diff --git a/spec/unit/matrixrtc/CallMembership.spec.ts b/spec/unit/matrixrtc/CallMembership.spec.ts
index cfa98ebd2bf..95c7140b131 100644
--- a/spec/unit/matrixrtc/CallMembership.spec.ts
+++ b/spec/unit/matrixrtc/CallMembership.spec.ts
@@ -19,6 +19,7 @@ import {
     CallMembership,
     type SessionMembershipData,
     DEFAULT_EXPIRE_DURATION,
+    type RtcMembershipData,
 } from "../../../src/matrixrtc/CallMembership";
 import { membershipTemplate } from "./mocks";
 
@@ -26,6 +27,7 @@ function makeMockEvent(originTs = 0): MatrixEvent {
     return {
         getTs: jest.fn().mockReturnValue(originTs),
         getSender: jest.fn().mockReturnValue("@alice:example.org"),
+        getId: jest.fn().mockReturnValue("$eventid"),
     } as unknown as MatrixEvent;
 }
 
@@ -40,12 +42,13 @@ describe("CallMembership", () => {
         });
 
         const membershipTemplate: SessionMembershipData = {
-            call_id: "",
-            scope: "m.room",
-            application: "m.call",
-            device_id: "AAAAAAA",
-            focus_active: { type: "livekit" },
-            foci_preferred: [{ type: "livekit" }],
+            "call_id": "",
+            "scope": "m.room",
+            "application": "m.call",
+            "device_id": "AAAAAAA",
+            "focus_active": { type: "livekit", focus_selection: "oldest_membership" },
+            "foci_preferred": [{ type: "livekit" }],
+            "m.call.intent": "voice",
         };
 
         it("rejects membership with no device_id", () => {
@@ -94,11 +97,271 @@ describe("CallMembership", () => {
         it("returns preferred foci", () => {
             const fakeEvent = makeMockEvent();
             const mockFocus = { type: "this_is_a_mock_focus" };
-            const membership = new CallMembership(
-                fakeEvent,
-                Object.assign({}, membershipTemplate, { foci_preferred: [mockFocus] }),
-            );
-            expect(membership.getPreferredFoci()).toEqual([mockFocus]);
+            const membership = new CallMembership(fakeEvent, { ...membershipTemplate, foci_preferred: [mockFocus] });
+            expect(membership.transports).toEqual([mockFocus]);
+        });
+
+        describe("getTransport", () => {
+            const mockFocus = { type: "this_is_a_mock_focus" };
+            const oldestMembership = new CallMembership(makeMockEvent(), membershipTemplate);
+            it("gets the correct active transport with oldest_membership", () => {
+                const membership = new CallMembership(makeMockEvent(), {
+                    ...membershipTemplate,
+                    foci_preferred: [mockFocus],
+                    focus_active: { type: "livekit", focus_selection: "oldest_membership" },
+                });
+
+                // if we are the oldest member we use our focus.
+                expect(membership.getTransport(membership)).toStrictEqual(mockFocus);
+
+                // If there is an older member we use its focus.
+                expect(membership.getTransport(oldestMembership)).toBe(membershipTemplate.foci_preferred[0]);
+            });
+
+            it("gets the correct active transport with multi_sfu", () => {
+                const membership = new CallMembership(makeMockEvent(), {
+                    ...membershipTemplate,
+                    foci_preferred: [mockFocus],
+                    focus_active: { type: "livekit", focus_selection: "multi_sfu" },
+                });
+
+                // if we are the oldest member we use our focus.
+                expect(membership.getTransport(membership)).toStrictEqual(mockFocus);
+
+                // If there is an older member we still use our own focus in multi sfu.
+                expect(membership.getTransport(oldestMembership)).toBe(mockFocus);
+            });
+            it("does not provide focus if the selection method is unknown", () => {
+                const membership = new CallMembership(makeMockEvent(), {
+                    ...membershipTemplate,
+                    foci_preferred: [mockFocus],
+                    focus_active: { type: "livekit", focus_selection: "unknown" },
+                });
+
+                // if we are the oldest member we use our focus.
+                expect(membership.getTransport(membership)).toBeUndefined();
+            });
+        });
+        describe("correct values from computed fields", () => {
+            const membership = new CallMembership(makeMockEvent(), membershipTemplate);
+            it("returns correct sender", () => {
+                expect(membership.sender).toBe("@alice:example.org");
+            });
+            it("returns correct eventId", () => {
+                expect(membership.eventId).toBe("$eventid");
+            });
+            it("returns correct slot_id", () => {
+                expect(membership.slotId).toBe("m.call#");
+                expect(membership.slotDescription).toStrictEqual({ id: "", application: "m.call" });
+            });
+            it("returns correct deviceId", () => {
+                expect(membership.deviceId).toBe("AAAAAAA");
+            });
+            it("returns correct call intent", () => {
+                expect(membership.callIntent).toBe("voice");
+            });
+            it("returns correct application", () => {
+                expect(membership.application).toStrictEqual("m.call");
+            });
+            it("returns correct applicationData", () => {
+                expect(membership.applicationData).toStrictEqual({ "type": "m.call", "m.call.intent": "voice" });
+            });
+            it("returns correct scope", () => {
+                expect(membership.scope).toBe("m.room");
+            });
+            it("returns correct membershipID", () => {
+                expect(membership.membershipID).toBe("0");
+            });
+            it("returns correct unused fields", () => {
+                expect(membership.getAbsoluteExpiry()).toBe(DEFAULT_EXPIRE_DURATION);
+                expect(membership.getMsUntilExpiry()).toBe(DEFAULT_EXPIRE_DURATION - Date.now());
+                expect(membership.isExpired()).toBe(true);
+            });
+        });
+    });
+
+    describe("RtcMembershipData", () => {
+        const membershipTemplate: RtcMembershipData = {
+            slot_id: "m.call#",
+            application: { "type": "m.call", "m.call.id": "", "m.call.intent": "voice" },
+            member: { user_id: "@alice:example.org", device_id: "AAAAAAA", id: "xyzHASHxyz" },
+            rtc_transports: [{ type: "livekit" }],
+            versions: [],
+            msc4354_sticky_key: "abc123",
+        };
+
+        it("rejects membership with no slot_id", () => {
+            expect(() => {
+                new CallMembership(makeMockEvent(), { ...membershipTemplate, slot_id: undefined });
+            }).toThrow();
+        });
+        it("rejects membership with invalid slot_id", () => {
+            expect(() => {
+                new CallMembership(makeMockEvent(), { ...membershipTemplate, slot_id: "invalid_slot_id" });
+            }).toThrow();
+        });
+        it("accepts membership with valid slot_id", () => {
+            expect(() => {
+                new CallMembership(makeMockEvent(), { ...membershipTemplate, slot_id: "m.call#" });
+            }).not.toThrow();
+        });
+
+        it("rejects membership with no application", () => {
+            expect(() => {
+                new CallMembership(makeMockEvent(), { ...membershipTemplate, application: undefined });
+            }).toThrow();
+        });
+
+        it("rejects membership with incorrect application", () => {
+            expect(() => {
+                new CallMembership(makeMockEvent(), {
+                    ...membershipTemplate,
+                    application: { wrong_type_key: "unknown" },
+                });
+            }).toThrow();
+        });
+
+        it("rejects membership with no member", () => {
+            expect(() => {
+                new CallMembership(makeMockEvent(), { ...membershipTemplate, member: undefined });
+            }).toThrow();
+        });
+
+        it("rejects membership with incorrect  member", () => {
+            expect(() => {
+                new CallMembership(makeMockEvent(), { ...membershipTemplate, member: { i: "test" } });
+            }).toThrow();
+            expect(() => {
+                new CallMembership(makeMockEvent(), {
+                    ...membershipTemplate,
+                    member: { id: "test", device_id: "test", user_id_wrong: "test" },
+                });
+            }).toThrow();
+            expect(() => {
+                new CallMembership(makeMockEvent(), {
+                    ...membershipTemplate,
+                    member: { id: "test", device_id_wrong: "test", user_id_wrong: "test" },
+                });
+            }).toThrow();
+            expect(() => {
+                new CallMembership(makeMockEvent(), {
+                    ...membershipTemplate,
+                    member: { id: "test", device_id: "test", user_id: "@@test" },
+                });
+            }).toThrow();
+            expect(() => {
+                new CallMembership(makeMockEvent(), {
+                    ...membershipTemplate,
+                    member: { id: "test", device_id: "test", user_id: "@test-wrong-user:user.id" },
+                });
+            }).toThrow();
+        });
+        it("rejects membership with incorrect sticky_key", () => {
+            expect(() => {
+                new CallMembership(makeMockEvent(), membershipTemplate);
+            }).not.toThrow();
+            expect(() => {
+                new CallMembership(makeMockEvent(), {
+                    ...membershipTemplate,
+                    sticky_key: 1,
+                    msc4354_sticky_key: undefined,
+                });
+            }).toThrow();
+            expect(() => {
+                new CallMembership(makeMockEvent(), {
+                    ...membershipTemplate,
+                    sticky_key: "1",
+                    msc4354_sticky_key: undefined,
+                });
+            }).not.toThrow();
+            expect(() => {
+                new CallMembership(makeMockEvent(), { ...membershipTemplate, msc4354_sticky_key: undefined });
+            }).toThrow();
+            expect(() => {
+                new CallMembership(makeMockEvent(), {
+                    ...membershipTemplate,
+                    msc4354_sticky_key: 1,
+                    sticky_key: "valid",
+                });
+            }).toThrow();
+            expect(() => {
+                new CallMembership(makeMockEvent(), {
+                    ...membershipTemplate,
+                    msc4354_sticky_key: "valid",
+                    sticky_key: "valid",
+                });
+            }).not.toThrow();
+            expect(() => {
+                new CallMembership(makeMockEvent(), {
+                    ...membershipTemplate,
+                    msc4354_sticky_key: "valid_but_different",
+                    sticky_key: "valid",
+                });
+            }).toThrow();
+        });
+
+        it("considers memberships unexpired if local age low enough", () => {
+            // TODO link prev event
+        });
+
+        it("considers memberships expired if local age large enough", () => {
+            // TODO link prev event
+        });
+
+        describe("getTransport", () => {
+            it("gets the correct active transport with oldest_membership", () => {
+                const oldestMembership = new CallMembership(makeMockEvent(), {
+                    ...membershipTemplate,
+                    rtc_transports: [{ type: "oldest_transport" }],
+                });
+                const membership = new CallMembership(makeMockEvent(), membershipTemplate);
+
+                // if we are the oldest member we use our focus.
+                expect(membership.getTransport(membership)).toStrictEqual({ type: "livekit" });
+
+                // If there is an older member we use our own focus focus. (RtcMembershipData always uses multi sfu)
+                expect(membership.getTransport(oldestMembership)).toStrictEqual({ type: "livekit" });
+            });
+        });
+        describe("correct values from computed fields", () => {
+            const membership = new CallMembership(makeMockEvent(), membershipTemplate);
+            it("returns correct sender", () => {
+                expect(membership.sender).toBe("@alice:example.org");
+            });
+            it("returns correct eventId", () => {
+                expect(membership.eventId).toBe("$eventid");
+            });
+            it("returns correct slot_id", () => {
+                expect(membership.slotId).toBe("m.call#");
+                expect(membership.slotDescription).toStrictEqual({ id: "", application: "m.call" });
+            });
+            it("returns correct deviceId", () => {
+                expect(membership.deviceId).toBe("AAAAAAA");
+            });
+            it("returns correct call intent", () => {
+                expect(membership.callIntent).toBe("voice");
+            });
+            it("returns correct application", () => {
+                expect(membership.application).toStrictEqual("m.call");
+            });
+            it("returns correct applicationData", () => {
+                expect(membership.applicationData).toStrictEqual({
+                    "type": "m.call",
+                    "m.call.id": "",
+                    "m.call.intent": "voice",
+                });
+            });
+            it("returns correct scope", () => {
+                expect(membership.scope).toBe(undefined);
+            });
+            it("returns correct membershipID", () => {
+                expect(membership.membershipID).toBe("xyzHASHxyz");
+            });
+            it("returns correct unused fields", () => {
+                expect(membership.getAbsoluteExpiry()).toBe(undefined);
+                expect(membership.getMsUntilExpiry()).toBe(undefined);
+                expect(membership.isExpired()).toBe(false);
+            });
         });
     });
 
