3.8.1 Mitigation Description
Persistent TCP state information can be used to overcome limitations
in the configuration of the initial state, and to automatically tune
TCP to environments using satellite links and to coordinate multiple
TCP connections sharing a satellite link.
TCP includes a variety of parameters, many of which are set to
initial values which can severely affect the performance of TCP
connections traversing satellite links, even though most TCP
parameters are adjusted later after the connection is established.
These parameters include initial size of cwnd and initial MSS size.
Various suggestions have been made to change these initial
conditions, to more effectively support satellite links. However, it
is difficult to select any single set of parameters which is
effective for all environments.
An alternative to attempting to select these parameters a-priori is
sharing state across TCP connections and using this state when
initializing a new connection. For example, if all connections to a
subnet result in extended congestion windows of 1 megabyte, it is
probably more efficient to start new connections with this value,
than to rediscover it by requiring the cwnd to increase using slow
start over a period of dozens of round-trip times.
Sharing state among connections brings up a number of questions such
as what information to share, with whom to share, how to share it,
and how to age shared information. First, what information is to be
shared must be determined. Some information may be appropriate to
share among TCP connections, while some information sharing may be
inappropriate or not useful. Next, we need to determine with whom to
share information. Sharing may be appropriate for TCP connections
sharing a common path to a given host. Information may be shared
among connections within a host, or even among connections between
different hosts, such as hosts on the same LAN. However, sharing
information between connections not traversing the same network may
not be appropriate. Given the state to share and the parties that
share it, a mechanism for the sharing is required. Simple state,
like MSS and RTT, is easy to share, but congestion window information
can be shared a variety of ways. The sharing mechanism determines
priorities among the sharing connections, and a variety of fairness
criteria need to be considered. Also, the mechanisms by which
information is aged require further study. See RFC 2140 for a
discussion of the security issues in both sharing state within a
single host and sharing state among hosts on a subnet. Finally, the
security concerns associated with sharing a piece of information need
to be carefully considered before introducing such a mechanism. Many
of these open research questions must be answered before state
sharing can be widely deployed.
The opportunity for such sharing, both among a sequence of
connections, as well as among concurrent connections, is described in
more detail in [Tou97]. The state management itself is largely an
implementation issue, however what information should be shared and
the specific ways in which the information should be shared is an
Sharing parts of the TCB state was originally documented in T/TCP
[Bra92], and is used there to aggregate RTT values across connection
instances, to provide meaningful average RTTs, even though most
connections are expected to persist for only one RTT. T/TCP also
shares a connection identifier, a sequence number separate from the
window number and address/port pairs by which TCP connections are
typically distinguished. As a result of this shared state, T/TCP
allows a receiver to pass data in the SYN segment to the receiving
application, prior to the completion of the three-way handshake,
without compromising the integrity of the connection. In effect, this
shared state caches a partial handshake from the previous connection,
which is a variant of the more general issue of TCB sharing.
Sharing state among connections (including transfers using non-TCP
protocols) is further investigated in [BRS99].
3.8.3 Implementation Issues
Sharing TCP state across connections requires changes to the sender's
TCP stack, and possibly the receiver's TCP stack (as in the case of
T/TCP, for example). Sharing TCP state may make a particular TCP
connection more aggressive. However, the aggregate traffic should be
more conservative than a group of independent TCP connections.
Therefore, sharing TCP state should be safe for use in shared
networks. Note that state sharing does not present any new security
problems within multiuser hosts. In such a situation, users can
steal network resources from one another with or without state
3.8.4 Topology Considerations
It is expected that sharing state across TCP connections may be
useful in all network environments presented in section 2.
3.8.5 Possible Interaction and Relationships with Other Research
The state sharing outlined above is very similar to the Congestion
Manager proposal [BRS99] that attempts to share congestion control
information among both TCP and UDP flows between a pair of hosts.
3.9 ACK Congestion Control
In highly asymmetric networks, a low-speed return link can restrict
the performance of the data flow on a high-speed forward link by
limiting the flow of acknowledgments returned to the data sender.
