Abstract

Web-based services are using Web technologies for implementing services on top of an existing networking infrastructure. The most important concepts are the basics of computer networking, in particular the way how networking protocols are arranged in layers, so that different functionalities required in a networking infrastructure can be isolated and implemented independently. On top of the networking infrastructure of the Internet, the Domain Name System (DNS) provides name-to-address mappings.

Services need Foundations

• Common Infrastructure
  • service always require information interchange
  • information services exchange nothing but information
  • real goods services exchange information about these goods
• Common Understanding
  • the information exchanged must have a well-defined meaning
  • without meaning, data interchange is pointless
• Trust
  • service peers must establish sufficient trust to engage in interactions
  • without trust, information interchange is pointless

Layering

• Layering is a popular computer science principle
  • it is used to encapsulate functionality
  • it is used to make complexity better manageable
  • every operating system is a layer on top of complex hardware
• Layers are hierarchically ordered
  • lower layers provide more concrete service
  • higher layers provide more abstract services
• Layered structures are well-ordered
  • a layer uses the service provided by the lower layer
  • a layer provides a service to an upper layer
  • layer implementation is hidden (and thus can be changed)

Layering in Systems

• lower layers provide low-level services
  1. a hard disk can read and write data to disk blocks
  2. memory management allows read and write of memory regions
• Higher layers provide more sophisticated services
  1. writing a file to a disk requires managing a file system
  2. controlling memory access requires management of process privileges
• New services require new layers
  1. distributed file systems must find the right node for a file
  2. virtual memory can be implemented by dynamically swapping memory contents

Layering in Communication Systems

• Requires the layering of any layered systems
  • lower layers offering lower-level services
  • higher layers using this service and implementing more abstract services
• In addition, communication systems use protocols
  • layers on different use a protocol for communications
  • the protocol enables them to provide their service
  • the protocol depends on the lower layers (and thus protocols)

Layers and Protocols

Middleware

• Computer network provide transport services
  • essential for facilitating data exchange in distributed systems
  • a very basic service not sufficient for supporting distributed systems
• Middleware is another layer on top of a network
  • it is assumed to hide the complexity of building distributed systems
  • it should provide services required for distributed systems
• Middleware should make software development easier
  • most middleware is a complex and expensive piece of software
  • most middleware results in lock-in
  • integration with other middleware frameworks is hard

Software Engineering

There are two ways of constructing a software design: One way is to make it so simple that there are obviously no deficiencies, and the other way is to make it so complicated that there are no obvious deficiencies. The first method is far more difficult.

C. A. R. Hoare, The Emperor's Old Clothes, 1980 Turing Award Lecture

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

Networks vs. Internetworks

• Specific networks use specific abstractions
  • how to address nodes
  • 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)

Internet vs. ISO/OSI

• Global network emerges by the end of the 80's
  • some kind of internetworking protocols were required
  • ARPANET had been running since the late 60's (1965: Berkeley-MIT)
• ISO/OSI (Open Systems Interconnection) was a new specification
  • the idea was to build something new
  • OSI was specified rather than developed and tested
• For some time, it was unclear what the global internetwork would be based on
  • Internet protocols were already established and running
  • OSI promised a fresh start with bigger is better protocols

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

Internet Principles

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

Jon Postel, RFC 1122

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

ISO/OSI (Open Systems Interconnection)

• Designed and specified in the 80's
  • a start-from-scratch approach to an internetworking protocol suite
  • many research contributions, many research publications
  • great for teaching principles of communications systems layers
• Problems with complexity and interoperability
  • some OSI protocols were never fully implemented
  • because of many optional features, interoperability was a problem
• Some of OSI's ideas had to be retrofitted to the Internet
  • session management (Layer 5) now is HTTP cookies
  • data representation (Layer 6) now is XML

OSI vs. Internet

Internet Protocols

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)

IP Addresses

• IP identifies nodes by an IP address
• IP addresses are globally unique (and can be geocoded)
• 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 addresses are well-organized
  • important for routing (i.e., sending packets to the target host)
  • not ideally suited for mobile or ad-hoc networks

IP Address Classes

TCP Features

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

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

TCP Window

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

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
  • DHCP provides IP address, 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)

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
• Names can have political relevance
  • ccTLDs are assigned based on the UNO's idea of what a country is
• Names can have symbolic relevance
  • Catalonia managed to get a domain of its own (cat)

Domain Name Space

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/ identified 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

DNS Request Processing

User Datagram Protocol (UDP)

• Transport protocol (based on IP, just like TCP)
  • very thin protocol, adds little features to IP
  • provides application addressing
• UDP is unreliable and connection-less
  • ideal for fast streaming media (delay is critical, lost packets are tolerable)
  • acceptable for one-packet applications (lightweight and fast)
  • not acceptable for reliable data transfer

Address Resolution Protocol (ARP)

• How to find an Internet host
  • hosts are configured (manually or by DHCP)
  • there is not externally controlled register of available hosts
• IP routing finds the network, but what about the host?
  • the sender broadcasts a request with the IP address
  • if there is such a host, it responds with its physical address
  • the sender can now send the IP packet to the physical address

ARP Operation

Internet Technologies

• The Internet can connect various underlying networks
• Internet Protocol (IP) transmits data between Internet hosts
• Transmission Control Protocol (TCP) provides reliable data transfer for applications
• Domain Name System (DNS) allows to use names rather than addresses

Web Technologies

• The Internet is a communications infrastructure
• The Web is an Internet-based distributed hypermedia system
• The Web's core network technology is Hypertext Transfer Protocol (HTTP)