mạng nâng cao

1. Internet l g? 5 thnh phn chc nng, 3 thnh phn vt l c bn, 3 c trng (nguyn l) hot ng, 2 dch v? +Internet l g: Mng my tnh l mt tp hp cc my tnh v cc thit b khc (cc nt), chng s dng mt giao thc mng chung chia s ti nguyn vi nhau nh cc phng tin truyn thng mng. +5 thnh phn chc nng: - Mobile network: 3G, WAP, GPRS - Home network: mng cp hoc khng dy, ADSL kt ni ti ISP. - Institutional network (mng cng ty): LAN, ethernet - Regional ISP: cc ISP tm khu vc nh VNPT, FPT, Viettel - Global ISP: cc ISP quc t nh Verizon, AT&T, Level3 Trong : Mobile network, Home network, Institutional network (mng cng ty) l mng ngoi vi (network edge). Regional ISP, Global ISP l mng li (network core). +3 thnh phn vt l: Cc thit b u cui (host / end system) kt ni vi nhau to thnh mng c th l cc my tnh (computer) hoc cc thit b khc. Ni chung hin nay ngy cng nhiu cc loi thit b c kh nng kt ni vo mng my tnh nh in thoi di ng, PDA, ti vi . . . . . Mi trng truyn (media / communication link) m truyn thng c thc hin qua . Mi trng truyn c th l cc loi dy dn (cp - cable), sng (i vi cc mng khng dy - wireless). V d: cp truyn thng c th l cp xon i (twisted-pair), cp ng trc (coaxial), v cp si quang (fiberoptic cable)... Truyn thng khng dy c th l sng radio (gm sng cc ngn hay vic truyn thng qua v tinh), bc x hng ngoi. Router l mt thit b dng ghp ni cc mng cc b vi nhau thnh mng rng. Router thc s l mt my tnh lm nhim v chn ng cho cc gi tin hng ra ngoi. Router c lp v phn cng v c th dng trn cc mng chy giao thc khc nhau

+3 c trng (nguyn l) hot ng: - Giao thc (protocol) l quy tc quy nh cch thc trao i d liu gia cc thc th. y khi nim thc th bao gm c cc thit b phn cng cng nh cc tin trnh phn mm. Giao thc mng cng nh ngha khun dng d liu c trao i gia cc bn. vd: TCP, IP, HTTP - Internet: network of network (thng qua mng trung gian) - Internet standards: IETF: Internet Engineering Task Force RFC: Request For Comment 1

+2 dch v: Phn cng (h tng) communication infrastructure hin thc vic phn phi cc ng dng: Web, VoIP, email, games, e-commerce, file sharing Phn mm (ng dng) communication services cung cp ng dng: reliable data delivery from source to destination: truyn tin cy d liu t ngun n ch best effort (unreliable) data delivery:n lc ti a (khng tin cy) vn chuyn d liu 2. Mng bin( network edge) cc m hnh, cc phng thc kt ni, m hnh kt ni h tng 3 lp, cc phng tin(media)? Mng bin: applications and hosts. c Mobile network, Home network, Institutional network. +Cc m hnh: end systems (hosts):chy chng trnh ng dng. vd: Web, email. ti cnh bin ca mng edge of network. client/server model: client host yu cu dch v, nhn dch v t my ch lun chy. vd: Web browser/server; email client/server peer-peer model: hn ch ti thiu hoc khng dng my ch chuyn dng (dedicate). vd: Skype, BitTorrent +M hnh kt ni h tng 3 lp:

Tng li (Core layer hay Backbone) Tng phn phi (Distribution layer) Tng truy cp (Access layer)

Mi tng c chc nng nhim v ring bit. Tuy nhin, ba tng ny ch l logic ch khng phi l phn cng c th no, v vy khi thc hin mng phn tng cc thit b c th l c nhiu thit b cho mt tng hoc ch mt thit b n l thc hin chc nng cho nhiu tng.

