1. Chapter 6 Packet Processing Functions

2. Outline Our Goal Packet Processing Timer Management
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

3. 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

4. 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

5. Address Lookup And Packet Forwarding
Forwarding：the 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

6. Error Detection And Correction
Most common forms：
Cyclic redundancy Check （CRC）
Checksum
Error checking can cause significant computation overhead
CRC：Often implemented with special-purpose hardware
Checksum：offers an alternative optimization～incremental 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

7. 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

8. 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

9. 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

10. 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

11. Demultiplexing V.S. Classification
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

12. 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

13. 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）

14. Queueing And Packet Discard
Queueing：The 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

15. 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

16. 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

17. 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

18. 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

19. Packet Discard
Refers to the policies and mechanisms used to handle the problem
Tail drop：discard 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
TCP：avoid 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

20. 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

21. Security: Authentication And Privacy
Authentication mechanisms
Ensure sender’s 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

22. 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 design：speed
Cost is high if performed in real-time

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

24. 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

25. 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

26. Illustration Of Traffic Shaper
Packets
Arrive in bursts
Leave at steady rate
Fordwards packets at a steady rate
Packet queue X
Packets arrive
Packets leave

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

28. QUESTION？