MPLS Forwarding

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Lecture 7
2
MPLS Forwarding

• MPLS forwarding can be described in terms of
• Label imposition
• Label disposition
• Label swapping

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Label Imposition

• The action of adding the label to an unlabeled
  packet is known as label imposition.
• For example, an edge LSR performs label
  imposition.
• Label imposition involves
• IP address lookup in the FIB to map the packet to
  a particular FEC
• FEC-to-NHLFE lookup in the LFIB

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Label Swapping

• Label swapping forwarding operation applies to
  transit nodes along an LSP.
• In the case of label swapping, the LSR
• uses top label in the incoming packet to identify
  ILM
• uses ILM to identify one or more NHLFE
• Uses the information in the NHLFE to Perform the
  required label stack operation
• encodes the new label stack and transmits the
  labeled packet on the appropriate outgoing
  interface
• Label swapping operation only uses LFIB table

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Label Disposition

• The action of removing label stack from a label
  stack is known as label disposition.
• Label disposition is performed in the egress node
  for an LSP.
• In the case of label disposition, the LSR
• uses top label in the incoming packet to identify
  ILM
• uses ILM to identify one or more NHLFE
• Uses the information in the NHLFE to Perform the
  required label stack operation.
• The label stack operation in this case indicates
  that the label stack needs to be removed.
• In some case (e.g., de-aggregation), an
  additional IP address lookup may be required to
  forward the unlabeled packet.

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MPLS Forwarding Architecture

• Edge LSRs make use of both FIB and LFIB
• Transit LSRs can forward labeled traffic based on
  LFIB
• Each LSR may carry a mix of IP and MPLS traffic
• In general, each LSR supports both conventional
  IP forwarding (i.e., FIB) and label switching
  based forwarding (i.e., LFIB) components.
• In MPLS, LIB is the counterpart of RIB and LFIB
  is the counterpart of the FIB.

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FIB and LFIB Synchronization

1. Exchange dynamic routing information and label
   information with peers
2. Process the routing information and build the
   FIB. Process the local/remote label and build the
   LIB.
3. Select the best paths. Build LFIB from LIB.
4. Distribute the FIB/LFIB to the line cards.
   Initially, entire database. Later, incremental.

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Label Distribution Protocol (LDP)
Big Picture
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Label Distribution Methods

• To establish label switch paths, LSRs must be
  able exchange label information.
• MPLS uses multiple protocols for this purpose,
  namely
• Label Distribution Protocol (LDP)
• RSVP-TE
• BGP
• Each label distribution protocol has certain
  characteristics that meets application specific
  requirements e.g
• LDP maps unicast IP destinations into labels and
  establishes a hop-by-hop routed LSP
• RSVP is used to establish a LSP along explicit
  path.
• BGP is used to assign and distribute labels for
  BGP routes.

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MPLS Applications

• MPLS enables a diverse set of applications such
  as
• IP over MPLS
• Traffic Engineering
• Virtual Private Networks (VPNs)
• IP over MPLS
• Use hop-by-hop routed LSP to route IP traffic
• Label distribution via LDP
• Traffic Engineering
• Use explicitly routed LSPs with QoS
• Label distribution via RSVP-TE
• VPN
• Share common transport network among multiple
  customers
• Label distribution via LDP and BGP

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Label Distribution Protocol (LDP)

• MPLS label distribution protocols are either
  extend existing protocols or define new
  protocols.
• For example, RSVP-TE and BGP label distribution
  protocols are based on extensions of the existing
  protocols.
• LDP is a protocols that is explicitly designed
  for distribution label information between
  directly or indirectly connected LSRs.

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Label Distribution Protocol (LDP)

• LDP assigns labels to IGP prefixes and as a
  result establishes hop-by-hop routed LSP.
• LSP that are established via LDP essentially
  traverse along path that would be followed by all
  packets in certain FEC if they were routed using
  conventional IP forwarding.
• In a nutshell, LDP assigns labels to the FEC. As
  a result, the LSPs that are established via LDP
  mimic or track the IGP Path.
• For example, if a next hop for a certain FEC
  changes, the corresponding LSP path also changes.

