背景不多说，来看看三次握手的tcpdump抓包：
11:56:59.825162 IP 127.0.0.1.51085 > 127.0.0.1.8080: Flags [S], seq 1973376034, win 65535, options [mss 16344,nop,wscale 5,nop,nop,TS val 783922422 ecr 0,sackOK,eol], length 0 0x0000: 0200 0000 4500 0040 0000 4000 4006 0000 ....E..@..@.@... 0x0010: 7f00 0001 7f00 0001 c78d 1f90 759f 5422 ............u.T" 0x0020: 0000 0000 b002 ffff fe34 0000 0204 3fd8 .........4....?. 0x0030: 0103 0305 0101 080a 2eb9 b4f6 0000 0000 ................ 0x0040: 0402 0000 .... 11:56:59.825172 IP 127.0.0.1.51085 > 127.0.0.1.8080: Flags [S], seq 1973376034, win 65535, options [mss 16344,nop,wscale 5,nop,nop,TS val 783922422 ecr 0,sackOK,eol], length 0 0x0000: 0200 0000 4500 0040 0000 4000 4006 0000 ....E..@..@.@... 0x0010: 7f00 0001 7f00 0001 c78d 1f90 759f 5422 ............u.T" 0x0020: 0000 0000 b002 ffff fe34 0000 0204 3fd8 .........4....?. 0x0030: 0103 0305 0101 080a 2eb9 b4f6 0000 0000 ................ 0x0040: 0402 0000 .... 11:56:59.825221 IP 127.0.0.1.8080 > 127.0.0.1.51085: Flags [S.], seq 3195185234, ack 1973376035, win 65535, options [mss 16344,nop,wscale 5,nop,nop,TS val 783922422 ecr 783922422,sackOK,eol], length 0 0x0000: 0200 0000 4500 0040 0000 4000 4006 0000 ....E..@..@.@... 0x0010: 7f00 0001 7f00 0001 1f90 c78d be72 a852 .............r.R 0x0020: 759f 5423 b012 ffff fe34 0000 0204 3fd8 u.T#.....4....?. 0x0030: 0103 0305 0101 080a 2eb9 b4f6 2eb9 b4f6 ................ 0x0040: 0402 0000 .... 11:56:59.825224 IP 127.0.0.1.8080 > 127.0.0.1.51085: Flags [S.], seq 3195185234, ack 1973376035, win 65535, options [mss 16344,nop,wscale 5,nop,nop,TS val 783922422 ecr 783922422,sackOK,eol], length 0 0x0000: 0200 0000 4500 0040 0000 4000 4006 0000 ....E..@..@.@... 0x0010: 7f00 0001 7f00 0001 1f90 c78d be72 a852 .............r.R 0x0020: 759f 5423 b012 ffff fe34 0000 0204 3fd8 u.T#.....4....?. 0x0030: 0103 0305 0101 080a 2eb9 b4f6 2eb9 b4f6 ................ 0x0040: 0402 0000 .... 11:56:59.825231 IP 127.0.0.1.51085 > 127.0.0.1.8080: Flags [.], ack 1, win 12759, options [nop,nop,TS val 783922422 ecr 783922422], length 0 0x0000: 0200 0000 4500 0034 0000 4000 4006 0000 ....E..4..@.@... 0x0010: 7f00 0001 7f00 0001 c78d 1f90 759f 5423 ............u.T# 0x0020: be72 a853 8010 31d7 fe28 0000 0101 080a .r.S..1..(...... 0x0030: 2eb9 b4f6 2eb9 b4f6 ........ 11:56:59.825232 IP 127.0.0.1.51085 > 127.0.0.1.8080: Flags [.], ack 1, win 12759, options [nop,nop,TS val 783922422 ecr 783922422], length 0 0x0000: 0200 0000 4500 0034 0000 4000 4006 0000 ....E..4..@.@... 0x0010: 7f00 0001 7f00 0001 c78d 1f90 759f 5423 ............u.T# 0x0020: be72 a853 8010 31d7 fe28 0000 0101 080a .r.S..1..(...... 0x0030: 2eb9 b4f6 2eb9 b4f6 ........怎么是“六次握手”呢？
原因： client和server位于同一机器。
不多说。
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