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Lecture 6

Lecture 6. Hierarchical LSP. Ingress LSR for LSP3. Ingress LSR for LSP1. LSP1. Egress LSR for LSP1. LSP3. LSP2. Hierarchical LSP. 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

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Lecture 6

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  1. Lecture 6

  2. Hierarchical LSP Ingress LSR for LSP3 Ingress LSR for LSP1 LSP1 Egress LSR for LSP1 LSP3 LSP2 Hierarchical LSP

  3. 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.

  4. 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

  5. Address Prefix and mask Next-Hop Interface In I/F In I/F In Lab In Lab Address Prefix Address Prefix Out I/F Out I/F Out Lab Out Lab Serial1 171.68.10/24 171.68.9.1 0 0 4 - 171.68/16 171.68/16 2 1 4 pop Next-Hop Next-Hop 171.68.44/24 171.68.12.1 Serial2 ... ... ... ... ... ... ... ... ... ... 171.68/16 ... Null Penultimate Hop Popping • 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 • route 171.68/16 • route 171.68/16 0 0 1 1 • Use label 4 for FEC 171.68/16 • Use label “implicit-null” for FEC 171.68/16 171.68.44/24 Egress LSR aggregates morespecific routes and advertises a label for the new FEC Aggregate route is propagate through the IGP and label is assigned by each LSR 171.68.10/24

  6. Address Prefix and mask Next-Hop Interface In I/F In I/F In Lab In Lab Address Prefix Address Prefix Out I/F Out I/F Out Lab Out Lab Serial1 171.68.10/24 171.68.9.1 0 0 - 4 171.68/16 171.68/16 2 1 pop 4 Next-Hop Next-Hop 171.68.44/24 171.68.12.1 Serial2 ... ... ... ... ... ... ... ... ... ... 171.68/16 ... Null Label = 4 • IP packetD=171.68.10.15 Penultimate Hop Popping 0 0 1 1 • IP packetD=171.68.10.15 • IP packetD=171.68.10.15 171.68.44/24 • IP packetD=171.68.10.15 171.68.10/24 Packet arrives without the labelat the egress LSR.Egress LSR needs to do an IP lookup to match more specific routes Packet is MPLS forwardedTop label is removed IP packet enters the MPLS networkIngress LSR assign a label andforward the packet

  7. 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.

  8. IGP domain with a label distribution protocol IGP domain with a label distribution protocol LSP follows IGP shortest path LSP diverges from IGP shortest path 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

  9. 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.

  10. ATM Cell Header GFC VPI VCI PTI CLP HEC DATA Label PPP Header Label Layer 3 Header PPP Header (Packet over SONET/SDH) Shim header LAN MAC Label Header Label MAC Header Layer 3 Header MPLS Label Encoding

  11. 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.

  12. 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

  13. AAL5 SAR Data Data L3 Hdr L3 Hdr Label Label PPP Eth VPI/VCI fields do not encode label Data Label L3 Hdr Packet over SONET/SDH Link Labeled Layer 3 Packet Hdr Cell ATM Link Frame-based LSR Hdr Cell 1 1 2 1 2 Ethernet Link As a Layer 2 frame containing labeled packet is received, the label is extracted and used to forward the packet. 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. Frame-based LSR

  14. 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

  15. Label Label LC-ATM Link Cell Hdr Data Data LC-ATM Link Cell Hdr Cell-based LSR LC-ATM Link Label 1 1 1 1 Cell Hdr Data A cell-based LSR forwards cells using label information encoded in the VPI/VCI fields. ATM-LSR

  16. Cells for control messages do not encode label in VPI/VCI AAL5 AAL5 Cell Hdr Data Control Plane Control Plane Cell-based forwarding Cell-based forwarding 1 1 Control VC Label Data VC Cell Hdr Data LC-ATM Link Cell-based LSR Cell-based LSR Over an LC-ATM link, labeled packets are is transmitted on data VCs and the unlabeled control packets on the control VC. ATM-LSR

  17. 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

  18. 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

  19. Label = 8 Label = 5 • IP packetD=171.68.10.15 • IP packetD=171.68.10.15 In I/F In Lab Address Prefix Out I/F Out Lab 0 5 171.68.10 2 7 Next-Hop 1 5 171.68.10 2 8 Per Interface Label space • Same label for FEC 171.168.10 is advertised to different upstream neighbours 1 2 0 Label = 7 Label = 5 Rtr-A 171.68.10/24 • IP packetD=171.68.10.12 • IP packetD=171.68.10.12

  20. In I/F In Lab Address Prefix Out I/F Out Lab Label = 5 Label = 8 0 4 5 5 171.68.40 171.68.10 2 3 7 9 • IP packetD=171.68.10.15 • IP packetD=171.68.10.15 Next-Hop 1 5 171.68.10 2 8 Label = 7 • IP packetD=171.68.10.12 Label = 9 Label = 5 Label = 5 • IP packetD=171.68.40.33 • IP packetD=171.68.40.33 • IP packetD=171.68.10.12 Per Interface Label space • Same label is assigned to different FECs if LSR is able to distinguish the upstream neighbours who sent the packet 1 0 2 3 4 171.68.10/24 171.68.40/24

  21. 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

  22. 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

  23. 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

  24. In I/F In Lab Address Prefix Out I/F Out Lab 1 5 171.68 0 3 2 8 171.68 0 3 ... ... ... ... ... 5 8 8 3 8 3 3 3 3 5 • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell DoD and ATM-LSR Downstream LSR do not know how to reassemble correctly cells into packets. VPI/VCI values are identical for all cells • IPPacket 171.68 • IPPacket

  25. In I/F In Lab Address Prefix Out I/F Out Lab 1 5 171.68 0 3 2 8 171.68 0 4 ... ... ... ... ... 5 8 8 3 8 4 4 3 4 5 • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell DoD and ATM-LSR ATM-LSR requested additional label for same FEC in order to distinguish between incoming interfaces (Downstream on Demand) • IPPacket 171.68 • IPPacket

  26. In I/F In Lab Address Prefix Out I/F Out Lab 1 5 171.68 0 3 2 8 171.68 0 3 ... ... ... ... ... 5 8 8 3 8 3 3 3 3 5 • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell • ATMcell ATM-LSRs and VC-Merge ATM-LSR transmitted cells in sequence in order for the downstream LSR to re-assembling correctly the cells into packets • IPPacket 171.68 • IPPacket

  27. 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)

  28. 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

  29. In Label Map (ILM) or FEC-to-NHLFE Map (FTN) Next Hop Label Forwarding Entry (NHLFE) In Label 1 In Label 2 Next Hop Next Hop Next Hop Next Hop Next Hop Next Hop Next Hop Label Stack Operation Label Stack Operation Label Stack Operation Label Stack Operation Label Stack Operation Label Stack Operation Label Stack Operation Out Label Out Label Out Label Out Label Out Label Out Label Out Label Layer 2 Encapsulation Layer 2 Encapsulation Layer 2 Encapsulation Layer 2 Encapsulation Layer 2 Encapsulation Layer 2 Encapsulation Layer 2 Encapsulation FEC 3 In Label FEC

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