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Versions: 00 01 rfc2682                                                 
MPLS Working Group                                      Indra Widjaja
                                       Fujitsu Network Communications
Internet Draft                                          Anwar Elwalid
Expiration: Dec 1997                   Bell Labs, Lucent Technologies
                                                            July 1997

          Performance Issues in VC-Merge Capable MPLS Switches

Status of this Memo

   This document is an Internet Draft. Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
   and its Working Groups. Note that other groups may also distribute
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   VC merging allows many routes to be mapped to the same VC label,
   thereby providing a scalable mapping method that can support tens of
   thousands of edge routers. VC merging requires reassembly buffers so
   that cells belonging to different packets intended for the same
   destination do not interleave with each other.  This document
   investigates the impact of VC merging on the additional buffer
   required for the reassembly buffers and other buffers.  The main
   result indicates that VC merging incurs a minimal overhead compared
   to non-VC merging in terms of additional buffering. Moreover, the
   overhead decreases as utilization increases, or as the traffic
   becomes more bursty.

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1.  Introduction

Recently some radical proposals to overhaul the legacy router architec-
tures have been presented by several organizations, notably the
Ipsilon's IP switching [1], Cisco's Tag switching [2], Toshiba's CSR
[3], IBM's ARIS [4], and IETF's MPLS [5].  Although the details of their
implementations vary, there is one fundamental concept that is shared by
all these proposals: map the route information to short fixed-length
labels so that next-hop routers can be determined quickly through index-
ing rather than some type of searching (or matching longest prefixes).

Although any layer 2 switching mechanism can in principle be applied,
the use of ATM switches in the backbone network is believed to be the
most attractive solution since ATM hardware switches have been exten-
sively studied and are widely available in many different architectures.
In this document, we will assume that layer 2 switching uses ATM tech-
nology. 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 doc-
ument, the destination can mean any of the above or other possible gran-

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 desti-
nation would be translated to the same outgoing VP value, thereby reduc-
ing 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.  For a

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given destination, the switch would encounter O(e) incoming VP labels ,
where e is the number of switch ports (typically, 8 to 16) which may
depend on the network size (or n).  If there are n destinations, each
switch would is now required to manage O(e*n) VP labels - a considerable
saving from O(n^2).  Although the number of label entries is consider-
ably reduced, VP merging  is not practical since the VP space is limited
to only 4,096 entries at the network-to-network interface.  A third
method, called  VC merging, maps incoming VC labels for the same desti-
nation 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.

It is worthy to mention that cell interleaving may be allowed if we use
the AAL 3/4 Message Identifier (MID) field to identify the sender
uniquely. However, this method has some serious drawbacks as: 1) the MID
size may not be sufficient to identify all senders, 2) the encapsulation
method is not efficient, 3) the CRC capability is not as powerful as in
AAL 5, and 4) AAL 3/4 is not as widely supported as AAL 5 in data commu-

Before VC merging with no cell interleaving can be qualified as the most
promising approach, two main issues need to be addressed.  First, the
feasibility of an ATM switch that is capable of merging VCs needs to be
investigated. Second, there is widespread concern that the additional
amount of buffering required to implement VC merging is excessive and
thus making the VC-merging method impractical.  Through analysis and
simulation, we will dispel these concerns in this document by showing
that the additional buffer requirement for VC merging is minimal for
most practical purposes.  Other performance related issues such addi-
tional delay due to VC merging will also be discussed.

2.  A VC-Merge Capable MPLS Switch Architecture

In principle, the reassembly buffers can be placed at the input or out-
put side of a switch. If they are located at the input, then the switch
fabric has to transfer all cells belonging to a given packet in an
atomic manner since cells are not allowed to interleave.  This  requires
the fabric to perform frame switching which is not flexible nor desir-
able when multiple QoSs need to be supported.  On the other hand, if the
reassembly buffers are located at the output, the switch fabric can

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forward each cell independently as in normal ATM switching. Placing the
reassembly buffers at the output makes an output-buffered ATM switch a
natural choice.

