face: allow GenericLinkService to override MTU

refs #5056

Change-Id: I8eb2dd732b1431e0e809deb53792a14be1698ff1
diff --git a/daemon/face/face-common.hpp b/daemon/face/face-common.hpp
index f233309..e2a0572 100644
--- a/daemon/face/face-common.hpp
+++ b/daemon/face/face-common.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,
@@ -54,6 +54,12 @@
 /// upper bound of reserved FaceIds
 const FaceId FACEID_RESERVED_MAX = 255;
 
+/** \brief Minimum MTU that may be set
+ *
+ *  This is done to ensure the NDNLPv2 fragmentation feature functions properly.
+ */
+const ssize_t MIN_MTU = 64;
+
 /** \brief Identifies a remote endpoint on the link.
  *
  *  This ID is only meaningful in the context of the same Transport.