Applications. Chong-kwon Kim. Running in end systems (hosts) over transport layer protocols Ex: , Web, FTP, instant messaging

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1 Applications Chong-kwon Kim Applications Network applications Running in end systems (hosts) over transport layer protocols Ex: , Web, FTP, instant messaging Application layer protocols Define app. layer message formats and actions SNU SCONE lab. 2

2 Electronic Mail Two types of components User agent(mail reader), Mail server Protocols Mail transfer protocol From sender agent to receiver s mail server SMTP(Simple Mail Transfer Protocol) Mail access protocol Receiver pulls mails from server to agent POP3, IMAP3, HTTP (webmail) Message format MIME (Multipurpose Internet Mail Extensions) Mail server SMTP Mail server User agent SMTP SMTP Mail server User agent User agent User agent SNU SCONE lab. 3 Mail Server & User Agent Mail servers Mailbox contains incoming messages for users Message queue of outgoing mail messages User agent Mail reader Composing, editing, reading mail messages E.g. Outlook, pine, SNU SCONE lab. 4

3 Example 1) A uses UA to compose to b@snu.ac.kr 2) A s UA sends message to A s mail server through SMTP; message in message queue 3) A s mail server opens a SMTP session with B s mail server 4) A s message is sent to B s mail server 5) B s mail server places message in B s mail box 6) B invokes user agent to read message 1 5 User agent 2 Mail server 3 4 Mail server 6 User agent SNU SCONE lab. 5 SMTP (Simple Mail Transfer Protocol) SMTP Forward messages between mail servers Client: sending mail server Server: receiving mail server, TCP port 25 Three phases Handshaking (greeting) Transfer of messages Closure Command/response interaction Commands: ASCII text Response: status code and phases SNU SCONE lab. 6

4 SMTP Example telnet mail.snu.ac.kr 25 SNU SCONE lab. 7 Mail Access Protocol Mail access protocol: retrieval from mailbox POP: Post Office Protocol Authorization and download IMAP: Internet Mail Access Protocol More features Manipulation of stored messages on server HTTP: Webmail User agent SMTP Mail server SMTP Mail server Access protocol User agent SNU SCONE lab. 8

5 HTTP and Web HTTP (Hyper-Text Transfer Protocol) Application protocol that requests and transfers hypertext messages HTML, XML,.. Web Applications using HTTP protocol Client/Server model Client: Brower that requests, receives and displays Web objects (e.g. IE, Netscape, Firefox, Chrome,..) Server: Sends objects in response to requests (e.g. Apache, MS IIS) HTTP1.0, HTTP1.1 HTTP request HTTP response SNU SCONE lab. 9 HTTP Request Human readable ASCII Request line (GET/POST/HEAD Commands) Header lines GET /somedir/page.html HTTP/1.1 User-grant: Mozilla/4.0 Connection: close Accept-language: en Method sp URL sp version cr lf request line Hdr fd name : value cr lf hdr lines Hdr fd name : value cr lf cr lf Entity Body SNU SCONE lab. 10

6 HTTP Response Status line (protocol status code status phrase) header lines Data, e.g. Requested HTML file HTTP/ OK Connection close Date: Thu, 27 May :00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 24 May 2004 Content-Length: 6821 Content-Type: text/html Data. SNU SCONE lab. 11 HTTP Connection TCP Client initiates TCP connection to server, port 80 HTTP messages exchanged between browser and Web server using TCP Non-persistent HTTP At most one object is sent over a TCP connection HTTP/1.0 Persistent HTTP Multiple objects can be sent over single TCP connection between client and server HTTP/1.1 SNU SCONE lab. 12

7 SNU SCONE lab. 13 FTP (File Transfer Protocol) Transfer file to/from remote host Client/Server Client: the side that initiates file transfer Server: remote host Server well-known port: TCP port 21 TCP control connection (#21) TCP data connection (#20) SNU SCONE lab. 14

8 FTP Connections TFP uses two connection one for control and the other for data transfer Control connection FTP client contacts FTP server at port 21 Client obtains authorization over control connection Client browses remote directory by sending commands over control connection Client request file transfer Data 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 data connection SNU SCONE lab. 15 DNS Name, Address & Routing Name: What you are Address: Where you are Routing: How to reach you. Address (IP address) Bit string for machine Embeds routing information Name Character string for human use Mnemonic SNU SCONE lab.

9 Domain Name & Name Resolution Domain names identify objects on the Internet Host Name server Mail server Information Name space Set of possible names Flat, hierarchical A name is associated with a set of values (Host name, IP address) (Mail server name, Host name) SNU SCONE lab. Name Resolution Find the value associated with a given name SNU SCONE lab.

10 Domain Hierarchy The Internet has too many objects for a single management center Partition the name space into a hierarchical tree edu com gov mil org net kr jp arizona...mit bellcore... dec nasa... nsf arpa... navy acm... ieee cs ece physic bas che opt Node: Domain Context in which additional domain/name can be defined Leaf node: Name SNU SCONE lab. Zone The name space hierarchy is partitioned into zones Each zone is an administrative authority Name assignment Name resolution edu com gov mil org net uk fr arizona...mit bellcore... dec nasa... nsf arpa... navy acm... ieee cs ece physic bas che opt SNU SCONE lab.

