Attached below is our short contribution for inclusion in the research
issues draft. It was written by my student Rohit Goyal and me. We
would appreciate any feedback you may have. Thanks.
-Raj Jain
=============================================================================
Mitigation description
----------------------
Several issues arise in optimizing the performance of TCP when ATM is
deployed over satellite links. ATM provides various traffic management
mechanisms including vanilla Unspecified Bit Rate (UBR) with tail
drop, UBR with intelligent buffer management, UBR with guaranteed
rate, Guaranteed Frame Rate (GFR), network feedback via the Available
Bit Rate (ABR) and ABR with virtual source/virtual destination (VS/VD).
Both, TCP congestion control algorithms, as well as ATM traffic
management mechanisms must be optimized to provide efficient and fair
TCP performance over satellite-ATM networks.
Research
--------
It has been shown [LI96] that vanilla TCP over the UBR service
category achieves low throughput and high unfairness over satellite
networks. Fast retransmit and recovery (without SACK) further hurts
TCP performance over UBR because of bursty packet losses during
congestion.
There are four ways to significantly improve the TCP throughput over
UBR: frame-level discard policies, intelligent buffer management
policies, 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]. For long delay paths, the throughput
improvement due to SACK is more than that from discard policies and
buffer management. When several TCP flows are multiplexed on to a
few VCs, fairness among the IP 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 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 TCP connections need a guaranteed minimum cell
rate (MCR) for reasonable performance [GOYAL97a]. MCR ensures that
the flow of TCP packets and acknowledgements is continuous and
prevents TCP timeouts due to temporary bandwidth starvation of
UBR. Minimum rate guarantees can be provided simply to the entire UBR
service category (UBR with guaranteed rate) or to each ATM VCs using
the GFR or the ABR service category.
For TCP over ABR, in addition to the four methods discussed above,
there are two more ways to improve the performance: VS/VD and ack
regulation. 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.
Ack regulation techniques require routers to control the flow of
TCP acknowelegements based on ABR feedback [SHIV98].
Implementation description
--------------------------
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.
Topology considerations
-----------------------
The enhancements suggested are expected to benefit all high
delay-bandwidth environments described in section 2.
References
-------------
[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, et. al. "Guaranteed Rate for Improving TCP
Performance on UBR+ over Terrestrial and Satellite Networks," ATM
Forum/97-0424, April 1997
[GOYAL97b] Rohit Goyal, Raj Jain, Sastri Kota et.al., Selective
Acknowledgments and UBR+ Drop Policies to Improve TCP/UBR
Performance over Terrestrial and Satellite Networks, Proceedings
of IC3N'97, September 1997.
[GOYAL98] Rohit Goyal, Raj Jain et. al., Per-VC rate allocation techniques
for ABR feedback in VS/VD networks, To appear, Proc. Globecom'98,
November 1998.
[SHIV98] Ramakrishna Satyavolu, Ketan Duvedi, Shivkumar Kalyanaraman,
"Explicit rate control of TCP applications," ATM_Forum/98-0152R1,
February 1998.
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