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Use total free memory as advertised window. This seems to be sufficient

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to avoid drops even on slow NICs.
This commit is contained in:
Michael Brown
2007-01-18 20:39:17 +00:00
parent 6d4e37cf42
commit c014f607a8
3 changed files with 31 additions and 29 deletions
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39
src/include/gpxe/tcp.h
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@@ -211,41 +211,30 @@ struct tcp_mss_option {
#define MIN_PKB_LEN MAX_HDR_LEN + 100 /* To account for padding by LL */
/**
* Advertised TCP window size
* Maxmimum 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. 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.
* We estimate the TCP window size as the amount of free memory we
* have. This is not strictly accurate (since it ignores any space
* already allocated as RX buffers), but it will do for now.
*
* Since we don't store out-of-order received packets, the
* retransmission penalty is that the whole window contents must be
* resent.
* resent. This suggests keeping the window size small, but bear in
* mind that the maximum bandwidth on any link is limited to
*
* 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.
* max_bandwidth = ( tcp_window / round_trip_time )
*
* With a 48kB window, which probably accurately reflects our amount
* of free memory, and a WAN RTT of say 200ms, this gives a maximum
* bandwidth of 240kB/s. This is sufficiently close to realistic that
* we will need to be careful that our advertised window doesn't end
* up limiting WAN download speeds.
*
* 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 ( 65536 - 4 )
#define TCP_MAX_WINDOW_SIZE ( 65536 - 4 )
/**
* Advertised TCP MSS