Transport Layer I

Question 1

Transport services and protocols

Answer 1

provide logical communication
between application processes
running on different hosts

transport protocols actions in end
systems:
* sender: breaks application messages
into segments, passes to network layer

• receiver: reassembles segments into
  messages, passes to application layer
  ▪two well known transport protocols
  available to Internet applications
  *TCP, UDP

Question 2

Transport vs. network layer services and protocols

Answer 2

transport layer:
communication between
processes
* relies on, enhances, network
layer services

network layer:
communication between
hosts

Question 3

Transport Layer Actions

Answer 3

Sender:
▪ is passed an application
layer message
▪ determines segment
header fields values
▪ creates segment
▪ passes segment to IP

Receiver:
▪extracts application-layer
message
▪checks header values
▪receives segment from IP
▪demultiplexes message up to application via socket

Question 4

Multiplexing/demultiplexing

Answer 4

multiplexing as sender:
handle data from multiple
sockets, add transport header (later used for demultiplexing)

demultiplexing as receiver:
use header info to deliver
received segments to correct sockets

Question 5

Connectionless demultiplexing

Answer 5

when receiving host receives
UDP segment:
* checks destination port # in segment
* directs UDP segment to
socket with that port #

IP/UDP datagrams with same dest. port #, but different source IP
addresses and/or source port numbers will be directed to same
socket at receiving host

Question 6

Two principal Internet transport protocols

Answer 6

▪TCP: Transmission Control Protocol
* reliable, in-order delivery
*congestion control
* flow control
*connection setup

UDP: User Datagram Protocol
* unreliable, unordered delivery
* no-frills extension of “best-effort” IP
▪services not available:
*delay guarantees
*bandwidth guarantees

Question 7

UDP: User Datagram Protocol

Answer 7

UDP: User Datagram Protocol
▪ “no frills,” “bare bones”
Internet transport protocol
▪ “best effort” service, UDP
segments may be:
* lost
* delivered out-of-order to app
▪ connectionless:
* no handshaking between UDP
sender, receiver→ Reduced RTT
* each UDP segment handled
independently of others

What it does:
Sends data from one device to another

Very simple and fast

🔹 What it does NOT do:
❌ No guaranteed delivery

❌ No error checking or correction

❌ No ordering (data may arrive out of order)

❌ No connection setup (sends data immediately)

Question 8

Connection-oriented demultiplexing

Answer 8

TCP socket identified by
4-tuple:
* source IP address
* source port number
* dest IP address
* dest port number

demux: receiver uses all
four values (4-tuple) to
direct segment to
appropriate socket
▪ server may support many
simultaneous TCP sockets:
* each socket identified by its
own 4-tuple
* each socket associated with
a different connecting client

Question 9

Internet checksum

Answer 9

Goal: detect errors (i.e., flipped bits) in transmitted segment

sender:
▪treat contents of UDP
segment (including UDP header
fields and IP addresses) as
sequence of 16-bit integers
▪checksum: addition (one’s
complement sum) of segment
content
▪checksum value put into
UDP checksum field

receiver:
▪compute checksum of received
segment
▪check if computed checksum equals
checksum field value:
* not equal - error detected