Chapter 2 Application Layer
Computer
Networking: A Top
Down Approach
6th edition
Jim Kurose, Keith Ross Addison- Wesley March 2012
Application Layer 2-1
Chapter 2: outline
2.1 principles of network applications
2.2 Web and HTTP
2.3 FTP
2.4 electronic mail
SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 socket programming with UDP and TCP
Application Layer 2-2
Chapter 2: application layer
our goals:
conceptual, implementation aspects of network application protocols
transport-layer service models
client-server paradigm
peer-to-peer paradigm
learn about protocols by examining popular application-level protocols
HTTP
FTP
SMTP / POP3 / IMAP DNS
creating network applications
socket API
Application Layer 2-3
Some network apps
e-mail
web
text messaging
remote login
P2P file sharing
multi-user network games
streaming stored video (YouTube, Hulu, Netflix)
voice over IP (e.g., Skype) real-time video
conferencing
social networking search
…
…
Application Layer 2-4
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
applications on end systems allows for rapid app development, propagation
application
transport
network
data link
physical
application
transport
network
application
transport
network
data link physical
data link
physical
Application Layer 2-5
Application architectures
possible structure of applications:
client-server
peer-to-peer (P2P)
Application Layer 2-6
Client-server architecture
client/server
server:
always-on host
permanent IP address data centers for scaling
clients:
communicate with server may be intermittently
connected
may have dynamic IP addresses
do not communicate directly with each other
Application Layer 2-7
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, as well as new service demands
peers are intermittently connected and change IP addresses
complex management
peer-peer
Application Layer 2-8
Processes communicating
process: program running within a host
within same host, two processes communicate using inter-process communication (defined by OS)
clients, servers
client process: process that initiates communication
server process: process that waits to be contacted
processes in different hosts
communicate by exchanging aside: applications with P2P
messages
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
application
process
transport
network
link
physical
application
process
transport
network
link
physical
socket
Internet
controlled by app developer
controlled by OS
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
App-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
allows for interoperability e.g., HTTP, SMTP proprietary protocols:
e.g., Skype
Application Layer 2-12
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 e-mail Web documents real-time audio/video
stored 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
few kbps up elastic
time sensitive
no
no
no
yes, 100’s msec
yes, few secs yes, 100’s msec
yes and no
Application Layer 2-14
Internet transport protocols services
TCP service:
reliable transport between sending and receiving process
flow control: sender won’t overwhelm receiver
congestion control: throttle sender when network overloaded
does not provide: timing, minimum throughput guarantee, security
connection-oriented: setup required between client and server processes
UDP service:
unreliable data transfer between sending and receiving process
does not provide: reliability, flow control, congestion control, timing, throughput guarantee, security, orconnection setup,
Q: why bother? Why is there a UDP?
Application Layer 2-15
Internet apps: application, transport protocols
application
e-mail remote terminal access
Web file transfer streaming multimedia
Internet telephony
application layer protocol
SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959]
HTTP (e.g., YouTube), RTP [RFC 1889]
SIP, RTP, proprietary (e.g., Skype)
underlying transport protocol
TCP
TCP
TCP
TCP
TCP or UDP
TCP or UDP
Application Layer 2-16
Securing TCP
TCP & UDP
no encryption
cleartext passwds sent into socket traverse Internet in cleartext
SSL/TLS
provides encrypted TCP connection
data integrity
end-point authentication
SSL/TLS is at app layer
Apps use SSL libraries, which “talk” to TCP
SSL socket API
cleartext passwds sent into socket traverse Internet encrypted
TLS is a newer version of SSL, it still operates in the application layer
Application Layer 2-17
Regular Web Connection
Application Layer 2-18
SSL/TLS Web Connection
Application Layer 2-19
Chapter 2: outline
2.1 principles of network applications
app architectures app requirements
2.2 Web and HTTP
2.3 FTP
2.4 electronic mail
SMTP, POP3, IMAP 2.5 DNS
2.6 P2P applications
2.7 socket programming with UDP and TCP
Application Layer 2-20
Web and HTTP
First, a review…
web page consists of objects
object can be HTML file, JPEG image, Java applet,
audio file,…
web page consists of base HTML-file which
includes several referenced objects
each object is addressable by a URL, e.g.,
www.someschool.edu/someDept/pic.gif
host name
path name
Application Layer 2-21
HTTP overview
HTTP: hypertext transfer protocol
Web’s 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
server running
Apache Web server
iphone running Safari browser
Application Layer 2-22
HTTP overview (continued)
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
aside
past history (state) must be maintained
if server/client crashes, their views of “state” may be inconsistent, must be reconciled
protocols that maintain “state” are complex!
