cs144 Midterm Review Fall 2010

Transcription

1 cs144 Midterm Review Fall 2010

2 Administrivia Lab 3 in flight. Due: Thursday, Oct 28 Midterm is this Thursday, Oct 21 (during class) Remember Grading Policy: - Exam grade = max (final, (final + midterm)/2) - Final grade will be computed as: max(2 exam + lab, exam + 2 lab) 3

3 Computer Networks Interconnected computers that move information The Internet: - the network of networks - (try) to deliver information from one host to another - Best Effort - The End to End Principle

4 Protocol Layering Application Presentation Session Transport Network Link Physical

5 Example Encapsulation Sending Receiving Application data Transport header IP header Link layer header

6 Addressing Each node typically has unique address - (or at least is made to think it does when there is shortage) Each layer can have its own addressing - Link layer: e.g., 48-bit Ethernet address (interface) - Network layer: 32-bit IP address (node) - Transport layer: 16-bit UDP/TCP port (service)

7 Physical Layer (Layer 1) Specifies the physical medium (e.g. Cat5, WiFi) Specifies the signal (physics to data (chips)) Specifies the bits (chips to bits) Pre L Data

8 Link Layer (Layer 2) Single-hop addressing (e.g. Ethernet address) Media Access Control - Link-layer congestion control - Collision detection/collision avoidance Single-hop acknowledgements Preamble 7 x Physical Link Layer 3 Link SFD Src Dest Type/ Len Payload 6 bytes 6 bytes 2 bytes bytes CRC 4 bytes Gap 96 ns, 960 ns, 9600 ns

9 Network Layer (Layer 3) Narrow waist of IP Glue lower-level networks unreliable, best-effort Packet forwarding Routing protocols FTP HTTP NV TFTP TCP UDP IP NET 1 NET 2 NET n

10 IPv4 packet format

11 Network Layer: IP Addressing Globally unique (or made to seem that way) Originally had Class prefixes Now, CIDR blocks (ip addr + prefix length) IPv4, IPv6 ICMP: Internet Control Message Protocol ARP: Address Resolution Protocol

12 Network Layer: IP Forwarding Passing packets from input to output ports Pick output port based on destination addr - Find out-port or out-port, MAC addr pair - Longest Prefix Matching Switches (shared bus, crossbar, Banyan)

13 Network Layer: IP Routing Process of populating forwarding table Routers exchange messages about network Find a route for every destination Intra-domain vs Inter-domain routing - Intra-: routers within an Autonomous System (AS) - Inter-: decentralized, the entire internet. Two kinds of Routing Protocols: - Link State: global view of network (OSPF) - Distance Vector: only local state (RIP)

14 Transport Layer (Layer 4) Provide multiplexing at the host Error detection UDP and TCP new narrow waist

15 Transport Layer: UDP user datagram protocol Unreliable and unordered Checksum on whole packet (+ pseudo header) No flow control, No Reliability

16 Transport Layer: TCP tansmission control protocol Full duplex, connection-oriented byte stream Provides: - Reliability - In Order - Flow control - Congestion control

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18 Application Layer (Layer 7) Sockets (just like files) HTTP, FTP, BitTorrent, Skype, etc.

19 Practice Problems Practice Midterm (posted on website) Book (questions at the end of each chapter) Some next up!

20 Question 1 Suppose Host A sends two TCP segments back to back to Host B over a TCP connection. The first segment has sequence number 90; the second has sequence number 110. How much data is in the first segment? Suppose that the first segment is lost but the second segment arrives at B. What will the ACK number be in the acknowledgment that Host B sends to Host A?

21 Answer 1 How much data is in the first segment? 20 bytes. Bytes are in the first segment. What will the ACK number be in the acknowledgment that Host B sends to Host A? 90. TCP might buffer the second packet, but the ACK is still requesting the first segment. (In order requirement)

22 Question 2 A 10-port hub is connected to 10 hosts using gigabit links. What is the maximum aggregate transfer rate of data flowing through this network?

23 Answer 2 A 10-port hub is connected to 10 hosts using gigabit links. What is the maximum aggregate transfer rate of data flowing through this network? All ports are part of the same collision domain only one device can send at a time. Therefore, peak bandwidth is one gigabit

24 Question 3: Routing with CIDR IP Address Netmask Next hop Interface eth eth eth eth3 What is the longest prefix matching for:

25 Answer 3: Routing with CIDR IP Address Netmask Next hop Interface eth eth eth eth3 What is the longest prefix matching for: / / /0

26 Question 4: Fragmentation When an IP packet needs to traverse a link with a lower MTU, it can be fragmented. Does fragmenting increase or reduce the likelihood of dropping the packet? Consider sending a 2100 byte datagram over a link with MTU of 700 bytes. How many fragments are generated? What fields are changed in the IP frame?

27 Answer 4: Fragmentation Does fragmenting increase or reduce the likelihood of dropping the packet? Increases it. Each fragment can be dropped independently of the others, yet the packet needs all fragments to be reconstructed. How many fragments are generated? 700 MTU = 680b of data + 20b IP header. = 3.08, so at least 4 fragments What fields are changed in the IP frame? Total Length, More flag, Fragment Offset, Checksum Identifier field is not changed, but used to reconstruct packet.

28 Question 5: Distance Vector