Transcript APS

Automatic
Protection Switching
Yaakov (J) Stein
CTO
RAD Data Communications
Mar 2012
Course Outline
• General protection switching principles
• Examples of protection mechanisms
• SONET/SDH
• Ethernet linear protection
• Ethernet ring protection
• MPLS fast reroute
• MPLS-TP APS
Y(J)S APS Slide 2
General principles
Definition
References
Traffic types
Network topologies
Triggers
Protection classes
Entities
Protection types
Signaling
Y(J)S APS Slide 3
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
Y(J)S APS Slide 4
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
Y(J)S APS Slide 5
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
Y(J)S APS Slide 6
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)
Y(J)S APS Slide 7
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)
Y(J)S APS Slide 8
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
Y(J)S APS Slide 9
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
Y(J)S APS Slide 10
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
Y(J)S APS Slide 11
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
working channel
protection channel
head-end
tail-end
Y(J)S APS Slide 12
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 headend and tail-end for APS purposes

Trail termination – function responsible for failure detection
including injection and extraction of OAM
working channel
head-end
tail-end
protection channel
(bridge)
(selector)
signaling channel
Y(J)S APS Slide 13
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
Y(J)S APS Slide 14
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
unidirectional
failure
unidirectional
protection working channel
protection channel in use
working channel
protection channel
bidirectional
failure
bidirectional
protection working channel
protection channel in use
working channel
protection channel in use
Y(J)S APS Slide 15
Uni- / bi- directional switching
Unidirectional switching may be advantageous

for 1+1 - 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
Y(J)S APS Slide 16
Protection types
We distinguish several different protection types

1+1

1:1

1:n

m:n

(1:1)n
Each type has its applicability, advantages, and disadvantages
and there are trade-offs between

