Network Working Group H. Balakrishnan
Request for Comments: 3449 MIT LCS
BCP: 69 V. N. Padmanabhan
Category: Best Current Practice Microsoft Research
G. Fairhurst
M. Sooriyabandara
University of Aberdeen, U.K.
December 2002
TCP Performance Implications
of Network Path Asymmetry
Status of this Memo
This document specifies an Internet Best Current Practices for the
Internet Community, and requests discussion and suggestions for
improvements. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document describes TCP performance problems that arise because
of asymmetric effects. These problems arise in several access
networks, including bandwidth-asymmetric networks and packet radio
subnetworks, for different underlying reasons. However, the end
result on TCP performance is the same in both cases: performance
often degrades significantly because of imperfection and variability
in the ACK feedback from the receiver to the sender.
The document details several mitigations to these effects, which have
either been proposed or evaluated in the literature, or are currently
deployed in networks. These solutions use a combination of local
link-layer techniques, subnetwork, and end-to-end mechanisms,
consisting of: (i) techniques to manage the channel used for the
upstream bottleneck link carrying the ACKs, typically using header
compression or reducing the frequency of TCP ACKs, (ii) techniques to
handle this reduced ACK frequency to retain the TCP sender's
acknowledgment-triggered self-clocking and (iii) techniques to
schedule the data and ACK packets in the reverse direction to improve
performance in the presence of two-way traffic. Each technique is
described, together with known issues, and recommendations for use.
A summary of the recommendations is provided at the end of the
document.
Balakrishnan et. al. Best Current Practice [Page 1]RFC 3449 PILC - Asymmetric Links December 2002
Table of Contents
1. Conventions used in this Document ...............................3
2. Motivation ....................................................4
2.1 Asymmetry due to Differences in Transmit
and Receive Capacity .........................................4
2.2 Asymmetry due to Shared Media in the Reverse Direction .......5
2.3 The General Problem ..........................................5
3. How does Asymmetry Degrade TCP Performance? .....................5
3.1 Asymmetric Capacity ..........................................5
3.2 MAC Protocol Interactions ....................................7
3.3 Bidirectional Traffic ........................................8
3.4 Loss in Asymmetric Network Paths ............................10
4. Improving TCP Performance using Host Mitigations ...............10
4.1 Modified Delayed ACKs .......................................11
4.2 Use of Large MSS ............................................12
4.3 ACK Congestion Control ......................................13
4.4 Window Prediction Mechanism .................................14
4.5 Acknowledgement based on Cwnd Estimation. ...................14
4.6 TCP Sender Pacing ...........................................14
4.7 TCP Byte Counting ...........................................15
4.8 Backpressure ................................................16
5. Improving TCP performance using Transparent Modifications ......17
5.1 TYPE 0: Header Compression ..................................18
5.1.1 TCP Header Compression ..................................18
5.1.2 Alternate Robust Header Compression Algorithms ..........19
5.2 TYPE 1: Reverse Link Bandwidth Management ...................19
5.2.1 ACK Filtering ...........................................20
5.2.2 ACK Decimation ..........................................21
5.3 TYPE 2: Handling Infrequent ACKs ............................22
5.3.1 ACK Reconstruction ......................................23
5.3.2 ACK Compaction and Companding ...........................25
5.3.3 Mitigating TCP packet bursts generated by
Infrequent ACKs .........................................26
5.4 TYPE 3: Upstream Link Scheduling ............................27
5.4.1 Per-Flow queuing at the Upstream Bottleneck Link ........27
5.4.2 ACKs-first Scheduling ...................................28
6. Security Considerations ........................................29
7. Summary ........................................................30
8. Acknowledgments ................................................32
9. References .....................................................32
10. IANA Considerations ...........................................37
Appendix: Examples of Subnetworks Exhibiting Network Path
Asymmetry ...............................................38
Authors' Addresses ................................................40
Full Copyright Statement ..........................................41
Balakrishnan et. al. Best Current Practice [Page 2]RFC 3449 PILC - Asymmetric Links December 2002
1. Conventions used in this Document
FORWARD DIRECTION: The dominant direction of data transfer over an
asymmetric network path. It corresponds to the direction with better
characteristics in terms of capacity, latency, error rate, etc. Data
transfer in the forward direction is called "forward transfer".
Packets travelling in the forward direction follow the forward path
through the IP network.
REVERSE DIRECTION: The direction in which acknowledgments of a
forward TCP transfer flow. Data transfer could also happen in this
direction (and is termed "reverse transfer"), but it is typically
less voluminous than that in the forward direction. The reverse
direction typically exhibits worse characteristics than the forward
direction. Packets travelling in the reverse direction follow the
reverse path through the IP network.
UPSTREAM LINK: The specific bottleneck link that normally has much
less capability than the corresponding downstream link. Congestion
is not confined to this link alone, and may also occur at any point
along the forward and reverse directions (e.g., due to sharing with
other traffic flows).
DOWNSTREAM LINK: A link on the forward path, corresponding to the
upstream link.
ACK: A cumulative TCP acknowledgment [RFC791]. In this document,
this term refers to a TCP segment that carries a cumulative
acknowledgement (ACK), but no data.
DELAYED ACK FACTOR, d: The number of TCP data segments acknowledged
by a TCP ACK. The minimum value of d is 1, since at most one ACK
should be sent for each data packet [RFC1122, RFC2581].
STRETCH ACK: Stretch ACKs are acknowledgements that cover more than 2
segments of previously unacknowledged data (d>2) [RFC2581]. Stretch
ACKs can occur by design (although this is not standard), due to
implementation bugs [All97b, RFC2525], or due to ACK loss [RFC2760].
NORMALIZED BANDWIDTH RATIO, k: The ratio of the raw bandwidth
(capacity) of the forward direction to the return direction, divided
by the ratio of the packet sizes used in the two directions [LMS97].
SOFTSTATE: Per-flow state established in a network device that is
used by the protocol [Cla88]. The state expires after a period of
time (i.e., is not required to be explicitly deleted when a session
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expires), and is continuously refreshed while a flow continues (i.e.,
lost state may be reconstructed without needing to exchange
additional control messages).
2. Motivation
Asymmetric characteristics are exhibited by several network
technologies, including cable data networks, (e.g., DOCSIS cable TV
networks [DS00, DS01]), direct broadcast satellite (e.g., an IP
service using Digital Video Broadcast, DVB, [EN97] with an
interactive return channel), Very Small Aperture satellite Terminals
(VSAT), Asymmetric Digital Subscriber Line (ADSL) [ITU02, ANS01], and
several packet radio networks. These networks are increasingly being
deployed as high-speed Internet access networks, and it is therefore
highly desirable to achieve good TCP performance. However, the
asymmetry of the network paths often makes this challenging.
Examples of some networks that exhibit asymmetry are provided in the
Appendix.
Asymmetry may manifest itself as a difference in transmit and receive
capacity, an imbalance in the packet loss rate, or differences
between the transmit and receive paths [RFC3077]. For example, when
capacity is asymmetric, such that there is reduced capacity on
reverse path used by TCP ACKs, slow or infrequent ACK feedback
degrades TCP performance in the forward direction. Similarly,
asymmetry in the underlying Medium Access Control (MAC) and Physical
(PHY) protocols could make it expensive to transmit TCP ACKs
(disproportionately to their size), even when capacity is symmetric.
2.1 Asymmetry due to Differences in Transmit and Receive Capacity
Network paths may be asymmetric because the upstream and downstream
links operate at different rates and/or are implemented using
different technologies.
The asymmetry in capacity may be substantially increased when best
effort IP flows carrying TCP ACKs share the available upstream
capacity with other traffic flows, e.g., telephony, especially flows
that have reserved upstream capacity. This includes service
guarantees at the IP layer (e.g., the Guaranteed Service [RFC2212])
or at the subnet layer (e.g., support of Voice over IP [ITU01] using
the Unsolicited Grant service in DOCSIS [DS01], or CBR virtual
connections in ATM over ADSL [ITU02, ANS01]).
When multiple upstream links exist the asymmetry may be reduced by
dividing upstream traffic between a number of available upstream
links.
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2.2 Asymmetry due to Shared Media in the Reverse Direction
In networks employing centralized multiple access control, asymmetry
may be a fundamental consequence of the hub-and-spokes architecture
of the network (i.e., a single base node communicating with multiple
downstream nodes). The central node often incurs less transmission
overhead and does not incur latency in scheduling its own downstream
transmissions. In contrast, upstream transmission is subject to
additional overhead and latency (e.g., due to guard times between
transmission bursts, and contention intervals). This can produce
significant network path asymmetry.
Upstream capacity may be further limited by the requirement that each
node must first request per-packet bandwidth using a contention MAC
protocol (e.g., DOCSIS 1.0 MAC restricts each node to sending at most
a single packet in each upstream time-division interval [DS00]). A
satellite network employing dynamic Bandwidth on Demand (BoD), also
consumes MAC resources for each packet sent (e.g., [EN00]). In these
schemes, the available uplink capacity is a function of the MAC
algorithm. The MAC and PHY schemes also introduce overhead per
upstream transmission which could be so significant that transmitting
short packets (including TCP ACKs) becomes as costly as transmitting
MTU-sized data packets.
2.3 The General Problem
Despite the technological differences between capacity-dependent and
MAC-dependent asymmetries, both kinds of network path suffer reduced
TCP performance for the same fundamental reason: the imperfection and
variability of ACK feedback. This document discusses the problem in
detail and describes several techniques that may reduce or eliminate
the constraints.
3. How does Asymmetry Degrade TCP Performance?
This section describes the implications of network path asymmetry on
TCP performance. The reader is referred to [BPK99, Bal98, Pad98,
FSS01, Sam99] for more details and experimental results.
3.1 Asymmetric Capacity
The problems that degrade unidirectional transfer performance when
the forward and return paths have very different capacities depend on
the characteristics of the upstream link. Two types of situations
arise for unidirectional traffic over such network paths: when the
upstream bottleneck link has sufficient queuing to prevent packet
(ACK) losses, and when the upstream bottleneck link has a small
buffer. Each is considered in turn.
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If the upstream bottleneck link has deep queues, so that this does
not drop ACKs in the reverse direction, then performance is a strong
function of the normalized bandwidth ratio, k. For example, for a 10
Mbps downstream link and a 50 Kbps upstream link, the raw capacity
ratio is 200. With 1000-byte data packets and 40-byte ACKs, the
ratio of the packet sizes is 25. This implies that k is 200/25 = 8.
Thus, if the receiver acknowledges more frequently than one ACK every
8 (k) data packets, the upstream link will become saturated before
the downstream link, limiting the throughput in the forward
direction. Note that, the achieved TCP throughput is determined by
the minimum of the receiver advertised window or TCP congestion
window, cwnd [RFC2581].
