Multiprotocol Label Switching
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Transcript of Multiprotocol Label Switching
Engineering Internet QoS 1
Multiprotocol Label Switching
Engineering Internet QoS 2
Overview
L3 Switching vs RoutingWhat is MPLS?MPLS on ATM: IssuesTraffic Engineering Using MPLSMPLS Simulation Results
Engineering Internet QoS 3
Introduction
MPLS stands for “Multiprotocol” label switching• techniques applicable to any L3 protocol
A router supporting MPLS is know as Label Switched Router (LSR)
IP routing/control software is combined with label swapping • similar to ATM VCI/VPI swapping
Engineering Internet QoS 4
Proprietary Protocols
The Cell Switching Router (CSR)• ftp://ftp.wide.toshiba.co.jp/pub/csr/
Ipsilon’s IP Switching• www.ipsilon.com
Cisco’s Tag Switching• www.cisco.com
IBM’s Aggregate Route-Based IP Switching (ARIS)• www.networking.ibm.com/isr/ip/
IETF MPLS working group• www.ietf.org/html.charters/mpls-charter.html
Engineering Internet QoS 5
Why label switching
Scalablity of L3 routing and simplicity of L2 forwarding• high performance• use of ATM hardware• faster than IP lookup• integration of IP and ATM
new Services such as VPNshierarchy of Routing
Engineering Internet QoS 6
Terminologies
flow : a single instance of an application to application flow of data
forwarding equivalence class: a group of IP packets which are forwarded along same path (or same treatment)
label: a short fixed length physically contiguous identifier which is used to identify a FEC (local significance)
label Switched Router: an MPLS node which is capable of forwarding L3 packets
Engineering Internet QoS 7
MPLS Domain
LSR: label switching router
LER: label edge router
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
Engineering Internet QoS 8
Conventional IP Routing
packet travels from one router to the next • an independent forwarding decision made at
each hop
At each hop packets assigned to a Forwarding Equivalence Class (FEC)• packets considered to be in same FEC if the
routing table contains some prefix X such that X is the longest match for each packet’s destination address
Engineering Internet QoS 9
MPLS Approach
assignments of packets to FEC done once• as the packet enters the network
FEC to which packet is assigned is encoded with a label
packet is forwarded to next hop with label• no further analysis of packet header at
next hop
Engineering Internet QoS 10
Label encoding
MPLS encapsulation should contain • the label stack field• time-to-live (TTL) field• a 3 bit Experimental field (earlier
COS)MPLS uses 32 bits for label encoding
Label: Label Value, 20 bits (0-16 reserved)Exp.: Experimental, 3 bits (earlier Class of Service)S: Bottom of Stack, 1 bit (1 = last entry in label stack)TTL: 8 bit Time to Live
label exp s ttl
Engineering Internet QoS 11
Reserved Labels
Multiprotocol = IP, IPX, etc. Protocol is inferred from the last label in the stack.
0 = IPv4 Explicit Null Label Pop this last label. Use IPv4 header.
1 = Router Alert Label Looked by the router software.Packet forwarded based on the next label in stackSimilar to “Router Alert Option” in IP Packets
2 = IPv5 Explicit Null Label 3 = Implicit Null Label
Used only for assignment and distributionShould not appear in any label stack in the packets
Engineering Internet QoS 12
TTL Handling
At the beginning TTL = TTL from IP headerAt every hop: TTL = TTL -1Drop the packet if TTL=0At the exit: TTL in IP Header = TTL from
labelIn some cases, entire MPLS domain may
be considered one hop, e.g., ATM
Engineering Internet QoS 13
MPLS Encapsulation
For ATM VPI/VCI field and for Frame Relay DLCI field used
a “shim” layer between link layer and network layer headers for non-ATM/Frame Relay network• protocol independent• shim layer may consist of sequence of label
stack entries• If PPP used, protocol field may identify
frames that carry labels
Engineering Internet QoS 14
Label Processing
label used as index into a table which specifies next hop and a new label• longest match calculation eliminated at
subsequent hops old label replaced with new label and packet
forwarded to next hop label may also contain class of service (COS)
• for scheduling/ discarding packets etc • again saves header processing
Engineering Internet QoS 15
Label Distribution
Who assigns labels for communication between A and B?• A, B, or someone else?• Downstream, upstream, ...
Where is the control for the entire path?A, B, ingress or egress LSR?
Separate protocol or existing route distribution mechanisms?• IETF’s Label Distribution Protocol (LDP)
based on Cisco’s TDP and IBM’s ARIS protocol
Engineering Internet QoS 16
Sample Network Topology
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
Engineering Internet QoS 17
Routing Tables
17
18
19
Table at R2
162.25.8 1
192.35.10 0Indexof table
Table at R4
162.25.8 1
192.35.10 1
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
Engineering Internet QoS 18
Label Binding Example
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
Engineering Internet QoS 19
Label allocation
Table at R4
162.25.8
In label
?
?
out label
17
18
Addr prefix
192.35.10
Interface
1
1
Table at R2
162.25.8
In label
17
18
out label
19
16
Addr prefix
192.35.10
Interface
0
1
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
Engineering Internet QoS 20
Label Switching
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
Engineering Internet QoS 21
Hierarchy Routing
IGP Routers (R3, R4) in transit domain A maintain all the routes providedby Interdomain routing
- necessary to forward transit traffic to/from domains B & C
BGP: Border Gateway Protocol (inter-domain)IGP: Interior Gateway Protocol (intra-domain)
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
Engineering Internet QoS 22
Hierarchy Routing
BGP entriesinlabel
outlabel
nexthop
R1
R2
R5
R6
….
….
….
….