We consider a generic output-buffered VC-merge capable MPLS switch with
VCI translation performed at the output. Other possible architectures
may also be adopted.  The switch consists of a non-blocking cell switch
fabric and multiple output modules (OMs), each is associated with an
output port.  Each arriving ATM cell is appended with two fields con-
taining an output port number and an input port number.  Based on the
output port number, the switch fabric forwards each cell to the correct
output port, just as in normal ATM switches.  If VC merging is not
implemented, then the OM consists of an output buffer. If VC merging is
implemented, the OM contains a number of reassembly buffers (RBs), fol-
lowed by a merging unit, and an output buffer. Each RB typically corre-
sponds to an incoming VC value. It is important to note that each buffer
is a logical buffer, and it is envisioned that a common pool of memory
for the reassembly buffers and the output buffer.

The purpose of the RB is to ensure that cells for a given packet do not
interleave with other cells that are merged to the same VC.  This mecha-
nism (called store-and-forward at the packet level) can be accomplished
by storing each incoming cell for a given packet at the RB until the
last cell of the packet arrives.  When the last cell arrives, all cells
in the packet are transferred in an atomic manner to the output buffer
for transmission to the next hop. It is worth pointing out that perform-
ing a cut-through mode at the RB is not recommended since it would
result in wastage of bandwidth if the subsequent cells are delayed.
During the transfer of a packet to the output buffer, the incoming VCI
is translated to the outgoing VCI by the merging unit.  To save VC
translation table space, different incoming VCIs are merged to the same
outgoing VCI during the translation process if the cells are intended
for the same destination.  If all traffic is best-effort, full-merging
where all incoming VCs destined for the same destination network are
mapped to the same outgoing VC, can be implemented.  However, if the
traffic is composed of multiple classes, it is desirable to implement
partial merging, where incoming VCs destined for the same (destination
network, QoS) are mapped to the same outgoing VC.

Regardless of whether full merging or partial merging is implemented,
the output buffer may consist of a single FIFO buffer or multiple
buffers each corresponds to a destination network or (destination net-
work, QoS).  If a single output buffer is used, then the switch essen-
tially tries to emulate frame switching.  If multiple output buffers are
used, VC merging is different from frame switching since cells of a
given packet are not bound to be transmitted back-to-back.  In fact,
fair queueing can be implemented so that cells from their respective
output buffers are served according to some QoS requirements. Note that

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cell-by-cell scheduling can be implemented with VC merging, whereas only
packet-by-packet scheduling can be implemented with frame switching.  In
summary, VC merging is more flexible than frame switching and supports
better QoS control.

3.  Performance Investigation of VC Merging

This section compares the VC-merging switch and the non-VC merging
switch. The non-VC merging switch is analogous to the traditional
output-buffered ATM switch, whereby cells of any packets are allowed to
interleave.  Since each cell is a distinct unit of information, the
non-VC merging switch is a work-conserving system at the cell level.  On
the other hand, the VC-merging switch is non-work conserving so its per-
formance is always lower than that of the non-VC merging switch.  The
main objective here is to study the effect of VC merging on performance
implications of MPLS switches such as additional delay, additional
buffer, etc., subject to different traffic conditions.

In the simulation, the arrival process to each reassembly buffer is an
independent ON-OFF process. Cells within an ON period form a single
packet. During an OFF periof, the slots are idle.

3.1  Effect of Utilization on Additional Buffer Requirement

We first investigate the effect of switch utilization on the additional
buffer requirement for a given overflow probability.  To carry the com-
parison, we analyze the VC-merging and non-VC merging case when the
average packet size is equal to 10 cells, using geometrically dis-
tributed packet sizes and packet interarrival times, with cells of a
packet arriving contiguously (later, we consider other distributions).
The results show, as expected, the VC-merging switch requires more
buffers than the non-VC merging switch. When the utilization is low,
there may be relatively many incomplete packets in the reassembly
buffers at any given time, thus wasting storage resource.  For example,
when the utilization is 0.3, VC merging requires an additional storage
of about 45 cells to achieve the same overflow probability.  However, as
the utilization increases to 0.9, the additional storage to achieve the
same overflow probability drops to about 30 cells.  The reason is that
when traffic intensity increases, the VC-merging system becomes more

