CS 620 Advanced Operating
Systems
Lecture 2 – Networking Overview
Professor Timothy Arndt
BU 331
Networking
 A computer network is a high-speed
communications medium connecting many,
possibly dissimilar, computers or hosts.
• A network is a combination of computer and
telecommunication hardware and software.
 In addition to host computers, the network may
involve many dedicated computers that perform
message switching (switches), relaying
between LANs (bridges), joining several
networks (hubs and routers) and joining
dissimilar networks (gateways).
Hosts and Dedicated
Computers
Types of Networks
 Networks are usually classified as either Wide
Area Networks (WAN) or Local Area
Networks (LAN)
 WANs span large distances and typically
connect networks rather than hosts.
• Speeds range from T1 (1.5 Mbps) to T3 (45 Mbps)
to T5 (400 Mbps).
 LANs connect hosts that are physically close
and operate at 10-100 Mbps or higher.
• The most widely used LAN is Ethernet running at
10 Mbps – 1 Gbps.
Networking Protocols
 Networking protocols are a set of rules and
conventions that hosts follow in order to
communicate on a network. They decide such
things as:
•
•
•
•
•
•
•
Address formats
Data formats
Manner of data transmission
Sequencing and addressing of messages
Initiating and terminating logical connections
Establishing remote services
Accessing remote services
Networking Protocols
• The Internet Protocol (IP) is the most
widely used networking protocol. IP is the
basic protocol for the Internet and is widely
supported by all major systems.
• As shown in the following slides, networks
are typically designed as a number of
layers.
 The higher layers use the services provided by
the lower level layers.
Networking Protocols
The OSI Reference Model
TCP/IP Reference Model
TCP/IP Model
Internet Basics
• The Internet is a global network that
connects IP networks.
 A network of networks is called an
internetwork.
• The Internet evolved from the ARPANET a
DARPA sponsored network that developed IP, TCP
and UDP. The concept was supposed to make the
military’s networks less vulnerable to attacks.
• The ARPANET became the Internet when the
government gave control of the network to its
component members.
Internet
Cisco CRS-1 Backbone Core
Router
Internet Backbone
• The Internet Backbone is operated by large
telecomm companies (upstream ISPs)
 AT&T, MCI, Sprint, Verizon, etc.
• The backbones operated by different orgs
are connected at Internet Exchange Points
(IXPs)
Network Addresses
 An addressing scheme is an important part of a
network and its protocol.
 For the Internet, each host has a unique IP
address represented by 4 bytes in a 32-bit
quantity.
 The IP address can be represented in dot
notation giving the decimal value of each byte:
137.148.216.15 is the IP address of grail.
 Each host may also have a domain-based name
composed of words: grail.cba.csuohio.edu
Network Addresses
IPv6
• The information on the previous slides
applies to IPv4
• A newer version, IPv6 is now coming into
use
 128 bit addresses (so no NAT needed)
• Classless Internet Domain Routing (CIDR) had been
introduced in IPv4 to more effectively use 32 bit
address space
 Also different header format
 Support for multicast
Packet Switching
 The Internet is an example of a packet
switched network. A packet is routed from one
host on the Internet to another. Data is sent as a
sequence of packets. Individual packets may
take different routes to the destination host.
 Use the command traceroute to see the route
that a packet takes to a particular host.
