Computer Networks

Course Code: CS – 3001

Fall 2024 Semester
Offered to BCS22 – 5K, BDS21 – 7C
Course Instructor: Aftab Alam

Lecture 6
September 05, 2024 (Thursday)
Application layer: overview
 P2P applications
 Principles of network  video streaming and content
applications distribution networks
 Web and HTTP  socket programming with
 E-mail, SMTP, IMAP UDP and TCP
 The Domain Name System
DNS

Application Layer: 2-2

Application layer: overview
Our goals:  learn about protocols by
examining popular
 conceptual and application-layer protocols
implementation aspects of and infrastructure
application-layer protocols • HTTP
• transport-layer service • SMTP, IMAP
models • DNS
• client-server paradigm • video streaming systems, CDNs
• peer-to-peer paradigm  programming network
applications
• socket API

Application Layer: 2-3

Some network apps
 social networking  voice over IP (e.g., Skype)
 Web  real-time video conferencing
 text messaging (e.g., Zoom)
 e-mail  Internet search
 multi-user network games  remote login
 streaming stored video …
(YouTube, Hulu, Netflix)
 P2P file sharing Q: your favorites?

Application Layer: 2-4

Creating a network app
application
transport
write programs that: mobile network
network
data link
physical
 run on (different) end systems national or global ISP

 communicate over network

 e.g., web server software
communicates with browser software
local or
no need to write software for regional ISP

network-core devices home network content

application
 network-core devices do not run user transport
network
provider
network datacenter
application
applications data link
physical
transport
network
network

 applications on end systems allows data link

physical

for rapid app development, enterprise

propagation network

Application Layer: 2-5

Client-server paradigm
server: mobile network
 always-on host national or global ISP

 permanent IP address
 often in data centers, for scaling
clients: local or
regional ISP
 contact, communicate with server
 may be intermittently connected home network content
provider
 may have dynamic IP addresses network datacenter
network

 do not communicate directly with

each other
enterprise
 examples: HTTP, IMAP, FTP network

Application Layer: 2-6

Peer-peer architecture
 no always-on server mobile network
 arbitrary end systems directly national or global ISP

communicate
 peers request service from other
peers, provide service in return to
other peers local or
regional ISP
• self scalability – new peers bring new
service capacity, as well as new service home network content
demands provider
network datacenter

 peers are intermittently connected network

and change IP addresses

• complex management enterprise
 example: P2P file sharing
network

Application Layer: 2-7

Processes communicating
process: program running clients, servers
within a host client process: process that
initiates communication
within same host, two server process: process
processes communicate that waits to be contacted
using inter-process
communication (defined by
OS)  note: applications with
P2P architectures have
processes in different hosts client processes &
communicate by exchanging server processes
messages
Application Layer: 2-8
Sockets
 process sends/receives messages to/from its socket
 socket analogous to door
• sending process shoves message out door
• sending process relies on transport infrastructure on other side of
door to deliver message to socket at receiving process
• two sockets involved: one on each side

application application
socket controlled by
process process app developer

transport transport
network network controlled
link by OS
link Internet
physical physical

Application Layer: 2-9

Addressing processes
 to receive messages, process  identifier includes both IP address
must have identifier and port numbers associated with
 host device has unique 32-bit process on host.
IP address  example port numbers:
 Q: does IP address of host on • HTTP server: 80
which process runs suffice for • mail server: 25
identifying the process?  to send HTTP message to
 A: no, many processes gaia.cs.umass.edu web server:
can be running on • IP address: 128.119.245.12
same host • port number: 80
 more shortly…
Application Layer: 2-10
An application-layer protocol defines:
 types of messages exchanged, open protocols:
• e.g., request, response  defined in RFCs, everyone
 message syntax: has access to protocol
• what fields in messages & definition
how fields are delineated  allows for interoperability
 message semantics  e.g., HTTP, SMTP
• meaning of information in proprietary protocols:
fields
 e.g., Skype, Zoom
 rules for when and how
processes send & respond to
messages
Application Layer: 2-11
What transport service does an app need?
data integrity throughput
 some apps (e.g., file transfer,  some apps (e.g., multimedia)
web transactions) require require minimum amount of
100% reliable data transfer throughput to be “effective”
 other apps (e.g., audio) can  other apps (“elastic apps”)
tolerate some loss make use of whatever
throughput they get
timing
 some apps (e.g., Internet security
telephony, interactive games)  encryption, data integrity,
require low delay to be “effective” …
Application Layer: 2-12
 Transport service requirements: common apps
application data loss throughput time sensitive?

file transfer/download no loss elastic no

e-mail no loss elastic no
Web documents no loss elastic no
real-time audio/video loss-tolerant audio: 5Kbps-1Mbps yes, 10’s msec
video:10Kbps-5Mbps
streaming audio/video loss-tolerant same as above yes, few secs
interactive games loss-tolerant Kbps+ yes, 10’s msec
text messaging no loss elastic yes and no
Application Layer: 2-13
Internet transport protocols services
TCP service: UDP service:
 reliable transport between sending  unreliable data transfer
and receiving process between sending and receiving
 flow control: sender won’t process
overwhelm receiver  does not provide: reliability,
 congestion control: throttle sender flow control, congestion
when network overloaded control, timing, throughput
guarantee, security, or
 connection-oriented: setup required connection setup.
between client and server processes
 does not provide: timing, minimum Q: why bother? Why
throughput guarantee, security is there a UDP?
Application Layer: 2-14
 Internet applications, and transport protocols
application
application layer protocol transport protocol

file transfer/download FTP [RFC 959] TCP

e-mail SMTP [RFC 5321] TCP
Web documents HTTP 1.1 [RFC 7320] TCP
Internet telephony SIP [RFC 3261], RTP [RFC TCP or UDP
3550], or proprietary
streaming audio/video HTTP [RFC 7320], DASH TCP
interactive games WOW, FPS (proprietary) UDP or TCP

Application Layer: 2-15

Securing TCP
Vanilla TCP & UDP sockets: TSL implemented in
 no encryption application layer
 cleartext passwords sent into socket  apps use TSL libraries, that
traverse Internet in cleartext (!) use TCP in turn
Transport Layer Security (TLS)  cleartext sent into “socket”
 provides encrypted TCP connections traverse Internet encrypted
 data integrity  more: Chapter 8
 end-point authentication

Application Layer: 2-16