diff --git a/spec/unit/matrixrtc/LivekitFocus.spec.ts b/spec/unit/matrixrtc/LivekitTransport.spec.ts
similarity index 53%
rename from spec/unit/matrixrtc/LivekitFocus.spec.ts
rename to spec/unit/matrixrtc/LivekitTransport.spec.ts
index 728d6a68de6..04f04a1357e 100644
--- a/spec/unit/matrixrtc/LivekitFocus.spec.ts
+++ b/spec/unit/matrixrtc/LivekitTransport.spec.ts
@@ -14,47 +14,51 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import { isLivekitFocus, isLivekitFocusActive, isLivekitFocusConfig } from "../../../src/matrixrtc/LivekitFocus";
+import {
+    isLivekitTransport,
+    isLivekitFocusSelection,
+    isLivekitTransportConfig,
+} from "../../../src/matrixrtc/LivekitTransport";
 
 describe("LivekitFocus", () => {
     it("isLivekitFocus", () => {
         expect(
-            isLivekitFocus({
+            isLivekitTransport({
                 type: "livekit",
                 livekit_service_url: "http://test.com",
                 livekit_alias: "test",
             }),
         ).toBeTruthy();
-        expect(isLivekitFocus({ type: "livekit" })).toBeFalsy();
+        expect(isLivekitTransport({ type: "livekit" })).toBeFalsy();
         expect(
-            isLivekitFocus({ type: "not-livekit", livekit_service_url: "http://test.com", livekit_alias: "test" }),
+            isLivekitTransport({ type: "not-livekit", livekit_service_url: "http://test.com", livekit_alias: "test" }),
         ).toBeFalsy();
         expect(
-            isLivekitFocus({ type: "livekit", other_service_url: "http://test.com", livekit_alias: "test" }),
+            isLivekitTransport({ type: "livekit", other_service_url: "http://test.com", livekit_alias: "test" }),
         ).toBeFalsy();
         expect(
-            isLivekitFocus({ type: "livekit", livekit_service_url: "http://test.com", other_alias: "test" }),
+            isLivekitTransport({ type: "livekit", livekit_service_url: "http://test.com", other_alias: "test" }),
         ).toBeFalsy();
     });
     it("isLivekitFocusActive", () => {
         expect(
-            isLivekitFocusActive({
+            isLivekitFocusSelection({
                 type: "livekit",
                 focus_selection: "oldest_membership",
             }),
         ).toBeTruthy();
-        expect(isLivekitFocusActive({ type: "livekit" })).toBeFalsy();
-        expect(isLivekitFocusActive({ type: "not-livekit", focus_selection: "oldest_membership" })).toBeFalsy();
+        expect(isLivekitFocusSelection({ type: "livekit" })).toBeFalsy();
+        expect(isLivekitFocusSelection({ type: "not-livekit", focus_selection: "oldest_membership" })).toBeFalsy();
     });
     it("isLivekitFocusConfig", () => {
         expect(
-            isLivekitFocusConfig({
+            isLivekitTransportConfig({
                 type: "livekit",
                 livekit_service_url: "http://test.com",
             }),
         ).toBeTruthy();
-        expect(isLivekitFocusConfig({ type: "livekit" })).toBeFalsy();
-        expect(isLivekitFocusConfig({ type: "not-livekit", livekit_service_url: "http://test.com" })).toBeFalsy();
-        expect(isLivekitFocusConfig({ type: "livekit", other_service_url: "oldest_membership" })).toBeFalsy();
+        expect(isLivekitTransportConfig({ type: "livekit" })).toBeFalsy();
+        expect(isLivekitTransportConfig({ type: "not-livekit", livekit_service_url: "http://test.com" })).toBeFalsy();
+        expect(isLivekitTransportConfig({ type: "livekit", other_service_url: "oldest_membership" })).toBeFalsy();
     });
 });
diff --git a/spec/unit/matrixrtc/MatrixRTCSession.spec.ts b/spec/unit/matrixrtc/MatrixRTCSession.spec.ts
index 70cbe927df0..3e394119a29 100644
--- a/spec/unit/matrixrtc/MatrixRTCSession.spec.ts
+++ b/spec/unit/matrixrtc/MatrixRTCSession.spec.ts
@@ -53,12 +53,12 @@ describe("MatrixRTCSession", () => {
 
             sess = MatrixRTCSession.sessionForRoom(client, mockRoom, callSession);
             expect(sess?.memberships.length).toEqual(1);
-            expect(sess?.memberships[0].sessionDescription.id).toEqual("");
+            expect(sess?.memberships[0].slotDescription.id).toEqual("");
             expect(sess?.memberships[0].scope).toEqual("m.room");
             expect(sess?.memberships[0].application).toEqual("m.call");
             expect(sess?.memberships[0].deviceId).toEqual("AAAAAAA");
             expect(sess?.memberships[0].isExpired()).toEqual(false);
-            expect(sess?.sessionDescription.id).toEqual("");
+            expect(sess?.slotDescription.id).toEqual("");
         });
 
         it("ignores memberships where application is not m.call", () => {
@@ -268,7 +268,6 @@ describe("MatrixRTCSession", () => {
                 type: "livekit",
                 focus_selection: "oldest_membership",
             });
-            expect(sess.getActiveFocus()).toBe(firstPreferredFocus);
             jest.useRealTimers();
         });
         it("does not provide focus if the selection method is unknown", () => {
@@ -288,7 +287,7 @@ describe("MatrixRTCSession", () => {
                 type: "livekit",
                 focus_selection: "unknown",
             });
-            expect(sess.getActiveFocus()).toBe(undefined);
+            expect(sess.memberships.length).toBe(0);
         });
     });
 
diff --git a/spec/unit/matrixrtc/MembershipManager.spec.ts b/spec/unit/matrixrtc/MembershipManager.spec.ts
index c22ab18390c..1e0e0b5e798 100644
--- a/spec/unit/matrixrtc/MembershipManager.spec.ts
+++ b/spec/unit/matrixrtc/MembershipManager.spec.ts
@@ -27,12 +27,11 @@ import {
 import {
     MembershipManagerEvent,
     Status,
-    type Focus,
-    type LivekitFocusActive,
+    type Transport,
     type SessionMembershipData,
+    type LivekitFocusSelection,
 } from "../../../src/matrixrtc";
 import { makeMockClient, makeMockRoom, membershipTemplate, mockCallMembership, type MockClient } from "./mocks";
-import { logger } from "../../../src/logger.ts";
 import { MembershipManager } from "../../../src/matrixrtc/MembershipManager.ts";
 
 /**
@@ -76,11 +75,11 @@ const callSession = { id: "", application: "m.call" };
 describe("MembershipManager", () => {
     let client: MockClient;
     let room: Room;
-    const focusActive: LivekitFocusActive = {
+    const focusActive: LivekitFocusSelection = {
         focus_selection: "oldest_membership",
         type: "livekit",
     };
-    const focus: Focus = {
+    const focus: Transport = {
         type: "livekit",
         livekit_service_url: "https://active.url",
         livekit_alias: "!active:active.url",
@@ -104,12 +103,12 @@ describe("MembershipManager", () => {
 
     describe("isActivated()", () => {
         it("defaults to false", () => {
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+            const manager = new MembershipManager({}, room, client, callSession);
             expect(manager.isActivated()).toEqual(false);
         });
 
         it("returns true after join()", () => {
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+            const manager = new MembershipManager({}, room, client, callSession);
             manager.join([]);
             expect(manager.isActivated()).toEqual(true);
         });
@@ -123,8 +122,8 @@ describe("MembershipManager", () => {
                 const updateDelayedEventHandle = createAsyncHandle<void>(client._unstable_updateDelayedEvent as Mock);
 
                 // Test
-                const memberManager = new MembershipManager(undefined, room, client, () => undefined, callSession);
-                memberManager.join([focus], focusActive);
+                const memberManager = new MembershipManager(undefined, room, client, callSession);
+                memberManager.join([focus], undefined);
                 // expects
                 await waitForMockCall(client.sendStateEvent, Promise.resolve({ event_id: "id" }));
                 expect(client.sendStateEvent).toHaveBeenCalledWith(
@@ -152,8 +151,45 @@ describe("MembershipManager", () => {
                 expect(client._unstable_sendDelayedStateEvent).toHaveBeenCalledTimes(1);
             });
 
+            it("sends a rtc membership event when using `useRtcMemberFormat`", async () => {
+                // Spys/Mocks
+
+                const updateDelayedEventHandle = createAsyncHandle<void>(client._unstable_updateDelayedEvent as Mock);
+
+                // Test
+                const memberManager = new MembershipManager({ useRtcMemberFormat: true }, room, client, callSession);
+                memberManager.join([], focus);
+                // expects
+                await waitForMockCall(client.sendStateEvent, Promise.resolve({ event_id: "id" }));
+                expect(client.sendStateEvent).toHaveBeenCalledWith(
+                    room.roomId,
+                    "org.matrix.msc4143.rtc.member",
+                    {
+                        application: { type: "m.call" },
+                        member: {
+                            user_id: "@alice:example.org",
+                            id: "_@alice:example.org_AAAAAAA_m.call",
+                            device_id: "AAAAAAA",
+                        },
+                        slot_id: "m.call#",
+                        rtc_transports: [focus],
+                        versions: [],
+                    },
+                    "_@alice:example.org_AAAAAAA_m.call",
+                );
+                updateDelayedEventHandle.resolve?.();
+                expect(client._unstable_sendDelayedStateEvent).toHaveBeenCalledWith(
+                    room.roomId,
+                    { delay: 8000 },
+                    "org.matrix.msc4143.rtc.member",
+                    {},
+                    "_@alice:example.org_AAAAAAA_m.call",
+                );
+                expect(client._unstable_sendDelayedStateEvent).toHaveBeenCalledTimes(1);
+            });
+
             it("reschedules delayed leave event if sending state cancels it", async () => {
-                const memberManager = new MembershipManager(undefined, room, client, () => undefined, callSession);
+                const memberManager = new MembershipManager(undefined, room, client, callSession);
                 const waitForSendState = waitForMockCall(client.sendStateEvent);
                 const waitForUpdateDelaye = waitForMockCallOnce(
                     client._unstable_updateDelayedEvent,
@@ -228,10 +264,9 @@ describe("MembershipManager", () => {
                         },
                         room,
                         client,
-                        () => undefined,
                         callSession,
                     );
-                    manager.join([focus], focusActive);
+                    manager.join([focus]);
 
                     await sendDelayedStateExceedAttempt.then(); // needed to resolve after the send attempt catches
                     await sendDelayedStateAttempt;
@@ -286,8 +321,8 @@ describe("MembershipManager", () => {
         describe("delayed leave event", () => {
             it("does not try again to schedule a delayed leave event if not supported", () => {
                 const delayedHandle = createAsyncHandle(client._unstable_sendDelayedStateEvent as Mock);
-                const manager = new MembershipManager({}, room, client, () => undefined, callSession);
-                manager.join([focus], focusActive);
+                const manager = new MembershipManager({}, room, client, callSession);
+                manager.join([focus]);
                 delayedHandle.reject?.(
                     new UnsupportedDelayedEventsEndpointError(
                         "Server does not support the delayed events API",
@@ -298,21 +333,15 @@ describe("MembershipManager", () => {
             });
             it("does try to schedule a delayed leave event again if rate limited", async () => {
                 const delayedHandle = createAsyncHandle(client._unstable_sendDelayedStateEvent as Mock);
-                const manager = new MembershipManager({}, room, client, () => undefined, callSession);
-                manager.join([focus], focusActive);
+                const manager = new MembershipManager({}, room, client, callSession);
+                manager.join([focus]);
                 delayedHandle.reject?.(new HTTPError("rate limited", 429, undefined));
                 await jest.advanceTimersByTimeAsync(5000);
                 expect(client._unstable_sendDelayedStateEvent).toHaveBeenCalledTimes(2);
             });
             it("uses delayedLeaveEventDelayMs from config", () => {
-                const manager = new MembershipManager(
-                    { delayedLeaveEventDelayMs: 123456 },
-                    room,
-                    client,
-                    () => undefined,
-                    callSession,
-                );
-                manager.join([focus], focusActive);
+                const manager = new MembershipManager({ delayedLeaveEventDelayMs: 123456 }, room, client, callSession);
+                manager.join([focus]);
                 expect(client._unstable_sendDelayedStateEvent).toHaveBeenCalledWith(
                     room.roomId,
                     { delay: 123456 },
@@ -329,11 +358,11 @@ describe("MembershipManager", () => {
                 { delayedLeaveEventRestartMs: RESTART_DELAY },
                 room,
                 client,
-                () => undefined,
+
                 callSession,
             );
             // Join with the membership manager
-            manager.join([focus], focusActive);
+            manager.join([focus]);
             expect(manager.status).toBe(Status.Connecting);
             // Let the scheduler run one iteration so that we can send the join state event
             await jest.runOnlyPendingTimersAsync();
@@ -367,11 +396,11 @@ describe("MembershipManager", () => {
                 { membershipEventExpiryMs: 1234567 },
                 room,
                 client,
-                () => undefined,
+
                 callSession,
             );
 
