Networking Exam 2

How are applications logically connected

By the transport layer

Data packets in the transport layer

segments

(sometimes called datagrams for UDP only)

Where is the transport layer implemented

on the host systems, not routers/switches

Services from the transport layer to the application layer

logical connection

services from the network layer to the transport layer

IP

UDP

User Datagram Protocol

TCP

Transmission Control Protocol

IP

internet protocol

best-effort delivery (unreliable)

Multiplexing

Combining data into one segment

Demultiplexing

separating data in a segment to deliver data to its sockets/ports

TCP common uses

email, remote access, web, file transfer

UDP common sources

remote file server, network manage, DNS, routing

Error detection

checksum

Checksum definition

attach the sum of all the data’s bits

if it doesn’t match, something was lost — resend

Receiver feedback

acknowledgements

Acknowledgements definition

when you get a packet, send back OK

Negative Acknowledgement

NOT used by TCP

acknowledging when a packet is not received

Sequence numbers

denotes order of packets

also identifies duplicate packets sent for redundancy

Time Out

if things are quiet for too long, assume all is lost and resend

Time Out Doubling

after the first timeout, wait twice the time for the next

Pipelining

sending many packets without waiting for each acknowledgement

acknowledgments use sequence numbers now

and they stop at the packet number before any were lost (GBN)

Go Back N size

as many packets as you pipeline at once

“N”

Go Back N function

when a packet is missed, you resend all packets in the buffer starting with that packet (“the nth packet”)

3 events for a GBN sender to respond to

invocation from above (app layer calls (rdt_send())

receipt of an ACK

timeout

Selective repeat

needs acknowledgements for all packets

only resends lost/corrupted ones

Selective repeat window meaning

how many packets can be sent before dealing with lost/corrupted ones

3 sender response events SR

Data received from above

timeout

ACK received

3 receiver response events SR

packet with number in the window is received (send ACK)

packet with number before the window is received (send ACK)

else, ignore packet

why is TCP connection oriented

the handshake

full duplex service

both parties establish a channel and communicate simultaneously

TCP ✅

Multicasting

one sender talks to many receivers

TCP ❌

Three way handshake

Client sends special segment

Server responds with a special segment

Client sends another special segment (has payload)

MSS

max segment size

max data than can be in a segment

MTU

maximum transmission unit

the largest segment + header (frame) that can be sent

RTT

round trip time

from send to receive ACK

estimated rtt

estimated based on the last actual RTT and the last estimated RTT

updates every packet

used for making time outs

Timeout interval

based on estimated RTT and deviation of RTT

flow control

speed matches how fast data can be read out of the receive buffer with how fast data is sent to the receiver

provided by the window (info about it is sent to sender)

3 nmap returns

SYN ACK - allowed and app running

TCP RST - allowed but no app running

Nothing - firewall blocked it