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Internet Engineering Task Force Hari Balakrishnan A new Request for Comments is now available in online RFC libraries.
Internet Draft MIT LCS
Document: draft-ietf-pilc-asym-07.txt Venkata N. Padmanabhan
Microsoft Research
Gorry Fairhurst
University of Aberdeen, U.K.
Mahesh Sooriyabandara
University of Aberdeen, U.K.
Category: Informational November 2001
TCP Performance Implications of Network Path Asymmetry
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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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.
This 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 table in section 8 provides a summary of
the recommendations.
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 .........4
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 ...............................9
4. Improving TCP Performance using Host Mitigations ................9
4.1 Modified Delayed ACKs .........................................10
4.2 Use of Large MSS ..............................................11
4.3 ACK Congestion Control ........................................12
4.4 Window Prediction Mechanism ...................................13
4.5 Acknowledgement based on Cwnd Estimation. .....................13
4.6 TCP Pacing ....................................................13
4.7 TCP Byte Counting .............................................14
4.8 Backpressure ..................................................15
5. Improving TCP performance using Transparent Modifications ......15
5.1 TYPE 0: Header Compression ....................................16
5.1.1 TCP Header Compression ......................................16
5.1.2 Alternate Robust Header Compression Algorithms ..............17
5.2 TYPE 1: Reverse Link Bandwidth Management .....................18
5.2.1 ACK Filtering ...............................................18
5.2.2 ACK Decimation ..............................................19
5.3 TYPE 2: Handling Infrequent ACKs ..............................20
5.3.1 ACK Reconstruction ..........................................21
5.3.2 ACK Compaction and Companding ...............................23
5.3.3 Mitigating the TCP packet bursts generated by Infrequent ACKs24
5.4 TYPE 3: Upstream Link Scheduling ..............................25
5.4.1 Per-Flow queuing at the Upstream Bottleneck Link ............25
5.4.2 ACKs-first Scheduling .......................................26
6. Security Considerations ........................................27
7. Summary ........................................................27
8. Acknowledgments ................................................29
9. References .....................................................30
10. Authors' Addresses ............................................33
11. IANA Considerations ...........................................34
Appendix I : Examples of Subnetworks Exhibiting Network Path
Asymmetry .........................................................35
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 traveling 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 it 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.
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. 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].
NORMALISED 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 which 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
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 which exhibit asymmetry are provided in
Appendix I.
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 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.
2.2 Asymmetry due to Shared Media in the Reverse Direction
In networks employing centralised 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 (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] 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.
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, considers the spacing between
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) 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 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. First, 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. Second, 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 (see section 5.7) 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. Third, 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). 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 synchronize (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 (for example, engaged in
a conversation with a different node). This leads to large and
variable communication latencies in packet-radio networks.
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), 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.
Bidirectional traffic makes the effects discussed in section 4.1
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 KB 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-synchronising 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,
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.
3.4 Loss in Asymmetric Network Paths
Loss may occur on either the forward or reverse path. 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 maximise 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 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 upsteam link buffer. This can
ultimately lead to a timeout and a premature end to the TCP Slow
Start [RFC2581]. This results in poor forward path throughput.
Section 6.2.1 describes a mitigation to counter this.
4. Improving TCP Performance using Host Mitigations
There are two key issues that need to be addressed in order to
improve TCP performance over asymmetric networks. The first issue 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 (5.1
5.4, 5.5), some require TCP sender changes (5.6, 5.7), and a pair
requires changes to both the TCP sender and receiver (5.2, 5.3). One
technique requires a sender modification at the receiving host
(5.8). The techniques may be used independently, however some sets
of techniques are complementary, for example, pacing (5.6) and byte
counting (5.7) which have been bundled into a single TCP Sender
Adaptation scheme [BPK99].
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, employing
split TCP. This document does not discuss the issues concerning PEPs
[RFC3135]. 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 (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-size segment does not arrive within a
given timeout (which must not exceed 500 ms [RFC1122], 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 implementor 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 reverse path.
RECOMMENDATION: A TCP receiver should send an ACK after receiving at
most every full-sized MSS volume of data (i.e. d=2) [RFC2581].