-            manager.join([focus], focusActive);
+            manager.join([focus]);
             await waitForMockCall(client.sendStateEvent);
             expect(client.sendStateEvent).toHaveBeenCalledWith(
                 room.roomId,
@@ -393,11 +422,11 @@ describe("MembershipManager", () => {
         });
 
         it("does nothing if join called when already joined", async () => {
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
-            manager.join([focus], focusActive);
+            const manager = new MembershipManager({}, room, client, callSession);
+            manager.join([focus]);
             await waitForMockCall(client.sendStateEvent);
             expect(client.sendStateEvent).toHaveBeenCalledTimes(1);
-            manager.join([focus], focusActive);
+            manager.join([focus]);
             expect(client.sendStateEvent).toHaveBeenCalledTimes(1);
         });
     });
@@ -405,16 +434,16 @@ describe("MembershipManager", () => {
     describe("leave()", () => {
         // TODO add rate limit cases.
         it("resolves delayed leave event when leave is called", async () => {
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
-            manager.join([focus], focusActive);
+            const manager = new MembershipManager({}, room, client, callSession);
+            manager.join([focus]);
             await jest.advanceTimersByTimeAsync(1);
             await manager.leave();
             expect(client._unstable_updateDelayedEvent).toHaveBeenLastCalledWith("id", "send");
             expect(client.sendStateEvent).toHaveBeenCalled();
         });
         it("send leave event when leave is called and resolving delayed leave fails", async () => {
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
-            manager.join([focus], focusActive);
+            const manager = new MembershipManager({}, room, client, callSession);
+            manager.join([focus]);
             await jest.advanceTimersByTimeAsync(1);
             (client._unstable_updateDelayedEvent as Mock<any>).mockRejectedValue("unknown");
             await manager.leave();
@@ -428,60 +457,16 @@ describe("MembershipManager", () => {
             );
         });
         it("does nothing if not joined", () => {
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+            const manager = new MembershipManager({}, room, client, callSession);
             expect(async () => await manager.leave()).not.toThrow();
             expect(client._unstable_sendDelayedStateEvent).not.toHaveBeenCalled();
             expect(client.sendStateEvent).not.toHaveBeenCalled();
         });
     });
 
-    describe("getsActiveFocus", () => {
-        it("gets the correct active focus with oldest_membership", () => {
-            const getOldestMembership = jest.fn();
-            const manager = new MembershipManager({}, room, client, getOldestMembership, callSession);
-            // Before joining the active focus should be undefined (see FocusInUse on MatrixRTCSession)
-            expect(manager.getActiveFocus()).toBe(undefined);
-            manager.join([focus], focusActive);
-            // After joining we want our own focus to be the one we select.
-            getOldestMembership.mockReturnValue(
-                mockCallMembership(
-                    {
-                        ...membershipTemplate,
-                        foci_preferred: [
-                            {
-                                livekit_alias: "!active:active.url",
-                                livekit_service_url: "https://active.url",
-                                type: "livekit",
-                            },
-                        ],
-                        user_id: client.getUserId()!,
-                        device_id: client.getDeviceId()!,
-                        created_ts: 1000,
-                    },
-                    room.roomId,
-                ),
-            );
-            expect(manager.getActiveFocus()).toStrictEqual(focus);
-            getOldestMembership.mockReturnValue(
-                mockCallMembership(
-                    Object.assign({}, membershipTemplate, { device_id: "old", created_ts: 1000 }),
-                    room.roomId,
-                ),
-            );
-            // If there is an older member we use its focus.
-            expect(manager.getActiveFocus()).toBe(membershipTemplate.foci_preferred[0]);
-        });
-
-        it("does not provide focus if the selection method is unknown", () => {
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
-            manager.join([focus], Object.assign(focusActive, { type: "unknown_type" }));
-            expect(manager.getActiveFocus()).toBe(undefined);
-        });
-    });
-
     describe("onRTCSessionMemberUpdate()", () => {
         it("does nothing if not joined", async () => {
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+            const manager = new MembershipManager({}, room, client, callSession);
             await manager.onRTCSessionMemberUpdate([mockCallMembership(membershipTemplate, room.roomId)]);
             await jest.advanceTimersToNextTimerAsync();
             expect(client.sendStateEvent).not.toHaveBeenCalled();
@@ -489,7 +474,7 @@ describe("MembershipManager", () => {
             expect(client._unstable_updateDelayedEvent).not.toHaveBeenCalled();
         });
         it("does nothing if own membership still present", async () => {
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+            const manager = new MembershipManager({}, room, client, callSession);
             manager.join([focus], focusActive);
             await jest.advanceTimersByTimeAsync(1);
             const myMembership = (client.sendStateEvent as Mock).mock.calls[0][2];
@@ -513,7 +498,7 @@ describe("MembershipManager", () => {
             expect(client._unstable_updateDelayedEvent).not.toHaveBeenCalled();
         });
         it("recreates membership if it is missing", async () => {
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+            const manager = new MembershipManager({}, room, client, callSession);
             manager.join([focus], focusActive);
             await jest.advanceTimersByTimeAsync(1);
             // clearing all mocks before checking what happens when calling: `onRTCSessionMemberUpdate`
@@ -531,7 +516,7 @@ describe("MembershipManager", () => {
         });
 
         it("updates the UpdateExpiry entry in the action scheduler", async () => {
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+            const manager = new MembershipManager({}, room, client, callSession);
             manager.join([focus], focusActive);
             await jest.advanceTimersByTimeAsync(1);
             // clearing all mocks before checking what happens when calling: `onRTCSessionMemberUpdate`
@@ -564,7 +549,6 @@ describe("MembershipManager", () => {
                 { delayedLeaveEventRestartMs: 10_000, delayedLeaveEventDelayMs: 30_000 },
                 room,
                 client,
-                () => undefined,
                 { id: "", application: "m.call" },
             );
             manager.join([focus], focusActive);
@@ -596,7 +580,7 @@ describe("MembershipManager", () => {
                 { membershipEventExpiryMs: expire, membershipEventExpiryHeadroomMs: headroom },
                 room,
                 client,
-                () => undefined,
+
                 { id: "", application: "m.call" },
             );
             manager.join([focus], focusActive);
@@ -621,14 +605,14 @@ describe("MembershipManager", () => {
 
     describe("status updates", () => {
         it("starts 'Disconnected'", () => {
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+            const manager = new MembershipManager({}, room, client, callSession);
             expect(manager.status).toBe(Status.Disconnected);
         });
         it("emits 'Connection' and 'Connected' after join", async () => {
             const handleDelayedEvent = createAsyncHandle<void>(client._unstable_sendDelayedStateEvent);
             const handleStateEvent = createAsyncHandle<void>(client.sendStateEvent);
 
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+            const manager = new MembershipManager({}, room, client, callSession);
             expect(manager.status).toBe(Status.Disconnected);
             const connectEmit = jest.fn();
             manager.on(MembershipManagerEvent.StatusChanged, connectEmit);
@@ -642,7 +626,7 @@ describe("MembershipManager", () => {
             expect(connectEmit).toHaveBeenCalledWith(Status.Connecting, Status.Connected);
         });
         it("emits 'Disconnecting' and 'Disconnected' after leave", async () => {
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+            const manager = new MembershipManager({}, room, client, callSession);
             const connectEmit = jest.fn();
             manager.on(MembershipManagerEvent.StatusChanged, connectEmit);
             manager.join([focus], focusActive);
@@ -658,7 +642,7 @@ describe("MembershipManager", () => {
             it("sends retry if call membership event is still valid at time of retry", async () => {
                 const handle = createAsyncHandle(client._unstable_sendDelayedStateEvent);
 
-                const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+                const manager = new MembershipManager({}, room, client, callSession);
                 manager.join([focus], focusActive);
                 expect(client._unstable_sendDelayedStateEvent).toHaveBeenCalledTimes(1);
 
@@ -685,7 +669,7 @@ describe("MembershipManager", () => {
                         new Headers({ "Retry-After": "1" }),
                     ),
                 );
-                const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+                const manager = new MembershipManager({}, room, client, callSession);
                 // Should call _unstable_sendDelayedStateEvent but not sendStateEvent because of the
                 // RateLimit error.
                 manager.join([focus], focusActive);
@@ -705,7 +689,7 @@ describe("MembershipManager", () => {
             it("abandons retry loop if leave() was called before sending state event", async () => {
                 const handle = createAsyncHandle(client._unstable_sendDelayedStateEvent);
 
-                const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+                const manager = new MembershipManager({}, room, client, callSession);
                 manager.join([focus], focusActive);
                 handle.reject?.(
                     new MatrixError(
@@ -740,7 +724,7 @@ describe("MembershipManager", () => {
                         new Headers({ "Retry-After": "1" }),
                     ),
                 );
-                const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+                const manager = new MembershipManager({}, room, client, callSession);
                 manager.join([focus], focusActive);
 
                 // Hit rate limit
@@ -773,7 +757,7 @@ describe("MembershipManager", () => {
                     new Headers({ "Retry-After": "2" }),
                 ),
             );
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+            const manager = new MembershipManager({}, room, client, callSession);
             manager.join([focus], focusActive, delayEventSendError);
 
             for (let i = 0; i < 10; i++) {
@@ -793,7 +777,7 @@ describe("MembershipManager", () => {
                     new Headers({ "Retry-After": "1" }),
                 ),
             );
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+            const manager = new MembershipManager({}, room, client, callSession);
             manager.join([focus], focusActive, delayEventRestartError);
 
             for (let i = 0; i < 10; i++) {
@@ -804,7 +788,7 @@ describe("MembershipManager", () => {
         it("falls back to using pure state events when some error occurs while sending delayed events", async () => {
             const unrecoverableError = jest.fn();
             (client._unstable_sendDelayedStateEvent as Mock<any>).mockRejectedValue(new HTTPError("unknown", 601));
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+            const manager = new MembershipManager({}, room, client, callSession);
             manager.join([focus], focusActive, unrecoverableError);
             await waitForMockCall(client.sendStateEvent);
             expect(unrecoverableError).not.toHaveBeenCalledWith();
@@ -817,7 +801,6 @@ describe("MembershipManager", () => {
                 { networkErrorRetryMs: 1000, maximumNetworkErrorRetryCount: 7 },
                 room,
                 client,
-                () => undefined,
                 callSession,
             );
             manager.join([focus], focusActive, unrecoverableError);
@@ -836,7 +819,7 @@ describe("MembershipManager", () => {
             (client._unstable_sendDelayedStateEvent as Mock<any>).mockRejectedValue(
                 new UnsupportedDelayedEventsEndpointError("not supported", "sendDelayedStateEvent"),
             );
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+            const manager = new MembershipManager({}, room, client, callSession);
             manager.join([focus], focusActive, unrecoverableError);
             await jest.advanceTimersByTimeAsync(1);
 
