Internet DRAFT - draft-hong-tcp-for-heterogeneous-networks
draft-hong-tcp-for-heterogeneous-networks
Network Working Group Choong Seon Hong
Internet-Draft Kyung Hee University
Expires: April 16, 2006 Hyung-Keun Ryu
KT Advanced Technology Lab
October 16, 2005
A TCP for Heterogeneous Networks
draft-hong-tcp-for-heterogeneous-networks-00.txt
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Abtract
Based on incorporating the ECN - Explicit Congestion Notification
[RFC3168] and the end-to-end Available Bandwidth Estimation,
TCP for heterogeneous networks accommodates a way for
TCP sender to be ability of distinguishing the wireless packet
losses from the congestion packet losses, as well as estimating of
the available bandwidth to adjust transmission rate when the neworks
become congested.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . 2
2. Proposed approaches for the last wireless hop . . . . . . . . 3
2.1 Link layer approachs . . . . . . . . . . . . . . . . . . . 3
2.2 Transport layer approach . . . . . . . . . . . . . . . . . 3
2.3 Split-connection approach . . . . . . . . . . . . . . . . 4
3. Explicit Congestion Notifications . . . . . . . . . . . . . 4
4. End-to-End bandwidth estimation . . . . . . . . . . . . . 5
5. TCP for heterogeneous networks . . . . . . . . . . . . . . . 6
5.1 Algorithm after receiving ACK or n DupACKs . . . . . . . 6
5.2 Algorithm after timeout expiration . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1 Normative references . . . . . . . . . . . . . . . . . . 8
6.2 Informative references . . . . . . . . . . . . . . . . . 8
7. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
This document specifies TCP protocol for heterogeneous networks.
TCP for heterogeneous networks is a congesion control mechanism
designed for TCP operating in heterogeneous networks where
wireless and wired-wireless hybrid networks exist.
TCP was originally developed for the wired networks, where
bit-error rate is trivial and packet losses are caused by network
congestion. However, cellular/wireless networks and wired-wireless
networks issue performance degradation challenges to TCP because TCP
congestion control cannot distinguish between packet losses caused
by random error of wireless link, signal fading and mobile handoff
processing, and those caused by network congestion.
At TCP sender, there are two state variables, congestion window size
(cwnd) and slow start threshold (ssthresh) are maintained by
congestion control to control the transmission rate. In slow start
phase, at beginning of connection, cwnd is increased by 1 for every
arrived ACK at the sender. Until cwnd reaches ssthresh, the sender
TCP congestion control goes to congestion avoidance phase, where cwnd
is increased by 1/cwnd for every arrived ACK at the sender. cwnd is
additively increased to reach the network bandwidth and is abundantly
decreased to one half of current value when the condition of network
congestion occurs by indicating received Duplicate ACKs at the sender.
TCP Reno then sets ssthresh to be the same as cwnd. If packet losses
occur by random errors before ssthresh reaches the network capacity,
ssthresh can be gotten a smaller value, thus blindly reducing the
sending rate (i.e degrading TCP performance).
TCP and many of its variants implement flow control and congestion
control algorithms [J88] based on the sliding window and additive
increase multiplicative decrease (AIMD)[CJ89] algorithms. TCP
versions, such as Reno[RFC2001] and NewReno[RFC2582] have showed
the heavy throughput degradation over links with high BER, fading,
and backout, such as wireless links, due to TCP's congestion control,
avoidance mechanism based on the assumption that all packet losses
are due to congestions.
1.1 Terminology
In this circumstance of document, we define the following
terminologies.
Heterogeneous network: In a heterogeneous network it is possible to
use a combination of several networks, each of which is wired
networks, wireless networks, or satellite networks. This means that
intermediate links may be wireless links or satellite links.
Last wireless hop, the term implies that accessing to Internet of the
mobile node is supported by a base station through the radio interface.
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2. Proposed approaches for the last wireless hop
2.1 Link layer approach
This approach tries to increase the quality of the lossy wireless
link. Thus, it hides the characteristics of the wireless link from
the transport layer and tries to solve the problem at the link layer.
The intuition behind link layer protocols is that the problem is
local, and hence should be solved locally. These protocols involve
forward error correction (FEC), retransmission in response to
automatic repeat request (ARQ) messages, or a hybrid of the two.
Another popular link-layer solution that uses knowledge of TCP is the
Snoop protocol [BSK95], which works to hide losses over the wireless
link from TCP. At a base station, a ‘snoop agent?keeps a cache of
transmitted TCP segments that have not been acknowledged by the
mobile user. When a link-local timeout or duplicate acknowledgements
indicate a packet loss over the link, the snoop agent accesses its
cache and resends the packet without notifying TCP. Thus TCP is
shielded from duplicate ACKs, which avoids unnecessary fast
retransmissions and initiation of congestion avoidance.
