7.4 IP Timestamp Option

The IP timestamp option is similar to the record route option. Figure 7.7 shows the format of the IP timestamp option (compare with Figure 7.3).

Figure 7.7. General format of timestamp option in IP header.

The code field is 0 —44 for the timestamp option. The two fields len and ptr are the same as for the record route option: the total length of the option (normally 36 or 40) and a pointer to the next available entry (5, 9, 13, etc.).

The next two fields are 4-bit values: OF is the overflow field and FL is a flags field. The operation of the timestamp option is driven by the flags field, as shown in Figure 7.8.

Figure 7.8. Meaning of the flags value for timestamp option.

If a router can't add a timestamp because there's no room left, it just increments the overflow field.

The preferred value for the timestamps is the number of milliseconds past midnight, UTC, similar to the ICMP timestamp request and reply (Section 6.4). If this format is not available to a router, it can insert whatever time representation that it uses, but must then turn on the high-order bit of the timestamp to indicate the nonstandard value.

Given the limitations that we encountered with the record route option, things get worse with the timestamp option. If we record both IP addresses and timestamps (a flags of 1), we can store only four of these pairs. Recording only timestamps is next to useless because we have no indication regarding which timestamp corresponds to which router (unless we have a fixed topology that never changes). A flags of 3 is better, as we can then select which routers insert their timestamp. A more fundamental problem is that you probably have no control over how accurate the timestamp is at any given router. This makes it fruitless to try to measure hop times between routers using this IP option. We'll see that the traceroute program (Chapter 8) provides a better way of measuring hop times between routers.