Programmable Networks with Synthesis

Transcription

1 Programmable Networks with Synthesis Ahmed ElHassany Petar Tsankov Laurent Vanbever Martin Vechev

2 Network Misconfigurations are Common

3 What Example Makes Network Configuration Hard? Low-level, local router configurations High-level, global routing B requirements C Network N2 Network N1 A D Network N3

4 Example OSPF + Static routes B C Network N2 Network N1 A D Network N3 R1: Packets from N1 to N2 must follow the path A D R2: Packets from N1 to N3 must follow the path A B C D

5 Example OSPF link cost B C Network N1 A D Network N2 Network N3 R1: Packets from N1 to N2 must follow the path A D R2: Packets from N1 to N3 must follow the path A B C D

6 Example B C Network N1 A Packets to N3 not forwarded correctly D Network N2 Network N3 R1: Packets from N1 to N2 must follow the path A D R2: Packets from N1 to N3 must follow the path A B C D

7 Example Static routes table Network NextHop N3 B B C Add a static route to A configuration Network N1 A D Configure A to prefer static routes over OSPF Network N2 Network N3 R1: Packets from N1 to N2 must follow the path A D R2: Packets from N1 to N3 must follow the path A B C D

8 Example B non-deterministically forwards packets for N3 to either A or C Static routes table Network NextHop N3 B B C Network N1 A D Network N2 Network N3 R1: Packets from N1 to N2 must follow the path A D R2: Packets from N1 to N3 must follow the path A B C D

9 Example Lower cost to 5 Static routes table Network NextHop N3 B B 5 C Network N1 A D Network N2 Network N3 R1: Packets from N1 to N2 must follow the path A D R2: Packets from N1 to N3 must follow the path A B C D

10 Example Static routes table 5 Network NextHop N3 B B Lower cost to 5 Router configurations must be such that: Network N2 1. A prefers static routes over OSPF 2. A has a static route to B for N3 3. A D N2 must A be lowest cost D from A to N2 Network 4. N1 B C D N3 must be lowest cost from B to N3 Network N3 C R1: Packets from N1 to N2 must follow the path A D R2: Packets from N1 to N3 must follow the path A B C D

11 Current Practice Initially not configured Network topology Routing requirements Operators manually configure each router All routers are configured

12 Current Practice Initially not configured Network topology Routing requirements Problems and Challenges Operators manually configure each router Diversity in protocol expressiveness Protocol dependencies No correctness guarantees All routers are configured

13 Wanted: Programmable Networks with Synthesis Network topology Routing requirements Operators manually configure each router

14 Wanted: Programmable Networks with Synthesis Network topology Routing requirements Operators Automatically manually configure configure routers with each synthesis router

15 Wanted: Programmable Networks with Synthesis Network topology Routing requirements How is the behavior of routers captured? Operators Automatically manually configure configure routers with each synthesis router How to find a configuration that conforms to the requirements? All routers are configured

16 Programmable Networks with Synthesis: Dimensions Deployment scenarios Datacenter Incremental ISP Enterprise Routing protocols Deterministic OSPF BGP Static routes MPLS Probabilistic ECMP Gossip Requirements Paths Isolation Reachability Waypointing Failures Congestion Synthesis Techniques Enumerative learning Probabilistic Symbolic execution (SyNET*) CEGIS Constraint solving *SyNET:

17 Capturing Network Behavior Key idea: Express routing protocols, along with their dependencies, in stratified Datalog

18 Datalog (Graph Reachability) Input link(n1, a) link(a, b) Program path(x, Y) link(x, Y) path X, Y link X, Z, path(z, Y) N1 B A C D N2 N3 Query path(n1, n2)? Can we capture the network s forwarding plane?

