Din pacate nu ne lamureste CCNA... nu e facut pentru ceva mai avansat decat conceptele de baza... si pe net m-am saturat pe moment de google-uit si nici nu am timp. numai chestii banale. In tanenbaum nu se trateaza cazul asta ci altele mai complicate. RFC-ul nu l-am citit. Poate incearca cineva totusi. Chiar m-a facut curios chestia asta.
> On Thursday 17 July 2003 13:02, Alex 'CAVE' Cernat wrote: > > > Nici asa. windowu precizeaza citi bytes (cel putin la TCP) > > > (dupa unii packeti)poate sa transmita sender . > > > Problema mea este ca poate bufferul nu se umple > > > si atunci cand se transmite ACK? > > > > hai ca am ramas toti in ceatza; are careva un tannenbaum la > > indemana ? > > > > Alex > > Asta ar cam fi explicatia din cursul CCNA pentru Networking > Academy... Imaginea (demo in flash) nu pot sa-l dau din motive de > "lene" (are nume aiurea, tb sa-l caut prin source la html si apoi > prin fisiere)... > > Connection oriented services involve three phases: > > Connection establishment phase > Data transfer phase > Connection termination phase > > In the connection establishment phase, a single path between the > source and destination is determined. Resources are typically > reserved at this time to ensure a consistent grade of service. During > the data transfer phase, data is transmitted sequentially over the > established path, arriving at the destination in the order in which > it was sent. The connection termination phase consists of terminating > the connection between the source and destination when it is no > longer needed. > > > TCP hosts establish a connection-oriented session with one another > using a three-way handshake. A three-way handshake/open connection > sequence synchronizes a connection at both ends before data is > transferred. This exchange of introductory sequence numbers during > the connection sequence is important, ensuring that any data that is > lost due to transmission problems can be recovered. > > > First, one host initiates a connection by sending a packet indicating > its initial sequence number of x with a certain bit in the header set > to indicate a connection request. Second, the other host receives the > packet, records the sequence number of x, replies with an > acknowledgment of x + 1, and includes its own initial sequence number > of y. The acknowledgment number of x + 1 means the host has received > all octets up to and including x, and is expecting x + 1 next. > > > Positive acknowledgment and retransmission, or PAR, is a common > technique many protocols use to provide reliability. With PAR, the > source sends a packet, starts a timer, and waits for an > acknowledgment before sending the next packet. If the timer expires > before the source receives an acknowledgment, the source retransmits > the packet and starts the timer over again. > > > Window size determines the amount of data that you can transmit at > one time before receiving an acknowledgment from the destination. The > larger the window size number (bytes), the greater the amount of data > that the host can transmit. After a host transmits the window-sized > number of bytes, the host must receive an acknowledgment that the > data has been received before it can send any more messages. For > example, with a window size of 1, each individual (1) segment must be > acknowledged before the source can send the next segment. > > > TCP uses expectational acknowledgments, meaning that the > acknowledgment number refers to the octet that is next expected. The > "sliding" part of the term sliding window refers to the fact that the > window size is negotiated dynamically during the TCP session. This > results in efficient use of bandwidth by the hosts. > > > Windowing is a flow control mechanism requiring that the source > device receive an acknowledgment from the destination after > transmitting a certain amount of data. For example, with a window > size of three, the source device can send three octets to the > destination. It must then wait for an acknowledgment. If the > destination receives the three octets, it sends an acknowledgment to > the source device, which can now transmit three more octets. If for > some reason the destination does not receive the three octets (for > example, due to overflowing buffers), it does not send an > acknowledgment. Because the source does not receive an > acknowledgment, it knows that the octets should be retransmitted, and > that the transmission rate should be slowed. > > > TCP provides sequencing of segments with a forward reference > acknowledgment. Each datagram is numbered before transmission. At the > receiving station, TCP reassembles the segments into a complete > message. If a sequence number is missing in the series, that segment > is re-transmitted. Segments that are not acknowledged within a given > time period result in re-transmission. -- "Let's be realistic and try the impossible." - Che Guevara
