Automatic Protection Switching

1
AutomaticProtection Switching

• Yaakov (J) Stein
• CTO
• RAD Data Communications

Mar 2012
2

• Course Outline
• General protection switching principles
• Examples of protection mechanisms
• SONET/SDH
• Ethernet linear protection
• Ethernet ring protection
• MPLS fast reroute
• MPLS-TP APS

3
General principles
Definition References Traffic types Network
topologies Triggers Protection classes Entities Pr
otection types Signaling
4
Definition

• Automatic Protection Switching (APS)
• is a functionality of carrier-grade transport
  networks
• is often called resilience
• since it enables service to quickly recover
  from failures
• is required to ensure high reliability and
  availability
• APS includes
• detection of failures (signal fail or signal
  degrade) on a working channel
• switching traffic transmission to a protection
  channel
• selecting traffic reception from the protection
  channel
• (optionally) reverting back to the working
  channel once failure is repaired
• Automatic means uses (at most) control plane
  protocols
• no management layer or manual operations
  needed

5
Some useful references

• G.808.1 generic linear protection
• G.808.2 generic ring protection (not yet
  written)
• G.841 and G.842 SDH
• G.774.3/4/9/10 SDH protection management
• G.870 and G.873.1 OTN
• G.8031 Ethernet linear protection
• G.8032 Ethernet ring protection
• G.8131 T-MPLS APS
• Y.1720 MPLS
• I.630 ATM
• M.495 analog signal protection
• G.781 clock selection (can be used to protect
  synchronization)
• RFC 4090 MPLS Fast ReRoute
• RFC 6372 MPLS-TP Survivability Framework
• RFC 6378 MPLS-TP Linear Protection

6
Traffic types

• In a network with APS capabilities, there are
  three types of traffic
• protected traffic
• traffic that may be rapidly switched to
  protection channel
• at any time it may be on the working channel or
  protection channel
• Nonpreemptible Unprotected Traffic (NUT)
• noncritical traffic that does not require
  protection mechanism
• not affected by protection mechanism
• somewhat less expensive to customer
• extra (preemptible) traffic
• best effort background traffic that runs on
  protection channel
• preempted (blocked) when protection channel is
  needed
• very inexpensive to customer

7
Network topologies

• APS can be defined for any topology with
  redundant links
• e.g., for tree topologies no protection is
  possible
• We will often discuss protection of individual
  links
• However, there are two topologies that are of
  particular interest
• rings
• protection is natural for rings
• although there are other reasons for using rings
  as well
• rings are so important that protection for other
  topologies
• is often called linear protection
• dense meshes
• for this topology multiple local bypasses can be
  preconfigured
• protection switching is similar to routing
  change, but faster
• often called Fast ReRoute (FRR)

8
Triggers

• Protection switching is usually triggered by a
  failure
• although the operator may manually force a
  protection switch
• A failure is declared when a fault condition
• persists long enough
• for the ability to perform the required function
• to be considered terminated
• Failures are Signal Fail (SF) or Signal Degrade
  (SD) (of various types)
• and may be
• detected by physical layer
• indicated by signaling (e.g. AIS)
• detected by OAM mechanisms
• When there is no SF or SD, the state is called No
  Request (NR)

9
Switching time (1)

• SONET/SDH protection switching takes place in
  under 50 ms
• Regarding multiplex section shared protection
  rings, G.841 states
• The following network objectives apply
• 1) Switch time In a ring with no extra traffic,
  all nodes in the idle state (no detected
  failures,
• no active automatic or external commands, and
  receiving only Idle K-bytes), and with less
• than 1200 km of fibre, the switch (ring and span)
  completion time for a failure on a single
• span shall be less than 50 ms. On rings under all
  other conditions, the switch completion
• time can exceed 50 ms (the specific interval is
  under study) to allow time to remove extra
• traffic, or to negotiate and accommodate
  coexisting APS requests.
• while for linear VC trail protection, it says
• The following network objectives apply
• 1) Switch time The APS algorithm for LO/HO VC
  trail protection shall operate as fast as
• possible. A value of 50 ms has been proposed as a
  target time. Concerns have been
• expressed over this proposed target time when
  many VCs are involved. This is for further
• study. Protection switch completion time excludes
  the detection time necessary to initiate the
• protection switch, and the hold-off time.
• There are similar statements in other clauses as
  well