simplicity

BW consumption

protection switch time

signaling requirements
Y(J)S APS Slide 17
1+1 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 unidirectional1+1 switching there is no need for APS signaling
If non-revertive
there is no distinction between working and protection channels
channel A
channel B
Y(J)S APS Slide 18
1:1 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
working channel
extra traffic
protection channel
APS signaling
Y(J)S APS Slide 19
1:n 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/(n+1) of total capacity is reserved for protection
working channels
protection channel
Y(J)S APS Slide 20
m:n protection
To enable protection of more than 1 channel
m protection channels are allocated for n working channels (m < n)
m simultaneous failures can be protected
Less protection capacity dedicated than for n times 1:1
When failure detected,
1 of the m protection channels need to be assigned and signaled
High complexity but conserves resources
working channels
protection channels
Y(J)S APS Slide 21
(1:1)n protection
This is like n times 1:1 but the n protection channels share bandwidth
Only 1 failed working channel can be protected
This is different from 1:n 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
Y(J)S APS Slide 22
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
Y(J)S APS Slide 23
Priority
Not every failure event / operator command results in a protection switch
For example
in 1:n 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
Y(J)S APS Slide 24
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 1+1 (nonrevertive)
Guard timer
– for rings – blockout time during which APS messages are ignored (since
they may be old and outdated)
Y(J)S APS Slide 25
APS signaling
In all types except unidirectional 1+1, 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 event’s 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
Y(J)S APS Slide 26
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 1+1 and unidirectional-(1:1)n
2-phase
(including 1:1)
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-1:n or m:n
3-phase 1) tailhead 2) headtail 3) tailhead (slowest)
–
works for all protection types (including m:n)
Y(J)S APS Slide 27
Examples of 1-phase
Example of when 1-phase signaling is possible is 1:1 or (1:1)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 1:1
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
Y(J)S APS Slide 28
Example of 2-phase
2-phase is useful for unidirectional 1:n 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
Y(J)S APS Slide 29
Example of 3-phase
3-phase signaling is imperative for bidirectional 1:n
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
Y(J)S APS Slide 30
For G.805 buffs
to add 1+1 trail protection to a trail - expand a trail termination function
we use a special transport processing function - the protection switch
unprotected
trail
protected trail
the unprotected TTs report status
to the protection switch
Y(J)S APS Slide 31
SONET/SDH APS
Y(J)S APS Slide 32
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
working channel
protection channel
head-end NE
tail-end NE
Y(J)S APS Slide 33
SONET/SDH layers
Path
Termination
ADM
regenerator
ADM
Line
Termination
Section
Termination
Line
Termination
Path
Termination
path
line
section
line (MS section)
section
section
line
section
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)
Y(J)S APS Slide 34
Line APS
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
9 rows
6 rows
3 rows
90 columns
Synchronous Payload Envelope
TOH
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
Y(J)S APS Slide 35
HO Path APS
J1
B3
C2
G1
F2
H4
F3
K3
N1
POH
POH is responsible for type, status, path performance monitoring, VCAT, trace
HO Path APS signaling uses 4 MSBs of byte K3
Y(J)S APS Slide 36
LO Path APS
1
30
59
87
V5
VC OH is responsible for
Timing, PM, REI, …
LO Path APS signaling is
4 MSBs of byte K4
V1
J2
V2
N2
V3
K4
V4
VC OH
Y(J)S APS Slide 37
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
head-end bridge
tail-end bridge
working channel
protection channel
signaling channel
Y(J)S APS Slide 38
Linear 1+1 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
working channel
protection channel
head-end NE
tail-end NE
Y(J)S APS Slide 39
Linear 1:1 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
Y(J)S APS Slide 40
Linear 1:N 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
working channels
protection channel
Y(J)S APS Slide 41
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
Y(J)S APS Slide 42
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
A-B
B
A-B
B
B-C
B-A
A
A
C-B
B-A
C
Y(J)S APS Slide 43
UPSR vs. BLSR (MS-SPRing)
UPSR
Unidirectional
Path switching
Two-fiber
BLSR
Bidirectional
Line switching
Four-fiber
Of all the possible combinations, only a few are in use
Unidirectional (routing) Path Switched Rings
protects tributaries
extension of 1+1 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 1:1 to ring topology
Y(J)S APS Slide 44
UPSR
Working channel is in one direction
protection channel in the opposite direction
All path traffic is “added” in both directions (1+1)
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”
2 rings
SONET ADM
Y(J)S APS Slide 45
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
wrap-around
Four-fiber version
full redundant OC-N devoted to protection
twice as many NEs as compared to two-fiber
2 rings
Example
recovery from unidirectional fiber cut
Y(J)S APS Slide 46
Ethernet linear APS
STP
LAG
G.8031
Y(J)S APS Slide 47
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
Y(J)S APS
Slide 48
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 N+N mode
The restoration time will be on the order of 1 second
Y(J)S APS
Slide 49
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
 1+1 and 1:1 protection types
 unidirectional and bidirectional switching for 1+1
 bidirectional switching for 1:1
 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)
Y(J)S APS
Slide 50
G.8031 signaling
The APS signaling message looks like this :
MEL
(3b)
VER=0
(5b)
req/state prot. type
(4b)
(4b)
OPCODE=39
FLAGS=0
OFFSET=4
(1B)
(1B)
(1B)
requested sig
bridged sig
reserved
(1B)
(1B)
(1B)
END=0
(1B)
–
–
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 (1+1, 1:1, uni/bidirectional, etc.)

requested and bridged signal identify incoming / outgoing traffic
since only 1+1 and 1:1 they are either null or traffic (all other values reserved)
Y(J)S APS
Slide 51
G.8031 1:1 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 bridged=normal
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
Y(J)S APS
Slide 52
Ethernet ring APS
G.8032
RPR
CLEER
Y(J)S APS Slide 53
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 don’t 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
Y(J)S APS Slide 54
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)
Y(J)S APS Slide 55
G.8032
Q9 of SG15 produced G.8032 between 2006 and 2008
G.8032 is similar to G.8031

strives for 50 ms protection (< 1200 km, < 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 (opcode=40)

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)
Y(J)S APS Slide 56
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
Y(J)S APS Slide 57
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 @ 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 @ 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
Y(J)S APS
Slide 58
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)
RPL
RPL
next
neighbor
Major differences :






