TCP Features

To provide the services discussed earlier—reliable communication, stream delivery, flow control, error control, and full-duplex communication—TCP uses several important features.

The TCP numbering system, which consists of:

1. Byte numbering
2. Sequence numbers
3. Acknowledgment numbers

These are fundamental to TCP's flow and error control.

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1. Numbering System

TCP keeps track of the data being transmitted and received.

An important point is that TCP does not number segments directly.

Instead, TCP uses two fields in the TCP header:

• Sequence Number
• Acknowledgment Number

These numbers refer to bytes (octets) rather than segments.

TCP Header
┌─────────────────────────┬─────────────────────────┐
│   Sequence Number       │ Acknowledgment Number   │
└─────────────────────────┴─────────────────────────┘

Therefore:

TCP is byte-oriented in its numbering system.

This numbering is used for:

• identifying data,
• error control,
• retransmission,
• flow control.

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2. Byte Numbering

TCP assigns a number to every byte of data transmitted in a connection.

For example, suppose an application wants to send:

6000 bytes of data

TCP assigns a number to every byte.

Importantly, TCP does not necessarily start numbering from 0.

TCP chooses an arbitrary starting number between:

0 and 232−1

Suppose TCP chooses:

1057

as the number of the first byte.

Then the 6000 bytes are numbered:

1057, 1058, 1059, ... , 7056

because:

$$1057+6000-1=7056$$

Why does TCP number bytes?

Byte numbering allows TCP to determine:

• which bytes have been received,
• which bytes are missing,
• which bytes need to be retransmitted,
• which bytes should be acknowledged.

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3. Sequence Number

After numbering the bytes, TCP assigns a sequence number to each segment.

The sequence number of a segment identifies the byte number of the first data byte carried by that segment.

There are two important rules.

Rule 1: First segment

The sequence number of the first segment is called the:

Initial Sequence Number (ISN)

The ISN is an arbitrarily generated/random number.

Rule 2: Subsequent segments

The sequence number of the next segment is:

$$\text{Sequence number of previous segment}+\text{number of bytes carried by previous segment}$$

Therefore, the sequence number does not simply increase by 1 for every segment.

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Example: Sequence Numbers

Suppose a TCP connection transfers 5000 bytes.

The first byte is numbered:

10001

The data is divided into five segments, each carrying 1000 bytes.

The sequence numbers are:

Segment	Bytes carried	Sequence number
1	1000	10001
2	1000	11001
3	1000	12001
4	1000	13001
5	1000	14001

Why?

The first segment carries bytes:

10001 to 11000

Therefore, the next segment begins with:

11001

Similarly:

Segment 1 → 10001
Segment 2 → 11001
Segment 3 → 12001
Segment 4 → 13001
Segment 5 → 14001

Key idea

The sequence number identifies the first byte of data in the segment.

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4. Sequence Number in Both Directions

TCP provides full-duplex communication.

Therefore, both sides independently number their bytes.

For example:

             TCP Connection

Host A ─────────────────────────→ Host B
       Sequence numbers: 5000, ...

Host A ←───────────────────────── Host B
       Sequence numbers: 9000, ...

The starting sequence number in one direction does not have to be the same as that in the other direction.

Thus:

TCP maintains an independent byte-numbering system for each direction.

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5. Control Segments and Sequence Numbers

Not every TCP segment carries application data.

Some segments carry only control information, such as those used for:

• connection establishment,
• connection termination,
• connection abortion.

Normally, a segment that carries no user data does not consume a sequence number.

However, some control segments are treated as if they carry one imaginary byte.

Therefore, they consume one sequence number.

For example:

Control segment
       ↓
Consumes 1 sequence number
       ↓
Next sequence number increases by 1

This allows the control segment itself to be acknowledged.

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6. Acknowledgment Number

TCP uses the acknowledgment number to inform the sender about the bytes successfully received.

But there is an important rule:

The acknowledgment number specifies the sequence number of the next byte that the receiver expects.

It does not indicate the number of the last byte received.

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Example

Suppose the receiver has successfully received bytes:

10001
10002
10003
...
11000

The next byte it expects is:

11001

Therefore, the receiver sends:

ACK = 11001

So:

Last byte received = 11000
Next byte expected  = 11001
ACK number          = 11001

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7. Cumulative Acknowledgment

TCP acknowledgment numbers are cumulative.

Suppose the receiver sends:

ACK = 5643

This means that the receiver has successfully received all bytes up to byte 5642.

Therefore:

ACK = 5643
        ↓
All bytes through 5642 have been received
        ↓
Next expected byte = 5643

The receiver is essentially saying:

"I have received everything up to byte 5642; please send me byte 5643 next."

Important

An acknowledgment number of 5643 does not mean that 5643 bytes have been received.

The first byte may have had a number other than 0.

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8. Sequence Number vs Acknowledgment Number

This distinction is very important for understanding TCP.

Feature	Sequence Number	Acknowledgment Number
Refers to	Bytes being sent	Bytes received
Meaning	Number of the first byte in the segment	Number of the next byte expected
Direction	Identifies transmitted data	Confirms received data
Nature	Byte-based	Byte-based
Example	SEQ = 10001	ACK = 11001

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Simple Example

Suppose Host A sends 1000 bytes beginning with byte number 5000.

Host A → Host B

SEQ = 5000
Data = 1000 bytes

The bytes are:

5000 ─────────────────── 5999

If Host B receives all these bytes correctly, it sends:

ACK = 6000

because byte 6000 is the next byte expected.

             1000 bytes
A ─────────────────────────→ B
   SEQ = 5000

A ←───────────────────────── B
   ACK = 6000

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9. Why TCP Uses Byte Numbering

Byte numbering allows TCP to provide reliable communication.

Suppose three segments are transmitted:

Segment 1: bytes 1000–1999
Segment 2: bytes 2000–2999
Segment 3: bytes 3000–3999

If Segment 2 is lost:

Segment 1 ✓
Segment 2 ✗
Segment 3 ✓

TCP can identify the missing portion because the byte numbers reveal the gap.

The receiver can use acknowledgment information to indicate what byte it expects.

Thus, byte numbering supports:

• error detection
• retransmission
• ordering of data
• cumulative acknowledgment
• flow control

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Summary of the TCP Numbering System

                 TCP Numbering
                      │
          ┌───────────┴───────────┐
          │                       │
    Byte Numbering          Acknowledgment
          │                       │
          ↓                       ↓
 Every byte gets a number    Next byte expected
          │                       │
          ↓                       ↓
   Sequence Number           Cumulative ACK
          │
          ↓
 First byte in a segment

Remember these three statements

1. TCP numbers bytes, not segments.
2. The sequence number identifies the first byte carried by a segment.
3. The acknowledgment number identifies the next byte expected and is cumulative.

These numbering mechanisms form the basis for TCP's reliable data transfer, error control, and flow control