Transcript chap2_2ed_5July02 - National Tsing Hua University
Chapter 7 The Application Layer (Cont’d)
Network Security Domain Name Server Network management Applications File Transfer Protocol, Email, Http, Telnet, ….
© All rights reserved. No part of these slides may be reproduced, in any form or by any means, without permission in writing from Professor Wen-Tsuen Chen (email: [email protected]).
2: Application Layer 1
Email Security
PGP: Pretty Good Privacy, by Phil Zimmermaun in 1995.
Support text compression, secrecy and digital signatures.
2: Application Layer 2
PGP message format
2: Application Layer 3
PEM: Privacy Enhanced Mail
An official Internet standard described in RFC 1421-1424.
Support privacy and authentication for RFC 822 based email systems.
The message together with its message digest is encrypted using DES with a one time key that is enclosed along with the message.
The key can be protected with RSA and certified by certification authorities.
2: Application Layer 4
2: Application Layer 5
DNS: Domain Name System
People: many identifiers: SSN, name, passport # Internet hosts, routers: IP address (32 bit) used for addressing datagrams “name”, e.g., gaia.cs.umass.edu - used by humans Q: map between IP addresses and name ?
Domain Name System:
A distributed database
implemented in hierarchy of many
name servers An application-layer protocol
that allows host, routers, name servers to communicate to
resolve
names (address/name translation) DNS provides a core Internet function, implemented as application layer protocol DNS is an example of the Internet design philosophy of placing complexity at network’s “edge” 2: Application Layer 6
DNS
DNS services Hostname to IP address translation Host aliasing Canonical and 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 7
Distributed, Hierarchical Database
Root DNS Servers com DNS servers org DNS servers edu DNS servers yahoo.com
DNS servers amazon.com
DNS servers pbs.org
DNS servers poly.edu
umass.edu
DNS servers DNS servers Client wants IP for www.amazon.com; 1 st 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
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DNS name servers
The DNS is a distributed design A large number of name servers organized in a hierarchical fashion and distributed around the world no server has all name-to-IP address mappings There are three types of name servers: (1) local name servers: each ISP, company has
local (default) name server
host DNS query first goes to local name server (2) root name servers: next) top level name servers ( to be explained (3) authoritative name servers: for a host: stores that host’s IP address, name can perform name/address translation for that host’s name 2: Application Layer 9
DNS: Root name servers
contacted by local name server that can not resolve name root name server: gets mapping returns mapping to local name server contacts authoritative name server if name mapping not known e NASA Mt View, CA f Internet Software C. Palo Alto, CA a NSI Herndon, VA c PSInet Herndon, VA d U Maryland College Park, MD g DISA Vienna, VA h ARL Aberdeen, MD j NSI (TBD) Herndon, VA k RIPE London i NORDUnet Stockholm m WIDE Tokyo b USC-ISI Marina del Rey, CA l ICANN Marina del Rey, CA 13 root name servers worldwide 2: Application Layer 10
TLD and Authoritative Servers
Top-level domain (TLD) servers:
for com, org, net, edu, etc, and all top-level country domains uk, fr, ca, jp.
responsible Network Solutions maintains servers for com TLD Educause for edu TLD
Authoritative DNS servers:
servers (e.g., Web and mail).
organization’s DNS servers, providing authoritative hostname to IP mappings for organization’s Can be maintained by organization or service provider 2: Application Layer 11
Local Name Server
Does not strictly belong to hierarchy Each ISP (residential ISP, company, university) has one.
Also called “default name server” When a host makes a DNS query, query is sent to its local DNS server Acts as a proxy, forwards query into hierarchy.
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Example
root DNS server Host at cis.poly.edu wants IP address for gaia.cs.umass.edu
2 local DNS server
dns.poly.edu
1 8 3 4 TLD DNS server 5 7 6 authoritative DNS server
dns.cs.umass.edu
requesting host
cis.poly.edu
gaia.cs.umass.edu
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Recursive queries
root DNS server recursive query: puts burden of name resolution on contacted name server heavy load?
iterated query: contacted server replies with name of server to contact “I don’t know this name, but ask this server” local DNS server
dns.poly.edu
1 2 8 requesting host
cis.poly.edu
7 6 5 3 4 EDU DNS server authoritative DNS server
dns.cs.umass.edu
gaia.cs.umass.edu
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Recursive and iterated queries
root DNS server Typically, all queries are iterated except for the query from the host to the local name server.
2 3 4 TLD DNS server 5 local DNS server
dns.poly.edu
iterated query recursive query 1 8 7 6 authoritative DNS server
dns.cs.umass.edu
requesting host
cis.poly.edu
gaia.cs.umass.edu
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DNS: caching and updating records
once (any) name server learns mapping, it
caches
mapping cache entries timeout (disappear) after some time (usually two days) Until recently, the contents of each DNS servers were configured statically from a configuration file created by a system manager.
