TCP/IP Tutorial and Technical Overview - IBM Redbooks

Timestamp Request (13) and Timestamp Reply (14)
These two messages are for debugging and performance measurements. They
are not used for clock synchronization.
The sender initializes the identifier and sequence number (which is used if
multiple time stamp requests are sent), sets the originate time stamp, and sends
the datagram to the recipient. The receiving host fills in the receive and transmit
time stamps, changes the type to time stamp reply, and returns it to the original
sender. The datagram has two time stamps if there is a perceptible time
difference between the receipt and transmit times. In practice, most
implementations perform the two (receipt and reply) in one operation. This sets
the two time stamps to the same value. Time stamps are the number of
milliseconds elapsed since midnight UT (GMT).
See Figure 3-37 for details.
Figure 3-37 ICMP: Timestamp Request and Timestamp Reply
Address Mask Request (17) and Address Mask Reply (18)
An address mask request is used by a host to determine the subnet mask used
on an attached network. Most hosts are configured with their subnet mask or
masks. However some, such as diskless workstations, must obtain this
information from a server. A host uses RARP (see 3.5, “Reverse Address
Resolution Protocol (RARP)” on page 124) to obtain its IP address. To obtain a
subnet mask, the host broadcasts an address mask request. Any host in the
network that has been configured to send address mask replies will fill in the
subnet mask, convert the packet to an address mask reply, and return it to the
sender. The ICMP header code field is zero.
See Figure 3-38 on page 117 for more details.
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identifier sequence number
originate timestamp
receive timestamp
transmit timestamp