| Internet-Draft | TCP ACK Rate Request Option | March 2022 |
| Gomez & Crowcroft | Expires 7 September 2022 | [Page] |
TCP Delayed Acknowledgments (ACKs) is a widely deployed mechanism that allows reducing protocol overhead in many scenarios. However, Delayed ACKs may also contribute to suboptimal performance. When a relatively large congestion window (cwnd) can be used, less frequent ACKs may be desirable. On the other hand, in relatively small cwnd scenarios, eliciting an immediate ACK may avoid unnecessary delays that may be incurred by the Delayed ACKs mechanism. This document specifies the TCP ACK Rate Request (TARR) option. This option allows a sender to request the ACK rate to be used by a receiver, and it also allows to request immediate ACKs from a receiver.¶
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Delayed Acknowledgments (ACKs) were specified for TCP with the aim to reduce protocol overhead [RFC1122]. With Delayed ACKs, a TCP delays sending an ACK by up to 500 ms (often 200 ms, with lower values in recent implementations such as ~50 ms also reported), and typically sends an ACK for at least every second segment received in a stream of full-sized segments. This allows combining several segments into a single one (e.g. the application layer response to an application layer data message, and the corresponding ACK), and also saves up to one of every two ACKs, under many traffic patterns (e.g. bulk transfers). The "SHOULD" requirement level for implementing Delayed ACKs in RFC 1122, along with its expected benefits, has led to a widespread deployment of this mechanism.¶
However, there exist scenarios where Delayed ACKs contribute to suboptimal performance. We next roughly classify such scenarios into two main categories, in terms of the congestion window (cwnd) size and the Maximum Segment Size (MSS) that would be used therein: i) "large" cwnd scenarios (i.e. cwnd >> MSS), and ii) "small" cwnd scenarios (e.g. cwnd up to ~MSS).¶
In "large" cwnd scenarios, increasing the number of data segments after which a receiver transmits an ACK beyond the typical one (i.e. 2 when Delayed ACKs are used) may provide significant benefits. One example is mitigating performance limitations due to asymmetric path capacity (e.g. when the reverse path is significantly limited in comparison to the forward path) [RFC3449]. Another advantage is reducing the computational cost both at the sender and the receiver, and reducing network packet load, due to the lower number of ACKs involved.¶
In many "small" cwnd scenarios, a sender may want to request the receiver to acknowledge a data segment immediately (i.e. without the additional delay incurred by the Delayed ACKs mechanism). In high bit rate environments (e.g. data centers), a flow's fare share of the available Bandwidth Delay Product (BDP) may be in the order of one MSS, or even less. For an accordingly set cwnd value (e.g. cwnd up to MSS), Delayed ACKs would incur a delay that is several orders of magnitude greater than the RTT, severely degrading performance. Note that the Nagle algorithm may produce the same effect for some traffic patterns in the same type of environments [RFC8490]. In addition, when transactional data exchanges are performed over TCP, or when the cwnd size has been reduced, eliciting an immediate ACK from the receiver may avoid idle times and allow timely continuation of data transmission and/or cwnd growth, contributing to maintaining low latency.¶
Further "small" cwnd scenarios can be found in Internet of Things (IoT) environments. Many IoT devices exhibit significant memory constraints, such as only enough RAM for a send buffer size of 1 MSS. In that case, if the data segment does not elicit an application-layer response, the Delayed ACKs mechanism unnecessarily contributes a delay equal to the Delayed ACK timer to ACK transmission. The sender cannot transmit a new data segment until the ACK corresponding to the previous data segment is received and processed.¶
With the aim to provide a tool for performance improvement in both "large" and "small" cwnd scenarios, this document specifies the TCP ACK Rate request (TARR) option. This option allows a sender to request the ACK rate to be used by a receiver, and it also allows to request immediate ACKs from a receiver.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL","SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].¶
A TCP endpoint announces that it supports the TARR option by including the TARR option format (with the appropriate Length value, see Section 4) in packets that have the SYN bit set.¶
