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[[File:Tftp-ack1.svg|thumb|(R3) Host A acknowledges data packet 1]]
A TFTP transfer is initiated by the client issuing a request to read or write a particular file on the server. The request can optionally include a set of negotiated transfer parameters proposed by the client under the terms specified by RFC 2347. If the server grants the request, the file is sent in fixed-length blocks of 512 bytes by default or the number specified in the blocksize negotiated option defined by RFC 2348. Each block of transferred data, which is usually carried within a single IP packet in order to avoid [[IP fragmentation]], must be acknowledged before the next block can be sent. A data packet of less than 512 bytes or the agreed blocksize option signals the termination of a transfer. If a packet gets lost in the network, the intended recipient will timeout and may retransmit their last packet (which may be data or an acknowledgment), thus causing the sender of the lost packet to retransmit that lost packet. The sender has to keep just one packet on hand for retransmission since the lock step acknowledgment guarantees that all older packets have been correctly received. Notice that both devices involved in a transfer are considered senders and receivers. One sends data and receives acknowledgments, the other sends acknowledgments and receives data.
TFTP defines three modes of transfer: netascii, octet, and mail.
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}}</ref> after 65535) which gives an essentially unlimited transfer file size.
Since TFTP utilizes UDP, it has to supply its own transport and session support. Each file transferred via TFTP constitutes an independent exchange. Classically, this transfer is performed in lock-step, with only one [[Packet (information technology)|packet]] (either a block of data or an acknowledgment) alternatively in flight on the network at any time. Due to this single data block strategy instead of sending a larger amount of uninterrupted data blocks before pausing the transfer to wait for the corresponding acknowledge (windowing), TFTP provides low [[throughput]] especially over high [[Latency (engineering)|latency]] links. Microsoft introduced windowed TFTP in Windows 2008 as part of their [[Windows Deployment Services]] (WDS), in January 2015 TFTP Windowsize Option RFC 7440 was published. This substantially improves performance for things like [[Preboot Execution Environment|PXE]] booting without the IP fragmentation side effect sometimes observed on Blocksize Option RFC 2348<ref>RFC 7440, page 1.</ref>
==Security considerations==
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