Restoration by Path Concatenation:
Fast Recovery of MPLS Paths
Anat Bremler-Barr Yehuda Afek Haim Kaplan
Tel-Aviv University
Edith Cohen
Michael Merritt
AT&T Labs-Research
Agenda
 MPLS
A
- quick introduction
fast restoration scheme for
MPLS
MPLS: Multi Protocol Label
Switching
Two motivating forces:

Fast forwarding (eliminate IP-lookup)

Traffic Engineering & QoS
IP Lookup forwarding
1011001101011011001111110101


Destination Address
Prefix
NxtHop
*
00*
011101110*
10000001*
10110*
101111*
10110011 *
10110011010*
4
12
3
3
3
5
2
4
Forwarding Table
IP lookup - given an IP address, determine the
next hop for reaching that destination
Fast Address lookup key component for high
performance routers
Multi Protocol Label Switching
 Label
MPLS Header
IP Packet
– Short, fixed-length packet identifier
– Label swapping (similar to forwarding algorithm
used in Frame Relay and ATM)
– Incoming Label Mapping (ILM)
Incoming Label Mapping
IP
In
Out
(port, label) (port, label)
8
Port 1
Port 2
Port 3
Port 4 IP 7
(1, 2)
(1, 6)
(1, 8)
(2, 13)
(2, 17)
(2, 21)
(4, 7)
(3, 32)
Label
Operation
Swap
Swap
Swap
Swap
MPLS Forward Equivalence Class (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 policy
MPLS Header
32-bits
IP Packet
MPLS Forwarding Example
134.5.6.1
ILM Table
In
Out
(2, 84)
2
(6, 3)
134.5.1.5
6
2
134.5.1.5
FEC Table
Destination Next Hop
134.5/16
(2, 84)
200.3.2/24
(3, 99)
3
1
2
ILM Table
In
Out
(1, 99)
(2, 56)
3
5
ILM Table
In
Out
(3, 56)
(5, 3)
200.3.2.1
200.3.2.7
MPLS Label Stack
– Stack of labels in the header
IP Packet
MPLS Label
MPLS Label
– Each LSR processes the top label
IP
21
IP
1
2
3
IP
21 12
2
21
3
6
IP
1
5
IP
21
4
56
2
5
3
ILM Table
In
Out
(1, 21)
(2, Push [12])
(3, 9)
(2, Push [12])
ILM Table
In
Out
(2, 12)
(6, 3 )
ILM Table
In
Out
(1, 3)
(5, Pop )
ILM Table
In
Out
(4, 21)
(2, 56)
(4, 9)
(5, 7)
In conclusion:

MPLS benefits:
+
+
+
-
No IP lookup
Traffic engineering
QoS
Restoration
Fault
Teardown
Calculate – loop free
Establish
Part II
Restoration by Path Concatenation:
Fast Recovery of MPLS Paths
Restoration By Path Concatenation
(RBPC)
Restore by concatenating existing paths
m
s
t
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.
Example
m
s
t
o
n
One edge failure - concatenation of two paths
Two edge failures - concatenation of three paths
Path Concatenation with MPLS
• Use the stack of labels mechanism:
• source pushes two labels (one fault)
200.3.2/24
27
87
s
t
1
30
Ingress Routing Table (FEC)
Destination
134.5/16
Next Hop
(1, 30)
(2, 27|87)
200.3.2/24
(2, 87)
134.5/16
No changes in ILM
tables
Concatenation mechanism in
ATM or WDM
•
Need an IP-lookup at m !!!
m
V87
s
V27
1
VC Table of S
Dest label (vci/vpi) port
t
V30 V87
1 2
m V87
2
V30
t
The restoration method
requirements
• Global knowledge at Ingress LSR
• Store the global view locally (on a disk)
Limitations 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
v
s
t
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 proof
Let p be the shortest path after removing k
edges. Let bypasses {bp1, bp2, bp3, bp4} be:
e1
e1
s
u
e1
s
e2
e3
p
x w
v
e2
e2
t
e3
t
Claim: There are at most k bypasses ==> Main claim
Proof by contradiction:
•Assume there are more than k bypasses
•Then exists p* (s->t), s.t., p* is shorter than p.
e1
e1 e2
e3
e2
p
s
t
constructing p*:
claim: exists a subset of bypasses, s.t., each
removed edge occurs in an even number of bypasses.
e1
x y
s
e1 e2
x y z w
e2
z w
t
p
Building blocks for the shortest path p*:
e1
x y
s
e1 e2
x y z w
e2
z w
t
p
Building blocks for the shortest path p*:
e1
x y
e1 e2
x y z w
e2
z w
s
t
x
P* must exist - Euler
z
e2
t
s
y
e1
p
w
e3
p*
Pre-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
benefits
• 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 results
Name
Nodes
Links
Avg. degree
ISP
~200
~400
~3.7
Internet
40,377
101,659
5.035
AS Graph
4,746
9,878
4.16
AS Graph
AS
After one link failure
Network
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 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 failure
Network
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
RBPC ILM table size / pre-provisioned t.s.
v
s
t
After two router failures
Network
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