Application Layer 2-23
HTTP connections
non-persistent HTTP
at most one object sent over TCP connection
connection then closed
downloading multiple objects required multiple connections
persistent HTTP
multiple objects can be sent over single TCP connection between client, server
Application Layer 2-24
Non-persistent HTTP
suppose user enters URL:
www.someSchool.edu/someDepartment/home.index
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
(contains text, references to 10
jpeg images)
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
time
Application Layer 2-25
Non-persistent HTTP (cont.)
5. HTTP client receives response message containing html file, displayshtml. Parsinghtmlfile, finds 10 referenced jpeg objects
time
4. HTTP server closes TCP connection.
6. Steps 1-5 repeated for each of 10 jpeg objects
Application Layer 2-26
Non-persistent HTTP: response time
RTT (definition): time for a small packet to travel from client to server and back
HTTP response time:
one RTT to initiate TCP connection
one RTT for HTTP request and first few bytes of HTTP response to return
initiate TCP connection
RTT
request file
RTT
time to transmit file
time
file transmission time file
non-persistent HTTP response time =
2RTT+ file transmission time
received
time
Application Layer 2-27
Persistent HTTP
non-persistent HTTP issues:
requires 2 RTTs per object
OS overhead for each TCP
connection
browsers often open parallel TCP connections to fetch referenced objects
persistent HTTP:
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
Application Layer 2-28
HTTP request message
two types of HTTP messages: request, response HTTP request message:
ASCII (human-readable format)
request line
(GET, POST, HEAD commands)
header lines
carriage return, line feed at start
of line indicates end of header lines
carriage return character line-feed character
GET /index.html HTTP/1.1\r\n
Host: www-net.cs.umass.edu\r\n
User-Agent: Firefox/3.6.10\r\n
Accept: text/html,application/xhtml+xml\r\n Accept-Language: en-us,en;q=0.5\r\n Accept-Encoding: gzip,deflate\r\n Accept-Charset: ISO-8859-1,utf-8;q=0.7\r\n Keep-Alive: 115\r\n
Connection: keep-alive\r\n \r\n
Application Layer 2-29
HTTP request message: general format
method
sp
URL
sp
version
cr
lf
header field name
value
cr
lf
~~ ~~
request line
header lines
header field name
value
cr
lf
cr
lf
~ entity body ~~
body
~
Application Layer 2-30
Uploading form input
POST method:
web page often includes form input
input is uploaded to server in entity body
URL method:
uses GET method
input is uploaded in URL
field of request line:
www.somesite.com/animalsearch?monkeys&banana
Application Layer 2-31
Method types
HTTP/1.0:
GET POST HEAD
asks server to leave requested object out of response
HTTP/1.1:
GET, POST, HEAD PUT
uploads file in entity body to path specified in URL field
DELETE
deletes file specified in the URL field
Application Layer 2-32
HTTP response message
status line (protocol status code status phrase)
header lines
HTTP/1.1 200 OK\r\n
Date: Sun, 26 Sep 2010 20:09:20 GMT\r\n Server: Apache/2.0.52 (CentOS)\r\n Last-Modified: Tue, 30 Oct 2007 17:00:02
GMT\r\n
ETag: “17dc6-a5c-bf716880″\r\n Accept-Ranges: bytes\r\n
Content-Length: 2652\r\n
Keep-Alive: timeout=10, max=100\r\n Connection: Keep-Alive\r\n
Content-Type: text/html; charset=ISO-8859-
1\r\n \r\n
data data data data data …
data, e.g., requested HTML file
Application Layer 2-33
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 msg 301 Moved Permanently
requested object moved, new location specified later in this msg (Location:)
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 2-34
Trying out HTTP (client side) for yourself
1. Telnet to your favorite Web server:
telnet cis.poly.edu 80
opens TCP connection to port 80
(default HTTP server port) at cis.poly.edu. anything typed in sent
to port 80 at cis.poly.edu
2. type in a GET HTTP request:
GET /~ross/ HTTP/1.1
Host: cis.poly.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 2-35
User-server state: cookies
example:
Susan always access Internet from PC
many Web sites use cookies
four components:
1) cookie header line of HTTP response message
2) cookie header line in next HTTP request message
3) cookie file kept on user’s host, managed by user’s browser
4) back-end database at Web site
visits specific e-commerce site for first time
when initial HTTP requests arrives at site, site creates:
unique ID
entry in backend database for ID
Application Layer 2-36
Cookies: keeping “state” (cont.)