Packet Switching
penguin:/users/faculty/arndt> /usr/local/etc/traceroute speedy.cs.pitt.edu
traceroute to speedy.cs.pitt.edu (136.142.79.2), 30 hops max, 40 byte packets
1 137.148.216.1 (137.148.216.1) 3 ms 2 ms 6 ms
2 137.148.254.16 (137.148.254.16) 5 ms 16 ms 3 ms
3 137.148.13.100 (137.148.13.100) 4 ms 25 ms 4 ms
4 clv1-sl9-1.cleveland.oar.net (199.18.113.5) 31 ms 47 ms 23 ms
5 oeb4-atm1-0.columbus.oar.net (199.18.202.14) 55 ms 79 ms 80 ms
6 Serial-0-0-1.GW1.CLE1.ALTER.NET (157.130.102.157) 75 ms 74 ms 80 ms
7 143.ATM2-0.XR1.CHI4.ALTER.NET (146.188.209.66) 104 ms 75 ms 97 ms
8 195.ATM8-0-0.GW1.CHI1.ALTER.NET (146.188.208.145) 53 ms 50 ms 34 ms
9 att-chicago-gw.customer.ALTER.NET (157.130.96.10) 45 ms 69 ms 76 ms
10 br1-a350s1.cgcil.ip.att.net (192.205.31.126) 77 ms 66 ms 58 ms
11 br2-p320.n54ny.ip.att.net (12.127.9.174) 74 ms 93 ms 59 ms
12 ar1-a3120s1.n54ny.ip.att.net (12.127.0.9) 95 ms 106 ms 73 ms
13 12.127.244.62 (12.127.244.62) 112 ms 75 ms 82 ms
14 pitt-gp.psc.net (198.32.224.8) 100 ms 95 ms 79 ms
15 fq.gw.pitt.edu (136.142.12.4) 98 ms 93 ms 76 ms
16 speedy.cs.pitt.edu (136.142.79.2) 134 ms 120 ms
penguin:/users/faculty/arndt>
TCP/IP
• The Transmission Control Protocol/Internet
Protocol (TCP/IP) suite is a standard set of
networking protocols.
 TCP/IP is the primary protocol of the Internet
and the World Wide Web.
 Almost all operating systems offer TCP/IP
support, and most large networks rely on
TCP/IP for much of their network traffic.
Media and Topologies
 TCP/IP networks can run over a variety of
physical media. Most current wired networks
use some sort of coaxial cable, twisted-pair
cable, or fiber-optic cable.
 Network adapters (also known as Network
Interface Cards or NICs) provide the interface
between a computer and the physical medium
comprising the network connection.
• They are usually built into the motherboard. May
also be part of a SoC or USB device.
• Network interfaces support one or more
communications protocols.
Media and Topologies
 All network adapters have a Media Access
Control (MAC) address, which is a unique
identifier for that individual unit (board).
 For Ethernet, wireless, and some other devices,
MAC addresses are 48-bit values expressed as
twelve hexadecimal digits: e.g. 00-00-F8-2331-91.
• A 64-bit scheme is also used
 Hosts within a LAN can be connected into a
variety of arrangements called topologies such
as a bus, ring, or star.
TCP/IP
• Core TCP/IP protocols include:





Transmission Control Protocol (TCP)
User Datagram Protocol (UDP)
Address Resolution Protocol (ARP)
Internet Control Message Protocol (ICMP)
Internet Group Management Protocol (IGMP)
Internet Protocol
• Internet Protocol (IP) provides packet
delivery for all other protocols in the suite.
• It is a best-effort, connectionless delivery
system for computer data.
• The protocol’s checksum confirms only the
IP header’s integrity. Responsibility for the
data within the IP packet (and the
sequencing) is assured only by using
higher-level protocols.
Transmission Control
Protocol
 Transmission Control Protocol provides a
reliable, connection-oriented packet delivery
service on top of IP.
 TCP guarantees the delivery of packets, proper
sequencing of packets, and provides a
checksum that validates the packet header and
data for accuracy.
 If the network corrupts or loses a TCP packet
during transmission, TCP is responsible for
retransmitting the faulty packet.
Transmission Control
Protocol
 TCP is used for session-based data
transmission, client-server applications, and
critical services.
 TCP headers require extra bits to provide
proper sequencing of information.
 To guarantee successful data delivery, the
protocol also requires that the recipient
acknowledge successful receipt of data.
• These ACKs generate additional network traffic,
diminishing data speed for accuracy.
User Datagram Protocol
 When reliability is not essential, User Datagram
Protocol (UDP), a TCP complement, offers a
connectionless datagram service that guarantees
neither delivery nor correct sequencing of
delivered packets.
• UDP checksums are optional, providing a way to
exchange data over highly reliable networks without
unnecessarily consuming network resources or
processing time. ACKS are not enforced.
• UDP also supports sending data from a single user
to multiple receivers.
Address Resolution Protocol
 Not directly related to data transport, the
Address Resolution Protocol (ARP) is one of
the maintenance protocols that supports the
TCP/IP suite.
 To communicate across a TCP/IP network, the
system sending the packet must map the IP
address of the target to the physical address of
the target.
