Crezi ce vrei dar trebuie sa stii ca exista si altceva decat bere, fotbal, politica si blonde la televizor.

http://www.dimensiuni.ro

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Beware Ethernet flow control

After having been recently bitten by Ethernet’s flow control mechanism, I decided to learn about this somewhat obscure but commonly used facet of modern networks. This post is a summary of what I discovered about it and its associated benefits and dangers.

What is flow control?

Ethernet flow control, or 802.3x, is a way for a network device to tell its immediate neighbor that it is overloaded with data, such as when a device is receiving data faster than it can process it. It allows for an overloaded device to send out a special Ethernet frame, called a pause frame, that asks the device on the other end of the wire to stop sending data temporarily. If the receiving device honors the pause frame then the sending device has time to catch up on the stack of received data that it hasn’t had time to process yet.

There also exists an older method for flow control called “back pressure” that is used in half-duplex environments (i.e. non-switched Ethernet). It consists of the overloaded device “jamming” the medium temporarily until it has the ability to accept more data. I don’t know much about half-duplex flow control, and thus I won’t mention it again; everything here applies solely to full-duplex flow control via 802.3x. Also, TCP has a mechanism for performing its own flow control that is entirely different from Ethernet’s flow control; I will not be fully explaining TCP’s flow control method here, as it would merit a lengthy discussion itself.

Rules of the game

When thinking about Ethernet flow control, it is important to keep several things in mind:

1. Flow control operates at a lower layer than TCP or IP, and thus is independent of them. Put another way, flow control is capable of being used regardless of what higher-level protocols are put on top of it. An important side-effect of this is that neither TCP nor IP know what Ethernet’s flow control is doing; they operate under the assumption that there is no flow control other than what they may or may not provide themselves.
2. Flow control functions between two directly connected network devices, and flow control frames are never forwarded between links. Thus, two computers that are connected via a switch will never send pause frames to each other, but could send pause frames to the switch itself (and vice versa: the switch can send pause frames to the two computers).
3. Pause frames have a limited duration; they will automatically “expire” after a certain amount of time. The expiration time is set by the device that transmits the pause frame.
4. A paused link is not a discriminator of protocols; it will prevent any data from being passed across the link other than more pause frames.

Perhaps you have begun to see some issues with flow control in light of some of the above points. Let’s start looking at them.

TCP breakage

Okay, it isn’t true, TCP doesn’t stop working when flow control is enabled. However, an important part of it does stop working correctly: its own flow control mechanism. TCP flow control uses a more complex mechanism of timeouts and acknowledgement segments to determine when a remote device is overloaded. It basically sends at a faster and faster pace until it sees that some of its sent data isn’t getting to the remote device and then slows down. This allows TCP to utilize network links in a somewhat intelligent manner, as an overloaded network or device will cause some TCP segments to be lost and thus cause the sender to send data at a slower rate.

Now consider what happens when Ethernet flow control is mixed with TCP flow control. Let’s assume that we have two directly connected computers, one of which is much slower than the other. The faster sending computer starts sending lots of data to the slower receiving computer. The receiver eventually notices that it is getting overloaded with data and sends a pause frame to the sender. The sender sees the pause frame and stops sending temporarily. Once the pause frame expires, the sender will resume sending its flood of data to the other computer. Unfortunately, the TCP engine on the sender will not recognize that the receiver is overloaded, as there was no lost data — the receiver will typically stop the sender before it loses any data. Thus, the sender will continue to speed up at an exponential rate; because it didn’t see any lost data, it will send data twice as fast as before! Because the receiver has a permanent speed disadvantage, this will require the receiver to send out pause frames twice as often. Things start snowballing until the receiver pauses the sender so often that the sender starts dropping its own data before it sends it, and thus finally sees some data being lost and slows down.

