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httpbis                                                      D. Stenberg
Internet-Draft                                                   Mozilla
Intended status: Best Current Practice                 December 21, 2015
Expires: June 23, 2016


                          TCP Tuning for HTTP
                     draft-stenberg-httpbis-tcp-01

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.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on June 23, 2016.

Copyright Notice

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

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   described in the Simplified BSD License.






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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Notational Conventions  . . . . . . . . . . . . . . . . .   3
   2.  Socket planning . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Number of open files  . . . . . . . . . . . . . . . . . .   3
     2.2.  Number of concurrent network messages . . . . . . . . . .   3
     2.3.  Number of incoming TCP SYNs allowed to backlog  . . . . .   4
     2.4.  Use the whole port range for local ports  . . . . . . . .   4
     2.5.  Lower the TCP FIN timeout . . . . . . . . . . . . . . . .   4
     2.6.  Reuse sockets in TIME_WAIT state  . . . . . . . . . . . .   4
     2.7.  TCP socket buffer sizes and Window Scaling  . . . . . . .   4
     2.8.  Set maximum allowed TCP window sizes  . . . . . . . . . .   5
     2.9.  Timers and timeouts . . . . . . . . . . . . . . . . . . .   5
   3.  TCP handshake . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  TCP Fast Open . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Initial Congestion Window . . . . . . . . . . . . . . . .   6
     3.3.  TCP SYN flood handling  . . . . . . . . . . . . . . . . .   6
   4.  TCP transfers . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Packet Pacing . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Explicit Congestion Control . . . . . . . . . . . . . . .   7
     4.3.  Nagle's Algorithm . . . . . . . . . . . . . . . . . . . .   7
     4.4.  Keep-alive  . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Re-using connections  . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Slow Start after Idle . . . . . . . . . . . . . . . . . .   7
     5.2.  TCP-Bound Authentications . . . . . . . . . . . . . . . .   8
   6.  Closing connections . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Half-close  . . . . . . . . . . . . . . . . . . . . . . .   8
     6.2.  Abort . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.3.  Close Idle Connections  . . . . . . . . . . . . . . . . .   8
     6.4.  Tail Loss Probes  . . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
     9.3.  URIs  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  10

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 the best
   current practice for using HTTP over TCP, with a focus on improving
   end-user perceived performance.




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   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 its 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
   those values 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 an 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>






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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

   High connection completion rates will consume ephemeral ports
   quickly.  Lower the time during which connections are in FIN-WAIT-2/
   TIME_WAIT states so that they can be purged faster and thus maintain
   a maximal number of available sockets.  The primitives for the
   assignment of these values were described in [RFC0793], however
   significantly lower values are commonly used.

   net.ipv4.tcp_fin_timeout = <number of seconds>

2.6.  Reuse sockets in TIME_WAIT state

   When running backend servers on a managed, low latency network you
   might allow the reuse of sockets in TIME_WAIT state for new
   connections when a protocol complete termination has occurred.  There
   is no RFC that covers this behaviour.

   net.ipv4.tcp_tw_reuse = 1

2.7.  TCP socket buffer sizes and Window Scaling

   Systems meant to handle and serve a huge number of TCP connections at
   high speeds need a significant amount of memory for TCP socket
   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.  Window Scaling is typically linked to socket buffer
   sizes and on a Linux system can be controlled with the values:

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





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   The minimum and default tend to require less proactive amendment than
   the maximum value.  When deriving maximum values for use, you should
   consider the BDP (Bandwidth Delay Product) of the target environment
   and clients.  Consider also that 'read' and 'write' values do not
   require to be synchronised, as the BDP requirements for a load
   balancer or middle-box might be very different when acting as a
   sender or receiver.

   Allowing needlessly high values beyond the expected limitations of
   the platform might increase the probability of retransmissions and
   buffer induced delays within the path.  Extensions such as ECN
   coupled with AQM can help mitigate this undesirable behaviour
   [RFC7141].

   [RFC7323] covers Window Scaling in greater detail.

