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   Internet Engineering Task Force                 Gagan L. Choudhury
   Internet Draft                                  Vera D. Sapozhnikova
   Expires in September, 2003                      AT&T
   Category: Best Current Practice
   draft-ietf-ospf-scalability-03.txt              Anurag S. Maunder
                                                   Sanera Systems

                                                   Vishwas Manral
                                                   Netplane Systems

                                                   March, 2003


           Prioritized Treatment of Specific OSPF
           Packets and Congestion Avoidance


Status of this Memo

   This document is an Internet-Draft and is in full conformance
   with all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
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        http://www.ietf.org/ietf/1id-abstracts.txt
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   Distribution of this memo is unlimited.


Abstract

   This document proposes methods that are intended to improve the
   scalability and stability of large networks using OSPF protocol.
   The methods include processing OSPF Hellos and LSA Acknowledgements
   at a higher priority compared to other OSPF packets, and other
   congestion avoidance procedures. Simulation results in support of
   some of the proposals are given in the appendix sections.




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Table of Contents

   1. Motivation.....................................................2
   2. The Proposals..................................................3
   3. Security Considerations........................................4
   4. Acknowledgments................................................4
   5. References.....................................................5
   6. Authors' Addresses.............................................5
   Appendix A. LSA Storm: Causes and Impact..........................6
   Appendix B. Simulation Study......................................8
   Appendix B.1. The Network Under Simulation........................8
   Appendix B.2. Simulation Results.................................11
   Appendix B.3. Observations on Simulation Results.................15
   Appendix C. Other Proposals......................................15


1. Motivation

   A large network running OSPF [Ref1] or OSPF-TE [Ref2] protocol may
   occasionally experience the simultaneous or near-simultaneous update
   of a large number of link-state-advertisement messages, or LSAs.
   We call this event, an LSA storm and it may be initiated by an
   unscheduled failure or a scheduled maintenance or upgrade event.
   The failure may be hardware, software, or procedural in nature.

   The LSA storm causes high CPU and memory utilization at the node
   processors causing incoming packets to be delayed or dropped.
   Delayed acknowledgements (beyond the retransmission timer value)
   results in retransmissions, and delayed Hello packets (beyond the
   router-dead interval) results in links being declared down.
   The retransmissions and additional LSA generations result in further
   CPU and memory usage, essentially causing a positive feedback loop,
   which, in the extreme case, may drive the network to an unstable
   state.

   The default value of retransmission timer is 5 seconds and that of
   the router-dead interval is 40 seconds.  However, recently there
   has been a lot of interest in significantly reducing OSPF convergence
   time and as part of that plan much shorter (subsecond) Hello and
   router-dead intervals have been proposed [Ref3].  In such a scenario
   it will be more likely for Hello packets to be delayed beyond
   the router-dead interval during a network congestion event
   caused by an LSA storm.

   Appendix A explains in more detail LSA storm generation scenarios,
   its impact, and points out a few real-life examples of control-message
   storm generation.  Appendix B presents a simulation study on this
   phenomenon.




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   In order to improve the scalability and stability of networks we
   propose steps for prioritizing critical OSPF packets and avoiding
   congestion. The details of the proposals are given
   in Section 2.  We also do a simulation study on a subset of the
   proposals in Appendix B and show that they indeed improve the
   scalability and stability of networks using OSPF protocol.

   Appendix C provides some further proposals with similar goals.


2. The Proposals

   The proposals below are intended to improve the scalability
   and stability of large networks using OSPF protocol.  During
   periods of network congestion they would reduce retransmissions,
   avoid an interface to be declared down due to Hello packets
   being delayed beyond the RouterDeadInterval, and take other
   congestion avoidance steps.

   Either all, or a subset of the proposals may be implemented by
   a Router.  It is also possible for some routers to implement
   them fully or partially, and others to not implement them at
   all.