client
ebay 8734
cookie file
ebay 8734 amazon 1678
server
Amazon server creates ID 1678 for user
cookie- specific action
access
cookie- specific action
usual http request msg
usual http response
set-cookie: 1678
backend entry database
access
create
usual http request msg
cookie: 1678
usual http response msg
one week later:
ebay 8734 amazon 1678
usual http request msg
cookie: 1678
usual http response msg
Application Layer 2-37
Cookies (continued)
what cookies can be used for:
authorization
shopping carts
recommendations
user session state (Web e-mail)
how to keep “state”:
protocol endpoints: maintain state at sender/receiver over multiple transactions
cookies: http messages carry state
cookies and privacy: aside cookies permit sites to
learn a lot about you
you may supply name and e-mail to sites
Application Layer 2-38
Web caches (proxy server)
goal: satisfy client request without involving origin server user sets browser: Web
accesses via cache
browser sends all HTTP requests to cache
object in cache: cache returns object
else cache requests object from origin server, then returns object to client
proxy server
client
origin server
client
origin server
Application Layer 2-39
More about Web caching
cache acts as both client and server
server for original requesting client
client to origin server
typically cache is installed by ISP (university, company, residential ISP)
why Web caching?
reduce response time for client request
reduce traffic on an institution’s access link
Internet dense with caches: enables “poor” content providers to effectively deliver content (so too does P2P file sharing)
Application Layer 2-40
Caching example:
assumptions:
avg object size: 100K bits
avg request rate from browsers to
origin servers:15/sec
avg data rate to browsers: 1.50 Mbps
RTT from institutional router to any origin server: 2 sec
access link rate: 1.54 Mbps
consequences:
LAN utilization: 15% problem!
access link utilization = 99%
total delay = Internet delay + access delay + LAN delay
= 2 sec + minutes + usecs
origin servers
institutional network
1 Gbps LAN
public Internet
1.54 Mbps access link
Application Layer 2-41
Caching example: fatter access link
assumptions:
avg object size: 100K bits
avg request rate from browsers to
origin servers:15/sec
avg data rate to browsers: 1.50 Mbps
RTT from institutional router to any origin server: 2 sec
access link rate: 1.54 Mbps 154 Mbps
origin servers
public Internet
consequences:
L AN utilization: 15%
access link utilization = 99%
1.54 Mbps access link
154 Mbps
total delay = Internet delay + access
delay + LAN delay
= 2 sec + minutes + usecs
msecs
9.9%
institutional network
1 Gbps LAN
Cost: increased access link speed (not cheap!)
Application Layer 2-42
Caching example: install local cache
assumptions:
avg object size: 100K bits
avg request rate from browsers to
origin servers:15/sec
avg data rate to browsers: 1.50 Mbps
RTT from institutional router to any origin server: 2 sec
access link rate: 1.54 Mbps
consequences:
origin servers
?
total delay = In?ternet delay + access
public Internet
L AN utilization: 15%
access link utilization = 100%
institutional network
1.54 Mbps access link
1 Gbps LAN
local web cache
delay + LAN delay
How to compute link
= 2 sec + minutes + usecs
utilization, delay?
Cost: web cache (cheap!)
