RFC 2682 (rfc2682) - Page 2 of 12

Performance Issues in VC-Merge Capable ATM LSRs

RFC 2682          Issues in VC Merge Capable ATM LSRs     September 1999


   architectures.  In this document, we will assume that layer 2
   switching uses ATM technology. In this case, each IP packet may be
   segmented to multiple 53-byte cells before being switched.
   Traditionally, AAL 5 has been used as the encapsulation method in
   data communications since it is simple, efficient, and has a powerful
   error detection mechanism.  For the ATM switch to forward incoming
   cells to the correct outputs, the IP route information needs to be
   mapped to ATM labels which are kept in the VPI or/and VCI fields.
   The relevant route information that is stored semi-permanently in the
   IP routing table contains the tuple (destination, next-hop router).
   The route information changes when the network state changes and this
   typically occurs slowly, except during transient cases.  The word
   "destination" typically refers to the destination network (or CIDR
   prefix), but can be readily generalized to (destination network,
   QoS), (destination host, QoS), or many other granularities. In this
   document, the destination can mean any of the above or other possible
   granularities.

   Several methods of mapping the route information to ATM labels exist.
   In the simplest form, each source-destination pair is mapped to a
   unique VC value at a switch. This method, called the non-VC merging
   case, allows the receiver to easily reassemble cells into respective
   packets since the VC values can be used to distinguish the senders.
   However, if there are n sources and destinations, each switch is
   potentially required to manage O(n^2) VC labels for full-meshed
   connectivity.  For example, if there are 1,000 sources/destinations,
   then the size of the VC routing table is on the order of 1,000,000
   entries.  Clearly, this method is not scalable to large networks.  In
   the second method called  VP merging, the VP labels of cells that are
   intended for the same destination would be translated to the same
   outgoing VP value, thereby reducing VP consumption downstream.  For
   each VP, the VC value is used to identify the sender so that the
   receiver can reconstruct packets even though cells from different
   packets are allowed to interleave.  Each switch is now required to
   manage O(n) VP labels - a considerable saving from O(n^2).  Although
   the number of label entries is considerably reduced, VP merging  is
   limited to only 4,096 entries at the network-to-network interface.
   Moreover, VP merging requires coordination of the VC values for a
   given VP, which introduces more complexity.  A third method, called
   VC merging, maps incoming VC labels for the same destination to the
   same outgoing VC label. This method is scalable and does not have the
   space constraint problem as in VP merging. With VC merging, cells for
   the same destination is indistinguishable at the output of a switch.
   Therefore, cells belonging to different packets for the same
   destination cannot interleave with each other, or else the receiver
   will not be able to reassemble the packets.  With VC merging, the
   boundary between two adjacent packets are identified by the "End-of-
   Packet" (EOP) marker used by AAL 5.



Widjaja & Elwalid            Informational