Internetworking

1
Internetworking
CS 105Tour of the Black Holes of Computing

• Topics
• Client-server programming model
• Networks
• Internetworks
• Global IP Internet
• IP addresses
• Domain names
• Connections

net1.ppt
2
A Client-Server Transaction

• Every network application is based on the
  client-server model
• A server process and one or more client processes
• Server manages some resource.
• Server provides service by manipulating resource
  for clients.
• Clients and servers are processes running on
  hosts - can be same or different hosts
• Different hosts require interconnection, I.e., a
  network

1. Client sends request
Client process
Server process
Resource
4. Client handles response
2. Server handles request
3. Server sends response
3
Computer Networks

• A network is a hierarchical system of boxes and
  wires organized by geographical proximity
• LAN (local area network) spans building or campus
• Ethernet is most prominent example
• 802.11 (wireless) has become important
• WAN (wide-area network) spans country or world
• Typically high-speed point-to-point copper or
  fiber lines
• Also microwave and satellite links in some
  situations
• An internetwork (internet) is an interconnected
  set of networks
• Global IP Internet (uppercase I) is most famous
  example of an internet (lowercase i)
• Lets look at how to build an internet from
  ground up

4
Suppose You Want to Build A Network

• What available technologies would serve as
  building blocks?
• What software architecture would you use?
• How would you incorporate the future?
• What principles would you enforce?
• Who would manage it?
• Who would control it?

5
Computer Networks

• Fundamental Requirement
• A Computer network must provide general,
  cost-effective, fair, robust, secure, and high
  performance connectivity among a large number of
  computers.
• Internetworking
• Abstraction that deals with complexity of
  multiple underlying communication technologies

6
Hardware Org of a Network Host
CPU chip
register file
ALU
system bus
memory bus
main memory
I/O bridge
MI
Expansion slots
I/O bus
USB controller
network adapter
disk controller
graphics adapter
mouse
keyboard
monitor
disk
network
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Original LAN

• A collection of hosts attached to a single common
  wire
• Ethernet now most known
• Token Ring, X25 alternatives

...
host
host
host
Ethernet
8
Lowest Level Ethernet Segment

• Ethernet segment consists of a collection of
  hosts connected by wires (twisted pairs) to a
  hub/switch - replaces a common wire or bus
• Spans room or floor in a building.
• Operation
• Each Ethernet adapter has a unique 48-bit (MAC)
  address.
• Hosts send bits to any other host in chunks
  called frames.
• Hub slavishly copies each bit from each port to
  every other port.
• Every adapter sees every bit chooses which
  frames to hand to the system.
• Hub replaces wire
• Hub Alternative switch copies bits only to
  proper destination

host
host
host
100 Mb/s
100 Mb/s
hub
Physical Ports/connections
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Next Level Bridged Network Segment

• Spans building or campus.
• Bridges/switches cleverly learn which hosts are
  reachable from which ports and then selectively
  copy frames from port to port. How? Frames have
  source and destination addresses (board).

A
B
host
host
host
host
host
X
hub
hub
bridge
100 Mb/s
100 Mb/s
1 Gb/s
host
host
100 Mb/s
100 Mb/s
hub
bridge
hub
Y
host
host
host
host
host
C
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Conceptual View of LANs

• For simplicity, hubs, bridges, and wires are
  often shown as a collection of hosts attached to
  a single wire

...
host
host
host
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Next Level internets

• Multiple incompatible LANs can be physically
  connected by specialized computers called routers
  (gateway).
• The connected networks are called an internet.

...
...
host
host
host
host
host
host
LAN 1
LAN 2
router
router
router
WAN
WAN
LAN 1 and LAN 2 might be completely different
technologies, totally incompatible LANs (e.g.,
Ethernet and ATM)
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Notion of an internet Protocol

• How is it possible to send bits across
  incompatible LANs and WANs?
• Solution protocol software running on each host
  and router that smoothes out the differences
  between the different networks.
• Implements an internet protocol (i.e., set of
  rules) that governs how hosts and routers should
  cooperate when they transfer data from network to
  network.
• TCP/IP is the protocol (family) for the global IP
  Internet.

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What Does an internet Protocol Do?

