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What is the difference between the Tunnel and Transport modes in ESP?

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Article ID: KB5302 KB Last Updated: 14 Dec 2017Version: 5.0
Summary:
This article provides information about the difference between the Tunnel and Transport modes in ESP.
Symptoms:
Information about the difference between the Tunnel and Transport modes in ESP.
Solution:

Tunnel mode:

  • Tunnel mode protects the internal routing information by encrypting the IP header of the original packet. The original packet is encapsulated by a another set of IP headers.

  • It is widely implemented in site-to-site VPN scenarios.

  • NAT traversal is supported with the tunnel mode.

  • Additional headers are added to the packet; so the payload MSS is less.

Transport mode:
 
  • The transport mode encrypts only the payload and ESP trailer; so the IP header of the original packet is not encrypted.

  • The IPsec Transport mode is implemented for client-to-site VPN scenarios.

  • NAT traversal is not supported with the transport mode.

  • MSS is higher, when compared to Tunnel mode, as no additional headers are required.

  • The transport mode is usually used when another tunneling protocol (such as GRE, L2TP) is used to first encapsulate the IP data packet, then IPsec is used to protect the GRE/L2TP tunnel packets.
Modification History:
2017-12-07: This is a general IPSec KB. Removing ScreenOS tag and modified related product list.
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