Question

Consider a TCP connection between Host A and Host B. Suppose that the TCP segments traveling from Host A to Host B have source port number $x$ and destination port number $y$. What are the source and destination port numbers for the segments traveling from Host B to Host A?

Short answer

Source port: $y$, Destination port: $x$.

Step-by-step solution

Understanding the Problem

We have a TCP connection established between Host A and Host B. Host A sends segments to Host B with a specific source and destination port configuration. We need to determine what the source and destination ports would be for segments traveling in the reverse direction, from Host B to Host A.

Identifying the TCP Port Numbers

In a TCP connection, the source port number is the sending host's port, while the destination port number is the receiving host's port. In this case, segments from Host A to Host B have a source port of number $x$ and a destination port of number $y$.

Reversing the Direction

For segments traveling from Host B to Host A, the source and destination ports are swapped. Host B will use the original destination port $y$ as its source port, and the original source port $x$ as its destination port.

Conclusion

Therefore, the source port number from Host B to Host A will be $y$, and the destination port number will be $x$.

Key concepts

TCP Connection

A TCP connection acts like a private-channel phone call between two hosts on the internet. It ensures that data sent over the network reaches its destination accurately and in the correct order. TCP, which stands for Transmission Control Protocol, provides reliable and ordered delivery of data packets. This protocol establishes a connection first through a handshake process before starting the data exchange. Think of it like setting up a dedicated communication line, ensuring both parties are ready to send and receive information accurately.

• The TCP connection ensures no data is missed or corrupted during transmission.
• It manages data flow to prevent network congestion and collapse.

During a TCP connection, each transfer is broken into segments. These segments are crucial for reassembling data at the receiving end in the exact form it was sent. This process makes TCP essential for applications that require reliable data transfer, such as web browsing, email, and file downloads.

Source Port

In the realm of networking, the source port number plays a significant role during data transmission. The source port is akin to a return address on a letter. It indicates where the data segment originated from on the sending host. Knowing the source port allows the receiving host to respond back to the correct application on the sending machine. The port number is a 16-bit field, allowing for approximately 65,000 different port numbers.

• Source ports often change with each new connection or session.
• They help in mapping the data to the correct service or application on the sending host.

This dynamic allocation helps manage multiple simultaneous connections easily, making sure that the right responses are directed back to the corresponding communications. For example, when Host A sends data to Host B in a TCP connection, it assigns a unique source port so the responses can be correctly routed.

Destination Port

The destination port number in a TCP connection designates the receiving service on the target host. This port number tells the receiving host which application should process the incoming data. For instance, if you are browsing a website, your request to view a webpage is directed to a destination port 80 on the web server, a common port for HTTP traffic.

• Some common destination ports include 80 for HTTP, 443 for HTTPS, and 25 for SMTP (email).
• These ports help in directing incoming data to the appropriate protocols or services.

In our context of Host A communicating with Host B, Host B will use port number `y` as the destination port. On the server, specific applications listen for incoming requests on these standard ports, ensuring the network traffic reaches the correct application, maintaining seamless connectivity.