Chapter 2 Application Layer
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Computer Networking: A Top Down Approach , 6th edition.
Jim Kurose, Keith Ross
2: Application Layer 1
Chapter 2: Application layer
2.1 Principles of 2.7 Socket programming network applications with TCP
2.2 Web and HTTP 2.5 DNS
2: Application Layer 2
Chapter 2: Application Layer
Our goals:
conceptual, implementation aspects of network application protocols
transport-layer service models
client-server paradigm
learn about protocols by examining popular application-level protocols
HTTP DNS
programming network applications
socket API
2: Application Layer 3
Some network apps
e-mail
web
instant messaging
remote login
P2P file sharing
multi-user network games
streaming stored video clips
voice over IP real-time video
conferencing grid computing
2: Application Layer 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
little software written for devices in network core
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 data link physical
application
transport network data link physical
2: Application Layer 5
Chapter 2: Application layer
2.1 Principles of network applications
app architectures app requirements
2.2 Web and HTTP 2.5 DNS
2.7 Socket programming with TCP
2: Application Layer 6
Application architectures
Client-server
2: Application Layer 7
Client-server architecture
client/server
server:
always-on host
permanent IP address
server farms for scaling
clients:
communicate with server
may be intermittently
connected
may have dynamic IP addresses
do not communicate directly with each other
2: Application Layer 8
What transport service does an app need?
Data loss
Bandwidth
some apps (e.g., multimedia) require minimum amount of bandwidth to be “effective”
other apps (“elastic apps”) make use of whatever bandwidth they get
some apps (e.g., audio) can tolerate some loss
other apps (e.g., file transfer, telnet) require 100% reliable data
transfer
Timing
some apps (e.g., Internet telephony, interactive games) require low delay to be “effective”
2: Application Layer 9
Transport service requirements of common apps
Application
file transfer e-mail Web documents real-time audio/video
stored audio/video interactive games instant messaging
Data loss
no loss
no loss
no loss loss-tolerant
loss-tolerant loss-tolerant no loss
Bandwidth
Time Sensitive
no
no
no
yes, 100’s msec
elastic
elastic
elastic
audio: 5kbps-1Mbps video:10kbps-5Mbps yes, few secs same as above
few kbps up elastic
yes, 100’s msec yes and no
2: Application Layer 10
Internet transport protocols services
TCP service:
connection-oriented: setup required between client and server processes
UDP service:
unreliable data transfer between sending and receiving process
does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee
Q: why bother? Why is there a UDP?
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 bandwidth guarantees
2: Application Layer 11
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] proprietary
(e.g. RealNetworks) proprietary
(e.g., Vonage,Dialpad)
Underlying transport protocol
TCP
TCP
TCP
TCP
TCP or UDP
typically UDP
2: Application Layer 12
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
Client process: process that initiates communication
Server process: process that waits to be contacted
2: Application Layer 13
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?
2: Application Layer 14
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?
identifier includes both IP address and port numbers associated with process on host.
A: No, many
processes can be
running on same 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…
2: Application Layer 15
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
Public-domain protocols:
defined in RFCs
allows for
interoperability
e.g., HTTP, SMTP Proprietary protocols: e.g., Skype
2: Application Layer 16
Chapter 2: Application layer
2.1 Principles of network applications
app architectures app requirements
2.2 Web and HTTP 2.5 DNS
2.7 Socket programming with TCP
2: Application Layer 17
Web and HTTP
First some jargon
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
Example URL:
www.someschool.edu/someDept/pic.gif
host name
path name
2: Application Layer 18
HTTP overview
HTTP: hypertext transfer protocol
Web’s application layer protocol
client/server model
client: browser that requests, receives, “displays” Web objects
server: Web server sends objects in response to requests
HTTP 1.0: RFC 1945
HTTP 1.1: RFC 2068
PC running Explorer
Mac running Navigator
Server running
Apache Web server
2: Application Layer 19
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
Protocols that maintain aside “state” are complex!
past history (state) must be maintained
if server/client crashes, their views of “state” may be inconsistent, must be reconciled
2: Application Layer 20
HTTP connections
Nonpersistent HTTP
At most one object is sent over a TCP connection.
HTTP/1.0 uses nonpersistent HTTP
Persistent HTTP
Multiple objects can be sent over single TCP connection between client and server.
HTTP/1.1 uses persistent connections in default mode
2: Application Layer 21
Nonpersistent HTTP
Suppose user enters URL
www.someSchool.edu/someDepartment/home.index
(contains 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
time
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
2: Application Layer 22
Nonpersistent HTTP (cont.)
time
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.
2: Application Layer 23
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 indicates end
of message
GET /somedir/page.html HTTP/1.1
Host: www.someschool.edu
User-agent: Mozilla/4.0
Connection: close
Accept-language:fr
(extra carriage return, line feed)
2: Application Layer 24
HTTP response message
status line (protocol status code status phrase)
header lines
data, e.g., requested HTML file
HTTP/1.1 200 OK
Connection close
Date: Thu, 06 Aug 1998 12:00:15 GMT
Server: Apache/1.3.0 (Unix)
Last-Modified: Mon, 22 Jun 1998 ……
Content-Length: 6821
Content-Type: text/html
data data data data data …
2: Application Layer 25
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!
