CS 672

1

Summer 2003

Lecture 6

CS 672

2

Summer 2003

Hierarchical LSP

LSP1

LSP2

LSP3

Ingress LSR for LSP1

Egress LSR for LSP1

Ingress LSR for LSP3

Hierarchical LSP

CS 672

3

Summer 2003

Penultimate Hop Popping

• From the definition of a LSP of level d (RFC3031), it follows that:• a LSR upstream of the egress LSR can pop the label, and• transmit packet with a label stack of depth d-1.

• For a certain level LSP,• the LSR preceding the egress LSR is called the penultimate hop for that LSP.• the process of removing top label on behalf of the egress LSR is referred to as

penultimate hop popping.• Penultimate hop popping is useful because it avoids extra lookups at the

egress LSR.

CS 672

4

Summer 2003

Penultimate Hop Popping

• The label at the top of the stack is removed (popped) by the upstream neighbour of the egress LSR

• The egress LSR requests the “popping” through the label distribution protocol

• Egress LSR advertises a implicit-null label

• The egress LSR will not have to do a lookup and remove itself the label

• One lookup is saved in the egress LSR

CS 672

5

Summer 2003

Penultimate Hop Popping

0 1

route 171.68/16

01

171.68.10/24

Next-HopNext-Hop

In In LabLab

Address Address PrefixPrefix

OutOutI/FI/F

Out Out LabLab

In In I/FI/F

44 171.68/16171.68/16 22 poppop00

...... ...... ...... ............Next-HopNext-Hop

In In LabLab

Address Address PrefixPrefix

OutOutI/FI/F

Out Out LabLab

In In I/FI/F

-- 171.68/16171.68/16 11 4400

...... ...... ...... ............

Aggregate route is propagate through the IGP and label is assigned by each LSR

Use label “implicit-null” for FEC 171.68/16

route 171.68/16

Use label 4 for FEC 171.68/16

• Egress LSR needs to do an IP lookup for finding more specific route• Egress LSR need NOT to receive a labelled packet

• labelled will have to be popped anyway

171.68.44/24

Address Address Prefix and maskPrefix and mask

171.68.10/24171.68.10/24

Next-HopNext-Hop

171.68.9.1171.68.9.1

InterfaceInterface

Serial1Serial1

171.68.44/24171.68.44/24 171.68.12.1171.68.12.1 Serial2Serial2

171.68/16171.68/16 ...... NullNull

Egress LSR aggregates morespecific routes and advertises a label for the new FEC

CS 672

6

Summer 2003

Penultimate Hop Popping

0 1

IP packetD=171.68.10.15

01

171.68.10/24

Next-HopNext-Hop

In In LabLab

Address Address PrefixPrefix

OutOutI/FI/F

Out Out LabLab

In In I/FI/F

44 171.68/16171.68/16 22 poppop00

...... ...... ...... ............Next-HopNext-Hop

In In LabLab

Address Address PrefixPrefix

OutOutI/FI/F

Out Out LabLab

In In I/FI/F

-- 171.68/16171.68/16 11 4400

...... ...... ...... ............

IP packetD=171.68.10.15

Label = 4

IP packetD=171.68.10.

15

Packet arrives without the labelat the egress LSR.Egress LSR needs to do an IP lookup to match more specific routes

IP packetD=171.68.10.

15

Packet is MPLS forwardedTop label is removedIP packet enters the MPLS network

Ingress LSR assign a label andforward the packet

171.68.44/24

Address Address Prefix and maskPrefix and mask

171.68.10/24171.68.10/24

Next-HopNext-Hop

171.68.9.1171.68.9.1

InterfaceInterface

Serial1Serial1

171.68.44/24171.68.44/24 171.68.12.1171.68.12.1 Serial2Serial2

171.68/16171.68/16 ...... NullNull

CS 672

7

Summer 2003

Hop-by-hop and Explicitly routed LSP

• A LSP whose path is hop-by-hop selected using IP routing protocols is known as hop-by-hop routed LSP.

• For example, hop-by-hop LSPs are established through LDP.• A LSP whose path is explicitly specified is referred to as explicitly routed

(or traffic engineered) LSP.• Explicitly routed (a.k.a source routed) LSPs are established using RSVP

or CR-LDP.

CS 672

8

Summer 2003

Hop-by-hop/explicitly routed LSPs

• FEC is determined in LSR-ingress• LSPs path derived from IGP routing information• LSPs may diverge from IGP shortest path

LSP tunnels (explicit routing) with Traffic Engineering

LSP follows IGP shortest path

IGP domain with a label distribution protocol

LSP diverges from IGP shortest path

IGP domain with a label distribution protocol

CS 672

9

Summer 2003

Label Encoding

• MPLS can be supported over different link layer protocols e.g:• Ethernet• Point-to-point protocol (PPP)• Frame Relay• ATM

• The encoding of label information varies depending upon the data links.

CS 672

10

Summer 2003

MPLS Label Encoding

PPP Header(Packet over SONET/SDH)

ATM Cell Header

LAN MAC Label Header

HECHEC DATADATACLPCLPPTIPTIVCIVCIGFCGFC VPIVPI

Label

PPP HeaderPPP Header Layer 3 HeaderLayer 3 HeaderLabel

Label Layer 3 HeaderLayer 3 HeaderMAC HeaderMAC Header

Shim header

CS 672

11

Summer 2003

Cell-mode and Frame-mode LSR

• As previously discussed, an LSR can be a based on router or ATM switch.

• An ATM switch that encodes label information in VPI/VCI fields is known as cell-based LSR or ATM-LSR.

• An LSR that encodes label information in the shim header on a frame or cell-based interface is known as frame-mode LSR.

• A cell-based interface that is controlled by MPLS label distribution protocols is known as label switching controlled ATM (LC-ATM) interface.

CS 672

12

Summer 2003

Frame-mode LSR

• Frame-mode LSR:• encodes label information in the shim header• forwards entire frames (as opposed to cells)• supports IP control plane• supports MPLS control plane

• Frame-mode LSR may have one or more cell-based interfaces. However, on cell-based interfaces:• in receive direction, cells are first reassembled before forwarding the frames• In transmit direction, frames are segmented to cells• labels are NOT encoded in the VPI/VCI fields

CS 672

13

Summer 2003

Frame-based LSR

ATM Link

Packet over SONET/SDH Link

Ethernet Link

DataL3 Hdr

LabelPPP

DataL3 Hdr

LabelEth

VPI/VCI fields do not encode label

1

1

Frame-based LSR

Data L3 Hdr Label AAL5 SAR

Labeled Layer 3 Packet

2

CellHdr CellHdr

1 As a Layer 2 frame containing labeled packet is received, the label is extracted and used to forward the packet.

2 On receive side, the labeled Layer 3 packet is first reassembled from cells using AAL5 SAR. The label is extracted and used to forward the packet. On the transmit side, the labeled layer 3 packet is segmented into cells using AAL5 SAR.

CS 672

14

Summer 2003

ATM-LSR

• In order to exchange labels, ATM-LSRs need to run an IP routing protocol as well as label distribution protocols

• ATM-LSRs support IP control plane• ATM-LSRs support label distribution protocols• ATM-LSRs behaves as ATM switches in terms of data plane• ATM-LSRs do not route packets based on routing table • Packet forwarding is based on label information• Control VC is used to exchange labels

• ATM switches use input port,VPI,VCI values and map them to output port,VPI,VCI values

• Label is encoded in same fields• VPI/VCI field used to carry label information• Existing software can work for label swapping

CS 672

15

Summer 2003

ATM-LSR

LC-ATM Link

LC-ATM Link

LC-ATM Link

DataCell Hdr

1

Label

DataCell

Hdr

1Label

Label

Data Cell Hdr1

Cell-based LSR

1 A cell-based LSR forwards cells using label information encoded in the VPI/VCI fields.

CS 672

16

Summer 2003

ATM-LSR

LC-ATM Link

1

Label

Cell Hdr Data

Control Plane

Cell-basedforwarding

AAL5

Control Plane

Cell-basedforwarding

AAL5

Cell Hdr Data

Cells for control messages do not encode label in VPI/VCI

Control VC

Data VC

Cell-based LSR Cell-based LSR

1 Over an LC-ATM link, labeled packets are is transmitted on data VCs and the unlabeled control packets on the control VC.

CS 672

17

Summer 2003

Label Space

• Label space refers to set of unique label values.• LSRs must be able to distinguish between labelled packets

A label is used to identify a particular FEC• LSR can distribute the same label/FEC mapping to different

neighbours• Same label can be assigned to different FECs if and only if the LSR

can distinguish the interface from which the packet will arrive• That is,the LSR can identify who us the upstream neighbours who insert the

label

CS 672

18

Summer 2003

Label Space• Label space refers to set of unique label values. Label allocation and

management can be implemented:• Per interface• System wide

• Per interface label space• Labels are unique in a per interface base• Used by ATM-LSRs in ATM interfaces

Label information is encoded in VPI/VCI fields• Downstream-on-demand (DoD)label distribution mode.

• System wide (global) label space• Labels are unique system wide (i.e, on all interfaces)• Used by frame-based LSRs

Label information encoded in shim header• DU or DoD label distribution mode

CS 672

19

Summer 2003

Per Interface Label space

171.68.10/24Rtr-A

Next-HopNext-Hop

In In LabLab

55

55

Address Address PrefixPrefix

171.68.10171.68.10

171.68.10171.68.10

OutOutI/FI/F

22

22

Out Out LabLab

77

88

In In I/FI/F

00

11

IP packetD=171.68.10.12

Label = 5

IP packetD=171.68.10.12

Label = 7

• Same label for FEC 171.168.10 is advertised to different upstream neighbours

1

02

IP packetD=171.68.10.15

Label = 5

IP packetD=171.68.10.15

Label = 8

CS 672

20

Summer 2003

Per Interface Label space

171.68.10/24

IP packetD=171.68.10.12

Label = 5 IP packet

D=171.68.10.12

Label = 7

1

0 2

IP packetD=171.68.10.15

Label = 5

IP packetD=171.68.10.15

Label = 8

Next-HopNext-Hop

In In LabLab

Address Address PrefixPrefix

OutOutI/FI/F

Out Out LabLab

In In I/FI/F

55 171.68.10171.68.10 22 7700

55 171.68.10171.68.10 22 8811

55 171.68.40171.68.40 33 9944

171.68.40/24

3

IP packetD=171.68.40.33

Label = 9

4

IP packetD=171.68.40.33

Label = 5

• Same label is assigned to different FECs if LSR is able to distinguish the upstream neighbours who sent the packet