It is important to note that ATM switches must be dimensioned at high
utilization value (in the range of 0.8-0.9) to withstand harsh traffic
conditions.  At the utilization of 0.9, a VC-merge ATM switch requires a
buffer of size 976 cells to provide an overflow probability of 10^{-5},

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whereas an non-VC merge ATM switch requires a buffer of size 946.  These
numbers translate the additional buffer requirement for VC merging to
about 3% - hardly an additional hardware cost.

3.2  Effect of Packet Size on Additional Buffer Requirement

We now vary the average packet size to see the impact on the buffer
requirement.  We fix the utilization to 0.5 and use two different aver-
age packet sizes; that is, B=10 and B=30. To achieve the same overflow
probability, VC merging requires an additional buffer of about 40 cells
(or 4 packets) compared to non-VC merging when B=10.  When B=30, the
additional buffer requirement is about 90 cells (or 3 packets).  In
terms of the number of packets, the additional buffer requirement does
not increase as the average packet size increases.

3.3  Additional Buffer Overhead Due to Packet Reassembly

There may be some concern that VC merging may require too much buffering
when the number of reassembly buffers increases, which would happen if
the switch size is increased or if cells for packets going to different
destinations are allowed to interleave.  We will show that the concern
is unfounded since buffer sharing becomes more efficient as the number
of reassembly buffers increases.

To demonstrate our argument, we consider the overflow probability for VC
merging for several values of reassembly buffers (N); i.e., N=4, 8, 16,
32, 64, and 128.  The utilization is fixed to 0.8 for each case, and the
average packet size is chosen to be 10.  For a given overflow probabil-
ity, the increase in buffer requirement becomes less pronounced as N
increases.  Beyond a certain value (N=32), the increase in buffer
requirement becomes insignificant.  The reason is that as N increases,
the traffic gets thinned and eventually approaches a limiting process.

3.4  Effect of Interarrival time Distribution on Additional Buffer

We now turn our attention to different traffic processes.  First, we use
the same ON period distribution and change the OFF period distribution
from geometric to hypergeometric which has a larger Square Coefficient
of Variation (SCV), defined to be the ratio of the variance to the
square of the mean.  Here we fix the utilization at 0.5.  As expected,
the switch performance degrades as the SCV increases in both the VC-
merging and non-VC merging cases.  To achieve a buffer overflow proba-
bility of 10^{-4}, the additional buffer required is about 40 cells when
SCV=1, 26 cells when SCV=1.5, and 24 cells when SCV=2.6.  The result
shows that VC merging becomes more work-conserving as SCV increases.  In
summary, as the interarrival time between packets becomes more bursty,
the additional buffer requirement for VC merging diminishes.

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3.5  Effect of Internet Packets on Additional Buffer Requirement

Up to now, the packet size has been modeled as a geometric distribution
with a certain parameter.  We now modify the packet size distribution to
a more realistic one. Since the initial deployment of VC-merge capable
ATM switches is likely to be in the core network, it is more realistic
to consider the packet size distribution in the Wide Area Network.  To
this end, we refer to the data given in [6].  The data  collected on Feb
10, 1996, in FIX-West network, is in the form of probability mass func-
tion versus packet size in bytes.  Data collected at other dates closely
resemble this one.

The distribution appears bi-modal with two big masses at 40 bytes (about
a third) due to TCP acknowledgment packets, and 552 bytes (about 22 per-
cent) due to Maximum Transmission Unit (MTU) limitations in many
routers. Other prominent packet sizes include 72 bytes (about 4.1 per-
cent), 576 bytes (about 3.6 percent), 44 bytes (about 3 percent), 185
bytes (about 2.7 percent), and 1500 bytes (about 1.5 percent) due to
Ethernet MTU. The mean packet size is  257 bytes, and the variance is
84,287 bytes^2. Thus, the SCV for the Internet packet size is about 1.1.