• IP acquires this physical address by broadcasting a
special inquiry packet (an ARP request packet)
containing the IP address of the target.
Address Resolution Protocol
 All ARP-enabled systems on the local IP
network detect these messages, and the system
that owns the IP address in question replies by
sending its physical address to the requester.
 The physical/IP address is then stored in the
ARP cache of the requesting system for
subsequent use.
 Because the ARP reply can also be broadcast to
the network, other systems on the network can
update their own ARP caches.
Internet Control Message
Protocol
• Internet Control Message Protocol (ICMP) is
another maintenance protocol. It allows two
systems on an IP network to share status and error
information.
• This information can be used by higher-level
protocols to recover from transmission problems.
 The ping program uses the ICMP echo request and
reply packets to determine whether a particular IP
system on a network is functional. The ping program
can be used to diagnose IP network or router failures.
Network Services
 Network operations are accomplished by a
number of network services, consisting of the
software needed to perform a specific type of
network task.
 Network services are defined by the
combination of transport protocol, IP address
and port number: a logical network
connection identified by a number.
• Network services are provided by server services or
daemon processes which listen for client requests.
Daemon Processes
UID
root
root
root
root
root
root
root
root
root
PID PPID C
STIME TTY
TIME COMMAND
88
1 0 03:03:20 ?
0:07 /etc/syncer
9439
165 0 09:30:40 ttyq1
0:00 telnetd
165
1 0 03:03:37 ?
0:17 /etc/inetd -l
129
1 0 03:03:26 ?
0:52 /usr/etc/ypserv
131
1 0 03:03:26 ?
0:01 /etc/ypbind
145
1 0 03:03:35 ?
0:00 /usr/etc/rpc.mountd
147
1 0 03:03:35 ?
0:13 /etc/nfsd 4
5626
165 0 07:58:31 ttyp5
0:00 rlogind
9063
1 0 09:25:00 ?
0:00 talkd
• Daemon processes generally have names which finish with
a ‘d’. The inetd daemon is a server process which listens
for TCP/IP requests and forks off the appropriate daemon
process to service the request. This approach avoids
multiple waiting daemon processes.
TCP/IP Diagnostic Tools
• arp: display or modify the IP-to-MAC address
tables.
• nslookup: interactively query a name server.
• lpq: examine/manipulate jobs in a printer queue.
• ping: perform a simple network connectivity test.
• route: display or modify the static routing tables.
• traceroute: view the route to a host.
• netstat: display various network statistics
• ipconfig: display information about the system’s
TCP/IP configuration.
Name Resolution Facilities
 For convenience, high-level user commands
generally use hostnames (rather than IP
addresses) to specify remote locations on the
network.
 TCP/IP provides two methods for performing
hostname-to-IP address translation (also called
name resolution).
 The IP address can be determined by looking
up the hostname in the /etc/hosts file.
The /etc/hosts File
grail:/users/faculty/arndt> more /etc/hosts
## Configured using SAM by root on Mon Jul 11 15:30:33 1994
127.0.0.1 localhost
Loopback address
#
# Host Database
Our name
#
#----IP address
# HP server
137.148.216.15 grail.cba.csuohio.edu
cba.csuohio.edu grail
# DELL PowerEdge server running Linux
137.148.216.45 arthur.cba.csuohio.edu arthur
# Sun SPARC5 file server
alias
137.148.216.17
shamu.cba.csuohio.edu shamu
# Sun SPARC20 with 4 CPUs; should be quadru-pus
137.148.216.18
octopus.cba.csuohio.edu octopus
hostname
# 11 Sun SPARC4 10 baseT
137.148.216.19
dolphin.cba.csuohio.edu dolphin
137.148.216.20
shark.cba.csuohio.edu shark
. . .
The Domain Name Service
 The Domain Name Service (DNS) facility
relies upon server processes running on various
network-accessible systems to provide
hostname-to-IP translation.
 DNS organizes groups of systems into units
known as domains.
• DNS domains are organized into a hierarchical
structure on an Internet-wide basis with the defined
suffixes constituting the top-level of the tree (.com,
.edu, .org, the two-character country codes, etc.).
The Domain Name Service
““
org
com
widgets
mfg
mrp2
research
cisco
edu
uk
csuohio
co
The Domain Name Service
 The administrative unit for DNS is the zone. A
zone is a subtree of the DNS database that is
administered as a single separate entity.
