Microloop Prevention Methods

1
Micro-loop Prevention Methods

• draft-bryant-shand-lf-conv-frmwk-00.txt
• draft-zinin-microloop-analysis-00.txt

2
Outline of Talk

• Convergence Strategy and Motivation
• Solution Taxonomy
• Existing Solution Space
• Summary

3
Traditional convergence strategy

• Switch to new as fast as you can independently
• Required for failures
• Strategy optimized for this case
• traffic to failed element is lost
• so speed is essential
• Used for everything else
• common method
• traffic can be lost due to loops
• Fast-Reroute prevents traffic loss due to failure
  but loops can still cause loss.

4
Micro-Loop Properties

• Independent decisions can cause micro-loops.
• Loops may occur between pairs of nodes or cycles
  of nodes.

• Duration depends on relative time to update FIBs.
• Implementation differences
• Number of affected destinations
• Propagation time

Loss due to Loop duration may be longer (an order
of magnitude) than Loss during the Fast Reroute
failover.
5
Controlled convergence

• Made feasible for failure case by fast reroute
• Traffic is not lost so can afford to take time
• Can use common method for both failure and
  management change events
• Traditional convergence optimized for failure
  case without fast-reroute.
• We can do better
• (but keep traditional as safe fall-back for
  single failure assumption violation.)

6
Solution taxonomy

• Controlled Information flow
• Incremental cost change
• Controlled Distributed Behavior
• Synchronized FIB installation
• Ordered FIB changes
• Path locking

7
Method Comparison
8
Incremental cost change

• A change in a link cost of x can only cause loops
  whose cyclic cost is ltx
• Minimum cycle is 2 (1 in each direction)
• Hence cost change of 1 can never cause a loop.
• Where minimum cycle is larger, larger increments
  can be used.
• Once cost reaches cost of alternate path no more
  loops possible.
• No Cooperation Required
• But Can Take Hours

9
Synchronized FIB swap

• Network synchronized change-over at predetermined
  time
• Signal/determine time to change
• Network Synchronized Time (NTP is there)
• Either Two FIBs for fast swap
• Substantial hardware implications
• Or FIB update fast-enough from change-over
  time.
• Dependent on NTP
• Conceptually simple with minimal signalling NTP
  dependency implementation concerns

10
Ordering by signalling alone

• On change, tell old primary neighbors to wait for
  you
• Wait for all neighbors as instructed, install
  FIB, and tell your old primary neighbors.
• Assumes a single non-SRLG failure
• Otherwise communication per destination is
  required
• No Estimation Required for FIB Compute/Install -
• Require Reliable Fast Signalling and
• Non-Trivial Protocol Extensions

11
Ordered FIB changes

• For any isolated link/node change
• Determine safe ordering for FIB installation
• bad news update from edge to failure,
• good news update from change to edge
• Each router computes its rank with respect to
  the change.
• Delays for a number of worst-case FIB
  compute/install times proportional to its rank.

12
Computing the ordering

• Single Reverse SPF rooted at change node
• Use old SPT to determine relevant node
• For bad news- count maximum depth of sub-tree
  below you
• For good news- count maximum hops to change

13
Delay Proportional to Network Diameter

• For Good News, rSPF gives necessary depth.
• For Bad News, rSPF is overly pessimistic for some
  topologies.
• Strategies to reduce unnecessary delay
• Prune rSPF by only considering the branch across
  the failure but still too pessimistic.
• Run SPF rooted at edge nodes to correctly prune
  them but doesnt scale.
• Compare rSPFs before and after failure

Avoids all micro-loops and requires single FIB
install. Delay dependent on network diameter so
may be unacceptable.
14
Signalling optimization to Reduce delay

• Use actual FIB compute/install instead of
  worst-case
• In many cases, actual delay is 0 b/c no change
  needed.
• Signal to parents in rSPF when
• Nothing to do, or
• Completed FIB changes
• Can change FIB when received signal from all
  children (or when delay expires)
• Only an optimization
• Loss of signals falls back to delay based

15
SRLG Concerns

• Diverse failures may require mutually
  incompatible ordering
• Different orderings for individual destination
  sets may help
• Need Rules to merge multiple rSPFs

16
Ordered SPF Summary

• No forwarding changes required.
• No signalling required at time of change.
• Complete prevention of loops for isolated node or
  link changes.
• Requires cooperation from all routers
• May delay re-convergence for tens of seconds
  (unless optional signalling used)
• SRLGs require per destination delays and may
  delay re-convergence more.

17
Path Locking Framework

• Obtain a fixed convergence delay regardless of
  network.
• Avoid ordering issue by providing transitional
  paths.
• Handles SRLGs
• Different methods to
• Determine/Create transitional paths
• Direct traffic to use transitional paths
• Standard trade-off of complexity versus coverage.
• Tunnels for Transitional Paths
• Safe Neighbors for Transitional Next-Hops
• Marked Packets to Use Transitional Topology
• U-turn Packets to Use New Topology

18
Time-Line of Convergence

• Change Discovery Time At this point, all
  routers know about the change. Routers install
  transitional path support.
• For some methods, immediately start use of
  self-determined transitional paths.
• Use Transitional Paths Time (1 worst-case FIB
  compute/install later) All routers use
  transitional paths, if available, and new primary
  next-hops otherwise.
• Lock to New Topology Time (1 worst-case FIB
  compute/install later) All routers use new
  primary next-hops.
• All micro-loops avoided if a transitional path
  always exists.

19
Create Tunnels

• Requires tunnel computation/creation at topology
  change
• Old topology Locking
• Tunnel to the upstream side of the failure
• Single tunnel for all affected destinations (if
  link/node failure).
• New topology locking
• Tunnel to first unaffected router on new primary
  path
• Tunnels provide a transitional path that can
  traverse non-supporting routers.
• Non-supporting routers can only loop locally
  originated traffic.

20
Safe Neighbors

• Find a safe neighbor to use as a transitional
  next-hop.
• Safety condition is a neighbor that is loop-free
  on old topology and a downstream path on new
  topology.
• If two neighboring routers dont have a safe
  neighbor, a micro-loop can form on that link.
• Analysis of real topologies shows pretty good
  coverage.
• Local micro-loops possible with non-supporting
  routers.

21
Typical Coverage
22
Packet Marking

• Can mark packets to force forwarding according to
  a particular topology.
• Topology can be new or old.
• All marking starts at the Use Transitional Paths
  Time
• If using new topology, traffic on new topology
  after 1 worst-case FIB compute/install delay.

23
U-turn Packet

• Create transitional next-hop by directing U-turn
  packets to the new primary next-hops.
• At Use Transitional Paths Time, send traffic to
  new primaries (potentially explicitly marked as
  U-turn packets).
• If implicitly determined U-turn packets, doesnt
  require marking.
• Explicit method for signalling support of U-turns
  

24
Lots of Possibilities

• What are important criteria?
• Time to be converged
• Affects single failure assumption
• Network Stability
• Ballpark requirement is 10s
• Simplicity
• Support for SRLGs
• No additional mechanisms beyond IP (but coverage
  may suffer)
• Common additional mechanisms for this and IPFRR
  advanced methods.
• Should also work for LDP

25
Conclusions Next Steps

• Incremental Cost Change is impractical.
• Synchronized FIB Swap what is the
  implementation complexity? Implications of
  coupling NTP to routing?
• Ordered SPF long delay and poor SRLG support.
  Is that enough to be an issue?
• Path Locking
• Seem most promising
• Many possibilities to get similar results
• Please send suggestions and comments to the list.
  This solution set may not be complete.