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+Cc phng tin (media): guided media: - Twisted-pair: gm 2 cp xon i Category 3: dy in thoi truyn thng (traditional phone wires), 10 Mbps Ethernet Category 5: 100 Mbps Ethernet - Coaxial cable (cp ng trc): + gm 2 dy dn ng tm + truyn dn hai chiu + baseband: - knh n trn cp - k tha Ethernet + broadband: - nhiu knh trn cp - HFC

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- Fiber optic cable (cp quang): + si thy tinh mang xung nh sng (light pulses), mi xung 1 bit + hot ng tc cao: truyn dn point-to-point tc cao (v d, 10's-100s Gps) + t l li thp: lp li khong cch xa, min dch vi nhiu in t. unguided media: (mng khng dy) - tn hiu mang trong ph in t - khng kt ni vt l bng dy - truyn dn hai chiu - nhng tc ng trong mi trng truyn: + phn nh + cn tr bi cc i tng + can thip - Cc loi lin kt v tuyn: Sng trn mt t e.g. up to 45 Mbps channels LAN (vd: Wifi) 2Mbps, 11Mbps, 54 Mbps wide-area (mng din rng) (vd: cellular) vd 3G: hng trm kbps satellite (v tinh) Kbps ti 45Mbps knh (hoc cc knh nh hn nhiu) 270 msec end-end delay geosynchronous versus low altitude +Network access (phng thc kt ni - truy cp mng): residential (khu dn c) access nets institutional access networks (school, company) mobile access networks cc cch truy cp mng: + Dialup via modem (Quay s qua modem) - ln n 56Kbps truy cp trc tip n b nh tuyn (thng t) - Khng c th lt web v in thoi ti cng mt thi im: khng th l "lun lun"

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+ ADSL: asymmetric digital subscriber line (ng dy thu bao s bt i xng) - ln n 1 Mbps upstream (ngy nay thng kt thc phin lm vic. - Trong ch th hai download and keep, user agent vn li th trn mail server sau khi ti v. + Giai on ba - cp nht, xy ra sau khi client ra lnh quit kt thc phin lm vic POP3 . Ti thi im mail server xo tt c th c nh du (nu ch "download and delete" ). - POP3 mc nh ch "download and delete". Do ngi dng khng th c li th nu i client khc v mail trn server b xa. - Trong ch download and keep, user agent vn li th trn mail server sau khi ti v (n copy th n client). - POP3 ch lu gi thng tin trng thi trong 1 phin lm vic (1 session). POP3 server khng lu gi trng thi ca mi ngi dng sau khi phin lm vic kt thc. +IMAP: - IMAP c thit k cho php ngi dng thao tc trn nhng hp th xa mt cch d dng. - IMAP gi tt c message trn server.

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- Cho php ngi dng to nhng th mc th khc nhau trong mailbox, c th t th vo trong th mc hay dch chuyn th t th mc ny n nhng th mc khc. - Tnh nng quan trng khc ca IMAP l cho php user agent ch ly mt s thnh phn ca bc th. Vd: c th ti trc mt phn cua bc th. - Nhng thng tin trng thi c mail server lu gi cho tt c cc phin lm vic. 10. DNS: cu trc DNS, hot ng, cc loi thng ip, bn ghi DNS +Chc nng, dch v ca DNS: - Chuyn i hostname sang IP - t b danh cho my tnh (Host aliasing) - t b danh cho mail server (Mail server aliasing) - Phn tn ti (Load distribution) +Hot ng ca DNS: (v hnh 2.21 /139 v hnh 2.22 /140 d hiu hn)

