11/23/2019 Outline • Circuit switching refresher • Virtual Circuits 15-441/641: Computer Networks Why virtual circuits? • Virtual Circuits, MPLS,VLAN How do they work? • • Today’s virtual circuits: MPLS 15-441 Fall 2019 Profs Peter Steenkiste & Justine Sherry • Virtual LANs How do they differ? • Fall 2019 https://computer-networks.github.io/fa19/ Circuit Switching Circuit Switching • Source first establishes a connection (circuit) to • Switches remembers how to forward the destination. Switch data Each router or switch along the way may reserve some • bandwidth for the data flow • No packets or addresses! • Source sends the data over the circuit. No destination address needed - routers know the path • Many options for switches • • The connection is torn down. • Connect specific wires (circuit = wire) • Example: traditional telephone network. • Forward on specific wire in specific Input Output timeslots (TDMA on each wire) Ports Ports • Forward to specific frequency on a specific wire (FDMA on each wire) 1
11/23/2019 Virtual Circuits Circuit Versus Packet Switching Each wire carries many “virtual” circuits • Circuit Switching Packet Switching Forwarding based on virtual circuit (VC) identifier in a packet header • • Fast switches can be built • Switch design is more IP header: source IP, destination IP, etc. • relatively inexpensively complex and expensive Virtual circuit header: VC ID, .. • • Inefficient for bursty data • Allows statistical A path through the network is set up when the VC is established • multiplexing Statistical multiplexing for efficiency, similar to IP • • Predictable performance Can support wide range of quality of service • (e.g. hard QoS) • Difficult to provide QoS No guarantees: best effort service guarantees • • Requires circuit Weak guarantees: delay < 300 msec, … • establishment before • Data can be sent without Strong guarantees: e.g. equivalent of physical circuit • communication signaling delay and overhead Can we get the benefits of both? Virtual Circuits Versus Virtual Circuit Forwarding Packet Switching A Switch C VC1 Many similarities: • 1 Forwarding based on “address” (VCID or destination address) • 3 B 2 VC2 D 4 Statistical multiplexing for efficiency • VC3 Must have buffers space on switches • F E Virtual circuit switching: • Address Next Hop Uses short connection identifiers to forward packets • • The address used for look up in the forwarding Switches maintain state for each connection so they can more easily implement features such VC1 3 • as quality of service table is a virtual circuit identifier (VC id) VC2 3 Switches are stateful: VC connection state cannot be lost • • Forwarding table entries are filled in during VC3 4 Packet switching: • signaling Uses full destination addresses for forwarding packets VC4 ? • • VC id is often shorter than destination address Can send data right away: no need to establish a connection first • VC5 ? and is typically “flat” (e.g., no CIDR) Switches are stateless: easier to recover from failures • Adding QoS is hard, kind of – see QoS lecture • 2
11/23/2019 VC versus Packets: How to Pick a New VC Id? Control over the End-to-End Path A Switch C VC1 = 1 Payload 1 VCI Payload Dst B 3 2 VC2 = 2 D 4 VC3 = 3 F 1 3 A E R2 Different paths to 2 1 3 4 1 3 same destination! R1 R4 Dst • When B establishes green virtual circuit, how does it know 2 4 (useful for traffic 2 4 1 3 B R3 what VC ids are available at all hops along the path? engineering!) 2 4 • Even worse: every VC id may already be used on a link along R1 VC table: R1 packet the path to the destination! forwarding table: VC 1 R2 • Solution: VC id swapping Dst R2 VC 2 R3 Connections and Signaling VC id Swapping • Permanent vs. switched virtual connections (PVC/SVC) A 1 Switch 1 C VC1 2 1 static vs. dynamic. PVCs last “a long time” • 2 2 1 3 1 E.g., connect two bank locations with a PVC B 3 • 2 VC2 D 1 2 4 2 1 SVCs are more like a phone call • VC3 2 F PVCs administratively configured (but not “manually”) E • 3 SVCs dynamically set up on a “per-call” basis • • Topology Address Next Hop Next id • Look up is based on VC id in header + incoming port number point to point, point to multipoint, multipoint to multipoint VC1 = 1 3 2 • • Challenges: How to configure these things? • Forwarding table specifies VC2 = 2 3 3 outgoing port and new VC id What VCI to use? VC3 = 1 4 1 • • VC id conflicts can be resolved Setting up the path • VC4 = 2 3 1 locally during signaling 3
11/23/2019 Basic SVC Connection Setup Virtual Circuits In Practice • Asynchronous Transfer Mode - ATM: Teleco approach calling network called Based on 53 byte “cells”, not packets (I am not kidding) • party party Kitchen sink: design driven by voice requirements, but supports file transfer, • video, etc., SETUP Intended as IP replacement. That didn’t happen. :) • Today: dead. • SETUP • MPLS: The “IP Heads” answer to ATM CONNECT Stole all the good ideas from ATM and integrated them into IP • Today: Used inside many transit networks to provide traffic engineering, VPN • support, .. CONNECT • Other networks just run IP. CONNECT • Older (ancient?) technology: Frame Relay ACK Only provided PVCs. Used for quasi-dedicated 56k/T1 links between offices, CONNECT • ACK etc. Slower, less flexible than ATM. Outline MPLS • Circuit switching refresher • Multi-Protocol Label Switching Layer 3 (IP) header • Virtual Circuits • Brings the virtual circuit concept into IP Layer 2 header • Driven by multiple forces Why virtual circuits? • QoS, traffic engineering • How do they work? • Simplifies packet forwarding • • Today’s virtual circuits: MPLS Layer 3 (IP) header • MPLS is implemented using an MPLS that sits • Virtual LANs between the IP and datalink header MPLS label VC ID is called an MPLS label How do they differ? • • Layer 2 header Some MPLS slides from H. Zhang 4
11/23/2019 Label Switched Router (LSR) Label Switched Paths (LSP) • Performs LSP setup and MPLS packet forwarding • Uni-directional path between two routers in an ISP’s network • Label Edge Router (LER): LSP ingress or egress • Forces packet along a specific path (set of routers) • Transit Router: forwards packet and swaps MPLS label New York Ingress New York San San Francisco Egress Francisco Transit MPLS Header MPLS Header • IP packet is encapsulated in MPLS header by the LSP ingress router and sent down a LSP Label CoS S TTL IP Packet • Label – 20 bits that identify LSP • Class of service 32-bit MPLS Header • Stacking bit Packets can be encapsulated in multiple MPLS headers • MPLS forward packet based on the label • • Time to live • IP packet is restored at end of LSP by egress router Decrement at each LSR, or • • TTL is adjusted, transit LSP routers count towards the TTL Pass through unchanged • • MPLS is an optimization – it does not affect IP semantics 5
11/23/2019 Forwarding Equivalence Classes Establishing LSPs • Use the Label Distribution Protocol (LDP) to establish paths LSR LSR LER LER based on IP forwarding tables LSP • Simple: IP routing protocols map forwarding of IP prefixes to IP1 IP1 LSPs as they fill in IP forwarding tables IP1 #L1 IP1 #L2 IP1 #L3 • Establish new LSPs as needed IP2 #L1 IP2 #L2 IP2 #L3 • MPLS packets follow the same path as IP – lose some MPLS IP2 IP2 benefits Packets are destined for different address prefixes, but can be • Explicitly establish LSPs to control flow of traffic mapped to common path • More work • FEC = “A subset of packets that are all treated the same way by a LSR” • Provides finer grain control over how traffic is distributed • The concept of FECs provides for a great deal of flexibility and scalability throughout the network • Can be used to force flows of different “sizes” (e.g., Mbps) to follow certain • Important tool for traffic engineering paths through the network – more flexible than traditional routing LSPs Driven by IP Routing MPLS Builds on Standard IP D e s t O u t 4 7 . 1 1 #216 D e s t O u t 4 7 . 2 2 #14 #311 4 7 . 1 1 4 7 . 3 3 4 7 . 2 2 #99 #311 4 7 . 3 3 1 47.1 #963 #311 1 2 3 D e s t O u t #963 2 4 7 .1 1 4 7 .2 2 #14 4 7 .3 3 1 #462 #612 47.2 3 47.3 #311 2 #5 #99 - A LSP is actually part of a sink tree with paths from every source to a destination (unidirectional). - A control protocol (e.g. Label Distribution Protocol, LDP) builds the tree based on the IP forwarding tables. • Destination based forwarding tables as built by OSPF, IS-IS, RIP, etc. 6
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