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Lecture 8 Network Verification

  • Network outages (停电 / 宕机) are common
  • Most network outages and performance issues result from misconfiguration
  • Most are Human Factors

Software Defined Network (SDN)

  1. Simplify the management and controlling of Networks
  2. Bug: Controller / Applications
  3. Some local problems may influence "total" Networks

Data Plane Verification

  1. Physical / Virtual Network (data / config table ......)
  2. Take a snapshot of the network
  3. Network Model

What to Verify

  • Pairwise Reachability（可达性）
  • Loop Freedom（环路检测）
  • Blackhole Freedom（数据包陷进黑洞）
  • Traffic Isolation（流量隔离性）

Header Space Analysis

  1. Model a packet (header) as a point in \(\{0,1\}^L\) space, i.e., the header space
    1. Flat space, Protocol oblivious
    2. Wildcard Expression(通配符表达式) (e.g., IP prefix) -> hyper cubes
  2. Model all networking boxes as transformers of header space
    1. Output = \(f (Match, Action)\)
    2. Transfer Function: \(T: (h,p) => \{(h_1, p_1),...,(h_n, p_n)\}\)

Theorems

  • Composition Theorem
    • Network Behavior = Composition of transfer functions
    • e.g., \(T_3(T_2(T_1(h,p)))\)
  • Inversion Theorem
    • given header h at destination p, we can invert to find (h’,p’): headers sent at source s’ to produce (h,p)

HSA Algebra

Bit by bit intersect using intersection (路口) table:

Case 1: Computing Reachability

Case 2: Checking Isolation

Problems with HSA

  • Only check a snapchat of network configuration
  • Networks are dynamically changing
  • What if a new rule is inserted?
    • checking the entire network’s state every time a new flow is wasteful and slow

What's VeriFlow?

Verifying Network Wide Invariants in Real Time

Controller set new rule:

  • Generate Equivalence Classes
  • Generate Forwarding Graphs
  • Run Queries

Challenging: Huge space of headers, impossible to enumerate

Step1: Generate Equivalence Classes

  • Equivalence Class（等价类）
    • packets experiencing the same forwarding actions throughout the network
    • 一个等价类，不管大小多少，其内部的转发行为是一模一样的
  • Data Structure for EC computation
    • multidimensional Prefix Tree ( Trie-Tree )
    • each branch in a node is "0/1/*", we don't care "wildcard"
    • (device, rule) pairs
    • the tree represents the "total" Network

  • C在AB的右侧，因为C的"匹配域"更长，说明"*"的含量更高，因此偏右！
  • A / B / C 可能并不是一个交换机上的规则

Step2: Generate Forwarding Graphs

  • Generate a forwarding graph for each EC
    • Define how packets within that EC will be forwarded through the network
    • Node: device, edge: forwarding rule
    • The graph will be "one-direction" between two nodes
    • Each node represents one EC
  • Let the EC traverse the Trie for the second time

source code

Step3: Run Queries (Verifying)

Check whether the forwarding graph of each EC satisfies

  • Reachability
  • Loop-freedom
  • Blackhole-freedom

Analysis: Inefficiency of VeriFlow

  1. High -> Low Priority

  1. Matching

上面可以很清晰地看出一个问题：在不同的等价类中，路径有很多重叠部分

Delta-net

Rather than re-computing forwarding graphs, it incrementally maintains a single edge-labelled graph! represents all packet flows.

操作原理及其示范：

  • 在上面的优先级更高
  • alpha2,3,4 从 r1 边 移动到 r4 边
  • 表示转发行为的变化（Graph Transformation）

Details: encode destination IP with ranges, binary search tree for rule insertion

Limitations

  • Only considers a single dimention, i.e., destination IP
  • Only works for IP ranges, e.g., IP prefixes
  • The # of atoms os not minimum