I came across the term HTTP. I have done some research and wanted to ensure that I correctly understood the term.
So, is it true that HTTP, in simple words, a letter containing information in the language that both client and server can understand.
Then, that letter is sent to the server thanks to TCP/IP which serves as a car that takes that letter to the server.
Then, after the letter is delivered to the server, the server reads the content of the letter and if it is GET request, the server takes the necessary data and ATTACHES that data to the letter and sends back to the client via again TCP/IP. But if it was POST request then the client ATTACHES the DATA to the letter and sends it to the server so that it saves that data in the database.
Is that true?
Basically, it is true.
However, the server can decide what to do if it is a GET or POST or any other request(it doesn't need to e.g. append it to a file).
I will show you some additional information/try to explain it in my words:
TCP is another communication protocol protocol. It allows a client to open a connection to a server and they can communicate afterwards.
HTTP(hyper text transfer protocol) builds up on TCP.
At first, the client opens a connection to the server.
After that, the client sends the HTTP Request. The first line contains the type of the request, the path and the version. For example, it could be GET / HTTP/1.1.
The next part of the request contains the Request parameters. Every parameter is a line. The parameters are sent like the following: paramName: paramValue
This part of the request ends with an empty line.
If it is a POST Request, query parameters are added next. If it is a GET Request, these query parameters are added with the path(e.g. /index.html?paramName=paramValue)
After rescieving the Request, the server sends a HTTP Response back to the client.
The first line of the response contains the HTTP version, the status code and the status message. For example, it could be HTTP/1.1 200 OK.
Then, just like in the request, the response parameters are following. For example Content-Length: 1024.
The response parameters also end with an empty line.
The last part of the response is the body/content. For example, this could be the HTML code of the website you are visiting.
Obviously, the length of the content/body of the response has to match the Content-Length parameter(in bytes).
After that, the connection will be closed(normally). If the client to e.g. request resources, it will send another request. The server has NO POSSIBILITY to send data to the client after that unless the client sends another request(websockets can bypass this issue).
GET is meant to get the content of a site A web browser will send a GET request if you type in a URL. POST can be used to update a site but in fact, the server can decide that. POST can be also used if the server doesn't want query parameters to be shown in the address bar.
There are other methods like PATCH or DELETE that are used by some APIs.
Some important status codes (and status messages) are:
200 OK (everything went well)
204 No content (like ok but there is no body in the response)
400 Bad Request (something is wrong with the Request)
404 Not found (the requested file(the path) was not found on the server)
500 Internal server error (An error occured while processing the request)
Every status code beginning with 1 is related to inform the client of something.
If it is starting with 2, everything went right.
Status code beginning with 3 forward the client to another site.
If it starts with 4, there is a error on the client side.
Codes starting with 5 represent an error that occured on the server side.
TCP is a network protocol that establishes a connection with the server over a network (or the Internet) and allows two-way communication. The HTTP will traffic inside this TCP tunnel. TCP is a very useful protocol that helps keep things sane, it ensures data packets are read in the correct order and that packets that went missing during transmission are sent again.
Sometimes there will be another protocol layer between HTTP and TCP, called SSL. It is responsible for encrypting the data that traffics over TCP, so that it is transmitted safely over unsafe networks. This is know as HTTPS, and is just HTTP but using this additional layer.
Although almost always true, HTTP doesn't necessarily uses TCP. UPnP requests use HTTP over UDP, a network protocol that uses standalone packets instead of a connection.
HTTP is a plain text protocol, meaning it's designed in such a way that a human can understand it without using any tools. This is very convenient for learning.
If you're using Firefox or Chrome, you can press Ctrl-Shift-C to open the Developer Tools, and under the Network tab you will see every HTTP request your browser is making, see exactly what's the request, what the server answered etc, and get a better view of how this protocol works.
Explaining it in details is... too extensive for this answer. But as you will see it's not that complicated.
Related
I have two machines, A and B.
A sends an HTTP request to B and asks for some document.
B responds back and sends the requested document and gives a 200 OK message, but machine A is complaining that the document is not received because of a network failure.
Does HTTP code 200 also work as acknowledgment that the document is received?
Does the HTTP 200 code also work as an acknowledgment that document has been received?
No. Not at all.
It is not even a guarantee that the document was completely transmitted.
The response code is in the first line of the response stream. The server could fail, or be disconnected from the client anywhere between sending the first line and the last byte of the response. The server may not even know this has happened.
