Session Resumption

Introduction

RFC 5723 defines an extension to IKEv2 (Internet Key Exchange v2) that allows a client to re-establish an IKE Security Association with a gateway in a highly efficient manner, utilizing a previously established IKE SA.

Resumption tickets solves the problem of re-establishing (many) IKE SAs between IPSEC peers after a network failure at the end of a VPN gateway which requires a lot of computational overhead, mostly in the Diffie-Hellman claculations. The Session Resumption avoids these expensive calculations.

Implementation

To allow the use of Session Resumption in IKEv2, the following main changes were made:

• Two new source code files were created: programs/pluto/ikev2_resume.{h,c} which defines the ticket by value structure and other helper functions for handling the session ticket.
• Added code in programs/pluto/ikev2_parent.c which deals with sending and processing of IKEv2 TICKET related Notify payloads.
• New state transitions were added in programs/pluto/ikev2.c to support the session Resumption Exchange.
• A new connection option session-resumption=yes was added that allows or disallows the session resumption.
• A new whack command was introduced which suspends a connection and delete states.

ipsec whack --suspend --name <connection name>

• The process of bringing a connection up was modified to check for a received resumption ticket. If available, it will automatically try Session Resumption.
• A new test case was added to test initiator and responder code for Session Resumption.

Issues encountered

• The IKE_AUTH exchange following an IKE_SESSION_RESUMPTION exchange is slightly different from the regular IKE_AUTH exchange. A new kind of KDF was created as described by the RFC. Libreswan can be compiled using native KDF's or KDF functions from the NSS cryptographic library. Code was added for native KDF support, and the NSS developers were contacted about modifying the NSS crypto library to add this new Resumption KDF.

• As the ticket does not have a fixed length and structure, it complicated receiving and parsing the ticket back on the responder side. After some discussion with other libreswan developers, this issue was resolved.

• The RFC did not address the issue of what happens with Delete/Notify messages are sent. The document claims that tickets should not be re-used after the connection is deleted by either server or client, but is not clear that it should exclude connection deletions triggered by liveness probes. Since a network outage would otherwise cause the server to delete its resumption tickets during a brief outage, and this is exactly the scenario where a server would benefit from using resumption if lots of clients connect to the server at the same time when the network outage is resolved. This has been discussed at the ietf IPsec Working Group and might result in an ERRATA to be issued for the RFC.

Current issues

• Code needs to be added to support connection instances, which typically happen for IKEv2 Exchanges with CP payloads (eg IKEv2 remote access clients). Once this support is added, interop testing can be done with ELVIS+
• The ikev2-resumption-01 test case is not completed, pending completion of the IKE_AUTH exchange modifications.
• The suspend command still needs to be modified to ensure it does not send Delete/Notify messages.

Future Work

• For ticket by value resumption tickets, the ticket must be encrypted by the responder before sending it to initiator in order to protect the keys and other sensitive data present inside it. The responder would will need to keep two encryption keys covering the ticket lifetimes and needs to regenerate these encryption keys (for example every 8h). By using an AEAD like AES_GCM, the encryption is also authenticated.

• Perform interop testing with ELVIS+

Source Code

Main Code commit : https://github.com/murex971/libreswan/commit/4b0bdd0c0f019059fb8402079a8c3f76233620da

Test commit : https://github.com/murex971/libreswan/commit/24b5ddb92881ebf50c9cf340f2328ce984c99308

A few issues are still being addressed, so more commits will appear in https://github.com/murex971/libreswan

The implementation of this feature is done by Nupur Agrawal (nupur202000@gmail.com) under the mentorship of Paul Wouters, Sahana Prasad and Tuomo Soini and sponsored by Google as part of Google Summer of Code 2020 Program. The code is not merged yet as it is a POC (proof of concept).