Call Setup
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A: The calling party sends an INVITE with SDP to SBC-A. The offer SDP contains an SDES crypto attribute within the SRTP media line.
B: Since Multi-system Selective SRTP Pass-through is enabled within the ingress realm, SD-A adds an a=X-acme-srtp-msm media attribute. The a=X-acme-srtp-msm attribute contains a cookie that includes an encryption of the SDES crypto attribute present in the SDP. The encryption is done using the shared secret configured for encrypting SRTP Pass-through information.
C: SBC-B receives the offer SDP that has the cookie sent by SBC-A. It is assumed that the proxies that forward the offer SDP sent by SBC-A preserve and forward the cookie added by SBC-A.
D: SBC-B checks if the egress realm has Multi-system Selective SRTP Pass-through enabled. If so, SBC-B decrypts the cookie using the shared secret to retrieve the SDES crypto attribute. SBC-B adds the SDES crypto attribute retrieved from the cookie to the offer SDP sent to UA-B.
E: The called party sends an answer SDP with a SDES crypto attribute on the SRTP media line to SBC-B.
F: SBC-B checks if it has received a cookie in the offer SDP and adds the cookie to the answer SDP. The cookie contains an encryption of the SDES crypto attribute received in the answer SDP from UA-B. SBC-B does not install any SA or media policy so that SRTP packets from/to UA-B can pass through SBC-B without any decryption/encryption.
G: SBC-A receives the answer SDP that has the cookie sent by SBC-B.
H: SBC-A decrypts the cookie using the shared secret to retrieve the SDES crypto attribute. SBC-A adds the SDES crypto attribute retrieved from the cookie to the answer SDP. SBC-A does not install any SA or media policy so that SRTP packets from/to UA-A can pass through SBC-A without any decryption/encryption.