More recently, an UPDATE option has been added to the DNS protocol to allow data to be added or deleted from the database via DNS messages.
DNS dynamic update mechanism is specified in RFC 2136 2: Application Layer 16
DNS records
DNS: distributed database storing resource records (RR) RR format:
(name, value, type, ttl)
Type=A
name value
is hostname is IP address Type=NS
name
is domain (e.g. foo.com)
value
is IP address of authoritative name server for this domain Type=CNAME
name
is alias name for some “cannonical” (the real) name www.ibm.com is really servereast.backup2.ibm.com
value
is cannonical name Type=MX
name
is alias name for some mail server
value
is the canonical name of the mail server 2: Application Layer 17
DNS protocol, messages
DNS protocol :
query message format
and
reply
messages, both with same message header identification: for query, reply to query uses same # 16 bit # flags: query or reply recursion desired recursion available reply is authoritative 2: Application Layer 18
DNS protocol, messages
Name, type fields for a query RRs in reponse to query A hostname can have multiple IP addresses records for other authoritative servers additional “helpful” information that may be used e.g. IP address for the canonical hostname of the mail server 2: Application Layer 19
Inserting records into DNS
Example: just created startup “Network Utopia” Register name networkuptopia.com at a registrar (e.g., Network Solutions) Need to provide registrar with names and IP addresses of your authoritative name server (primary and secondary) Registrar inserts two RRs into the com TLD server: (networkutopia.com, dns1.networkutopia.com, NS) (dns1.networkutopia.com, 212.212.212.1, A) Put in authoritative server Type A record for www.networkuptopia.com and Type MX record for networkutopia.com
How do people get the IP address of your Web site?
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Network Management Systems
A network management system is a collection of tools for network monitoring and control.
It has the following key elements: Management station, or manager.
Agent in managed nodes, equipments etc.
Management Information base (MIB).
Network management protocol.
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2: Application Layer 22
The management station will have A set of management applications for data analysis, fault recovery etc.
An Interface by which the network manager may monitor and control the network.
The capability of translating the network manager’s requirements into the actual monitoring and control of remote elements in the network.
A database of network management information extracted from the database of all the managed entities in the network.
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The agent is an active element residing in hosts, bridges, routers, and hubs etc. that responds to requests for information or actions from a management station, and may provide the management station (through trap) with important but unsolicited information.
An MIB is a collection of objects which are resources in the network that may be managed.
Network management protocol for TCP/IP networks is SNMP (Simple network management protocol), and for OSI-based networks is CMIP (Common Management Information Protocol).
2: Application Layer 24
SNMP: Simple Network Management Protocol First Proposed in 1988, RFC 1028, RFC 1067 Version 1 of SNMP in May 1990, RFC 1157 SNMPv2 issued in 11993, RFCs 1441 to 1452.
SNMP provides the infrastructure for network management applications.
The object definition language ASN.1 (Abstract Syntax Notation One), taken from OSI, is used for defining objects in MIBs.
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Structure of Management Information
The SMI defines the general framework within which an MIB can be defined and constructed.
The SMI identifies the data types (only simple) the scalars and two-dimensional arrays of scalars, called tables) that can be used in the MIB, and how resources within the MIB are represented and named.
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2: Application Layer 27
SNMP Protocol
Provides a basic mechanism for the exchange of management information between manager and agent.
Get-bulk-request, Inform-request, Response, and SNMPv2-trap are SNMPv2 specific.
An SNMPv2-trap is generated by an agent for reporting unusual events.
Inform-request is sent by a manager on behalf of an application, to another manager for providing management information to an application.
The manager receiving an Inform Request acknowledges receipt with Response.
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in lexicographic order Response Acknowledgement of receipt by Inform-request 2: Application Layer 29
Internet apps: application, transport protocols
Application Application layer protocol
e-mail remote terminal access Web file transfer streaming multimedia Internet telephony SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] proprietary (e.g. RealNetworks) proprietary (e.g., Dialpad)
Underlying transport protocol
TCP TCP TCP TCP TCP or UDP typically UDP 2: Application Layer 30
FTP: the file transfer protocol
user at host FTP user interface FTP client local file system file transfer FTP server remote file system transfer file to/from remote host client/server model
Client side:
the side that initiates transfer (either to/from remote)
Server side:
ftp: RFC 959 remote host ftp server: port 21 2: Application Layer 31
FTP: separate control, data connections
FTP client contacts FTP server at port 21, specifying TCP as transport protocol Client obtains authorization over control connection – username, password Client browses remote directory by sending commands over control connection.
