CODEWITHSUNDEEP
httppacket
SunyGirl
SunyGirl

Reputation: 419

counting HTTP packets

What is relation between number of HTTP packets and number of objects in a web page?

Upvotes: 1

Views: 3398

Answers (2)

Stephen C
Stephen C

Reputation: 718698

What is relation between number of HTTP packets and number of objects in a web page?

The short answer is there is obviously some relation, but there is no way you can accurately predict one from the other.

For a longer answer, we first need to correct some misconceptions in the question:

  • There is no such thing as an "HTTP packet". HTTP is a message oriented application protocol with one request message and one response message per "resource" fetched). This sits on top of a reliable byte stream protocol (with flow control, etc) called TCP. This in turn sits on top of a packet switching protocol called IP. An HTTP request/response exchange takes an unpredictable number of IP packets ... depending on message sizes AND network conditions. Other HTTP features such as compression, keeping connections alive, caching and so on make things even more complicated.

  • The idea of an "object" is ill-defined. An "object" could have a one-to-one correspondence between HTTP request / response pairs (i.e. a "resource" in the above) then that part is simple. OTOH, a "resource" could be a rendering of multiple "objects" in the application domain of the webserver.

  • On top of that, you've also got to account for the fact that a typical HTML resource has references to other resources (Scripts, CSS, images, etc) and may even involve Ajax callbacks. Each of these is a "resource", that may or may not need to be fetched ... depending on caching, etc.

  • Finally, there is an implicit assumption that all "objects" are the same size. This might be true in some application domains, but it is not true in general.

So to summarize, there are far to many variables and unknowns for it to be feasible to predict the number of network packets required to fetch a certain number of "objects".

A more practical approach is to attach a packet-level network analyser to your network and get it to count the number of packets sent and received.

If you make the following assumptions:

  • "HTTP packets" are HTTP messages,
  • "objects" are resources,
  • a resource doesn't require other resources (Scripts, CSS, images, etc) to render,
  • there is no caching,
  • the server is not doing redirects.

then one "object" requires two "HTTP packets".

But frankly, you've simplified the problem to a point where the answer is next to useless for predicting actual performance of real web-servers. (For instance, any one of those "objects" could be tiny ... or huge. And if you allow for arbitrary javascript, or content such as links to video streams, then the number of "packets" of one kind or another is potentially unbounded.)

Upvotes: 5

PEdroArthur
PEdroArthur

Reputation: 884

A GET request is issued for every file referred in a HTML page, all of which, usually, fit in one TCP stream segment. HTTP is a state machine, so, many requests/response can be pipelined in one request/response.

The number of packets sent in response vary in the size of the objects and in caching parameters. For example, if a file is already in the browser cache, it will make a conditional get and will receive a HTTP/1.1 304 Not Modified response code, which does not contain any data. Moreover, many HTTP/1.1 304 can be issued in one segment, as this response is very tiny compared to segments' maximum size. Another example, if a file is bigger than the maximum segment size, the file may (and it probably will) be divided in many segments.

Is this what you wish to know?

Upvotes: 2

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