To convert the IP packet size in bytes to ATM cells, we assume AAL 5
using null encapsulation where the additional overhead in AAL 5 is 8
bytes long [7].  Using the null encapsulation technique, the average
packet size is about 6.2 ATM cells.

We examine the buffer overflow probability against the buffer size using
the Internet packet size distribution. The OFF period is assumed to have
a geometric distribution.  Again, we find that the same behavior as
before, except that the buffer requirement drops with Internet packets
due to smaller average packet size.

3.6  Effect of Correlated Interarrival Times on Additional Buffer

To model correlated interarrival times, we use the DAR(p) process (dis-
crete autoregressive process of order p) [8], which has been used to
accurately model video traffic (Star Wars movie) in [9].  The DAR(p)
process is a p-th order (lag-p) discrete-time Markov chain. The state of
the process at time n depends explicitly on the states at times (n-1),
...,  (n-p).

We examine the overflow probability for the case where the interarrival
time between packets is geometric and independent, and the case where
the interarrival time is geometric and correlated to the previous one
with coefficient of correlation equal to 0.9. The empirical distribution
of the Internet packet size from the last section is used. The utiliza-
tion is fixed to 0.5 in each case.  Although, the overflow probability

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increases as p increases, the additional amount of buffering actually
decreases for VC merging as p, or equivalently the correlation,
increases.  One can easily conclude that higher-order correlation or
long-range dependence will result in similar qualitative performance.

3.7  Slow Sources

The discussions up to now have assumed that cells within a packet arrive
back-to-back. With slow sources, adjacent cells would typically be
spaced  by idle slots. Adjacent cells within the same packet may also be
perturbed and spaced as these cells travel downstream due to the merging
and splitting of cells at preceding nodes.

Here, we assume that each source transmits at the rate of r (0 < r <
1), in units of link speed, to the ATM switch.  To capture the merging
and splitting of cells as they travel in the network, we will also
assume that the cell interarrival time within a packet is randomly per-
turbed.  To model this perturbation, we stretch the original ON period
by 1/r, and  flip a Bernoulli coin with parameter r during the
stretched ON period. In other words, a slot would contain a cell with
probability r, and would be idle with probability 1-r during the ON
period. By doing so, the average packet size remains the same as r is
varied.  We simulated slow sources on the VC-merge ATM switch using the
Internet packet size distribution with r=1 and r=0.2.  The packet
interarrival time is assumed to be geometrically distributed.  Reducing
the source rate in general reduces the stresses on the ATM switches
since the traffic becomes smoother.  With VC merging, slow sources also
have the effect of increasing the reassembly time. At utilization of
0.5, the reassembly time is more dominant and causes the slow source
(with r=0.2) to require more buffering than the fast source (with
r=1).  At utilization of 0.8, the smoother traffic is more dominant
and causes the slow source (with r=0.2) to require less buffering than
the fast source (with r=1).  This result again has practical conse-
quences in ATM switch design where buffer dimensioning is performed at
reasonably high utilization. In this situation, slow sources only help.

3.8  Packet Delay

It is of interest to see the impact of cell reassembly on packet delay.
Here we consider the delay at one node only; end-to-end delays are sub-
ject of ongoing work.  We define the delay of a packet as the time
between the arrival of the first cell of a packet at the switch and the
departure of the last cell of the same packet.  We study the average
packet delay as a function of utilization for both VC-merging and non-VC
merging switches for the case r=1 (back-to-back cells in a packet).
Again, the Internet packet size distribution is used to adopt the more
realistic scenario. The interarrival time of packets is geometrically

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distributed.  Although the difference in the worst-case delay between
VC-merging and non-VC merging can be theoretically very large, we
observe that the difference in average delays of the two systems to be
consistently about one average packet time for a wide range of utiliza-
tion. The difference is due to the average time needed to reassemble a

To see the effect of cell spacing in a packet, we again simulate the
average packet delay for r=0.2. We observe that the difference in
average delays of VC merging and non-VC merging increases to a few
packet times (approximately 20 cells at high utilization).  It should be
noted that when a VC-merge capable ATM switch reassembles packets, in
effect it performs the task that the receiver has to do otherwise.
>From practical point-of-view, an increase in 20 cells translates to
about 60 micro seconds at OC-3 link speed.  This additional delay should
be insignificant for most applications.  For delay-sensitive traffic,
the additional delay can be reduced by using smaller packets.