10
Switching time (2)

• This 50 ms time has become the golden standard
• and new protection schemes are expected to meet
  this objective
• However, studying the literature that lead up to
  SONET/SDH standards
• shows that the objective was to attain the
  minimum possible time
• for the sum of
• persistent (i.e. non-transient) failure detection
• speed of light propagation
• signaling protocol time
• regaining sync alignment
• and 50 ms was the minimum that was considered
  practical !
• Many modern standards have built in 50 ms
• and much marketing literature boasts faster
  than 50 ms
• But there is really nothing special about 50 ms
• 50 ms gaps in voiced speech are noticeable,
• but not fatal if infrequent
• 50 ms of data at high rates can not be stored and
  later forwarded
• timing circuits can withstand much more than 50
  ms without clock

11
Protection classes

• It is useful to distinguish two different
  protection classes
• path protection (AKA trail protection, end-to-end
  protection)
• when a failure is detected on the end-to-end path
• we switch to an alternative end-to-end path
• the failure is usually detected by end-to-end OAM
• local protection (AKA local restoration, SNC
  protection, bypass, detour)
• we protect individual network elements, links, or
  groups of same
• when such an entity fails
• only that local entity is bypassed
• the failure may be detected by link OAM or
  physical layer means

12
APS entities (1)

• The following entities are important in APS
• working channel channel used when no failure
  exists
• protection channel channel used when a failure
  exists
• head-end entity transmitting data to
  working/protection channel
• tail-end entity receiving data from the
  working/protection channel
• Note we will usually consider traffic to be
  bidirectional
• so that the head-end for one direction
• is the tail-end for the opposite direction

13
APS entities (2)

• Bridge function at head-end that connects
  traffic (including extra traffic) to the working
  and protection channels
• Selector function at tail-end that extracts
  traffic (perhaps extra traffic) from the working
  or protection channel
• APS signaling channel channel used to
  communicate between head-end and tail-end for APS
  purposes
• Trail termination function responsible for
  failure detection
• including injection and extraction of OAM

working channel
tail-end (selector)
head-end (bridge)
protection channel
signaling channel
14
Revertive operation

• Reversion means returning to use the working
  channel
• after the failure has been rectified
• Protection mechanisms can be revertive or
  nonrevertive
• Revertive mechanisms may be preferable
• when the working channel has better performance
  (free BW, BER, delay)
• when there are frequent switches (easier to
  manage)
• when there is extra traffic
• but nonrevertive also has advantages
• only one service disruption due to protection
  switching
• may be simpler to implement

15
Uni/bi-directional

• We will usually consider bidirectional traffic
• but even then the failures can be uni- or bi-
  directional
• and for unidirectional failures there can be uni-
  or bi- directional switching

16
Uni- / bi- directional switching

• Unidirectional switching may be advantageous
• for 11 - faster and no signaling channel is
  needed
• no unnecessary service disruption for direction
  without failure
• higher chance of protection under multiple
  failures
• easier to implement for local protection
• maintains extra traffic in direction without
  failure
• But bidirectional may be preferable
• easier management since directions traverse same
  network elements
• does not disrupt delay balance between direction
• may simplify repair since failed spans are unused

17
Protection types

• We distinguish several different protection types
• 11
• 11
• 1n
• mn
• (11)n
• Each type has its applicability, advantages, and
  disadvantages
• and there are trade-offs between
• simplicity
• BW consumption
• protection switch time
• signaling requirements

18
11 protection

• Simplest and fastest form of protection
• but wasteful - only 50 of actual physical
  capacity is used
• Head-end bridge always sends data on both
  channels
• Tail-end selector chooses channel to use (based
  on BER, dLOS, etc.)
• For unidirectional11 switching there is no need
  for APS signaling
• If non-revertive
• there is no distinction between working and
  protection channels

19
11 protection

• Head-end bridge usually sends data on working
  channel
• When failure detected it starts sending data over
  protection channel
• and tail-end needs to select the protection
  channel
• When not in use, protection channel can be used
  for extra traffic
• However, since failure is detected by tail-end,
  APS signaling is needed
• Protection channel should have OAM running to
  ensure its functionality

20
1n protection

• One protection channel is allocated for n working
  channels
• Only can protect one working channel at a time
• but improbable that more than 1 working channel
  will simultaneously fail
• Only 1/(n1) of total capacity is reserved for
  protection

21
mn protection

• To enable protection of more than 1 channel
• m protection channels are allocated for n working
  channels (m lt n)
• m simultaneous failures can be protected
• Less protection capacity dedicated than for n
  times 11
• When failure detected,
• 1 of the m protection channels need to be
  assigned and signaled
• High complexity but conserves resources

