If sending data on a one Giga-byte per second Gbps network, the sequence numbers could wrap in as little as 34 seconds. If a packet is delayed, a different packet could potentially exist with the same sequence number.
To avoid confusion of duplicate sequence numbers, the TCP timestamp is used as an extension to the sequence number. Packets have current and progressing time stamps. An old packet has an older time stamp and is discarded. Before SACK, a receiver could only acknowledge the latest sequence number of a contiguous data stream that had been received, or the "left edge" of the receive window. With SACK enabled, the receiver continues to use the ACK number to acknowledge the left edge of the receive window, but it can also acknowledge other blocks of received data individually.
The data blocks are identified using the sequence number at the start and at the end of that block of data. It's also known as the left and right edge of the block of data. Length is the length in bytes of this TCP option. With SACK enabled default , a packet or series of packets can be dropped.
The receiver informs the sender which data has been received, and where there may be "holes" in the data. The sender can then selectively retransmit the missing data without a retransmission of blocks of data that have already been received successfully.
The SackOpts value in the following registry key can be edited to control the use of selective acknowledgments:. The valid binary value is 0 or 1, the default value is 1. The following Network Monitor trace illustrates a host acknowledging all data up to sequence number , plus the data from sequence number The missing data is from to As a review of normal retransmission behavior, TCP starts a retransmission timer when each outbound segment is handed down to the Internet Protocol IP.
If no acknowledgment has been received for the data in a given segment before the timer expires, then the segment is retransmitted. The timer for a given segment is doubled after each retransmission of that segment. Using this algorithm, TCP tunes itself to the normal delay of a connection. TCP retransmits data before the retransmission timer expires under some circumstances. The most common cause is a feature known as fast retransmit.
When a receiver that supports fast retransmit receives data with a sequence number beyond the current expected one, some data was likely dropped. To help inform the sender of this event, the receiver immediately sends an ACK, with the ACK number set to the sequence number that it was expecting.
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Manage all your internet downloads with this easy-to-use manager. It features a simple interface with many customizable options:. Download multiple files at one time Download large files quickly and reliably Suspend active downloads and resume downloads that have failed. Yes, install Microsoft Download Manager recommended No, thanks. Windows Server updates are distributed in service packs. Service packs help keep Windows Server current.
Additionally, service packs extend and update the functionality of your computer. Service packs include updates, system administration tools, drivers, and additional components. These components are conveniently bundled for easy downloading. The global version of this hotfix installs files that have the attributes that are listed in the following tables.
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