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- Khi cn phn gii tn hay IP, client s truy vn n DNS server. DNS c thit k phn tn. DNS s dng nhiu DNS server t chc phn cp v phn tn trn ton cu. Khng c DNS server no cha tt c tn v a ch IP cc my tnh trn Internet nhng thng tin ny c phn tn trn nhiu DNS server. - C ba lp DNS server: root DNS server, top-level domain (TLD) server v authoritative DNS server. Cc DNS server trao i thng tin vi nhau v vi cc my tnh khc. + Root DNS server: Trn Intemet c 13 root DNS server, hu ht t ti Bc M. + Top-level domain server: nhng server phn hi cho cc top-level domain nh com, org, net, edu, gov v tt c cc top-level domain quc gia nh uk, fr, ca, jp. + Authoritative DNS server: Mi my tnh phi ng k ti mt Authoritative name server. Thng thng authoriative name server ca mt my tnh l name server trong min ISP ca my tnh (thc t mi my tnh phi c t nht hai authoritative name server, phng trng hp mt name server b hng). C th nh ngha, Authoriative name server ca mt my tnh l nameserver lun lu tr bn ghi DNS cho php xc nh a ch IP ca my tnh t tn. - Ngoi ra cn 1 loi DNS server quan trng khc l local DNS server: Mi t chc nh cc ISP, trng i hc, cng ty u c local DNS server (cn c gi l default name server). Khi my tnh trong c quan to ra mt thng ip truy vn DNS th u tin thng ip c gi ti local DNS server ca t chc. a ch IP ca local DNS server phi c cu hnh trong my tnh. - Nu mt local DNS server khng th tr li truy vn ca 1 my tnh, n s truy vn ln cc DNS server cp cao hn bn ngoi, chng hn nh root DNS server. Nu root DNS server c thng tin ca my tnh c hi, n s gi mt thng ip hi m DNS ti local DNS server v sau thng tin ny c local DNS server gi tr li cho my tnh yu cu. Nu root DNS server khng c thng tin, n s truy vn ngc xung cc TLD server v authoritative server khc ly thng tin v, sau gi tr li cho local DNS server yu cu truy vn. +DNS caching: - khi DNS Server nhn c tr li DNS (nh x hostname sang IP), n lu li kt qu . - Cc TLD server mc nh c cache trong cc local DNS server. +Bn ghi DNS (resource record): - Bn ghi ti nguyn gm 4 trng sau: (Name, Value, Type, TTL) + TTL l thi gian tn ti ca bn ghi ti nguyn, dng xc nh thi im c th xo bn ghi ti nguyn khi b nh lu tr. ngha ca trng Name v Value ph thuc vo trng Type:

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Nu Type = A th Name l tn my v Value l a ch IP ca my . Bn ghi kiu A l nh x Tn my - a ch IP chun. V d, (relay1.bar.foo.com, 145.37.93.126, A) l mt bn ghi Type A. Nu Type = NS th Name l mt min (nh l foo.com) v Value l tn my ca authoriative name server ca cc my tnh trong min . Bn ghi ny thng c s dng gi tip cc truy vn DNS. V d 1 bn ghi Type NS: (foo.com, dns.foo.com, NS) Nu Type = CNAME th Name l tn b danh (alias) v Value l tn y ca my c tn b danh t trong Name. Bn ghi kiu ny cho php xc nh tn y ca mt my tnh t tn b danh. V d mt bn ghi CNAME: (foo.com, relay1.bar.foo.com, CNAME). Nu Type = MX th Value l tn my ca mail server c tn b danh t trong Name. V d, bn ghi kiu MX (foo.com, mail.bar.foo.com, MX). Bn ghi MX cho php mail server c tn b danh n gin. +Cc loi thng ip DNS: Gm hai loi l query v reply. Chng c cng nh dng.

-Header: 12 byte, gm 6 trng, mi trng 16 bit (1byte=8bit) +Identification: Trng u tin l mt nh danh 16 bit cho mi thng ip yu cu. 16 bt nh anh ny c ghi li vo thng ip tr li, cho php client xc nh c y l cu tr li cho thng ip yu cu no. +Flags: - 1 bit query/reply ch ra khi no message l query(0) hay reply(1). 24

