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Increase window size to 64kB. Line rate downloads on a 100Mbps link,

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anyone?
This commit is contained in:
Michael Brown 2007-01-18 16:39:55 +00:00
parent bd95927386
commit 5b00fbade3
1 changed files with 30 additions and 9 deletions
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39
src/include/gpxe/tcp.h
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@ -213,18 +213,39 @@ struct tcp_mss_option {
/**
* Advertised TCP window size
*
*
* Our TCP window is actually limited by the amount of space available
* for RX packets in the NIC's RX ring; we tend to populate the rings
* with far fewer descriptors than a typical driver. Since we have no
* way of knowing how much of this RX ring space will be available for
* received TCP packets (consider, for example, that they may all be
* consumed by a series of unrelated ARP requests between other
* machines on the network), it is actually not even theoretically
* possible for us to specify an accurate window size. We therefore
* guess an arbitrary number that is empirically as large as possible
* while avoiding retransmissions due to dropped packets.
* with far fewer descriptors than a typical driver. This would
* result in a desperately small window size, which kills WAN download
* performance; the maximum bandwidth on any link is limited to
*
* max_bandwidth = ( tcp_window / round_trip_time )
*
* With a 4kB window, which probably accurately reflects our amount of
* buffer space, and a WAN RTT of say 200ms, this gives a maximum
* achievable bandwidth of 20kB/s, which is not acceptable.
*
* We therefore aim to process packets as fast as they arrive, and
* advertise an "infinite" window. If we don't process packets as
* fast as they arrive, then we will drop packets and have to incur
* the retransmission penalty.
*
* Since we don't store out-of-order received packets, the
* retransmission penalty is that the whole window contents must be
* resent.
*
* We choose to compromise on a window size of 64kB (which is the
* maximum that can be represented without using TCP options). This
* gives a maximum bandwidth of 320kB/s at 200ms RTT, which is
* probably faster than the actual link bandwidth. It also limits
* retransmissions to 64kB, which is reasonable.
*
* Finally, since the window goes into a 16-bit field and we cannot
* actually use 65536, we use a window size of (65536-4) to ensure
* that payloads remain dword-aligned.
*/
#define TCP_WINDOW_SIZE 4096
#define TCP_WINDOW_SIZE ( 65536 - 4 )
/**
* Advertised TCP MSS