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Crowcroft 5 Expires: January 2, 2020 University of Cambridge 6 July 1, 2019 8 TCP ACK Pull 9 draft-gomez-tcpm-ack-pull-00 11 Abstract 13 Delayed Acknowledgments (ACKs) allow reducing protocol overhead in 14 many scenarios. However, in some cases, Delayed ACKs may 15 significantly degrade network and device performance in terms of link 16 utilization, latency, memory usage and/or energy consumption. This 17 document defines the TCP ACK Pull (AKP) mechanism, which allows a 18 sender to request the ACK for a data segment to be sent without 19 additional delay by the receiver. AKP makes use of one of the 20 reserved bits in the TCP header, which is defined in this 21 specification as the AKP flag. 23 Status of This Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at https://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on January 2, 2020. 40 Copyright Notice 42 Copyright (c) 2019 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents 47 (https://trustee.ietf.org/license-info) in effect on the date of 48 publication of this document. Please review these documents 49 carefully, as they describe your rights and restrictions with respect 50 to this document. Code Components extracted from this document must 51 include Simplified BSD License text as described in Section 4.e of 52 the Trust Legal Provisions and are provided without warranty as 53 described in the Simplified BSD License. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 58 2. Conventions used in this document . . . . . . . . . . . . . . 3 59 3. ACK Pull Mechanism . . . . . . . . . . . . . . . . . . . . . 4 60 4. The ACK Pull Flag . . . . . . . . . . . . . . . . . . . . . . 4 61 5. IANA Actions . . . . . . . . . . . . . . . . . . . . . . . . 4 62 6. Security Considerations . . . . . . . . . . . . . . . . . . . 5 63 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 5 64 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5 65 8.1. Normative References . . . . . . . . . . . . . . . . . . 5 66 8.2. Informative References . . . . . . . . . . . . . . . . . 5 67 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6 69 1. Introduction 71 Delayed Acknowledgments (ACKs) were specified with the aim to reduce 72 protocol overhead [RFC1122]. With Delayed ACKs, a TCP delays sending 73 an ACK by up to 500 ms (typically, 200 ms), and typically sends an 74 ACK for at least every second segment received in a stream of full- 75 sized segments. This allows combining several segments into a single 76 one (e.g. the application layer response to an application layer data 77 message, and the corresponding ACK), and it also saves up to one of 78 every two ACKs under many traffic patterns (e.g. bulk transfers). 79 The "SHOULD" requirement level for implementing Delayed ACKs in RFC 80 1122, along with its expected benefits, has led to a widespread 81 deployment of this mechanism. 83 However, there exist traffic patterns and scenarios for which Delayed 84 ACKs can actually be detrimental to performance. When a segment 85 carrying a message of a size up to one Maximum Segment Size (MSS) is 86 transferred, if the message does not elicit an application-layer 87 response, and a second data segment is not transferred earlier than 88 the Delayed ACK timeout, the ACK is unnecessarily delayed, with a 89 number of negative consequences. 91 When the Nagle algorithm is used, in some cases the sender may be 92 prevented from sending more data while awaiting the Delayed ACK. In 93 some high bit rate environment (e.g. Gigabit Ethernet) use cases, 94 such a delay may be very large, and link utilitzation may be 95 dramatically reduced, as the Delayed ACK timeout is several orders of 96 magnitude greater than the Round Trip Time (RTT) [RFC8490]. 98 Delayed ACKs are also detrimental in Internet of Things (IoT) 99 scenarios, where TCP is being increasingly used 100 [I-D.ietf-lwig-tcp-constrained-node-networks]. Many IoT devices, 101 such as sensors, transfer small messages (e.g. containing sensor 102 readings) rather infrequently, therefore if the receiver uses Delayed 103 ACKs, the ACK will often be unnecessarily delayed. The sender cannot 104 release the memory resources associated to a transferred data segment 105 until the ACK is received and processed. This may be a problem for 106 many IoT devices, which are typically memory-constrained, and may 107 even lead to subsequent packet drops if their scarce memory resources 108 are blocked while awaiting an ACK. Moreover, if the IoT device uses 109 a radio interface for communication, in some scenarios Delayed ACKs 110 will lead to increased energy consumption (e.g. with the radio 111 interface of the device staying in receive mode while awaiting the 112 ACK). Since many IoT devices run on small batteries, the device 113 lifetime may be significantly decreased. Furthermore, the delay 114 suffered by the ACK may interact negatively with layer two 115 mechanisms, especially in wireless network technologies where devices 116 remain in low-power states for long intervals [RFC 8352], potentially 117 leading to a further exacerbated delay (by even one or more orders of 118 magnitude). 