Practical Exercises in Computer Networks and Distributed Systems

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(V..6, Nv 2) Practical Exercises in Cmputer Netwrks and Distributed Systems Stream Sckets and the Client/Server mdel (C language, W) 2-, Jsé María F Mrán This

Als, I intrduce the Client/Server cmputing mdel and the relevant APIs such as Sckets and undertake the structured design f a simple, Scket-based C server prgram and

A brief recllectin f / and Berkeley sckets The prtcl mdule in a cmputer system f tday ffers sme essential servi t user prgrams such as reliable transmissin, flw and

This variety f bjectives makes cmplex, thus, in rder t islate the applicatin prgrammer frm the cmplexity f, prgramming languages incrprate APIs that allw prgrams t

In the Java language, ur language f chice fr this curse, ffers Sckets, an API derived frm the riginal C-language Berkeley scket API (Applicatin Prgramming Interface).

. Cnnectin analgy t telephne dialing is cnnectin-riented, that means that every time we want t transmit infrmatin reliably by using it, the prtcl mdule will have t

1 (V..6, Nv 2) Practical Exercises in Cmputer Netwrks and Distributed Systems Stream Sckets and the Client/Server mdel (C language, W) 2-, Jsé María F Mrán This practical illustrates basic cncepts prtcl by creating a simple C/S pair and analyzing its behavir with the Wireshark prtcl analyzer. Als, I intrduce the Client/Server cmputing mdel and the relevant APIs such as Sckets and undertake the structured design f a simple, Scket-based C server prgram and the crrespnding Client prgram. A brief recllectin f / and Berkeley sckets The prtcl mdule in a cmputer system f tday ffers sme essential servi t user prgrams such as reliable transmissin, flw and cngestin cntrl. This variety f bjectives makes cmplex, thus, in rder t islate the applicatin prgrammer frm the cmplexity f, prgramming languages incrprate APIs that allw prgrams t cmmunicate via. In the Java language, ur language f chice fr this curse, ffers Sckets, an API derived frm the riginal C-language Berkeley scket API (Applicatin Prgramming Interface). Berkeley Sckets Berkeley Sckets netwrk, internet Fig.. Cnnectin analgy t telephne dialing is cnnectin-riented, that means that every time we want t transmit infrmatin reliably by using it, the prtcl mdule will have t establish a cnnectin with the remte cmputer, the result f this cnnectin is that the cmmunicating cmputers share state, a series f data structures that permit the executin f the sliding windw, flw cntrl and cngestin cntrl algrithms f. The Berkeley Scket API, riginally prgrammed in C has been exprted t ther languages where the similarities are clear, this was s that interperability culd be guaranteed, fr example, when a scket-based server prgram written in Java cmmunicates with a client written in Pythn, we want bth prgrams t interperate crrectly. The prgrammer s task in this case cnsists f implementing the applicatin-level prtcl crrectly by sending the relevant prtcl messages ver the client and server sckets. The whle prs, frm cnnectin setup thrugh

2 (V..6, Nv 2) cnnectin teardwn is illustrated in Fig. 2. scket() Welcme scket bind() 2 scket() listen() accept() Delegate scket cnnect() Client scket read() write() write() read() clse() Fig. 2. Berkeley scket cnnectin, cmmunicatin and teardwn The cnnectin prtcl, knwn as three-way handshake, is initiated when the client prgram creates a client scket and cnnects it with a listening server scket that is accepting cnnectins. The three-way handshake implies the exchange f three messages between client and server and is nrmally started by the client in what is knwn as an active pen. When this prs has finished, the server scket and the client scket can begin t exchange appprtcl messages reliably. clse() (Stream Scket) SYN (Stream Scket) Cnnectin indicatin Cnnectin requested SYN and ACK Cnnectin acknwledged ACK + Data Cnnectin established Fig.. Three-way handshake started by the client (Active pen) The scket created by the server prgram waits fr cnnectin requests cming frm clients, when a new cnnectin request is received and is sucsfully cmpleted, this welcme scket generates a new scket (The delegate scket) which purpse cnsists f carrying ut the bidirectinal data transfers with the client scket. The welcme scket is frever waiting fr new cnnectin requests. Time 2

such that, nce a scket is created, it effectively identifies the prgram that created it, that is, if we knw the scket we will be able t identify the unique prs that created it.

