Network Working Group V. Paxson, Editor
Internet Draft M. Allman
S. Dawson
J. Griner
I. Heavens
K. Lahey
J. Semke
B. Volz
Expiration Date: Feburary 1999 August 1998
Known TCP Implementation Problems
<draft-ietf-tcpimpl-prob-04.txt>
1. Status of this Memo
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of six
months, and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet Drafts as reference
material or to cite them other than as ``work in progress''.
To view the entire list of current Internet-Drafts, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern
Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific
Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
2. Introduction
This memo catalogs a number of known TCP implementation problems.
The goal in doing so is to improve conditions in the existing
Internet by enhancing the quality of current TCP/IP implementations.
It is hoped that both performance and correctness issues can be
resolved by making implementors aware of the problems and their
solutions. In the long term, it is hoped that this will provide a
reduction in unnecessary traffic on the network, the rate of
connection failures due to protocol errors, and load on network
Paxson, Editor [Page 1]
ID Known TCP Implementation Problems August 1998
servers due to time spent processing both unsuccessful connections
and retransmitted data. This will help to ensure the stability of
the global Internet.
Each problem is defined as follows:
Name of Problem
The name associated with the problem. In this memo, the name is
given as a subsection heading.
Classification
One or more problem categories for which the problem is
classified. Categories used so far: "congestion control",
"performance", "reliability", "resource management". Others
anticipated: "security", "interoperability", "configuration".
Description
A definition of the problem, succinct but including necessary
background material.
Significance
A brief summary of the sorts of environments for which the
problem is significant.
Implications
Why the problem is viewed as a problem.
Relevant RFCs
Brief discussion of the RFCs with respect to which the problem
is viewed as an implementation error. These RFCs often qualify
behavior using terms such as MUST, SHOULD, MAY, and others
written capitalized. See RFC 2119 for the exact interpretation
of these terms.
Trace file demonstrating the problem
One or more ASCII trace files demonstrating the problem, if
applicable. These may in the future be replaced with URLs to
on-line traces.
Trace file demonstrating correct behavior
One or more examples of how correct behavior appears in a trace,
if applicable. These may in the future be replaced with URLs to
on-line traces.
References
References that further discuss the problem.
Paxson, Editor [Page 2]
ID Known TCP Implementation Problems August 1998
How to detect
How to test an implementation to see if it exhibits the problem.
This discussion may include difficulties and subtleties
associated with causing the problem to manifest itself, and with
interpreting traces to detect the presence of the problem (if
applicable). In the future, this may include URLs for
diagnostic tools.
How to fix
For known causes of the problem, how to correct the
implementation.
Implementation specifics
If it is viewed as beneficial to document particular
implementations exhibiting the problem, and if the corresponding
implementors approve, then this section gives the specifics of
those implementations, along with a contact address for the
implementors.
3. Known implementation problems
3.1.
Name of Problem
No initial slow start
Classification
Congestion control
Description
When a TCP begins transmitting data, it is required by RFC 1122,
4.2.2.15, to engage in a "slow start" by initializing its
congestion window, cwnd, to one packet (one segment of the maximum
size). (Note that an experimental change to TCP, documented in
[Allman98], allows an initial value somewhat larger than one
packet.) It subsequently increases cwnd by one packet for each ACK
it receives for new data. The minimum of cwnd and the receiver's
advertised window bounds the highest sequence number the TCP can
transmit. A TCP that fails to initialize and increment cwnd in
this fashion exhibits "No initial slow start".
Significance
In congested environments, detrimental to the performance of other
connections, and possibly to the connection itself.
Paxson, Editor [Page 3]
ID Known TCP Implementation Problems August 1998
Implications
A TCP failing to slow start when beginning a connection results in
traffic bursts that can stress the network, leading to excessive
queueing delays and packet loss.
Implementations exhibiting this problem might do so because they
suffer from the general problem of not including the required
congestion window. These implementations will also suffer from "No
slow start after retransmission timeout".
There are different shades of "No initial slow start". From the
perspective of stressing the network, the worst is a connection
that simply always sends based on the receiver's advertised window,
with no notion of a separate congestion window. Another form is
described in "Uninitialized CWND" below.
Relevant RFCs
RFC 1122 requires use of slow start. RFC 2001 gives the specifics
of slow start.
Trace file demonstrating it
Made using tcpdump/BPF recording at the connection responder. No
losses reported.
10:40:42.244503 B > A: S 1168512000:1168512000(0) win 32768
<mss 1460,nop,wscale 0> (DF) [tos 0x8]
10:40:42.259908 A > B: S 3688169472:3688169472(0)
ack 1168512001 win 32768 <mss 1460>
10:40:42.389992 B > A: . ack 1 win 33580 (DF) [tos 0x8]
10:40:42.664975 A > B: P 1:513(512) ack 1 win 32768
10:40:42.700185 A > B: . 513:1973(1460) ack 1 win 32768
10:40:42.718017 A > B: . 1973:3433(1460) ack 1 win 32768
10:40:42.762945 A > B: . 3433:4893(1460) ack 1 win 32768
10:40:42.811273 A > B: . 4893:6353(1460) ack 1 win 32768
10:40:42.829149 A > B: . 6353:7813(1460) ack 1 win 32768
10:40:42.853687 B > A: . ack 1973 win 33580 (DF) [tos 0x8]
10:40:42.864031 B > A: . ack 3433 win 33580 (DF) [tos 0x8]
After the third packet, the connection is established. A, the
connection responder, begins transmitting to B, the connection
initiator. Host A quickly sends 6 packets comprising 7812 bytes,
even though the SYN exchange agreed upon an MSS of 1460 bytes
(implying an initial congestion window of 1 segment corresponds to
1460 bytes), and so A should have sent at most 1460 bytes.
The ACKs sent by B to A in the last two lines indicate that this
trace is not a measurement error (slow start really occurring but
Paxson, Editor [Page 4]
ID Known TCP Implementation Problems August 1998
the corresponding ACKs having been dropped by the packet filter).
A second trace confirmed that the problem is repeatable.
Trace file demonstrating correct behavior
Made using tcpdump/BPF recording at the connection originator. No
losses reported.
12:35:31.914050 C > D: S 1448571845:1448571845(0) win 4380 <mss 1460>
12:35:32.068819 D > C: S 1755712000:1755712000(0) ack 1448571846 win 4096
12:35:32.069341 C > D: . ack 1 win 4608
12:35:32.075213 C > D: P 1:513(512) ack 1 win 4608
12:35:32.286073 D > C: . ack 513 win 4096
12:35:32.287032 C > D: . 513:1025(512) ack 1 win 4608
12:35:32.287506 C > D: . 1025:1537(512) ack 1 win 4608
12:35:32.432712 D > C: . ack 1537 win 4096
12:35:32.433690 C > D: . 1537:2049(512) ack 1 win 4608
12:35:32.434481 C > D: . 2049:2561(512) ack 1 win 4608
12:35:32.435032 C > D: . 2561:3073(512) ack 1 win 4608
12:35:32.594526 D > C: . ack 3073 win 4096
12:35:32.595465 C > D: . 3073:3585(512) ack 1 win 4608
12:35:32.595947 C > D: . 3585:4097(512) ack 1 win 4608
12:35:32.596414 C > D: . 4097:4609(512) ack 1 win 4608
12:35:32.596888 C > D: . 4609:5121(512) ack 1 win 4608
12:35:32.733453 D > C: . ack 4097 win 4096
References
This problem is documented in [Paxson97].
How to detect
For implementations always manifesting this problem, it shows up
immediately in a packet trace or a sequence plot, as illustrated
above.
How to fix
If the root problem is that the implementation lacks a notion of a
congestion window, then unfortunately this requires significant
work to fix. However, doing so is important, as such
implementations also exhibit "No slow start after retransmission
timeout".
Paxson, Editor [Page 5]
ID Known TCP Implementation Problems August 1998
3.2.
Name of Problem
No slow start after retransmission timeout
Classification
Congestion control
Description
When a TCP experiences a retransmission timeout, it is required by
RFC 1122, 4.2.2.15, to engage in "slow start" by initializing its
congestion window, cwnd, to one packet (one segment of the maximum
size). It subsequently increases cwnd by one packet for each ACK
it receives for new data until it reaches the "congestion
avoidance" threshold, ssthresh, at which point the congestion
avoidance algorithm for updating the window takes over. A TCP that
fails to enter slow start upon a timeout exhibits "No slow start
after retransmission timeout".
Significance
In congested environments, severely detrimental to the performance
of other connections, and also the connection itself.
Implications
Entering slow start upon timeout forms one of the cornerstones of
Internet congestion stability, as outlined in [Jacobson88]. If
TCPs fail to do so, the network becomes at risk of suffering
"congestion collapse" [RFC896].
Relevant RFCs
RFC 1122 requires use of slow start after loss. RFC 2001 gives the
specifics of how to implement slow start. RFC 896 describes
congestion collapse.
The retransmission timeout discussed here should not be confused
with the separate "fast recovery" retransmission mechanism
discussed in RFC 2001.
Trace file demonstrating it
Made using tcpdump/BPF recording at the sending TCP (A). No losses
reported.
10:40:59.090612 B > A: . ack 357125 win 33580 (DF) [tos 0x8]
10:40:59.222025 A > B: . 357125:358585(1460) ack 1 win 32768
10:40:59.868871 A > B: . 357125:358585(1460) ack 1 win 32768
Paxson, Editor [Page 6]
ID Known TCP Implementation Problems August 1998
10:41:00.016641 B > A: . ack 364425 win 33580 (DF) [tos 0x8]
10:41:00.036709 A > B: . 364425:365885(1460) ack 1 win 32768
10:41:00.045231 A > B: . 365885:367345(1460) ack 1 win 32768
10:41:00.053785 A > B: . 367345:368805(1460) ack 1 win 32768
10:41:00.062426 A > B: . 368805:370265(1460) ack 1 win 32768
10:41:00.071074 A > B: . 370265:371725(1460) ack 1 win 32768
10:41:00.079794 A > B: . 371725:373185(1460) ack 1 win 32768
10:41:00.089304 A > B: . 373185:374645(1460) ack 1 win 32768
10:41:00.097738 A > B: . 374645:376105(1460) ack 1 win 32768
10:41:00.106409 A > B: . 376105:377565(1460) ack 1 win 32768
10:41:00.115024 A > B: . 377565:379025(1460) ack 1 win 32768
10:41:00.123576 A > B: . 379025:380485(1460) ack 1 win 32768
10:41:00.132016 A > B: . 380485:381945(1460) ack 1 win 32768
10:41:00.141635 A > B: . 381945:383405(1460) ack 1 win 32768
10:41:00.150094 A > B: . 383405:384865(1460) ack 1 win 32768
10:41:00.158552 A > B: . 384865:386325(1460) ack 1 win 32768
10:41:00.167053 A > B: . 386325:387785(1460) ack 1 win 32768
10:41:00.175518 A > B: . 387785:389245(1460) ack 1 win 32768
10:41:00.210835 A > B: . 389245:390705(1460) ack 1 win 32768
10:41:00.226108 A > B: . 390705:392165(1460) ack 1 win 32768
10:41:00.241524 B > A: . ack 389245 win 8760 (DF) [tos 0x8]
The first packet indicates the ack point is 357125. 130 msec after
receiving the ACK, A transmits the packet after the ACK point,
357125:358585. 640 msec after this transmission, it retransmits
357125:358585, in an apparent retransmission timeout. At this
point, A's cwnd should be one MSS, or 1460 bytes, as A enters slow
start. The trace is consistent with this possibility.
B replies with an ACK of 364425, indicating that A has filled a
sequence hole. At this point, A's cwnd should be 1460*2 = 2920
bytes, since in slow start receiving an ACK advances cwnd by MSS.
However, A then launches 19 consecutive packets, which is
inconsistent with slow start.
A second trace confirmed that the problem is repeatable.
Trace file demonstrating correct behavior
Made using tcpdump/BPF recording at the sending TCP (C). No losses
reported.
12:35:48.442538 C > D: P 465409:465921(512) ack 1 win 4608
12:35:48.544483 D > C: . ack 461825 win 4096
12:35:48.703496 D > C: . ack 461825 win 4096
12:35:49.044613 C > D: . 461825:462337(512) ack 1 win 4608
12:35:49.192282 D > C: . ack 465921 win 2048
12:35:49.192538 D > C: . ack 465921 win 4096
Paxson, Editor [Page 7]
ID Known TCP Implementation Problems August 1998
12:35:49.193392 C > D: P 465921:466433(512) ack 1 win 4608
12:35:49.194726 C > D: P 466433:466945(512) ack 1 win 4608
12:35:49.350665 D > C: . ack 466945 win 4096
12:35:49.351694 C > D: . 466945:467457(512) ack 1 win 4608
12:35:49.352168 C > D: . 467457:467969(512) ack 1 win 4608
12:35:49.352643 C > D: . 467969:468481(512) ack 1 win 4608
12:35:49.506000 D > C: . ack 467969 win 3584
After C transmits the first packet shown to D, it takes no action
in response to D's ACKs for 461825, because the first packet
already reached the advertised window limit of 4096 bytes above
461825. 600 msec after transmitting the first packet, C
retransmits 461825:462337, presumably due to a timeout. Its
congestion window is now MSS (512 bytes).
