The TCPIP architecture: an Introduction

1
The TCP/IP architecturean Introduction

• Benoît MACQ
• macq_at_tele.ucl.ac.be
• www.tele.ucl.ac.be

2
The elementary communication resource

• Circuits (ISDN, SDH, SONET) with a fixed bit
  rate
• resource reservation procedure (centralised)
• wasted bandwidth (no statistical multiplexing)
• Two ways to use packets
• connection oriented carry only an VC identifier
  (X25, ATM)
• connectionless carry entire destination address
  in header (Internet Protocol IP)

Data
Sample ATM cell Datagram
Data
VCI
Data
Addr.
3
Connection vs. connectionless

• A connection allows resource reservation
• Quality of Service (voice, data, multimedia)
• A connection allows to establish a virtual
  circuit
• guaranteed path (security)
• elementary information of small size (filling
  time of the payload, easier statistical
  multiplexing and switching)
• Connectionless datagrams communications are
  suited for non reliable networks
• Connectionless communicatons allows a
  decentralised, lower cost and more scalable
  implementation

4
Internet requirements
5
Internet for what ?

• Make computer programs communicating together,
  e.g.
• FTP file transferts from a server to a client
• HTTP World Wide Web navigation (a navigator is
  requesting pages from a server)
• POP, SMTP e-mail receiving and sending
• and other dedicated applications
• Make those computer programs reachable and
  available from everywhere in the Universe
• Synchronise and make reliable communications

6
Internet requirements

• Reliable highly connected network (suited to
  survive the day after (cold war))
• Decentralised, datagram based
• Unique format of the information unit (datagram)
• Dump network

7
TCP-IP key-concepts
8
TCP-IP key-concepts

• Internet Protocol (IP) transmit datagrams
  between two machines
• communication nodes routers feed the datagram
  and transmit to the best neighbouring connected
  node
• routing table
• entries endpoint destinations
• output corresponding optimum neighbour
• routing protocol messages and procedures between
  routers to update the outing tables from a
  knowledge of status of the network
• IP address the address of a machine (computer or
  router in the network) 4 bytes (e.g.
  130.194.128.35)

9
Datagram

• IP best effort network
• each datagram is treated as better as possible
  regardless the nature of its contents
• If a Datagram transmission fails, send a control
  message back to the source
• Each datagram of a communication is send
  independly from the others

32 bits
Control data
Source address
Destination address
Control data
Payload
10
3 Types of messages at the IP level

• Datagrams
• Control messages (failure of transmission,
  unreachable destination, ) ICMP
• Routing protocols messages (RIP or OSPF)

11
The role of the Transmission Control Protocol
(TCP)

• The need for a datagram to be directed towards a
  specific program running on the destination
  computer, and possibly run several communicating
  programs on the same computer (i.e. the same IP
  address)multiplexing
• communicating programs are numbered according to
  the port number
• The need to establish virtual connections
• need for synchronising the source and
  destination communicating programs
• establish synchronised buffers at the source and
  the destination
• need for reliable communication
• numbering and checksum of TCP segments

12
Datagram vs. TCP segment
32 bits
Control data
Source address
Destination address
32 bits
Control data
Source port
Destin.port
Sequence number
Acknowledgement num.
Control data
Payload
13
A global view on the Internet
14
Economy Huge growth!

• The Internet has doubled in size every year since
  1969
• Soon, everyone who has a phone is likely to also
  have an email account
• More and more, telephone directories will include
  email addresses in white pages and vice-versa
• Moores law the amount of functions implemented
  into a chip double every 18 months
• Metcalfes law the utility of a network is a
  function of the square of the amount of its
  subscriber

15
Technologydatagram transmission, endpoints
services

• The information is sent through datagrams
• Datagram header (Dest. Adr. Orig. Adr.)
  data
• Address IP (Internet Protocol) address
  (virtual address)
• ARP (address resolution protocol) how to find a
  physical address (Ethernet, ISDN, ATM, SDH, )
  corresponding to the IP address
• Best effort connectionless network
• Practical services (e-mail, WWW, files transfer,
  ) provided by software running at endpoints
  (layering like OSI, but only a the endpoints)