- C authoritiative flag c t trong thng ip tr li khi DNS server l authoriative DNS server ca tn my tnh cn xc nh a ch IP. - C mong mun quy (recursive-desired query) c t khi client (my tnh hay DNS server ) mong mun DNS server thc hin truy vn quy khi n khng c bn ghi . - C chp nhn quy (recursion-available flag) c t trong thng ip tr li nu name server h tr quy. Trong phn tiu cng c 4 trng s lng, cc trng ny xc nh s lng cc bn ghi trong 4 phn d liu sau phn tiu : +Number of questions: gm thng tin l (1): tn cn truy vn ,(2): loi truy vn. Vd: google.com,type A +Number of answer RRs: cha cc bn ghi ti nguyn cho tn c yu cu trc . Ch rng mi bn ghi ti nguyn c 4 trng: Type (A, NS, CNAME, MX), Name, Value, TTL. Thng ip tr li c th c nhiu bn ghi ti nguyn v tn my tnh c th ng vi nhiu a ch IP. +Number of authority RRs (Mc thm quyn): cha cc bn ghi ca cc authoritative server. +Number of additional RRs (mc ph tr): cha cc bn ghi "hu ch" khc. V d trng tr li trong thng ip tr li mt truy vn MX s cha tn y ca mail server c tn b danh t trong Name. Phn ph tr c th c mt bn ghi kiu A cung cp a ch IP cho chnh mail server . 11. Dch v ca tng giao vn - Nm gia tng ng dng v tng mng, tng giao vn l tng trung tm trong kin trc phn tng vi nhim v cung cp dch v truyn thng gia cc tin trnh ng dng chy trn cc my tnh khc nhau. - Giao thc tng giao vn cung cp mt knh truyn logic (o) gia cc tin trnh ng dng chy trn my tnh khc nhau. Gi l logic v khng tn ti mt ng truyn vt l thc s gia hai tin trnh. - Cc giao thc tng giao vn chy trn cc thit b cui: + Bn gi: Chia cc thng ip ng dng thnh nhiu on (segment), y qua cho tng network. + Bn nhn: Kt hp cc phn on (segment) thnh thng ip hon chnh, sau y qua cho tng ng dng. - Trn mng my tnh c th c nhiu giao thc hot ng tng giao vn. Mi giao thc c th cung cp cc dch v vi cht lng khc nhau cho ng dng. - Tt c giao thc tng giao vn u cung cp dch v dn knh (multiplex) v tch knh (demultiplex), ngoi dch v dn knh / tch knh, tng giao vn cn c th cung cp cc dch v khc cho tin trnh ng dng nh truyn d liu tin cy, m bo bng thng hay gii hn tr. 25

- Tng giao vn c hai giao thc quan trng l TCP (tin cy) v UDP (khng tin cy). - im khc bit gia tng giao vn v tng network l: + Tng network giao tip logic gia cc host. + Tng transport giao tip logic gia cc tin trnh. 12. UDP: cu trc, ngha cc trng trong header v cch tnh checksum - UDP c t trong RFC 768 l giao thc giao vn cc k n gin. Bn cnh chc nng dn knh/phn knh, UDP c thm c ch pht hin li n gin. C th ni nu s dng UDP th gn nh ng dng lm vic trc tip vi tng mng IP. - L dch v n lc ti a (best effort), nn cc UDP segment c th b mt hoc gi khng theo trt t ti ng dng. - UDP c xem l dch v khng hng ni hay khng lin kt trc (connectionless), v: + UDP khng i hi thc th bn gi v bn nhn phi lin kt trc khi trao i d liu. (khng c handshaking gia UDP sender v UDP receive) + Mi segment l c lp vi cc segment khc. + khng duy tr trng thi kt ni. - Thng dng cho cc lung ng dng multimedia v khng c giai on thit lp kt ni nn s nhanh. Cc giao thc DNS v SNMP (giao thc qun l mng) thng chy trn nn UDP cng v l do ny. - UDP chuyn d liu khng tin cy, khng c iu khin tc nghn (no congestion control). (phn trn ny c cho bit thi, b cng c) +Cu trc mt UDP segment v ngha cc trng trong header:

- Header length l 8 byte. - Tiu (header) UDP c bn trng, ln mi trng l 2 byte (=16 bits): + Source port + Destination port + Length: di (tnh theo byte) ca ton b segment (bao gm c header)