22
(11)n protection

• This is like n times 11 but the n protection
  channels share bandwidth
• Only 1 failed working channel can be protected
• This is different from 1n since
• n protection channels are preconfigured
• n working channels need not be of the same type
• Protection bandwidth must be at least that of the
  largest working channel

23
APS algorithm

• We have seen that protection switching is a
  tricky business
• So it is not surprising that network elements
  that support APS
• run an APS algorithm
• This algorithm inputs
• configuration (protection type, revertive?,
  available channels, )
• failure indications (NR, SF, SD)
• operator commands
• APS signaling (more on that soon)
• and makes switching decisions
• The algorithm maintains state information for
  head-end and tail-end
• APS algorithms are detailed in standards documents

24
Priority

• Not every failure event / operator command
  results in a protection switch
• For example
• in 1n protection the protection channel may
  already be in use !
• Conflicts are resolved by assigning priorities to
  events/commands
• When an event is detected or a command received
• the APS algorithm will not act
• if an event/command or equal or higher priority
  is already in effect
• True failure conditions usually have higher
  priority than manual commands

25
Timers

• Even failure events with priority are not acted
  upon immediately
• to do so would cause unnecessary switches after
  transient defects
• The APS algorithm may maintains several timers,
  such as
• Holdoff timers
• the time between detection of a SF or SD event
• and the APS algorithm acting upon this even
• the algorithm usually used is called peek twice
• i.e., the condition is checked again after the
  timer expires
• Wait To Restore timer
• for revertive switching, the time between
  detection of the failure being cleared and the
  APS algorithm acting upon this event
• also used in SDH optimized bidirectional 11
  (nonrevertive)
• Guard timer
• for rings blockout time during which APS
  messages are ignored (since they may be old and
  outdated)

26
APS signaling

• In all types except unidirectional 11, some APS
  signaling is needed
• APS signaling is used to synchronize between
  head-end and tail-end
• It is critical that head-end and tail-end always
  be in the same state
• Example messages include
• No Request (NR)
• by tail-end to inform head-end of Signal Failure
  (SF)
• by head-end to confirm the events priority
• by head-end to report the particular protection
  channel
• by head-end to inform tail-end of Reverse
  (bidirectional) Request (RR)
• by tail-end after failure cleared to Wait To
  Restore (WTR)
• by tail-end after failure cleared to Do Not
  Revert (DNR) for nonrevertive

27
APS signaling phases

• When APS signaling is used, it needs to be as
  rapid as possible
• Depending on the scenario it may be
• 1-phase tail?head (fastest)
• tail-end informs head-end of failure
• both ends uniquely know the protection channel to
  be used
• only for 11 and unidirectional-(11)n
  (including 11)
• 2-phase 1) tail?head 2) head?tail
• tail-end informs head-end of failure
• head-end signals that it has switched to
  protection channel
• not for bidirectional-1n or mn
• 3-phase 1) tail?head 2) head?tail 3) tail?head
  (slowest)
• works for all protection types (including mn)

28
Examples of 1-phase

• Example of when 1-phase signaling is possible is
  11 or (11)n
• 1. upon detection of failure the tail-end sends
  SF to the head-end
• and immediately changes its selector (blind
  switch)
• upon receipt the head-end changes the bridge
  setting
• (no priority is checked)
• 1-phase can also be used for bidirectional 11
• 1. upon detection of failure the tail-end sends
  SF to the head-end
• and immediately changes both its selector and
  bridge
• upon receipt the head-end changes its bridge and
  selector

29
Example of 2-phase

• 2-phase is useful for unidirectional 1n with
  priority checking
• 1. upon detection of failure the tail-end sends
  SF to the head-end
• but does not change its selector
• 2. the head-end checks priority
• sends confirmation to tail-end (with identity of
  working channel)
• the bridge setting is changed
• 3. the tail-end changes its selector

30
Example of 3-phase

• 3-phase signaling is imperative for bidirectional
  1n
• 1. upon detection of failure the tail-end sends
  SF to the head-end
• but does not change its selector
• 2. the head-end checks priority, and sends
  confirmation to tail-end
• head-end changes its bridge setting
• and also sends a reverse request
• 3. the tail-end changes selector
• checks priority and sends confirmation to
  head-end
• tail-end changes its bridge setting (as head-end
  of opposite direction)
• head-end receives confirmation and changes its
  selector

31
For G.805 buffs

• to add 11 trail protection to a trail - expand a
  trail termination function
• we use a special transport processing function -
  the protection switch

the unprotected TTs report status to the
protection switch
32
SONET/SDH APS
33
SONET protection ?

