Data plane algorithms in routers

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Data plane algorithms in routers

• From prefix lookup to deep packet inspection
• Cristian Estan, University of Wisconsin-Madison

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What is the data plane?

• The part of the router handling the traffic
• Data plane algorithms applied to every packet
• Successive packets typically treated
  independently
• Example deciding on which link to send a packet
• Throughput defined as number of packets or bytes
  handled per second is very important
• Line speed keeping up with the rate at which
  traffic can be transmitted over the wire or fiber
• Example 10Gbps router has 32 ns to handle 40
  byte packet
• Memory usage limited by technology and costs
• Can afford at most tens of megabits of fast
  on-chip memory

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A generic data plane problem

• Router has many directives composed of a guard,
  and an associated action (all guards distinct)
• There is a simple procedure for testing how well
  a guard matches a packet
• For each packet, find the guard that matches
  best and take the associated action
• Example routing table lookup
• Each guard is an IP prefix (between 0 and 32
  bits)
• Matching procedure is the guard a prefix of the
  32 bit destination IP address
• Best defined as longest matching prefix

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The rules of the game

• Matching against all guards in sequence is too
  slow
• We build a data structure that captures the
  semantics of all guards and use it for matching
• Primary metrics
• How fast the matching algorithm is
• How much memory the data structure needs
• Time to build data structure also has some
  importance
• We can cheat (but we wont today) by
• Using binary or ternary content-addressable
  memories
• Using other forms of hardware support

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Measuring algorithm complexity

• Execution cost measured in number of memory
  accesses to read data structure
• Actual data manipulation operations typically
  very simple
• On some platforms we can read wide words
• Worst case performance most important
• Worst case defined with respect to input, not
  guards
• Caching has been proven ineffective for many
  settings
• Using algorithms with good amortized complexity,
  but bad worst case requires large buffers

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Overview

• Longest matching prefix
• Trie-based algorithms
• Uni-bit and multi-bit tries (fixed stride and
  variable stride)
• Leaf pushing
• Bitmap compression of multi-bit trie nodes
• Tree bitmap representation for multi-bit trie
  nodes
• Binary search on ranges
• Binary search on prefix lengths
• Classification on multiple fields
• Signature matching

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Longest matching prefix

• Used in routing table lookup (a.k.a. forwarding)
  for finding the link on which to send a packet
• Guard a bit string of 0 to w bits called IP
  prefix
• Action a single byte interface identifier
• Input a w-bit string representing the
  destination IP address of the packet (w is 32 for
  IPv4,128 for IPv6)
• Output the interface associated with the longest
  guard matching the input
• Size of problem hundreds of thousands of prefixes

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Controlled prefix expansion with stride 3
Leaf pushing
Multi-bit trie with fixed stride
Routing table
Multi-bit trie with variable stride
Leaf pushing reduces memory usage but increases
update time
Uni-bit trie
Given a maximum trie height h and a routing table
of size n dynamic programming algorithm computes
optimal variable stride trie in O(nw2h)
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Controlled prefix expansion with stride 3
Leaf pushing
Multi-bit trie with fixed stride
Routing table
Input
Multi-bit trie with variable stride
Leaf pushing reduces memory usage but increases
update time
Longest matching prefix
Uni-bit trie
Given a maximum trie height h and a routing table
of size n dynamic programming algorithm computes
optimal variable stride trie in O(nw2h)
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Controlled prefix expansion with stride 3
Leaf pushing
Multi-bit trie with fixed stride
Routing table
Input
Multi-bit trie with variable stride
Leaf pushing reduces memory usage but increases
update time
Longest matching prefix
Uni-bit trie
Given a maximum trie height h and a routing table
of size n dynamic programming algorithm computes
optimal variable stride trie in O(nw2h)
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Lulea bitmap compression
Bitmap supporting fast counting
Compressed node
When the compression bitmaps are large it is
expensive to count bits during lookup. The bitmap
is divided into chunks and a pre-computed
auxiliary array stores the number of bits set
before each chunk. The lookup algorithm needs to
count only bits set within one chunk.
Repeating entries are stored only once in the
compressed array. An auxiliary bitmap is needed
to find the right entry in the compressed node.
It stores a 0 for positions that do not differ
from the previous one.
Input
Representing node as tree bitmap
Pointers to children and prefixes are stored in
separate structures. Prefixes of all lengths are
stored, thus leaf pushing is not needed and
update is fast. Bitmaps have 1s corresponding to
entries that are not empty.
Longest matching prefix
13013
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Binary search on ranges

