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Midterm Review. Lab 4: dynamic routing protocols.

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Presentation on theme: "Midterm Review. Lab 4: dynamic routing protocols."— Presentation transcript:

1 Midterm Review

2 Lab 4: dynamic routing protocols

3 3 Typo on Page 144 Table 4.2 –Should be router 4 instead of router 1

4 4 How Cost is Set on a Router The default cost each router adds is 1. When Router B announces to Router C the network, the cost metric is 1. Offset-list 0 in value Iface (increases the metric of incoming RIP packets) Offset-list 0 out value Iface (increases the metric of outgoing RIP packets) At router B, if we run offset-list 0 out 10 eth1, then the metric for is 11; if we run offset-list 0 in 10 eth1, then the metric for the network announced by C becomes 11. Router ARouter B Router C eth0eth1

5 5 Exercise (4B): count-to-infinity Time consuming to reproduce, but interesting. Why does count-to-infinity still exist with split horizon? Lab report due after midterm Router2 Router4 Router3 Router1 10 1 1 1 1 1

6 6 Why does count-to-infinity still exist with split horizon? Router2 Router4 Router3 Router1 10 1 1 1 1 X 1 Router3’s routing table: ?? 1 Router2’s routing table: ?? 1 Router4’s routing table: Router3 3 Router2 is not Router4’s next hop. Router4 sends to router2 the routing update Router2’s routing table: Router 4 4 This lie will be told to Router3 and Circulates in the system  count-to-infinity Suppose updates happen in the following sequence: 1.The update from PC3 arrives at Router 3 2.The update from Router 3 arrives at Router 2 3.The update from Router 4 arrives at Router 2 PC3

7 Midterm review

8 8 What you’ll be tested on Basic lab commands –E.g., ping, traceroute, tcpdump, ethereal, ifconfig, how to copy a file, how to list a directory Basic trouble shooting –E.g., I cannot ping, why? Basic networking concepts –E.g., layering principle, multiplexing, and encapsulation Protocols we’ve covered so far –ARP –ICMP –IP How to design a protocol

9 9 Address translation protocol What is it used for? What is an ARP cache used for? Proxy ARP ARP is “hop-by-hop”

10 10 Address Translation with ARP ARP Request: Argon broadcasts an ARP request to all stations on the network: “What is the hardware address of”

11 11 Address Translation with ARP ARP Reply: Router 137 responds with an ARP Reply which contains the hardware address

12 12 ARP Packet Format

13 13 Example ARP Request from Argon: Source hardware address: 00:a0:24:71:e4:44 Source protocol address: Target hardware address: 00:00:00:00:00:00 Target protocol address: ARP Reply from Router137: Source hardware address: 00:e0:f9:23:a8:20 Source protocol address: Target hardware address: 00:a0:24:71:e4:44 Target protocol address:

14 14 ARP Cache Since sending an ARP request/reply for each IP datagram is inefficient, hosts maintain a cache (ARP Cache) of current entries. The entries expire after a time interval. Contents of the ARP Cache: ( at 00:10:4B:C5:D1:15 [ether] on eth0 ( at 00:B0:D0:E1:17:D5 [ether] on eth0 ( at 00:B0:D0:DE:70:E6 [ether] on eth0 ( at 00:05:3C:06:27:35 [ether] on eth1 ( at 00:B0:D0:E1:17:DB [ether] on eth0 ( at 00:B0:D0:E1:17:DF [ether] on eth0

15 15 Proxy ARP Proxy ARP: Host or router responds to ARP Request that arrives from one of its connected networks for a host that is on another of its connected networks.

16 16 ICMP What is it used for? –E.g. error reporting, route redirect When will an ICMP message be triggered? Application programs that use ICMP –Ping, traceroute

17 17 IP Network order versus host order Structure of an IP address CIDR addressing Route aggregation Longest prefix match Fragmentation

18 18 An IP address is often written in dotted decimal notation Each byte is identified by a decimal number in the range [0..255]: 10001111100000001000100110010000 1 st Byte = 128 2 nd Byte = 143 3 rd Byte = 137 4 th Byte = 144

19 19 Structure of an IP address network prefixhost number An IP address encodes both a network number (network prefix) and an interface number (host number). –network prefix identifies a network –the host number identifies a specific host (actually, an interface on the network). –All hosts on the same single segment network have the same network prefix. 0 31

20 20 Network prefix is variable length A network mask specifies the number of bits used to identify a network in an IP address. 10001111100000001000100110010000 11111111 111111100000000 128143 137 144 255 0 Addr Mask

21 21 CIDR notation CIDR notation of an IP address: – –/24 is the prefix length. It states that the first 24 bits are the network prefix of the address (and the remaining 8 bits are available for specific host addresses)

22 22 Network prefix versus address prefix CIDR notation can nicely express blocks of addresses –An address block –[,] –can be represented by an address prefix –All addresses whose first 16 bits are the same as those in are in the address block or match the address prefix –How many addresses are there in a /x address block? 2 (32-x) A network prefix is the first n bits in an IP address that identifies a single-segment network.

23 23 How to assign network prefixes from an address prefix An organization obtains an address prefix The organization has two LANS. LAN1 has at most 500 hosts; LAN2 has at most 100 hosts. Assign network prefixes to each LAN. Algorithm 1.Figure out the length of the network prefix –2 32-x 1 ¸ 500  x 1 = 23 –2 32-x 2 ¸ 100  x 2 = 25 2.Allocate subdivisions of to each LAN – [,]  LAN1 – [,]  LAN2

24 24 Protocol Design human protocols: “what’s the time?” “I have a question” introductions … specific msgs sent … specific actions taken when msgs received, or other events network protocols: machines rather than humans all communication activity in Internet governed by protocols protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt

25 25 What’s a protocol? a human protocol and a computer network protocol: Q: Other human protocols? Hi Got the time? 2:00 TCP connection req TCP connection response Get time

26 26 How to review Lecture notes Lab and pre-lab Sample midterm

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