Application Level Protocols

Name: Application Level Protocols
Uploaded: 2017-11-23T10:39:44+00:00
Duration: PTM33S36
Channel: Virgil Cameron
Description: Application Level Protocols

Application Level Protocols

Application-Level Protocols
HTTP (web) FTP (file transfer) SMTP (mail) DNS (name lookup) Not really applications by OSI standards, but higher than level 4. Level 5 or 6?

Themes Representation at different levels ASCII protocols Name Lookup
Text-based How Messages are structured Request/response nature of these protocols Name Lookup Division of concerns (e.g. zones) Name to number mapping Reverse map Caching

Application-Level overview
Layer-4 provides a byte-stream Infinite, ordered stream of 8-bit bytes HTTP, SMTP, FTP use text messages built on layer-4 byte streams “simple ASCII protocols” Messages are a sequence of text-based commands Like Java string, but each character is in 7 or 8-bit ASCII, not 16-bit Unicode Control and data typically separated by a “return” (e.g., control/line feed pair of bytes)

Representation by Level
Host A Host B ASCII Text Strings Layer 7 “GET index.html” Layer 7 Layer 6 Layer 6 Layer 5 Layer 5 Byte Stream Layer 4 71,69,84,32,105,110 … Layer 4 Discrete Packets Layer 3 71,69,84 32,105,110 Layer 3 Discrete Packets Layer 2 71,69,84 32,105,110 Layer 2 , , … Bit Sequence Layer 1 Layer 1 Physical Medium

HTTP (Hyper Text Transfer Protocol)

Overview Application Protocol for browsers, web-servers
Simple ASCII protocol Additionally, HTTP has a notion of invoking “methods” on a named resources Resource can be anything named in a Uniform Resource Locator (URL) Most often, an HTML file (but doesn’t have to be!) sometimes it’s the output of a program

URL Naming What does a URL refer to? HTML files? PDF documents
Runnable programs (scripts) Java objects + methods?

Path of an HTTP request DNS Server Web Server Client
Client – Server Architecture

HTTP Protocol Summary Client connects to server
Client sends HTTP message request With GET, POST or HEAD methods Server sends HTTP message as a response

HTTP Messages initial line zero or more header lines a blank line
method or response code + version zero or more header lines Information about message content a blank line optional message body a file, or client input, or server output

HTTP request message: general format

Common Response codes 2XX success codes 200 OK 3XX redirection codes
301 moved 4XX client errors 404 not found 5XX server errors 502 service overloaded

Example Client Message
GET /newacct.html HTTP/1.0 From: User-Agent: Mozilla-linux/4.7 (blank line here)

Example Server Response
HTTP/ Not Found (blank line here)

Example Client Message
GET /newaccount.html HTTP/1.0 From: User-Agent: Mozilla-linux/4.7 (blank line here)

Example Server Response
response code HTTP/ OK Date: Sun, 17 Sep :12:51 GMT Server: Apache/1.3.3 (Unix) Last-Modified: Wed, 30 Aug :12:01 GMT ETag: "1ac6-9c1-39ac6d71" Accept-Ranges: bytes Content-Length: 2497 Connection: close Content-Type: text/html <html> <head> <title>Building new accounts</title> </head> <body> <center> <img src="images/sample.jpg"> … header Blank line separating header/body body

MIME Headers Responses from servers to complete GET requests contain MIME information MIME = Multipurpose Internet Mail Extensions MIME allows media types other than simple ASCII text to be encoded into a message The “Content-Type:” line in the MIME header indicates what type of data (type/subtype) is contained in the message Examples: Content-Type: text/html Content-Type: Image/GIF

POST Method What a browser submits in when a form is sent to the server Stylized way of passing form data 2 ways to encode form data: “Fat URL” via GET for older systems that didn’t support POST POST method

POST Requests Most commonly used by browsers to send large “form” responses to servers Forms are web pages that contain fields that the browser user can edit or change

POST Requests (cont’d)
POST /index.html HTTP/1.1 language=any&message=this+is+a+message+to+the+server+being+sent+by+the+browser+with+a+POST+request