   (1) Classify all OSPF packets in two classes: a "high priority"
       class comprising of OSPF Hello packets and Link State
       Acknowledgement packets, and a "low priority" class
       comprising of all other packets. The classification is
       accomplished by examining the OSPF packet header. While
       receiving a packet from a neighbor and while transmitting
       a packet to a neighbor, try to process a "high priority"
       packet ahead of a "low priority" packet.

   (2) Reset the Inactivity Timer for an interface whenever any OSPF
       packet is received over that interface (currently this is
       done only for the Hello packet).
       So OSPF would declare the interface to be down only if no
       OSPF packet is received over that interface for a period
       equaling or exceeding the RouterDeadInterval.

   (3) Use an Exponential Backoff algorithm for determining the value
       of the LSA retransmission interval (RxmtInterval).  Let R(i)
       represent the RxmtInterval value used during the i-th
       retransmission of an LSA.  Use the following algorithm to
       compute R(i)

                    R(1) = Rmin
                    R(i+1) = Min(KR(i),Rmax)  for i>=1



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       where K, Rmin and Rmax are constants and the function
       Min(.,.) represents the minimum value of its two arguments.
       Example values for K, Rmin and Rmax may be 2, 5
       seconds and 40 seconds respectively.

   (4) Implicit Congestion Detection and Action Based on That:
       If there is control message congestion at a node, its
       neighbors do not know about that explicitly.  However, they
       can implicitly detect it based on the number of unacknowledged
       LSAs to this node.  If this number exceeds a certain "high
       water mark" then the rate at which LSAs are sent to this node
       should be reduced.  At a future time, if the number of
       unacknowledged LSAs to this node falls below a certain "low
       water mark" then the normal rate of sending LSAs to this
       node should be resumed.  An example value for the "high
       water mark" may be 20 unacknowledged LSAs and that for the "low
       water mark" may be 10 unacknowledged LSAs.  An example
       value for the rate on exceeding the "high water mark" may be
       50% the normal rate.

   (5) Throttling Adjacencies to be Brought Up Simultaneously:
       If a node tries to bring up a large number of adjacencies to
       its neighbors simultaneously then that may cause severe
       congestion due to database synchronization and LSA flooding
       activities.  It is recommended that during such a situation
       no more than "n" adjacencies should be brought up
       simultaneously.  Once a subset of adjacencies have been brought
       up successfully, newer adjacencies may be brought up as long as
       the number of simultaneous adjacencies being brought up does not
       exceed "n". An example value for "n" may be 4.

3. Security Considerations

   This memo does not create any new security issues for the OSPF
   protocol.  Security considerations for the base OSPF protocol are
   covered in [Ref1].

4. Acknowledgments

   We would like to acknowledge the support of OSPF WG chairs
   Rohit Dube, Acee Lindem, and John Moy.  We also acknowledge
   Jerry Ash, Margaret Chiosi, Elie
   Francis, Jeff Han, Beth Munson, Roshan Rao, Moshe Segal, Mike
   Wardlow, and Pat Wirth for collaboration and encouragement in
   our scalability improvement efforts for Link-State-Protocol based
   networks.






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5. References

   [Ref1] J. Moy, "OSPF Version 2", RFC 2328, April, 1998.

   [Ref2] D. Katz, D. Yeung, K. Kompella, "Traffic Engineering
   Extension to OSPF Version 2," Work in Progress.

   [Ref3] C. Alaettinoglu, V. Jacobson and H. Yu, "Towards Milli-
   second IGP Convergence," Work in Progress.

   [Ref4] Pappalardo, D., "AT&T, customers grapple with ATM net
   outage," Network World, February 26, 2001.

   [Ref5] "AT&T announces cause of frame-relay network outage," AT&T
   Press Release, April 22, 1998.

   [Ref6] Cholewka, K., "MCI Outage Has Domino Effect," Inter@ctive
   Week, August 20, 1999.

   [Ref7] Jander, M., "In Qwest Outage, ATM Takes Some Heat," Light
   Reading, April 6, 2001.

