Zolemba malire Kufala Unit (MTU)

Zolemba malire Kufala Unit (MTU)

Pogwiritsa ntchito makompyuta, mawu akuti Maximum Transmission Unit (MTU) amatanthauza kukula (mwa ma byte) a PDU yayikulu kwambiri yomwe njira yolumikizirana imatha kupitilira. Magawo a MTU nthawi zambiri amawoneka mogwirizana ndi njira yolumikizirana (NIC, doko loyeserera, ndi zina zambiri). MTU itha kukhazikitsidwa ndi miyezo (monga momwe zimakhalira ndi Ethernet) kapena kusankha nthawi yolumikizira (monga zimakhalira ndi maulalo aku point-to-point). MTU wapamwamba umabweretsa kuchita bwino kwambiri chifukwa paketi iliyonse imakhala ndi zidziwitso zambiri za ogwiritsa ntchito pomwe ma protocol ambiri, monga mutu kapena kuchedwa kwa paketi kumakhalabe kosasunthika, ndipo magwiridwe antchito apamwamba amatanthauza kusintha pang'ono pakulowerera kwa protocol yambiri. Komabe, mapaketi akulu amatha kukhala ndi cholumikizira pang'onopang'ono kwakanthawi, kuchititsa kuchedwa kwakukulu kutsatira mapaketi ndikuwonjezera kutsalira komanso kuchepa kwanthawi. Mwachitsanzo, paketi ya 1500 byte, yayikulu kwambiri yololedwa ndi Ethernet pamaneti (ndipo chifukwa chake ambiri pa intaneti), imatha kumanga modemu ya 14.4k kwa sekondi imodzi.

Njira ya MTU kupeza
Internet Protocol imatanthauzira "njira MTU" yanjira yotumizira pa intaneti ngati MTU yaying'ono kwambiri yamtundu uliwonse wa "njira" ya IP pakati pa komwe imachokera ndikupita. Ikani njira ina, njira MTU ndiye kukula kwakukulu paketi yomwe imadutsa njirayi osagawanika.

RFC 1191 imalongosola "Njira ya MTU kupeza", njira yodziwira njira ya MTU pakati pa magulu awiri a IP. Imagwira ndikukhazikitsa njira ya DF (Do Not Fragment) pamutu wa IP wamapaketi omwe akutuluka. Chida chilichonse panjira yomwe MTU ndi yaying'ono kuposa paketiyo chitha kugwetsa mapaketiwo ndikubwezeretsanso uthenga wa ICMP "Destination Unreachable (Datagram Too Big)" womwe umakhala ndi MTU wake, kulola wopezayo kuti achepetse njira yake yoyendetsera MTU moyenera. Njirayi imabwereza mpaka MTU ikhale yaying'ono mokwanira kuti ingodutsa njira yonse popanda kugawanika.

Tsoka ilo, kuchuluka kwama netiweki kumasiya kuchuluka kwa ICMP (mwachitsanzo, kupewa kukana kulowa ntchito), komwe kumalepheretsa kupeza kwa MTU kugwira ntchito. Nthawi zambiri munthu amazindikira kutsekereza koteroko ngati kulumikizana kumagwirira ntchito zotsika koma amangomangirira pomwe wolandirayo atumiza deta yayikulu nthawi imodzi. Mwachitsanzo, ndi IRC kasitomala wolumikiza amatha kuwona uthenga wa ping, koma osayankhidwa pambuyo pake. Izi ndichifukwa choti uthenga waukulu wolandila umatumizidwa m'mapaketi akulu kuposa MTU weniweni. Komanso, mu netiweki ya IP, njira yochokera ku adilesi yoyambira kupita ku adilesi yakomwe mukupitako imasinthidwa mwamphamvu, poyankha zochitika zosiyanasiyana (kusanjikiza katundu, kuchulukana, zotuluka, ndi zina zambiri) - izi zitha kubweretsa kusintha kwa MTU (nthawi zina mobwerezabwereza) panthawi yamagetsi, yomwe imatha kuyambitsa mapaketi ena asanafike wolandila MTU watsopano.

Ma LAN ambiri a Ethernet amagwiritsa ntchito MTU ya ma 1500 byte (ma LAN amakono amatha kugwiritsa ntchito mafelemu a Jumbo, kulola MTU mpaka ma 9000 byte), komabe malamulo akumalire ngati PPPoE achepetsa izi. Izi zimapangitsa kuti kupezeka kwa MTU kuyambike kugwira ntchito ndi zotsatira zomwe zingachitike pakupanga masamba ena kumbuyo kwa makhoma otentha osafikirika. Wina akhoza kugwira ntchito mozungulira izi, kutengera gawo lomwe maukonde amalamulira; Mwachitsanzo munthu amatha kusintha ma MSS (kukula kwamagawo akulu) mu paketi yoyamba yomwe imakhazikitsa kulumikizana kwa TCP pamakoma amoto.