If ACKs are not dropped (at the upstream bottleneck link) and k > 1
or k > 0.5 when delayed ACKs are used [RFC1122], TCP ACK-clocking
breaks down. Consider two data packets transmitted by the sender in
quick succession. En route to the receiver, these packets get spaced
apart according to the capacity of the smallest bottleneck link in
the forward direction. The principle of ACK clocking is that the
ACKs generated in response to receiving these data packets reflects
this temporal spacing all the way back to the sender, enabling it to
transmit new data packets that maintain the same spacing [Jac88]. ACK
clocking with delayed ACKs, reflects the spacing between data packets
that actually trigger ACKs. However, the limited upstream capacity
and queuing at the upstream bottleneck router alters the inter-ACK
spacing of the reverse path, and hence that observed at the sender.
When ACKs arrive at the upstream bottleneck link at a faster rate
than the link can support, they get queued behind one another. The
spacing between them when they emerge from the link is dilated with
respect to their original spacing, and is a function of the upstream
bottleneck capacity. Thus the TCP sender clocks out new data packets
at a slower rate than if there had been no queuing of ACKs. The
performance of the connection is no longer dependent on the
downstream bottleneck link alone; instead, it is throttled by the
rate of arriving ACKs. As a side effect, the sender's rate of cwnd
growth also slows down.
A second side effect arises when the upstream bottleneck link on the
reverse path is saturated. The saturated link causes persistent
queuing of packets, leading to an increasing path Round Trip Time
(RTT) [RFC2998] observed by all end hosts using the bottleneck link.
This can impact the protocol control loops, and may also trigger
false time out (underestimation of the path RTT by the sending host).
A different situation arises when the upstream bottleneck link has a
relatively small amount of buffer space to accommodate ACKs. As the
transmission window grows, this queue fills, and ACKs are dropped. If
the receiver were to acknowledge every packet, only one of every k
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ACKs would get through to the sender, and the remaining (k-1) are
dropped due to buffer overflow at the upstream link buffer (here k is
the normalized bandwidth ratio as before). In this case, the reverse
bottleneck link capacity and slow ACK arrival rate are not directly
responsible for any degraded performance. However, the infrequency
of ACKs leads to three reasons for degraded performance:
1. The sender transmits data in large bursts of packets, limited only
by the available cwnd. If the sender receives only one ACK in k,
it transmits data in bursts of k (or more) packets because each
ACK shifts the sliding window by at least k (acknowledged) data
packets (TCP data segments). This increases the likelihood of
data packet loss along the forward path especially when k is
large, because routers do not handle large bursts of packets well.
2. Current TCP sender implementations increase their cwnd by counting
the number of ACKs they receive and not by how much data is
actually acknowledged by each ACK. The later approach, also known
as byte counting (section 4.7), is a standard implementation
option for cwnd increase during the congestion avoidance period
[RFC2581]. Thus fewer ACKs imply a slower rate of growth of the
cwnd, which degrades performance over long-delay connections.
3. The sender TCP's Fast Retransmission and Fast Recovery algorithms
[RFC2581] are less effective when ACKs are lost. The sender may
possibly not receive the threshold number of duplicate ACKs even
if the receiver transmits more than the DupACK threshold (> 3
DupACKs) [RFC2581]. Furthermore, the sender may possibly not
receive enough duplicate ACKs to adequately inflate its cwnd
during Fast Recovery.
3.2 MAC Protocol Interactions
The interaction of TCP with MAC protocols may degrade end-to-end
performance. Variable round-trip delays and ACK queuing are the main
symptoms of this problem.
One example is the impact on terrestrial wireless networks [Bal98]. A
high per-packet overhead may arise from the need for communicating
link nodes to first synchronise (e.g., via a Ready To Send / Clear to
Send (RTS/CTS) protocol) before communication and the significant
turn-around time for the wireless channel. This overhead is
variable, since the RTS/CTS exchange may need to back-off
exponentially when the remote node is busy (e.g., engaged in a
conversation with a different node). This leads to large and
variable communication latencies in packet-radio networks.
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An asymmetric workload (more downstream than upstream traffic) may
cause ACKs to be queued in some wireless nodes (especially in the end
host modems), exacerbating the variable latency. Queuing may also
occur in other shared media, e.g., cable modem uplinks, BoD access
systems often employed on shared satellite channels.
Variable latency and ACK queuing reduces the smoothness of the TCP
data flow. In particular, ACK traffic can interfere with the flow of
data packets, increasing the traffic load of the system.
TCP measures the path RTT, and from this calculates a smoothed RTT
estimate (srtt) and a linear deviation, rttvar. These are used to
estimate a path retransmission timeout (RTO) [RFC2988], set to srtt +
4*rttvar. For most wired TCP connections, the srtt remains constant
or has a low linear deviation. The RTO therefore tracks the path
RTT, and the TCP sender will respond promptly when multiple losses
occur in a window. In contrast, some wireless networks exhibit a
high variability in RTT, causing the RTO to significantly increase
(e.g., on the order of 10 seconds). Paths traversing multiple
wireless hops are especially vulnerable to this effect, because this
increases the probability that the intermediate nodes may already be
engaged in conversation with other nodes. The overhead in most MAC
schemes is a function of both the number and size of packets.
However, the MAC contention problem is a significant function of the
number of packets (e.g., ACKs) transmitted rather than their size.
In other words, there is a significant cost to transmitting a packet
regardless of packet size.
Experiments conducted on the Ricochet packet radio network in 1996
and 1997 demonstrated the impact of radio turnarounds and the
corresponding increased RTT variability, resulting in degraded TCP
performance. It was not uncommon for TCP connections to experience
timeouts of 9 - 12 seconds, with the result that many connections
were idle for a significant fraction of their lifetime (e.g.,
sometimes 35% of the total transfer time). This leads to under-
utilization of the available capacity. These effects may also occur
in other wireless subnetworks.
3.3 Bidirectional Traffic
Bidirectional traffic arises when there are simultaneous TCP
transfers in the forward and reverse directions over an asymmetric
network path, e.g., a user who sends an e-mail message in the reverse
direction while simultaneously receiving a web page in the forward
direction. To simplify the discussion, only one TCP connection in
each direction is considered. In many practical cases, several
simultaneous connections need to share the available capacity,
increasing the level of congestion.
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Bidirectional traffic makes the effects discussed in section 3.1 more
pronounced, because part of the upstream link bandwidth is consumed
by the reverse transfer. This effectively increases the degree of
bandwidth asymmetry. Other effects also arise due to the interaction
between data packets of the reverse transfer and ACKs of the forward
transfer. Suppose at the time the forward TCP connection is
initiated, the reverse TCP connection has already saturated the
bottleneck upstream link with data packets. There is then a high
probability that many ACKs of the new forward TCP connection will
encounter a full upstream link buffer and hence get dropped. Even
after these initial problems, ACKs of the forward connection could
get queued behind large data packets of the reverse connection. The
larger data packets may have correspondingly long transmission times
(e.g., it takes about 280 ms to transmit a 1 Kbyte data packet over a
28.8 kbps line). This causes the forward transfer to stall for long
periods of time. It is only at times when the reverse connection
loses packets (due to a buffer overflow at an intermediate router)
and slows down, that the forward connection gets the opportunity to
make rapid progress and build up its cwnd.
When ACKs are queued behind other traffic for appreciable periods of
time, the burst nature of TCP traffic and self-synchronizing effects
can result in an effect known as ACK Compression [ZSC91], which
reduces the throughput of TCP. It occurs when a series of ACKs, in
one direction are queued behind a burst of other packets (e.g., data
packets traveling in the same direction) and become compressed in
time. This results in an intense burst of data packets in the other
direction, in response to the burst of compressed ACKs arriving at
the server. This phenomenon has been investigated in detail for
bidirectional traffic, and recent analytical work [LMS97] has
predicted ACK Compression may also result from bi-directional
transmission with asymmetry, and was observed in practical asymmetric
satellite subnetworks [FSS01]. In the case of extreme asymmetry
(k>>1), the inter-ACK spacing can increase due to queuing (section
3.1), resulting in ACK dilation.
In summary, sharing of the upstream bottleneck link by multiple flows
(e.g., IP flows to the same end host, or flows to a number of end
hosts sharing a common upstream link) increases the level of ACK
Congestion. The presence of bidirectional traffic exacerbates the
constraints introduced by bandwidth asymmetry because of the adverse
interaction between (large) data packets of a reverse direction
connection and the ACKs of a forward direction connection.
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3.4 Loss in Asymmetric Network Paths
Loss may occur in either the forward or reverse direction. For data
transfer in the forward direction this results respectively in loss
of data packets and ACK packets. Loss of ACKs is less significant
than loss of data packets, because it generally results in stretch
ACKs [CR98, FSS01].
In the case of long delay paths, a slow upstream link [RFC3150] can
lead to another complication when the end host uses TCP large windows
[RFC1323] to maximize throughput in the forward direction. Loss of
data packets on the forward path, due to congestion, or link loss,
common for some wireless links, will generate a large number of
back-to-back duplicate ACKs (or TCP SACK packets [RFC2018]), for each
correctly received data packet following a loss. The TCP sender
employs Fast Retransmission and Recovery [RFC2581] to recover from
the loss, but even if this is successful, the ACK to the
retransmitted data segment may be significantly delayed by other
duplicate ACKs still queued at the upstream link buffer. This can
ultimately lead to a timeout [RFC2988] and a premature end to the TCP
Slow Start [RFC2581]. This results in poor forward path throughput.
Section 5.3 describes some mitigations to counter this.
4. Improving TCP Performance using Host Mitigations
There are two key issues that need to be addressed to improve TCP
performance over asymmetric networks. The first is to manage the
capacity of the upstream bottleneck link, used by ACKs and possibly
other traffic. A number of techniques exist which work by reducing
the number of ACKs that flow in the reverse direction. This has the
side effect of potentially destroying the desirable self-clocking
property of the TCP sender where transmission of new data packets is
triggered by incoming ACKs. Thus, the second issue is to avoid any
adverse impact of infrequent ACKs.
Each of these issues can be handled by local link-layer solutions
and/or by end-to-end techniques. This section discusses end-to-end
modifications. Some techniques require TCP receiver changes
(sections 4.1 4.4, 4.5), some require TCP sender changes (sections
4.6, 4.7), and a pair requires changes to both the TCP sender and
receiver (sections 4.2, 4.3). One technique requires a sender
modification at the receiving host (section 4.8). The techniques may
be used independently, however some sets of techniques are
complementary, e.g., pacing (section 4.6) and byte counting (section
4.7) which have been bundled into a single TCP Sender Adaptation
scheme [BPK99].
