Multiprotocol Label Switching The future of IP Backbone Technology

Transcription

1 Multiprotocol Label Switching The future of IP Backbone Technology Computer Network Architecture For Postgraduates Chen Zhenxiang School of Information Science and Technology. University of Jinan (c) Chen Zhenxiang. University of Jinan 1

2 1. Background Why Label Switching? (c) Chen Zhenxiang. University of Jinan 2

3 Conventional IP Networks & Routing Client networks are connected to backbone via edge routers LAN, PSTN, ADSL,HFC,ATM/Ethernet. Data packets are routed based on IP address and other information in the header Functional components Forwarding responsible for actual forwarding across a router consists of set of procedures to make forwarding decisions Control (Routing Decision) responsible for construction and maintenance of the forwarding table consists of routing protocols such as OSPF, BGP,RIP Both based Layer-3 (This means you MUST check the IP header when every packet received). (c) Chen Zhenxiang. University of Jinan 3

4 The Need for Multi-protocol Label Switching (MPLS) Forwarding function of a conventional router a capacity demanding procedure constitutes a bottle neck with increase in line speed MPLS simplifies forwarding function by taking a totally different approach by introducing a connection oriented mechanism inside the connectionless IP networks Also a combination of Circuit Switch and Packet Switch in solutions such as ATM/IP switching. (c) Chen Zhenxiang. University of Jinan 4

5 Label Switching: Bring the speed of layer 2 switching to layer 3 Decomposition of network layer routing into control and forwarding components applicable Label switching forwarding component algorithm uses forwarding table label carried in the packet (Inserted into IPv4 Packet, Consider 20-bits Flow Label in IPv6 Header?) What is a Label? Short fixed length entity Label: Label Value,20 Bits Exp: Experiment Use, 3 Bits S: Bottom of the Stack TTL: Time To Live, 8Bits (c) Chen Zhenxiang. University of Jinan 5

6 2. MPLS: The Basic Idea RFC-3031 The goal is to avoid executing packet forwarding algorithm for each and every packet and replace it with switching in hardware. The result is faster and less expensive IP network with Integrated Traffic Engineering Mechanisms. (c) Chen Zhenxiang. University of Jinan 6

7 MPLS -- Basic Idea MPLS is a hybrid model adopted by IETF to incorporate best properties in both packet routing & circuit switching IP Router MPLS ATM Switch Control: IP Router Software Forwarding: Longest-match Lookup Control: IP Router Software Forwarding: Label Swapping Control: ATM Forum Software Forwarding: Label Swapping A Label Switched Path (LSP) is set up for each route A LSP for a particular packet P is a sequence of routers, <R1,R2..Rn> for all i, 1< i < n, R i transmits P to R i+1 by means of a label Edge routers analyze the IP header to decide which LSP to use add a corresponding local Label Switched Path Identifier, in the form of a label forward the packet to the next hop (This Forward is hardware-based) (c) Chen Zhenxiang. University of Jinan 7

8 MPLS Basics (continued.) Packets are switched, not routed, based on labels Labels are filled in the packet header Basic operation: Ingress LER (Label Edge Router) pushes a label in front of the IP header (Between the Data-link Header and Network Layer Header) LSR (Label Switch Router) does label swapping (switching) Egress LER removes the label The key : establish the forwarding table Link state routing protocols Exchange network topology information for path selection OSPF-TE, IS-IS-TE (TE: Traffic Engineering) Signaling/Label distribution protocols: Set up LSPs (Label Switched Path) LDP, RSVP-TE, CR-LDP (c) Chen Zhenxiang. University of Jinan 8

9 Basic Model for MPLS Network Internet LSR LER LER IP LSR MPLS LSR LSR MPLS LSR = Label Switched Router LER = Label Edge Router LER (c) Chen Zhenxiang. University of Jinan 9 IP

10 MPLS Operation 1a. Routing protocols (e.g. OSPF-TE, IS-IS-TE) exchange reachability to destination networks 1b. Label Distribution Protocol (LDP) establishes label mappings to destination network 4. LER at egress removes label and delivers packet IP IP 10 IP 20 IP 40 IP 2. Ingress LER receives packet and label s packets 3. LSR forwards packets using label swapping (c) Chen Zhenxiang. University of Jinan 10

