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CS 672
Summer 2003
Lecture 5
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IP Router Architecture
• A multiservice router provides layer 2 and layer 3 services:
• Layer 2 services include Ethernet, FR, ATM
• Layer 3 services include IP/MPLS VPN
• Contains several types of line cards and interfaces e.g:
• Electrical – DS1/E1, DS3/E3, etc.
• Optical – Channelized OC-N (e.g., N=3,12,48,192)
• Ethernet – 10/100/1000 Mbps
• Each interface may carry different types of traffic e.g:
• TDM, Packet, Cell and/or Frames
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IP Router Architecture
Interfaces
Forwarding
Backplane
Control Plane
Forwarding
Control Processor
Forwarding
Switch Fabric
Line Card
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Cisco Multiservice Routers
Cisco 7600
Cisco 10000
(ESR)
C12000
(GSR)
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IP Control Plane
• IP router contains two interrelated but largely independent
components:
• Control Plane
• Forwarding Plane
• IP control plane consists routing protocols such as:
• IGP – OSPF, IS-IS, …
• EGP – BGP4, …
• IP control plane provides routing information which is used to build
the forwarding information base or forwarding table.
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IP Forwarding Plane
• IP forwarding component uses forwarding table to obtained
information necessary to transmit packets toward their destinations.
• Forwarding plane performs performs time-sensitive operations such
as:
• IP address lookup
• IP address lookup is usually one the most common bottlenecks in
high performance routers.
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IP Control and Forwarding
To/from routing peers
IP Control Plane
BGP
OSPF
IS-IS
Routing Information Base (RIB)
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IP Forwarding
• One of the main functions of IP routers is to forward packets.
• To perform this task, routers make forwarding decision on each
packet.
• IP Forwarding decision involves forwarding table lookup, using IP
destination address as the search key, to determine:
• Next hop address
• Outgoing link
• Using forwarding information, the packet is forwarded towards its
destination.
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IP Forwarding
Forwarding Decision
IP Packet
Switching
Scheduling
header
Ingress Link
Egress Link
Switch Fabric
Address lookup
Next Hop, Out Link
Prefix
Next hop Out Link
Forwarding Table
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Centralized Forwarding Architecture
• Earlier generation routers used centralized forwarding models.
• Control and forwarding components were centrally implemented on processor
• Packet forwarding was done in software using central routing table.
• The central forwarding model worked satisfactorily with slower link speeds
• As the link speed increased, the forwarding performance of the central
forwarding architecture has to be increased.
• To exploit temporal and spatial of the IP traffic, routers heavily relied on
route cache.
• Temporal locality means that there is a high probability of re-using a
given IP address within short duration e.g.:
• a sequence of packets for the same destination exhibit temporal locality
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Cache-based Centralized Forwarding
• Spatial locality means that there is high probability of referencing
address in certain range e.g.,
• A sequence of packets with the same destination subnet exhibit spatial locality
• Cache is smaller but faster (access time < 50 ns) than the main
memory (access time < 100ns)
• With route cache,
• forwarding of one or more initial packets for a destination is based on slower
routing table. The result of IP address lookup are saved in the route cache
• All subsequent packets for the same destination are the forwarded using faster
route cache
• A common route cache performance metric is known as hit ratio
• Hit ratio indicates the percentage address lookups successfully found in the round
cache
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Cache-based Centralized Forwarding
• In general, hit ratio of a route cache depends upon:
• degree of temporal and spatial locality of the IP traffic being forwarded, and
• size of the route cache
• Route cache-based forwarding may be better suited for enterprise
environment, where IP traffic exhibits more temporal/spatial locality and
routing changes occur very infrequently.
• Performance of a route-based forwarding severely degrades in the
Internet core.This is because:
• Traffic in the core shows much less temporal/spatial locality
• Routing changes are frequent (e.g., in the order of 100 route changes/sec)
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Limitations of Cache-based
Centralized Forwarding
• If the route changes too frequently,
• Route cache entries need to be invalidated which amounts to reduction in hit ratio
• Some studies have shown, hit ratio as low as around 50% to 70%.
• A lower hit ratio means:
• A larger portion of traffic is forwarded based on slower routing table
• The amount of redirected traffic could easily overload the processor
• And processor overload in turn can cause system failure
• With the increase in Internet traffic,
• routing table size,
• link data rates, and
• aggregate switching bandwidth requirement have also increased by several folds.
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Limitations of Cache-based
Centralized Forwarding
• Although link speeds have kept with the increasing traffic, however,
forwarding capacity have not been able to do so.
