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					 Full Packet Monitoring Sensors:
Hardware and Software Challenges




         Vladimír Smotlacha
              CESNET
                 High-speed network monitoring


Scalability limited by:


• throughput of local bus
   - flow at 10 Gb/s exceeds throughput of PCI-X 64/133
• CPU performance
• data handling in RAM
• disk systems
   - amount of stored data
   - sustained write speed
                          Flow based monitoring

Motivation: describe dynamics of link traffic

• Elementary flow specified by
   - source and host IP address
   - transport protocol
   - source and destination port (if applicable)
   - start and end time (Timeouts ! )

• Flow data aggregation
   - end point - host, network, AS
   - time granularity

• Example: NetFlow
   - implemented in routers
   - database of open flows
   - statistics of each flow
                    Packet based monitoring

Motivation: describe dynamics of selected connections

• Flow specification
   - all packets that match arbitrary criteria (e.g., “all UDP and TCP
   packets sent to port 456”)
   - flow is dealt as generalized socket
   - filter is expressed in a special language (e.g., BPF, FPL, C library)

• Example: pcap
   - based on BPF
   - used in tcpdump, snort, ntop, ngrep, ethereal, ...
   - intuitive way of writing filters
                         Software optimization

• Performance
   - effective filters - CPU instructions/packets
    - optimal manipulation with packets - memory mapping
    - parallelism in packet processing


 examples:
• FFPF
    - new extensible language
    - intensive computation pushed into kernel
    - support of network processors

• nCap
    - handle full 1 Gbps data flow
                        Monitoring API


Basic abstraction: network flow
   - create & terminate the flow
   - read packets from the flow
   - apply functions to the flow
   - read results of functions


MAPI functions
   - filtering ( BPF filters)
   - logging
   - accounting
   - sampling
   - cooking (IP defragmentation & TCP reassembly)
   - string search
                     Hardware-software codesign


Putting functionality down to the hardware

• FFPF
   - support of network processors

• MAPI
   - utilizes available functionality
    - DAG cards
    - SCAMPI cards
                   Intelligent hardware adapters



Goals
  - reduce the amount of data passing local bus
  - reduce CPU load and memory request
  - do complex classification of packets
  - move computational intensive algorithms to adapter
  - introduce new parallel algorithms
  - accurate timestamps
                         Adapters functionality


• Timestamping
    - unique accurate timestamp to each packet
    - clock synchronization required


• Header based filtering
   - rule to specify passing through packets
                or
• Header based classification
   - one rule per each class
    - disjunctive rules - packets belongs to one class
    - non-disjunctive rules - packet can belong to more classes
                    Adapters functionality (cont)


• Packet shrinking
    - cut unnecessary payload to reduce data

• Sampling
   - reduction of packet number
    - deterministic x probabilistic

• Calculation of statistics
   - based on packet length x time interval between packets

• String searching
    - packets containing string pass the unit
SCAMPI adapter
                          Packet classification

CAM - matching a (sub)field with a constant value
(e.g., IP address, network address, protocol)

Processing unit - arithmetic comparison with a constant
value (e.g., port, interval of port values)
Whenever possible, comparison is done in CAM
Pair (C,P)
    • C - CAM row (with “don’t care” bits)
    • P - sequence of comparison (conditional jump) instructions

Semantics
    • matching row C of CAM points to an instruction sequence P
    • instruction result:
         • assign packet to a class & stop (packet classified)
         • stop without assigning (not classified)
         • continue with next instruction
                              Filter language - FL

• Primitive operation: comparison of an arbitrary header field
with a constant

•Filter specification: expression consisting of primitive
operations, ‘and’, ‘or’, ‘not’ and brackets

•Implementation
   • expression is transformed to DNF
       example: „A and (C or D) and (E or F) or G and H“
               is equal to   „ACE or ACF or ADE or ADF or GH“
   • each primitive operation or a conjunction of them is translated to
   max. one pair (C, P)
   • FL expression in DNF is translated to a number of pairs (C, P)
                           Searching of string


• CAM with 272 bits wide row

• Algorithm implemented in hardware:
   - 16 byte long string stored in 16 rows CAM, shifted by 0,1,2,... 15
   bytes
   - comparison with 32 bytes of payload in one CAM
   - in next cycle, payload is shifted for 16 bytes

• Implementation in Scampi
    - search of more then 100 strings simultaneously
   - designed throughput 3 Gb/s

• Issues
   - finds only first occurrence of any string
   - in case of longer strings lot of false positives -> additional software
   verification
                                 Open problems



• Searched string occurs on border of two packets
   - solution: flow cooking in adapter


• Dealing with non-disjunctive classes
   - solution: evaluation of all intersections -> possibly exponential
    number of new pairs (C, P)

				
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