ECE 526

1
ECE 526 Network Processing Systems Design

• Packet Processing I  algorithms and data
  structures
• Chapter 5 D. E. Comer

2
Goals

• Understand basic packet processing operations
• Learn how to design efficient network processing
  system by
• Optimized data structures
• Optimized processing algorithms
• Get ready for Lab1

3
Outline

• Packet processing data structures
• Data storage
• Linked list
• http//cslibrary.stanford.edu/103/LinkedListBasics
  .pdf
• Hash
• http//en.wikipedia.org/wiki/Hash_table
• Packet processing algorithms
• Ethernet bridge
• Table lookup and hashing (used for lab1)
• Packet flow classification (used for lab1)
• IP forwarding, fragmentation and reassembly --
  next
• TCP connection recognition and splicing - next
• Lab 1

4
Data Storage coping buffering

• Used when packet moved from one memory location
  to another for protocol processing
• Expensive, why?
• What can be done to reduce buffer copies?
• Must be avoided whenever possible
• Leave packet in buffer
• Pass buffer address among threads/layers
• Buffer allocation
• Large, fixed buffer
• Varied-size buffers
• Linked list of fixed-size blocks
• What size should buffers be?

5
Buffer Sizes

• Max IP packet size is 64KB (total length field in
  IP format 16 bit)
• Almost no packets larger than 1500 bytes

6
Ethernet Bridge

• Used between a pair of Ethernets
• Provides transparent connections
• Listens in promiscuous mode
• Forwards frames in both directions
• Uses source address in frames to identify
  computers on each network
• Uses addresses to filter
• Uses destination address to decide whether to
  forward frame

7
Learning Bridge Algorithm
Forwarding unless the destination is known lie on
the segment over which frames arrives
8
Hash Table Lookup

• Table lookup
• Need high speed
• Software-based systems use hashing for table
  lookup
• Hashing
• Scheme for providing rapid access to data items
  which are distinguished by some key
• Hash table and hash function
• Hashing function
• Function assigns a data item distinguished by
  some key into one of a number of possible hash
  buckets in a hash table
• Ideal hash function should distribute items
  evenly between the buckets
• Hashing collision
• Two different items hash to the same location in
  a hash table
• Double hashing one of popular collision
  resolution technique
• Two values one as a starting value and one as
  interval between successive values

9
Hashing Algorithm
10
Packet Classification

• Process of mapping a packet to one of the finite
  sets of flows or categories
• Flow set of packets that share common
  characteristics, for example
• to the same destination
• use the same procedure to process
• Packets can be classified according to packet
  header information

11
Example Web Traffic

• Ethernet frame contains IP datagram
• IP datagram contains TCP segment
• TCP segment has destination port 80 (HTTP)

12
Flow Creation

• 5-tuple most commonly used version (also for
  lab1)

13
Flow Creation major steps

• Extract the five tuples from packet header
• Calculate the hash value of the five tuples using
  function
• Follows the concept of hashing algorithm
• to find the right slot either empty or the one
  has the same hashing value but also same five
  tuples
• different way to compute hash value
• different way to resolve the collision

14
Lab 1

• TCP/IP flow identification
• Your program should read a trace of packets and
  tell the following statistics at the end of the
  run
• Number of flows observed. That is the number of
  unique 5-tuples consisting of IP source and
  destination, layer 4 protocol number, and source
  and destination ports.
• We provide you with support functions to read and
  write packet trace
• You just need to write partial packet processing
  code based on the guidance.

15
Lab 1

• Modify file flowid.c only
• Three key functions
• Init_flowid()
• Initialize any data structures that you need
• You dont need modify it, but you need understand
  it
• Int flowid(packet pass_packet)
• Packet processing code
• You need modify the partial code
• Finish_flowid()
• Called at the end of trace, so you can output
  results
• You need modify this code

16
Lab 1 Help

• Help
• Dr. Weng Office Hours (MW 230 pm to 330 pm)
• TAs Kajal P. Patil and Mini Mathew
  kajal_at_siu.edu, mini10_at_siu.edu
• Help Hours Tu 1100am-100pm Tr 930-1130am
  and Fr 1-3pm

17
For Next Class

• Read Comer Chapter 5

18
Quiz

• Note (Please write clearly as you can only a
  few key words required for each question)
• Why packet processing is more and more important
  compared with other sources of packet delay?
  Hint think about the technology/application
  trend, and the way to reduce each of them.
•  Why is hashing table efficient for table lookup?
  Can hashing table be used for layer 2 lookup? How
  about for layer 3? Please explain to support why
  and why not. 
• What is the basic idea of optimize NIC? List of
  three techniques of using this idea.