Application Layer 2-43
Caching example: install local cache
Calculating access link utilization, delay with cache:
suppose cache hit rate is 0.4 40% requests satisfied at cache,
60% requests satisfied at origin
access link utilization:
60% of requests use access link
data rate to browsers over access link = 0.6*1.50 Mbps = .9 Mbps
utilization = 0.9/1.54 = .58 total delay
= 0.6 * (delay from origin servers) +0.4 * (delay when satisfied at cache)
= 0.6 (2.01) + 0.4 (~msecs)
= ~ 1.2 secs
less than with 154 Mbps link (and cheaper too!)
origin servers
institutional network
1 Gbps LAN
local web cache
public Internet
1.54 Mbps access link
Application Layer 2-44
Conditional GET
Goal: don’t send object if cache has up-to-date cached version
no object transmission delay
lower link utilization
cache: 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
client
server
object not
modified before
HTTP request msg
If-modified-since:
HTTP response
HTTP/1.0 304 Not Modified
HTTP request msg
If-modified-since:
object modified
after
HTTP response
HTTP/1.0 200 OK
Application Layer 2-45
Chapter 2: outline
2.1 principles of network applications
app architectures app requirements
2.2 Web and HTTP
2.3 FTP
2.4 electronic mail
SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 socket programming with UDP and TCP
Application Layer 2-46
FTP: the file transfer protocol
FTP
user interface
FTP client
FTP server
user at host
file transfer
local file system
remote file system
transfer file to/from remote host client/server model
client: side that initiates transfer (either to/from remote)
server: remote host ftp: RFC 959
ftp server: port 21
Application Layer 2-47
FTP: separate control, data connections
FTP client contacts FTP server at port 21, using TCP
client authorized over control connection
client browses remote directory, sends commands over control connection
when server receives file transfer command, server opens 2nd TCP data connection (for file) to client
after transferring one file, server closes data connection
FTP client
TCP control connection, server port 21
TCP data connection, FTP
server port 20
server
server opens another TCP data connection to transfer another file
control connection: “out of band”
FTP server maintains “state”: current directory, earlier authentication
Application Layer 2-48
FTP commands, responses
sample commands:
sent as ASCII text over control channel
USER username
PASS password
LIST return list of file in current directory
RETR filename retrieves (gets) file
STOR filename stores (puts) file onto remote host
sample return codes
status code and phrase (as in HTTP)
331 Username OK, password required
125 data connection already open; transfer starting
425 Can’t open data connection
452 Error writing file
Application Layer 2-49
Chapter 2: outline
2.1 principles of network applications
app architectures app requirements
2.2 Web and HTTP
2.3 FTP
2.4 electronic mail
SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 socket programming with UDP and TCP
Application Layer 2-50
Electronic mail
Three major components:
user agents
mail servers
simple mail transfer protocol: SMTP
User Agent
a.k.a. “mail reader”
composing, editing, reading
mail messages
e.g., Outlook, Thunderbird, iPhone mail client
outgoing, incoming messages stored on server
SMTP
SMTP
SMTP
outgoing message queue
user mailbox
user agent
mail server
user agent
mail server
user agent
user agent
mail server
user agent
user agent
Application Layer 2-51
Electronic 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
SMTP
SMTP
user agent
mail server
user agent
mail server
user agent
SMTP
user agent
mail server
user agent
user agent
Application Layer 2-52
Electronic Mail: SMTP [RFC 2821]
uses TCP to reliably transfer email message from client to server, port 25
direct transfer: sending server to receiving server
three phases of transfer handshaking (greeting)
transfer of messages
closure
command/response interaction (like HTTP, FTP) commands: ASCII text
response: status code and phrase
messages must be in 7-bit ASCI
Application Layer 2-53
Scenario: Alice sends message to Bob
1) Alice uses UA to compose message “to” bob@someschool.edu
2) Alice’s UA sends message to her mail server; message placed in message queue
3) client side of SMTP opens TCP connection with Bob’s mail server
4) SMTP client sends Alice’s message over the TCP connection
5) Bob’s mail server places the message in Bob’s mailbox
6) Bob invokes his user agent to read message
user agent
1 user agent
2
4
6
mail server
mail server
3
Alice’s mail server
5
Bob’s mail server
Application Layer 2-54
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 alice@crepes.fr… Sender ok
C: RCPT TO:
S: 250 bob@hamburger.edu … 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 2-55
Try SMTP interaction for yourself:
telnet servername 25
see 220 reply from server
enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands
above lets you send email without using email client (reader)
Application Layer 2-56
SMTP: final words
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
comparison with HTTP:
HTTP: pull
SMTP: push
both have ASCII command/response interaction, status codes
HTTP: each object encapsulated in its own response msg
SMTP: multiple objects sent in multipart msg
Application Layer 2-57
Mail message format
SMTP: protocol for exchanging email msgs
RFC 822: standard for text message format:
header lines, e.g., To:
From:
Subject:
different from SMTP MAIL FROM, RCPT TO: commands!
