DHCP
- 简单的来说，DHCP是一个局域网协议，使用UDP协议进行工作，它的作用就是动态的分配IP地址，Gateway地址，DNS服务器地址等信息，一旦租约到期，那么路由器就会把当前的这个ip分配给其他设备使用，所以，对于设备而言要定期请求DHCP Server来更新ip地址信息，保证ip地址有效可用
DHCP租约到期，设备不会自动进行续租而仍然使用旧的ip地址
- https://www.net.princeton.edu/android/android-stops-renewing-lease-keeps-using-IP-address-11236.html
DHCP租约到期，设备向DHCP Server发起请求，但是没有收到Ack
- 遇到如上情况会导致ip无法更新，所以设备仍然会使用旧的ip，对于tcp连接来说，使用旧的过期ip就意味着连接不到远程服务器，从我们的日志信息中可以得知，当TCP使用过期的ip去连接远程服务器的时候会报如下异常：java.net.NoRouteToHostException: No route to host，意思是说没有可达Host的路由，确实是这样的，设备连接无线网是连接到路由器上的，而路由器上分配给设备的ip已经过期不可用，那么设备到路由器的链路是通的，但是路由器到远程主机的链路肯定是不通的，所以会报如上错误
- 正常情况下，我们的Android设备在处于熄屏的状态下，DHCP的租约是1个小时，所以会每隔半小时就请求DHCP Server来更新ip地址：
Line 30221: 09-11 12:58:51.016 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 30227: 09-11 12:58:51.246 1835 3791 D DhcpClient: Received packet: 00:0a:f5:43:d8:7c ACK: your new IP /172.28.88.48, netmask /255.255.248.0, gateways [/172.28.88.254] DNS servers: /114.114.114.114 /172.28.1.28 /8.8.8.8 /172.28.1.26 , lease time 3600
Line 30228: 09-11 12:58:51.247 1835 3789 D DhcpClient: Renewed lease: IP address 172.28.88.48/21 Gateway 172.28.88.254 DNS servers: [ 114.114.114.114 172.28.1.28 8.8.8.8 172.28.1.26 ] Domains dw.gdbbk.com DHCP server /1.1.1.1 Vendor info ������ lease 3600 seconds
Line 30228: 09-11 12:58:51.247 1835 3789 D DhcpClient: Renewed lease: IP address 172.28.88.48/21 Gateway 172.28.88.254 DNS servers: [ 114.114.114.114 172.28.1.28 8.8.8.8 172.28.1.26 ] Domains dw.gdbbk.com DHCP server /1.1.1.1 Vendor info ������ lease 3600 seconds
Line 30243: 09-11 12:58:51.252 1835 3789 D DhcpClient: Scheduling renewal in 1799s
Line 30244: 09-11 12:58:51.253 1835 3789 D DhcpClient: Scheduling rebind in 3149s
Line 30245: 09-11 12:58:51.253 1835 3789 D DhcpClient: Scheduling expiry in 3599s
Line 30718: 09-11 13:28:51.280 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 30718: 09-11 13:28:51.280 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 30722: 09-11 13:28:51.306 1835 3791 D DhcpClient: Received packet: 00:0a:f5:43:d8:7c ACK: your new IP /172.28.88.48, netmask /255.255.248.0, gateways [/172.28.88.254] DNS servers: /114.114.114.114 /172.28.1.28 /8.8.8.8 /172.28.1.26 , lease time 3600
Line 30723: 09-11 13:28:51.307 1835 3789 D DhcpClient: Renewed lease: IP address 172.28.88.48/21 Gateway 172.28.88.254 DNS servers: [ 114.114.114.114 172.28.1.28 8.8.8.8 172.28.1.26 ] Domains dw.gdbbk.com DHCP server /1.1.1.1 Vendor info ������ lease 3600 seconds
Line 30723: 09-11 13:28:51.307 1835 3789 D DhcpClient: Renewed lease: IP address 172.28.88.48/21 Gateway 172.28.88.254 DNS servers: [ 114.114.114.114 172.28.1.28 8.8.8.8 172.28.1.26 ] Domains dw.gdbbk.com DHCP server /1.1.1.1 Vendor info ������ lease 3600 seconds
Line 30747: 09-11 13:28:51.316 1835 3789 D DhcpClient: Scheduling renewal in 1799s
Line 30748: 09-11 13:28:51.316 1835 3789 D DhcpClient: Scheduling rebind in 3149s
Line 30749: 09-11 13:28:51.316 1835 3789 D DhcpClient: Scheduling expiry in 3599s
Line 31114: 09-11 13:58:53.153 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 31114: 09-11 13:58:53.153 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 31120: 09-11 13:58:53.220 1835 3791 D DhcpClient: Received packet: 00:0a:f5:43:d8:7c ACK: your new IP /172.28.88.48, netmask /255.255.248.0, gateways [/172.28.88.254] DNS servers: /114.114.114.114 /172.28.1.28 /8.8.8.8 /172.28.1.26 , lease time 3600
