leading edge routing
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Leading Edge Routing. MPLS Enhancements to Support Layer 2 Transport Services. Jeremy Brayley [email protected]. Agenda. Introduction Why Layer 2 service over MPLS? Provisioning Signaling Layer 2 encapsulations Summary and future work. What is MPLS?. - PowerPoint PPT PresentationTRANSCRIPT
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
Copyright © 2001, Laurel Networks, Inc.
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
Copyright © 2001, Laurel Networks, Inc.
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)
Copyright © 2001, Laurel Networks, Inc.
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
Copyright © 2001, Laurel Networks, Inc.
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
Copyright © 2001, Laurel Networks, Inc.
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
Copyright © 2001, Laurel Networks, Inc.
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
Copyright © 2001, Laurel Networks, Inc.
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
Copyright © 2001, Laurel Networks, Inc.
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
Copyright © 2001, Laurel Networks, Inc.
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
Copyright © 2001, Laurel Networks, Inc.
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
Copyright © 2001, Laurel Networks, Inc.
Layer 2 encapsulation
• Martini drafts define the following encapsulations over MPLS
– Frame Relay– Ethernet port / 802.1q VLAN– ATM AAL5– ATM cell– PPP/HDLC
Copyright © 2001, Laurel Networks, Inc.
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
Copyright © 2001, Laurel Networks, Inc.
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
Copyright © 2001, Laurel Networks, Inc.
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
Copyright © 2001, Laurel Networks, Inc.
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
Copyright © 2001, Laurel Networks, Inc.
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
Copyright © 2001, Laurel Networks, Inc.
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
Copyright © 2001, Laurel Networks, Inc.
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
Copyright © 2001, Laurel Networks, Inc.
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