The transport layer. Transport-layer services. Transport layer runs on top of network layer. In other words,

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The transprt layer An intrductin t prcess t prcess cmmunicatin CS242 Cmputer Netwrks Department f Cmputer Science Wellesley Cllege Transprt-layer services Prvides fr lgical cmmunicatin* between

*By lgical cmmunicatin we mean that frm an applicatin s perspective, it is as if the hsts directly cnnected. Transprt layer 8-2 In ther wrds, We are nw wrking n the ther side f the scket.

1 The transprt layer An intrductin t prcess t prcess cmmunicatin CS242 Cmputer Netwrks Department f Cmputer Science Wellesley Cllege Transprt-layer services Prvides fr lgical cmmunicatin* between applicatin prcesses running n different hsts. Transprt-layer prtcls are implemented in the end systems, nt in the netwrk ruters. *By lgical cmmunicatin we mean that frm an applicatin s perspective, it is as if the hsts directly cnnected. Transprt layer 8-2 In ther wrds, We are nw wrking n the ther side f the scket. Questins n this side include Hw des UDP find the right scket? Hw des TCP make it appear t the applicatinlayer that there is a direct virtual pipe frm client and server sckets? And hw des it keep all thse pipes straight? Transprt layer runs n tp f netwrk layer Transprt-layer prtcls prvide lgical cmmunicatin between prcesses. Netwrk-layer prtcls prvides prvide lgical cmmunicatin between hsts. Transprt layer 8-3 Transprt layer 8-4 1

Transprt layer versus netwrk layers There are sme imprtant differences Applicatin Letters in messages envelpes Prcesses Peple Hsts Huses Transprt- Randy & layer prtcl Marla Netwrk- Pstal layer prtcl

Marla r I may decide t d smething abut this (fr example, resend lst letter), r we may nt.

2 Transprt layer versus netwrk layers There are sme imprtant differences Applicatin Letters in messages envelpes Prcesses Peple Hsts Huses Transprt- Randy & layer prtcl Marla Netwrk- Pstal layer prtcl service A netwrk-layer prtcl may nt guarantee delivery f the mail. Even if the mail is delivered, it may cme ut f rder r even damaged. Marla r I may decide t d smething abut this (fr example, resend lst letter), r we may nt. Transprt layer 8-5 Transprt layer 8-6 Internet prtcl (IP) The vacuus transprt prtcl Lgical cnnectin between hsts (the Internet s netwrk-layer service) is prvided by IP. The IP service mdel is best-effrt delivery, but it makes n guarantees. Packets may nt arrive n time, may arrive ut f rder, damaged, r nt at all. Questin: What's the least we get can away with at the transprt layer. Answer: D nthing, we simply pass messages frm the applicatin prcess directly t the netwrk layer. On the receiving side, we haul back whatever appears n ur drstep. Transprt layer 8-7 Transprt layer 8-8 2

The prblem with ding nthing Sckets* Prperly addressed, the netwrk prtcl can get the message t the right hst,.

Fr example, suppse a segment cntaining mail arrives while the hst is running STMP, FTP, DNP, and HTTP. Hw des the message find the prper prcess?

Transprt layer 8-9 *Again Transprt layer 8-10 Srting and gathering Surce and destinatin prts The task f gathering data chunks frm different sckets, packaging each chunk with header infrmatin,

3 The prblem with ding nthing Sckets* Prperly addressed, the netwrk prtcl can get the message t the right hst,... Each prcess has a scket thrugh which data passes frm the prcess t the netwrk and back.... but we still need t get it t the prper prcess. Fr example, suppse a segment cntaining mail arrives while the hst is running STMP, FTP, DNP, and HTTP. Hw des the message find the prper prcess? The transprt layer in the receiving hst des nt deliver data directly t a prcess, but t a scket. Transprt layer 8-9 *Again Transprt layer 8-10 Srting and gathering Surce and destinatin prts The task f gathering data chunks frm different sckets, packaging each chunk with header infrmatin, and passing it t the netwrk is called multiplexing. A scket is identified with its prt number.* Applicatin data are encapsulated int segments which are addressed t the scket they are t be delivered. 32 bits surce prt # dest prt # ther header fields applicatin data (message) The task f unpacking and delivering the data t the crrect scket is called demultiplexing. Transprt layer 8-11 *Prt numbers range frm 0 t with 0 t 1023 reserved fr well-knwn applicatins. Transprt layer

UDP cnnectinless multiplexing A Pythn prgram creates a UDP scket using either clientscket = scket(af_inet, SOCK_DGRAM) r serverscket = scket(af_inet, SOCK_DGRAM) serverscket.

