1. M hnh OSI Lp Lp ng dng Lp Trnh din Lp phin Lp giao vn Lp mng
Lp lin kt d liu Lp vt l V d HTTP, FTP, Telnet ASCII SSL, PPP TCP,
UDP IP, IPX Ethernet, Token Ring, FDDI UPT Cable, coax, voltage
levels, signaling
2. Lp mng: Giao thc Internet Giao thc lin mng IP l mt trong nhng
giao thc quan trng nht ca b giao thc TCP/IP. Mc ch ca giao thc lin
mng IP l cung cp kh nng kt ni cc mng con thnh lin mng truyn d liu.
IP l giao thc cung cp dch v gi datagram theo kiu khng lin kt v khng
tin cy ngha l khng cn c giai on thit lp lin kt trc khi truyn d liu,
khng m bo rng IP datagram s ti ch v khng duy tr bt k thng tin no v
nhng datagram gi i. Khun dng n v d liu dng trong IP c th hin trn
hnh 1
Hnh 1: Khun dng d liu trong IP ngha cc tham s trong IP
header:
-
-
-
Version (4 bit): ch phin bn (version) hin hnh ca IP c ci t. IHL
(4 bit): ch di phn header tnh theo n v t m (word - 32 bit). Gi tr
nh nht cho tiu l 5. Type of Service (8 bit): c t tham s v yu cu dch
v Total length (16 bit): ch di ton b IP datagram tnh theo byte. Da
vo trng ny v trng header length ta tnh c v tr bt u ca d liu trong
IP datagram. Indentification (16 bit): l trng nh danh, cng cc tham
s khc nh a ch ngun (Source address) v a ch ch (Destination address)
nh danh duy nht cho mi datagram c gi i bi 1 trm. Thng thng phn nh
danh (Indentification) c tng thm 1 khi 1 datagram c gi i. Flags (3
bit): cc c, s dng trong khi phn on cc datagram. 0 01 02
R DF MF o R, reserved. 1 bit. Cha s dng, c gi tr 0 o DF, Don't
Fragment. 1 bit. : iu khin vic phn mnh gi tin Gi tr M t 0 Phn mnh
nu cn 1 Khng phn mnh o MF, More fragments. 1 bit.ch th gi tin phn
mnh hay khng Gi tr 0 1 M r y l mnh cui cng Vn cn nhiu mnh pha
sau.
-
Fragment Offset (13 bit): ch v tr ca on phn mnh (Fragment) trong
datagram tnh theo n v 64 bit. TTL (8 bit): thit lp thi gian tn ti
ca datagram trnh tnh trng datagram b qun trn mng. TTL thng c gi tr
32 hoc 64 c gim i 1 khi d liu i qua mi router. Khi trng ny bng 0
datagram s b hy b v s khng bo li cho trm gi. Protocol (8 bit): ch
giao thc lp trn k tip Header checksum (16 bit): kim sot li cho vng
IP header. Source address (32 bit): a ch IP trm ngun Destination
address (32 bit): a ch IP trm ch Option ( di thay i): khai bo cc ty
chn do ngi gi yu cu, thng l: an ton v bo mt, Bng ghi tuyn m
datagram i qua c ghi trn ng truyn, Time stamp,
Xc nh danh sch a ch IP m datagram phi qua nhng datagram khng bt
buc phi truyn qua router nh trc, Xc nh tuyn trong cc router m IP
datagram phi c i qua. - Padding: di ty bin. Nhm n thm thng tin trng
m bo phn d liu c bt u ngay sau khi kt thc 32 bt cui ca phn tiu gi
tin. 3. Lp truyn ti/ Ethernet 3.1. Cu trc khung Ethernet:
Hnh 2. Cu trc khung Ethernet Preamble: trng ny nh du s xut hin
ca khung bit, n lun mang gi tr 10101010. T nhm bit ny, pha nhn c th
to ra xung ng h 10 Mhz. SFD (start frame delimiter): trng ny mi thc
s xc nh s bt u ca 1 khung. N lun mang gi tr 10101011. Cc trng
Destination v Source: mang a ch vt l ca cc trm nhn v gi khung, xc
nh khung c gi t u v s c gi ti u. LENGTH/TYPE: xc nh giao thc mng lp
cao no c mang trong khung. Data/ Payload l trng quan trng nht v n
mang d liu cn truyn. Kch thc ti thiu l 46 byte v ti a l 1500 byte.
CRC (Chui kim tra khung): Nm cui khung cha 32 bit. y l mt c ch kim
tra s ton vn ca khung khi ti ch. CRC c to ra bng cch thm mt a thc
bit to khung khi truyn i. a thc cng c s dng bn nhn kim tra ni dung
ca khung c b thay i trong qu trnh truyn hay khng. 3.2. C ch CSMA/CD
Phng thc iu khin truy nhp CSMA/CD quy nh hot ng ca h thng Ethernet.