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. It is currently not recommended for
use within the Internet.
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 B
(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 the 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 forward path [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.
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.
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, for example,
detect incipient congestion 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) 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 [RFC2481]. 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 [RFC2481], 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 subse-
quent 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 sender's 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. Otherwise, to prevent the
sender from stalling until the receiver's delayed ACK timer triggers
an ACK to be sent.
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. It 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 that [CLP98] uses a dynamic ACK delay factor (varying d)
resembling the ACC scheme (section 5.3). 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
subject of ongoing research. It 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. It is not recommended for
use within the Internet in its current form.
4.6 TCP 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. Congestion may also lead to ACK Compression
[ZSC91] under some conditions.
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 TCP connection's transmission rate. The
transmission rate may be estimated from the ratio cwnd/srtt. Thus,
large bursts of data packets get broken up into smaller bursts
spread out 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 (see section 6.3.3).
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 remain an area of ongoing
research. Use of TCP sender pacing is not expected to introduce new
problems, and may be used by TCP hosts, however it is not currently
widely deployed.
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 (section
5.6), and ACC (section 5.3) associated with a duplicate ACK
threshold at the receiver.)
There are issues associated with this approach. The main issue is
that the scheme 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].
It is strongly recommended [RFC2581; RFC2760] that any byte counting
scheme should also include a mechanism to prevent excessive
transmission bursts (e.g. TCP Pacing (section 5.6), ABC [abc-ID]).
RECOMMENDATION: TCP Byte Counting requires a small TCP sender
modification.
The simplest modification may generate large bursts of TCP data
packets, particularly when Stretch ACKs are received. Unlimited byte
counting is therefore not recommended [RFC2581; RFC2760] for use
within the Internet without a method to mitigate the potentially
large bursts of TCP data packets the algorithm can cause. If the
burst size or sending rate of the TCP sender can be controlled (e.g.
by Pacing), 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
A technique to enhance the performance of bidirectional traffic has
been proposed 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. (The host
where backpressure is implemented is assumed to be connected
directly to the upstream bottleneck link.) 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 which may be implemented in
hosts/routers are discussed in section 6.4.
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
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 5. 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 6.4). The techniques (with
one exception, ACK Decimation (section 6.2.2) require:
(i) Visibility of an unencrypted IP and TCP packet header (e.g.
no use of IPSEC with payload encryption)
(ii) Knowledge of IP/TCP options/tunnels (or ability 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 6 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].
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
[RFC1144] (sometimes known as V-J compression) is a Proposed
Standard describing TCP header compression for use over low capacity
links running SLIP or PPP. 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 4.2).
(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 4.3).
(iii) TCP header compression cannot be used with packets that have
IP or TCP options (including IPSEC, 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.
(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. 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. The
technique benefits paths that have a low-to-medium bandwidth
asymmetry (e.g. k<10). The scheme is widely implemented and
deployed and used over Internet links.
In the form described in [RFC1144], it provides very poor
performance when used over links (or paths) that may exhibit
appreciable rates of packet loss. The scheme on its own may not
provide significant improvement for links with bidirectional
traffic. It also offers no benefit for flows employing IPSEC.
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 (k) and may
be robust to packet loss.
Selection of suitable compression algorithms remain 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 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 high 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.
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 (see section
5.1). 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 (sync, reset,
urgent, and final). 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) [RFC2481].
A range of policies to filter ACKs may be used. These may be either
deterministic or random (similar to a random-drop gateway, but
taking the semantics of the items in the queue into consideration).
Algorithms have also been suggested to ensure a minimum ACK rate to
guarantee the sender's window is updated [Sam99, FSS01], and 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 4.4) can be
reduced by deleting duplicate ACKs up to a threshold value [MJW00,
CLP98] allowing Fast Retransmission, but avoiding early TCP
timeouts, which may otherwise result from excessive queuing of
DupACKs.
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 reverse path delay. Selection of
suitable algorithms remains an ongoing area of research.
RECOMMENDATION: ACK Filtering requires a modification to the
upstream link interface. It benefits paths that have an arbitrary
bandwidth asymmetry. 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. At high 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 TCP sender burst size is therefore strongly recommended in
combination with ACK Filtering.