Encoding form data with POST
General form is: &variable1=value1&variable2=value2… Spaces changed to “+” Other characters encoded(I.e. escaped) via “%”

Example: Client POST request
POST /cgi-bin/rats.cgi HTTP/1.0 Referer: Connection: Keep-Alive User-Agent: Mozilla/4.73 [en] (X11; U; Linux i686) Host: nes:8192 Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, image/png, */* Accept-Encoding: gzip Accept-Language: en Accept-Charset: iso ,*,utf-8 Content-type: application/x-www-form-urlencoded Content-length: 93 Account=cs111fall&First=richard&Last=martin&SSN= &Bday= &.State=CreateAccount

HTTP in context Time Client W.X.Y.Z Server A.B.C.D:80
ss= serverSocket(port 80); cc = socket(A.B.C.D, 80); sc = ss.accept; out.print(“GET /newaccount.html http/1.0)”); Time read input from socket parse header read data find resource build response header send resource write to socket read header read input display HTML

Why loading pages seems slow
Potential problems Client is overloaded DNS takes a long time Network overloaded Dropped packets => TCP windows Large pages Server is overloaded Solutions: proxy servers, “Flow” servers

Caching Proxies Web Server Proxy Server Clients GET foo.html
Store foo.html

“Flow” Approach Re-write URLs in web pages
Point URL to “nearest” server for the data HTML from main server Images, sound, animations point to closer servers Requires knowledge of network topology! Used by Akamai

Flow Approach (cont) Web Server GET Image01.gif GET Index.html Client

HTTP 1.0 Simple protocol Client issues 1 operation per TCP connection
Connnect(); Get index.html ; close() Connect(); Get image01.html; close () … How long does it take to retrieve a whole page? Concurrency by using multiple connections can speed this up, but…

HTTP 1.1 Client keeps connection open to server
Makes multiple requests per connection Get foo.html, get image02.gif …. Length of time socket stays up? # of open connections on server? 1.0 allows server to close connections faster Not clear if 1.1 is better from the server’s perspective

Web Server Scripting A URL may refer to a static web page or a server-side script Script is just a program that is run in response to a HTTP request Server-side scripts produce web page content as output This is what a” dynamic” web page is Standard argument passing convention between the web server and the program: Common Gateway Interface (CGI) CGI scripts may be written in any language (Perl Python, sh, csh, Java.) CGI scripts are commonly used to produce responses to Web page form input from client browsers

Client Side Embedded Web Page Scripts and Programs
Web pages may also contain scripts or programs within the HTML code to be run on the client Unlike server scripts, web page scripts and programs run on the browser machine’s processor, not on the server’s processor Examples: Javascript VBScript Java applets Example non-trivial program: Takes Rutgers campus bus positions as input Client side plots different routes on a map

HTML (Hyper Text Markup Language)
The text is surrounded by tags which describe the formatting and layout of the text on the browser window Allows for data input also – using FORMS Documentations/Tutorials View source code of any page you visit in the browser

SMTP (Simple Mail Transfer Protocol)

Email Email is transferred from one host to another using the
Simple Mail Transfer Protocol (SMTP) Like HTTP, SMTP has a similar ASCII command and reply set to transfer messages between machines Think of a set of request strings and reply strings sent over the network SMTP transfers occur between: sending host and dedicated server dedicated servers They do not occur between receiving hosts and servers These are POP or IMAP protocols

SMTP Protocol 220 hill.com SMTP service ready HELO town.com
250 hill.com Hello town.com, pleased to meet you MAIL FROM: 250 Sender ok RCPT TO: 250 Recipient ok DATA 354 Enter mail, end with “.” on a line by itself From: To: Subject: Please fetch me a pail of water Jill, I’m not feeling up to hiking today. Will you please fetch me a pail of water? . 250 message accepted QUIT 221 hill.com closing connection

SMTP Direct Mode Direct mode:
Sending from to SMTP Messages town. com Server SMTP Responses for hill.com town.com first finds IP address for hill.com server using DNS request (type=MS) town.com opens TCP connection on SMTP port 25 and initiates SMTP protocol to transfer message