19 Datalog (Shortest-path Routing) Input Add OSPF cost to links link(nn 4, a, ) Program path Router, Net, Net, Cost link Router, Net, Cost path Router, Net, NextHop, C 4 + C B link Router, NextHop, C 4, path NextHop, Net, X, C B sp Router, Net, NextHop, min C path Router, Net, NextHop, C fwd Router, Net, NextHop sp Router, Net, NextHop, C Query fwd(a, a, n B,? ) Captures the router configuration Checks a property on the forwarding plane N1 Captures how routers compute their forwarding entries using routing protocols B A 5 C D N2 N3

20 Routing Requirements Paths Packets for traffic class TC must follow the path fwd r 4, tc, r B fwd r KN4, tc, r K r 4 r K

21 Routing Requirements Paths Packets for traffic class TC must follow the path r 4 r K fwd r 4, tc, r B fwd r KN4, tc, r K Traffic isolation The paths for two distinct traffic classes tc 4 and tc B do not share links in the same direction R 4, R B. fwd R 4, tc 4, R B fwd R 4, tc B, R B

22 Routing Requirements Paths Packets for traffic class TC must follow the path r 4 r K fwd r 4, tc, r B fwd r KN4, tc, r K Traffic isolation The paths for two distinct traffic classes tc 4 and tc B do not share links in the same direction R 4, R B. fwd R 4, tc 4, R B fwd R 4, tc B, R B Reachability Packets for traffic class tc can reach router r B from router r 4 reach(r 4, tc, r B )

23 Routing Requirements Paths Packets for traffic class TC must follow the path r 4 r K fwd r 4, tc, r B fwd r KN4, tc, r K Traffic isolation The paths for two distinct traffic classes tc 4 and tc B do not share links in the same direction R 4, R B. fwd R 4, tc 4, R B fwd R 4, tc B, R B Reachability Packets for traffic class tc can reach router r B from router r 4 reach(r 4, tc, r B ) Loop-freeness The forwarding plane has no loops TC, R. reach(r, TC, R)

24 Network-wide Configuration Synthesis

25 Network-wide Configuration Synthesis Network specification N (OSPF, BGP, MPLS, ) Routing requirements R (isolation, reachability, reliability) Datalog program P Datalog query Q Synthesis problem: Find a configuration C such that N configured with C satisfies R (Input) Synthesis problem: Find an input I such that P, I Q Network-wide configuration C (protocol configurations for routers) Datalog input I

26 Network-wide Configuration Synthesis Network specification N (OSPF, BGP, MPLS, ) Routing requirements R (isolation, reachability, reliability) Datalog program P Datalog query Q Problems: Synthesis problem: No input synthesis tools (Input) for Datalog Synthesis problem: Find a configuration C such that N Problem is undecidablefind an input I such that P, I Q configured with C satisfies R Network-wide configuration C (protocol configurations for routers) Datalog input I

27 Input Synthesis for Datalog Key idea: Reduce to solving logical constraints

28 Input Synthesis for Datalog (via Constraint Solving) Datalog program P Datalog query Q path X, Y link X, Y path X, Y link X, Z, path(z, Y) X, Y. path 4 X, Y link X, Y X, Y. path B X, Y path B a, c link a, c Generate constraints path a, c link a, c link X, Y Z. (link X, Z path 4 Z, Y Constraint solving Bounded unrolling for recursive rules Constraints ψ link a, b, link b, c, path 4 a, b, path 4 b, c path B a, b, path B b, c, path B a, c link a, b, link b, c Derive input Model M ψ Datalog input I

29 Input Synthesis for Datalog (via Constraint Solving) Datalog program P path X, Y link X, Y path X, Y link X, Z, path(z, Y) X, Y. path 4 X, Y link X, Y X, Y. path B X, Y link a, b, link b, c Datalog query Q path a, c link a, c link X, Y Z. (link X, Z path 4 Z, Y path B a, c link a, c Theorem: M ψ if Constraint and only solving if P, I Q link a, b, link b, c, path 4 a, b, path 4 b, c path B a, b, path B b, c, path B a, c Generate constraints Derive input Bounded unrolling for recursive rules Constraints ψ Model M ψ Datalog input I