• One of the main reasons for this can be attributed the bottleneck caused
by IP address lookup operations.
• In addition to poor performance due to route cache, centralized forwarding
architecture does not scale with the increase in
• number of line cards,
• link data rates, and
• aggregate switching capacity
• This is because to be able to handle all traffic, centralized architecture
would require increase in forwarding capacity in proportion to the
aggregate switching capacity.
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Distributed Forwarding Architecture
• Therefore, IP address easily becomes system bottleneck and limits
aggregate forwarding capacity.
• Therefore, modern high-end routers are normally based on distributed
forwarding architecture.
• In a distributed architecture, address lookup is implemented in each card.
• Distributed forwarding scales better than centralized model because:
• each card only need to support forwarding that matches link rates rather than system
aggregate capacity,
• line card forwarding capacity is a small fraction of system aggregate forwarding
capacity, and
• address lookup becomes relatively easier
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Distributed Forwarding Architecture
• Distributed forwarding architecture relies on separation of time-critical
forwarding plane and less time-critical control plane tasks to achieve
scalability.
• Thus distributed forwarding model makes it possible to:
• decentralize time-critical IP forwarding plane tasks, and
• centralize less time-critical IP control plane tasks
• This means, forwarding plane tasks can be optimized independently on
each line card according to the link rates.
• As a result, router forwarding capacity scales with the aggregate switching
bandwidth and link data rates.
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Routing Information Base (RIB)
• The decoupling of control/forwarding and distributed nature of forwarding
means:
• Router needs to maintain two separate databases namely RIB and FIB
• The RIB (or routing table) maintains user supplied static and routing
protocol provided dynamic routes
• RIB may contain multiple routes for a destination
• routes from different protocols or
• routes from the same protocol but different metric values
• A path consists of outgoing interface to a next hop.
• RIB may have one or more paths for each destination.
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Forwarding Information Base (FIB)
• Not all routes maintained by RIB are feasible routes.
• FIB is a subset of RIB since it only maintains feasible routes that can be
actually used for forwarding.
• FIB is differs from route cache in following aspects:
• Route cache only maintains a small number of most recently used routes
• FIB maintains all accessible routes (i.e., it is a mirror image of RIB)
• Route cache may need to invalidate its entries frequently in a dynamic routing
environment resulting in performance degradation.
• Because FIB mirrors the RIB, its performance does not degrades in a dynamic
routing environment.
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Forwarding Information Base (FIB)
• A FIB entry contains all the necessary information for forwarding a packet
towards its destination e.g.:
• IP destination address, Next hop, Out Interface Layer 2 Header information
• A FIB entry maps an destination IP address to a single or multiple paths
• With multiple paths, traffic for a given destination can be forwarded over
multiple paths.
• The capability to forward traffic over multiple paths is known as loadbalancing.
• Standard packet scheduling algorithms such as RR and WRR can be
used to load-balance traffic over multiple paths.
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Forwarding Information Base (FIB)
• FIB forms the time-critical forwarding plane path through a router.
• To have a scalable forwarding architecture, in terms of number of address
prefixes (or FIB entries) and data rates, an efficient representation and
lookup scheme is crucial for the FIB.
• In particular, we would like to have a scheme with fastest lookup time and
lowest memory consumption.
• In contrast, RIB needs to be optimized for smallest updates times to handle route
changes quickly.
• The adaptability of Mtrie-based schemes for wide range of HW/SW
environments, make them attractive for RIB and FIB implementations.
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Routing information to/from peers
BGP
IP Control Plane
OSPF
IS-IS
1
RIB
2
FIB
3
Control Processor Card
Switch Fabric
4
4
FIB
Forwarding Plane
FIB
Line Cards
Links
Links
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Distributed FIB
1.
2.
3.
4.
Receive routes information's
Select the best feasible routes
Distribute the feasible routes to FIB on the line cards
Initially, synchronize the entire FIB. Afterwards, resynchronize
incrementally as routing info changes
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Forwarding Equivalence Class (FEC)
•
Conventional IP forwarding makes hop-by-hop forwarding decision along
the path traversed by the packet.
•
•
•
IP forwarding maps each destination IP address to a next hop router address and
an outgoing link
Multiple IP destination address are mapped to a next hop/outgoing link (i.e., manyto-one mapping).
As far as IP forwarding is concerned, group of IP destination addresses
that get mapped to the the same set of IP destination addresses are
indistinguishable.