Transprt layer 8-13 Identifying a UDP scket frm the ther side A UDP scket is fully identified by a tw-tuple cnsisting f a destinatin IP address and a destinatin prt number.

4 UDP cnnectinless multiplexing A Pythn prgram creates a UDP scket using either clientscket = scket(af_inet, SOCK_DGRAM) r serverscket = scket(af_inet, SOCK_DGRAM) serverscket.bind(, 12000) Typically, the client side f the applicatin lets the transprt layer pick its prt number, while the server side applicatin assigns a specific prt number.** *Why? Transprt layer 8-13 Identifying a UDP scket frm the ther side A UDP scket is fully identified by a tw-tuple cnsisting f a destinatin IP address and a destinatin prt number. Hst A/UDP sends chunk t hst B/UDP Transprt layer delivers segment S why include a surce prt number? Hst B/UDP respnds by sending segment t hst A/UDP Imprtant: This scheme wrks precisely because UDP is cnnectinless Transprt layer 8-14 TCP cnnectin-riented multiplexing* A TCP server applicatin establishes a welcming scket and waits fr a cnnectin. The TCP client generates a cnnectin-establishment segment using servername = servername serverprt = clientscket = scket(af_inet, SOCK_STREAM) clientscket.cnnect(servername, serverprt) When the server receives cnnectinrequest, it infrms the server prcess, which creates a cnnectin scket. cnnectinscket, addr = serverscket.accept() *Things get a little mre cmplicated. Transprt layer 8-15 Hw des TCP keep all thse cnnectins straight? Fr starters, a TCP scket is identified by a 4-tuple: Surce IP; Surce prt number; Destinatin IP; Destinatin prt number. In particular, arriving TCP segments with different surce IP addresses r surce prt numbers may g t different sckets. surce prt # dest prt # surce IP Headers Data dest. IP: Transprt layer

.. Transprt layer 8-17 Transprt layer 8-18 Internet prtcls: Tw extremes UDP des as little as pssible The remainder f this lecture is devted t UDP.

5 Multiple virtual cnnectins Multiple virtual cnnectins Three segments are sent with identical destinatin IPs and prt numbers... Web servers typically spawn either 1) a new prcess, r 2) a new thread fr each new client cnnectin. Preferences... Transprt layer 8-17 Transprt layer 8-18 Internet prtcls: Tw extremes UDP des as little as pssible The remainder f this lecture is devted t UDP.* We'll need a bit mre preparatin t discuss TCP, starting with principles f reliable data transfer. That's up next Aside frm multiplexing and demultiplexing and sme light errr checking, UDP adds nthing t IP. S why use it? N cnnectin establishment. N cnnectin state. Small packet header verhead. Finer applicatin-level cntrl ver what data is sent. *Frankly, there's nt a whle lt mre t say. Transprt layer 8-19 Transprt layer

6 UDP is ften the prtcl f chice Fr example, DNS... * * DNS queries a name server then waits fr a reply. If nne within a given time, DNS either queries anther name server r infrms the invking applicatin f n reply. Chice f DNS results in a cnsiderable savings in RRTs.* *Actually, these days bth UDP and TCP are used with multimedia applicatins. Transprt layer 8-21 *As yu calculated in ps2. Transprt layer 8-22 UDP segment structure UDP segment ges int addressed envelpe Length, in bytes f UDP segment, including header 32 bits surce prt # dest prt # length Applicatin data (message) checksum 1 s cmplement f the sum f 16 bits in segment. frm scket imprt * servername = 'hstname' serverprt = clientscket = scket(af_inet, SOCK_DGRAM) message = raw_input('input lwercase sentence:') clientscket.sendt(message,(servername, serverprt)) Drp UDP segment int Netwrk Layer datagram UDP segment frmat Transprt layer 8-23 Transprt layer

7 UDP checksum Sending End: 1. Add the bits f all 16 bit wrds in segment (wrapping the verflw bit (if any). 2. Take the ne's cmplement f the result wraparund sum checksum Receiving End: 1. Add bits f all 16 bit wrds in received segment (wrapping if necessary). 2. Add result t checksum and cmpare with Transprt layer

CCNA 1 Chapter v5.1 Answers 100%

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