11 Name Server Each zone has at least two name servers for name resolution Reliability The name server manages the information on objects as resource records <Name, Value, Type, Class, TTL> Type: Specifies the types of the value A: IP address NS: Name server CNAME: Canonical name MX: Mail server SNU SCONE lab. Name Server A name server for a zone contains resource records of Name-to-value bindings for objects in the zone IP addresses of the next low level zone name servers IP addresses of the root name server Root name server contains <edu, a3.nstld.com, NS, IN> < a3.nstld.com, , A, IN> <com, > SNU SCONE lab.

12 Name Server Hierarchy SNU SCONE lab. 23 Name Resolution SNU SCONE lab.

13 Multimedia Applications Realtime Video/Audio conference Differences from POTS (Plain Old Telephone Service) Voice only One to one Homogeneous devices Number mapped to device Multimedia Many to many Heterogeneous devices Name mapped tomobile person With multiple devices There were efforts called IN (Intelligent Networks) to add functions to POTS SNU SCONE lab. 25 Session Control Protocols Suite of protocols that make multimedia applications possible SDP (Session Description Protocol) SAP (Session Announcement Protocol) SIP (Session Initiation Protocol) SCCP (Simple Conference Control Protocol) SDP describes the attributes of a session Attributes of a session Multicast address, media and encoding rules, initiator, title, owner, time, SAP announces sessions to the public SIP locates callees and makes connections to them SNU SCONE lab. 26

14 Session Directory Session owner publicizes the session directory using SAP Along with session description SNU SCONE lab. 27 SIP, H323 Both protocols from IETF and ITU handle call (session) control SIP is simpler and thus much more popular Functions User location User mobility Multi devices User availability User capability Session setup Session mgmt SNU SCONE lab. 28

15 SIP SNU SCONE lab. 29 SIP Message INVITE sip: SIP/2.0 Via: SIP/2.0/UDP bsd-pc.cisco.com;branch=z9hg4bk433yte4 To: Larry <sip : larry@princeton.edu> From: Bruce <sip : bruce@cisco.com>; tag=55123 Call-ID: xy74jj210re3@bsd-pc.cisco.com Cseq: INVITE Contact: <sip : bsd-pc.cisco.com;branch> Content-Type: application/sdp Content-Length: 142 SNU SCONE lab. 30

16 Overlay Networks Overlay network Build abstracted networks on top of a base network Why overlay networks? To provide services that commercial routers do not support Requires special packet forwarding & processing Multicast, IPv6, Where to provide special services Routers Modify or add special capabilities to routers Efficient but difficult Applications Easy but inefficient SNU SCONE lab. 31 Multicast Mbone Network level multicast Form a multicast overlay consists of multicast enabled routers Use IP tunneling to connect Mbone-enabled routers over legacy routers SNU SCONE lab. 32

17 End System Multicast Hosts performs multicast specific packet forwarding In application level, all hosts are directly connected Naïve multicast tree construction yields inefficient routing SNU SCONE lab. 33 End System Multicast - 2 Multicast tree construction Guess multicast mesh from a complete graph consists of member nodes Construct multicast tree Management Join: new member sends JOIN msg to one or more member nodes to add branch to the multicast tree Leave: May cause network partitioning Join MSG from one partition to another SNU SCONE lab. 34

18 End System Multicast - 3 How to enhance the quality of multicast tree Link addition Periodically, each host randomly selects a target and evaluate the utility of adding a link between them Link deletion Periodically evaluate the cost of deleting links SNU SCONE lab. 35 P2P (Peer To Peer) Purpose Decentralized file sharing Community members exchange files (mostly music and video) without the centralized aids Issues How to locate objects of interest How to download files Without centralized control for millions of users SNU SCONE lab. 36

19 Gnutella Overlay network Link is established between nodes that know each other Object location Query flooding over the overlay Use TTL and QID to control flooding Overlay maintenance Should know at least one neighbor when join Lean more during QUERY processing Scalable? SNU SCONE lab. 37 Structured Overlays Unstructured overlay Like Gnutella, object locating is random (or centralized) Inefficient & unreliable Structured overlay Structured graph for reliable, predictable and efficient object locating Mechanism Store certain objects to a specific host (node) hash(x) n How to find the node that contains object SNU SCONE lab. 38

20 Structured Overlays - Pastry Consistent hashing A range of hash values are mapped to a bin Support dynamic changes of the number of bins H1(name) OID H2(IP_Addr) NID OIDs are mapped to the closest NID SNU SCONE lab. 39 Distributed Hash Tables (DHT) Given OID, how to find the node that keeps the object? Route toward the node until meet the closest node DHT SNU SCONE lab. 40

21 DHT - 2 Each node has a routing table and leaf set Leaf set A set of numerically larger or smaller nodes If D is within range of my leaf set forward to numerically closest member in leaf set else let l = length of shared prefix let d = value of l-th digit in D s address if RouteTab[l,d] exists forward to RouteTab[l,d] else forward to known node with at least as long a shared prefix and numerically closer than this node SNU SCONE lab. 41 Node Addition Should know at least one node Similar to routing Build its routing table as move forward to its position SNU SCONE lab. 42

22 BitTorrent New ideas Swarm Incentive for good behavior SNU SCONE lab. 43 Content Distribution Networks (CDN) Replicate content to geographically distributed surrogates (caches) to alleviate traffic concentration and network burden SNU SCONE lab. 44