Network Working Group R. Ludwig
Request for Comments: 3522 M. Meyer
Category: EXPerimental EriCsson Research
April 2003

The Eifel Detection Algorithm for TCP

Status of this Memo

This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

The Eifel detection algorithm allows a TCP sender to detect a
posteriori whether it has entered loss recovery unnecessarily. It
requires that the TCP Timestamps option defined in RFC1323 be
enabled for a connection. The Eifel detection algorithm makes use of
the fact that the TCP Timestamps option eliminates the retransmission
ambiguity in TCP. Based on the timestamp of the first acceptable ACK
that arrives during loss recovery, it decides whether loss recovery
was entered unnecessarily. The Eifel detection algorithm provides a
basis for future TCP enhancements. This includes response algorithms
to back out of loss recovery by restoring a TCP sender's congestion
control state.

Terminology

The keyWords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC2119].

We refer to the first-time transmission of an octet as the 'original
transmit'. A subsequent transmission of the same octet is referred
to as a 'retransmit'. In most cases, this terminology can likewise
be applied to data segments as opposed to octets. However, with
repacketization, a segment can contain both first-time transmissions
and retransmissions of octets. In that case, this terminology is
only consistent when applied to octets. For the Eifel detection
algorithm, this makes no difference as it also operates correctly
when repacketization occurs.

We use the term 'acceptable ACK' as defined in [RFC793]. That is an
ACK that acknowledges previously unacknowledged data. We use the
term 'duplicate ACK', and the variable 'dupacks' as defined in
[WS95]. The variable 'dupacks' is a counter of duplicate ACKs that
have already been received by a TCP sender before the fast retransmit
is sent. We use the variable 'DupThresh' to refer to the so-called
duplicate acknowledgement threshold, i.e., the number of duplicate
ACKs that need to arrive at a TCP sender to trigger a fast
retransmit. Currently, DupThresh is specified as a fixed value of
three [RFC2581]. Future TCPs might implement an adaptive DupThresh.

1. IntrodUCtion

The retransmission ambiguity problem [Zh86], [KP87] is a TCP sender's
inability to distinguish whether the first acceptable ACK that
arrives after a retransmit was sent in response to the original
transmit or the retransmit. This problem occurs after a timeout-
based retransmit and after a fast retransmit. The Eifel detection
algorithm uses the TCP Timestamps option defined in [RFC1323] to
eliminate the retransmission ambiguity. It thereby allows a TCP
sender to detect a posteriori whether it has entered loss recovery
unnecessarily.

This added capability of a TCP sender is useful in environments where
TCP's loss recovery and congestion control algorithms may often get
falsely triggered. This can be caused by packet reordering, packet
duplication, or a sudden delay increase in the data or the ACK path
that results in a spurious timeout. For example, such sudden delay
increases can often occur in wide-area wireless Access networks due
to handovers, resource preemption due to higher priority traffic
(e.g., voice), or because the mobile transmitter traverses through a
radio coverage hole (e.g., see [Gu01]). In such wireless networks,
the often unnecessary go-back-N retransmits that typically occur
after a spurious timeout create a serious problem. They decrease
end-to-end throughput, are useless load upon the network, and waste
transmission (battery) power. Note that across such networks the use
of timestamps is recommended anyway [RFC3481].

Based on the Eifel detection algorithm, a TCP sender may then choose
to implement dedicated response algorithms. One goal of such a
response algorithm would be to alleviate the consequences of a
falsely triggered loss recovery. This may include restoring the TCP
sender's congestion control state, and avoiding the mentioned
unnecessary go-back-N retransmits. Another goal would be to adapt
protocol parameters such as the duplicate acknowledgement threshold
[RFC2581], and the RTT estimators [RFC2988]. This is to reduce the
risk of falsely triggering TCP's loss recovery again as the
connection progresses. However, such response algorithms are outside

the scope of this document. Note: The original proposal, the "Eifel
algorithm" [LK00], comprises both a detection and a response
algorithm. This document only defines the detection part. The
response part is defined in [LG03].

