1. EECB 473 Data Network Architecture and Electronics Lecture 3 Packet Processing Functions

2. Aims of this lecture Identify functions that occur in packet processing Devise set of operations sufficient for all packet processing Find an efficient implementation for the operations

3. Packet Processing Functions We Will Consider 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, policing, and shaping

4. Address Lookup And Packet Forwarding Forwarding requires address lookup Lookup is table driven Two types Exact match (typically layer 2) Longest-prefix match (typically layer 3) Cost depends on size of table and type of lookup

5. Error Detection And Correction Data sent with packet used as verification Checksum CRC Cost proportional to size of packet Often implemented with special-purpose hardware

6. 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.

7. Fragmentation, Segmentation, And Reassembly IP fragments and reassembles datagrams ATM segments and reassembles AAL5 packets Same idea; details differ Cost is high because State must be kept and managed Unreassembled fragments occupy memory

8. Frame And Protocol Demultiplexing Traditional technique used in layered protocols Type appears in each header Assigned on output Used on input to select ‘‘next’’ protocol Cost of demultiplexing proportional to number of layers

9. Packet Classification Alternative to demultiplexing Crosses multiple layers Achieves lower cost More on classification later in the course which is inclusive of Static and Dynamic Classification Demultiplexing versus Classification Optimized Packet Processing Classification Languages

10. Queueing And Packet Discard General paradigm is store-and-forward Incoming packet placed in queue Outgoing packet placed in queue When queue is full, choose packet to discard Affects throughput of higher-layer protocols

11. 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

12. Examples Of Queueing Disciplines 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

13. Examples Of Queueing Disciplines (continued) 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

14. Examples Of Queueing Disciplines (continued) 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

15. Scheduling And Timing Important mechanisms Used to coordinate parallel and concurrent tasks Processing on multiple packets Processing on multiple protocols Multiple processors Scheduler attempts to achieve fairness

16. Security: Authentication And Privacy Authentication mechanisms Ensure sender’s identity Confidentiality mechanisms Ensure that intermediaries cannot interpret packet contents Note: in common networking terminology, privacy refers to confidentiality E.g. Virtual Private Networks

17. Traffic Measurement And Policing Used by network managers Can measure aggregate traffic or per-flow traffic Often related to Service Level Agreement (SLA) Cost is high if performed in real-time

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

19. 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

20. Example Traffic Shaping Mechanisms (2) Token bucket Sends steady number of bits per second Rate depends on number of bits waiting Achieves steady data rate More difficult to implement

21. Illustration Of Traffic Shaper Packets Arrive in bursts Leave at steady rate

22. 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

23. Summary Primary packet processing functions are Address lookup and forwarding Error detection and correction Fragmentation and reassembly Demultiplexing and classification Queueing and discard Scheduling and timing Security functions Traffic measurement, policing, and shaping