1. CPEG512 Advanced Computer Networks
University of Nizwa
Electrical and computer Engineering
CPEG512 Advanced Computer Networks
Week8
Fall 2014/2015
Atef Abu Salim

2. Data link layer
The Data Link layer of a model typically has the following responsibilities:
1. Creates a frame
2. Creates an error-free logical connection
Error control
Flow control
3. Makes sure the receiver stays synchronized with the incoming data stream.
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3. Flow Control
Ensuring the sending entity does not overwhelm the receiving entity:
Preventing buffer overflow.
Propagation time.
Transmission time.
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4. Error Control Detection errors Lost frames Damaged frames
Automatic repeat request
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5. The network layer is responsible:
The delivery of individual packets from the original source to the final destination .
Logical addressing:
if the packet passes the network boundary we need another addressing system to help (source to destination) connection.
Routing :
route or switch the packet to final destination.
Source-to-destination delivery (End-to-End).
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6. Internet Protocol
Provides information about how and where data should be delivered
Subprotocol that enables TCP/IP to internetwork
To internetwork is to traverse more than one LAN segment and more than one type of network through a router
In an internetwork, the individual networks that are joined together are called subnetworks
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7. Internet Protocol cont.
IP is an unreliable, connectionless protocol, which means it does not guarantee delivery of data
Connectionless
Allows protocol to service a request without requesting verified session and without guaranteeing delivery of data
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8. Position of IP in TCP/IP protocol suite
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9. Datagram
A packet in the IP layer is called a datagram, a variable- length packet consisting of two parts: header and data.
The header is 20 to 60 bytes in length and contains information essential to routing and delivery.
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10. IP datagram
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11. IP datagram fields VER - version numbers, 4 and 6
HLEN - header length in 4-byte words. Value of 5 means 20 byte header
DS (differentiated services or Service type ) -
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12. IP datagram fields cont.
Precedence bits - never used. Similar to PRI bits in IPv6.
TOS (Type of Service) bits -
If you want to send a packet
with a special type of service,
use one of the 5 bit sets.
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13. IP datagram fields cont.
If we call these 8 bits Differentiated Services (and not the older Service Type), then the first six bits are called code- points
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14. IP datagram fields cont.
The total length field defines the total length of the datagram Including the header.
Total length field is 16 bits, or 65,535 bytes. Of which 20 to 60 bytes are the header.
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15. IP datagram fields cont.
Identification, Flags, and Fragmentation offset are all used to perform fragmentation.
Time to Live - 8-bit field, so Time to Live can be set to 255.
As it passes thru a router, the router decrements the counter. When counter hits 0, the datagram is deleted (and ICMP sends an error message back to the source).
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16. IP datagram fields cont.
The Protocol field (8 bits) identifies the upper layer protocol that is using IP for transmission of its data.
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17. IP datagram fields cont.
These are some common values that are used in the Protocol field
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18. IP datagram fields cont.
Header Checksum: Over the IP header only. Each 16-bit separated, then all added using one’s complement. Then taking the one’s complement of the result. Answer should be 1s.
Upon checksum error, datagram is dropped and no indication is sent back.
Options:
Security and handling restrictions
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19. IP Addressing IP Address Total: 232 = 4,294,967,296
Logical address used in TCP/IP networking
Unique 32-bit number
Divided into four groups of octets (8-bit bytes) that are separated by periods.
E.g., (
Total: 232 = 4,294,967,296
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20. IP Addressing cont. Loopback address
IP address reserved for communicating from a node to itself (localhost)
Value of the loopback address is always
Internet Corporation for Assigned Names and Numbers (ICANN)
Non-profit organization to maintain and assign IP addresses
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21. Why IP Addresses?
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22. Why IP Addresses? Cont.
Unique addressing allows communication between end stations
Unicast (one-to-one)
Broadcast (one-to-all)
Multicast (one-to-many)
Path choice is based on location
Location is represented by an address
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23. IP Addresses Classful Addressing
8 bits
8 bits
8 bits
8 bits
Class A:
Class B:
Class C:
Class D: Multicast
Class E: Research
Network
Host
Network
Host
Network
Host
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24. IP
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25. 4.2 CLASSFUL ADDRESSING CLASSFUL ADDRESSING
IP addresses, when started a few decades ago, used the concept of classes. This architecture is called classful addressing. In the mid-1990s, a new architecture, called classless addressing, was introduced and will eventually supersede the original architecture. However, part of the Internet is still using classful addressing, but the migration is very fast.
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26. Address Space Class A addresses cover ½ the address space!!
Millions of class A addresses are wasted!
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27. Addresses per class
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28. Finding the class in binary notation
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29. Netid and hostid
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30. Note
In classful addressing, the network address (the first address in the block) is the one that is assigned to the organization. The range of addresses can automatically be inferred from the network address.
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31. Masking concept
Given an address from a block of addresses, we can find the network address by ANDing with a mask.
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32. AND operation
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33. Default masks
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34. Note
The network address is the beginning address of each block. It can be found by applying the default mask to any of the addresses in the block (including itself). It retains the netid of the block and sets the hostid to zero.
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35. MULTIHOMED
A computer that is connected to different networks is called a multihomed computer and will have more than one address, each possibly belonging to a different class. Routers are multihomed too.
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36. Special addresses
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37. Network address
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38. Example of direct broadcast address
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39. Example of limited broadcast address
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40. Examples of “this host on this network
Example: starting a dial-up connection with DHCP.
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41. Example of “specific host on this network”
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42. Addresses for private networks
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43. Classless Addressing
Classless addressing, announced in 1996, allows an ISP to assign as few or as many IP addresses as requested
The entire 2^32 address space is divided into variable-sized blocks, which are multiples of powers of 2
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44. Mask In classful addressing, the mask for each block is implicit/8
/16
/24
In classless addressing, we need the address and the mask to find the block the address belongs to
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45. Format of classless addressing address
An address in classless addressing usually has this format:
The n after the slash defines the number of bits that are the
same in every address in the block. So if n is 20, it means
the twenty leftmost bits are identical in each address.
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46. Prefix and Prefix Length
Two terms often used in classless addressing
Prefix – another name for the common part of the address range (netid)
Prefix length – the length of the prefix
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47. Prefix lengths
The addresses in color are the default masks for classes A, B, and C.
Thus, classful addressing is a special case of classless addressing.
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48. Address Resolution Protocol (ARP)
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49. Position of ARP and RARP in TCP/IP protocol suite
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50. ARP
ARP table
Layer 2 to layer 3 mapping
Database that lists the associated MAC and IP addresses
Contains two types of entries:
Dynamic ARP table entries
Static ARP table entries
ARP utility provides a way of obtaining information from and manipulating a device’s ARP table
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51. How does ARP work?
99:88:77:22:22:22
99:88:77:11:11:11
99:88:77:33:33:33
1. Who is ?
2a.. It is not me.
2b. It is me.
4. OK. Let us create an ARP entry
:88:77:33:33:33
3. My MAC address is 99:88:77:33:33:33.
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52. ARP operation
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53. ARP Table
99:88:77:22:22:22
99:88:77:11:11:11
99:88:77:33:33:33
:88:77:22:22:22
:88:77:33:33:33
:88:77:11:11:11
:88:77:22:22:22
:88:77:11:11:11
:88:77:33:33:33
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54. Internet Control Message Protocol (ICMP)
ICMP is a companion protocol for IP.
ICMP is an error reporting protocol.
ICMP messages are encapsulated in an IP datagram
PING uses ICMP message with ECHO
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