For example, if the data sender uses 1500 byte segments, and the
receiver generates 40 byte acknowledgments (IPv4, TCP without
options), the reverse link will congest with ACKs for asymmetries of
more than 75:1 if delayed ACKs are used, and 37:1 if every segment is
acknowledged. For a 1.5 Mb/second data link, ACK congestion will
occur for reverse link speeds below 20 kilobits/sec. These levels of
asymmetry will readily occur if the reverse link is shared among
multiple satellite receivers, as is common in many VSAT satellite
networks. If a terrestrial modem link is used as a reverse link, ACK
congestion is also likely, especially as the speed of the forward
link is increased. Current congestion control mechanisms are aimed
at controlling the flow of data segments, but do not affect the flow
In [KVR98] the authors point out that the flow of acknowledgments can
be restricted on the low-speed link not only by the bandwidth of the
link, but also by the queue length of the router. The router may
limit its queue length by counting packets, not bytes, and therefore
begin discarding ACKs even if there is enough bandwidth to forward
3.9.1 Mitigation Description
ACK Congestion Control extends the concept of flow control for data
segments to acknowledgment segments. In the method described in
[BPK97], any intermediate router can mark an acknowledgment with an
Explicit Congestion Notification (ECN) bit once the queue occupancy
in the router exceeds a given threshold. The data sender (which
receives the acknowledgment) must "echo" the ECN bit back to the data
receiver (see section 3.3.3 for a more detailed discussion of ECN).
The proposed algorithm for marking ACK segments with an ECN bit is
Random Early Detection (RED) [FJ93]. In response to the receipt of
ECN marked data segments, the receiver will dynamically reduce the
rate of acknowledgments using a multiplicative backoff. Once
segments without ECN are received, the data receiver speeds up
acknowledgments using a linear increase, up to a rate of either 1 (no
delayed ACKs) or 2 (normal delayed ACKs) data segments per ACK. The
authors suggest that an ACK be generated at least once per window,
and ideally a few times per window.
As in the RED congestion control mechanism for data flow, the
bottleneck gateway can randomly discard acknowledgments, rather than
marking them with an ECN bit, once the queue fills beyond a given
[BPK97] analyze the effect of ACK Congestion Control (ACC) on the
performance of an asymmetric network. They note that the use of ACC,
and indeed the use of any scheme which reduces the frequency of
acknowledgments, has potential unwanted side effects. Since each ACK
will acknowledge more than the usual one or two data segments, the
likelihood of segment bursts from the data sender is increased. In
addition, congestion window growth may be impeded if the receiver
grows the window by counting received ACKs, as mandated by
[Ste97,APS99]. The authors therefore combine ACC with a series of
modifications to the data sender, referred to as TCP Sender
Adaptation (SA). SA combines a limit on the number of segments sent
in a burst, regardless of window size. In addition, byte counting
(as opposed to ACK counting) is employed for window growth. Note
that byte counting has been studied elsewhere and can introduce
side-effects, as well [All98].
The results presented in [BPK97] indicate that using ACC and SA will
reduce the bursts produced by ACK losses in unmodified (Reno) TCP.
In cases where these bursts would lead to data loss at an
intermediate router, the ACC and SA modification significantly
improve the throughput for a single data transfer. The results
further suggest that the use of ACC and SA significantly improve
fairness between two simultaneous transfers.
ACC is further reported to prevent the increase in round trip time
(RTT) that occurs when an unmodified TCP fills the reverse router
queue with acknowledgments.
In networks where the forward direction is expected to suffer losses
in one of the gateways, due to queue limitations, the authors report
at best a very slight improvement in performance for ACC and SA,
compared to unmodified Reno TCP.
3.9.3 Implementation Issues
Both ACC and SA require modification of the sending and receiving
hosts, as well as the bottleneck gateway. The current research
suggests that implementing ACC without the SA modifications results
in a data sender which generates potentially disruptive segment
bursts. It should be noted that ACC does require host modifications
if it is implemented in the way proposed in [BPK97]. The authors
note that ACC can be implemented by discarding ACKs (which requires
only a gateway modification, but no changes in the hosts), as opposed
to marking them with ECN. Such an implementation may, however,
produce bursty data senders if it is not combined with a burst
mitigation technique. ACC requires changes to the standard ACKing
behavior of a receiving TCP and therefore is not recommended for use
in shared networks.
3.9.4 Topology Considerations
Neither ACC nor SA require the storage of state in the gateway.
These schemes should therefore be applicable for all topologies,
provided that the hosts using the satellite or hybrid network can be
modified. However, these changes are expected to be especially
beneficial to networks containing asymmetric satellite links.
3.9.5 Possible Interaction and Relationships with Other Research
Note that ECN is a pre-condition for using ACK congestion control.
Additionally, the ACK Filtering algorithm discussed in the next
section attempts to solve the same problem as ACC. Choosing between
the two algorithms (or another mechanism) is currently an open
3.10 ACK Filtering
ACK Filtering (AF) is designed to address the same ACK congestion
effects described in 3.9. Contrary to ACC, however, AF is designed
to operate without host modifications.
3.10.1 Mitigation Description
AF takes advantage of the cumulative acknowledgment structure of TCP.
The bottleneck router in the reverse direction (the low speed link)
must be modified to implement AF. Upon receipt of a segment which
represents a TCP acknowledgment, the router scans the queue for
redundant ACKs for the same connection, i.e. ACKs which acknowledge
portions of the window which are included in the most recent ACK.