In fact, there is no way that the server can know if the client received a complete (or partial) HTTP response. There is no provision for an acknowledgment in the HTTP protocol.
Now you could implement an application protocol over the top of HTTP in which the client is required to send a second HTTP request to the server to say "yes, I got the document". But this would involve some "application logic" implemented in the user's browser; e.g. in Javascript.
Absolutely not.
HTTP 200 is generated by the server, and only means that it understood the request and thinks it is able to fulfill it (e.g. the file is actually there).
All sorts of errors may occur during the transmission of the full response document (network connection breaking, packet loss, etc) which will not show up in the HTTP response, but need to be detected separately.
A pretty good guide to the HTTP protocol is found here: http://blog.catchpoint.com/2010/09/17/anatomyhttp/
You should make a distinction between the HTTP protocol and the underlying stream transport protocol, which should be reliable for HTTP purposes. The stream transport protocol will ACKnowledge all data transmission, including the response, so that both ends of exchange will affirm that the data is transmitted correctly. If the transport stream fails, then you will get a 'network failure' or similar error. When this happens, the HTTP protocol cannot continue; the data is no longer reliable or even complete.
What a 200 OK message means, at the HTTP level, is that the server has the document you're after and is about to transmit it to you. Normally you will get a content-length header as well, so you will be able to ascertain if/when the body is complete as an additional check on top of the stream protocol. From the HTTP protocol perspective, a response receives no acknowledgement, so once a response has been sent there is no verification.
However, as the stream transport is reliable, the act of sending the response will either be successful or result in an error. This does verify whether the document has been received by the network target (as noted by TripeHound, in the case of non-direct connection, e.g. a proxy, this is not a guarantee of delivery to the final target).
It's very simple to see that the 200 OK response code can't be a guarantee of anything about the response document. It's sent before the document is transmitted, so only a violation of causality could allow it to be dependent on successful reception of the document. It only serves as an indicator that the request was received properly and the server believes that it's able to fulfill the request. If the request requires extra processing (e.g. running a script), rather than just returning a static document, the response code should generally be sent after this has been completed, so it's normally an indicator that this was successful (but there are situations where this is not feasible, such as requests with persistent connections and push notifications -- the script could fail later).
On a more general level, it's never possible to provide an absolute guarantee that all messages have been received in any protocol, due to the Two Generals Problem. No acknowledgement system can get around this, because at some point there has to be a last acknowledgement; there's no way to know if this is received successfully, because that would require another acknowledgement, contradicting the premise that it was the last one.
HTTP is designed with an awareness of the possibility of various sorts of "middleboxes" - proxies operating with or without the knowledge of the client.
If there is a proxy involved, then even knowing that the server had transmitted all the data and recieved an normal close connection would not tell you anything about whether the document has been received by the machine who generated the HTTP request.
A sends a request to B. There may be all kinds of obstacles in the way that prevent the request from reaching B. In the case of https, the request may be reaching B but be rejected and it counts as if it hadn't reached B. In all these cases, B will not send any status at all.
Once the request reaches B, and there are no bugs crashing B, and no hardware failure etc. B will examine the request and determine what to do and what status to report. If A requested a file that is there and A is allowed access, B will start sending a "status 200" together with the file data.
Again all kinds of things can go wrong. A may receive nothing, or the "status 200" with no data or incomplete data etc. (By "receive" I mean that data arrives on the Ethernet cable, or through WiFi).
Usually the user of A will use some library that handles the ugly bits. With some decent library, the user can expect that they either get some error, or a status complete with the corresponding data. If a status 200 arrives at A with only half the data, the user will (depending on the design of the library) receive an error, not a status, and definitely not a status 200.
Or you may have a library that reports the status 200 and tells you "here's the first 2,000 bytes", "here's the next 2,000 bytes" and so on, and at some point when things go wrong, you might be told "sorry, there was an error, the data is incomplete".
But in general, the case that the user gets a status 200, and no data, will not happen.
Say i make the web request(www.amazon.com) to amazon web server through browser. Browser makes the connection with Internet through Internet service providers.
Request reaches to amazon server which process it and send back the response. Two questions here :-
Does Amazon server makes new connection with internet to send the response back or incoming request(initiated by me) waits on socket till amazon process the response ?
Once my browser receives the response how does it map the response(sent from amazon) back to particular request . I believe there must be some unique identifier like
requestId must be present in response through which browser must be mapping to request. Is that correct ?
Does Amazon server makes new connection with internet to send the response back or incoming request(initiated by me) waits on socket
till amazon process the response ?