• SONET/SDH networks need to be highly reliable
  (five nines)
• Down-time should be minimal (less than 50 msec)
• So systems must repair themselves (no time for
  manual intervention)
• Upon detection of a failure (dLOS, dLOF, high
  BER)
• the network must reroute traffic (protection
  switching)
• from working channel to protection channel
• SDH APS is unidirectional
• SDH APS may be revertive

34
SONET/SDH layers

• Between regenerators there are sections
  (regenerator sections)
• Between ADMs there are lines (multiplex sections)
• Between path terminations there are paths
• Protection can be at OC-n level (different
  physical fibers)
• or at STM/VC level
• or end-to-end path (trail protection)

35
Line APS
90 columns
Synchronous Payload Envelope
3 rows
9 rows
9 rows
6 rows
TOH
A1 A2 J0
B1 E1 F1
D1 D2 D3
H1 H2 H3
B2 K1 K2
D4 D5 D6
D7 D8 D9
DA DB DC
S1 M0 E2

• TOH consists of
• 3 rows of section overhead - frame sync, trace,
  EOC,
• 6 rows of line overhead - pointers, SSM, FEBE,
  and
• Line APS signaling uses bytes K1 and K2

36
HO Path APS

• POH is responsible for type, status, path
  performance monitoring, VCAT, trace
• HO Path APS signaling uses 4 MSBs of byte K3

37
LO Path APS

• VC OH is responsible for
• Timing, PM, REI,
• LO Path APS signaling is
• 4 MSBs of byte K4

38
How does it work?

• Head-end and tail-end NEs have bridges (muxes)
• Head-end and tail-end NEs maintain bidirectional
  signaling channel
• Signaling is contained in K bytes of protection
  channel
• For line APS
• K1 tail-end status and requests
• K2 head-end status

39
Linear 11 protection

• Can be at OC-n level (different physical fibers)
• or at STM/VC level (SubNetwork Connection
  Protection)
• or end-to-end path (called trail protection)
• Head-end bridge always sends data on both
  channels
• Tail-end chooses channel to use based on BER,
  dLOS, etc.
• No need for signaling
• If non-revertive
• there is no distinction between working and
  protection channels

40
Linear 11 protection

• Head-end bridge usually sends data on working
  channel
• When tail-end detects failure it signals (using
  K1) to head-end
• Head-end then starts sending data over protection
  channel
• When not in use
• protection channel can be used for (discounted)
  extra traffic
• (pre-emptible unprotected traffic)
• May be at any layer (but only OC-n level protects
  against fiber cuts)

working channel
extra traffic
protection channel
41
Linear 1N protection

• In order to save BW
• we allocate 1 protection channel for every N
  working channels
• N limited to 14
• 4 bits in K1 byte from tail-end to head-end
• 0 protection channel
• 1-14 working channels
• 15 extra traffic channel

42
Two fiber vs. Four-fiber rings

• Ring based protection is popular in North America
  (100K rings)
• Full protection against physical fiber cuts
• Simpler and less expensive than mesh topologies
• Protection at line (multiplexed section) or path
  layer
• Four-fiber rings
• fully redundant at OC level
• can support bidirectional routing at line layer
• Two-fiber rings
• support unidirectional routing at line layer

2 fibers in opposite directions
43
Unidirectional vs. bidirectional

• Unidirectional routing
• working channel B-A same direction (e.g.
  clockwise) as A-B
• management simplicity A-B and B-A can occupy
  same timeslots
• Inefficient waste in ring BW and excessive delay
  in one direction
• Bidirectional routing
• A-B and B-1 are opposite in direction
• both using shortest route
• spatial reuse timeslots can be reused in other
  sections

44
UPSR vs. BLSR (MS-SPRing)
Path switching Line switching
Two-fiber Four-fiber
Unidirectional Bidirectional
UPSR
BLSR

• Of all the possible combinations, only a few are
  in use
• Unidirectional (routing) Path Switched Rings
• protects tributaries
• extension of 11 to ring topology
• Bidirectional (routing) Line Switched Rings
  (two-fiber and four-fiber versions)
• called Multiplex Section Shared Protection Ring
  in SDH
• simultaneously protects all tributaries in STM
• extension of 11 to ring topology

