Matsakaicin Matsayin Rarrabawa (MTU)

Matsakaicin Matsayin Rarrabawa (MTU)

A cikin sadarwar komputa, kalmar Maximum Transmission Unit (MTU) tana nufin girman (a cikin bytes) na PDU mafi girma wanda layin da aka bayar na ladabi na sadarwa zai iya wucewa. Sigogin MTU galibi suna bayyana a cikin haɗin gwiwa tare da ƙirar sadarwa (NIC, tashar jiragen ruwa, da sauransu). Ana iya gyara MTU ta ƙa'idodi (kamar yadda yake tare da Ethernet) ko yanke shawara a lokacin haɗawa (kamar yadda galibi haka yake tare da hanyoyin haɗin kai-da-aya). MTU mafi girma yana kawo ingantaccen aiki saboda kowane fakiti yana ɗauke da ƙarin bayanan mai amfani yayin da ƙa'idojin yarjejeniya, kamar kanun labarai ko jinkiri na kowane fakiti ya kasance a tsaye, kuma mafi girman inganci yana nufin ɗan ingantawa a cikin tsarin yarjejeniya mai yawa. Koyaya, manyan fakitoci na iya mamaye hanyar haɗin yanar gizo na ɗan lokaci, yana haifar da jinkiri mafi girma ga bin fakitoci da haɓaka lag da ƙaramin latency. Misali, fakiti na baiti 1500, mafi girma da Ethernet ya ba da izini a layin cibiyar sadarwa (don haka yawancin Intanet), zai ɗaure modem 14.4k na kusan daƙiƙa ɗaya.

Hanyar gano MTU
Yarjejeniyar Intanit tana bayyana "hanyar MTU" na hanyar watsawa ta Intanet azaman ƙaramin MTU na kowane hops na IP na "hanya" tsakanin tushe da manufa. Sanya wata hanya, hanyar MTU ita ce girman fakiti mafi girma wanda ke ratsa wannan hanyar ba tare da rarrabuwar kai ba.

RFC 1191 yana bayyana "gano hanyar MTU", dabara don tantance hanyar MTU tsakanin runduna biyu na IP. Yana aiki ta hanyar saita zaɓin DF (Kada Ku Tsinke) a cikin jigon IP na fakitoci masu fita. Duk wata na’urar da ke kan hanyar da MTU ta yi ƙasa da fakiti za ta sauke irin waɗannan fakitoci kuma ta mayar da saƙon ICMP “Manufa Ba Za a Iya Ba (Datagram Too Big)” da ke ɗauke da MTU ɗin ta, wanda ke ba wa mai masaukin damar rage hanyar da ya ɗauka MTU daidai. Tsarin yana maimaitawa har sai MTU yayi ƙanƙantar da zai ratsa dukkan hanyar ba tare da rarrabuwa ba.

Abin takaici, adadin lambobin sadarwa suna raguwa da zirga-zirgar ICMP (misali don hana kai hare-hare na sabis), wanda ke hana gano hanyar MTU daga aiki. Mutum yakan gano irin wannan toshewar a lokutan da haɗin ke aiki don ƙananan bayanai amma yana rataye da zaran mai masaukin ya aika babban toshewar bayanai a lokaci guda. Misali, tare da IRC abokin ciniki mai haɗawa zai iya gani har zuwa saƙon ping, amma bai sami amsa ba bayan hakan. Wannan saboda manyan saƙo na maraba ana aika su cikin fakiti mafi girma fiye da ainihin MTU. Hakanan, a cikin hanyar sadarwar IP, hanyar daga adireshin tushe zuwa adireshin da ake kaiwa galibi ana canza ta da ƙarfi, a cikin martani ga abubuwan da suka faru daban-daban (daidaita nauyi, cunkoso, fitarwa, da sauransu)-wannan na iya haifar da hanyar MTU ta canza (wani lokacin maimaita) yayin watsawa, wanda zai iya gabatar da ƙarin fakiti kafin faifai ya sami sabon MTU mai lafiya.

Yawancin LANs na Ethernet suna amfani da MTU na baiti 1500 (LANs na zamani na iya amfani da firam ɗin Jumbo, suna ba da izinin MTU har zuwa 9000 bytes), duk da haka ƙa'idodin kan iyaka kamar PPPoE za su rage wannan. Wannan yana haifar da hanyar gano hanyar MTU ta fara aiki tare da yuwuwar sakamakon sanya wasu rukunin yanar gizo a bayan abubuwan da ba a iya daidaita su ba. Mutum na iya yin aiki a kusa da wannan, ya danganta da wani ɓangaren cibiyar sadarwa wanda yake sarrafawa; misali mutum zai iya canza MSS (matsakaicin girman sashi) a cikin fakiti na farko wanda ke saita haɗin TCP a tafin wuta na mutum.