• 1. Provides a naming scheme
• An internet protocol defines a uniform format for
  host addresses.
• Each network node (host, switch, router) is
  assigned at least one of these internet addresses
  that uniquely identifies it.
• 2. Provides a delivery mechanism
• An internet protocol defines a standard transfer
  unit (datagram, frame, message, packet)
• Packet consists of header and payload
• Header contains info such as packet size, source
  and destination addresses.
• Payload contains data bits sent from source
  host.
• Encapsulation - key to network messages

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Transferring Data via an internet
Host A
Host B
client
server
(1)
(8)
data
data
protocol software
protocol software
internet packet
(2)
(7)
data
PH
data
PH
LAN1 adapter
LAN2 adapter
Frame
Router
(3)
(6)
data
PH
data
PH
FH2
FH1
LAN1 adapter
LAN2 adapter
LAN1
LAN2
LAN2 frame
(4)
data
PH
FH1
(5)
data
PH
FH2
different media
protocol software
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Other Issues

• We are glossing over a number of important
  questions
• What if different networks have different maximum
  frame sizes? (segmentation)
• How do routers know where to forward frames?
• How are routers informed when the network
  topology changes?
• What if packets get lost?
• These (and other) questions are addressed by the
  area of systems known as computer networking CS
  125.

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Global IP Internet

• Most famous example of an internet.
• Administered by a Unique Group - IETF
• Based on the TCP/IP protocol family
• IP (Internet protocol)
• Provides basic naming (addressing) scheme and
  unreliable, best effort delivery capability of
  packets (datagrams) from host-to-host.
• UDP (Unreliable Datagram Protocol)
• Uses IP to provide unreliable datagram delivery
  from process-to-process.
• TCP (Transmission Control Protocol)
• Uses IP to provide reliable byte streams from
  process-to-process over connections.
• Accessed via a mix of Unix file I/O and functions
  from the sockets interface.

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Hardware and Software Org of an Internet
Application
Internet client host
Internet server host
Client
Server
User code
Sockets interface (system calls)
TCP/IP
TCP/IP
Kernel code
Hardware interface (interrupts)
Hardware and firmware
Network adapter
Network adapter
Global IP Internet
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Basic Internet Components

• An Internet backbone is a collection of routers
  (nationwide or worldwide) connected by high-speed
  point-to-point networks.
• A Network Access Point (NAP) is a router that
  connects multiple backbones (sometimes referred
  to as peers).
• Regional networks are smaller backbones that
  cover smaller geographical areas (e.g., cities or
  states)
• A point of presence (POP) is a machine that is
  connected to the Internet.
• Internet Service Providers (ISPs) provide dial-up
  or direct access to POPs.

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The Internet Circa 1993

• In 1993, the Internet consisted of one backbone
  (NSFNET) that connected 13 sites via 45 Mbs T3
  links.
• Merit (Univ of Mich), NCSA (Illinois), Cornell
  Theory Center, Pittsburgh Supercomputing Center,
  San Diego Supercomputing Center, John von Neumann
  Center (Princeton), BARRNet (Palo Alto), MidNet
  (Lincoln, NE), WestNet (Salt Lake City),
  NorthwestNet (Seattle), SESQUINET (Rice), SURANET
  (Georgia Tech).
• Connecting to the Internet involved connecting
  one of your routers to a router at a backbone
  site, or to a regional network that was already
  connected to the backbone.

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NSFNET Internet Backbone
source www.eef.org
US Centric, only one backbone
21
Enter Al Gore

• Myth Al Gore claimed to have invented the
  Internet
• Fact In a 1999 interview, Al Gore said, During
  my service in the United States Congress, I took
  the initiative in creating the Internet
• Fact Dave Farber, Vint Cerf, and Bob Metcalfe
  have all supported the statement
• Fact Al Gore introduced and supported many bills
  funding the shift from a primarily US research
  network to a worldwide commercial one
• Farber The guy used an inappropriate word. If
  he had said he was instrumental in the
  development of what it is now, he'd be accurate.

22
Current NAP-Based Internet Arch

• In the early 90s commercial outfits were
  building their own high-speed backbones,
  connecting to NSFNET, and selling access to their
  POPs to companies, ISPs, and individuals.
• In 1995, NSF decommissioned NSFNET, and fostered
  creation of a collection of NAPs to connect the
  commercial backbones.
• Currently in the US there are about 50 commercial
  backbones connected by 12 NAPs (peering points).
  NANOG
• Similar architecture worldwide connects national
  networks to the Internet.

