Application Layer and Socket Programming Hakim Weatherspoon

Application Layer and Socket Programming Hakim Weatherspoon Assistant Professor, Dept of Computer Science CS 5413: High Performance Systems and Networking September 3, 2014 Slides used and adapted judiciously from Computer Networking, A Top-Down Approach

Goals for Today Application Layer – Example network applications – conceptual, implementation aspects of network application protocols – client-server paradigm – transport-layer service models Socket Programming – Client-Server Example Backup Slides – Web Caching – DNS (Domain Name System)

Some network apps e-mail web text messaging remote login P2P file sharing multi-user network games streaming stored video (YouTube, Hulu, Netflix) voice over IP (e.g., Skype) real-time video conferencing social networking search

Creating a network app 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 app development, propagation application transport network data link physical application transport network data link physical application transport network data link physical

Client-Server Architecture server: always-on host permanent IP address data centers for scaling clients: client/server communicate with server may be intermittently connected may have dynamic IP addresses do not communicate directly with each other

Network Applications Communicating Processes process: program running within a host clients, servers within same host, two processes communicate using inter-process communication (defined by OS) processes in different hosts communicate by exchanging messages client process: process that initiates communication server process: process that waits to be contacted aside: applications with P2P architectures have client processes & server processes

Network Applications 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 application process socket application process transport transport network network link physical Internet link physical controlled by app developer controlled by OS

Network Applications How to identify network applications? to receive messages, process must have identifier host device has unique 32bit IP address Q: does IP address of host on which process runs suffice for identifying the process? A: 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 to send HTTP message to www.cs.cornell.edu web server: – IP address: 128.84.154.137 – port number: 80

Network Applications App-Layer protocols define: types of messages exchanged, – e.g., request, response message syntax: – what fields in messages & how fields are delineated message semantics – meaning of information in fields rules for when and how processes send & respond to messages open protocols: defined in RFCs allows for interoperability e.g., HTTP, SMTP proprietary protocols: e.g., Skype

Communicating Network Applications Processes What transport layer services does an app 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,

Network Applications What transport layer services does an app need? application data loss throughput file transfer e-mail Web documents real-time audio/video no loss no loss no loss loss-tolerant stored audio/video interactive games text messaging loss-tolerant loss-tolerant no loss elastic no elastic no elastic no audio: 5kbps-1Mbps yes, 100’s msec video:10kbps-5Mbps same as above yes, few secs few kbps up yes, 100’s msec elastic yes and no time sensitive

Network Applications Transport Protocol Services TCP service: 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 UDP service: 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?

Network Applications Transport Protocol Services 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] HTTP (e.g., YouTube), RTP [RFC 1889] SIP, RTP, proprietary (e.g., Skype) TCP TCP TCP TCP TCP or UDP TCP or UDP

Network Applications: Securing TCP TCP & UDP no encryption cleartext passwds sent into socket traverse Internet in cleartext SSL provides encrypted TCP connection data integrity end-point authentication SSL is at app layer Apps use SSL libraries, which “talk” to TCP SSL socket API cleartext passwds sent into socket traverse Internet encrypted See Chapter 7

Goals for Today Application Layer – Example network applications – conceptual, implementation aspects of network application protocols – client-server paradigm – transport-layer service models Socket Programming – Client-Server Example Backup Slides – Web Caching – DNS (Domain Name System)

Socket Programming goal: learn how to build client/server applications that communicate using sockets socket: door between application process and end-end-transport protocol application process socket application process transport transport network network link physical Internet link physical controlled by app developer controlled by OS

Socket Programming Two socket types for two transport services: – UDP: unreliable datagram – TCP: reliable, byte stream-oriented Application Example: 1. Client reads a line of characters (data) from its keyboard and sends the data to the server. 2. The server receives the data and converts characters to uppercase. 3. The server sends the modified data to the client. 4. The client receives the modified data and displays the line on its screen.

