diff --git a/package.json b/package.json
index f772f89..cfdd949 100644
--- a/package.json
+++ b/package.json
@@ -8,7 +8,6 @@
   "type": "module",
   "scripts": {
     "build": "aegir build",
-    "clean": "rm -rfv node_modules dist *.lock *-lock.json ",
     "test": "aegir test",
     "format": "prettier --write src/*.ts"
   },
@@ -27,6 +26,7 @@
     "@libp2p/interfaces": "^3.0.3",
     "@libp2p/logger": "^2.0.0",
     "@libp2p/peer-id": "^1.1.15",
+    "@multiformats/multiaddr": "../js-multiaddr/",
     "abortable-iterator": "^4.0.2",
     "it-merge": "^1.0.4",
     "p-defer": "^4.0.0",
diff --git a/src/error.ts b/src/error.ts
new file mode 100644
index 0000000..c973d34
--- /dev/null
+++ b/src/error.ts
@@ -0,0 +1,13 @@
+export class WebRTCTransportError extends Error {
+    constructor(msg: string) {
+        super(msg);
+        this.name = 'WebRTCTransportError';
+    }
+}
+
+export class InvalidArgumentError extends WebRTCTransportError {
+    constructor(msg: string) {
+        super(msg);
+        this.name = 'WebRTC/InvalidArgumentError';
+    }
+}
\ No newline at end of file
diff --git a/src/sdp.ts b/src/sdp.ts
index b8c3189..65f131d 100644
--- a/src/sdp.ts
+++ b/src/sdp.ts
@@ -1,9 +1,10 @@
+import { InvalidArgumentError } from './error.js'
 import { logger } from '@libp2p/logger';
 import { Multiaddr } from '@multiformats/multiaddr';
 
 const log = logger('libp2p:webrtc:sdp');
 
-const P_XWEBRTC: number = 0x115;
+const CERTHASH_CODE: number = 466;
 const ANSWER_SDP_FORMAT: string = `
 v=0
 o=- 0 0 IN %s %s
@@ -37,38 +38,44 @@ function port(ma: Multiaddr): number {
   return ma.toOptions().port;
 }
 function certhash(ma: Multiaddr): string {
-  let webrtc_value = ma
-    .stringTuples()
-    .filter((tup) => tup[0] == P_XWEBRTC)
+  let tups = ma.stringTuples();
+  let certhash_value = tups
+    .filter((tup) => tup[0] == CERTHASH_CODE)
     .map((tup) => tup[1])[0];
-  if (webrtc_value) {
-    return webrtc_value.split('/')[1];
+  if (certhash_value) {
+    return certhash_value;
   } else {
-    throw new Error("Couldn't find a webrtc component of multiaddr:" + ma.toString());
+    throw new Error("Couldn't find a certhash component of multiaddr:" + ma.toString());
   }
 }
 
 function ma2sdp(ma: Multiaddr, ufrag: string): string {
-  return ANSWER_SDP_FORMAT.replace('/%s/', ipv(ma))
-    .replace('/%s/', ip(ma))
-    .replace('/%s/', ipv(ma))
-    .replace('/%s/', ip(ma))
-    .replace('/%s/', port(ma).toString())
-    .replace('/%s/', ufrag)
-    .replace('/%s/', ufrag)
-    .replace('/%s/', certhash(ma));
+  return ANSWER_SDP_FORMAT
+    .replace('%s', ipv(ma))
+    .replace('%s', ip(ma))
+    .replace('%s', ipv(ma))
+    .replace('%s', ip(ma))
+    .replace('%d', port(ma).toString())
+    .replace('%s', ufrag)
+    .replace('%s', ufrag)
+    .replace('%s', certhash(ma));
 }
 
 export function fromMultiAddr(ma: Multiaddr, ufrag: string): RTCSessionDescriptionInit {
   return {
-    type: 'offer',
+    type: 'answer',
     sdp: ma2sdp(ma, ufrag),
   };
 }
 
 export function munge(desc: RTCSessionDescriptionInit, ufrag: string): RTCSessionDescriptionInit {
-  //TODO
-  // desc.sdp.replaceAll(/^a=ice-ufrag=(.*)/, 'a=ice-ufrag=' + ufrag);
-  // desc.sdp.replaceAll(/^a=ice-pwd=(.*)/, 'a=ice-pwd=' + ufrag);
-  return desc;
+  if (desc.sdp) {
+    desc.sdp = desc.sdp
+      .replace(/\na=ice-ufrag:[^\n]*\n/, '\na=ice-ufrag:' + ufrag + '\n')
+      .replace(/\na=ice-pwd:[^\n]*\n/, '\na=ice-pwd:' + ufrag + '\n')
+      ;
+      return desc;
+  } else {
+    throw new InvalidArgumentError("Can't munge a missing SDP");
+  }
 }
diff --git a/test/node.js b/test/node.js
deleted file mode 100644
index 5c78a05..0000000
--- a/test/node.js
+++ /dev/null
@@ -1,6 +0,0 @@
-/* eslint-env mocha */
-
-export {}
-
-describe('noop', () => {
-})
diff --git a/test/sdp.spec.ts b/test/sdp.spec.ts
new file mode 100644
index 0000000..f576a9e
--- /dev/null
+++ b/test/sdp.spec.ts
@@ -0,0 +1,51 @@
+import { Multiaddr } from '@multiformats/multiaddr';
+import { expect } from 'chai';
+import * as underTest from '../src/sdp.js';
+
+const an_sdp = `
+v=0
+o=- 0 0 IN IP4 192.168.0.152
+s=-
+c=IN IP4 192.168.0.152
+t=0 0
+m=application 2345 UDP/DTLS/SCTP webrtc-datachannel
+a=mid:0
+a=ice-options:ice2
+a=ice-ufrag:MyUserFragment
+a=ice-pwd:MyUserFragment
+a=fingerprint:mTXVsdGliYXNlIGlzIGF3ZXNvbWUhIFxvLw
+a=setup:actpass
+a=sctp-port:5000
+a=max-message-size:100000
+`;
+
+describe('SDP creation', () => {
+  it('handles simple blue sky easily enough', async () => {
+    let ma = new Multiaddr('/ip4/192.168.0.152/udp/2345/webrtc/certhash/zYAjKoNbau5KiqmHPmSxYCvn66dA1vLmwbt');
+    let ufrag = 'MyUserFragment';
+    let sdp = underTest.fromMultiAddr(ma, ufrag);
+    expect(sdp.sdp).to.equal(an_sdp);
+  });
+});
+
+describe('SDP munging', () => {
+    it('does a simple replacement', () => {
+        let result = underTest.munge({type:'answer',sdp: an_sdp},'someotheruserfragmentstring');
+        expect(result.sdp).to.equal(`
+v=0
+o=- 0 0 IN IP4 192.168.0.152
+s=-
+c=IN IP4 192.168.0.152
+t=0 0
+m=application 2345 UDP/DTLS/SCTP webrtc-datachannel
+a=mid:0
+a=ice-options:ice2
+a=ice-ufrag:someotheruserfragmentstring
+a=ice-pwd:someotheruserfragmentstring
+a=fingerprint:mTXVsdGliYXNlIGlzIGF3ZXNvbWUhIFxvLw
+a=setup:actpass
+a=sctp-port:5000
+a=max-message-size:100000
+`);
+    });
+});