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It is normally envisaged that these changes would occur in the end
hosts using the asymmetric path, however they could, and have, been
used in a middle-box or Protocol Enhancing Proxy (PEP) [RFC3135]
employing split TCP. This document does not discuss the issues
concerning PEPs. Section 4 describes several techniques, which do
not require end-to-end changes.
4.1 Modified Delayed ACKs
There are two standard methods that can be used by TCP receivers to
generate acknowledgments. The method outlined in [RFC793] generates
an ACK for each incoming data segment (i.e., d=1). [RFC1122] states
that hosts should use "delayed acknowledgments". Using this
algorithm, an ACK is generated for at least every second full-sized
segment (d=2), or if a second full-sized segment does not arrive
within a given timeout (which must not exceed 500 ms [RFC1122], and
is typically less than 200 ms). Relaxing the latter constraint
(i.e., allowing d>2) may generate Stretch ACKs [RFC2760]. This
provides a possible mitigation, which reduces the rate at which ACKs
are returned by the receiver. An implementer should only deviate
from this requirement after careful consideration of the implications
[RFC2581].
Reducing the number of ACKs per received data segment has a number of
undesirable effects including:
(i) Increased path RTT
(ii) Increased time for TCP to open the cwnd
(iii) Increased TCP sender burst size, since cwnd opens in larger
steps
In addition, a TCP receiver is often unable to determine an optimum
setting for a large d, since it will normally be unaware of the
details of the properties of the links that form the path in the
reverse direction.
RECOMMENDATION: A TCP receiver must use the standard TCP algorithm
for sending ACKs as specified in [RFC2581]. That is, it may delay
sending an ACK after it receives a data segment [RFC1122]. When ACKs
are delayed, the receiver must generate an ACK within 500 ms and the
ACK should be generated for at least every second full sized segment
(MSS) of received data [RFC2581]. This will result in an ACK delay
factor (d) that does not exceed a value of 2. Changing the algorithm
would require a host modification to the TCP receiver and awareness
by the receiving host that it is using a connection with an
asymmetric path. Such a change has many drawbacks in the general
case and is currently not recommended for use within the Internet.
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4.2 Use of Large MSS
A TCP sender that uses a large Maximum Segment Size (MSS) reduces the
number of ACKs generated per transmitted byte of data.
Although individual subnetworks may support a large MTU, the majority
of current Internet links employ an MTU of approx 1500 bytes (that of
Ethernet). By setting the Don't Fragment (DF) bit in the IP header,
Path MTU (PMTU) discovery [RFC1191] may be used to determine the
maximum packet size (and hence MSS) a sender can use on a given
network path without being subjected to IP fragmentation, and
provides a way to automatically select a suitable MSS for a specific
path. This also guarantees that routers will not perform IP
fragmentation of normal data packets.
By electing not to use PMTU Discovery, an end host may choose to use
IP fragmentation by routers along the path in the forward direction
[RFC793]. This allows an MSS larger than smallest MTU along the
path. However, this increases the unit of error recovery (TCP
segment) above the unit of transmission (IP packet). This is not
recommended, since it can increase the number of retransmitted
packets following loss of a single IP packet, leading to reduced
efficiency, and potentially aggravating network congestion [Ken87].
Choosing an MSS larger than the forward path minimum MTU also permits
the sender to transmit more initial packets (a burst of IP fragments
for each TCP segment) when a session starts or following RTO expiry,
increasing the aggressiveness of the sender compared to standard TCP
[RFC2581]. This can adversely impact other standard TCP sessions
that share a network path.
RECOMMENDATION:
A larger forward path MTU is desirable for paths with bandwidth
asymmetry. Network providers may use a large MTU on links in the
forward direction. TCP end hosts using Path MTU discovery may be
able to take advantage of a large MTU by automatically selecting an
appropriate larger MSS, without requiring modification. The use of
Path MTU discovery [RFC1191] is therefore recommended.
Increasing the unit of error recovery and congestion control (MSS)
above the unit of transmission and congestion loss (the IP packet) by
using a larger end host MSS and IP fragmentation in routers is not
recommended.
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4.3 ACK Congestion Control
ACK Congestion Control (ACC) is an experimental technique that
operates end to end. ACC extends congestion control to ACKs, since
they may make non-negligible demands on resources (e.g., packet
buffers, and MAC transmission overhead) at an upstream bottleneck
link. It has two parts: (a) a network mechanism indicating to the
receiver that the ACK path is congested, and (b) the receiver's
response to such an indication.
A router feeding an upstream bottleneck link may detect incipient
congestion, e.g., using an algorithm based on RED (Random Early
Detection) [FJ93]. This may track the average queue size over a time
window in the recent past. If the average exceeds a threshold, the
router may select a packet at random. If the packet IP header has
the Explicit Congestion Notification Capable Transport (ECT) bit set,
the router may mark the packet, i.e., sets an Explicit Congestion
Notification (ECN) [RFC3168] bit(s) in the IP header, otherwise the
packet is normally dropped. The ECN notification received by the end
host is reflected back to the sending TCP end host, to trigger
congestion avoidance [RFC3168]. Note that routers implementing RED
with ECN, do not eliminate packet loss, and may drop a packet (even
when the ECT bit is set). It is also possible to use an algorithm
other than RED to decide when to set the ECN bit.
ACC extends ECN so that both TCP data packets and ACKs set the ECT
bit and are thus candidates for being marked with an ECN bit.
Therefore, upon receiving an ACK with the ECN bit set [RFC3168], a
TCP receiver reduces the rate at which it sends ACKs. It maintains a
dynamically varying delayed-ACK factor, d, and sends one ACK for
every d data packets received. When it receives a packet with the
ECN bit set, it increases d multiplicatively, thereby
multiplicatively decreasing the frequency of ACKs. For each
subsequent RTT (e.g., determined using the TCP RTTM option [RFC1323])
during which it does not receive an ECN, it linearly decreases the
factor d, increasing the frequency of ACKs. Thus, the receiver
mimics the standard congestion control behavior of TCP senders in the
manner in which it sends ACKs.
The maximum value of d is determined by the TCP sender window size,
which could be conveyed to the receiver in a new (experimental) TCP
option. The receiver should send at least one ACK (preferably more)
for each window of data from the sender (i.e., d < (cwnd/mss)) to
prevent the sender from stalling until the receiver's delayed ACK
timer triggers an ACK to be sent.
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RECOMMENDATION: ACK Congestion Control (ACC) is an experimental
technique that requires TCP sender and receiver modifications. There
is currently little experience of using such techniques in the
Internet. Future versions of TCP may evolve to include this or
similar techniques. These are the subject of ongoing research. ACC
is not recommended for use within the Internet in its current form.
4.4 Window Prediction Mechanism
The Window Prediction Mechanism (WPM) is a TCP receiver side
mechanism [CLP98] that uses a dynamic ACK delay factor (varying d)
resembling the ACC scheme (section 4.3). The TCP receiver
reconstructs the congestion control behavior of the TCP sender by
predicting a cwnd value. This value is used along with the allowed
window to adjust the receiver's value of d. WPM accommodates for
unnecessary retransmissions resulting from losses due to link errors.
RECOMMENDATION: Window Prediction Mechanism (WPM) is an experimental
TCP receiver side modification. There is currently little experience
of using such techniques in the Internet. Future versions of TCP may
evolve to include this or similar techniques. These are the subjects
of ongoing research. WPM is not recommended for use within the
Internet in its current form.
4.5 Acknowledgement based on Cwnd Estimation.
Acknowledgement based on Cwnd Estimation (ACE) [MJW00] attempts to
measure the cwnd at the TCP receiver and maintain a varying ACK delay
factor (d). The cwnd is estimated by counting the number of packets
received during a path RTT. The technique may improve accuracy of
prediction of a suitable cwnd.
RECOMMENDATION: Acknowledgement based on Cwnd Estimation (ACE) is an
experimental TCP receiver side modification. There is currently
little experience of using such techniques in the Internet. Future
versions of TCP may evolve to include this or similar techniques.
These are the subject of ongoing research. ACE is not recommended
for use within the Internet in its current form.
4.6 TCP Sender Pacing
Reducing the frequency of ACKs may alleviate congestion of the
upstream bottleneck link, but can lead to increased size of TCP
sender bursts (section 4.1). This may slow the growth of cwnd, and
is undesirable when used over shared network paths since it may
significantly increase the maximum number of packets in the
bottleneck link buffer, potentially resulting in an increase in
network congestion. This may also lead to ACK Compression [ZSC91].
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TCP Pacing [AST00], generally referred to as TCP Sender pacing,
employs an adapted TCP sender to alleviating transmission burstiness.
A bound is placed on the maximum number of packets the TCP sender can
transmit back-to-back (at local line rate), even if the window(s)
allow the transmission of more data. If necessary, more bursts of
data packets are scheduled for later points in time computed based on
the transmission rate of the TCP connection. The transmission rate
may be estimated from the ratio cwnd/srtt. Thus, large bursts of
data packets get broken up into smaller bursts spread over time.
A subnetwork may also provide pacing (e.g., Generic Traffic Shaping
(GTS)), but implies a significant increase in the per-packet
processing overhead and buffer requirement at the router where
shaping is performed (section 5.3.3).
RECOMMENDATIONS: TCP Sender Pacing requires a change to
implementation of the TCP sender. It may be beneficial in the
Internet and will significantly reduce the burst size of packets
transmitted by a host. This successfully mitigates the impact of
receiving Stretch ACKs. TCP Sender Pacing implies increased
processing cost per packet, and requires a prediction algorithm to
suggest a suitable transmission rate. There are hence performance
trade-offs between end host cost and network performance.
Specification of efficient algorithms remains an area of ongoing
research. Use of TCP Sender Pacing is not expected to introduce new
problems. It is an experimental mitigation for TCP hosts that may
control the burstiness of transmission (e.g., resulting from Type 1
techniques, section 5.1.2), however it is not currently widely
deployed. It is not recommended for use within the Internet in its
current form.
4.7 TCP Byte Counting
The TCP sender can avoid slowing growth of cwnd by taking into
account the volume of data acknowledged by each ACK, rather than
opening the cwnd based on the number of received ACKs. So, if an ACK
acknowledges d data packets (or TCP data segments), the cwnd would
grow as if d separate ACKs had been received. This is called TCP
Byte Counting [RFC2581, RFC2760]. (One could treat the single ACK as
being equivalent to d/2, instead of d ACKs, to mimic the effect of
the TCP delayed ACK algorithm.) This policy works because cwnd
growth is only tied to the available capacity in the forward
direction, so the number of ACKs is immaterial.
This may mitigate the impact of asymmetry when used in combination
with other techniques (e.g., a combination of TCP Pacing
(section4.6), and ACC (section 4.3) associated with a duplicate ACK
threshold at the receiver.)
