Communication Networks (5)

Internet transport protocols services

TCP service

• reliable transport
• flow control
  • sender won’t overwhelm receiver
• congestion control
  • throttle sender when network overloaded
• does not provide
  • timing
  • minimum throughput guarantee
  • security
• connection-oriented

UDP service

• unreliable data transfer between sending and receiving process
• does not provide
  • reliability
  • flow control
  • congestion control
  • timing
  • throughput guarantee
  • security
  • connection setup

Applications & transport protocols

Application	Application Layer Protocol	Underlying Transport Protocol
e-mail	SMTP [RFC 2821]	TCP
remote terminal access	Telnet [RFC 854], SSH	TCP
Web	HTTP [RFC 2616]	TCP
file transfer	FTP [RFC 959]	TCP
streaming multimedia	HTTP (e.g., YouTube), RTP [RFC 1889]	TCP or UDP
Internet telephony	SIP, RTP, proprietary (e.g., Skype)	TCP or UDP
naming	DNS	UDP (and TCP)

Socket programming with UDP

• no handshaking
• sender explicitly attaches IP destination address and port # to each packet
• receiver extracts sender IP address and port# from received packet

Socket programming with TCP

server must

• first be running
• have created socket (door) that welcomes client’s contact

client contacts server by:

• Creating TCP socket, specifying IP address, port number of server process

server TCP creates new socket

• for server process to communicate with that particular client

UDP

TCP

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Web and HTTP (hypertext transfer protocol)

• each object is addressable by a URL:

• use TCP on port 80
• HTTP is stateless: server maintains no information about past client requests

Non-persistent HTTP

RTT (Round Trip Time): time for a small packet to travel from client to server and back

non-persistent HTTP response time = 2RTT+ file transmission time

Persistent HTTP

server leaves connection open after sending response

Other Optimizations

Pipelining

Send several requests at once

HTTP/2

Push resources (send file ahead that clients may request)

QUIC

Eliminate first RTT

HTTP request message

two types of HTTP messages: request, response.

HTTP request message:

• GET method: input is in URL field.
• POST method: inpit is in entity body.
• HEAD method: asks server to leave requested object out of response
• (HTTP1.1) PUT method: uploads file in entity body to path specified in URL field
• (HTTP1.1) DELETE method: deletes file specified in the URL field

HTTP response message

code	message	meaning
200	OK	request succeeded,
301	Moved Permanently	requested object moved, new location specified later in this msg (Location:)
400	Bad Request	request msg not understood by server
404	Not Found	requested document not found on this server
505	HTTP Version Not Supported

User-server state: cookies

1. Cookie header line of HTTP response message
2. Cookie header line in next HTTP request message
3. Cookie file kept on user’s host, managed by user’s browser
4. Back-end database at Website

Server creats a unique ID as cookies and a entry in back-end database. Cookies can be used for:

• authorization
• shopping carts
• recommendations
• user session state (Web e-mail)