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Abstracted Internet Hiearchy
Private peering agreements between two
backbone companies often bypass NAP
NAP
NAP
NAP
Colocation sites
Backbone
Backbone
Backbone
Backbone
POP
POP
POP
POP
POP
POP
POP
T3
Regional net
Big Business
ISP
POP
POP
POP
POP
POP
POP
POP
dialup
dialup
T1
T1
Pgh employee
DC employee
Claremont Collegs
ISP (for individuals)
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Network Access Points (NAPs)
Note Peers in this context are commercial
backbones..droh
Source Boardwatch.com
25
MCI/UUNET Global Backbone Example of Nested
Internet
Source Boardwatch.com
26
A Programmers View of the Internet

• 1. Hosts are mapped to a set of 32-bit IP
  addresses.
• 128.2.203.179
• Class structure A, B, C, Now Cider
• Running out of IPv4 addresses, no more in general
  pool, allocated to locate registeries
• 2. The set of IP addresses is mapped to a set of
  identifiers called Internet domain names.
• 128.2.203.179 is mapped to www.cs.cmu.edu
• 134.173.42.2 is mapped to www.cs.hmc.edu
• 3. A process on one Internet host can communicate
  with a process on another Internet host over a
  connection -- IP Address, Port Number

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1. IP Addresses

• 32-bit IP addresses are stored in an IP address
  struct
• IP addresses are always stored in memory in
  network byte order (big-endian byte order)
• True in general for any integer transferred in a
  packet header from one machine to another.
• E.g., the port number used to identify an
  Internet connection.

/ Internet address structure / struct in_addr
unsigned int s_addr / network byte order
(big-endian) /
Handy network byte-order conversion functions
(NoOps on some machines) x86 is
little-endian htonl convert long int from host
to network byte order. htons convert short int
from host to network byte order. ntohl convert
long int from network to host byte order. ntohs
convert short int from network to host byte order.
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Dotted Decimal Notation

• By convention, each byte in a 32-bit IP address
  is represented by its decimal value and separated
  by a period
• IPv4 address 0x8002C2F2 128.2.194.242
• IPv6 address 200118783010902321868bfffef9a4
  07
• Functions for converting between binary IP
  addresses and dotted decimal strings
• inet_aton converts a dotted decimal string to
  an IP address in network byte order.
• inet_ntoa converts an IP address in network
  byte order to its corresponding dotted decimal
  string.
• n denotes network representation. a denotes
  application representation.

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2. Internet Domain Names
unnamed root
TLD - Top Level Domains First-level domain names
mil
edu
gov
com
Second-level domain names
hmc
berkeley
mit
amazon
Third-level domain names
cs
math
www 208.216.181.15
Wilkes 134.173.42.167
Turing 134.173.42.99

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Properties of DNS Host Entries

• Each host entry is an equivalence class of domain
  names and IP addresses.
• Each host has a locally defined domain name
  localhost which always maps to the loopback
  address 127.0.0.1
• Different kinds of mappings are possible
• Simple case 1-1 mapping between domain name and
  IP addr
• turing.cs.hmc.edu. maps to 134.173.42.99
• Multiple domain names mapped to the same IP
  address
• cs.hmc.edu and www.cs.hmc.edu both map to
  134.173.42.2
• Multiple domain names mapped to multiple IP
  addresses
• aol.com and www.aol.com map to multiple IP
  addresses
• Some valid domain names dont map to any IP
  address
• for example research.cs.hmc.edu

31
Domain Naming System (DNS)

• The Internet maintains a mapping between IP
  addresses and domain names in a huge worldwide
  distributed database called DNS.
• Conceptually, programmers can view the DNS
  database as a collection of millions of host
  entry structures, OLD STRUCT
• Old Functions for retrieving host entries from
  DNS
• gethostbyname query key is a DNS domain name.
• gethostbyaddr query key is an IP address.

/ DNS host entry structure / struct hostent
char h_name / official domain name
of host / char h_aliases /
null-terminated array of domain names / int
h_addrtype / host address type (AF_INET)
/ int h_length / length of an
address, in bytes / char h_addr_list /
null-terminated array of in_addr structs /
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man gethostbyname

• struct hostent
• char h_name / official
  name of host /
• char h_aliases / alias
  list /
• int h_addrtype / host
  address type /
• int h_length / length of
  address /
• char h_addr_list / list of
  addresses from name server /
• define h_addr h_addr_list0 / address,
  for backward compatibility /
• The members of this structure are
• h_name Official name of the host.
• h_aliases A NULL-terminated array of
  alternate names for the host.
• h_addrtype The type of address being
  returned usually AF_INET.
• h_length The length, in bytes, of the
  address.
• h_addr_list A NULL-terminated array of
  network addresses for the host.
• Host addresses are returned in
  network byte order.
• h_addr The first address in
  h_addr_list this is for backward com-
• patibility.