Mt s khi nim c bn lin quan n qu trnh truyn khung Ethernet: Khi tn
hiu ang c truyn trn knh truyn, knh truyn lc ny bn v ta gi trng thi
ny l c sng mang carrier. Khi ng truyn ri: khng c sng mang absence
carrier. Nu hai trm cng truyn khung ng thi th chng s pht hin ra s
xung t v phi thc hin li qu trnh truyn khung.
Khong thi gian mt giao tip mng khi phc li sau mi ln nhn khung c
gi l khong trng lin khung ( interframe gap) k hiu IFG. Ethernet
10Mb/s: IFG = 9,6 us Ethernet 100Mb/s: IFG = 960 ns Ethernet
1000Mb/s: IFG = 96 ns
Gi tr ca IFG bng 96 ln thi gian ca mt bit.
Cch thc truyn khung v pht hin xung t din ra nh sau: Khi pht hin
ng truyn ri, my trm s i thm mt khong thi gian bng IFG, sau n thc
hin ngay vic truyn khung. Nu truyn nhiu khung th gia cc khung phi
cch nhau khong IFG. Trong trng hp ng truyn bn, my trm s tip tc lng
nghe ng truyn cho n khi ng truyn ri th thc hin li 1. Trng hp khi qu
trnh truyn khung ang din ra th my trm pht hin thy s xung t, my trm
s phi tip tc truyn 32 bit d liu. Nu s xung t c pht hin ngay khi mi
bt u truyn khung th my trm s phi truyn ht trng preamble v thm 32
bit na , vic truyn nt cc bit ny (ta xem nh l cc bit bo hiu tc nghn)
m bo tn hiu s tn ti trn ng truyn lu cho php cc trm khc ( trong cc
trm gy ra xung t) nhn ra c s xung t v x l : o Sau khi truyn ht cc
bit bo hiu tc nghn, my trm s i trong mt khong thi gian ngu nhin hy
vng sau s khng gp xung t v thc hin li vic truyn khung nh bc 1. o
Trong ln truyn khung tip theo ny m vn gp xung t, my trm buc phi i
thm ln na vi khong thi gian ngu nhin nhng di hn. Khi mt trm truyn
thnh cng 512 bit (khng tnh trng preamble), ta xem nh knh truyn b
chim. iu ny cng c ngha l khng th c xung t xy ra na. Khong thi gian
ng vi thi gian ca 512 bit c gi l slotTime. y l tham s quan trng
quyt nh nhiu ti vic thit k. Do bn cht cng chia s knh truyn, ti mt
thi im ch c mt trm c php truyn khung. Cng c nhiu trm trong phn on
mng th s xung t cng xy ra nhiu, khi tc truyn b gim xung. S xung t l
hin tng xy ra bnh thng trong hot ng ca mng Ethernet ( t xung t d gy
hiu nhm l mng b s c hay l hot ng sai, hng hc). 3.3. Giao thc phn
gii a ch ARP ARP (Address Resolution Protocol) l giao thc gii (tra)
a ch t a ch mng xc nh c a ch lin kt d liu (a ch MAC). V d: khi IP
gi mt gi d liu cho mt h thng khc trn cng mng vt l Ethernet, IP cn
bit a ch Ethernet ca h thng ch tng lin kt d liu xy dng khung. Thng
thng , c th xc nh a ch trong bng a ch IP a ch MAC mi h thng. Nu
khng, c th s dng ARP lm vic ny. Trm lm vic gi yu cu ARP
(ARP_Request) n my phc v ARP Server, my phc v ARP tm trong bng a ch
IP MAC ca mnh v tr li bng ARP_Response cho trm lm vic. Nu khng, my
phc v chuyn tip yu cu nhn c di dng qung b cho tt c cc trm lm vic
trong mng. Trm no c trng a ch IP c yu cu s tr li vi a ch MAC ca
mnh. 4. Truyn IP qua Ethernet IP ch l mt giao th thun ty logic v
lun cn mi trng vt l. Ethernet l topo mng vt l ph bin nht c s dng.