Body: the “message” ASCII characters only
blank line
header
body
Application Layer 2-58
Mail access protocols
SMTP SMTP
sender’s mail server
mail access protocol
(e.g., POP, IMAP)
user agent
user agent
SMTP: delivery/storage to receiver’s server mail access protocol: retrieval from server
POP: Post Office Protocol [RFC 1939]: authorization, download
IMAP: Internet Mail Access Protocol [RFC 1730]: more features, including manipulation of stored msgs on server
HTTP: gmail, Hotmail, Yahoo! Mail, etc.
receiver’s mail server
Application Layer 2-59
POP3 protocol
authorization phase
client commands:
user: declare username pass: password
server responses +OK
-ERR
transaction phase, client:
list: list message numbers
retr: retrieve message by number
dele:delete
quit
S: +OK POP3 server ready C: user bob
S: +OK
C: pass hungry
S: +OK user successfully logged on
C: list
S: 1 498
S: 2 912
S: .
C: retr 1
S:
C: dele 1
C: retr 2
S:
C: dele 2
C: quit
S: +OK POP3 server signing off
Application Layer 2-60
POP3 (more) and IMAP
more about POP3
IMAP
keeps all messages in one place: at server
allows user to organize messages in folders
keeps user state across sessions:
names of folders and mappings between message IDs and folder name
previous example uses POP3 “download and delete” mode
Bob cannot re-read e- mail if he changes client
POP3 “download-and- keep”: copies of messages on different clients
POP3 is stateless across sessions
Application Layer 2-61
Chapter 2: outline
2.1 principles of network applications
app architectures app requirements
2.2 Web and HTTP
2.3 FTP
2.4 electronic mail
SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 socket programming with UDP and TCP
Application Layer 2-62
DNS: domain name system
people: many identifiers:
SSN, name, passport #
Internet hosts, routers:
IP address (32 bit) – used for addressing datagrams
“name”, e.g., www.yahoo.com – used by humans
Q: how to map between IP address and name, and vice versa ?
Domain Name System:
distributed database implemented in hierarchy of many name servers
application-layer protocol: hosts, name servers communicate to resolve names (address/name translation)
note: core Internet function, implemented as application- layer protocol
complexity at network’s “edge”
Application Layer 2-63
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
why not centralize DNS?
single point of failure
traffic volume
distant centralized database maintenance
A: doesn’t scale!
Application Layer 2-64
DNS: a distributed, hierarchical database
com DNS servers
yahoo.com DNS servers
Root DNS Servers
……
org DNS servers
pbs.org DNS servers
edu DNS servers
poly.edu umass.edu DNS serversDNS servers
amazon.com DNS servers
client wants IP for www.amazon.com; 1st approx:
client queries root server to find com DNS server
client queries .com DNS server to get amazon.com DNS server
client queries amazon.com DNS server to get IP address for www.amazon.com
Application Layer 2-65
DNS: root name servers
contacted by local name server that can not resolve name
root name server:
contacts authoritative name server if name mapping not known gets mapping
returns mapping to local name server
c. Cogent, Herndon, VA (5 other sites) d. U Maryland College Park, MD
h. ARL Aberdeen, MD
j. Verisign, Dulles VA (69 other sites )
e. NASA Mt View, CA
f. Internet Software C.
