The Echo Pair

Identifier and Sequence

Identifier and sequence fields make the tiny request and reply pair line up.

highlighted = computed this step

The pair keeps two fields

The identifier and sequence connect the reply back to the request. Here both messages carry the same pair.

id=0x1234,seq=1\text{id}=0x1234,\quad \text{seq}=1
Identifier and sequenceThe request and reply fields are decoded from exact ICMP bytes.Echo request64 bits / 8 bytes0x0800e5ca12340001byte 0byte 1byte 2byte 300001000000000001110010111001010byte 4byte 5byte 6byte 700010010001101000000000000000001type8code0checksum0xe5caidentifier0x1234sequence1Echo reply64 bits / 8 bytes0x0000edca12340001byte 0byte 1byte 2byte 300000000000000001110110111001010byte 4byte 5byte 6byte 700010010001101000000000000000001type0code0checksum0xedcaidentifier0x1234sequence1

Identifier matches

The identifier is 0x1234 in the request and 0x1234 in the reply.

0x1234=0x12340x1234=0x1234
Identifier and sequenceThe request and reply fields are decoded from exact ICMP bytes.Echo request64 bits / 8 bytes0x0800e5ca12340001byte 0byte 1byte 2byte 300001000000000001110010111001010byte 4byte 5byte 6byte 700010010001101000000000000000001type8code0checksum0xe5caidentifier0x1234sequence1Echo reply64 bits / 8 bytes0x0000edca12340001byte 0byte 1byte 2byte 300000000000000001110110111001010byte 4byte 5byte 6byte 700010010001101000000000000000001type0code0checksum0xedcaidentifier0x1234sequence1

Sequence matches

The sequence number is 1 in both messages. The field is small here, but it is still checked from bytes.

sequence=1\text{sequence}=1
Identifier and sequenceThe request and reply fields are decoded from exact ICMP bytes.Echo request64 bits / 8 bytes0x0800e5ca12340001byte 0byte 1byte 2byte 300001000000000001110010111001010byte 4byte 5byte 6byte 700010010001101000000000000000001type8code0checksum0xe5caidentifier0x1234sequence1Echo reply64 bits / 8 bytes0x0000edca12340001byte 0byte 1byte 2byte 300000000000000001110110111001010byte 4byte 5byte 6byte 700010010001101000000000000000001type0code0checksum0xedcaidentifier0x1234sequence1

Summary

The reply is paired to the request by exact identifier and sequence fields. No timing or operating-system ping behavior is modeled.

same id and sequence=1\text{same id and sequence}=1
Identifier and sequenceThe request and reply fields are decoded from exact ICMP bytes.Echo request64 bits / 8 bytes0x0800e5ca12340001byte 0byte 1byte 2byte 300001000000000001110010111001010byte 4byte 5byte 6byte 700010010001101000000000000000001type8code0checksum0xe5caidentifier0x1234sequence1Echo reply64 bits / 8 bytes0x0000edca12340001byte 0byte 1byte 2byte 300000000000000001110110111001010byte 4byte 5byte 6byte 700010010001101000000000000000001type0code0checksum0xedcaidentifier0x1234sequence1