Is this a problem? In some ways it isn’t. Because TCP is a reliable protocol, nothing is ever really “lost”; it is simply retransmitted and life goes on. Ethernet flow control accomplishes the same thing as TCP flow control in this situation, as they both slow down the data transmission to the speed that the slower device can handle. There are some arguments to be made for there being an awkward overlap between the two flow control mechanisms, but it could be worse.

Unfortunately, it does get worse.

Head-of-line blocking

In the last example, I considered the case where two computers were directly connected to each other. This example is too simplistic to be of much use — when was the last time you saw two directly connected computers? It is a bit of a rarity. Let’s now look at what happens when you introduce a switch into the mix. For our purposes, let us assume that the switch fully supports Ethernet flow control and that it is willing to use it. Our new setup will consist of two desktop computers and one file server, all of which are attached to the switch. It isn’t any fun to make everything perfect, so let’s also say that one of the desktops has a 10 Mbps connection to the switch while the other desktop and the server have 100 Mbps connections.

This setup is usually fine — the 10 Mbps connection will be slower than the others, but it doesn’t cause too many problems, just slower service to the one desktop. Things could get ugly, though, if Ethernet flow control is enabled on the switch. Imagine that the 10 Mbps desktop requests a large file from the file server. The file server begins to send the file to the desktop initially at a slow rate, but quickly picks up steam. Eventually, the file server will start to send data to the desktop at 11 Mbps, which is more than the poor 10 Mbps connection can handle. Without flow control enabled on the switch, the switch would start to simply drop data segments destined to the desktop, which the file server would notice and start to throttle back its sending rate.

With flow control enabled on the switch, though, the switch takes a very different approach; it will send out its own pause frames to any port that is sending data to the now-overloaded 10 Mbps port. This means that the file server will receive a pause frame from the switch, requesting it to cease all transmissions for a certain amount of time. Is this a problem? Yes! Because pause frames cease all transmissions on the link, any other data that the file server is sending will be paused as well, including data that may be destined to the 100 Mbps desktop computer. Eventually the pause will expire and the file server will continue sending out data. Unfortunately, the TCP mechanism on the file server will not know that anything is wrong and will continue sending out data at faster and faster speeds, thus overloading the 10 Mbps desktop again. As before, the cycle will keep repeating itself until the file server starts dropping its own data. Unlike the previous situation, the innocent 100 Mbps desktop bystander is penalized and will see its transfers from the file server drop to 10 Mbps speeds.

This situation is called head-of-line blocking, and it is the major reason why Ethernet flow control is somewhat dangerous to use. When enabled on network switches, it can create situations where one slow link in a network can bring the rest of the network to a crawl. It gets especially bad if the backbones in your network have flow control enabled; it should be obvious by this point just how bad that could get.

When to enable flow control

So what should you do? Should you completely disable flow control on all computers and switches? Not necessarily. It is generally safe to leave flow control enabled on computers. Switches, though, should either have flow control disabled or configured such that they will honor received pause frames but will never send out new pause frames. Some Cisco switches are even permanently configured this way — they can receive pause frames but never emit them. To be honest, the complete answer to flow control is somewhat more complicated than this (e.g. you could probably enable pause frame emission if a switch port is connected to a slow backplane), but the safest bet is to disable flow control when given the option.

ref: http://virtualthreads.blogspot.com/2006/02/beware-ethernet-flow-control.html

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Join Telecom ofera servicii de colocare de servere si/sau de hosting partajat. Acest lucru presupune existenta unui spatiu racit in mod adecvat, alimentat de la surse de energie redundante sau existenta de UPS-uri de mare putere, existenta rack-urilor de 19″, acces limitat in incinta si, bineinteles, acces la linii de internet dedicate.