   [Ref8] A. Zinin and M. Shand, "Flooding Optimizations in Link-State
   Routing Protocols," Work in Progress.

   [Ref9] J. Moy, "Flooding over Parallel Point-to-Point Links," Work in
   progress.

   [Ref10] P. Pillay-Esnault, "OSPF Refresh and flooding reduction in
   stable topologies," Work in progress.

   [Ref11] J. Ash, G. Choudhury, V. Sapozhnikova, M. Sherif, A.
   Maunder, V. Manral, "Congestion Avoidance & Control for OSPF
   Networks", Work in Progress.

   [Ref12] B. M. Waxman, "Routing of Multipoint Connections," IEEE
   Journal on Selected Areas in Communications, 6(9):1617-1622, 1988.


6. Authors' Addresses

   Gagan L. Choudhury
   AT&T
   Room D5-3C21
   200 Laurel Avenue
   Middletown, NJ, 07748
   USA
   Phone: (732)420-3721
   email: gchoudhury@att.com


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   Vera D. Sapozhnikova
   AT&T
   Room C5-2C29
   200 Laurel Avenue
   Middletown, NJ, 07748
   USA
   Phone: (732)420-2653
   email: sapozhnikova@att.com

   Anurag S. Maunder
   Sanera Systems
   370 San Aleso Ave.
   Second Floor
   Sunnyvale, CA 94085
   Phone: (408)734-6123
   email: amaunder@sanera.net

   Vishwas Manral
   NetPlane
   189, Prashasan Nagar,
   Road Number 72
   Jubilee Hills, Hyderabad
   India
   email: Vishwasm@netplane.com




Appendix A. LSA Storm: Causes and Impact

   An LSA storm may be initiated due to many reasons.  Here
   are some examples:

   (a) one or more link failures due to fiber cuts,

   (b) one or more node failures for some reason, e.g., software
       crash or some type of disaster (including power outage)
       in an office complex hosting many nodes,

   (c) Link/node flapping,

   (d) requirement of taking down and later bringing back many
       nodes during a software/hardware upgrade,

   (e) near-synchronization of the once-in-30-minutes refresh instants
       of a subset of LSAs,

   (f) refresh of all LSAs in the system during a change in software
       version,



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   (g) injecting a large number of external routes to OSPF due to
       a procedural error.

   In addition to the LSAs generated as a direct result of link/node
   failures, there may be other indirect LSAs as well.  One example
   in MPLS networks is traffic engineering LSAs generated at other
   links as a result of significant change in reserved bandwidth
   resulting from rerouting of Label Switched Paths (LSPs) that went
   down during the link/node failure.

   The LSA storm causes high CPU and memory utilization at the node
   processors causing incoming packets to be delayed or dropped.
   Delayed acknowledgements (beyond the retransmission timer value)
   results in retransmissions, and delayed Hello packets (beyond the
   Router-Dead interval) results in links being declared down.  A
   trunk-down event causes Router LSA generation by its end-point
   nodes.  If traffic engineering LSAs are used for each link then
   that type of LSAs would also be generated by the end-point nodes
   and potentially elsewhere as well due to significant changes in
   reserved bandwidths at other links caused by the failure and reroute
   of LSPs originally using the failed trunk.  Eventually, when the
   link recovers that would also trigger additional Router and traffic
   engineering LSAs.

   The retransmissions and additional LSA generations result in further
   CPU and memory usage, essentially causing a positive feedback loop.
   We define the LSA storm size as the number of LSAs in the original
   storm and not counting any additional LSAs resulting from the
   feedback loop described above.  If the LSA storm is too large then

   the positive feedback loop mentioned above may be large enough to
   indefinitely sustain a large CPU and memory utilization at many
   network nodes, thereby driving the network to an unstable state.
   In the past, network
   outage events have been reported in IP and ATM networks using
   link-state protocols such as OSPF, IS-IS, PNNI or some proprietary
   variants.  See, for example [Ref4-Ref7].  In many of these examples,
   large scale flooding of LSAs or other similar control messages
   (either naturally or triggered by some bug or inappropriate
   procedure) have been partly or fully responsible for network
   instability and outage.

