Question

HTP provides two connections methods: persistent and non-persistent. While there are clear advantages of persistent-HTTP, it is rather unclear whether non-persistent can be of any use. Conduct a small research to find out why these two modes of connections are provided. In specific, you should find out some of the advantages and disadvantages of each of them, hence justifying their concurrent existence.

Answer 1

HTTP Overview:

HTTP stands for Hyper Text Transfer Protocol, the protocol used to deliver all resources on the World Wide Web. HTTP defines how the messages are formatted and transmitted.

When a client requests some information (say, clicks on a hyperlink), the browser sends a request message to the HTTP server for the requested objects. The server receives the requests and sends the response message with the objects. However, the HTTP server maintains no information about the clients, and if the client asks for the same object again, the server resends the object. Therefore, HTTP is called a stateless protocol.

HTTP can use both non persistent connections and persistent connections. A non persistent connection is the one that is closed after the server sends the requested object to the client. In other words, the connection is used exactly for one request and one response.

With persistent connections, the server leaves the TCP connection open after sending responses and hence the subsequent requests and responses between the same client and server can be sent. The server closes the connection only when it is not used for a certain configurable amount of time. With persistent connections, the performance is improved by 20%.

Non persistent connections are the default mode for HTTP/1.0 and persistent connections are the default mode for HTTP/1.1.

HTTP Non-Persistent & Persistent Connection:

The Hypertext Transfer Protocol (HTTP) is an application-level protocol that uses TCP as an underlying transport and typically runs on port 80. HTTP is a stateless protocol i.e. server maintains no information about past client requests.

HTTP Connections

1. Non-Persistent
2. Persistent

Before starting with persistent and non-persistent HTTP connection lets know what is a RTT.

RTT-> Time for a small packet to travel from client to server and back.

RTT= 2*propagation time

1. For an connection Persistent or Non-persistent it is sure that to initiate TCP connection one RTT is used.
2. One RTT is used for HTTP request and first few bytes to HTTP response to return.

So in order to know total file transmission time->

 total = 2RTT+transmit time

Difference between Persistent & Non-Persistent connection.

Persistent HTTP Nonpersistent HTTP issues: Persistent HTTP requires 2 RTTs per object server leaves connection open after sending response subsequent HTTP messages OS overhead for each TCP connection O browsers often open parallel between same client/server sent over open connection client sends requests as soon as it encounters a referenced object as little as one RTT for all the referenced objects TCP connections to fetch referenced objects

Non-Persistent Connection

1. Without parallel connection
2. With parallel connection

Without parallel connection Non-Persistent
Each objection takes two RTT (assuming no window limit) one for TCP connection and other for HTTP image/text file.

With parallel connection Non-Persistent

Non-persistent & Parallel connections initiate TCP connection RTT request Base file or reference page RTT File/base page received RTT Request objects RTT Object revd time time O Non-persistent

Persistent connection

1. Non-Pipelined
2. Pipelined

In Non-pipeline connection we first establish connection which takes two RTT then we send all the objects images/text files which takes 1 RTT each (TCP for each object is not required).

In Pipelined connection 2RTT for connection establishment and then 1RTT(assuming no window limit) for all the objects i.e. images/text.

Advantages of persistent connections :
1) Lower CPU and memory usage because there are less number of connections.
2) Allows HTTP pipelining of requests and responses.
3) Reduced network congestion (fewer TCP connections).
4) Reduced latency in subsequent requests (no handshaking).
5) Errors can be reported without the penalty of closing the TCP connection.

Disadvantages of persistent connections :
Resources may be be kept occupied even when not needed and may not be available to others.

Most of the modern browsers like Chrome, Firefox and Internet Explorer use persistent connections.