In afara serviciilor standard enumerate mai sus, Join Telecom ofera si servicii de consultanta si implementare de solutii complete, bazate pe sisteme Linux sau Windows, de:

• webhosting (web, mail, ftp);
• radio hosting (shoutcast, icecast);
• acces shell sau root (ssh, vnc );
• servere de ceas de timp real (ntp);
• tunneling;
• servere virtuale;
• Autoritati de certificare (CA) pentru certificate digitale;

Indiferent de solutia aleasa, Join Telecom ofera consultanta si implementeaza solutii dei firewall bazate pe echipamente hardware sau software (iptables), monitorizare permanenta cu cacti (cu acces online pe baza de username/parola);

Serverele pot fi proprietatea clientului sau pot fi inchiriate de la Join Telecom. In cazul serverelor proprietate, nu se accepta decat servere de 19″/1,5m adancime, inaltime maxim 2U.

Conectivitatea este efectuata de regula pe cabluri UTP categoria 5-6, Fast Ethernet. La cerere se pot asigura si conexiuni Giga Ethernet. Sunt de asemenea posibile si legaturile interne direct pe fibra optica. Sunt acceptate numai mediaconvertoare CTC Union (Fast Ethernet, Giga Ethernet cu GBIC sau minibgic).

Legaturile de date se efectueaza pe swich-uri DLink sau direct in module Cisco Catalyst 6500.

Temperatura in centrul de colocare este monitorizata si mentinuta la o temperatura de 23-25 C.

Tags: acces root, acces shell, autoritate de certificare, banda dedicata, cacti, catalyst, ceas de timp real, certificat digital, Cisco, Cisco Catalist 6500, clamav, colocare, CTC, CTC Union, DKSign, DomainKeys, Fast Ethernet, firewall, gbic, Giga Ethernet, Hosting, icecast, imap, ipsec, iptables, mail hosting, minigbic, monitorizare, mysql, ntp, openssl, oracle, pop3, postgresql, qmail, radio live, Radio Online, server partajat, servere, servere colocate, servere dedicate, shoutcast, smtp, spam assassin, spamassassin, ssh, ssl, tunelling, vhost, vlan, vnc, webhosting, windows

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Tehnologia VoIP utilizeaza mediul Internet pentru stabilirea de comunicatii audio sau video, similare cu cele din telefonia clasica PSTN.

Prezentare functionare

Datorita utilizarii protocolului IP, tehnologia VoIP este extrem de versatila, limitele fiind impuse numai de imaginatia arhitectului de infrastructura si a implementatorului.

Sistemul de comunicatii VoIP Eurovoice, utilizat de catre Join Telecom, este o implementare inteligenta care permite comunicatii de calitate excelenta in orice domeniu de utilizare: rezidential sau corporate.

Tehnologia SIP (Signaling Protocol) care sta la baza dezvoltarii VoIP, este formata in esenta din doua protocoluri distincte:

• protocolul SIP, de semnalizare a evenimentelor (a fost initiata convorbirea, s-a ridicat telefonul din furca, exista un apel telefonic etc);
• protocolul RTM (Real-Time Media) care permite transportul comunicatiei in sine (audio sau video) si care la randul sau se bazeaza pe codec-uri cu diferite capacitati de compresie.

Sistemul SIP a permis crearea de aparatura dedicata, echipamente hardware care contin toate componentele necesare transferului vocii sau a imaginilor de la un microfon/camera video direct catre Internet si invers. Astfel este posibila utilizarea acestui gen de echipamente (numite generic VoIP gateway) cu telefoane sau videotelefoane clasice, functionale si pe liniile PSTN. Pentru cei obisnuiti sa aiba in casa un telefon, este solutia ideala de utilizare a telefoniei SIP.

Sistemul SIP permite de asemenea implementarea de solutii software de la cele mai simple (XLite, Zoiper) pana la cele foarte complicate, transformand un calculator clasic inttr-o adevarat centrala telefonica, cu zeci si chiar sute de conexiuni, interconectari, convorbiri simultane, roboti telefonici, etc.

Sistemul SIP este posibil de a fi transportat catre consumatori direct prin linia de Internet din casa sau, pentru domeniul corporate, separat prin fibra optica, cu ajutorul unor multiplexoare de linie E1 (se pot aduce pana la 16 fluxuri de 30 de numere pe o singura fibra optica), pentru utilizarea in centrale telefonice inteligente (de genul celor de la Alcatel).