Upon reception of a SYN segment carrying the TARR option, a TARR- option-capable endpoint MUST include the TARR option in the SYN-ACK segment sent in response.¶
The next two subsections define the sender and receiver behaviors for devices that support the TARR option, respectively.¶
A TCP sender MUST NOT include the TARR option in TCP segments to be sent if the TCP receiver does not support the TARR option.¶
A TCP sender MAY request a TARR-option-capable receiver to modify the ACK rate of the latter to one ACK every R data segments received from the sender. This request is performed by the sender by including the TARR option in the TCP header of a segment. The TARR option carries the R value requested by the sender (see section 4).¶
When a TCP sender needs a data segment to be acknowledged immediately by a TARR-option-capable receiving TCP, the sender includes the TARR option in the TCP header of the data segment, with a value of R equal to 1.¶
A TCP segment carrying retransmitted data is not required to include a TARR option.¶
A TCP sender that uses time-based loss detection (e.g. RACK-TLP [RFC8985] MAY use the Ignore Order feature, by which the sender indicates that it has a reordering tolerance of R packets. Otherwise, such feature SHOULD NOT be used. The Ignore Order feature can be used by setting the Ignore Order field of the TARR option to True (see Section 4).¶
A receiving TCP conforming to this specification MUST process a TARR option present in a received segment.¶
A TARR-option-capable receiving TCP SHOULD modify its ACK rate to one ACK every R received data segments from the sender. If a TARR-option-capable TCP receives a segment carrying the TARR option with R=1, the receiving TCP SHOULD send an ACK immediately.¶
If packet reordering occurs, a TCP receiver should send an immediate duplicate ACK when an out-of-order segment arrives [RFC2581], thus in such cases the TCP receiver will not comply with the request. Note also that the receiver might be unable to send ACKs at the requested rate (e.g., due to lack of resources); on the other hand, the receiver might opt not to fulfill a request for security reasons (e.g., to avoid or mitigate an attack by which a large number of senders request disabling delayed ACKs simultaneously and send a large number of data segments to the receiver).¶
The request to modify the ACK rate of the receiver holds until the next segment carrying a TARR option is received.¶
A TARR-option-capable TCP that receives a TARR option with the Ignore Order (I) field set to True (see Section 4), MUST NOT send an ACK after each reordered data segment. Instead, it MUST continue to send one ACK every R received data segments. Otherwise (i.e., Ignore Order = False), such a receiver will need to send an ACK after each reordered data segment received.¶
The TARR option presents two different formats that can be identified by the corresponding format length. For packets that have the SYN bit set, the TARR option has the format shown in Fig. 1.¶
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Kind | Length | ExID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Kind: The Kind field value is TBD.¶
Length: The Length field value is 4 bytes.¶
ExID: The experiment ID field size is 2 bytes, and its value is 0x00AC.¶
For packets that do not have the SYN bit set, the TARR option has the format and content shown in Fig. 2.¶
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Kind | Length | ExID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | R |V|I| +-+-+-+-+-+-+-+-+
Kind: The Kind field value is TBD.¶
Length: The Length field value is 5 bytes.¶
ExID: The experiment ID field size is 2 bytes, and its value is 0x00AC.¶
R: The size of this field is 6 bits. The field carries the ACK rate requested by the sender. The minimum value of R is 1.¶
OPTION 1: the R field corresponds to the binary encoding of the requested ACK rate. The maximum value of R is 63. A receiver MUST ignore an R field with all bits set to zero.¶
OPTION 2: the R field is composed of two subfields: the 4 leftmost bits represent a mantissa (m) and the 2 rightmost bits represent an exponent (e). The value of the requested ACJ rate is obtained as R = (m+1)*2^(2*e). The maximum value of R is 1024.¶
ReserVed (V): The size of this field is 1 bit. This bit is reserved for future use.¶
Ignore Order (I): The size of this field is 1 bit. This field either has the value 1 ("True") or 0 ("False"). When this field is set to True, the receiver MUST NOT send an ACK after each reordered data segment. Instead, it SHOULD continue to send one ACK every R received data segments.¶