   In Appendix B, we use a simulation model to show that there
   is a certain LSA storm
   size threshold above which the network may show unstable behavior
   caused by large number of retransmissions, link failures due to
   missed Hello packets and subsequent link recoveries.  We also show
   that the LSA storm size causing instability may be substantially
   increased by providing prioritized treatment to Hello and LSA
   Acknowledgment packets and by using an exponential backoff


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   algorithm for determining the LSA retransmission interval.
   Furthermore, if we prioritize Hello
   packets then even when the network operates somewhat above the
   stability threshold, links are not declared down due to missed
   Hellos.  This implies that even though there is
   control plane congestion due to many retransmissions, the data plane
   stays up and no new LSAs are generated (besides the ones in the
   original storm and the refreshes).  These observations are the
   basis of the first three proposals in Section 2.

   One might argue that the scalability issue of large networks should
   be solved solely by dividing the network hierarchically into
   multiple areas so that flooding of LSAs remains localized within
   areas.  However, this approach increases the network management
   and design complexity and may result in less optimal routing between
   areas. Also, ASE LSAs are flooded throughout the AS and it may be
   a problem if there are large numbers of them.  Furthermore,
   a large number of summary LSAs may need to be flooded across
   Areas and their numbers would increase significantly if
   multiple Area Border Routers are employed for the purpose of
   reliability. Thus it is important to allow the network to grow
   towards as large a size as possible under a single area.

   Our proposal here is synergistic with a broader set of scalability
   and stability improvement proposals. [Ref8, Ref9] proposes flooding
   overhead reduction in case more than one interface goes to the same
   neighbor.  [Ref10] proposes a mechanism for
   greatly reducing LSA refreshes in stable topologies.
   [Ref11] proposes a wide range of congestion control and failure
   recovery mechanisms.

Appendix B. Simulation Study

   The main motivation of this study is to show the network congestion
   and instability caused by large LSA storms and the improvement in
   stability and scalability that can be achieved by following the
   proposals in this memo.

Appendix B.1. The Network Under Simulation

   We generate a random network over a rectangular grid using a
   modified version of Waxman's algorithm [Ref12] that ensures that
   the network is connected and has a pre-specified number of nodes,
   links, maximum number of neighbors per node, and maximum number
   of adjacencies per node. The rectangular grid resembles the
   continental U.S.A. with maximum one-way propagation delay of 30 ms
   in the East-West direction and maximum one-way propagation delay of
   15 ms in the North-South direction.  We consider two different
   network sizes as explained in Section B.2.

   The network has a flat, single-area topology.

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   Each node is a Router and each link is a point-to-point link
   connecting two routers.

   We assume that node CPU and memory (not the link bandwidth) is the
   main bottleneck in the LSA flooding process.  This will typically
   be true for high speed links (e.g., OC3 or above) and/or links
   where OSPF traffic gets an adequate Quality of Service (QoS)
   compared to other traffic.

   Different Timers:
     LSA refresh interval = 1800 seconds,
     Hello refresh interval = 10 Seconds,
     Router-Dead interval = 40 seconds,
     LSA retransmission interval: two values are considered, 10 seconds
       and 5 Seconds (note that a retransmission is disabled on the
       receipt of either an explicit acknowledgment or a duplicate LSA
       over the same interface that acts as an implicit acknowledgment)
     Minimum time between successive generation of the same LSA = 5
       seconds,
     Minimum time between successive Dijkstra SPF calculations
       is 1 second.

   Packing of LSAs: It is assumed that for any given node, the LSAs
   generated over a 1-second period are packed together to form an LSU
   but no more than 3 LSAs are packed in one LSU.