16
The Internet microcosm

• Internet Architecture Board (IAB)
• The IAB is responsible for defining the overall
  architecture of the Internet, providing guidance
  and broad direction to the IETF.
• The IAB also serves as the technology advisory
  group to the Internet Society, and oversees a
  number of critical activities in support of the
  Internet.
• Internet Engineering Task Force (IETF)
• The IETF is the protocol engineering and
  development arm of the Internet. Establish and
  develop the RFC documents approved by IAB
• Take a look at www.ietf.org and www.ietf.org/rfc

17
IP RFC 791

• INTERNET PROTOCOL (September 1981)
  
• TABLE OF CONTENTS
• PREFACE ......................................
  .................. iii
• 1. INTRODUCTION .................................
  .................... 1
• 1.1 Motivation ................................
  .................... 1
• 1.2 Scope .....................................
  .................... 1
• 1.3 Interfaces ................................
  .................... 1
• 1.4 Operation .................................
  .................... 2
• 2. OVERVIEW .....................................
  .................... 5
• 2.1 Relation to Other Protocols
  ................................... 9
• 2.2 Model of Operation ........................
  .................... 5
• 2.3 Function Description ......................
  .................... 7
• 2.4 Gateways ..................................
  .................... 9
• 3. SPECIFICATION ................................
  ................... 11
• 3.1 Internet Header Format ....................
  ................... 11
• 3.2 Discussion ................................
  ................... 23
• 3.3 Interfaces ................................
  ................... 31

18
TCP RFC 793

• TRANSMISSION CONTROL PROTOCOL(September 1981)
• 1. INTRODUCTION .................................
  .................... 1
• 2. PHILOSOPHY ...................................
  .................... 7
• 2.1 Elements of the Internetwork System
  ........................... 7
• 2.2 Model of Operation ........................
  .................... 7
• 2.3 The Host Environment ......................
  .................... 8
• 2.4 Interfaces ................................
  .................... 9
• 2.5 Relation to Other Protocols
  ................................... 9
• 2.6 Reliable Communication ....................
  .................... 9
• 2.7 Connection Establishment and Clearing
  ........................ 10
• 2.8 Data Communication ........................
  ................... 12
• 2.9 Precedence and Security ...................
  ................... 13
• 2.10 Robustness Principle ......................
  ................... 13
• 3. FUNCTIONAL SPECIFICATION .....................
  ................... 15
• 3.1 Header Format .............................
  ................... 15
• 3.2 Terminology ...............................
  ................... 19
• 3.3 Sequence Numbers ..........................
  ................... 24
• 3.4 Establishing a connection
  .................................... 30
• 3.5 Closing a Connection ......................
  ................... 37

19
The Internet microcosm

• The Internet Engineering Steering Group (IESG)
• The IESG is directly responsible for the actions
  associated with entry into and movement along the
  Internet "standards track," including final
  approval of specifications as Internet Standards.
  
• Internet Society (ISOC)
• The Internet Society is a professional membership
  organization of Internet experts that comments on
  policies and practices and oversees a number of
  other boards and task forces dealing with network
  policy issues.

20
The Internet microcosm

• Internet Assigned Numbers Authority (IANA)
• Based at the University of Southern California's
  Information Sciences Institute, IANA is in charge
  of all "unique parameters" on the Internet,
  including IP (Internet Protocol) addresses. Each
  domain name is associated with a unique IP
  address, a numerical name consisiting of four
  blocks of up to three digits each, e.g.
  204.146.46.8, which systems use to direct
  information through the network.

