IPsec

AWS, Azure, GCP - all clouds offer managed IPsec VPN for on-premise connectivity. It's been the de facto standard for enterprise VPN for 30 years. Understanding IPsec opens doors to the world of corporate networks and multi-cloud architectures.

• **Cloud connectivity:** AWS Site-to-Site VPN, Azure VPN Gateway, GCP Cloud VPN - all IPsec under the hood
• **IKEv2 on mobile:** iOS and Android have built-in IKEv2 clients. Many corporate VPNs use it instead of OpenVPN
• **Network equipment:** Cisco, Juniper, Fortinet - IPsec is the lingua franca for site-to-site between different vendors

Предварительные знания

• VPN: Virtual Networks

IPsec Modes

**IPsec** - a suite of protocols for protecting IP traffic. Operates at the network layer (Layer 3), protecting any protocols above: TCP, UDP, ICMP. Two modes: **Transport** and **Tunnel**.

**Tunnel Mode by default:** Most VPN configurations use Tunnel Mode. Transport Mode is for specific scenarios (L2TP/IPsec, host-to-host).

IPsec components:

ESP - Encapsulating Security Payload

**ESP (Protocol 50)** - the primary IPsec protocol. Provides encryption (confidentiality), authentication (integrity), and replay attack protection.

**SPI (Security Parameter Index):** A 32-bit identifier linking a packet to a specific SA (Security Association). The receiver uses the SPI to look up decryption keys.

ESP encryption algorithms:

**Replay Protection:** Sequence Number increments with each packet. The receiver maintains a sliding window (~32-64 packets). Packets with already-seen or too-old sequence numbers are dropped.

AH - Authentication Header

**AH (Protocol 51)** provides only authentication and integrity - **no encryption**. Also protects immutable fields in the IP header. Almost never used today.

**Why AH died:** NAT changes IP addresses, which breaks AH's ICV. ESP with NAT-T (UDP encapsulation) passes through NAT. ESP also provides encryption, which AH lacks.

AH's only advantage - protecting the outer IP header. But this is rarely needed:

IKE - Internet Key Exchange

**IKE** - the parameter negotiation and key exchange protocol for IPsec. Runs over UDP port 500 (or 4500 with NAT-T). Two versions: **IKEv1** (complex) and **IKEv2** (recommended).

**IKEv1 vs IKEv2:** IKEv1 required 6-9 messages (Main Mode + Quick Mode). IKEv2 - only 4. Plus IKEv2 has built-in NAT-T, MOBIKE, and better cryptography.

IKE authentication methods:

Security Associations (SA)

**Security Association (SA)** - a contract between two devices: which algorithms to use, which keys, which traffic to protect. An SA is identified by its **SPI** (Security Parameter Index).

**SAs always come in pairs:** Bidirectional traffic requires two SAs: one for outbound, one for inbound. Each with its own SPI and keys.

**SAD (Security Association Database)** and **SPD (Security Policy Database)**:

Key Ideas

• **Transport Mode** protects payload, **Tunnel Mode** encapsulates the entire packet. VPN typically uses Tunnel
• **ESP** (Protocol 50) - encryption + authentication. AES-GCM recommended. **AH** (Protocol 51) - auth only, almost never used
• **IKEv2** negotiates parameters in 4 messages (2 RTT). Supports PSK, certificates, EAP
• **SA** - a contract with keys and parameters. Always in pairs (inbound/outbound). Stored in SAD, policies in SPD

Related Topics

IPsec is the foundation of many VPN solutions:

• VPN Concepts — IPsec implements Site-to-Site and Remote Access VPN at Layer 3
• WireGuard — Modern alternative to IPsec with simplified cryptography

Вопросы для размышления

• Why did ESP win over AH, despite AH protecting more (including the IP header)?
• How does Perfect Forward Secrecy (PFS) affect the security of long-lived VPNs?
• Why do cloud providers use IPsec instead of WireGuard for managed VPN?

Связанные уроки

• crypto-32-tls