View
221
Download
0
Category
Tags:
Preview:
Citation preview
Restoration by Path Restoration by Path Concatenation:Concatenation: Fast Recovery of MPLS PathsFast Recovery of MPLS Paths
Anat Bremler-Barr Yehuda Afek Haim Kaplan Tel-Aviv University
Edith Cohen Michael Merritt AT&T Labs-Research
AgendaAgenda
MPLS - quick introduction
A fast restoration scheme for MPLS
MPLS: Multi Protocol MPLS: Multi Protocol Label SwitchingLabel Switching
Fast forwarding (eliminate IP-
lookup)
Traffic Engineering & QoS
Two motivating forces:Two motivating forces:
IP Lookup forwardingIP Lookup forwarding
IP lookup - given an IP address, determine the next hop for reaching that destination
Fast Address lookup key component for high performance routers
1011001101011011001111110101 Destination Address
Prefix NxtHop* 400* 12011101110* 310000001* 3 10110* 3101111* 510110011 * 210110011010* 4
Forwarding Table
Multi Protocol Label Switching
Label– Short, fixed-length packet identifier
–Label swapping (similar to forwarding algorithm used in Frame Relay and ATM)
IP PacketIP PacketMPLS Header
Incoming Label Mapping
In(port, label)
Out(port, label)
(1, 2)(1, 6)(1, 8)
(2, 13)
(2, 17)(2, 21)(4, 7)
(3, 32)
LabelOperation
Swap
SwapSwap
Swap
8IP
7IP
–Incoming Label Mapping (ILM)
Port 3
Port 1
Port 4
Port 2
MPLS Forward Equivalence Class MPLS Forward Equivalence Class (FEC)(FEC) The same label to a stream/flow of IP
packets:
– Forwarded over the same path
– Treated in the same manner
FEC/label binding mechanism
– Currently based on destination IP address
prefix
– Future mappings based on TE-defined policyIP PacketIP Packet
32-bits
MPLS Header
134.5.1.5
200.3.2.7
1 2
2 6
3 5
200.3.2.1
134.5.6.1
FEC Table
Destination Next Hop
134.5/16
200.3.2/24
(2, 84)
(3, 99)
ILM Table
In Out
(1, 99) (2, 56)
ILM Table
In Out
(3, 56) (5, 3)
ILM Table
In Out
(2, 84) (6, 3)
2
3
MPLS Forwarding ExampleMPLS Forwarding Example
134.5.1.5
134.5.1.5
8484134.5.1.5 33134.5.1.5
3
MPLS Label StackMPLS Label Stack
In OutILM Table
(2, Push [12])(1, 21)
(3, 9) (2, Push [12])
In OutILM Table
(6, 3 )(2, 12)
In OutILM Table
(5, Pop )(1, 3)
In OutILM Table
(2, 56)(4, 21)
(4, 9) (5, 7)
3
1
–Each LSR processes the top label
–Stack of labels in the header
IP PacketIP PacketMPLS Label MPLS Label
IP 21
2 2 6 1 5 4 5
2IP 21 12 IP 21 3 IP 21 IP 56
In conclusion:In conclusion: MPLS benefits:
+No IP lookup+Traffic engineering+QoS- Restoration
Fault TeardownCalculate – loop freeEstablish
Restoration by Path Restoration by Path Concatenation:Concatenation: Fast Recovery of MPLS PathsFast Recovery of MPLS Paths
Part IIPart II
Restoration By Path Restoration By Path ConcatenationConcatenation ( (RBPC)RBPC)
Restore by concatenating existing paths
s t
m
Main claim:Main claim: Unweighted case: Any shortest path after k edge failures is a
concatenation of at most k+1 original surviving shortest paths.
Weighted case: k+1 paths and k edges
The basic set of Paths: Either All shortest paths or One shortest path for each pair of routers.
ExampleExample
st
Two edge failures - concatenation of three paths
One edge failure - concatenation of two paths
no
m
Path Concatenation with MPLSPath Concatenation with MPLS• Use the stack of labels mechanism:• source pushes two labels (one fault)
30
8727
Ingress Routing Table (FEC)Destination Next Hop
134.5/16
200.3.2/24
(1, 30)
(2, 87)
1
2
134.5/16
(2, 27|87)
200.3.2/24
No changes in ILM
tables
s t
Concatenation mechanism inConcatenation mechanism in ATM or WDM ATM or WDM
VC Table of S
t
• Need an IP-lookup at m !!!
