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GATE CSE 2017 | Set 2 | Question: 20

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The maximum number of $\textsf{IPv4}$ router addresses that can be listed in the record route (RR) option field of an $\textsf{IPv4}$ header is______.

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A record-route (RR) option is used to record the Internet routers that handle the datagram. It is listed in OPTIONS of $\textsf{IPv4}.$

According to $\textsf{RFC 791},$ there are two cases for the format of an option:

  • Case $1:$ A single octet of option-type.
  • Case $2:$ An option-type octet, an option-length octet, and the actual option-data octets.

In both the cases, first $16$ bits of OPTIONS field is used. Therefore, out off $40$ Bytes only $38$ Bytes are remaining for storing $\textsf{IPv4}$ addresses. In $38$ Bytes we can store $9\;\textsf{ IPv4}$ addresses as each $\textsf{IPv4}$ address is of $4$ Bytes.

$\therefore 9$ should be answer.

Reference: https://tools.ietf.org/html/rfc791

$\large \textbf{Fig: Option format}$
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Record Route is an optional field used to record address. 
In IPv4 header, 40 bytes are reserved for OPTIONS. For Record Route to stores, 1 byte is used to store type of option, 1 byte for length and 1 byte for pointer. Out of 40, 37 bytes are left. 
An IP4 address takes 32 bits or 4 bytes. 
We can store at most floor(37/4) = 9 router addresses. 

Alternate Solution
In IPV4 frame format there are 40 bytes of option field, out of which only 38 bytes can be used, 2 bytes are reserved . Also one IPV4 adress is of 4 bytes 
Thus maxmimum IPV4 address that can be hold <= 38/4 = 9 

3 3 votes

  1. IPv4 Header:

    • An IPv4 header has a variable-length section called the Options field. This field is used for special purposes, such as recording route information, security parameters, or timestamps.
    • The entire IPv4 header, including options, must not exceed 60 bytes. Typically, the base IPv4 header is 20 bytes, leaving 40 bytes for the Options field.

  2. Record Route (RR) Option:

    • The RR option is used to record the IP addresses of routers along the path of a packet.
    • The RR option has a fixed overhead of 4 bytes:
      • 1 byte for the option type.
      • 1 byte for the option length.
      • 1 byte for the pointer (indicating the next location for recording).
      • 1 byte reserved for padding.
    • The remaining space in the Options field is used to store the IP addresses of routers.

  3. IP Address Size:

    • Each IPv4 address is 4 bytes (32 bits).

Calculating the Maximum Number of Router Addresses:

  • Maximum size of the Options field: 40 bytes.
  • Fixed overhead for the RR option: 4 bytes.
  • Remaining space for IP addresses: 40−4=36 bytes.
  • Each IP address takes 4 bytes.

Thus, the maximum number of router addresses that can be recorded is:

Maximum addresses=Remaining space/Size of one IP address=36/4=9.

Answer:

The maximum number of router addresses that can be listed in the RR option field of an IPv4 header is 9.

1 1 vote
Record Routing Information Is Stored In The Option Field of the IPV4 Header .

The Option Field At Max Is of 40B . In It , The First 2B are reserved for option type(8 bits) and the length(8 bits) , hence leaving 38B for storing IPV4 Addresses.

Each IPV4 Address Is of 4B , Hence in the available 38B , we can store ONLY 9 IPV4 addresses .
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