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+ Checksum: c bn nhn s dng kim tra trong segment c li hay khng. Trn thc t, k c tiu ca gi d liu IP cng c tnh checksum. +Cch tnh checksum: - UDP checksum c s dng pht hin li. Checksum c tnh nh sau: tnh gi tr b mt ca tng cc t 16 bit trong segment, gi tr nhn c c t vo trng checksum trong gi d liu UDP segment. Gi s c ba t 16 bit sau y: 0110011001100110 0101010101010101 0000111100001111 Tng hai t u l: 0110011001100110 0101010101010101 1011101110111011 Cng t th ba vo, ta c: 1011101110111011 0000111100001111 1100101011001010 Cch ly b mt l o 0 thnh 1 v 1 thnh 0. V vy kt qu php ly b mt ca 1100101011001010 l 0011010100110101 v chnh l gi tr checksum. Ti pha nhn, tt c bn t (k c checksum) c cng li. Nu d liu khng c li th tng nhn c l 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1. Nu c mt bit no bng 0 th ta bit d liu nhn c c li. - Mc d UDP c th pht hin li nhng n khng lm g khi pht hin ra li. C th n s loi b segment b li, c th n s chuyn segment b li cho ng dng nhn cng vi mt thng ip cnh bo. 13. nguyn l truyn tin cy 2.0, 2.1, 2.2 v 3.0 * Ch thch hnh: S kin kch hot vic chuyn trng thi c t pha trn ng k nm ngang, l nhn s kin. Pha bn di ng k nm ngang l nhng hnh ng m thc th phi thc hin ngay khi s kin xy ra (thc hin trc khi thc th chuyn sang trng thi mi). +Truyn d liu tin cy trn knh truyn c li bit: giao thc rdt 2.0: - c ch mi trong rdt2.0 (so vi rdt1.0): pht hin li, nhn thng tin phn hi (ACK / NAK)

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Trong giao thc rdt 2.0, pha gi c hai trng thi. trng thi th nht, pha gi i d liu t tng trn. Trong trng thi th hai, pha gi i phn hi ACK hoc NAK t pha nhn. Nu nhn c ACK (rdt_rcv(rcvpkt ) && isACK(rcvpkt) trong hnh 3.10 tng ng vi s kin ny), pha gi bit c gi d liu chuyn n ch an ton, v vy n tr v trng thi i d liu t tng trn chuyn tip. Nu nhn c NAK, pha gi gi li gi d liu ri quay li trng thi i phn hi ACK hoc NAK cho gi d liu va gi li. Ch rng khi pha gi trong trng thi ch phn hi (ACK hoc NAK), n khng th nhn thm d liu t tng trn a xung. N ch chp nhn d liu khi nhn c ACK v chuyn trng thi. Pha gi khng gi d liu cho n khi n chc chn rng pha nhn nhn ng gi d liu gi. Giao thc rdt 2.0 vi hnh vi nh vy thuc kiu dng v ch (stop and wait). FSM bn nhn trong giao thc rdt 2.0 ch c mt trng thi duy nht. Khi nhn c gi d liu (packet), pha nhn gi thng ip phn hi ACK hoc NAK, ph thuc vo gi d liu nhn c li hay khng. Trong hnh 3.10, rdt_rcv (rcvpkt) && corrups(rcvpkt) tng ng vi s kin gi d liu nhn c b li. - Giao thc rdt 2.0 vn cn nhc im: chng ta cha tnh n kh nng chnh gi ACK hoc NAK c li. +Giao thc rdt 2.1: x l ACK hoc NAK b li: - Gii quyt c trng hp duplicate packet do ACK hay NAK li bng cch thm s sequence number vo gi tin. - Pha nhn ch cn kim tra s th t xc nh gi d liu nhn c l gi mi hay gi truyn li. Vi giao thc stop and wait n gin, ch cn mt bit s th t. Bn nhn c th xc nh bn gi gi li gi d liu gi ln trc (s th t ca gi d

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liu nhn c trng vi s th t vi gi d liu nhn c ln trc) hay gi d liu mi (c s th t khc nhau, tng ln theo module 2).

Trong rdt 2.1, FSM ca bn gi v nhn u c s trng thi tng gp i. l v trng thi giao thc phi biu din gi d liu c gi (bi bn gi) v gi d liu c i (ti bn nhn) c s th t l 0 hay 1. Ch rng cc hnh ng trong trng 29

thi gi d liu c s th t 0 c gi (pha gi) hoc c mong i (pha nhn) ngc vi trng thi gi d liu c s th t 1 c gi hay c i. +Giao thc rdt 2.2: khng s dng NAK: - Ging chc nng ca 2.1, nhng gi nh thay v dng NAK, bn nhn gi ACK cho packet cui cng nhn c. Nu nhn hai ACK cho cng mt gi d liu (hin tng trng ACK - duplicate ACK) bn gi xc nh c bn nhn khng nhn ng gi d liu sau gi d liu bin nhn ACK hai ln. + Bn gi:

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+ Bn nhn:

+Giao thc rdt 3.0: Truyn d liu tin cy trn knh truyn m d liu bi mt, li: - Gi nh: knh c bn cng c th mt cc gi tin (d liu hoc ACKs):ngha l truyn trong knh va mt va b li, checksum, s th t, ccACK, cc vic truyn li s h tr nhng khng . - Cch gii quyt: bn gi ch ACK trong khong thi gian chp nhn c, truyn li nu khng nhn ACK trong khong thi gian ny. Nu gi (hoc ACK) ch tr (khng mt): truyn li s gy trng, nhng dng s th t s gii quyt c. Bn nhn phi xc nh s th t ca gi ACK va gi. Ci tin l: yu cu ng h m ngc (timer).

