LIS508 lecture 8: tcpip and domains

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LIS508 lecture 8tcp/ip and domains

• Thomas Krichel
• 2003-11-25

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Structure

• IP
• TCP
• Domains
• discussion mostly based on Tanenbaums computer
  networks classic book.

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Internet Protocol IP

• LAN protocols are not part of the Internet.
• IP operates on top of LANs to make them
  interoperable.
• IP sets up a set of logical addresses that locate
  a physical device over the whole range of
  locations over the Internet.
• It provides for the means to send datagrams from
  one address to the other
• Such datagrams are called packets. The IP service
  is not reliable!

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in the words of rfc791

• The Internet protocol is designed for use in
  interconnected systems of packet-switched
  computer connection networks. The Internet
  protocol provides for transmitting blocks of data
  called datagrams from sources to destinations.
  The Internet protocol also provides for
  fragmentation and reassembly of long datagram, if
  necessary, for transmission through 'small
  packet' networks.
• There are no mechanisms to augment end-to-end
  data reliability, flow control, sequencing, or
  other services commonly found in host-to-host
  protocols.

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IP address

• 32 bit address, usually written as a sequence of
  four decimal numbers between 0 and 255.
• Contains a network part and a host part
• dependent on the network mask
• depending on the class of the network
• All traffic that is not on the local network is
  sent to a gateway

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finding the IP address

• A host is on the Internet when it has an IP
  address that others can reach. Usually that
  implies knowing
• IP address of the machine
• network mask
• IP address of the gateway
• There can be written into a file on the machine.
  This is usually referred to as a static address.

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reading the IP address

• An IP address has a network component and a host
  component.
• Class A address a binary number that starts with
  0, then has 7bit for network information and 24
  for the host on that network
• Class B address is a binary number that starts
  with 10, then has 14 bit for the network and then
  16 bits for the host.
• Class C address is a binary number starts with
  110, then has 21 bit for the network and then 8
  bit for the host.

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address allocation

• traditionally organizations have been given a
  network, belonging to one of the classes, and
  then have given out hosts in the organization
• Some organizations don't need a big block of
  addresses
• Others are short (and they are usually not in the
  US!)

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address shortage

• IP classes waste addresses.
• They were set up to simplify routing of packets.
• IP version 6 will have 128 bit addresses and a
  completely different routing algorithm, but until
  this is implemented, we have address shortage.

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static and dynamic addresses

• Static IP addresses have problems
• waste a lot of address space on machines that are
  switched off
• have to be rewritten when device moves
• dynamic addressing has become more common
• the most widely used protocol is the dynamic host
  configuration protocol

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dhcp

• When connected to a network a client software on
  a host machine sends out a request on the LAN it
  is attached to.
• A dhcp server will answer and lease an IP address
  to the host, sending it the network mask and the
  gateway address as well.
• Internet providers work like that to save address
  space.
• Problem some services require IP addresses to be
  stable. They can not be run in such a setting.

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routing

• using IP, all datagrams travel individually.
• they may travel through many networks in order to
  get to their destination
• networks are interconnected through routers who
  make the decision where to send the packet to
  next.
• Such decision are based on routing protocols such
  as OSPF or BGP etc. This is a very complicated
  piece of engineering.

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frame / packet / segment

• The Ethernet frame on the LAN contains the IP
  packet.
• A gateway between two networks takes the packet
  out of the Ethernet frame of one network, and
  wraps it in a frame used on another network.
• Inside the IP packet, there is a TCP segment. The
  TCP segment tells the destination machine what to
  do with the data.

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transmission control protocol

• provides a reliable service of communication
• stream orientation bytes come out from the
  sender, arrive in the same order at the receiver
• packet buffering fast arrived data is stored at
  the destination until it can be processed
• full duplex communication in two ways.
• if a packet has gone astray, TCP retransmit it.
• uses port numbers as addresses to tell the host
  what to do with arriving packets.

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applications and ports

• TCP uses port numbers to detect which application
  protocol to send the data to.
• Some 0-1023 ports are well-known
• 80 for http -- 22 for ssh
• 23 for telnet -- 53 for dns
• firewalls can block traffic for specific ports on
  specific machines and therefore allow or prevent
  services.

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summary TCP/IP model

• Application layer
• http, smtp, ftp, dns, ntp
• Transport layer
• tcp, udp
• Network layer
• IP
• Host-to-host layer
• LAN and WAN protocols

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Critique of tcp/ip

• Not a model but a protocol stack
• No proper distinction between
• Service
• Protocol
• Interface
• Ad hoc set of application protocols

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introduction to NAT

• The biggest problem facing the Internet is that
  it is running out of addresses for IPv.4 and
  that IPv6 is not implemented everywhere.
• dhcp relieves this for machines that are on and
  off.
• But many home users want to connect a number of
  machines independently to the Internet, without
  having to request a new IP for each new machine.