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The main issue is that TCP byte counting may generate undesirable
long bursts of TCP packets at the sender host line rate. An
implementation must also consider that data packets in the forward
direction and ACKs in the reverse direction may both travel over
network paths that perform some amount of packet reordering.
Reordering of IP packets is currently common, and may arise from
various causes [BPS00].
RECOMMENDATION: TCP Byte Counting requires a small TCP sender
modification. In its simplest form, it can generate large bursts of
TCP data packets, particularly when Stretch ACKs are received.
Unlimited byte counting is therefore not allowed [RFC2581] for use
within the Internet.
It is therefore strongly recommended [RFC2581, RFC2760] that any byte
counting scheme should include a method to mitigate the potentially
large bursts of TCP data packets the algorithm can cause (e.g., TCP
Sender Pacing (section 4.6), ABC [abc-ID]). If the burst size or
sending rate of the TCP sender can be controlled then the scheme may
be beneficial when Stretch ACKs are received. Determining safe
algorithms remain an area of ongoing research. Further
experimentation will then be required to assess the success of these
safeguards, before they can be recommended for use in the Internet.
4.8 Backpressure
Backpressure is a technique to enhance the performance of
bidirectional traffic for end hosts directly connected to the
upstream bottleneck link [KVR98]. A limit is set on how many data
packets of upstream transfers can be enqueued at the upstream
bottleneck link. In other words, the bottleneck link queue exerts
'backpressure' on the TCP (sender) layer. This requires a modified
implementation, compared to that currently deployed in many TCP
stacks. Backpressure ensures that ACKs of downstream connections do
not get starved at the upstream bottleneck, thereby improving
performance of the downstream connections. Similar generic schemes
that may be implemented in hosts/routers are discussed in section
5.4.
Backpressure can be unfair to a reverse direction connection and make
its throughput highly sensitive to the dynamics of the forward
connection(s).
RECOMMENDATION: Backpressure requires an experimental modification to
the sender protocol stack of a host directly connected to an upstream
bottleneck link. Use of backpressure is an implementation issue,
rather than a network protocol issue. Where backpressure is
implemented, the optimizations described in this section could be
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desirable and can benefit bidirectional traffic for hosts.
Specification of safe algorithms for providing backpressure is still
a subject of ongoing research. The technique is not recommended for
use within the Internet in its current form.
5. Improving TCP performance using Transparent Modifications
Various link and network layer techniques have been suggested to
mitigate the effect of an upstream bottleneck link. These techniques
may provide benefit without modification to either the TCP sender or
receiver, or may alternately be used in conjunction with one or more
of the schemes identified in section 4. In this document, these
techniques are known as "transparent" [RFC3135], because at the
transport layer, the TCP sender and receiver are not necessarily
aware of their existence. This does not imply that they do not
modify the pattern and timing of packets as observed at the network
layer. The techniques are classified here into three types based on
the point at which they are introduced.
Most techniques require the individual TCP connections passing over
the bottleneck link(s) to be separately identified and imply that
some per-flow state is maintained for active TCP connections. A link
scheduler may also be employed (section 5.4). The techniques (with
one exception, ACK Decimation (section 5.2.2) require:
(i) Visibility of an unencrypted IP and TCP packet header (e.g., no
use of IPSec with payload encryption [RFC2406]).
(ii) Knowledge of IP/TCP options and ability to inspect packets with
tunnel encapsulations (e.g., [RFC2784]) or to suspend
processing of packets with unknown formats.
(iii) Ability to demultiplex flows (by using address/protocol/port
number, or an explicit flow-id).
[RFC3135] describes a class of network device that provides more than
forwarding of packets, and which is known as a Protocol Enhancing
Proxy (PEP). A large spectrum of PEP devices exists, ranging from
simple devices (e.g., ACK filtering) to more sophisticated devices
(e.g., stateful devices that split a TCP connection into two separate
parts). The techniques described in section 5 of this document
belong to the simpler type, and do not inspect or modify any TCP or
UDP payload data. They also do not modify port numbers or link
addresses. Many of the risks associated with more complex PEPs do
not exist for these schemes. Further information about the operation
and the risks associated with using PEPs are described in [RFC3135].
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5.1 TYPE 0: Header Compression
A client may reduce the volume of bits used to send a single ACK by
using compression [RFC3150, RFC3135]. Most modern dial-up modems
support ITU-T V.42 bulk compression. In contrast to bulk
compression, header compression is known to be very effective at
reducing the number of bits sent on the upstream link [RFC1144]. This
relies on the observation that most TCP packet headers vary only in a
few bit positions between successive packets in a flow, and that the
variations can often be predicted.
5.1.1 TCP Header Compression
TCP header compression [RFC1144] (sometimes known as V-J compression)
is a Proposed Standard describing use over low capacity links running
SLIP or PPP [RFC3150]. It greatly reduces the size of ACKs on the
reverse link when losses are infrequent (a situation that ensures
that the state of the compressor and decompressor are synchronized).
However, this alone does not address all of the asymmetry issues:
(i) In some (e.g., wireless) subnetworks there is a significant
per-packet MAC overhead that is independent of packet size
(section 3.2).
(ii) A reduction in the size of ACKs does not prevent adverse
interaction with large upstream data packets in the presence
of bidirectional traffic (section 3.3).
(iii) TCP header compression cannot be used with packets that have
IP or TCP options (including IPSec [RFC2402, RFC2406], TCP
RTTM [RFC1323], TCP SACK [RFC2018], etc.).
(iv) The performance of header compression described by RFC1144 is
significantly degraded when compressed packets are lost. An
improvement, which can still incur significant penalty on
long network paths is described in [RFC2507]. This suggests
it should only be used on links (or paths) that experience a
low level of packet loss [RFC3150].
(v) The normal implementation of Header Compression inhibits
compression when IP is used to support tunneling (e.g., L2TP,
GRE [RFC2794], IP-in-IP). The tunnel encapsulation
complicates locating the appropriate packet headers. Although
GRE allows Header Compression on the inner (tunneled) IP
header [RFC2784], this is not recommended, since loss of a
packet (e.g., due to router congestion along the tunnel path)
will result in discard of all packets for one RTT [RFC1144].
RECOMMENDATION: TCP Header Compression is a transparent modification
performed at both ends of the upstream bottleneck link. It offers no
benefit for flows employing IPSec [RFC2402, RFC2406], or when
additional protocol headers are present (e.g., IP or TCP options,
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and/or tunnel encapsulation headers). The scheme is widely
implemented and deployed and used over Internet links. It is
recommended to improve TCP performance for paths that have a low-to-
medium bandwidth asymmetry (e.g., k<10).
In the form described in [RFC1144], TCP performance is degraded when
used over links (or paths) that may exhibit appreciable rates of
packet loss [RFC3150]. It may also not provide significant
improvement for upstream links with bidirectional traffic. It is
therefore not desirable for paths that have a high bandwidth
asymmetry (e.g., k>10).
5.1.2 Alternate Robust Header Compression Algorithms
TCP header compression [RFC1144] and IP header compression [RFC2507]
do not perform well when subject to packet loss. Further, they do
not compress packets with TCP option fields (e.g., SACK [RFC2018] and
Timestamp (RTTM) [RFC1323]). However, recent work on more robust
schemes suggest that a new generation of compression algorithms may
be developed which are much more robust. The IETF ROHC working group
has specified compression techniques for UDP-based traffic [RFC3095]
and is examining a number of schemes that may provide improve TCP
header compression. These could be beneficial for asymmetric network
paths.
RECOMMENDATION: Robust header compression is a transparent
modification that may be performed at both ends of an upstream
bottleneck link. This class of techniques may also be suited to
Internet paths that suffer low levels of re-ordering. The techniques
benefit paths with a low-to-medium bandwidth asymmetry (e.g., k>10)
and may be robust to packet loss.
Selection of suitable compression algorithms remains an area of
ongoing research. It is possible that schemes may be derived which
support IPSec authentication, but not IPSec payload encryption. Such
schemes do not alone provide significant improvement in asymmetric
networks with a high asymmetry and/or bidirectional traffic.
5.2 TYPE 1: Reverse Link Bandwidth Management
Techniques beyond Type 0 header compression are required to address
the performance problems caused by appreciable asymmetry (k>>1). One
set of techniques is implemented only at one point on the reverse
direction path, within the router/host connected to the upstream
bottleneck link. These use flow class or per-flow queues at the
upstream link interface to manage the queue of packets waiting for
transmission on the bottleneck upstream link.
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This type of technique bounds the upstream link buffer queue size,
and employs an algorithm to remove (discard) excess ACKs from each
queue. This relies on the cumulative nature of ACKs (section 4.1).
Two approaches are described which employ this type of mitigation.
5.2.1 ACK Filtering
ACK Filtering (AF) [DMT96, BPK99] (also known as ACK Suppression
[SF98, Sam99, FSS01]) is a TCP-aware link-layer technique that
reduces the number of ACKs sent on the upstream link. This technique
has been deployed in specific production networks (e.g., asymmetric
satellite networks [ASB96]). The challenge is to ensure that the
sender does not stall waiting for ACKs, which may happen if ACKs are
indiscriminately removed.
When an ACK from the receiver is about to be enqueued at a upstream
bottleneck link interface, the router or the end host link layer (if
the host is directly connected to the upstream bottleneck link)
checks the transmit queue(s) for older ACKs belonging to the same TCP
connection. If ACKs are found, some (or all of them) are removed
from the queue, reducing the number of ACKs.
Some ACKs also have other functions in TCP [RFC1144], and should not
be deleted to ensure normal operation. AF should therefore not
delete an ACK that has any data or TCP flags set (SYN, RST, URG, and
FIN). In addition, it should avoid deleting a series of 3 duplicate
ACKs that indicate the need for Fast Retransmission [RFC2581] or ACKs
with the Selective ACK option (SACK)[RFC2018] from the queue to avoid
causing problems to TCP's data-driven loss recovery mechanisms.
Appropriate treatment is also needed to preserve correct operation of
ECN feedback (carried in the TCP header) [RFC3168].
A range of policies to filter ACKs may be used. These may be either
deterministic or random (similar to a random-drop gateway, but should
take into consideration the semantics of the items in the queue).
Algorithms have also been suggested to ensure a minimum ACK rate to
guarantee the TCP sender window is updated [Sam99, FSS01], and to
limit the number of data packets (TCP segments) acknowledged by a
Stretch ACK. Per-flow state needs to be maintained only for
connections with at least one packet in the queue (similar to FRED
[LM97]). This state is soft [Cla88], and if necessary, can easily be
reconstructed from the contents of the queue.
The undesirable effect of delayed DupACKs (section 3.4) can be
reduced by deleting duplicate ACKs above a threshold value [MJW00,
CLP98] allowing Fast Retransmission, but avoiding early TCP timeouts,
which may otherwise result from excessive queuing of DupACKs.
