Complication of NAT in SDP for a SIP Call

D.

When a SIP call is initiated between two endpoints, the endpoints exchange Session Description Protocol (SDP) messages to negotiate the media capabilities and parameters of the call. The SDP messages contain information such as the media type, codec, IP addresses, and ports for the media streams.

However, when one or both of the endpoints are located behind a NAT device, the NAT can introduce several complications in the SDP negotiation. NAT works by translating the private IP addresses of the internal devices into a public IP address that can be routed on the internet. This translation process can affect the SDP negotiation in the following ways:

C. The IP address specified in the connection data field may be an unrouteable internal address. When the endpoint behind the NAT device sends its SDP message, it includes the private IP address in the connection data field. However, this private IP address is not routable on the public internet, so the other endpoint cannot establish a direct connection to it. Instead, the NAT device must perform a translation to replace the private IP address with a public IP address that can be routed on the internet.

A. Additional headers due to NAT encapsulation can cause the packet size to exceed the MTU. When a packet is sent through a NAT device, the NAT adds additional headers to the packet to perform the translation. These additional headers can increase the size of the packet, which can cause the packet to exceed the Maximum Transmission Unit (MTU) of the network. This can result in fragmentation and reassembly of the packet, which can introduce delay and packet loss in the call.

D. The encryption keys advertised in the SDP are only valid for clients not behind a NAT. If the endpoints are using encryption for the media streams, the encryption keys are exchanged in the SDP messages. However, if one or both of the endpoints are behind a NAT device, the encryption keys may not be valid because they are based on the private IP addresses of the endpoints. This can result in the call failing or media streams being encrypted with different keys, which can cause quality issues.

B. When the client is behind a NAT they may be unable to determine the appropriate offset due to time zones. This answer is not relevant to the complications introduced by NAT in SDP negotiation. The time zone offset is not related to NAT and does not affect the SDP negotiation process.