2.8.  Set maximum allowed TCP window sizes

   You may have to increase the largest allowed window size.  Window
   scaling must be accommodated within the maximal values, however it is
   not uncommon to see the maximum definable higher than the scalable
   limit; these values can statically defined within socket parameters
   (SO_RCVBUF,SO_SNDBUF)

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

2.9.  Timers and timeouts

   On a modern shared platform it can be common to plan for both long
   and short lived connections on the same implementation.  However, the
   delivery of static assets and a 'web push' or 'long poll' service
   provide very different quality of service promises.

   Fail 'fast': TCP resources can be highly contended.  For fault
   tolerance reasons a server needs to be able to determine within a
   reasonable time frame whether a connection is still active or
   required. e.g.  If static assets typically return in 100s of
   milliseconds, and users 'switch off' after <10s keeping timeouts of
   >30s make little sense and defining a 'quality of service'
   appropriate to the target platform is encouraged.  On a shared
   platform with mixed session lifetimes, applications that require
   longer render times have various options to ensure the underlying
   service and upstream servers in the path can identify the session as
   not failed: HTTP continuations, Redirects, 202s or sending data.

   Clients and servers typically have many timeout options, a few
   notable options are: Connect(client), time to request(server), time



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   to first byte(client), between bytes(server/client), total connection
   time(server/client).  Some implementations merge these values into a
   single 'timeout' definition even when statistics are reported
   individually.  All should be considered as the defaults in many
   implementations are highly underiable, even infinite timeouts have
   been observed.

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).  It should be noted that
   TFO requires a secret to be defined on the server to mitigate
   security vulnerabilities it introduces.  TFO therefore requires more
   server side deployment planning than other enhancements.

   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 initcwnd (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.  Many implementations allow initcwnd to be applied to
   specific routes which allows a greater degree of flexibility than
   some other TCP parameters.

   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.








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4.  TCP transfers

4.1.  Packet Pacing

   TBD

4.2.  Explicit Congestion Control

   Apple deploying in iOS and OSX [1].

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



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   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
   lifetime.  This discrepancy with how other HTTP authentications work
   makes it important to handle these connections with care.

6.  Closing connections

6.1.  Half-close

   The 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 reuse is
   key for many HTTP clients' performance.  The value of an existing
   connection quickly degrades and after only a few minutes the chance
   that a connection will successfully get reused by a web browser is
   slim.

6.4.  Tail Loss Probes

   draft [2]








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7.  IANA Considerations

   This document does not require action from IANA.

8.  Security Considerations

   TBD

9.  References

9.1.  Normative References

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <http://www.rfc-editor.org/info/rfc793>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <http://www.rfc-editor.org/info/rfc7230>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <http://www.rfc-editor.org/info/rfc7540>.

9.2.  Informative References

   [RFC0896]  Nagle, J., "Congestion Control in IP/TCP Internetworks",
              RFC 896, DOI 10.17487/RFC0896, January 1984,
              <http://www.rfc-editor.org/info/rfc896>.

   [RFC4987]  Eddy, W., "TCP SYN Flooding Attacks and Common
              Mitigations", RFC 4987, DOI 10.17487/RFC4987, August 2007,
              <http://www.rfc-editor.org/info/rfc4987>.

   [RFC6928]  Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
              "Increasing TCP's Initial Window", RFC 6928,
              DOI 10.17487/RFC6928, April 2013,
              <http://www.rfc-editor.org/info/rfc6928>.






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   [RFC7141]  Briscoe, B. and J. Manner, "Byte and Packet Congestion
              Notification", BCP 41, RFC 7141, DOI 10.17487/RFC7141,
              February 2014, <http://www.rfc-editor.org/info/rfc7141>.

   [RFC7323]  Borman, D., Braden, B., Jacobson, V., and R.
              Scheffenegger, Ed., "TCP Extensions for High Performance",
              RFC 7323, DOI 10.17487/RFC7323, September 2014,
              <http://www.rfc-editor.org/info/rfc7323>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
              <http://www.rfc-editor.org/info/rfc7413>.

9.3.  URIs

   [1] https://developer.apple.com/videos/wwdc/2015/?id=719

   [2] http://tools.ietf.org/html/draft-dukkipati-tcpm-tcp-loss-probe-01

Appendix A.  Acknowledgments

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

Author's Address

   Daniel Stenberg
   Mozilla

   Email: daniel@haxx.se
   URI:   http://daniel.haxx.se




















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