@@ -850,7 +833,7 @@ describe("MembershipManager", () => {
                 { delayedLeaveEventDelayMs: 10000 },
                 room,
                 client,
-                () => undefined,
+
                 callSession,
             );
             const { promise: stuckPromise, reject: rejectStuckPromise } = Promise.withResolvers<EmptyObject>();
@@ -904,7 +887,7 @@ describe("MembershipManager", () => {
 
     describe("updateCallIntent()", () => {
         it("should fail if the user has not joined the call", async () => {
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+            const manager = new MembershipManager({}, room, client, callSession);
             // After joining we want our own focus to be the one we select.
             try {
                 await manager.updateCallIntent("video");
@@ -913,7 +896,7 @@ describe("MembershipManager", () => {
         });
 
         it("can adjust the intent", async () => {
-            const manager = new MembershipManager({}, room, client, () => undefined, callSession);
+            const manager = new MembershipManager({}, room, client, callSession);
             manager.join([]);
             expect(manager.isActivated()).toEqual(true);
             const membership = mockCallMembership({ ...membershipTemplate, user_id: client.getUserId()! }, room.roomId);
@@ -926,7 +909,7 @@ describe("MembershipManager", () => {
         });
 
         it("does nothing if the intent doesn't change", async () => {
-            const manager = new MembershipManager({ callIntent: "video" }, room, client, () => undefined, callSession);
+            const manager = new MembershipManager({ callIntent: "video" }, room, client, callSession);
             manager.join([]);
             expect(manager.isActivated()).toEqual(true);
             const membership = mockCallMembership(
@@ -944,7 +927,7 @@ it("Should prefix log with MembershipManager used", () => {
     const client = makeMockClient("@alice:example.org", "AAAAAAA");
     const room = makeMockRoom([membershipTemplate]);
 
-    const membershipManager = new MembershipManager(undefined, room, client, () => undefined, callSession, logger);
+    const membershipManager = new MembershipManager(undefined, room, client, callSession);
 
     const spy = jest.spyOn(console, "error");
     // Double join
diff --git a/src/@types/event.ts b/src/@types/event.ts
index 6e4d0ddff19..1364d9ca75f 100644
--- a/src/@types/event.ts
+++ b/src/@types/event.ts
@@ -58,7 +58,7 @@ import {
     type ICallNotifyContent,
 } from "../matrixrtc/types.ts";
 import { type M_POLL_END, type M_POLL_START, type PollEndEventContent, type PollStartEventContent } from "./polls.ts";
-import { type SessionMembershipData } from "../matrixrtc/CallMembership.ts";
+import { type RtcMembershipData, type SessionMembershipData } from "../matrixrtc/CallMembership.ts";
 import { type LocalNotificationSettings } from "./local_notifications.ts";
 import { type IPushRules } from "./PushRules.ts";
 import { type SecretInfo, type SecretStorageKeyDescription } from "../secret-storage.ts";
@@ -151,6 +151,7 @@ export enum EventType {
     GroupCallMemberPrefix = "org.matrix.msc3401.call.member",
 
     // MatrixRTC events
+    RTCMembership = "org.matrix.msc4143.rtc.member",
     CallNotify = "org.matrix.msc4075.call.notify",
     RTCNotification = "org.matrix.msc4075.rtc.notification",
     RTCDecline = "org.matrix.msc4310.rtc.decline",
@@ -369,7 +370,7 @@ export interface StateEvents {
     // MSC3401
     [EventType.GroupCallPrefix]: IGroupCallRoomState;
     [EventType.GroupCallMemberPrefix]: IGroupCallRoomMemberState | SessionMembershipData | EmptyObject;
-
+    [EventType.RTCMembership]: RtcMembershipData | EmptyObject;
     // MSC3089
     [UNSTABLE_MSC3089_BRANCH.name]: MSC3089EventContent;
 
diff --git a/src/matrixrtc/CallMembership.ts b/src/matrixrtc/CallMembership.ts
index 00e42baa17a..59ff3778e7a 100644
--- a/src/matrixrtc/CallMembership.ts
+++ b/src/matrixrtc/CallMembership.ts
@@ -14,12 +14,14 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import { type MatrixEvent } from "../matrix.ts";
+import { MXID_PATTERN } from "../models/room-member.ts";
 import { deepCompare } from "../utils.ts";
-import { type Focus } from "./focus.ts";
-import { isLivekitFocusActive } from "./LivekitFocus.ts";
-import { type SessionDescription } from "./MatrixRTCSession.ts";
-import { type RTCCallIntent } from "./types.ts";
+import { type LivekitFocusSelection } from "./LivekitTransport.ts";
+import { slotDescriptionToId, slotIdToDescription, type SlotDescription } from "./MatrixRTCSession.ts";
+import type { RTCCallIntent, Transport } from "./types.ts";
+import { type IContent, type MatrixEvent } from "../models/event.ts";
+import { type RelationType } from "../@types/event.ts";
+import { logger } from "../logger.ts";
 
 /**
  * The default duration in milliseconds that a membership is considered valid for.
@@ -29,6 +31,106 @@ import { type RTCCallIntent } from "./types.ts";
 export const DEFAULT_EXPIRE_DURATION = 1000 * 60 * 60 * 4;
 
 type CallScope = "m.room" | "m.user";
+type Member = { user_id: string; device_id: string; id: string };
+
+export interface RtcMembershipData {
+    "slot_id": string;
+    "member": Member;
+    "m.relates_to"?: {
+        event_id: string;
+        rel_type: RelationType.Reference;
+    };
+    "application": {
+        type: string;
+        // other application specific keys
+        [key: string]: unknown;
+    };
+    "rtc_transports": Transport[];
+    "versions": string[];
+    "msc4354_sticky_key"?: string;
+    "sticky_key"?: string;
+}
+
+const checkRtcMembershipData = (
+    data: IContent,
+    errors: string[],
+    referenceUserId: string,
+): data is RtcMembershipData => {
+    const prefix = " - ";
+
+    // required fields
+    if (typeof data.slot_id !== "string") {
+        errors.push(prefix + "slot_id must be string");
+    } else {
+        if (data.slot_id.split("#").length !== 2) errors.push(prefix + 'slot_id must include exactly one "#"');
+    }
+    if (typeof data.member !== "object" || data.member === null) {
+        errors.push(prefix + "member must be an object");
+    } else {
+        if (typeof data.member.user_id !== "string") errors.push(prefix + "member.user_id must be string");
+        else if (!MXID_PATTERN.test(data.member.user_id)) errors.push(prefix + "member.user_id must be a valid mxid");
+        // This is not what the spec enforces but there currently are no rules what power levels are required to
+        // send a m.rtc.member event for a other user. So we add this check for simplicity and to avoid possible attacks until there
+        // is a proper definition when this is allowed.
+        else if (data.member.user_id !== referenceUserId) errors.push(prefix + "member.user_id must match the sender");
+        if (typeof data.member.device_id !== "string") errors.push(prefix + "member.device_id must be string");
+        if (typeof data.member.id !== "string") errors.push(prefix + "member.id must be string");
+    }
+    if (typeof data.application !== "object" || data.application === null) {
+        errors.push(prefix + "application must be an object");
+    } else {
+        if (typeof data.application.type !== "string") {
+            errors.push(prefix + "application.type must be a string");
+        } else {
+            if (data.application.type.includes("#")) errors.push(prefix + 'application.type must not include "#"');
+        }
+    }
+    if (data.rtc_transports === undefined || !Array.isArray(data.rtc_transports)) {
+        errors.push(prefix + "rtc_transports must be an array");
+    } else {
+        // validate that each transport has at least a string 'type'
+        for (const t of data.rtc_transports) {
+            if (typeof t !== "object" || t === null || typeof (t as any).type !== "string") {
+                errors.push(prefix + "rtc_transports entries must be objects with a string type");
+                break;
+            }
+        }
+    }
+    if (data.versions === undefined || !Array.isArray(data.versions)) {
+        errors.push(prefix + "versions must be an array");
+    } else if (!data.versions.every((v) => typeof v === "string")) {
+        errors.push(prefix + "versions must be an array of strings");
+    }
+
+    // optional fields
+    if ((data.sticky_key ?? data.msc4354_sticky_key) === undefined) {
+        errors.push(prefix + "sticky_key or msc4354_sticky_key must be a defined");
+    }
+    if (data.sticky_key !== undefined && typeof data.sticky_key !== "string") {
+        errors.push(prefix + "sticky_key must be a string");
+    }
+    if (data.msc4354_sticky_key !== undefined && typeof data.msc4354_sticky_key !== "string") {
+        errors.push(prefix + "msc4354_sticky_key must be a string");
+    }
+    if (
+        data.sticky_key !== undefined &&
+        data.msc4354_sticky_key !== undefined &&
+        data.sticky_key !== data.msc4354_sticky_key
+    ) {
+        errors.push(prefix + "sticky_key and msc4354_sticky_key must be equal if both are defined");
+    }
+    if (data["m.relates_to"] !== undefined) {
+        const rel = data["m.relates_to"] as RtcMembershipData["m.relates_to"];
+        if (typeof rel !== "object" || rel === null) {
+            errors.push(prefix + "m.relates_to must be an object if provided");
+        } else {
+            if (typeof rel.event_id !== "string") errors.push(prefix + "m.relates_to.event_id must be a string");
+            if (rel.rel_type !== "m.reference") errors.push(prefix + "m.relates_to.rel_type must be m.reference");
+        }
+    }
+
+    return errors.length === 0;
+};
 
 /**
  * MSC4143 (MatrixRTC) session membership data.
@@ -56,13 +158,13 @@ export type SessionMembershipData = {
     /**
      * The focus selection system this user/membership is using.
      */
-    "focus_active": Focus;
+    "focus_active": LivekitFocusSelection;
 
     /**
-     * A list of possible foci this uses knows about. One of them might be used based on the focus_active
+     * A list of possible foci this user knows about. One of them might be used based on the focus_active
      * selection system.
      */
-    "foci_preferred": Focus[];
+    "foci_preferred": Transport[];
 
     /**
      * Optional field that contains the creation of the session. If it is undefined the creation
@@ -77,7 +179,7 @@ export type SessionMembershipData = {
 
     /**
      * If the `application` = `"m.call"` this defines if it is a room or user owned call.
-     * There can always be one room scroped call but multiple user owned calls (breakout sessions)
+     * There can always be one room scoped call but multiple user owned calls (breakout sessions)
      */
     "scope"?: CallScope;
 
@@ -95,16 +197,26 @@ export type SessionMembershipData = {
     "m.call.intent"?: RTCCallIntent;
 };
 
-const checkSessionsMembershipData = (
-    data: Partial<Record<keyof SessionMembershipData, any>>,
-    errors: string[],
-): data is SessionMembershipData => {
-    const prefix = "Malformed session membership event: ";
+const checkSessionsMembershipData = (data: IContent, errors: string[]): data is SessionMembershipData => {
+    const prefix = " - ";
     if (typeof data.device_id !== "string") errors.push(prefix + "device_id must be string");
     if (typeof data.call_id !== "string") errors.push(prefix + "call_id must be string");
     if (typeof data.application !== "string") errors.push(prefix + "application must be a string");
     if (typeof data.focus_active?.type !== "string") errors.push(prefix + "focus_active.type must be a string");
-    if (!Array.isArray(data.foci_preferred)) errors.push(prefix + "foci_preferred must be an array");
+    if (data.focus_active === undefined) {
+        errors.push(prefix + "focus_active has an invalid type");
+    }
+    if (
+        data.foci_preferred !== undefined &&
+        !(
+            Array.isArray(data.foci_preferred) &&
+            data.foci_preferred.every(
+                (f: Transport) => typeof f === "object" && f !== null && typeof f.type === "string",
+            )
+        )
+    ) {
+        errors.push(prefix + "foci_preferred must be an array of transport objects");
+    }
     // optional parameters
     if (data.created_ts !== undefined && typeof data.created_ts !== "number") {
         errors.push(prefix + "created_ts must be number");
@@ -120,109 +232,278 @@ const checkSessionsMembershipData = (
     return errors.length === 0;
 };
 
+type MembershipData = { kind: "rtc"; data: RtcMembershipData } | { kind: "session"; data: SessionMembershipData };
+// TODO: Rename to RtcMembership once we removed the legacy SessionMembership from this file.
 export class CallMembership {
-    public static equal(a: CallMembership, b: CallMembership): boolean {
-        return deepCompare(a.membershipData, b.membershipData);
+    public static equal(a?: CallMembership, b?: CallMembership): boolean {
+        return deepCompare(a?.membershipData, b?.membershipData);
     }
-    private membershipData: SessionMembershipData;
 