•
For example, they are forwarded along the same path
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Forwarding Equivalence Class (FEC)
•
•
A group of packets (possibly belonging to different IP destination) that are
identically with respect to forwarding decision (e.g., forwarded in the
same way and along the same path) is said to form a FEC.
In IP forwarding, routers assign and reassign each packets to certain
FEC on every hop.
•
•
•
For example, two packets with different IP destination addresses are mapped to the
same FEC if there is a common longest prefix entry.
These packets would continue to traverse the same path as long as they share a
common FEC.
When a set of packets with different destination addresses traverse a
network, each router may assign them to the same or different FEC(s)
based on forwarding information.
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d1 and d2 mapped to the same FEC
Prefix
d1,d2
d1 and d2 mapped to the same FEC
Prefix
d1,d2
d1 and d2 mapped to different FECs
Next hop Out Link
R4
Link 2
R2
d2
Prefix
d1
d2
d1
d2
d1
Next hop Out Link
R6
Link 1
R5
Link 2
R4
Next hop Out Link
R2
Link 1
FEC
R6
R1
d1
d2
d1
Destination = d1
Destination = d2
R3
R5
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IP Packet forwarding
• IP forwarding
• IP forwarding is done independently at every hop
• IP forwarding decision is made on:
 Packet header
 Routing algorithm output (routing table)
• Each IP hop runs its own instance of the routing algorithm
• Each IP hop makes its own forwarding decisions
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IP Address Lookup (bottleneck)
•
•
•
•
IP lookup forms a major bottleneck in high performance routers.
This is further made worse by the fact that IP forwarding requires
complex lookup operation at every hop along the path.
So, we could identify the FECs and associate each packet to certain
FEC, we would be able to either avoid or simplify IP address lookup
operation.
Supposing such a scheme does exist, then, a router would be able to
make equivalent forwarding decision using new information other than IP
destination address.
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MPLS
•
•
•
In this context, one method to avoid IP address lookup bottleneck is to
carry extra information, known as label, in the packet header.
The forwarding scheme (technology) that uses label information for
making packet forwarding decision is called Multiprotocol Label
Switching (MPLS).
Thus MPLS avoids (ignoring the ingress/egress routers) the IP address
lookup operation for make forwarding decision.
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MPLS
•
MPLS: Multi Protocol Label Switching
•
•
•
•
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Packet forwarding decision is made using label-based lookups
MPLS nodes forward packets using label rather than the IP destination address
Labels are assigned to packets at the ingress to the MPLS network
In the MPLS network (i.e., MPLS transit nodes) packets are forwarded based on
label information.
 No further packet analysis
 Label swapping (label in/label out)
Labels are carried in packet header
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MPLS forwarding: FEC and Next-Hop
• MPLS makes use of FECs
• MPLS nodes assign a label to each FEC
• Packet classification (into a FEC) is done where the packet enters the
MPLS network
• No sub-sequent packet classification in the MPLS network
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MPLS Labels
•
•
•
•
•
Short fixed length value that identifies a particular FEC.
The association of a label to a FEC is known as label-to-FEC binding.
Label of an incoming packet is known as in or local label
Label of an outgoing packet is known as an out or remote label.
A packet may contain:
•
•
•
Both in and out labels
In label but no out label
Out label but no in label
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MPLS Labels
0
1
2
3
01234567890123456789012345678901
Label
| Exp|S|
TTL
Label = 20 bits
Exp = Experimental, 3 bits
S = Bottom of stack, 1bit
TTL = Time to live, 8 bits
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Label Stack
•
•
•
•
•
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A MPLS packet may contain one or more labels which are referred to as
label stack.
Labels in a label stack are organized Last-In-First-Out (LIFO) ordered
sequence.
The number of entries in the label stack represent the stack depth.
Label of an incoming packet is known as in or local label
A label stack of depth d represents an ordered sequence of labels
<1,2,3,…,d-1,d>, where 1 is at the bottom of the stack and d at the top.
An unlabeled packet may be viewed as a packet with label stack of depth
0.
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Label Stack
Top of the Stack
Bottom of the Stack
Label Stack Entry
Level d
Label Stack Entry
Level d-1
Label Stack Entry
Level 2
Label Stack Entry
Level 1
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Label Stack Operations
•
•
•
MPLS always forward packets based on the value of the in label at the
top of the stack
For an incoming packet, other than examining the top label of the stack
to index the label forwarding table no other operations are performed.
