Transport Mode is a method of sending data over the Internet where the data is encrypted but the original IP address information is not.
The Encapsulating Security Payload (ESP) operates in Transport Mode or Tunnel Mode. In Transport Mode, ESP encrypts the data but the IP header information is viewable.
Together, the Internet Security (IPsec) protocol uses ESP and Authentication Header (AH) to protect packets as they travel over the Internet. ESP handles encryption of the data (its confidentiality) and some authentication of data and AH provides authentication only. These two protocols may be used independently or they may be bundled as IPsec, useful for virtual private networks (VPNs).
“ESP transport mode is a way of sending data over the Internet so that IP header info is still visible, so if you want to hide the source of information such as when using a VPN, you would want to use ESP tunnel mode.“