To see in the application layer, according to the current requirements and actions, combined with the application layer of the Protocol, we have determined to send the data content, we put this data in a buffer, and then formed the application layer of the message data. Transport layer This data is sent through the transport layer, such as the TCP protocol. So they will be sent to the transport layer processing, where the message is punched in the header of the transmission head, mainly including the port number, as well as TCP's various system information, this information is directly obtained, because the interface needs to specify the port. This makes up the TCP data Transfer Unit segment. TCP is an end-to-end protocol, the use of such information, such as the TCP header ordinal confirmation sequence number, according to these numbers, the sending party constantly send a wait for confirmation, send a data segment, will open a counter, only after receiving confirmation will send the next, If the excess count time is still not received, the resend is sent, and if the receiving side receives the error data, it is discarded, which causes the send side to time out to resend. Through the TCP protocol, it controls the generation of the sending sequence of packets, continuously adjusts the sending sequence, realizes the flow control and data integrity. The network layer then sends the data segment to the network layer, in the network layer is packaged, so encapsulation on the network layer of the Baotou, the Baotou inside contains the source and destination IP address, the layer of data transmission unit is called packet. The network layer is responsible for transferring such packets over the network, passing through the router and finally reaching the destination address. Here, depending on the destination IP address, you need to find the address of the next hop route. First on this machine, to find the local routing table, run route print on Windows to see the current routing table contents, as follows: Active Routes Default Route persistent route. The entire lookup process is like this: (1) According to the destination address, get the destination network number, if it is in the same intranet, it can be sent directly. (2) If not, query the routing table to find a route. (3) If no clear route is found, there will also be a default gateway in the routing table, also known as the default gateway, IP with the default gateway address to the next designated router, the gateway may also be a router, or it may only be the network to a specific router to transfer data gateway. (4) After the router receives the data, it routes the remote host or network query again, and if no route is found, the packet is sent to the router's default gateway address. The packet contains a maximum number of hops, and if the hop is exceeded, the packet is discarded, which prevents infinite delivery. After the router receives the packet, it only looks at the packet data of the network layer, the destination IP. So it's working in the network layer, and the data for the transport layer is transparent to it. If none of the above steps are successful, then the datagram cannot be transmitted. If datagrams that cannot be delivered come from the native, then the application that generates the datagram is generallyThe program returns a "host unreachable" or "Network unreachable" error. Take the routing table of the host under Windows as an example to see the route lookup process ======================================================================active Routes:network Destination netmask &NBS P gateway interface & nbsp metric0.0.0.0 &NBS P 0.0.0.0 192.168.1 .2 192.168.1.101 10127.0.0.0 &N Bsp 255.0.0.0 127.0.0.1 127.0.0.1   1192.168.1.0 &NBS P 255.255.255.0 192.168.1.101 192.168.1.101 &NB Sp 10192.168.1.101 255.255.255.255 127.0.0.1 127.0.0.1 &NB Sp 10192.168.1.255 255.255.255.255 192.168.1.101 192.168.1.101 10 224.0.0. 0 240.0.0.0 192.168.1.101 192.168.1.101 &N Bsp 10255.255.255.255 255.255.255.255 192.168.1. 101 192.168.1.101 1Default Gateway: &N Bsp 192.168.1.2network Destination destination network segment netmask Subnet mask gateway Next-hop router ingress IP, Routers define a link to the next router via the interface and gateway, and typically, the interface and gateway are the same network segment. Interface to the destination of the router's export IP (for our personal pc, usually by machine a network card, with the IP address of the network card identification, of course, a PC can also have multiple network cards). The concept of gateway, mainly for the interaction between the different subnets, when the host, A, a and a two subnets in the network to communicate, first A to send data to its local gateway, and then the gateway then send the data to B's gateway, and then send the gateway to B. Default gateway, when the destination network segment of a packet is not in your routing record, then your router should send that packet to where! The gateway for the default route is determined by the default gateway on your connection, which is the value that we typically configure in the network connection. Usually interface and gateway are in a subnet, and for routers, because they may have different interface, when a packet arrives, it looks for a matching entry based on the network destination, if found, The interface indicates that the router should be out of that interface, and the gateway represents that subnet's address. First 0.0.0.0 0.0.0.0 192.168.1.2 192.168.1.101 100.0.0.0 represents the default route. The route record means: When I receive a packet of destination network segment is not in my routing record, I will pass the packet 192.168.1.101 thisThe interface is sent to the 192.168.1.2 address, which is an interface to the next router so that the packet can be delivered to the next router, regardless of me. The route record has a line quality of 10. When multiple entries match, the one with the smaller metric value is selected. Section III 192.168.1.0 255.255.255.0 192.168.1.101 192.168.1.101 10 routing records for Straight-line segments: What to do