In some scenarios, especially when conditions are rather stable and/or predictable, setting the ACK rate once for the whole lifetime of a TCP connection may be suitable. However, there are also cases where it may be desirable to modify the ACK rate during the lifetime of a connection.¶
The ACK rate to be used may depend on the cwnd value used by the sender, which can change over the lifetime of a connection. cwnd will start at a low value and grow rapidly during the slow-start phase, then settle into a reasonably consistent range for the congestion-avoidance phase - assuming the underlying bandwidth-delay product (BDP) remains constant. Phenomena such as routing updates, link capacity changes or path load changes may modify the underlying BDP significantly; the cwnd should be expected to change accordingly, prompting the need for ACK rate updates.¶
TARR can also be used to suppress Delayed ACKs in order to allow measuring the RTT of each packet in specific intervals (e.g., during flow start-up), and allow a different ACK rate afterwards.¶
A Linux receiver has a heuristic to detect slow start and suppress Delayed ACKs just for that period. However, some slow start variants (e.g., HyStart, HyStart++, etc.) may alter the ending of slow start, thus confusing the heuristics of the receiver. To avoid slow start sender behavior ossification, an explicit signal such as TARR may be useful.¶
Another reason to modify the ACK rate might be reducing the ACK load. The sender may notice that the ACKs it receives cover more segments than the ACK rate requested, indicating that ACK decimation is occurring en route. The sender may then decide to reduce the ACK frequency to reduce receiver workload and network load up to the ACK decimation point.¶
Future TCP specifications may also permit Congestion Experienced (CE) marks to appear on pure ACKs [I-D.ietf-tcpm-generalized-ecn]. This might involve more frequent ACK rate updates (e.g., once an RTT), as the sender probes around an operating point.¶
This document specifies a new TCP option (TCP ACK Rate Request) that uses the shared experimental options format [RFC6994], with ExID in network-standard byte order.¶
The authors plan to request the allocation of ExID value 0x00AC for the TCP option specified in this document.¶
The TARR option opens the door to new security threats. This section discusses such new threats, and suggests mitigation techniques.¶
An attacker might be able to impersonate a legitimate sender, and forge an apparently valid packet intended for the receiver, in order to intentionally communicate a bad R value to the latter with the aim to damage communication or device performance. For example, in a small cwnd scenario, using a too high R value may lead to exacerbated RTT increase and throughput decrease. In other scenarios, a too low R value may contribute to depleting the energy of a battery-operated receiver at a faster rate or may lead to increased network packet load.¶
While Transport Layer Security (TLS) [RFC8446] is strongly recommended for securing TCP-based communication, TLS does not protect TCP headers, and thus cannot protect the TARR option fields carried by a segment. One approach to address the problem is using network-layer protection, such as Internet Protocol Security (IPsec) [RFC4301]. Another solution is using the TCP Authentication Option (TCP-AO), which provides TCP segment integrity and protection against replay attacks [RFC5925].¶
While it is relatively hard for an off-path attacker to attack an unprotected TCP session, it is RECOMMENDED for a TARR receiver to use the guidance and attack mitigation given in [RFC5961]. The TARR option MUST be ignored on a packet that is deemed invalid.¶
A TARR receiver might opt not to fulfill a request to avoid or mitigate an attack by which a large number of senders request disabling delayed ACKs simultaneously and send a large number of data segments to the receiver.¶
Bob Briscoe, Jonathan Morton, Richard Scheffenegger, Neal Cardwell, Michael Tuexen, Yuchung Cheng, Matt Mathis, Jana Iyengar, Gorry Fairhurst, and Stuart Cheshire provided useful comments and input for this document. Jana Iyengar suggested including a field to allow a sender communicate its tolerance to reordering. Jonathan Morton and Bob Briscoe provided the main input for Section 5.¶
Carles Gomez has been funded in part by the Spanish Government through project PID2019-106808RA-I00, and by Secretaria d'Universitats i Recerca del Departament d'Empresa i Coneixement de la Generalitat de Catalunya 2017 through grant SGR 376.¶