Peers is a very minimalist SIP User-Agent, it is based on RFC3261. The only supported codec is G711 PCMU, sample rate 8kHz. The reference RFCs for SDP and RTP have been RFC4566 and RFC3550. Here is a quick list of basic SIP features that are not implemented for the moment:
The author of this stack is Yohann Martineau. This stack has been implemented in 2007, 2008 and 2009.