D acks 465921, indicating that C's retransmission filled a sequence
hole. This ACK advances C's cwnd from 512 to 1024. Very shortly
after, D acks 465921 again in order to update the offered window
from 2048 to 4096. This ACK does not advance cwnd since it is not
for new data. Very shortly after, C responds to the newly enlarged
window by transmitting two packets. D acks both, advancing cwnd
from 1024 to 1536. C in turn transmits three packets.
References
This problem is documented in [Paxson97].
How to detect
Packet loss is common enough in the Internet that generally it is
not difficult to find an Internet path that will force
retransmission due to packet loss.
If the effective window prior to loss is large enough, however,
then the TCP may retransmit using the "fast recovery" mechanism
described in RFC 2001. In a packet trace, the signature of fast
recovery is that the packet retransmission occurs in response to
the receipt of three duplicate ACKs, and subsequent duplicate ACKs
may lead to the transmission of new data, above both the ack point
and the highest sequence transmitted so far. An absence of three
duplicate ACKs prior to retransmission suffices to distinguish
between timeout and fast recovery retransmissions. In the face of
only observing fast recovery retransmissions, generally it is not
difficult to repeat the data transfer until observing a timeout
retransmission.
Once armed with a trace exhibiting a timeout retransmission,
determining whether the TCP follows slow start is done by computing
Paxson, Editor [Page 8]
ID Known TCP Implementation Problems August 1998
the correct progression of cwnd and comparing it to the amount of
data transmited by the TCP subsequent to the timeout rtransmission.
How to fix
If the root problem is that the implementation lacks a notion of a
congestion window, then unfortunately this requires significant
work to fix. However, doing so is critical, for reasons outlined
above.
3.3.
Name of Problem
Uninitialized CWND
Classification
Congestion control
Description
As described above for "No initial slow start", when a TCP
connection begins cwnd is initialized to one segment (or perhaps a
few segments, if experimenting with [Allman98]). One particular
form of "No initial slow start", worth separate mention as the bug
is fairly widely deployed, is "Uninitialized CWND". That is, while
the TCP implements the proper slow start mechanism, it fails to
initialize cwnd properly, so slow start in fact fails to occur.
The particular bug occurs when, during the connection establishment
handshake, the SYN ACK packet arrives without an MSS option. The
faulty implementation uses receipt of the MSS option to initialize
cwnd to one segment; if the option fails to arrive, then cwnd is
instead initialized to a very large value.
Significance
In congested environments, detrimental to the performance of other
connections, and likely to the connection itself. The burst can be
so large (see below) that it has deleterious effects even in
uncongested environments.
Implications
A TCP exhibiting this behavior is stressing the network with a
large burst of packets, which can cause loss in the network.
Relevant RFCs
RFC 1122 requires use of slow start. RFC 2001 gives the specifics
Paxson, Editor [Page 9]
ID Known TCP Implementation Problems August 1998
of slow start.
Trace file demonstrating it
This trace was made using tcpdump/BPF running on host A. Host A is
the sender and host B is the receiver. The advertised window and
timestamp options have been omitted for clarity, except for the
first segment sent by host A. Note that A sends an MSS option in
its initial SYN but B does not include one in its reply.
16:56:02.226937 A > B: S 237585307:237585307(0) win 8192
<mss 536,nop,wscale 0,nop,nop,timestamp[|tcp]>
16:56:02.557135 B > A: S 1617216000:1617216000(0)
ack 237585308 win 16384
16:56:02.557788 A > B: . ack 1 win 8192
16:56:02.566014 A > B: . 1:537(536) ack 1
16:56:02.566557 A > B: . 537:1073(536) ack 1
16:56:02.567120 A > B: . 1073:1609(536) ack 1
16:56:02.567662 A > B: P 1609:2049(440) ack 1
16:56:02.568349 A > B: . 2049:2585(536) ack 1
16:56:02.568909 A > B: . 2585:3121(536) ack 1
[54 additional burst segments deleted for brevity]
16:56:02.936638 A > B: . 32065:32601(536) ack 1
16:56:03.018685 B > A: . ack 1
After the three-way handshake, host A bursts 61 segments into the
network, before duplicate ACKs on the first segment cause a
retransmission to occur. Since host A did not wait for the ACK on
the first segment before sending additional segments, it is
exhibiting "Uninitialized CWND"
Trace file demonstrating correct behavior
See the example for "No initial slow start".
References
This problem is documented in [Paxson97].
How to detect
This problem can be detected by examining a packet trace recorded
at either the sender or the receiver. However, the bug can be
difficult to induce because it requires finding a remote TCP peer
that does not send an MSS option in its SYN ACK.
Paxson, Editor [Page 10]
ID Known TCP Implementation Problems August 1998
How to fix
This problem can be fixed by ensuring that cwnd is initialized upon
receipt of a SYN ACK, even if the SYN ACK does not contain an MSS
option.
3.4.
Name of Problem
Inconsistent retransmission
Classification
Reliability
Description
If, for a given sequence number, a sending TCP retransmits
different data than previously sent for that sequence number, then
a strong possibility arises that the receiving TCP will reconstruct
a different byte stream than that sent by the sending application,
depending on which instance of the sequence number it accepts.
Such a sending TCP exhibits "Inconsistent retransmission".
Significance
Critical for all environments.
Implications
Reliable delivery of data is a fundamental property of TCP.
Relevant RFCs
RFC 793, section 1.5, discusses the central role of reliability in
TCP operation.
Trace file demonstrating it
Made using tcpdump/BPF recording at the receiving TCP (B). No
losses reported.
12:35:53.145503 A > B: FP 90048435:90048461(26) ack 393464682 win 4096
4500 0042 9644 0000
3006 e4c2 86b1 0401 83f3 010a b2a4 0015
055e 07b3 1773 cb6a 5019 1000 68a9 0000
data starts here>504f 5254 2031 3334 2c31 3737*2c34 2c31
2c31 3738 2c31 3635 0d0a
12:35:53.146479 B > A: R 393464682:393464682(0) win 8192
12:35:53.851714 A > B: FP 90048429:90048463(34) ack 393464682 win 4096
4500 004a 965b 0000
3006 e4a3 86b1 0401 83f3 010a b2a4 0015
Paxson, Editor [Page 11]
ID Known TCP Implementation Problems August 1998
055e 07ad 1773 cb6a 5019 1000 8bd3 0000
data starts here>5041 5356 0d0a 504f 5254 2031 3334 2c31
3737*2c31 3035 2c31 3431 2c34 2c31 3539
0d0a
The sequence numbers shown in this trace are absolute and not
adjusted to reflect the ISN. The 4-digit hex values show a dump of
the packet's IP and TCP headers, as well as payload. A first sends
to B data for 90048435:90048461. The corresponding data begins
with hex words 504f, 5254, etc.
B responds with a RST. Since the recording location was local to
B, it is unknown whether A received the RST.
A then sends 90048429:90048463, which includes six sequence
positions below the earlier transmission, all 26 positions of the
earlier transmission, and two additional sequence positions.
The retransmission disagrees starting just after sequence 90048447,
annotated above with a leading '*'. These two bytes were
originally transmitted as hex 2c34 but retransmitted as hex 2c31.
Subsequent positions disagree as well.
This behavior has been observed in other traces involving different
hosts. It is unknown how to repeat it.
In this instance, no corruption would occur, since B has already
indicated it will not accept further packets from A.
A second example illustrates a slightly different instance of the
problem. The tracing again was made with tcpdump/BPF at the
receiving TCP (D).
22:23:58.645829 C > D: P 185:212(27) ack 565 win 4096
4500 0043 90a3 0000
3306 0734 cbf1 9eef 83f3 010a 0525 0015
a3a2 faba 578c 70a4 5018 1000 9a53 0000
data starts here>504f 5254 2032 3033 2c32 3431 2c31 3538
2c32 3339 2c35 2c34 330d 0a
22:23:58.646805 D > C: . ack 184 win 8192
4500 0028 beeb 0000
3e06 ce06 83f3 010a cbf1 9eef 0015 0525
578c 70a4 a3a2 fab9 5010 2000 342f 0000
22:31:36.532244 C > D: FP 186:213(27) ack 565 win 4096
4500 0043 9435 0000
3306 03a2 cbf1 9eef 83f3 010a 0525 0015
a3a2 fabb 578c 70a4 5019 1000 9a51 0000
data starts here>504f 5254 2032 3033 2c32 3431 2c31 3538
Paxson, Editor [Page 12]
ID Known TCP Implementation Problems August 1998
2c32 3339 2c35 2c34 330d 0a
In this trace, sequence numbers are relative. C sends 185:212, but
D only sends an ACK for 184 (so sequence number 184 is missing). C
then sends 186:213. The packet payload is identical to the
previous payload, but the base sequence number is one higher,
resulting in an inconsistent retransmission.
Neither trace exhibits checksum errors.
Trace file demonstrating correct behavior
(Omitted, as presumably correct behavior is obvious.)
References
None known.
How to detect
This problem unfortunately can be very difficult to detect, since
available experience indicates it is quite rare that it is
manifested. No "trigger" has been identified that can be used to
reproduce the problem.
How to fix
In the absence of a known "trigger", we cannot always assess how to
fix the problem.
In one implementation (not the one illustrated above), the problem
manifested itself when (1) the sender received a zero window and
stalled; (2) eventually an ACK arrived that offered a window larger
than that in effect at the time of the stall; (3) the sender
transmitted out of the buffer of data it held at the time of the
stall, but (4) failed to limit this transfer to the buffer length,
instead using the newly advertised (and larger) offered window.
Consequently, in addition to the valid buffer contents, it sent
whatever garbage values followed the end of the buffer. If it then
retransmitted the corresponding sequence numbers, at that point it
sent the correct data, resulting in an inconsistent retransmission.
Note that this instance of the problem reflects a more general
problem, that of initially transmitting incorrect data.
3.5.
Name of Problem
Failure to retain above-sequence data
Paxson, Editor [Page 13]
ID Known TCP Implementation Problems August 1998
Classification
Congestion control, performance
Description
When a TCP receives an "above sequence" segment, meaning one with a
sequence number exceeding RCV.NXT but below RCV.NXT+RCV.WND, it
SHOULD queue the segment for later delivery (RFC 1122, 4.2.2.20).
A TCP that fails to do so is said to exhibit "Failure to retain
above-sequence data".
It may sometimes be appropriate for a TCP to discard above-sequence
data to reclaim memory. If they do so only rarely, then we would
not consider them to exhibit this problem. Instead, the particular
concern is with TCPs that always discard above-sequence data.
Significance
In environments prone to packet loss, detrimental to the
performance of both other connections and the connection itself.
Implications
In times of congestion, a failure to retain above-sequence data
will lead to numerous otherwise-unnecessary retransmissions,
aggravating the congestion and potentially reducing performance by
a large factor.
Relevant RFCs
RFC 1122 revises RFC 793 by upgrading the latter's MAY to a SHOULD
on this issue.
Trace file demonstrating it
Made using tcpdump/BPF recording at the receiving TCP. No losses
reported.
B is the TCP sender, A the receiver. A exhibits failure to retain
above sequence data:
10:38:10.164860 B > A: . 221078:221614(536) ack 1 win 33232 [tos 0x8]
10:38:10.170809 B > A: . 221614:222150(536) ack 1 win 33232 [tos 0x8]
10:38:10.177183 B > A: . 222150:222686(536) ack 1 win 33232 [tos 0x8]
10:38:10.225039 A > B: . ack 222686 win 25800
Here B has sent up to (relative) sequence 222686 in-sequence, and A
accordingly acknowledges.