when the router receives packets destined for a straight-line segment, in which case the interface and gateway of the routing record are the same. When I receive the destination segment of a packet is 192.168.1.0, I will send the packet through 192.168.1.101 this interface directly, because this port directly connected to the 192.168.1.0 this segment, the route recorded by the quality of the line 10 (Because interface and gateway are the same, it means that the packet is routed directly to the destination address and does not need to be forwarded to the router). In general, the two cases, the destination address and the current router interface is on the same subnet. If it is sent directly, no need to transfer to the router, otherwise it will need to forward to the next router to continue processing. After finding the next hop IP address, you also need to know its MAC address, which is loaded into the link layer header as the link layer data. The ARP protocol is required, and the process is to look for ARP buffering, which runs arp-a under Windows to view the current ARP buffer content. If the MAC address of the corresponding IP is included, it will be returned directly. Otherwise, an ARP request is required, which contains the IP and MAC address of the source, the IP address of the destination, and the broadcast within the network, all the main opportunities to check whether their IP is the same as the destination IP in the request, and if it happens, return the MAC address and save the requestor's IP mac. This gets the MAC address of the destination IP. The link layer adds the MAC address and the link layer control information to the data packet, forms the Frame,frame under the link layer protocol, completes the adjacent node data transmission, completes the connection establishment, the control transmission speed, the data integrity. The physical layer physical line is responsible for the data being transferred from the host to the next destination in bit units. After the next destination receives the data, the data is obtained from the physical layer and then goes through the layer-wise unpacking to the link layer to the network layer, then begins the above processing, and then encapsulates the data in the physical layer of the network Layer link layer and continues to pass the following address. In the above procedure, you can see that there is a routing table query process, and the establishment of this routing table depends on the routing algorithm. This means that the routingmethod is actually used only to update maintenance routing tables between routers, the real data transfer process does not execute this algorithm, only view the routing table. This concept is also important and requires understanding of commonly used routing algorithms. And the whole TCP protocol is more complex, and the link layer protocol is somewhat similar, there are some important mechanisms or concepts need to be carefully understood, such as number and confirmation, flow control, resend mechanism, send acceptance window.  TCP/IP basic models and concepts physical layer devices, repeaters (repeater), hubs (hub). For this layer, data received from one port is forwarded to all ports. Link Layer protocol: SDLC (Synchronous Data Link Control) HDLC (High-level Data Link Control) PPP protocol independent link device The most common is the NIC, the bridge is also a link product. Some of the functions of the hub modem are considered to belong to the link layer, and there are some controversies that belong to the physical layer device. In addition, all switches need to work at the data link layer, but only the two-layer switch that works on the data link layer. Others like layer three switches, four layer switches, and seven layer switches can work on the three, four, and seven layers of the OSI, but the two-tier feature is still their basic function. Because of the MAC address table, the conflict is fully avoided because the switch knows which port to forward this data to by the destination MAC address. Instead of being forwarded to all drop ports, like the hub. Therefore, the switch is able to divide the conflict domain drops. Network layer four main protocols: Internet Protocol IP: Responsible for addressing and routing packets between host and network. Address Resolution Protocol ARP: Obtain the hardware host address in the same physical network. Internetwork Control Message Protocol ICMP: sends a message and reports a delivery error on the packet. Interconnect Group Management Protocol IGMP: is taken by the IP host to report host group members to the local multicast router. The layer device has a three layer switch, router. Transport Layer Two important protocols TCP and UDP. Port concept: TCP/UDP uses an IP address to identify the web host, using the port number to identify the application process, that is, TCP/UDP uses the host IP address and the port number assigned to the application process to identify the application process. The port number is a 16-bit unsigned integer, and the port number of TCP and the port number of UDP are two separate sequences. Despite the independence of each other, if TCP and UDP provide some well-known service at the same time, two protocols usually choose the same port number. This is purely for ease of use, not agreement.Their own requirements. With the port number, multiple processes on one host can use the transport service provided by TCP/UDP at the same time, and the traffic is end-to-end, and its data is passed by IP, regardless of the delivery path of the IP datagram. In network communication, a ternary group can be used globally to uniquely flag an application process: (protocol, local address, local port number). This means that TCP and UDP can use the same port. Can be seen through (protocol, source port, source IP, destination port, destination IP) can be used to fully identify a set of network connections. The application tier is based on the tcp:telnet FTP SMTP DNS http based on Udp:rip NTP (Subnet Time Protocol) and DNS (DNS also uses TCP) SNMP TFTP
Browser Access detailed procedures inside a Web page