+    private membershipData: MembershipData;
+
+    /** The parsed data from the Matrix event.
+     * To access checked eventId and sender from the matrixEvent.
+     * Class construction will fail if these values cannot get obtained. */
+    private readonly matrixEventData: { eventId: string; sender: string };
     public constructor(
-        private parentEvent: MatrixEvent,
-        data: any,
+        /** The Matrix event that this membership is based on */
+        private readonly matrixEvent: MatrixEvent,
+        data: IContent,
     ) {
+        const eventId = matrixEvent.getId();
+        const sender = matrixEvent.getSender();
+
+        if (eventId === undefined) throw new Error("parentEvent is missing eventId field");
+        if (sender === undefined) throw new Error("parentEvent is missing sender field");
+
         const sessionErrors: string[] = [];
-        if (!checkSessionsMembershipData(data, sessionErrors)) {
-            throw Error(
-                `unknown CallMembership data. Does not match MSC4143 call.member (${sessionErrors.join(" & ")}) events this could be a legacy membership event: (${data})`,
-            );
+        const rtcErrors: string[] = [];
+        if (checkSessionsMembershipData(data, sessionErrors)) {
+            this.membershipData = { kind: "session", data };
+        } else if (checkRtcMembershipData(data, rtcErrors, sender)) {
+            this.membershipData = { kind: "rtc", data };
         } else {
-            this.membershipData = data;
+            const details =
+                sessionErrors.length < rtcErrors.length
+                    ? `Does not match MSC4143 m.call.member:\n${sessionErrors.join("\n")}\n\n`
+                    : `Does not match MSC4143 m.rtc.member:\n${rtcErrors.join("\n")}\n\n`;
+            const json = "\nevent:\n" + JSON.stringify(data).replaceAll('"', "'");
+            throw Error(`unknown CallMembership data.\n` + details + json);
         }
+        this.matrixEventData = { eventId, sender };
     }
 
-    public get sender(): string | undefined {
-        return this.parentEvent.getSender();
+    /** @deprecated use userId instead */
+    public get sender(): string {
+        return this.userId;
+    }
+    public get userId(): string {
+        const { kind, data } = this.membershipData;
+        switch (kind) {
+            case "rtc":
+                return data.member.user_id;
+            case "session":
+            default:
+                return this.matrixEventData.sender;
+        }
     }
 
-    public get eventId(): string | undefined {
-        return this.parentEvent.getId();
+    public get eventId(): string {
+        return this.matrixEventData.eventId;
     }
 
     /**
-     *  @deprecated Use sessionDescription.id instead.
+     * The ID of the MatrixRTC slot that this membership belongs to (format `{application}#{id}`).
+     * This is computed in case SessionMembershipData is used.
      */
-    public get callId(): string {
-        return this.membershipData.call_id;
+    public get slotId(): string {
+        const { kind, data } = this.membershipData;
+        switch (kind) {
+            case "rtc":
+                return data.slot_id;
+            case "session":
+            default:
+                return slotDescriptionToId({ application: this.application, id: data.call_id });
+        }
     }
 
     public get deviceId(): string {
-        return this.membershipData.device_id;
+        const { kind, data } = this.membershipData;
+        switch (kind) {
+            case "rtc":
+                return data.member.device_id;
+            case "session":
+            default:
+                return data.device_id;
+        }
     }
 
     public get callIntent(): RTCCallIntent | undefined {
-        return this.membershipData["m.call.intent"];
+        const { kind, data } = this.membershipData;
+        switch (kind) {
+            case "rtc": {
+                const intent = data.application["m.call.intent"];
+                if (typeof intent === "string") {
+                    return intent;
+                }
+                logger.warn("RTC membership has invalid m.call.intent");
+                return undefined;
+            }
+            case "session":
+            default:
+                return data["m.call.intent"];
+        }
     }
 
-    public get sessionDescription(): SessionDescription {
-        return {
-            application: this.membershipData.application,
-            id: this.membershipData.call_id,
-        };
+    /**
+     * Parsed `slot_id` (format `{application}#{id}`) into its components (application and id).
+     */
+    public get slotDescription(): SlotDescription {
+        return slotIdToDescription(this.slotId);
     }
 
-    public get application(): string | undefined {
-        return this.membershipData.application;
+    public get application(): string {
+        const { kind, data } = this.membershipData;
+        switch (kind) {
+            case "rtc":
+                return data.application.type;
+            case "session":
+            default:
+                return data.application;
+        }
+    }
+    public get applicationData(): { type: string; [key: string]: unknown } {
+        const { kind, data } = this.membershipData;
+        switch (kind) {
+            case "rtc":
+                return data.application;
+            case "session":
+            default:
+                return { "type": data.application, "m.call.intent": data["m.call.intent"] };
+        }
     }
 
+    /** @deprecated scope is not used and will be removed in future versions. replaced by application specific types.*/
     public get scope(): CallScope | undefined {
-        return this.membershipData.scope;
+        const { kind, data } = this.membershipData;
+        switch (kind) {
+            case "rtc":
+                return undefined;
+            case "session":
+            default:
+                return data.scope;
+        }
     }
 
     public get membershipID(): string {
         // the createdTs behaves equivalent to the membershipID.
-        // we only need the field for the legacy member envents where we needed to update them
+        // we only need the field for the legacy member events where we needed to update them
         // synapse ignores sending state events if they have the same content.
-        return this.createdTs().toString();
+        const { kind, data } = this.membershipData;
+        switch (kind) {
+            case "rtc":
+                return data.member.id;
+            case "session":
+            default:
+                return (this.createdTs() ?? "").toString();
+        }
     }
 
     public createdTs(): number {
-        return this.membershipData.created_ts ?? this.parentEvent.getTs();
+        const { kind, data } = this.membershipData;
+        switch (kind) {
+            case "rtc":
+                // TODO we need to read the referenced (relation) event if available to get the real created_ts
+                return this.matrixEvent.getTs();
+            case "session":
+            default:
+                return data.created_ts ?? this.matrixEvent.getTs();
+        }
     }
 
     /**
      * Gets the absolute expiry timestamp of the membership.
      * @returns The absolute expiry time of the membership as a unix timestamp in milliseconds or undefined if not applicable
      */
-    public getAbsoluteExpiry(): number {
-        // TODO: calculate this from the MatrixRTCSession join configuration directly
-        return this.createdTs() + (this.membershipData.expires ?? DEFAULT_EXPIRE_DURATION);
+    public getAbsoluteExpiry(): number | undefined {
+        const { kind, data } = this.membershipData;
+        switch (kind) {
+            case "rtc":
+                return undefined;
+            case "session":
+            default:
+                // TODO: calculate this from the MatrixRTCSession join configuration directly
+                return this.createdTs() + (data.expires ?? DEFAULT_EXPIRE_DURATION);
+        }
     }
 
     /**
      * @returns The number of milliseconds until the membership expires or undefined if applicable
      */
-    public getMsUntilExpiry(): number {
-        // Assume that local clock is sufficiently in sync with other clocks in the distributed system.
-        // We used to try and adjust for the local clock being skewed, but there are cases where this is not accurate.
-        // The current implementation allows for the local clock to be -infinity to +MatrixRTCSession.MEMBERSHIP_EXPIRY_TIME/2
-        return this.getAbsoluteExpiry() - Date.now();
+    public getMsUntilExpiry(): number | undefined {
+        const { kind } = this.membershipData;
+        switch (kind) {
+            case "rtc":
+                return undefined;
+            case "session":
+            default:
+                // Assume that local clock is sufficiently in sync with other clocks in the distributed system.
+                // We used to try and adjust for the local clock being skewed, but there are cases where this is not accurate.
+                // The current implementation allows for the local clock to be -infinity to +MatrixRTCSession.MEMBERSHIP_EXPIRY_TIME/2
+                return this.getAbsoluteExpiry()! - Date.now();
+        }
     }
 
     /**
      * @returns true if the membership has expired, otherwise false
      */
     public isExpired(): boolean {
-        return this.getMsUntilExpiry() <= 0;
+        const { kind } = this.membershipData;
+        switch (kind) {
+            case "rtc":
+                return false;
+            case "session":
+            default:
+                return this.getMsUntilExpiry()! <= 0;
+        }
     }
 
-    public getPreferredFoci(): Focus[] {
-        return this.membershipData.foci_preferred;
+    /**
+     * ## RTC Membership
+     * Gets the primary transport to use for this RTC membership (m.rtc.member).
+     * This will return the primary transport that is used by this call membership to publish their media.
+     * Directly relates to the `rtc_transports` field.
+     *
+     * ## Legacy session membership
+     * In case of a legacy session membership (m.call.member) this will return the selected transport where
+     * media is published. How this selection happens depends on the `focus_active` field of the session membership.
+     * If the `focus_selection` is `oldest_membership` this will return the transport of the oldest membership
+     * in the room (based on the `created_ts` field of the session membership).
+     * If the `focus_selection` is `multi_sfu` it will return the first transport of the `foci_preferred` list.
+     * (`multi_sfu` is equivalent to how `m.rtc.member` `rtc_transports` work).
+     * @param oldestMembership For backwards compatibility with session membership (legacy). Unused in case of RTC membership.
+     * Always required to make the consumer not care if it deals with RTC or session memberships.
+     * @returns The transport this membership uses to publish media or undefined if no transport is available.
+     */
+    public getTransport(oldestMembership: CallMembership): Transport | undefined {
+        const { kind, data } = this.membershipData;
+        switch (kind) {
+            case "rtc":
+                return data.rtc_transports[0];
+            case "session":
+                switch (data.focus_active.focus_selection) {
+                    case "multi_sfu":
+                        return data.foci_preferred[0];
+                    case "oldest_membership":
+                        if (CallMembership.equal(this, oldestMembership)) return data.foci_preferred[0];
+                        if (oldestMembership !== undefined) return oldestMembership.getTransport(oldestMembership);
+                        break;
+                }
+        }
+        return undefined;
     }
 
-    public getFocusSelection(): string | undefined {
-        const focusActive = this.membershipData.focus_active;
-        if (isLivekitFocusActive(focusActive)) {
-            return focusActive.focus_selection;
+    /**
+     * The focus_active filed of the session membership (m.call.member).
+     * @deprecated focus_active is not used and will be removed in future versions.
+     */
+    public getFocusActive(): LivekitFocusSelection | undefined {
+        const { kind, data } = this.membershipData;
+        if (kind === "session") return data.focus_active;
+        return undefined;
+    }
+    /**
+     * The value of the `rtc_transports` field for RTC memberships (m.rtc.member).
+     * Or the value of the `foci_preferred` field for legacy session memberships (m.call.member).
+     */
+    public get transports(): Transport[] {
+        const { kind, data } = this.membershipData;
+        switch (kind) {
+            case "rtc":
+                return data.rtc_transports;
+            case "session":
+            default:
+                return data.foci_preferred;
         }
     }
 }
diff --git a/src/matrixrtc/IMembershipManager.ts b/src/matrixrtc/IMembershipManager.ts
index 320654543d9..7826fa9d16e 100644
--- a/src/matrixrtc/IMembershipManager.ts
+++ b/src/matrixrtc/IMembershipManager.ts
@@ -15,8 +15,7 @@ limitations under the License.
 */
 
 import type { CallMembership } from "./CallMembership.ts";
-import type { Focus } from "./focus.ts";
-import type { RTCCallIntent, Status } from "./types.ts";
+import type { RTCCallIntent, Status, Transport } from "./types.ts";
 import { type TypedEventEmitter } from "../models/typed-event-emitter.ts";
 
 export enum MembershipManagerEvent {
@@ -80,10 +79,13 @@ export interface IMembershipManager
     /**
      * Start sending all necessary events to make this user participate in the RTC session.
      * @param fociPreferred the list of preferred foci to use in the joined RTC membership event.
-     * @param fociActive the active focus to use in the joined RTC membership event.
+     * If multiSfuFocus is set, this is only needed if this client wants to publish to multiple transports simultaneously.
+     * @param multiSfuFocus the active focus to use in the joined RTC membership event. Setting this implies the
+     * membership manager will operate in a multi-SFU connection mode. If `undefined`, an `oldest_membership`
+     * transport selection will be used instead.
      * @throws can throw if it exceeds a configured maximum retry.
      */
-    join(fociPreferred: Focus[], fociActive?: Focus, onError?: (error: unknown) => void): void;
+    join(fociPreferred: Transport[], multiSfuFocus?: Transport, onError?: (error: unknown) => void): void;
     /**
      * Send all necessary events to make this user leave the RTC session.
      * @param timeout the maximum duration in ms until the promise is forced to resolve.
@@ -95,11 +97,6 @@ export interface IMembershipManager
      * Call this if the MatrixRTC session members have changed.
      */
     onRTCSessionMemberUpdate(memberships: CallMembership[]): Promise<void>;
-    /**
-     * The used active focus in the currently joined session.
-     * @returns the used active focus in the currently joined session or undefined if not joined.
-     */
-    getActiveFocus(): Focus | undefined;
 