• Divide w-bit address space into maximal
  continuous ranges covered by same prefix
• Build array or balanced (binary) search tree with
  boundaries of ranges
• At lookup time perform O(log(n)) search
• Not better than multi-bit tries with compression,
  but it is not covered by patents

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Binary search on prefix lengths

• Core idea for each prefix length represented in
  the routing table, have a hash table with the
  prefixes
• Can find longest matching prefix after looking up
  in each hash table the prefix of the address with
  corresponding length
• Binary search on prefix lengths is faster
• Simple but wrong algorithm if you find prefix at
  length x store it as best match and look for
  longer matching prefixes, otherwise look for
  shorter prefixes
• Problem what if there is both a shorter and a
  longer prefix, but no prefix at length x?
• Solution insert marker at length x when there
  are longer prefixes. Must store with marker
  longest matching shorter prefix. Markers lead to
  moderate increase in memory usage.
• Promising algorithm for IPv6 (w128)

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Papers on longest matching prefix

• G. Varghese Network algorithmics an
  interdisciplinary approach to designing fast
  networked devices, chapter 11, Morgan Kaufmann
  2005
• V. Srinivasan, G. Varghese Faster IP lookups
  using controlled prefix expansion, ACM Trans. on
  Comp. Sys., Feb. 1999
• M. Degermark, A. Brodnik, S. Carlsson, S. Pink
  Small forwarding tables for fast routing
  lookups, ACM SIGCOMM, 1997
• W. Eatherton, Z. Dittia, G. Varghese Tree Bitmap
  Hardware / Software IP Lookups with Incremental
  Updates, http//www-cse.ucsd.edu/varghese/PAPERS
  /willpaper.pdf
• B. Lampson, V. Srinivasan, G. Varghese IP
  lookups using multiway and multicolumn search,
  IEEE Infocom, 1998
• M. Waldvogel, G. Varghese, J. Turner, B.
  Plattner, Scalable high-speed IP lookups, ACM
  Trans. on Comp. Sys., Nov. 2001

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Overview

• Longest matching prefix
• Classification on multiple fields
• Solution for two-dimensional case grid of tries
• Bit vector linear search
• Cross-producting
• Decision tree approaches
• Signature matching

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Packet classification problem

• Required for security, recognizing packets with
  quality of service requirements
• Guard prefixes or ranges for k header fields
• Typically source and destination prefix, source
  and destination port range, and exact value or
  for protocol
• All fields must match for rule to apply
• Action drop, forward, map to a certain traffic
  class
• Input a tuple with the values of the k header
  fields
• Output the action associated with the first rule
  that matches the packet (rules are strictly
  ordered)
• Size of problem thousands of classification rules

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Example of classification rule set
External time server TO
Router that filters traffic
Mail gateway M
Internet
Net
Internal time server TI
Secondary name server S
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A geometric view of packet classification
R1
R1
R3
R3
Destination address space
R2
R2
Source address space
Source address space

• In theory number of regions defined can be much
  larger than number of rules
• Any algorithm that guarantees O(n) space for all
  rule sets of size n needs O(log(n)k-1) time for
  classification

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The two dimensional case source and destination
IP addresses

• For each destination prefix in rule set, link to
  corresponding node in destination IP trie a trie
  with source prefixes of rules using this
  destination prefix
• Matching algorithm must use backtracking to visit
  all source tries
• Grid of tries by pre-computing switch pointers
  in destination tries and propagating some
  information about more general rules, matching
  may proceed without backtracking
• Memory used proportional to number of rules
• Matching time O(w) with constant depending on
  stride
• Extended grid of tries handles 5 fields and has
  good run time and memory in practice

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R8

• Bit vector approaches do linear search through
  rule set
• For each field we pre-compute a structure (e.g.
  trie) to find most specific prefix or range
  distinguished by rule set
• For each rule, a single bit represents whether a
  given most specific prefix matches rule or not
• We associate with each range a bitmap of size n
  encoding which of the rules may match a packet in
  that prefix or range
• Classification algorithm first computes for each
  field of the packet the most specific
  prefix/range it belongs to
• By then AND-ing together the k bitmaps of size n
  we find matching rules
• Works well for hardware solutions that allow wide
  memory reads
• Scales poorly to large rule sets

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312033046131478
Cross-producting performs longest prefix matching
separately for all fields and combines the
results in a single step by looking up the
matching rule in a pre-computed table explicitly
listing the first matching rule for each element
of the cross-product. The size of this table is
the product of the numbers of recognized
prefixes/ranges for the individual fields. Due to
its memory requirements this method is not
feasible.

