TCP Tuning for HTTP
draft-stenberg-httpbis-tcp-00

Abstract

This document records current best practice for using all versions of HTTP over TCP.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on May 9, 2016.

Copyright Notice

Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 1.1. Notational Conventions
• 2. Socket planning
• 2.1. Number of open files
• 2.2. Number of concurrent network messages
• 2.3. Number of incoming TCP SYNs allowed to backlog
• 2.4. Use the whole port range for local ports
• 2.5. Lower the TCP FIN timeout
• 2.6. Re-use sockets in TIME_WAIT state
• 2.7. Give the the TCP stack enough memory
• 2.8. Set maximum allowed TCP window sizes
• 2.9. Timers and time-outs
• 3. TCP handshake
• 3.1. TCP Fast Open
• 3.2. Initial Congestion Window
• 3.3. TCP SYN flood handling
• 4. TCP transfers
• 4.1. Packet Pacing
• 4.2. Explicit Congestion Control
• 4.3. Nagle’s Algorithm
• 4.4. Keep-alive
• 5. Re-using connections
• 5.1. Slow Start after Idle
• 5.2. TCP-Bound Authentications
• 6. Closing connections
• 6.1. Half-close
• 6.2. Abort
• 6.3. Close Idle Connections
• 6.4. Tail Loss Probes
• 7. IANA Considerations
• 8. Security Considerations
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Appendix A. Acknowledgments
• Author's Address

1. Introduction

HTTP version 1.1 [RFC7230] as well as HTTP version 2 [RFC7540] are defined to use TCP [RFC0793], and their performance can depend greatly upon how TCP is configured. This document records best current practice for using HTTP over TCP, with a focus on improving end-user perceived performance.

These practices are generally applicable to HTTP/1 as well as HTTP/2, although some may note particular impact or nuance regarding a particular protocol version.

There are countless scenarios, roles and setups where HTTP is being using so there can be no single specific “Right Answer” to most TCP questions. This document intends only to cover the most important areas of concern and suggest possible actions.

1.1. Notational Conventions

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].

2. Socket planning

Your HTTP server or intermediary may need configuration changes to some system tunables and timeout periods to perform optimally. Actual values will depend on how you are scaling the platform, horizontally or vertically, and other connection semantics. Changing system limits and altering thresholds will change the behavior of your web service and it’s dependencies, these dependencies are usually common to other services running on the same system so good planning and testing is advised.

This is a list of values to consider and some general advice on how they can be modified on Linux systems.

2.1. Number of open files

A modern HTTP server will serve a large number of TCP connections and in most systems each open socket equals on open file. Make sure that limit isn’t a bottle neck. In Linux, the limit can be raised like this:

fs.file-max = <number of files>

2.2. Number of concurrent network messages

Raise the number of packets allowed to get queued when a particular interface receives packets faster than the kernel can process them. In Linux, this limit can be raised like this:

net.core.netdev_max_backlog = <number of packets>

2.3. Number of incoming TCP SYNs allowed to backlog

The number of new connection requests that are allowed to queue up in the kernel. In Linux, this limit can be raised like this:

net.core.somaxconn = <number>

2.4. Use the whole port range for local ports

To make sure the TCP stack can take full advantage of the entire set of possible sockets, give it a larger range of local port numbers to use.

net.ipv4.ip_local_port_range = 1024 65535

2.5. Lower the TCP FIN timeout

Lower the timeouts during which connections are in FIN-WAIT-2 state so that they can be re-used faster and thus increase number of simultaneous connections possible.

net.ipv4.tcp_fin_timeout = <number of seconds>

2.6. Re-use sockets in TIME_WAIT state

Especially when running backend servers that are having edge servers fronting them to the Internet, allow reuse of sockets in TIME_WAIT state for new connections when it is safe from the network stack’s perspective.

net.ipv4.tcp_tw_reuse = 1

2.7. Give the the TCP stack enough memory

Systems meant to handle and serve a huge number of TCP connections at high speeds need a significant amount of memory for TCP stack buffers. On some systems you can tell the TCP stack what default buffer sizes to use and how much they are allowed to dynamically grow and shrink. On a Linux system, you can control it like:

net.ipv4.tcp_wmem = <minimum size> <default size> <max size in bytes>
net.ipv4.tcp_rmem = <minimum size> <default size> <max size in bytes>

2.8. Set maximum allowed TCP window sizes

You may have to increase the largest allowed window size.

net.core.rmem_max = <number of bytes>
net.core.wmem_max = <number of bytes>

2.9. Timers and time-outs

Fail fast. Do not allow very long time-outs. Wasting several minutes for various network related attempts won’t make any users happy.