Particularitati Join Telecom

Join Telecom a dorit ca sistemul SIP propriu sa fie extrem de versatil, avand in vedere ca majoritatea clientilor sunt la nivelul corporate si pretentiile pentru solutii flexibile sunt ridicate.

Astfel:

In vederea obtinerii unei calitati maxime a convorbirilor, legaturile le efectueaza centralizat catre centrale pe linii de mare viteza, metropolitane. In momentul in care se apeleaza un numar din afara Romaniei, centrala utilizeaza linii dedicate externe pentru convorbirile telefonice, legaturi pe care nu circula Internet si nu apar congestii de trafic, fiind permanent monitorizate. Aceste linii dedicate trimit informatia de voce/video catre cele mai importante noduri de Internet din lume (Internet Exchange-uri), de unde se transmit mai departe in retelele unde se afla destinatarii apelati.

Clienti multumiti ai Jointelecom se afla astazi pe toate continentele (in afara de Antarctica).

- convorbirile din cadrul retelei Eurovoice si Join Telecom sunt complet gratuite (clientii platesc deja pentru accesul internet, suprataxarea nu are sens in acest caz);

- abonamentele sunt simbolice pentru clientii Join Telecom (0-2 EUR/numar de telefon), pentru a acoperi cheltuielile operationale ale sistemului;

- nu exista notiunea de minute incluse sau gratuite (orice reclama de acest gen ascunde costurile in cadrul abonamentului), fiecare convorbire apare detaliat in interfata de telefonie, direct pe internet, cu durata, cost si alte informatii detaliate.

- fiecare numar de telefon poate fi setat pe mai multe echipamente (sau calculatoare) simultan. In momentul apelului, suna toate echipamentele, iar cel de la care se preia convorbirea o si realizeaza. Echipamentele pot fi uriunde pe Internet: unul acasa, unul la birou si unul pe telefonul mobil sau in alt birou in strainatate. Acest lucru realizeaza o portabilitate deosebita, utila oamenilor de afaceri care nu au timp sa se ocupe de solutii complicate, si care doresc sa fie apelati la acelasi numar de telefon indiferent de locul in care se afla, fara a cara echipamente de colo-colo;

- un numar de telefon poate fi configurat in sistem hunting, util pentru call-centere, unde mai multe persoane suna simultan catre acelasi numar de telefon si totusi se doreste realizarea de mai multe convorbiri simultan.

- orice numar de telefon poate fi redirijat catre orice alt numar de telefon functie de criterii multiple:

• neconditionat;
• dupa un anumit numar de apeluri;
• dupa un anumit numar de secunde daca nu s-a raspuns;

- in final, este posibila implementarea de numere scurte, de la 3-4 cifre in sus, pentru a putea fi utilizate in cadru restrans. Odata cu trecerea tuturor numerelor de telefon la numerotatia nationala de 10 cifre, Join Telecom realizeaza trecerea la mai putine cifre pentru grupuri de lucru. Echipamentul pe care este setat un numar de telefon poate fi apelat atat pe numarul scurt (din interiorul grupului de lucru) cat si pe numerotatia nationala de 10 cifre.

Tags: apel pe internet, apeluri pe internet, eurovoice, flux E1, IAX, internet call, numar de telefon scurt, numar scurt, numerotare scurta, PSTN, SIP, teleconferinta, telefon internet, telefonie, Voice over IP, VoIP

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Am inceput redactarea si postarea online de manuale de utilizare pentru diferite programe pe care le consideram deosebit de utile pentru utilizatorii Join Telecom.

In masura posibilitatilor si a eficientei programelor, incercam promovarea de “free software”.

Uneori acest lucru nu este posibil, caz in care clientii Join Telecom vor trebui sa isi procure o licenta pentru programele descrise, daca doresc sa le si utilizeze.

