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+# CFP-47089: Gateway API Session Persistence
+
+## Meta
+
+SIG: SIG-ServiceMesh
+Sharing: Public
+Begin Design Discussion: July 10, 2026
+End Design Discussion: TBD
+Cilium Release: 1.20
+Authors: tim <thorn3r@cisco.com>
+
+## Summary
+
+This proposal extends Cilium's Gateway API implementation to support session
+persistence via the Route Rule API as defined in
+[GEP-1619](https://gateway-api.sigs.k8s.io/geps/gep-1619/#achieving-session-persistence).
+Session persistence is part of Gateway API's experimental release channel and
+is subject to change. However, the scope of this proposal is deliberately narrow
+and focuses on delivering the core features required by Gateway API that are
+widely adopted by other implementations. Cilium will support cookie-based
+persistence on `HTTPRoute` and `GRPCRoute` rules with session-based lifetime.
+
+## Motivation
+
+Some applications require requests in a client session to continue reaching the
+same backend endpoint. Gateway API exposes this through `sessionPersistence` on
+route rules. Cilium already translates Gateway API routes into an internal HTTP
+model and Envoy xDS resources, but currently ignores route-rule session
+persistence.
+
+## Goals
+
+- Support `sessionPersistence` on `HTTPRoute` and `GRPCRoute` rules.
+- Support cookie-based persistence.
+- Support session-based lifetime for cookies.
+- Reject unsupported session persistence fields with route status.
+
+## Non-Goals
+
+- `BackendTrafficPolicy` session persistence.
+- Header-based persistence.
+- Permanent cookies or `absoluteTimeout`.
+- Configurable cookie security attributes.
+- Signed or encrypted upstream-host cookies.
+
+## Proposal
+
+### Supported API Surface
+
+The following fields are supported on `HTTPRoute` and
+`GRPCRoute` rules:
+
+```yaml
+sessionPersistence:
+  type: Cookie
+  sessionName: gateway-session
+  cookieConfig:
+    lifetimeType: Session
+```
+
+Unsupported fields:
+
+- `sessionPersistence.absoluteTimeout`
+
+Unsupported values:
+
+- `sessionPersistence.type: Header`
+- `sessionPersistence.cookieConfig.lifetimeType: Permanent`
+
+Routes using unsupported fields or values receive `Accepted=False` with reason
+`UnsupportedValue`.
+
+### Rendering and Status
+
+Cilium will translate supported route-rule persistence into
+`envoy.filters.http.stateful_session` with per-route
+`envoy.http.stateful_session.cookie` configuration. Since the only supported
+lifetime type is `Session`, Cilium will set the Envoy cookie `ttl` to `0s`,
+causing Envoy to generate a session cookie rather than a persistent cookie.
+Cilium will set `Secure`, `HttpOnly`, and `SameSite=Strict`, as suggested by
+upstream documentation.
+
+The stateful session configuration will use non-strict behavior. If the endpoint
+encoded in the cookie is unavailable, Envoy will fall back to normal load
+balancing instead of failing the request.
+
+Because Envoy HTTP filters are configured at listener scope, Cilium will
+explicitly disable the stateful-session filter on routes that do not use session
+persistence, including redirect and direct-response routes that do not select an
+upstream backend.
+
+During ingestion, session persistence fields are validated and stored in the
+internal model. If the route contains unsupported fields or values, the route
+status will be updated to indicate that it was rejected. Session persistence
+settings are included in the route match key so routes with the same match but
+different persistence settings are not merged. Cilium derives the cookie path
+from exact or prefix path matches, falling back to "/" if the path can't be
+determined.
+
+```
+type HTTPSessionPersistence struct {
+ Cookie *HTTPCookieSessionPersistence `json:"cookie,omitempty"`
+}
+
+type HTTPCookieSessionPersistence struct {
+ Name     string `json:"name,omitempty"`
+ Path     string `json:"path,omitempty"`
+ Secure   bool   `json:"secure,omitempty"`
+ HTTPOnly bool   `json:"httpOnly,omitempty"`
+ SameSite string `json:"sameSite,omitempty"`
+}
+```
+
+## Key Questions
+
+### Key question #1: Should Cilium support policy attachment?
+
+Policy attachment through the `BackendTrafficPolicy` API introduces additional
+complexity in precedence and conflict handling and does not seem widely
+supported in other Gateway API implementations.
+
+Recommendation: Support route rules only for this phase of development.
+
+### Key question #2: Should Cilium make the upstream host cookie tamper-proof?
+
+Envoy's session state stores the upstream host address in the cookie. Making the
+cookie tamper-proof would likely require upstream work in Envoy to support
+signing or encryption, alongside key management. This is out of scope for the
+current proposal and can be revisited as a follow-up.
+
+## Future Milestones
+
+- Support header persistence.
+- Support permanent cookies and `absoluteTimeout`.
+- Support configurable cookie attributes if the secure defaults are too
+  restrictive for real deployments.
+- Evaluate signed or encrypted upstream-host cookies.