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UCLA CS 118 Winter 2021
Instructor: Giovanni Pau TAs:
Hunter Dellaverson Eric Newberry
Chapter 2 Application Layer
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Thanks and enjoy! JFK/KWR
All material copyright 1996-2016
J.F Kurose and K.W. Ross, All Rights Reserved
Computer Networking: A Top Down Approach
7th edition
Jim Kurose, Keith Ross Pearson/Addison Wesley April 2016
Application Layer 2-2
Application layer: overview
Principles of network applications
Web and HTTP
E-mail, SMTP, IMAP
The Domain Name System DNS
P2P applications
video streaming and content
distribution networks
socket programming with UDP and TCP
Application Layer: 2-3
Application layer: overview
Our goals:
conceptual and implementation aspects of application-layer protocols transport-layer service
models
client-server paradigm peer-to-peer paradigm
learn about protocols by examining popular application-layer protocols and infrastructure
HTTP
SMTP, IMAP
DNS
video streaming systems, CDNs
programming network applications
socket API
Application Layer: 2-4
34
1
Some network apps
social networking
Web
text messaging
e-mail
multi-user network games
streaming stored video
voice over IP (e.g., Skype)
real-time video conferencing
(e.g., Zoom)
Internet search remote login
Creating a network app
write programs that:
run on (different) end systems
communicate over network
e.g., web server software communicates with browser software
no need to write software for network-core devices
network-core devices do not run user applications
applicationsonendsystems allows for rapid app development, propagation
application
transport
mobile nentewtwork data link physical
national
or global ISP
content
provider
network
(YouTube, Hulu, Netflix) P2P file sharing
home network
application
transport network data link physical
enterprise network
datacenter
Q:yourfavorites? 56
Client-server paradigm
server:
always-on host
permanent IP address
often in data centers, for scaling
clients:
contact, communicate with server may be intermittently connected may have dynamic IP addresses do not communicate directly with
each other
examples: HTTP, IMAP, FTP
mobile network
Peer-peer architecture
no always-on server arbitrary end systems directly
communicate
peers request service from other
peers, provide service in return to other peers
self scalability new peers bring new service capacity, as well as new service demands
peers are intermittently connected and change IP addresses
complex management example: P2P file sharing
mobile network
78
home network
enterprise network
content provider network
datacenter network
home network
enterprise network
content provider network
datacenter network
local or regional ISP
national or global ISP
national
local or regional ISP
or global ISP
Application Layer: 2-5
Application Layer: 2-6
Application Layer: 2-7
Application Layer: 2-8
local or regional ISP
application
transport network data link physical
network
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2
9 10
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Processes communicating
process: program running within a host
within same host, two processes communicate using inter-process communication (defined by OS)
processes in different hosts communicate by exchanging messages
clients, servers
client process: process that
initiates communication
server process: process that waits to be contacted
note: applications with P2P architectures have client processes & server processes
Application Layer: 2-9
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
process
socket
Internet
controlled by app developer
controlled by OS
application
process
transport
transport
network
network
link physical
link physical
Application Layer: 2-10
Addressing processes
to receive messages, process must have identifier
host device has unique 32-bit IP address
Q: does IP address of host on which process runs suffice for identifying the process?
A: no, many processes
can be running on same host
identifier includes both IP address and port numbers associated with process on host.
example port numbers: HTTP server: 80
mail server: 25
to send HTTP message to gaia.cs.umass.edu web server:
IP address: 128.119.245.12 port number: 80
more shortly
Application Layer: 2-11
An application-layer protocol defines:
types of messages exchanged, e.g., request, response
message syntax:
what fields in messages &
how fields are delineated
message semantics
meaning of information in
fields
rules for when and how processes send & respond to messages
open protocols:
defined in RFCs, everyone has access to protocol definition
allows for interoperability e.g., HTTP, SMTP proprietary protocols: e.g., Skype, Zoom
Application Layer: 2-12
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3
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What transport service does an app need?
data integrity
some apps (e.g., file transfer, web transactions) require 100% reliable data transfer
other apps (e.g., audio) can tolerate some loss
timing
some apps (e.g., Internet telephony, interactive games) require low delay to be effective
throughput
some apps (e.g., multimedia) require minimum amount of throughput to be effective
other apps (elastic apps) make use of whatever throughput they get
security
encryption, data integrity,
Application Layer: 2-13
Transport service requirements: common apps
application
file transfer/download e-mail Web documents real-time audio/video
streaming audio/video interactive games text messaging
data loss
no loss
no loss
no loss loss-tolerant
loss-tolerant loss-tolerant no loss
throughput
elastic
elastic
elastic
audio: 5Kbps-1Mbps video:10Kbps-5Mbps same as above Kbps+
elastic
time sensitive?
no
no
no
yes, 10s msec
yes, few secs yes, 10s msec yes and no
Application Layer: 2-14
Internet transport protocols services
TCP service:
reliable transport between sending and receiving process
flow control: sender wont overwhelm receiver
congestion control: throttle sender when network overloaded
connection-oriented: setup required between client and server processes
does not provide: timing, minimum throughput guarantee, security
UDP service:
unreliable data transfer between sending and receiving process
does not provide: reliability, flow control, congestion control, timing, throughput guarantee, security, or connection setup.
Q: why bother? Why is there a UDP?
Application Layer: 2-15
Internet applications, and transport protocols
application
file transfer/download e-mail Web documents Internet telephony
streaming audio/video interactive games
application layer protocol
FTP [RFC 959]
SMTP [RFC 5321]
HTTP 1.1 [RFC 7320]
SIP [RFC 3261], RTP [RFC 3550], or proprietary HTTP [RFC 7320], DASH WOW, FPS (proprietary)
transport protocol
TCP TCP TCP
TCP or UDP
TCP UDP or TCP
Application Layer: 2-16
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Securing TCP
Vanilla TCP & UDP sockets:
no encryption
cleartext passwords sent into socket
traverse Internet in cleartext (!)
