Final Review CS1652

1. Final Review CS1652 Jack Lange University of Pittsburgh

2. Final • Thurs 12th: 12-2PM • This room • Not Cumulative • One page of notes

3. Chapter 4: Network Layer • Network layer service model • Forwarding vs. Routing • IP addressing, NAT & DHCP • Router architecture • Routing algorithms • Handling scale - CIDR and BGP • ICMP, traceroute

4. Network layer service model • Unreliable, connectionless data delivery • Q: is UDP more reliable than IP? • Q: do TCP packets receive more special treatment than UDP packets? • Host-to-host packet delivery • Q: Difference from transport layer? • All systems implement network layer • End systems + routers • Q: Why not routers implement transport layer?

5. Forwarding vs. Routing • Definition? • Switch vs. router? • How routing affects forwarding? • Routing algorithms • Intradomain routing algorithms • Distance vector, Link state • Interdomain routing algorithms • BGP • Longest prefix match

6. IP addressing, DHCP and NAT • 32-bit IPv4 address • Subnet part + host part • Subnet part is used for forwarding decision • CIDR • Subnet portion can be an arbitrary size • Why CIDR? • IP allocation efficiency & supernetting • DHCP • How it works? Where is it useful? • NAT • Pros. and Cons.

7. IP protocol version number 32 bits total datagram length (bytes) header length (bytes) type of service head. len ver length for fragmentation/ reassembly fragment offset “type” of data flgs 16-bit identifier max number remaining hops (decremented at each router) upper layer time to live header checksum 32 bit source IP address 32 bit destination IP address upper layer protocol to deliver payload to E.g. timestamp, record route taken, specify list of routers to visit. Options (if any) data (variable length, typically a TCP or UDP segment) IP datagram format how much overhead with TCP? • 20 bytes of TCP • 20 bytes of IP • = 40 bytes + app layer overhead

8. Router Architecture Two key router functions: • Run routing algorithms/protocol (RIP, OSPF, BGP) • Forwarding datagrams from incoming to outgoing link

9. Routers • Input ports (line cards) • Forwarding table lookup – line speed • Queuing packets if switching fabric is busy • Head-of-line blocking? • Switching fabric • Via memory, bus, special interconnection • Output ports • Buffering & scheduling

10. Link state algorithm • Broadcast local link info to all routers • Dijkstra’s algorithm • Greedy algorithm • Compute the least cost path to every node • Each loop finds at least one node whose least cost path is found • Algorithm complexity? O(nlogn) • Oscillation problem

11. Distance vector algorithm • Distribute one’s view of network to neighbors • Bellman-ford algorithm • Dynamic programming • Asynchronous update • Problem? • Count-to-infinity & routing loops • Possible solution?

12. Border Gateway Protocol (BGP) • Autonomous System (AS) • AS number – 16 bit id • BGP contains full path from src to dest AS • AS PATH – list of AS numbers • How to prevent routing loops? • Hot potato routing? • One reason for routing path asymmetry • How to deliver a packet from one AS to another? • Intradomain (Intra-AS) routing • Interdomain routing • Forwarding table (FIB)

13. Chapter 5: Link Layer • Delivering frames to a direct neighbor • Error detection and correction • Sharing a broadcast channel • Reliable data transfer & flow control • Hop-by-hop vs. end-to-end

14. Error detection & correction • Parity checking • Single bit vs. two-dimentional bit parity • Odd/even parity • Internet checksum – IP/TCP layer • Why is error checking needed in the upper layer? • Cyclic Redunancy Check (CRC) • CRC32 is widely used (e.g., Ethernet)

15. Multiple Access Protocol • How to share a broadcasting media • Medium Acccess Control (MAC) protocol • Channel Partitioning • Random Access • Taking turns

16. Random Access Protocol • Slotted ALOHA • Fixed time slot – synchronized • If collision, retransmit with a probability of p for each slot • ALOHA • No slot synchronization • CSMA, CSMA/CD, CSMA/CA • CSMA – sense carrier before sending • CD – detect collision while sending and cancel it • CA – avoid collision by getting permission first • Ethernet and Wi-Fi?

17. MAC addresses • Ethernet: 48-bit MAC addresses • Burned into hardware • Globally uniquely assigned • Why not use MAC address instead of IP? • Address Resolution Protocol (ARP) • Determining MAC address with IP address • ARP table = <IP address, MAC address, TTL> • Broadcasting mechanism (make sure to know!)

18. Ethernet & Switch • Access protocol: CSMA/CD • Old Ethernet hubs used to share access • Understand the exponential back-off algorithm • Most current devices are switches • Connectionless, unreliable • Frame format • Preamble, 2 MAC address, type, data, CRC32 • Switch • Make sure you understand self-learning algorithm

19. Chapter 6: Wireless • Difference from wired environment • Signal attenuation • Hidden terminal problem • Interference from other sources (phone, microwave) • SNR (Signal-to-Noise ratio) • The larger, the better • BER (Bit error rate) • CDMA

20. Wi-Fi • Access protocol : CSMA/CA • Reserve the channel first before sending • No collision detection – why? • Understand the sending protocol • Access Point(AP) • Link layer device (may run DHCP) • Passive/Active scanning for association • 802.11 frame format • Why we need three MAC addresses?

21. Chapter 7: Multimedia • Quality of service guarantee • Providing performance guarantee required by app • Current Internet does not directly support it • Delay sensitive, loss tolerant application • Video streaming vs. Email? • What is jitter? • Multimedia application • Stored streaming • Live streaming • Real-time interactive

22. Internet Phone, CDN • Internet phone • Network loss vs. delay loss • Content distribution networks (CDNs) • Definition? • DNS redirection for finding the near server?