diff --git a/channels/chan_sip.c b/channels/chan_sip.c index 60ab323f07d180238b421bcb1a8b7c62389042a4..fa03d9c43b97c70453f32705531f4aacdc189ce1 100644 --- a/channels/chan_sip.c +++ b/channels/chan_sip.c @@ -23079,7 +23079,14 @@ static int handle_incoming(struct sip_pvt *p, struct sip_request *req, struct so req->ignore = 1; ast_debug(3, "Ignoring SIP message because of retransmit (%s Seqno %d, ours %d)\n", sip_methods[p->method].text, p->icseq, seqno); } - + + /* RFC 3261 section 9. "CANCEL has no effect on a request to which a UAS has + * already given a final response." */ + if (!p->pendinginvite && (req->method == SIP_CANCEL)) { + transmit_response(p, "481 Call/Transaction Does Not Exist", req); + return res; + } + if (seqno >= p->icseq) /* Next should follow monotonically (but not necessarily incrementally -- thanks again to the genius authors of SIP --