It uses the same connection. Most of the time it's not even possible to connect back to a web browser due to firewall restrictions or Network Address Translation (NAT).
Once my browser receives the request how does it map the response(sent from amazon) back to particular request . I believe
there must be some unique identifier like requestId must be present in
response through which browser must be mapping to request. Is that
correct ?
It receives the response on the same socket. So the socket is the identifier. If HTTP2 multiplexing is used, then each multiplexed stream has a stream identifier, which is used to map the response back to the request.
The client opens a TCP-connection to the server, sends an HTTP-request and the server sends the response using the same connection. So, the browser knows from the connection that the response belongs to a specific request. This applies to basic HTTP 1.
This has to be distinguished from the programming model of an AJAX web application which is asynchronous and not synchronous. The application does not actively wait for a response. It is instead triggered later when the response arrives. The connection handling described above is what happens "under the hood".
Back to the connection handling: There are optimizations of HTTP that make things more complicated. HTTP 1.1 has a feature called "keep alive" and HTTP 2 goes further into this direction. The idea is to send more data over a single TCP-connection because establishing a TCP-connection is expensive (-> three way handshake, slow start). So, multiple requests and responses are sent over a single TCP-connection. Your question arises again in case of this optimization. If e. g. there is a sequence of requests A, B and a sequence of corresponding responses B, A within a single HTTP-connection how does the browser know the request a response belongs to? HTTP 2 introduces the concept of streams (RFC 7540, section 5):
A single HTTP/2 connection can contain multiple concurrently open
streams, with either endpoint interleaving frames from multiple
streams.
The order in which frames are sent on a stream is significant.
Streams are identified by an integer.
So, the stream identifier and the order within a stream can be used by the browser to find out the request a response belongs to.
HTTP 2 introduces another interesting feature which is called "push". The client can proactively send resources to the client that the client has not even requested. So, resources like e. g. images can be already sent when the HTML is requested avoiding another communication roundtrip.
HTTP uses Transfer Control Protocol. This is how it happens-
Does Amazon server makes new connection with internet to send the response back or incoming request(initiated by me) waits on socket till amazon process the response ?
No. Most browsers use HTTP 1.1 so the connection between client and server is established only once until closed (Persistent connection).
Once my browser receives the request how does it map the response(sent from amazon) back to particular request . I believe there must be some unique identifier like requestId must be present in response through which browser must be mapping to request. Is that correct ?
There is a protocol(HTTP) on how the messages are exchanged. HTTP dictates that responses must arrive in the order they were requested. So it goes like-
Request;Response;Request;Response;Request;Response;...
And there is also a specific format of HTTP request (from your browser- HTTP client) and HTTP response message (from amazon HTTP server). There are response status codes that let the browser know if their request has been succeeded, otherwise tell the errors.
A few sample codes-
Is a http end point suppose to respond to requests from a particular client in order that they are received?
What about if it doesn't make sense to in the case of requests handled by cluster behind a proxy or in requests handled with NIO where one request is finished faster than the other?
Is there a standard way of associating a unique id with each http request to associate with the response? How is this handled in clients like http componenets httpclient or curl?
The question comes down to the following case:
Suppose, I am downloading a file from a server and the request is not finished. Is a client capable of completing other requests on the same keep-alive connection?
Whenever a TCP connection is opened, the connection is recognized by the source and destination ports and IP addresses. So if I connect to www.google.com on destination port 80 (default for HTTP), I need a free source port which the OS will generate.
The reply of the web server is then sent to the source port (and IP). This is also how NAT works, remembering which source port belongs to which internal IP address (and vice versa for incoming connections).
As for your edit: no, a single http connection can execute one command (GET/POST/etc) at the same time. If you send another command while you are retreiving data from a previously issued command, the results may vary per client and server implementation. I guess that Apache, for example, will transmit the result of the second request after the data of the first request is sent.
I won't re-write CodeCaster's answer because it is very well worded.
In response to your edit - no. It is not. A single persistent HTTP connection can only be used for one request at once, or it would get very confusing. Because HTTP does not define any form of request/response tracking mechanism, it simply would not be possible.
It should be noted that there are other protocols which use a similar message format (conforming to RFC822), which do allow for this (using mechanisms such as SIP's cSeq header), and it would be possible to implement this in a custom HTTP app, but HTTP does not define any standard mechanism for doing this, and therefore nothing can be done that could be assumed to work everywhere. It would also present a problem with the response for the second message - do you wait for the first response to finish before sending the second response, or try and pause the first response while you send the second response? How will you communicate this in a way that guarantees messages won't become corrupted?
