GEP-2643: TLSRoute

[rikatz]

What type of PR is this?
/kind gep

What this PR does / why we need it:
Adds the TLSRoute GEP, which is a document aggregating all of the existing TLSRoute implementations and also adding some disambiguation discussions

Which issue(s) this PR fixes:
Fixes #2643

Does this PR introduce a user-facing change?:

TLSRoute gep creation

This GEP is targeting v1.5

[rikatz]

I will add some comments that needs some clarification before we decide to merge this, the comments were discussed between me, @candita and @Miciah and we realized that they need to be clarified with the community

[rikatz]

Suggested change

	// TLSRoute hostnames that do not match the Listener hostname MUST be
	// TLSRoute hostnames that do not match any Listener hostname MUST be

This is on the current API, so would need to be fixed there as well

[rostislavbobo]

Why "any"? A Listener has only one hostname, hence "the hostname"

[rikatz]

because you can be attaching to a Gateway that has multiple Listeners, and not specifying a sectionName on the parentRef will make the route try to attach to any Listener, so any Listener hostname

[rikatz]

cc @rostislavbobo so we can discuss a bit more about it :)

[rikatz]

This part needs some discussion/attention:

• Is Multiplexing support a core feature, or a implementation specific feature?
• Previously we state that a Gateway with a TLS termination can only have TLSRoutes, but here we say that multiple listeners on the same port, for different types can be accepted. So the conditions and conflict management should be changed to reflect this (and the conformance tests), if we agree that this case is possible
• What conditions should an implementation add when this is not supported? We need to word it explicitly on the GEP and the expected conformance tests

[rostislavbobo]

1. Let's keep it implementation-specific.
   • It's a niche feature, so I don't expect many implementations to support it or have a need for it.
2. Let's move protocol multiplexing into a separate GEP.
   • It goes beyond TLSRoute scope and spans multiple protocols.

[rikatz]

I think we can add initially the multiplexing as implementation specific as is.

I did a test with Istio and was able to check that it works OOTB, I expect that at least any other envoy, haproxy and nginx implementation can work with it.

[mikemorris]

I think multiplexing should likely be extended conformance.

The current docs at https://gateway-api.sigs.k8s.io/reference/spec/#gatewayspec allude to it being permissible and definitely not required as it may be difficult for some implementations, but I think we can and should have conformance tests for this to ensure the behavior is deterministic and predictable.

This probably does merit its' own GEP though, and should likely pull context from https://github.com/kubernetes/enhancements/tree/master/keps/sig-network/1435-mixed-protocol-lb too.

[rikatz]

From a discussion that we had: is this for any listener, for the one added later? If there is a conflict listener on the TLSRoute case, do we want to mark all of Listeners as conflicted? Should we stop serving in this case, and mark Accepted=false?

Per @Miciah comment:

The GatewaySpec godoc is explicit: 'If a set of Listeners contains Listeners that are not distinct, then those Listeners are Conflicted, and the implementation MUST set the "Conflicted" condition in the Listener Status to "True"', and, "The implementation MUST NOT pick one conflicting Listener as the winner."

The godoc for ListenerConditionOverlappingTLSConfig re-iterates: "This condition MUST be set on all Listeners with overlapping TLS config."

• what if we have a conflict? Do we really want all of the listeners to be gone?
• what if we are using ListenerSet?

[rikatz]

I have changed here for now as:

* When a Gateway does not support [Multiplexing](#multiplexing-support) and contains 
a listener with `protocol=TLS`, the Gateway MUST NOT allow any other kind of 
listener on the same port, and any violating Listener should have a Condition `OverlappingTLSConfig=True`
with the reason `OverlappingProtocols`.

This is a new condition that we should be adding

[mikemorris]

Hmm, the suggestion to mark all listeners as conflicted feels at odds with the typical conflict resolution guidance in https://gateway-api.sigs.k8s.io/guides/api-design/#conflicts, but I think maybe we aren't able to follow that because listener config is all batched together as an atomic update to the Gateway (and trying to be "stateful" rather than reflecting the current YAML is an anti-pattern)?

(I think the most granular breakdown achievable might be one entire ListenerSet attached to a Gateway becomes conflicted, but other ListenerSets attached to the same Gateway remain functional.)

