Polycom HDA50 Administrator'S Manual Download

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Service Providers

Polycom, Inc.

Proxy Server is configured, it is used instead of the SIP Proxy Server or SIP Registration Server. The
resolution of the server domain name into IP address is performed in the following manner:

●

Try looking up the name as DNS A Record. If not found,

●

Try looking up the name as DNS SRV Record. If not found,

●

Try looking up the name as DNS SRV Record with “_sip._udp.“ prepended to the host name. If not

found, fail the request.

If the result from the DNS query is an SRV record, the server port is taken from that record also. The server
port value configured on the device is ignored. Otherwise, the server port is taken from the configured value
or uses port 5060 if none is specified.

NAT Traversal Considerations

If the device sits behind a NAT router (typically the case), it can discover the mapped external address
corresponding to its local SIP contact address as seen by the server in one of the following ways:

●

From the “received=” and “rport=” parameters of the VIA header of the REGISTER response sent by

the server. These two parameters tell the device its mapped IP address and port number. This method

is used if periodic registration is enabled on the device.

●

From the response to a STUN binding request the device sent to a STUN server. This method is used

by enabling

X_KeepAliveEnable

and setting

X_KeepAliveMsgType

to “stun”. In that case, the

STUN server is taken from

X_KeepAliveServer

, if it is specified. Otherwise, the keep-alive messages

are sent to the same server where a REGISTER request would be sent to. The latter is the most

effective way of using STUN to discover the mapped external contact address.

The device always uses the mapped external contact address in all outbound SIP requests instead of its
local contact address if one is discovered by either method discovered above.

SIP Proxy Server Redundancy and Dual REGISTRATION

Server Redundancy specifically refers to the device’s capability to a) look for a working server to REGISTER
with from among a list of candidates, and b) switch to another server once the server that it currently
registers with becomes unresponsive. In other words, device registration must be enabled in order to use
the server redundancy feature. Other SIP requests, such as INVITE or SUBSCRIBE, are sent to the same
server that the device currently registers with.

If Outbound Proxy Server is provided, server redundancy is applied to the Outbound Proxy Server instead
of the REGISTRATION server. Server redundancy behavior is enabled by enabling the

ITSP Profile X –

SIP::

X_ProxyServerRedundancy

parameter, which is disabled by default.

Another requirement for using the server redundancy feature is that the underlying server must be
configured in the device as a domain name instead of an IP address. This allows the device to collect a list
of candidate servers based on DNS query.

The domain name can be looked up as DNS A record or DNS SRV record. For A records, all the IP
addresses returned by the DNS server are considered to have the same priority. For SRV records, the hosts
returned by the DNS server can be each assigned a different priority.

After a list of candidate servers are obtained, the device first looks for a working server according to the
stated priority. A

working server

means one that the device can successfully register with. This is known as

the

Primary Server

. Subsequently, the device maintains registration with the primary server the usual way.

However, if no working server is found after traversing the entire list, the device takes a short break and
repeats the search in the same order.