Internet Engineering Task Force S. Dawkins
INTERNET DRAFT G. Montenegro
M. Kojo
V. Magret
September 1, 1999
Performance Implications of Link-Layer Characteristics:
Slow Links
draft-ietf-pilc-slow-01.txt
Status of This Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC 2026.
Comments should be submitted to the PILC mailing list at
pilc@grc.nasa.gov.
Distribution of this memo is unlimited.
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Abstract
The PILC (Performance Implications of Link-Layer Characteristics)
Working Group in IETF was chartered to develop a series of
recommendations for improved protocol performance in network paths
that traverse "extreme" link conditions. This document is part
of the PILC series, and focuses on network paths that traverse "very
low bit-rate" links.
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"Very low bit-rate" implies "slower than we would like". This
recommendation may be used in any network where hosts can saturate
available bandwidth, but the design space for this recommendation
explicitly includes connections that traverse 4800 bit-per-second
links.
This document discusses general-purpose mechanisms. Where
application-specific mechanisms can outperform the relevant
general-purpose mechanism, we point this out and explain why.
Changes since last draft:
Include discussion of TCP timestamp option interaction with header
compression.
Use 296-byte MTU in header compression examples.
Include discussion of specifying small receive windows to prevent
continued probing on slow links.
Include discussion of experimental duplicate acknowledgement
mechanisms.
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Table of Contents
1.0 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.0 Description of Optimizations . . . . . . . . . . . . . . . . . . 3
2.1 Header Compression Alternatives . . . . . . . . . . . . . . 3
2.2 Payload Compression Alternatives . . . . . . . . . . . . . 5
2.3 Interactions with TCP Congestion Avoidance . . . . . . . . 6
2.4 Small Window Effects (Experimental) . . . . . . . . . . . . 7
3.0 Summary of Recommended Optimizations . . . . . . . . . . . . . . 8
4.0 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
5.0 References . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' addresses . . . . . . . . . . . . . . . . . . . . . . . . . 10
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1.0 Introduction
The Internet protocol stack was designed to span a wide range of
link speeds, and has met this design goal with only a limited number
of enhancements (for example, the use of RFC 1323 TCP window
scaling for very-high-bandwidth connections).
Pre-World Wide Web application protocols tended to be either
interactive applications sending very little data (Telnet) or bulk
transfer applications that did not require interactive response
(File Transfer Protocol, Network News).
The World Wide Web has given us traffic that is both interactive and
"bulky", including images, sound, and video.
The World Wide Web has also popularized the Internet, so that there
is significant interest in accessing the World Wide Web over link
speeds that are much "slower" than typical desktop host speeds.
In order to provide the best interactive response for these "bulky"
transfers, implementors may wish to minimize the number of bits
actually transmitted over these "slow" connections.
There are two areas that can be considered - compressing the bits
that make up the overhead associated with the connection, and
compressing the bits that make up the payload being transported
over the connection.
In addition, implementors may wish to consider TCP receive window
settings and queuing mechanisms as techniques to improve performance
over low-speed links. While these techniques don't involve protocol
changes, they are included in this document for completeness.
2.0 Description of Optimizations
This section describes optimizations which have been suggested
for use in situations where hosts can saturate their links. The
next section summarizes recommendations about the use of these
optimizations.
2.1 Header Compression Alternatives
Mechanisms for TCP and IP header compression defined in
[RFC1144, RFC2507, RFC2508, RFC2509] provide the following
benefits:
- Improve interactive response time
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- Allow using small packets for bulk data with good line
efficiency
- Allow using small packets for delay sensitive low data-rate
traffic
- Decrease header overhead (for a typical dialup MTU of 296
bytes, the overhead of TCP/IP headers can decrease from
about 13 percent with typical 40-byte headers to 1-1.5
percent with with 3-5 byte compressed headers, for most
packets)
- Reduce packet loss rate over lossy links.
Van Jacobson (VJ) header compression [RFC1144] describes a
Proposed Standard for TCP Header compression that is widely
deployed. It uses TCP timeouts to detect a loss of
synchronization between the compressor and decompressor. [RFC2507]
includes an explicit request for retransmission of an uncompressed
packet to allow resynchronization without waiting for a TCP
timeout (and executing congestion avoidance procedures).
Recommendation: Implement [RFC2507], in particular as it relates to
IPv4 tunnels and Minimal Encapsulation for Mobile IP, as well as
TCP header compression for lossy links and links that reorder
packets. PPP capable devices should implement [RFC2509].
[RFC1144] header compression should only be enabled when operating
over reliable "slow" links, because even a single bit error results
in a full TCP window being dropped, followed by a costly recovery
via slow-start.
[RFC1323] defines a "TCP Timestamp" option, used to improve TCP RTT
estimates by providing unambiguous TCP roundtrip timings. Use of TCP
Timestamps prevents header compression, because the timestamps are
sent as TCP options. This means that each timestamped header has TCP
options that differ from the previous header, and successive headers
with different headers are sent uncompressed.