   LSU/Ack/Hello Processing Times: All processing times are expressed
   in terms of the parameter T.  Two values of T are considered, 1 ms
   and 0.5 ms.

   In the case of a dedicated processor for processing OSPF packets the
   processing time reported represents the true processing time. If the
   processor does other work and only a fraction of its capacity can be
   dedicated to OSPF processing then we have to inflate the processing
   time appropriately to get the effective processing time and in that
   case it is assumed that the inflation factor is already taken into
   account as part of the reported processing time.

   The fixed time to send or receive any LSU, Ack or Hello packet is T.
   In addition, a variable processing time is used for LSU and Ack
   depending on the number and types of LSAs packed.  No variable
   processing time is used for Hello.
   Variable processing time per Router LSA is (0.5 + 0.17L)T where L is
   the number of adjacencies advertised by the Router LSA.  For other
   LSA types (e.g., ASE LSA or a "Link" LSA carrying traffic
   engineering information about a link), the variable processing time
   per LSA is 0.5T.

   Variable processing time for an Ack is 25% that of the corresponding
   LSA.


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   It is to be noted that if multiple LSAs are packed in a single LSU
   packet then the fixed processing time is needed only once but the
   variable processing time is needed for every component of the
   packet.

   The processing time values we use are roughly in the same range of
   what has been observed in an operational network.

   LSU/Ack/Hello Priority: Two non-preemptive priority levels and
   three priority scenarios are considered. Within each priority level
   processing is FIFO with new packets of lower priority being
   dropped when the lower priority queue is full.  The higher priority
   packets are never dropped.
      In Priority scenario 1, all LSUs/Acks/Hellos received at a node
      are queued at the lower priority.
      In Priority scenario 2, Hellos received at a node are queued at
      the higher priority but LSUs/Acks are queued at lower priority.
      In Priority scenario 3, Hellos and Acks received at a node are
      queued at the higher priority but LSUs are queued at lower
      priority.
   All packets generated internally to a node (usually triggered by
   a timer) are processed at the higher priority.  This includes the
   initial LSA storm, LSA refresh, Hello refresh, LSA retransmission
   and new LSA generation after detection of a failure or recovery.

   Buffer Size for Incoming LSUs/Acks/Hellos (lower priority): Buffer
   size is assumed to be 2000 packets where a packet is either an Ack,
   LSU, or Hello.

   LSA Refresh: Each LSA is refreshed once in 1800 seconds and the
   refresh instants of various LSAs in the LSDB are assumed to be
   uniformly distributed over the 1800 seconds period, i.e., they are
   completely unsynchronized.  If however, an LSA is generated as part
   of the initial LSA storm then it goes on a new refresh schedule of
   once in 1800 seconds starting from its generation time.

   LSA Storm Generation: As defined earlier, "LSA storm" is the
   simultaneous or near simultaneous generation of a large number of
   LSAs. In the case of only Router and ASE LSAs we normally assume
   that the number of ASE LSAs in the storm is about 4 times that of
   the Router LSAs, but the ratio is allowed to change if either the
   Router or the ASE LSAs have reached their maximum possible value.
   In the case of only Router and Link LSAs (carrying traffic
   engineering information) we normally assume that the number of Link
   LSAs in the storm is about 4 times that of the Router LSAs, but the
   ratio is allowed to change if either the Router or the Link LSAs
   have reached their maximum possible value.  For any given LSA storm
   we keep generating LSAs starting from Node index 1 and moving
   upwards and stop until the correct number of LSAs of each type have
   been generated.  The LSAs generated at any given node is assumed to
   start at an instant uniformly distributed between 20 and 30 seconds

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   from the start of the simulation.  Successive LSA generations at a
   node are assumed to be spaced apart by 400 ms. It is to be noted
   that during the period of observation there are other LSAs
   generated besides the ones in the storm.  These include refresh of
   LSAs that are not part of the storm and LSAs generated due to
   possible link failures and subsequent possible link recoveries.

