Session is a semi-permanent interactive information interchange, also known as a dialogue, a protocol conversation or a meeting, or Communication between two or more communicating Entities.

Typically, the Protocol Client initiates a Session with a Protocol Server but may also, perhaps, incorrectly, refer to a peer-to-peer Session.

Communication Transport may be implemented as part of protocols and services at the Application Layer, at the Session Layer or at the Transport Layer in the OSI-Model.

Application Layer examples:

  • HTTP Session, which allow associating information with individual visitors
  • A telnet remote login session

Session Layer example:

  • A Session Initiation Protocol (SIP) based Internet phone call

Transport Layer example:

  • A TCP Session, which is synonymous to a TCP virtual circuit, a TCP connection, or an established TCP socket.

HTTP/1.0 was thought to only allow a single Request-Response during one Web/HTTP Session. Protocol version HTTP/1.1 improved this by completing the Common Gateway Interface (CGI), making it easier to maintain the Web Session and supporting HTTP Cookies and file uploads.

Most Client-Server Exchange sessions are maintained by the Transport Layer - a single connection for a single session. However each transaction phase of a Web/HTTP session creates a separate connection. Maintaining session continuity between phases requires a Session ID. The Session ID is embedded within the <A HREF> or <FORM> links of dynamic web pages so that it is passed back to the CGI. CGI then uses the Session ID to ensure session continuity between transaction phases. One advantage of one connection-per-phase is that it works well over low bandwidth (modem) connections.

Server-side web Session#

Server-side sessions are handy and efficient, but can become difficult to handle in conjunction with load-balancing/high-availability systems and are not usable at all in some embedded systems with no storage. The load-balancing problem can be solved by using shared storage or by applying forced peering between each client and a single server in the cluster, although this can compromise system efficiency and load distribution.

A method of using server-side sessions in systems without mass-storage is to reserve a portion of RAM for storage of session data. This method is applicable for servers with a limited number of clients (e.g. router or access point with infrequent or disallowed access to more than one client at a time).

The use of Tokens, especially JSON Web Tokens is also a method of maintaining Sessions across Load Balancing

Client-side web Session#

Client-side sessions use cookies and cryptographic techniques to maintain state without storing as much data on the server. When presenting a dynamic web page, the server sends the current state data to the client (web browser) in the form of a cookie. The client saves the cookie in memory or on disk. With each successive request, the client sends the cookie back to the server, and the server uses the data to "remember" the state of the application for that specific client and generate an appropriate response.

This mechanism may work well in some contexts; however, data stored on the client is vulnerable to tampering by the user or by software that has access to the client computer. To use client-side sessions where confidentiality and integrity are required, the following must be guaranteed:

  • Confidentiality: Nothing apart from the server should be able to interpret session data.
  • Data integrity: Nothing apart from the server should manipulate session data (accidentally or maliciously).
  • Authenticity: Nothing apart from the server should be able to initiate valid sessions.
To accomplish this, the server needs to encrypt the session data before sending it to the client, and modification of such information by any other party should be prevented via cryptographic means.

Transmitting state back and forth with every request is only practical when the size of the cookie is small. In essence, client-side sessions trade server disk space for the extra bandwidth that each web request will require. Moreover, web browsers limit the number and size of cookies that may be stored by a web site. To improve efficiency and allow for more session data, the server may compress the data before creating the cookie, decompressing it later when the cookie is returned by the client.

HTTP Session token#

A session token is a unique identifier that is generated and sent from a server to a client to identify the current interaction session. The client usually stores and sends the token as an HTTP cookie and/or sends it as a parameter in HTTP GET or HTTP POST queries. The reason to use session tokens is that the client only has to handle the identifier—all session data is stored on the server (usually in a database, to which the client does not have direct access) linked to that identifier.

Examples of the names that some programming languages use when naming their HTTP cookie include:

More Information#

There might be more information for this subject on one of the following: