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Multiprotocol Label Switching
(MPLS)
中正大學資工系
黃仁竑
Outline
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Introduction
Label Encoding
Label Assignment
Label Distribution
Label Swapping
Label Merging
Conclusion
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Introduction
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

IETF Multiprotocol Label Switching Working Group created
in 1997
Integrates the label swapping forwarding paradigm with
network layer routing
 Use a short, fixed-length label
Examples of MPLS
 Tag Switching (Cisco)
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Why MPLS
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MPLS versus Datagram Routed Network
 Simplified forwarding
 Efficient explicit routing
 Traffic engineering
 QoS routing
 Complex mapping from IP packets to FEC (Forwarding
Equivalence Class)
MPLS versus ATM
 Scaling of the routing protocol
 Common operation over packet and cell media
 Easier management
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Basics of MPLS

Semantics assigned to a stream label
 Labels are associated with specific streams of data (FEC)

Forwarding Methods
 Short fixed length labels to identify streams
 Looking up a label in a table, swapping labels, and possibly
decreasing and checking a TTL
 May make direct use of layer 2 forwarding (e.g. ATM)

Label Distribution Methods
 Allow nodes to determine which labels to use for specific streams
 May use some sort of control exchange, or be piggybacked on a
routing protocol
 By LDP
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Next Hop Label Forwarding Entry

NHLFE is used when forwarding a labeled packet,
it contains
 next hop
 operations
 replace the label at the top of the label stack with a new
label
 pop the label stack
 replace and push one or more new labels
 data link encapsulation
 how to encode the label stack
 other information for properly dispose of the packet
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Label Stack
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Label stack
 A labeled packet may carry a number of labels
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A Label
 A short fixed length significant identifier
 Based on the stream or forwarding equivalence class (FEC)
 Only have local significance

Label encoding
 MPLS generic encapsulation mechanism
 ATM SVC, SVP, SVP multipoint encoding methods
 Others
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Label Switched Path (LSP)
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Begins with an LSR (LSP Ingress) that pushes on a
level m label
Intermediate LSRs make their forwarding decision
by label switching on a level m label
Ends (LSP Egress) when forwarding decision is
made by label switching on a level m-k label (k>0)
or when a forwarding decision is made by nonMPLS forwarding procedures
The label stack may be popped at the penultimate
LSR of the LSP, rather than at the LSP Egress
 reduce times of label lookup at LSP egress
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Label Encoding

Generic MPLS encapsulation
 Between the data link layer and network layer headers
 Network layer protocol independent
 A label contains
 Label Stack
 A sequence of label stack entries
 Time-to-Live (TTL)
 Similar to what is provided by IP (e.g. traceroute)
 Class of Service (CoS)
 Allows multiple service classes within the same label
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Label Stack Entry
0
20
Label

Exp
23
S
31
TTL
Label(20bits)
 carries the actual value of the label
 0/2:IPv4/v6 Explicit NULL Label
 must be sole label stack entry (forward based on IPv4/v6)
 1:Router Alert Label;(need software process)
 3:Implicit NULL Label
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Exp(3bits):reserved
S(1bits):Bottom of Stack
TTL(8bits):Time to Live
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Label Encoding
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ATM Switches as LSRs
 SVC Encoding
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Use the VPI/VCI field to encode the label
Each LSP is realized as an ATM SVC
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ATM-LSR cannot perform PUSH or POP
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 SVP Encoding
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VPI : Top of label stack
VCI : Second label on the stack
Permits the use of ATM VP switching
can’t include a non-MPLS ATM network
 SVP Multipoint Encoding
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VPI : Top of label stack
VCI : Part for the second label on the stack, the remainder to identify
the LSP ingress
Multipoint-to-point VPs
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Label Assignment

Topology driven (Tag)
 In response to normal processing of routing protocol
control traffic
 Labels are pre-assigned; no label setup latency at
forwarding time

Request driven (RSVP)
 In response to normal processing of request based control
traffic
 May require a large number of labels to be assigned

Traffic driven (Ipsilon)
 The arrival of data at an LSR triggers label assignment and
distribution
 Label setup latency; potential for packet reordering
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Label Distribution
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Explicit Label Distribution
 Downstream label allocation
 label allocation is done by the downstream LSR
 most natural mechanism for unicast traffic
 Upstream label allocation
 label allocation is done by the upstream LSR
 may be used for optimality for some multicast traffic
 A unique label for an egress LSR within the MPLS domain
 Any stream to a particular MPLS egress node could use
the label of that node.
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Label Distribution

Explicit Label Distribution Protocol (LDP)
 Reliability : by transport protocol (TCP) or as part of LDP
 Separate routing computation and label distribution
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Piggybacking on Other Control Messages
 Use existing routing/control protocol for distributing
routing/control and label information
 OSPF, BGP, RSVP, PIM
 Combine routing and label distribution

Label purge mechanisms
 By time out
 Exchange of MPLS control packets
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Label Distribution Protocol
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LDP Peer:
 Two LSRs that exchange label/stream mapping information via LDP
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LDP messages
 Discovery messages
 announce and maintain the presence of LSR
 via UDP
 Session messages
 maintain session between LDP peers
 Advertisement message
 label operation (Label distribution)
 Notification message
 advisory information and signal error information
 Error notification:signal fatal errors
 Advisory notification: status of the LDP session or some
previous message received from the peer.
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Label Swapping
Example : Forwarding a Labeled Packet

Labeled Packet
 Map the incoming label to an
next hop label, determines
where to forward the packet
 Encodes the new label stack
into the packet, and then
forwards it

Unlabeled Packet
Incoming Label Map (ILM)
Input
Port Label
1
4
Output
Port Label
2
6
Label Switching Router
(LSR)
L3 Header
 LSR analyzes the L3 header,
to determine the packet’s
stream
 Map the stream to a next
hop, determines where to
forward the packet
 Encodes the new label stack
into the packet, and then
forwards it
L2 Header
IP Router
Module
Label
Dat
H3
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H2
Dat
1
H3
6
H2
2
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Use of MPLS in a Hierarchy
Swap
L1
L4
OSPF
R2
R1
L2
Push
IN
OUT
IN
OUT
L2
L3
L3
L1
Swap
OUT
L1
R3
IN
OUT
L1
L4
R5
R6
R4
Pop
L2
L3
L3
L1
L1
L1
L1
BGP
L2
L1
Domain 1
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Domain 2
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Route Selection
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Hop by hop routing
 Like conventional IP routing
 Each hop makes independent choice of next hop
 Repair of a failed route done locally
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Explicit routing
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Manual or based on dynamic routing
The LSP next hop is chosen by a single node
Useful for policy routing and/or traffic engineering
If an explicit route is specified for an LSP, then that route
must be followed
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Loop Handling
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Loop Survival
 minimizes the impact of loops
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Loop Detection
 allows loops to be set up, but detects them and eliminates
them later
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Loop Prevention
 avoiding setting up a loop
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Loop Survival
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Allow the network to operate well even though
short term transient loops may be formed by the
routing protocol
Possible solutions
 Use of TTL to limit the hops that a packet traversed
 Use of dynamic routing protocol which converges fast
 looping packets may cause congestion which may then
affect the converge speed of routing protocol
 Use of fair queueing to limit the impact of looping packets
on normal packets
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Loop Detection
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Loop may be set up, but will be subsequently
detected.
Possible solutions
 Loop Detection Control Protocol (LDCP)
 transmit LDCP packet when route change
 LDCP is forwarded towards destination until
 destination
 TTL exceeded
 return to a node which originally transmitted it
 Path Vector Control message: list of LSRs on the path
 hop count to each egress node (like RIP?)
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Loop Prevention
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Ensure loops are never set up
Possible solutions
 labels are propagated from the egress switch, control
packets which propagate the labels also include the path
 diffusion mechanism when route changes
 colored mechanism
 a color consist of address of the node that created the
color and a local id that is unique within the node
 a node that finds a change in the next hop creates a
color and passes it to the new next hop
 stops when a loop or a loop free path is found
 explicit routing
 configured
 use routing protocol
(link state or path vector)
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Diffusion Algorithm
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On a route change, R ask N for a
label and the associated LSR ID for
that stream
R looks in the LSR ID list
 If R is in the list (route loop), the old
LSP will continue to be used until
the route protocol break the loop
 If R is not in the list, R will start a
diffusion computation


Diffusion computation prunes tree
of paths that would loop if R
switches to new LSP
When the diffusion completes, R
switches to new LSP and discards
old LSP
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R
New Path
Old Path
N
E
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Diffusion Computation
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An extension of Path Vector mechanism
An LSR, D, detects the next hop for an FEC has
changed, transmits a query message with a Path
Vector containing its id to its upstream
A LSR, U, that receives a query will determine if D
is the next hop for the given FEC
 if not, then U return OK message
 if so, then U checks if the Path Vector already contains it id
 if yes, a loop is detected, U responds with a LOOP msg
 if not, U adds its id to the Path Vector and propagates
the query message to its upstream neighbors
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What to Do if a Loop is Detected

If a loop is know to exist
 L2 label-swapped path is not setup
 Packet is forwarding using normal L3 forwarding
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Problems :
 Nodes which are not capable of L3 forwarding
 discard packet
 L2 forwarding faster than L3 forwarding
 node will not be capable of forwarding the same
volume of traffic at l3, some packets will be discarded
 packet lost cause TCP to backoff, which will in turn
reduce the load and allow the network to stabilize until
the label binding is reestablished again.
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Label Merging

Label merging
 An LSR may want to bind multiple incoming labels to a
particular FEC
 once packets are transmitted, the information that they
arrived with different labels is not

Non-merging LSRs
 In ATM, label merging may cause interleaving of cells
from various packets
 MPLS support procedures which allow ATM switches to
function as merging LSRs
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Merge over ATM

VP Merge (SVP multipoint encoding)
 Packet from different sources are distinguished by using
different VCs within the VP
 Advantage : no new hardware
 Disadvantage : requires coordination of the VCI space

VC Merge
 Switches are required to buffer cells from one packet until
the entire packet is received
 Advantage : straightforward application of VC switching
 Disadvantage :
 New hardware (based on per-VC queuing)
 Delays at the merge points
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Conclusion
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MPLS

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A more general forwarding mechanism
Cooperates with routing/control protocol
Provides Integrated service, Differentiated service
Allows flow aggregation (FEC) for QoS routing
Support Multicast?
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