2: Application Layer 26
HTTP request message: general format
2: Application Layer 27
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
2: Application Layer 28
HTTP response status codes
In first line in server->client response message. A few 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 (Location:)
400 Bad Request
request message not understood by server 404 Not Found
requested document not found on this server 505 HTTP Version Not Supported
2: Application Layer 29
Non-Persistent HTTP: Response time (performance)
Definition of RTT: time to send a small packet to travel from client to server and back.
Response time:
one RTT to initiate TCP connection
initiate TCP connection
RTT
request file
RTT
file received
time
time to transmit file
time
one RTT for HTTP request and first few bytes of HTTP response to return
file transmission time total = 2RTT+transmit time
2: Application Layer 30
Persistent HTTP
Nonpersistent 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
Persistent without pipelining:
client issues new request only when previous response has been received
one RTT for each referenced object
Persistent with pipelining:
default in HTTP/1.1
client sends requests as soon as it encounters a referenced object
as little as one RTT for all the referenced objects
2: Application Layer 31
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
origin server
client
Proxy server
object in cache: cache returns object
else cache requests object from origin server, then returns object to client
client
origin server
2: Application Layer 32
More about Web caching
cache acts as both client and 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 (but so does P2P file sharing)
2: Application Layer 33
Caching example
Assumptions
average object size = 100,000 bits
avg. request rate from institution’s browsers to origin servers = 15/sec
delay from institutional router to any origin server and back to router = 2 sec
Consequences
utilization on LAN = 15%
utilization on access link = 100%
total delay = Internet delay + access delay + LAN delay
= 2 sec + minutes + milliseconds
origin servers
institutional network
1.5 Mbps access link
10 Mbps LAN
institutional cache
public Internet
2: Application Layer 34
Caching example (cont)
possible solution
increase bandwidth of access link to, say, 10 Mbps
consequence
utilization on LAN = 15%
utilization on access link = 15%
Total delay = Internet delay + access delay + LAN delay
= 2 sec + msecs + msecs
often a costly upgrade
origin servers
institutional network
10 Mbps access link
10 Mbps LAN
institutional cache
public Internet
2: Application Layer 35
Caching example (cont)
possible solution: install cache
suppose hit rate is 0.4 consequence
40% requests will be satisfied almost immediately
60% requests satisfied by origin server
utilization of access link reduced to 60%, resulting in negligible delays (say 10 msec)
total avg delay = Internet delay + access delay + LAN delay = .6*(2.01) secs + .4*milliseconds < 1.4 secs
origin servers
institutional network
1.5 Mbps access link
10 Mbps LAN
institutional cache
public Internet
2: Application Layer 36
Conditional GET
cache
server
object not
modified
Goal: don’t send object if cache has up-to-date cached version
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
object modified
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
2: Application Layer 37
Chapter 2: Application layer
2.1 Principles of 2.7 Socket programming network applications with TCP
2.2 Web and HTTP 2.5 DNS
2: Application Layer 38
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: map between IP addresses and name ?
Domain Name System:
distributed database implemented in hierarchy of many name servers
application-layer protocol host, routers, name servers to communicate to resolve names (address/name translation)
note: core Internet function, implemented as application-layer protocol
complexity at network’s “edge”
2: Application Layer 39
DNS
DNS services
hostname to IP address translation
host aliasing
Canonical, alias names
mail server aliasing
load distribution
replicated Web servers: set of IP addresses for one canonical name
Why not centralize DNS?
single point of failure
traffic volume
distant centralized database
maintenance doesn’t scale!
2: Application Layer 40
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 a 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
2: Application Layer 41
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
e NASA Mt View, CA
f Internet Software C. Palo Alto, CA (and 36 other locations)
a Verisign, Dulles, VA
c Cogent, Herndon, VA (also LA) d U Maryland College Park, MD g US DoD Vienna, VA
h ARL Aberdeen, MD
j Verisign, ( 21 locations)
k RIPE London (also 16 other locations)
i Autonomica, Stockholm (plus 28 other locations)
m WIDE Tokyo (also Seoul, Paris, SF)
13 root name servers worldwide
b USC-ISI Marina del Rey, CA l ICANN Los Angeles, CA
2: Application Layer 42
TLD and Authoritative Servers
Top-level domain (TLD) servers:
responsible for com, org, net, edu, etc, and all
top-level country domains uk, fr, ca, jp.
