Ubuningi Transmission Unit (MTU)

Ubuningi Transmission Unit (MTU)

Ekuxhumaneni kwamakhompiyutha, igama elithi Maximum Transmission Unit (MTU) libhekisa kusayizi (ngama-byte) we-PDU enkulu kunazo zonke ukuthi ungqimba olunikeziwe lwephrothokholi yezokuxhumana lungadlulela phambili. Imingcele ye-MTU imvamisa ivela ngokuhlangana ne-interface yokuxhumana (i-NIC, i-serial port, njll.). I-MTU ingahle ilungiswe ngamazinga (njengoba kunjalo nge-Ethernet) noma inqunywe ngesikhathi sokuxhuma (njengoba kujwayele ukwenzeka ngezixhumanisi ze-serial point-to-point). I-MTU ephakeme iletha ukusebenza kahle okukhulu ngoba iphakethe ngalinye lithwala idatha eyengeziwe yomsebenzisi ngenkathi amaphrothokholi angaphezulu, njengamakhanda noma ukulibaziseka kwephakethe ngakunye kuhlala kulungisiwe, futhi ukusebenza kahle okuphezulu kusho ukuthuthuka okuncane kokungena kwe-protocol yenqwaba. Kodwa-ke, amaphakethe amakhulu angahlala kusixhumanisi esihamba kancane isikhathi esithile, abangele ukubambezeleka okukhulu ekulandeleni amaphakethe nokwandisa i-lag kanye ne-latency encane. Isibonelo, iphakethe le-1500 byte, elikhulu kunawo wonke avunyelwe i-Ethernet kusendlalelo senethiwekhi (yingakho iningi le-Intanethi), lingabopha imodemu eyi-14.4k cishe umzuzwana owodwa.

Ukutholwa kwe-Path MTU
I-Internet Protocol ichaza “i-path MTU” yendlela yokudlulisela i-Inthanethi njenge-MTU encane kunazo zonke zohlobo lwe-IP “lwendlela” phakathi komthombo nendawo okuyiwa kuyo. Beka ngenye indlela, indlela i-MTU ingusayizi omkhulu wepakethe onqamula le ndlela ngaphandle kokuhlakazeka.

I-RFC 1191 ichaza "Ukutholwa kwe-Path MTU", inqubo yokunquma indlela ye-MTU phakathi kwama-IP host amabili. Isebenza ngokusetha inketho ye-DF (Do not Fragment) kuzihloko ze-IP zamaphakethe aphumayo. Noma iyiphi idivayisi esendleleni i-MTU yayo incane kunepakethe izolahla amaphakethe anjalo bese ibuyisela emuva umyalezo we-ICMP "Destination Unreachable (Datagram Too Big)" oqukethe i-MTU yayo, okuvumela umphathi womthombo ukuthi anciphise indlela yakhe ayicabangayo i-MTU ngokufanele. Inqubo iyaphinda kuze kube yilapho i-MTU incane ngokwanele ukuba ihambe yonke indlela ngaphandle kokuhlukaniswa.

Ngeshwa, inani elikhulayo lamanethiwekhi lilahla ithrafikhi ye-ICMP (isib. Umuntu uvame ukuthola ukuvimba okunjalo ezimweni lapho uxhumano lusebenzela khona idatha yevolumu ephansi kepha ilenga ngokushesha lapho umphathi esethumela ibhlokhi enkulu yedatha ngasikhathi. Isibonelo, nge-IRC iklayenti elixhuma lingabona umlayezo we-ping, kepha lingatholi mpendulo ngemuva kwalokho. Lokhu kungenxa yokuthi isethi enkulu yemilayezo yokwamukela ithunyelwa ngamaphakethe amakhulu kune-MTU yangempela. Futhi, kunethiwekhi ye-IP, indlela esuka ekhelini lomthombo eya ekhelini oya kuyo ivame ukuguqulwa ngamandla, ekuphenduleni imicimbi eyahlukahlukene (ukulinganisa umthwalo, isiminyaminya, imiphumela, njll.) - lokhu kungaholela ekushintsheni kwe-MTU (kwesinye isikhathi okuphindaphindwayo) ngesikhathi sokudluliswa, okungaletha amanye amaconsi ephakethe ngaphambi kokuthi umphathi asithole i-MTU entsha ephephile.

Ama-LAN amaningi we-Ethernet asebenzisa i-MTU yama-byte ayi-1500 (ama-LAN anamuhla angasebenzisa amafreyimu e-Jumbo, avumela i-MTU kuze kufike kuma-byte ayi-9000), kepha izivumelwano zomngcele ezifana ne-PPPoE zizokunciphisa lokhu. Lokhu kubangela ukutholakala kwendlela ye-MTU kuqale ukusebenza nomphumela ongaba khona wokwenza amanye amasayithi ngemuva kwe-firewalls amiswe kabi angatholakali. Umuntu kungenzeka asebenze azungeze lokhu, kuya ngokuthi iyiphi inethiwekhi elawula eyodwa; ngokwesibonelo umuntu angashintsha i-MSS (ubukhulu bengxenye enkulu) ephaketheni lokuqala elisetha ukuxhumana kwe-TCP ku-firewall yomuntu.

