633
Figure 658 Call redirection procedure for UAs
This is a common application. Fundamentally, a redirect server can respond with the address of a proxy
server as well. The subsequent call procedures are the same as the call procedures involving proxy
servers.
Support for transport layer protocols
As an application layer protocol, SIP supports three transport layer protocols, including:
• UDP—UDP is a connectionless protocol and does not provide reliability. Therefore, SIP connections
established over UDP are unreliable.
• TCP—Ensures transmission reliability for SIP messages. TCP provides connection-oriented and
reliable transmission for SIP-based VoIP communications. Using TCP, SIP need not consider packet
loss and retransmission issues.
• Transport layer security (TLS)—Ensures transmission security for SIP messages. For more information,
see "Signaling encryption."
The above three transport layer protocols have their own benefits, and allow you to select a protocol
based on your network environment. The system does not support transport layer protocol switchover
during communication.
SIP security
This section provides information on signaling encryption, media flow encryption, and TLS-SRTP
combinations.
Internet
User agent User agent
Redirect Server
INVITE
100 Trying
302 Moved Temporarily
ACK
100 Trying
200 OK
INVITE
ACK
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