diff --git a/components/script/dom/mediastream.rs b/components/script/dom/mediastream.rs
index a5bfb0c183d..49550c97521 100644
--- a/components/script/dom/mediastream.rs
+++ b/components/script/dom/mediastream.rs
@@ -2,6 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+use crate::dom::bindings::cell::DomRefCell;
 use crate::dom::bindings::codegen::Bindings::MediaStreamBinding;
 use crate::dom::bindings::reflector::reflect_dom_object;
 use crate::dom::bindings::root::DomRoot;
@@ -9,19 +10,20 @@ use crate::dom::eventtarget::EventTarget;
 use crate::dom::globalscope::GlobalScope;
 use dom_struct::dom_struct;
 use servo_media::webrtc::MediaStream as BackendMediaStream;
+use std::mem;
 
 #[dom_struct]
 pub struct MediaStream {
     eventtarget: EventTarget,
     #[ignore_malloc_size_of = "defined in servo-media"]
-    tracks: Vec<Box<BackendMediaStream>>,
+    tracks: DomRefCell<Vec<Box<BackendMediaStream>>>,
 }
 
 impl MediaStream {
     pub fn new_inherited(tracks: Vec<Box<BackendMediaStream>>) -> MediaStream {
         MediaStream {
             eventtarget: EventTarget::new_inherited(),
-            tracks,
+            tracks: DomRefCell::new(tracks),
         }
     }
 
@@ -32,4 +34,12 @@ impl MediaStream {
             MediaStreamBinding::Wrap,
         )
     }
+
+    pub fn get_tracks(&self) -> Vec<Box<BackendMediaStream>> {
+        // XXXManishearth we have hard ownership constraints here so we actually empty the vec,
+        // ideally we should only have a media stream id here, or servo-media hands
+        // out Arcs
+        let mut tracks = self.tracks.borrow_mut();
+        mem::replace(&mut *tracks, vec![])
+    }
 }
diff --git a/components/script/dom/rtcpeerconnection.rs b/components/script/dom/rtcpeerconnection.rs
index 6105bdd81eb..2ef603594a7 100644
--- a/components/script/dom/rtcpeerconnection.rs
+++ b/components/script/dom/rtcpeerconnection.rs
@@ -22,6 +22,7 @@ use crate::dom::bindings::root::{DomRoot, MutNullableDom};
 use crate::dom::event::{Event, EventBubbles, EventCancelable};
 use crate::dom::eventtarget::EventTarget;
 use crate::dom::globalscope::GlobalScope;
+use crate::dom::mediastream::MediaStream;
 use crate::dom::promise::Promise;
 use crate::dom::rtcicecandidate::RTCIceCandidate;
 use crate::dom::rtcpeerconnectioniceevent::RTCPeerConnectionIceEvent;
@@ -390,6 +391,15 @@ impl RTCPeerConnectionMethods for RTCPeerConnection {
             }).into());
         p
     }
+
+    // https://w3c.github.io/webrtc-pc/#legacy-interface-extensions
+    fn AddStream(&self, stream: &MediaStream) {
+        let mut tracks = stream.get_tracks();
+
+        for track in tracks.drain(..) {
+            self.controller.borrow().as_ref().unwrap().add_stream(track);
+        }
+    }
 }
 
 impl From<SessionDescription> for RTCSessionDescriptionInit {
diff --git a/components/script/dom/webidls/RTCPeerConnection.webidl b/components/script/dom/webidls/RTCPeerConnection.webidl
index c68efd7e2b0..c79dba097bf 100644
--- a/components/script/dom/webidls/RTCPeerConnection.webidl
+++ b/components/script/dom/webidls/RTCPeerConnection.webidl
@@ -34,6 +34,9 @@ interface RTCPeerConnection : EventTarget {
     //          attribute EventHandler           oniceconnectionstatechange;
     //          attribute EventHandler           onicegatheringstatechange;
     //          attribute EventHandler           onconnectionstatechange;
+
+    // removed from spec, but still shipped by browsers
+    void addStream (MediaStream stream);
 };
 
 dictionary RTCConfiguration {