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Downstream and Upstream LSRs
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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 label-to-FEC bindings without
  explicit requests from the upstream LSR is known
  as downstream-unsolicited (DU).

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Downstream-on-Demand (DoD)

• LSRs assign a label to each FEC (usually
  triggered when IGP learns about a prefix)
• Upstream LSRs request labels to downstream
  neighbors
• Downstream LSRs distribute labels upon request

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Downstream Unsolicited (DU)

• LSRs assign a label to each FEC
• Downstream LSRs distribute labels unsolicited

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Label Information Base (LIB)

• LDP maintains all locally assigned (i.e.,
  incoming) and remotely learned (i.e., outgoing)
  labels in a Label Information Base (LIB).
• Each entry in the LIB consists of
• destination address,
• incoming label (assigned by the LSR itself)
• one or more pair of the form (LDP Identifier, out
  label).
• For LSP established using LDP, incoming and
  outgoing label information is used to populate
  Label Forwarding Information Base (LFIB).

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LIB/LFIB
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Downstream-on-Demand (DoD)
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Downstream Unsolicited (DU)
R4 learns about 172.32.0.0/16 via IGP. Assigns a
local label. L1. And advertises unsolicited this
label-to-FEC mapping to LDP peers
Here is FEC/L3
Here is FEC/L1
R2
Here is FEC/L2
FEC
172.32.0.0/16
IGP
R1
R4
R3
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Label Retention Modes

• There are two modes to retain labels from peers
  namely liberal and conservative retention modes.
• In liberal retention mode, LSR retains labels
  from all neighbors (whether valid next hops or
  not for the associated FEC)
• Pros faster convergence and LSP adaptation to
  next hop changes. This is because in label/FEC
  mappings from a new next hop already exist and
  dont need to be requested.
• Cons - Require more memory and larger label space
• In conservative retention mode, LSR retains
  labels only from valid next-hops neighbors (i.e.,
  discards all labels/FEC mappings except received
  from valid next-hops)
• Pros Free memory and label space
• Cons Slower LSP adaptation to changes in next
  hops.

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Label Retention Modes
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Label Distribution Control (Ordered vs.
Independent)

• There are two modes for LSP control ordered and
  independent.
• In ordered LSP control, a LSR only binds and
  advertise a label for a particular FEC if
• it is the egress LSR for that FEC or
• it has already received a label binding from its
  next-hop
• Pros packets are not forwarded onto the LSP
  until the complete LSP is established
• Cons Slower. Introduces dependencies in LSP
  establishment process. For example, a LSP may
  have a local label/FEC mappings but cannot
  advertise it because it must wait for the remote
  (out) label/FEC mappings.

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Example- DoD/Ordered Control
2
Request label for 172.32.0.0/16
Request label for 172.32.0.0/16
1
3
Use label 7
Use label 5
4
R2
FEC
172.32.0.0/16
IGP
R1
R4
R3
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Label Distribution Control (Ordered vs.
Independent)

• In independent LSP control, a LSR binds a label
  to a FEC independently from the label it has to
  receive from its next-hop
• Pros Faster. Does not introduce extra latency.
• Cons - An LSR may label forward packet to a
  next-hop that does not have yet label information
  for that FEC. Packets may be dropped in the core
  until LSP set-up is complete.

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DoD/Independent LSP Control
4
Request label for 172.32.0.0/16
Request label for 172.32.0.0/16
1
5
Use label 7
Use label 5
2
R2
FEC
172.32.0.0/16
R1 starts sending the traffic for 172.32.0.0/16
with label 7
3
IGP
R1
R4
R3
Between R2-R4 traffic is forwarded using IP.
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Independent LSP Control
Depending upon the amount of traffic is routed on
IP forwarding, the forwarding performance of some
LSRs may suffer.
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Commonly used LDP Modes

• Frame-based LSR typically use
• DU label advertisement
• Conservative label retention mode
• Ordered label distribution
• Cell-based LSR typically use
• DoD label advertisement mode
• Conservative label retention mode
• Ordered label distribution