10:38:10.268131 B > A: . 223222:223758(536) ack 1 win 33232 [tos 0x8]
10:38:10.337995 B > A: . 223758:224294(536) ack 1 win 33232 [tos 0x8]
Paxson, Editor [Page 14]
ID Known TCP Implementation Problems August 1998
10:38:10.344065 B > A: . 224294:224830(536) ack 1 win 33232 [tos 0x8]
10:38:10.350169 B > A: . 224830:225366(536) ack 1 win 33232 [tos 0x8]
10:38:10.356362 B > A: . 225366:225902(536) ack 1 win 33232 [tos 0x8]
10:38:10.362445 B > A: . 225902:226438(536) ack 1 win 33232 [tos 0x8]
10:38:10.368579 B > A: . 226438:226974(536) ack 1 win 33232 [tos 0x8]
10:38:10.374732 B > A: . 226974:227510(536) ack 1 win 33232 [tos 0x8]
10:38:10.380825 B > A: . 227510:228046(536) ack 1 win 33232 [tos 0x8]
10:38:10.387027 B > A: . 228046:228582(536) ack 1 win 33232 [tos 0x8]
10:38:10.393053 B > A: . 228582:229118(536) ack 1 win 33232 [tos 0x8]
10:38:10.399193 B > A: . 229118:229654(536) ack 1 win 33232 [tos 0x8]
10:38:10.405356 B > A: . 229654:230190(536) ack 1 win 33232 [tos 0x8]
A now receives 13 additional packets from B. These are above-
sequence because 222686:223222 was dropped. The packets do however
fit within the offered window of 25800. A does not generate any
duplicate ACKs for them.
The trace contributor (V. Paxson) verified that these 13 packets
had valid IP and TCP checksums.
10:38:11.917728 B > A: . 222686:223222(536) ack 1 win 33232 [tos 0x8]
10:38:11.930925 A > B: . ack 223222 win 32232
B times out for 222686:223222 and retransmits it. Upon receiving
it, A only acknowledges 223222. Had it retained the valid above-
sequence packets, it would instead have ack'd 230190.
10:38:12.048438 B > A: . 223222:223758(536) ack 1 win 33232 [tos 0x8]
10:38:12.054397 B > A: . 223758:224294(536) ack 1 win 33232 [tos 0x8]
10:38:12.068029 A > B: . ack 224294 win 31696
B retransmits two more packets, and A only acknowledges them. This
pattern continues as B retransmits the entire set of previously-
received packets.
A second trace confirmed that the problem is repeatable.
Trace file demonstrating correct behavior
Made using tcpdump/BPF recording at the receiving TCP (C). No
losses reported.
09:11:25.790417 D > C: . 33793:34305(512) ack 1 win 61440
09:11:25.791393 D > C: . 34305:34817(512) ack 1 win 61440
09:11:25.792369 D > C: . 34817:35329(512) ack 1 win 61440
09:11:25.792369 D > C: . 35329:35841(512) ack 1 win 61440
09:11:25.793345 D > C: . 36353:36865(512) ack 1 win 61440
09:11:25.794321 C > D: . ack 35841 win 59904
Paxson, Editor [Page 15]
ID Known TCP Implementation Problems August 1998
A sequence hole occurs because 35841:36353 has been dropped.
09:11:25.794321 D > C: . 36865:37377(512) ack 1 win 61440
09:11:25.794321 C > D: . ack 35841 win 59904
09:11:25.795297 D > C: . 37377:37889(512) ack 1 win 61440
09:11:25.795297 C > D: . ack 35841 win 59904
09:11:25.796273 C > D: . ack 35841 win 61440
09:11:25.798225 D > C: . 37889:38401(512) ack 1 win 61440
09:11:25.799201 C > D: . ack 35841 win 61440
09:11:25.807009 D > C: . 38401:38913(512) ack 1 win 61440
09:11:25.807009 C > D: . ack 35841 win 61440
(many additional lines omitted)
09:11:25.884113 D > C: . 52737:53249(512) ack 1 win 61440
09:11:25.884113 C > D: . ack 35841 win 61440
Each additional, above-sequence packet C receives from D elicits a
duplicate ACK for 35841.
09:11:25.887041 D > C: . 35841:36353(512) ack 1 win 61440
09:11:25.887041 C > D: . ack 53249 win 44032
D retransmits 35841:36353 and C acknowledges receipt of data all
the way up to 53249.
References
This problem is documented in [Paxson97].
How to detect
Packet loss is common enough in the Internet that generally it is
not difficult to find an Internet path that will result in some
above-sequence packets arriving. A TCP that exhibits "Failure to
retain ..." may not generate duplicate ACKs for these packets.
However, some TCPs that do retain above-sequence data also do not
generate duplicate ACKs, so failure to do so does not definitively
identify the problem. Instead, the key observation is whether upon
retransmission of the dropped packet, data that was previously
above-sequence is acknowledged.
Two considerations in detecting this problem using a packet trace
are that it is easiest to do so with a trace made at the TCP
receiver, in order to unambiguously determine which packets arrived
successfully, and that such packets may still be correctly
discarded if they arrive with checksum errors. The latter can be
tested by capturing the entire packet contents and performing the
IP and TCP checksum algorithms to verify their integrity; or by
confirming that the packets arrive with the same checksum and
Paxson, Editor [Page 16]
ID Known TCP Implementation Problems August 1998
contents as that with which they were sent, with a presumption that
the sending TCP correctly calculates checksums for the packets it
transmits.
It is considerably easier to verify that an implementation does NOT
exhibit this problem. This can be done by recording a trace at the
data sender, and observing that sometimes after a retransmission
the receiver acknowledges a higher sequence number than just that
which was retransmitted.
How to fix
If the root problem is that the implementation lacks buffer, then
then unfortunately this requires significant work to fix. However,
doing so is important, for reasons outlined above.
3.6.
Name of Problem
Extra additive constant in congestion avoidance
Classification
Congestion control / performance
Description
RFC 1122 section 4.2.2.15 states that TCP MUST implement Jacobson's
"congestion avoidance" algorithm [Jacobson88], which calls for
increasing the congestion window, cwnd, by:
MSS * MSS / cwnd
for each ACK received for new data [RFC2001]. This has the effect
of increasing cwnd by approximately one segment in each round trip
time.
Some TCP implementations add an additional fraction of a segment
(typically MSS/8) to cwnd for each ACK received for new data
[Stevens94, Wright95]:
(MSS * MSS / cwnd) + MSS/8
These implementations exhibit "Extra additive constant in
congestion avoidance".
Paxson, Editor [Page 17]
ID Known TCP Implementation Problems August 1998
Significance
May be detrimental to performance even in completely uncongested
environments (see Implications).
In congested environments, may also be detrimental to the
performance of other connections.
Implications
The extra additive term allows a TCP to more aggressively open its
congestion window (quadratic rather than linear increase). For
congested networks, this can increase the loss rate experienced by
all connections sharing a bottleneck with the aggressive TCP.
However, even for completely uncongested networks, the extra
additive term can lead to diminished performance, as follows. In
congestion avoidance, a TCP sender probes the network path to
determine its available capacity, which often equates to the number
of buffers available at a bottleneck link. With linear congestion
avoidance, the TCP only probes for sufficient capacity (buffer) to
hold one extra packet per RTT.
Thus, when it exceeds the available capacity, generally only one
packet will be lost (since on the previous RTT it already found
that the path could sustain a window with one less packet in
flight). If the congestion window is sufficiently large, then the
TCP will recover from this single loss using fast retransmission
and avoid an expensive (in terms of performance) retransmission
timeout.
However, when the additional additive term is used, then cwnd can
increase by more than one packet per RTT, in which case the TCP
probes more aggressively. If in the previous RTT it had reached
the available capacity of the path, then the excess due to the
increase will again be lost, but now this will result in multiple
losses from the flight instead of a single loss. TCPs that do not
utilize SACK [RFC2018] generally will not recover from multiple
losses without incurring a retransmission timeout [Fall96,Hoe96],
significantly diminishing performance.
Relevant RFCs
RFC 1122 requires use of the "congestion avoidance" algorithm. RFC
2001 outlines the fast retransmit/fast recovery algorithms. RFC
2018 discusses the SACK option.
Trace file demonstrating it
Paxson, Editor [Page 18]
ID Known TCP Implementation Problems August 1998
Recorded using tcpdump running on the same FDDI LAN as host A.
Host A is the sender and host B is the receiver. The connection
establishment specified an MSS of 4,312 bytes and a window scale
factor of 4. We omit the establishment and the first 2.5 MB of
data transfer, as the problem is best demonstrated when the window
has grown to a large value. At the beginning of the trace excerpt,
the congestion window is 31 packets. The connection is never
receiver-window limited, so we omit window advertisements from the
trace for clarity.
11:42:07.697951 B > A: . ack 2383006
11:42:07.699388 A > B: . 2508054:2512366(4312)
11:42:07.699962 A > B: . 2512366:2516678(4312)
11:42:07.700012 B > A: . ack 2391630
11:42:07.701081 A > B: . 2516678:2520990(4312)
11:42:07.701656 A > B: . 2520990:2525302(4312)
11:42:07.701739 B > A: . ack 2400254
11:42:07.702685 A > B: . 2525302:2529614(4312)
11:42:07.703257 A > B: . 2529614:2533926(4312)
11:42:07.703295 B > A: . ack 2408878
11:42:07.704414 A > B: . 2533926:2538238(4312)
11:42:07.704989 A > B: . 2538238:2542550(4312)
11:42:07.705040 B > A: . ack 2417502
11:42:07.705935 A > B: . 2542550:2546862(4312)
11:42:07.706506 A > B: . 2546862:2551174(4312)
11:42:07.706544 B > A: . ack 2426126
11:42:07.707480 A > B: . 2551174:2555486(4312)
11:42:07.708051 A > B: . 2555486:2559798(4312)
11:42:07.708088 B > A: . ack 2434750
11:42:07.709030 A > B: . 2559798:2564110(4312)
11:42:07.709604 A > B: . 2564110:2568422(4312)
11:42:07.710175 A > B: . 2568422:2572734(4312) *
11:42:07.710215 B > A: . ack 2443374
11:42:07.710799 A > B: . 2572734:2577046(4312)
11:42:07.711368 A > B: . 2577046:2581358(4312)
11:42:07.711405 B > A: . ack 2451998
11:42:07.712323 A > B: . 2581358:2585670(4312)
11:42:07.712898 A > B: . 2585670:2589982(4312)
11:42:07.712938 B > A: . ack 2460622
11:42:07.713926 A > B: . 2589982:2594294(4312)
11:42:07.714501 A > B: . 2594294:2598606(4312)
11:42:07.714547 B > A: . ack 2469246
11:42:07.715747 A > B: . 2598606:2602918(4312)
11:42:07.716287 A > B: . 2602918:2607230(4312)
11:42:07.716328 B > A: . ack 2477870
11:42:07.717146 A > B: . 2607230:2611542(4312)
11:42:07.717717 A > B: . 2611542:2615854(4312)
Paxson, Editor [Page 19]
ID Known TCP Implementation Problems August 1998
11:42:07.717762 B > A: . ack 2486494
11:42:07.718754 A > B: . 2615854:2620166(4312)
11:42:07.719331 A > B: . 2620166:2624478(4312)
11:42:07.719906 A > B: . 2624478:2628790(4312) **
11:42:07.719958 B > A: . ack 2495118
11:42:07.720500 A > B: . 2628790:2633102(4312)
11:42:07.721080 A > B: . 2633102:2637414(4312)
11:42:07.721739 B > A: . ack 2503742
11:42:07.722348 A > B: . 2637414:2641726(4312)
11:42:07.722918 A > B: . 2641726:2646038(4312)
11:42:07.769248 B > A: . ack 2512366
The receiver's acknowledgment policy is one ACK per two packets
received. Thus, for each ACK arriving at host A, two new packets
are sent, except when cwnd increases due to congestion avoidance,
in which case three new packets are sent.
With an ack-every-two-packets policy, cwnd should only increase one
MSS per 2 RTT. However, at the point marked "*" the window
increases after 7 ACKs have arrived, and then again at "**" after 6
more ACKs.
While we do not have space to show the effect, this trace suffered
from repeated timeout retransmissions due to multiple packet losses
during a single RTT.
Trace file demonstrating correct behavior
Made using the same host and tracing setup as above, except now A's
TCP has been modified to remove the MSS/8 additive constant.
Tcpdump reported 77 packet drops; the excerpt below is fully self-
consistent so it is unlikely that any of these occurred during the
excerpt.
We again begin when cwnd is 31 packets (this occurs significantly
later in the trace, because the congestion avoidance is now less
aggressive with opening the window).
14:22:21.236757 B > A: . ack 5194679
14:22:21.238192 A > B: . 5319727:5324039(4312)
14:22:21.238770 A > B: . 5324039:5328351(4312)
14:22:21.238821 B > A: . ack 5203303
14:22:21.240158 A > B: . 5328351:5332663(4312)
14:22:21.240738 A > B: . 5332663:5336975(4312)
14:22:21.270422 B > A: . ack 5211927
14:22:21.271883 A > B: . 5336975:5341287(4312)
Paxson, Editor [Page 20]
ID Known TCP Implementation Problems August 1998
14:22:21.272458 A > B: . 5341287:5345599(4312)
14:22:21.279099 B > A: . ack 5220551
14:22:21.280539 A > B: . 5345599:5349911(4312)
14:22:21.281118 A > B: . 5349911:5354223(4312)
14:22:21.281183 B > A: . ack 5229175
14:22:21.282348 A > B: . 5354223:5358535(4312)
14:22:21.283029 A > B: . 5358535:5362847(4312)
14:22:21.283089 B > A: . ack 5237799
14:22:21.284213 A > B: . 5362847:5367159(4312)
14:22:21.284779 A > B: . 5367159:5371471(4312)
14:22:21.285976 B > A: . ack 5246423
14:22:21.287465 A > B: . 5371471:5375783(4312)
14:22:21.288036 A > B: . 5375783:5380095(4312)
14:22:21.288073 B > A: . ack 5255047
14:22:21.289155 A > B: . 5380095:5384407(4312)
14:22:21.289725 A > B: . 5384407:5388719(4312)
14:22:21.289762 B > A: . ack 5263671
14:22:21.291090 A > B: . 5388719:5393031(4312)
14:22:21.291662 A > B: . 5393031:5397343(4312)
14:22:21.291701 B > A: . ack 5272295
14:22:21.292870 A > B: . 5397343:5401655(4312)
14:22:21.293441 A > B: . 5401655:5405967(4312)
14:22:21.293481 B > A: . ack 5280919
14:22:21.294476 A > B: . 5405967:5410279(4312)
14:22:21.295053 A > B: . 5410279:5414591(4312)
14:22:21.295106 B > A: . ack 5289543
14:22:21.296306 A > B: . 5414591:5418903(4312)
14:22:21.296878 A > B: . 5418903:5423215(4312)
14:22:21.296917 B > A: . ack 5298167
14:22:21.297716 A > B: . 5423215:5427527(4312)
14:22:21.298285 A > B: . 5427527:5431839(4312)
14:22:21.298324 B > A: . ack 5306791
14:22:21.299413 A > B: . 5431839:5436151(4312)
14:22:21.299986 A > B: . 5436151:5440463(4312)
14:22:21.303696 B > A: . ack 5315415
14:22:21.305177 A > B: . 5440463:5444775(4312)
14:22:21.305755 A > B: . 5444775:5449087(4312)
14:22:21.308032 B > A: . ack 5324039
14:22:21.309525 A > B: . 5449087:5453399(4312)
14:22:21.310101 A > B: . 5453399:5457711(4312)
14:22:21.310144 B > A: . ack 5332663 ***
14:22:21.311615 A > B: . 5457711:5462023(4312)
14:22:21.312198 A > B: . 5462023:5466335(4312)
14:22:21.341876 B > A: . ack 5341287
14:22:21.343451 A > B: . 5466335:5470647(4312)
14:22:21.343985 A > B: . 5470647:5474959(4312)
14:22:21.350304 B > A: . ack 5349911
Paxson, Editor [Page 21]
ID Known TCP Implementation Problems August 1998
14:22:21.351852 A > B: . 5474959:5479271(4312)
14:22:21.352430 A > B: . 5479271:5483583(4312)
14:22:21.352484 B > A: . ack 5358535
14:22:21.353574 A > B: . 5483583:5487895(4312)
14:22:21.354149 A > B: . 5487895:5492207(4312)
14:22:21.354205 B > A: . ack 5367159
14:22:21.355467 A > B: . 5492207:5496519(4312)
14:22:21.356039 A > B: . 5496519:5500831(4312)
14:22:21.357361 B > A: . ack 5375783
14:22:21.358855 A > B: . 5500831:5505143(4312)
14:22:21.359424 A > B: . 5505143:5509455(4312)
14:22:21.359465 B > A: . ack 5384407
14:22:21.360605 A > B: . 5509455:5513767(4312)
14:22:21.361181 A > B: . 5513767:5518079(4312)
14:22:21.361225 B > A: . ack 5393031
14:22:21.362485 A > B: . 5518079:5522391(4312)
14:22:21.363057 A > B: . 5522391:5526703(4312)
14:22:21.363096 B > A: . ack 5401655
14:22:21.364236 A > B: . 5526703:5531015(4312)
14:22:21.364810 A > B: . 5531015:5535327(4312)
14:22:21.364867 B > A: . ack 5410279
14:22:21.365819 A > B: . 5535327:5539639(4312)
14:22:21.366386 A > B: . 5539639:5543951(4312)
14:22:21.366427 B > A: . ack 5418903
14:22:21.367586 A > B: . 5543951:5548263(4312)
14:22:21.368158 A > B: . 5548263:5552575(4312)
14:22:21.368199 B > A: . ack 5427527
14:22:21.369189 A > B: . 5552575:5556887(4312)
14:22:21.369758 A > B: . 5556887:5561199(4312)
14:22:21.369803 B > A: . ack 5436151
14:22:21.370814 A > B: . 5561199:5565511(4312)
14:22:21.371398 A > B: . 5565511:5569823(4312)
14:22:21.375159 B > A: . ack 5444775
14:22:21.376658 A > B: . 5569823:5574135(4312)
14:22:21.377235 A > B: . 5574135:5578447(4312)
14:22:21.379303 B > A: . ack 5453399
14:22:21.380802 A > B: . 5578447:5582759(4312)
14:22:21.381377 A > B: . 5582759:5587071(4312)
14:22:21.381947 A > B: . 5587071:5591383(4312) ****
"***" marks the end of the first round trip. Note that cwnd did
not increase (as evidenced by each ACK eliciting two new data
packets). Only at "****", which comes near the end of the second
round trip, does cwnd increase by one packet.
This trace did not suffer any timeout retransmissions. It
transferred the same amount of data as the first trace in about
half as much time. This difference is repeatable between hosts A
Paxson, Editor [Page 22]
ID Known TCP Implementation Problems August 1998
and B.
References
[Stevens94] and [Wright95] discuss this problem. The problem of
Reno TCP failing to recover from multiple losses except via a
retransmission timeout is discussed in [Fall96,Hoe96].
How to detect
If source code is available, that is generally the easiest way to
detect this problem. Search for each modification to the cwnd
variable; (at least) one of these will be for congestion avoidance,
and inspection of the related code should immediately identify the
problem if present.
The problem can also be detected by closely examining packet traces
taken near the sender. During congestion avoidance, cwnd will
increase by an additional segment upon the receipt of (typically)
eight acknowledgements without a loss. This increase is in
addition to the one segment increase per round trip time (or two
round trip times if the receiver is using delayed ACKs).
Furthermore, graphs of the sequence number vs. time, taken from
packet traces, are normally linear during congestion avoidance.
When viewing packet traces of transfers from senders exhibiting
this problem, the graphs appear quadratic instead of linear.
Finally, the traces will show that, with sufficiently large
windows, nearly every loss event results in a timeout.
How to fix
This problem may be corrected by removing the "+ MSS/8" term from
the congestion avoidance code that increases cwnd each time an ACK
of new data is received.
3.7.
Name of Problem
Initial RTO too low
Classification
Performance
Paxson, Editor [Page 23]
ID Known TCP Implementation Problems August 1998
Description
When a TCP first begins transmitting data, it lacks the RTT
measurements necessary to have computed an adaptive retransmission
timeout (RTO). RFC 1122, 4.2.3.1, states that a TCP SHOULD
initialize RTO to 3 seconds. A TCP that uses a lower value
exhibits "Initial RTO too low".
Significance
In environments with large RTTs (where "large" means any value
larger than the initial RTO), TCPs will experience very poor
performance.
Implications
Whenever RTO < RTT, very poor performance can result as packets are
unnecessarily retransmitted (because RTO will expire before an ACK
for the packet can arrive) and the connection enters slow start and
congestion avoidance. Generally, the algorithms for computing RTO
avoid this problem by adding a positive term to the estimated RTT.
However, when a connection first begins it must use some estimate
for RTO, and if it picks a value less than RTT, the above problems
will arise.
Furthermore, when the initial RTO < RTT, it can take a long time
for the TCP to correct the problem by adapting the RTT estimate,
because the use of Karn's algorithm (mandated by RFC 1122, 4.2.3.1)
will discard many of the candidate RTT measurements made after the
first timeout, since they will be measurements of retransmitted
segments.
Relevant RFCs
RFC 1122 states that TCPs SHOULD initialize RTO to 3 seconds and
MUST implement Karn's algorithm.
Trace file demonstrating it
The following trace file was taken using tcpdump at host A, the
data sender. The advertised window and SYN options have been
omitted for clarity.
07:52:39.870301 A > B: S 2786333696:2786333696(0)
07:52:40.548170 B > A: S 130240000:130240000(0) ack 2786333697
07:52:40.561287 A > B: P 1:513(512) ack 1
07:52:40.753466 A > B: . 1:513(512) ack 1
07:52:41.133687 A > B: . 1:513(512) ack 1
07:52:41.458529 B > A: . ack 513
Paxson, Editor [Page 24]
ID Known TCP Implementation Problems August 1998
07:52:41.458686 A > B: . 513:1025(512) ack 1
07:52:41.458797 A > B: P 1025:1537(512) ack 1
07:52:41.541633 B > A: . ack 513
07:52:41.703732 A > B: . 513:1025(512) ack 1
07:52:42.044875 B > A: . ack 513
07:52:42.173728 A > B: . 513:1025(512) ack 1
07:52:42.330861 B > A: . ack 1537
07:52:42.331129 A > B: . 1537:2049(512) ack 1
07:52:42.331262 A > B: P 2049:2561(512) ack 1
07:52:42.623673 A > B: . 1537:2049(512) ack 1
07:52:42.683203 B > A: . ack 1537
07:52:43.044029 B > A: . ack 1537
07:52:43.193812 A > B: . 1537:2049(512) ack 1
Note from the SYN/SYN-ack exchange, the RTT is over 600 msec.
However, from the elapsed time between the third and fourth lines
(the first packet being sent and then retransmitted), it is
apparent the RTO was initialized to under 200 msec. The next line
shows that this value has doubled to 400 msec (correct exponential
backoff of RTO), but that still does not suffice to avoid an
unnecessary retransmission.
Finally, an ACK from B arrives for the first segment. Later two
more duplicate ACKs for 513 arrive, indicating that both the
original and the two retransmissions arrived at B. (Indeed, a
concurrent trace at B showed that no packets were lost during the
entire connection). This ACK opens the congestion window to two
packets, which are sent back-to-back, but at 07:52:41.703732 RTO
again expires after a little over 200 msec, leading to an
unnecessary retransmission, and the pattern repeats. By the end of
the trace excerpt above, 1536 bytes have been successfully
transmitted from A to B, over an interval of more than 2 seconds,
reflecting terrible performance.
Trace file demonstrating correct behavior
The following trace file was taken using tcpdump at host C, the
data sender. The advertised window and SYN options have been
omitted for clarity.
17:30:32.090299 C > D: S 2031744000:2031744000(0)
17:30:32.900325 D > C: S 262737964:262737964(0) ack 2031744001
17:30:32.900326 C > D: . ack 1
17:30:32.910326 C > D: . 1:513(512) ack 1
17:30:34.150355 D > C: . ack 513
17:30:34.150356 C > D: . 513:1025(512) ack 1
17:30:34.150357 C > D: . 1025:1537(512) ack 1
17:30:35.170384 D > C: . ack 1025
Paxson, Editor [Page 25]
ID Known TCP Implementation Problems August 1998
17:30:35.170385 C > D: . 1537:2049(512) ack 1
17:30:35.170386 C > D: . 2049:2561(512) ack 1
17:30:35.320385 D > C: . ack 1537
17:30:35.320386 C > D: . 2561:3073(512) ack 1
17:30:35.320387 C > D: . 3073:3585(512) ack 1
17:30:35.730384 D > C: . ack 2049
The initital SYN/SYN-ack exchange shows that RTT is more than 800
msec, and for some subsequent packets it rises above 1 second, but
C's retransmit timer does not ever expire.
References
This problem is documented in [Paxson97].
How to detect
This problem is readily detected by inspecting a packet trace of
the startup of a TCP connection made over a long-delay path. It
can be diagnosed from either a sender-side or receiver-side trace.
Long-delay paths can often be found by locating remote sites on
other continents.
How to fix
As this problem arises from a faulty initialization, one hopes
fixing it requires a one-line change to the TCP source code.
3.8.
Name of Problem
Failure of window deflation after loss recovery
Classification
Congestion control / performance
Description
The fast recovery algorithm allows TCP senders to continue to
transmit new segments during loss recovery. First, fast
retransmission is initiated after a TCP sender receives three
duplicate ACKs. At this point, a retransmission is sent and cwnd
is halved. The fast recovery algorithm then allows additional
segments to be sent when sufficient additional duplicate ACKs
arrive. Some implementations of fast recovery compute when to send
additional segments by artificially incrementing cwnd, first by
Paxson, Editor [Page 26]
ID Known TCP Implementation Problems August 1998
three segments to account for the three duplicate ACKs that
triggered fast retransmission, and subsequently by 1 MSS for each
new duplicate ACK that arrives. When cwnd allows, the sender
transmits new data segments.
