Sip Over Tcp; Sip Proxy Redundancy; Dual Registration - Cisco 7800 Series Administration Manual

Cisco IP Phone Installation

SIP Over TCP

To guarantee state-oriented communications, the Cisco IP Phone can use TCP as the transport protocol for
SIP. This protocol provides guaranteed delivery that assures that lost packets are retransmitted. TCP also
guarantees that the SIP packages are received in the same order that they were sent.
TCP overcomes the problem of UDP port-blocking by corporate firewalls. With TCP, new ports do not need
to be open or packets dropped, because TCP is already in use for basic activities, such as internet browsing
or e-commerce.

SIP Proxy Redundancy

An average SIP Proxy Server can handle tens of thousands of subscribers. A backup server allows an active
server to be temporarily switched out for maintenance. Cisco phones support the use of backup SIP Proxy
Servers to minimize or eliminate service disruption.
A static list of proxy servers is not always adequate. If your user agent serves different domains, for example,
you do not want to configure a static list of proxy servers for each domain into every Cisco IP Phone.
A simple way to support proxy redundancy is to configure a SIP Proxy Server in the Cisco IP Phone
configuration profile. The DNS SRV records instruct the phones to contact a SIP Proxy Server in a domain
named in SIP messages. The phone consults the DNS server. If configured, the DNS server returns an SRV
record that contains a list of SIP Proxy Servers for the domain, with their hostnames, priority, listening ports,
and so forth. The Cisco IP Phone tries to contact the hosts in the order of their priority.
If the Cisco IP Phone currently uses a lower-priority proxy server, the phone periodically probes the
higher-priority proxy and switches to the higher-priority proxy when available.

Dual Registration

The phone always registers to both primary (or primary outbound) and alternate (or alternate outbound)
proxies. After registration, the phone sends out Invite and Non-Invite SIP messages through primary proxy
first. If there is no response for the new INVITE from the primary proxy, after timeout, the phone attempts
to connect with the alternate proxy. If the phone fails to register to the primary proxy, it sends an INVITE to
the alternate proxy without trying the primary proxy.
Dual registration is supported on a per-line basis. Three added parameters can be configured through web
user interface and remote provisioning:
• Alternate Proxy—Default is empty.
• Alternate Outbound Proxy—Default is empty.
• Dual Registration—Default is NO (turned off).
After you configure the parameters, reboot the phone for the feature to take effect.
Note Specify a value for primary proxy (or primary outbound proxy) and alternate proxy (or alternate outbound
proxy) for the feature to function properly.
Dual Registration and DNS SRV Limitations
• When Dual Registration is enabled, DNS SRV Proxy Fallback or Recovery must be disabled.
Cisco IP Phone 7800 Series Multiplatform Phones Administration Guide
SIP Over TCP
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