IP protocol 50 is Encapsulation Security Payload (ESP), and IP protocol 51 is Authentication Header (AH).
Understanding IP Protocols 50 and 51
These numbers are assigned by the Internet Assigned Numbers Authority (IANA) to uniquely identify the next-level protocol encapsulated within the IP payload. Unlike widely known transport protocols like TCP and UDP, protocols 50 and 51 operate differently within the network stack, primarily serving security functions.
IP Protocol 50: Encapsulation Security Payload (ESP)
According to the provided reference from Ruijie Community, IP Protocol 50 corresponds to ESP (Encapsulation Security Payload).
Key aspects of ESP:
- As stated in the reference, it is not a TCP and UDP based protocol. Instead, it is an IP-layer protocol directly following the IP header.
- ESP is a fundamental component of the IPsec (IP Security) suite.
- Its primary functions include providing data confidentiality (encryption), data origin authentication, connectionless data integrity, and anti-replay protection for IP packets.
- The reference specifically highlights that "IP protocol ID 50 should be set to allow IPSec Encapsulating Security Protocol (ESP) traffic to be forwarded," indicating that firewalls or routers must be configured to permit this protocol number for IPsec tunnels utilizing ESP to function correctly.
IP Protocol 51: Authentication Header (AH)
The reference also identifies IP Protocol 51 as AH (Authentication Header).
Key aspects of AH:
- Similar to ESP, the reference notes that AH is not a TCP and UDP based protocol.
- AH is another core protocol within the IPsec suite.
- Its main purpose is to provide data origin authentication, connectionless data integrity, and anti-replay protection.
- Crucially, unlike ESP, AH does not provide confidentiality (encryption) of the data payload itself. It authenticates the data and certain parts of the IP header.
Role in IPsec
Both ESP and AH are vital for implementing secure communications using IPsec, commonly found in VPN (Virtual Private Network) solutions.
- When setting up IPsec VPNs or secure tunnels, network devices need to identify packets belonging to these secure associations. The IP protocol number in the packet header (50 for ESP, 51 for AH) signals how the rest of the packet should be interpreted and processed for security.
- Proper configuration of network devices (like routers or firewalls) to recognize and allow traffic with IP protocol IDs 50 and 51 is essential for enabling IPsec connectivity, as emphasized by the note about forwarding ESP traffic (Protocol 50) in the reference.
Summary
Here is a concise overview of these two protocols:
| IP Protocol ID | Name | Primary IPsec Functions | Not TCP/UDP Based? (Ref.) | Provides Encryption? | Provides Authentication/Integrity? |
|---|---|---|---|---|---|
| 50 | ESP (Encapsulation Security Payload) | Authentication, Integrity, Encryption, Anti-Replay | Yes | Yes | Yes |
| 51 | AH (Authentication Header) | Authentication, Integrity, Anti-Replay (No Encryption) | Yes | No | Yes |
Reference: L2TP Passthrough & IP Protocol 50 & 51 - Router - Ruijie Community
These protocols enable the secure exchange of data over potentially insecure networks by providing authentication, integrity, and optionally, confidentiality.