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 6.2.1) 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. 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 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 approach is however suboptimal, in
that 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 a high 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 hosts use standard TCP sender
implementations (e.g. those not implementing the techniques in
sections 5.6, 5.7).
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 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 6.2.1), or ACK Decimation (6.2.2).
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 compacted ACK). Identification of the sender could
be accomplished by appropriately configured packet filters, by
tunnel encryption procedures. 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 6.2.1). 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 (see section 6.3) 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 to standard TCP delayed-ACK policy (d=2). Thus, if
successive ACKs arrive separated by deltaa, 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 ACK 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 5.6 and 5.7).
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 or
encryption), 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 or the TCP RTTM option [RFC1323]).
An ACK Reconstructor operates correctly (i.e. generates no spurious
ACKs and preserving the end-to-end semantics of the connection),
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 (AH or ESP)).
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. The scheme requires modification of
equipment after the upstream link (including maintaining per-flow
soft state). The scheme introduces 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 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, JSK99] are
experimental techniques that operate at a point on the reverse path
following the constrained ACK bottleneck. Like AR (section 6.3.1),
ACK Compaction and ACK Companding are both used in conjunction with
an AF technique (section 6.2.1) 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 be
subsequently 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 [JSK99].
To encode the prefix requires the subsequent expander to recognise 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).
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 (see
section 7) 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 and 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 which
may act as a packet amplifier. These need to be addressed by
appropriate security techniques. The technique has not, at the time
of writing been widely deployed, and 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 the 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 5.6).
These 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.
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].
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 the reverse flow(s), while enabling the forward
direction flow(s) to achieve high 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 KB 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 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.) 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 5.3) to regulate the volume of ACKs, it could lead to
starvation of data packets. This is a performance penalty
experiences by 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 memo 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 Internet Draft
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 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) With IPSEC AH or TF-ESP, the TCP header can be read, but not
modified, by intermediaries. This rules out ACK
Reconstruction, but may in future allow extensions to support
ACK Filtering and ACK Compaction. The enhanced header
compression scheme discussed in [RFC2507] would also work
with 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, for
example, 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 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 BoD 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.
The following table summarises the current recommendations.
Mechanisms are classified as recommended (REC), not recommended (NOT
REC) or experimental (EXP). Experimental techniques may not be well
specified, and 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 | 5.1 | TCP R |
| Large MSS & NO FRAG | REC | 5.2 | TCP S |
| Large MSS & IP FRAG | NOT REC | 5.2 | TCP S |
| ACK Congestion Control | EXP | 5.3 | TCP SR |
| Window Pred. Mech (WPM)| NOT REC | 5.4 | TCP R |
| Window Cwnd. Est. (ACE)| NOT REC | 5.5 | TCP R |
| TCP Sender Pacing | EXP *1 | 5.6 | TCP S |
| Byte Counting | NOT REC *2 | 5.7 | TCP S |
| Backpressure | EXP *1 | 5.8 | 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 summarised 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).
+------------------------+-------------+------------+--------+
| Mechanism | Use | Section | Type |
+------------------------+-------------+------------+--------+
| Header Compr. (V-J) | REC | 6.1.1 | 0 SR |
| Header Compr. (ROHC) | REC *1 | 6.1.2 | 0 SR |
+------------------------+-------------+------------+--------+
| ACK Filtering (AF) | EXP *2 | 6.2.1 | 1 S |
| ACK Decimation | EXP *2 | 6.2.2 | 1 S |
+------------------------+-------------+------------+--------+
| ACK Reconstruction (AR)| NOT REC | 6.3.1 | 2 *3 |
| ACK Compaction/Compand.| EXP | 6.3.2 | 2 S *3 |
| Gen. Traff. Shap. (GTS)| REC | 6.3.3 | 2 *4 |
+------------------------+-------------+------------+--------+
| Fair Queueing (FQ) | REC | 6.4.1 | 3 S |
| ACKs-First Scheduling | NOT REC | 6.4.2 | 3 S |
+------------------------+-------------+------------+--------+
Table 2: Recommendations concerning transparent modifications.