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Future schemes may include more advanced rules allowing removal of
selected SACKs [RFC2018]. Such a scheme could prevent the upstream
link queue from becoming filled by back-to-back ACKs with SACK
blocks. Since a SACK packet is much larger than an ACK, it would
otherwise add significantly to the path delay in the reverse
direction. Selection of suitable algorithms remains an ongoing area
of research.
RECOMMENDATION: ACK Filtering requires a modification to the upstream
link interface. The scheme has been deployed in some networks where
the extra processing overhead (per ACK) may be compensated for by
avoiding the need to modify TCP. ACK Filtering can generate Stretch
ACKs resulting in large bursts of TCP data packets. Therefore on its
own, it is not recommended for use in the general Internet.
ACK Filtering when used in combination with a scheme to mitigate the
effect of Stretch ACKs (i.e., control TCP sender burst size) is
recommended for paths with appreciable asymmetry (k>1) and/or with
bidirectional traffic. Suitable algorithms to support IPSec
authentication, SACK, and ECN remain areas of ongoing research.
5.2.2 ACK Decimation
ACK Decimation is based on standard router mechanisms. By using an
appropriate configuration of (small) per-flow queues and a chosen
dropping policy (e.g., Weighted Fair Queuing, WFQ) at the upstream
bottleneck link, a similar effect to AF (section 5.2.1) may be
obtained, but with less control of the actual packets which are
dropped.
In this scheme, the router/host at the bottleneck upstream link
maintains per-flow queues and services them fairly (or with
priorities) by queuing and scheduling of ACKs and data packets in the
reverse direction. A small queue threshold is maintained to drop
excessive ACKs from the tail of each queue, in order to reduce ACK
Congestion. The inability to identify special ACK packets (c.f., AF)
introduces some major drawbacks to this approach, such as the
possibility of losing DupACKs, FIN/ACK, RST packets, or packets
carrying ECN information [RFC3168]. Loss of these packets does not
significantly impact network congestion, but does adversely impact
the performance of the TCP session observing the loss.
A WFQ scheduler may assign a higher priority to interactive traffic
(providing it has a mechanism to identify such traffic) and provide a
fair share of the remaining capacity to the bulk traffic. In the
presence of bidirectional traffic, and with a suitable scheduling
policy, this may ensure fairer sharing for ACK and data packets. An
increased forward transmission rate is achieved over asymmetric links
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by an increased ACK Decimation rate, leading to generation of Stretch
ACKs. As in AF, TCP sender burst size increases when Stretch ACKs
are received unless other techniques are used in combination with
this technique.
This technique has been deployed in specific networks (e.g., a
network with high bandwidth asymmetry supporting high-speed data
services to in-transit mobile hosts [Seg00]). Although not optimal,
it offered a potential mitigation applicable when the TCP header is
difficult to identify or not visible to the link layer (e.g., due to
IPSec encryption).
RECOMMENDATION: ACK Decimation uses standard router mechanisms at the
upstream link interface to constrain the rate at which ACKs are fed
to the upstream link. The technique is beneficial with paths having
appreciable asymmetry (k>1). It is however suboptimal, in that it
may lead to inefficient TCP error recovery (and hence in some cases
degraded TCP performance), and provides only crude control of link
behavior. It is therefore recommended that where possible, ACK
Filtering should be used in preference to ACK Decimation.
When ACK Decimation is used on paths with an appreciable asymmetry
(k>1) (or with bidirectional traffic) it increases the burst size of
the TCP sender, use of a scheme to mitigate the effect of Stretch
ACKs or control burstiness is therefore strongly recommended.
5.3 TYPE 2: Handling Infrequent ACKs
TYPE 2 mitigations perform TYPE 1 upstream link bandwidth management,
but also employ a second active element which mitigates the effect of
the reduced ACK rate and burstiness of ACK transmission. This is
desirable when end hosts use standard TCP sender implementations
(e.g., those not implementing the techniques in sections 4.6, 4.7).
Consider a path where a TYPE 1 scheme forwards a Stretch ACK covering
d TCP packets (i.e., where the acknowledgement number is d*MSS larger
than the last ACK received by the TCP sender). When the TCP sender
receives this ACK, it can send a burst of d (or d+1) TCP data
packets. The sender is also constrained by the current cwnd.
Received ACKs also serve to increase cwnd (by at most one MSS).
A TYPE 2 scheme mitigates the impact of the reduced ACK frequency
resulting when a TYPE 1 scheme is used. This is achieved by
interspersing additional ACKs before each received Stretch ACK. The
additional ACKs, together with the original ACK, provide the TCP
sender with sufficient ACKs to allow the TCP cwnd to open in the same
way as if each of the original ACKs sent by the TCP receiver had been
forwarded by the reverse path. In addition, by attempting to restore
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the spacing between ACKs, such a scheme can also restore the TCP
self-clocking behavior, and reduce the TCP sender burst size. Such
schemes need to ensure conservative behavior (i.e., should not
introduce more ACKs than were originally sent) and reduce the
probability of ACK Compression [ZSC91].
The action is performed at two points on the return path: the
upstream link interface (where excess ACKs are removed), and a point
further along the reverse path (after the bottleneck upstream
link(s)), where replacement ACKs are inserted. This attempts to
reconstruct the ACK stream sent by the TCP receiver when used in
combination with AF (section 5.2.1), or ACK Decimation (section
5.2.2).
TYPE 2 mitigations may be performed locally at the receive interface
directly following the upstream bottleneck link, or may alternatively
be applied at any point further along the reverse path (this is not
necessarily on the forward path, since asymmetric routing may employ
different forward and reverse internet paths). Since the techniques
may generate multiple ACKs upon reception of each individual Stretch
ACK, it is strongly recommended that the expander implements a scheme
to prevent exploitation as a "packet amplifier" in a Denial-of-
Service (DoS) attack (e.g., to verify the originator of the ACK).
Identification of the sender could be accomplished by appropriately
configured packet filters and/or by tunnel authentication procedures
(e.g., [RFC2402, RFC2406]). A limit on the number of reconstructed
ACKs that may be generated from a single packet may also be
desirable.
5.3.1 ACK Reconstruction
ACK Reconstruction (AR) [BPK99] is used in conjunction with AF
(section 5.2.1). AR deploys a soft-state [Cla88] agent called an ACK
Reconstructor on the reverse path following the upstream bottleneck
link. The soft-state can be regenerated if lost, based on received
ACKs. When a Stretch ACK is received, AR introduces additional ACKs
by filling gaps in the ACK sequence. Some potential Denial-of-
Service vulnerabilities may arise (section 6) and need to be
addressed by appropriate security techniques.
The Reconstructor determines the number of additional ACKs, by
estimating the number of filtered ACKs. This uses implicit
information present in the received ACK stream by observing the ACK
sequence number of each received ACK. An example implementation
could set an ACK threshold, ackthresh, to twice the MSS (this assumes
the chosen MSS is known by the link). The factor of two corresponds
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to standard TCP delayed-ACK policy (d=2). Thus, if successive ACKs
arrive separated by delta, the Reconstructor regenerates a maximum of
((delta/ackthresh) - 2) ACKs.
To reduce the TCP sender burst size and allow the cwnd to increase at
a rate governed by the downstream link, the reconstructed ACKs must
be sent at a consistent rate (i.e., temporal spacing between
reconstructed ACKs). One method is for the Reconstructor to measure
the arrival rate of ACKs using an exponentially weighted moving
average estimator. This rate depends on the output rate from the
upstream link and on the presence of other traffic sharing the link.
The output of the estimator indicates the average temporal spacing
for the ACKs (and the average rate at which ACKs would reach the TCP
sender if there were no further losses or delays). This may be used
by the Reconstructor to set the temporal spacing of reconstructed
ACKs. The scheme may also be used in combination with TCP sender
adaptation (e.g., a combination of the techniques in sections 4.6 and
4.7).
The trade-off in AR is between obtaining less TCP sender burstiness,
and a better rate of cwnd increase, with a reduction in RTT
variation, versus a modest increase in the path RTT. The technique
cannot perform reconstruction on connections using IPSec (AH
[RFC2402] or ESP [RFC2406]), since it is unable to generate
appropriate security information. It also cannot regenerate other
packet header information (e.g., the exact pattern of bits carried in
the IP packet ECN field [RFC3168] or the TCP RTTM option [RFC1323]).
An ACK Reconstructor operates correctly (i.e., generates no spurious
ACKs and preserves the end-to-end semantics of TCP), providing:
(i) the TCP receiver uses ACK Delay (d=2) [RFC2581]
(ii) the Reconstructor receives only in-order ACKs
(iii) all ACKs are routed via the Reconstructor
(iv) the Reconstructor correctly determines the TCP MSS used by
the session
(v) the packets do not carry additional header information (e.g.,
TCP RTTM option [RFC1323], IPSec using AH [RFC2402]or ESP
[RFC2406]).
RECOMMENDATION: ACK Reconstruction is an experimental transparent
modification performed on the reverse path following the upstream
bottleneck link. It is designed to be used in conjunction with a
TYPE 1 mitigation. It reduces the burst size of TCP transmission in
the forward direction, which may otherwise increase when TYPE 1
schemes are used alone. It requires modification of equipment after
the upstream link (including maintaining per-flow soft state). The
scheme introduces implicit assumptions about the network path and has
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potential Denial-of-Service vulnerabilities (i.e., acting as a packet
amplifier); these need to be better understood and addressed by
appropriate security techniques.
Selection of appropriate algorithms to pace the ACK traffic remains
an open research issue. There is also currently little experience of
the implications of using such techniques in the Internet, and
therefore it is recommended that this technique should not be used
within the Internet in its current form.
5.3.2 ACK Compaction and Companding
ACK Compaction and ACK Companding [SAM99, FSS01] are techniques that
operate at a point on the reverse path following the constrained ACK
bottleneck. Like AR (section 5.3.1), ACK Compaction and ACK
Companding are both used in conjunction with an AF technique (section
5.2.1) and regenerate filtered ACKs, restoring the ACK stream.
However, they differ from AR in that they use a modified AF (known as
a compactor or compressor), in which explicit information is added to
all Stretch ACKs generated by the AF. This is used to explicitly
synchronize the reconstruction operation (referred to here as
expansion).
The modified AF combines two modifications: First, when the
compressor deletes an ACK from the upstream bottleneck link queue, it
appends explicit information (a prefix) to the remaining ACK (this
ACK is marked to ensure it is not subsequently deleted). The
additional information contains details the conditions under which
ACKs were previously filtered. A variety of information may be
encoded in the prefix. This includes the number of ACKs deleted by
the AF and the average number of bytes acknowledged. This may
subsequently be used by an expander at the remote end of the tunnel.
