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Lecture 6 Overlay Networks

Lecture 6 Overlay Networks. CPE 401/601 Computer Network Systems. slides are modified from Jennifer Rexford. Overlay Networks. Overlay Networks. Focus at the application level. A. B. E. F. A. B. E. F. IP Tunneling to Build Overlay Links. IP tunnel is a virtual point-to-point link

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Lecture 6 Overlay Networks

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  1. Lecture 6Overlay Networks CPE 401/601 Computer Network Systems slides are modified from Jennifer Rexford

  2. Overlay Networks

  3. Overlay Networks Focus at the application level

  4. A B E F A B E F IP Tunneling to Build Overlay Links • IP tunnel is a virtual point-to-point link • Illusion of a direct link between two separated nodes • Encapsulation of the packet inside an IP datagram • Node B sends a packet to node E • … containing another packet as the payload tunnel Logical view: Physical view:

  5. Tunnels Between End Hosts B Src: A Dest: B Src: C Dest: B Src: A Dest: B A C Src: A Dest: C Src: A Dest: B

  6. Overlay Networks • A logical network built on top of a physical network • Overlay links are tunnels through the underlying network • Many logical networks may coexist at once • Over the same underlying network • And providing its own particular service • Nodes are often end hosts • Acting as intermediate nodes that forward traffic • Providing a service, such as access to files • Who controls the nodes providing service? • The party providing the service • Distributed collection of end users

  7. Overlays for Incremental Deployment

  8. Using Overlays to Evolve the Internet • Internet needs to evolve • IPv6 • Security • Mobility • Multicast • But, global change is hard • Coordination with many ASes • “Flag day” to deploy and enable the technology • Instead, better to incrementally deploy • And find ways to bridge deployment gaps

  9. Flow: X Src: A Dest: F data Flow: X Src: A Dest: F data Flow: X Src: A Dest: F data Flow: X Src: A Dest: F data D C F E B A F E B A Src:B Dest: E Src:B Dest: E 6Bone: Deploying IPv6 over IP4 tunnel Logical view: IPv6 IPv6 IPv6 IPv6 Physical view: IPv6 IPv6 IPv6 IPv6 IPv4 IPv4 A-to-B: IPv6 E-to-F: IPv6 B-to-C: IPv6 inside IPv4 B-to-C: IPv6 inside IPv4

  10. Secure Communication Over Insecure Links • Encrypt packets at entry and decrypt at exit • Eavesdropper cannot snoop the data • … or determine the real source and destination

  11. Communicating With Mobile Users • A mobile user changes locations frequently • So, the IP address of the machine changes often • The user wants applications to continue running • So, the change in IP address needs to be hidden • Solution: fixed gateway forwards packets • Gateway has a fixed IP address • … and keeps track of the mobile’s address changes www.cnn.com gateway

  12. IP Multicast • Multicast • Delivering the same data to many receivers • Avoiding sending the same data many times • IP multicast • Special addressing, forwarding, and routing schemes • Pretty complicated stuff unicast multicast

  13. MBone: Multicast Backbone • A catch-22 for deploying multicast • Router vendors wouldn’t support IP multicast • … since they weren’t sure anyone would use it • And, since it didn’t exist, nobody was using it • Idea: software implementing multicast protocols • And unicast tunnels to traverse non-participants

  14. Multicast Today • Mbone applications starting in early 1990s • Primarily video conferencing, but no longer operational • Still many challenges to deploying IP multicast • Security vulnerabilities, business models, … • Application-layer multicast is more prevalent • Tree of servers delivering the content • Collection of end hosts cooperating to delivery video • Some multicast within individual ASes • Financial sector: stock tickers • Within campuses or broadband networks: TV shows • Backbone networks: IPTV

  15. Case Study: Narada - End System Multicast

  16. Narada: End System Multicast Gatech Stanford Stan1 Stan2 CMU Berk1 Berk2 Berkeley Overlay Tree Stan1 Gatech Stan2 CMU Berk1 Berk2

  17. Performance Concerns Delay from CMU to Berk1 increases Stan1 Gatech Stan2 CMU Berk2 Berk1 Duplicate Packets: Bandwidth Wastage Gatech Stanford Stan1 Stan2 CMU Berk1 Berk2 Berkeley

  18. The delay between the source and receivers is small Ideally, the number of redundant packets on any physical link is low Heuristic: Every member in the tree has a small degree Degree chosen to reflect bandwidth of connection to Internet Overlay Tree CMU CMU CMU Stan2 Stan2 Stan2 Stan1 Stan1 Stan1 Gatech Gatech Berk1 Berk1 Berk1 Gatech Berk2 Berk2 Berk2 High latency High degree (unicast) “Efficient” overlay

  19. Stan1 Stan2 Berk1 Gatech Berk2 Overlay Trees • Source routed minimum spanning tree on mesh • Desired properties • Members have low degree • Small delays from source to receivers CMU Stan2 Stan1 Gatech Berk2 Berk1

  20. Case Study: Resilient Overlay Networks

  21. Premise: by building application overlay network, can increase performance and reliability of routing RON: Resilient Overlay Networks Princeton Yale application-layer router Two-hop (application-level) Berkeley-to-Princeton route Berkeley UNR

  22. RON Circumvents Policy Restrictions • IP routing depends on AS routing policies • But hosts may pick paths that circumvent policies USLEC ISP Patriot PU me My home computer

  23. RON Adapts to Network Conditions B • Start experiencing bad performance • Then, start forwarding through intermediate host A C

  24. RON Customizes to Applications B • VoIP traffic: low-latency path • Bulk transfer: high-bandwidth path voice A bulk transfer C

  25. How Does RON Work? • Keeping it small to avoid scaling problems • A few friends who want better service • Just for their communication with each other • E.g., VoIP, gaming, collaborative work, etc. • Send probes between each pair of hosts B A C

  26. How Does RON Work? • Exchange the results of the probes • Each host shares results with every other host • Essentially running a link-state protocol! • So, every host knows the performance properties • Forward through intermediate host when needed B B A C

  27. RON Works in Practice • Faster reaction to failure • RON reacts in a few seconds • BGP sometimes takes a few minutes • Single-hop indirect routing • No need to go through many intermediate hosts • Typically, an extra hop circumvents the problems • Better end-to-end paths • Circumventing routing policy restrictions • Sometimes the RON paths are actually shorter

  28. RON Limited to Small Deployments • Extra latency through intermediate hops • Software delays for packet forwarding • Propagation delay across the access link • Overhead on the intermediate node • Imposing CPU and I/O load on the host • Consuming bandwidth on the access link • Overhead for probing the virtual links • Bandwidth consumed by frequent probes • Trade-off between probe overhead and detection speed • Possibility of causing instability • Moving traffic in response to poor performance • May lead to congestion on the new paths

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