     /**
      * Update the intent of a membership on the call (e.g. user is now providing a video feed)
diff --git a/src/matrixrtc/LivekitFocus.ts b/src/matrixrtc/LivekitFocus.ts
deleted file mode 100644
index 66d8a0a50be..00000000000
--- a/src/matrixrtc/LivekitFocus.ts
+++ /dev/null
@@ -1,39 +0,0 @@
-/*
-Copyright 2023 New Vector Ltd
-
-Licensed under the Apache License, Version 2.0 (the "License");
-you may not use this file except in compliance with the License.
-You may obtain a copy of the License at
-
-    http://www.apache.org/licenses/LICENSE-2.0
-
-Unless required by applicable law or agreed to in writing, software
-distributed under the License is distributed on an "AS IS" BASIS,
-WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-See the License for the specific language governing permissions and
-limitations under the License.
-*/
-
-import { type Focus } from "./focus.ts";
-
-export interface LivekitFocusConfig extends Focus {
-    type: "livekit";
-    livekit_service_url: string;
-}
-
-export const isLivekitFocusConfig = (object: any): object is LivekitFocusConfig =>
-    object.type === "livekit" && "livekit_service_url" in object;
-
-export interface LivekitFocus extends LivekitFocusConfig {
-    livekit_alias: string;
-}
-
-export const isLivekitFocus = (object: any): object is LivekitFocus =>
-    isLivekitFocusConfig(object) && "livekit_alias" in object;
-
-export interface LivekitFocusActive extends Focus {
-    type: "livekit";
-    focus_selection: "oldest_membership";
-}
-export const isLivekitFocusActive = (object: any): object is LivekitFocusActive =>
-    object.type === "livekit" && "focus_selection" in object;
diff --git a/src/matrixrtc/LivekitTransport.ts b/src/matrixrtc/LivekitTransport.ts
new file mode 100644
index 00000000000..eda11f554e5
--- /dev/null
+++ b/src/matrixrtc/LivekitTransport.ts
@@ -0,0 +1,46 @@
+/*
+Copyright 2025 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { type Transport } from "./types.ts";
+
+export interface LivekitTransportConfig extends Transport {
+    type: "livekit";
+    livekit_service_url: string;
+}
+
+export const isLivekitTransportConfig = (object: any): object is LivekitTransportConfig =>
+    object.type === "livekit" && "livekit_service_url" in object;
+
+export interface LivekitTransport extends LivekitTransportConfig {
+    livekit_alias: string;
+}
+
+export const isLivekitTransport = (object: any): object is LivekitTransport =>
+    isLivekitTransportConfig(object) && "livekit_alias" in object;
+
+/**
+ * @deprecated, this is just needed for the old focus active / focus fields of a call membership.
+ * Not needed for new implementations.
+ */
+export interface LivekitFocusSelection extends Transport {
+    type: "livekit";
+    focus_selection: "oldest_membership" | "multi_sfu";
+}
+/**
+ * @deprecated see LivekitFocusSelection
+ */
+export const isLivekitFocusSelection = (object: any): object is LivekitFocusSelection =>
+    object.type === "livekit" && "focus_selection" in object;
diff --git a/src/matrixrtc/MatrixRTCSession.ts b/src/matrixrtc/MatrixRTCSession.ts
index 020f768558d..9a61a7238b4 100644
--- a/src/matrixrtc/MatrixRTCSession.ts
+++ b/src/matrixrtc/MatrixRTCSession.ts
@@ -24,17 +24,17 @@ import { KnownMembership } from "../@types/membership.ts";
 import { type ISendEventResponse } from "../@types/requests.ts";
 import { CallMembership } from "./CallMembership.ts";
 import { RoomStateEvent } from "../models/room-state.ts";
-import { type Focus } from "./focus.ts";
 import { MembershipManager } from "./MembershipManager.ts";
 import { EncryptionManager, type IEncryptionManager } from "./EncryptionManager.ts";
 import { deepCompare, logDurationSync } from "../utils.ts";
-import {
-    type Statistics,
-    type RTCNotificationType,
-    type Status,
-    type IRTCNotificationContent,
-    type ICallNotifyContent,
-    type RTCCallIntent,
+import type {
+    Statistics,
+    RTCNotificationType,
+    Status,
+    IRTCNotificationContent,
+    ICallNotifyContent,
+    RTCCallIntent,
+    Transport,
 } from "./types.ts";
 import { RoomKeyTransport } from "./RoomKeyTransport.ts";
 import {
@@ -103,10 +103,17 @@ export interface SessionConfig {
 /**
  * The session description is used to identify a session. Used in the state event.
  */
-export interface SessionDescription {
+export interface SlotDescription {
     id: string;
     application: string;
 }
+export function slotIdToDescription(slotId: string): SlotDescription {
+    const [application, id] = slotId.split("#");
+    return { application, id };
+}
+export function slotDescriptionToId(slotDescription: SlotDescription): string {
+    return `${slotDescription.application}#${slotDescription.id}`;
+}
 
 // The names follow these principles:
 // - we use the technical term delay if the option is related to delayed events.
@@ -185,6 +192,7 @@ export interface MembershipConfig {
      * but only applies to calls to the `_unstable_updateDelayedEvent` endpoint with a body of `{action:"restart"}`.)
      */
     delayedLeaveEventRestartLocalTimeoutMs?: number;
+    useRtcMemberFormat?: boolean;
 }
 
 export interface EncryptionConfig {
@@ -240,8 +248,6 @@ export class MatrixRTCSession extends TypedEventEmitter<
 > {
     private membershipManager?: IMembershipManager;
     private encryptionManager?: IEncryptionManager;
-    // The session Id of the call, this is the call_id of the call Member event.
-    private _callId: string | undefined;
     private joinConfig?: SessionConfig;
     private logger: Logger;
 
@@ -279,33 +285,53 @@ export class MatrixRTCSession extends TypedEventEmitter<
      *
      * It can be undefined since the callId is only known once the first membership joins.
      * The callId is the property that, per definition, groups memberships into one call.
+     * @deprecated use `slotId` instead.
      */
     public get callId(): string | undefined {
-        return this._callId;
+        return this.slotDescription?.id;
+    }
+    /**
+     * The slotId of the call.
+     * `{application}#{appSpecificId}`
+     * It can be undefined since the slotId is only known once the first membership joins.
+     * The slotId is the property that, per definition, groups memberships into one call.
+     */
+    public get slotId(): string | undefined {
+        return slotDescriptionToId(this.slotDescription);
     }
 
     /**
      * Returns all the call memberships for a room that match the provided `sessionDescription`,
      * oldest first.
      *
-     * @deprecated Use `MatrixRTCSession.sessionMembershipsForRoom` instead.
+     * @deprecated Use `MatrixRTCSession.sessionMembershipsForSlot` instead.
      */
     public static callMembershipsForRoom(
         room: Pick<Room, "getLiveTimeline" | "roomId" | "hasMembershipState">,
     ): CallMembership[] {
-        return MatrixRTCSession.sessionMembershipsForRoom(room, {
+        return MatrixRTCSession.sessionMembershipsForSlot(room, {
             id: "",
             application: "m.call",
         });
     }
 
+    /**
+     * @deprecated use `MatrixRTCSession.slotMembershipsForRoom` instead.
+     */
+    public static sessionMembershipsForRoom(
+        room: Pick<Room, "getLiveTimeline" | "roomId" | "hasMembershipState">,
+        sessionDescription: SlotDescription,
+    ): CallMembership[] {
+        return this.sessionMembershipsForSlot(room, sessionDescription);
+    }
+
     /**
      * Returns all the call memberships for a room that match the provided `sessionDescription`,
      * oldest first.
      */
-    public static sessionMembershipsForRoom(
+    public static sessionMembershipsForSlot(
         room: Pick<Room, "getLiveTimeline" | "roomId" | "hasMembershipState">,
-        sessionDescription: SessionDescription,
+        slotDescription: SlotDescription,
     ): CallMembership[] {
         const logger = rootLogger.getChild(`[MatrixRTCSession ${room.roomId}]`);
         const roomState = room.getLiveTimeline().getState(EventTimeline.FORWARDS);
@@ -335,12 +361,16 @@ export class MatrixRTCSession extends TypedEventEmitter<
             if (membershipContents.length === 0) continue;
 
             for (const membershipData of membershipContents) {
+                if (!("application" in membershipData)) {
+                    // This is a left membership event, ignore it here to not log warnings.
+                    continue;
+                }
                 try {
                     const membership = new CallMembership(memberEvent, membershipData);
 
-                    if (!deepCompare(membership.sessionDescription, sessionDescription)) {
+                    if (!deepCompare(membership.slotDescription, slotDescription)) {
                         logger.info(
-                            `Ignoring membership of user ${membership.sender} for a different session:  ${JSON.stringify(membership.sessionDescription)}`,
+                            `Ignoring membership of user ${membership.sender} for a different slot:  ${JSON.stringify(membership.slotDescription)}`,
                         );
                         continue;
                     }
@@ -379,26 +409,29 @@ export class MatrixRTCSession extends TypedEventEmitter<
      * This method is an alias for `MatrixRTCSession.sessionForRoom` with
      * sessionDescription `{ id: "", application: "m.call" }`.
      *
-     * @deprecated Use `MatrixRTCSession.sessionForRoom` with sessionDescription `{ id: "", application: "m.call" }` instead.
+     * @deprecated Use `MatrixRTCSession.sessionForSlot` with sessionDescription `{ id: "", application: "m.call" }` instead.
      */
     public static roomSessionForRoom(client: MatrixClient, room: Room): MatrixRTCSession {
-        const callMemberships = MatrixRTCSession.sessionMembershipsForRoom(room, { id: "", application: "m.call" });
+        const callMemberships = MatrixRTCSession.sessionMembershipsForSlot(room, { id: "", application: "m.call" });
         return new MatrixRTCSession(client, room, callMemberships, { id: "", application: "m.call" });
     }
 
+    /**
+     * @deprecated Use `MatrixRTCSession.sessionForSlot` instead.
+     */
+    public static sessionForRoom(client: MatrixClient, room: Room, slotDescription: SlotDescription): MatrixRTCSession {
+        return this.sessionForSlot(client, room, slotDescription);
+    }
+
     /**
      * Return the MatrixRTC session for the room.
      * This returned session can be used to find out if there are active sessions
-     * for the requested room and `sessionDescription`.
+     * for the requested room and `slotDescription`.
      */
-    public static sessionForRoom(
-        client: MatrixClient,
-        room: Room,
-        sessionDescription: SessionDescription,
-    ): MatrixRTCSession {
-        const callMemberships = MatrixRTCSession.sessionMembershipsForRoom(room, sessionDescription);
+    public static sessionForSlot(client: MatrixClient, room: Room, slotDescription: SlotDescription): MatrixRTCSession {
+        const callMemberships = MatrixRTCSession.sessionMembershipsForSlot(room, slotDescription);
 
-        return new MatrixRTCSession(client, room, callMemberships, sessionDescription);
+        return new MatrixRTCSession(client, room, callMemberships, slotDescription);
     }
 
