CSIS 350 Data comms

TCP service

Reliable, flow control, congestion control, connection - oriented •but bigger than UDP so slower

UDP service

Unreliable, no connection set up, no guarantees. But smaller so faster

Stateless

Server keeps no info about past clients requests

Nonpersistent HTTP

A separate TCP connection is established for each requested object (HTML file or image). One connection closes after the object is transferred • requires approximately 2 RTTS and file transmission time per object

Cookies

State kept across http transactions I set cookie header in response cookie header in next request, cookie file on the user's host, back-end database at the site

Cookies

State kept across http transactions I set cookie header in response cookie header in next request, cookie file on the user's host, back-end database at the site

DNS

Distributed, hierarical database mapping names to IPs. Records: A ( hosts → IP).NS( domain → authoritative server), name calls → canonical name ),mx (mail server)

Stop and wait protocol

A protocol where the sender transmits one packet and then pauses waiting fora receiver acknowledgement before sending the next packet - when if packet is lost then waits for time out to retransmit packet

1. Application layer

Provides network services to applications and defines how application processes send and receive messages. Ex: HTTP, DNS, SMTP, IMAP. Addresses through domain names, URLs, and application specific identifiers where applicable

2. Transport layer

Provides logical communication between application processes running on different hosts. TCP offers reliable delivery, control, ordered delivery, and congestion control. UDP provides connectionless delivery. Addresses by port numbers

Transport protocol actions in end systems

Sender: breaks application messages into segments, basses to network layer Receiver: reassembles segments into messages passes to application layer

3. Network layer

Delivers packets between hosts across networks through forwarding and routing • addresses by IP addresses. Logical communication between hosts.

4. Link layer

Transfers frames between neighboring nodes on the same link handing frame and link level error detection

5. Physical layer

Transmits individual bits as electrical, optical, or radio signals across physical media

Multiplexing at sender

Handle data from multiple sockets, add transport header later used for demultiplexing)

Demultiplexing at receiver

Use header info to deliver received segments to correct socket

How demultiplexing works

Host receives ip datagrams • each datagram has source and destination IP addresses. Each datagram carries one transport layer segment • each segment has source, destination past number • host uses IP address and port number to direct segment to appropriate socket

Connectionless demultiplexing

Datagram to upp socket needs destination I p address and destination port. Datagrams with same destination part numbers but different source IP or port numbers will be directed to the same socket.

Connection oriented demultiplexing

Identified by all 4: source IP, source port, destination IP, destination port number . Needs all 4 values to direct segment to appropriate socket

Stop and wait protocol (rat 3.0)

The sender sends one packet and waits for its acknowledgement before sending the next packet. Uses alternating sequences Co and 1) •reliable because sender doesn't advance until acknowledged. When packet is lost then times out and waits to resend until successfully acknowledged. Inefficient because waiting and time outs can be slow and can only have one outstanding packet,

Go back N

Allows multiple packets to be sent before ack returns (called pipelining). N is max number of unacknowledged packets allowed in the sender's window. Uses timer for the oldest outstanding packet. Receiver discards out of order packets and repeats the ack for the last correctly received in order packet. Time out retransmits the oldest unacknowledged packet and al subsequent unacknowledged packets in the window.

Selective repeat ( r dt)

Sender can nave multiple packets sent, but receiver can accept and buffer out of order packets. Sender retransmits only the specific packet that needs retransmission