![]() ![]() ![]() Step-2: Detects if there is a NAT device along the path. Step-1: Detects if both VPN Devices RTR-Site1 and RTR-Site2 support NAT-T As this new UDP header is NOT encrypted and is treated as just like a normal UDP packet, the NAT device can make the required changes and process the message, NAT Traversal adds a UDP header which encapsulates the IPSec ESP header. ![]() It is clear NAT and IPSec are incompatible with each other, and to resolve this NAT Traversal was developed. Without NAT Traversal and new UDP Encapsulation of ESP packets with source port 4500 and destination 4500, the NAT Device cannot do anything. (I decrypted the packet to see how it looks the packet inside ESP after decryption at the RTR-Site2. This is a difference from ISAKMP which uses UDP port 500 as its UDP layer 4.īecause ESP is a protocol without ports and at the other side the L4 information the, The NAT device can not change these encrypted headers and cannot perform PAT translation at the L4 level.īelow the telnet packet captured from PC-1 to PC-2, the Source port 30206 and the Destination Port 23 are encapsulated by ESP and both are encrypted. Tocol but there is no port number (Layer 4). IPsec uses ESP to encrypt all packet, encapsulating the 元/L4 headers within an ESP header. See below interesting details about NAT Traversal In IPSEC VPN. This interesting concept is discussed in many vendors courses, blogs and forums and this additional article I wrote is to demystify it with packet captures and simple words. One of the biggest concept in VPN Technologies is NAT Traversal, like NAT Traversal in VOIP deployment with SIP Protocol, the history is always inside the payload to solve the Incompatibility between NAT and IPSEC like the Incompatibility between SIP protocol and NAT. ![]()
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