Transport Layer Security (TLS) provides encrypted TCP connections data integrity
end-point authentication
TSL implemented in application layer
apps use TSL libraries, that use TCP in turn
cleartext sent into socket traverse Internet encrypted
more: Chapter 8
Application Layer: 2-17
Application layer: overview
Principles of network applications
Web and HTTP
E-mail, SMTP, IMAP
The Domain Name System DNS
P2P applications
video streaming and content
distribution networks
socket programming with UDP and TCP
Application Layer: 2-18
17 18
Web and HTTP
First, a quick review
web page consists of objects, each of which can be stored on different Web servers
object can be HTML file, JPEG image, Java applet, audio file, web page consists of base HTML-file which includes several
referenced objects, each addressable by a URL, e.g., www.someschool.edu/someDept/pic.gif
host name
path name
Application Layer: 2-19
HTTP overview
HTTP: hypertext transfer protocol
Webs application-layer protocol client/server model:
client: browser that requests, receives, (using HTTP protocol) and displays Web objects
server: Web server sends (using HTTP protocol) objects in response to requests
PC running Firefox browser
iPhone running Safari browser
server running Apache Web server
Application Layer: 2-20
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5
HTTP request
HTTP response
HTTP request
HTTP response
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HTTP overview (continued)
HTTP uses TCP:
client initiates TCP connection (creates socket) to server, port 80
server accepts TCP connection from client
HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server)
TCP connection closed
HTTP is stateless server maintains no
information about past client requests
protocols that maintain state are complex!
past history (state) must be maintained
if server/client crashes, their views of state may be inconsistent, must be reconciled
Application Layer: 2-21
aside
HTTP connections: two types
Non-persistent HTTP
1. TCPconnectionopened 2. atmostoneobjectsent over TCP connection
3. TCPconnectionclosed
downloading multiple objects required multiple connections
Persistent HTTP
TCPconnectionopenedto a server
multiple objects can be sent over single TCP connection between client, and that server
TCP connection closed
Application Layer: 2-22
21 22
Non-persistent HTTP: example
User enters URL: www.someSchool.edu/someDepartment/home.index (containing text, references to 10 jpeg images)
1a. HTTP client initiates TCP connection to HTTP server (process) at www.someSchool.edu on port 80
2. HTTP client sends HTTP request message (containing URL) into TCP connection socket. Message indicates
that client wants object someDepartment/home.index
1b. HTTP server at host www.someSchool.edu waiting for TCP
connection at port 80 accepts connection, notifying client
3. HTTP server receives request message, forms response message containing
requested object, and sends message into its socket
Application Layer: 2-23
time
Non-persistent HTTP: example (cont.)
User enters URL: www.someSchool.edu/someDepartment/home.index (containing text, references to 10 jpeg images)
5. HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 referenced jpeg objects
6. Steps 1-5 repeated for each of 10 jpeg objects
4. HTTP server closes TCP connection.
time
Application Layer: 2-24
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Non-persistent HTTP: response time
RTT (definition): time for a small packet to travel from client to server and back
HTTP response time (per object):
one RTT to initiate TCP connection one RTT for HTTP request and first few
bytes of HTTP response to return obect/file transmission time
initiate TCP connection
RTT
request file
RTT
file received
time to transmit file
Non-persistent HTTP response time = 2RTT+ file transmission time
time time
Application Layer: 2-25
Persistent HTTP (HTTP 1.1)
Non-persistent HTTP issues:
requires 2 RTTs per object OS overhead for each TCP
connection
browsers often open multiple parallel TCP connections to fetch referenced objects in parallel
Persistent HTTP (HTTP1.1):
server leaves connection open after sending response
subsequent HTTP messages between same client/server sent over open connection
client sends requests as soon as it encounters a referenced object
as little as one RTT for all the referenced objects (cutting response time in half)
Application Layer: 2-26
HTTP request message
two types of HTTP messages: request, response HTTP request message:
ASCII (human-readable format)
carriage return character line-feed character
request line (GET, POST, HEAD commands)
header lines
carriage return, line feed
at start of line indicates end of header lines
GET /index.html HTTP/1.1r
Host: www-net.cs.umass.edur
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X
10.15; rv:80.0) Gecko/20100101 Firefox/80.0 r
Accept: text/html,application/xhtml+xmlr
Accept-Language: en-us,en;q=0.5r
Accept-Encoding: gzip,deflater
Connection: keep-aliver
r
* Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive/
Application Layer: 2-27
HTTP request message: general format
method
sp
URL
sp
version
cr
lf
header field name
value
cr
lf
~
~
~
request line
header lines
~ body
header field name
value
cr
lf
cr
lf
entity body
Application Layer: 2-28
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Other HTTP request messages
POST method:
web page often includes form input
user input sent from client to server in entity body of HTTP POST request message
GET method (for sending data to server):
include user data in URL field of HTTP GET request message (following a ?):
HEAD method:
requests headers (only) that would be returned if specified URL were requested with an HTTP GET method.
PUT method:
uploads new file (object) to server
completely replaces file that exists at specified URL with content in
entity body of POST HTTP request message
Application Layer: 2-29
www.somesite.com/animalsearch?monkeys&banana
HTTP response message
status line (protocol status code status phrase)
header lines
HTTP/1.1 200 OK
Date: Tue, 08 Sep 2020 00:53:20 GMT Server: Apache/2.4.6 (CentOS)
OpenSSL/1.0.2k-fips PHP/7.4.9
mod_perl/2.0.11 Perl/v5.16.3
Last-Modified: Tue, 01 Mar 2016 18:57:50 GMT
data, e.g., requested HTML file
ETag: a5b-52d015789ee9e
Accept-Ranges: bytes
Content-Length: 2651
Content-Type: text/html; charset=UTF-8
r
data data data data data
* Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/interactive/
Application Layer: 2-30
HTTP response status codes
status code appears in 1st line in server-to-client response message. some sample codes:
200 OK
request succeeded, requested object later in this message 301 Moved Permanently
requested object moved, new location specified later in this message (in Location: field)
400 Bad Request
request msg not understood by server 404 Not Found
requested document not found on this server 505 HTTP Version Not Supported
Application Layer: 2-31
Trying out HTTP (client side) for yourself
1. netcat to your favorite Web server:
% nc -c -v gaia.cs.umass.edu 80
2. type in a GET HTTP request:
opens TCP connection to port 80 (default HTTP server port) at gaia.cs.umass. edu.
anything typed in will be sent to port 80 at gaia.cs.umass.edu
GET /kurose_ross/interactive/index.php HTTP/1.1
Host: gaia.cs.umass.edu
by typing this in (hit carriage return twice), you send this minimal (but complete) GET request to HTTP server
3. look at response message sent by HTTP server!