45
UPSR

• Working channel is in one direction
• protection channel in the opposite direction
• All path traffic is added in both directions
  (11)
• decision as to which to use is made at drop point
  (no signaling)
• Normally non-revertive, so effectively two
  diversity paths
• Good match for access networks
• 1 access resilient ring
• less expensive than fiber pair per customer
• Inefficient for core networks
• no spatial reuse
• every signal in every span
• in both directions
• node needs to continuously monitor
• every tributary to be dropped

46
BLSR

• Switch at line level less monitoring
• When failure detected tail-end NE signals
  head-end NE
• Works for unidirectional/bidirectional fiber
  cuts, and NE failures
• Two-fiber version
• half of OC-N capacity devoted to protection
• only half capacity available for traffic
• Four-fiber version
• full redundant OC-N devoted to protection
• twice as many NEs as compared to two-fiber

Example recovery from unidirectional fiber cut
47
Ethernet linear APS

• STP
• LAG
• G.8031

48
STP

• The original Spanning Tree Protocol automatically
  removed loops
• from arbitrary networks (with loops)
• However, its convergence was very slow (about a
  minute)
• STP can not be used as a protection mechanism
• since its reconvergence time is very long
• due to a cumbersome protocol
• and long holdoff timer settings
• An evolutionary update called Rapid STP 802.1w
• was incorporated into 802.1D-2004 clause 17
• that converges in about the same time as STP
• but can reconverge after a topology change in
  less than 1 second
• RSTP can be used to detect failures and
  reconverge
• and thus can be used as a primitive protection
  mechanism
• However, the switching time will be many tens of
  ms to 100s of ms

49
Use of LAG

• Ethernet link aggregation (AKA bonding,
  Ethernet trunk, inverse mux, NIC teaming)
• enables bonding several ports together as single
  uplink
• Defined by 802.3ad task force and folded into
  802.3-2000 as clause 43
• Binding of ports to Link Aggregation Groups
  (LAGs) distributed via
• Link Aggregation Control Protocol (LACP)
• LACP uses slow protocol frames (up to 5 per
  second)
• Links may be dynamically added/removed from LAG
• and LACP continuously monitors to detect if
  changes needed
• Upon link failure LAG delivers traffic at a
  reduced rate
• Thus LAG can be used as a primitive protection
  mechanism
• When used this way it is called worker/standby or
  NN mode
• The restoration time will be on the order of 1
  second

50
G.8031

• Q9 of SG15 in the ITU-T is responsible for
  protection switching
• In 2006 it produced G.8031 Linear Ethernet
  Protection Switching
• G.8031 uses standard Ethernet formats, but is
  incompatible with STP
• The standard addresses
• point-to-point VLAN connections
• SNC (local) protection class
• 11 and 11 protection types
• unidirectional and bidirectional switching for
  11
• bidirectional switching for 11
• revertive and nonrevertive modes
• 1-phase signaling protocol
• G.8031 uses Y.1731 OAM CCM messages in order to
  detect failures
• G.8031 defines a new OAM opcode (39) for APS
  signaling messages
• Switching times should be under 50 ms (only
  holdoff timers when groups)

51
G.8031 signaling

• The APS signaling message looks like this
• regular APS messages are sent 1 per 5 seconds
• after change 3 messages are sent at max rate (300
  per sec)
• where
• req/state identifies the message (NR, SF, WTR,
  SD, forced switch, etc)
• prot. type identifies the protection type (11,
  11, uni/bidirectional, etc.)
• requested and bridged signal identify incoming /
  outgoing traffic
• since only 11 and 11 they are either null or
  traffic (all other values reserved)

52
G.8031 11 revertive operation

• In the normal (NR) state
• head-end and tail-end exchange CCM (at 300 per
  second rate)
• on both working and protection channels
• head-end and tail-end exchange NR APS messages
• on the protection channel (every 5 seconds)
• When a failure appears in the working channel
• tail-end stops receiving 3 CCM messages on
  working channel
• tail-end enters SF state
• tail-end sends 3 SF messages at 300 per second on
  the APS channel
• tail-end switches selector (bi-d and bridge) to
  the protection channel
• head-end (receiving SF) switches bridge (bi-d and
  selector) to protection channel
• tail-end continues sending SF messages every 5
  seconds
• head-end sends NR messages but with
  bridgednormal
• When the failure is cleared
• tail-end leaves SF state and enters WTR state
  (typically 5 minutes, 5..12 min)
• tail-end sends WTR message to head-end (in
  nonrevertive - DNR message)
• tail-end sends WTR every 5 seconds
• when WTR expires both sides enter NR state

