2015년 9월 23일 수요일

THE PING PROCESS

  1. The source host generates an ICMP protocol data unit.
  2. The ICMP PDU is encapsulated in an IP datagram, with the source and destination IP addresses in the IP header. At this point the datagram is most properly referred to as an ICMP ECHO datagram, but we will call it an IP datagram from here on since that's what it looks like to the networks it is sent over.
  3. The source host notes the local time on it's clock as it transmits the IP datagram towards the destination. Each host that receives the IP datagram checks the destination address to see if it matches their own address or is the all hosts address (all 1's in the host field of the IP address).
  4. If the destination IP address in the IP datagram does not match the local host's address, the IP datagram is forwarded to the network where the IP address resides.
  5. The destination host receives the IP datagram, finds a match between itself and the destination address in the IP datagram.
  6. The destination host notes the ICMP ECHO information in the IP datagram, performs any necessary work then destroys the original IP/ICMP ECHO datagram.
  7. The destination host creates an ICMP ECHO REPLY, encapsulates it in an IP datagram placing it's own IP address in the source IP address field, and the original sender's IP address in the destination field of the IP datagram.
  8. The new IP datagram is routed back to the originator of the PING. The host receives it, notes the time on the clock and finally prints PING output information, including the elapsed time

What are the series of steps that happen when an URL is requested from the address field of a browser?

This is a question whose answer could grow into an entire course on networking, so here's a version that only details some of the cases. There could probably be followup questions.


  1. The browser extracts the domain name from the URL.
  2. The browser queries DNS for the IP address of the URL. Generally, the browser will have cached domains previously visited, and the operating system will have cached queries from any number of applications. If neither the browser nor the OS have a cached copy of the IP address, then a request is sent off to the system's configured DNS server. The client machine knows the IP address for the DNS server, so no lookup is necessary.
  3. The request sent to the DNS server is almost always smaller than the maximum packet size, and is thus sent off as a single packet. In addition to the content of the request, the packet includes the IP address it is destined for in its header. Except in the simplest of cases (network hubs), as the packet reaches each piece of network equipment between the client and server, that equipment uses a routing table to figure out what node it is connected to that is most likely to be part of the fastest route to the destination. The process of determining which path is the best choice differs between equipment and can be very complicated.
  4. The is either lost (in which case the request fails or is reiterated), or makes it to its destination, the DNS server.
  5. If that DNS server has the address for that domain, it will return it. Otherwise, it will forward the query along to DNS server it is configured to defer to. This happens recursively until the request is fulfilled or it reaches an authoritative name server and can go no further. (If the authoritative name server doesn't recognize the domain, the response indicates failure and the browser generally gives an error like "Can't find the server atwww.lkliejafadh.com".) The response makes its way back to the client machine much like the request traveled to the DNS server.
  6. Assuming the DNS request is successful, the client machine now has an IP address that uniquely identifies a machine on the Internet. The web browser then assembles an HTTP request, which consists of a header and optional content. The header includes things like the specific path being requested from the web server, the HTTP version, any relevant browser cookies, etc. In the case in question (hitting Enter in the address bar), the content will be empty. In other cases, it may include form data like a username and password (or the content of an image file being uploaded,  
  7. This HTTP request is sent off to the web server host as some number of packets, each of which is routed in the same was as the earlier DNS query. (The packets have sequence numbers that allow them to be reassembled in order even if they take different paths.) Once the request arrives at the webserver, it generates a response (this may be a static page, served as-is, or a more dynamic response, generated in any number of ways.) The web server software sends the generated page back to the client.
  8. Assuming the response HTML and not an image or data file, then the browser parses the HTML to render the page. Part of this parsing and rendering process may be the discovery that the web page includes images or other embedded content that is not part of the HTML document. The browser will then send off further requests (either to the original web server or different ones, as appropriate) to fetch the embedded content, which will then be rendered into the document as well.
Copied from https://www.quora.com/What-are-the-series-of-steps-that-happen-when-an-URL-is-requested-from-the-address-field-of-a-browser