(or use Wireshark to look at captured HTTP request/response)
Application Layer: 2-32
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Maintaining user/server state: cookies
Recall: HTTPGET/response interaction is stateless
no notion of multi-step exchanges of HTTP messages to complete a Web transaction
no need for client/server to track state of multi-step exchange
all HTTP requests are independent of each other
no need for client/server to recover from a partially-completed-but-never- completely-completed transaction
a stateful protocol: client makes two changes to X, or none at all
X
X
X
X
X
Q: what happens if network connection or client crashes at t ?
Application Layer: 2-33
t
time time
Maintaining user/server state: cookies
Web sites and client browser use Example:
cookies to maintain some state between transactions
four components:
1) cookie header line of HTTP response message
2) cookie header line in next HTTP request message
3) cookie file kept on users host, managed by users browser
4) back-end database at Web site
Susan uses browser on laptop, visits specific e-commerce site for first time
when initial HTTP requests arrives at site, site creates:
unique ID (aka cookie)
entry in backend database
for ID
subsequent HTTP requests from Susan to this site will contain cookie ID value, allowing site to identify
Susan Application Layer: 2-34
33 34
Maintaining user/server state: cookies
client
ebay 8734
server
usual HTTP request msg
Amazon server cookie file creates ID
backend database
Application Layer: 2-35
usual HTTP response
set-cookie: 1678
ebay 8734 amazon 1678
one week later:
1678 for user
cookie- specific action
cookie- specific action
create entry
access access
usual HTTP request msg
cookie: 1678
usual HTTP response msg
ebay 8734 amazon 1678
usual HTTP request msg
cookie: 1678
usual HTTP response msg
time time
HTTP cookies: comments
Whatcookiescanbeusedfor:
authorization
shopping carts
recommendations
user session state (Web e-mail)
Challenge: How to keep state?
at protocol endpoints: maintain state at sender/receiver over multiple transactions
in messages: cookies inHTTP messages carry state
cookiesandprivacy: aside cookies permit sites to
learn a lot about you on
their site.
third party persistent
cookies (tracking cookies) allow common identity (cookie value) to be
tracked across multiple web sites
Application Layer: 2-36
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OK OK
OK OK
lock data record X update X X
update X X unlock X
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Web caches
Goal: satisfy client requests without involving origin server
user configures browser to point to a (local) Web cache
browser sends all HTTP requests to cache
if object in cache: cache returns object to client
else cache requests object from origin server, caches received object, then returns object to client
client
client
Web cache
origin server
Application Layer: 2-37
Web caches (aka proxy servers)
Web cache acts as both client and server
server for original requesting client
client to origin server
server tells cache about objects allowable caching in response header:
Why Web caching?
reduce response time for client request
cache is closer to client
reduce traffic on an institutions
access link
Internet is dense with caches
enables poor content providers to more effectively deliver content
Application Layer: 2-38
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Caching example
Scenario:
access link rate: 1.54 Mbps
RTT from institutional router to server: 2 sec
web object size: 100K bits
average request rate from browsers to origin
servers: 15/sec
avg data rate to browsers: 1.50 Mbps
origin servers
1 Gbps LAN
Application Layer: 2-39
Performance:
access link utilization = .97
LAN utilization: .0015
public Internet
1.54 Mbps access link
problem: large queueing delays at high utilization!
institutional network
end-end delay = Internet delay +
access link delay + LAN delay
= 2 sec + minutes + usecs
Option 1: buy a faster access link
Scenario: 154 Mbps
access link rate: 1.54 Mbps
RTT from institutional router to server: 2 sec
web object size: 100K bits
average request rate from browsers to origin
servers: 15/sec
avg data rate to browsers: 1.50 Mbps
origin servers
154 Mbps
1.54 Mbps access link
1 Gbps LAN
Application Layer: 2-40
39 40
end-end delay = Internet delay +
access link delay + LAN delay
= 2 sec + minutes + usecs
Cost: faster access link (expensive!) msecs
public Internet
Performance:
access link utilization = .97
LAN utilization: .0015
.0097
institutional network
10
HTTP request
HTTP request
HTTP response
HTTP response
HTTP request
HTTP response
Option 2: install a web cache
Scenario:
access link rate: 1.54 Mbps
RTT from institutional router to server: 2 sec
web object size: 100K bits
average request rate from browsers to origin
servers: 15/sec
avg data rate to browsers: 1.50 Mbps
Cost: web cache (cheap!)
Performance:
public Internet
origin servers
LAN utilization: .?
access link utilization = ?
average end-end delay = ?
institutional network
1 Gbps LAN
local web cache
Application Layer: 2-41
1.54 Mbps access link
How to compute link utilization, delay?
