--- 1/draft-ietf-tcpm-cubic-01.txt 2016-08-07 08:16:56.219667983 -0700 +++ 2/draft-ietf-tcpm-cubic-02.txt 2016-08-07 08:16:56.251668788 -0700 @@ -1,25 +1,25 @@ TCP Maintenance and Minor Extensions (TCPM) WG I. Rhee Internet-Draft NCSU Intended status: Informational L. Xu -Expires: July 21, 2016 UNL +Expires: February 6, 2017 UNL S. Ha Colorado A. Zimmermann L. Eggert R. Scheffenegger NetApp - January 18, 2016 + August 5, 2016 CUBIC for Fast Long-Distance Networks - draft-ietf-tcpm-cubic-01 + draft-ietf-tcpm-cubic-02 Abstract CUBIC is an extension to the current TCP standards. The protocol differs from the current TCP standards only in the congestion window adjustment function in the sender side. In particular, it uses a cubic function instead of a linear window increase of the current TCP standards to improve scalability and stability under fast and long distance networks. BIC-TCP, a predecessor of CUBIC, has been a default TCP adopted by Linux since year 2005 and has already been @@ -42,21 +42,21 @@ Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - This Internet-Draft will expire on July 21, 2016. + This Internet-Draft will expire on February 6, 2017. Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents @@ -79,29 +79,29 @@ 3.6. Fast convergence . . . . . . . . . . . . . . . . . . . . 7 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 8 4.1. Fairness to standard TCP . . . . . . . . . . . . . . . . 8 4.2. Using Spare Capacity . . . . . . . . . . . . . . . . . . 10 4.3. Difficult Environments . . . . . . . . . . . . . . . . . 11 4.4. Investigating a Range of Environments . . . . . . . . . . 11 4.5. Protection against Congestion Collapse . . . . . . . . . 11 4.6. Fairness within the Alternative Congestion Control Algorithm. . . . . . . . . . . . . . . . . . . . . . . . 11 4.7. Performance with Misbehaving Nodes and Outside Attackers 11 - 4.8. Responses to Sudden or Transient Events . . . . . . . . . 11 - 4.9. Incremental Deployment . . . . . . . . . . . . . . . . . 11 + 4.8. Behavior for Application-Limited Flows . . . . . . . . . 11 + 4.9. Responses to Sudden or Transient Events . . . . . . . . . 11 + 4.10. Incremental Deployment . . . . . . . . . . . . . . . . . 12 5. Security Considerations . . . . . . . . . . . . . . . . . . . 12 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12 8.1. Normative References . . . . . . . . . . . . . . . . . . 12 8.2. Informative References . . . . . . . . . . . . . . . . . 13 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14 1. Introduction The low utilization problem of TCP in fast long-distance networks is well documented in [K03][RFC3649]. This problem arises from a slow increase of congestion window following a congestion event in a network with a large bandwidth delay product (BDP). Our experience [HKLRX06] indicates that this problem is frequently observed even in the range of congestion window sizes over several hundreds of packets @@ -214,35 +214,36 @@ 3. CUBIC Congestion Control The unit of all window sizes in this document is segments of the maximum segment size (MSS), and the unit of all times is seconds. 3.1. Window growth function CUBIC maintains the acknowledgment (ACK) clocking of Standard TCP by increasing congestion window only at the reception of ACK. The protocol does not make any change to the fast recovery and retransmit - of TCP-NewReno [RFC6582] and TCP-SACK [RFC2018]. During congestion - avoidance after fast recovery, CUBIC changes the window update - algorithm of Standard TCP. Suppose that W_max is the window size - before the window is reduced in the last fast retransmit and + of TCP, such as TCP-NewReno [RFC6582] and TCP-SACK [RFC2018]. During + congestion avoidance after fast recovery, CUBIC changes the window + update algorithm of Standard TCP. Suppose that W_max is the window + size before the window is reduced in the last fast retransmit and recovery. The window growth function of CUBIC uses the following function: W_cubic(t) = C*(t-K)^3 + W_max (Eq. 1) where C is a constant fixed to determine the aggressiveness of window growth in high BDP networks, t is the elapsed time from the last - window reduction,and K is the time period that the above function - takes to increase the current window size to W_max when there is no - further loss event and is calculated by using the following equation: + window reduction (measured right after the fast recovery), and K is + the time period that the above function takes to increase the current + window size to W_max if there is no further loss event and is + calculated by using the following equation: K = cubic_root(W_max*(1-beta_cubic)/C) (Eq. 2) where beta_cubic is the CUBIC multiplication decrease factor, that is, when a packet loss occurs, CUBIC reduces its current window cwnd to cwnd*beta_cubic. We discuss how we set C in the next Section in more details. Upon receiving an ACK during congestion avoidance, CUBIC computes the window growth rate during the next RTT period using Eq. 1. It sets @@ -254,26 +255,26 @@ then CUBIC is in the TCP friendly region (we describe below how to determine this window size of Standard TCP in term of time t). Otherwise, if cwnd is less than W_max, then CUBIC is the concave region, and if cwnd is larger than W_max, CUBIC is in the convex region. Below, we describe the exact actions taken by CUBIC in each region. 3.2. TCP-friendly region When receiving an ACK in congestion avoidance, we first check whether - the protocol is in the TCP region or not. This is done as follows. - We can analyze the window size of a TCP-friendly AIMD in terms of the - elapsed time t. Using a simple analysis in [FHP00], we can analyze - the average window size of additive increase and multiplicative - decrease (AIMD) with an additive factor alpha_aimd and a - multiplicative factor beta_aimd with the following function: + the protocol is in the TCP region or not. This is done by estimating + the average rate of the Standard TCP using a simple analysis + described in [FHP00]. It considers the Standard TCP as a special + case of an Additive Increase and Multiplicative Decrease algorithm + (AIMD), which has an additive factor alpha_aimd and a multiplicative + factor beta_aimd with the following function: AVG_W_aimd = [ alpha_aimd * (1+beta_aimd) / (2*(1-beta_aimd)*p) ]^0.5 (Eq. 3) By the same analysis, the average window size of Standard TCP is (1.5/p)^0.5, as the Standard TCP is a special case of AIMD with alpha_aimd=1 and beta_aimd=0.5. Thus, for Eq. 3 to be the same as that of Standard TCP, alpha_aimd must be equal to 3*(1-beta_aimd)/(1+beta_aimd). As AIMD increases its window by alpha_aimd per RTT, we can get the window size of AIMD in terms of @@ -504,26 +505,34 @@ in the same bottleneck links to an equal bandwidth share. When competing flows have different RTTs, their bandwidth shares are linearly proportional to the inverse of their RTT ratios. This is true independent of the level of statistical multiplexing in the link. 4.7. Performance with Misbehaving Nodes and Outside Attackers This is not considered in the current CUBIC. -4.8. Responses to Sudden or Transient Events +4.8. Behavior for Application-Limited Flows + + CUBIC does not raise its congestion window size if the flow is + currently limited by the application instead of the congestion + window. In cases of idle periods, t in Eq. 1 should not include the + idle time; otherwise, W_cubic(t) might be very high after restarting + from a long idle time. + +4.9. Responses to Sudden or Transient Events In case that there is a sudden congestion, a routing change, or a mobility event, CUBIC behaves the same as Standard TCP. -4.9. Incremental Deployment +4.10. Incremental Deployment CUBIC requires only the change of TCP senders, and does not require any assistant of routers. 5. Security Considerations This proposal makes no changes to the underlying security of TCP. 6. IANA Considerations