A key feature of the Eifel detection algorithm is that it already
detects, upon the first acceptable ACK that arrives during loss
recovery, whether a fast retransmit or a timeout was spurious. This
is crucial to be able to avoid the mentioned go-back-N retransmits.
Another feature is that the Eifel detection algorithm is fairly
robust against the loss of ACKs.

Also the DSACK option [RFC2883] can be used to detect a posteriori
whether a TCP sender has entered loss recovery unnecessarily [BA02].
However, the first ACK carrying a DSACK option usually arrives at a
TCP sender only after loss recovery has already terminated. Thus,
the DSACK option cannot be used to eliminate the retransmission
ambiguity. Consequently, it cannot be used to avoid the mentioned
unnecessary go-back-N retransmits. Moreover, a DSACK-based detection
algorithm is less robust against ACK losses. A recent proposal based
on neither the TCP timestamps nor the DSACK option does not have the
limitation of DSACK-based schemes, but only addresses the case of
spurious timeouts [SK03].

2. Events that Falsely Trigger TCP Loss Recovery

The following events may falsely trigger a TCP sender's loss recovery
and congestion control algorithms. This causes a so-called spurious
retransmit, and an unnecessary reduction of the TCP sender's
congestion window and slow start threshold [RFC2581].

- Spurious timeout

- Packet reordering

- Packet duplication

A spurious timeout is a timeout that would not have occurred had the
sender "waited longer". This may be caused by increased delay that
suddenly occurs in the data and/or the ACK path. That in turn might
cause an acceptable ACK to arrive too late, i.e., only after a TCP
sender's retransmission timer has expired. For the purpose of
specifying the algorithm in Section 3, we define this case as SPUR_TO
(equal 1).

Note: There is another case where a timeout would not have
occurred had the sender "waited longer": the retransmission timer
expires, and afterwards the TCP sender receives the duplicate ACK

that would have triggered a fast retransmit of the oldest
outstanding segment. We call this a 'fast timeout', since in
competition with the fast retransmit algorithm the timeout was
faster. However, a fast timeout is not spurious since apparently
a segment was in fact lost, i.e., loss recovery was initiated
rightfully. In this document, we do not consider fast timeouts.

Packet reordering in the network may occur because IP [RFC791] does
not guarantee in-order delivery of packets. Additionally, a TCP
receiver generates a duplicate ACK for each segment that arrives
out-of-order. This results in a spurious fast retransmit if three or
more data segments arrive out-of-order at a TCP receiver, and at
least three of the resulting duplicate ACKs arrive at the TCP sender.
This assumes that the duplicate acknowledgement threshold is set to
three as defined in [RFC2581].

Packet duplication may occur because a receiving IP does not (cannot)
remove packets that have been duplicated in the network. A TCP
receiver in turn also generates a duplicate ACK for each duplicate
segment. As with packet reordering, this results in a spurious fast
retransmit if duplication of data segments or ACKs results in three
or more duplicate ACKs to arrive at a TCP sender. Again, this
assumes that the duplicate acknowledgement threshold is set to three.

The negative impact on TCP performance caused by packet reordering
and packet duplication is commonly the same: a single spurious
retransmit (the fast retransmit), and the unnecessary halving of a
TCP sender's congestion window as a result of the subsequent fast
recovery phase [RFC2581].

The negative impact on TCP performance caused by a spurious timeout
is more severe. First, the timeout event itself causes a single
spurious retransmit, and unnecessarily forces a TCP sender into slow
start [RFC2581]. Then, as the connection progresses, a chain
reaction gets triggered that further decreases TCP's performance.
Since the timeout was spurious, at least some ACKs for original
transmits typically arrive at the TCP sender before the ACK for the
retransmit arrives. (This is unless severe packet reordering
coincided with the spurious timeout in such a way that the ACK for
the retransmit is the first acceptable ACK to arrive at the TCP
sender.) Those ACKs for original transmits then trigger an implicit
go-back-N loss recovery at the TCP sender [LK00]. Assuming that none
of the outstanding segments and none of the corresponding ACKs were
lost, all outstanding segments get retransmitted unnecessarily. In
fact, during this phase, a TCP sender violates the packet
conservation principle [Jac88]. This is because the unnecessary go-
back-N retransmits are sent during slow start. Thus, for each packet
that leaves the network and that belongs to the first half of the

original flight, two useless retransmits are sent into the network.
In addition, some TCPs suffer from a spurious fast retransmit. This
is because the unnecessary go-back-N retransmits arrive as duplicates
at the TCP receiver, which in turn triggers a series of duplicate
ACKs. Note that this last spurious fast retransmit could be avoided
with the careful variant of 'bugfix' [RFC2582].

More detailed explanations, including TCP trace plots that visualize
the effects of spurious timeouts and packet reordering, can be found
in the original proposal [LK00].

3. The Eifel Detection Algorithm

3.1 The Idea

The goal of the Eifel detection algorithm is to allow a TCP sender to
detect a posteriori whether it has entered loss recovery
unnecessarily. Furthermore, the TCP sender should be able to make
this decision upon the first acceptable ACK that arrives after the
timeout-based retransmit or the fast retransmit has been sent. This
in turn requires extra information in ACKs by which the TCP sender
can unambiguously distinguish whether that first acceptable ACK was
sent in response to the original transmit or the retransmit. Such
extra information is provided by the TCP Timestamps option [RFC1323].
Generally speaking, timestamps are monotonously increasing "serial
numbers" added into every segment that are then echoed within the
corresponding ACKs. This is exploited by the Eifel detection
algorithm in the following way.

Given that timestamps are enabled for a connection, a TCP sender
always stores the timestamp of the retransmit sent in the beginning
of loss recovery, i.e., the timestamp of the timeout-based retransmit
or the fast retransmit. If the timestamp of the first acceptable
ACK, that arrives after the retransmit was sent, is smaller then the
stored timestamp of that retransmit, then that ACK must have been
sent in response to an original transmit. Hence, the TCP sender must
have entered loss recovery unnecessarily.

The fact that the Eifel detection algorithm decides upon the first
acceptable ACK is crucial to allow future response algorithms to
avoid the unnecessary go-back-N retransmits that typically occur
after a spurious timeout. Also, if loss recovery was entered
unnecessarily, a window worth of ACKs are outstanding that all carry
a timestamp that is smaller than the stored timestamp of the
retransmit. The arrival of any one of those ACKs is sufficient for
the Eifel detection algorithm to work. Hence, the solution is fairly

robust against ACK losses. Even the ACK sent in response to the
retransmit, i.e., the one that carries the stored timestamp, may get
lost without compromising the algorithm.

3.2 The Algorithm

Given that the TCP Timestamps option [RFC1323] is enabled for a
connection, a TCP sender MAY use the Eifel detection algorithm as
defined in this subsection.

If the Eifel detection algorithm is used, the following steps MUST be
taken by a TCP sender, but only upon initiation of loss recovery,
i.e., when either the timeout-based retransmit or the fast retransmit
is sent. The Eifel detection algorithm MUST NOT be reinitiated after
loss recovery has already started. In particular, it must not be
reinitiated upon subsequent timeouts for the same segment, and not
upon retransmitting segments other than the oldest outstanding
segment, e.g., during selective loss recovery.

(1) Set a "SpuriousRecovery" variable to FALSE (equal 0).

(2) Set a "RetransmitTS" variable to the value of the
Timestamp Value field of the Timestamps option included in
the retransmit sent when loss recovery is initiated. A
TCP sender must ensure that RetransmitTS does not get
overwritten as loss recovery progresses, e.g., in case of
a second timeout and subsequent second retransmit of the
same octet.

(3) Wait for the arrival of an acceptable ACK. When an
acceptable ACK has arrived, proceed to step (4).

(4) If the value of the Timestamp Echo Reply field of the
acceptable ACK's Timestamps option is smaller than the
value of RetransmitTS, then proceed to step (5),

else proceed to step (DONE).