SMTP Relay Mode Relay mode:
Sending from to town. com Server Server for town.com for hill.com town.com is configured to send all messages through a local server The local server buffers messages and forwards them to other servers

Retrieving Email from a desktop
Users retrieve from their assigned server retrieval does NOT use the SMTP protocol 3 common protocols for retrieval server adds received messages to a file stored on a shared file system (e.g., /var/mail/jill) downloaded via the POP3 protocol accessed via the IMAP protocol

FTP (File Transfer Protocol )

FTP Download/upload files between a client and server
One of the first Internet protocols More complex than SMTP ASCII control connection Separate data connection performs presentation functions E.g, formats and converts data depending on type Sends passwords in plain ASCII text Eavesdropper can recover passwords Fatal flaw, turned off at a lot of sites Replaced with scp, sftp instead

FTP Client/Server Client Program User Interface Client protocol
interpreter Client data transfer function User Server Program Server protocol interpreter client file system Server data Transfer function server file system

Sample FTP Command Set LIST list directory GET get a file (download)
MGET get multiple files STOR store (upload) a file TYPE set the data transfer type USER set the username QUIT End the session

Sample FTP Replies 200 Command OK 214 Help Message
331 Username OK, password required 425 Can’t open data connection 452 Error writing file 500 Syntax error (unrecognized command) 502 Unimplemented MODE

Sample FTP Session %ftp ftp.rutgers.edu
Connected to kublai.td.Rutgers.EDU. 220 ftp.rutgers.edu FTP server (Version wu-2.6.2(9) Thu Feb 7 13:31:16 EST 2002) ready. Name (ftp.rutgers.edu:rmartin): anonymous 331 Guest login ok, send your complete address as password. Password: 230 Guest login ok, access restrictions apply. Remote system type is UNIX. ftp> cd /pub/redhat/linux/9/en/os/i386/images ftp> get bootdisk.img local: bootdisk.img remote: bootdisk.img 227 Entering Passive Mode (165,230,246,3,149,67) 150 Opening BINARY mode data connection for bootdisk.img ( bytes). 226 Transfer complete. bytes received in 00:01 ( KB/s) ftp> quit

Domain Name System (DNS)

Domain Name System (DNS)
Problem statement: Average brain can easily remember 7 digits On average, IP addresses have 12 digits We need an easier way to remember IP addresses Solution: Use alphanumeric names to refer to hosts Add a distributed, hierarchical protocol (called DNS) to map between alphanumeric host names and binary IP addresses We call this Address Resolution

Domain Name Hierarchy ... ... Country Domains Generic Domains com edu
net gov int mil org ae us zw yahoo cnn rutgers yale Country Domains cs eng Generic Domains

Domain Name Management
The domain name hierarchy is divided into zones Zone: A separate portion of the DNS hierarchy No two zones should overlap Name servers In each zone, there is a primary name server and one or more secondary name servers Name servers contain two kinds of address mappings: Authoritative mappings: For hosts within the zone Cached mappings: For previously requested mappings to hosts not in the zone

Domain Name Hierarchy ... ... com edu net gov int mil org ae us zw
yahoo cnn rutgers yale cs eng

DNS Protocol When client wants to know an IP address for a host name
Client sends a DNS query to the primary name server in its zone If name server contains the mapping, it returns the IP address to the client Otherwise, the name server forwards the request to the root name server The request works its way down the tree toward the host until it reaches a name server with the correct mapping

DNS Protocol Example Scenario: remus.rutgers.edu tries to
resolve an IP address for venus.cs.yale.edu using a recursive query 1 8 ns-lcsr.rutgers.edu 2 7 a.root-servers.net 3 6 yale.edu 4 5 cs.yale.edu

DNS Protocol Another Example
remus.rutgers.edu Scenario: remus.rutgers.edu tries to resolve an IP address for venus.cs.yale.edu using an iterative query 1 2 ns-lcsr.rutgers.edu 3 4 a.root-servers.net 5 6 yale.edu 7 8 cs.yale.edu