Calculating access link utilization, end-end delay with cache:
suppose cache hit rate is 0.4:
40% requests served by cache, with low
(msec) delay
60% requests satisfied at origin
rate to browsers over access link =0.6*1.50Mbps = .9Mbps
accesslinkutilization=0.9/1.54=.58means low (msec) queueing delay at access link
average end-end delay:
= 0.6 * (delay from origin servers)
+ 0.4 * (delay when satisfied at cache) = 0.6 (2.01) + 0.4 (~msecs) = ~ 1.2 secs
public Internet
origin servers
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Conditional GET
Goal: dont send object if cache has up-to-date cached version
no object transmission delay (or use of network resources)
client: specify date of cached copy in HTTP request
If-modified-since:
server: response contains no object if cached copy is up-to-date:
HTTP/1.0 304 Not Modified
HTTP/2
client
server
object not modified before
object modified after
Application Layer: 2-43
Key goal: decreased delay in multi-object HTTP requests HTTP1.1: introduced multiple, pipelined GETs over single TCP
connection
server responds in-order (FCFS: first-come-first-served scheduling) to GET requests
with FCFS, small object may have to wait for transmission (head-of- line (HOL) blocking) behind large object(s)
HTTP request msg
If-modified-since:
HTTP response
HTTP/1.0
304 Not Modified
HTTP request msg
If-modified-since:
HTTP response
HTTP/1.0 200 OK
loss recovery (retransmitting lost TCP segments) stalls object transmission
institutional network
1.54 Mbps access link
1 Gbps LAN
local web cache
41
42
lower average end-end delay than with 154 Mbps link (and cheaper too!)
Application Layer: 2-42
Application Layer: 2-44
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HTTP/2
Key goal: decreased delay in multi-object HTTP requests HTTP/2: [RFC 7540, 2015] increased flexibility at server in sending
objects to client:
methods, status codes, most header fields unchanged from HTTP 1.1
transmission order of requested objects based on client-specified object priority (not necessarily FCFS)
push unrequested objects to client
divide objects into frames, schedule frames to mitigate HOL blocking
Application Layer: 2-45
HTTP/2: mitigating HOL blocking
HTTP 1.1: client requests 1 large object (e.g., video file) and 3 smaller
objects
server
GET O4
GET O3
GET O2
GET O1
object data requested
O1
O2 O3 O4
client
O1 O2 O3O4
objects delivered in order requested: O2, O3, O4 wait behind O1
Application Layer: 2-46
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HTTP/2: mitigating HOL blocking
HTTP/2: objects divided into frames, frame transmission interleaved
server
GET O4
GET O3
GET O2
GET O1
object data requested
client
O2 O4
O3
O1
O1
O2 O3 O4
O2, O3, O4 delivered quickly, O1 slightly delayed
Application Layer: 2-47
HTTP/2 to HTTP/3
HTTP/2 over single TCP connection means:
recovery from packet loss still stalls all object transmissions as in HTTP 1.1, browsers have incentive to open multiple parallel
TCP connections to reduce stalling, increase overall throughput
no security over vanilla TCP connection
HTTP/3: adds security, per object error- and congestion- control (more pipelining) over UDP
more on HTTP/3 in transport layer
Application Layer: 2-48
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E-mail
Three major components: mail
user agent
user agent
user agent
outgoing message queue
user mailbox
Application Layer: 2-50
user agents
mail servers
simple mail transfer protocol: SMTP
User Agent
server
SMTP
a.k.a. mail reader
composing, editing, reading mail messages e.g., Outlook, iPhone mail client outgoing, incoming messages stored on
server
mail server
user agent
SMTP SMTP
user agent
user agent
mail server
49 50
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Application layer: overview
Principles of network applications
Web and HTTP
E-mail, SMTP, IMAP
The Domain Name System DNS
P2P applications
video streaming and content
distribution networks
socket programming with UDP and TCP
Application Layer: 2-49
E-mail: mail servers
mail servers:
mailbox contains incoming messages for user
message queue of outgoing (to be sent) mail messages
SMTP protocol between mail servers to send email messages
client: sending mail server server: receiving mail server
mail server
SMTP
mail server
user agent
user agent
user agent
mail user server agent
user agent
SMTP SMTP
user agent
outgoing message queue
user mailbox
Application Layer: 2-51
SMTP RFC (5321)
uses TCP to reliably transfer email message
from client (mail server initiating
connection) to server, port 25 RTT
direct transfer: sending server (acting like client) TCP connection to receiving server initiated
three phases of transfer
SMTP handshaking (greeting) SMTP SMTP transfer of messages handshaking SMTP closure
server SMTP server
220
HELO 250 Hello
client SMTP server
initiate TCP connection
command/response interaction (like HTTP) commands: ASCII text
response: status code and phrase
SMTP transfers
time
Application Layer: 2-52
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Scenario: Alice sends e-mail to Bob
1) Alice uses UA to compose e-mail message to [email protected]
2) Alices UA sends message to her mail server using SMTP; message placed in message queue
3) client side of SMTP at mail server opens TCP connection with Bobs mail server
4) SMTP client sends Alices message over the TCP connection
1 user agent
mail server
5) Bobs mail server places the message in Bobs mailbox
6) Bob invokes his user agent to read message
mail server
2
346
Alices mail server
5
Bobs mail server
user agent
Application Layer: 2-53
Sample SMTP interaction
S: 220 hamburger.edu
C: HELO crepes.fr
S: 250 Hello crepes.fr, pleased to meet you
C: MAIL FROM:
S: 250 [email protected] Sender ok
C: RCPT TO:
S: 250 [email protected] Recipient ok
C: DATA
S: 354 Enter mail, end with . on a line by itself C: Do you like ketchup?
C: How about pickles?
C: .
S: 250 Message accepted for delivery
C: QUIT
S: 221 hamburger.edu closing connection
Application Layer: 2-54
SMTP: observations
comparison with HTTP:
HTTP: client pull
SMTP: client push
both have ASCII command/response interaction, status codes
HTTP: each object encapsulated in its own response message
SMTP: multiple objects sent in multipart message
SMTP uses persistent connections
SMTP requires message (header & body) to be in 7-bit ASCII
SMTP server uses CRLF.CRLF to determine end of message
Application Layer: 2-55
Mail message format
SMTP: protocol for exchanging e-mail messages, defined in RFC 5321 (like RFC 7231 defines HTTP)
RFC 2822 defines syntax for e-mail message itself (like HTML defines syntax for web documents)
header lines, e.g., To:
From:
Subject:
these lines, within the body of the email message area different from SMTP MAIL FROM:, RCPT TO: commands!
