Re: [rtcweb] SRTP not mandatory-to-use

[Harald Alvestrand <[e-mail]>]

At the risk of resurrecting a dead horse

On 12/14/2011 03:41 PM, Xavier Marjou wrote:
> Is there any scientific reference comparing the performances (cpu) of
> a SRTP communication vs an RTP communication on a smartphone device,
> or on an interworking network server? The feedback I have is that SRTP
> overhead is significant, at least on interworking network servers
> handling several calls in parallel. One example is
> http://trac.pjsip.org/repos/wiki/PJMEDIA-MIPS  which gives figures,
> perhaps non-optimized, but figures anyway:
>
>
> 00:59:38.531 os_core_unix.c pjlib 0.9.0-trunk for POSIX initialized
> MIPS test, with CPU=180Mhz,  198.0 MIPS
> Clock  Item                                      Time     CPU    MIPS
>   Rate                                           (usec)    (%)
>   8KHz codec encode/decode - L16/8000/1           1704    0.170    0.34
>   8KHz stream TX/RX - G.711                       6786    0.679    1.34
>   8KHz stream TX/RX - G.711 SRTP 32bit           21688    2.169    4.29
>   8KHz stream TX/RX - G.711 SRTP 32bit +auth     33501    3.350    6.63
>   8KHz stream TX/RX - G.711 SRTP 80bit           21725    2.172    4.30
>   8KHz stream TX/RX - G.711 SRTP 80bit +auth     33551    3.355    6.64
Note that the codec used here (G.711) is hardly a complex encoder; it 
can be implemented using a lookup table, which is probably small enough 
to serve out of cache.
The first line seems to have been included by a copy/paste error.

A different cut'n'paste from the same table:

  8KHz codec encode/decode - G.711                2701    0.270    0.53
  8KHz codec encode/decode - GSM                 75750    7.575   15.00
  8KHz codec encode/decode - iLBC              2856203  285.620  565.53
  8KHz codec encode/decode - Speex 8Khz         436162   43.616   86.36
  8KHz codec encode/decode - L16/8000/1           1704    0.170    0.34
  8KHz stream TX/RX - G.711                       6786    0.679    1.34
  8KHz stream TX/RX - G.711 SRTP 32bit           21688    2.169    4.29
  8KHz stream TX/RX - G.711 SRTP 32bit +auth     33501    3.350    6.63
  8KHz stream TX/RX - G.711 SRTP 80bit           21725    2.172    4.30
  8KHz stream TX/RX - G.711 SRTP 80bit +auth     33551    3.355    6.64
  8KHz stream TX/RX - GSM                        82035    8.203   16.24
  8KHz stream TX/RX - GSM SRTP 32bit             90890    9.089   18.00
  8KHz stream TX/RX - GSM SRTP 32bit + auth      99334    9.933   19.67
  8KHz stream TX/RX - GSM SRTP 80bit             90893    9.089   18.00
  8KHz stream TX/RX - GSM SRTP 80bit + auth      99356    9.936   19.67


So if someone wishes to do GSM, the overhead of SRTP is on the order of 
10%; if they do iLBC, the overhead is microscopic. On this particular 
platform and implementation.