Computer Systems Application Layer
Dr. Mian M. Hamayun
m.m.hamayun@bham.ac.uk
Based on material and slides from
Computer Networking: A Top Down
Approach, 7th Edition – Chapter 2 Jim Kurose, Keith Ross Pearson/Addison Wesley
Lecture Objective
The objective of this lecture is to understand the conceptual and implementation aspects of network application protocols
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Lecture Outline
Principles of Network Applications Web & HTTP
Electronic Mail (SMTP)
Domain Name System (DNS)
Summary
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Recap – Network Layers
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Some Network Applications
E-Mail & Web
Text Messaging
Remote login (SSH, Telnet, …)
P2P File sharing (Bittorrents, …)
Multi-user network games
Streaming stored video (YouTube, Netflix, …) Voice over IP (VoIP) (Skype, Hangouts, …) Social Networking (Facebook, Twitter, …)
Search (Google, Yahoo, Bing, …)
…
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Creating a Network Application
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 application development, propagation
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Network Application Architectures
Possible structure of network applications
Client Server
Peer-to-Peer (P2P)
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Client-Server Architecture
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
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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
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Communicating Processes
Process: A 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: A process that initiates communication
Server Process: A process that waits to be contacted
Applications with P2P architectures have both client processes & server processes
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Sockets – Process to Network Interface
A 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
Socket
Process
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Sockets – Process to Network Interface
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How do we Address Processes on a Host?
To receive messages, each process must have an identifier (!=PID)
Every host device has unique 32-bit IP address
Q: Does IP address of host on which process runs suffice for identifying the process?
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
…
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What Transport Services does an Application 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, …
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Common Transport Services Requirements
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Internet Transport Protocol Services – TCP
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
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Internet Transport Protocol Services – UDP
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?
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Internet Applications – Transport Protocols
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Web and HTTP
A web page consists of objects
An object can be HTML file, JPEG image, Java applet,
audio file, …
A 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
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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
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HTTP request
HTTP response
HTTP request
HTTP response
HTTP Overview (cont’d)
HTTP uses TCP Protocol
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 does not maintains information about past client requests
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Non-persistent HTTP – Example (1 of 2)
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 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
time
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Non-persistent HTTP – Example (2 of 2)
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 the 10 jpeg objects
4. HTTP server closes TCP connection.
(waits until the response message is received by the client)
time
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Non-persistent HTTP: Response Time
RTT: Round-Trip Time:
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
File transmission time
Non-persistent HTTP Response time = 2RTT+ file transmission time
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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
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HTTP Request Message
ASCII (Human-readable format)
request line
(GET, POST, HEAD commands)
header lines
\r\n
carriage return character line-feed character
GET /somedir/page.html HTTP/1.1 \r\n Host: www.someschool.edu \r\n Connection: close \r\n
User-agent: Mozilla/5.0 \r\n Accept-language: fr \r\n
carriage return, line feed at start
of line indicates end of header lines
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HTTP Request Message: General Format
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HTTP Response Message
ASCII (Human-readable format)
status line (protocol, status code, status phrase)
header lines
HTTP/1.1 200 OK\r\n
Connection: close\r\n
Date: Tue, 09 Aug 2011 15:44:04 GMT\r\n
Server: Apache/2.2.3 (CentOS)\r\n
Last-Modified: Tue, 09 Aug 2011 15:11:03 GMT\r\n Content-Length: 6821\r\n
Content-Type: text/html\r\n
\r\n
data data data data data …
Data, e.g. requested HTML file
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HTTP Response Message: General Format
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HTTP Response Status Codes
Status code appears in 1st line in server-to-client response message.
Some sample codes are:
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 message not understood by server
404 Not Found
requested document not found on this server
505 HTTP Version Not Supported
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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
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Electronic Mail
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:sendingmailserver
“server”:receivingmailserver
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Electronic Mail : SMTP
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 ASCII
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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
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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
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Try SMTP Interaction for Yourself
telnet servername 25
see 220 reply from server
enter HELO, MAIL FROM, RCPT TO, DATA, QUIT commands
The above lets you send email without using email client!
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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 message
SMTP: multiple objects sent in multipart messages
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Mail Access Protocols
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 message on server
HTTP: gmail, Hotmail, Yahoo! Mail, etc.
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POP3 Protocol
Authorization phase
client commands:
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
user: declare username
pass: password
server responses +OK
-ERR
Transaction phase,
client:
list: list message numbers
retr: retrieve message by number
dele: delete quit
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POP3 and IMAP
More about POP3
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
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
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DNS: Domain Name System
The Internet’s Directory Service
People have 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 ?
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DNS: Domain Name System
The Internet’s Directory Service
DNS is a distributed database implemented in hierarchy of many name servers
Application-layer protocol: hosts, name servers
communicate to resolve names (address/name
translation)
Note: DNS is a core Internet function and it is implemented as an application-layer protocol
Complexity at network’s “edge”
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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!
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DNS: A Distributed Hierarchical Database
Top-Level Domain (TLD) DNS servers
Authoritative 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
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DNS: Root Name Servers
There are 13 root DNS servers (labelled A to M) worldwide Each “server” is actually a network of replicated servers
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DNS: TLD & Authoritative Servers
Top-level Domain (TLD) servers:
Responsible for com, org, net, edu, gov and all top-level country
domains, e.g.: uk, fr, ca, jp
Verisign Global Registry Services maintains the TLD servers for the
com top-level domain. 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
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DNS: Local DNS Name Server
Does not strictly belong to hierarchy
Each ISP (residential ISP, company, university) has its own local DNS server
Also called “default name server”
When a host makes DNS query, the 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
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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”
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DNS: Name Resolution Example
host at cis.poly.edu wants IP address for gaia.cs.umass.edu
Recursive Query:
Puts burden of name resolution on contacted name server
Heavy load at upper levels of hierarchy?
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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, it may not be known Internet- wide until all TTLs expire
Update/notify mechanisms proposed IETF standard RFC 2136 (https://tools.ietf.org/html/rfc2136)
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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=CNAME
name is alias name for some “canonical” (the real) name
type=NS
name is domain (e.g., foo.com)
value is hostname of authoritative name server for this domain
type=MX
value is name of mailserver associated with name
www.ibm.com is really servereast.backup2.ibm.com
value is canonical name
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DNS Protocol, Messages
Query and Reply messages, both with same message format Message Header
identification: 16 bit # for query, reply to query uses same #
flags:
query or reply
recursion desired
recursion available
reply is authoritative
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DNS Protocol, Messages
Query and Reply messages, both with same message format
name, type fields for a query
RRs in response to query
records for authoritative servers
additional “helpful” info that may be used
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Inserting Records into DNS
Example: new start-up “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
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Summary
We have studied:
Application architectures client-server
P2P
Application service
requirements:
reliability, bandwidth, delay, security
Internet transport service model
connection-oriented, reliable: TCP
unreliable, datagrams: UDP Specific protocols:
HTTP
SMTP, POP, IMAP DNS
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Summary
Most Importantly: We have learned about protocols!
Typical request/reply message exchange:
Important themes:
control vs. data msgs
centralized vs. decentralized service stateless vs. stateful
client requests info or server responds with
data, status code
reliable vs. unreliable message transfer
Message formats: “complexity at network edge” headers: fields giving
info about data
data: info being communicated
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References / Links
Chapter #2: Application Layer, Computer
Networking: A Top-Down Approach (7th edition) by Kurose & Ross
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