Detailed internal procedures for browser access to Web pages

Source: Internet
Author: User

Let's look at what happens when we enter in the browser, behind the scenes.

First HTTP is an application layer of protocol, in this layer of protocol, is just a communication specification, that is, because the two sides to communicate, we have to agree to a specification beforehand. 1. Connection when we enter such a request, the first to establish a socket connection, because the socket is established through the IP and port, so there is a DNS resolution process, the the IP, if the URL does not contain the port number, The default port number for the protocol is used. The process of DNS is this: first we know that our local machine in the configuration of the network will be filled with DNS, so that the machine will send this URL to the configured DNS server, if you can find the appropriate URL to return its IP, otherwise the DNS will continue to send the resolution request to the superior DNS, The entire DNS can be considered a tree structure, and the request will be sent to the root until the result is obtained. Now you have the target IP and port number so that we can open the socket connection. 2. Once the request connection is successfully established, the request is sent to the Web server, which is typically a GET or post command (post is used for the pass of the form parameter). The Get command is in the form of a Get path/file name http/1.0 The file name that is accessed, and http/1.0 indicates the HTTP version used by the Web browser. You can now send the GET command: get/mytest/index.html http/1.0,3. The answering Web server receives this request for processing. Search for subdirectories mytest file index.html from its document space. If the file is found, the Web server transmits the file content to the appropriate Web browser. In order to inform the browser, the Web server first transmits some HTTP header information and then transmits the specific content (that is, the HTTP body information), the HTTP header information and the HTTP body information separated by a blank line. Common HTTP header information is: ①http 1.0 OK this is the first line of the Web server answer that lists the HTTP version number and the answer code that the server is running.  The code "OK" indicates that the request is complete.  ②mime_version:1.0 it indicates the version of the MIME type. ③content_type: Type This header information is very important, it indicates the MIME type of the HTTP body information.  Such as: Content_type:text/html indicates that the transmitted data is an HTML document. ④content_length: Length value It indicates the length of the HTTP body information (in bytes).  4. Closing the connection: When the answer is complete, the Web browser and Web server must be disconnected to ensure that the other Web browser is able to work with the WThe EB server establishes the connection.   Below we specifically analyze the data packets in the network roaming experience in the network hierarchy, the layers are strictly one-way dependent. A service is an abstract concept that describes the relationships between layers, a set of operations that are provided to the upper layer in the network. The lower layer is the service provider, and the upper layer is the user requesting the service. The representation of a service is a primitive (primitive), such as a system call or library function. A system call is a service primitive provided by the operating system kernel to a network application or a high-level protocol. The n layer in the network always provides a more complete service to the N+1 layer than the n-1 layer, otherwise there is no value for n layer. The transport layer realizes "end-to-end" communication, introduces the concept of inter-network process communication, and also solves the problems of error control, flow control, data sorting (message sequencing), connection management, etc., and provides different service modes for this purpose. Typically the transport layer's service is provided by means of a system call, in the form of a socket. For the client, to establish a socket connection, you need to call some of the functions socket () bind () connect () and then send the data via Send (). Now look at the process of packet traffic in the network: Application layer First we can see in the application layer, according to the current requirements and actions, combined with the application layer of protocol, we determine the content of the data sent, we put the data into 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, in this machine, to find the local routing table, run route print on Windows to see the current routing table content, with the followingEntry: Active Routes Default Route persistent route. The entire lookup process is this: (1) According to the destination address, get the destination network number, if in the same intranet, you can send 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 it is common to return a "host unreachable" or "Network unreachable" error to the application that generated the datagram.   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       &NBSp         &NBSP ;       10127.0.0.0                         &NBSP ;                  &NBS P                   1192.168.1.0       & nbsp         &NB Sp           10192.168.1.101             and nbsp               127.0 .0.1                   10192.168.1.255           & nbsp         10                                      10255.255.255.255                1Default Gateway:       Destination Purpose Network segment   Netmask Subnet Mask  gateway The IP of the next-hop router entry, where the router defines a link to the next router via interface and gateway, typically interface and gateway are the same 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 subnet, A/b to communicate, first A to send the data to its local gateway, and then the gateway sends the data to B's gateway, and then the gatewaySent 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 represents the default route. The route record means: When I receive a packet of the destination network segment is not in my routing record, I will send the packet through the this interface to this address, which is an interface to the next router, so that the packet can be delivered to the next router processing, and I have nothing to do. The route record has a line quality of 10. When multiple entries match, the one with the smaller metric value is selected. Section III   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, I will send the packet through this interface directly, because this port directly connected to the 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 find aRP Buffering, run arp-a under Windows can 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. That is, the routing algorithm is actually used only to update the maintenance routing table 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 mainProtocol:    Internet Protocol IP: Responsible for addressing and routing packets between the host and the 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 and not for the requirements of the agreement itself. 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

Detailed internal procedures for browser access to Web pages

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