[rikatz]

This is what exists on the current API, but IMO the hostnames should be part of a TLSRouteRule and not its own field on the spec.

It doesn't make much sense that the rules are an array, that contain an array of backendRefs, but the hostnames are outside of it, but maybe there's some more context here.

Maybe it should be something like:

rules:
- hostnames: 
  - abc.com
  - def.com
  backendRefs:
  - name: tls-backend
    port: 443

As hostnames is a filter that will direct for the backendRefs, and we don't expect soon to have any additional filter for TLSRoutes

[kl52752]

If there are no filters how the backend from a list of BackendRefs should be chosen? Should we require the weight field to be uniquely set for the backend here?

[rostislavbobo]

It doesn't make much sense that the rules are an array, that contain an array of backendRefs

There are two reasons for that.

1. TLSRouteRule at some point might have TLSRouteMatch with ALPN match

kind: TLSRoute
spec:
  hostnames:
  - "example.com"
  rules:
  - matches:
    - alpn:
      - h2
    backendRefs:
    - name: example-backend
      port: 443

2. BackendRefs has weight, which we're not supporting with TLSRouteRule yet

kind: TLSRoute
spec:
  hostnames:
  - "example.com"
  rules:
  - backendRefs:
    - name: example-backend-1
      port: 443
      weight: 0.5
    - name: example-backend-2
      port: 443
      weight: 0.5

[rostislavbobo]

IMO the hostnames should be part of a TLSRouteRule and not its own field on the spec.

@howardjohn and @hbagdi , what was the motivation for #682 moving hostnames out of TLSRouteRule (besides aligning with HTTPRoute)? TLSRoute doesn't have many matching options compared to HTTPRoute.

So now, instead of having a single TLSRoute that fans out to multiple backends

kind: TLSRoute
spec:
  rules:
  - matches:
    - hostnames:
      - "example.com"
    backendRefs:
    - name: example-backend
      port: 443
  - matches:
    - hostnames:
      - "*.com"
    backendRefs:
    - name: fallback-backend
      port: 443

Everyone now needs to set up multiple TLSRoutes for a single backend:

kind: TLSRoute
spec:
  hostnames:
  - "example.com"
  rules:
    backendRefs:
    - name: example-backend
      port: 443

kind: TLSRoute
spec:
  hostnames:
  - "*.com"
  rules:
    backendRefs:
    - name: fallback-backend
      port: 443

[rikatz]

Based on https://kubernetes.slack.com/archives/CR0H13KGA/p1756935423971129

The GEP should contain that:

• As of today/GA just a single backendRef is supported on TLSRoute
• Eventually we will support other matchers like ALPN and this may change in the future

[kl52752]

I think it would be good to precise that Listener's protocol needs to be of type: TLS and that TLSRoutes only applies to Listeners with this protocol. Is this said somewhere in the doc/spec?

[rostislavbobo]

Yes, we wanted to clearly cover the Listener protocol and XRoutes compatibilities.

[rikatz]

agreed. As a heads up, the API here was copied exactly from the existing code, so the idea is that we also make the proper updates on the API based on the comments here.

That said, we do not explicitly say anywhere here that TLSRoute is attacheable to Listeners of type TLS and Passthrough, just on places like https://gateway-api.sigs.k8s.io/guides/tls/

I will make this explicit on this doc

[rikatz]

To be added: #3541

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[candita]

/assign

[rostislavbobo]

why not "v1alpha3" everywhere?

[rikatz]

probably my mistake, as I was checking old code. Changing here

[rostislavbobo]

Let's add a bullet point for Gateway not supporting mixed protocol termination. This is a niche capability, similar to protocol multiplexing, that most implementations won't need.

[rikatz]

you mean two listeners, one terminating and other not? I am adding this, but let me know if I misunderstood it

[rikatz]

A side note to myself, I want to open 2 more GEPs thjat will complement this:

• Extend BackendTLSPolicy to be used by TLSRoute (when on Terminate mode)
• Add support for multiplexing (probably a memorandum GEP) explaining how the listeners should behave on each case (I guess we have some, will need to check with Nick)

[k8s-ci-robot]

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Test name	Commit	Details	Required	Rerun command
pull-gateway-api-verify	1eea2ab	link	true	/test pull-gateway-api-verify

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