COMS 414 - Prelim 2 Review Session

1. COMS 414 - Prelim 2 Review Session Biswanath Panda bpanda@cs.cornell.edu

2. Outline • Networks • Memory • Homework

3. < DNS > • DNS == Domain Name System • Why do we need Domain Name? • Domain names are easier to remember than IP addresses • IP addresses can be dynamically changing. • IP addresses may not be unique. • Why do we need DNS? • Mapping between domain name and IP addresses.

4. Biswanath Panda: DNS information distrbuted across all hosts. Each domain level has its own name server. Caching for faster lookups. TTL field so that stale entires are removed and rechecked < DNS > • By maintaining distributed host table • Scalability !!! • How changes to [domain name – IP address] mapping will be updated? • Caching… • TTL…

5. < DNS > • Name resolution commands • NSLookup [ipaddress | sitename] • ping -a • Query scheme is simple • Query( domain name, RR type ) • Answer( values, additional RRs ) • RR == Resource record

6. DNS tree structure NS RR “pointers” . edu. com. jp. us. cornell.edu. cmu.edu. mit.edu. cs.cornell.edu. eng.cornell.edu. foo.cs.cornell.edu A 10.1.1.1 bar.cs.cornell.edu A 10.1.1.1

7. < CDN > • CDN == Content Distribution Networks • Replication of web servers • CDN V.S. Centralized server • Less latency, better performance • More robust service availability

8. < CDN > • Cached CDN – cache contents on cache miss • Pushed CDN – push contents up-front • Issues • Difficulty with dynamic contents • Cache performance V.S. Content synchronization.

9. < UDP > • Unreliable / Out-of-order message delivery. • Connection-less. • Datagram based. • Messages > MTU will be dropped. • MTU == Maximum Transmission Unit • Default ~1460bytes with Cisco routers • No flow control • No congestion control

10. < TCP > • Reliable / In-order message delivery. • Connection-oriented. • Stream based - thus no restriction on transmission size • Flow control • Congestion control

11. TCP connection establishment SYN, SeqNum=x • Three-way handshake • 1. SYN • 2. ACK + SYN • 3. ACK • Connection established only after all three steps. • If not, time-out. SYN+ACK, SeqNum=y, Ack=x+1 ACK, Ack=y+1 Client (active) Server (passive)

12. TCP-SYN Attack • Classic DOS (Denial of Service) attack. • Attack by creating myriads of half-established connections.

13. TCP Sliding Window • This is how below TCP properties come to life • Reliable delivery • In-order delivery • Any size message (stream based) • Flow control • Sliding window can’t slide if messages in the window didn’t get through.

14. TCP Sliding Window • Advertisement of Window size via ACK • Small sliding window • Low performance due to delay waiting on ACK. • Bad with network with large RTT (Round Trip Time) • Large sliding window • Send data as a bulk, waiting ACK as a bulk. • Bad if network congestion, as bulk transfer will make circumstance worse.

15. TCP Congestion Control • Interpret dropped packets as congestion • Maintain congestion window size • Additive Increase/Multiplicative Decrease TCP sawtooth pattern KB Time (seconds)

16. Wireless environment • Issues • High RTT(Round Trip Time) • Message loss pattern differs from wired network • What do ‘dropped packets’ indicate? • TCP assumes congestion. • But it could be just lossy medium. • How will UDP/TCP behave on wireless?