4.  Security Considerations

Security considerations are not addressed in this document.

5.  Conclusion

This document has investigated the impacts of VC merging on an ATM
switch performance. We experimented with various traffic processes to
understand the detailed behavior of VC-merge capable MPLS switches.  Our
main finding indicates that VC merging incurs a minimal overhead com-
pared to non-VC merging in terms of additional buffering. Moreover, the
overhead decreases as utilization increases, or as the traffic becomes
more bursty.  This fact has important practical consequences  since
switches are dimensioned for high utilization and stressful traffic con-
ditions.  We have considered the case where the output buffer uses a
FIFO scheduling. Future work will focus on fair queueing and variations.
Fair queueing essentially has the effect of spacing the incoming cells
and increasing the number of reassembly buffers.  Our earlier results
indicate that these two factors do not have a significant impact on the
amount of buffering. However, additional delay due to fair queueing
requires further investigation.  Network-wide performance implications
resulting from interconnecting many VC-merge capable ATM switches also
need further study.

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6.  Acknowledgment:

The authors thank Debasis Mitra for his penetrating questions during the
internal talks and discussions.

7.  References

   [1] P. Newman, Tom Lyon and G. Minshall,
   ``Flow Labelled IP: Connectionless ATM Under IP,''
   in Proceedings of INFOCOM'96, San-Francisco, Apr. 1996.

   [2] Y. Rekhter, B. Davie, D. Katz, E. Rosen and
   G. Swallow, ``Cisco Systems' Tag Switching Architecture Overview,''
   RFC 2105, Feb. 1997.

   [3] Y. Katsube, K. Nagami and H. Esaki,
   ``Toshiba's Router Architecture Extensions for ATM: Overview,''
   RFC 2098, Feb. 1997.

   [4] A. Viswanathan, N. Feldman, R. Boivie and R. Woundy,
   ``ARIS: Aggregate Route-Based IP Switching,''
   Internet Draft <draft-viswanathan-aris-overview-00.txt>, Mar. 1997.

   [5] R. Callon, P. Doolan, N. Feldman, A. Fredette,
   G. Swallow and A. Viswanathan,
   ``A Framework for Multiprotocol Label Switching,''
   Internet Draft <draft-ietf-mpls-framework-00.txt>, May 1997.

   [6] WAN Packet Size Distribution,

   [7] J. Heinanen,
   ``Multiprotocol Encapsulation over ATM Adaptation Layer 5,''
   RFC 1483, Jul. 1993.

   [8] P. Jacobs and P. Lewis,
   ``Discrete Time Series Generated by Mixtures III:
   Autoregressive Processes (DAR(p)),'' Technical Report NPS55-78-022,
   Naval Postgraduate School, 1978.

   [9] B.K. Ryu and A. Elwalid,
   ``The Importance of Long-Range Dependence of VBR Video Traffic
   in ATM Traffic Engineering,''
   ACM SigComm'96, Stanford, CA, pp. 3-14, Aug. 1996.

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Authors' Address:

   Indra Widjaja
   Fujitsu Network Communications, Inc.
   4403 Bland Road
   Raleigh, NC 27609, USA
   Phone: 919 790 2037
   Email: i_widjaja@fujitsu-fnc.com

   Anwar Elwalid
   Bell Labs, Lucent Technologies
   600 Mountain Ave. Rm 2C-124
   Murray Hill, NJ 07974, USA
   Phone: 908 582-7589
   Email: anwar@lucent.com

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