2.2 Payload Compression Alternatives
Compression of IP payloads is also desirable. "IP Payload
Compression Protocol (IPComp)" [RFC2393] defines a framework where
common compression algorithms can be applied to arbitrary IP
segment payloads.
IP payload compression is something of a niche optimization.
It is necessary because IP-level security converts IP payloads
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to random bitstreams, defeating commonly-deployed link-layer
compression mechanisms which are faced with payloads that have
no redundant "information" that can be more compactly represented.
However, many IP payloads are already compressed (images, audio,
video, "zipped" files being FTPed), or are already encrypted above
the IP layer (SSL/TLS, etc.). These payloads will not "compress"
further, limiting the benefit of this optimization.
For uncompressed HTTP payload types, HTTP/1.1 [RFC2068] also
includes Content-Encoding and Accept-Encoding headers, supporting
a variety of compression algorithms for common compressible MIME
types like text/plain. This leaves only the HTTP headers
themselves uncompressed.
The current HTTP-NG proposal [HTTP-NG] replaces the text-based HTTP
header representation with a binary representation for compactness.
In general, application-level compression can often outperform
IPComp, because of the opportunity to use compression dictionaries
based on knowledge of the specific data being compressed.
All these compression techniques will reduce the need for IPComp,
especially for WWW users.
Recommendation: IPComp may optionally be implemented. Track
HTTP-NG standardization and deployment for now.
2.3 Interactions with TCP Congestion Avoidance
In many cases, TCP connections that traverse slow links have the
slow link as an "access" link, with higher-speed links in use for
most of the connection path. One common configuration might be a
laptop computer using dialup access to a terminal server,
with an HTTP server on a high-speed LAN "behind" the terminal server.
The HTTP server may be able to place packets on a directly-attached
high-speed LAN at a higher rate than the terminal server can forward
them on the low-speed link. The consequence of this action is that
the terminal server will be unable to buffer unlimited traffic
intended for the low-speed link, and will begin to "drop" the
excess packets. The self-clocking nature of TCP's slow start and
congestion avoidance algorithms prevent this buffer overrun from
continuing, but these algorithms also allow senders to "probe"
for available bandwidth - cycling through an increasing rate of
transmission until loss occurs, followed by a dramatic (50-percent)
drop in transmission rate. This happens when a host directly
connected to a low-speed link offers a receive window that is
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unrealistically large for the low-speed link. The peer host
continues to probe for available bandwidth, trying to fill the
receive window, until packet loss occurs.
Hosts that are directly connected to low-speed links should
limit the receive windows they advertise. This recommendation
takes two forms:
- Modern operating systems are using increasingly larger default
TCP receive buffers, in order to maximize throughput on
high-speed links. Users should be aware of the default receive
window in use - typically a system-wide parameter.
- Application developers should consider the possibility of users
connecting via low-speed links before increasing the receive
buffer in use for a single connection - typically a socket option.
For example - in the case (described in RFC 2416) where a modem
has only three buffers, whenever the HTTP server returns four
back-to-back packets, one will be dropped. If this bottleneck link
causes the TCP window to be less than four to five segments, it will
not be possible to receive three duplicate acknowledgements, so
Fast Retransmit/Fast Recovery will never happen, and TCP recovery
will take place with full RTO and slow start.
In this case, the common MTU of 296 bytes gives an MSS of 256
bytes, so an appropriate receive buffer size would be 768 bytes -
any value larger would allow unproductive probing for non-existent
bandwidth.
2.4 Small Window Effects (Experimental)
If a TCP connection stabilizes with a window of only a few
segments, the sender isn't sending enough segments to generate
three duplicate acknowledgements, triggering fast retransmit/
fast recovery. This means that a course-grained TCP recovery is
performed - dropping the TCP connection to a window with only
one segment.
[TCPB98] and [TCPF98] observe that (in studies of network
traces) dataset) it is relatively common for TCP coarse-grained
timeouts to occur even when some duplicate acknowledgements are
being sent. The challenge is to use these duplicate acknowledgements
to trigger fast retransmit/fast recovery without injecting
traffic into the network unecessarily - and especially not
injecting traffic in ways that will result in instability.
In these situations, it may be desireable to trigger fast
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retransmit/fast recovery more aggressively. [TCPB98] and
[TCPF98] suggest suggest sending a new segment whenever a
duplicate acknowledgement is received, so that the receiver
will continue to generate duplicate acknowledgements until the
TCP retransmit threshhold is reached, triggering fast
retransmit/fast recovery.
We note that a maximum of two additional new segments will be
sent before the receiver sends either an acknowledgement
advancing the window or two additional duplicate acknowledgements,
triggering fast retransmit/fast recovery, and that these new
segments will be acknowledgement-clocked, not back-to-back.
The alternative, lowering the fast retransmit/fast recovery
threshold, is more likely to inject unnecessary retransmissions
when the duplicate acknowledgements are the result of out-of-order
delivery to the far-end TCP.
3.0 Summary of Recommended Optimizations
This section summarizes our recommendations regarding the previous
mechanisms, for end nodes that are capable of saturating available
bandwidth.
