We discuss the video recommendation system in use at YouTube, the world's most popular online video community. The system recommends personalized sets of videos to users based on their activity on the site. We discuss some of the unique challenges that the system faces and how we address them. In addition, we provide details on the experimentation and evaluation framework used to test and tune new algorithms. We also present some of the findings from these experiments.

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This RFC suggests a proposed protocol for the ARPA-Internet community, and requests discussion and suggestions for improvements. This is a preliminary discussion of the NETBLT protocol. It is published for discussion and comment, and does not constitute a standard. As the proposal may change, implementation of this document is not advised. Distribution of this memo is unlimited. INTRODUCTIONNETBLT (Network Block Transfer) is a transport level protocol intended for the rapid transfer of a large quantity of data between computers. It provides a transfer that is reliable and flow controlled, and is structured to provide maximum throughput over a wide variety of networks.The protocol works by opening a connection between two clients the sender and the receiver), transferring the data in a series of large data aggregates called buffers, and then closing the connection. Because the amount of data to be transferred can be arbitrarily large, the client is not required to provide at once all the data to the protocol module. Instead, the data is provided by the client in buffers. The NETBLT layer transfers each buffer as a sequence of packets, but since each buffer is composed of a large number of packets, the per-buffer interaction between NETBLT and its client is far more efficient than a per-packet interaction would be.In its simplest form, a NETBLT transfer works as follows. The sending client loads a buffer of data and calls down to the NETBLT layer to transfer it. The NETBLT layer breaks the buffer up into packets and sends these packets across the network in Internet datagrams. The receiving NETBLT layer loads these packets into a matching buffer provided by the receiving client. When the last packet in the buffer has been transmitted, the receiving NETBLT checks to see that all packets in that buffer have arrived. If some packets are missing, the receiving NETBLT requests that they be resent. When the buffer has been completely transmitted, the receiving client is notified by its NETBLT layer. The receiving client disposes of the buffer and provides a new buffer to receive more data. The receiving NETBLT notifies the sender that the buffer arrived, and the sender prepares and sends the next buffer in the

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