All of these "earlier" ACKs are removed from the queue and discarded.
The router does not store state information, but does need to
implement the additional processing required to find and remove
segments from the queue upon receipt of an ACK.
[BPK97] analyzes the effects of AF. As is the case in ACC, the use
of ACK filtering alone would produce significant sender bursts, since
the ACKs will be acknowledging more previously-unacknowledged data.
The SA modifications described in 3.9.2 could be used to prevent
those bursts, at the cost of requiring host modifications. To
prevent the need for modifications in the TCP stack, AF is more
likely to be paired with the ACK Reconstruction (AR) technique, which
can be implemented at the router where segments exit the slow reverse
AR inspects ACKs exiting the link, and if it detects large "gaps" in
the ACK sequence, it generates additional ACKs to reconstruct an
acknowledgment flow which more closely resembles what the data sender
would have seen had ACK Filtering not been introduced. AR requires
two parameters; one parameter is the desired ACK frequency, while the
second controls the spacing, in time, between the release of
consecutive reconstructed ACKs.
In [BPK97], the authors show the combination of AF and AR to increase
throughput, in the networks studied, over both unmodified TCP and the
ACC/SA modifications. Their results also strongly suggest that the
use of AF alone, in networks where congestion losses are expected,
decreases performance (even below the level of unmodified TCP Reno)
due to sender bursting.
AF delays acknowledgments from arriving at the receiver by dropping
earlier ACKs in favor of later ACKs. This process can cause a slight
hiccup in the transmission of new data by the TCP sender.
3.10.3 Implementation Issues
Both ACK Filtering and ACK Reconstruction require only router
modification. However, the implementation of AR requires some
storage of state information in the exit router. While AF does not
require storage of state information, its use without AR (or SA)
could produce undesired side effects. Furthermore, more research is
required regarding appropriate ranges for the parameters needed in
3.10.4 Topology Considerations
AF and AR appear applicable to all topologies, assuming that the
storage of state information in AR does not prove to be prohibitive
for routers which handle large numbers of flows. The fact that TCP
stack modifications are not required for AF/AR makes this approach
attractive for hybrid networks and networks with diverse types of
hosts. These modifications, however, are expected to be most
beneficial in asymmetric network paths.
On the other hand, the implementation of AF/AR requires the routers
to examine the TCP header, which prohibits their use in secure
networks where IPSEC is deployed. In such networks, AF/AR can be
effective only inside the security perimeter of a private, or virtual
private network, or in private networks where the satellite link is
protected only by link-layer encryption (as opposed to IPSEC). ACK
Filtering is safe to use in shared networks (from a congestion
control point-of-view), as the number of ACKs can only be reduced,
which makes TCP less aggressive. However, note that while TCP is
less aggressive, the delays that AF induces (outlined above) can lead
to larger bursts than would otherwise occur.
3.10.5 Possible Interaction and Relationships with Other Research
ACK Filtering attempts to solve the same problem as ACK Congestion
Control (as outlined in section 3.9). Which of the two algorithms is
more appropriate is currently an open research question.
This document outlines TCP items that may be able to mitigate the
performance problems associated with using TCP in networks containing
satellite links. These mitigations are not IETF standards track
mechanisms and require more study before being recommended by the
IETF. The research community is encouraged to examine the above
mitigations in an effort to determine which are safe for use in
shared networks such as the Internet.
5 Security Considerations
Several of the above sections noted specific security concerns which
a given mitigation aggravates.
Additionally, any form of wireless communication link is more
susceptible to eavesdropping security attacks than standard wire-
based links due to the relative ease with which an attacker can watch
the network and the difficultly in finding attackers monitoring the
Our thanks to Aaron Falk and Sally Floyd, who provided very helpful
comments on drafts of this document.
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8 Authors' Addresses
NASA Glenn Research Center/BBN Technologies
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Cleveland, OH 44135
Richardson, TX 75083-3805
NASA Glenn Research Center
21000 Brookpark Rd. MS 3-6
Cleveland, OH 44135
NASA Glenn Research Center
21000 Brookpark Rd. MS 54-2
Cleveland, OH 44135
NASA Glenn Research Center
21000 Brookpark Rd. MS 54-2
Cleveland, OH 44135
University of California at Berkeley
Phone: +1 (510) 642-8919
University of Southern California/Information Sciences Institute
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University of Southern California/Information Sciences Institute
4676 Admiralty Way
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Phone: +1 310-448-9151
Fax: +1 310-823-6714
J. Warren McClure School of Communication Systems Management
9 S. College Street
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School of Electrical Engineering and Computer Science
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Athens, OH 45701
Phone: (740) 593-1234
The MITRE Corporation
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Pittsburgh, PA 15213
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