When an ACK arrives that covers new data, cwnd is to be reduced by
the amount by which it was artificially increased. However, some
TCP implementations fail to "deflate" the window, causing an
inappropriate amount of data to be sent into the network after
recovery. One cause of this problem is the "header prediction"
code, which is used to handle incoming segments that require little
work. In some implementations of TCP, the header prediction code
does not check to make sure cwnd has not been artificially
inflated, and therefore does not reduce the artificially increased
cwnd when appropriate.
Significance
TCP senders that exhibit this problem will transmit a burst of data
immediately after recovery, which can degrade performance, as well
as network stability. Effectively, the sender does not reduce the
size of cwnd as much as it should (to half its value when loss was
detected), if at all. This can harm the performance of the TCP
connection itself, as well as competing TCP flows.
Implications
A TCP sender exhibiting this problem does not reduce cwnd
appropriately in times of congestion, and therefore may contribute
to congestive collapse.
Relevant RFCs
RFC 2001 outlines the fast retransmit/fast recovery algorithms.
[Brakmo95] outlines this implementation problem and offers a fix.
Trace file demonstrating it
The following trace file was taken using tcpdump at host A, the
data sender. The advertised window (which never changed) has been
omitted for clarity, except for the first packet sent by each host.
08:22:56.825635 A.7505 > B.7505: . 29697:30209(512) ack 1 win 4608
08:22:57.038794 B.7505 > A.7505: . ack 27649 win 4096
08:22:57.039279 A.7505 > B.7505: . 30209:30721(512) ack 1
08:22:57.321876 B.7505 > A.7505: . ack 28161
08:22:57.322356 A.7505 > B.7505: . 30721:31233(512) ack 1
08:22:57.347128 B.7505 > A.7505: . ack 28673
Paxson, Editor [Page 27]
ID Known TCP Implementation Problems August 1998
08:22:57.347572 A.7505 > B.7505: . 31233:31745(512) ack 1
08:22:57.347782 A.7505 > B.7505: . 31745:32257(512) ack 1
08:22:57.936393 B.7505 > A.7505: . ack 29185
08:22:57.936864 A.7505 > B.7505: . 32257:32769(512) ack 1
08:22:57.950802 B.7505 > A.7505: . ack 29697 win 4096
08:22:57.951246 A.7505 > B.7505: . 32769:33281(512) ack 1
08:22:58.169422 B.7505 > A.7505: . ack 29697
08:22:58.638222 B.7505 > A.7505: . ack 29697
08:22:58.643312 B.7505 > A.7505: . ack 29697
08:22:58.643669 A.7505 > B.7505: . 29697:30209(512) ack 1
08:22:58.936436 B.7505 > A.7505: . ack 29697
08:22:59.002614 B.7505 > A.7505: . ack 29697
08:22:59.003026 A.7505 > B.7505: . 33281:33793(512) ack 1
08:22:59.682902 B.7505 > A.7505: . ack 33281
08:22:59.683391 A.7505 > B.7505: P 33793:34305(512) ack 1
08:22:59.683748 A.7505 > B.7505: P 34305:34817(512) ack 1
08:22:59.684043 A.7505 > B.7505: P 34817:35329(512) ack 1
08:22:59.684266 A.7505 > B.7505: P 35329:35841(512) ack 1
08:22:59.684567 A.7505 > B.7505: P 35841:36353(512) ack 1
08:22:59.684810 A.7505 > B.7505: P 36353:36865(512) ack 1
08:22:59.685094 A.7505 > B.7505: P 36865:37377(512) ack 1
The first 12 lines of the trace show incoming ACKs clocking out a
window of data segments. At this point in the transfer, cwnd is 7
segments. The next 4 lines of the trace show 3 duplicate ACKs
arriving from the receiver, followed by a retransmission from the
sender. At this point, cwnd is halved (to 3 segments) and
artificially incremented by the three duplicate ACKs that have
arrived, making cwnd 6 segments. The next two lines show 2 more
duplicate ACKs arriving, each of which increases cwnd by 1 segment.
So, after these two duplicate ACKs arrive the cwnd is 8 segments
and the sender has permission to send 1 new segment (since there
are 7 segments outstanding). The next line in the trace shows this
new segment being transmitted. The next packet shown in the trace
is an ACK from host B that covers the first 7 outstanding segments
(all but the segment sent during recovery). This should cause cwnd
to be reduced to 3 segments and 2 segments to be transmitted (since
there is already 1 outstanding segment in the network). However,
as shown by the last 7 lines of the trace, cwnd is not reduced,
causing a line-rate burst of 7 new segments.
Trace file demonstrating correct behavior
The trace would appear identical to the one above, only it would
stop after:
08:22:59.683748 A.7505 > B.7505: P 34305:34817(512) ack 1
Paxson, Editor [Page 28]
ID Known TCP Implementation Problems August 1998
because at this point host A would correctly reduce cwnd after
recovery, allowing only 2 segments to be transmited, rather than
producing a burst of 7 segments.
References
This problem is documented and the performance implications
analyzed in [Brakmo95].
How to detect
Failure of window deflation after loss recovery can be found by
examining sender-side packet traces recorded during periods of
moderate loss (so cwnd can grow large enough to allow for fast
recovery when loss occurs).
How to fix
When this bug is caused by incorrect header prediction, the fix is
to add a predicate to the header prediction test that checks to see
whether cwnd is inflated; if so, the header prediction test fails
and the usual ACK processing occurs, which (in this case) takes
care to deflate the window.
3.9.
Name of Problem
Excessively short keepalive connection timeout
Classification
Reliability
Description
Keep-alive is a mechanism for checking whether an idle connection
is still alive. According to RFC-1122, keepalive should only be
invoked in server applications that might otherwise hang
indefinitely and consume resources unnecessarily if a client
crashes or aborts a connection during a network failure.
RFC-1122 also specifies that if a keep-alive mechanism is
implemented it MUST NOT interpret failure to respond to any
specific probe as a dead connection. The RFC does not specify a
particular mechanism for timing out a connection when no response
is received for keepalive probes. However, if the mechanism does
not allow ample time for recovery from network congestion or delay,
Paxson, Editor [Page 29]
ID Known TCP Implementation Problems August 1998
connections may be timed out unnecessarily.
Significance
In congested networks, can lead to unwarranted termination of
connections.
Implications
It is possible for the network connection between two peer machines
to become congested or to exhibit packet loss at the time that a
keep-alive probe is sent on a connection. If the keep-alive
mechanism does not allow sufficient time before dropping
connections in the face of unacknowledged probes, connections may
be dropped even when both peers of a connection are still alive.
Relevant RFCs
RFC 1122 specifies that the keep-alive mechanism may be provided.
It does not specify a mechanism for determining dead connections
when keepalive probes are not acknowledged.
Trace file demonstrating it
Made using the Orchestra tool at the peer of the machine using
keep-alive. After connection establishment, incoming keep-alives
were dropped by Orchestra to simulate a dead connection.
22:11:12.040000 A > B: 22666019:0 win 8192 datasz 4 SYN
22:11:12.060000 B > A: 2496001:22666020 win 4096 datasz 4 SYN ACK
22:11:12.130000 A > B: 22666020:2496002 win 8760 datasz 0 ACK
(more than two hours elapse)
00:23:00.680000 A > B: 22666019:2496002 win 8760 datasz 1 ACK
00:23:01.770000 A > B: 22666019:2496002 win 8760 datasz 1 ACK
00:23:02.870000 A > B: 22666019:2496002 win 8760 datasz 1 ACK
00:23.03.970000 A > B: 22666019:2496002 win 8760 datasz 1 ACK
00:23.05.070000 A > B: 22666019:2496002 win 8760 datasz 1 ACK
The initial three packets are the SYN exchange for connection
setup. About two hours later, the keepalive timer fires because
the connection has been idle. Keepalive probes are transmitted a
total of 5 times, with a 1 second spacing between probes, after
which the connection is dropped. This is problematic because a 5
second network outage at the time of the first probe results in the
connection being killed.
Trace file demonstrating correct behavior
Paxson, Editor [Page 30]
ID Known TCP Implementation Problems August 1998
Made using the Orchestra tool at the peer of the machine using
keep-alive. After connection establishment, incoming keep-alives
were dropped by Orchestra to simulate a dead connection.
16:01:52.130000 A > B: 1804412929:0 win 4096 datasz 4 SYN
16:01:52.360000 B > A: 16512001:1804412930 win 4096 datasz 4 SYN ACK
16:01:52.410000 A > B: 1804412930:16512002 win 4096 datasz 0 ACK
(two hours elapse)
18:01:57.170000 A > B: 1804412929:16512002 win 4096 datasz 0 ACK
18:03:12.220000 A > B: 1804412929:16512002 win 4096 datasz 0 ACK
18:04:27.270000 A > B: 1804412929:16512002 win 4096 datasz 0 ACK
18:05:42.320000 A > B: 1804412929:16512002 win 4096 datasz 0 ACK
18:06:57.370000 A > B: 1804412929:16512002 win 4096 datasz 0 ACK
18:08:12.420000 A > B: 1804412929:16512002 win 4096 datasz 0 ACK
18:09:27.480000 A > B: 1804412929:16512002 win 4096 datasz 0 ACK
18:10:43.290000 A > B: 1804412929:16512002 win 4096 datasz 0 ACK
18:11:57.580000 A > B: 1804412929:16512002 win 4096 datasz 0 ACK
18:13:12.630000 A > B: 1804412929:16512002 win 4096 datasz 0 RST ACK
In this trace, when the keep-alive timer expires, 9 keepalive
probes are sent at 75 second intervals. 75 seconds after the last
probe is sent, a final RST segment is sent indicating that the
connection has been closed. This implementation waits about 11
minutes before timing out the connection, while the first
implementation shown allows only 5 seconds.
References
This problem is documented in [Dawson97].
How to detect
For implementations manifesting this problem, it shows up on a
packet trace after the keepalive timer fires if the peer machine
receiving the keepalive does not respond. Usually the keepalive
timer will fire at least two hours after keepalive is turned on,
but it may be sooner if the timer value has been configured lower,
or if the keepalive mechanism violates the specification (see
Insufficient interval between keepalives problem). In this
example, suppressing the response of the peer to keepalive probes
was accomplished using the Orchestra toolkit, which can be
configured to drop packets. It could also have been done by
creating a connection, turning on keepalive, and disconnecting the
network connection at the receiver machine.
How to fix
This problem can be fixed by using a different method for timing
Paxson, Editor [Page 31]
ID Known TCP Implementation Problems August 1998
out keepalives that allows a longer period of time to elapse before
dropping the connection. For example, the algorithm for timing out
on dropped data could be used. Another possibility is an algorithm
such as the one shown in the trace above, which sends 9 probes at
75 second intervals and then waits an additional 75 seconds for a
response before closing the connection.
3.10.
Name of Problem
Failure to back off retransmission timeout
Classification
Congestion control / reliability
Description
The retransmission timeout is used to determine when a packet has
been dropped in the network. When this timeout has expired without
the arrival of an ACK, the segment is retransmitted. Each time a
segment is retransmitted, the timeout is adjusted according to an
exponential backoff algorithm, doubling each time. If a TCP fails
to receive an ACK after numerous attempts at retransmitting the
same segment, it terminates the connection. A TCP that fails to
double its retransmission timeout upon repeated timeouts is said to
exhibit "Failure to back off retransmission timeout".
Significance
Backing off the retransmission timer is a cornerstone of network
stability in the presence of congestion. Consequently, this bug
can have severe adverse affects in congested networks. It also
affects TCP reliability in congested networks, as discussed in the
next section.
Implications
It is possible for the network connection between two TCP peers to
become congested or to exhibit packet loss at the time that a
retransmission is sent on a connection. If the retransmission
mechanism does not allow sufficient time before dropping
connections in the face of unacknowledged segments, connections may
be dropped even when, by waiting longer, the connection could have
continued.
Paxson, Editor [Page 32]
ID Known TCP Implementation Problems August 1998
Relevant RFCs
RFC 1122 specifies mandatory exponential backoff of the
retransmission timeout, and the termination of connections after
some period of time (at least 100 seconds).