Body: the message , ASCII characters only
header
blank line
Application Layer: 2-56
body
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Retrieving email: mail access protocols
user agent
SMTP SMTP
e-mail access user
agent
SMTP: delivery/storage of e-mail messages to receivers server mail access protocol: retrieval from server
IMAP: Internet Mail Access Protocol [RFC 3501]: messages stored on server, IMAP provides retrieval, deletion, folders of stored messages on server
HTTP: gmail, Hotmail, Yahoo!Mail, etc. provides web-based interface on top of STMP (to send), IMAP (or POP) to retrieve e-mail messages
Application Layer: 2-57
protocol
(e.g., IMAP, HTTP)
senders e-mail receivers e-mail server server
Application Layer: Overview
Principles of network applications
Web and HTTP
E-mail, SMTP, IMAP
The Domain Name System DNS
P2P applications
video streaming and content
distribution networks
socket programming with UDP and TCP
Application Layer: 2-58
DNS: Domain Name System
people: many identifiers:
SSN, name, passport #
Internet hosts, routers:
IP address (32 bit) used for addressing datagrams
name, e.g., cs.umass.edu used by humans
Q: how to map between IP address and name, and vice versa ?
Domain Name System (DNS):
distributed database implemented in hierarchy of many name servers
application-layer protocol: hosts, DNS servers communicate to resolve names (address/name translation)
note: core Internet function, implemented as application-layer protocol
complexity at networks edge
Application Layer: 2-59
DNS: services, structure
DNS services:
hostname-to-IP-address translation host aliasing
canonical, alias names mail server aliasing
load distribution
replicated Web servers: many IP addresses correspond to one name
Q: Why not centralize DNS?
single point of failure traffic volume
distant centralized database maintenance
A: doesnt scale!
Comcast DNS servers alone:
600B DNS queries/day
Akamai DNS servers alone:
2.2T DNS queries/day
Application Layer: 2-60
59 60
15
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Thinking about the DNS
humongous distributed database: ~ billion records, each simple
handles many trillions of queries/day: many more reads than writes performance matters: almost every
Internet transaction interacts with DNS msecs count!
organizationally, physically decentralized:
millions of different organizations responsible for their records
bulletproof: reliability, security
Application Layer: 2-61
DNS: a distributed, hierarchical database
Root DNS Servers
.com DNS servers .org DNS servers .edu DNS servers
yahoo.com amazon.com pbs.org nyu.edu umass.edu DNS servers DNS servers DNS servers DNS servers DNS servers
Client wants IP address for www.amazon.com; 1st approximation:
client queries root server to find .com DNS server
client queries .com DNS server to get amazon.com DNS server
Root
Top Level Domain Authoritative
client queries amazon.com DNS server to get IP address for www.amazon.com Application Layer: 2-62
DNS: root name servers
official, contact-of-last-resort by name servers that can not resolve name
Application Layer: 2-63
DNS: root name servers
official, contact-of-last-resort by name servers that can not resolve name
incredibly important Internet function
Internet couldnt function without it!
DNSSEC provides security
(authentication, message integrity)
ICANN (Internet Corporation for Assigned Names and Numbers) manages root DNS domain
13 logical root name servers worldwide each server replicated many times (~200 servers in US)
Application Layer: 2-64
63 64
16
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Top-Level Domain, and authoritative servers
Top-Level Domain (TLD) servers:
responsible for .com, .org, .net, .edu, .aero, .jobs, .museums, and all top-level country domains, e.g.: .cn, .uk, .fr, .ca, .jp
Network Solutions: authoritative registry for .com, .net TLD Educause: .edu TLD
authoritative DNS servers:
organizations own DNS server(s), providing authoritative hostname to IP mappings for organizations named hosts
can be maintained by organization or service provider
Application Layer: 2-65
Local DNS name servers
when host makes DNS query, it is sent to its local DNS server Local DNS server returns reply, answering:
from its local cache of recent name-to-address translation pairs (possibly out of date!)
forwarding request into DNS hierarchy for resolution
each ISP has local DNS name server; to find yours:
MacOS: % scutil dns Windows: >ipconfig /all
local DNS server doesnt strictly belong to hierarchy
Application Layer: 2-66
65 66
DNS name resolution: iterated query
Example: host at engineering.nyu.edu wants IP address for gaia.cs.umass.edu
root DNS server
Iterated query:
contacted server replies with name of server to contact
I dont know this name, but ask this server
2
3 14
8 5
TLD DNS server
gaia.cs.umass.edu
requesting host at
engineering.nyu.edu
local DNS server
dns.nyu.edu
7
6
authoritative DNS server
dns.cs.umass.edu
Application Layer: 2-67
DNS name resolution: recursive query
Example: host at engineering.nyu.edu wants IP address for gaia.cs.umass.edu
root DNS server
3 7
4
Recursive query:
puts burden of name resolution on contacted name server
heavy load at upper levels of hierarchy?
2
6
requesting host at
engineering.nyu.edu
1 8
local DNS server
dns.nyu.edu
TLD DNS server
gaia.cs.umass.edu
authoritative DNS server
dns.cs.umass.edu
Application Layer: 2-68
5
67 68
17
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Caching DNS Information
once (any) name server learns mapping, it caches mapping, and immediately returns a cached mapping in response to a query
caching improves response time
cache entries timeout (disappear) after some time (TTL) TLD servers typically cached in local name servers
cached entries may be out-of-date
if named host changes IP address, may not be known Internet-
wide until all TTLs expire!
best-effort name-to-address translation!
