Answer to Question #317688 in Computer Networks for vavanshu

Question

Q2: (a) Two 8 bit numbers in a hexadecimal are 05 and 06. Obtain the checksum. (5 marks )    

(b) Explain in detail each field of IPv6 header. (5 marks)

Answer 1

a)Hexadecimal Numbers is a more complex system than using just binary or decimal and is mainly used when dealing with computers and memory address locations. By dividing a binary number up into groups of 4 bits, each group or set of 4 digits can now have a possible value of between “0000” (0) and “1111” ( 8+4+2+1 = 15 ) giving a total of 16 different number combinations from 0 to 15. Don’t forget that “0” is also a valid digit.

We remember from our first tutorial about Binary Numbers that a 4-bit group of digits is called a “nibble”. As 4-bits are also required to produce a hexadecimal number, a hex digit can also be thought of as a nibble, or half-a-byte. Thus two hexadecimal numbers are required to produce one full-byte ranging from 00 to FF.

Also, since 16 in the decimal system is the fourth power of 2 ( or 2⁴ ), there is a direct relationship between the numbers 2 and 16 so one hex digit has a value equal to four binary digits so now q is equal to “16”.

b)IPv6 Header

The IPv6 header is a streamlined version of the IPv4 header. It eliminates fields that are either unneeded or rarely used, and it adds a field that provides better support for real-time traffic. The figure below shows the structure of the IPv6 header as described in RFC 2460.

Following is a list of the fields in the IPv6 header:

• Version The Version field indicates the version of IP and is set to 6. The size of this field is 4 bits. While the purpose of the Version field is defined in the same way for both IPv4 and IPv6, its value is not used to pass the packet to an IPv4 or IPv6 protocol layer. This identification is done through a protocol identification field in the link-layer header. For example, a common link-layer encapsulation for Ethernet, called Ethernet II, uses a 16-bit EtherType field to identify the Ethernet frame payload. For IPv4 packets, the EtherType field is set to 0x800. For IPv6 packets, the EtherType field is set to 0x86DD. Thus, the determination of the protocol of the Ethernet payload occurs before the packet is passed to the appropriate protocol layer.
• Traffic Class The Traffic Class field indicates the IPv6 packet’s class or priority. The size of this field is 8 bits. This field provides functionality similar to the IPv4 Type of Service field. Like the Type of Service field in the IPv4 header, the first 6 bits of the Traffic Class field represent the DSCP field as defined in RFC 2474, and the last 2 bits are used for ECN as defined in RFC 3168.
• Flow Label The Flow Label field indicates that this packet belongs to a specific sequence of packets between a source and destination, requiring special handling by intermediate IPv6 routers. The size of this field is 20 bits. The flow label is used for prioritized delivery, such as delivery needed by real-time data (voice and video). For default router handling, the Flow Label field is set to 0. To distinguish a given flow, an intermediate router can use the packet’s source address, destination address, and flow label. Therefore, there can be multiple flows between a source and destination, as distinguished by separate non-zero flow labels. The details of the use of the Flow Label field are described in RFC 3697.
• Payload Length The Payload Length field indicates the length of the IPv6 payload. The size of this field is 16 bits. The Payload Length field includes the extension headers and the upper-layer PDU. With 16 bits, an IPv6 payload of up to 65,535 bytes can be indicated. For payload lengths greater than 65,535 bytes, the Payload Length field is set to 0 and the Jumbo Payload option is used in the Hop-by-Hop Options extension header, which is described in the "Hop-by-Hop Options Header" section in this chapter.
• Next Header The Next Header field indicates either the type of the first extension header (if present) or the protocol in the upper-layer PDU (such as TCP, UDP, or ICMPv6). The size of this field is 8 bits. When indicating an upper-layer protocol, the Next Header field uses the same values that are used in the IPv4 Protocol field.
• Hop Limit The Hop Limit field indicates the maximum number of links over which the IPv6 packet can travel before being discarded. The size of this field is 8 bits. The Hop Limit field is similar to the IPv4 TTL field except that there is no historical relation to the amount of time (in seconds) that the packet is queued at the router. When Hop Limit equals 0 at a router, the router sends an ICMPv6 Time Exceeded-Hop Limit Exceeded in Transit message to the source and discards the packet.
• Source Address The Source Address field indicates the IPv6 address of the originating host. The size of this field is 128 bits.
• Destination Address The Destination Address field indicates the IPv6 address of the current destination node. The size of this field is 128 bits. In most cases, the Destination Address field is set to the final destination address. However, if a Routing extension header is present, the Destination Address field might be set to the address of the next intermediate destination.
  

NETWORK MONITOR CAPTURE

Here is an example of an IPv6 header, as displayed by Network Monitor 3.4 (capture 04_01 in the companion content for this book):

 Frame:
+ Ethernet: Etype = IPv6
- Ipv6: Next Protocol = ICMPv6, Payload Length = 40
  - Versions: IPv6, Internet Protocol, DSCP 0
     Version:  (0110............................) IPv6, Internet Protocol, 6(0x6)
     DSCP:     (....000000......................) Differentiated services codepoint 0
     ECT:      (..........0.....................) ECN-Capable Transport not set
     CE:       (...........0....................) ECN-CE not set
     FlowLabel: (............00000000000000000000) 0
    PayloadLength: 40 (0x28)
    NextProtocol: ICMPv6, 58(0x3a)
    HopLimit: 128 (0x80)
    SourceAddress: FE80:0:0:0:260:97FF:FE02:6E8F
    DestinationAddress: FE80:0:0:0:260:97FF:FE02:6D3D
+ Icmpv6: Echo request, ID = 0x0, Seq = 0x18

This ICMPv6 Echo Request packet uses the default Traffic Class and Flow Label and a Hop Limit of 128, and it is sent between two hosts using link-local addresses.

Values of the Next Header Field

Table 4-1 lists typical values of the Next Header field for an IPv6 header or an IPv6 extension header. Each of the IPv6 extension headers is covered later in the chapter.