In masura timpului disponibil, incercam sa redactam aceste manuale astfel incat sa acoperim cele mai dese probleme intalnite in practica.

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Solutia de Internet Rezidential sau Internet pentru Acasa, livrat de catre Join Telecom, este o solutie completa si complexa, care tine cont de mai multe aspecte precum:

• intervalele orare lucru/timp liber/odihna;
• nivelul de incarcare al retelei;
• ore de varf;
• impartire de banda;
• destinatii internet clasice (Yahoo, Google, You Tube, bloguri, ziare, entertainment etc).

Solutia de Internet pentru Acasa se bazeaza in principal pe orientarea majoritara a abonatilor catre continutul localizat si restrans. Accesul in infrastructura Interlan si in alte retele, prin legaturile directe efectuate, permit viteze uriase de descarcare in mediul metropolitan si national. Accesul la Internet in afara Romaniei se realizeaza prin aceiasi furnizori ca si cei corporate, pe legaturi si banda separate. Acest mod de gandire aduce principalele avantaje fata de alte retele care se bazeaza pe furnizori ieftinisi slab calitativi:

• uptime ridicat;
• utilizarea de echipamente moderne si de calitate ridicata;
• viteze de descarcare foarte mari.

Retelele rezidentiale, concentrate in cartiere sau arii restranse, permit impartirea costurilor de distributie la mai multi abonati. Intretinerea retelelor se efectueaza direct de catre Join Telecom in intervalul orar 10-19 zilnic, fara interventii de noapte decat in cazuri urgente, cand avaria afecteaza un numar insemnat de abonati. Telul final pe care il dorim este mentinerea unui uptime de peste 99% (fata de cel de 97% contractual), tel indeplinit cu succes pana in momentul de fata (uptime-ul general calculat in ultimul trimestru 2007 a fost de 99,986%). Toti acesti parametri conduc catre scaderea drastica a preturilor, in conditiile mentinerii unui nivel calitativ foarte ridicat.

Join Telecom incearca de asemenea sa utilizeze avantajele celebrelor “retele de cartier”, avand disponibile in toate cartierele unde opereaza personal de intretinere si incasari abonamente. In acelasi timp, parametrii de functionare sunt net superiori celor mai cunoscute “retele de cartier” datorita experientei de lucru cu mediul corporate implementata in cadrul retelelor rezidentiale.

Tags: acces internet rezidential, Internet pentru acasa, retea de cartier, uptime

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Serviciile de acces Internet Corporate sunt livrate de catre Join Telecom incepand cu anul 2004 catre societi comerciale si agentii nationale in conditii de maxima calitate.

Multa lume intreaba de ce solutiile corporate costa mult fata de cele rezidentiale. Intrebarea este perfect justificata, avand in vedere preturile care sunt si de 10-20 de ori mai mari.

O explicatie simpla nu exista, costurile suplimentare decurgand dintr-o suma de factori, dintre care cei mai importanti sunt:

• banda de internet livrata catre societati este garantata 100% din timp, astfel toate costurile din upstream si costurile de intretinere se cumuleaza iar rezultatul este pe masura;
• mediul de livrare corporate este format exclusiv din fibra optica, care permite modificari de banda rapide, la cerere, din configurarea aparatelor, fara a necesita instalari sau echipamente suplimentare;
• locatiile corporate sunt monitorizate permanent pentru a facilita actiunile proactive in caz de avarii;
• locatiile corporate utilizeaza echipamente high-end, de anduranta;
• consultanta de orice natura pentru domeniul corporate este gratuita in limitele contractuale, fara exceptii;
• in marea majoritate a cazurilor, domeniul corporate necesita o solutie de comunicatii si nu numai acces internet, incluzand, dar fara a se limita la:

1. acces internet cu uptime peste 99,95%;
2. acces intranet securizat (intre mai multe sedii);
3. sistem de telefonie VoIP gratuit intre sedii;
4. sistem de monitorizare audio/video;
5. sistem de teleconferinta;
6. hosting web si email cu facilitati speciale;
7. colocare servere pe banda separata;
8. securizare si secretizare de informatii transportate intre sedii sau via Internet.