V30
V87 V27
m
s
Dest label (vci/vpi) port
t V30 1
m V87 2
V87 2
21
The restoration method The restoration method requirementsrequirements
• Global knowledge at Ingress LSR
• Store the global view locally (on a disk)
Limitations of RBPCLimitations of RBPC•Bandwidth reservation: have not yet dealt with
•Non shortest paths: Requires T.E. Algorithms at the source
•Theory does not apply to node failure
•Does not, in general work in directed graphs
s t
v
Main claim:Main claim: Unweighted case: Any shortest path after k edge failures is a
concatenation of at most k+1 original surviving shortest paths.
Weighted case: k+1 paths and k edges
The basic set of Paths: Either All shortest paths or One shortest path for each pair of routers.
Unweighted case: sketch of Unweighted case: sketch of proofproof
Let p be the shortest path after removing k edges. Let bypasses {bp1, bp2, bp3, bp4} be:
s t
s t
e1 e2 e3
Claim: There are at most k bypasses ==> Main claim
e1
p
e2e1 e3 e2
u v x w
Proof by contradiction:
•Assume there are more than k bypasses•Then exists p* (s->t), s.t., p* is shorter than p.
constructing p*:
claim: exists a subset of bypasses, s.t., each removed edge occurs in an even number of bypasses.
s tp
e1 e1 e2 e2
s t
x y x y z w z w
p
e1 e1 e2 e3 e2
e1 e1 e2 e2
s t
Building blocks for the shortest path p*:
p
x y x y z w z w
e1 e1 e2 e2
s tp
x y x y z w z w
P* must exist - Euler st
x
y
z
w
e1 e2 e3
p*
Building blocks for the shortest path p*:
Pre-provisioned methodPre-provisioned method For each link & LSP (label swapping path) going over it maintain (pre-
provision) a restoration path
Similarly, for each two links in an LSP maintain a restoration path
Huge O/H: ILM tables
Not scalable
The restoration method The restoration method benefitsbenefits• Fast restoration
• Static set of paths
• No messages for tearing down and setting up
• Static & Small ILM tables
• Only one router changes the FEC table.
•Speed and simplicity of pre-provisioned restoration paths without the associated overhead.
Empirical resultsEmpirical results
NameName NodesNodes LinksLinks Avg. degreeAvg. degree
ISPISP ~200 ~400 ~3.7
InternetInternet 40,377 101,659 5.035
AS GraphAS Graph 4,746 9,878 4.16
AS
AS Graph
After one link failureAfter one link failureNetwork max ILM Avg ILM. Avg. Concate Length. savings savings s.factor
ISP weighted 12.5% 25.6% 2.05 1.15
ISP unweighted 20.0% 32.3% 2 1.14
Internet 16.7% 22.8% 2 1.08
AS graph 25.0% 32.7% 2 1.19
RBPC ILM table size / pre-provisioned t.s.
After two link failuresAfter two link failures
Network max ILM Avg ILM. Avg. PC length Length. savings savings s.factor
ISP weighted 2.3% 6.1% 2.38 1.77
ISP unweighted 3.6% 8.5% 2.20 1.34
Internet 3.0% 4.7% 2.06 1.15
AS graph 7.1% 16.4% 2.09 1.32
RBPC ILM table size / pre-provisioned t.s.
After one router failureAfter one router failureNetwork max ILM Avg ILM. Avg. PC length Length.
savings savings s.factor
ISP weighted 25.0% 43.7% 2.10 1.38
ISP unweighted 20.0% 36.8% 2.03 1.18
Internet 12.5% 21.1% 2.02 1.08
AS graph 25.0% 38.5% 2.03 1.26
s t
v
RBPC ILM table size / pre-provisioned t.s.
After two router failuresAfter two router failuresNetwork max ILM Avg ILM. Avg. PC length Length.
savings savings s.f.
ISP weighted 5.26% 11.1% 2.43 1.57
ISP unweighted 6.67% 13.3% 2.21 1.44
Internet 2.50% 4.1% 2.23 1.17
AS graph 8.33% 18.5% 2.17 1.31
RBPC ILM table size / pre-provisioned t.s.
End End
Recommended