IP Header Flashcards
What information is contained in the Version (Protocol) field of the IP header? How long is the field?
Which version of the Internet Protocol was used for the packet in question, IPv4 or IPv6.
4 bits
I wonder why 3 bits wouldn’t be sufficient:
111 = 7
Maybe they had future IP versions in mind.
What information is contained in the Header Length field in the IP header? How long is the field?
The length of that packet’s IP header. The length of an IP header can vary because some fields are optional, and the information contained in fields can vary in length from packet to packet.
4 bits
What is the purpose of the Type of Service (Differentiated Services) field of the IP header? How long is the field?
This field is used to indicate the quality of service a packet should receive.
8 bits:
“The first three bits of this field are known as precedence bits and are ignored as of today. The next 4 bits represent type of service and the last bit is left unused. The 4 bits that represent TOS are : minimize delay, maximize throughput, maximize reliability and minimize monetary cost.” http://www.thegeekstuff.com/2012/03/ip-protocol-header/
What information is contained in the Total Length field in the IP header? How long is the field?
The content of the TL field indicates the length of the entire packet.
16 bits
What information is contained in the Identification field in the IP header? How long is the field?
The ID field contains a 16 bit value that identifies the packet for sequencing purposes (and probably other purposes as well). The ID value is incremented with each packet sent to a destination host.
16 bits
What information is contained in the Flags field in the IP header? How long is the field?
The Flags field is a 3 bit that’s used to indicate that a packet either should not be fragmented, or that it is a fragment and more fragments will follow. The first bit is reserved, and is not used. The second bit is the “don’t fragment” bit. The third bit is the “more fragment” bit.
If the “don’t fragment” bit is turned on, the packet will never be fragmented, and will be dropped if the need for fragmentation arises. If the “more fragment” bit is on, this indicates that the current packet is a fragment of an IP datagram, and that more fragments comprising the same datagram are on the way.
3 bits
What is the purpose of the Fragment Offset field in the IP header? How long is the field?
“In case of fragmented IP data grams, this field contains the offset( in terms of 8 bytes units) from the start of IP datagram. So again, this field is used in reassembly of fragmented IP datagrams.” http://www.thegeekstuff.com/2012/03/ip-protocol-header
13 bits
What is the purpose of the Time to Live field in the IP header? How long is the field?
The TTL field implements a sort of maximum hop count for IP packets. The TTL starts at 255 (I think) and is decremented every time the packet is routed (switches do not alter the TTL). If this value reaches 0 the packet is dropped regardless of whether it’s reached its destination. This prevents packets from becoming eternally stuck in a routing loop.
8 bits (easy to remember, since the value starts at 255)
What is the purpose of the Protocol (type) field in the IP header? How long is the field?
The Protocol field indicates which Transport Layer protocol was used in the creation of the packet on the source device, and thus which protocol should be used to handle the segment when it reaches the destination device.
8 bits
What is the purpose of the Header Checksum field in the IP header? How long is the field?
The source device applies an algorithm to the length of the header, and fills in this field with the resulting value. When the destination device receives the packet, it applies the same algorithm to the header and compares the result. If the values don’t match, it means that the packet has been damaged or corrupted.
16 bits
What is the purpose of the Options field in the IP header? How long is the field?
The field identifies various options that may be active for an IP datagram. (Nobody seems to know what this field is for. I certainly don’t. Maybe it’s a secret.)
We don’t even know how long the options field is - that’s how mysterious it is.
What are the fields in an IPv4 header?
Version (Protocol) Header Length (IHL)
Type of Service (TOS or Differentiated Services)
Total Length (TL)
Identification (ID)
Flags
Fragment Offset
Time to Live (TTL)
Protocol (Protocol Type)
Checksum
Source IP
Destination IP
Options
Data