     /**
@@ -444,14 +477,14 @@ export class MatrixRTCSession extends TypedEventEmitter<
         >,
         public memberships: CallMembership[],
         /**
-         * The session description is used to define the exact session this object is tracking.
-         * A session is distinct from another session if one of those properties differ: `roomSubset.roomId`, `sessionDescription.application`, `sessionDescription.id`.
+         * The slot description is a virtual address where participants are allowed to meet.
+         * This session will only manage memberships that match this slot description.
+         * Sessions are distinct if any of those properties are distinct: `roomSubset.roomId`, `slotDescription.application`, `slotDescription.id`.
          */
-        public readonly sessionDescription: SessionDescription,
+        public readonly slotDescription: SlotDescription,
     ) {
         super();
         this.logger = rootLogger.getChild(`[MatrixRTCSession ${roomSubset.roomId}]`);
-        this._callId = memberships[0]?.sessionDescription.id;
         const roomState = this.roomSubset.getLiveTimeline().getState(EventTimeline.FORWARDS);
         // TODO: double check if this is actually needed. Should be covered by refreshRoom in MatrixRTCSessionManager
         roomState?.on(RoomStateEvent.Members, this.onRoomMemberUpdate);
@@ -490,14 +523,18 @@ export class MatrixRTCSession extends TypedEventEmitter<
      * This will not subscribe to updates: remember to call subscribe() separately if
      * desired.
      * This method will return immediately and the session will be joined in the background.
-     *
-     * @param fociActive - The object representing the active focus. (This depends on the focus type.)
-     * @param fociPreferred - The list of preferred foci this member proposes to use/knows/has access to.
-     *                        For the livekit case this is a list of foci generated from the homeserver well-known, the current rtc session,
-     *                        or optionally other room members homeserver well known.
+     * @param fociPreferred the list of preferred foci to use in the joined RTC membership event.
+     * If multiSfuFocus is set, this is only needed if this client wants to publish to multiple transports simultaneously.
+     * @param multiSfuFocus the active focus to use in the joined RTC membership event. Setting this implies the
+     * membership manager will operate in a multi-SFU connection mode. If `undefined`, an `oldest_membership`
+     * transport selection will be used instead.
      * @param joinConfig - Additional configuration for the joined session.
      */
-    public joinRoomSession(fociPreferred: Focus[], fociActive?: Focus, joinConfig?: JoinSessionConfig): void {
+    public joinRoomSession(
+        fociPreferred: Transport[],
+        multiSfuFocus?: Transport,
+        joinConfig?: JoinSessionConfig,
+    ): void {
         if (this.isJoined()) {
             this.logger.info(`Already joined to session in room ${this.roomSubset.roomId}: ignoring join call`);
             return;
@@ -508,8 +545,7 @@ export class MatrixRTCSession extends TypedEventEmitter<
                 joinConfig,
                 this.roomSubset,
                 this.client,
-                () => this.getOldestMembership(),
-                this.sessionDescription,
+                this.slotDescription,
                 this.logger,
             );
 
@@ -571,7 +607,7 @@ export class MatrixRTCSession extends TypedEventEmitter<
         this.pendingNotificationToSend = this.joinConfig?.notificationType;
 
         // Join!
-        this.membershipManager!.join(fociPreferred, fociActive, (e) => {
+        this.membershipManager!.join(fociPreferred, multiSfuFocus, (e) => {
             this.logger.error("MembershipManager encountered an unrecoverable error: ", e);
             this.emit(MatrixRTCSessionEvent.MembershipManagerError, e);
             this.emit(MatrixRTCSessionEvent.JoinStateChanged, this.isJoined());
@@ -606,16 +642,23 @@ export class MatrixRTCSession extends TypedEventEmitter<
 
         return await leavePromise;
     }
-
     /**
-     * Get the active focus from the current CallMemberState event
-     * @returns The focus that is currently in use to connect to this session. This is undefined
-     * if the client is not connected to this session.
+     * This returns the focus in use by the oldest membership.
+     * Do not use since this might be just the focus for the oldest membership. others might use a different focus.
+     * @deprecated use `member.getTransport(session.getOldestMembership())` instead for the specific member you want to get the focus for.
      */
-    public getActiveFocus(): Focus | undefined {
-        return this.membershipManager?.getActiveFocus();
+    public getFocusInUse(): Transport | undefined {
+        const oldestMembership = this.getOldestMembership();
+        return oldestMembership?.getTransport(oldestMembership);
     }
 
+    /**
+     * The used focusActive of the oldest membership (to find out the selection type multi-sfu or oldest membership active focus)
+     * @deprecated does not work with m.rtc.member. Do not rely on it.
+     */
+    public getActiveFocus(): Transport | undefined {
+        return this.getOldestMembership()?.getFocusActive();
+    }
     public getOldestMembership(): CallMembership | undefined {
         return this.memberships[0];
     }
@@ -646,20 +689,6 @@ export class MatrixRTCSession extends TypedEventEmitter<
         await this.membershipManager?.updateCallIntent(callIntent);
     }
 
-    /**
-     * This method is used when the user is not yet connected to the Session but wants to know what focus
-     * the users in the session are using to make a decision how it wants/should connect.
-     *
-     * See also `getActiveFocus`
-     * @returns The focus which should be used when joining this session.
-     */
-    public getFocusInUse(): Focus | undefined {
-        const oldestMembership = this.getOldestMembership();
-        if (oldestMembership?.getFocusSelection() === "oldest_membership") {
-            return oldestMembership.getPreferredFoci()[0];
-        }
-    }
-
     /**
      * Re-emit an EncryptionKeyChanged event for each tracked encryption key. This can be used to export
      * the keys.
@@ -777,9 +806,7 @@ export class MatrixRTCSession extends TypedEventEmitter<
      */
     private recalculateSessionMembers = (): void => {
         const oldMemberships = this.memberships;
-        this.memberships = MatrixRTCSession.sessionMembershipsForRoom(this.room, this.sessionDescription);
-
-        this._callId = this._callId ?? this.memberships[0]?.sessionDescription.id;
+        this.memberships = MatrixRTCSession.sessionMembershipsForSlot(this.room, this.slotDescription);
 
         const changed =
             oldMemberships.length != this.memberships.length ||
diff --git a/src/matrixrtc/MatrixRTCSessionManager.ts b/src/matrixrtc/MatrixRTCSessionManager.ts
index cc25105d977..792132d273e 100644
--- a/src/matrixrtc/MatrixRTCSessionManager.ts
+++ b/src/matrixrtc/MatrixRTCSessionManager.ts
@@ -20,7 +20,7 @@ import { TypedEventEmitter } from "../models/typed-event-emitter.ts";
 import { type Room } from "../models/room.ts";
 import { type RoomState, RoomStateEvent } from "../models/room-state.ts";
 import { type MatrixEvent } from "../models/event.ts";
-import { MatrixRTCSession, type SessionDescription } from "./MatrixRTCSession.ts";
+import { MatrixRTCSession, type SlotDescription } from "./MatrixRTCSession.ts";
 import { EventType } from "../@types/event.ts";
 
 export enum MatrixRTCSessionManagerEvents {
@@ -56,7 +56,7 @@ export class MatrixRTCSessionManager extends TypedEventEmitter<MatrixRTCSessionM
     public constructor(
         rootLogger: Logger,
         private client: MatrixClient,
-        private readonly sessionDescription: SessionDescription = { id: "", application: "m.call" }, // Default to the Matrix Call application
+        private readonly slotDescription: SlotDescription = { application: "m.call", id: "" }, // Default to the Matrix Call application
     ) {
         super();
         this.logger = rootLogger.getChild("[MatrixRTCSessionManager]");
@@ -66,7 +66,7 @@ export class MatrixRTCSessionManager extends TypedEventEmitter<MatrixRTCSessionM
         // We shouldn't need to null-check here, but matrix-client.spec.ts mocks getRooms
         // returning nothing, and breaks tests if you change it to return an empty array :'(
         for (const room of this.client.getRooms() ?? []) {
-            const session = MatrixRTCSession.sessionForRoom(this.client, room, this.sessionDescription);
+            const session = MatrixRTCSession.sessionForRoom(this.client, room, this.slotDescription);
             if (session.memberships.length > 0) {
                 this.roomSessions.set(room.roomId, session);
             }
@@ -102,7 +102,7 @@ export class MatrixRTCSessionManager extends TypedEventEmitter<MatrixRTCSessionM
         if (!this.roomSessions.has(room.roomId)) {
             this.roomSessions.set(
                 room.roomId,
-                MatrixRTCSession.sessionForRoom(this.client, room, this.sessionDescription),
+                MatrixRTCSession.sessionForRoom(this.client, room, this.slotDescription),
             );
         }
 
diff --git a/src/matrixrtc/MembershipManager.ts b/src/matrixrtc/MembershipManager.ts
index 9e50e92d448..45f0a5ce9fc 100644
--- a/src/matrixrtc/MembershipManager.ts
+++ b/src/matrixrtc/MembershipManager.ts
@@ -15,20 +15,28 @@ limitations under the License.
 */
 import { AbortError } from "p-retry";
 
-import { EventType } from "../@types/event.ts";
+import { EventType, RelationType } from "../@types/event.ts";
 import { UpdateDelayedEventAction } from "../@types/requests.ts";
-import { type MatrixClient } from "../client.ts";
-import { UnsupportedDelayedEventsEndpointError } from "../errors.ts";
+import type { MatrixClient } from "../client.ts";
 import { ConnectionError, HTTPError, MatrixError } from "../http-api/errors.ts";
 import { type Logger, logger as rootLogger } from "../logger.ts";
 import { type Room } from "../models/room.ts";
-import { type CallMembership, DEFAULT_EXPIRE_DURATION, type SessionMembershipData } from "./CallMembership.ts";
-import { type Focus } from "./focus.ts";
-import { isMyMembership, type RTCCallIntent, Status } from "./types.ts";
-import { isLivekitFocusActive } from "./LivekitFocus.ts";
-import { type SessionDescription, type MembershipConfig, type SessionConfig } from "./MatrixRTCSession.ts";
+import {
+    type CallMembership,
+    DEFAULT_EXPIRE_DURATION,
+    type RtcMembershipData,
+    type SessionMembershipData,
+} from "./CallMembership.ts";
+import { type Transport, isMyMembership, type RTCCallIntent, Status } from "./types.ts";
+import {
+    type SlotDescription,
+    type MembershipConfig,
+    type SessionConfig,
+    slotDescriptionToId,
+} from "./MatrixRTCSession.ts";
 import { ActionScheduler, type ActionUpdate } from "./MembershipManagerActionScheduler.ts";
 import { TypedEventEmitter } from "../models/typed-event-emitter.ts";
+import { UnsupportedDelayedEventsEndpointError } from "../errors.ts";
 import {
     MembershipManagerEvent,
     type IMembershipManager,
@@ -36,12 +44,11 @@ import {
 } from "./IMembershipManager.ts";
 
 /* MembershipActionTypes:
-                           
 On Join:  ───────────────┐   ┌───────────────(1)───────────┐
                          ▼   ▼                             │
                    ┌────────────────┐                      │
                    │SendDelayedEvent│ ──────(2)───┐        │
-                   └────────────────┘             │        │ 
+                   └────────────────┘             │        │
                            │(3)                   │        │
                            ▼                      │        │
                     ┌─────────────┐               │        │
@@ -52,9 +59,9 @@ On Join:  ───────────────┐   ┌─────
 ┌────────────┐  │                  │ ┌───────────────────┐ │
 │UpdateExpiry│ (s)                (s)|RestartDelayedEvent│ │
 └────────────┘  │                  │ └───────────────────┘ │
-          │     │                  │      │        │       │       
-          └─────┘                  └──────┘        └───────┘ 
-     
+          │     │                  │      │        │       │
+          └─────┘                  └──────┘        └───────┘
+
 On Leave: ─────────  STOP ALL ABOVE
                            ▼
             ┌────────────────────────────────┐
@@ -169,18 +176,21 @@ export class MembershipManager
     /**
      * Puts the MembershipManager in a state where it tries to be joined.
      * It will send delayed events and membership events
-     * @param fociPreferred
-     * @param focusActive
+     * @param fociPreferred the list of preferred foci to use in the joined RTC membership event.
+     * If multiSfuFocus is set, this is only needed if this client wants to publish to multiple transports simultaneously.
+     * @param multiSfuFocus the active focus to use in the joined RTC membership event. Setting this implies the
+     * membership manager will operate in a multi-SFU connection mode. If `undefined`, an `oldest_membership`
+     * transport selection will be used instead.
      * @param onError This will be called once the membership manager encounters an unrecoverable error.
      * This should bubble up the the frontend to communicate that the call does not work in the current environment.
      */
-    public join(fociPreferred: Focus[], focusActive?: Focus, onError?: (error: unknown) => void): void {
+    public join(fociPreferred: Transport[], multiSfuFocus?: Transport, onError?: (error: unknown) => void): void {
         if (this.scheduler.running) {
             this.logger.error("MembershipManager is already running. Ignoring join request.");
             return;
         }
         this.fociPreferred = fociPreferred;
-        this.focusActive = focusActive;
+        this.rtcTransport = multiSfuFocus;
         this.leavePromiseResolvers = undefined;
         this.activated = true;
         this.oldStatus = this.status;
@@ -266,25 +276,6 @@ export class MembershipManager
         return Promise.resolve();
     }
 