21
31
4131
41
21 R5
R6
IGP entriesinlabel
outlabel
nexthop
R3
R4
R5
R5
?
?
52
52 62
7262
72
R2
R4
R3
R5
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
Engineering Internet QoS 23
Stack of labels
Packets carry several labels organised as a label stack packet forwarded from one domain to other contains one label packet forwarded through a transit domain contains two labels LSRs use label from top of stack
• labels pushed (ingress) and poped (egress) at domain boundaries
Engineering Internet QoS 24
push/pop on stack
BGP entriesinlabel
outlabel
nexthop
R1
R2
R5
R6
….
….
….
….
21
31
4131
41
21 R5
R6
push
IGP entriesinlabel
outlabel
nexthop
R3
R4
R5
R5
?
?
52
52 62
7262
72
R2
R4
R3
R5
pop
swap label 21 to 31R5 not directly connectedFind direct conn. and labelpush label52 and send to R3
next hop R5 (self)pop off the top label of the stackswap this label 31 to 41 (BGP)
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
Engineering Internet QoS 25
MPLS over ATM
ATM switches performing label switching are called ATM-LSRs
labels need to be encoded in VCI/VPI fields MPLS forwarding similar to “label swapping” in
ATM • same ATM user plane may be used• control plane to be changed (MPLS)
protocols BGP, OSPF .. replaces UNI, PNNI
Engineering Internet QoS 26
MPLS over ATM
Packets have a shim with Label=0. Actual label is encoded in VPI/VCI.
No TTL decrementn-to-1 merge or n-to-n multipoint not
supportedVCI = 0 through 32 should not be used in
labelsIf LSRs are connected via SVCs, VCI/VPIs
cannot be used as labels. LSRs exchange VCIDs.
Engineering Internet QoS 27
Encoding labels in ATM header
SVC Encoding• VPI/VCI field to encode the label from top of
stack• ATM signaling can be used
SVP Encoding• VPI encodes top label• VCI encodes 2nd label
SVP Multipoint Encoding
Engineering Internet QoS 28
Cell Interleave Problem
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
Engineering Internet QoS 29
Request Separate labels
R1
R2
ATM R3
162.25
inlabel
outlabel
addrprefix interface
162.25
162.25
51
41
21
21
1
1
0
2
ininterface
if0
if2
get a label for 162.25
get a label for 162.25
request separate labels for R1 and R2 from R3
Predefined VPI/VCIused for Label binding
if1
Engineering Internet QoS 30
VC - Merge
Delay cells from one frame(AAL5 EOF marker)
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
Engineering Internet QoS 31
MPLS on ATM: Issues
VCI field is sufficient for one level taggingVPI may be used for the 2nd level
LSR switches need to participate in network layer routing protocols (OSPF, BGP)
Multiple tags per destination may be used to avoid frame merging
VPI/VCI space may be segmented for label switching and normal ATM switching
Engineering Internet QoS 32
Traffic Engineering Using MPLS
Traffic Engineering • Performance Optimization• Efficient resource allocation• Constrained routing / Load balancing• Maximum throughput, Min delay, min
loss Quality of service
• Meet policy requirements of operators
Engineering Internet QoS 33
Shortest-Path Routing
A
B
C
MPLS Backbone
R3
R1 R5
R2
R4 R6
R2->R5->R6 link 45MbpsR2->R4->R6 link 15Mbps
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
Engineering Internet QoS 34
Constrained Routed LSP
Hard to do non-shortest path routing with connectionless forwarding
Constrained Routed label-switched paths allows selective non-shortest path routing• load balancing across path possible• Signaling protocols for such path
establishment being developed by IETF M-RSVP is based on Resource Reservation
Protocol RSVP (soft-state model) CR-LDP defines hard-state signaling protocols
Engineering Internet QoS 35
CR-LSP Example
A
B
C
MPLS Backbone
R1
R2
R5
R6R4
R3
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
Engineering Internet QoS 36
Traffic Trunk
In MPLS networks: “Traffic Trunks” = SVCsA set of traffic parameters can be specified to
determine the “Forwarding Equivalence Class (FEC)” or set of packets assigned to that trunk
Features of trunks • Multiple trunks can be used in parallel to the same
egress.• Traffic trunks are routable entities like VCs• Each traffic trunk can have a set of associated
characteristics, e.g., priority, preemption, policing, overbooking
Engineering Internet QoS 37
Traffic Trunks Features
Traffic Trunk Features• Trunk paths are setup based on policies or
specified resource availability.• A traffic trunk can have alternate sets of paths
in case of failure of the main path. Trunks can be rerouted.
• Some trunks may preempt other trunks. A trunk can be preemptor, non-preemptor, preemptable, or non-preemptable.
• Each trunk can have its own overbooking rate
Engineering Internet QoS 38
Flows, Trunks, LSPs, and Links
Label Switched Path (LSP): All packets with the same label
Trunk: Same Label+Exp Flow: Same MPLS+IP+TCP headers
DL Label Exp S TTL IP TCP
Reprinted with Permission from “Engineering Internet QoS - Jha & Hassan, Artech House Publishing, Norwood, MA, USA. www.artechhouse.com
Engineering Internet QoS 39
MPLS Simulation Results
Total network throughput improves significantly with proper traffic engineering
Congestion-unresponsive flows affect congestion- responsive flows• Separate trunks for different types of
flowsTrunks should be end-to-end
• Trunk + No Trunk = No Trunk
Engineering Internet QoS 40
Summary
Simplified forwarding based on exact match of fixed length label
Separation of routing and forwarding in IP networks
Facilitates the integration of ATM and IPEnables the use of explicit routing/source
routing in IP networksImproved routing scalability (hierarchy)
through stacking of labelsTraffic Engineering Benefits