Further timing information may also be added to control the pacing of
the regenerated ACKs [FSS01]. The temporal spacing of the filtered
ACKs may also be encoded.
To encode the prefix requires the subsequent expander to recognize a
modified ACK header. This would normally limit the expander to
link-local operation (at the receive interface of the upstream
bottleneck link). If remote expansion is needed further along the
reverse path, a tunnel may be used to pass the modified ACKs to the
remote expander. The tunnel introduces extra overhead, however
networks with asymmetric capacity and symmetric routing frequently
already employ such tunnels (e.g., in a UDLR network [RFC3077], the
expander may be co-located with the feed router).
Balakrishnan et. al. Best Current Practice [Page 25]
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ACK expansion uses a stateless algorithm to expand the ACK (i.e.,
each received packet is processed independently of previously
received packets). It uses the prefix information together with the
acknowledgment field in the received ACK, to produce an equivalent
number of ACKs to those previously deleted by the compactor. These
ACKs are forwarded to the original destination (i.e., the TCP
sender), preserving normal TCP ACK clocking. In this way, ACK
Compaction, unlike AR, is not reliant on specific ACK policies, nor
must it see all ACKs associated with the reverse path (e.g., it may
be compatible with schemes such as DAASS [RFC2760]).
Some potential Denial-of-Service vulnerabilities may arise (section
6) and need to be addressed by appropriate security techniques. The
technique cannot perform reconstruction on connections using IPSec,
since they are unable to regenerate appropriate security information.
It is possible to explicitly encode IPSec security information from
suppressed packets, allowing operation with IPSec AH, however this
remains an open research issue, and implies an additional overhead
per ACK.
RECOMMENDATION: ACK Compaction and Companding are experimental
transparent modifications performed on the reverse path following the
upstream bottleneck link. They are designed to be used in
conjunction with a modified TYPE 1 mitigation and reduce the burst
size of TCP transmission in the forward direction, which may
otherwise increase when TYPE 1 schemes are used alone.
The technique is desirable, but requires modification of equipment
after the upstream bottleneck link (including processing of a
modified ACK header). Selection of appropriate algorithms to pace
the ACK traffic also remains an open research issue. Some potential
Denial-of-Service vulnerabilities may arise with any device that may
act as a packet amplifier. These need to be addressed by appropriate
security techniques. There is little experience of using the scheme
over Internet paths. This scheme is a subject of ongoing research
and is not recommended for use within the Internet in its current
form.
5.3.3 Mitigating TCP packet bursts generated by Infrequent ACKs
The bursts of data packets generated when a Type 1 scheme is used on
the reverse direction path may be mitigated by introducing a router
supporting Generic Traffic Shaping (GTS) on the forward path [Seg00].
GTS is a standard router mechanism implemented in many deployed
routers. This technique does not eliminate the bursts of data
generated by the TCP sender, but attempts to smooth out the bursts by
employing scheduling and queuing techniques, producing traffic which
resembles that when TCP Pacing is used (section 4.6). These
Balakrishnan et. al. Best Current Practice [Page 26]
RFC 3449 PILC - Asymmetric Links December 2002
techniques require maintaining per-flow soft-state in the router, and
increase per-packet processing overhead. Some additional buffer
capacity is needed to queue packets being shaped.
To perform GTS, the router needs to select appropriate traffic
shaping parameters, which require knowledge of the network policy,
connection behavior and/or downstream bottleneck characteristics. GTS
may also be used to enforce other network policies and promote
fairness between competing TCP connections (and also UDP and
multicast flows). It also reduces the probability of ACK Compression
[ZSC91].
The smoothing of packet bursts reduces the impact of the TCP
transmission bursts on routers and hosts following the point at which
GTS is performed. It is therefore desirable to perform GTS near to
the sending host, or at least at a point before the first forward
path bottleneck router.
RECOMMENDATIONS: Generic Traffic Shaping (GTS) is a transparent
technique employed at a router on the forward path. The algorithms
to implement GTS are available in widely deployed routers and may be
used on an Internet link, but do imply significant additional per-
packet processing cost.
Configuration of a GTS is a policy decision of a network service
provider. When appropriately configured the technique will reduce
size of TCP data packet bursts, mitigating the effects of Type 1
techniques. GTS is recommended for use in the Internet in
conjunction with type 1 techniques such as ACK Filtering (section
5.2.1) and ACK Decimation (section 5.2.2).
5.4 TYPE 3: Upstream Link Scheduling
Many of the above schemes imply using per flow queues (or per
connection queues in the case of TCP) at the upstream bottleneck
link. Per-flow queuing (e.g., FQ, CBQ) offers benefit when used on
any slow link (where the time to transmit a packet forms an
appreciable part of the path RTT) [RFC3150]. Type 3 schemes offer
additional benefit when used with one of the above techniques.
5.4.1 Per-Flow queuing at the Upstream Bottleneck Link
When bidirectional traffic exists in a bandwidth asymmetric network
competing ACK and packet data flows along the return path may degrade
the performance of both upstream and downstream flows [KVR98].
Therefore, it is highly desirable to use a queuing strategy combined
with a scheduling mechanism at the upstream link. This has also been
called priority-based multiplexing [RFC3135].
Balakrishnan et. al. Best Current Practice [Page 27]
RFC 3449 PILC - Asymmetric Links December 2002
On a slow upstream link, appreciable jitter may be introduced by
sending large data packets ahead of ACKs [RFC3150]. A simple scheme
may be implemented using per-flow queuing with a fair scheduler
(e.g., round robin service to all flows, or priority scheduling). A
modified scheduler [KVR98] could place a limit on the number of ACKs
a host is allowed to transmit upstream before transmitting a data
packet (assuming at least one data packet is waiting in the upstream
link queue). This guarantees at least a certain minimum share of the
capacity to flows in the reverse direction, while enabling flows in
the forward direction to improve TCP throughput.
Bulk (payload) compression, a small MTU, link level transparent
fragmentation [RFC1991, RFC2686] or link level suspend/resume
capability (where higher priority frames may pre-empt transmission of
lower priority frames) may be used to mitigate the impact (jitter) of
bidirectional traffic on low speed links [RFC3150]. More advanced
schemes (e.g., WFQ) may also be used to improve the performance of
transfers with multiple ACK streams such as http [Seg00].
RECOMMENDATION: Per-flow queuing is a transparent modification
performed at the upstream bottleneck link. Per-flow (or per-class)
scheduling does not impact the congestion behavior of the Internet,
and may be used on any Internet link. The scheme has particular
benefits for slow links. It is widely implemented and widely
deployed on links operating at less than 2 Mbps. This is recommended
as a mitigation on its own or in combination with one of the other
described techniques.
5.4.2 ACKs-first Scheduling
ACKs-first Scheduling is an experimental technique to improve
performance of bidirectional transfers. In this case data packets
and ACKs compete for resources at the upstream bottleneck link
[RFC3150]. A single First-In First-Out, FIFO, queue for both data
packets and ACKs could impact the performance of forward transfers.
For example, if the upstream bottleneck link is a 28.8 kbps dialup
line, the transmission of a 1 Kbyte sized data packet would take
about 280 ms. So even if just two such data packets get queued ahead
of ACKs (not an uncommon occurrence since data packets are sent out
in pairs during slow start), they would shut out ACKs for well over
half a second. If more than two data packets are queued up ahead of
an ACK, the ACKs would be delayed by even more [RFC3150].
A possible approach to alleviating this is to schedule data and ACKs
differently from FIFO. One algorithm, in particular, is ACKs-first
scheduling, which accords a higher priority to ACKs over data
packets. The motivation for such scheduling is that it minimizes the
idle time for the forward connection by minimizing the time that ACKs
Balakrishnan et. al. Best Current Practice [Page 28]
RFC 3449 PILC - Asymmetric Links December 2002
spend queued behind data packets at the upstream link. At the same
time, with Type 0 techniques such as header compression [RFC1144],
the transmission time of ACKs becomes small enough that the impact on
subsequent data packets is minimal. (Subnetworks in which the per-
packet overhead of the upstream link is large, e.g., packet radio
subnetworks, are an exception, section 3.2.) This scheduling scheme
does not require the upstream bottleneck router/host to explicitly
identify or maintain state for individual TCP connections.
ACKs-first scheduling does not help avoid a delay due to a data
packet in transmission. Link fragmentation or suspend/resume may be
beneficial in this case.
RECOMMENDATION: ACKs-first scheduling is an experimental transparent
modification performed at the upstream bottleneck link. If it is
used without a mechanism (such as ACK Congestion Control (ACC),
section 4.3) to regulate the volume of ACKs, it could lead to
starvation of data packets. This is a performance penalty
experienced by end hosts using the link and does not modify Internet
congestion behavior. Experiments indicate that ACKs-first scheduling
in combination with ACC is promising. However, there is little
experience of using the technique in the wider Internet. Further
development of the technique remains an open research issue, and
therefore the scheme is not currently recommended for use within the
Internet.
6. Security Considerations
The recommendations contained in this document do not impact the
integrity of TCP, introduce new security implications to the TCP
protocol, or applications using TCP.
Some security considerations in the context of this document arise
from the implications of using IPSec by the end hosts or routers
operating along the return path. Use of IPSec prevents, or
complicates, some of the mitigations. For example:
(i) When IPSec ESP [RFC2406] is used to encrypt the IP payload, the
TCP header can neither be read nor modified by intermediate
entities. This rules out header compression, ACK Filtering, ACK
Reconstruction, and the ACK Compaction.
(ii) The TCP header information may be visible, when some forms of
network layer security are used. For example, using IPSec AH
[RFC2402], the TCP header may be read, but not modified, by
intermediaries. This may in future allow extensions to support
ACK Filtering, but rules out the generation of new
Balakrishnan et. al. Best Current Practice [Page 29]RFC 3449 PILC - Asymmetric Links December 2002
packets by intermediaries (e.g., ACK Reconstruction). The
enhanced header compression scheme discussed in [RFC2507] would
also work with IPSec AH.
There are potential Denial-of-Service (DoS) implications when using
Type 2 schemes. Unless additional security mechanisms are used, a
Reconstructor/expander could be exploited as a packet amplifier. A
third party may inject unauthorized Stretch ACKs into the reverse
path, triggering the generation of additional ACKs. These ACKs would
consume capacity on the return path and processing resources at the
systems along the path, including the destination host. This
provides a potential platform for a DoS attack. The usual
precautions must be taken to verify the correct tunnel end point, and
to ensure that applications cannot falsely inject packets that expand
to generate unwanted traffic. Imposing a rate limit and bound on the
delayed ACK factor(d) would also lessen the impact of any undetected
exploitation.