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Network Address Translation

• Your provider gives you one IP address for a hub
  that connects you with the outside world.
• Internally, you connect a number of machines to
  the hub. Each machine has an internet address,
  but these addresses are only valid for your LAN.
  
• 10.0.0.0 -- 172.16.0.0 -- 192.168.0.0
• have been set aside for that. The hub uses dhcp
  to assign such addresses

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NAT at work

• The hub has two IP addresses, an internal one and
  an external one .
• When a host on the LAN wants to connect a remote
  (non-LAN) service, it tells the hub
• its internal address and port
• the destination address and port
• The hub exchanges the internal IP with the
  external one and uses a special port for this
  connection.
• When the response comes to the special port on
  the hub, it is forwarded to the LAN host.

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NAT critique

• violates the principle that each machine on the
  Internet has its own IP address
• brings the Internet back to the stage of
  connection-oriented networking
• protocol layering is screwed IP layer makes
  assumptions about TCP, which is an upper layer.
• some protocols, such as ftp use IP addresses in a
  way that NAT does not know about. They can not
  run with NAT, unless further tweaking is done.

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Application layer

• This is the top level of the network,
  applications that run on it.
• In fact, the Domain Name System occupies a
  special role because most other application layer
  protocols relay on it.
• Off the top of my head, only peer-to-peer
  communication systems dont really use DNS

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Host names

• A host name associates a human-friendly name with
  an IP address.
• Example arcano.openlib.org 148.4.16.229
• Finding an IP for a name is called a name lookup.
  The reverse is a reverse lookup.
• Names are a sequence of labels, separated by dot.
• Names may contain letters, numbers and hyphens.
  They may not start with a hyphen.
• Names solve from right to left, contrary to
  addresses, that resolve from left to right.

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purpose

• Allows to keep constant name for
• changing machines
• changing the location of the machine.
• Makes it easier for humans to remember access
  points to services.
• Establish brand names and have an economic value

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History of DNS

• In the 70s, one single file HOSTS.TXT was
  maintained at SRI-NIC, downloaded frequently by
  all hosts on the Internet.
• Problems
• traffic and load
• name collisions
• Consistency
• 1984, Paul Mockapetris releases RFC822 and RFC883
  that describe the Domain Name System DNS.
• Names are words separated by dots.

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DNS and domains

• DNS is
• distributed database
• client server architecture
• Name servers have information about names.
• general purpose
• Allows a lot of different properties to be
  associated with names
• hierarchical structure
• Top component of name is to the right.
• independent of physical structure

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Berkeley Internet Name Domain

• BIND is an implementation of the Domain Name
  System (DNS) protocols and provides an openly
  redistributable reference implementation of the
  major components of the Domain name system,
  including
• a Domain Name System server (named)
• a Domain Name System resolver library
• tools for verifying the proper operation of the
  DNS server

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Top level domains

• For the US, delimited by function .com, .net,
  .org, .int, .biz, .info etc.
• For other countries, use name of the country,
  .to, .su, .ru.
• Below that you can register names, such as
  myprof.com
• And then, you can create your own names like
  krichel.myprof.com, daluca.myprof.com, and
  associate properties with them.

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To register names

• There is a market of name registrars around.
• You have to check that the name is not already
  taken, e.g. whois openlib.org
• Domain Name OPENLIB.ORG
• Registrar NETWORK SOLUTIONS, INC.
• Whois Server whois.networksolutions.com
• Referral URL http//www.networksolutions.com
• Name Server UTSERV.MCC.AC.UK
• Name Server FAFNER.OPENLIB.ORG
• Many registrars will run a server for you, I run
  my own.

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• openlib.org. IN SOA wotan.liu.edu.
  tkrichel.wotan.liu.edu. (
• 2001111300 Serial
• 10800 Refresh
  after 3 hours
• 3600 Retry
  after 1 hour
• 640800 Expire
  after 1 week
• 86400 Minimum
  ttl of 1 day
• )
• openlib.org. IN NS
  wotan.liu.edu.
• primary server, the one which holds the
  authoritative info (this file)
• openlib.org. IN NS
  utserv.mcc.ac.uk.
• secondary servers, At least one is necssesary.
• openlib.org. IN A
  131.227.9.154
• mail.openlib.org. IN CNAME
  wotan.liu.edu.
• openlib.org. IN MX 1
  mail.openlib.org.
• trabbi.openlib.org. IN TXT "hello
  world"

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http//openlib.org/home/krichel

• Thank you for your attention!