44523480
Equivalenced cross-producting (a.k.a. recursive
flow classification or RFC) combines the results
of the per-field longest matching prefix
operations two by two. The pairs of values are
grouped in equivalence classes and in general
there are much fewer equivalence classes than
pairs of values. This leads to significant memory
savings as compared to simple cross-producting.
This algorithm provides fast packet
classification, but compared to other algorithms,
the memory requirements are relatively large (but
feasible in some settings).
16 entries, 8 distinct classes
Src IP
Dest IP
Src Port
Dest Port
Proto
Final result
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Decision tree approaches

• At each node of the tree test a bit in a field or
  perform a range test
• Large fan-out leads to shallow trees and fast
  classification
• Leaves contain a few rules traversed linearly
• Interior nodes may contain rules that match also
• Tests may look at bits from multiple fields
• A rule may appear in multiple nodes of the
  decision tree this can lead to increased memory
  usage
• Tree built using heuristics that pick fields to
  compare on that divide remaining rules relatively
  evenly among descendants
• Fast and compact on rule sets used today

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Papers on packet classification

• G. Varghese Network algorithmics , chapter 12
• V. Srinivasan, G. Varghese, S. Suri, M.
  Waldvogel, Fast and Scalable Layer Four
  Switching, ACM SIGCOMM, Sep. 1998
• F. Baboescu, S. Singh, G. Varghese, Packet
  classification for core routers Is there an
  alternative to CAMs?, IEEE Infocom, 2003
• P. Gupta, N. McKeown, Packet classification on
  multiple fields, ACM SIGCOMM 1999
• T. Woo, A modular approach to packet
  classification Algorithms and results, IEEE
  Infocom, 2000
• S. Singh, F. Baboescu, G. Varghese, Packet
  classification using multidimensional cutting,
  SIGCOMM, 2003

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Overview

• Longest matching prefix
• Classification on multiple fields
• Signature matching
• String matching
• Regular expression matching w/ DFAs and D2FAs

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Signature matching

• Used in intrusion prevention/detection,
  application classification, load balancing
• Guard a byte string or a regular expression
• Action drop packet, log alert, set priority,
  direct to specific server
• Input byte string from the payload of packet(s)
• Hence the name deep packet inspection
• Output the positions at which various signatures
  match or the identifier of the highest priority
  signature that matches
• Size of problem hundreds of signatures per
  protocol

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String matching

• Most widely used early form of deep packet
  inspection, but the more expressive regular
  expressions have superceded strings by now
• Still used as pre-filter to more expensive
  matching operations by popular open source
  IDS/IPS Snort
• Matching multiple strings a well-studied problem
• A. Aho, M. Corasick. Efficient string matching
  An aid to bib- liographic search, Communications
  of the ACM, June 1975
• Many hardware-based solutions published in last
  decade
• Matching time independent of number of strings,
  memory requirements proportional to sum of their
  sizes

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Regular expression matching

• Deterministic and non-deterministic finite
  automata (DFAs and NFAs) can match regular
  expressions
• NFAs more compact but require backtracking or
  keeping track of sets of states during matching
• Both representations used in hardware and
  software solutions, but only DFA based solutions
  can guarantee throughput in software
• DFAs have a state space explosion problem
• From DFAs recognizing individual signatures we
  can build a DFA that recognizes entire signature
  set in a single pass
• Size of combined DFA much larger than sum of
  sizes for DFAs recognizing individual signatures
• Multiple combined DFAs are used to match
  signature set

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S. Kumar, S. Dharmapurikar, F. Yu, P. Crowley, J.
Turner, Algorithms to Accelerate Multiple
Regular Expressions Matching for Deep Packet
Inspection, ACM SIGCOMM, September 2006
Delayed Input DFA (D2FA)
Deterministic finite automaton (DFA)
State 0
State 1
State 2
State 3
State 0
State 1
State 2
State 3
Default transitions


Input
If the current state variable meets an
acceptance condition (e.g. whether the state
identifier is larger than a given threshold), the
automaton raises an alert.
Crt. state
D2FAs build on the observation that for many
pairs of states, the transition tables are very
similar and it is enough to store the
differences. The lookup algorithm may need to
follow multiple default transitions until it
finds a state that explicitly stores a pointer to
the next state it needs to transition to. Since
this is a throughput concern, the algorithm for
constructing D2FAs allows the user to set a limit
on the length of the maximum default path.
The memory columns report the ratio between the
number of transitions used by the D2FA and the
corresponding DFA.
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Conclusions

• Networking devices implement more and more
  complex data plane processing to better control
  traffic
• The algorithms and data structures used have big
  performance impact
• Often set of rules to be matched against has
  specific structure
• Algorithms exploiting this structure may give
  good performance even if it is impossible to find
  an algorithm that gives good performance on all
  possible rule sets

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Thats all folks!