CS 672

21

Summer 2003

Label assignment and distribution

• Labels have local significance Each LSR binds his own label mappings

• Each LSR assign labels to his FECs• Labels are assigned and exchanged between adjacent LSRs

Downstream to Upstream• Applications may require non-adjacent neighbors

Traffic Engineering (TE) VPN

CS 672

22

Summer 2003

DU vs. DoD

• Downstream on demand is used by ATM-LSRs and in traffic engineering with RSVP

• ATM switches forward cells, not packets• Label is carried in VPI/VCI field• SAR process need to distinguish packets

Cells belonging to different packets but having the same label need to be re-assembled correctly

• Label need to be assigned based:• on IP destination and incoming interfaces• Traffic Engineering mechanisms

CS 672

23

Summer 2003

DoD and VC-Merge

• VC-Merge allows correct packet re-assembling• Sequencing of cells by buffering • Receiving (downstream) ATM-LSR can securely re-assemble cells

into packets Even cells of different packets use same VPI/VCI value Save label space on ATM-LSRs

CS 672

24

Summer 2003

DoD and ATM-LSR

In In LabLab

55

88

......

Address Address PrefixPrefix

171.68171.68

171.68171.68

......

OutOutI/FI/F

00

00

......

Out Out LabLab

33

33

......

In In I/FI/F

11

22

......

171.68

IPPacket

IPPacket AT

Mcell

5

ATM

cell

8

ATM

cell

8

ATM

cell

8

ATM

cell

5

ATM

cell

3 ATM

cell

3 ATM

cell

3 ATM

cell

3 ATM

cell

3

Downstream LSR do not know how to reassemble correctly cells into packets. VPI/VCI values are identical for all cells

CS 672

25

Summer 2003

DoD and ATM-LSR

In In LabLab

55

88

......

Address Address PrefixPrefix

171.68171.68

171.68171.68

......

OutOutI/FI/F

00

00

......

Out Out LabLab

33

44

......

In In I/FI/F

11

22

......

171.68

IPPacket

IPPacket AT

Mcell

5

ATM

cell

8

ATM

cell

8

ATM

cell

8

ATM

cell

5

ATM

cell

3 ATM

cell

4 ATM

cell

4 ATM

cell

4 ATM

cell

3

ATM-LSR requested additional label for same FEC in order to distinguish between incoming interfaces (Downstream on Demand)

CS 672

26

Summer 2003

ATM-LSRs and VC-Merge

In In LabLab

55

88

......

Address Address PrefixPrefix

171.68171.68

171.68171.68

......

OutOutI/FI/F

00

00

......

Out Out LabLab

33

33

......

In In I/FI/F

11

22

......

171.68

IPPacket

IPPacket AT

Mcell

5

ATM

cell

8

ATM

cell

8

ATM

cell

8

ATM

cell

5

ATM

cell

3 ATM

cell

3 ATM

cell

3 ATM

cell

3 ATM

cell

3

ATM-LSR transmitted cells in sequence in order for the downstream LSR to re-assembling correctly the cells into packets

CS 672

27

Summer 2003

LFIB

• IP forwarding decision is made via address lookup in the FIB.• MPLS maintains a label forwarding information base (LFIB)• LFIB contains entries of type:

• Next Hop Label Forwarding Entry (NHLFE)• Incoming Label Map (ILM)• FEC-to-NHLFE Map (FTN)

CS 672

28

Summer 2003

LFIB

• NHLFE entry contains information such as:• Next hop address, label stack operation, outgoing interface, layer 2 header

encapsulation• ILM maps each incoming label to one or more NHLFEs

• ILM entry is used by a labeled packet• FTN maps each FEC to one or more NHLFEs

• FTN entry is used to label an unlabeled packet

CS 672

29

Summer 2003

Label Stack Operation Out LabelNext Hop Layer 2 Encapsulation

Label Stack Operation Out LabelNext Hop Layer 2 Encapsulation

Label Stack Operation Out LabelNext Hop Layer 2 Encapsulation

Label Stack Operation Out LabelNext Hop Layer 2 Encapsulation

In Label

In Label

Label Stack Operation Out LabelNext Hop Layer 2 EncapsulationIn Label

FEC

Next Hop Label Forwarding Entry (NHLFE)In Label Map (ILM) orFEC-to-NHLFE Map (FTN)

1

2

3

Label Stack Operation Out LabelNext Hop Layer 2 Encapsulation

Label Stack Operation Out LabelNext Hop Layer 2 Encapsulation

FEC