The Retransmit

Same Bytes Again

The retransmitted segment repeats the missing bytes with the same sequence number.

highlighted = computed this step

Client sends the same segment again

The retransmitted segment has the same sequence 1001 and the same payload bytes.

retrans seq=1001\text{retrans seq}=1001
Retransmitted segment bytesThe retransmitted segment is decoded from the same exact bytes as the missing segment.Retransmit header160 bits / 20 bytes0x9c4001bb000003e9000013895018100000000000byte 0byte 1byte 2byte 310011100010000000000000110111011byte 4byte 5byte 6byte 700000000000000000000001111101001byte 8byte 9byte 10byte 1100000000000000000001001110001001byte 12byte 13byte 14byte 1501010000000110000001000000000000byte 16byte 17byte 18byte 1900000000000000000000000000000000source-port40000destination-port443sequence-number1001acknowledgment-number5001data-offset5reserved0x0flags0x18window0x1000checksum0x0000urgent-pointer0Payload16 bits / 2 bytes0x6869byte 0byte 10110100001101001payload0x6869

The bytes are identical

The helper checks that the retransmitted segment bytes match the lost segment bytes exactly.

segments identical=1\text{segments identical}=1
Retransmitted segment bytesThe retransmitted segment is decoded from the same exact bytes as the missing segment.Retransmit header160 bits / 20 bytes0x9c4001bb000003e9000013895018100000000000byte 0byte 1byte 2byte 310011100010000000000000110111011byte 4byte 5byte 6byte 700000000000000000000001111101001byte 8byte 9byte 10byte 1100000000000000000001001110001001byte 12byte 13byte 14byte 1501010000000110000001000000000000byte 16byte 17byte 18byte 1900000000000000000000000000000000source-port40000destination-port443sequence-number1001acknowledgment-number5001data-offset5reserved0x0flags0x18window0x1000checksum0x0000urgent-pointer0Payload16 bits / 2 bytes0x6869byte 0byte 10110100001101001payload0x6869

Timeline shows the resend

The timeline shows the client sending the data again before the ACK can move forward.

retransmit messages=1\text{retransmit messages}=1
Same bytes againA toy TCP timeline shows the retransmit followed by the ACK that advances.Same bytes againClientESTABLISHEDESTABLISHEDESTABLISHEDESTABLISHEDServerwaitingwaitingreceivedreceivedDATA hi lost seq=1001 ACK, PSHduplicate ACK seq=5001 ACKDATA hi again seq=1001 ACK, PSHACK advances seq=5001 ACK

Summary

Retransmission means the missing bytes are sent again with the same sequence number.

sequence=1001\text{sequence}=1001
Retransmitted segment bytesThe retransmitted segment is decoded from the same exact bytes as the missing segment.Retransmit header160 bits / 20 bytes0x9c4001bb000003e9000013895018100000000000byte 0byte 1byte 2byte 310011100010000000000000110111011byte 4byte 5byte 6byte 700000000000000000000001111101001byte 8byte 9byte 10byte 1100000000000000000001001110001001byte 12byte 13byte 14byte 1501010000000110000001000000000000byte 16byte 17byte 18byte 1900000000000000000000000000000000source-port40000destination-port443sequence-number1001acknowledgment-number5001data-offset5reserved0x0flags0x18window0x1000checksum0x0000urgent-pointer0Payload16 bits / 2 bytes0x6869byte 0byte 10110100001101001payload0x6869