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**Models and techniques for verification of Software Defined Networks**

Victor Altukhov Eugene Chemeritsky Vladislav Podymov Vladimir Zakharov Applied Research Center for Computer Networks

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**Outline Introduction Software Defined Networks**

SDN Packet Forwarding Policies PFP Model Policy language FO[TC] Verifying monitor Vermont Experiments & Comparison

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**What is Software Defined Network?**

What is SDN? Conventional network Application Forwarding state Host Port Link Switch B A

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**What is SDN? … Conventional network B A Task How to forward a packet**

Application Topology … Forwarding state Packet Packet Packet B A

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**What is SDN? Conventional network decentralized control non-uniformity**

Application Forwarding state decentralized control non-uniformity App App FS FS B App App FS A FS

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**What is SDN? Conventional network SDN Controller centralized control**

decentralized control uniformity non-uniformity App App FS FS B App App FS A FS

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**What is SDN? SDN Controller centralized control uniformity**

Application centralized control uniformity Control plane OpenFlow Data plane FS FS B FS A FS

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**What is SDN? SDN Controller centralized control uniformity**

Application Upd Upd centralized control uniformity Control plane OpenFlow Ok, I can do it Don’t know what to do Data plane FS FS B FS A FS

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**What is Packet Forwarding Policy?**

What is PFP? What is Packet Forwarding Policy? Example: Requirements imposed on a network to guarantee that its behavior is safe correct secure …

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**What is PFP? Example: Reachability B A**

Packets from the host A will eventually reach the host B B A

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**What is PFP? Example: No topological loops B A**

Packets do not traverse the same switch twice B A

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**What is PFP? Example: Short routes only B A**

1 2 3 4 All hosts are reached in at most 3 hops B A

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**What is PFP? Static Timeline Why ? Hardware errors**

Software (application) errors We want to check if PFPs hold in a real SDN Static and consider PFPs Timeline w.r.t. to

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**How to check PFPs? Fast! Fast! ⊧ Policies Network M P Network model**

Formal specification Fast! ⊧ M P ~ 10μs Model checking

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**Packet state Switch #2 Port #1 Header #h2 Switch #1 Port #1 Switch #4**

B h1 h4 A Switch #4 Port #3 Header #h4

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**Packet state Switch #2 Port #1 Header #h2 Switch #w Port #p Header #h**

B A Switch #4 Port #3 Header #h4

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**Packet state S sizew sizep sizeh is the set of all packet states … 1**

Switch #w Port #p Header #h Switch #w Port #p Header #h … 1 sizew sizep sizeh S is the set of all packet states

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**Raw model (p1, h1) (p2, h2) (p, h) … (pk, hk) rule**

is an explicit description of key SDN components such as: (p1, h1) (p2, h2) rule (p, h) … (pk, hk)

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**Raw model table (p1, h1) (p2, h2) (p, h) … (pk, hk) rule rule rule**

is an explicit description of key SDN components such as: table (p1, h1) rule (p2, h2) rule (p, h) rule … rule (pk, hk) default

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**Raw model table table … (p1, h1) (p2, h2) … (p, h) … … (pk, hk) Switch**

is an explicit description of key SDN components such as: Switch … (p1, h1) table table (p2, h2) … (p, h) … … (pk, hk)

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Relational model Step ⊆ S x S In ⊆ S Out ⊆ S

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**Relational model Step In Out ⊆ S x S ⊆ S ⊆ S (x, y) BDD (x) BDD (x)**

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**PFP Specification Language: syntax**

Step In Out (x, y) (x) (x) Atoms: First order logic constructors: ⋁ & ⌝ ∀ ∃ x = y x = const State equalities: x.w = y.w x.w = const x.p = y.p x.p = const x.h = y.h x.h = const Closure constructors: + F (x, y) – transitive closure [i1, i2] F (x, y) – bounded transitive closure

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**PFP SL: semantics (Step, In, Out, …) F RF ⊆ S × … × S n times**

Given a relational model F (x1, …, xn) a PFP SL formula defines a relation RF ⊆ S × … × S n times How?

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**PFP SL: semantics (Step, In, Out, …) F RF Step In Out ⊆ S × … × S**

Given a relational model F (x1, …, xn) a PFP SL formula defines a relation RF ⊆ S × … × S n times How? Step In (x, y) (x) Obvious Out (x) … = …

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**PFP SL: semantics (Step, In, Out, …) F RF F1 F2 F1 F2 F ⊆ S × … × S**

Given a relational model F (x1, …, xn) a PFP SL formula defines a relation RF ⊆ S × … × S n times F1 F2 (…) (…) How? ⋁ Union F1 F2 (…) (…) & Intersection F ⌝ (…) Complement

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**PFP SL: semantics (Step, In, Out, …) F RF F F ⊆ S × … × S n times**

Given a relational model F (x1, …, xn) a PFP SL formula defines a relation RF ⊆ S × … × S n times How? F ∀ x (…) Universal projection F ∃ x (…) Existential projection

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**PFP SL: semantics (Step, In, Out, …) F RF F F ⊆ S × … × S n times**

Given a relational model F (x1, …, xn) a PFP SL formula defines a relation RF ⊆ S × … × S n times + How? F (x, y) Transitive closure [i1, i2] F (x, y) Bounded transitive closure

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**PFP SL: examples A B Step In Step Step In Out Step Step Reachability y**

∀ x (x) ∃ (y) Step * & (x, y) No topological loops In Step * ⌝ ∃ x,y,z (x) & (x, y) & + Step (y, z) & y.w = z.w Short routes only In Out ⌝ ∃ x,y (x) & (y) & + Step Step [1, 3] (x, y) ⌝ & (x, y)

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**What else? adequate Model update Model Model Network**

continuously changes adequate Model should be at every instant We should be able to update Model on-line Model The update rate for Network should surpass the update rate for We can do it not discussed (to some extent)

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**How does it work? Checker Controller Loader Proxy Network**

Main usage now: Checker Proxy Network

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**We tested it for Stanford University Network 16 switches**

Fat Tree topology 48 tables forw. rules 1500 ACL rules >100 VLAN

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**(strict superset of others)**

Tool comparison Tool Build (ms.) Update Policies OpenFlow concepts VERMONT (2014) 4600 FO[TC] (strict superset of others) Full NetPlumber (2013) 37000 CTL Partial VeriFlow (2013) > 4000 68-100 Small fixed set Minimal AP Verifier (2013) 1000 0.1 FlowChecker (2010) Anteater (2011) 400000 ??? No

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The End Me:

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