1. Recitation 6
Midterm 1’s solution
Project 2
IP Forwarding
CIDR

2. Project 2 Environment Expand your project 1
1 tracker, 2 seeds and 2 leeches
Expand your project 1
Download from multiple peers.
Be able to upload.
Be able to accept incoming connections.
Connect to 2 leeches and upload your available pieces to leeches.

3. Protocol State machine:
am_choking: this client is choking the peer
am_interested: this client is interested in the peer
peer_choking: peer is choking this client
peer_interested: peer is interested in this client
bitfield: <len=0001+X><id=5><bitfield> SEND+RECV
keep-alive: <length prefix> is 0
choke: <len=0001><id=0>
unchoke: <len=0001><id=1>
interested: <len=0001><id=2>
not interested: <len=0001><id=3>
have: <len=0005><id=4><piece index>
request: <len=0013><id=6><index><begin><length> SEND+RECV
piece: <len=0009+X><id=7><index><begin><block> SEND+RECV

4. Notes
After handshake, seeds should send the bitfield with “all of bit 1s”. Leeches may not send “bitfields” or bitfield with “all of bit 0s”
After downloading & verifying a piece, your program should advertise the piece by sending a “have” to peers. Don’t send a “have” to peers having it already.

5. Sample Question: Subnetting
Consider a conventional class B network. A network administrator decides to give all subnets in the class B network a sub-net mask of
(5 points) How many sub-nets can the administrator use if all sub-nets use this mask?
(5 points) How many hosts are possible on each sub-net?
(5 points) The administrator just heard that she only needs 16 sub-nets for the class B address. What sub-net mask maximizes the number of hosts on each sub-net?

6. ARP (from IP add. to MAC add.)
Application doesn’t know MAC address !!!
Application works with IP address:
ssh (ssh cereal.rutgers.edu)
ping
Recall: Ethernet Frame Structure
“A” knows IP X of “C”, how can A send an ethernet frame to C? How does A know MAC of C??
MAC Dest. address
MAC Source address
Type
DATA ….
CRC
Send broadcast
MAC A -> FF: …FF, Type=ARP
Who has IP X ????
HUB
NIC -> EISA, ISA, PCI card
Maybe separate into several slide !!!
MAC C -> MAC A, type=ARP
Hey guy, I have IP X !!!
MAC A-> MAC C, type=IP
IP packet A B C D

7. Multi-Hop Forwarding Example
X sends an IP packet to Y??
3. R2b->Y
1.X-> R1a
2. R1b->R2a a b a b R1 R2 N1 N2 N3 x Y
1. X sends to its gateway (IP of R1a)
MAC X -> MAC R1a
IP X -> IP Y
DATA
2. R1 forwards the packet to R2
MAC R1b -> MAC R2a
IP X -> IP Y
DATA
3. R2 forwards the packet to Y
MAC R2b -> MAC Y
IP X -> IP Y
DATA

8. Multi-Hop Forwarding Example
Dest
Next hop N1 N2 N3 N4 R1
Deliver directly (ARP) R3
Routing R2
Actual routing table contains IP addresses, Flags indicating type of entries, net mask etc.

9. IP forwarding ROUTING TABLE AT A ROUTER:
Destination Gateway Genmask Flags Metric Ref Use Iface
A U eth0
B U eth1
U lo
UG eth0
(default) UG eth0
Question1: Packet with dest IP = , what is the next-hop (gateway)?
For each entry:
if ( <dest IP> AND <netmask> == <destination field>) {
choose the next hop = the corresponding gateway;
break; }
Question2: How many networks does the router connect to?

10. Network programming in Java TCP/IP stack
Application
Layer
Transport
Network
Host-to-
Net Layer
Host A
Host B
Application Protocol
Application
Layer
Data
Transport Protocols (UDP and TCP)
Transport
Layer
TCP/UDP header
Data IP IP IP
Network
Layer
Network
Layer
Network
Layer
IP header
TCP/UDP header
Data
Host-to-
Net Layer
Host-to-
Net Layer
Host-to-
Net Layer

11. Sample Question: IP Forwarding
The following table is a routing table using CIDR. Address bytes are in hexadecimal. The notation “/12” defines the length of the mask in bits. For example, for C /12, the mask is 12 bits, or FF.F0.0.0 in hexadecimal notation.
For each of the following destination IP addresses (in hexadecimal), state what the next hop will be by using the routing table above (2 points each):
C4.4B.31.2E :
C4.5E :
C4.4D.31.2E :
C4.5E :
C4.5E.7E.12 :
C4.5E.D1.02 :
Dest.
Next Hop
C4.5E.2.0/23 A
C4.5E.4.0/22 B
C4.5E.C0.0/19 C
C4.5E.40.0/18 D
C4.4C.0.0/14 E
C /2 F /1 G
default H

12. Reducing Routing Table Size
Without CIDR:
service
provider
…..
Routing
table
With CIDR:
…..
service
provider
Routing
table
/16

13. Sample Question: CIDR
The company X has 4 offices, each office has one subnet. All subnets connect to a router (Router 1 in the figure). This router connects to the ISP router. The subnets are described in the figure.

14. Sample Question: CIDR (cont.)
A. Fill out the routing table for the router 1:
Destination
Netmask
Nexthop
Interface
Eth1
Eth2
Eth3
Eth4
Eth0

15. Sample Question: CIDR (cont.)
B. Assume that the ISP router forwards to the router 1 an IP packet that has the destination address Which subnet will the packet be forwarded to? Based on the routing table in (1), what is the interface that the router 1 will forward the packet through?
C. Assume that the ISP router implement CIDR. What is the routing entry for all subnets of the company X in the routing table of the ISP router?
Destination
Netmask
Nexthop
Interface
Eth1