For an outgoing packet, however, MPLS node performs one or more
label operations such as:
•
Pop
Remove the top label
Swap
 Replace the top label with a new one
Push
 Add a specified number of label entries at the top of the stack.

•
•
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Label Stack
In
I/F
0
In
Lab
Address
Prefix
5 171.68.10
... ...
Out Out
I/F Lab
1
Next-Hop...
...
7
...
171.68.10/24
Label = 5
LSR1
Label = 7
Label = 21
Label = 21
IP packet
D=171.68.10.12
IP packet
D=171.68.10.12
• LSR1 forwards the labelled packet based on the label at the top of the
label stack
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Label Switching Router (LSR)
•
Router that support MPLS are known as label switching routers (LSRs).
•
LSR can be ATM switch or a router
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Downstream and Upstream LSRs
Upstream
Downstream
172.68.10/24
LSR1
LSR2
Packets with IP destination = 172.68.10/10
and carrying label L1
• Note: an LSR may be upstream with respect to certain FEC and
downstream for another.
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Label Distribution Modes
•
•
In MPLS, it is always the downstream LSR that assigns the FEC-to-label
bindings and distribute them to the upstream LSR.
The downstream LSR distributes bindings in two ways:
•
•
•
•
On explicit request from the upstream LSR
Unsolicited
The label distribution mode when the upstream LSR makes an explicit
request for label-to-FEC bindings is known as downstream-on-demand
(DoD).
The label distribution mode when the downstream LSR distributes labelto-FEC bindings without explicit requests from the upstream LSR is
known as downstream-unsolicited (DU).
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Downstream on demand distribution
Use label 5 for destination
171.68.10/24
172.68.10/24
LSR1
LSR2
Request label for
destination 172.68.10/24
• LSRs assign a label to each FEC
• Upstream LSRs request labels to downstream neighbors
• Downstream LSRs distribute labels upon request
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Downstream unsolicited
Use label 5 for destination
171.68.10/24
172.68.10/24
LSR1
LSR2
• LSRs assign a label to each FEC
• Downstream LSRs distribute labels unsolicited
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Label Distribution Protocols
•
•
The protocols that are used to request and distribute label-to-FEC
bindings are known as label distribution protocols.
MPLS architecture defines several label distribution protocols i.e.:
•
LDP

•
Maps unicast FECs (e.g., IP destinations) into labels
BGP
External labels (e.g., VPN routes)
RSVP
 Traffic engineering and resource reservation
CR-LDP
 Traffic engineering and resource reservation

•
•
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Edge and Core LSRs
•
MPLS network consists of interconnection of LSRs.
•
•
The LSRs at the edge of the MPLS network are referred to as edge
LSRs. Edge LSRs perform following functions:
•
•
•
The contiguous set of LSRs form a MPLS domain
label imposition as traffic enters the MPLS network
label disposition as traffic leaves the MPLS network
The non-edge LSRs are known as core (or transit) LSRs.
•
Core LSRs only forward labeled packets.
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MPLS Network
Edge LSR
Core LSR
MPLS Network
Edge LSR
IGP domain with a label
distribution protocol
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Label Switched Path (LSP)
•
Each labelled packet
•
•
•
•
•
enters the MPLS network in the ingress LSR
exits the MPLS network in the egress LSR
LSP is the sequence of LSRs through which the labelled packets have to
go through in order to reach the egress LSR.
LSPs are unidirectional, therefore, for bi-directional traffic flows, two
LSPs are required.
LSPs are established via label distribution protocols such as LDP, RSVPTE, and BGP.
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Label Switched Path (LSP)
LSP
Ingress-LSR
Egress-LSR
IGP domain with a label
distribution protocol
• LSR-ingress to LSR-egress path is the same for packets of the same
FEC
• LSPs are unidirectional
• Return traffic requires another LSP
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Hierarchical LSPs
•
•
Using label stack mechanism, it is possible to nest one or more LSPs
within another LSPs.
LSPs that contain other LPS are referred to as hierarchical LSPs.
•
•
Hierarchical LSPs are similar to Virtual Paths in ATM.
A LSP of depth d is defined as a sequence of LSRs which:
•
•
starts at an LSR (i.e., LSP ingress) that pushes on a level d label, and
terminates at an LSR (i.e., LSP egress) where packet is forwarded based on a label
stack of depth d-1.
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Hierarchical LSP
Hierarchical LSP (Label stack)
Ingress LSR for LSP3
Ingress LSR for LSP1
LSP1
LSP3
Egress LSR for LSP1
LSP2
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