Trace file demonstrating it
Made using tcpdump on an intermediate host:
16:51:12.671727 A > B: S 510878852:510878852(0) win 16384
16:51:12.672479 B > A: S 2392143687:2392143687(0) ack 510878853 win 16384
16:51:12.672581 A > B: . ack 1 win 16384
16:51:15.244171 A > B: P 1:3(2) ack 1 win 16384
16:51:15.244933 B > A: . ack 3 win 17518 (DF)
<receiving host disconnected>
16:51:19.381176 A > B: P 3:5(2) ack 1 win 16384
16:51:20.162016 A > B: P 3:5(2) ack 1 win 16384
16:51:21.161936 A > B: P 3:5(2) ack 1 win 16384
16:51:22.161914 A > B: P 3:5(2) ack 1 win 16384
16:51:23.161914 A > B: P 3:5(2) ack 1 win 16384
16:51:24.161879 A > B: P 3:5(2) ack 1 win 16384
16:51:25.161857 A > B: P 3:5(2) ack 1 win 16384
16:51:26.161836 A > B: P 3:5(2) ack 1 win 16384
16:51:27.161814 A > B: P 3:5(2) ack 1 win 16384
16:51:28.161791 A > B: P 3:5(2) ack 1 win 16384
16:51:29.161769 A > B: P 3:5(2) ack 1 win 16384
16:51:30.161750 A > B: P 3:5(2) ack 1 win 16384
16:51:31.161727 A > B: P 3:5(2) ack 1 win 16384
16:51:32.161701 A > B: R 5:5(0) ack 1 win 16384
The initial three packets are the SYN exchange for connection
setup, then a single data packet, to verify that data can be
transferred. Then the connection to the destination host was
disconnected, and more data sent. Retransmissions occur every
second for 12 seconds, and then the connection is terminated with a
RST. This is problematic because a 12 second pause in connectivity
could result in the termination of a connection.
Trace file demonstrating correct behavior
Again, a tcpdump taken from a third host:
16:59:05.398301 A > B: S 2503324757:2503324757(0) win 16384
16:59:05.399673 B > A: S 2492674648:2492674648(0) ack 2503324758 win 16384
16:59:05.399866 A > B: . ack 1 win 17520
Paxson, Editor [Page 33]
ID Known TCP Implementation Problems August 1998
16:59:06.538107 A > B: P 1:3(2) ack 1 win 17520
16:59:06.540977 B > A: . ack 3 win 17518 (DF)
<receiving host disconnected>
16:59:13.121542 A > B: P 3:5(2) ack 1 win 17520
16:59:14.010928 A > B: P 3:5(2) ack 1 win 17520
16:59:16.010979 A > B: P 3:5(2) ack 1 win 17520
16:59:20.011229 A > B: P 3:5(2) ack 1 win 17520
16:59:28.011896 A > B: P 3:5(2) ack 1 win 17520
16:59:44.013200 A > B: P 3:5(2) ack 1 win 17520
17:00:16.015766 A > B: P 3:5(2) ack 1 win 17520
17:01:20.021308 A > B: P 3:5(2) ack 1 win 17520
17:02:24.027752 A > B: P 3:5(2) ack 1 win 17520
17:03:28.034569 A > B: P 3:5(2) ack 1 win 17520
17:04:32.041567 A > B: P 3:5(2) ack 1 win 17520
17:05:36.048264 A > B: P 3:5(2) ack 1 win 17520
17:06:40.054900 A > B: P 3:5(2) ack 1 win 17520
17:07:44.061306 A > B: R 5:5(0) ack 1 win 17520
In this trace, when the retransmission timer expires, 12
retransmissions are sent at exponentially-increasing intervals,
until the interval value reaches 64 seconds, at which time the
interval stops growing. 64 seconds after the last retransmission,
a final RST segment is sent indicating that the connection has been
closed. This implementation waits about 9 minutes before timing
out the connection, while the first implementation shown allows
only 12 seconds.
References
None known.
How to detect
A simple transfer can be eaily interrupted by disconnecting the
receiving host from the network. tcpdump or another appropriate
tool should show the retransmissions being sent. Several trials in
a low-rtt environment may be required to demonstrate the bug.
How to fix
For one of the implementations studied, this problem seemed to be
the result of an error introduced with the addition of the Brakmo-
Peterson RTO algorithm [Brakmo95], which can return a value of zero
where the older Jacobson algorithm would always have a minimum
value of three. Brakmo and Peterson specified an additional step
Paxson, Editor [Page 34]
ID Known TCP Implementation Problems August 1998
of min(rtt + 2, RTO) to avoid problems with this. Unfortunately,
in the implementation this step was omitted when calculating the
exponential backoff for the RTO. This results in an RTO of 0
seconds being multiplied by the backoff, yielding again zero, and
then being subjected to a later MAX operation that increases it to
1 second, regardless of the backoff factor.
A similar TCP persist failure has the same cause.
3.11.
Name of Problem
Insufficient interval between keepalives
Classification
Reliability
Description
Keep-alive is a mechanism for checking whether an idle connection
is still alive. According to RFC-1122, keep-alive may be included
in an implementation. If it is included, the interval between
keep-alive packets MUST be configurable, and MUST default to no
less than two hours.
Significance
In congested networks, can lead to unwarranted termination of
connections.
Implications
According to RFC-1122, keep-alive is not required of
implementations because it could: (1) cause perfectly good
connections to break during transient Internet failures; (2)
consume unnecessary bandwidth ("if no one is using the connection,
who cares if it is still good?"); and (3) cost money for an
Internet path that charges for packets. Regarding this last point,
we note that in addition the presence of dial-on-demand links in
the route can greatly magnify the cost penalty of excess
keepalives, potentially forcing a full-time connection on a link
that would otherwise only be connected a few minutes a day.
If keepalive is provided the RFC states that the required inter-
keepalive distance MUST default to no less than two hours. If it
does not, the probability of connections breaking increases, the
Paxson, Editor [Page 35]
ID Known TCP Implementation Problems August 1998
bandwidth used due to keepalives increases, and cost increases over
paths which charge per packet.
Relevant RFCs
RFC 1122 specifies that the keep-alive mechanism may be provided.
It also specifies the two hour minimum for the default interval
between keepalive probes.
Trace file demonstrating it
Made using the Orchestra tool at the peer of the machine using
keep-alive. Machine A was configured to use default settings for
the keepalive timer.
11:36:32.910000 A > B: 3288354305:0 win 28672 datasz 4 SYN
11:36:32.930000 B > A: 896001:3288354306 win 4096 datasz 4 SYN ACK
11:36:32.950000 A > B: 3288354306:896002 win 28672 datasz 0 ACK
11:50:01.190000 A > B: 3288354305:896002 win 28672 datasz 0 ACK
11:50:01.210000 B > A: 896002:3288354306 win 4096 datasz 0 ACK
12:03:29.410000 A > B: 3288354305:896002 win 28672 datasz 0 ACK
12:03:29.430000 B > A: 896002:3288354306 win 4096 datasz 0 ACK
12:16:57.630000 A > B: 3288354305:896002 win 28672 datasz 0 ACK
12:16:57.650000 B > A: 896002:3288354306 win 4096 datasz 0 ACK
12:30:25.850000 A > B: 3288354305:896002 win 28672 datasz 0 ACK
12:30:25.870000 B > A: 896002:3288354306 win 4096 datasz 0 ACK
12:43:54.070000 A > B: 3288354305:896002 win 28672 datasz 0 ACK
12:43:54.090000 B > A: 896002:3288354306 win 4096 datasz 0 ACK
The initial three packets are the SYN exchange for connection
setup. About 13 minutes later, the keepalive timer fires because
the connection is idle. The keepalive is acknowledged, and the
timer fires again in about 13 more minutes. This behavior
continues indefinitely until the connection is closed, and is a
violation of the specification.
Trace file demonstrating correct behavior
Made using the Orchestra tool at the peer of the machine using
keep-alive. Machine A was configured to use default settings for
the keepalive timer.
17:37:20.500000 A > B: 34155521:0 win 4096 datasz 4 SYN
Paxson, Editor [Page 36]
ID Known TCP Implementation Problems August 1998
17:37:20.520000 B > A: 6272001:34155522 win 4096 datasz 4 SYN ACK
17:37:20.540000 A > B: 34155522:6272002 win 4096 datasz 0 ACK
19:37:25.430000 A > B: 34155521:6272002 win 4096 datasz 0 ACK
19:37:25.450000 B > A: 6272002:34155522 win 4096 datasz 0 ACK
21:37:30.560000 A > B: 34155521:6272002 win 4096 datasz 0 ACK
21:37:30.570000 B > A: 6272002:34155522 win 4096 datasz 0 ACK
23:37:35.580000 A > B: 34155521:6272002 win 4096 datasz 0 ACK
23:37:35.600000 B > A: 6272002:34155522 win 4096 datasz 0 ACK
01:37:40.620000 A > B: 34155521:6272002 win 4096 datasz 0 ACK
01:37:40.640000 B > A: 6272002:34155522 win 4096 datasz 0 ACK
03:37:45.590000 A > B: 34155521:6272002 win 4096 datasz 0 ACK
03:37:45.610000 B > A: 6272002:34155522 win 4096 datasz 0 ACK
The initial three packets are the SYN exchange for connection
setup. Just over two hours later, the keepalive timer fires
because the connection is idle. The keepalive is acknowledged, and
the timer fires again just over two hours later. This behavior
continues indefinitely until the connection is closed.
References
This problem is documented in [Dawson97].
How to detect
For implementations manifesting this problem, it shows up on a
packet trace. If the connection is left idle, the keepalive probes
will arrive closer together than the two hour minimum.
3.12.
Name of Problem
Stretch ACK violation
Classification
Congestion Control/Performance
Description
To improve efficiency (both computer and network) a data receiver
may refrain from sending an ACK for each incoming segment,
Paxson, Editor [Page 37]
ID Known TCP Implementation Problems August 1998
according to [RFC1122]. However, an ACK should not be delayed an
inordinate amount of time. Specifically, ACKs MUST be sent for
every second full-sized segment that arrives. If a second full-
sized segment does not arrive within a given timeout (of no more
than 0.5 seconds), an ACK must be transmitted, according to
[RFC1122]. A TCP receiver which does not generate an ACK for every
second full-sized segment exhibits a "Stretch ACK Violation".
Significance
TCP receivers exhibiting this behavior will cause TCP senders to
generate burstier traffic, which can degrade performance in
congested environments. In addition, generating fewer ACKs
increases the amount of time needed by the slow start algorithm to
open the congestion window to an appropriate point, which
diminishes performance in environments with large bandwidth-delay
products. Finally, generating fewer ACKs may cause needless
retransmission timeouts in lossy environments, as it increases the
possibility that an entire window of ACKs is lost, forcing a
retransmission timeout.
Implications
When not in loss recovery, every ACK received by a TCP sender
triggers the transmission of new data segments. The burst size is
determined by the number of previously unacknowledged segments each
ACK covers. Therefore, a TCP receiver ACKing more than 2 segments
at a time causes the sending TCP to generate a larger burst of
traffic upon receipt of the ACK. This large burst of traffic can
overwhelm an intervening gateway, leading to higher drop rates for
both the connection and other connections passing through the
congested gateway.
In addition, the TCP slow start algorithm increases the congestion
window by 1 segment for each ACK received. Therefore, increasing
the ACK interval (thus decreasing the rate at which ACKs are
transmitted) increases the amount of time it takes slow start to
increase the congestion window to an appropriate operating point,
and the connection consequently suffers from reduced performance.
This is especially true for connections using large windows.
Relevant RFCs
RFC 1122 outlines delayed ACKs as a recommended mechanism.
Trace file demonstrating it
Trace file taken using tcpdump at host B, the data receiver (and
Paxson, Editor [Page 38]
ID Known TCP Implementation Problems August 1998
ACK originator). The advertised window (which never changed) and
timestamp options have been omitted for clarity, except for the
first packet sent by A:
12:09:24.820187 A.1174 > B.3999: . 2049:3497(1448) ack 1
win 33580 <nop,nop,timestamp 2249877 2249914> [tos 0x8]
12:09:24.824147 A.1174 > B.3999: . 3497:4945(1448) ack 1
12:09:24.832034 A.1174 > B.3999: . 4945:6393(1448) ack 1
12:09:24.832222 B.3999 > A.1174: . ack 6393
12:09:24.934837 A.1174 > B.3999: . 6393:7841(1448) ack 1
12:09:24.942721 A.1174 > B.3999: . 7841:9289(1448) ack 1
12:09:24.950605 A.1174 > B.3999: . 9289:10737(1448) ack 1
12:09:24.950797 B.3999 > A.1174: . ack 10737
12:09:24.958488 A.1174 > B.3999: . 10737:12185(1448) ack 1
12:09:25.052330 A.1174 > B.3999: . 12185:13633(1448) ack 1
12:09:25.060216 A.1174 > B.3999: . 13633:15081(1448) ack 1
12:09:25.060405 B.3999 > A.1174: . ack 15081
This portion of the trace clearly shows that the receiver (host B)
sends an ACK for every third full sized packet received. Further
investigation of this implementation found that the cause of the
increased ACK interval was the TCP options being used. The
implementation sent an ACK after it was holding 2*MSS worth of
unacknowledged data. In the above case, the MSS is 1460 bytes so
the receiver transmits an ACK after it is holding at least 2920
bytes of unacknowledged data. However, the length of the TCP
options being used [RFC1323] took 12 bytes away from the data
portion of each packet. This produced packets containing 1448
bytes of data. But the additional bytes used by the options in the
header were not taken into account when determining when to trigger
an ACK. Therefore, it took 3 data segments before the data
receiver was holding enough unacknowledged data (>= 2*MSS, or 2920
bytes in the above example) to transmit an ACK.