*1 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.
*2 Use in the Internet is dependent on a scheme for preventing
excessive TCP transmission burst (see section 6.2).
*3 Performed at a point along the reverse path after the upstream
bottleneck link.
*4 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, 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
[abc-ID] Allman, M., draft-allman-tcp-abc-00.txt, Internet Draft,
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.
[Bal98] Balakrishnan, H., "Challenges to Reliable Data Transport
over Heterogeneous Wireless Networks", Ph.D. Thesis, University of
California at Berkeley, USA, August 1998.
http://www.cs.berkeley.edu/~hari/thesis/
[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 MOBICOM '97.
[BPS00] Bennett, J. C., Partridge, C., and N. Schectman, "Packet
Reordering is Not Pathological Network Behaviour," IEEE/ACM
Transactions on Networking, Vol. 7, 2000, pp.789-798.
[Cla88] Clark, D.D, "The Design Philosophy of the DARPA Internet
Protocols", Proc. ACM SIGCOMM, Stanford, CA, 1988, pp.106-114.
[CLC99] Clausen, H., Linder, H., and B. Collini-Nocker, "Internet
over Broadcast Satellites", IEEE Commun. Mag. 1999, pp.146-151.
[CLP98] Calveras, A., Linares, J., and J. Paradells, "Window
Prediction Mechanism for Improving TCP in Wireless Asymmetric
Links". Proc. IEEE 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, February 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," Proc. ACM MOBICOM, New York, USA, November
1996, pp.15-26.
[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, August 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 Proc.
Commun, Vol.148, No.2, April 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, CCR Vol.18, No.4, August 1988, pp.314-329.
[JSK99] Johansson, G.L., Shakargi, H., Kanljung, C., and J.
Kullander, "ACKNOWLEDGEMENT Compression Rev B", Technical Report
December 1999.
[Ken87] Kent C.A., and J. C. Mogul, _Fragmentation Considered
Harmful", Proc. ACM SIGCOMM, USA, 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 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, CCR Vol.27, No.4, 1997, pp.78-89.
[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 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.research.microsoft.com/~padmanab/phd-thesis.html
[RFC793] Postel, J., "Transmission Control Protocol", RFC791.
[RFC1122] B. Braden, ed., "Requirements for Internet Hosts -
Communication Layers", RFC 1122.
[RFC1144] Jacobson, V., "Compressing TCP/IP Headers for Low-Speed
Serial Links", RFC1144.
[RFC1323] Jacobson, V., Braden, R., Borman, D., "TCP Extensions for
High Performance", RFC 1323.
[RFC1191] Mogul, J., Deering, S., "Path MTU Discovery", RFC 1191.
[RFC2018] Mathis, B., Mahdavi, J., Floyd, S., Romanow, A., "TCP
Selective Acknowledgment Options", RFC2018.
[RFC2481] Ramakrishnan K., and S. Floyd, "A Proposal to add Explicit
Congestion Notification (ECN) to IP,", Experimental RFC2481.
[RFC2507] Degermark, M., Nordgren, B., and Pink, S., "IP Header
Compression", RFC2507.
[RFC2525] Paxson, V., Allman, M., Dawson, S., Heavens, I., and B.
Volz, "Known TCP Implementation Problems", RFC2525.
[RFC2581] Allman, M., Paxson, V., and W. Stevens, "TCP Congestion
Control", RFC2581.
[RFC2686] Bormann, C., "The Multi-Class Extension to Multi-Link
PPP", RFC2686.
[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", RFC2760.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., Traina, P.,
"Generic Routing Encapsulation (GRE)", RFC2784.
[RFC3077] Duros, E., Dabbous, W., Izumiyama, H., Fujii, N., and Y.
Zhang, "A link Layer tunneling mechanism for unidirectional links",
RFC3077.
[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., Zheng, H., "RObust Header Compression (ROHC): Framework and four
profiles: RTP, UDP ESP and uncompressed", RFC3095.
[RFC3135] Border, J., Kojo, M., Griner, J., Montenegro, G., and Z.