   Failure/Recovery of Links: If no Hello is received over a link (due
   to CPU/memory congestion) for longer than Router-Dead Interval then
   the link is declared down.  At a later time, if Hellos are received
   then the link would be declared up.  Whenever a link is declared
   up or down, one Router LSA is generated by each Router on the
   two sides of the point-to-point link.  If "Link LSAs" carrying
   traffic engineering information is used then it is assumed that each
   Router would also generate a Link LSA.  In this case it is also
   assumed that due to rerouting of LSPs, three other links in the
   network (selected randomly in the simulation) would have significant
   change in reserved bandwidth which would result in one Link LSA
   being generated by the routers on the two ends of each such link.


Appendix B.2. Simulation Results

   In this section we study the relative performance of the three
   priority scenarios defined earlier (no priority to Hello or Ack,
   priority to Hello only, and priority to both Hello and Ack) with a
   range of Network sizes, LSA retransmission timer values, LSA types,
   processing time values and Hello/Router-Dead-Interval values:

   Network size: Two networks are considered.  Network 1 has 100 nodes,
   1200 links, maximum number of neighbors per node is 30 and maximum
   number of adjacencies per node is 50 (same neighbor may have more
   than one adjacencies).   Network 2 has 50 nodes, 600 links, maximum
   number of neighbors per node is 25 and maximum number of adjacencies
   per node is 48. Dijkstra SPF calculation time for Network 1 is
   assumed to be 100 ms and that for Network 2 is assumed to be 70 ms.

   LSA Type: Each node has 1 Router LSA (Total of 100 for Network 1 and
   50 for Network 2). There are no Network LSAs since all links are
   point-to-point links and no Summary LSAs since the network has only
   one area. Regarding other LSA types we consider two situations.  In
   Situation 1 we assume that there are no ASE LSAs and each link has
   one "Link" LSA carrying traffic engineering information (Total of
   2400 for Network 1 and 1200 for Network 2). In Situation 2 we assume
   that there are no "Link" LSAs and half of the nodes are ASA-Border
   nodes and each border node has 10 ASE LSAs (Total of 500 for
   Network 1 and 250 for Network 2).  We identify Situation 1 as "Link
   LSAs" and Situation 2 as "ASE LSAs".

   LSA retransmission timer value: Two values are considered, 10
   seconds and 5 seconds (default value).

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   Processing time values: Processing times for LSUs, Acks and Hello
   packets have been previously expressed in terms of a common
   parameter T.  Two values are considered for T, which are 1 ms
   and 0.5 ms respectively.

   Hello/Router-Dead-Interval: It is assumed that Router-Dead interval
   is four times the Hello interval.  In one case it is assumed that
   Hello interval is 10 seconds and Router-Dead-Interval is 40
   seconds (default values), and in the other case it is assumed that
   Hello interval is 2 seconds and Router-Dead-Interval is 8 seconds.

   Based on Network size, LSA type and processing time values we
   develop 6 Test cases as follows:

   Case 1: Network 1, Link LSAs, retransmission timer = 10 sec.,
           T = 1 ms, Hello/Router-Dead-Interval = 10/40 sec.

   Case 2: Network 1, ASE LSAs, retransmission timer = 10 sec.,
           T = 1 ms, Hello/Router-Dead-Interval = 10/40 sec.

   Case 3: Network 1, Link LSAs, retransmission timer = 5 sec.,
           T = 1 ms, Hello/Router-Dead-Interval = 10/40 sec.

   Case 4: Network 1, Link LSAs, retransmission timer = 10 sec.,
           T = 0.5 ms, Hello/Router-Dead-Interval = 10/40 sec.

   Case 5: Network 1, Link LSAs, retransmission timer = 10 sec.,
           T = 1 ms, Hello/Router-Dead-Interval = 2/8 sec.

   Case 6: Network 2, Link LSAs, retransmission timer = 10 sec.,
           T = 1 ms, Hello/Router-Dead-Interval = 10/40 sec.

