TRANSPORT CONTROL PROTOCOL (TCP)

TCP is a very clever design! One can quickly notice the ressemblance between TCP and a coherent telephone conversation between two or more people.

The sender starts with "Hello. May I speak with Alex ?". The recipient replies "Sure. May I ask who's calling ?". And the sender answers "This is Peter". This 3 step process is how TCP initiates a transmission. A SYN packet including the sending address is sent, the recipient answers with and acknowledge-syn packet (ACK SYN) including his address and the sender acknowledge with an ACK packet. Note that the recipient answer "Yes. This is Alex speaking" is not allowed since the sender has not identified himself at this point.

From here, the conversation can follow in both directions, provided that either one of the parties has clearly understood what the other had to say. TCP does this through the Sequence (SEQ) and Acknowledge (ACK) numbers. For simple explanatory purpose, every sent packet has a SEQ number which is equal with the number of octets sent (and acknowledged to be received) and an ACK number equal to the number of octets received up to the current packet. For the receiver, these numbers reverse. If these numbers don't match the packet is re-transmitted or the transmission stops in case the error cannot be corrected. Just as for a real conversation one part can send a bulk of packets before receiving an acknowledge. And even more, "What did you just say ?", "Can you say it again ?" or "Slow down a little, I can't follow" are usual situations implemented by TCP.

The conversation can finish with "Bye" followed by "OK, I'll talk to you later" from the receiver which in TCP terms it's called graceful. TCP implements a graceful end by sending a FIN packet followed by a received ACK FIN packet. Or, the conversation may have a not-so-graceful end when one part just hangs-up. In this case TCP sends a RST (reset) packet closing the connection.

TCP is the protocol mostly utilized on the Internet. It's usually called TCP/IP because in all cases TCP packets are encapsulated in IP packets.

TCP HEADER FORMAT

OCTET 1,2 Source Port(SRC_PORT)

OCTET 3,4 Destination Port(DEST_PORT)

OCTET 5,6,7,8 Sequence Number(SEQ)

OCTET 9,10,11,12 Acknowledgement Number(ACK)

OCTET 13,14 Data Offset (4 bit)+Reserved (6 bit)+

Control Flags (6 bit) (DTO, FLG)

OCTET 15,16 Window(WIN)

OCTET 17,18 Checksum(TCP_SUM)

OCTET 19,20 Urgent Pointer(URP)

OCTET 21,22,23 Options(OPT)

OCTET 24 Padding

OCTET 25,26…Data

TCP/IP Packet Example:

7E 21 45 00 00 4B 57 49 40 00 FA 06 85 77 C7 B6 78 0E CE D6 95 50 00 6E 04 9F 74 5B EE A2 59 9A 00 0E 50 18 24 00 E3 2A 00 00 2B 4F 4B 20 50 61 73 73 77 6F 72 64 20 72 65 71 75 69 72 65 64 20 66 6F 72 20 61 6C 65 78 75 72 2E 0D 0A 67 B2 7E

Start 7E

SEP 21

IP Header 45 00 00 4B 57 49 40 00 FA 06 85 77 C7 B6 78 0E CE D6 95 50

TCP Header 00 6E 04 9F 74 5B EE A2 59 9A 00 0E 50 18 24 00 E3 2A 00 00

Data

2B 4F 4B 20 50 61 73 73 77 6F 72 64 20 72 65 71 75 69 72 65 64 20 66 6F 72 20 61 6C 65 78 75 72 2E 0D 0A

FCS 67 B2

Stop 7E

TCP Header:

SRC_PORT=110 DEST_PORT=1183 SEQ=745BEEA2 ACK=599A000E DTO=5 FLG=18 WIND=9216

TCP_SUM=E32A URP=0000 (No Options)

Data:

+OK Password required for alexur\r\n

Control Flags (FLG=18):

FLG=00011000 Urgent Pointer URG=0

Acknowledgment ACK=1

Push FunctionPSH=1

Reset connection RST=0

Synchronization SYN=0

Finished data FIN=0

State = ACK-PSH

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