(5) If the acceptable ACK carries a DSACK option [RFC2883],
then proceed to step (DONE),

else if during the lifetime of the TCP connection the TCP
sender has previously received an ACK with a DSACK option,
or the acceptable ACK does not acknowledge all outstanding
data, then proceed to step (6),

else proceed to step (DONE).

(6) If the loss recovery has been initiated with a timeout-
based retransmit, then set
SpuriousRecovery <- SPUR_TO (equal 1),

else set
SpuriousRecovery <- dupacks+1

(RESP) Do nothing (Placeholder for a response algorithm).

(DONE) No further processing.

The comparison "smaller than" in step (4) is conservative. In
theory, if the timestamp clock is slow or the network is fast,
RetransmitTS could at most be equal to the timestamp echoed by an ACK
sent in response to an original transmit. In that case, it is
assumed that the loss recovery was not falsely triggered.

Note that the condition "if during the lifetime of the TCP connection
the TCP sender has previously received an ACK with a DSACK option" in
step (5) would be true in case the TCP receiver would signal in the
SYN that it is DSACK-enabled. But unfortunately, this is not
required by [RFC2883].

3.3 A Corner Case: "Timeout due to loss of all ACKs" (step 5)

Even though the oldest outstanding segment arrived at a TCP receiver,
the TCP sender is forced into a timeout if all ACKs are lost.
Although the resulting retransmit is unnecessary, such a timeout is
unavoidable. It should therefore not be considered spurious.
Moreover, the subsequent reduction of the congestion window is an
appropriate response to the potentially heavy congestion in the ACK
path. The original proposal [LK00] does not handle this case well.
It effectively disables this implicit form of congestion control for
the ACK path, which otherwise does not exist in TCP. This problem is
fixed by step (5) of the Eifel detection algorithm as explained in
the remainder of this section.

If all ACKs are lost while the oldest outstanding segment arrived at
the TCP receiver, the retransmit arrives as a duplicate. In response
to duplicates, RFC1323 mandates that the timestamp of the last
segment that arrived in-sequence should be echoed. That timestamp is
carried by the first acceptable ACK that arrives at the TCP sender
after loss recovery was entered, and is commonly smaller than the
timestamp carried by the retransmit. Consequently, the Eifel
detection algorithm misinterprets such a timeout as being spurious,
unless the TCP receiver is DSACK-enabled [RFC2883]. In that case,
the acceptable ACK carries a DSACK option, and the Eifel algorithm is
terminated through the first part of step (5).

Note: Not all TCP implementations strictly follow RFC1323. In
response to a duplicate data segment, some TCP receivers echo the
timestamp of the duplicate. With such TCP receivers, the corner
case discussed in this section does not apply. The timestamp
carried by the retransmit would be echoed in the first acceptable
ACK, and the Eifel detection algorithm would be terminated through
step (4). Thus, even though all ACKs were lost and independent of
whether the DSACK option was enabled for a connection, the Eifel
detection algorithm would have no effect.

With TCP receivers that are not DSACK-enabled, disabling the
mentioned implicit congestion control for the ACK path is not a
problem as long as data segments are lost, in addition to the entire
flight of ACKs. The Eifel detection algorithm misinterprets such a
timeout as being spurious, and the Eifel response algorithm would
reverse the congestion control state. Still, the TCP sender would
respond to congestion (in the data path) as soon as it finds out
about the first loss in the outstanding flight. I.e., the TCP sender
would still halve its congestion window for that flight of packets.
If no data segment is lost while the entire flight of ACKs is lost,
the first acceptable ACK that arrives at the TCP sender after loss
recovery was entered acknowledges all outstanding data. In that
case, the Eifel algorithm is terminated through the second part of
step (5).