   For each case and for each Priority scenario we study the network
   stability as a function of the size of the LSA storm.  The stability
   is determined by looking at the number of non-converged LSUs as a
   function of time. An example is shown in Table 1 for Case 1 and
   Priority scenario 1 (No priority to Hellos or Acks).















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=========|==========================================================
         | Number of Non-Converged LSUs in the Network at Time(in sec)
    LSA  |
   STORM |====|=====|=====|=====|=====|=====|=====|=====|========|==
   SIZE  |10s | 20s | 30s | 35s | 40s | 50s | 60s | 80s | 100s   |
=========|====|=====|=====|=====|=====|=====|=====|=====|========|==
    100  | 0  |  0  | 24  | 29  | 24  |  1  |  0  |  1  |  1     |
 (Stable)|    |     |     |     |     |     |     |     |        |
---------|----|-----|-----|-----|-----|-----|-----|-----|--------|--
    140  | 0  |  0  | 35  | 48  | 46  | 27  | 14  |  1  |  1     |
 (Stable)|    |     |     |     |     |     |     |     |        |
---------|----|-----|-----|-----|-----|-----|-----|-----|--------|--
    160  | 0  |  0  | 38  | 57  | 55  | 40  | 26  | 65  | 203    |
(Unstable)    |     |     |     |     |     |     |     |        |
=========|==========================================================

           Table 1: Network Stability Vs. LSA Storm
              (Case 1, No priority to Hello/Ack)

   The LSA storm starts a little after 20 seconds and so for some
   period of time after that the number of non-converged LSUs should
   stay high and then come down for a stable network.
   This happens for LSA storms of sizes 100 and 140.  With an LSA storm
   of size 160, the number of non-converged LSUs stay high indefinitely
   due to repeated retransmissions, link failures due to missed Hellos
   for more than the Router-Dead interval which generates additional
   LSAs and also due to subsequent link recoveries which again
   generate additional LSAs.  We define network stability threshold as
   the maximum allowable LSA storm size for which the number of
   non-converged LSUs come down to a low level after some time. It
   turns out that for this example the stability threshold is
   150.

   The network behavior as a function of the LSA storm size can
   be categorized as follows:

   (1) If the LSA storm is well below the stability threshold then
       the CPU/memory congestion lasts only for a short period and
       during this period there are very few retransmissions, very
       few dropped OSPF packets and no link
       failures due to missed Hellos.  This type of LSA storms are
       observed routinely in operational networks and networks
       recover from them easily.

   (2) If the LSA storm is just below the stability threshold then
       the CPU/memory congestion lasts for a longer period and during
       this period there may be considerable amount of retransmissions
       and dropped OSPF packets.  If Hello packets are not given
       priority then there may also be some link failures due to


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       missed Hellos.  However, the network does go back to a stable
       state eventually. This type of LSA storm may happen rarely in
       operational networks and they recover from it with some
       difficulty.

   (3) If the LSA storm is above the stability threshold then
       the CPU/memory congestion may last indefinitely unless
       some special procedure for relieving congestion is followed.
       During this period there are considerable amount of
       retransmissions and dropped OSPF packets.  If Hello packets are
       not given priority then there would also be link failures due
       to missed Hellos.  This type of LSA storm may happen very rarely
       in operational networks and usually some manual procedure such
       as taking down adjacencies in heavily congested nodes is needed.

   (4) If Hello packets are given priority then the network stability
       threshold increases, i.e., the network can withstand a larger
       LSA storm. Furthermore, even if the network operates at or
       somewhat above this higher stability threshold, Hellos are
       still not missed and so there are no link failures.  So even
       if there is congestion in the control plane due to increased
       retransmissions requiring some special procedures for congestion
       reduction, the data plane remains unaffected.

   (5) If both Hello and Acknowledgement packets are given priority
       then the stability threshold increases even further.