Socket Programming w/ UDP UDP: no “connection” between client & server no handshaking before sending data sender explicitly attaches IP destination address and port # to each packet rcvr extracts sender IP address and port# from received packet UDP: transmitted data may be lost or received out-of-order Application viewpoint: UDP provides unreliable transfer of groups of bytes (“datagrams”) between client and server

Socket Programming w/ UDP server (running on serverIP) create socket, port x: serverSocket socket(AF INET,SOCK DGRAM) read datagram from serverSocket write reply to serverSocket specifying client address, port number client create socket: clientSocket socket(AF INET,SOCK DGRAM) Create datagram with server IP and port x; send datagram via clientSocket read datagram from clientSocket close clientSocket

Socket Programming w/ UDP Python UDPClient include Python’s socket library create UDP socket for server get user keyboard input Attach server name, port to message; send into socket read reply characters from socket into string print out received string and close socket from socket import * serverName ‘hostname’ serverPort 12000 clientSocket socket(socket.AF INET, socket.SOCK DGRAM) message raw input(’Input lowercase sentence:’) clientSocket.sendto(message,(serverName, serverPort)) modifiedMessage, serverAddress clientSocket.recvfrom(2048) print modifiedMessage clientSocket.close()

Socket Programming w/ UDP Python UDPServer create UDP socket bind socket to local port number 12000 loop forever Read from UDP socket into message, getting client’s address (client IP and port) send upper case string back to this client from socket import * serverPort 12000 serverSocket socket(AF INET, SOCK DGRAM) serverSocket.bind(('', serverPort)) print “The server is ready to receive” while 1: message, clientAddress serverSocket.recvfrom(2048) modifiedMessage message.upper() serverSocket.sendto(modifiedMessage, clientAddress)

Socket Programming w/ TCP client must contact server server process must first be running server must have created socket (door) that welcomes client’s contact client contacts server by: Creating TCP socket, specifying IP address, port number of server process when client creates socket: client TCP establishes connection to server TCP when contacted by client, server TCP creates new socket for server process to communicate with that particular client – allows server to talk with multiple clients – source port numbers used to distinguish clients (more in Chap 3) application viewpoint: TCP provides reliable, in-order byte-stream transfer (“pipe”) between client and server

Socket Programming w/ TCP server (running on hostid) client create socket, port x, for incoming request: serverSocket socket() wait for incoming TCP connection request connectionSocket connection serverSocket.accept() read request from connectionSocket write reply to connectionSocket close connectionSocket setup create socket, connect to hostid, port x clientSocket socket() send request using clientSocket read reply from clientSocket close clientSocket

Socket Programming w/ TCP Python TCPClient create TCP socket for server, remote port 12000 No need to attach server name, port from socket import * serverName ’servername’ serverPort 12000 clientSocket socket(AF INET, SOCK STREAM) clientSocket.connect((serverName,serverPort)) sentence raw input(‘Input lowercase sentence:’) clientSocket.send(sentence) modifiedSentence clientSocket.recv(1024) print ‘From Server:’, modifiedSentence clientSocket.close()

Socket Programming w/ TCP Python TCPServer create TCP welcoming socket server begins listening for incoming TCP requests loop forever server waits on accept() for incoming requests, new socket created on return read bytes from socket (but not address as in UDP) close connection to this client (but not welcoming socket) from socket import * serverPort 12000 serverSocket socket(AF INET,SOCK STREAM) serverSocket.bind((‘’,serverPort)) serverSocket.listen(1) print ‘The server is ready to receive’ while 1: connectionSocket, addr serverSocket.accept() sentence connectionSocket.recv(1024) capitalizedSentence sentence.upper() connectionSocket.send(capitalizedSentence) connectionSocket.close()

Perspective application architectures – client-server – P2P application service requirements: – reliability, bandwidth, delay Internet transport service model – connection-oriented, reliable: TCP – unreliable, datagrams: UDP specific protocols: HTTP FTP SMTP, POP, IMAP DNS P2P: BitTorrent, DHT socket programming: TCP, UDP sockets Application Layer is the same in a data center!

