(4) Network History

• First regarded as a convenient workaround for floppy disks
  • real computer scientists write compilers
  • the value of computer networks depends on their size
• Early networking solutions were vendor-specific islands
  • DECnet for Digital Equipment Corporation (DEC) customers
  • XNS for Xerox customers
  • SNA for IBM customers
  • transmitting data between these networks was very cumbersome
• Bridging networks transparently became increasingly important
  • more computers and networks increase the benefit of interconnections
  • layering being used for internetworks, not only for networks

(5) Networks vs. Internetworks

• Specific networks use specific abstractions
  • how to address nodes (computers, phones, PDAs, RFID tags)
  • how to address applications on these nodes
  • how to transmit data to these applications
• Internetworks provide a network-independent abstraction
  • nodes are addressed uniformly (IP addresses)
  • applications are identified uniformly (ports)
  • data transmission uses one set of protocols (TCP/UDP)

(6) Internet

• Very early start and a lot of experience
  • pragmatic and evolutionary approach
  • if it's not broken, don't fix it
• Standardization by independent technical experts
  • avoids the designed by committee effect of consortiums
  • conservative and concentrating on stability
  • implementations are required to prove technical feasibility
  • simplicity whenever possible

(7) Internet Principles

Be liberal in what you accept, and conservative in what you send.

Jon Postel [http://www.postel.org/postel.html], RFC 1122 [http://tools.ietf.org/html/rfc1122]

Whenever possible, communications protocol operations should be defined to occur at the end-points of a communications system, or as close as possible to the resource being controlled.

J. Saltzer, D. Reed, D. Clark, End-to-end Arguments in System Design [http://dret.net/biblio/reference/sal84]

(8) Internet Protocols

(9) Network Convergence

(11) IP Features

• End-to-end data transfer (IP addresses)
• Hiding lower-level heterogeneity
• Connection-less (each packet routed individually)
• Unreliable (packets may be lost or duplicated)

(12) IP Address

• IP identifies nodes by an IP address
• IP addresses are globally unique (and can be geocoded [http://api.hostip.info/get_html.php?position=true])
• IP uses 4 bytes for addresses (e.g., 128.32.226.29)
  • maximum number of addresses: 2³² = 4 billion
  • IPv6 extends the address format to 16 bytes (2¹²⁸ addresses)
• IP address shortage led to the some trickery using IP addresses
  • Dynamic Host Configuration Protocol (DHCP) [http://en.wikipedia.org/wiki/Dhcp] is used to assign addresses on-demand
  • Network Address Translation (NAT) [http://en.wikipedia.org/wiki/Network_address_translation] uses one IP address for more than one device
• IP addresses are well-organized
  • important for routing (i.e., sending packets to the target host)
  • not ideally suited for mobile or ad-hoc networks

(14) TCP Features

• Flow-controlled (avoiding congestion)
• Reliable (no data lost or duplicated)
• Connection-oriented
• Application addressing

(15) Reliable Connections

• IP may drop or duplicate packets
  • TCP adds serial numbers in data packets
  • if problems are detected, TCP recovers automatically
• TCP avoids network congestion and system overload
  • slow start avoid flooding receivers with data they cannot process
  • fast retransmit for avoiding timeouts when losing data
  • a sliding window for controlling the amount of outstanding packets

(16) TCP Window

(18) Naming vs. Addressing

• IP addresses depend on network topology and organization
  • reorganizing a network may change all IP addresses
  • identifying important hosts should not be address-based
• Names are supposed to be more stable than addresses
  • a name is an abstract identification of something
  • names can be used to obtain more information
• Network services should use names instead of addresses
  • before using the service, a mapping has to be performed
  • the Domain Name System (DNS) is providing this service

(19) DNS Properties

• DNS has a bootstrap problem
  • DNS provides a service and should thus be identified by a name
  • for resolving names into addresses, the DNS service is required
• DNS configuration is part of basic Internet configuration
  • Dynamic Host Configuration Protocol (DHCP) provides IP Address [IP Address (1)], netmask, gateway, and DNS server address
• DNS names are hierarchically structured
  • ischool.berkeley.edu, edu is the Top-Level Domain (TLD)
  • TLDs are either generic (gTLD) or country code (ccTLD)
  • subdomains are federated (e.g., edu, us, uk, tv)

(20) Names Matter

• Names are not unique and namespaces are finite
  • name disputes arise which were irrelevant before the Web
  • cybersquatting as a popular way to make money
• Names can be worth a lot of money
  • business.com was sold for $7.5 million
• Name inflation can be used to generate money
  • aero, biz, coop, info, jobs, mobi, museum, name, pro, travel
  • starting 2009, user-defined top-level domains will go on sale [http://dret.typepad.com/dretblog/2008/06/dret.html]
• Names can have political significance
  • ccTLDs are assigned based on the UNO's idea of what a country is
• Names can have symbolic significance
  • Catalonia managed to get a domain of its own (cat)

(21) Domain Name Space

(22) DNS Namespace Organization

• Domain owners can organize the assignment of subdomains
  • berkeley.edu [http://www.berkeley.edu/] is an U.S. educational institution
  • ethz.ch [http://www.ethz.ch/] is a Swiss university
  • imperial.ac.uk [http://www.imperial.ac.uk/] is a British university
  • uts.edu.au [http://www.uts.edu.au/] is an Australian university
• Special rules may apply (Germany does not assign car license plate names)
• Organizations may be countries or companies
  • countries have national organizations for assigning names
  • companies may create an internal multi-level namespace (www.ischool.berkeley.edu [http://www.ischool.berkeley.edu/])

(23) Using DNS

• DNS is used by virtually all Internet applications
  • names are more stable than addresses
• E-mail has some dedicated features built into DNS
  • special entries (MX records) identify the e-mail server for a domain
  • fallback entries help dealing with failing e-mail servers
• most URIs are based on DNS names
  • http://ischool.berkeley.edu/ identifies the access protocol and the host
  • the browser first performs a DNS lookup
  • a TCP connection is then established to the address returned by the DNS