11-Virtual-Circuits.Pdf

11-Virtual-Circuits.Pdf

Outline • Circuit switching refresher 15-441 • Virtual Circuits - general 15-441 Computer Networking 15-641 • Why virtual circuits? • How virtual circuits? -- tag switching! • Two modern implementations Lecture 11: Virtual Circuits • ATM - teleco-style virtual circuits Peter Steenkiste • MPLS - IP-style virtual circuits Fall 2016 www.cs.cmu.edu/~prs/15-441-F16 Circuit Switching Circuit Switching • Source first establishes a connection (circuit) to • Switch remembers the destination. Switch how to forward data • Each router or switch along the way may reserve some bandwidth for the data flow • No addresses! • Source sends the data over the circuit. • Many options • No destination address needed - routers know the path • Between specific wires • The connection is torn down. (circuit = wire) • Example: traditional telephone network. • Between timeslots Input Output (TDMA on each wire) Ports Ports • Between frequencies (FDMA on each wire) 1 Virtual Circuits Circuit Versus Packet Switching • Each wire carries many “virtual” circuits. Circuit Switching Packet Switching • Forwarding based on virtual circuit (VC) identifier • Fast switches can be • Switch design is more • IP header: src, dst, etc. • Virtual circuit header: just “VC” built relatively complex and expensive • A path through the network is set up when the VC is established inexpensively • Allows statistical • Can eue statistical multiplexing for efficiency • Inefficient for bursty data multiplexing • Can support wide range of quality of service. • Predictable performance • Difficult to provide QoS • No guarantees: best effort service (e.g. hard QoS) guarantees • Weak guarantees: delay < 300 msec, … • Requires circuit • Data can be sent • Strong guarantees: e.g. equivalent of physical circuit establishment before without signaling delay communication and overhead Can we get the benefits of both? Virtual Circuits Versus Packet Switching Virtual Circuit Forwarding • Many similarities: A Switch VC1 C • Forwarding based on “address” (VCID or dest address) 1 • Statistical multiplexing for efficiency B 2 3 VC2 • Must have buffers space on switches 4 D VC3 • Virtual circuit switching: F • Uses short circuit identifiers to forward packets E • Switches keep (hard) state for each circuit, so they can more easily Address Next Hop implement features such as quality of service VC1 3 • Failures result in loss of virtual circuit • Address used for look up is a virtual • Packet switching: VC2 3 circuit identifier (VC id) • Use full destination addresses for forwarding packets VC3 4 • Forwarding table entries are filled in • Can send data right away: no need to establish a connection first during signaling • Switches are stateless: easier to recover from failures VC4 ? • Adding QoS is hard VC5 ? • VC id is often shorter than destination address 8 2 VC versus Packets: Control over Path How to Pick a VC Id? VCI A Switch Payload Payload Dst VC1 = 1 1 C 3 R1 packet B 2 4 VC2 = 2 D 1 3 A R2 forwarding VC3 = 3 1 3 2 4 F 1 3 table: E R1 R4 Dst 2 4 2 4 Dst R2 B 1 R3 3 • When B establishes green virtual circuit, how does it 2 4 know what VC ids are available? Different paths to R1 VC table: • Even worse: every VC id may already be used on a same destination! VC 1 R2 link along the path to the destination (useful for traffic • Solution: VC id swapping VC 2 R3 engineering!) 10 VC id Swapping Connections and Signaling A 1 Switch VC1 1 C 1 2 • Permanent vs. switched virtual connections 2 2 B 2 3 1 3 VC2 1 12 4 2 D (PVC/SVC) 1 VC3 2 • PVCs last “a long time” F 3 E • E.g., connect two bank locations with a PVC or for traffic engineering Address Next Hop Next id • Look up is based on VC id in • PVCs administratively configured VC1 = 1 3 2 header + incoming port number • SVCs dynamically set up on a “per-call” basis • A bit more like a phone call VC2 = 2 3 3 • Forwarding table specifies • Significant signaling overhead for each connection outgoing port and new VC id VC3 = 1 4 1 • Topology VC4 = 2 3 1 • VC id conflicts can be resolved locally during signaling • point to point, point to multipoint, multipoint to multipoint 11 3 SVC Connection Setup Virtual Circuits In Practice calling network called party party • Older tech: Frame Relay • Only provided PVCs. Used for quasi-dedicated 56k/T1 links SETUP between offices, etc. • Asynchronous Transfer Mode - ATM: Telco SETUP approach CONNECT • Kitchen sink: voice, file transfer, video, etc. • Intended as IP replacement. That didn’t happen. :) CONNECT • Today: rarely used. • MPLS: The “IP Heads” answer to ATM CONNECT ACK • Stole good ideas from ATM CONNECT • Integrates well with IP ACK • Today: Used inside many transit networks to provide traffic engineering, VPN support, simplify core. Outline ATM Background • Circuit switching refresher • Telephone companies supported voice telephony: 4 kHz analog, 64 kbs digital • Virtual Circuits - general • They provided lines for data networking • Why virtual circuits? • ISDN: 64 kbps and faster channels • How virtual circuits? -- tag switching! • T1 (1.544 Mbps) • Two modern implementations • T3 (44.736 Mbps) • Wanted to become the primary service provider for • ATM - teleco-style virtual circuits data networking services • MPLS - IP-style virtual circuits • Internet traffic, voice, multimedia, .. • Virtual LANs • ATM is based on small, fixed sized cells • How do they differ? • Voice needs small “packets” – reduces latency • Efficient technology: simple buffer management, fast forwarding based on flat VCIDs • Initial version ran at 155 Mbs (versus 10 Mbs Ethernet) 4 ATM Features ATM Discussion • Fixed size cells (53 bytes) • Vision: ATM is a replacement for IP. • Could carry both traditional telephone traffic (constant bit rate • 5 byte header, 48 byte payload GFC VPI circuits) and other traffic (data, variable BR) VPI VCI • Virtual circuit technology using • Simple switching core: forwarding based on VC identifiers hierarchical virtual circuits (VP,VC). VCI • Better than IP, since it supports QoS, traffic engineering • Support for multiple traffic classes VCI PT CLP • But: the technology is very complex by adaptation layer. HEC • Signaling software is very complex • E.g. voice channels, data traffic • Supporting connection-less service model on connection-based technology is painful • Elaborate signaling stack. • Deploying ATM in LAN is complex (e.g. broadcast) • Backwards compatible with respect to the payload • With IP over ATM, a lot of functionality is replicated telephone standards • Reality: Too little benefit compare with IP, although • Standards defined by ATM Forum. traffic engineering benefits were attractive • Organization of manufacturers, providers, • Fast VCI lookup became less critical over time users MPLS – Multi-Protocol Label Switched Path (LSP) Label Switching • Bringing virtual circuit concept into IP • Simplex path through interior network • Packets forwarded by label-switched routers(LSR) • Driven by multiple forces • Performs LSP setup and MPLS packet forwarding • Traffic engineering • Label Edge Router (LER): LSP ingress or egress Layer 3 (IP) header • Transit Router: swaps MPLS label, forwards packet • High performance forwarding • VPN Layer 2 header • Quality of service New York Ingress Egress Layer 3 (IP) header San Francisco Transit Some MPLS slides from H. Zhang MPLS label Layer 2 header 5 MPLS Header Forwarding Equivalence Classes IP Packet FEC = “A subset of packets that are all treated the same way by a LSR” 32-bit MPLS Header Label CoS S TTL LSR LSR LER LER LSP • IP packet is encapsulated in MPLS header • Label IP1 IP1 • Class of service IP1 #L1 IP1 #L2 IP1 #L3 • Stacking bit: if next header is an MPLS header IP2 #L1 IP2 #L2 IP2 #L3 • Time to live: decremented at each LSR, or pass through IP2 IP2 • IP packet is restored at end of LSP by egress router • TTL is adjusted, transit LSP routers count towards the TTL Packets are destined for different address prefixes, but can be mapped to common path • MPLS is an optimization – does not affect IP semantics Establishing LSPs LSPs Driven by IP Routing #216 #14 #311 #99 #311 • Use a Label Distribution Protocol (LDP) to #963 #311 establish paths based on IP forwarding tables • Simple #963 • MPLS packets follow the same path as IP #14 #462 • But offers only limited benefits #612 • Explicitly route circuits #5 #99 #311 • More work • Provides finer grain control over how traffic is - A LSP is actually part of a tree from all sources to a distributed throughout the network destination (unidirectional). - A control protocol (e.g. Label Distribution Protocol, LDP) • Important tool for traffic engineering builds the tree based on the IP forwarding tables. 23 6 MPLS Builds on Standard IP Label Switched Path (LSP) Dest Out 47.1 1 Dest Out 47.2 2 Intf Label Dest Intf Label Intf Label Dest Intf 47.1 1 47.3 3 In In Out 47.2 2 In In Out Out 47.3 3 3 50 47.1 1 40 3 40 47.1 1 1 47.1 IP 47.1.1.11 47.1 1 2 Intf Dest Intf Label 3 3 3 In Out Out Dest Out 2 47.1 1 3 47.1 1 50 2 47.2 2 1 47.3 3 1 1 2 47.3 3 47.2 47.3 3 47.2 2 2 IP 47.1.1.1 Destination based forwarding tables as built by OSPF, IS-IS, RIP, etc. Explicityly Routed - ER-LSP Explicitly Routed LSP - Example Route= {A,B,C} Intf Label Dest Intf Label Intf Label Dest Intf In In Out Out In In Out #216 3 50 47.1 1 40 3 40 47.1 1 #14 #972 Intf Dest Intf Label AB In Out Out IP 47.1.1.11 47.1 3 47.1.1 2 33 3 3 3 47.1 1 50 #14 2 1 1 #972 C 2 47.3 3 47.2 2 #462 IP 47.1.1.1 ER-LSP follows route that source chooses.

View Full Text

Details

  • File Type
    pdf
  • Upload Time
    -
  • Content Languages
    English
  • Upload User
    Anonymous/Not logged-in
  • File Pages
    7 Page
  • File Size
    -

Download

Channel Download Status
Express Download Enable

Copyright

We respect the copyrights and intellectual property rights of all users. All uploaded documents are either original works of the uploader or authorized works of the rightful owners.

  • Not to be reproduced or distributed without explicit permission.
  • Not used for commercial purposes outside of approved use cases.
  • Not used to infringe on the rights of the original creators.
  • If you believe any content infringes your copyright, please contact us immediately.

Support

For help with questions, suggestions, or problems, please contact us