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Can we not simply make network delays predictable?

Note that a circuit in a telephone network is very different from a TCP connection: a circuit is a fixed amount of reserved bandwidth which nobody else can use while the circuit is established, whereas the packets of a TCP connection opportunistically use whatever network bandwidth is available. You can give TCP a variable-sized block of data (e.g., an email or a web page), and it will try to transfer it in the shortest time possible. While a TCP connection is idle, it doesn’t use any bandwidth.11

If datacenter networks and the internet were circuit-switched networks, it would be possible to establish a guaranteed maximum round-trip time when a circuit was set up. However, they are not: Ethernet and IP are packet-switched protocols, which suffer from queueing and thus unbounded delays in the network. These protocols do not have the concept of a circuit.

Why do datacenter networks and the internet use packet switching? The answer is that they are optimized for bursty traffic. A circuit is good for an audio or video call, which needs to transfer a fairly constant number of bits per second for the duration of the call. On the other hand, requesting a web page, sending an email, or transferring a file doesn’t have any particular bandwidth requirement—we just want it to complete as quickly as possible.

If you wanted to transfer a file over a circuit, you would have to guess a bandwidth allocation. If you guess too low, the transfer is unnecessarily slow, leaving network capacity unused. If you guess too high, the circuit cannot be set up (because the network cannot allow a circuit to be created if its bandwidth allocation cannot be guaranteed). Thus, using circuits for bursty data transfers wastes network capacity and makes transfers unnecessarily slow. By contrast, TCP dynamically adapts the rate of data transfer to the available network capacity.

There have been some attempts to build hybrid networks that support both circuit switching and packet switching, such as Atm.[1] [2] InfiniBand has some similarities [35]: it implements end-to-end flow control at the link layer, which reduces the need for queueing in the network, although it can still suffer from delays due to link congestion [36]. With careful use of quality of service (QoS, prioritization and scheduling of packets) and admission control (rate-limiting senders), it is possible to emulate circuit switching on packet networks, or provide statistically bounded delay [25, 32].

  • [1] ii. Except perhaps for an occasional keepalive packet, if TCP keepalive is enabled.
  • [2] Asynchronous Transfer Mode (ATM) was a competitor to Ethernet in the 1980s [32], but it didn’t gainmuch adoption outside of telephone network core switches. It has nothing to do with automatic tellermachines (also known as cash machines), despite sharing an acronym. Perhaps, in some parallel universe, theinternet is based on something like ATM—in that universe, internet video calls are probably a lot more reliable than they are in ours, because they don’t suffer from dropped and delayed packets.
 
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