face: allow GenericLinkService to override MTU

refs #5056

Change-Id: I8eb2dd732b1431e0e809deb53792a14be1698ff1
diff --git a/daemon/face/link-service.hpp b/daemon/face/link-service.hpp
index 415c185..fd35723 100644
--- a/daemon/face/link-service.hpp
+++ b/daemon/face/link-service.hpp
@@ -1,6 +1,6 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /*
- * Copyright (c) 2014-2019,  Regents of the University of California,
+ * Copyright (c) 2014-2020,  Regents of the University of California,
  *                           Arizona Board of Regents,
  *                           Colorado State University,
  *                           University Pierre & Marie Curie, Sorbonne University,
@@ -109,6 +109,9 @@
   virtual const Counters&
   getCounters() const;
 
+  virtual ssize_t
+  getEffectiveMtu() const;
+
 public: // upper interface to be used by forwarding
   /** \brief Send Interest to \p endpoint
    *  \pre setTransport has been called
@@ -225,6 +228,12 @@
   return *this;
 }
 
+inline ssize_t
+LinkService::getEffectiveMtu() const
+{
+  return m_transport->getMtu();
+}
+
 inline void
 LinkService::receivePacket(const Block& packet, const EndpointId& endpoint)
 {