   In Table 2 we show the network stability threshold for the five
   different cases and for the three different priority scenarios
   defined earlier.

|===========|========================================================|
|           |    Maximum Allowable LSA Storm Size For                |
|   Case    |=================|==================|===================|
|  Number   | No Priority to  |Priority to Hello | Priority to Hello |
|           |  Hello or Ack   |      Only        |   and Ack         |
|===========|=================|==================|===================|
|   Case 1  |        150      |        190       |        250        |
|___________|_________________|__________________|___________________|
|   Case 2  |        185      |        215       |        285        |
|___________|_________________|__________________|___________________|
|   Case 3  |        115      |        127       |        170        |
|___________|_________________|__________________|___________________|
|   Case 4  |        320      |        375       |        580        |
|___________|_________________|__________________|___________________|
|   Case 5  |        120      |        175       |        225        |
|___________|_________________|__________________|___________________|
|   Case 6  |        185      |        224       |        285        |
|___________|_________________|__________________|___________________|

       Table 2: Maximum Allowable LSA Storm for a Stable Network

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   We also considered one more scenario with priority to Hello and Ack
   and with a truncated binary exponential backoff of the
   retransmission interval with an upper limit of 40 seconds (for the
   same LSA, each successive retransmission interval
   is doubled but not to exceed 40 seconds).  The maximum allowed
   LSA storm size for this scenario significantly exceeded the numbers
   given in the third column.

Appendix B.3. Observations on Simulation Results

   Table 2 shows that in all cases prioritizing Hello packets increases
   the network stability threshold, and in addition, prioritization of
   LSA Acknowledgment packets increases the stability threshold even
   further.  The reasons for the above observations are as follows.
   The main sources of sustained CPU/memory congestion (or positive
   feedback loop) following an LSA storm are (1) LSA retransmissions
   and (2) links being declared down due to missed Hellos which in
   turn causes further LSA generation and future recovery of the link
   causing even more LSA generation.
   Prioritizing Hello packets avoids and practically eliminates the
   second source of congestion.  Prioritizing Acknowledgements
   significantly reduces the first source of congestion, i.e.,
   LSA retransmissions.  It is to be noted that retransmissions can
   not be completely eliminated due to the following reasons. Firstly,
   only the explicit Acknowledgments are prioritized but duplicate
   LSAs carrying implicit Acknowledgments are still served at the
   lower priority.  Secondly, LSAs may get greatly delayed or dropped
   at the input queue of receivers and therefore Acknowledgments may
   not even get generated in which case prioritizing Acks would not
   help. Another factor to keep in mind is that since Hellos and Acks
   are prioritized, the LSAs see bigger delay and potential for
   dropping. However, the simulation results show that on the whole
   prioritizing Hello and LSA Acks are always beneficial and
   significantly improve the network stability threshold.

   As stated in Section B.2, exponenetial backoff of LSA retransmission
   interval further increases the network stability threshold.

   Our simulation study also showed that in each of the cases, instead
   of prioritizing Hello packets if we treat any packet received over
   a link as a surrogate for a Hello packet (an implicit Hello) then
   we get about the same stability threshold as obtained with
   prioritizing Hello packets.


Appendix C. Other Proposals

   (1) Explicit Marking:  In Section 2 we proposed that OSPF packets
       be classified to "high" and "low" priority classes based on
       examining the OSPF packet header.  In some cases (particularly

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       in the receiver) this examination may be computationally
       costly.  An alternative would be the
       use of different TOS (DSCP) bits marking for high and low
       priority OSPF packets respectively.  The exact specification
       of this marking is for further study.

   (2) Other High Priority OSPF Packets: Besides the packets designated
       as high priority in Section 2 there may be other packets with
       a need for high priority designation.  One example is the
       Database Description (DBD) packet from a slave (during the
       database synchronization process) that is used as an
       acknowledgement.  A second example is an LSA carrying
       intra-area topology change information (this may trigger
       SPF calculation and rerouting of Label Switched paths and so
       fast processing of this packet may improve OSPF/LDP convergence
       times).




































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