diff --git a/CHANGELOG.md b/CHANGELOG.md
index 3babf8f5..eb5c4394 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -27,7 +27,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [0.3.12] - 2019-11-22
 ### Added
-- New command line parameters `-address` and `-subnet` for getting the address/subnet from the config file, for use with `-useconffile` or `-useconf` 
+- New API functions `SetMaximumSessionMTU` and `GetMaximumSessionMTU`
+- New command line parameters `-address` and `-subnet` for getting the address/subnet from the config file, for use with `-useconffile` or `-useconf`
 - A warning is now produced in the Yggdrasil output at startup when the MTU in the config is invalid or has been adjusted for some reason
 
 ### Changed
diff --git a/src/tuntap/tun.go b/src/tuntap/tun.go
index d5bed523..1e994ea5 100644
--- a/src/tuntap/tun.go
+++ b/src/tuntap/tun.go
@@ -35,6 +35,7 @@ const tun_ETHER_HEADER_LENGTH = 14
 // you should pass this object to the yggdrasil.SetRouterAdapter() function
 // before calling yggdrasil.Start().
 type TunAdapter struct {
+	core        *yggdrasil.Core
 	writer      tunWriter
 	reader      tunReader
 	config      *config.NodeState
@@ -131,6 +132,7 @@ func (tun *TunAdapter) Init(core *yggdrasil.Core, config *config.NodeState, log
 	if !ok {
 		return fmt.Errorf("invalid options supplied to TunAdapter module")
 	}
+	tun.core = core
 	tun.config = config
 	tun.log = log
 	tun.listener = tunoptions.Listener
@@ -181,6 +183,7 @@ func (tun *TunAdapter) _start() error {
 	if tun.MTU() != current.IfMTU {
 		tun.log.Warnf("Warning: Interface MTU %d automatically adjusted to %d (supported range is 1280-%d)", current.IfMTU, tun.MTU(), MaximumMTU(tun.IsTAP()))
 	}
+	tun.core.SetMaximumSessionMTU(uint16(tun.MTU()))
 	tun.isOpen = true
 	go tun.handler()
 	tun.reader.Act(nil, tun.reader._read) // Start the reader
@@ -230,6 +233,12 @@ func (tun *TunAdapter) UpdateConfig(config *config.NodeConfig) {
 	// Replace the active configuration with the supplied one
 	tun.config.Replace(*config)
 
+	// If the MTU has changed in the TUN/TAP module then this is where we would
+	// tell the router so that updated session pings can be sent. However, we
+	// don't currently update the MTU of the adapter once it has been created so
+	// this doesn't actually happen in the real world yet.
+	//   tun.core.SetMaximumSessionMTU(...)
+
 	// Notify children about the configuration change
 	tun.Act(nil, tun.ckr.configure)
 }
diff --git a/src/yggdrasil/api.go b/src/yggdrasil/api.go
index 6dd70b8e..7f82c260 100644
--- a/src/yggdrasil/api.go
+++ b/src/yggdrasil/api.go
@@ -363,6 +363,27 @@ func (c *Core) SetNodeInfo(nodeinfo interface{}, nodeinfoprivacy bool) {
 	c.router.nodeinfo.setNodeInfo(nodeinfo, nodeinfoprivacy)
 }
 
+// GetMaximumSessionMTU returns the maximum allowed session MTU size.
+func (c *Core) GetMaximumSessionMTU() uint16 {
+	var mtu uint16
+	phony.Block(&c.router, func() {
+		mtu = c.router.sessions.myMaximumMTU
+	})
+	return mtu
+}
+
+// SetMaximumSessionMTU sets the maximum allowed session MTU size. The default
+// value is 65535 bytes. Session pings will be sent to update all open sessions
+// if the MTU has changed.
+func (c *Core) SetMaximumSessionMTU(mtu uint16) {
+	phony.Block(&c.router, func() {
+		if c.router.sessions.myMaximumMTU != mtu {
+			c.router.sessions.myMaximumMTU = mtu
+			c.router.sessions.reconfigure()
+		}
+	})
+}
+
 // GetNodeInfo requests nodeinfo from a remote node, as specified by the public
 // key and coordinates specified. The third parameter specifies whether a cached
 // result is acceptable - this results in less traffic being generated than is
diff --git a/src/yggdrasil/session.go b/src/yggdrasil/session.go
index 2f5e0af3..743dd7a4 100644
--- a/src/yggdrasil/session.go
+++ b/src/yggdrasil/session.go
@@ -55,10 +55,6 @@ type sessionInfo struct {
 	callbacks     []chan func()       // Finished work from crypto workers
 }
 
-func (sinfo *sessionInfo) reconfigure() {
-	// This is where reconfiguration would go, if we had anything to do
-}
-
 // Represents a session ping/pong packet, andincludes information like public keys, a session handle, coords, a timestamp to prevent replays, and the tun/tap MTU.
 type sessionPing struct {
 	SendPermPub crypto.BoxPubKey // Sender's permanent key
@@ -121,6 +117,7 @@ type sessions struct {
 	lastCleanup      time.Time
 	isAllowedHandler func(pubkey *crypto.BoxPubKey, initiator bool) bool // Returns true or false if session setup is allowed
 	isAllowedMutex   sync.RWMutex                                        // Protects the above
+	myMaximumMTU     uint16                                              // Maximum allowed session MTU
 	permShared       map[crypto.BoxPubKey]*crypto.BoxSharedKey           // Maps known permanent keys to their shared key, used by DHT a lot
 	sinfos           map[crypto.Handle]*sessionInfo                      // Maps handle onto session info
 	byTheirPerm      map[crypto.BoxPubKey]*crypto.Handle                 // Maps theirPermPub onto handle
@@ -133,12 +130,19 @@ func (ss *sessions) init(r *router) {
 	ss.sinfos = make(map[crypto.Handle]*sessionInfo)
 	ss.byTheirPerm = make(map[crypto.BoxPubKey]*crypto.Handle)
 	ss.lastCleanup = time.Now()
+	ss.myMaximumMTU = 65535
 }
 
 func (ss *sessions) reconfigure() {
-	for _, session := range ss.sinfos {
-		session.reconfigure()
-	}
+	ss.router.Act(nil, func() {
+		for _, session := range ss.sinfos {
+			sinfo, mtu := session, ss.myMaximumMTU
+			sinfo.Act(ss.router, func() {
+				sinfo.myMTU = mtu
+			})
+			session.ping(ss.router)
+		}
+	})
 }
 
 // Determines whether the session with a given publickey is allowed based on
@@ -187,9 +191,7 @@ func (ss *sessions) createSession(theirPermKey *crypto.BoxPubKey) *sessionInfo {
 	sinfo.mySesPriv = *priv
 	sinfo.myNonce = *crypto.NewBoxNonce()
 	sinfo.theirMTU = 1280
-	ss.router.core.config.Mutex.RLock()
-	sinfo.myMTU = uint16(ss.router.core.config.Current.IfMTU)
-	ss.router.core.config.Mutex.RUnlock()
+	sinfo.myMTU = ss.myMaximumMTU
 	now := time.Now()
 	sinfo.timeOpened = now
 	sinfo.time = now