leading edge routing

Leading Edge Routing

MPLS Enhancements to Support Layer 2 Transport Services

Jeremy [email protected]

Copyright © 2001, Laurel Networks, Inc.

Agenda

• Introduction– Why Layer 2 service over MPLS?

• Provisioning

• Signaling

• Layer 2 encapsulations

• Summary and future work


What is MPLS?

Multiprotocol Label Switching

• Label switching mechanism with IP control plane initially designed to increase forwarding performance

• Label stacking allows tunnel hierarchy for superior scalability

• New signaling protocols (LDP, RSVP-TE, CR-LDP, and even mBGP)

• First application - IP Traffic Engineering• Subsequently several new applications

have been proposed


New applications for MPLS

• MPLS has been viewed as an IP traffic engineering technology– Allows a carrier to increase

operational efficiency, but service remains the same

• Layer 2 transport is a new application of MPLS– MPLS becomes forwarding

infrastructure for a number of services• IP services• Private Data (Frame Relay, ATM, Ethernet)


What problems are we solving?

• Network consolidation– For carriers offering Private Data and IP

services

• Additional service revenue opportunities– For carriers currently only offering IP services

• Scalability– Core switches MPLS tunnels and manages far

fewer connections

• Ease of provisioning– Touch only edge devices


Methods of providing layer 2 services over MPLS

• MPLS-based Layer 2VPNs (L2VPN)– draft-kompella-mpls-12vpn-02.txt

• L2VPN method eases provisioning of full mesh VPNs• Policies can be formed to provision hub and spoke

topologies

• Transport of Layer 2 frames over MPLS– draft-martini-l2circuit-trans-mpls-05.txt

• Defines point-to-point transport using LDP– draft-martini-l2circuit-encap-mpls-01.txt

• Defines encapsulations for multiple layer 2 services– Full or partial mesh provisioning requires automated

management tools

• Both techniques use label stacking for scalability


Scalability through label stacking

IP/MPLS network

Tunnel to 2.2.2.2

Layer 2 VCs

1.1.1.1

2.2.2.2

Tunnel label determines path to remote edgeVC label designates connection at tunnel endpoint

Tunnel labelVC labelVC label

VC label

P

PPEPE


Martini method for L2 transport

• All services look like a Virtual Circuit to MPLS network

• Provision service by associating each endpoint with a common VC Identifier(VCID)

• Network automatically determines VC label and Tunnel label to push on L2 frame

Tunnel label

VC label Layer 2 frame

Port/DLCI Port/DLCI

VCID


Provisioning a Layer 2 transport service

1.1.1.1 2.2.2.2

1A, 100 1B, 200

Port 1A, VLAN 100 -> peer 2.2.2.2, VCID 50

Port 1B, VLAN 200 -> peer 1.1.1.1, VCID 50

LDP advertises label 501 for VCID 50LDP advertises VC label 500 for VCID 50

PE chooses tunnel to 2.2.2.2, label 600

600 501 Ethernet

PE chooses tunnel to 1.1.1.1, label 601

601 500 Ethernet

600 601

indirect LDP sessionto advertise VC labelsPE PE

P

P


LDP in review

• VC label established via indirect LDP session

• New Virtual Circuit FEC element defined– Used in Label Mapping and Label Withdraw

messages

• PE binds VC label to VC Identifier(VCID) and advertises to remote peer

• LDP message includes:– Local port identifier– VC type (FR, ATM, VLAN, etc)– Local MTU


Tunnel creation and selection

• Tunnels must exist between PE endpoints before transport connection may be established

• Same tunnel may be used for IP and L2 transport traffic– Eases provisioning– Scalable

• Service determined by tunnel creation– RSVP-TE tunnels allow traffic engineering and

resource reservation– LDP tunnels are plug & play


Withdrawing labels to indicate connection status

1.1.1.12.2.2.2

DCLI 100DCLI 200

X

PE withdraws VC label for connection ID 50

Incoming frames on DLCI 200 are dropped until a new VC label is received

LMI indicates status=down

VCID is down

PE PE

P

P


Layer 2 encapsulation

• Martini drafts define the following encapsulations over MPLS

– Frame Relay– Ethernet port / 802.1q VLAN– ATM AAL5– ATM cell– PPP/HDLC


Frame Relay encapsulation

• Ingress device strips the Frame Relay header and FCS and appends label stack and control word

• Control word carries FECN, BECN, DE, C/R bits plus PDU length• Sequence number is optional. It is used to guarantee in-order

delivery of frames

Tunnellabel

VC label

4 octets 4 octetsControlword Frame Relay PDU

4 octets

Rsvd B F D C

bits 4 1 1 1 1

Length

8

Sequence Number

16

Control Word

payloadQ.922 address FCS

Frame Relay frame

Frame Relay over MPLS


Ethernet encapsulation

• Ingress device strips the Ethernet preamble and CRC, but transports the entire header

• Control word is not used• 802.1q VLAN ID may be overwritten at egress

Tunnel label

VC label

4 octets 4 octetsEthernet header

Ethernetpayload

payloadDA SA T FCS

Ethernet frame

Ethernet over MPLS


ATM AAL5 encapsulation

• Ingress reassembles AAL5 frames and strips 8 octet AAL5 trailer

• Required control word includes:– Transport type (AAL5 CPCS-PDU or ATM cell)– EFCI, CLP, and C/R bits (CPCS-UU’s LSB)– (CPCS-PDU + control word) length– Sequence number

Tunnel label

VC label

4 octets 4 octetsControlword AAL5 CPCS-PDU

4 octets

Rsvd T E L C

bits 4 1 1

Length

8

Sequence Number

16

Control Word

AAL5 over MPLS1 1


ATM cell mode

• Ingress performs no reassembly• Control word is optional:

– Length may be used to infer number of cells– Flags set to zero

Tunnel label

VC label

4 octets 4 octetsControlword

ATM cell #1minus HCS

4 octets

Rsvd

bits 4

Length

8

Sequence Number

16

Control Word

Flags

4

52 octetsATM cell #2minus HCS

52 octets

…

ATM cells over MPLS


Control word in review

• Layer 2 header fields may be discarded at ingress

• Control word carries variable “flag” bits – (FR FECN, BECN, C/R, DE, ATM CLP, EFCI, etc)

• Length required when padding small frames on links which have a minimum frame size

• Sequence number is optional. It is used to detect out of order delivery of frames.

Rsvd

bits 4

Length

8

Sequence Number

16

Control Word

Flags

4


Class of Service considerations

• Ingress device may set MPLS EXP bits to denote class of service on MPLS network

• Should set EXP on both Tunnel and VC labels– Tunnel label stripped before egress if PHP is

used– Allows user to keep 802.1p classification across

MPLS network

Set Tunnel and VC label EXP to 010

Treat according to VC label EXP

Treat according to Tunnel label EXP

PE PE

P

P P P


Future work

• Interworking between different frame types– Frame Relay / ATM service interworking

(FRF.8.1)– FR/ATM/Ethernet interworking for IP

• OA&M improvements & SLA measurement– In-band performance monitoring and continuity

check


Summary

• L2 transport is a new application of MPLS– Not just for traffic engineering anymore

• Allows a service provider to expand IP/MPLS network while offering Layer 2 services

• MPLS label stacking mechanism allows for core network scalability– Far fewer connections to manage in core– Services provisioned at edge

leading edge routing

Documents