Application Layer: 2-69
DNS records
type=A
name is hostname value is IP address
type=NS
type=CNAME
name is alias name for some canonical (the real) name
www.ibm.com is really servereast.backup2.ibm.com value is canonical name
DNS: distributed database storing resource records (RR) RR format: (name, value, type, ttl)
name is domain (e.g., foo.com) value is hostname of
authoritative name server for this domain
type=MX
value is name of SMTP mail server associated with name
Application Layer: 2-70
DNS protocol messages
DNS query and reply messages, both have same format:
2 bytes
2 bytes
identification
flags
# questions
# answer RRs
# authority RRs
# additional RRs
questions (variable # of questions)
answers (variable # of RRs)
authority (variable # of RRs)
additional info (variable # of RRs)
message header:
identification: 16 bit # for query,
reply to query uses same #
flags:
query or reply
recursion desired
recursion available
reply is authoritative
Application Layer: 2-71
DNS protocol messages
DNS query and reply messages, both have same format:
2 bytes
2 bytes
identification
flags
# questions
# answer RRs
# authority RRs
# additional RRs
questions (variable # of questions)
answers (variable # of RRs)
authority (variable # of RRs)
additional info (variable # of RRs)
name, type fields for a query RRs in response to query
records for authoritative servers
additional helpful info that may be used
Application Layer: 2-72
71 72
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Getting your info into the DNS
example: new startup Network Utopia
register name networkuptopia.com at DNS registrar (e.g., Network Solutions)
provide names, IP addresses of authoritative name server (primary and secondary)
registrar inserts NS, A RRs into .com TLD server: (networkutopia.com, dns1.networkutopia.com, NS) (dns1.networkutopia.com, 212.212.212.1, A)
create authoritative server locally with IP address 212.212.212.1 typeArecordforwww.networkuptopia.com
typeMXrecordfornetworkutopia.com
Application Layer: 2-73
DNS security
DDoS attacks
bombard root servers with traffic
not successful to date
traffic filtering
local DNS servers cache IPs of TLD
servers, allowing root server bypass
bombard TLD servers
potentially more dangerous
Spoofing attacks
intercept DNS queries, returning bogus replies
DNS cache poisoning
RFC 4033: DNSSEC authentication services
Application Layer: 2-74
73 74
Application Layer: Overview
Principles of network applications
Web and HTTP
E-mail, SMTP, IMAP
The Domain Name System DNS
P2P applications
video streaming and content
distribution networks
socket programming with UDP and TCP
Application Layer: 2-75
Peer-to-peer (P2P) architecture no always-on server
arbitrary end systems directly communicate
peers request service from other peers, provide service in return to other peers
self scalability new peers bring new
service capacity, and new service demands
peers are intermittently connected and change IP addresses
complex management
examples: P2P file sharing (BitTorrent), streaming (KanKan), VoIP (Skype)
mobile network
national or global ISP
local or regional ISP
home network
enterprise network
content provider network
da er ne
tacent twork
Application Layer: 2-76
75 76
19
File distribution: client-server vs P2P
Q: how much time to distribute file (size F) from one server to N peers?
peer upload/download capacity is limited resource us: server upload
capacity
us
di: peer i download capacity
di bandwidth) ui
ui: peer i upload capacity
file, size F
server
uN dN
u1 d1
u2 d2
network (with abundant
Introduction: 1-77
File distribution time: client-server
server transmission: must sequentially send (upload) N file copies:
time to send one copy: F/us time to send N copies: NF/us
client: each client must download file copy
dmin = min client download rate
min client download time: F/dmin
F us
network
di ui
time to distribute F
to N clients using Dc-s > max{NF/us,,F/dmin} client-server approach
increases linearly in N
Introduction: 1-78
77 78
1/5/21
File distribution time: P2P
server transmission: must upload at least one copy:
time to send one copy: F/us
client: each client must download file copy
F
us
max upload rate (limiting max download rate) is us + Sui
increases linearly in N
but so does this, as each peer brings service capacity
min client download time: F/dmin
clients: as aggregate must download NF bits
network di ui
time to distribute F to N clients using P2P approach
DP2P > max{F/us,,F/dmin,,NF/(us + Sui)}
Application Layer: 2-79
Client-server vs. P2P: example
client upload rate = u, F/u = 1 hour, us = 10u, dmin us 3.5
P2P
Client-Server
3 2.5 2 1.5 1 0.5 0
0 5 10 15 20 25 30 35 N
Application Layer: 2-80
79 80
20
Minimum Distribution Time
P2P file distribution: BitTorrent
file divided into 256Kb chunks
peers in torrent send/receive file chunks
tracker: tracks peers participating in torrent
Alice arrives
obtains list
of peers from tracker
and begins exchanging
file chunks with peers in torrent
torrent: group of peers exchanging chunks of a file
Application Layer: 2-81
P2P file distribution: BitTorrent
peer joining torrent:
has no chunks, but will accumulate them over time from other peers
registers with tracker to get list of peers, connects to subset of peers (neighbors)
while downloading, peer uploads chunks to other peers
peer may change peers with whom it exchanges chunks
churn: peers may come and go
once peer has entire file, it may (selfishly) leave or (altruistically) remain in torrent
Application Layer: 2-82
81 82
1/5/21
BitTorrent: requesting, sending file chunks
Requesting chunks:
at any given time, different peers have different subsets of file chunks
periodically, Alice asks each peer for list of chunks that they have
Alice requests missing chunks from peers, rarest first
Sending chunks: tit-for-tat
Alice sends chunks to those four peers currently sending her chunks at highest rate
other peers are choked by Alice (do not receive chunks from her)
re-evaluate top 4 every10 secs
every 30 secs: randomly select another peer, starts sending
chunks
optimistically unchoke this peer newly chosen peer may join top 4
Application Layer: 2-83
BitTorrent: tit-for-tat
(1) Alice optimistically unchokes Bob
(2) Alice becomes one of Bobs top-four providers; Bob reciprocates (3) Bob becomes one of Alices top-four providers
higher upload rate: find better trading partners, get file faster !