53
Ethernet ring APS

• G.8032
• RPR
• CLEER

54
Ethernet rings ?

• Ethernet has become carrier grade
• deterministic connection-oriented forwarding
• OAM
• synchronization
• The only thing missing to completely replace SDH
  is ring protection
• However, Ethernet and ring architectures dont go
  together
• Ethernet has no TTL, so looped traffic will loop
  forever
• STP builds trees out of any architecture no
  loops allowed
• There are two ways to make an Ethernet ring
• open loop
• cut the ring by blocking some link
• when protection is required - block the failed
  link
• closed loop
• disable STP (but avoid infinite loops in some way
  !)
• when protection is required - steer and/or wrap
  traffic

55
Ethernet ring protocols

• Open loop methods
• G.8032 (ERPS)
• rSTP (ex 802.1w)
• RFER (RAD)
• ERP (NSN)
• RRST (based on RSTP)
• REP (Cisco)
• RRSTP (Alcatel)
• RRPP (Huawei)
• EAPS (Extreme, RFC 3619)
• EPSR (Allied Telesis)
• PSR (Overture)
• Closed loop methods
• RPR (IEEE 802.17)
• CLEER and NERT (RAD)

56
G.8032

• Q9 of SG15 produced G.8032 between 2006 and 2008
• G.8032 is similar to G.8031
• strives for 50 ms protection (lt 1200 km, lt 16
  nodes)
• but here this number is deceiving as MAC table is
  flushed
• standard Ethernet format but incompatible with
  STP
• uses Y.1731 CCM for failure detection
• employs Y.1731 extension for R-APS signaling
  (opcode40)
• R-APS message format similar to APS of G.8031
• (but between every 2 nodes and to MAC address
  01-19-A7-00-00-01)
• revertive and nonrevertive operation defined
• However, G.8032 is more complex due to
• requirement to avoid loop creation under any
  circumstances
• need to localize failures
• need to maintain consistency between all nodes on
  ring
• existence of a special node (RPL owner)

57
RPL

• G.8032v1 defines the Ring Protection Link (RPL)
• as the link to be blocked (to avoid closing the
  loop) in NR state
• One of the 2 nodes connected to the RPL
• is designated the RPL owner
• Unlike RFER
• there is only one RPL owner
• the RPL and owner are designated before setup
• operation is usually revertive
• All ring nodes are simultaneously in 1 of 2 modes
  idle or protecting
• in idle mode the RPL is blocked
• in protecting mode the failed link is blocked and
  RPL is unblocked
• in revertive operation
• once the failure is cleared the block link is
  unblocked
• and the RPL is blocked again

58
G.8032 revertive operation

• In the idle state
• adjacent nodes exchange CCM at 300 per second
  rate (including over RPL)
• exchange NR RB (RPL Blocked) messages in
  dedicated VLAN every 5 seconds (but not over RPL)
• R-APS messages are never forwarded
• When a failure appears between 2 nodes
• node(s) missing CCM messages peek twice with
  holdoff time
• node(s) block failed link and flush MAC table
• node(s) send SF message (3 times _at_ max rate, then
  every 5 sec)
• node receiving SF message will check priority and
  unblock any blocked link
• node receiving SF message will send SF message to
  its other neighbor
• in stable protecting state SF messages over every
  unblocked link
• When the failure is cleared
• node(s) detect CCM and start guard timer (blocks
  acting on R-APS messages)
• node(s) send NR messages to neighbors (3 times _at_
  max rate, then every 5 sec)
• RPL owner receiving NR starts WTR timer
• when WTR expires RPL owner blocks RPL, flushes
  table, and sends NR RB
• node receiving NR RB flushes table, unblocks any
  blocked ports, sends NR RB