Shelby, "Performance Enhancing Proxies Intended to Mitigate Link-
Related Degradations", RFC3135.
[RFC3150] S. Dawkins, G. Montenegro, M. Kojo, V. Magret, "End-to-end
Performance Implications of Slow Links", RFC3150.
[Sam99] Samaraweera, N.K.G, "Return Link Optimization for Internet
Service Provision Using DVB-S Networks", ACM 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. Aug. 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, 1991, pp.133-147.
10. 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 BCP 69
Department of Engineering RFC 3449
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 Title: TCP Performance Implications of Network Path
Department of Engineering Asymmetry
Fraser Noble Building Author(s): H. Balakrishnan, V. Padmanabhan, G. Fairhurst,
University of Aberdeen M. Sooriyabandara
Aberdeen AB24 3UE Status: Standards Track
UK Date: December 2002
Email: mahesh@erg.abdn.ac.uk Mailbox: hari@lcs.mit.edu, padmanab@microsoft.com,
Web: http://www.erg.abdn.ac.uk/users/mahesh gorry@erg.abdn.ac.uk, mahesh@erg.abdn.ac.uk
Full Copyright Statement Pages: 41
Characters: 108839
See Also: BCP 69
"Copyright (C) The Internet Society (date). All Rights Reserved. I-D Tag: draft-ietf-pilc-asym-08.txt
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The limited permissions granted above are perpetual and will not be URL: ftp://ftp.rfc-editor.org/in-notes/rfc3449.txt
revoked by the Internet Society or its successors or assigns.
11. IANA Considerations 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.
There are no IANA considerations associated with this draft. 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.
Appendix I : Examples of Subnetworks Exhibiting Network Path Asymmetry This document is a product of the Performance Implications of Link
Characteristics Working Group of the IETF.
This appendix provides a list of some subnetworks which are known to This document specifies an Internet Best Current Practices for the
experience network path asymmetry. The asymmetry in capacity of Internet Community, and requests discussion and suggestions for
these network paths can require mitigations to provide acceptable improvements. Distribution of this memo is unlimited.
overall performance. Examples include the following:
- IP service over some wide area and local area wireless networks. This announcement is sent to the IETF list and the RFC-DIST list.
In such networks, the predominant network path asymmetry arises Requests to be added to or deleted from the IETF distribution list
from the hub-and-spokes architecture of the network (e.g., a should be sent to IETF-REQUEST@IETF.ORG. Requests to be
single base station that communicates with multiple mobile added to or deleted from the RFC-DIST distribution list should
stations), this requires a Ready To Send / Clear To Send (RTS/CTS) be sent to RFC-DIST-REQUEST@RFC-EDITOR.ORG.
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 utilising Digital Video Details on obtaining RFCs via FTP or EMAIL may be obtained by sending
Broadcast (DVB) transmission [EN97] (e.g. 38-45 Mbps), and a an EMAIL message to rfc-info@RFC-EDITOR.ORG with the message body
slower upstream link using terrestrial network technology (e.g. help: ways_to_get_rfcs. For example:
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- To: rfc-info@RFC-EDITOR.ORG
transit or isolated hosts [Seg00] using a high capacity downstream Subject: getting rfcs
satellite link (e.g. 2-3 Mbps) with a narrowband upstream link
(e.g. 2.4-9.6kbps) 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 help: ways_to_get_rfcs
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 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 Requests for special distribution should be addressed to either the
downstream link transmission rate which is higher than that of the author of the RFC in question, or to RFC-Manager@RFC-EDITOR.ORG. Unless
upstream link. The available rates depend upon channel quality and specifically noted otherwise on the RFC itself, all RFCs are for
system configuration. For example, one widely deployed ADSL unlimited distribution.echo
technology [ITU02, ANS01] operates at rates that are multiples of Submissions for Requests for Comments should be sent to
32 kbps (up to 6.144 Mbps) in the downstream link, and up to 640 RFC-EDITOR@RFC-EDITOR.ORG. Please consult RFC 2223, Instructions to RFC
kbps for the upstream link. The network path asymmetry experienced Authors, for further information.
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.
 End of changes. 14 change blocks. 
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