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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 onetime 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 applicationlayer 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 yahoo.com amazon.com DNS servers DNS servers org DNS servers pbs.org DNS servers edu DNS servers poly.edu umass.edu DNS serversDNS 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 8 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: top level name servers ( to be explained next) (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 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 e NASA Mt View, CA f Internet Software C. Palo Alto, CA 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: 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 and mail). 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. 2: Application Layer 12 Example root DNS server Host at cis.poly.edu 2 wants IP address for gaia.cs.umass.edu 3 TLD DNS server 4 5 local DNS server dns.poly.edu 1 8 requesting host 7 6 authoritative DNS server dns.cs.umass.edu cis.poly.edu gaia.cs.umass.edu 2: Application Layer 13 Recursive queries recursive query: 2 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” root DNS server 3 7 6 EDU DNS server local DNS server dns.poly.edu 1 5 4 8 requesting host authoritative DNS server dns.cs.umass.edu cis.poly.edu gaia.cs.umass.edu 2: Application Layer 14 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 TLD DNS server 4 5 local DNS server dns.poly.edu recursive query 1 8 requesting host 7 6 iterated query authoritative DNS server dns.cs.umass.edu cis.poly.edu gaia.cs.umass.edu 2: Application Layer 15 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, Type=A name is hostname value is IP address value, type, ttl) Type=CNAME name is alias name for some “cannonical” (the real) name www.ibm.com is really servereast.backup2.ibm.com Type=NS value is cannonical name name is domain (e.g. foo.com) Type=MX value is IP address of name is alias name for some authoritative name server for this domain mail server value is the canonical name of the mail server 2: Application Layer 17 DNS protocol, messages DNS protocol : 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 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? 2: Application Layer 20 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. 2: Application Layer 21 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. 2: Application Layer 23 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. 2: Application Layer 25 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. 2: Application Layer 26 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. 2: Application Layer 28 in lexicographic order Response Acknowledgement of receipt by Inform-request 2: Application Layer 29 Internet apps: application, transport protocols Application e-mail remote terminal access Web file transfer streaming multimedia Internet telephony Application layer protocol Underlying transport protocol SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] proprietary (e.g. RealNetworks) proprietary (e.g., Dialpad) TCP TCP TCP TCP TCP or UDP typically UDP 2: Application Layer 30 FTP: the file transfer protocol user at host FTP FTP user client interface file transfer local file system 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: remote host ftp: RFC 959 ftp server: port 21 2: Application Layer 31 FTP: separate control, data connections TCP control connection port 21 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. 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: Sample return codes sent as ASCII text over control status code and phrase (as in channel HTTP) 331 Username OK, USER username password required PASS password 125 data connection LIST -- return list of file in already open; current directory transfer starting RETR filename -- retrieves 425 Can’t open data (gets) file connection STOR filename -- stores 452 Error writing (puts) file onto remote host file 2: Application Layer 33 Electronic Mail outgoing message queue user mailbox Three major components of a mail system: user agents user agent mail server mail servers SMTP simple mail transfer protocol: SMTP 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 SMTP user agent mail server user agent user agent user agent 2: Application Layer 34 Electronic Mail: mail servers outgoing message queue user mailbox user agent Mail Servers mailbox contains incoming mail server messages for user message queue of outgoing (to be sent) mail messages SMTP protocol between mail SMTP servers to send email messages client: sending mail mail server server “server”: receiving mail server message queue user mailbox SMTP SMTP user agent mail server user agent user agent user agent 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: ASCII text response: status code and phrase messages must be in 7-bit ASCII 2: Application Layer 36 Scenario: Alice sends message to Bob 1) Alice uses user agent to compose message and “to” [email protected] 2) Alice’s 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 mail server (server side) 1 user agent 2 message queue mail server 3 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 mail server 4 mailbox 5 6 user agent 2: Application Layer 37 Sample SMTP interaction The following transcript begins as soon as the TCP connection is established: server S: 220 hamburger.edu C: HELO crepes.fr client S: 250 Hello crepes.fr, pleased to meet you C: MAIL FROM: <[email protected]> S: 250 [email protected]... Sender ok C: RCPT TO: <[email protected]> S: 250 [email protected] ... 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 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 7bit 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! header blank line body body the “message”, ASCII characters only 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 encoded data 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 42 MIME types Content-Type: type/subtype; parameters Currently, seven types are (5)Application defined: other data that must be processed (1) Text by reader before “viewable” example subtypes: plain, html (2) Image example subtypes: jpeg, gif example subtypes: msword, octet-stream (6) Multipart one or more different sets of data exampe subtypes: basic (8- are combined in a single body example subtypes: mixed, alternative (alternative version of the same information) (4) Video example subtypes: rfc822, partial (3) Audio bit mu-law encoded), 32kadpcm (32 kbps Adaptive Differential Pulse Code (7) Message Modulation coding) encapsulate another mail message example subtypes: mpeg, quicktime 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 sender’s mail server Mail access user protocol agent 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. 2: Application Layer 46 POP3 protocol (1) Client opens a TCP connection to the mail server on port 110 (2) authorization phase client commands: user: declare username pass: password server responses +OK -ERR (3) transaction phase, client: list: list message numbers retr: retrieve message by number dele: delete Quit (4) update phase : mail server deletes the messages marked for deletion S: C: S: C: S: +OK POP3 server ready user bob +OK pass hungry +OK user successfully logged C: S: S: S: C: S: S: C: C: S: S: C: C: S: list 1 498 Message size 2 912 . Message number retr 1 <message 1 contents> . dele 1 retr 2 <message 1 contents> . dele 2 quit +OK POP3 server signing off 2: Application Layer on 47 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” mode: copies of messages on different clients 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 and several referenced objects The base HTML file references the other objects in the page with the object’s URLs (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 Server running Apache Web server Mac running Navigator 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 (applicationlayer 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 Protocols that maintain “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 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 (contains text, references to 10 www.someSchool.edu/someDepartment/home.index jpeg images) Suppose user enters URL 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 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 2: Application Layer 53 Nonpersistent HTTP (cont.) 4. HTTP server closes TCP 5. HTTP client receives response connection. message containing html file, displays html. Parsing html file, finds 10 references to the 10 jpeg objects time 6. Steps 1-5 repeated for each of 10 jpeg objects 2: Application Layer 54 Response time modeling Definition of RRT: time to send a small packet to travel from client to server and back. 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 total = 2RTT+transmit time initiate TCP connection RTT request file time to transmit file RTT file received time 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: 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 56 HTTP request message two types of HTTP messages: request, response HTTP request message: ASCII (human-readable format) request line (GET, POST, HEAD commands) GET /somedir/page.html HTTP/1.1 Host: www.someschool.edu User-agent: Mozilla/4.0 header Connection: close lines 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 2: Application Layer 58 HTTP request message: general format 2: Application Layer 59 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) HTTP/1.1 200 OK Connection close Date: Thu, 06 Aug 1998 12:00:15 GMT Server: Apache/1.3.0 (Unix) header Last-Modified: Mon, 22 Jun 1998 …... lines Content-Length: 6821 Content-Type: text/html blank line data, e.g., requested HTML file data data data data data ... 2: Application Layer 62 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 63 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