3 (V..6, Nv 2) Welcme Scket º: Three-way handshake 2 Client scket 2º: Full-duplex, reliable channel established Delegate Scket Fig.. After the three-way handshake, the delegate scket and the client scket begin the data transfer Sckets as names f prses A scket is created at the request f an applicatin prgram executing as a prs, such that, nce a scket is created, it effectively identifies the prgram that created it, that is, if we knw the scket we will be able t identify the unique prs that created it. This prs is the receiving end f the infrmatin cnveyed thrugh the scket. Since a scket gives a prs a name, what is, then, the difference between any tw sckets? pr pr pr pr pr Applicatins GDT Sesión (Sckets) Demux key is: (Src Ip, Dst Ip, Src prt, Dst prt) segment Packet Fig.. Layer- demultiplexing keys in Tw sckets within an perating system differentiate by their prt numbers, these are 6-bit integers cnstitute ne f the cmpnents f the multiplexing keys at the transprt layer,. Fr example, when a web server

4 (V..6, Nv 2) perfrms a passive pen t establish its Welcme Scket it specifies prt number, the well-knwn (Standardized) prt number fr acsing the web server frm the internet. Well-knwn prts and their crrespnding servi are related in file /etc/servi in Unix and Linux and in a similar file in Windws. All in all, the prt number applied t a scket is ne f the fur cmpnents f the mux key (Src Ip, src, prt, dst Ip, dst prt) turning the scket interface int a name space fr prses within a system, this namespace will allw client sckets t cnnect with server sckets and thereby with their creatr prses, end-t-end. Figures and 6 illustrate this cncept f sckets as a namespace fr prses. pr web firefx server pr pr pr pr pr pr Applicatins Applicatins ( ,, 2..2., 2) Mux/Demux key fr pr Sessin (Sckets) 2 Sessin (Sckets) internet Fig.. Example f a demultiplexing key value that identifies a Web client-t-server cnnectin A simple Stream-Scket Server We are starting with tw base prgrams written in C, ne fr the client and the ther ne fr the server. Please, fllw the steps belw t have the C/S applicatin running:. Request a terminal and dwnlad the server s surce cde: $ wget 2. Cmpile server prgram: $ gcc server -Server.c. Run the server (N need t sud since this practical uses Stream sckets which require n particular permissins) $./server Server tight lp restarted

5 (V..6, Nv 2). Have a class mate at a hst ther than yurs run the $ nc cmmand. This cmmand allws yu t cnnect with a server ( r UDP) and send it character strings; it results handy when checking simple clients and servers. It can als functin as a server. Befre running $ nc, yu ll have t find ut yur address (The prt is number as included in the server prgram): $ nc <server address> (Yu can type any string yu want here; read the server s surce cde and test all the pssibilities). Nw, send the CERRAR string t the server; it will clse the server scket and end the prgram executin 6. Nw, run Wireshark n the active netwrk interface (Run $ ifcnfig and chse an interface that has an ). When Wireshark is running, apply a display filter that suppresses all frames but thse belnging t the flw between ur Client and Server (Capture this filter string within the Filter textbx: tcp.prt == ) 8. Run the server again: Ntice that when the server scket (Welcme scket) is created, n traffic is generated 9. With the server active, cnnect t it with $nc like abve and bserve the three-way handshake Observe SYN segment frm C t S Des this segment cntain any ptins (MSS, fr example). MSS is the Maximum Segment Size. Any ther ptins cntained in the the first segment sent by client? Observe the respnse frm the server; take nte f any ptins. Observe whether the ACK number is cnsistent with the received Initial Sequence Number. Finally, yu ll see an ACK segment frm C t S. When that segment arrives at S, the cnnectin is in the ESTABLISHED state (C and S are in the ESTABLISHED state).. The Sliding Windw (ESTBLISHED STATE) will gvern all the transfers between C/S and yu ll be able t watch them in yur Wireshark Check that the sequence numbers and acknwledgement numbers are the expected nes accrding t the prtcl. Finish the server by cnnecting t it with $ nc cmmand and then sending the CERRAR string. Carefully bserve the segments exchanged by C and S upn cnnectin clse 2. Write a client prgram using the Sckets API prvided abve and the server prgram written in C Yu may want t dwnlad a base client prgram which is cmmented but incmplete: $ wget Once yu have finished it, cmpile it and run it and check the applicatin prtcl with Wireshark like we did abve. There shuld be n substantial difference between $ nc and yur client prgram when cnnecting with the server.

DS LABS DISTRIBUTED SYSTEMS PRACTICAL EXERCISES

DS LABS DISTRIBUTED SYSTEMS PRACTICAL EXERCISES Java Streams and TCP Sckets Jsé María F Mrán, 2013 Creative Cmmns License 9 OCTUBRE 2013 1 JAVA SOCKETS (TCP) 7 web server 7 firefx Applicatins Applicatins