Note that there is little concern about violating the packet
conservation principle when entering slow start after an unavoidable
timeout caused by the loss of an entire flight of ACKs, i.e., when
the Eifel detection algorithm was terminated through step (5). This
is because in that case, the acceptable ACK corresponds to the
retransmit, which is a strong indication that the pipe has drained
entirely, i.e., that no more original transmits are in the network.
This is different with spurious timeouts as discussed in Section 2.

3.4 Protecting Against Misbehaving TCP Receivers (the Safe Variant)

A TCP receiver can easily make a genuine retransmit appear to the TCP
sender as a spurious retransmit by forging echoed timestamps. This
may pose a security concern.

Fortunately, there is a way to modify the Eifel detection algorithm
in a way that makes it robust against lying TCP receivers. The idea
is to use timestamps as a segment's "secret" that a TCP receiver only
gets to know if it receives the segment. Conversely, a TCP receiver
will not know the timestamp of a segment that was lost. Hence, to
"prove" that it received the original transmit of a segment that a
TCP sender retransmitted, the TCP receiver would need to return the
timestamp of that original transmit. The Eifel detection algorithm

could then be modified to only decide that loss recovery has been
unnecessarily entered if the first acceptable ACK echoes the
timestamp of the original transmit.

Hence, implementers may choose to implement the algorithm with the
following modifications.

Step (2) is replaced with step (2'):

(2') Set a "RetransmitTS" variable to the value of the
Timestamp Value field of the Timestamps option that was
included in the original transmit corresponding to the
retransmit. Note: This step requires that the TCP sender
stores the timestamps of all outstanding original
transmits.

Step (4) is replaced with step (4'):

(4') If the value of the Timestamp Echo Reply field of the
acceptable ACK's Timestamps option is equal to the value
of the variable RetransmitTS, then proceed to step (5),

else proceed to step (DONE).

These modifications come at a cost: the modified algorithm is fairly
sensitive against ACK losses since it relies on the arrival of the
acceptable ACK that corresponds to the original transmit.

Note: The first acceptable ACK that arrives after loss recovery
has been unnecessarily entered should echo the timestamp of the
original transmit. This assumes that the ACK corresponding to the
original transmit was not lost, that that ACK was not reordered in
the network, and that the TCP receiver does not forge timestamps
but complies with RFC1323. In case of a spurious fast
retransmit, this is implied by the rules for generating ACKs for
data segments that fill in all or part of a gap in the sequence
space (see section 4.2 of [RFC2581]) and by the rules for echoing
timestamps in that case (see rule (C) in section 3.4 of
[RFC1323]). In case of a spurious timeout, it is likely that the
delay that has caused the spurious timeout has also caused the TCP
receiver's delayed ACK timer [RFC1122] to expire before the
original transmit arrives. Also, in this case the rules for
generating ACKs and the rules for echoing timestamps (see rule (A)
in section 3.4 of [RFC1323]) ensure that the original transmit's
timestamp is echoed.

A remaining problem is that a TCP receiver might guess a lost
segment's timestamp from observing the timestamps of recently
received segments. For example, if segment N was lost while segment
N-1 and N+1 have arrived, a TCP receiver could guess the timestamp
that lies in the middle of the timestamps of segments N-1 and N+1,
and echo it in the ACK sent in response to the retransmit of segment
N. Especially if the TCP sender implements timestamps with a coarse
granularity, a misbehaving TCP receiver is likely to be successful
with such an approach. In fact, with the 500 ms granularity
suggested in [WS95], it even becomes quite likely that the timestamps
of segments N-1, N, N+1 are identical.

One way to reduce this risk is to implement fine grained timestamps.
Note that the granularity of the timestamps is independent of the
granularity of the retransmission timer. For example, some TCP
implementations run a timestamp clock that ticks every millisecond.
This should make it more difficult for a TCP receiver to guess the
timestamp of a lost segment. Alternatively, it might be possible to
combine the timestamps with a nonce, as is done for the Explicit
Congestion Notification (ECN) [RFC3168]. One would need to take
care, though, that the timestamps of consecutive segments remain
monotonously increasing and do not interfere with the RTT timing
defined in [RFC1323].

4. IPR Considerations

The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights at http://www.ietf.org/ipr.