Before Next time Project Group: Make sure that you are part of one Finish Lab0 No required reading and review due But, review chapter 3 from the book, Transport Layer – We will also briefly discuss – Data center TCP (DCTCP), Mohammad Alizadeh, Albert Greenberg, David A. Maltz, Jitendra Padhye, Parveen Patel, Balaji Prabhakar, Sudipta Sengupta, and Murari Sridharan. ACM SIGCOMM Computer Communications Review, Volumne 40, Issue 4 (August 2010), pages 63-74. Check website for updated schedule

Goals for Today Application Layer – Example network applications – conceptual, implementation aspects of network application protocols – client-server paradigm – transport-layer service models Socket Programming – Client-Server Example Backup Slides – Web Caching – DNS (Domain Name System)

Web Caches (proxies) goal: satisfy client request without involving origin server user sets browser: Web accesses via cache browser sends all HTTP requests to cache – object in cache: cache returns object – else cache requests object from origin server, then returns object to client HT TP H client TTP req res p proxy server ues t ons e t s ue q e e Pr ns T o T p H es r TP T H client st e u req P T se n o HT p origin res P T server HT origin server

Web Caches (proxies) cache acts as both client and server – server for original requesting client – client to origin server typically cache is installed by ISP (university, company, residential ISP) why Web caching? reduce response time for client request reduce traffic on an institution’s access link Internet dense with caches: enables “poor” content providers to effectively deliver content (so too does P2P file sharing)

Web Caching Example assumptions: avg object size: 100K bits avg request rate from browsers to origin servers:15/sec avg data rate to browsers: 1.50 Mbps RTT from institutional router to any origin server: 2 sec access link rate: 1.54 Mbps problem! consequences: LAN utilization: 15% access link utilization 99% total delay Internet delay access delay LAN delay 2 sec minutes usecs origin servers public Internet 1.54 Mbps access link institutional network 1 Gbps LAN

Web Caching Example: Fatter access Link assumptions: avg object size: 100K bits avg request rate from browsers to origin servers:15/sec avg data rate to browsers: 1.50 Mbps RTT from institutional router to any origin server: 2154 sec access link rate: 1.54 Mbps Mbps consequences: 9.9% LAN utilization: 15% access link utilization 99% total delay Internet delay access delaymsecs LAN delay 2 sec minutes usecs public Internet origin servers 1.54 Mbps 154 Mbps access link institutional network Cost: increased access link speed (not cheap!) 1 Gbps LAN

Web Caching Example: Install Local Cache assumptions: avg object size: 100K bits avg request rate from browsers to origin servers:15/sec avg data rate to browsers: 1.50 Mbps RTT from institutional router to any origin server: 2 sec access link rate: 1.54 Mbps consequences: LAN utilization: 15% ? access link utilization 100% ? Internet delay total delay access delay LAN delay How to compute link 2 sec minutes usecs utilization, delay? Cost: web cache (cheap!) origin servers public Internet 1.54 Mbps access link institutional network 1 Gbps LAN local web cache

Web Caching Example: Install Local Cache Calculating access link utilization, delay with cache: origin servers suppose cache hit rate is 0.4 – 40% requests satisfied at cache, 60% requests satisfied at origin access public Internet link utilization: 60% of requests use access link data rate to browsers over access link 0.6*1.50 Mbps .9 Mbps utilization 0.9/1.54 .58 total delay 0.6 * (delay from origin servers) 0.4 * (delay when satisfied at cache) 0.6 (2.01) 0.4 ( msecs) 1.2 secs less than with 154 Mbps link (and cheaper too!) 1.54 Mbps access link institutional network 1 Gbps LAN local web cache

Web Caching Example: Conditional GET server client Goal: don’t send object if cache has up-to-date cached version – no object transmission delay – lower link utilization cache: specify date of cached copy in HTTP request If-modified-since: date server: response contains no object if cached copy is up-to-date: HTTP/1.0 304 Not Modified HTTP request msg If-modified-since: date HTTP response HTTP/1.0 304 Not Modified HTTP request msg If-modified-since: date HTTP response HTTP/1.0 200 OK data object not modified before date object modified after date

Goals for Today Application Layer – Example network applications – conceptual, implementation aspects of network application protocols – client-server paradigm – transport-layer service models Socket Programming – Client-Server Example Backup Slides – Web Caching – DNS (Domain Name System)

DNS (Domain Name System) people: 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 ? Domain Name System: distributed database implemented in hierarchy of many name servers application-layer protocol: hosts, name servers communicate to resolve names (address/name translation) – note: core Internet function, implemented as applicationlayer protocol – complexity at network’s “edge”

DNS Structure DNS services why not centralize DNS? 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 single point of failure traffic volume distant centralized database maintenance A: doesn’t scale!

