Page 385 - Asterisk™: The Future of Telephony

advantages of DNS, whereas disabling them removes the ability to place SIP calls
based on domain names.

Currently, the support for SRV records in Asterisk is somewhat
lacking. If multiple SRV records are returned, Asterisk will use only
the first record.

Using DNS SRV record lookups is highly recommended. To enable them, set
srvlookup=yes in the [general] section of sip.conf:
srvlookup=yes

t1min
This is the minimum round-trip time for messages to monitored hosts in
milliseconds. Defaults to 100 milliseconds:

t1min=100
subscribecontext
Limits SUBSCRIBE requests to the specified context. This is useful if you want to
limit subscriptions to internal extensions, for example.
This option may also be set on a per-user or per-peer basis:
subscribecontext=internal
t38pt_udptl
Setting t38pt_udptl to yes enables T.38 fax (UDPTL) passthrough on SIP-to-SIP
calls, provided both parties have T.38 support. This setting must be enabled in the
general section for all devices to work. You can then disable it on a per-device basis:

t38pt_udptl=yes|no
T.38 fax passthrough works only in SIP-to-SIP calls, without any
local or agent channel being used. Asterisk cannot currently orig-
inate or terminate T.38 fax calls; it can only passthrough UDPTL
from one device to another.

tos_sip, tos_audio, and tos_video
Asterisk can set the TOS bits in the IP header to help improve performance on
routers that respect TOS bits in their routing calculations. The tos_sip,
tos_audio, and tos_video settings control the TOS bits for the SIP messages, the
RTP audio, and RTP video respectively. Valid: CS0, CS1, CS2, CS3, CS4, CS5, CS6, CS7,
AF11, AF12, AF13, AF21, AF22, AF23, AF31, AF32, AF33, AF41, AF42, AF43, and ef (expe-
dited forwarding). You may also use a numeric value for the TOS bits.
For more information, see the doc/ip-tos.txt file in the Asterisk source directory.

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