2D1490 p MPLS, RSVP, etc. Olof Hagsand KTHNOC/NADA

Transcription

1 2D1490 p MPLS, RSVP, etc Olof Hagsand KTHNOC/NADA

2 Literature Handouts: MPLS-Enabled applications (Minei, Lucek). Parts of Section 1. JunOS Cookbook: Chapter 14

3 Background MPLS - Multiprotocol Label Switching Originally thought to simplify IP forwarding Roots in ATM (Asynchronous Transfer Mode) Small label lookup instead of longest prefix match Early 90s, most telecoms thought ATM would take over all dataand tele-communication MPLS was standardized in IETF in mid 90s.

4 MPLS Advantages Originally, the motivation was speed and cost. But routers does IP lookup in hardware at very high speeds. Current advantages: Label switching can be used for traffic engineering Aggregating a class of traffic and treating it in a specific way Control of traffic in a network Labels can be used to forward using other fields than destination address Label switching can be used to support VPNs virtual private networks The generalized form of MPLS: GMPLS is ideal for optical network management

5 MPLS Terminology MPLS uses some new terminology (MPLS ~ IP) LSR: Label Switching Router ~ Router LER: Label Edge Router ~ Border router Alternative: PE - Provider Edge / CE Customer Edge Also: Egress/Ingress/Transit LSR LSP: Label Switched Path ~ Tunnel FEC: Forwarding Equivalence Class ~ Flow Label distribution ~ Routing LFIB: Label Forwarding Information Base ~ FIB

6 Control and Data Planes LSR Label Distribution Control Plane (Binding Layer) Label Distribution OSPF IS-IS BGP Data Plane (Forwarding Layer) Control Plane (Routing Layer) OSPF IS-IS BGP FIB LFIB MPLS packet Router MPLS packet IP packet Data Plane (Forwarding Layer) IP packet

7 Labels A label is an integer identifying a FEC (or a flow). You cannot have globally or network- unique labels Too complex to negotiate Too large labels Labels are unique only between two nodes Labels reserved by the IETF. Labels change at each node as a packet traverses a path You can set labels manually(worse than static routing), or use label distribution Example of Label Forwarding Information Base LFIB (MPLS forwarding table): 20-bit label In-Interface in-label op out-label out-interface if 3 if if1 if swap swap

8 Encapsulation 3-bit exp 20-bit label Eth hdr Label hdr IP hdr 1-bit stack 8-bit TTL IP payload MPLS uses a 32-bit shim header: Label: value for table lookup in router TTL: Time To Live (resembles IP TTL) Exp: experimental, several ways to support QoS has been defined Stack: indicates that the bottom of a stack of labels has been reached Shim headers may be concatenated into stacks

9 Forwarding Equivalence Class (FEC) Sort packets into different classes classify them. Example1: All packets to one destination. Example2: All UDP packets with the ToS field set to 0x42 from sub-network /24 Such a subset is called a Forwarding Equivalence Class MPLS binds labels to different FECs Labelling FECs granularity Coarse good for scalability Fine good for flexibility FEC label FEC1 FEC

10 Label assignment LER pkt pkt FEC op out-label interface F1 F2 push push if1 if2 LSP The border router (LER) classifies packets into FECs 99 Classification is a more general form of lookup Binds a label to the packet Or to an LSP, which in turn is mapped to a label Pushes an MPLS header on the packet And sends it on

11 Label swapping LSP 99 pkt 203 pkt In-Interface in-label op out-label out-interface if 3 if if1 if swap swap LSP A router (LSR) makes a label lookup and swaps the labels Rewrites the MPLS header And sends it further on the LSP

12 Label popping LER LSP 203 pkt pkt In-Interface in-label op out-label out-interface if 3 if if pop swap The border router (LER) pops the MPLS packet And then forwards it as usual depending on the packets protocol Example: pkt is an IP packet --> pkt is sent to IP forwarding Thus both MPLS and IP forwarding on same node!

13 Pen-ultimate popping Pen-ultimate LSR LSP 203 pkt pkt MPLS pop LER IP routing To make it easier for the border router, pop the label on the previous router (pen-ultimate) The pen-ultimate LSR does MPLS pop The LER does only IP routing

14 Label Switched Path LSP An LSP is an abstraction of a set of labels an MPLS tunnel LSP IP pkt IP pkt l=x IP pkt IP pkt l=z Push label IP pkt l=y IP pkt l=v Pop label Swap label Eth hdr Label hdr IP hdr 20-bit label IP payload 3-bit exp 1-bit stack 8-bit TTL