Palo Alto, CA (and 48 other sites)
a. Verisign, Los Angeles CA (5 other sites)
b. USC-ISI Marina del Rey, CA l. ICANN Los Angeles, CA
k. RIPE London (17 other sites)
i. Netnod, Stockholm (37 other sites)
m. WIDE Tokyo (5 other sites)
13 root name “servers” worldwide
(41 other sites)
g. US DoD Columbus, OH (5 other sites)
Application Layer 2-66
TLD, authoritative servers
top-level domain (TLD) servers:
responsible for com, org, net, edu, aero, jobs, museums, and all top-level country domains, e.g.: uk, fr, ca, jp
Network Solutions maintains servers for .com TLD
Educause for .edu TLD
authoritative DNS servers:
organization’s own DNS server(s), providing authoritative hostname to IP mappings for organization’s named hosts
can be maintained by organization or service provider Application Layer 2-67
Local DNS name server
does not strictly belong to hierarchy
each ISP (residential ISP, company, university) has
one
also called “default name server”
when host makes DNS query, query is sent to its local DNS server
has local cache of recent name-to-address translation pairs (but may be out of date!)
acts as proxy, forwards query into hierarchy
Application Layer 2-68
DNS name resolution example
host at cis.poly.edu wants IP address for gaia.cs.umass.edu
iterated query:
contacted server replies with name of server to contact
“I don’t know this name, but ask this server”
root DNS server
2
3
4 5
TLD DNS server
local DNS server
dns.poly.edu
1 8
requesting host
cis.poly.edu
7 6
authoritative DNS server
dns.cs.umass.edu
gaia.cs.umass.edu
Application Layer 2-69
DNS name resolution example
recursive query:
puts burden of name resolution on contacted name server
heavy load at upper levels of hierarchy?
root DNS server
3 7
2
6
local DNS server
dns.poly.edu
18
requesting host
cis.poly.edu
TLD DNS server
4
authoritative DNS server
dns.cs.umass.edu
gaia.cs.umass.edu
5
Application Layer 2-70
DNS: caching, updating records
once (any) name server learns mapping, it caches mapping
cache entries timeout (disappear) after some time (TTL) TLD servers typically cached in local name servers
• thus root name servers not often visited
cached entries may be out-of-date (best effort name-to-address translation!)
if name host changes IP address, may not be known Internet-wide until all TTLs expire
update/notify mechanisms proposed IETF standard RFC 2136
Application Layer 2-71
DNS records
DNS: distributed db storing resource records (RR)
RR format: (name, value, type, ttl)
type=A
name is hostname
value is IP address
type=NS
name is domain (e.g., foo.com)
value is hostname of authoritative name server for this domain
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 type=MX
value is name of mailserver associated with name
Application Layer 2-72
DNS protocol, messages
query and reply messages, both with same message
format
msg header
identification: 16 bit # for query, reply to query uses same #
flags:
query or reply
recursion desired
recursion available
reply is authoritative
2 bytes
2 bytes
identification
# questions
# authority RRs
questions (variable # of questions)
answers (variable # of RRs)
authority (variable # of RRs)
additional info (variable # of RRs)
flags
# answer RRs
# additional RRs
Application Layer 2-73
DNS protocol, messages
2 bytes
2 bytes
identification
# questions
# authority RRs
questions (variable # of questions)
answers (variable # of RRs)
authority (variable # of RRs)
additional info (variable # of RRs)
flags
# answer RRs
# additional 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 2-74
Inserting records into 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 two RRs into .com TLD server: (networkutopia.com, dns1.networkutopia.com, NS)
(dns1.networkutopia.com, 212.212.212.1, A)
create authoritative server type A record for www.networkuptopia.com; type MX record for networkutopia.com
Application Layer 2-75
Attacking DNS
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
Redirect attacks
Man-in-middle
Intercept queries
DNS poisoning
Send bogus relies to DNS server, which caches
Exploit DNS for DDoS
Send queries with spoofed source address: target IP
Requires amplification Application Layer 2-76
Chapter 2: outline
2.1 principles of network applications
app architectures app requirements
2.2 Web and HTTP
2.3 FTP
2.4 electronic mail
SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 socket programming with UDP and TCP
Application Layer 2-77
Pure P2P architecture
no always-on server arbitrary end systems
directly communicate
peers are intermittently connected and change IP addresses
examples:
file distribution (BitTorrent)
Streaming (KanKan) VoIP (Skype)
Application Layer 2-78
File distribution: client-server vs P2P
Question: how much time to distribute file (size F) from one server to N peers?