-    public getActiveFocus(): Focus | undefined {
-        if (this.focusActive) {
-            // A livekit active focus
-            if (isLivekitFocusActive(this.focusActive)) {
-                if (this.focusActive.focus_selection === "oldest_membership") {
-                    const oldestMembership = this.getOldestMembership();
-                    return oldestMembership?.getPreferredFoci()[0];
-                }
-            } else {
-                this.logger.warn("Unknown own ActiveFocus type. This makes it impossible to connect to an SFU.");
-            }
-        } else {
-            // We do not understand the membership format (could be legacy). We default to oldestMembership
-            // Once there are other methods this is a hard error!
-            const oldestMembership = this.getOldestMembership();
-            return oldestMembership?.getPreferredFoci()[0];
-        }
-    }
-
     public async updateCallIntent(callIntent: RTCCallIntent): Promise<void> {
         if (!this.activated || !this.ownMembership) {
             throw Error("You cannot update your intent before joining the call");
@@ -302,7 +293,6 @@ export class MembershipManager
      * @param joinConfig
      * @param room
      * @param client
-     * @param getOldestMembership
      */
     public constructor(
         private joinConfig: (SessionConfig & MembershipConfig) | undefined,
@@ -315,8 +305,7 @@ export class MembershipManager
             | "_unstable_sendDelayedStateEvent"
             | "_unstable_updateDelayedEvent"
         >,
-        private getOldestMembership: () => CallMembership | undefined,
-        public readonly sessionDescription: SessionDescription,
+        public readonly slotDescription: SlotDescription,
         parentLogger?: Logger,
     ) {
         super();
@@ -325,7 +314,9 @@ export class MembershipManager
         if (userId === null) throw Error("Missing userId in client");
         if (deviceId === null) throw Error("Missing deviceId in client");
         this.deviceId = deviceId;
-        this.stateKey = this.makeMembershipStateKey(userId, deviceId);
+        // this needs to become a uuid so that consecutive join/leaves result in a key rotation.
+        // we keep it as a string for now for backwards compatibility.
+        this.memberId = this.makeMembershipStateKey(userId, deviceId);
         this.state = MembershipManager.defaultState;
         this.callIntent = joinConfig?.callIntent;
         this.scheduler = new ActionScheduler((type): Promise<ActionUpdate> => {
@@ -371,9 +362,10 @@ export class MembershipManager
     }
     // Membership Event static parameters:
     private deviceId: string;
-    private stateKey: string;
-    private fociPreferred?: Focus[];
-    private focusActive?: Focus;
+    private memberId: string;
+    /** @deprecated This will be removed in favor or rtcTransport becoming a list of actively used transports */
+    private fociPreferred?: Transport[];
+    private rtcTransport?: Transport;
 
     // Config:
     private delayedLeaveEventDelayMsOverride?: number;
@@ -406,6 +398,9 @@ export class MembershipManager
     private get delayedLeaveEventRestartLocalTimeoutMs(): number {
         return this.joinConfig?.delayedLeaveEventRestartLocalTimeoutMs ?? 2000;
     }
+    private get useRtcMemberFormat(): boolean {
+        return this.joinConfig?.useRtcMemberFormat ?? false;
+    }
     // LOOP HANDLER:
     private async membershipLoopHandler(type: MembershipActionType): Promise<ActionUpdate> {
         switch (type) {
@@ -472,9 +467,9 @@ export class MembershipManager
                 {
                     delay: this.delayedLeaveEventDelayMs,
                 },
-                EventType.GroupCallMemberPrefix,
+                this.useRtcMemberFormat ? EventType.RTCMembership : EventType.GroupCallMemberPrefix,
                 {}, // leave event
-                this.stateKey,
+                this.memberId,
             )
             .then((response) => {
                 this.state.expectedServerDelayLeaveTs = Date.now() + this.delayedLeaveEventDelayMs;
@@ -659,9 +654,9 @@ export class MembershipManager
         return await this.client
             .sendStateEvent(
                 this.room.roomId,
-                EventType.GroupCallMemberPrefix,
+                this.useRtcMemberFormat ? EventType.RTCMembership : EventType.GroupCallMemberPrefix,
                 this.makeMyMembership(this.membershipEventExpiryMs),
-                this.stateKey,
+                this.memberId,
             )
             .then(() => {
                 this.setAndEmitProbablyLeft(false);
@@ -705,9 +700,9 @@ export class MembershipManager
         return await this.client
             .sendStateEvent(
                 this.room.roomId,
-                EventType.GroupCallMemberPrefix,
+                this.useRtcMemberFormat ? EventType.RTCMembership : EventType.GroupCallMemberPrefix,
                 this.makeMyMembership(this.membershipEventExpiryMs * nextExpireUpdateIteration),
-                this.stateKey,
+                this.memberId,
             )
             .then(() => {
                 // Success, we reset retries and schedule update.
@@ -731,7 +726,12 @@ export class MembershipManager
     }
     private async sendFallbackLeaveEvent(): Promise<ActionUpdate> {
         return await this.client
-            .sendStateEvent(this.room.roomId, EventType.GroupCallMemberPrefix, {}, this.stateKey)
+            .sendStateEvent(
+                this.room.roomId,
+                this.useRtcMemberFormat ? EventType.RTCMembership : EventType.GroupCallMemberPrefix,
+                {},
+                this.memberId,
+            )
             .then(() => {
                 this.resetRateLimitCounter(MembershipActionType.SendLeaveEvent);
                 this.state.hasMemberStateEvent = false;
@@ -746,7 +746,7 @@ export class MembershipManager
 
     // HELPERS
     private makeMembershipStateKey(localUserId: string, localDeviceId: string): string {
-        const stateKey = `${localUserId}_${localDeviceId}_${this.sessionDescription.application}${this.sessionDescription.id}`;
+        const stateKey = `${localUserId}_${localDeviceId}_${this.slotDescription.application}${this.slotDescription.id}`;
         if (/^org\.matrix\.msc(3757|3779)\b/.exec(this.room.getVersion())) {
             return stateKey;
         } else {
@@ -757,20 +757,45 @@ export class MembershipManager
     /**
      * Constructs our own membership
      */
-    private makeMyMembership(expires: number): SessionMembershipData {
-        const hasPreviousEvent = !!this.ownMembership;
-        return {
-            // TODO: use the new format for m.rtc.member events where call_id becomes session.id
-            "application": this.sessionDescription.application,
-            "call_id": this.sessionDescription.id,
-            "scope": "m.room",
-            "device_id": this.deviceId,
-            expires,
-            "focus_active": { type: "livekit", focus_selection: "oldest_membership" },
-            "foci_preferred": this.fociPreferred ?? [],
-            "m.call.intent": this.callIntent,
-            ...(hasPreviousEvent ? { created_ts: this.ownMembership?.createdTs() } : undefined),
-        };
+    private makeMyMembership(expires: number): SessionMembershipData | RtcMembershipData {
+        const ownMembership = this.ownMembership;
+        if (this.useRtcMemberFormat) {
+            const relationObject = ownMembership?.eventId
+                ? { "m.relation": { rel_type: RelationType.Reference, event_id: ownMembership?.eventId } }
+                : {};
+            return {
+                application: {
+                    type: this.slotDescription.application,
+                    ...(this.callIntent ? { "m.call.intent": this.callIntent } : {}),
+                },
+                slot_id: slotDescriptionToId(this.slotDescription),
+                rtc_transports: this.rtcTransport ? [this.rtcTransport] : [],
+                member: { device_id: this.deviceId, user_id: this.client.getUserId()!, id: this.memberId },
+                versions: [],
+                ...relationObject,
+            };
+        } else {
+            const focusObjects =
+                this.rtcTransport === undefined
+                    ? {
+                          focus_active: { type: "livekit", focus_selection: "oldest_membership" } as const,
+                          foci_preferred: this.fociPreferred ?? [],
+                      }
+                    : {
+                          focus_active: { type: "livekit", focus_selection: "multi_sfu" } as const,
+                          foci_preferred: [this.rtcTransport, ...(this.fociPreferred ?? [])],
+                      };
+            return {
+                "application": this.slotDescription.application,
+                "call_id": this.slotDescription.id,
+                "scope": "m.room",
+                "device_id": this.deviceId,
+                expires,
+                "m.call.intent": this.callIntent,
+                ...focusObjects,
+                ...(ownMembership !== undefined ? { created_ts: ownMembership.createdTs() } : undefined),
+            };
+        }
     }
 
     // Error checks and handlers
diff --git a/src/matrixrtc/focus.ts b/src/matrixrtc/focus.ts
deleted file mode 100644
index cf9836dd450..00000000000
--- a/src/matrixrtc/focus.ts
+++ /dev/null
@@ -1,25 +0,0 @@
-/*
-Copyright 2023 The Matrix.org Foundation C.I.C.
-
-Licensed under the Apache License, Version 2.0 (the "License");
-you may not use this file except in compliance with the License.
-You may obtain a copy of the License at
-
-    http://www.apache.org/licenses/LICENSE-2.0
-
-Unless required by applicable law or agreed to in writing, software
-distributed under the License is distributed on an "AS IS" BASIS,
-WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-See the License for the specific language governing permissions and
-limitations under the License.
-*/
-
-/**
- * Information about a MatrixRTC conference focus. The only attribute that
- * the js-sdk (currently) knows about is the type: applications can extend
- * this class for different types of focus.
- */
-export interface Focus {
-    type: string;
-    [key: string]: unknown;
-}
diff --git a/src/matrixrtc/index.ts b/src/matrixrtc/index.ts
index 40ab6919f5b..9f52bec6f27 100644
--- a/src/matrixrtc/index.ts
+++ b/src/matrixrtc/index.ts
@@ -15,8 +15,7 @@ limitations under the License.
 */
 
 export * from "./CallMembership.ts";
-export type * from "./focus.ts";
-export * from "./LivekitFocus.ts";
+export * from "./LivekitTransport.ts";
 export * from "./MatrixRTCSession.ts";
 export * from "./MatrixRTCSessionManager.ts";
 export type * from "./types.ts";
diff --git a/src/matrixrtc/types.ts b/src/matrixrtc/types.ts
index b344a22d8b4..08c32a20628 100644
--- a/src/matrixrtc/types.ts
+++ b/src/matrixrtc/types.ts
@@ -156,3 +156,11 @@ export type Statistics = {
 
 export const isMyMembership = (m: CallMembership, userId: string, deviceId: string): boolean =>
     m.sender === userId && m.deviceId === deviceId;
+
+/**
+ *  A RTC transport is a JSON object that describes how to connect to a RTC member.
+ */
+export interface Transport {
+    type: string;
+    [key: string]: unknown;
+}
diff --git a/src/models/room-member.ts b/src/models/room-member.ts
index 6cf702e8aa3..afe72d4ef51 100644
--- a/src/models/room-member.ts
+++ b/src/models/room-member.ts
@@ -388,7 +388,7 @@ export class RoomMember extends TypedEventEmitter<RoomMemberEvent, RoomMemberEve
     }
 }
 
-const MXID_PATTERN = /@.+:.+/;
+export const MXID_PATTERN = /@.+:.+/;
 const LTR_RTL_PATTERN = /[\u200E\u200F\u202A-\u202F]/;
 
 function shouldDisambiguate(selfUserId: string, displayName?: string, roomState?: RoomState): boolean {