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Most Linux systems use a unique Identifier for every ping process, and Sequence number is an increasing number within that process. Windows uses a fixed Identifier, which varies between Windows versions, and a Sequence number that is only reset at boot time. The Echo Reply is returned as:
Protocol Number Keyword Protocol References/RFC; 0x00 0 HOPOPT IPv6 Hop-by-Hop Option: RFC 8200: 0x01 1 ICMP Internet Control Message Protocol: RFC 792: 0x02 2 IGMP Internet Group Management Protocol: RFC 1112: 0x03 3 GGP Gateway-to-Gateway Protocol: RFC 823: 0x04 4 IP-in-IP IP in IP (encapsulation) RFC 2003: 0x05 5 ST Internet Stream Protocol ...
The client sets the segment's sequence number to a random value A. SYN-ACK: In response, the server replies with a SYN-ACK. The acknowledgment number is set to one more than the received sequence number i.e. A+1, and the sequence number that the server chooses for the packet is another random number, B.
Identifier and Sequence number can be used by the client to match the reply with the request that caused the reply. Originate timestamp is the time the sender last touched the message before sending it. Receive timestamp is the time the echoer first touched it on receipt. Transmit timestamp is the time the echoer last touched the message on ...
The checksum calculation is defined as follows: [5] The checksum field is the 16 bit one's complement of the one's complement sum of all 16 bit words in the header. For purposes of computing the checksum, the value of the checksum field is zero.
[1]: section 3.2.9 An alternative is to calculate a CRC using the right shifting CRC-32 (polynomial = 0xEDB88320, initial CRC = 0xFFFFFFFF, CRC is post complemented, verify value = 0x2144DF1C), which will result in a CRC that is a bit reversal of the FCS, and transmit both data and the CRC least significant bit first, resulting in identical ...
This is followed by a 2-bit type, a poll/final bit, and a 3-bit sequence number. (Or a 4-bit padding field followed by a 7-bit sequence number.) The first (least significant) 2 bits mean it is an S-frame. All S frames include a P/F bit and a receive sequence number as described above.
So, if we simply re-cast sequence numbers as 2's complement integers and allow there to be one more sequence number considered "less than" than there are sequence numbers considered "greater than", we should be able to use simple signed arithmetic comparisons instead of the logically incomplete formula proposed by the RFC.