TCP Basics

Noted on January 18, 2022

About

TCP (Transmission Control Protocol) is defined in RFC793.
TCP implements the following to ensure reliability over an unreliable underlying protocol:

Arrival acknowledgement
Error checking
Retransmission
Order of arrival
Flow control
QoS

History

Before standardized

RFC 33: New HOST-HOST Protocol

It was finalized in RFC 33 in early 1970, and deployed to all nodes on the ARPANET in December 1970.

First paper “A Protocol for Packet Network Intercommunication.”

Vinton Cerf and Robert Kahn submitted this at the IEEE Transactions on Communications conference in 1974.

RFC 675: SPECIFICATION OF INTERNET TRANSMISSION CONTROL PROGRAM

RFC 675 was published in December 1974, but it was not fully functional in 1974.

RFC 793: TRANSMISSION CONTROL PROTOCOL

RFC 675 was revised and two separate RFCs: - RFC 791 “Internet Protocol” - RFC 793 “Transmission Control Protocol”

After standardized (some feature added and implemented)

RFC 879: The TCP Maximum Segment Size

RFC 1011: OFFICIAL INTERNET PROTOCOLS

RFC 1122: Requirements for Internet Hosts – Communication Layers

RFC 2581: TCP Congestion Control

RFC 2873: TCP Processing of the IPv4 Precedence Field

RFC 5681: TCP Congestion Control

RFC 6013: TCP Cokkie Transactions (TCPCT)

RFC 6093: On the Implementation of the TCP Urgent Mechanism

RFC 6298: Computing TCP’s Retransmission Timer

RFC 6429: TCP Sender Clarification for Persist Condition

RFC 6528: Defending against Sequence Number Attacks

RFC 6691: TCP Options and Maximum Segment Size (MSS)

RFC 7413: TCP Fast Open

RFC 793bis: Transmission Control Protocol (TCP) Specification

“IPv4” means 4th version of the original RFC ever published on the internet.

TCP specification

Datagram

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port          |       Destination Port        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sequence Number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Acknowledgment Number                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Data |           |U|A|P|R|S|F|                               |
   | Offset| Reserved  |R|C|S|S|Y|I|            Window             |
   |       |           |G|K|H|T|N|N|                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Checksum            |         Urgent Pointer        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Options                    |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             data                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Source Port / Destination Port

A port number is a 16-bit integer ranging from 0 to 65535.

0 - 1023: System Ports

1024 - 49151: User Ports

49152 - 65535: Dynamic and/or Private Ports

NOTE: Different OS are known to use different ephemeral port ranges.
Linux: 32768 - 6100 controlled by sysctl knob net.ipv4.ip_local_port_range and selected sequentially (not randomization)
MacOS: 49152-65535 (IANA suggestion)
Windows (until XP): 1025-5000
Windows (later Vista, 7, Server 2008): 49152-65535 (IANA suggestion)

Sequence Number / Acknowledgement Number

Flags

NS - ECN-nonce

CWR - Congestion window reduced

ECE - ECN-Echo

URG - Urgent

ACK - Acknowledgement

PSH - Push

RST - Reset

SYN - Synchronization

FIN - Finish

Window Size

Options (without reserved, obsoleted)

Kind	Length	Description	Reference
0	-	End of Option List	[RFC793]
1	-	No-Operation	[RFC793]
2	4	Maximum Segment Size	[RFC793]
3	3	Window Scale	[RFC7323]
4	2	SACK Permitted	[RFC2018]
5	N	SACK	[RFC2018]
8	10	Timestamps	[RFC7323]
16		Skeeter	[Stev_Knowles]
17		Bubba	[Stev_Knowles]
18	3	Trailer Checksum Option	[Subbu_Subramaniam][Monroe_Bridges]
20		SCPS Capabilities	[Keith_Scott]
21		Selective Negative Acknowledgements	[Keith_Scott]
22		Record Boundaries	[Keith_Scott]
23		Corruption experienced	[Keith_Scott]
24		SNAP	[Vladimir_Sukonnik]
26		TCP Compression Filter	[Steve_Bellovin]
27	8	Quick-Start Response	[RFC4782]
28	4	User Timeout Option (also, other known unauthorized use) [***][1]	[RFC5482]
29		TCP Authentication Option (TCP-AO)	[RFC5925]
30	N	Multipath TCP (MPTCP)	[RFC8684]
34	variable	TCP Fast Open Cookie	[RFC7413]
69	N	Encryption Negotiation (TCP-ENO)	[RFC8547]
253	N	RFC3692-style Experiment 1 (also improperly used for shipping products) [*]	[RFC4727]
254	N	RFC3692-style Experiment 2 (also improperly used for shipping products) [*]	[RFC4727]

(Source of the table: https://www.iana.org/assignments/tcp-parameters/tcp-parameters.xhtml)

MSS

The maximum segment size (MSS) is a parameter of the options field of the TCP header that specifies the largest amount of data.

NOTE: MSS Clamping

Timestamp

SACK Permitted

SACK

Window Scaling

TCP State Machine

Features

3-wayhandshake

Flow Control

Flow control is a function that notifies each other of the amount that can be received and avoids a buffer overflow and a heavy load on the receiver.

Window Scaling (RFC 1323) window-size = window(0~65535) * 2^shift.cnf(0~14 or -1)
rwnd (receive window size)

sysctl net.ipv4.tcp_window_scaling