TCP/IP Tutorial and Technical Overview - IBM Redbooks

Where:
– 0: Reserved, must be zero.
– DF (Do not Fragment): 0 means allow fragmentation; 1 means do not
allow fragmentation.
– MF (More Fragments): 0 means that this is the last fragment of the
datagram; 1 means that additional fragments will follow.
► Fragment Offset: This is used to aid the reassembly of the full datagram. The
value in this field contains the number of 64-bit segments (header bytes are
not counted) contained in earlier fragments. If this is the first (or only)
fragment, this field contains a value of zero.
► Time to Live: This field specifies the time (in seconds) the datagram is
allowed to travel. Theoretically, each router processing this datagram is
supposed to subtract its processing time from this field. In practise, a router
processes the datagram in less than 1 second. Therefore, the router
subtracts one from the value in this field. The TTL becomes a hop-count
metric rather than a time metric. When the value reaches zero, it is assumed
that this datagram has been traveling in a closed loop and is discarded. The
initial value should be set by the higher-level protocol that creates the
datagram.
► Protocol Number: This field indicates the higher-level protocol to which IP
should deliver the data in this datagram. These include:
– 0: Reserved
– 1: Internet Control Message Protocol (ICMP)
– 2: Internet Group Management Protocol (IGMP)
– 3: Gateway-to-Gateway Protocol (GGP)
– 4: IP (IP encapsulation)
– 5: Stream
– 6: Transmission Control Protocol (TCP)
– 8: Exterior Gateway Protocol (EGP)
– 9: Interior Gateway Protocol (IGP)
– 17: User Datagram Protocol (UDP)
– 41:Simple Internet Protocol (SIP)
– 50: SIPP Encap Security Payload (ESP)
– 51: SIPP Authentication Header (AH)
– 89: Open Shortest Path First (OSPF) IGP
The complete list is in STD 2 – Assigned Internet Numbers.
► Header Checksum: This field is a checksum for the information contained in
the header. If the header checksum does not match the contents, the
datagram is discarded.
► Source IP Address: The 32-bit IP address of the host sending this datagram.
Chapter 3. Internetworking protocols 101