Hi, I am connected to a 3GPP IMS with TLS and media plane security (SRTP for voice, end-to-access edge (e2ae) media security), so VoSIP is used.
I am sending a fax to another subscriber connected to this IMS, who is not using VoSIP. After the session is established with RTP/SRTP G.711 audio, the other subscriber initiates a switch to T.38 with UDPTL as transport layer. This Re-Invite arrives at my UA like this: INVITE sip:+x...@xxx.xxx.xxx.xxx:15005;transport=tcp SIP/2.0 Max-Forwards: 62 Via: SIP/2.0/TLS xxx.xxx.xxx.xxx:5061;branch=z9hG4bKg3Zqkv7ibd7l36xooj61rm62ln9i9uxs5 To: "+xxx" <sip:+x...@xxx.xxx;user=phone>;tag=-1010720271--310328935 From: <sip:x...@xxx.xxx ;user=phone>;tag=h7g4Esbg_p65540t1542904057m599244c37907839s1_1481021629-511233563 Call-ID: 2079428692@00a0572a649c CSeq: 102 INVITE Contact: <sip:xxx@xxx :5061;transport=tls>;+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel" Min-Se: 900 Session-Expires: 1800;refresher=uac Supported: timer Supported: 100rel Supported: replaces Content-Type: application/sdp Content-Length: 314 Allow-Events: refer Allow-Events: message-summary Allow-Events: dialog Allow: REGISTER, REFER, NOTIFY, SUBSCRIBE, INFO, UPDATE, PRACK, INVITE, ACK, OPTIONS, CANCEL, BYE v=0 o=- 598991827 1486619746 IN IP4 xxx.xxx.xxx.xxx s=SIP call c=IN IP4 xxx.xxx.xxx.xxx t=0 0 m=image 40396 RTP/SAVP t38 a=T38FaxVersion:0 a=T38MaxBitRate:14400 a=T38FaxMaxBuffer:512 a=T38FaxMaxDatagram:96 a=3ge2ae:applied a=crypto:104 AES_CM_128_HMAC_SHA1_80 inline:cP2BqzBxUTnmZVki1+GgudPICTXQrf6Ahu/a5Dsp My question is whether the media type image can be used in conjunction with RTP/SRTP. In the ITU-T Rec. 38 I only found media type audio when RTP/SRTP is used https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-T.38-201511-I!!PDF-E&type=items, PDF-Page 68: INVITE sip:+1-212-555-1...@bell-tel.com SIP/2.0 Via: SIP/2.0/UDP kton.bell-tel.com From: A. Bell <sip:+1-519-555-1...@bell-tel.com> To: T. Watson <sip:+1-212-555-1...@bell-tel.com> Call-ID: 3298420...@kton.bell-tel.com Cseq: 1 INVITE Subject: Mr. Watson, here is a fax Content-Type: application/sdp Content-Length: … v=0 o=faxgw1 2890844526 2890842807 IN IP4 128.59.19.68 e=+1-212-555-1...@bell-tel.com t=2873397496 0 c=IN IP4 128.59.19.68 m=audio 49170 RTP/AVP 100 101 a=rtpmap:100 t38/8000 a=fmtp:100 T38FaxRateManagement=transferredTCF a=rtpmap:101 parityfec/8000 a=fmtp:101 49173 IN IP4 128.59.19.68 m=image 49172 tcp t38 a=T38FaxRateManagement:localTCF In RFC4612 "Real-Time Facsimile (T.38) - audio/t38 MIME Sub-type Registration" it is saying: With these considerations in mind, the ITU-T amended T.38 [1] to allow RTP to be used to transport T.38. With that, a new MIME registration (audio/t38) is needed to allow for T.38 to be switched along with audio within the same RTP session. In ITU-T Rec. T,38 it also says: https://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-T.38-201511-I!!PDF-E&type=items PDF-Page 63: D.2.3.4 Use of either TCP or UDP In support of ITU-T T.38 choice of UDP or TCP transport, SDP extensions: [...] - indicate RTP/AVP (real-time protocol/audio-video profile) as a valid transport value (third field); - indicate RTP/SAVP (real-time protocol/secure audio-video profile) as a valid transport value (third field); [...] - include t38 as a valid format type value (fourth field). This value is used when the transport value is UDPTL or TCP; - include an RTP payload type as a valid format type value (fourth field). This value is used when the transport value is RTP/AVP or RTP/SAVP. This payload type is mapped via an 'rtpmap' attribute to the MIME type "audio/t38". From my perspective, the initial presented signalling is not correct. The media type needs to be audio, when SRTP is used. Best Regards Philipp Schöning _______________________________________________ Sip-implementors mailing list Sip-implementors@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/sip-implementors