11 MPLS Benefits Comparing MPLS with existing IP core and IP/ATM technologies, MPLS has many advantages and benefits: The performance characteristics of layer 2 networks The connectivity and network services of layer 3 networks Improves the price/performance of network layer routing Improved scalability Improves the possibilities for traffic engineering Supports the delivery of services with QoS guarantees Avoids need for coordination of IP and ATM address allocation and routing information Facilitate the virtual private networks (VPNs) (MPLS-VPN) (c) Chen Zhenxiang. University of Jinan 11

12 Necessity of L3 Forwarding The Reason for NOT using MPLS For security To allow packet filtering at firewalls Requires examination of packet contents, including the IP header For forwarding at the initial router - used when hosts don t do MPLS For Scaling Forward on a finer granularity than the labels can provide (c) Chen Zhenxiang. University of Jinan 12

13 3. MPLS: Implementation (c) Chen Zhenxiang. University of Jinan 13

14 How to Carrying a Label Certain link layer technologies can carry label as a part of their link layer header e.g ATM & Frame Relay(Using VC/VP as label) Link layers that do not support labels in their header carry them in a shim label header Link layer header Shim label header Network layer header Network layer data (c) Chen Zhenxiang. University of Jinan 14

15 MPLS packet formatting Ethernet (Shim Header) ATM Cell Header (c) Chen Zhenxiang. University of Jinan 15

16 Label Swapping (Switching) (c) Chen Zhenxiang. University of Jinan 16

17 Establishing Label Switched Path LSPs are generated and maintained in a distributed fashion Each LSR negotiates a label for each Forwarding Equivalence Class (FEC) with its upstream and downstream neighbors using a distribution method Label Information Base (LIB) - Result of negotiation, stored in each LSRs. (Forwarding Table) (c) Chen Zhenxiang. University of Jinan 17

18 How to distribute Label: LDP Terminology Label Distribution Protocol (LDP) set of procedures by which LSRs establish LSPs mapping between network-layer routing information directly to data-link layer switched paths LDP peers: two LSRs which use LDP to exchange label/stream mapping information exchange known as LDP Session (c) Chen Zhenxiang. University of Jinan 18

19 LDP Message Exchange Discovery messages - used to announce and maintain the presence of a LSR Session messages - used to establish, maintain and terminate sessions between LDP peers Advertisement messages - used to create, change, and delete label mappings Notification messages - used to provide advisory information and to signal error information (c) Chen Zhenxiang. University of Jinan 19

20 LDP Message Format (Header not included) U Message Type Message Length Message ID Mandatory Parameters Optional Parameters (c) Chen Zhenxiang. University of Jinan 20

21 LDP Protocol Data Units (PDUs) LDP message exchanges are accomplished by sending LDP PDUs Each LDP PDU is an LDP header followed by LDP message The LDP header is: Version PDU Length LDP Identifier (c) Chen Zhenxiang. University of Jinan 21

22 LDP Discovery A mechanism that enables a LSR to discover potential LDP peers Avoids unnecessary explicit configuration of LSR label switching peers Two variants of the discovery mechanism basic discovery mechanism: used to discover LSR neighbors that are directly connected at the link level extended discovery mechanism: used to locate LSRs that are not directly connected at the link level (c) Chen Zhenxiang. University of Jinan 22

23 LDP Discovery (continued.) Basic discovery mechanism To engage - send LDP Hellos periodically LDP Hellos sent as UDP packets for all routers on that subnet Extended discovery mechanism To engage - send LDP targeted Hellos periodically Targeted Hellos are sent to a specific address Targeted LSR decides whether to respond or to ignore the targeted Hello LDP Link Hello sent by an LSR carries the LDP identifier for the label space the LSR intends to use for the interface (c) Chen Zhenxiang. University of Jinan 23

24 Label Information Base LSR maintains learned labels in Label Information Base (LIB) Each entry of LIB associates an FEC with an (LDP Identifier, label) pair and a next hop When next hop changes for a FEC, LSR will retrieve the label for the new next hop from the LIB (c) Chen Zhenxiang. University of Jinan 24

25 4.Tag Switching (Developed by CISCO) (c) Chen Zhenxiang. University of Jinan 25