Wannan matsalar ta sake fitowa akai -akai tun lokacin gabatarwar Windows Vista wanda ke gabatar da 'TCP/IP Stack na gaba'. Wannan yana aiwatar da "Karɓi Window ta atomatik wanda ke ci gaba da ƙayyade mafi kyawun girman girman taga ta hanyar auna samfurin jinkiri na bandwidth da ƙimar dawo da aikace-aikacen, kuma yana daidaita matsakaicin girman girman taga bisa la’akari da canjin yanayin cibiyar sadarwa.” [2] An ga wannan ya kasa aiki tare da tsoffin magudanan ruwa da firewalls waɗanda suka bayyana suna aiki tare da sauran tsarin aiki. An fi ganin sa a cikin magudanar ADSL kuma galibi ana iya gyara shi ta hanyar sabunta firmware.

Kashin baya na ATM, misalin gyaran MTU
Wani lokaci yana da fifiko daga mahangar inganci don ayyana taɓarɓarewar MTU a cikin software da ke ƙasa madaidaicin matsakaicin matsakaicin da za a iya tallafawa. Misali ɗaya na wannan shine yanayin da ake ɗaukar zirga -zirgar IP akan cibiyar sadarwar ATM (Asynchronous Transfer Mode). Wasu masu ba da sabis, musamman waɗanda ke da asalin wayar tarho, suna amfani da ATM akan hanyar sadarwar kashin bayan su.

Ana amfani da ATM a mafi kyawun inganci lokacin da fakiti fakitin ya kasance mai lamba 48. Wannan saboda ana aika da ATM azaman rafi na fakiti mai tsayayyen tsayi (wanda aka sani da 'sel'), kowannensu na iya ɗaukar nauyin adadin bayanan mai amfani 48 na bayanan mai amfani tare da baiti 5 na sama don jimlar farashin 53 bytes kowace sel. Don haka jimlar tsawon bayanan da aka watsa shine 53 * ncells bytes, inda ncells = adadin sel da ake buƙata na = INT ((payload_length+47)/48). Don haka a cikin mafi munin yanayi, inda jimlar tsawon = (48*n+1), ana buƙatar ƙarin tantanin halitta don watsa siginar ƙimar ƙarshe ta ƙarshe, sel na ƙarshe yana kashe ƙarin 53 da ake watsawa bytes 47 daga cikinsu padding ne. A saboda wannan dalili, ayyana taɓarɓarewar MTU a cikin software yana haɓaka ƙimar yarjejeniya a cikin layin ATM ta hanyar sanya adadin AAL5 na jimlar biyan kuɗi ya zama ninki 48 na baiti a duk lokacin da zai yiwu.

Misali, cibiyoyin ATM guda 31 da suka cika gaba daya suna dauke da nauyin 31*48 = 1488 bytes. Thisauki wannan adadi na 1488 da cirewa daga cikin sa duk wani abin da aka ba da gudummawa ta duk ƙa'idodin ƙa'idodin da suka dace za mu iya samun ƙimar da aka ba da shawarar don rage ƙirar MTU mafi kyau. A cikin yanayin da mai amfani zai saba aika fakiti na baiti 1500, aikawa tsakanin 1489 zuwa 1536 yana buƙatar ƙarin ƙayyadadden farashi na 53 bytes da aka watsa, a cikin nau'in sel guda na ATM.

Don misalin IP akan haɗin DSL ta amfani da PPPoA/VC-MUX, sake zaɓar cika 31 ATM ATM kamar da, muna samun adadi mafi ƙarancin adadi na MTU na 1478 = 31*48-10 la'akari da saman 10 bytes wanda ya ƙunshi na Yarjejeniyar-zuwa-Point Protocol sama da 2 bytes, da AAL5 sama na 8 bytes. Wannan yana ba da jimlar farashin 31*53 = 1643 bytes da ake watsawa ta ATM daga fakiti 1478 da aka wuce zuwa PPPoA. Dangane da IP da aka aika akan ADSL ta amfani da PPPoA adadi na 1478 zai zama jimlar fakitin IP gami da taken IP. Don haka a cikin wannan misalin ci gaba da rage MTU na 1478 da kansa ya saɓa wa aika fakitoci na IP na jimlar tsawon 1500 yana adana 53 baiti a kowane fakiti a cikin layin ATM a kan rage ragin byte 22 na fakitin IP.

Matsakaicin MTU don haɗin PPPoE/DSL shine 1492, a kowane RFC 2516: 6 bytes shine PPPoE header, yana barin isasshen ɗaki don biyan kuɗaɗen kuɗaɗen 1488, ko kuma cikakken cibiyoyin ATM 31.

A karshe: Daidaitaccen darajar MTU shine ya zama 1492 .... kuma idan akwai matsalolin lilo ko matsalolin haɗin haɗin MSN yakamata a rage zuwa ƙimar 1422 da 1420.

reference: wikipedia

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