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14. cc phng thc GBN v SR + GBN (Go-Back-End): - Trong giao thc GBN, pha gi phi p ng ba s kin sau: C d liu t trn chuyn xung : khi rdt_send() c pha trn s dng chuyn d liu xung, pha gi phi kim tra xem ca s y cha (tc l c N gi d liu gi i cha c bin nhn khng). Nu ca s cha y, pha gi to ra v sau gi gi d liu ng thi cp nht cc bin. Nu ca s y, pha gi khng chp nhn d liu t tng trn v thng bo ca s y. Khi , tng trn s phi gi li. Trn thc t, pha gi s a d liu vo vng m (nhng cha gi ngay) hoc c c ch ng b (s dng semaphore hay c) ch cho php tng ng dng s dng rdt_send() khi ca s cha y. Nhn c mt ACK: trong giao thc GBN, gi tr bin nhn gi tin c s th t n s mang gi tr tch lu, ngha l ton b gi d liu c s th t nh hn hoc bng n u c pha nhn nhn ng. Chng ta s quay li vn ny khi xem xt pha nhn trong giao thc GBN. Ht thi gian i (timeout): tn giao thc - Go-Back-N bt ngun t hnh vi ca pha gi khi d liu b mt hay b tr. Ging nh trong giao thc stop and wait, timer c s dng x l vic mt gi d liu hay gi phn hi. Khi ht thi gian i (timeout), pha gi s gi li tt c cc gi d liu

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c gi i trc nhng cha c bin nhn. Nu ACK no c nhn nhng vn cn gi d liu gi i cha c bin nhn th timer s c khi ng li. Nu tt c cc gi d liu gi u c bin nhn th c th ngng timer. - Cc hnh ng ca pha nhn trong giao thc GBN n gin. Nu nhn c ng gi d liu v gi ny ng th t th pha nhn gi ACK cho gi nhn c v chuyn d liu trong gi d liu ny ln trn. Trong tt c cc trng hp cn li, pha nhn loi b gi d liu v gi li ACK cho gi d liu ng th t cui cng n nhn c. Ch rng gi d liu c chuyn ln tng trn mt ln duy nht nn nu gi d liu th k c nhn v chuyn ln trn th ngha l tt c cc gi d liu c s th t nh hn k cng c chuyn ln. S dng ACK tch lu l s la chn tuyt vi cho giao thc GBN. (on trn ny c cho hiu thi) >>>Tm li, hot ng ca GBN nh sau: Bn gi s gi gi tin cho bn nhn, gi s ang gi gi N no . Khi bn nhn nhn c gi tin th N (nu N ng th t), n s gi tr 1 ACK th N cho bn gi bin nhn cho gi th N. Nu gi N trong qu trnh gi b mt hay b tr, (Timer c s dng pht hin mt gi tin), bn gi s vn gi tip cc gi tin tip theo (N+1, N+2,). V gi th N cha ti c bn nhn (hoc ti tr), nn r rng bn nhn s pht hin cc gi th N+1, N+2 l sai th t (ang thiu gi th N hoc gi th N n sau), c mi gi nhn sai th t bn nhn s loi b v gi li ACK ca gi ng th t cui cng n nhn c ( trng hp ny l ACK th N-1). Khi ht thi gian i (timeout), pha gi s gi li tt c cc gi d liu c gi i trc nhng cha c bin nhn. Nu ACK no c nhn nhng vn cn gi d liu gi i cha c bin nhn th timer s c khi ng li. Nu tt c cc gi d liu gi u c bin nhn th c th ngng timer.

