Avoid Unnecessary Ajax Traffic with Session State
http://www.ibm.com/developerworks/web/library/wa-aj-resttip.html?S_TACT=105AGX08&S_CMP=EDU

Mertz, David
IBM
2007

Abstract:

Where possible, creating Web applications -- including Ajax-based applications -- in a RESTful way avoids a large class of bugs. However, a pitfall of REST (REpresentational State Transfer) is sending duplicate data across similar XMLHttpRequests. This tip shows how the moderate use of session cookies can maintain just enough server-side state to significantly reduce client-server traffic, while still allowing fallback to cookie-free operation.