1 Network-wide Decision Making: Toward a Wafer-thin Control Plane Jennifer Rexford, Albert Greenberg, Gisli Hjalmtysson ATT Labs Research David A. Maltz, Andy Myers, Geoffrey Xie, Jibin Zhan, Hui Zhang Carnegie Mellon University

2 A Well-Studied Architecture Question Smart hosts, dumb network Network moves IP packets between hosts Services implemented on hosts Keep state at the edges Edge Network à IP ! How to partition function vertically?

3 Inside a Single Network Data Plane Distributed routers Forwarding, filtering, queueing Based on FIB or labels Management Plane Figure out what is happening in network Decide how to change it Shell scripts Traffic Eng Databases Planning tools OSPF SNMPnetflowmodems Configs OSPF BGP Link metrics OSPF BGP OSPF BGP Control Plane Multiple routing processes on each router Each router with different configuration program Huge number of control knobs: metrics, ACLs, policy FIB Routing policies Packet filters

4 Inside a Single Network Data Plane Distributed routers Forwarding, filtering, queueing Based on FIB or labels Management Plane Figure out what is happening in network Decide how to change it Shell scripts Traffic Eng Databases Planning tools OSPF SNMPnetflowmodems Configs OSPF BGP Link metrics OSPF BGP OSPF BGP Control Plane Multiple routing processes on each router Each router with different configuration program Huge number of control knobs: metrics, ACLs, policy FIB Routing policies Packet filters State everywhere! Dynamic state in FIBs Configured state in settings, policies, packet filters Programmed state in magic constants, timers Many dependencies between bits of state State updated in uncoordinated, decentralized way!

5 Example – Traffic Engineering - 1 Routers make uncoordinated changes to their routes Poor stability, traffic thrashing Network-wide view needed for a network-wide goal Load sensitive routing OSPF distributes load info Paths computed to avoid hotspots Management Plane Control Plane Data Plane

6 Example – Traffic Engineering - 2 Must predict & undo effects of control plane Must translate solution into settings of control plane knobs Need ability to express desired solution Route planning Learn topology Estimate traffic matrix Compute OSPF weights Reconfigure routers Management Plane Control Plane Data Plane OSPF Load info

7 An Architecture Question to Study How should the functionality that controls a network be divided up? Important: everyone hates net outages Practical: solutions can be implemented without changing IP or end-hosts Relevant: trends toward separating decision-making from forwarding Unsolved: problem is not solved by running BGP/OSPF on faster servers

8 Our Proposal: Dissemination and Decision Planes What functions require a view of entire network and network objectives? Path selection and traffic engineering Reachability control and VPNs ! Decision plane What functions must be on every router to support creation of a network-wide view? Topology discovery Report measurements, status, resources Install state (e.g., FIBs, ACLs) into data-plane ! Dissemination plane

9 Good Abstractions Reduce Complexity All decision making logic lifted out of control plane Eliminates duplicate logic in management plane Dissemination plane provides a control channel to/from data plane Management Plane Control Plane Data Plane Decision Plane Dissemination Data Plane Configs FIBs, ACLs

10 Many Implementations Possible Decision Plane Centralized, or Distributed Dissemination Plane In-band, or Out-of-band Choice based on reliability requirements Data plane evolution should be driven by needs of decision and dissemination planes

11 Example – Traffic Engineering Reprise Network-wide view provided by Dissemination Plane All policy, goals, decision logic located in Decision Plane Consistent network-wide solution constructed Decision plane can directly express desired solution Decision Plane Dissemination Plane Data Plane Traffic Matrix Path Computation Topology Load info FIBs

12 Example – Traffic Isolation Prevent some hosts/apps from communicating with others Routing policy is very coarse grained Packet filters are very expensive in the data plane Missing filters can allow packets to leak, violating isolation Reachability control Create routing design Configure routing protocols Add packet filters to patch holes where needed Management Plane Control Plane Data Plane Route attrs

13 Example – Traffic Isolation Reprise Reachability matrix directly expresses intended goal Path computation can jointly optimize traffic load and obey reachability constraints Packet filters installed only where needed, and changed whenever routing changes Decision Plane Dissemination Plane Data Plane Traffic Matrix Path Computation Topology Load info FIBs, ACLs Reachability matrix

14 Challenges Scalability for a single network Back-of-the-envelope calculations ! no show-stoppers Responsiveness Reacting to unplanned failure takes an extra ms; OSPF/iBGP reconvergence today measured in seconds Preplanning for failure is easier with network-wide view Coordination Minimize by having single active decision engine Leverage distributed computing work

Challenges Dissemination plane robustness Must survive failures of links, but be less complicated than the routing protocols it tries to replace Hierarchy How do two Decision Planes inter-network? How is the boundary of the Dissemination Plane defined? 14.25

Related Work Separation of forwarding elements and control elements –IETF: FORCES, GSMP, GMPLS –SoftRouter [Lakshman] Driving network operation from network-wide views –Traffic Engineering, Traffic Matrix computation Centralization of decision making logic –RCP [Feamster], PCE [Farrel] –SS7 [Ma Bell] 14.75

Summary How to partition functionality inside the systems that control a network? Dissemination and Decision Planes Power of solution comes from: Locating all decision making in one plane Providing that plane with network-wide views Directly write forwarding state to data plane Benefits Network-wide views Focus on network issues, less on distributed protocols Coordinated state updates ! better reliability 15