When server receives a command for a file transfer, the server opens a TCP data connection to client After transferring one file, server closes connection.
TCP control connection port 21 FTP client TCP data connection port 20 FTP server Server opens a second TCP data connection to transfer another file.
Control connection: “out of band” FTP server maintains “state”: current directory, earlier authentication 2: Application Layer 32
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
(gets) file - retrieves
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
2: Application Layer 33
Electronic Mail
outgoing message queue user mailbox Three major components of a mail system: user agents mail servers simple mail transfer protocol: SMTP mail server User Agent Also known as “mail reader” composing, editing, reading mail messages e.g., Eudora, Outlook, elm, Netscape Messenger outgoing, incoming messages stored on server SMTP mail server user agent user agent SMTP SMTP user agent mail server user agent user agent user agent 2: Application Layer 34
Electronic Mail: mail servers
outgoing message queue user mailbox Mail Servers mailbox contains incoming messages for user message queue of outgoing (to be sent) mail messages SMTP protocol servers to send email messages between mail client: sending mail server “server”: receiving mail server message queue mailbox mail server SMTP mail server user agent user agent SMTP SMTP user agent mail server user agent user agent user agent 2: Application Layer 35
Electronic Mail: SMTP [RFC 2821]
Simple Mail Transfer Protocol (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 commands: response: ASCII text status code and phrase messages must be in 7-bit ASCII 2: Application Layer 36
Scenario: Alice sends message to Bob
1) Alice uses 2) Alice’s user agent to compose message and “to” [email protected]
user agent sends message to her mail server ; message placed in message queue 3) Alice’s mail server (Client side) of SMTP opens TCP connection with Bob’s server (server side) mail 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 outgoing message queue user mailbox 1 user agent 2 message queue mail server 3 4 mailbox mail server 5 6 user agent 2: Application Layer 37
Sample SMTP interaction
The following transcript begins as soon as the TCP connection is established: S: 220 hamburger.edu C: HELO crepes.fr
server client
S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM:
2: Application Layer 38
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) 2: Application Layer 39
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 protocol (client’s point of view) SMTP: push protocol both have ASCII command/response interaction, status codes HTTP does not require message to be in 7-bit ASCII HTTP: one object in one response message SMTP: multiple objects can be sent in one message 2: Application Layer 40
Mail message format
SMTP: protocol for exchanging email messages RFC 822: standard for text message format: header lines, e.g., To: From: Subject:
different from SMTP
commands!
body the “message”, ASCII characters only header body blank line 2: Application Layer 41
Message format: multimedia extensions
MIME (Multipurpose Internet Mail extensions) : multimedia mail extension, RFC 2045, 2056 additional lines in message header declare MIME content type MIME version method used to encode data multimedia data type, subtype, parameter declaration
From: [email protected] To: [email protected] Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data ..... ......................... ......base64 encoded data
encoded data 2: Application Layer 42
MIME types
Content-Type: type/subtype; parameters
Currently, seven types are defined: (1) Text (5)Application other data that must be processed by reader before “viewable” example subtypes:
plain, html
example subtypes:
msword, octet-stream
(2) Image example subtypes:
jpeg, gif
(6) Multipart one or more different sets of data are combined in a single body (3) Audio exampe subtypes:
basic
(8 bit mu-law encoded),
32kadpcm
(32 kbps Adaptive Differential Pulse Code Modulation coding) (4) Video example subtypes:
mpeg, quicktime
example subtypes: mixed, alternative (alternative version of the same information) (7) Message encapsulate another mail message example subtypes: rfc822, partial 2: Application Layer 43
Example of Multipart Type
From: [email protected] To: [email protected] Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Type: multipart/mixed; boundary=StartOfNextPart --StartOfNextPart Dear Bob, Please find a picture of a crepe.
--StartOfNextPart Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data ..... ......................... ......base64 encoded data --StartOfNextPart Do you want the reciple?
2: Application Layer 44
Header line inserted by the receiving server
Received: from crepes.fr by hamburger.edu; 12 Oct 98 15:27:39 GMT From: [email protected] To: [email protected] Subject: Picture of yummy crepe. MIME-Version: 1.0 Content-Transfer-Encoding: base64 Content-Type: image/jpeg base64 encoded data ..... ......................... ......base64 encoded data
2: Application Layer 45
Mail access protocols
user agent SMTP SMTP Mail access protocol user agent sender’s mail server receiver’s mail server SMTP: delivery/storage to receiver’s server Mail access protocol: retrieval from server POP3: Post Office Protocol, version 3 [RFC 1939] • authorization (agent <--> server) and download IMAP: Internet Mail Access Protocol [RFC 2060] • more features (more complex) • manipulation of stored messages on server HTTP: Hotmail , Yahoo! Mail, etc.