33
Domain Naming System (DNS)

• Internet tracks mapping between IP addresses and
  domain names in huge worldwide distributed
  database called DNS.
• Conceptually, programmers can view DNS database
  as collection of millions of address information
  structures NEW STRUCT
• New Functions for retrieving host entries from
  DNS
• getaddrinfo query key is DNS domain name
• getnameinfo query key is IP address (V4 or V6)

/ Address information structure (DNS only has
entries) / struct addrinfo int
ai_flags / Various options / int
ai_family / AF_INET or AF_INET6 / int
ai_socktype / Preferred socket type
/ int ai_protocol / Preferred
protocol / size_t ai_addrlen /
Length of address / struct sockaddr
ai_addr / Encoded IP address / char
ai_canonname / Canonical host name
/ struct addrinfo ai_next / Link to next
answer /
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man getaddrinfo

• This structure can be used to provide hints
  concerning the type of socket that the caller
  supports or wishes to use. The caller can supply
  the following structure elements in hints
• ai_family The protocol family that
  should be used. When ai_family
• is set to PF_UNSPEC, it means
  the caller will accept any
• protocol family supported by
  the operating system.
• ai_socktype Denotes the type of socket
  that is wanted SOCK_STREAM,
• SOCK_DGRAM, or SOCK_RAW.
  When ai_socktype is zero the
• caller will accept any socket
  type.
• ai_protocol Indicates which transport
  protocol is desired,
• IPPROTO_UDP or IPPROTO_TCP.
  If ai_protocol is zero the
• caller will accept any
  protocol.
• ai_flags The ai_flags field to which
  the hints parameter points
• shall be set to zero or be
  the bitwise-inclusive OR of
• one or more of the values
  AI_ADDRCONFIG, AI_ALL,
• AI_CANONNAME, AI_NUMERICHOST,
  AI_NUMERICSERV, AI_PASSIVE,
• and AI_V4MAPPED.
• AI_ADDRCONFIG If the
  AI_ADDRCONFIG bit is set, IPv4
• addresses
  shall be returned only if an

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A Program That Queries DNS - NEW
int main(int argc, char argv) / argv1 is a
domain name / struct addrinfo hints, host,
firsthost NULL struct sockaddr_in addr
char buf80 memset(hints, 0, sizeof
hints) hints.ai_flags AI_CANONNAME
hints.ai_family AF_UNSPEC / Or AF_INET or
AF_INET6 / if (getaddrinfo(argv1, NULL,
hints, firsthost) ! 0) exit(1)
printf("official hostname s\n",
firsthost-gtai_canonname) for (host
firsthost host ! NULL host host-gtai_next)
addr (struct sockaddr_in )host-gtai_addr pr
intf("address s\n", inet_ntop(addr-gtsin_family,
addr-gtsin_addr, buf, sizeof buf))
exit(0)
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Querying DNS from the Command Line

• Domain Information Groper (dig) provides a
  scriptable command line interface to DNS.

linuxgt dig short kittyhawk.cmcl.cs.cmu.edu
128.2.194.242 linuxgt dig short -x
128.2.194.242 KITTYHAWK.CMCL.CS.CMU.EDU. linuxgt
dig short aol.com 205.188.145.215
205.188.160.121 64.12.149.24 64.12.187.25
linuxgt dig short -x 64.12.187.25
aol-v5.websys.aol.com.
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3. Internet Connections

• Clients and servers communicate by sending
  streams of bytes over connections
• Point-to-point, full-duplex (2-way
  communication), and reliable.
• A socket is an endpoint of a connection
• Socket address is an IPaddress/port pair
• A port is a 16-bit integer that identifies a
  process
• Ephemeral port Assigned automatically on client
  when client makes a connection request
• Well-known port Associated with some service
  provided by a server (e.g., port 80 is associated
  with Web servers)
• A connection is uniquely identified by the socket
  addresses of its endpoints (socket pair)
• (cliaddrcliport, servaddrservport)

38
Putting it all Together Anatomy of an Internet
Connection
Client socket address 128.2.194.24251213
Server socket address 208.216.181.1580
Server (port 80)
Client
Connection socket pair (128.2.194.24251213,
208.216.181.1580)
Client host address 128.2.194.242
Server host address 208.216.181.15
39
Next Time

• How to use the sockets interface to establish
  Internet connections between clients and servers

40
A Program That Queries DNS - OLD
int main(int argc, char argv) / argv1 is a
domain name char pp
or dotted decimal IP addr / struct in_addr
addr struct hostent hostp if
(inet_aton(argv1, addr) ! 0 // got address
hostp Gethostbyaddr((const char )addr,
sizeof(addr), AF_INET)
else hostp Gethostbyname(argv1) //
got name printf("official hostname s\n",
hostp-gth_name) for (pp
hostp-gth_aliases pp ! NULL pp)
printf("alias s\n", pp) for (pp
hostp-gth_addr_list pp ! NULL pp)
addr.s_addr ((unsigned int )pp)
printf("address s\n", inet_ntoa(addr))