21
IANA

• Internet Protocol (IP) addresses (under the
  current version 4) are 32-bit numbers often
  expressed as 4 octets in "dotted decimal"
  notation (for example, 192.168.45.230).
• If you need an IP address or block of addresses,
  please contact your Internet service provider
  (ISP).
• Internet Service Providers (ISPs) should contact
  their upstream registry or their appropriate
  regional registry at one of the following
  addresses
• APNIC (Asia-Pacific Network Information Center)
  lthttp//www.apnic.netgt
• ARIN (American Registry for Internet Numbers )
  lthttp//www.arin.netgt
• RIPE NCC (Reseau IP Europeens)
  lthttp//www.ripe.netgt
• RFC 2050 - Internet Registry IP Allocation
  Guidelines
• RFC 1918 - Address Allocation for Private
  Internets
• RFC 1518 - An Architecture for IP Address
  Allocation with CIDR

22
What does Internet look like?

• Loose collection of networks organized into a
  multilevel hierarchy
• 10-100 machines connected to a hub or a router
  (gateway)
• service providers also provide direct dialup
  access
• or over a wireless link
• 10s of routers on a department backbone
• 10s of department backbones connected to campus
  backbone
• 10s of campus backbones connected to regional
  service providers
• 100s of regional service providers connected by
  national backbone
• 10s of national backbones connected by
  international trunks

23
Internet services

• Ftp file transfer protocol
• e-mail electronic mail
• World Wide Web
• Direct communication routines ((win)sockets
  library)
• Naming service (DNS Domain Name Service)
• Certification procedures (Public Key
  Infrastructures)
• Network management tools (ping, traceroute, )

24
Example of message routing

• Détermination de l'itinéraire vers
  DANDELION-PATCH.MIT.EDU 18.181.0.31
• avec un maximum de 30 sauts
• 1 1698 ms 799 ms 799 ms
  PMHalles1.sri.ucl.ac.be 130.104.1.15
• 2 794 ms 799 ms 799 ms
  CsHalles.sri.ucl.ac.be 130.104.1.60
• 3 1195 ms 799 ms 799 ms
  c7206vxr-lln.belnet.net 130.104.254.174
• 4 796 ms 698 ms 800 ms
  pvc1-76.c7513.brussels.belnet.net
  193.190.61.178
• 5 807 ms 700 ms 700 ms
  g0-0-0.c7507.brussels.belnet.net 193.190.182.1
• 6 1695 ms 800 ms 800 ms
  s4-1-0.bru-bbr-01.carrier1.net 212.4.203.1
• 7 795 ms 800 ms 800 ms 212.4.199.194
• 8 785 ms 800 ms 700 ms
  s0-0-0.ham-bbr-01.carrier1.net 212.4.199.54
• 9 895 ms 799 ms 1199 ms
  s1-1-0.nyc-bbr-01.carrier1.net 212.4.199.25
• 10 998 ms 1199 ms 899 ms
  h2-0.nyc4-cr3.bbnplanet.net 4.1.73.1
• 11 895 ms 900 ms 899 ms
  p4-1.nyc4-nbr3.bbnplanet.net 4.0.1.109
• 12 895 ms 900 ms 899 ms
  p4-1.bstnma1-ba2.bbnplanet.net 4.24.4.237
• 13 795 ms 799 ms 799 ms
  p2-3.cambridge1-nbr1.bbnplanet.net 4.0.2.166
• 14 895 ms 899 ms 999 ms
  p1-0-0.cambridge1-br1.bbnplanet.net 4.0.1.22
• 15 960 ms 899 ms 899 ms DANDELION-PATCH.MIT
  .EDU 18.181.0.31
• Itinéraire déterminé

25
Intranet, Internet, and Extranet

• Intranets are administered by a single entity
• e.g. Louvain-la-Neuve campus network
• Internet is administered by a coalition of
  entities
• name services, backbone services, routing
  services etc.
• Extranet is a marketing term
• refers to exterior customers who can access
  privileged Intranet services
• e.g. Louvain-la-Neuve could provide extranet
  services to UCL St Luc

26
What holds the Internet together?

• Addressing
• how to refer to a machine on the Internet
• Routing
• how to get there
• Internet Protocol (IP)
• what to speak to be understood

27
Example joining the Internet

• How can people talk to you?
• get an IP address from your administrator
• How do you know where to send your data?
• if you only have a single external connection,
  then no problem
• otherwise, need to speak a routing protocol to
  decide next hop
• How to format data?
• use the IP format so that intermediate routers
  can understand the destination address
• If you meet these criteria--youre on the
  Internet!
• Decentralized, distributed, and chaotic
• but it scales (why?)