15 Label Stacking IP pkt inner=3outer=6 IP pkt inner=3outer=7 IP pkt inner=5outer=6 IP pkt inner=5outer=7 IP pkt inner=3 IP pkt inner=3 IP pkt inner=5 IP pkt inner=5 Transit Routing Domain Used when packet forwarded through transit routing domain An inner label is transferred through the domain Push outer label onto stack at ingress LSR, swap outer label in domain pop outer label at egress LSR

16 Label operations Label operations are interface-specific Push a label (typically at ingress) Double push (label stacking) Swap a label (typically at transit or pen-ultimate) Pop a label (typically at egress or pen-ultimate)

17 Special labels operations 0: IPv4 explicit NULL Pop unconditionally when packet is received Popped packet is an IPv4 datagram 1: Router alert Deliver to control plane do not forward 2: IPv6 explicit-null Pop unconditionally when packet is received Popped packet is an IPv6 datagram 3: Implicit-NULL Downstream router should pop does not appear on link only in signaling protocol

18 Label Distribution Labels need to be assigned And LFIBs programmed A signaling protocol distributes labels New protocol Creates an LSP through an MPLS network LDP Label Distribution Protocol Extend existing protocols BGP Border Gateway Protocol RSVP Resource Reservation Protocol These protocols all distributes labels But they are somewhat different and can be combined to transfer different labels, eg BGP+RSVP

19 Label distribution and IGP Label distribution protocols typically rely on an IGP (eg OSPF) to find the shortest path of their LSP. Although RSVP may use source-routing as well They then assign labels to the LSP on the shortest path. Labels are assigned in upstream direction normally During setup of LSPs, traffic may be black-holed IGP and label distribution Same is true during reconvergence

20 Resource Reservation Protocol RSVP is a network control protocol used to express quality of service (QoS). Binds a QoS request to a flow RSVP is a receiver oriented protocol. RSVP delivers QoS reservations along a path from source to destination(s). no routing IGP computes path uses soft state periodically refresh With MPLS, RSVP-TE is used for traffic engineering To guarantee a quality for a FEC Aggregated flows Extended to carry MPLS labels

21 RSVP Building Blocks RSVP carries the following information: Classification information Flows with QoS requirements must be recognized within the network Includes src and dst IP addr and UDP/TCP port numbers Or MPLS labels Traffic specification (TSpecs) QoS requirements (FlowSpecs) RSVP is explicitly designed to support multicast but MPLS is not,...

22 RSVP Model Receiver PATH msg (TSpec) Sender RESV msg (FlowSpec) Receiver Two types of messages to set up reservations: PATH message From a sender to one or several receivers, carrying TSpec and classification information provided by sender RESV message From receiver, carrying FlowSpec indicating QoS required by receiver

23 RSVP and Router Operation At each router, RSVP applies admission control If the router cannot reserve resources the traffic an error message is returned. PATH messages flowing downstreams indicate how the traffic will flow. RESV messages flowing upstreams negotiate the actual reservation And MPLS labels

24 MPLS Support for RSVP PATH Ingess Egress RESV lbl=9 RESV lbl=5 RESV RSVP has been successfully adapted to support MPLS New objects defined carrying labels RSVP can express QoS (Tspec, FlowSpec) Each LSR associates QoS resources with an LSP The ingress router needs to classify and bind traffic to an LSP Network resources are then bound to that LSP throughout the network.

25 Objects in PATH messages Label Request Object Explicit Route Object (ERO) Addresses through which the LSP must pass. Used for source routing Record Route Object (RRO) Request an MPLS label for a path Record the path of the PATH message Sender Traffic Specification (Tspec) A specification of the traffic of this LSP Eg: 2Mbps

26 Objects in RESV messages Label Object Contains a label that was requested Labels are thus allocated upstreams Record Route Object (RRO) Record the path pf the RESV message Flow Specification (Flowspec) What was actually reserved

27 Traffic engineering with RSVP-TE/MPLS RSVP can reserve resources in the network RSVP can signal alternative paths using Strict source routing Loose source routing Why? Traffic distribution, alternate paths Difficult to do with regular routing protocols. Example: LSP loose source routing: via D (20Mbps) LSP strict source routing: A,B,C,D,E (15 Mbps) B D A E C

28 Fast reroute and BFD With alternative paths already computed, switchover can be fast No need to recompute a new best path Link failures can be detected in many ways Loss of signal RSVP Path/Resv loss BFD Bidirectional Forwarding Detection BFD can detect link failures in ~100ms Based on a simple ping method Is also used for other protocols: OSPF, BGP, etc But it is complex to admin several alternate paths in a large network

29 Lab: Static MPLS and RSVP Gives a basic understanding of MPLS label swapping No signaling manually assign labels like static routing Understand configuration, forwarding tables, and debugging of MPLS LSP-RTX1-RTX4 LSP-RTX4-RTX1 LSP-RTX2-RTX3 LSP-RTX3-RTX2