How to set the SRTCP Index value during Server failover/switchover case ?

[rajanikanth1234]

Currently using libsrtp version - v2.4.2.

We need to support server failover/switchover scenarios. When a server transitions from the standby state to the active state, the SRTCP Index value must be passed to the newly active server.

Currently, I don’t see an existing API in libsrtp to set the SRTCP Index value.

To handle this, I created a new function srtp_set_stream_srtcp_index() and call it before invoking srtp_protect_rtcp().
In this function, I set the updated SRTCP Index value i.e. 'srtcp_index_to_set' value to 'stream->rtcp_rdb.window_start'.

/******************** new code below *********************************************/
srtp_err_status_t srtp_set_stream_srtcp_index(srtp_t ctx,
void *rtcp_hdr,
uint32_t srtcp_index_to_set)
{
srtp_stream_ctx_t *stream;
srtcp_hdr_t *hdr = (srtcp_hdr_t *)rtcp_hdr;

stream = srtp_get_stream(ctx, hdr->ssrc);
if (stream == NULL)
    return srtp_err_status_bad_param;

stream->rtcp_rdb.window_start = srtcp_index_to_set;

return srtp_err_status_ok;

}
/******************** new code above *********************************************/

Could you please review this approach? Do you see any potential issues or risks with this implementation?

[pabuhler]

I guess something similar exists for setting the ROC for SRTP although fail over was not the use case in mind.
An alternative is to change the SSRC in a fail over case but I guess that is not always possible.

I do not think the fail over case has been considered very much, you would need potentially more state to support it properly in both the RX and TX direction.
You would also need to track this state some where outside the failing server, does not sound very practical.

I think the code you have would work but ideally this would be part of the stream setup and not something that could be changed arbitrarily.

[rajanikanth1234]

Thanks for the reply. srtp_set_stream_roc ( ) and srtp_get_stream_roc ( ) are the available APIs to set/get the ROC for SRTP.

ROC for SRTP:

/crypto/include/rdbx.h

/*

• An srtp_rdbx_t is a replay database with extended range; it uses an
• xtd_seq_num_t and a bitmask of recently received indices.
  */
  typedef struct {
  srtp_xtd_seq_num_t index;
  bitvector_t bitmask;
  } srtp_rdbx_t;

Here srtp_xtd_seq_num_t index; 'index' maintains the ROC for SRTP.

SRTCP Index value:

/crypto/include/rdb.h

/*

• if the ith least significant bit is one, then the packet index
• window_end-i is in the database
  */

typedef struct {
uint32_t window_start; /* packet index of the first bit in bitmask */
v128_t bitmask;
} srtp_rdb_t;

Here 'window_start' maintains the SRTCP Index value.

srtp_rdb_get_value( ) is the available API to get the SRTCP Index value.

uint32_t srtp_rdb_get_value(const srtp_rdb_t *rdb)
{
return rdb->window_start;
}

Do you see any issues or risks, if we change the value of 'stream->rtcp_rdb.window_start' arbitrarily ?

Additionally, if the failover scenario has not been addressed in the current implementation, can we treat it as a new feature request and plan for its inclusion in a future release?