Question

The maximum payload of a TCP segment is 65,495 bytes. Why was such a strange number chosen?

Short answer

The maximum payload of 65,495 bytes is chosen from subtracting the typical IP and TCP header sizes from their maximum datagram sizes and considering implementation choices.

Step-by-step solution

Understanding TCP Header

In the Transmission Control Protocol (TCP), each segment contains a header and a data section. The TCP header is typically 20 bytes in size, although it can be larger if options are included. This header size must be subtracted from the total size to find the maximum payload.

Understanding IP Maximum Datagram Size

The maximum size of an IP datagram is 65,535 bytes. This size is fixed by the IPv4 specifications and represents the largest packet size (including header and data) that can be handled by IP.

Calculating Maximum Payload Size

To find the maximum payload size, we subtract the typical TCP header size from the maximum IP datagram size. Hence, the maximum TCP payload size is given by \(65,535 - 20 = 65,515\) bytes. This subtraction accounts for the minimum TCP header size.

Considering Fragmentation

Networks may add additional headers for tunneling, options, or might require segmentation for compatibility, reducing the effective payload size. Theoretical maximum payload (65,515 bytes) considers ideal conditions without additional overhead.

Confirming the Result

While the theoretical calculations show a payload size of 65,515 bytes, experience and practice with actual payload configuration may result in differing practical values, such as 65,495 bytes, due to protocol implementations or equipment limits.

Key concepts

Transmission Control Protocol

The Transmission Control Protocol (TCP) is a core component of internet communication. It ensures reliable delivery of data between two computers. This protocol breaks information into smaller packets to be transmitted over the network.
TCP guarantees ordered delivery by numbering the packets, so they arrive in the right sequence. This makes it possible to detect missing packets and request their retransmission. Furthermore, TCP checks each packet for errors via a checksum.
When a sender transmits data using TCP, it establishes a connection with the receiver. This process is known as the three-way handshake. Once the connection is set, data transfer begins, and eventually, the connection is gracefully terminated after successful delivery. Packets that move through a network using TCP adhere to various networking layers, including IP, making the relationship between TCP and IP crucial for seamless data exchange.

IP Datagram Size

In the realm of networking, an IP datagram is like the vehicle transporting data across the web. It's composed of a header and data payload. In IPv4, the maximum size an IP datagram can be is 65,535 bytes.
There are two parts to understand within this size: the header and the payload. The header typically takes up 20 bytes, although it can be larger if options are added. This portion contains essential information, such as source and destination addresses and flags crucial for routing the packet.
Understanding IP datagram size is essential, as it determines how large a packet can be without getting fragmented along the way. Fragmentation causes packets to be divided into smaller parts, which can complicate packet management.

TCP Header

All TCP packets have a header, which is the same for every packet used to transfer data. This header lays the foundation for managing data transmission.
The standard TCP header is 20 bytes in length, carrying critical information like sequence numbers, acknowledgment numbers, and flags for managing connections. However, options in the header can make it larger when additional features are necessary. For instance, timestamps or enhancements like window scaling can add more bytes to the TCP header.
When we determine payload size, the header length must be considered. Knowing the usual size helps optimize data transfer, allowing room for data within the maximum datagram limit.

Fragmentation

Fragmentation is an inevitable process when dealing with large packets that exceed a network's Maximum Transmission Unit (MTU). When this happens, IP takes on the responsibility of breaking down packets into smaller pieces to fit through an MTU.
Each fragment retains part of the original IP header and is reassembled at the destination to rebuild the original message. Unfortunately, fragmentation increases overhead, latency, and reduces efficiency, as more packets mean more handling.
Networks try to avoid fragmentation through path MTU discovery methods. By learning and adapting the packet size to the smallest MTU on a transmission path, fragmentation is reduced, enhancing overall network performance.

Protocol Implementations

Protocol implementations refer to how communication protocols like TCP/IP are realized in real-life hardware and applications. Each implementation can introduce minor variations, which might affect things like maximum payload sizes.
While theoretical values are calculated, practical implementations might vary due to different operating systems or network equipment limitations. For example, a network's specific equipment might not handle the maximum calculated limit, so payload sizes might be reduced slightly.
This emphasizes the importance of understanding both theory and practice. Testing actual networks helps see how different implementations can influence data transfer and adjustments needed to achieve optimal performance.