peer upload/download capacity is limited resource us: server upload
file, size F
capacity
us
u d
1 1 u2 d2
network (with abundant bandwidth)
di: peer i download capacity
di ui
ui: peer i upload capacity
server
uN dN
Application Layer 2-79
File distribution time: client-server
server transmission: must sequentially send (upload) N F 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
us
network
di
ui
time to distribute F to N clients using
client-server approach
Dc-s > max{NF/us,,F/dmin}
increases linearly in N
Application Layer 2-80
File distribution time: P2P
server transmission: must
upload at least one copy F
time to send one copy: F/us
client: each client must download file copy
us
di network ui
min client download time: F/dmin
clients: as aggregate must download NF bits
max upload rate (limting max download rate) is us + Sui
time to distribute F to N clients using
P2P approach
DP2P > max{F/us,,F/dmin,,NF/(us + Sui)}
increases linearly in N …
… but so does this, as each peer brings service capacity
Application Layer 2-81
Client-server vs. P2P: example
client upload rate = u, F/u = 1 hour, us = 10u, dmin ≥ us
3.5 3 2.5 2 1.5 1 0.5 0
P2P Client-Server
0 5 10 15 20 25 30 35 N
Application Layer 2-82
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 2-83
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 2-84
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 2-85
BitTorrent: tit-for-tat
(1) Alice “optimistically unchokes” Bob
(2) Alice becomes one of Bob’s top-four providers; Bob reciprocates (3) Bob becomes one of Alice’s top-four providers
higher upload rate: find better trading partners, get file faster !
Application Layer 2-86
Distributed Hash Table (DHT)
Hash table
DHT paradigm
Circular DHT and overlay networks Peer churn
Simple Database
Simple database with(key, value) pairs:
• key: human name; value: social security #
Key
Value
John Washington
132-54-3570
Diana Louise Jones
761-55-3791
Xiaoming Liu
385-41-0902
Rakesh Gopal
441-89-1956
Linda Cohen
217-66-5609
…….
………
Lisa Kobayashi
177-23-0199
• key: movie title; value: IP address
Hash Table
• More convenient to store and search on numerical representation of key
• key = hash(original key)
Original Key
Key
Value
John Washington
8962458
132-54-3570
Diana Louise Jones
7800356
761-55-3791
Xiaoming Liu
1567109
385-41-0902
Rakesh Gopal
2360012
441-89-1956
Linda Cohen
5430938
217-66-5609
…….
………
Lisa Kobayashi
9290124
177-23-0199
Distributed Hash Table (DHT)
Distribute (key, value) pairs over millions of peers pairs are evenly distributed over peers
Any peer can query database with a key
database returns value for the key
To resolve query, small number of messages exchanged among peers
Each peer only knows about a small number of other peers
Robust to peers coming and going (churn)
Assign key-value pairs to peers
rule: assign key-value pair to the peer that has the closest ID.
convention: closest is the immediate successor of the key.
e.g., ID space {0,1,2,3,…,63}
suppose 8 peers: 1,12,13,25,32,40,48,60 If key = 51, then assigned to peer 60
If key = 60, then assigned to peer 60
If key = 61, then assigned to peer 1
•
Circular DHT
each peer only aware of immediate successor and predecessor.
1
12
13 25
60
48
40
32
“overlay network”
Resolving a query
1
What is the value associated with key 53
12
value
60
48
O(N) messages
on avgerage to resolve query, when there
are N peers
40
13
25 32
?
Circular DHT with shortcuts
1
value
What is the value for key 53
13 25
12
60
48
40
32
• each peer keeps track of IP addresses of predecessor, successor, short cuts.
• reduced from 6 to 3 messages.