Network Solutions maintains servers for com TLD Educause for edu TLD
Authoritative DNS servers:
organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s servers (e.g., Web, mail).
can be maintained by organization or service provider
2: Application Layer 43
Local 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
acts as proxy, forwards query into hierarchy
2: Application Layer 44
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
6
dns.cs.umass.edu
gaia.cs.umass.edu
TLD DNS server
local DNS server
dns.poly.edu
7
18
authoritative DNS server
requesting host
cis.poly.edu
2: Application Layer 45
DNS: caching and updating records
once (any) name server learns mapping, it caches mapping
cache entries timeout (disappear) after some time
TLD servers typically cached in local name servers
• Thus root name servers not often visited
update/notify mechanisms under design by IETF
RFC 2136
http://www.ietf.org/html.charters/dnsind-charter.html
2: Application Layer 46
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 (resource records) into com TLD server:
(networkutopia.com, dns1.networkutopia.com, NS)
(dns1.networkutopia.com, 212.212.212.1, A)
create authoritative server Type A record (IP address) for www.networkuptopia.com; Type NS record (name servers) for networkutopia.com
How do people get IP address of your Web site?
2: Application Layer 47
Chapter 2: Application layer
2.1 Principles of 2.7 Socket programming network applications with TCP
2.2 Web and HTTP 2.5 DNS
2: Application Layer 48
Socket programming
Goal: learn how to build client/server application that communicate using sockets
Socket API
introduced in BSD4.1 UNIX, 1981
explicitly created, used, released by apps
client/server paradigm
two types of transport
service via socket API:
unreliable datagram
reliable, byte stream- oriented
socket
a host-local, application-created, OS-controlled interface (a “door”) into which application process can both send and receive messages to/from another application process
2: Application Layer 49
Socket-programming using TCP
Socket: a door between application process and end- end-transport protocol (UCP or TCP)
TCP service: reliable transfer of bytes from one process to another
controlled by application developer
controlled by operating system
host or server
internet
controlled by application developer
controlled by operating system
host or server
process
process
socket
socket
TCP with buffers, variables
TCP with buffers, variables
2: Application Layer 50
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 client-local 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 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 transfer of bytes (“pipe”) between client and server
2: Application Layer 51
Client/server socket interaction: TCP
Server (running on hostid)
Client
create socket,
port=x, for
incoming request:
welcomeSocket = ServerSocket()
TCP connection setup
create socket,
connect to hostid, port=x clientSocket =
Socket()
send request using
clientSocket
read reply from
clientSocket
close
clientSocket
wait for incoming connection request connectionSocket = welcomeSocket.accept()
read request from
connectionSocket
write reply to
connectionSocket
close
connectionSocket
2: Application Layer 52
Stream jargon
A stream is a sequence of characters that flow into or out of a process.
An input stream is attached to some input source for the process, e.g., keyboard or socket.
An output stream is attached to an output source, e.g., monitor or socket.
keyboard
input stream
monitor
Client
Process
process
output stream
input stream
client TCP
clientSocket
socket TCP socket
to network from network
2: Application Layer 53
outToServer inFromUser
inFromServer
Socket programming with TCP
Example client-server app:
1) client reads line from standard input (inFromUser stream) , sends to server via socket (outToServer stream)
2) server reads line from socket
3) server converts line to uppercase, sends back to client
4) client reads, prints modified line from socket (inFromServer stream)
2: Application Layer 54
Example: Java client (TCP)
import java.io.*; import java.net.*; class TCPClient {
Create client socket,
connect to server
Create input stream
Create output stream attached to socket
public static void main(String argv[]) throws Exception
{
String sentence;
String modifiedSentence;
Socket clientSocket = new Socket(“hostname”, 6789);
BufferedReader inFromUser =
new BufferedReader(new InputStreamReader(System.in));
DataOutputStream outToServer =
new DataOutputStream(clientSocket.getOutputStream());
2: Application Layer 55
Example: Java client (TCP), cont.
Create input stream attached to socket
Send line to server
Read line from server
} }
BufferedReader inFromServer =
new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
//————————————— sentence = inFromUser.readLine();
outToServer.writeBytes(sentence + ‘\n’); modifiedSentence = inFromServer.readLine(); System.out.println(“FROM SERVER: ” + modifiedSentence); clientSocket.close();
2: Application Layer 56
Example: Java server (TCP)
Create welcoming socket at port 6789
Wait, on welcoming socket for contact by client
Create input stream, attached
to socket
{ String clientSentence; String capitalizedSentence;
ServerSocket welcomeSocket = new ServerSocket(6789); while(true) {
Socket connectionSocket = welcomeSocket.accept();
BufferedReader inFromClient =
new BufferedReader(new InputStreamReader(connectionSocket.getInputStream()));
import java.io.*; import java.net.*;
class TCPServer {
public static void main(String argv[]) throws Exception
2: Application Layer 57
Example: Java server (TCP), cont
Create output stream, attached to socket
Read in line from socket
Write out line to socket
}}
DataOutputStream outToClient =
new DataOutputStream(connectionSocket.getOutputStream());
clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase() + ‘\n’; } outToClient.writeBytes(capitalizedSentence);
End of while loop,
loop back and wait for another client connection
2: Application Layer 58
Chapter 2: Summary
our study of network apps now complete!
application architectures client-server
application service requirements:
reliability, bandwidth, delay
Internet transport service model
connection-oriented, reliable: TCP
unreliable, datagrams: UDP
specific protocols: HTTP
DNS
socket programming
2: Application Layer 59
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
2: Application Layer 60