+SR (Selective Repeat): - ng nh tn gi, giao thc lp li c la chn (SR - Selective Repeat) trnh vic truyn li khng cn thit bng cch bn gi ch gi li cc gi tin m n cho l c li (hoc mt). truyn li tng gi tin khi cn thit, bn nhn cn bin nhn cho tng gi tin nhn ng. - SR vn dng timer, tuy nhin, mi gi tin gi i c mt timer ring, bi v ch c duy nht mt gi tin c gi li khi ht thi gian i. - Bn nhn Selective Repeat s bin nhn cho bt k gi tin nhn ng, cho d khng theo ng th t. Gi tin khng ng th t vn c lu gi li cho n khi tt c cc gi tin cn thiu (gi tin c s th t nh hn) c chuyn n, khi tt c cc gi tin s c chuyn ln tng trn theo ng th t.

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15. giao thc TCP: cu trc TCP header v RTT +TCP segment structure:

- TCp header gm 20 byte: (32 bit word x5) - 16 bit source port,16 bit destination port - 32 bit sequence number - 32 bit acknowledge number, - 4 bit header length, 6 bit not used, 6 bit flag ,16 bit receive window: dng iu khin dng. - 16 bit checksum,16 bit urgendatapointer Trng s th t (sequence number) 32 bit v trng s bin nhn (acknowledge number) 32 bit c bn gi v bn nhn s dng trong vic cung cp dch v truyn d liu tin cy, s c cp k hn trong phn di y. Trng ln ca s (window size) 16 bit c s dng kim sot lu lng. y chnh l s lng byte d liu ti a m bn nhn c th chp nhn c. Trng di tiu (length field) 4 bit xc nh di ca tiu TCP theo n v l cc t 32 bit. Tiu TCP c th c di thay i ph thuc trng option (Nu trng option rng, th chiu di ca tiu TCP l 20 byte).

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Trng option l tu chn, c th thay i tu . Trng ny c s dng khi bn gi, bn nhn c th thng lng v gi tr MMS hoc gi tr gia tng ca ca s trong mng cao tc. La chn nhn thi gian (timestamping) cng c nh ngha. Trng c (flag) gm 6 bit. Bit ACK c s dng ch ra rng gi tr t trong trng bin nhn l ng. Cc bit RST, SYN v FIN c s dng trong vic thit lp hay ng kt ni. Khi bit PSH c bt, th y l du hiu yu cu bn nhn phi chuyn d liu ln tng trn ngay lp tc. Cui cng, bit URG c dng bo hiu d liu trong segment c thc th tng trn pha gi to ra l khn cp. V tr byte cui cng ca d liu khn cp c xc nh bi con tr d liu khn 16 bit (ptr to urgent data). TCP phi bo cho tng trn bit c d liu khn v t con tr vo cui d liu khn (Trong thc t, PSH, URG v con tr d liu khn khng c s dng) - URG - ACK - PSH - RST - SYN - FIN C cho trng Urgent pointer C cho trng Acknowledgement Hm Push Thit lp li ng truyn ng b li s th t Khng gi thm s liu

+RTT (Round Trip Time): - Thi gian chuyn i vng quanh (RTT) l di ca thi gian cn cho mt tn hiu c gi cng vi chiu di ca thi gian cn cho mt s tha nhn tn hiu c nhn. Do , thi gian tr ny bao gm thi gian truyn ti gia hai im ca mt tn hiu. - SampleRTT l khong tthi gian t khi gi segment n khi nhn c ack cho segment ny. - Thi gian k vng: EstimatedRTT = (1-)*EstimatedRTT + *SampleRTT Trong : =0.125 - lch chun: DevRTT = (1-)*DevRTT + *|SampleRTT - EstimatedRTT| Trong : =0.25 - Thi gian ch: TimeoutInterval = EstimateRTt + 4*DevRTT >>Hay: thi gian ch = thi gian k vng + 4 ln lch chun