Le nkinga ivele kaningi selokhu kwethulwe iWindows Vista eyethula 'Next Generation TCP / IP Stack'. Lokhu kusebenza "Thola i-Window Auto-Tuning eqhubeka inquma ubukhulu bewindi lokuthola kahle ngokulinganisa umkhiqizo wokubambezeleka komkhawulokudonsa kanye nesilinganiso sokubuyisa uhlelo, futhi ilungisa ubukhulu besayizi lokuthola iwindi ngokuya ngezimo zenethiwekhi eziguqukayo." [2] Lokhu kubonakale kwehluleka ukuhlangana nama-routers amadala kanye nezicishamlilo ezibonakala zisebenza nezinye izinhlelo zokusebenza. Kuvame ukubonwa kuma-routers we-ADSL futhi kaningi kungalungiswa ngesibuyekezo se-firmware.

Imigogodla ye-ATM, isibonelo sokusethwa kwe-MTU
Kwesinye isikhathi kunconyelwa kusukela ekubukeni kokusebenza kahle ukumemezela ngokungeyikho i-MTU encishisiwe kusoftware engaphansi kobude beqiniso obuningi obusekelwayo obusekelwayo. Isibonelo esisodwa salokhu kwenzeka lapho ithrafikhi ye-IP ithwalwa ngenethiwekhi ye-ATM (Asynchronous Transfer Mode). Abanye abahlinzeki, ikakhulukazi labo abanesizinda sefoni, basebenzisa i-ATM kunethiwekhi yabo yomgogodla yangaphakathi.

Kusetshenziswa i-ATM ngokusebenza kahle kakhulu kutholakala lapho ubude bephakethe bungamabhayithi angama-48. Lokhu kungenxa yokuthi i-ATM ithunyelwa njengomfudlana wamaphakethe obude obumisiwe (owaziwa ngokuthi ama-'maseli '), ngalinye lalo lingathwala umthwalo wama-byte angama-48 wemininingwane yomsebenzisi enama-byte amahlanu ngaphezulu ngenani eliphelele lama-byte angama-5 kuseli ngalinye. Ngakho-ke ingqikithi yobude bedatha edlulisiwe ingama-53 * ncells bytes, lapho i-ncells = inani lamaseli adingekayo we = INT ((payload_length + 53) / 47). Ngakho-ke esimweni esibi kakhulu, lapho ubude bebonke = (48 * n + 48) amabhayithi, iseli elilodwa elengeziwe liyadingeka ukuhambisa ibhayithi lokugcina lomthwalo okhokhelwayo, iseli lokugcina libiza amanye ama-byte adluliselwe angama-1 angama-53 awo. Ngalesi sizathu, ukumemezela ngokungeyikho i-MTU encishisiwe kusoftware kukhulisa ukusebenza kahle kweprothokholi kusendlalelo se-ATM ngokwenza ubude be-ATM AAL47 obuphelele bokulayishwa kube ngamabhayithi angama-5 lapho kungenzeka khona.

Isibonelo, amaseli we-ATM angama-31 agcwaliswe ngokuphelele athwala umthwalo okhokhelwayo wama-byte angama-31 * 48 = 1488. Ukuthatha lesi sibalo sika-1488 futhi sisusa kuso noma yimaphi ama-overhead anikelwe yizo zonke izivumelwano eziphakeme ezifanelekile singathola inani eliphakanyisiwe le-MTU encishiswe ngokufanelekile. Esimweni lapho umsebenzisi ajwayele ukuthumela amaphakethe ama-byte ayi-1500, ukuthumela phakathi kuka-1489 no-1536 amabhayithi kudinga izindleko ezingeziwe ezingamabhayithi angama-53 adlulisiwe, ngesimo seseli elilodwa le-ATM.

Isibonelo se-IP ngaphezulu kokuxhumeka kwe-DSL kusetshenziswa i-PPPoA / VC-MUX, siphinde sikhethe ukugcwalisa amaseli we-ATM angama-31 njengakuqala, sithola isibalo se-MTU esincishiswe ngokufanelekile esingu-1478 = 31 * 48-10 sicabangela ubukhulu bamabhayithi ayi-10 aqukethe kwe-Protocol-to-Point Protocol ngaphezulu kwama-byte ama-2, kanye ne-AAL5 ngaphezulu kwama-byte ayi-8. Lokhu kunikeza inani eliphelele lama-byte angama-31 * 53 = 1643 adluliswa nge-ATM kusuka kwiphakethe le-1478 byte elidluliselwe ku-PPPoA. Endabeni ye-IP ethunyelwe nge-ADSL isebenzisa i-PPPoA isibalo esingu-1478 kungaba ubude obuphelele bepakethe le-IP kufaka phakathi izihloko ze-IP. Ngakho-ke kulesi sibonelo ukugcina i-MTU ezinciphise ngokwakho ye-1478 ngokungafani nokuthumela amaphakethe we-IP wobude obuphelele i-1500 isindisa ama-byte angama-53 iphakethe ngalinye kusendlalelo se-ATM ngentengo yokwehliswa kwama-byte angama-22 ubude bamaphakethe we-IP.

I-MTU ephezulu yokuxhuma kwe-PPPoE / DSL ingu-1492, nge-RFC 2516: 6 byte eyinhloko ye-PPPoE, ishiya indawo eyanele yokulayishwa kwe-byte ka-1488, noma amaseli we-ATM agcwele angama-31.

Ekugcineni: Inani elijwayelekile le-MTU kuzoba yi-1492 .... futhi uma kunezinkinga zokuphequlula noma izinkinga zokuxhuma ze-MSN kufanele yehliswe ibe ngamanani ayi-1422 no-1420.

Izikhombo: Wikipedia

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