TCP Network – Why is Packet Size Limited?

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I was reading: this answer to "Maximum packet size for a TCP connection", where it says:

The absolute limitation on TCP packet size is 64K (65535 bytes), but
in practicality this is far larger than the size of any packet you
will see, because the lower layers (e.g. ethernet) have lower packet
sizes.

A few questions remain unclear for me:

  1. Why don't we just send one single packet? Why do we need to split content into multiple packets (ignoring the size limit)?

  2. If a lower layer (like the internet layer) has a lower packet size what does this have to do with TCP packet size limitations? A higher layer (than the internet layer) can add as much data as it wants to.

Best Answer

Why we don't just send one single packet? why we need to split content into multiple pockets (ignoring size limit).

That would just lead back to circuit-switched networks like the original PSTN (Public Switched Telephone Network). The government funded research into packet-switched networks (result: Internet) to overcome the limitations of circuit-switched networks.

In a circuit-switched network, or what you propose, one caller or packet would monopolize the circuit or path until it is done, not giving anyone or any other process process a chance to use the circuit or path until it is done. Breaking things up into smaller packets means that you can share the circuit among callers or processes. Each IP packet is routed independently, so a packet follows a path to the destination, regardless of the path any other packet took to the same destination. If the path loses a link, then the routers in the path can reroute packets to a different path to the destination, and the sender does not know or care.

The big driver of the government funding was the threat of disaster (including nuclear war, which was a big threat in the 1960s and 1970s). If you are making a call (say to respond to ICBM launches), and the telephone company central office is destroyed, then you lose the call and need to start all over, manually rerouting the call. The same holds true for a giant data packet. If you break things up into smaller packets, and there is an interruption in the path, the rest of the packets can automatically be re-routed around the damage.

So, in the simple case you get to share the circuit or path, and you lose very little in the event of a circuit or path interruption.

If lower layer (like internet) have lower packet size why this has to do with TCP packet size limitations? a higher layer (than internet) can add as much data as it wants to.

TCP takes a stream of data (can be very large) and segments it into PDUs (Protocol Data Units) we call segments. The segments fit into the IP packets, which fit into the data-link protocol frames. TCP is a very large subject, far too large to explain it all in a site like this.

Once you understand the reasons for the different layers in the network stack (abstraction and encapsulation), you will see how that works. Basically, the data-link protocol is responsible for delivering frames in the local network, IP is responsible for delivering packets between networks, and a transport protocol like TCP is responsible for delivering datagrams between host processes on different hosts.

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