17. VPN == Virtual Private Network • remote client can communicate with the company network securely over the public network as if it resided on the internal LAN

18. NAT == Network Address Translation • allows an IP-based network to manage its public (Internet) addresses separately from its private (intranet) addresses. • popular technology DSL or cable LANs

19. Network Failure • Packet drop or packet delay • System Crash / halt • Byzantine failure • Some systems behaves incorrectly or unexpectedly • Could be a malicious attacker • Network Partition • Also known as “Split Brain Syndrome” • Some nodes in a cluster no longer communicate with each other

20. IP Multicast • Reduces overhead for sender • Reduces bandwidth consumption in network • Useful in small subnet • I.e.) virtual meeting broadcast within a corporate network

21. 0: 1: CPU N-1: < Virtual Memory Overview > Memory Page Table Virtual Addresses Physical Addresses 0: 1: P-1: Disk Address Translation: Hardware converts virtual addresses to physical addresses via an OS-managed lookup table (page table)

22. Virtual Memory yet another picture.. Virtual Page Number Valid Physical Memory 1 1 0 1 1 1 0 1 0 Disk Storage (swap file or regular file system file) 1 Memory resident page table (physical page or disk address)

23. Multi-Level Page Tables • multi-level page tables • Level 1 table: • 1024 entries, each of which points to a Level 2 page table. • Level 2 table: • 1024 entries, each of which points to a page ... Level 1 Table Level 2 Tables

24. Page Faults • PTE == Page Table Entry • Each entry is (pointer to physical address, flags) • If a process tries to access a page not memory •  Page Fault Interrupt •  OS exception handler “page-fault trap”

25. Paging and swapping Before fault After fault Memory Memory Page Table Page Table Virtual Addresses Physical Addresses Virtual Addresses Physical Addresses CPU CPU Disk Disk

26. < Page replacement schemes > • FIFO– first in first out • OPT - (or MIN) optimal page replacement • LRU– least recently used • LRU Approximation • Mimicking LRU when no hardware support for LRU • Reference bits • Additional reference bits algorithm • Second chance algorithm • LFU– least frequently used • MFU– most frequently used

27. FIFO and Belady's Anomaly  For some page replacement algorithms, the page fault rate may increase as the number of allocated frames increases.

28. OPT (or MIN) • Assumes knowledge for future requirement. • Replace the page that will not be used for the longest period of time • Doesn’t show Belady’s anomaly • But practically too difficult to implement !

29. LRU • Assume pages used recently will be used again • throw away page not used for longest time • Popular policy to be taken • Doesn’t show Belady’s anomaly • Implementation options • Counters • Stack

30. Second-chance • LRU Approximation • Reference Bits + FIFO • if set, a page will be granted for second chance. • If a page used often enough, it will never be replaced. • Implementation by “Circular Queue” • Bad if all bits are set  degenerates to FIFO.

31. LFU • Assumes pages used actively will be used again. • What about a page used heavily only in the beginning? •  shift count by 1 at regular intervals

32. Virtual Memory Programmer’s view • Large “flat” address space • Can allocate large blocks of contiguous addresses • Processor “owns” machine • Has private address space • Unaffected by behavior of other processes

33. Virtual Memory System’s view • virtual address space created by page mapping • Address space need not be contiguous • Allocated dynamically • Multi-processing performance • Switching to other processes when servicing disk I/O for page fault

34. CPU C a c h e regs Levels in Memory Hierarchy virtual memory cache Memory disk 8 B 32 B 4 KB Register Cache Memory Disk Memory size: speed: $/Mbyte: line size: 32 B 1 ns 8 B 32 KB-4MB 2 ns $100/MB 32 B 128 MB 50 ns $1.00/MB 4 KB 20 GB 8 ms $0.006/MB larger, slower, cheaper

35. Virtual Memory+ Cache miss VA PA Trans- lation Cache Main Memory CPU hit data • Problem? • Performs Address Translation before eachcache lookup • Which may involve memory access itself (of the PTE) • We could cache page table entries…

36. hit miss VA PA TLB Lookup Cache Main Memory CPU miss hit Trans- lation data Virtual Memory+ Cache + TLB  Speed up Address translation

37. < How to Prepare Prelim > • Make sure to review homework problem sets. • Practice writing synchronization code on your own !! • Sleep well and have your brain ready to think ! • http://www.cs.cornell.edu/Courses/cs414/2004fa/ • http://www.cs.cornell.edu/Courses/cs414/2003fa/