2.2 Transport layer approach
One modification to TCP that would improve performance over wireless
links is the use of Selective Acknowledgement (SACK)[RFC2018]. The
use of cumulative ACKs in traditional TCP result in poor performance
when multiple packets are lost during one transmission window.
The cumulative ACKs do not provide information quickly enough to
allow for fast recovery.
Explicit loss notification (ELN) is a solution aimed at providing the
sender with information to distinguish between losses due to
congestion and those due to errors on the wireless link.
2.3 Split-connection approach
As wireless networks usually connect to a wired infrastructure,
a split-connection approach can be used whereby distinct connections
are set up for the wired and wireless links. A TCP session could be
used on each link, or a specialized transport protocol that is suited
to a wireless environment could be used for the wireless link.
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The basic idea behind MTCP[BS96] is to protect the long connection
over the wired network from the impact of the erratic behavior of the
short connection over the wireless link and also recover quickly from
errors over the wireless link.
If a mobile host need to communicate to a fixed host using
I-TCP[BB94], a request is sent to the current base station to open
a TCP connection with the fixed host on behalf of the mobile host.
The mobile host communicates with its base station on a separate
connection using a variation of TCP that is tuned for wireless links
and is aware of mobility.
3. Explicit Congestion Notifications
Dectecting packet loss with explicit notification from the network
it is possible to determine when a loss is due to congestion.
Several proposals along these lines include:
- Explicit Loss Notification (ELN) [BPSK96]
- Explicit Bad State Notification (EBSN) [BBKVP96]
- Explicit Loss Notification to the Receiver (ELNR), and Explicit
Delayed Dupack Activation Notification (EDDAN) (notifications
to mobile receiver) [MV97]
- Explicit Congestion Notification (ECN) [RFC3168]
Of these proposals, Explicit Congestion Notification (ECN) seems
closest to deployment on the Internet, and will provide some benefit
for TCP connections on long thin networks (as well as for all other
TCP connections).
ECN uses the TOS byte in the IP header to carry congestion
information (ECN-capable and Congestion-encountered). This byte is
not encrypted in IPSEC, so ECN can be used on TCP connections that
are encrypted using IPSEC.
ECN is an explicit signaling mechanism designed to convey network
congestion information from routers to end stations; however, since
the signaling only uses one bit for such congestion information,
the information conveyed is not quantitative. For TCP flow control,
ECN works by configuring the intermediate router to mark packets
with congestion experienced (CE) bit in the IP header when the
router’s average queue occupancy exceeds a threshold, so that the
TCP receiver can echo this information back to the sender via ACK
by setting the explicit congestion echo (ECE) bit in the TCP header.
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4. End-to-End bandwidth estimation
Regular TCP detects the available bandwidth of the bottleneck link
by continuously increasing the window size until the network is
congested and then decrease the window size multiplicatively,
e.g., the AIMD algorithm. However, the decrement of window size
is rather heuristic and coarse, i.e., halving the current window
size. This is because regular TCP scheme lacks the ability to
quantitatively estimate the available bandwidth before
the congestion happens. Regular TCP scheme’s congestion control
mechanisms are reactive rather than proactive and preventive.
We propose a stable accurate rapid bandwidth estimate (SARBE)
algorithm incorporating ECN to improve TCP performance over
heterogeneous networks.
The SARBE algorithm estimates the bandwidth sample of the
forward path of connection by monitoring the sending time intervals
of ACKs sent by the receiver. By using the stability-based filter,
the current bandwidth samples are eliminated from transient changes,
while keeping reasonably persistent changes in the absence of noise.
In SARBE approach, the estimate of the forward path is not be
affected by ACK compression [ZSC91] that results in overestimate.
When the kth ACK arrives at the sender, the sender simply uses
information of the kth ACK to compute an available bandwidth sample
as Bw[k] = L[k]/(ts[k] - ts[k-1]) (1)
where L[k] is the amount of data acknowledged by the kth ACK,
ts[k] is timestamp of the kth ACK, ts[k-1] is the timestamp of
the previous ACK arrived at the sender. Obviously, sample Bwk
represents the current network condition, which faces noises.
So the bandwidth estimator has to eliminate transient noise but
responds rapidly to persistent changes.
The stability-based filter [KN00] is similar to the EWMA filter,
except using a measure function of the samples?large variance
to dynamically change the gain in the EWMA filter. After computing
the bandwidth sample Bwk from (1), the stability-based filter is
computed as
U[k] = beta*U[k-1] + (1 - beta)*|Bw[k] - Bw[k-1]| (2)
Umax = max(U[k-N],...U[k-1],U[k])
alpha = U[k]/Umax (3)
eBw[k]= alpha*eBw[k-1] + (1-alpha)*Bw[k] (4)
where U[k] is the network instability (beta=0.8); Umax is the largest
network instability observed in the last N instabilities (N=8); and
eBw[k] is the estimated smoothed bandwidth, eBw[k-1] is the previous
estimate and the gain alpha is computed dynamically as (3) when
the bandwidth samples vary largely.
