The Internet Protocol
(Part II)
LESSON 9

IP Datagram Format:
Options

• IP OPTIONS is a variable length field used primarily for testing and debugging
• This capability must always be implemented with IP.
• Options appear contiguously in the field (not separated) and consist of a single octet option code for one octet options, or vary in length for longer options.
• COPY (1-bit):
  • Determines how gateways treat options during fragmentation:
    • 1 => copy option to ALL fragments.
    • 0 => copy options to first fragment only.

• OPTION CLASS (2-bits):
  • Specifies the general class of the option.
• OPTION NUMBER (5-bits):
  • Determines the specific option within that class.

IP Options:

• Record Route Option:
  • Allows the source to create an empty list of IP addresses and arrange for each gateway that handles the datagram to add it’s IP address to the list.

Record Route Option:

• CODE (8-bits):
  • Code field should equal 7 for record route.
• LENGTH (8-bits):
  • Total length of the option field, including the header
• POINTER (8-bits):
  • Incremented in 4-octet intervals, it points to the next available address slot in the option field.
• IP ADDRESS (variable length):
  • Allocated space in blocks of 32-bits are to hold each IP address encountered.
  • Since the header is a limited size, the number of IP addresses recorded is also limited.
  • All gateways that handle the datagram add their IP addresses (outgoing link only) and if space is full, simply forward the datagram without making any changes.

Source Route Option:

• Source Route Option:
  • Provides a way for the sender to dictate a path through the internet.
  • Source route can force a datagram to traverse a path, even if it would not normally be chosen by a gateway.
  • Good for testing a physical path and debugging problems with it.

• CODE (8-bits):
  • Code field should equal 37 for record route.
• LENGTH (8-bits):
  • Total length of the option field, including the header
• POINTER (8-bits):
  • Incremented in 4-octet intervals, it points to the next available address slot in the option field.
• IP ADDRESS OF NEXT HOP (variable length):
  • Allocated space in blocks of 32-bits are to hold each IP address that a datagram must traverse
  • Since the header is a limited size, the number of hops is limited.
  • Usually five or less gateways are included in the next hop addresses, due to the limited size of the header fields.
• Two forms of Source Routing:
  1. Strict Source Routing - specifies path via included IP addresses in the option field. Must strictly follow the path, and errors occur if any path is NOT specified. All addresses must be included.
  2. Loose Source Routing - also specifies the path, but allows for multiple hops between each specified address (more flexible).

Timestamp Option:

• Timestamp Option:
  • Records each gateway’s IP address (32-bits) along the path and a 32-bit timestamp generated by that gateway.
  • Similar to Record Route Option, but can only record half as many addresses.

• CODE (8-bits):
  • Code field should equal 68 for record route.
• LENGTH (8-bits):
  • Total length of the option field, including the header
• POINTER (8-bits):
  • Incremented in 4-octet intervals, it points to the next available address slot in the option field.
• OFLOW (4-bits):
  • Integer count of gateways who could not supply the timestamp due to lack of header space.
• FLAGS (4-bits):
  • Specifies the exact format of the timestamp and option fields.

Timestamp Option Flags:

• FIRST IP ADDRESS (32-bits):
  • Allocated space to hold the IP address of a gateway that is required to produce a timestamp value.
• FIRST TIMESTAMP (32-bits):
  • A 32-bit timestamp is produced and recorded at each participating gateway hop.
• Timestamps give the time (in milliseconds) and date (Universal Time - at prime meridian)
• Recording IP addresses with timestamps eliminates ambiguity as to whom set the timestamp.

Security Option:

• Security Option:
  • Allow a datagram to have different levels of secure status.
  • The option values are assigned by the U.S. Defense Intelligence Agency, and range from unclassified to top secret.
  • Routers detect the security status and based on the value, determine whether packets should route beyond certain gateway boundaries.

COPY Flag Settings:

• The COPY flag should NOT be set for a Record Route since many different paths may result.
• The COPY flag should be set for a Source Route since all fragments must follow a specific route, thus all fragments need this option code copied to their datagrams.

IP Input and Output:

• As mentioned at the beginning of this lesson, IP has two major parts:
  • Input processing module
  • Output processing module

Pseudo-Network Interface:

• IP supports a pseudo-network interface which is used for:
  • Communication with the local host.
  • Debugging purposes (Loopback Address).

Datagram Formation and Extraction:

• Datagrams are formed by:
  • Encapsulating data passed from a transport layer protocol into a datagram.
  • Prefixing header information.
• The previously mentioned header fields are prepended to the data to allow correct routing and controlling of the datagram.
• When a datagram arrives:
  • The header and data portions are split.
  • Header information is processed to determine if the datagram has reached it’s correct destination.
    • If so, the remaining portion of the header is processed and the data is passed to the transport layer.
    • If not, the datagram is either re-routed or dropped.

Examples of Fragmentation in Gateways:

• Datagrams can be fragmented numerous times during their traversal over an Internet:

All work was written, produced, and is copyrighted by Daniel Z. Tabor Jr.
Page created by Daniel Z. Tabor Jr. Copyright ©1995 Illusion Industries Inc.