• possible to design shortcuts with O(log N) neighbors, O(log N) messages in query
Peer churn
handling peer churn:
peers may come and go (churn) each peer knows address of its
two successors
each peer periodically pings its
1
3
15
12
10
8
4 two successors to check aliveness
5
example: peer 5 abruptly leaves
if immediate successor leaves, choose next successor as new immediate successor
Peer churn
handling peer churn:
peers may come and go (churn) each peer knows address of its
two successors
each peer periodically pings its
15
12
10
1
3
8
example: peer 5 abruptly leaves
4 two successors to check aliveness
if immediate successor leaves, choose next successor as new immediate successor
peer 4 detects peer 5’s departure; makes 8 its immediate successor
4 asks 8 who its immediate successor is; makes 8’s immediate successor its second successor.
Chapter 2: outline
2.1 principles of network applications
app architectures app requirements
2.2 Web and HTTP
2.3 FTP
2.4 electronic mail
SMTP, POP3, IMAP
2.5 DNS
2.6 P2P applications
2.7 socket programming with UDP and TCP
Application Layer 2-97
Socket programming
goal: learn how to build client/server applications that communicate using sockets
socket: door between application process and end- end-transport protocol
application
process
transport
network
link
physical
application
process
transport
network
link
physical
socket
Internet
controlled by app developer
controlled by OS
Application Layer 2-98
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 the data to the server.
2. The server receives the data and converts characters to uppercase.
3. The server sends the modified data to the client.
4. The client receives the modified data and displays the line on its screen.
Application Layer 2-99
Socket programming with UDP
UDP: no “connection” between client & server
no handshaking before sending data
sender explicitly attaches IP destination address and port # to each packet
rcvr 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
Application Layer 2-100
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 server IP and port=x; send datagram via clientSocket
read datagram from
clientSocket
close
clientSocket
Application 2-101
Example app: UDP client
include Python’s socket library
create UDP socket for server
get user keyboard input
Attach server name, port to message; send into socket
read reply characters from socket into string
print out received string and close socket
Python UDPClient
from socket import *
serverName = ‘hostname’
serverPort = 12000
clientSocket = socket(socket.AF_INET,
socket.SOCK_DGRAM) message = raw_input(’Input lowercase sentence:’)
clientSocket.sendto(message,(serverName, serverPort)) modifiedMessage, serverAddress =
clientSocket.recvfrom(2048) print modifiedMessage
clientSocket.close()
Application Layer 2-102
Example app: UDP server
create UDP socket
bind socket to local port number 12000
loop forever
Read from UDP socket into message, getting client’s 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 1:
message, clientAddress = serverSocket.recvfrom(2048) modifiedMessage = message.upper() serverSocket.sendto(modifiedMessage, clientAddress)
Application Layer 2-103
Socket programming with TCP
client must contact server
server process must first be running
server must have created socket (door) that welcomes client’s 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
Application Layer 2-104
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 2-105
Example app: TCP client
create TCP socket for server, remote port 12000
Python TCPClient
from socket import *
serverName = ’servername’
serverPort = 12000
clientSocket = socket(AF_INET, SOCK_STREAM) clientSocket.connect((serverName,serverPort)) sentence = raw_input(‘Input lowercase sentence:’) clientSocket.send(sentence)
modifiedSentence = clientSocket.recv(1024)
print ‘From Server:’, modifiedSentence clientSocket.close()
No need to attach server name, port
Application Layer 2-106
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 1:
connectionSocket, addr = serverSocket.accept()
sentence = connectionSocket.recv(1024) capitalizedSentence = sentence.upper() connectionSocket.send(capitalizedSentence) connectionSocket.close()
Application Layer 2-107
Chapter 2: summary
our study of network apps 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
FTP
SMTP, POP, IMAP
DNS
P2P: BitTorrent, DHT
socket programming: TCP, UDP sockets
Application Layer 2-108
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 being communicated
important themes:
control vs. data msgs
in-band, out-of-band
centralized vs. decentralized
stateless vs. stateful
reliable vs. unreliable msg transfer
“complexity at network edge”
Application Layer 2-109
Chapter 1 Additional Slides
Introduction 1-110
packet analyzer
packet capture (pcap)
application (www browser,
email client)
application OS
Transport (TCP/UDP)
Network (IP)
Link (Ethernet)
copy of all Ethernet frames sent/receive d
Physical