Chapter 6 Packet Processing Functions

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Chapter 6Packet Processing Functions
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Outline

• Our Goal
• Packet Processing
• Address Lookup And Packet Forwarding
• Error Detection And Correction
• Fragmentation, Segmentation, And Reassembly
• Frame And Protocol Demultiplexing
• Packet Classification
• Queueing And Packet Discard
• Scheduling And Timing
• Security Authentication And Privacy
• Traffic Measurement And Policing
• Traffic Shaping
• Timer Management

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Our Goal

• Identify functions that occur in packet
  processing
• Devise set of operations sufficient for all
  packet processing
• Find an efficient implementation for the
  operations

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Packet Processing

• Address Lookup And Packet Forwarding
• Error Detection And Correction
• Fragmentation, Segmentation, And Reassembly
• Frame And Protocol Demultiplexing
• Packet Classification
• Queueing And Packet Discard
• Scheduling And Timing
• Security Authentication And Privacy
• Traffic Measurement And Policing
• Traffic Shaping

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Address Lookup And Packet Forwarding

• Forwardingthe process of sending a packet on
  toward its destination
• Two types
• Exact match (typically layer 2)
• Longest-prefix match (typically layer 3)
• Cost depends on size of table and type of lookup

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Error Detection And Correction

• Most common forms
• Cyclic redundancy Check (CRC)
• Checksum
• Error checking can cause significant computation
  overhead
• CRCOften implemented with special-purpose
  hardware
• Checksumoffers an alternative optimizationincrem
  ental update
• Error correction provides additional redundancy
  that can be used to correct corrupted bits
• Values sent to perform error correction as known
  as Error Correcting Codes(ECCs)- Audio Video

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An Important Note About Cost

• The cost of an operation is proportional to the
  amount of data processed
• An operation such as checksum computation that
  requires examination of all the data in a packet
  is among the most expensive

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Fragmentation, Segmentation, And Reassembly

• IP defines a fragmentation and reassembly
  datagrams
• ATM defines a segmentation and reassembly AAL5
  packets
• Cost is high because
• State must be kept and managed
• Unreassembled fragments occupy memory

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Frame And Protocol Demultiplexing

• The concept pervades packet processing, and
  occurs at each layer of the stack
• Type appears in each header
• Assigned on output
• Used on input to select next protocol
• Cost of demultiplexing proportional to number of
  layers

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Packet Classification

• Mapping a packet to one of a finite set of flows
  or categories
• Static Classification
• TCP, UDP, ICMP, and other
• Dynamically Classification
• Use the IP source address in a packet to
  determine the flow

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Demultiplexing V.S. Classification

• Demultiplexing
• Always a stateless operation in the sense
• Uses a global type system
• Operates one layer at a time
• Classification
• Not guaranteed to be stateless
• Does not need to use a global type system, and it
  does require the sender to participate
• Can span multiple layers of the stack

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Optimized Packet Processing

• Proponents of classification claim that its
  ability to bypass traditional layering gives
  classification potential for higher performance
• Unlike a traditional layering scheme, where
  processing is restricted to a sequential tour
  through the layers, classification spans multiple
  layers in one step

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Classification Languages

• Designed to allow engineers to write packet
  classification rules that are precise and
  unambiguous
• Agere Systems has designed a classification
  language named Functional Programming
  Language(FPL)
• Intel has adopted the Network Classification
  Language(NCL)

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Queueing And Packet Discard

• QueueingThe policies, data structure, and
  algorithm related to storing and selection
  packets
• General paradigm is store-and-forward
• Incoming packet placed in queue
• Outgoing packet placed in queue
• In the simplest case, a queue is literally a
  First-In-First-Out(FIFO)
• When queue is full, choose packet to discard
• Affects throughput of higher-layer protocols

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Queueing Priorities

• Multiple queues used to enforce priority among
  packets
• Incoming packet
• Assigned priority as function of contents
• Placed in appropriate priority queue
• Queueing discipline
• Examines priority queues
• Chooses which packet to send

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Priority Queueing

• Assign unique priority number to each queue
• Choose packet from highest priority queue that is
  nonempty
• Known as strict priority queueing
• Can lead to starvation

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Weighted Round Robin (WRR)

• Assign unique priority number to each queue
• Process all queues round-robin
• Compute N, max number of packets to select from a
  queue proportional to priority
• Take up to N packets before moving to next queue
• Works well if all packets equal size

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Weighted Fair Queueing (WFQ)

• Make selection from queue proportional to
  priority
• Use packet size rather than number of packets
• Allocates priority to amount of data from a queue
  rather than number of packets

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Packet Discard

• Refers to the policies and mechanisms used to
  handle the problem
• Tail dropdiscard an arriving packet when memory
  is full
• Random Early Detection(RED)used a probabilistic
  approach that increases the probability of
  discard as the memory fills
• TCPavoid global synchronization of
  retransmission
• When discard an ATM cell, the Early Packet
  Discard(EPD)technique identifies other cell that
  are part of the same packet, and discards all
  pieces at the same time

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Scheduling And Timing

• Important mechanisms
• Used to coordinate parallel and concurrent tasks
• Processing on multiple packets
• Processing on multiple protocols
• Multiple processors
• Multiple interfaces
• Scheduling is related to timer management,
  traffic shaping, and queueing
• Scheduler attempts to achieve fairness

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Security Authentication And Privacy

• Authentication mechanisms
• Ensure senders identity
• Confidentiality mechanisms
• Ensure that intermediaries cannot interpret
  packet contents Encryption
• Authentication mechanisms also rely on encryption
• Note in common networking terminology, privacy
  refers to confidentiality
• Example Virtual Private Networks

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Traffic Measurement And Policing

• Used by network managers
• Can measure aggregate traffic or per-flow traffic
• Often related to Service Level Agreement (SLA)
• Traffic policing refer to active enforcement in
  which traffic that exceeds specified bounds is
  marked as a candidate for discard or explicitly
  dropped
• One aspect of traffic policing is important is
  system designspeed
• Cost is high if performed in real-time

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Traffic Shaping

• Make traffic conform to statistical bounds
• Typical use
• Smooth bursts
• Avoid packet trains
• Only possibilities
• Discard packets (seldom used)
• Delay packets

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Example Traffic Shaping Mechanisms-- Leaky bucket

• Easy to implement
• Popular
• Sends steady number of packets per second
• Rate depends on number of packets waiting
• Does not guarantee steady data rate

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Example Traffic Shaping Mechanisms-- Token bucket

• Sends steady number of bits per second
• Rate depends on number of bits waiting
• Achieves steady data rate
• More difficult to implement

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Illustration Of Traffic Shaper

• Packets
• Arrive in bursts
• Leave at steady rate

Fordwards packets at a steady rate
Packet queue
Packetsleave
Packetsarrive
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Timer management

• Fundamental piece of network system
• Needed for
• Scheduling
• Traffic shaping
• Other protocol processing (e.g., retransmission)
• Cost
• Depends on number of timer operations (e.g., set,
  cancel)
• Can be high

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• QUESTION?