Application Layer: 2-84
83 84
21
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Application layer: overview
Principles of network applications
Web and HTTP
E-mail, SMTP, IMAP
The Domain Name System DNS
P2P applications
video streaming and content
distribution networks
socket programming with UDP and TCP
Application Layer: 2-85
Video Streaming and CDNs: context
stream video traffic: major
consumer of Internet bandwidth Netflix, YouTube, Amazon Prime: 80% of
residential ISP traffic (2020) challenge: scale-howtoreach
~1B users?
challenge: heterogeneity
different users have different capabilities (e.g., wired
versus mobile; bandwidth rich versus bandwidth poor)
solution: distributed, application-level infrastructure
Application Layer: 2-86
Multimedia: video
video: sequence of images displayed at constant rate
e.g., 24 images/sec digital image: array of pixels
each pixel represented by bits
coding: use redundancy within and between images to decrease # bits used to encode image
spatial (within image)
temporal (from one image to
next)
spatial coding example: instead of sending N values of same color (all purple), send only two values: color value (purple) and number of repeated values (N)
.. ..
frame i
temporal coding example:
instead of sending complete frame at i+1, send only differences from frame i
frame i+1
Application Layer: 2-87
Multimedia: video
CBR: (constant bit rate): video encoding rate fixed
VBR: (variable bit rate): video encoding rate changes as amount of spatial, temporal coding changes
examples:
MPEG 1 (CD-ROM) 1.5 Mbps
MPEG2 (DVD) 3-6 Mbps
MPEG4 (often used in Internet, 64Kbps12Mbps)
spatial coding example: instead of sending N values of same color (all purple), send only two values: color value (purple) and number of repeated values (N)
.. ..
frame i
temporal coding example:
instead of sending complete frame at i+1, sendonlydifferencesfrom frame i
framei+1
Application Layer: 2-88
87 88
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Streaming stored video
simple scenario:
Internet
video server (stored video)
Main challenges:
client
server-to-client bandwidth will vary over time, with changing network congestion levels (in house, access network, network core, video server)
packet loss, delay due to congestion will delay playout, or result in
poor video quality
Application Layer: 2-89
Streaming stored video
1. video recorded (e.g., 30 frames/sec)
2. video sent
3.videoreceived,playedoutatclient (30 frames/sec)
network delay (fixed in this example)
time
streaming: at this time, client playing out early part of video, while server still sending later part of video
Application Layer: 2-90
Streaming stored video: challenges
continuous playout constraint: during client video playout, playout timing must match original timing
but network delays are variable (jitter), so will need client-side buffer to match continuous playout constraint
other challenges:
client interactivity: pause, fast-forward, rewind, jump through video
video packets may be lost, retransmitted
Application Layer: 2-91
Streaming stored video: playout buffering
constant bit rate video transmission
variable
client video reception
constant bit rate video
playout at client
network delay
client playout delay
time
client-side buffering and playout delay: compensate for network-added delay, delay jitter
Application Layer: 2-92
91 92
23
Cumulative data
Cumulative data
buffered video
93 94
1/5/21
Streaming multimedia: DASH
server:
divides video file into multiple chunks
each chunk encoded at multiple different rates different rate encodings stored in different files
files replicated in various CDN nodes
manifest file: provides URLs for different chunks
client:
periodically estimates server-to-client bandwidth consulting manifest, requests one chunk at a time
?
client
Application Layer: 2-93
Dynamic, Adaptive Streaming over HTTP
chooses maximum coding rate sustainable given current bandwidth
can choose different coding rates at different points in time (depending on available bandwidth at time), and from different servers
Streaming multimedia: DASH
intelligence at client: client determines
when to request chunk (so that buffer starvation, or overflow does not occur)
what encoding rate to request (higher quality when more bandwidth available)
where to request chunk (can request from URL server that is close to client or has high available bandwidth)
?
client
Streaming video = encoding + DASH + playout buffering
Application Layer: 2-94
Content distribution networks (CDNs)
challenge: how to stream content (selected from millions of videos) to hundreds of thousands of simultaneous users?
option 1: single, large mega- server
single point of failure
point of network congestion
long (and possibly congested) path to distant clients
.quite simply: this solution doesnt scale
Application Layer: 2-95
Content distribution networks (CDNs)
challenge: how to stream content (selected from millions of videos) to hundreds of thousands of simultaneous users?
option 2: store/serve multiple copies of videos at multiple geographically distributed sites (CDN)
enter deep: push CDN servers deep into many access networks close to users
Akamai: 240,000 servers deployed
in > 120 countries (2015)
bring home: smaller number (10s) of
larger clusters in POPs near access nets used by Limelight
Application Layer: 2-96
95 96
24
Content distribution networks (CDNs)
CDN: stores copies of content (e.g. MADMEN) at CDN nodes
subscriber requests content, service provider returns manifest using manifest, client retrieves content at highest supportable rate
may choose different rate or copy if network path congested
Content distribution networks (CDNs)
OTT: over the top
Internet host-host communication as a service
OTT challenges: coping with a congested Internet from the edge what content to place in which CDN node?
from which CDN node to retrieve content? At which rate?
manifest file
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98
99
100
wheres Madmen?
