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 0 1 171.68.10/24 Next-Hop InLabAddressPrefixOutI/FOutLabInI/F 4171.68/162pop0............... Next-Hop InLabAddressPrefixOutI/FOutLabInI/F -171.68/16140............... 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 Prefix and mask 171.68.10/24 Next-Hop 188.8.131.52InterfaceSerial1 171.68.44/24184.108.40.206Serial2 171.68/16...Null Egress LSR aggregates more specific routes and advertises a label for the new FEC
CS 672 6 Summer 2003 Penultimate Hop Popping 0 1 IP packet D=220.127.116.11 0 1 171.68.10/24 Next-Hop InLabAddressPrefixOutI/FOutLabInI/F 4171.68/162pop0............... Next-Hop InLabAddressPrefixOutI/FOutLabInI/F -171.68/16140............... IP packet D=18.104.22.168 Label = 4 IP packet D=171.68.10. 15 Packet arrives without the label at the egress LSR. Egress LSR needs to do an IP lookup to match more specific routes IP packet D=171.68.10. 15 Packet is MPLS forwarded Top label is removed IP packet enters the MPLS network Ingress LSR assign a label and forward the packet 171.68.44/24 Address Prefix and mask 171.68.10/24 Next-Hop 22.214.171.124InterfaceSerial1 171.68.44/24126.96.36.199Serial2 171.68/16...Null
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 HEC DATA CLP PTI VCI GFC VPI Label PPP Header Layer 3 Header Label Layer 3 Header MAC 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 Data L3 Hdr Label PPP Data L3 Hdr Label Eth 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. 2On 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 Data Cell Hdr 1 Label Data Cell Hdr 1 Label Data Cell Hdr 1 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-based forwarding AAL5 Control Plane Cell-based forwarding AAL5 Cell Hdr Data Cells for control messages do not encode label in VPI/VCI Control VC Data VC Cell-based LSR 1Over 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 171.68.10/24 Rtr-A Next-Hop InLab 5 5 AddressPrefix 171.68.10 171.68.10 OutI/F 2 2 OutLab 7 8 InI/F 0 1 IP packet D=188.8.131.52 Label = 5 IP packet D=184.108.40.206 Label = 7 Same label for FEC 171.168.10 is advertised to different upstream neighbours 1 0 2 IP packet D=220.127.116.11 Label = 5 IP packet D=18.104.22.168 Label = 8
CS 672 20 Summer 2003 Per Interface Label space 171.68.10/24 IP packet D=22.214.171.124 Label = 5 IP packet D=171.68.10. 12 Label = 7 1 0 2 IP packet D=126.96.36.199 Label = 5 IP packet D=188.8.131.52 Label = 8 Next-Hop InLabAddressPrefixOutI/FOutLabInI/F 5171.68.10270 5171.68.10281 5171.68.40394 171.68.40/24 3 IP packet D=184.108.40.206 Label = 9 4 IP packet D=220.127.116.11 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-LSRInLab 5 8... AddressPrefix 171.68 171.68... OutI/F 0 0... OutLab 3 3... InI/F 1 2... 171.68 IP Packet AT M cell 5 8 8 8 5 3 3 3 3 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-LSRInLab 5 8... AddressPrefix 171.68 171.68... OutI/F 0 0... OutLab 3 4... InI/F 1 2... 171.68 IP Packet AT M cell 5 8 8 8 5 3 4 4 4 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 InLab 5 8... AddressPrefix 171.68 171.68... OutI/F 0 0... OutLab 3 3... InI/F 1 2... 171.68 IP Packet AT M cell 5 8 8 8 5 3 3 3 3 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