Internet DRAFT - draft-goyal-tcpsat-tcpatm
draft-goyal-tcpsat-tcpatm
Internet Engineering Task Force Rohit Goyal
INTERNET DRAFT Raj Jain
File: draft-goyal-tcpsat-tcpatm-00.txt November, 1998
Expires: May, 1999
Optimizing TCP over Satellite ATM Networks
Status of this Memo
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NOTE
This document is not to be taken as a finished product. Some of the
sections are rough and are included in order to obtain comments from
the community that will benefit future iterations of this document.
This is simply a step in the ongoing conversation about this
document. Finally, all the authors of this draft do not necessarily
agree with and/or advocate all the mechanisms outlined in this
document.
Abstract
This document discusses techniques to improve the performance of TCP
over satellite-ATM networks. ATM provides the ABR, UBR and GFR
service categories for data traffic. TCP experiences poor performance
over UBR due to bursty packet losses during congestion. Buffer
management and guaranteed rate techniques to improve TCP performance
over UBR are presented. The performance of TCP over ABR and the
interaction of ABR congestion control mechanisms with TCP congestion
control mechanisms are also described. This document is intended to
be in informational document for the efficient transport of TCP over
satellite-ATM networks.
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1. Introduction
Several issues arise in optimizing the performance of TCP when ATM is
deployed over satellite links. This document emphasizes that both
TCP mechanisms as well as ATM mechanisms should be used to improve
TCP performance over long-delay ATM networks.
ATM technology provides at least 3 service categories for data: UBR,
ABR, and GFR [ATMF]. Each of these categories can be improved by a
number of mechanisms. Some examples of these categories and
improvements are:
- Unspecified Bit Rate (UBR) with tail drop,
- UBR with intelligent buffer management,
- UBR with guaranteed rate,
- Available Bit Rate (ABR) with network feedback, and
- ABR with virtual source/virtual destination (VS/VD).
- Guaranteed Frame Rate (GFR),
In addition, TCP provides several congestion control mechanisms
including:
- Vanilla TCP with slow start and congestion avoidance,
- TCP Reno with fast retransmit and recovery,
- TCP New Reno
- TCP with selective acknowledgements (SACK)
Satellite-ATM network designers as well as service providers must
choose the optimal ATM services for efficient TCP transport. In the
presence of TCP performance enhancing proxies at the edges of the
satellite-ATM network, the service providers also have the ability to
control TCP mechanisms so as to provide good performance. This
document describes the design choices and performance analysis
results of various options available to TCP over satellite-ATM
networks.
2. Techniques for Improving TCP Performance over ATM
It has been shown [LI96] that vanilla TCP over the UBR service
category achieves low throughput and high unfairness over satellite
networks. This is because during packet loss, TCP loses time waiting
for its coarse granularity retransmission timeout.
In the presence of bursty packet losses, fast retransmit and recovery
(FRR) (without SACK) further hurts TCP performance over UBR for long
delay-bandwidth product networks. This is because after two fast
retransmissions, the congestion window is too small to send out new
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packets that trigger duplicate acks. In the absence of duplicate
acks, the third lost packet is not retransmitted, and a timeout
occurs at a small window. This results in congestion avoidance with a
small window, which is very slow for long delay networks.
There are four ways to significantly improve the TCP throughput over
UBR:
- frame-level discard policies,
- intelligent buffer management policies,
- TCP New Reno,
- TCP SACK, and
- guaranteed rates.
Frame level discard policies such as early packet discard (EPD)
improve the throughput significantly over cell-level discard
policies. However, the fairness is not guaranteed unless intelligent
buffer management using per-VC accounting is used [GOYAL97b].
Throughput increases further with more aggressive New Reno and SACK
[SACK]. SACK gives the best performance in terms of throughput. It
has been found that for long delay paths, the throughput improvement
due to SACK is more than that from discard policies and buffer
management [GOYAL97b]. When several TCP flows are multiplexed on to
a few VCs, fairness among the TCP flows can be provided by the
routers at the edges of the ATM network, while VC level fairness must
be provided by the ATM network using either buffer management or per-
VC queuing.