Vutoli lawonekera pafupipafupi kuyambira kukhazikitsidwa kwa Windows Vista yomwe imayambitsa 'Next Generation TCP / IP Stack'. Izi zimagwiritsa ntchito "Landirani Mawindo Othandizira Pazowonekera omwe amawunikira kukula kwa zenera poyerekeza chinthu chochedwetsa chiwongolero ndi kuchuluka kwa ntchito, ndikusinthanso kukula kwazenera kutengera kusintha kwa netiweki." [2] Izi zawoneka kuti zikulephera molumikizana ndi ma routers akale ndi zotchingira moto zomwe zimawoneka kuti zikugwira ntchito ndi machitidwe ena. Nthawi zambiri zimawoneka m'ma routers a ADSL ndipo nthawi zambiri zimakonzedwanso ndi pulogalamu ya firmware.

Mafupa am'mbuyo a ATM, chitsanzo chakukonzekera kwa MTU
Nthawi zina zimakhala bwino kuchokera pakuwona bwino kuti zilengeze MTU yocheperako mu pulogalamuyo pansi pazowona zenizeni zotheka. Chitsanzo chimodzi cha izi ndi momwe IP traffic imanyamulidwira pa nchitoyi ya ATM (Asynchronous Transfer Mode). Omwe amapereka, makamaka omwe ali ndi foni, amagwiritsa ntchito ATM pamakina awo am'mbuyo.

Kugwiritsa ntchito ATM moyenera kwambiri kumatheka ngati kutalika kwa mapaketi kuli ma 48 byte. Izi ndichifukwa choti ATM imatumizidwa ngati mapaketi azitali zazitali (omwe amadziwika kuti 'ma cell'), iliyonse yomwe imatha kunyamula zolipira za ma byte 48 azosuta ndi ma 5 ma bedi pamtengo wokwanira wa ma byte 53 pa selo. Kotero kutalika kwa kutalika kwa deta ndi 53 * ncells bytes, kumene ncells = chiwerengero cha maselo ofunikira a = INT ((payload_length + 47) / 48). Chifukwa chake choyipa kwambiri, pomwe kutalika kwa ma = (48 * n + 1) mabatani, selo limodzi lowonjezera likufunika kuti lipereke gawo limodzi lomaliza la zolipira, khungu lomalizira lidalipira mabowo 53 owonjezera omwe 47 ndi padding. Pachifukwa ichi, kulengeza mwachangu kuchepa kwa MTU mu pulogalamuyo kumakulitsa magwiridwe antchito pamtambo wa ATM ndikupangitsa kuti ATM AAL5 yonse yolipira kulipira ikhale ma byte 48 ngati zingatheke.

Mwachitsanzo, maselo a ATM okwana 31 amakhala ndi 31 * 48 = 1488 byte. Kutenga chiwerengerochi cha 1488 ndikuchotsapo pamutu uliwonse womwe waperekedwa ndi ma protocol onse ofunikira titha kupeza phindu lochepetsera bwino MTU. Ngati wogwiritsa ntchito amatumiza mapaketi a 1500 byte, kutumiza pakati pa 1489 ndi 1536 mabayiti kumafunikira ndalama zowonjezera zowonjezera ma 53 mabatani opatsirana, ngati selo limodzi la ATM.

Mwachitsanzo pa IP yolumikizana ndi DSL pogwiritsa ntchito PPPoA / VC-MUX, posankhanso kudzaza maselo 31 ATM monga kale, timapeza MTU yocheperako ya 1478 = 31 * 48-10 poganizira kuchuluka kwa ma 10 ya Protocol-to-Point Protocol yomwe ili pamwamba pa ma 2 byte, ndi AAL5 pamwamba pa 8 byte. Izi zimapereka mtengo wokwanira 31 * 53 = 1643 byte opatsirana kudzera pa ATM kuchokera paketi ya 1478 byte yomwe idaperekedwa ku PPPoA. Pankhani ya IP yotumizidwa pa ADSL pogwiritsa ntchito PPPoA chiwerengero cha 1478 chikhoza kukhala kutalika kwa paketi ya IP kuphatikiza ma IP. Chifukwa chake mchitsanzo ichi kusunga MTU yodzichepetsera ya 1478 motsutsana ndi kutumiza mapaketi a IP azitali zonse 1500 amasunga ma 53 mabatani paketi iliyonse pa ATM wosanjikiza pamtengo wotsika wa 22 byte wa kutalika kwa mapaketi a IP.

MTU wokwanira kulumikizana kwa PPPoE / DSL ndi 1492, pa RFC 2516: 6 byte kukhala mutu wa PPPoE, kusiya malo okwanira kulipira 1488 byte, kapena ma cell a ATM 31.

Pomaliza: Mtengo woyenera wa MTU uyenera kukhala 1492 .... pakakhala zovuta kusakatula kapena kulumikizana kwa MSN ziyenera kuchepetsedwa kufikira 1422 ndi 1420.

Tsamba: Wikipedia

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