17_MPLS_TE
-
Upload
huma-nadeem -
Category
Documents
-
view
216 -
download
0
Transcript of 17_MPLS_TE
-
8/7/2019 17_MPLS_TE
1/44
Introduction to MPLS
and TrafficEngineering
-
8/7/2019 17_MPLS_TE
2/44
2
Outline
Traditional IP Routing Forwarding and routing Problems with IP routing
Motivations behind MPLS MPLS Terminology and Operation
MPLS Label, LSR and LSP, LFIB Vs FIB Transport of an IP packet over MPLS More MPLS terminology
Traffic Engineering [with MPLS] Nomenclature Requirements Examples
-
8/7/2019 17_MPLS_TE
3/44
3
Outline
Traditional IP Routing Forwarding and routing Problems with IP routing
Motivations behind MPLS MPLS Terminology and Operation
MPLS Label, LSR and LSP, LFIB Vs FIB Transport of an IP packet over MPLS More MPLS terminology
Traffic Engineering [with MPLS] Nomenclature Requirements Examples
-
8/7/2019 17_MPLS_TE
4/44
-
8/7/2019 17_MPLS_TE
5/44
5
IP versus MPLS routing
IP routing Each IP datagram is routed independently Routing and forwarding is destination-based
Routers look at the destination addresses May lead to congestion in parts of the network
MPLS routing A path is computed in advance and a virtual
circuit is established from ingress to egress An MPLS path from ingress to egress node is
called a label switched path (LSP)
-
8/7/2019 17_MPLS_TE
6/44
6
How IP routing works
Searching
Longest
Prefix Match
in FIB (Too
Slow)
-
8/7/2019 17_MPLS_TE
7/44
7
Problems with IP routing
Too slow IP lookup (longest prefix matching) was a
major bottleneck in high performance routers This was made worse by the fact that IP
forwarding requires complex lookup operationat every hop along the path
Too rigid no flexibility Routing decisions are destination-based
Not scalable in some desirable applications When mapping IP traffic onto ATM
-
8/7/2019 17_MPLS_TE
8/44
8
IP routing rigidity example
Packet 1: Destination A Packet 2: Destination B S computes shortest paths to A and B; finds D as next hop Both packets will follow the same path
Leads to IP hotspots!
Solution? Try to divert the traffic onto alternate paths
1 1
1 2
A B
C
A
B
S
D
-
8/7/2019 17_MPLS_TE
9/44
9
IP routing rigidity example
Increase the cost of link DA from 1 to 4
Traffic is diverted away from node D
A new IP hotspot is created!
Solution(?): Network Engineering Put more bandwidth where the traffic is!
Leads to underutilized links; not suitable for large networks
1 4
1 2
A B
C
S
A
B
D
-
8/7/2019 17_MPLS_TE
10/44
10
Motivations behind MPLS
Avoid [slow] IP lookup Led to the development of IP switching in 1996
Provide some scalability for IP over ATM
Evolve routing functionality Control was too closely tied to forwarding
Evolution of routing functionality led to someother benefits Explicit path routing
Provision of service differentiation (QoS)
-
8/7/2019 17_MPLS_TE
11/44
11
IP routing versus MPLS
routing
Traditional IP RoutingMultiprotocol Label Switching (MPLS)
S D
543
21
MPLS allows overriding shortest paths!
-
8/7/2019 17_MPLS_TE
12/44
12
Outline
Traditional IP Routing Forwarding and routing Problems with IP routing Motivations behind MPLS
MPLS Terminology and Operation MPLS Label, LSR and LSP, LFIB Vs FIB Transport of an IP packet over MPLS More MPLS terminology
Traffic Engineering [with MPLS] Nomenclature Requirements Examples
-
8/7/2019 17_MPLS_TE
13/44
13
MPLS label
To avoid IP lookup MPLS packets carryextra information called Label
Packet forwarding decision is made usinglabel-based lookups
Labels have local significance only!
How routing along explicit path works?
IP DatagramLabel
-
8/7/2019 17_MPLS_TE
14/44
14
Routing along explicit paths
Idea: Let the source make the complete routingdecision
How is this accomplished? Let the ingress attach a label to the IP packet and let
intermediate routers make forwarding decisions only
On what basis should you choose different pathsfor different flows? Define some constraints and hope that the constraints
will take some traffic away from the hotspot!