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POP3 protocol
(1) Client opens a TCP connection to the mail server on port 110 (2) authorization phase client commands:
user: pass:
declare username password server responses
+OK
(3) transaction phase,
-ERR list:
client: list message numbers
retr:
number retrieve message by
dele:
delete
Quit
(4) update phase : deletes the messages marked for deletion mail server
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:
Message size Message number
POP3 server signing off
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POP3 (more) and IMAP
More about POP3 Previous example uses “ download and delete ” mode.
Bob cannot re-read e-mail if he changes client “ Download-and-keep on different clients ” mode: copies of messages POP3 is stateless across sessions IMAP Keep all messages in one place: the server Allows user to organize messages in folders IMAP keeps user state across sessions: names of folders and mappings between message IDs and folder name 2: Application Layer 48
Web and HTTP
First some jargon Web page consists of objects An object is a file such as an HTML file, a JPEG image, a Java applet, an audio file,… A Web page consists of a base HTML-file several referenced objects and The base HTML file references the other objects in the page with the object’s URL s (Uniform Resource Locators) Example URL: www.someschool.edu/someDept/pic.gif
host name path name 2: Application Layer 49
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 2616 PC running Explorer Mac running Navigator Server running Apache Web server 2: Application Layer 50
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 “state” are complex!
aside past history (state) must be maintained if server/client crashes, their views of “state” may be inconsistent, must be reconciled 2: Application Layer 51
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.
A new connection need not be set up for the transfer of each Web object HTTP/1.1 uses persistent connections in default mode – can be configured to use nonpersistent connection 2: Application Layer 52
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 1b.
HTTP server at host www.someSchool.edu client waiting for TCP connection at port 80. “accepts” connection, notifying 2.
HTTP client sends HTTP
request message
(containing URL) into TCP connection socket. Message indicates that client wants object someDepartment/home.index
3.
HTTP server receives request message, forms
message response
containing requested object, and sends message into its socket time 2: Application Layer 53
Nonpersistent HTTP (cont.)
time 5 .
HTTP client receives response message containing html file, displays html. Parsing html file, finds 10 references to the 10 jpeg objects 6.
Steps 1-5 repeated for each of 10 jpeg objects 4.
HTTP server closes TCP connection. 2: Application Layer 54
Response time modeling
Definition of RRT: Response time: time to send a small packet to travel from client to server and back.
one RTT to initiate TCP connection one RTT for HTTP request and first few bytes of HTTP response to return file transmission time total = 2RTT+transmit time initiate TCP connection RTT request file RTT file received time time to transmit file time 2: Application Layer 55
Persistent HTTP
Nonpersistent HTTP issues: requires 2 RTTs per object OS must work and allocate host resources for each TCP connection but 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 are sent over connection Two versions of persistent connections: Persistent without pipelining: one RTT for each referenced object Persistent with pipelining: client issues new request only when previous response has been received 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 56
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
GET /somedir/page.html HTTP/1.1
Host: www.someschool.edu User-agent: Mozilla/4.0
Connection: close Accept-language:fr
Carriage return, line feed indicates end of message (extra carriage return, line feed) 2: Application Layer 57
Explanation of the example
GET /somedir/page.html HTTP/1.1
-- Request to return the object
/somedir/page.html
-- The browser implements version HTTP/1.1
Host: www.someschool.edu
-- Specifies the host on which the object resides User-agent: Mozilla/4.0
-- Specifies the browser type that is making the request
Connection: close
-- Indicates that the connection SHOULD NOT be considered `persistent‘. It wants the server to close the connection after the current request/response is complete
Accept-language:fr -- Indicates that the user prefers to receive a French version of the object
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HTTP request message: general format
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Method types
HTTP/1.0
GET : Return the object POST : Send information to be stored on the server HEAD Return only information about the object, such as how old it is, but not the object itself HTTP/1.1
GET, POST, HEAD PUT Uploads a new copy of existing object in entity body to path specified in URL field DELETE deletes object specified in the URL field 2: Application Layer 60
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 2: Application Layer 61
HTTP response message
An HTTP response consists of the following: 1.
2.
3.
A status line , which indicates the success or failure of the request Header lines : A description of the information in the response. This is the metadata or meta information The actual information requested status line (protocol status code status phrase) header lines blank line
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, e.g., requested HTML file
data data data data data ...
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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
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Trying out HTTP (client side) for yourself
1. Telnet to your favorite Web server:
telnet www.eurecom.fr 80
Opens TCP connection to port 80 (default HTTP server port) at www.eurecom.fr.
Anything typed is sent to port 80 at www.eurecom.fr
2. Type in a GET HTTP request:
GET /~ross/index.html HTTP/1.0
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 64