Trace file demonstrating correct behavior
Trace file taken using tcpdump at host B, the data receiver (and
ACK originator), again with window and timestamp information
omitted except for the first packet:
12:06:53.627320 A.1172 > B.3999: . 1449:2897(1448) ack 1
win 33580 <nop,nop,timestamp 2249575 2249612> [tos 0x8]
12:06:53.634773 A.1172 > B.3999: . 2897:4345(1448) ack 1
12:06:53.634961 B.3999 > A.1172: . ack 4345
12:06:53.737326 A.1172 > B.3999: . 4345:5793(1448) ack 1
12:06:53.744401 A.1172 > B.3999: . 5793:7241(1448) ack 1
12:06:53.744592 B.3999 > A.1172: . ack 7241
Paxson, Editor [Page 39]
ID Known TCP Implementation Problems August 1998
12:06:53.752287 A.1172 > B.3999: . 7241:8689(1448) ack 1
12:06:53.847332 A.1172 > B.3999: . 8689:10137(1448) ack 1
12:06:53.847525 B.3999 > A.1172: . ack 10137
This trace shows the TCP receiver (host B) ack'ing every second
full-sized packet, according to [RFC1122]. This is the same
implementation shown above, with slight modifications that allow
the receiver to take the length of the options into account when
deciding when to transmit an ACK.
References
This problem is documented in [Allman97] and [Paxson97].
How to detect
Stretch ACK violations show up immediately in receiver-side packet
traces of bulk transfers, as shown above. However, packet traces
made on the sender side of the TCP connection may lead to
ambiguities when diagnosing this problem due to the possibility of
lost ACKs.
3.13.
Name of Problem
Retransmission sends multiple packets
Classification
Congestion control
Description
When a TCP retransmits a segment due to a timeout expiration or
beginning a fast retransmission sequence, it should only transmit a
single segment. A TCP that transmits more than one segment
exhibits "Retransmission Sends Multiple Packets".
Instances of this problem have been known to occur due to
miscomputations involving the use of TCP options. TCP options
increase the TCP header beyond its usual size of 20 bytes. The
total size of header must be taken into account when retransmitting
a packet. If a TCP sender does not account for the length of the
TCP options when determining how much data to retransmit, it will
send too much data to fit into a single packet. In this case, the
correct retransmission will be followed by a short segment
(tinygram) containing data that may not need to be retransmitted.
A specific case is a TCP using the RFC 1323 timestamp option, which
Paxson, Editor [Page 40]
ID Known TCP Implementation Problems August 1998
adds 12 bytes to the standard 20-byte TCP header. On
retransmission of a packet, the 12 byte option is incorrectly
interpreted as part of the data portion of the segment. A standard
TCP header and a new 12-byte option is added to the data, which
yields a transmission of 12 bytes more data than contained in the
original segment. This overflow causes a smaller packet, with 12
data bytes, to be transmitted.
Significance
This problem is somewhat serious for congested environments because
the TCP implementation injects more packets into the network than
is appropriate. However, since a tinygram is only sent in response
to a fast retransmit or a timeout, it does not effect the sustained
sending rate.
Implications
A TCP exhibiting this behavior is stressing the network with more
traffic than appropriate, and stressing routers by increasing the
number of packets they must process. The redundant tinygram will
also elicit a duplicate ack from the receiver, resulting in yet
another unnecessary transmission.
Relevant RFCs
RFC 1122 requires use of slow start after loss; RFC 2001 explicates
slow start; RFC 1323 describes the timestamp option that has been
observed to lead to some implementations exhibiting this problem.
Trace file demonstrating it
Made using tcpdump/BPF recording at a machine on the same subnet as
Host A. Host A is the sender and Host B is the receiver. The
advertised window and timestamp options have been omitted for
clarity, except for the first segment sent by host A. In addition,
portions of the trace file not pertaining to the packet in question
have been removed (missing packets are denoted by ``[...]'' in the
trace).
11:55:22.701668 A > B: . 7361:7821(460) ack 1
win 49324 <nop,nop,timestamp 3485348 3485113>
11:55:22.702109 A > B: . 7821:8281(460) ack 1
[...]
11:55:23.112405 B > A: . ack 7821
11:55:23.113069 A > B: . 12421:12881(460) ack 1
Paxson, Editor [Page 41]
ID Known TCP Implementation Problems August 1998
11:55:23.113511 A > B: . 12881:13341(460) ack 1
11:55:23.333077 B > A: . ack 7821
11:55:23.336860 B > A: . ack 7821
11:55:23.340638 B > A: . ack 7821
11:55:23.341290 A > B: . 7821:8281(460) ack 1
11:55:23.341317 A > B: . 8281:8293(12) ack 1
11:55:23.498242 B > A: . ack 7821
11:55:23.506850 B > A: . ack 7821
11:55:23.510630 B > A: . ack 7821
[...]
11:55:23.746649 B > A: . ack 10581
The second line of the above trace shows the original transmission
of a segment which is later dropped. After 3 duplicate ACKs, line
9 of the trace shows the dropped packet (7821:8281), with a 460-
byte payload, being retransmitted. Immediately following this
retransmission, a packet with a 12-byte payload is unnecessarily
sent.
Trace file demonstrating correct behavior
The trace file would be identical to the one above, with a single
line:
11:55:23.341317 A > B: . 8281:8293(12) ack 1
omitted.
References
[Brakmo95]
How to detect
This problem can be detected by examining a packet trace of the TCP
connections of a machine using TCP options, during which a packet
is retransmitted.
3.14.
Name of Problem
Failure to send FIN notification promptly
Paxson, Editor [Page 42]
ID Known TCP Implementation Problems August 1998
Classification
Performance
Description
When an application closes a connection, the corresponding TCP
should send the FIN notification promptly to its peer (unless
prevented by the congestion window). If a TCP implementation
delays in sending the FIN notification, for example due to waiting
until unacknowledged data has been acknowledged, then it is said to
exhibit "Failure to send FIN notification promptly".
Also, while not strictly required, FIN segments should include the
PSH flag to ensure expedited delivery of any pending data at the
receiver.
Significance
The greatest impact occurs for short-lived connections, since for
these the additional time required to close the connection
introduces the greatest relative delay.
The additional time can be significant in the common case of the
sender waiting for an ACK that is delayed by the receiver.
Implications
Can diminish total throughput as seen at the application layer,
because connection termination takes longer to complete.
Relevant RFCs
RFC 793 indicates that a receiver should treat an incoming FIN flag
as implying the push function.
Trace file demonstrating it
Made using tcpdump (no losses reported).
10:04:38.68 A > B: S 1031850376:1031850376(0) win 4096
<mss 1460,wscale 0,eol> (DF)
10:04:38.71 B > A: S 596916473:596916473(0) ack 1031850377
win 8760 <mss 1460> (DF)
10:04:38.73 A > B: . ack 1 win 4096 (DF)
10:04:41.98 A > B: P 1:4(3) ack 1 win 4096 (DF)
10:04:42.15 B > A: . ack 4 win 8757 (DF)
10:04:42.23 A > B: P 4:7(3) ack 1 win 4096 (DF)
10:04:42.25 B > A: P 1:11(10) ack 7 win 8754 (DF)
Paxson, Editor [Page 43]
ID Known TCP Implementation Problems August 1998
10:04:42.32 A > B: . ack 11 win 4096 (DF)
10:04:42.33 B > A: P 11:51(40) ack 7 win 8754 (DF)
10:04:42.51 A > B: . ack 51 win 4096 (DF)
10:04:42.53 B > A: F 51:51(0) ack 7 win 8754 (DF)
10:04:42.56 A > B: FP 7:7(0) ack 52 win 4096 (DF)
10:04:42.58 B > A: . ack 8 win 8754 (DF)
Machine B in the trace above does not send out a FIN notification
promptly if there is any data outstanding. It instead waits for
all unacknowledged data to be acknowledged before sending the FIN
segment. The connection was closed at 10:04.42.33 after requesting
40 bytes to be sent. However, the FIN notification isn't sent
until 10:04.42.51, after the (delayed) acknowledgement of the 40
bytes of data.
Trace file demonstrating correct behavior
Made using tcpdump (no losses reported).
10:27:53.85 C > D: S 419744533:419744533(0) win 4096
<mss 1460,wscale 0,eol> (DF)
10:27:53.92 D > C: S 10082297:10082297(0) ack 419744534
win 8760 <mss 1460> (DF)
10:27:53.95 C > D: . ack 1 win 4096 (DF)
10:27:54.42 C > D: P 1:4(3) ack 1 win 4096 (DF)
10:27:54.62 D > C: . ack 4 win 8757 (DF)
10:27:54.76 C > D: P 4:7(3) ack 1 win 4096 (DF)
10:27:54.89 D > C: P 1:11(10) ack 7 win 8754 (DF)
10:27:54.90 D > C: FP 11:51(40) ack7 win 8754 (DF)
10:27:54.92 C > D: . ack 52 win 4096 (DF)
10:27:55.01 C > D: FP 7:7(0) ack 52 win 4096 (DF)
10:27:55.09 D > C: . ack 8 win 8754 (DF)
Here, Machine D sends a FIN with 40 bytes of data even before the
original 10 octets have been acknowledged. This is correct behavior
as it provides for the highest performance.
References
This problem is documented in [Dawson97].
How to detect
For implementations manifesting this problem, it shows up on a
packet trace.
Paxson, Editor [Page 44]
ID Known TCP Implementation Problems August 1998
3.15.
Name of Problem
Failure to send a RST after Half Duplex Close
Classification
Resource management
Description
RFC 1122 4.2.2.13 states that a TCP SHOULD send a RST if data is
received after "half duplex close", i.e. if it cannot be delivered
to the application. A TCP that fails to do so is said to exhibit
"Failure to send a RST after Half Duplex Close".
Significance
Potentially serious for TCP endpoints that manage large numbers of
connections, due to exhaustion of memory and/or process slots
available for managing connection state.
Implications
Failure to send the RST can lead to permanently hung TCP
connections. This problem has been demonstrated when HTTP clients
abort connections, common when users move on to a new page before
the current page has finished downloading. The HTTP client closes
by transmitting a FIN while the server is transmitting images,
text, etc. The server TCP receives the FIN, but its application
does not close the connection until all data has been queued for
transmission. Since the server will not transmit a FIN until all
the preceding data has been transmitted, deadlock results if the
client TCP does not consume the pending data or tear down the
connection: the window decreases to zero, since the client cannot
pass the data to the application, and the server sends probe
segments. The client acknowledges the probe segments with a zero
window. As mandated in RFC1122 4.2.2.17, the probe segments are
transmitted forever. Server connection state remains in
CLOSE_WAIT, and eventually server processes are exhausted.
Note that there are two bugs. First, probe segments should be
ignored if the window can never subsequently increase. Second, a
RST should be sent when data is received after half duplex close.
Fixing the first bug, but not the second, results in the probe
segments eventually timing out the connection, but the server
remains in CLOSE_WAIT for a significant and unnecessary period.
Paxson, Editor [Page 45]
ID Known TCP Implementation Problems August 1998
Relevant RFCs
RFC 1122 sections 4.2.2.13 and 4.2.2.17.