Use CSPF instead of SPF (shortest path first)
-
8/7/2019 17_MPLS_TE
15/44
15
Label, LSP and LSR
Label
Router that supports MPLS is known as labelswitching router (LSR)
Path which is followed using labels is called LSP
Label = 20 bits
Exp = Experimental, 3 bits
S = Bottom of stack, 1bit
TTL = Time to live, 8 bits
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
Label | Exp|S| TTL
-
8/7/2019 17_MPLS_TE
16/44
16
LFIB versus FIB
Labels are searched in LFIB whereas normal IPRouting uses FIB to search longest prefix matchfor a destination IP address
Why switching based on labels is faster? LFIB has fewer entries
Routing table FIB has larger number of entries???
In LFIB, label is an exact match In FIB, IP is longest prefix match
-
8/7/2019 17_MPLS_TE
17/44
17
MPLS Flow Progress
LSR1
LSR2
LSR3
LSR5
LSR6
R1 R2LSR4D
1 - R1 receives a packet for destination D connected to R2
R1 and R2 are
regular routers
D
destination
-
8/7/2019 17_MPLS_TE
18/44
18
MPLS Flow Progress
LSR1
LSR2
LSR3
LSR5
LSR6
R1 R2LSR4D
2 - R1 determines the next hop as LSR1 and forwards the packet
(Makes a routing as well as a forwarding decision)
D
destination
-
8/7/2019 17_MPLS_TE
19/44
19
MPLS Flow Progress
LSR1
LSR2
LSR3
LSR5
LSR6
R1 R2LSR4
D
3 LSR1establishes a path to LSR6 and PUSHES/ATTACHES a label
(Makes a routing as well as a forwarding decision)
D
destination
31
-
8/7/2019 17_MPLS_TE
20/44
20
MPLS Flow Progress
LSR1
LSR2
LSR3
LSR5
LSR6
R1 R2LSR4
D
4 LSR3 just looks at the incoming label
LSR3 SWAPS with another label before forwarding
D
destination
17
Labels have local
signifacance!
-
8/7/2019 17_MPLS_TE
21/44
21
MPLS Flow Progress
LSR1
LSR2
LSR3
LSR5
LSR6
R1 R2LSR4
D
5 LSR6 looks at the incoming label
LSR6 POPS/REMOVES the label before forwarding to R2
D
destination
17
Path within MPLS cloudis pre-established:
LSP (label-switched path)
-
8/7/2019 17_MPLS_TE
22/44
22
MPLS and explicit routing
recap
Who establishes the LSPs in advance? Ingress routers
How do ingress routers decide not to always takethe shortest path? Ingress routers use CSPF (constrained shortest path
first) instead of SPF
Examples of constraints:
Do not use links left with less than 7Mb/s bandwidth Do not use blue-colored links for this request
Use a path with delay less than 130ms
-
8/7/2019 17_MPLS_TE
23/44
23
CSPF
What is the mechanism? (in typical cases!) First prune all links not fulfilling constrains
Now find shortest path on the rest of the topology
Requires some reservation mechanism Changing state of the network must also be
recorded and propagated For example, ingress needs to know how much
bandwidth is left on links The information is propagated by means of routing
protocols and their extensions
-
8/7/2019 17_MPLS_TE
24/44
-
8/7/2019 17_MPLS_TE
25/44
25
Label advertisement
Always downstream to upstream labeladvertisement and distribution
171.68.32/24
LSR1
LSR2
Use label 5 for destination
171.68.32/24
MPLS Data Packet
with label 5 travels
Upstream Downstream
-
8/7/2019 17_MPLS_TE
26/44
26
Label advertisement
Label advertisement can be downstreamunsolicited or downstream on-demand
171.68.32/24
LSR1 LSR2
Sends label
Without any Request
Upstream Downstream
171.68.32/24
LSR1 LSR2
Sends label ONLY after
receiving request
Request For label
Upstream Downstream
-
8/7/2019 17_MPLS_TE
27/44
27
Label distribution
Label distribution can be ordered or unordered
First we see an example of ordered label distribution
Ingress LSREgress LSR
Label
-
8/7/2019 17_MPLS_TE
28/44
28
Label distribution
Label distribution can be ordered or unordered
Next we see an example of unordered label distribution
Ingress LSREgress LSR
Label
Label
-
8/7/2019 17_MPLS_TE
29/44
29
Label operations
Advertisement Downstream unsolicited Downstream on-demand
Distribution Ordered Unordered
-
8/7/2019 17_MPLS_TE
30/44
30
Outline
Traditional IP Routing Forwarding and routing Problems with IP routing Motivations behind MPLS
MPLS Terminology and Operation MPLS Label, LSR and LSP, LFIB Vs FIB Transport of an IP packet over MPLS More MPLS terminology
Traffic Engineering [with MPLS] Nomenclature Requirements Examples
-
8/7/2019 17_MPLS_TE
31/44
Traffic Engineering
Traffic Engineering with MPLS
(Application of CSPF)
-
8/7/2019 17_MPLS_TE
32/44
32
What is traffic engineering?