Line 31121: 09-11 13:58:53.221 1835 3789 D DhcpClient: Renewed lease: IP address 172.28.88.48/21 Gateway 172.28.88.254 DNS servers: [ 114.114.114.114 172.28.1.28 8.8.8.8 172.28.1.26 ] Domains dw.gdbbk.com DHCP server /1.1.1.1 Vendor info ������ lease 3600 seconds
Line 31121: 09-11 13:58:53.221 1835 3789 D DhcpClient: Renewed lease: IP address 172.28.88.48/21 Gateway 172.28.88.254 DNS servers: [ 114.114.114.114 172.28.1.28 8.8.8.8 172.28.1.26 ] Domains dw.gdbbk.com DHCP server /1.1.1.1 Vendor info ������ lease 3600 seconds
Line 31133: 09-11 13:58:53.226 1835 3789 D DhcpClient: Scheduling renewal in 1799s
Line 31134: 09-11 13:58:53.226 1835 3789 D DhcpClient: Scheduling rebind in 3149s
Line 31135: 09-11 13:58:53.226 1835 3789 D DhcpClient: Scheduling expiry in 3599s
以上是发起请求后有正常收到Ack的情况，但是接下来设备发起了请求并没有收到Ack，所以导致设备一直不断的去发起DHCP请求，而且此时设备并没有网络切换广播，并且设备本地判断网络是正常可用的，原因是设备到路由器的网络是通的，只是路由器到远程服务器的链路是不通的：
33540: 09-11 14:28:54.007 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 33540: 09-11 14:28:54.007 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 33557: 09-11 14:28:59.012 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 33557: 09-11 14:28:59.012 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 33585: 09-11 14:29:04.021 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 33585: 09-11 14:29:04.021 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 33701: 09-11 14:29:12.088 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 33701: 09-11 14:29:12.088 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 33724: 09-11 14:29:28.065 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 33724: 09-11 14:29:28.065 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 33779: 09-11 14:30:03.317 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 33779: 09-11 14:30:03.317 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 33815: 09-11 14:31:13.838 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 33815: 09-11 14:31:13.838 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 51658: 09-11 14:33:33.786 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 51658: 09-11 14:33:33.786 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 53369: 09-11 14:35:32.229 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 53369: 09-11 14:35:32.229 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 53714: 09-11 14:37:46.146 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 53714: 09-11 14:37:46.146 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 55611: 09-11 14:40:02.569 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 55611: 09-11 14:40:02.569 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 57692: 09-11 14:42:01.271 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
Line 57692: 09-11 14:42:01.271 1835 3789 D DhcpClient: Unicasting DHCPREQUEST ciaddr=172.28.88.48 request=0.0.0.0 serverid=null to /1.1.1.1:67
在这段时间内，TCP连接是一直报java.net.NoRouteToHostException: No route to host异常，如果TCP当前正处于连接中，那么DHCP的更新可能会导致TCP断线，等到后面设备发起的DHCP请求收到Ack之后，TCP连接立刻恢复正常，并且此时收到了网络切换广播，针对以上问题，如何恢复呢？搜索网上资料是说应用程序每次重新建立一条新的TCP的时候会触发设备请求DHCP Server，但是从我们的log查看发现并没有，请求DHCP Server没有Ack后设备会间隔重试，当设备被触发亮屏后，网络马上恢复正常，DHCP请求也收到Ack，同时也收到网络切换广播