Note also that SIP (usually) operates over UDP, which does not guarantee packet ordering, making the cSeq system more of a necessity.
If you want to send a request to a server while another transaction is still in progress, you will need to create a new connection to the server, and hence a new TCP stream.
Facebook did some research into this while they were building their CDN, and they concluded that you can efficiently have 2 or 3 open HTTP streams at any one time, but any more than that reduces overall transfer time because of the extra packet overhead cost. I would link to the blog entry if I could find the link...
If I make multiple HTTP Get Requests to the same server and get HTTP 200 OK responses to each one how do I tell which request maps to which response using Wireshark?
Currently it looks like an http request is made, and the next HTTP 200 OK response is quickly received so everything is in a the proper sequence. I have seen things to the contrary however. For example using the Google Maps API v2 I've made several requests for location information and then the information is received in an arbitrary order (closely resembling the order in which I requested it, but not necessarily perfect.)
So my intuition is I cannot assume that my responses will be received in a specific order, even though they may be in order most of the time. So I'm wondering how I can determine this order from the response.
Update: Clarification as to what I need. I just need to know that the server has received the request. It seems like I need to do this by looking at sequence numbers and perhaps even ACKS. The reasoning behind this approach is I'm basically observing a web app and checking it is sending the information and the information is being received.
Update: This has nothing to do with wireshark specifically. I believe it is confusing people so I removing it from the title. It has to do with the HTTP protocol on top of the TCP/IP protocol and how we map responses to requests.
Thanks.
After you have stopped capturing packets follow this steps:
position the cursor on a GET request
Open the Analyze menu
click "Follow TCP Stream"
You get a new window with requests and responses in sequence.
While I was googling for a complete different question, I saw this one and I think I can provide a more complete answer :
HTTP dictates that responses must arrive in the order they were requested, Therefore, if you are looking at a single TCP connection at a given time you should be seeing :
Request ; Response ; Request ; Response ...
Also in HTTP/1.1, there is support for "Pipeline" where the client doesn't have to wait for responses to arrive in order to issue the next request. What could be observed in such cases is :
Request ; Response ; Request ; Request ; Response ; Response ; Request ; Response
In the HTTP response itself, there is no reference to the specific request that triggered it.
Filipo's suggestion is classic when debugging / observing a single TCP connection, but, when observing multiple TCP connections, you can't click the follow TCP Stream because you'd have to do it for each connection.
If you have many TCP connections, and many requests/responses you will have to look at TCP Source port in the request packet, and the TCP dest port in the response packet to know which response is related to each tcp connection, and then apply the HTTP request/response order rules.
Also, Wireshark CAN decompress the response body, and it will do it automatically if all the response body has arrived, but it will do so NOT in the Follow TCP Stream.
I always use Wireshark to debug HTTP.
Seems like this ability is not provided by the HTTP protocol at the application layer so I must go down to the transportation layer to determine this. In my case the TCP/IP layer using sequence numbers.
HTTP only presumes a reliable
transport; any protocol that provides
such guarantees can be used; the
mapping of the HTTP/1.1 request and
response structures onto the
transport data units of the protocol
in question is outside the scope of
this specification.
Read more:
http://www.faqs.org/rfcs/rfc2616.html#ixzz0e20kxKcz
Don't use Wireshark to debug HTTP, use an HTTP debugger such as Fiddler2
I need a way to detect a missing response to a long running HTTP POST request. This problem arises when the network infrastructure (firewalls, proxies, unplugged cables, etc.) drops the response packets. The server may detect this failure, but the client cannot send additional bytes after the POST to probe the state of the TCP connection. The failure may be limited to a single TCP connection. For example I may be able to subsequently open a new TCP connection to the server.
I'm looking for a solution that still uses HTTP POST and does not change the duration of the server side processing.
Some solutions that I can think of are:
Provide a side channel interface to retrieve request & response history. If the history lists the response as having been send (presumably resulting in a TCP error) but I have not yet received it within a reasonable time I can generate a local error.
Use an X header to request that the server deliver "spurious" 100 Continue provisional responses on a regular interval. If I fail to see an expected 100 Continue or a non-provisional response I can generate a local error.
Is there a state of the art solution for this problem?
It sounds to me like you are using Soap for something that would be much better done using a stateful connection, or a server side push technology.