Các tùy chọn Internet Dấu thời gian được sử dụng để ghi lại thời gian mà một gói IP đến tại mỗi bộ định tuyến IP trong đường dẫn giữa các nguồn và máy chủ đích. Dấu thời gian tùy chọn Internet là tương tự như tùy chọn Route Record trong đó các nút gửi | Chapter 5 Internet Protocol IP 121 Internet Timestamp Option Code Option Length Next Slot Pointer Overflow Flags First IP Address First Timestamp lllllll 68 mini III III 111 111 num IIIIIII IIIIIII IIIIIII IIIIIII IIIIIII IIIIIII IIIIIII num num num num The Internet Timestamp option is used to record the time that an IP datagram arrived at each IP router in the path between the source and destination host. The Internet Timestamp option is similar to the Record Route option in that the sending node creates blank entries in the IP header that routers fill out as the packet travels through the IP internetwork. Each entry consists of the router s IP address and a 32-bit integer timestamp that indicates the number of milliseconds since midnight Universal Time. If Universal Time is not being used the high-order bit of the timestamp field is set to 1. Note To use Internet timestamps Internet timestamping must be enabled on all the routers in the path between the source and destination hosts. It is common for routers to either not support Internet timestamping or have it disabled. The Internet Timestamp option contains the following fields Option Code Set to 68 Copy Bit 0 Option Class 2 Option Number 4 . Option Length Set by the sending host to the number of bytes in the Internet Timestamp option. Next Slot Pointer Set to the byte offset starting at 1 within the Internet Timestamp option of the next slot for the recording of the IP address and timestamp. The Next Slot Pointer field s minimum value is 5. Overflow Set by routers to indicate the number of routers that were unable to record their IP address and timestamp. Flags Set by the sending host to indicate the format of the IP Address Timestamp slots. When Flags is set to 0 the IP address is omitted. This allows up to nine timestamps to be recorded. When Flags is set to 1 the IP address is recorded allowing up to four IP address timestamp pairs to be recorded. The Internet Timestamp option format shown assumes Flags is