7. Summary
This document considers several TCP performance constraints that
arise from asymmetry in the properties of the forward and reverse
paths across an IP network. Such performance constraints arise,
e.g., as a result of both bandwidth (capacity) asymmetry, asymmetric
shared media in the reverse direction, and interactions with Media
Access Control (MAC) protocols. Asymmetric capacity may cause TCP
Acknowledgments (ACKs) to be lost or become inordinately delayed
(e.g., when a bottleneck link is shared between many flows, or when
there is bidirectional traffic). This effect may be exacerbated with
media-access delays (e.g., in certain multi-hop radio subnetworks,
satellite Bandwidth on Demand access). Asymmetry, and particular
high asymmetry, raises a set of TCP performance issues.
A set of techniques providing performance improvement is surveyed.
These include techniques to alleviate ACK Congestion and techniques
that enable a TCP sender to cope with infrequent ACKs without
destroying TCP self-clocking. These techniques include both end-to-
end, local link-layer, and subnetwork schemes. Many of these
techniques have been evaluated in detail via analysis, simulation,
and/or implementation on asymmetric subnetworks forming part of the
Internet. There is however as yet insufficient operational
experience for some techniques, and these therefore currently remain
items of on-going research and experimentation.
Balakrishnan et. al. Best Current Practice [Page 30]RFC 3449 PILC - Asymmetric Links December 2002
The following table summarizes the current recommendations.
Mechanisms are classified as recommended (REC), not recommended (NOT
REC) or experimental (EXP). Experimental techniques may not be well
specified. These techniques will require further operational
experience before they can be recommended for use in the public
Internet.
The recommendations for end-to-end host modifications are summarized
in table 1. This lists each technique, the section in which each
technique is discussed, and where it is applied (S denotes the host
sending TCP data packets in the forward direction, R denotes the host
which receives these data packets).
+------------------------+-------------+------------+--------+
| Technique | Use | Section | Where |
+------------------------+-------------+------------+--------+
| Modified Delayed ACKs | NOT REC | 4.1 | TCP R |
| Large MSS & NO FRAG | REC | 4.2 | TCP SR |
| Large MSS & IP FRAG | NOT REC | 4.2 | TCP SR |
| ACK Congestion Control | EXP | 4.3 | TCP SR |
| Window Pred. Mech (WPM)| NOT REC | 4.4 | TCP R |
| Window Cwnd. Est. (ACE)| NOT REC | 4.5 | TCP R |
| TCP Sender Pacing | EXP *1 | 4.6 | TCP S |
| Byte Counting | NOT REC *2 | 4.7 | TCP S |
| Backpressure | EXP *1 | 4.8 | TCP R |
+------------------------+-------------+------------+--------+
Table 1: Recommendations concerning host modifications.
*1 Implementation of the technique may require changes to the
internal design of the protocol stack in end hosts.
*2 Dependent on a scheme for preventing excessive TCP transmission
burst.
The recommendations for techniques that do not require the TCP sender
and receiver to be aware of their existence (i.e., transparent
techniques) are summarized in table 2. Each technique is listed
along with the section in which each mechanism is discussed, and
where the technique is applied (S denotes the sending interface prior
to the upstream bottleneck link, R denotes receiving interface
following the upstream bottleneck link).
Balakrishnan et. al. Best Current Practice [Page 31]RFC 3449 PILC - Asymmetric Links December 2002
+------------------------+-------------+------------+--------+
| Mechanism | Use | Section | Type |
+------------------------+-------------+------------+--------+
| Header Compr. (V-J) | REC *1 | 5.1.1 | 0 SR |
| Header Compr. (ROHC) | REC *1 *2 | 5.1.2 | 0 SR |
+------------------------+-------------+------------+--------+
| ACK Filtering (AF) | EXP *3 | 5.2.1 | 1 S |
| ACK Decimation | EXP *3 | 5.2.2 | 1 S |
+------------------------+-------------+------------+--------+
| ACK Reconstruction (AR)| NOT REC | 5.3.1 | 2 *4 |
| ACK Compaction/Compand.| EXP | 5.3.2 | 2 S *4 |
| Gen. Traff. Shap. (GTS)| REC | 5.3.3 | 2 *5 |
+------------------------+-------------+------------+--------+
| Fair Queueing (FQ) | REC | 5.4.1 | 3 S |
| ACKs-First Scheduling | NOT REC | 5.4.2 | 3 S |
+------------------------+-------------+------------+--------+
Table 2: Recommendations concerning transparent modifications.
*1 At high asymmetry these schemes may degrade TCP performance, but
are not considered harmful to the Internet.
*2 Standardisation of new TCP compression protocols is the subject of
ongoing work within the ROHC WG, refer to other IETF RFCs on the
use of these techniques.
*3 Use in the Internet is dependent on a scheme for preventing
excessive TCP transmission burst.
*4 Performed at a point along the reverse path after the upstream
bottleneck link.
*5 Performed at a point along the forward path.
8. Acknowledgments
This document has benefited from comments from the members of the
Performance Implications of Links (PILC) Working Group. In
particular, the authors would like to thank John Border, Spencer
Dawkins, Aaron Falk, Dan Grossman, Randy Katz, Jeff Mandin, Rod
Ragland, Ramon Segura, Joe Touch, and Lloyd Wood for their useful
comments. They also acknowledge the data provided by Metricom Inc.,
concerning operation of their packet data network.
9. References
References of the form RFCnnnn are Internet Request for Comments
(RFC) documents available online at http://www.rfc-editor.org/.
Balakrishnan et. al. Best Current Practice [Page 32]RFC 3449 PILC - Asymmetric Links December 2002
9.1 Normative References
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[RFC1144] Jacobson, V., "Compressing TCP/IP Headers for Low-Speed
Serial Links", RFC 1144, February 1990.
[RFC1191] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191,
November 1990.
[RFC2581] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion
Control", RFC 2581, April 1999.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D. and P. Traina,
"Generic Routing Encapsulation (GRE)", RFC 2784, March
2000.
[RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G. and Z.
Shelby, "Performance Enhancing Proxies Intended to Mitigate
Link-Related Degradations", RFC 3135, June 2001.
9.2 Informative References
[abc-ID] Allman, M., "TCP Congestion Control with Appropriate Byte
Counting", Work in Progress.
[All97b] Allman, M., "Fixing Two BSD TCP Bugs", Technical Report
CR-204151, NASA Lewis Research Center, October 1997.
[ANS01] ANSI Standard T1.413, "Network to Customer Installation
Interfaces - Asymmetric Digital Subscriber Lines (ADSL)
Metallic Interface", November 1998.
[ASB96] Arora, V., Suphasindhu, N., Baras, J.S. and D. Dillon,
"Asymmetric Internet Access over Satellite-Terrestrial
Networks", Proc. AIAA: 16th International Communications
Satellite Systems Conference and Exhibit, Part 1,
Washington, D.C., February 25-29, 1996, pp.476-482.
[AST00] Aggarwal, A., Savage, S., and T. Anderson, "Understanding
the Performance of TCP Pacing", Proc. IEEE INFOCOM, Tel-
Aviv, Israel, V.3, March 2000, pp. 1157-1165.
Balakrishnan et. al. Best Current Practice [Page 33]RFC 3449 PILC - Asymmetric Links December 2002
[Bal98] Balakrishnan, H., "Challenges to Reliable Data Transport
over Heterogeneous Wireless Networks", Ph.D. Thesis,
University of California at Berkeley, USA, August 1998.
http://nms.lcs.mit.edu/papers/hari-phd/
[BPK99] Balakrishnan, H., Padmanabhan, V. N., and R. H. Katz, "The
Effects of Asymmetry on TCP Performance", ACM Mobile
Networks and Applications (MONET), Vol.4, No.3, 1999, pp.
219-241. An expanded version of a paper published at Proc.
ACM/IEEE Mobile Communications Conference (MOBICOM), 1997.
[BPS00] Bennett, J. C., Partridge, C., and N. Schectman, "Packet
Reordering is Not Pathological Network Behaviour", IEEE/ACM
Transactions on Networking, Vol. 7, Issue. 6, 2000,
pp.789-798.
[Cla88] Clark, D.D, "The Design Philosophy of the DARPA Internet
Protocols", ACM Computer Communications Review (CCR), Vol.
18, Issue 4, 1988, pp.106-114.
[CLC99] Clausen, H., Linder, H., and B. Collini-Nocker, "Internet
over Broadcast Satellites", IEEE Communications Magazine,
Vol. 37, Issue. 6, 1999, pp.146-151.
[CLP98] Calveras, A., Linares, J., and J. Paradells, "Window
Prediction Mechanism for Improving TCP in Wireless
Asymmetric Links". Proc. IEEE Global Communications
Conference (GLOBECOM), Sydney Australia, November 1998,
pp.533-538.
[CR98] Cohen, R., and Ramanathan, S., "Tuning TCP for High
Performance in Hybrid Fiber Coaxial Broad-Band Access
Networks", IEEE/ACM Transactions on Networking, Vol.6,
No.1, 1998, pp.15-29.
[DS00] Cable Television Laboratories, Inc., Data-Over-Cable
Service Interface Specifications---Radio Frequency
Interface Specification SP-RFIv1.1-I04-00407, 2000
[DS01] Data-Over-Cable Service Interface Specifications, Radio
Frequency Interface Specification 1.0, SP-RFI-I05-991105,
Cable Television Laboratories, Inc., November 1999.
[DMT96] Durst, R., Miller, G., and E. Travis, "TCP Extensions for
Space Communications", ACM/IEEE Mobile Communications
Conference (MOBICOM), New York, USA, November 1996, pp.15-
26.
Balakrishnan et. al. Best Current Practice [Page 34]RFC 3449 PILC - Asymmetric Links December 2002
[EN97] "Digital Video Broadcasting (DVB); DVB Specification for
Data Broadcasting", European Standard (Telecommunications
series) EN 301 192, 1997.
[EN00] "Digital Video Broadcasting (DVB); Interaction Channel for
Satellite Distribution Systems", Draft European Standard
(Telecommunications series) ETSI, Draft EN 301 790, v.1.2.1
[FJ93] Floyd, S., and V. Jacobson, "Random Early Detection
gateways for Congestion Avoidance", IEEE/ACM Transactions
on Networking, Vol.1, No.4, 1993, pp.397-413.
[FSS01] Fairhurst, G., Samaraweera, N.K.G, Sooriyabandara, M.,
Harun, H., Hodson, K., and R. Donardio, "Performance Issues
in Asymmetric Service Provision using Broadband Satellite",
IEE Proceedings on Communication, Vol.148, No.2, 2001,
pp.95-99.
[ITU01] ITU-T Recommendation E.681, "Traffic Engineering Methods
For IP Access Networks Based on Hybrid Fiber/Coax System",
September 2001.
[ITU02] ITU-T Recommendation G.992.1, "Asymmetrical Digital
Subscriber Line (ADSL) Transceivers", July 1999.