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5. TCP for Heterogeneous Networks
TCP scheme for heterogeneous networks incorporates ECN and
available bandwidth estimation algorithm SARBE.
The optimal congestion window (ownd) size is computed based on
bandwidth estimation of SARBE, following:
ownd = eBw[k]*RTTmin/seg_size
The RTTmin is the smallest RTT sample observed over the duration
of the connection. Seg_size is the length of the TCP segment.
5.1 Algorithm after receiving ACK or n DupACKs
if (ACK is received)
if (ECE == 1 ) /* the network has been congested */
ssthresh = ownd;
endif
endif
if ( n DupACKs are received)
ssthresh = ownd;
if (ECE == 0) /* the network has not been congested */
/* the loss is occured by random error of wireless links */
/* in the congestion avoidance phase */
if (cwnd > ssthresh )
cwnd = ssthresh;
endif
endif
endif
When n DUPACKs are received without the sign of the congested
network, TCP-aware of the packet drop is caused by a
wireless link, and therefore it does not adjust the window size.
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5.2 Algorithm after timeout expiration
if ( timeout expires)
ssthresh = ownd;
if (ECE == 1) /* the network has been congested */
if (ssthresh < 2)
ssthresh = 2;
endif
cwnd = 1; /* go to Slow Start phase */
else /* the timeout is occured by random error of wireless links */
/* in the congestion avoidance phase */
if (cwnd > ssthresh )
cwnd = ssthresh;
endif
endif
endif
After the timer expires, if the network has been congested by
notifying from an intermediate router, cwnd and ssthresh are
set equal to 1 and ownd, respectively. Otherwise, the timeout
is occured by random error of wireless links, forcing TCP congestion
control into beginning of the congestion avoidance phase during
it has been being in the congestion avoidance phase.
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6. References
6.1 Normative references
[KN00] M. Kim and B. D. Noble, "SANE: stable agile network
estimation," Technical Report CSE-TR-432-00, University
of Michigan, Department of Electrical Engineering and
Computer Science, Ann Arbor, MI, August 2000.
[MV97] Mehta, M., Vaidya, N., "Delayed Duplicate-
Acknowledgements: A Proposal to Improve Performance
of TCP on Wireless Links," Texas A&M University,
December 24, 1997.
[BBKVP96] Bakshi, B., P., Krishna, N., Vaidya, N., Pradhan,
D.K., "Improving Performance of TCP over Wireless
Networks," Technical Report 96-014, Texas A&M
University, 1996.
[BPSK96] Balakrishnan, H., Padmanabhan, V., Seshan, S., Katz,R.,
"A Comparison of Mechanisms for Improving TCP
Performance over Wireless Links," in ACM SIGCOMM,
Stanford, California, August 1996.
[BS96] Brown, K. Singh, S., "A Network Architecture for
Mobile Computing," Proc. IEEE INFOCOM'96, pp. 1388-
1396, March 1996.
6.2 Informative references
[J88] V. Jacobson, "Congestion avoidance and control", in Proc.
ACM SIGCOMM, Aug. 1988, pp. 314- 329.
[CJ89] D. Chiu and R. Jain, "Analysis of the increase/decrease
algorithms for congestion avoidance in computer networks",
J. Comput. Networks, vol. 17, no. 1, pp. 1-14, June 1989.
[RFC3168] K. Ramakrishnan, S. Floyd, and D. Black, "The Addition of
Explicit Congestion Notification (ECN) to IP", RFC 3168,
September 2001.
[RFC2582] S. Floyd, T. Henderson, "The NewReno Modification to TCP's
Fast Recovery Algorithm", RFC 2582, April 1999.
[RFC2001] W. Stevens, "TCP slow start, congestion avoidance,
fast retransmit and fast recovery algorithms", RFC 2001,
1997.
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[RFC2018] M. Mathis, J. Mahdavi, S. Floyd, and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[BSK95] H.Balakrishnan, S.Seshan, and R.H.Katz, "Improving reliable
transport and handoff performance in cellular wireless
networks," ACM Wireless Networks, vol.1, Dec 1995.
[BB94] Ajay Bakre, and B.R. Badrinath, "I-TCP: Indirect TCP
for Mobile Hosts," October, 1994.
[ZSC91] L. Zhang, S. Shenker, and D. Clark, "Observations on
the Dynamics of a Congestion Control Algorithm: The Effects
of Two-Way Traffic," Proc. SIGCOMM Symp. Comm.
Architectures and Protocols, pp. 133-147, Sept. 1991.
7. Authors' Addresses
Questions about this memo can be directed to:
Choong Seon Hong
Department of Computer Engineering
Kyung Hee University
1 Seocheon, Giheung, Yongin, Gyeongi-Do, 449-701, Korea
E-mail : cshong@khu.ac.kr
Hyung-Keun Ryu
KT Advanced Technology Lab
Woomyun, Seocho, Seoul, Korea
Email : hkryu@kt.co.kr
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