Proposals or Transform Sets - Setup IPSec tunnel VPN Guide
Proposals / Transform Sets
VPN Proposals or Transform sets is a set of protocols and algorithms specified on a gateway to secure data. The three factors that make up a proposal or transform set are data encryption, data authentication and the encapsulation mode. A proposal/transform set is like a profile with a specific combination of protocols and algorithms that an end user may choose to use for their VPN\IPSec security parameters.
For example a VPN gateway at a bank with highly confidential data I may want to use;
AES 256 bit for encryption,
Sha-1 for Authentication
Diffie-Hellman key group 5 - For public key cryptography.
The above would be my proposal or transform set for a bank. I may name this proposal "High-Security" for my own reference. However the peer device connecting to my bank must also use the exact same settings as above to successfully create a VPN tunnel. Although if a remote peer does not use the same settings you can configure other proposals with alternative settings to fall back on. So you may specify your ideal protocol and algorithms to use in your first proposal and then below this a fallback proposal which could be used if a remote peer has a different combination of protocols and algorithms specified as it’s proposal.
Another way to describe proposals or transform sets are to describe them to mobile phone profiles. Usually your outdoor profile would be on the highest volume, constant ringing, keypad tone will be enabled and so on. To select from silent to loud you would just select the profile named "Outdoor", and all the settings within this profile would be enabled. Well this would be the same for my VPN profile "High-Security" mentioned above. I may use "High-Security for a specific VPN connection and whatever settings it holds in it's proposal would be enabled for that VPN connection.
Wikipedia's guide to IPSec