26 Tag Switching (Developed by CISCO) Terminologies Tags Tag Switching Router (TSR) Tag Edge Router (TER) Tag Forwarding Information Base (TFIB) Tag Distribution Protocol (TDP) Analogies in Label Switching Labels Label Switching Router Edge Label Switching Router Label Switching Forwarding Table Label Distribution Protocol (c) Chen Zhenxiang. University of Jinan 26

27 5. IP Switching (c) Chen Zhenxiang. University of Jinan 27

28 IP Switching Introduced by Ipsilon (1996) Already been tested in the field Significant Innovation: Defined a switch management protocol (GSMP) along with label binding protocol called Ipsilon Flow Management Protocol (IFMP) General Switch Management Protocol (GSMP) - allows an ATM switch to be controlled by an IP switch controller (c) Chen Zhenxiang. University of Jinan 28

29 IP Switching Overview IP over ATM models are complex and inefficient - involve running two control planes ATM Forum signaling and routing IP routing and address resolution on top In contrast IP Switching uses IP component plus label binding protocol completely removes ATM control plane Goal: To integrate ATM switches and IP routing in a simple and efficient way (c) Chen Zhenxiang. University of Jinan 29

30 Removing ATM Control Plane IP ATM MARS NHRP ARP PNNI Q.2931 ATM hardware (a) IP IFMP ATM hardware (b) (a) IP over Standard ATM (b) IP Switching IFMP: Ipsilon Flow Management Protocol (c) Chen Zhenxiang. University of Jinan 30

31 IP Switching Basics IP Switching relies on IP protocols to establish routing information to determine next hop Flow classification and control module selects flows from incoming traffic IP flow refers to a sequence of datagrams from one source to one destination, identified by the ordered pair <source address, destination address> can also refer to a flow at finer granularity, e.g., different applications between same pair of machines, identified by < source address, source port, destination address, destination port> (c) Chen Zhenxiang. University of Jinan 31

32 Flow Redirection Redirection: Process of binding labels to flows and establishing label switched paths Example: data is flowing from A via B to C on default VC B sends a redirect to A specifying flow y and the label (VPI/VCI) on which it expects to receive If C issues a redirect to B for flow y, B forwards y on the VPI/VCI specified by C Since same flow y enters B on one VC and leaves on another, B uses GSMP to inform its switching element to set up the appropriate switching path GSMP: Generic Switch Management Protocol (c) Chen Zhenxiang. University of Jinan 32

33 Appendix: 术语解释与对照 标签交换路由器 (LSR) LSR 类似一个通用 IP 交换机, 它是 MPLS 中负责第三层转发分组和第二层标签交换分组的设备 标签边缘路由器 (LER) LER 是从一个 MPLS 域转发分组的传统路由器 它的作用是分析 IP 分组头, 用于决定相应的传送级别和标签交换路径 (LSP) 标签 (Label) 标签是一个包含在每个分组中的短固定的数值, 用于通过网络转发分组 一对 LSR 在标签的数值和意义上一致 标签分发协议 (LDP) LDP 是 MPLS 的控制协议, 用于在 LSR 之间交换 FEC/ 标签绑定信息 标签交换路径 (LSP) LSP 是指具有一个特定的 FEC 的分组, 在传输经过的标签交换路由器集合构成的传输通路 它由 MPLS 节点建立, 目的是采用一个标签交换转发机制转发一个特定的 FEC 分组 标签信息库 (LIB) LIB 是保存在一个 LSR(LER) 中的连接表, 在 LSR 中包含有 FEC/ 标签绑定信息和关联端口以及媒体的封装信息 LIB 通常包括下面内容 : 入 出口端口 ; 入 出口标签 ;FEC 标识符 ; 下一跳 LSR; 出口链路层封装等 转发等价类 (FEC) MPLS 采用 FEC 作为标签索引来处理 IP 分组 具有相同 FEC 的包在相同路径上转发, 以相同方式处理并因此被一个 LSR 映射到一个单一标签的一组 IP 分组 流束 (Stream) 属于同一个 FEC 的一组分组流, 它们流经同一个节点, 从相同的通道传输, 并以相同方式转发到目的地, 它们在 MPLS 里被称为 流束 (c) Chen Zhenxiang. University of Jinan 33