28
What lies at the heart?

• Two key technical innovations
• packets
• store and forward

29
Packets

• Self-descriptive data
• packet data metadata (header)
• Packet vs. sample
• samples are not self descriptive
• to forward a sample, we have to know where it
  came from and when
• cant store it!
• hard to handle bursts of data

30
Store and forward

• Metadata allows us to forward packets when we
  want
• E.g. letters at a post office headed for main
  post office
• address labels allow us to forward them in
  batches
• Efficient use of critical resources
• Three problems
• hard to control delay within network
• switches need memory for buffers
• convergence of flows can lead to congestion

31
Key features of the Internet

• Addressing
• Routing
• Endpoint control

32
Addressing

• Internet addresses are called IP addresses
• Refer to a host interface need one IP address
  per interface
• Addresses are structured as a two-part hierarchy
• network number
• host number

135.105.53
100
33
An interesting problem

• How many bits to assign to host number and how
  many to network number?
• If many networks, each with a few hosts, then
  more bits to network number
• And vice versa
• But designers couldnt predict the future
• Decided three sets of partitions of bits
• class A 8 bits network, 24 bits host
• class B 16 bits each
• class C 24 bits network, 8 bits host

34
Addressing (contd.)

• To distinguish among them
• use leading bit
• first bit 0gt class A
• first bits 10 gt class B
• first bits 110 gt class C
• (what class address is 135.104.53.100?)
• Problem
• if you want more than 256 hosts in your network,
  need to get a class B, which allows 64K hosts gt
  wasted address space
• Solution
• associate every address with a mask that
  indicates partition point
• CIDR

35
Routing

• How to get to a destination given its IP address?
• We need to know the next hop to reach a
  particular network number
• this is called a routing table
• computing routing tables is non-trivial
• Simplified example

36
Default routes

• Strictly speaking, need next hop information for
  every network in the Internet
• gt 800,000 now
• Instead, keep detailed routes only for local
  neighborhood
• For unknown destinations, use a default router
• Reduces size of routing tables at the expense of
  non-optimal paths

37
Endpoint control

• Key design philosophy
• do as much as possible at the endpoint
• dumb network
• exactly the opposite philosophy of telephone
  network
• Layer above IP compensates for network defects
• Transmission Control Protocol (TCP)
• Can run over any available link technology
• but no quality of service
• modification to TCP requires a change at every
  endpoint
• (how does this differ from telephone network?)

38
Challenges

• IP address space shortage
• because of free distribution of inefficient Class
  B addresses
• decentralized control gt hard to recover
  addresses, once handed out
• Decentralization
• allows scaling, but makes reliability next to
  impossible
• cannot guarantee that a route exists, much less
  bandwidth or buffer resources
• single points of failure can cause a major
  disaster
• and there is no control over who can join!
• hard to guarantee security
• end-to-end encryption is a partial solution
• who manages keys?

39
Challenges (contd.)

• Decentralization (contd.)
• no uniform solution for accounting and billing
• cant even reliably identify individual users
• no equivalent of white or yellow pages
• hard to reliably discover a users email address
• nonoptimal routing
• each administrative makes a locally optimal
  decision

40
Challenges (contd).

• Multimedia
• requires network to support quality of service of
  some sort
• hard to integrate into current architecture
• store-and-forward gt shared buffers gt traffic
  interaction gt hard to provide service quality
• requires endpoint to signal to the network what
  it wants
• but Internet does not have a simple way to
  identify streams of packets
• nor are are routers required to cooperate in
  providing quality
• and what about pricing!