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Summary of LDP modes
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LDP Mechanics

• Neighbor Discovery
• Session Establishment
• TCP Connection Establishment
• LDP Session Initialization
• Session Maintenance
• Label Advertisement

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LDP Discovery

• Before establishing LDP sessions, the potential
  LDP neighbors are discovered.
• To discover directly connected neighbours
• LSR periodically transmits Hellos as UDP packets
  over each LDP interface
• LDP Link Hellos are addresses to all routers on
  the subnet
• Hello messages use UDP port 646.
• Reception of Hello on an interface establishes a
  Hello Adjacency on the that interface with the
  sender .
• A separate Hello Adjacency is on each interface.

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Session Establishment

• Creation of Hello Adjacency triggers
  establishment of TCP connection.
• For establishing LDP sessions, TCP port 646.
• Once TCP connection has been established, two
  LSRs exchange LDP Initialisation message to
  negotiate parameters.
• After successful negotiation of parameters, two
  LSRs are LDP peers and ready for label
  advertisement messages.

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Session Establishment

• LSR1 receives Hello LSR20 on link.
• LSR1 attempts to open LDP TCP connection to LSR2.
• LSR1 and LSR2 negotiate session parameters via
  exchange of Initialization messages e.g.,
  advertisement mode, label range.

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Initialization Message
The encoding for the Initialization Message
is   0 1
2 3 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
-------------------------
------- 0 Initialization (0x0200)
Message Length
-------------------------
------- Message
ID
-------------------------
------- Common
Session Parameters TLV
-------------------------
------- Optional
Parameters
-------------------------
-------
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 ---------------
----------------- 00
Common Sess Parms (0x0500) Length
-----------------
--------------- Protocol
Version KeepAlive Time
-------------------
------------- AD Reserved
PVLim Max PDU Length
---------------------
-----------
Receiver LDP Identifier

----------------
----------
------
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Initialization Message
ATM Session Parameters Used when an LDP session
manages label exchange for an ATM link to specify
ATM-specific session parameters.   0
1 2
3 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
-------------------------
------- 00 ATM Sess Parms
(0x0501) Length
-------------------------
------- M N D
Reserved
-------------------------
------- ATM Label
Range Component 1
-------------------------
-------




-------------------------
------- ATM Label
Range Component N
-------------------------
-------
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Hello Adjacencies and Sessions

• Recall, label space refers to set of unique label
  values.
• Label space be implemented system wide (global)
  and/or per interface
• Per interface label space is used on LC-ATM
  interfaces
• System wide label space is implemented on
  frame-based LSRs
• A LSR may have one or more Hello adjacencies.
  However, as long as parallel links use global
  label space, only one LDP session is established.
• On ATM interfaces, however, a separate LDP
  session is established per interface.

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Label Space Identification

• LDP label space is identified by LDP Identifier
  (LDP Id).
• LDP Id is six bytes in length
• first 4 bytes represent router ID
• last 2 bytes indicate label space
• Global label space, last 2 bytes contain 0
• 172.32.23.120
• Per interface label space, last 2 bytes contain gt
  0
• 172.32.23.125

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LDP Session Between Directly Connected LSRs
Global label space
LSR 20
LSR 10
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LDP Session Between Directly Connected LSRs
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LDP Session Between Non-Directly Connected LSRs
LSR7
LSR1
R1
R4
LSR1 and LSR7 uses to discover each other and
establish hello adjacency
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Session Maintenance

• A Hello adjacency is created on receiving the
  first hello.
• Once created, LSR maintains a Hello adjacency by
  periodic transmission of Hellos
• LSR keeps a hold timer for each Hello adjacency.
  If no Hello is received before expiration of the
  hold timer, adjacency is deleted.
• As long as there is at least one adjacency, the
  corresponding LDP session is retained. Upon
  deletion of last adjacency, the LDP session is
  removed.
• Once LDP session is established, the integrity of
  LDP session is maintained by periodic
  transmission of KeepAlive messages.

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