The Session Initiation Protocol Core (SIPCore) working group is chartered to maintain and continue the development of the core SIP specifications, currently defined as proposed standard RFCs 3261, 3262, 3263, 3264, and 3265. The SIPCore working group will concentrate on specifications that update or replace the core SIP specifications. In this context, "update" means replacing or modifying protocol elements in the above listed RFCs in ways that would affect most or all implementations of those RFCs alone. Extensions to SIP that add new functionality that would not be required of all implementations will be done outside of this WG. The process and requirements for such extensions are documented in RFC 3427bis, "Change Process for the Session Initiation Protocol". Throughout its work, the group will strive to maintain the basic model and architecture defined by SIP. In particular: 1. Services and features are provided end-to-end whenever possible. 2. Reuse of existing Internet protocols and architectures and integrating with other Internet applications is crucial. The primary source of change requirements will be a) interoperability problems that stem from ambiguous, or under-defined specification, and b) requirements from other working groups in the RAI Area. Although in general the WG will not work on extensions to SIP, it may take on some previous work items from the SIP working group to allow for a smooth transition. The adoption of new items requires explicit agreement from the AD or rechartering.