Header compression should be implemented. [RFC1144] header
compression can be enabled over robust network connections. [RFC2507]
should be used over network connections that are expected to
experience loss due to corruption as well as loss due to congestion.
[RFC1323] TCP timestamps must be turned off to allow header
compression.
IP Payload Compression [RFC2393] should be implemented, although
compression at higher layers of the protocol stack (examples:
[RFC 2068, HTTP-NG]) may make this mechanism less useful.
For HTTP/1.1 environments, [RFC2068] payload compression should be
implemented and should be used for payloads that are not already
compressed.
Users at the end of low-speed links should be aware of the default
TCP receive window size on their hosts.
Application developers should consider the possibility that an
application will be used on a host that is directly connected to a
low-speed link, before increasing the TCP receive window size beyond
the default for TCP connections used by this application.
All of the mechanisms described above are stable standards-track
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RFCs (at Proposed Standard status, as of this writing), with the
exception of [HTTP-NG], which is included for completeness.
In addition, implementors may wish to experiment with injecting
new traffic into the network when duplicate acknowledgements are
being received, as described in [TCPB98] and [TCPF98]. This is
not a standards-track TCP mechanism.
Of the above mechanisms, only Header Compression (for IP and TCP)
ceases to work in the presence of end-to-end IPSEC.
4.0 Acknowledgements
This recommendation has grown out of the Internet Draft "TCP Over
Long Thin Networks", which was in turn based on work done in the
IETF TCPSAT working group.
5.0 References
[RFC1144] Jacobson, V., "Compressing TCP/IP Headers for
Low-Speed Serial Links," RFC 1144, February 1990. (Proposed
Standard)
[RFC1323] Jacobson, V., Braden, R., Borman, D., "TCP Extensions
for High Performance", RFC 1323, May 1992. (Proposed Standard)
[RFC2068] R. Fielding, J. Gettys, J. Mogul, H. Frystyk,
T. Berners-Lee. "Hypertext Transfer Protocol -- HTTP/1.1",
RFC 2068, January 1997. (Proposed Standard)
[HTTP-NG] H. Frystyk Nielsen, Mike Spreitzer, Bill Janssen, Jim
Gettys, "HTTP-NG Overview", draft-frystyk-httpng-overview-00.txt,
November 17, 1998, expired, but also available from
http://www.w3.org/Protocols/HTTP-NG/1998/11/.
[RFC2393] A. Shacham, R. Monsour, R. Pereira, M. Thomas, "IP
Payload Compression Protocol (IPComp)," RFC 2393, December
1998. (Proposed Standard)
[RFC2416] T. Shepard, C. Partridge, "When TCP Starts Up With
Four Packets Into Only Three Buffers", RFC 2416, September 1998.
[RFC2507] Mikael Degermark, Bjorn Nordgren, Stephen Pink. "IP
Header Compression," RFC 2507, February 1999. (Proposed
Standard)
[RFC2508] S. Casner, V. Jacobson. "Compressing IP/UDP/RTP
Headers for Low-Speed Serial Links," RFC 2508, February 1999.
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(Proposed Standard)
[RFC2509] Mathias Engan, S. Casner, C. Bormann. "IP Header
Compression over PPP," RFC 2509, February 1999. (Proposed
Standard)
[TCPB98] Hari Balakrishnan, Venkata N. Padmanabhan, Srinivasan
Seshan, Mark Stemm, Randy H. Katz, "TCP Behavior of a Busy
Internet Server: Analysis and Improvements", IEEE Infocom,
March 1998. Available from:
http://www.cs.berkeley.edu/~hari/papers/infocom98.ps.gz
[TCPF98] Dong Lin and H.T. Kung, "TCP Fast Recovery Strategies:
Analysis and Improvements", IEEE Infocom, March 1998.
Available from: http://www.eecs.harvard.edu/networking/papers/
infocom-tcp-final-198.pdf
Authors' addresses
Questions about this document may be directed to:
Spencer Dawkins
Nortel Networks
3 Crockett Ct
Allen, TX 75002
Voice: +1-972-684-4827
Fax: +1-972-685-3292
E-Mail: sdawkins@nortelnetworks.com
Gabriel E. Montenegro
Sun Labs Networking and Security Group
Sun Microsystems, Inc.
901 San Antonio Road
Mailstop UMPK 15-214
Mountain View, California 94303
Voice: +1-650-786-6288
Fax: +1-650-786-6445
E-Mail: gab@sun.com
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Markku Kojo
University of Helsinki/Department of Computer Science
P.O. Box 26 (Teollisuuskatu 23)
FIN-00014 HELSINKI
Finland
Voice: +358-9-7084-4179
Fax: +358-9-7084-4441
E-Mail: kojo@cs.helsinki.fi
Vincent Magret
Corporate Research Center
Alcatel Network Systems, Inc
1201 Campbell
Mail stop 446-310
Richardson Texas 75081 USA
M/S 446-310
Voice: +1-972-996-2625
Fax: +1-972-996-5902
E-mail: vincent.magret@aud.alcatel.com
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