Re: [IPsec] [furry13/ipsecme-esp-ping] Abandoning non-reserved SPIs (PR #6)

Michael Richardson <> Tue, 27 February 2024 21:12 UTC

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From: Michael Richardson <>
To: Jen Linkova <>
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Date: Tue, 27 Feb 2024 16:12:28 -0500
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Subject: Re: [IPsec] [furry13/ipsecme-esp-ping] Abandoning non-reserved SPIs (PR #6)
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Jen Linkova <> wrote:
    > On Wed, Feb 28, 2024 at 7:12 AM Michael Richardson
    > <> wrote:
    >> In github issue:
    >> I said: >I am not in favour of any link to IKE.
    >> I don't think it's useful to signal in IKE that the host/gateway
    >> supports SPI=7.  I believe in many debug situations, the person doing
    >> the diagnostics won't have a credential that they can use in IKE, and
    >> the person that has that credential (it could be a physical token)
    >> won't know how to do the debugging.
    >> Consider a scenario where some branch office regularly has an external
    >> auditor physically present.  Said auditor expects to operate their
    >> Remote Access VPN from their laptop, for which they got permission
    >> some years before.  Central IT turns on IPv6, and everything is great,
    >> but they didn't think to enable ESP (because NAT44 rules, right?).
    >> Assume auditor's VPN hub supports IPv6.

    > [skip]

    > In your scenario announcing ESP Ping capability in IKE is useless but
    > harmless.

I mostly agree.

    > You seem to assume that ESP ping is only used by humans
    > troubleshooting the network, but think about other case: a device (a
    > phone, for example) is going to establish an IPsec session to a VPN
    > server or, even better, a voice gateway (as WiFi Calling requires
    > IPSec).


    > Currently, if IKE succeeds but ESP fails, it's very hard for
    > the device to detect the issue.  If the peer supports ESP ping and
    > announces it in IKE, the device can use ESP ping to find out
    > that..ooops, the corporate WiFi the device is connected to is having
    > issues with end2end ESP connectivity, and switch back to the mobile
    > network, instead of blackholing the data packets.

If there is an IKE SA that works and is up, then we can easily negotiate the right TS to enable
an appropriate ESP-encrypted ICMPv6 (echo request) PING to a target system.  This is not
spoofable or even detectable by an adversary.

I would encourage someone to write a really short I-D that defines an IKEv2
NOTIFY that contained an IP address at which a gateway is willing to
entertain ICMPv6 Echo Requests to.   That would fit into the defined TSr.
In many many environments, that would be the IPv6 address of the inside interface
of the gateway.
But, in some esoteric situations, it would need to be some other address.
In such an I-D, we could argue whether or not the address has to fit into TSi/TSr.
(There are arguments both ways)

That wouldn't be an SPI=7 thing though, so I'd want it another document.

    > So I do not see any harm in advertising it in IKE but I do see some
    > benefits - but I clearly might be missing smth here.

Michael Richardson <>   . o O ( IPv6 IøT consulting )
           Sandelman Software Works Inc, Ottawa and Worldwide