Avoid long-going TCP flows that are (seemingly) idle. Use HTTP continuations instead, or redirects, 202s or similar.

3. TCP handshake

3.1. TCP Fast Open

TCP Fast Open (a.k.a. TFO, [RFC7413]) allows data to be sent on the TCP handshake, thereby allowing a request to be sent without any delay if a connection is not open.

TFO requires both client and server support, and additionally requires application knowledge, because the data sent on the SYN needs to be idempotent. Therefore, TFO can only be used on idempotent, safe HTTP methods (e.g., GET and HEAD), or with intervening negotiation (e.g, using TLS).

Support for TFO is growing in client platforms, especially mobile, due to the significant performance advantage it gives.

3.2. Initial Congestion Window

[RFC6928] specifies an initial congestion window of 10, and is now fairly widely deployed server-side. There has been experimentation with larger initial windows, in combination with packet pacing.

IW10 has been reported to perform fairly well even in high volume servers.

3.3. TCP SYN flood handling

TCP SYN Flood mitigations [RFC4987] are necessary and there will be thresholds to tweak.

4. TCP transfers

4.1. Packet Pacing

TBD

4.2. Explicit Congestion Control

Apple deploying in iOS and OSX.

4.3. Nagle’s Algorithm

Nagle’s Algorithm [RFC0896] is the mechanism that makes the TCP stack hold (small) outgoing packets for a short period of time so that it can potentially merge that packet with the next outgoing one. It is optimized for throughput at the expense of latency.

HTTP/2 in particular requires that the client can send a packet back fast even during transfers that are perceived as single direction transfers. Even small delays in those sends can cause a significant performance loss.

HTTP/1.1 is also affected, especially when sending off a full request in a single write() system call.

In POSIX systems you switch it off like this:

int one = 1;
setsockopt(fd, IPPROTO_TCP, TCP_NODELAY, &one, sizeof(one));

4.4. Keep-alive

TCP keep-alive is likely disabled - at least on mobile clients for energy saving purposes. App-level keep-alive is then required for long-lived requests to detect failed peers or connections reset by stateful firewalls etc.

5. Re-using connections

5.1. Slow Start after Idle

Slow-start is one of the algorithms that TCP uses to control congestion inside the network. It is also known as the exponential growth phase. Each TCP connection will start off in slow-start but will also go back to slow-start after a certain amount of idle time.

In Linux systems you can prevent the TCP stack from going back to slow-start after idle by settting

net.ipv4.tcp_slow_start_after_idle = 0

5.2. TCP-Bound Authentications

There are several HTTP authentication mechanisms in use today that are used or can be used to authenticate a connection rather than a single HTTP request. Two popular ones are NTLM and Negotiate.

If such an authentication has been negotiated on a TCP connection, that connection can remain authenticated throughout the rest of its life time. This discrepancy with how other HTTP authentications work makes it important to handle these connections with care.

6. Closing connections

6.1. Half-close

Client or server is free to half-close after a request or response has been completed; or when there is no pending stream in HTTP/2.

Half-closing is sometimes the only way for a server to make sure it closes down connections cleanly so that it doesn’t accept more requests while still allowing clients to receive the ongoing responses.

6.2. Abort

No client abort for HTTP/1.1 after the request body has been sent. Delayed full close is expected following an error response to avoid RST on the client.

6.3. Close Idle Connections

Keeping open connections around for subsequent connection re-use is key for many HTTP clients’ performance. The value of an existing connection quickly degrades and already after a few minutes the chance that a connection will successfully get re-used by a web browser is slim.

6.4. Tail Loss Probes

draft

7. IANA Considerations

This document does not require action from IANA.

8. Security Considerations

TBD

9. References

9.1. Normative References

9.2. Informative References

Appendix A. Acknowledgments

This specification builds upon previous work and help from Mark Nottingham, Craig Taylor

Author's Address