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4.2. Generating Acknowledgments

Delayed ACK Algorithm​

The delayed ACK algorithm specified in [RFC1122] SHOULD be used by a TCP receiver.

When using delayed ACKs, a TCP receiver MUST NOT excessively delay acknowledgments. Specifically:

  • An ACK SHOULD be generated for at least every second full-sized segment
  • An ACK MUST be generated within 500 ms of the arrival of the first unacknowledged packet

Clarification on SHOULD vs MUST​

The requirement that an ACK "SHOULD" be generated for at least every second full-sized segment is listed in [RFC1122] in one place as a SHOULD and another as a MUST. Here we unambiguously state it is a SHOULD.

We also emphasize that this is a SHOULD, meaning that an implementor should indeed only deviate from this requirement after careful consideration of the implications. See the discussion of "Stretch ACK violation" in [RFC2525] and the references therein for a discussion of the possible performance problems with generating ACKs less frequently than every second full-sized segment.

Full-Sized Segment Agreement​

In some cases, the sender and receiver may not agree on what constitutes a full-sized segment. An implementation is deemed to comply with this requirement if it sends at least one acknowledgment every time it receives 2*RMSS bytes of new data from the sender, where RMSS is the Maximum Segment Size specified by the receiver to the sender (or the default value of 536 bytes, per [RFC1122], if the receiver does not specify an MSS option during connection establishment).

Path MTU Discovery Consideration​

The sender may be forced to use a segment size less than RMSS due to:

  • The maximum transmission unit (MTU)
  • The path MTU discovery algorithm
  • Other factors

For instance, consider the case when the receiver announces an RMSS of X bytes but the sender ends up using a segment size of Y bytes (Y < X) due to path MTU discovery (or the sender's MTU size). The receiver will generate stretch ACKs if it waits for 2*X bytes to arrive before an ACK is sent. Clearly this will take more than 2 segments of size Y bytes.

Therefore, while a specific algorithm is not defined, it is desirable for receivers to attempt to prevent this situation, for example, by acknowledging at least every second segment, regardless of size.

Maximum Delay​

Finally, we repeat that an ACK MUST NOT be delayed for more than 500 ms waiting on a second full-sized segment to arrive.

Out-of-Order Segments​

Out-of-order data segments SHOULD be acknowledged immediately, in order to accelerate loss recovery.

To trigger the fast retransmit algorithm, the receiver SHOULD send an immediate duplicate ACK when it receives a data segment above a gap in the sequence space.

To provide feedback to senders recovering from losses, the receiver SHOULD send an immediate ACK when it receives a data segment that fills in all or part of a gap in the sequence space.

ACK Frequency Limit​

A TCP receiver MUST NOT generate more than one ACK for every incoming segment, other than to update the offered window as the receiving application consumes new data (see [RFC813] and page 42 of [RFC793]).

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