RPL
RPL
owner
neighbor
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)
subring ring
– ladders (For Further Study in v1)
– multi-ring
ring topology discovery
virtual channel based on VLAN or MAC address
subring
ladder
Y(J)S APS Slide 59
RPR – 802.17
Resilient Packet Rings
 are compatible with standard Ethernet, but different frame format
 are robust (lossless, <50ms protection, OAM)
 are fair (based on client throttling)
 support QoS (3 classes – A, B, C)
 are efficient (full spatial reuse)
ringlet0
 are plug and play (automatic station autodiscovery)
 extend use of existing fiber rings
counter-rotating add/drop ringlets, running


ringlet1
SONET/SDH (any rate, PoS, GFP or LAPS) or
“packetPHY” (1 or 10 Gb/s ETH PHY)
developed by 802.17 WG
based on Cisco’s Spatial Reuse Protocol (RFC 2892)
ringlet selection
Y(J)S APS
Slide 60
Basic RPR queuing
traffic going around ring
traffic for local sink
placed in output buffer
according to service class
A
B
C
placed into internal buffer
in dual-transit queue mode
placed into 1 of 2 buffers
according to service class
sent according to fairness
PTQ
STQ
fairness
A
B
C
Primary/Secondary Transit Queue
traffic from local source
sent according to fairness
first sent to ringlet selection
Y(J)S APS
Slide 61
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,
low
No
bounded
No
reclaimable
B-CIR near RT allocated,
reclaimable
B-EIR
near RT opportunistic unbounded Yes
C
BE
opportunistic unbounded Yes
Y(J)S APS
Slide 62
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
Y(J)S APS
Slide 63
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
Y(J)S APS
Slide 64
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
ring nodes
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
switches
Y(J)S APS
Slide 65
MPLS fast reroute
IP FRR
RFC 4090
Y(J)S APS Slide 66
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
Y(J)S APS Slide 67
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
not
discussed in
RFC 4090
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
Y(J)S APS Slide 68
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
ingress
LER
PLR
MP
egress
LER
Y(J)S APS Slide 69
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
PLR
Facility backup

backup tunnel for multiple LSPs

bypass tunnel created at each potential PLR

uses label stacking
PLR
MP
MP
Y(J)S APS Slide 70
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)
PLR
MP
In order to bypass a single failed node, we need an alternative path to the
next next hop (NNHOP)
PLR
MP
Y(J)S APS Slide 71
MPLS TP APS
RFC 6372 (MPLS-TP Survivability Framework)
RFC 6378 (MPLS-TP Linear Protection)
draft-ietf-mpls-tp-ring-protection
Y(J)S APS Slide 72
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
Y(J)S APS Slide 73
Linear protection
from RFC 6378 (ex draft-ietf-mpls-tp-linear-protection)
• 1+1, 1:1, 1:n and uni/bidi are supported
• APS signaling protocol (for all modes except 1+1 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
Y(J)S APS
Slide 74
PSC message format
GAL Label (13)
0001
VER
00000000
Ver Request PT R
Res
TC
S=1
TTL
PSC channel type
FPath
TLV Length
GAL
GACh
Path
Res
PSC
Optional TLVs
Request : NR, SF, SD, manual switch, forced switch, lockout, WTR, DNR
PT = Protection Type : uni 1+1, bidi 1+1, bidi 1:1/1:n
R = Revertive
FPath = which path has fault Path = which data path is on protection channel
Y(J)S APS Slide 75
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
Y(J)S APS Slide 76
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)
Y(J)S APS Slide 77
Linear protection – ITU style
from draft-zulr-mpls-tp-linear-protection-switching
Similar to previous, but uses Y.1731/G.8031 format (no surprise!)
GAL Label (13)
0001
VER
00000000
MEL
VER
OPCODE=39
req
state
prot
type
requested
sig
TC
S=1
TTL
allocated channel type
FLAGS=0
bridged
sig
GAL
GACh
OFFSET=4
G.8031
reserved
END=0
Y(J)S APS
Slide 78
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-helvoort-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
Y(J)S APS
Slide 79