59
G.8032-2010

• After coming out with G.8032 in 2008 (G.8032v1)
• the ITU came out with G.8032-2010 (G.8032v2) in
  2010
• This new version is not backwards-compatible with
  v1
• but a v2 node must support v1 as well (but then
  operation is according to v1)
• Major differences
• 2 designated nodes RPL owner node and RPL
  neighbor node
• and for optional flush-optimization
  next neighbor node
• significant changes to
• state machine
• priority logic
• commands (forced/manual/clear) and protocol
• new Wait To Block timer
• supports more general topologies (sub-rings)
• ladders (For Further Study in v1)
• multi-ring
• ring topology discovery
• virtual channel based on VLAN or MAC address

ring
subring
subring
ladder
60
RPR 802.17

• Resilient Packet Rings
• are compatible with standard Ethernet, but
  different frame format
• are robust (lossless, lt50ms protection, OAM)
• are fair (based on client throttling)
• support QoS (3 classes A, B, C)
• are efficient (full spatial reuse)
• are plug and play (automatic station
  autodiscovery)
• extend use of existing fiber rings
• counter-rotating add/drop ringlets, running
• SONET/SDH (any rate, PoS, GFP or LAPS) or
• packetPHY (1 or 10 Gb/s ETH PHY)
• developed by 802.17 WG
• based on Ciscos Spatial Reuse Protocol (RFC
  2892)

ringlet selection
61
Basic RPR queuing
traffic going around ring placed into internal
buffer in dual-transit queue mode placed into 1
of 2 buffers according to service class sent
according to fairness
traffic for local sink placed in output buffer
according to service class

• traffic from local source
• sent according to fairness
• first sent to ringlet selection

Primary/Secondary Transit Queue
62
RPR service classes

• RPR defines 3 main classes
• class A real time (low latency/FDV)
• class B near real time (bounded predictable
  latency/FDV)
• class C best effort

class use info rate D/FDV FE
A0 RT reserved low No
A1 RT allocated, reclaimable low No
B-CIR near RT allocated, reclaimable bounded No
B-EIR near RT opportunistic unbounded Yes
C BE opportunistic unbounded Yes
63
RPR Class use

• A0 ring BW is reserved not reclaimed even if no
  traffic
• in dual-transit queue mode
• class A frames from the ring are queued in PTQ
• class B, C in STQ
• priority for egress
• frames in PTQ
• local class A frames
• local class B (when no frames in PTQ)
• frames in STQ
• local class C (when no PTQ, STQ, local A or B)
• Notes
• class A have minimal delay
• class B have higher priority than STQ transit
  frames, so bounded delay/FDV
• classes B and C share STQ, so once in ring have
  similar delay

64
RPR - protection

• rings give inherent protection against single
  point of failure
• RPR specifies 2 mechanisms
• steering
• wrapping (optional)
• (implementations may also do wrapping then
  steering)

steering info
wrap
65
NERT and CLEER

• New Ethernet Ring Technology / Closed Loop
  Encapsulated Ethernet Ring
• Similar to RPR but uses real Ethernet format
• NERT and CLEER distinguish between
• ring nodes
• switches connected to ring nodes
• Traffic in ring is MAC-in-MAC encapsulated
• External MACs are of ring node
• Internal MACs are original
• Unexpected external MACs discarded
• External MACs learned as in 1ah
• Ring nodes forward according to table
• NERT floods, CLEER never floods
• Protection switch only involves changing table
• so service restoration is fast

66
MPLS fast reroute

• IP FRR
• RFC 4090

67
IP FRR

• True protection mechanisms do not exist for
  connectionless IP
• In practice, routing protocols discover breaks
  and recalculate routes
• but this usually takes a long time
• Link-state IGPs detect link-down state using
  hellos
• for OSPF - typically every 10 sec, and detection
  after 40 sec
• and then Dijkstra algorithm avoids the failed
  link
• BFD can be used to speed up the detection
• However,
• the information still has to be propagated
  further (seconds?)
• and FIBs updated (100s of ms)
• Various IP Fast ReRoute (IP FRR) mechanisms have
  been proposed
• but true protection is best done at the MPLS
  level

68
MPLS fast reroute

• RSVP-TE enables MPLS traffic engineering by fine
  control over placement
• specifies explicit path using information
  gathered from IGP
• resources may be reserved at LSRs along the way
• RFC 4090 defines extensions to RSVP-TE Fast
  ReRoute (FRR)
• LSRs along the path preconfigure local bypasses
  (detours)
• Upon detection of failure by
• BFD (specified in microseconds, typically 10s of
  ms) or
• RSVP hellos (RFC default is 5 ms) or
• RESV / PATH messages (driven by IGP)
• upstream LSR simply enables the detour
• Since this is a local action, it should be fast
• RFC 4090 only discusses adding FRR to RSVP-TE
  network
• but its use with LDP is possible if there is a
  single label generator

not discussed in RFC 4090
69
PLRs and MPs

• A fundamental entities in MPLS FRR are
• Point of Local Repair (PLR)
• Merge Point (MP)
• A PLR is the LSR before the failed element (link
  or node)
• All LSRs except the egress LER can be PLRs
• The PLR is solely responsible for the FRR (no
  explicit APS signaling)
• During path setup, potential PLRs create detours
  towards the egress LER
• A MP is the LSR where the detour rejoins the LSP
• All LSRs except the ingress LER can be MPs

