Which complication does a NAT introduce in SDP for a SIP call?

Question

Options

• AAdditional headers due to NAT encapsulation can cause the packet size to exceed the MTU.
• BWhen the client is behind a NAT they may be unable to determine the appropriate offset due to
• CThe IP address specified in the connection data field may be an unrouteable internal address.
• DThe encryption keys advertised in the SDP are only valid for clients not behind a NAT.

• A
  4% (1)
• C
  92% (23)
• D
  4% (1)

Explanation

SDP (Session Description Protocol) is embedded in SIP messages and describes the media session, including where to send audio/video (the 'c=' connection data line containing an IP address and the 'm=' media lines containing ports). When an endpoint is behind a NAT, its SDP advertises its private, internal IP address - such as 192.168.1.10 - in the connection data field. This private address is not routable on the public internet, so the remote party cannot send media back to it. The NAT device translates addresses at the IP/transport layer but does not modify the SDP payload, leaving a mismatch between the transport-layer source address and the address advertised in SDP. Solutions include STUN, TURN, ICE, or an application-layer gateway like Cisco Expressway.

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