Network Applications

Write programs that:

• run on different end systems
• communicate over network
• no need to write software for network core devices

Architectures

• client/server
• peer-to-peer

Client-server

Server

• always on
• static IP

Clients

• communicated with server
• intermittently connected
• dynamic IP
• doesn't directly communicate with other clients

Peer-to-peer

• no always-on server
• arbitrary end systems can directly communicate
• all peers intermittently connected, can change IP

Processes communicating

• Process: program running on host
• within a host, two processes communicate using inter process communication, defined by the OS
• processes on different hosts communicate by exchanging messages
• Processes send and receive messages to/from its unique socket
• to receive messages, processes must have an identifier, a unique 32-bit IP address

Application layer protocol

• types of messages (request, response)
• message syntax
• message semantics
• rules for when and how processes send and respond

What transport service is needed?

• data integrity: must be reliable, or is some loss acceptable?
• timing: does it need low latency?
• throughput: does it need a minimum amount or can it be elastic?
• security: encryption, data integrity, etc

TCP

Transmission Control Protocol

• reliable transport
• flow control: sender won't overwhelm receiver
• congestion control
• Does not provide timing, throughput, security guarantees
• Connection-oriented: requires setup
• Adding security: use SSL
  • provides encrypted TCP connection
  • endpoint authentication
  • resides in the application layer, using SSL libraries that talk to TCP
  • cleartext passwords sent into socket traverse the internet encrypted

UDP

User Datagram Protocol

• Unreliable data transfer
• does not provide flow control, congestion control, timing, throughput, security, or connection setup
• Faster than TCP, no overhead

Web and HTTP

• Web is made of objects
• Web page consists of a base HTML file object which references other objects
• Each object is addressable by a URL (Uniform Resource Locator)
• Client: e.g. browser
• Server: web server sends objects in response to requests
• Stateless: server maintains no information about past client requests
• Uses TCP
• Connection types
  • Non persistent
    • At most one object sent over TCP connection, which is then closed
    • 2 RTT + transmission time
  • Persistent
    • Multiple objects sent over single TCP connection between client and server
    • RTT + 1 transmission time per object
• HTTP/1.0:
  • GET
  • POST
  • HEAD (gets header only, no response)
• HTTP/1.1:
  • PUT
  • DELETE
• Conditional GET: don't send obhect if cache has up-to-date version
  • Send If-modified0since: <date> in header
  • Server can send a 304 not modified if the cache is valid

DNS

• Provides mapping between address and name
• Distributed database implemented by hierarchy of many name servers
• Hosts resolve name (application layer protocol)
• Services
  • Hostname to IP translation
  • Host aliasing
  • Mail server aliasing
  • Load distribution
• Local DNS: acts as a local DNS cache, acting as a proxy
• Caching
  • Once a nameserver learns a mapping, it caches it
  • Caches time out after some time (TTL)
  • Cached entries may be out of date: won't fully propagate changes until TTL
• DNS records:
  • Format: (name, value, type, ttl)
  • A: name is a hostname, value is an IP address
  • NS: name is a domain (e.g. foo.com), value is hostname of authoritative name server for the domain
  • CNAME: name is an alias for some real name, value is the real (canonical) name
  • MX: value is the name of the mailserver associated with the name

Query and reply have the same format: