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

Goals of Today’s Lecture r Motivations for overlay networks m Incremental deployment of new protocols m Customized routing and forwarding solutions r Overlays for partial deployments m 6Bone, Mbone, security, mobility, … r Resilient Overlay Network (RON) m Adaptive routing through intermediate node r Distributed Hash Table (DHT) m Overlay for look-up of pairs 2

Overlay Networks 3

4

5 Focus at the application level

IP Tunneling to Build Overlay Links r IP tunnel is a virtual point-to-point link m Illusion of a direct link between two separated nodes r Encapsulation of the packet inside an IP datagram m Node B sends a packet to node E m … containing another packet as the payload 6 A B E F tunnel Logical view: Physical view: A B E F

Tunnels Between End Hosts 7 A C B Src: A Dest: B Src: A Dest: B Src: A Dest: C Src: A Dest: B Src: C Dest: B

Overlay Networks r A logical network built on top of a physical network m Overlay links are tunnels through the underlying network r Many logical networks may coexist at once m Over the same underlying network m And providing its own particular service r Nodes are often end hosts m Acting as intermediate nodes that forward traffic m Providing a service, such as access to files r Who controls the nodes providing service? m The party providing the service m Distributed collection of end users 8

Overlays for Incremental Deployment 9

Using Overlays to Evolve the Internet r Internet needs to evolve m IPv6 m Security m Mobility m Multicast r But, global change is hard m Coordination with many ASes m “Flag day” to deploy and enable the technology r Instead, better to incrementally deploy m And find ways to bridge deployment gaps 10

6Bone: Deploying IPv6 over IP4 11 A B E F IPv6 tunnel Logical view: Physical view: A B E F IPv6 C D IPv4 Flow: X Src: A Dest: F data Flow: X Src: A Dest: F data Flow: X Src: A Dest: F data Src:B Dest: E Flow: X Src: A Dest: F data Src:B Dest: E A-to-B: IPv6 E-to-F: IPv6 B-to-C: IPv6 inside IPv4 B-to-C: IPv6 inside IPv4

Secure Communication Over Insecure Links r Encrypt packets at entry and decrypt at exit r Eavesdropper cannot snoop the data r … or determine the real source and destination 12

Communicating With Mobile Users r A mobile user changes locations frequently m So, the IP address of the machine changes often r The user wants applications to continue running m So, the change in IP address needs to be hidden r Solution: fixed gateway forwards packets m Gateway has a fixed IP address m … and keeps track of the mobile’s address changes 13 gateway

IP Multicast r Multicast m Delivering the same data to many receivers m Avoiding sending the same data many times r IP multicast m Special addressing, forwarding, and routing schemes m Pretty complicated stuff (see Section 4.4) 14 unicastmulticast

MBone: Multicast Backbone r A catch-22 for deploying multicast m Router vendors wouldn’t support IP multicast m … since they weren’t sure anyone would use it m And, since it didn’t exist, nobody was using it r Idea: software implementing multicast protocols m And unicast tunnels to traverse non-participants 15

Multicast Today r Mbone applications starting in early 1990s m Primarily video conferencing, but no longer operational r Still many challenges to deploying IP multicast m Security vulnerabilities, business models, … r Application-layer multicast is more prevalent m Tree of servers delivering the content m Collection of end hosts cooperating to delivery video r Some multicast within individual ASes m Financial sector: stock tickers m Within campuses or broadband networks: TV shows m Backbone networks: IPTV 16

Case Study: Resilient Overlay Networks 17

RON: Resilient Overlay Networks 18 Premise: by building application overlay network, can increase performance and reliability of routing Two-hop (application-level) Berkeley-to-Princeton route application-layer router Princeton Yale Berkeley UNR

RON Circumvents Policy Restrictions r IP routing depends on AS routing policies m But hosts may pick paths that circumvent policies 19 USLEC PU Patriot ISP me My home computer

RON Adapts to Network Conditions r Start experiencing bad performance m Then, start forwarding through intermediate host 20 A C B

RON Customizes to Applications r VoIP traffic: low-latency path r Bulk transfer: high-bandwidth path 21 A C B voice bulk transfer

How Does RON Work? r Keeping it small to avoid scaling problems m A few friends who want better service m Just for their communication with each other m E.g., VoIP, gaming, collaborative work, etc. r Send probes between each pair of hosts 22 A C B

How Does RON Work? r Exchange the results of the probes m Each host shares results with every other host m Essentially running a link-state protocol! m So, every host knows the performance properties r Forward through intermediate host when needed 23 A C B B

RON Works in Practice r Faster reaction to failure m RON reacts in a few seconds m BGP sometimes takes a few minutes r Single-hop indirect routing m No need to go through many intermediate hosts m One extra hop circumvents the problems r Better end-to-end paths m Circumventing routing policy restrictions m Sometimes the RON paths are actually shorter 24

RON Limited to Small Deployments r Extra latency through intermediate hops m Software delays for packet forwarding m Propagation delay across the access link r Overhead on the intermediate node m Imposing CPU and I/O load on the host m Consuming bandwidth on the access link r Overhead for probing the virtual links m Bandwidth consumed by frequent probes m Trade-off between probe overhead and detection speed r Possibility of causing instability m Moving traffic in response to poor performance m May lead to congestion on the new paths 25