120 One approach that cannot be recommended as a general solution to 121 solve the described problems is disabling Delayed ACKs at the 122 receiving TCP. In fact, the latter may interact with a wide variety 123 of devices and many of those may still benefit from the advantages of 124 Delayed ACKs. In addition, in some cases, a sender may offer a mixed 125 traffic pattern comprising single data segments that will lead to 126 unnecessarily delayed ACKs, with other data segments upon which 127 Delayed ACKs will act as intended. Therefore, the solution has to be 128 provided at a per-segment granularity. 130 This document defines the TCP ACK Pull (AKP) mechanism and an AKP 131 flag in the TCP header. AKP allows a sender to request an ACK to be 132 sent by a receiving TCP without additional delay upon reception of a 133 data segment, by setting the AKP flag in that data segment. The AKP 134 flag uses one of the reserved bits in the TCP header. More 135 specifically, the AKP flag uses bit 6 of byte 13 of the TCP header. 137 2. Conventions used in this document 139 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL","SHALL NOT", 140 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 141 document are to be interpreted as described in [RFC2119]. 143 3. ACK Pull Mechanism 145 When a TCP sender needs a data segment to be acknowledged by the 146 receiving TCP without additional delay, the sender sets the AKP flag 147 of the data segment TCP header. A receiving TCP conforming to this 148 specification MUST process the AKP flag of a received segment. If 149 the AKP flag is set, the receiving TCP MUST send an ACK without 150 additional delay, regardless of whether the receiving TCP uses the 151 Delayed ACKs mechanism. 153 4. The ACK Pull Flag 155 The AKP flag is defined as bit number 6 of the 13th byte of the TCP 156 header. Figure 1 illustrates bytes 13 and 14 of the TCP header, 157 including the AKP flag. 159 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 160 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 161 | | | A | R | C | E | U | A | P | R | S | F | 162 | Header Length |Reservd| K | v | W | C | R | C | S | S | Y | I | 163 | | | P | d | R | E | G | K | H | T | N | N | 164 +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 166 Figure 1: Definition of the AKP field within bytes 13 and 14 of the 167 TCP Header. 169 (Note: as of the writing, bit 7 in the above figure is reserved, 170 although this may change with the publication of 171 [I-D.ietf-tcpm-accurate-ecn].) 173 5. IANA Actions 175 This document assigns bit 6 of the TCP header flags to the AKP flag. 176 This flag will be defined as shown in Figure 2: 178 +-----+-------------------+-----------+ 179 | Bit | Name | Reference | 180 +-----+-------------------+-----------+ 181 | 6 | AKP (ACK Pull) | RFC XXXX | 182 +-----+-------------------+-----------+ 184 Figure 2 186 [TO BE REMOVED: IANA is requested to update the existing entry in the 187 Transmission Control Protocol (TCP) Header Flags registration 188 (https://www.iana.org/assignments/tcp-header-flags/tcp-header- 189 flags.xhtml#tcp-header-flags-1) for Bit 6 to 'AKP (ACK Pull)'.] 191 6. Security Considerations 193 TCP ACK Pull introduces a possible Denial of Service (DoS) attack on 194 a resource-constrained receiver. An attacker might send a large 195 number of messages to a victim node, requesting an immediate ACK in 196 response to each one of them. This attack is easily avoided by 197 ignoring the TCP ACK Pull flag. 199 7. Acknowledgments 201 Stuart Cheshire, Ted Lemon, Michael Scharf, and Christoph Paasch 202 participated in a discussion that was seminal to this document. 204 Carles Gomez has been funded in part by the Spanish Government 205 (Ministerio de Ciencia, Innovacion y Universidades) through the Jose 206 Castillejo grant CAS18/00170 and by European Regional Development 207 Fund (ERDF) and the Spanish Government through project 208 TEC2016-79988-P, AEI/FEDER, UE. His contribution to this work has 209 been carried out during his stay as a visiting scholar at the 210 Computer Laboratory of the University of Cambridge. 212 8. References 214 8.1. Normative References 216 [RFC1122] Braden, R., Ed., "Requirements for Internet Hosts - 217 Communication Layers", STD 3, RFC 1122, 218 DOI 10.17487/RFC1122, October 1989, 219 . 221 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 222 Requirement Levels", BCP 14, RFC 2119, 223 DOI 10.17487/RFC2119, March 1997, 224 . 226 8.2. Informative References 228 [I-D.ietf-lwig-tcp-constrained-node-networks] 229 Gomez, C., Crowcroft, J., and M. Scharf, "TCP Usage 230 Guidance in the Internet of Things (IoT)", draft-ietf- 231 lwig-tcp-constrained-node-networks-08 (work in progress), 232 June 2019. 234 [I-D.ietf-tcpm-accurate-ecn] 235 Briscoe, B., Kuehlewind, M., and R. Scheffenegger, "More 236 Accurate ECN Feedback in TCP", draft-ietf-tcpm-accurate- 237 ecn-08 (work in progress), March 2019. 239 [RFC8352] Gomez, C., Kovatsch, M., Tian, H., and Z. Cao, Ed., 240 "Energy-Efficient Features of Internet of Things 241 Protocols", RFC 8352, DOI 10.17487/RFC8352, April 2018, 242 . 244 [RFC8490] Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S., 245 Lemon, T., and T. Pusateri, "DNS Stateful Operations", 246 RFC 8490, DOI 10.17487/RFC8490, March 2019, 247 . 249 Authors' Addresses 251 Carles Gomez 252 UPC 253 C/Esteve Terradas, 7 254 Castelldefels 08860 255 Spain 257 Email: carlesgo@entel.upc.edu 259 Jon Crowcroft 260 University of Cambridge 261 JJ Thomson Avenue 262 Cambridge, CB3 0FD 263 United Kingdom 265 Email: jon.crowcroft@cl.cam.ac.uk