IP v Internet c tch hp bng cch nhng mi gi IP vo mng d liu ca gi
Ethernet
Hnh 3. Ghp IP trong khung Ethernet 4.1. Truyn mt khung qua
Ethernet
Hnh 4. Th tc ghp gi IP vo khung Ethernet Gi Ip c t trong mt
khung Ethernet nh sau: a. a ch IP Broadcast/Multicast: a ch IP ch c
kim tra xc nh h thng c nn nhn bn sao ca gi hay khng. iu ny xy ra nu
y l mt a ch mng broadcast (hay mt a ch multicast c dng l a ch ca mt
trong cc b lc IP multicast c ng k do b thu IP thit lp). Nu cn ti mt
bn sao, n c gi ti giao din vng lp (loopback) v sau c chuyn trc tip
ti u vo IP. Gi tin ban u tip tc c x l. b. a ch IP Unicast: Tip
theo, a ch IP ch c kim tra bit c a ch ny c phi l a ch IP Unicast
ngun ca h thng gi hay khng. Cc gi ny c gi trc tip ti giao din vng
lp (loopback) (tc l khng bao gi ti c giao din Ethernet vt l) c. a
ch IP Nexthop. Bn gi sau xc nh a ch next hop tc l a ch IP ca h thng
trung gian/ h thng u cui tip theo nhn gi tin. Khi a ch ny c xc nh,
giao thc phn gii a ch ARP c s dng tm a ch MAC ph hp cho khung
Ethernet. y l giai on th hai: (i) b nh cache arp c s dng, nu a ch
MAC c bit, a ch chnh xc c thm vo v gi c chuyn ti hng i gi i. (ii)
Nu a ch MAC cha c trong b m, giao thc ARP c s dng yu cu a ch, v gi
c xp hng cho ti khi c c p ng ph hp (hoc ht thi gian ch). d. MTU:
Kch thc ca gi c kim tra da trn MTU (kch thc gi cc i cho php) ca lin
kt m n c gi ti. (MTU ca giao din vng lp c th khc vi MTU ca
Ethernet). Nu c yu cu, c th thc hin phn
mnh IP hoc gi i mt bn tin li ICMP bn tin ny s kch hot Pht hin ng
MTU Host gi. di ti thiu ca trng d liu ca gi c gi qua Ethernet l 46
octet. Nu cn thit, trng ny c th c n (vi cc octet ton 0) m bo kch
thc khung ti thiu. Phn n thm ny khng nm trong gi IP v khng c tnh
trong trng di ca tiu gi IP. di ti thiu ca trng d liu gi c gi qua
Ethernet l 1500 octet, do di ti a ca mt datagram IP c gi qua
Ethernet l 1500 octet. Cc datagram trn Ethernet c th di hn kch thc
gi Internet ti a ni chung (576 octet). Cc host c ni ti Ethernet cn
bit iu ny khi gi cc datagram ti host cc Ethernet khc nhau. C th gi
cc datagram nh hn trnh s phn mnh khng cn thit cc cng trung gian. e.
ng gi: Khung Ethernet c hon thnh bng vic chn cc trng Kiu Ethernet,
ngun v ch. Khi Tag c s dng, Tag 802.1pQ ph hp c chn vo theo tiu MAC
(trng u tin trong Tag co th c gi theo gi tr IP DSCP). f. Truyn:
khung c gi i theo th tc MAC cho Ethernet 4.2. Nhn mt khung t
Ethernet
Qu trnh c thc hin bi im cui nhn ca mng IP.
Hnh 5. Nhn mt khung t Ethernet Cc khung nhn c c x l nh sau: a.
Giao thc MAC: B iu khin Ethernet trong card giao din mng kim tra
khung m bo: Khng ngn hn di khung ti thiu hoc khng di hn di khung ti
a
b. c. d. e. f. g.
Cha gi tr CRC hp l ti im cui Khng cha phn d (tc l cc bit khng to
thnh mt byte) a ch MAC: khung c lc theo a ch MAC ch v ch c nhn nu:
L mt khung broadcast (tc l tt c cc bit ca trng a ch ch l 1) L mt
khung multicast gi ti mt a ch nhm MAC c ng k L mt khung unicast gi
ti a ch MAC ca nt. Hay giao din ang hot ng ch hn n (promiscuous)
(tc l nh mt cu) MAC SAP: Khung sau c ghp li theo kiu gi MAC c th
(SAP) Cc khung mang mt Tag IEEE 802.1pQ s c thng tin LAN o c kim
tra v x l, trc khi b qua trng Tag v c trng EtherType tip theo. N c
gi ti lp giao thc ph hp (nh LLC, ARP, IP) Cc khung mang Gi IP c mt
trng kiu c gi tr 0x0800 v cc khung cho ARP c gi tr 0x0806. Kim tra
IP: Tiu gi IP c kim tra, gm c: Kim tra kiu giao thc = 4 (tc l phin
bn IP hin ti) Kim tra tng kim tra tiu Kim tra di gi tiu a ch IP: a
ch mng IP ch sau c kim tra: Nu n trng vi a ch IP ca nt th n c nhn
Nu n l gi broadcast mng gi ti mng ca nt th n c nhn Nu n l gi
multicast ti mt a ch IP multicast ang c s dng th n c nhn Nu khng
phi cc gi ny, n c gi tip i theo bng nh tuyn (nu c) hoc b loi b Phn
mnh IP: Cc gi cho nt c kim tra xem c cn ghp li hay khng: Gi tr
fragment offset v nhiu c c kim tra Cc mnh c t trong mt b m cho ti
khi cc mnh khc c nhn hon thin gi IP SAP: trng giao thc IP (SAP) c
kim tra Trng SAP xc nh giao thc truyn ti (v d 1=ICMP; 6 =TCP;
17=UDP) Gi hon thin c gi ti giao thc lp truyn ti ph hp.