DNS Packets Clients communicate with DNS servers using either TCP or UDP on port 53 Transaction Identification Flags Number of Questions Number of Answer RRs Number of Authoritative RRs Number of Additional RRs Questions (variable length) Answer Resource Records (variable length) Authoritative Resource Records (variable length) Additional Resource Records (variable length)

DNS Packet Fields Transaction Identification: Random number used to match client queries with name server responses Flags: QR: 0=Query, 1=Response opcode: 0=standard query, 1=inverse query, 2=status request AA: Authoritative answer TC: Truncated DNS packet RD: Recursion desired RA: Recursion available rcode: Return code. 0=no error, 3=name error QR opcode AA TC RD RA (unused) rcode

DNS Packet Fields (cont’d)
Transaction Identification: Random number used to match client queries with name server responses Number of Questions: Number of DNS queries in the packet Not supported in many DNS servers! Number of Answer RRs: Number of non-authoritative DNS responses in the packet Number of Authoritative RRs: Number of authoritative DNS responses in the packet Number of Additional RRs: Number of other DNS responses in the packet (usually contains other DNS servers in domain) Questions & Answers: Variable length fields to store DNS queries and DNS server responses

DNS Packet Question field contains a sequence of queries:
DNS Queries DNS Packet Question field contains a sequence of queries: Query name (variable length) Query Type Query Class Query Name: Contains an encoded form of the name for which we are seeking an IP address Query Type: 1=IP address, 2=name server, 12=pointer record, etc. Query Class: 1=Internet address

Encoding Query Names DNS queries must be encoded in a special way
Divide host address into segments whenever a period appears For each segment, store a byte representing the length of the segment followed by the letters in the segment Store a zero byte at the end of the query

Encoding Query Names Example
remus.rutgers.edu remus rutgers edu 5 e r m u s 7 t g 3 d NOTE: These count fields are not the ASCII characters “5”, “7”, “3” and “0”!!!

DNS Packet RR fields contain a sequence of resource records:
DNS Responses DNS Packet RR fields contain a sequence of resource records: Domain name (variable length) Type Class Time-to-live Resource Data (variable length) Resource data length Domain Name: Encoded domain name for query Type & Class: Same as for query (1=IP; 1=Internet) Time-to-Live: How long this responses will be useful Resource Data: Contains the four-byte IP address

DNS Caching Going to the root server and then down the tree every time we need to resolve an address is inefficient Introduce address caching at name servers Store host-to-IP-address mappings from recently requested host names at name server When the same address is requested later, use the cached version at the local name server instead of recursively querying other name servers again

DNS Caching Example First time:
remus.rutgers.edu First time: remus.rutgers.edu tries to resolve an IP address for venus.cs.yale.edu using a recursive query 1 8 Later: venus.cs.yale.edu has been cached at ns-lcsr. remus.rutgers.edu (and any other host that uses ns-lcsr) will receive the cached IP address for venus.cs.yale.edu ns-lcsr.rutgers.edu remus.rutgers.edu 2 7 1 2 a.root-servers.net ns-lcsr.rutgers.edu 3 6 yale.edu 4 5 cs.yale.edu

Interface to DNS The “dig” and “nslookup” programs provide an interface to DNS dig remus.rutgers.edu Server: ns-lcsr.rutgers.edu Address: Name: remus.rutgers.edu Address:

Bootstrapping DNS How does a host contact the name server if all it has is the name and no IP address? IP address of at least 1 nameserver must be given a priori or with another protocol (DHCP, bootp) File /etc/resolv.conf in unix Start -> settings-> control panel-> network ->TCP/IP -> properties in windows

Default Domains When Host issues a query to DNS server, can add the default domain. Default domain added to end of ever DNS query E.g.: default domain is rutgers.edu Machine “eden” automatically extended to eden.rutgers.edu

Reverse DNS We have the IP address, but want the name
Use DNS to perform the lookup function Special domain, “in-addr.arpa” domain for reverse lookups Internet address is reversed in the lookup E.g in-addr.arpa == remus Follows least-> most specific convention

Application Level Protocols

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