30 Generating Constraints link(n1, A,? ) path Router, Net, Net, Cost link Router, Net, Cost path Router, Net, NextHop, C 4 + C B link Router, NextHop, C 4, path NextHop, Net, X, C B X, Y. path 4 X, Y link X, Y X, Y. path B X, Y link X, Y Z. (link X, Z path 4 Z, Y path B a, c link a, c sp Router, Net, NextHop, min C path Router, Net, NextHop, C fwd Router, Net, NextHop sp Router, Net, NextHop, C fwd a, n B, d ^ fwd a, n`, b ^ 3030

31 Generating Constraints link(n1, A,? ) path Router, Net, Net, Cost link Router, Net, Cost path Router, Net, NextHop, C 4 + C B link Router, NextHop, C 4, path NextHop, Net, X, C B Input Box 1 sp Router, Net, NextHop, min C path Router, Net, NextHop, C Box 2 fwd Router, Net, NextHop sp Router, Net, NextHop, C Box 3 fwd a, n B, d ^ fwd a, n`, b ^ Requirements 31

32 Synthesis Algorithm (Naïve Approach) Input I 1 Box 1 (ψ 1 ) I 2 Box 2 (ψ 2 ) I 3 Box 3 (ψ 3 ) M 1 ψ 1 ^ I 2 M 2 ψ 2 ^ I 3 M 3 ψ 3 ^ Reqs Requirements 32

33 Synthesis Algorithm (Naïve Approach) Box 1 Naïve Box 2 approach Box 3 does NOT Box 4 work Box 5 1. Not all valid SMT models are valid network configurations. 2. The search space of valid inputs is huge. M box5 ψ 33

34 Synthesis Algorithm (Our Approach) Input Constraints Ensure the synthesized configurations are valid Ex. OSPF Link costs are positive numbers 34

35 Synthesis Algorithm (Our Approach) Input Constraints Ensure the synthesized configurations are valid Ex. OSPF Link costs are positive numbers Domain specific constraints Reduce the search space Ex. Routers only forward to neighbours 35

36 Synthesis Algorithm (Our Approach) Input Constraints Ensure the synthesized configurations are valid Ex. OSPF Link costs are positive numbers Domain specific constraints Reduce the search space Learn from mistakes Don t reuse the same invalid input Ex. Routers only forward to neighbours M boxi ψ ^ I i+1 ^ I`i 29

37 Synthesis Algorithm (Our Approach) Input Constraints Ensure the synthesized configurations are valid Ex. OSPF Link costs are positive numbers Domain specific constraints Reduce the search space Learn from mistakes Don t reuse the same invalid input Ex. Routers only forward to neighbours M boxi ψ ^ I i+1 ^ I`i Partial evaluation Don t compute already known values 30

38 Network-wide Configuration Synthesis Network topology Constraints on the forwarding plane computed by the routers Routing requirements Encode as a Datalog program Automatically configure routers with synthesis Via reduction to input synthesis for Datalog Datalog input identifies correct configuration

39 Implementation

40 Implementation The SyNET system ( Written in Python ( 4K lines of code) Protocols encoded in stratified Datalog ( 0 rules) Uses the Z3 constraint solver Outputs CISCO configurations Supports BGP, OSPF, and static routes! A snippet from router A interface f0/1 ip address ip ospf cost description "To B" interface f0/0 ip address ip ospf cost description "To C" interface f1/0 ip address ip ospf cost description "To D"! CISCO configuration output by SyNET

41 Experiments

42 Experiment US-based network connecting major universities and research institutes Protocols / # Traffic classes 1 class 5 classes classes Static 1.3s 2.0s 4.0s Static + OSPF 9.0s 21.3s 49.3s Static + OSPF + BGP 13.3s 22.7s 1m19.7s Synthesis Times

43 Scalability Experiment Grid topologies with up to 64 routers Requirements for traffic classes Synthesis Time [s] < 24h for Static + OSPF Static Static + OSPF Static + OSPF + BGP < 1h for static routes # of routers 36

44 Summary: Programmable Networks with Synthesis High-level, global routing requirements B Low-level, local router configurations Global requirements vs local configurations Network-wide configuration synthesis Approach scales to realistic problems