IPSec (IP Security)
This is not a description on how IPSec works. Please read:
I would also suggest this:
If you're interested in cryptography, I would recommend this excellent introductory lecture series (there is also a book, but you'll have to buy it, IMHO it's worth it):
IPSec VPNs come in two flavours; policy and route based, the difference is how the Security Association (SA) is chosen.
Route Base VPNs
There are two aspects of a route based VPN; all packets to a particular peer are encrypted by the same SA and routing decides the peer to which to forward traffic (as routing always does). Therefore, routing is choosing the SA. Of course the same must be true in reverse, that all packets from a given peer are decrypted with the same SA. Another way of expressing this is to say a peer is 'protected' by this SA (really a pair of SAs; one for rx and tx).
The 'standard'  way of representing this protected peer is by using a point-to-point virtual interface to which the peer is attached and the SA pair is associated. Prefixes that require protection are routed through this virtual interface and hence implicitly to the peer.
There are three components to the model:
The SAs; An ipsec_sa_t, use the force, read the source.
The virtual interface
The protection - the association of the SAs to the interface.
The protection is represented by a ipsec_tun_protect_t. The "tun" part comes from the fact that the protected interface is usually a tunnel. IMO It would have been better if the author had not assumed this . The protection associates a single TX SA and up to four RX SAs to an interface. Four is as many as can fit on one cache-line. Multiple RX SAs mean that a peer can be using any SA in the set, this is particularly useful during rekeying because it is not possible for the peers to swap their RX and TX SAs at exactly the same moment in the traffic stream. Instead they can add the new RX immediately, then swap the TX after a short delay, then remove the old RX after another short delay. This will minimize, if not eliminate, packet loss.
The virtual interface can be represented in two ways:
interface + encap + SA = (interface + encap) + SA = ipip-interface + SA transport mode
interface + encap + SA = interface + (encap + SA) = IPSec-interface + SA tunnel mode
It's a question of where you add the parenthesis, from the perspective of the external user the effect is identical.
The IPSec interface serves as the encap-free interface to be used in conjunction with an encap-describing tunnel mode SA. VPP supports both models.
A route based VPN could impose 0, 1 or 2 encaps. the support matrix for these use cases is:
| 0 | 1 | 2 | -------------------------- ipip | N | Y | Y | ipsec | P | Y | P |
Where P = potentially.
Ipsec could potentially support 0 encap (i.e. transport mode) since neither the interface nor the SA requires encap. However, for a route based VPN to use transport mode is probably wrong since one shouldn't use transport mode for transit traffic, since without encap it is not guaranteed to return. IPSec could potentially support 2 encaps, but that would require the SA to describe both, something it does not do at this time.
Internally the difference is that the mid-chain adjacency for the IPSec interface has no associated encap (whereas for an ipip tunnel it describes the peer). Consequently, features on the output arc see packets without any encap. Since the protecting SAs are in tunnel mode, they apply the encap. The mid-chain adj is stacked only once the protecting SA is known, since only then is the peer known. Otherwise the VLIB graph nodes used are the same:
(routing) --> ipX-michain --> espX-encrypt --> adj-midchain-tx --> (routing) where X = 4 or 6.
Some benefits to the ipsec interface:
it is slightly more efficient since the encapsulating IP header has its checksum updated only once.
even when the interface is admin up traffic cannot be sent to a peer unless the SA is available (since it's the SA that determines the encap). With ipip interfaces a client must use the admin state to prevent sending until the SA is available.
The best recommendations I can make are:
pick a model that supports your use case
make sure any other features you wish to use are supported by the model
choose the model that best fits your control plane's model.
As mentioned above route based VPNs protect all packets destined to a given peer with the same SA pair. This protection was modelled using a virtual p2p interface, so one could legitimately reason that all traffic through the interface is protected with the SA pair or all traffic to the peer is protected, since they are one in the same. However, when we consider multi-point interfaces, we have to think of protection applying to the peers on the link.
When using IPSec protection on a P2MP link the ipsec_tun_protect_t will be specific to a particular peer (in the P2P case this peer is the usual special all zero address).
All other aspects of using route based VPNs remains the same. The routes are resolved via specific peers on the interface, i.e.
ip route add 10.0.0.0/8 via 192.168.1.1 mipip0
ip route add 10.0.0.0/8 via ipip0
but one should always use a next-hop on a multi-access interface, so this is not a restriction.
The data-path is unchanged, in both P2P and P2MP case the SA to use for TX comes from the adjacency, and for RX it's the SPI that matches to the SA and interface.
Policy Based VPNs
At the risk of stating the obvious, in a policy based VPN the SA is chosen based on a specific IPSec policy. A policy describes what attributes of the packets to match and what action to take if matched. Actions are:
bypass: Ignore it
discard: Drop it
protect: Either encrypt or decrypt with a specific SA
The 'resolve' action which (as per-RFC4301) states that an IKE session should be initiated, is not supported.
Policies are stored in a security policy database (SPD). An SPD is attached to an interface. Packets that ingress and egress the interface are matched against the policies in the attached SPD. This is IPSec as described in RFC4301.