| Network Working Group | S. Bensley |
| Internet-Draft | Microsoft |
| Intended status: Standards Track | L. Eggert |
| Expires: August 18, 2014 | NetApp |
| D. Thaler | |
| Microsoft | |
| February 14, 2014 |
Datacenter TCP (DCTCP): TCP Congestion Control for Datacenters
draft-bensley-tcpm-dctcp-00
This memo describes Datacenter TCP (DCTCP), an improvement to TCP congestion control for datacenter traffic. DCTCP enhances Explicit Congestion Notification (ECN) processing to estimate the fraction of bytes that encounter congestion, rather than simply detecting that some congestion has occurred. DCTCP then scales the TCP congestion window based on this estimate. This method achieves high burst tolerance, low latency, and high throughput with shallow-buffered switches.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
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 August 18, 2014.
Copyright (c) 2014 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 carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Large datacenters necessarily need a large number of network switches to interconnect the servers in the datacenter. Therefore, a datacenter can greatly reduce its capital expenditure by leveraging low cost switches. However, low cost switches tend to have limited queue capacities and thus are more susceptible to packet loss due to congestion.
Network traffic in the datacenter is often a mix of short and long flows, where the short flows require low latency and the long flows require high throughput. Datacenters also experience incast bursts, where many endpoints send traffic to a single server at the same time. For example, this is a natural consequence of MapReduce algorithms. The worker nodes complete at approximately the same time, and all reply to the master node concurrently.
These factors place some conflicting demands on the switch's queue occupancy:
Standard TCP congestion control [RFC5681] relies on segment loss to detect congestion. This does not meet the demands described above. First, the short flows will start to experience unacceptable latencies before packet loss occurs. Second, by the time TCP congestion control kicks in on the sender, most of the incast burst has already been dropped.
[RFC3168] describes a mechanism for using Explicit Congestion Notification from the switch for early detection of congestion, rather than waiting for segment loss to occur. However, this method only detects the presence of congestion, not the extent. In the presence of mild congestion, it reduces the TCP congestion window too aggressively and unnecessarily affects the throughput of long flows.
Datacenter TCP (DCTCP) enhances Explicit Congestion Notification (ECN) processing to estimate the fraction of bytes that encounter congestion, rather than simply detecting that some congestion has occurred. DCTCP then scales the TCP congestion window based on this estimate. This method achieves high burst tolerance, low latency, and high throughput with shallow-buffered switches.
There are three components involved in the DCTCP algorithm:
The switch indicates congestion to the end nodes by setting the CE codepoint in the IP header as specified in Section 5 of [RFC3168]. For example, the switch may be configured with a congestion threshold. When a packet arrives at the switch and the switch's queue length is greater than the congestion threshold, the switch sets the CE codepoint in the packet. However, the actual algorithm for marking congestion is an implementation detail of the switch and will generally not be known to the sender and receiver.
According to Section 6.1.3 of [RFC3168], the receiver sets the ECE flag if any of the packets being acknowledged had the CE code point set. The receiver then continues to set the ECE flag until it receives a packet with the Congestion Window Reduced (CWR) flag set. However, the DCTCP algorithm requires more detailed congestion information. In particular, the sender must be able to determine the number of sent bytes that encountered congestion. Thus, the scheme described in [RFC3168] does not suffice.
One possible solution is to ACK every packet and set the ECE flag in the ACK if and only if the CE code point was set in the packet being acknowledged. However, this prevents the use of delayed ACKs, which are an important performance optimization in datacenters. Instead, we introduce a new Boolean TCP state variable, DCTCP Congestion Encountered (DCTCP.CE), which is initialized to false and stored in the Transmission Control Block (TCB). When sending an ACK, the ECE flag is set if and only if DCTCP.CE is true. When receiving packets, the CE codepoint is processed as follows:
The sender estimates the fraction of sent bytes that encountered congestion. The current estimate is stored in a new TCP state variable, DCTCP.Alpha, which is initialized to 1 and updated as follows:
where
Whenever the TCP congestion estimate is updated, the sender also updates the TCP congestion window as follows:
Thus, when there is no congestion at all, Alpha equals zero, and the congestion window is left unchanged. When there is total congestion, Alpha equals one, and the congestion window is reduced by half. Lower levels of congestion will result in correspondingly lesser reductions to the congestion window.
In order to update DCTCP.Alpha, we make use of the TCP state variables defined in [RFC0793], and introduce three additional TCP state variables:
The congestion estimator on the sender processes acceptable ACKs as follows:
As noted in Section 2.3, the implementation must choose a suitable estimation gain. [DCTCP10] provides a theoretical basis for selecting the gain. However, it may be more practical to use experimentation to select a suitable gain for a particular network and workload. The Microsoft implementation of DCTCP in Windows Server 2012 uses a fixed estimation gain of 1/16.
The implementation must also decide when to use DCTCP. Datacenter servers may need to communicate with endpoints outside the datacenter, where DCTCP is unsuitable or unsupported. Thus, a global configuration setting to enable DCTCP will generally not suffice. DCTCP may be configured based on the IP address of the remote endpoint. Microsoft Windows Server 2012 also supports automatic selection of DCTCP if the estimated RTT is less than 10 msec, under the assumption that if the RTT is low, then the two endpoints are likely on the same datacenter network.
Since DCTCP relies on congestion marking by the switch, DCTCP can only be deployed in datacenters where the network infrastructure supports ECN. The switches may also support configuration of the congestion threshold used for marking. [DCTCP10] provides a theoretical basis for selecting the congestion threshold, but as with estimation gain, it may be more practical to rely on experimentation or simply to use the device's default configuration.
DCTCP requires changes on both the sender and the receiver, so in a heterogeneous datacenter, all the endpoints should support DCTCP and should be configured to use it.
DCTCP enhances ECN and thus inherits the security considerations discussed in [RFC3168]. The processing changes introduced by DCTCP do not exacerbate these considerations or introduce new ones. In particular, with either algorithm, the network infrastructure or the remote endpoint can falsely report congestion and thus cause the sender to reduce its congestion window. However, this is no worse than what can be achieved by simply dropping packets.
This document has no actions for IANA.
The DCTCP algorithm was originally proposed and analyzed in [DCTCP10] by Mohammad Alizadeh, Albert Greenberg, Dave Maltz, Jitu Padhye, Parveen Patel, Balaji Prabhakar, Sudipta Sengupta, and Murari Sridharan.
| [RFC0793] | Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981. |
| [RFC3168] | Ramakrishnan, K., Floyd, S. and D. Black, "The Addition of Explicit Congestion Notification (ECN) to IP", RFC 3168, September 2001. |
| [RFC5681] | Allman, M., Paxson, V. and E. Blanton, "TCP Congestion Control", RFC 5681, September 2009. |
| [DCTCP10] | Alizadeh, M., Greenberg, A., Maltz, D.A., Padhye, J., Patel, P., Prabhakar, B., Sengupta, S. and M. Sridharan, "Data Center TCP (DCTCP)", December 2010. |