49 lines
1.9 KiB
Plaintext
49 lines
1.9 KiB
Plaintext
// SPDX-License-Identifier: GPL-2.0
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//
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// RFC 5961 Section 5.2 / RFC 793 Section 3.9: an incoming segment's
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// ACK value must lie in [SND.UNA - MAX.SND.WND, SND.NXT]; otherwise
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// the receiver MUST discard the segment and send a challenge ACK
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// back. Exercise both edges of that window in a single connection.
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`./defaults.sh
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sysctl -q net.ipv4.tcp_invalid_ratelimit=0
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`
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0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
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+0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
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+0 bind(3, ..., ...) = 0
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+0 listen(3, 1) = 0
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// Three-way handshake. Peer advertises rwnd = 1000 (no wscale), so
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// MAX.SND.WND is tracked as 1000.
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+0 < S 0:0(0) win 1000 <mss 1000,sackOK,nop,nop,nop,wscale 0>
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+0 > S. 0:0(0) ack 1 <...>
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+.1 < . 1:1(0) ack 1 win 1000
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+0 accept(3, ..., ...) = 4
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// ---- Upper edge: SEG.ACK > SND.NXT --------------------------------
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// Server has sent nothing yet, so SND.UNA = SND.NXT = 1.
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// Peer sends a pure ACK with SEG.ACK = 2, beyond SND.NXT.
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+0 < . 1:1(0) ack 2 win 1000
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// Expect a challenge ACK: <SEQ = SND.NXT = 1, ACK = RCV.NXT = 1>.
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+0 > . 1:1(0) ack 1
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// Advance SND.UNA past MAX.SND.WND so that the lower edge becomes
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// reachable. Issue two 1-MSS writes so each skb is exactly one MSS
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// and PSH is set by tcp_push() at the end of each sendmsg, keeping
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// the setup independent of the TSO / tcp_fragment split path.
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+0 write(4, ..., 1000) = 1000
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+0 > P. 1:1001(1000) ack 1
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+.01 < . 1:1(0) ack 1001 win 1000
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+0 write(4, ..., 1000) = 1000
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+0 > P. 1001:2001(1000) ack 1
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+.01 < . 1:1(0) ack 2001 win 1000
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// Now SND.UNA = SND.NXT = 2001, MAX.SND.WND = 1000, bytes_acked = 2000.
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// ---- Lower edge: SEG.ACK < SND.UNA - MAX.SND.WND ------------------
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// SND.UNA - MAX.SND.WND = 2001 - 1000 = 1001, so SEG.ACK = 1000 falls
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// below the acceptable range.
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+0 < . 1:1(0) ack 1000 win 1000
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// Expect a challenge ACK: <SEQ = SND.NXT = 2001, ACK = RCV.NXT = 1>.
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+0 > . 2001:2001(0) ack 1
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