8.1.1. Use of the <From, To> for re-keying

8.1.1. Use of the <From, To> for re-keying

In addition to the use of the MKI, SRTP defines another optional mechanism for master key retrieval, the <From, To>. The <From, To> specifies the range of SRTP indices (a pair of sequence number and ROC) within which a certain master key is valid, and is (when used) part of the crypto context. By looking at the 48-bit SRTP index of the current SRTP packet, the corresponding master key can be found by determining which From-To interval it belongs to. For SRTCP, the most recently observed/used SRTP index (which can be obtained from the cryptographic context) is used for this purpose, even though SRTCP has its own (31-bit) index (see caveat below).

This method, compared to the MKI, has the advantage of identifying the master key and defining its lifetime without adding extra bits to each packet. This could be useful, as already noted, for some wireless links that do not cater for added bits. However, its use SHOULD be limited to specific, very simple scenarios. We recommend to limit its use when the RTP session is a simple unidirectional or bi-directional stream. This is because in case of multiple streams, it is difficult to trigger the re-key based on the <From, To> of a single RTP stream. For example, if several streams share a master key, there is no simple one-to-one correspondence between the index sequence space of a certain stream, and the index sequence space on which the <From, To> values are based. Consequently, when a master key is shared between streams, one of these streams MUST be designated by key management as the one whose index space defines the re-keying points. Also, the re-key triggering on SRTCP is based on the correspondent SRTP stream, i.e., when the SRTP stream changes the master key, so does the correspondent SRTCP. This becomes obviously more and more complex with multiple streams.

The default values for the <From, To> are "from the first observed packet" and "until further notice". However, the maximum limit of SRTP/SRTCP packets that are sent under each given master/session key (Section 9.2) MUST NOT be exceeded.

In case the <From, To> is used as key retrieval, then the MKI is not inserted in the packet (and its indicator in the crypto context is zero). However, using the MKI does not exclude using <From, To> key lifetime simultaneously. This can for instance be useful to signal at the sender side at which point in time an MKI is to be made active.