Application Layer: Overview
Socket programming
goal: learn how to build client/server applications that communicate using sockets
Principles of network applications
Web and HTTP E-mail, SMTP, IMAP
The Domain Name System DNS
P2P applications
video streaming and content distribution networks
socket programming with UDP and TCP
Application Layer: 2-99
socket: door between application process and end-end-transport protocol
Application Layer: 2-97
Application Layer: 2-98
application
process
transport network link physical
socket
Internet
application
process
transport network link physical
controlled by app developer
controlled by OS
Application Layer: 2-100
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Socket programming
Two socket types for two transport services:
UDP: unreliable datagram
TCP: reliable, byte stream-oriented
Application Example:
1. client reads a line of characters (data) from its keyboard and sends data to server
2. server receives the data and converts characters to uppercase
3. server sends modified data to client
4. client receives modified data and displays line on its screen
Application Layer: 2-101
Socket programming with UDP
UDP: no connection between client and server:
no handshaking before sending data
sender explicitly attaches IP destination
address and port # to each packet
receiver extracts sender IP address and
port# from received packet
UDP: transmitted data may be lost or received out-of-order
Application viewpoint:
UDP provides unreliable transfer of groups of bytes (datagrams)
between client and server processes
Application Layer: 2-102
101 102
Client/server socket interaction: UDP
server (running on serverIP)
create socket, port= x:
serverSocket = socket(AF_INET,SOCK_DGRAM)
read datagram from
serverSocket
write reply to
serverSocket
specifying client address, port number
client
create socket:
clientSocket = socket(AF_INET,SOCK_DGRAM)
Create datagram with serverIP address And port=x; send datagram via clientSocket
read datagram from
clientSocket
close
clientSocket
Application Layer: 2-103
Example app: UDP client
include Pythons socket library
Python UDPClient
from socket import *
serverName = hostname
serverPort = 12000
create UDP socket for server clientSocket = socket(AF_INET,
SOCK_DGRAM)
get user keyboard input message = raw_input(Input lowercase sentence:)
attach server name, port to message; send into socket read reply characters from socket into string print out received string and close socket
clientSocket.sendto(message.encode(), (serverName, serverPort))
modifiedMessage, serverAddress = clientSocket.recvfrom(2048)
print modifiedMessage.decode() clientSocket.close()
Application Layer: 2-104
103 104
26
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Example app: UDP server
create UDP socket bind socket to local port number 12000
loop forever
Read from UDP socket into message, getting clients address (client IP and port)
send upper case string back to this client
Python UDPServer
from socket import *
serverPort = 12000
serverSocket = socket(AF_INET, SOCK_DGRAM) serverSocket.bind((, serverPort))
print (The server is ready to receive)
while True:
message, clientAddress = serverSocket.recvfrom(2048) modifiedMessage = message.decode().upper() serverSocket.sendto(modifiedMessage.encode(),
clientAddress)
Application Layer: 2-105
Socket programming with TCP
Client must contact server
server process must first be running
server must have created socket (door) that welcomes clients contact
Client contacts server by:
Creating TCP socket, specifying IP address, port number of server process
when client creates socket: client
TCP establishes connection to server TCP
when contacted by client, server TCP creates new socket for server process to communicate with that particular client
allows server to talk with multiple clients
source port numbers used to distinguish clients (more in Chap 3)
Application viewpoint
TCP provides reliable, in-order byte-stream transfer (pipe) between client and server processes
Application Layer: 2-106
105 106
Client/server socket interaction: TCP
server (running on hostid)
create socket,
port=x, for incoming request:
serverSocket = socket()
wait for incoming connection request connectionSocket = serverSocket.accept()
read request from
connectionSocket write reply to
connectionSocket
close
connectionSocket
client
TCP connection setup
create socket,
connect to hostid, port=x clientSocket = socket()
send request using
clientSocket
read reply from
clientSocket
close
clientSocket
Application Layer: 2-107
Example app: TCP client
Python TCPClient
from socket import * serverName = servername serverPort = 12000
create TCP socket for server, clientSocket = socket(AF_INET, SOCK_STREAM)
remote port 12000
clientSocket.connect((serverName,serverPort)) sentence = raw_input(Input lowercase sentence:) clientSocket.send(sentence.encode())
No need to attach server name, port modifiedSentence = clientSocket.recv(1024)
print (From Server:, modifiedSentence.decode())
clientSocket.close()
Application Layer: 2-108
107 108
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Example app: TCP server
create TCP welcoming socket
server begins listening for incoming TCP requests
loop forever
server waits on accept() for incoming requests, new socket created on return
read bytes from socket (but not address as in UDP)
close connection to this client (but not welcoming socket)
Python TCPServer
from socket import *
serverPort = 12000
serverSocket = socket(AF_INET,SOCK_STREAM) serverSocket.bind((,serverPort)) serverSocket.listen(1)
print The server is ready to receive
while True:
connectionSocket, addr = serverSocket.accept()
sentence = connectionSocket.recv(1024).decode() capitalizedSentence = sentence.upper() connectionSocket.send(capitalizedSentence.
connectionSocket.close()
encode())
Application Layer: 2-109
Chapter 2: Summary
our study of network application layer is now complete!
application architectures client-server
P2P
application service requirements:
reliability, bandwidth, delay
Internet transport service model
connection-oriented, reliable: TCP unreliable, datagrams: UDP
specific protocols: HTTP
SMTP, IMAP
DNS
P2P: BitTorrent
video streaming, CDNs socket programming:
TCP, UDP sockets
Application Layer: 2-110
Chapter 2: Summary
Most importantly: learned about protocols!
typical request/reply message exchange:
client requests info or service
server responds with data, status code
message formats:
headers: fields giving info about data
data: info(payload) being communicated
important themes:
centralized vs. decentralized stateless vs. stateful
scalability
reliable vs. unreliable
message transfer complexity at network
edge
Application Layer: 2-111
Additional Chapter 2 slides
Application Layer: 2-112
111 112
28
CDN content access: a closer look
Bob (client) requests video http://netcinema.com/6Y7B23V video stored in CDN at http://KingCDN.com/NetC6y&B23V
1. Bob gets URL for video http://netcinema.com/6Y7B23V from netcinema.com web page
2. resolve http://netcinema.com/6Y7B23V via Bobs local DNS
Bobs local DNS server
4
KingCDN authoritative DNS
1
2 5
6. request video from KINGCDN server, streamed via HTTP
3. netcinemas DNS returns CNAME for http://KingCDN.com/NetC6y&B23V
3
netcinemas
authoratative DNS KingCDN.com
netcinema.com
Application Layer: 2-114
Case study: Netflix
Netflix registration, accounting servers
1
Bob manages Netflix account
upload copies of multiple versions of video to CDN servers
Manifest file,
requested
3 returned for
specific video
4
DASH server selected, contacted, streaming begins
1/5/21
Sample SMTP interaction
S: 220 hamburger.edu
C: HELO crepes.fr
S: 250 Hello crepes.fr, pleased to meet you
C: MAIL FROM:
S: 250 [email protected] Sender ok
C: RCPT TO:
S: 250 [email protected] Recipient ok
C: DATA
S: 354 Enter mail, end with . on a line by itself C: Do you like ketchup?
C: How about pickles?
C: .
S: 250 Message accepted for delivery
C: QUIT
S: 221 hamburger.edu closing connection
Application Layer: 2-113
113
114
Amazon cloud
CDN server
Bob browses Netflix video
CDN server
CDN server
2
115
Application Layer: 2-115
29
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