70
Methods

• RFC 4090 defines two different protection methods
• Usually one or the other is employed in a given
  network
• One-to-one backup
• each LSP protected separately
• detour LSP created for each LSP at each potential
  PLR
• no labels pushed
• Facility backup
• backup tunnel for multiple LSPs
• bypass tunnel created at each potential PLR
• uses label stacking

71
NHOP and NNHOP

• MPLS FRR can bypass a failed link or a failed
  node
• In order to bypass a single failed link
• we need an alternative path to the next hop
  (NHOP)
• In order to bypass a single failed node, we need
  an alternative path to the next next hop (NNHOP)

72
MPLS TP APS

• RFC 6372 (MPLS-TP Survivability Framework)
• RFC 6378 (MPLS-TP Linear Protection)
• draft-ietf-mpls-tp-ring-protection

73
MPLS-TP resilience

• Since it strives to be a carrier-grade transport
  network
• TP has strong protection switching requirements
• APS has been almost as contentious issue as OAM
• and indeed the arguments are inter-related
• RFC 6372 gives a general framework
• and differentiates between
• linear
• shared-mesh and
• ring protection

74
Linear protection

• from RFC 6378 (ex draft-ietf-mpls-tp-linear-protec
  tion)
• 11, 11, 1n and uni/bidi are supported
• APS signaling protocol (for all modes except 11
  uni)
• is single-phase
• and called the Protection State Coordination
  protocol
• PSC messages are sent over the protection
  channel
• APS messages are sent over the GACh with a
  single channel type
• message functions identified by a request field
  
• 6 states normal, protecting due to failure,
  admin protecting,
• WTR, protection path
  unavailable, DNR
• when revertive, a WTR timer is used

75
PSC message format

• Request NR, SF, SD, manual switch, forced
  switch, lockout, WTR, DNR
• PT Protection Type uni 11, bidi 11, bidi
  11/1n
• R Revertive
• FPath which path has fault Path which data
  path is on protection channel

76
PSC control logic states

• Normal state - no trigger events reported
• Unavailable state - protection path is
  unavailable
• Protecting failure state
• traffic is being transported on the protection
  path
• Protecting administrative state
• operator issued command switching traffic to
  protection path
• Wait-to-Restore state - recovering from working
  path SF/SD
• WTR timer not up
• Do-not-Revert state - recovered from a protecting
  state
• but operator has configured DNR

77
PSC local requests

• In order from highest to lowest priority
• 1. Clear (operator command)
• 2. Lockout of protection (operator command)
• 3. Forced Switch (operator command)
• 4. Signal Fail on protection (OAM / control-plane
  / server indication)
• 5. Signal Fail on working (OAM / control-plane /
  server indication)
• 6. Signal Degrade on working (OAM / control-plane
  / server indication)
• 7. Clear Signal Fail/Degrade (OAM / control-plane
  / server indication)
• 8. Manual Switch (operator command)
• 9. WTR Expires (WTR timer)
• 10. No Request (default)

78
Linear protection ITU style
from draft-zulr-mpls-tp-linear-protection-switchin
g Similar to previous, but uses Y.1731/G.8031
format (no surprise!)
79
Ring protection
once again there were two drafts, both supporting
p2p and p2mp, wrapping and steering, link/node
failures draft-ietf-mpls-tp-ring-protection (not
yet RFC) Between any 2 LSRs can define a Sub-Path
Maintenance Entity So between 2 LSRs on a ring
there are 2 SPMEs we define 1 as the
working channel and 1 as the protection
channel Now we re-use the linear protection
mechanisms, including the PSC protocol draft-helvo
ort-mpls-tp-ring-protection-switching Both
counter-rotating rings carry working and
protection traffic The bandwidth on each ring is
divided X BW is dedicated to working traffic
and Y dedicated to protection traffic The
protection bandwidth of one ring is used to
protect the other ring Each node should have
information about the sequence of ring nodes
MPLS-TP Ring Protection Switching is
G.8032-like, but forwards non-NR msgs