RFC 3147 (rfc3147) - Page 2 of 8


Generic Routing Encapsulation over CLNS Networks



Alternative Format: Original Text Document



RFC 3147    Generic Routing Encapsulation over CLNS Networks   July 2001


   Many vendors have already started to offer SONET and SDH products
   that are managed by IP instead of CLNS and a general migration from
   CLNS towards IP is anticipated within the industry.

   Part of any migration strategy from CLNS to IP should provide for the
   co-existence of both CLNS managed and IP managed network elements in
   the same network.

   Such a migration strategy should foresee the need to manage existing
   CLNS managed network elements that become isolated by a new IP
   network.  Such a scenario may be tackled by tunnelling CLNP PDUs over
   IP using the existing GRE standard RFC 2784 [1] and informational RFC
   1702 [2].  Networks have already been deployed that use this method.

   Such a migration strategy should also foresee the need to manage new
   IP managed network elements that are installed on the far side of
   existing CLNS managed network.  Such a scenario requires a method for
   tunnelling IP over CLNS.

2. GRE over CLNS advantages

   Using GRE to tunnel IP over CLNS offers some advantages.

      In the absence of a standard for tunnelling IP over CLNS, GRE as
      specified in RFC 2784 [1] is the most applicable standard that
      exists.

      The move from CLNS to IP comes at a time when IP is itself
      migrating from IPv4 to IPv6.  GRE defines a method to tunnel any
      protocol that has an Ethernet Protocol Type.  Therefore by
      defining a method for CLNS to transport GRE, a method will then
      exist for CLNS to transport any other protocol that has an
      Ethernet Protocol Type defined in RFC 1700 [5].  Thus GRE over
      CLNS can be used to tunnel both IPv4 and IPv6.

      GRE is already commonly used to tunnel CLNP PDUs over IP and so
      using GRE to tunnel IP over CLNS gives a common approach to
      tunnelling and may simplify software within network elements that
      initiate and terminate tunnels.

   The only disadvantage of using GRE is the extra minimum of four bytes
   that will be used between CLNP header and IP payload packet.  Given
   the large size of CLNP headers this will not make a  significant
   difference to the performance of any network that has IP over CLNP
   PDUs present on it.






Christian                    Informational