• solutionarea problemelor se rezolva in regim de outsourcing de catre o alta societate specializata;
• solutionarea problemelor se efectueaza foarte rapid, practic in regim de urgenta, 24h/24;

In momentul de fata, Join Telecom livreaza servicii corporate in Romania in orasele Bucuresti, Constanta, TImisoara si Cluj. De asemenea livram servicii corporate utilizand benzi inchiriate si in Milano si Lisabona.

Sistemul de telefonie corporate Join Telecom este activ in Romania, Franta, Anglia, Portugalia, Spania, America de Nord (SUA si Canada) si in Chile.

Tags: Acces Internet Business, Acces Internet Corporate, colocare, Hosting, monitorizare, Secretizare, Securizare, servere, teleconferinta, telefonie, uptime, VoIP

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Testele de viteza online ofera o imagine despre capacitatea unei retele de a oferi trafic unui potential client, precum este si un instrument de verificare pentru clientii existenti.

De ceva vreme incoace am tot vrut sa fac disponibil un test de viteza pe Join Telecom, dar scripturile peste care am dat erau ori prost concepute, ori prea scumpe.

OOkla Net Metrics a pus recent, gratuit, la dispozitie, un Mini Speed Test pentru a fi pus pe situri. Motorul de verificare este identic cu cel comercial (pe care il gasiti la http://speedtest.net), doar ca mai putin aratos.

Astfel, am instalat pe unul din serverele disponibile in noc acest test de banda, pe o interfata de 100 Mbps. In momentul de fata, totul pare OK.

Testul de viteza este disponibil direct pe pagina Join Telecom dar si separat, la adresa http://bandtest.jointelecom.ro

Happy testing!

Tags: band test, servere, serveur, Speed Test, test de banda, test de bande passante, test de vitesse, Test de Viteza

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Serverul NTP (Network Time Protocol) este utilizat pentru sincronizarea orei a mai multor calculatoare client, indiferent de tip, marca sau sistem de operare.

Am in casa trei ceasuri digitale care arata trei ore diferite.

Telefonul (ca asa e cool) suna, evident, la o alta ora.

Imi beau cafeaua in fata calculatorului din casa care arata si el, alta ora.

Ajung la birou, unde laptopul imi arata si el alta ora.

Incepe o zi de lucru in care umblu prin jde servere, din noc sau de la clienti, unde de fiecare data dau de alta ora.

Sambata m-am saturat. Dupa cateva configurari si setari, am pornit un server NTP (ceas de timp real) la care se sincronizeaza toate serverele pe care le administrez. Tot la el se sincronizeaza si statia mea cu XP de acasa si cea cu Vista de la birou.

Daca va pasioneaza sa stiti ora exacta la secunda, folositi cu incredere: ntp.jointelecom.ro

Tags: ceas de timp real, ntp, Server NTP, servere

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Din cele cateva servere achizitionate recent, am directionat unul catre un server de radio live. Desi initial am vrut sa fac streaming video live, am realizat ca necesarul de echipamente m-ar fi sufocat pur si simplu. Asa ca am facut un server de streaming radio asteptand zile mai bune.

Preiau frumos direct de la sursele originale si retransmit. Mai greu a fost cu DI si Sky FM dar, alegand judicios benzile de 24kbps AAC am reusit sa “inghesui” posturi cat pentru toate gusturile. Restul de surse vin din metropolitan si nici nu se resimt la traficul total.

Am reusit dupa 3 zile de cautari sa strang 50 de posturi de radio/muzica de cele mai diverse genuri, sper pentru toate gusturile.

Un playlist cat se poate de usor de utilizat din Winamp se regaseste la adresa http://radio.jointelecom.ro

Auditie placuta!

Tags: live radio, radio live, Radio Online, servere

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