The fourth method of improving the UBR performance is the so called
"guaranteed rate" (GR) in which a small fraction of the bandwidth is
reserved in the switches for the UBR service. This bandwidth is
shared by all UBR VCs. Using guaranteed rates helps in the presence
of a high load of higher priority traffic such as Constant Bit Rate
(CBR) or Variable Bit Rate (VBR) traffic. It has been found that
reserving just a small fraction, say 10%, of the bandwidth for UBR
significantly improves TCP performance [GOYAL97a]. This is because
the reserved bandwidth ensures that the flow of TCP packets and
acknowledgements is continuous and prevents TCP timeouts due to
temporary bandwidth starvation of UBR. Note that this mechanism is
different from the GFR service category where each VC (rather than
the entire UBR class) has a minimum rate guarantee.
For TCP over ABR, in addition to the four methods discussed above,
there are two more ways to improve the performance:
- ABR with virtual source/virtual destination (VS/VD), and
- ack regulation.
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Studies [GOYAL98] have indicated that VS/VD can be used to isolate
long-delay segments from terrestrial segments. This helps in
efficiently sizing buffers in routers and ATM switches. As a result,
terrestrial switches only need to have buffers proportional to the
bandwidth-delay products of the terrestrial segment of the TCP path.
Switches connected to the satellite VS/VD loops must have buffers
proportional to the satellite delay-bandwidth products.
Ack regulation techniques require routers to control the flow of TCP
acknowledgements based on ABR feedback [SHIV98]. These techniques
can be applied in ATM edge devices or in Internet routers. These
techniques can considerably improve fairness, throughput and end-to-
end delay properties of TCP applications.
3. Summary
In this document, we have briefly described TCP mechanisms and ATM
policies to improve TCP performance over satellite-ATM networks. The
TCP enhancements such as SACK and large windows require changes to
the TCP stack. The ATM mechanisms such as buffer management, ABR
feedback controls, guaranteed rates, and VS/VD do not require any
changes to the TCP stack.
4. References
[ATMF] The ATM Forum Technical Committee, "Traffic Management
Specification Version 4.0," April 1996,
ftp://ftp.atmforum.com/pub/approved-specs/af-tm-0056.000.pdf
[LI96] H. Li, K.Y. Siu, H.T. Tzeng, C. Ikeda and H. Suzuki, "TCP over
ABR and UBR Services in ATM,'' Proc. IPCCC'96, March 1996.
[GOYAL97a] Rohit Goyal, Raj Jain, Shiv Kalyanaraman, Sonia Fahmy,
Bobby Vandalore, Xiangrong Cai, Seong-Cheol Kim, Sastri Kota,
"Guaranteed Rate for Improving TCP Performance on UBR+ over
Terrestrial and Satellite Networks," ATM Forum/97-0424, April 1997,
http://www.cis.ohio-state.edu/~jain/atmf/a97-0424.htm
[GOYAL97b] Rohit Goyal, Raj Jain, Shivkumar Kalyanaraman, Sonia
Fahmy, Bobby Vandalore, Sastri Kota, "TCP Selective Acknowledgments
and UBR Drop Policies to Improve ATM-UBR Performance over Terrestrial
and Satellite Networks", Proc. ICCCN97, Las Vegas, September 1997,
pp17-27. http://www.cis.ohio-state.edu/~jain/papers/ic3n97.htm
[GOYAL98] Rohit Goyal, Xiangrong Cai, Raj Jain, Sonia Fahmy, Bobby
Vandalore " Per-VC Rate Allocation Techniques for ATM-ABR Virtual
Source Virtual Destination Networks," Proceedings of Globecom'98,
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November 1998, http://www.cis.ohio-
state.edu/~jain/papers/globecom98.htm
[SACK] Sally Floyd, "Issues of TCP with SACK" Lawerence Berkeley
Laboratory, Technical Report, December 1995.
[SHIV98] Ramakrishna Satyavolu, Ketan Duvedi, Shivkumar Kalyanaraman,
"Explicit rate control of TCP applications," ATM_Forum/98-0152R1,
February 1998.
Author's address
Rohit Goyal
Department of Computer and Information Science
The Ohio State University
2015 Neil Ave, DL395
Columbus, OH 43210
Phone: +1 614-688-4482
Email: goyal@cis.ohio-state.edu
Raj Jain
Department of Computer and Information Science
The Ohio State University
2015 Neil Ave, DL395
Columbus, OH 43210
Phone: +1 614-292-3989
Email: jain@cis.ohio-state.edu
Expires: May, 1999
Goyal & Jain [Page 5]