Performance optimization of operational networks optimizing resource utilization optimizing traffic performance reliable network operation
How is traffic engineered? measurement, modeling, characterization, and
control of Internet traffic
Why? high cost of network assets service differentiation
-
8/7/2019 17_MPLS_TE
33/44
33
Traffic engineering
Recall the IP hotspot problem
The ability to move traffic away from the
shortest path calculated by the IGP (such asOSPF) to a less congested path
IP: changing a metric will cause ALL the traffic
to divert to the less congested path MPLS: allows explicit routing (using CSPF) and
setup of such explicitly computed LSPs
-
8/7/2019 17_MPLS_TE
34/44
34
MPLS-TE: How to do it?
LSPs are set up by LSRs based on informationthey learn from routing protocols (IGPs)
This defeats the purpose! If we were to use shortest path, IGP was okay
-
8/7/2019 17_MPLS_TE
35/44
35
MPLS TE: How we actually do
it?
MPLS TE Requires: Enhancements to routing protocols
OSPF-TE ISIS-TE
Enhancement to signaling protocols to allowexplicit constraint based routing RSVP-TE and CR-LDP
Constraint based routing Explicit route selection Recovery mechanisms defined
-
8/7/2019 17_MPLS_TE
36/44
36
Signaling mechanisms
RSVP-TE Extensions to RSVP for traffic engineering
BGP-4 Carrying label information in BGP-4
CR-LDP A label distribution protocol that distributes labels
determined based on constraint based routing
RSVP-TE and CR-LDP both do label distributionand path reservation use any one of them!
-
8/7/2019 17_MPLS_TE
37/44
37
RSVP-TE
Basic flow of LSP set-up using RSVP
-
8/7/2019 17_MPLS_TE
38/44
38
RSVP-TE PATH Message
PATH message is used to establish state andrequest label assignment
R1 transmits a PATH message addressed to R9
-
8/7/2019 17_MPLS_TE
39/44
39
RSVP-TE RESV Message
RESV is used to distribute labels after reserving resources
R9 transmits a RESV message, with label=3, to R8 R8 and R4 store outbound label and allocate an inbound label.
They also transmit RESV with inbound label to upstream LSR R1 binds label to forwarding equivalence class (FEC)
-
8/7/2019 17_MPLS_TE
40/44
40
Rerouting LSP tunnels
When a more optimal route/pathbecomes available
When a failure of a resource occurs alonga TE LSP
Make-before-break mechanism
Adaptive, smooth rerouting and traffictransfer beforetearing down the old LSP Not disruptive to traffic
-
8/7/2019 17_MPLS_TE
41/44
41
Recovering LSP tunnels
LSP Set-up
-
8/7/2019 17_MPLS_TE
42/44
42
Protection LSP set up
-
8/7/2019 17_MPLS_TE
43/44
43
Protection LSP
-
8/7/2019 17_MPLS_TE
44/44
44
References
http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/120newft/120limit/120s/120s5/mpls_te.htm
RFC 2702 Requirements for Traffic EngineeringOver MPLS
RFC 3031 Multiprotocol Label SwitchingArchitecture
RFC 3272 Overview and Principles of Internet
Traffic Engineering RFC 3346 Applicability Statement for Traffic
Engineering with MPLS MPLS Forum (http://www.mplsforum.org)