• It can consist of a single domain or a domain with
subdomains.
 With the exception of the root, each node in the
DNS database has a name of up to 63
characters. Each subdomain must have a unique
name within its parent domain. Node names are
concatenated and separated with a period to
form a fully qualified domain name (FQDN).
Name Resolution
 Name resolution is provided by name servers
which interpret the information in a FQDN to
find its specific address.
 A resolver passes a query to its local name
server. If the name server does not have the
data requested in the query, it queries other
name servers on behalf of the resolver.
• In the worst-case scenario, the local name server
starts at the top of the DNS tree with one of the root
name servers and works its way down until the
address is found.
Name Resolution
 DNS name resolution consists of three key
concepts:
• Recursion: a resolver typically passes a recursive
resolution request to its local name server. This puts
the burden on the name server.
• Iteration: If the local name server cannot fully
resolve the query, it enlists the aid of other DNS
name servers, passing an iterative resolution request
(the result may only be a pointer to another server).
• Caching: As local name servers process recursive
requests, they discover a lot of information about the
DNS name space. This information is cached.
Name Resolution
grail:/etc> cat resolv.conf
search cba.csuohio.edu csuohio.edu
nameserver 137.148.49.10
nameserver 137.148.5.26
grail:/etc> nslookup
Default Name Server: csu-a.csuohio.edu
Address: 137.148.49.10
> 137.148.5.26
Name Server: csu-a.csuohio.edu
Address: 137.148.49.10
Name:
csu-b.csuohio.edu
Address: 137.148.5.26
>
IP Routing
 The Internet and other TCP/IP networks are
connected by routers, which are devices that
pass IP packets from one network to another.
 It is impractical to maintain addresses for all
computers on an IP network with which the
local computer communicates.
 Instead, a default gateway (route) which is a
computer connected to the local subnet and to
other networks is used.
• It has knowledge of the network IDs of the other
networks in the internetwork and how to reach them.
IP Routing
 When IP prepares to send a packet, it inserts the
source IP address and the destination address of
the packet in the IP header. It then examines a
locally maintained route table and does one of
the following:
• Pass the packet up to a protocol layer above IP on
the local host.
• Forward the packet through one of the locally
attached network adapters.
• Discard the packet.
IP Routing
 The search for a match for the destination
address in the route table proceeds from the
specific to the general as follows:
• Examine table for exact match.
• Strip the host portion and examine table for a match
(subnet route).
• Strip subnet portion and examine table for a match
(network route).
• Use default gateway.
• If no default gateway specified, discard packet.
IP Routing
 The default gateway contains info about the
network Ids of the other networks in the
internetwork, so it can forward the packet to
other routers until the packet is eventually
delivered to a router connected to the specified
destination.
 This process is known as routing.
Dynamic Host Configuration
Protocol
 The Dynamic Host Configuration Protocol
(DHCP) facility assigns IP addresses to
network hosts dynamically.
 This decreases the individual workstation
configuration necessary for a system to be
successfully connected to the network.
 It is especially suited for computers that change
network locations frequently (e.g. laptops).
 A system is identified as a DHCP client via the
General tab of its TCP/IP Properties.
Dynamic Host Configuration
Protocol
 DHCP works as follows
• When a DHCP-prepared PC starts, it sends a
DHCPDISCOVER broadcast message on the LAN.
 If the client doesn’t receive an answer, it keeps sending
messages four times a minute for the next five minutes, or
until it receives an answer.
• Each DHCP server that receives a
DHCPDISCOVER message searches its tables for
an allocation for this PC.
 If it finds one, it answers with a DHCPOFFER message,
which contains the IP address the lease period, and other
info.
Dynamic Host Configuration
Protocol
 If no previous allocation exists, one or more DHCP
servers answer with IP addresses belonging to the subnet
on which the PC is placed.
• When the PC wants to accept an IP address, it
returns a DHCPREQUEST message with an
identification of the chosen DHCP server.
• Clients equipped with hard disks keep the allocated
TCP/IP address with each subsequent start.
• The client will try to renew the lease when half of
the lease period is over.
 If the lease period expires without a resubscription, the PC
must stop using the IP address and start a new negotiation
using the process described above.