[Jac88] Jacobson, V., "Congestion Avoidance and Control", Proc. ACM
SIGCOMM, Stanford, CA, ACM Computer Communications Review
(CCR), Vol.18, No.4, 1988, pp.314-329.
[Ken87] Kent C.A., and J. C. Mogul, "Fragmentation Considered
Harmful", Proc. ACM SIGCOMM, USA, ACM Computer
Communications Review (CCR), Vol.17, No.5, 1988, pp.390-
401.
[KSG98] Krout, T., Solsman, M., and J. Goldstein, "The Effects of
Asymmetric Satellite Networks on Protocols", Proc. IEEE
Military Communications Conference (MILCOM), Bradford, MA,
USA, Vol.3, 1998, pp.1072-1076.
[KVR98] Kalampoukas, L., Varma, A., and Ramakrishnan, K.K.,
"Improving TCP Throughput over Two-Way Asymmetric Links:
Analysis and Solutions", Proc. ACM SIGMETRICS, Medison,
USA, 1998, pp.78-89.
[LM97] Lin, D., and R. Morris, "Dynamics of Random Early
Detection", Proc. ACM SIGCOMM, Cannes, France, ACM Computer
Communications Review (CCR), Vol.27, No.4, 1997, pp.78-89.
Balakrishnan et. al. Best Current Practice [Page 35]RFC 3449 PILC - Asymmetric Links December 2002
[LMS97] Lakshman, T.V., Madhow, U., and B. Suter, "Window-based
Error Recovery and Flow Control with a Slow Acknowledgement
Channel: A Study of TCP/IP Performance", Proc. IEEE
INFOCOM, Vol.3, Kobe, Japan, 1997, pp.1199-1209.
[MJW00] Ming-Chit, I.T., Jinsong, D., and W. Wang,"Improving TCP
Performance Over Asymmetric Networks", ACM SIGCOMM, ACM
Computer Communications Review (CCR), Vol.30, No.3, 2000.
[Pad98] Padmanabhan, V.N., "Addressing the Challenges of Web Data
Transport", Ph.D. Thesis, University of California at
Berkeley, USA, September 1998 (also Tech Report UCB/CSD-
98-1016). http://www.cs.berkeley.edu/~padmanab/phd-
thesis.html
[RFC1323] Jacobson, V., Braden, R. and D. Borman, "TCP Extensions for
High Performance", RFC 1323, May 1992.
[RFC2018] Mathis, B., Mahdavi, J., Floyd, S. and A. Romanow, "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[RFC2402] Kent, S. and R. Atkinson, "IP Authentication Header", RFC
2402, November 1998.
[RFC2406] Kent, S. and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC 2406, November 1998.
[RFC2507] Degermark, M., Nordgren, B. and S. Pink, "IP Header
Compression", RFC 2507, February 1999.
[RFC2525] Paxson, V., Allman, M., Dawson, S., Heavens, I. and B.
Volz, "Known TCP Implementation Problems", RFC 2525, March
1999.
[RFC2686] Bormann, C., "The Multi-Class Extension to Multi-Link PPP",
RFC 2686, September 1999.
[RFC2760] Allman, M., Dawkins, S., Glover, D., Griner, J., Henderson,
T., Heidemann, J., Kruse, H., Ostermann, S., Scott, K.,
Semke, J., Touch, J. and D. Tran, "Ongoing TCP Research
Related to Satellites", RFC 2760, February 2000.
[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.
[RFC3077] Duros, E., Dabbous, W., Izumiyama, H., Fujii, N. and Y.
Zhang, "A link Layer tunneling mechanism for unidirectional
links", RFC 3077, March 2001.
Balakrishnan et. al. Best Current Practice [Page 36]RFC 3449 PILC - Asymmetric Links December 2002
[RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
Hannu, H., Jonsson, E., Hakenberg, R., Koren, T., Le, K.,
Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K., Wiebke,
T., Yoshimura, T. and H. Zheng, "RObust Header Compression
(ROHC): Framework and four profiles: RTP, UDP ESP and
uncompressed", RFC 3095, July 2001.
[RFC3150] Dawkins, S., Montenegro, G., Kojo, M. and V. Magret, "End-
to-end Performance Implications of Slow Links", BCP 48, RFC
3150, July 2001.
[RFC3168] Ramakrishnan K., Floyd, S. and D. Black, "A Proposal to add
Explicit Congestion Notification (ECN) to IP", RFC 3168,
September 2001.
[Sam99] Samaraweera, N.K.G, "Return Link Optimization for Internet
Service Provision Using DVB-S Networks", ACM Computer
Communications Review (CCR), Vol.29, No.3, 1999, pp.4-19.
[Seg00] Segura R., "Asymmetric Networking Techniques For Hybrid
Satellite Communications", NC3A, The Hague, Netherlands,
NATO Technical Note 810, August 2000, pp.32-37.
[SF98] Samaraweera, N.K.G., and G. Fairhurst. "High Speed Internet
Access using Satellite-based DVB Networks", Proc. IEEE
International Networks Conference (INC98), Plymouth, UK,
1998, pp.23-28.
[ZSC91] Zhang, L., Shenker, S., and D. D. Clark, "Observations and
Dynamics of a Congestion Control Algorithm: The Effects of
Two-Way Traffic", Proc. ACM SIGCOMM, ACM Computer
Communications Review (CCR), Vol 21, No 4, 1991, pp.133-
147.
10. IANA Considerations
There are no IANA considerations associated with this document.
Balakrishnan et. al. Best Current Practice [Page 37]RFC 3449 PILC - Asymmetric Links December 2002
Appendix - Examples of Subnetworks Exhibiting Network Path Asymmetry
This appendix provides a list of some subnetworks which are known to
experience network path asymmetry. The asymmetry in capacity of
these network paths can require mitigations to provide acceptable
overall performance. Examples include the following:
- IP service over some wide area and local area wireless networks.
In such networks, the predominant network path asymmetry arises
from the hub-and-spokes architecture of the network (e.g., a
single base station that communicates with multiple mobile
stations), this requires a Ready To Send / Clear To Send (RTS/CTS)
protocol and a Medium Access Control (MAC) protocol which needs to
accommodate the significant turn-around time for the radios. A
high per-packet transmission overhead may lead to significant
network path asymmetry.
- IP service over a forward satellite link utilizing Digital Video
Broadcast (DVB) transmission [EN97] (e.g., 38-45 Mbps), and a
slower upstream link using terrestrial network technology (e.g.,
dial-up modem, line of sight microwave, cellular radio) [CLC99].
Network path asymmetry arises from a difference in the upstream
and downstream link capacities.
- Certain military networks [KSG98] providing Internet access to
in-transit or isolated hosts [Seg00] using a high capacity
downstream satellite link (e.g., 2-3 Mbps) with a narrowband
upstream link (e.g., 2.4-9.6 kbps) using either Demand Assigned
Multiple Access (DAMA) or fixed rate satellite links. The main
factor contributing to network path asymmetry is the difference in
the upstream and downstream link capacities. Some differences
between forward and reverse paths may arise from the way in which
upstream link capacity is allocated.
- Most data over cable TV networks (e.g., DOCSIS [ITU01, DS00]),
where the analogue channels assigned for upstream communication
(i.e., in the reverse direction) are narrower and may be more
noisy than those assigned for the downstream link. As a
consequence, the upstream and downstream links differ in their
transmission rate. For example, in DOCSIS 1.0 [DS00], the
downstream transmission rate is either 27 or 52 Mbps. Upstream
transmission rates may be dynamically selected to be one of a
series of rates which range between 166 kbps to 9 Mbps. Operators
may assign multiple upstream channels per downstream channel.
Physical layer (PHY) overhead (which accompanies upstream
transmissions, but is not present in the downstream link) can also
increase the network path asymmetry. The Best Effort service,
which is typically used to carry TCP, uses a
Balakrishnan et. al. Best Current Practice [Page 38]RFC 3449 PILC - Asymmetric Links December 2002
contention/reservation MAC protocol. A cable modem (CM) sending
an isolated packet (such as a TCP ACK) on the upstream link must
contend with other CMs to request capacity from the central cable
modem termination system (CMTS). The CMTS then grants timeslots
to a CM for the upstream transmission. The CM may "piggyback"
subsequent requests onto upstream packets, avoiding contention
cycles; as a result, spacing of TCP ACKs can be dramatically
altered due to minor variations in load of the cable data network
and inter-arrival times of TCP DATA packets. Numerous other
complexities may add to, or mitigate, the asymmetry in rate and
access latency experienced by packets sent on the upstream link
relative to downstream packets in DOCSIS. The asymmetry
experienced by end hosts may also change dynamically (e.g., with
network load), and when best effort services share capacity with
services that have symmetric reserved capacity (e.g., IP telephony
over the Unsolicited Grant service) [ITU01].
- Asymmetric Digital Subscriber Line (ADSL), by definition, offers a
downstream link transmission rate that is higher than that of the
upstream link. The available rates depend upon channel quality
and system configuration. For example, one widely deployed ADSL
technology [ITU02, ANS01] operates at rates that are multiples of
32 kbps (up to 6.144 Mbps) in the downstream link, and up to 640
kbps for the upstream link. The network path asymmetry
experienced by end hosts may be further increased when best effort
services, e.g., Internet access over ADSL, share the available
upstream capacity with reserved services (e.g., constant bit rate
voice telephony).
Balakrishnan et. al. Best Current Practice [Page 39]RFC 3449 PILC - Asymmetric Links December 2002
Authors' Addresses
Hari Balakrishnan
Laboratory for Computer Science
200 Technology Square
Massachusetts Institute of Technology
Cambridge, MA 02139
USA
Phone: +1-617-253-8713
EMail: hari@lcs.mit.edu
Web: http://nms.lcs.mit.edu/~hari/
Venkata N. Padmanabhan
Microsoft Research
One Microsoft Way
Redmond, WA 98052
USA
Phone: +1-425-705-2790
EMail: padmanab@microsoft.com
Web: http://www.research.microsoft.com/~padmanab/
Godred Fairhurst
Department of Engineering
Fraser Noble Building
University of Aberdeen
Aberdeen AB24 3UE
UK
EMail: gorry@erg.abdn.ac.uk
Web: http://www.erg.abdn.ac.uk/users/gorry
Mahesh Sooriyabandara
Department of Engineering
Fraser Noble Building
University of Aberdeen
Aberdeen AB24 3UE
UK
EMail: mahesh@erg.abdn.ac.uk
Web: http://www.erg.abdn.ac.uk/users/mahesh
Balakrishnan et. al. Best Current Practice [Page 40]
RFC 3449 PILC - Asymmetric Links December 2002
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Balakrishnan et. al. Best Current Practice [Page 41]
