IPSec VPN Tunnel Creation
What is IPSec?
IPsec (Internet Protocol, secure) is a group of protocols that are used together to set up encrypted connections between devices.
How does IPsec work?
IPsec connections include the following steps:
Keys are necessary for encryption; a key is a string of random characters that can be used to “lock” (encrypt) and “unlock” (decrypt) messages. IPsec sets up keys with a key exchange between the connected devices, so that each device can decrypt the other device’s messages.
Packet headers and trailers: All data that is sent over a network is broken down into smaller pieces called packets. Packets contain both a payload, or the actual data being sent, and headers, or information about that data so that computers receiving the packets know what to do with them. IPsec adds several headers to data packets containing authentication and encryption information. IPsec also adds trailers, which go after each packet’s payload instead of before.
IPsec provides authentication for each packet, like a stamp of authenticity on a collectible item. This ensures that packets are from a trusted source and not an attacker.
IPsec encrypts the payloads within each packet and each packet’s IP header (unless transport mode is used instead of tunnel mode: see below). This keeps data sent over IPsec secure and private.
Encrypted IPsec packets travel across one or more networks to their destination using a transport protocol. At this stage, IPsec traffic differs from regular IP traffic in that it most often uses UDP as its transport protocol, rather than TCP. TCP, the Transmission Control Protocol, sets up dedicated connections between devices and ensures that all packets arrive. UDP, the User Datagram Protocol, does not set up these dedicated connections. IPsec uses UDP because this allows IPsec packets to get through firewalls.
At the other end of the communication, the packets are decrypted, and applications (e.g. a browser) can now use the delivered data.
IKE Phases
What protocols are used in IPsec?
The AH protocol ensures that data packets are from a trusted source and that the data has not been tampered with, like a tamper-proof seal on a consumer product. These headers do not provide any encryption; they do not help conceal the data from attackers.
ESP encrypts the IP header and the payload for each packet — unless transport mode is used, in which case it only encrypts the payload. ESP adds its own header and a trailer to each data packet.
SA refers to several protocols used for negotiating encryption keys and algorithms. One of the most common SA protocols is Internet Key Exchange (IKE).
Finally, while the Internet Protocol (IP) is not part of the IPsec suite, IPsec runs directly on top of IP.
IPsec tunnel mode is used between two dedicated routers, with each router acting as one end of a virtual “tunnel” through a public network. In IPsec tunnel mode, the original IP header containing the destination of the packet is encrypted, in addition to the packet payload. To tell intermediary routers where to forward the packets, IPsec adds a new IP header. At each end of the tunnel, the routers decrypt the IP headers to deliver the packets to their destinations.
In transport mode, the payload of each packet is encrypted, but the original IP header is not. Intermediary routers are thus able to view the destination of each packet — unless a separate tunneling protocol (such as GRE) is used.
A network port is the virtual location where data goes in a computer. Ports are how computers keep track of different processes and connections; if data goes to a certain port, the computer’s operating system knows which process it belongs to. IPsec usually uses port 500.
MSS and MTU are two measurements of packet size. Packets can only reach a certain size (measured in bytes) before computers, routers, and switches cannot handle them. MSS measures the size of each packet’s payload, while MTU measures the entire packet, including headers. Packets that exceed a network’s MTU may be fragmented, meaning broken up into smaller packets and then reassembled. Packets that exceed the MSS are simply dropped.
IPsec protocols add several headers and trailers to packets, all of which take up several bytes. For networks that use IPsec, either the MSS and MTU must be adjusted accordingly, or packets will be fragmented and slightly delayed. Usually, the MTU for a network is 1,500 bytes. A normal IP header is 20 bytes long, and a TCP header is also 20 bytes long, meaning each packet can contain 1,460 bytes of payload. However, IPsec adds an Authentication Header, an ESP header, and associated trailers. These add 50-60 bytes to a packet, or more.
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