DNS Structure A 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 umass.edu cornell.edu DNS serversDNS 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

DNS Structure Root name servers contacted by local name server that can not resolve name root name server: – contacts authoritative name server if name mapping not known – gets mapping – returns mapping to local name server c. Cogent, Herndon, VA (5 other sites) d. U Maryland College Park, MD h. ARL Aberdeen, MD j. Verisign, Dulles VA (69 other sites ) e. NASA Mt View, CA f. Internet Software C. Palo Alto, CA (and 48 other sites) a. Verisign, Los Angeles CA (5 other sites) b. USC-ISI Marina del Rey, CA l. ICANN Los Angeles, CA (41 other sites) g. US DoD Columbus, OH (5 other sites) k. RIPE London (17 other sites) i. Netnod, Stockholm (37 other sites) m. WIDE Tokyo (5 other sites) 13 root name “servers” worldwide

DNS Structure Top-Level Domain (TLD) and Authoritative Servers top-level domain (TLD) servers: – responsible for com, org, net, edu, aero, jobs, museums, and all top-level country domains, e.g.: uk, fr, ca, jp – Network Solutions maintains servers for .com TLD – 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

DNS Structure Local DNS Name Servers does not strictly belong to hierarchy each ISP (residential ISP, company, university) has one – also called “default name server” when host makes DNS query, 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

DNS Structure: Resolution example root DNS server 2 host at cis.poly.edu wants IP address for www.cs.cornell.edu iterated query: contacted server replies with name of server to contact “I don’t know this name, but ask this server” 3 4 TLD DNS server 5 local DNS server dns.poly.edu 1 8 requesting host 7 6 authoritative DNS server dns.cs.cornell.edu cis.poly.edu www.cs.cornell.edu

DNS Structure: Resolution example root DNS server 2 recursive query: puts burden of name resolution on contacted name server heavy load at upper levels of hierarchy? 3 7 6 TLD DNS server local DNS server dns.poly.edu 1 5 4 8 requesting host authoritative DNS server dns.cs.cornell.edu cis.poly.edu www.cs.cornell.edu

DNS Structure Caching and 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 nameto-address translation!) – if name host changes IP address, may not be known Internet-wide until all TTLs expire update/notify mechanisms proposed IETF standard – RFC 2136

DNS Structure DNS Records DNS: distributed db storing resource records (RR) RR format: (name, type A name is hostname value is IP address type NS – name is domain (e.g., foo.com) – value is hostname of authoritative name server for this domain value, type, ttl) type CNAME name is alias name for some “canonical” (the real) name www.ibm.com is really servereast.backup2.ibm.com value is canonical name type MX value is name of mailserver associated with name

DNS Structure DNS Protocol and Messages query and reply messages, both with same message format msg header identification: 16 bit # for query, reply to query uses same # flags: query or reply recursion desired recursion available reply is authoritative 2 bytes 2 bytes identification flags # questions # answer RRs # authority RRs # additional RRs questions (variable # of questions) answers (variable # of RRs) authority (variable # of RRs) additional info (variable # of RRs)

DNS Structure DNS Protocol and Messages name, type fields for a query RRs in response to query records for authoritative servers additional “helpful” info that may be used 2 bytes 2 bytes identification flags # questions # answer RRs # authority RRs # additional RRs questions (variable # of questions) answers (variable # of RRs) authority (variable # of RRs) additional info (variable # of RRs)

DNS Structure Inserting Records into DNS example: new startup “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

Attacking DNS DDoS attacks Bombard root servers with traffic – Not successful to date – Traffic Filtering – Local DNS servers cache IPs of TLD servers, allowing root server bypass Bombard TLD servers – Potentially more dangerous Redirect attacks Man-in-middle – Intercept queries DNS poisoning – Send bogus relies to DNS server, which caches Exploit DNS for DDoS Send queries with spoofed source address: target IP Requires amplification