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The Network Path to a Feed Handler

The systems fundamentals round asks about TCP, UDP and multicast, and a trading system uses all three in one place: the feed handler that turns exchange packets into an order book, and the gateway that sends orders back. This lesson follows a packet along that path and names the places where it can be delayed or lost.

Two protocols, two jobs

TCP
DeliveryGuaranteed, with retransmission
OrderPreserved
After a lost packetEvery later byte waits for the retransmission
Receivers per sendOne connection
Used forOrder entry and drop copies
UDP multicast
DeliveryNot guaranteed
OrderNot preserved
After a lost packetOnly that packet is gone
Receivers per sendEvery host that joined the group
Used forMarket data

A trading system needs both: every order must arrive exactly once, and a late price is worth less than the next one.

TCP's guarantee has a cost that matters on a feed. When one segment is lost, the receiver cannot hand any later bytes to the application until the retransmission arrives. That is head-of-line blocking, and during a busy open it would hold the whole feed behind one lost packet. UDP multicast avoids it and scales with subscribers: the exchange sends each packet once to a group address, and every host that has joined the group receives it.

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