The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
oBTain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.

5. Security Considerations

There do not seem to be any security considerations associated with
the Eifel detection algorithm. This is because the Eifel detection
algorithm does not alter the existing protocol state at a TCP sender.
Note that the Eifel detection algorithm only requires changes to the
implementation of a TCP sender.

Moreover, a variant of the Eifel detection algorithm has been
proposed in Section 3.4 that makes it robust against lying TCP
receivers. This may become relevant when the Eifel detection
algorithm is combined with a response algorithm such as the Eifel
response algorithm [LG03].

Acknowledgments

Many thanks to Keith Sklower, Randy Katz, Stephan Baucke, Sally
Floyd, Vern Paxson, Mark Allman, Ethan Blanton, Andrei Gurtov, Pasi
Sarolahti, and Alexey Kuznetsov for useful discussions that
contributed to this work.

Normative References

[RFC2581] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion
Control", RFC2581, April 1999.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.

[RFC2883] Floyd, S., Mahdavi, J., Mathis, M., Podolsky, M. and A.
Romanow, "An Extension to the Selective Acknowledgement
(SACK) Option for TCP", RFC2883, July 2000.

[RFC1323] Jacobson, V., Braden, R. and D. Borman, "TCP Extensions for
High Performance", RFC1323, May 1992.

[RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP
Selective Acknowledgement Options", RFC2018, October 1996.

[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, September 1981.

Informative References

[BA02] Blanton, E. and M. Allman, "Using TCP DSACKs and SCTP
Duplicate TSNs to Detect Spurious Retransmissions", Work in
Progress.

[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC1122, October 1989.

[RFC2582] Floyd, S. and T. Henderson, "The NewReno Modification to
TCP's Fast Recovery Algorithm", RFC2582, April 1999.

[Gu01] Gurtov, A., "Effect of Delays on TCP Performance", In
Proceedings of IFIP Personal Wireless Communications,
August 2001.

[RFC3481] Inamura, H., Montenegro, G., Ludwig, R., Gurtov, A. and F.
Khafizov, "TCP over Second (2.5G) and Third (3G) Generation
Wireless Networks", RFC3481, February 2003.

[Jac88] Jacobson, V., "Congestion Avoidance and Control", In
Proceedings of ACM SIGCOMM 88.

[KP87] Karn, P. and C. Partridge, "Improving Round-Trip Time
Estimates in Reliable Transport Protocols", In Proceedings
of ACM SIGCOMM 87.

[LK00] Ludwig, R. and R. H. Katz, "The Eifel Algorithm: Making TCP
Robust Against Spurious Retransmissions", ACM Computer
Communication Review, Vol. 30, No. 1, January 2000.

[LG03] Ludwig, R. and A. Gurtov, "The Eifel Response Algorithm for
TCP", Work in Progress.

[RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
Timer", RFC2988, November 2000.

[RFC791] Postel, J., "Internet Protocol", STD 5, RFC791, September
1981.

[RFC3168] Ramakrishnan, K., Floyd, S. and D. Black, "The Addition of
Explicit Congestion Notification (ECN) to IP", RFC3168,
September 2001.

[SK03] Sarolahti, P. and M. Kojo, "F-RTO: A TCP RTO Recovery
Algorithm for Avoiding Unnecessary Retransmissions", Work
in Progress.

[WS95] Wright, G. R. and W. R. Stevens, "TCP/IP Illustrated,
Volume 2 (The Implementation)", Addison Wesley, January
1995.

[Zh86] Zhang, L., "Why TCP Timers Don't Work Well", In Proceedings
of ACM SIGCOMM 86.

Authors' Addresses

Reiner Ludwig
Ericsson Research
Ericsson Allee 1
52134 Herzogenrath, Germany

EMail: Reiner.Ludwig@eed.ericsson.se

Michael Meyer
Ericsson Research
Ericsson Allee 1
52134 Herzogenrath, Germany

EMail: Michael.Meyer@eed.ericsson.se

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