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Protocol

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Details

RTP (Real-time Transport Protocol) is a protocol which delivers audio or video over an IP network, ie. for VoIP.
For low latency it uses UDP, and there's no standard port defined. It just has to be an even port above 1024.

RTP works in conjunction with the RTCP (Real-time Control Protocol), which uses an odd port above 1024. While RTP
provides the audio or video stream using an arbitary Codec the RTCP protocol provides stream control and status feedback.

A secure variant of RTP is the SRTP protocol.

FRITZ!OS

RTP

In FRITZ!OS RTP is used for VoIP and T.38 Fax and is performed by voipd using librtpstream.so.

For every Fritzbox with configured Telephony the 40 ports Port-7078-7109-udp are open to the Internet.
This is required for a maximum of 20 SIP accounts, 20 even ports for RTP / SRTP and 20 odd ports for RTCP / SRTCP.

There's are some global RTP related options in voip.cfg:

use_krtp = yes;                      # use krtp.ko
rtp_prio = 0;                        #
...
rtpstream {
    voice_activity_detection {       # VAD - Voice Activity Detection
       vad_enabled = vadenabled_no;
       vad_threshold = 10000;
    }
    plc {                            # PLC - Packet Loss Concealment
       in_the_stack = yes;
    }
    jitter {                         # Jitter buffering to mitigate PDV - Packet Delay Variation 
       auto_on = yes;
       in_ms = 50;
       in_packets = 0;
    }
    rtcp_enabled = yes;              # separate RTCP or multiplexing with RTP
    silence_detection = no;
}

Besides this voip.cfg also contains some RTP related options per SIP account:

ua1 {
    ...
    dtmfcfg = dtmfcfg_rtp_or_inband; # DTMF transfer method
    rtpevent_keep_packetrate = no;
    xrtp_periodic = 0;
    add_rtpmap_for_all_codecs = no;
    srtp_supported = no;             # SRTP - Secure RTP
}

Codecs

librtpstream.so uses the RTPAudioEncoder in capi_codec.ko which uses builtin and hardware Codecs in pcmlink.ko.

The Kernel-Config contains these Codec related variables:

There's a global option in voip.cfg to limit the Codecs used:

root@fritz1:/var/mod/root# grep audiocodecs /var/flash/voip.cfg

use_audiocodecs = no;
audiocodecs = "PCMA", "PCMU", "G726-32", "G726-40", "G726-24";

The currently activated Codecs could be queried from voipd - fw 7.29:

root@fritz1:/var/mod/root# aicmd voipd show codeclist | sort

CLEARMODE/8000/1: 106000 bits/second (encaplen=32,20ms)
CLEARMODE/8000/1: 98933 bits/second (encaplen=32,30ms)
G722/8000/1: 106000 bits/second (encaplen=32,20ms)
G722/8000/1: 98933 bits/second (encaplen=32,30ms)
G726-24/8000/1: 56533 bits/second (encaplen=32,30ms)
G726-24/8000/1: 63600 bits/second (encaplen=32,20ms)
G726-32/8000/1: 70666 bits/second (encaplen=32,30ms)
G726-32/8000/1: 84800 bits/second (encaplen=32,20ms)
G726-40/8000/1: 70666 bits/second (encaplen=32,30ms)
G726-40/8000/1: 84800 bits/second (encaplen=32,20ms)
PCMA/16000/1: 197866 bits/second (encaplen=32,30ms)
PCMA/16000/1: 212000 bits/second (encaplen=32,20ms)
PCMA/8000/1: 106000 bits/second (encaplen=32,20ms)
PCMA/8000/1: 98933 bits/second (encaplen=32,30ms)
PCMU/16000/1: 197866 bits/second (encaplen=32,30ms)
PCMU/16000/1: 212000 bits/second (encaplen=32,20ms)
PCMU/8000/1: 106000 bits/second (encaplen=32,20ms)
PCMU/8000/1: 98933 bits/second (encaplen=32,30ms)
iLBC/8000/1: 42400 bits/second (encaplen=32,30ms)
iLBC/8000/1: 63600 bits/second (encaplen=32,20ms)
rtploopback/8000/1: 106000 bits/second (encaplen=32,20ms)
rtploopback/8000/1: 98933 bits/second (encaplen=32,30ms)

See also

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