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16. giao thc TCP: hot ng v kim sot lung, bt tay 3 bc +TCP: - TCP l dch v hng ni (connection-oriented). - TCP hng ni v trc khi tin trnh ng dng c th bt u gi d liu ti mt tin trnh khc, hai tin trnh ny phi c th tc bt tay vi nhau, ngha l chng phi gi mt s gi segment c bit xc nh cc tham s m bo cho qu trnh truyn d liu. - Trong giai on thit lp kt ni TCP, hai bn s khi to nhiu bin trng thi TCP cho kt ni. - TCP s dng nhiu nguyn l truyn d liu tin cy, bao gm c ch pht hin li, truyn li, bin nhn tch lu, timer, trng tiu cho s th t v s bin nhn. - Vn c chc nng dn knh, phn knh. +Hot ng ca TCP: cc kt ni TCP c ba pha: - thit lp kt ni - truyn d liu - kt thc kt ni - Thit lp kt ni: thit lp mt kt ni, TCP s dng mt quy trnh 3 bc (3-way handshake) Trc khi client th kt ni vi mt server, server phi ng k mt cng v m cng cho cc kt ni: y c gi l m b ng. Mt khi m b ng c thit lp th mt client c th bt u m ch ng. - Truyn d liu: hai bc u tin trong ba bc bt tay, hai my tnh trao i mt s th t gi ban u (Initial Sequence Number -ISN). S ny c th chn mt cch ngu nhin. S th t ny c dng nh du cc khi d liu gi t mi my tnh. Sau mi byte c truyn i, s ny li c tng ln. Nh vy ta c th sp xp li chng khi ti my tnh kia bt k cc gi ti ni theo th t th no. C cc c ch: + Truyn d liu khng li (do c c ch sa li/truyn li) + Truyn cc gi d liu theo ng th t + Truyn li cc gi d liu mt trn ng truyn + Loi b cc gi d liu trng lp + C ch hn ch tc nghn ng truyn - Kt thc kt ni:

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kt thc kt ni hai bn s dng qu trnh bt tay 4 bc v chiu ca kt ni kt thc c lp vi nhau. Khi mt bn mun kt thc, n gi i mt gi tin FIN v bn kia gi li tin bo nhn ACK. V vy, mt qu trnh kt thc tiu biu s c 2 cp gi tin trao i. +Kim sot lung (Flow control): Cp thit b u cui mi pha ca kt ni TCP u c b m d liu (buffer). Khi kt ni TCP nhn c ng mt dng byte lin tc (ng th t), n s t dng byte ny vo b m nhn (receive buffer). Tin trnh ng dng nhn tng ng s c d liu t b m ny, nhng khng nht thit l phi c ngay khi d liu n. C th tin trnh ng dng nhn phi thc hin nhiu tc v khc nn cha c c d liu trong b m. Nu ng dng c d liu chm th bn gi c th lm trn b m nhn do d liu c gi qu nhiu v qu nhanh. Chnh v l do ny TCP cung cp dch v kim sot lung (flow control) trnh hin tng bn gi lm trn b m bn nhn. - Kim sot lung l qu trnh lm tng thch (matching) v tc : tng thch gia tc bn gi gi vi tc nhn ca bn nhn.

- cung cp c ch kim sot lu lng, TCP bn gi s dng bin receive window. y l gi tr m bn nhn bo cho bn gi bit ln vng m cn ri ca n. - Trong kt ni hai hng, mi pha kt ni c gi tr receive window phn bit. - Gi tr receive window ng, c ngha l n s thay i trong thi gian kt ni. - Gi s my A gi mt file ln ti my B qua kt ni TCP. My B s khi to b m cho kt ni ny vi ln RcvBuffer. Tin trnh ng dng trn B c d liu t b m. Chng ta nh ngha mt s bin sau:

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+LastByteread = s th t ca byte cui cng trong dng d liu m tin trnh ng dng trong my B c t buffer. +LastByteRcvd = s byte cui cng trong dng d liu n t mng v c trong receive buffer ca my B. +V TCP khng c php trn b m nn chng ta phi c : LastByteRcvd LastByteread < RcvBuffer +Receive window l gi tr RcvWindow, l ln vng m ri: RcvWindow = RcvBuffer - [LastByteRcvd LastByteread] +My A cng c hai bin LastByteSent v LastByteAcked. lch gia hai bin ny, LastByteSent - LastByteAcked l s lng d liu cha c bin nhn m A gi qua kt ni. Bng cch khng ch s lng d liu cha c bin nhn nh hn gi tr RcvWindow, A m bo khng lm trn b m ti B. Do vy trong sut thi gian kt ni, A phi m bo: LastByteSent - LastByteAcked < !

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