RFC 2682 (rfc2682) - Page 2 of 12
Performance Issues in VC-Merge Capable ATM LSRs
Alternative Format: Original Text Document
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