bits)
Source IP address (32 bits)
Destination IP address (32 bits)
Ethernet Header
IP Header
TCP Header
Ethernet frame
Application data
Ethernet Trailer
Encapsulation & Demultiplexing: IP Header
32 bits
0x4
0x5
0x0
0x0
9d08
12810
4410
0102
00000000000002
0x06
8bff
128.143.137.144
128.143.71.21
Ethernet Header
IP Header
TCP Header
Ethernet frame
Application data
Ethernet Trailer
Encapsulation & Demultiplexing: TCP
Header
32 bits
Source Port Number
Destination Port Number
Sequence number (32 bits)
Acknowledgement number (32 bits)
header
length
0
Flags
TCP checksum
option
type
length
window size
urgent pointer
Max. segment size
Option:
maximum
segment size
Ethernet Header
IP Header
TCP Header
Ethernet frame
Application data
Ethernet Trailer
Encapsulation & Demultiplexing: TCP
Header
32 bits
162710
8010
60783510
010
610
0000002
0000102
0x598e
210
Ethernet Header
IP Header
819210
00002
410
TCP Header
Ethernet frame
146010
Application data
Ethernet Trailer
Encapsulation & Demultiplexing: Application
data
Ethernet Header
IP Header
TCP Header
Ethernet frame
Application data
Ethernet Trailer
Different Views of Networking

Different Layers of the protocol stack have a
different view of the network. This is HTTP’s and
Argon
Neon
TCP’s128.143.137.144
view of the network.
128.143.71.21
HTTP client
HTTP
server
HTTP
server
TCP client
TCP server
TCP server
IP Network
Network View of IP Protocol
Network View of Ethernet

Ethernet’s view of the network