Trace file demonstrating it
Made using an unknown network analyzer. No drop information
available.
client.1391 > server.8080: S 0:1(0) ack: 0 win: 2000 <mss: 5b4>
server.8080 > client.1391: SA 8c01:8c02(0) ack: 1 win: 8000 <mss:100>
client.1391 > server.8080: PA
client.1391 > server.8080: PA 1:1c2(1c1) ack: 8c02 win: 2000
server.8080 > client.1391: [DF] PA 8c02:8cde(dc) ack: 1c2 win: 8000
server.8080 > client.1391: [DF] A 8cde:9292(5b4) ack: 1c2 win: 8000
server.8080 > client.1391: [DF] A 9292:9846(5b4) ack: 1c2 win: 8000
server.8080 > client.1391: [DF] A 9846:9dfa(5b4) ack: 1c2 win: 8000
client.1391 > server.8080: PA
server.8080 > client.1391: [DF] A 9dfa:a3ae(5b4) ack: 1c2 win: 8000
server.8080 > client.1391: [DF] A a3ae:a962(5b4) ack: 1c2 win: 8000
server.8080 > client.1391: [DF] A a962:af16(5b4) ack: 1c2 win: 8000
server.8080 > client.1391: [DF] A af16:b4ca(5b4) ack: 1c2 win: 8000
client.1391 > server.8080: PA
server.8080 > client.1391: [DF] A b4ca:ba7e(5b4) ack: 1c2 win: 8000
server.8080 > client.1391: [DF] A b4ca:ba7e(5b4) ack: 1c2 win: 8000
client.1391 > server.8080: PA
server.8080 > client.1391: [DF] A ba7e:bdfa(37c) ack: 1c2 win: 8000
client.1391 > server.8080: PA
server.8080 > client.1391: [DF] A bdfa:bdfb(1) ack: 1c2 win: 8000
client.1391 > server.8080: PA
[ HTTP client aborts and enters FIN_WAIT_1 ]
client.1391 > server.8080: FPA
[ server ACKs the FIN and enters CLOSE_WAIT ]
server.8080 > client.1391: [DF] A
[ client enters FIN_WAIT_2 ]
server.8080 > client.1391: [DF] A bdfa:bdfb(1) ack: 1c3 win: 8000
[ server continues to try to send its data ]
client.1391 > server.8080: PA < window = 0 >
server.8080 > client.1391: [DF] A bdfa:bdfb(1) ack: 1c3 win: 8000
client.1391 > server.8080: PA < window = 0 >
server.8080 > client.1391: [DF] A bdfa:bdfb(1) ack: 1c3 win: 8000
Paxson, Editor [Page 46]
ID Known TCP Implementation Problems August 1998
client.1391 > server.8080: PA < window = 0 >
server.8080 > client.1391: [DF] A bdfa:bdfb(1) ack: 1c3 win: 8000
client.1391 > server.8080: PA < window = 0 >
server.8080 > client.1391: [DF] A bdfa:bdfb(1) ack: 1c3 win: 8000
client.1391 > server.8080: PA < window = 0 >
[ ... repeat ad exhaustium ... ]
Trace file demonstrating correct behavior
Made using an unknown network analyzer. No drop information
available.
client > server D=80 S=59500 Syn Seq=337 Len=0 Win=8760
server > client D=59500 S=80 Syn Ack=338 Seq=80153 Len=0 Win=8760
client > server D=80 S=59500 Ack=80154 Seq=338 Len=0 Win=8760
[ ... normal data omitted ... ]
client > server D=80 S=59500 Ack=14559 Seq=596 Len=0 Win=8760
server > client D=59500 S=80 Ack=596 Seq=114559 Len=1460 Win=8760
[ client closes connection ]
client > server D=80 S=59500 Fin Seq=596 Len=0 Win=8760
server > client D=59500 S=80 Ack=597 Seq=116019 Len=1460 Win=8760
[ client sends RST (RFC1122 4.2.2.13) ]
client > server D=80 S=59500 Rst Seq=597 Len=0 Win=0
server > client D=59500 S=80 Ack=597 Seq=117479 Len=1460 Win=8760
client > server D=80 S=59500 Rst Seq=597 Len=0 Win=0
server > client D=59500 S=80 Ack=597 Seq=118939 Len=1460 Win=8760
client > server D=80 S=59500 Rst Seq=597 Len=0 Win=0
server > client D=59500 S=80 Ack=597 Seq=120399 Len=892 Win=8760
client > server D=80 S=59500 Rst Seq=597 Len=0 Win=0
server > client D=59500 S=80 Ack=597 Seq=121291 Len=1460 Win=8760
client > server D=80 S=59500 Rst Seq=597 Len=0 Win=0
"client" sends a number of RSTs, one in response to each incoming
packet from "server". One might wonder why "server" keeps sending
data packets after it has received a RST from "client"; the
explanation is that "server" had already transmitted all five of
the data packets before receiving the first RST from "client", so
it is too late to avoid transmitting them.
Paxson, Editor [Page 47]
ID Known TCP Implementation Problems August 1998
How to detect
The problem can be detected by inspecting packet traces of a large,
interrupted bulk transfer.
3.16.
Name of Problem
Failure to RST on close with data pending
Classification
Resource management
Description
When an application closes a connection in such a way that it can
no longer read any received data, the TCP SHOULD, per section
4.2.2.13 of RFC 1122, send a RST if there is any unread received
data, or if any new data is received. A TCP that fails to do so
exhibits "Failure to RST on close with data pending".
Note that, for some TCPs, this situation can be caused by an
application "crashing" while a peer is sending data.
We have observed a number of TCPs that exhibit this problem. The
problem is less serious if any subsequent data sent to the now-
closed connection endpoint elicits a RST (see illustration below).
Significance
This problem is most significant for endpoints that engage in large
numbers of connections, as their ability to do so will be curtailed
as they leak away resources.
Implications
Failure to reset the connection can lead to permanently hung
connections, in which the remote endpoint takes no further action
to tear down the connection because it is waiting on the local TCP
to first take some action. This is particularly the case if the
local TCP also allows the advertised window to go to zero, and
fails to tear down the connection when the remote TCP engages in
"persist" probes (see example below).
Relevant RFCs
RFC 1122 section 4.2.2.13. Also, 4.2.2.17 for the zero-window
Paxson, Editor [Page 48]
ID Known TCP Implementation Problems August 1998
probing discussion below.
Trace file demonstrating it
Made using tcpdump. No drop information available.
13:11:46.04 A > B: S 458659166:458659166(0) win 4096
<mss 1460,wscale 0,eol> (DF)
13:11:46.04 B > A: S 792320000:792320000(0) ack 458659167
win 4096
13:11:46.04 A > B: . ack 1 win 4096 (DF)
13:11.55.80 A > B: . 1:513(512) ack 1 win 4096 (DF)
13:11.55.80 A > B: . 513:1025(512) ack 1 win 4096 (DF)
13:11:55.83 B > A: . ack 1025 win 3072
13:11.55.84 A > B: . 1025:1537(512) ack 1 win 4096 (DF)
13:11.55.84 A > B: . 1537:2049(512) ack 1 win 4096 (DF)
13:11.55.85 A > B: . 2049:2561(512) ack 1 win 4096 (DF)
13:11:56.03 B > A: . ack 2561 win 1536
13:11.56.05 A > B: . 2561:3073(512) ack 1 win 4096 (DF)
13:11.56.06 A > B: . 3073:3585(512) ack 1 win 4096 (DF)
13:11.56.06 A > B: . 3585:4097(512) ack 1 win 4096 (DF)
13:11:56.23 B > A: . ack 4097 win 0
13:11:58.16 A > B: . 4096:4097(1) ack 1 win 4096 (DF)
13:11:58.16 B > A: . ack 4097 win 0
13:12:00.16 A > B: . 4096:4097(1) ack 1 win 4096 (DF)
13:12:00.16 B > A: . ack 4097 win 0
13:12:02.16 A > B: . 4096:4097(1) ack 1 win 4096 (DF)
13:12:02.16 B > A: . ack 4097 win 0
13:12:05.37 A > B: . 4096:4097(1) ack 1 win 4096 (DF)
13:12:05.37 B > A: . ack 4097 win 0
13:12:06.36 B > A: F 1:1(0) ack 4097 win 0
13:12:06.37 A > B: . ack 2 win 4096 (DF)
13:12:11.78 A > B: . 4096:4097(1) ack 2 win 4096 (DF)
13:12:11.78 B > A: . ack 4097 win 0
13:12:24.59 A > B: . 4096:4097(1) ack 2 win 4096 (DF)
13:12:24.60 B > A: . ack 4097 win 0
13:12:50.22 A > B: . 4096:4097(1) ack 2 win 4096 (DF)
13:12:50.22 B > A: . ack 4097 win 0
Machine B in the trace above does not drop received data when the
socket is "closed" by the application (in this case, the
application process was terminated). This occured at approximately
13:12:06.36 and resulted in the FIN being sent in response to the
close. However, because there is no longer an application to
deliver the data to, the TCP should have instead sent a RST.
Note: Machine A's zero-window probing is also broken. It is
resending old data, rather than new data. Section 3.7 in RFC 793
Paxson, Editor [Page 49]
ID Known TCP Implementation Problems August 1998
and Section 4.2.2.17 in RFC 1122 discuss zero-window probing.
Trace file demonstrating better behavior
Made using tcpdump. No drop information available.
Better, but still not fully correct, behavior, per the discussion
below. We show this behavior because it has been observed for a
number of different TCP implementations.
13:48:29.24 C > D: S 73445554:73445554(0) win 4096
<mss 1460,wscale 0,eol> (DF)
13:48:29.24 D > C: S 36050296:36050296(0) ack 73445555
win 4096 <mss 1460,wscale 0,eol> (DF)
13:48:29.25 C > D: . ack 1 win 4096 (DF)
13:48:30.78 C > D: . 1:1461(1460) ack 1 win 4096 (DF)
13:48:30.79 C > D: . 1461:2921(1460) ack 1 win 4096 (DF)
13:48:30.80 D > C: . ack 2921 win 1176 (DF)
13:48:32.75 C > D: . 2921:4097(1176) ack 1 win 4096 (DF)
13:48:32.82 D > C: . ack 4097 win 0 (DF)
13:48:34.76 C > D: . 4096:4097(1) ack 1 win 4096 (DF)
13:48:34.84 D > C: . ack 4097 win 0 (DF)
13:48:36.34 D > C: FP 1:1(0) ack 4097 win 4096 (DF)
13:48:36.34 C > D: . 4097:5557(1460) ack 2 win 4096 (DF)
13:48:36.34 D > C: R 36050298:36050298(0) win 24576
13:48:36.34 C > D: . 5557:7017(1460) ack 2 win 4096 (DF)
13:48:36.34 D > C: R 36050298:36050298(0) win 24576
In this trace, the application process is terminated on Machine D
at approximately 13:48:36.34. Its TCP sends the FIN with the
window opened again (since it discarded the previously received
data). Machine C promptly sends more data, causing Machine D to
reset the connection since it cannot deliver the data to the
application. Ideally, Machine D SHOULD send a RST instead of
dropping the data and re-opening the receive window.
Note: Machine C's zero-window probing is broken, the same as in the
example above.
Trace file demonstrating correct behavior
Made using tcpdump. No losses reported.
14:12:02.19 E > F: S 1143360000:1143360000(0) win 4096
14:12:02.19 F > E: S 1002988443:1002988443(0) ack 1143360001
win 4096 <mss 1460> (DF)
14:12:02.19 E > F: . ack 1 win 4096
Paxson, Editor [Page 50]
ID Known TCP Implementation Problems August 1998
14:12:10.43 E > F: . 1:513(512) ack 1 win 4096
14:12:10.61 F > E: . ack 513 win 3584 (DF)
14:12:10.61 E > F: . 513:1025(512) ack 1 win 4096
14:12:10.61 E > F: . 1025:1537(512) ack 1 win 4096
14:12:10.81 F > E: . ack 1537 win 2560 (DF)
14:12:10.81 E > F: . 1537:2049(512) ack 1 win 4096
14:12:10.81 E > F: . 2049:2561(512) ack 1 win 4096
14:12:10.81 E > F: . 2561:3073(512) ack 1 win 4096
14:12:11.01 F > E: . ack 3073 win 1024 (DF)
14:12:11.01 E > F: . 3073:3585(512) ack 1 win 4096
14:12:11.01 E > F: . 3585:4097(512) ack 1 win 4096
14:12:11.21 F > E: . ack 4097 win 0 (DF)
14:12:15.88 E > F: . 4097:4098(1) ack 1 win 4096
14:12:16.06 F > E: . ack 4097 win 0 (DF)
14:12:20.88 E > F: . 4097:4098(1) ack 1 win 4096
14:12:20.91 F > E: . ack 4097 win 0 (DF)
14:12:21.94 F > E: R 1002988444:1002988444(0) win 4096
When the application terminates at 14:12:21.94, F immediately sends
a RST.
Note: Machine E's zero-window probing is (finally) correct.
How to detect
The problem can often be detected by inspecting packet traces of a
transfer in which the receiving application terminates abnormally.
When doing so, there can be an ambiguity (if only looking at the
trace) as to whether the receiving TCP did indeed have unread data
that it could now no longer deliver. To provoke this to happen, it
may help to suspend the receiving application so that it fails to
consume any data, eventually exhausting the advertised window. At
this point, since the advertised window is zero, we know that the
receiving TCP has undelivered data buffered up. Terminating the
application process then should suffice to test the correctness of
the TCP's behavior.
3.17.
Name of Problem
Options missing from TCP MSS calculation
Classification
Reliability / performance
Paxson, Editor [Page 51]
ID Known TCP Implementation Problems August 1998
Description
When a TCP determines how much data to send per packet, it
calculates a segment size based on the MTU of the path. It must
then subtract from that MTU the size of the IP and TCP headers in
the packet. If IP options and TCP options are not taken into
account correctly in this calculation, the resulting segment size
may be too large. TCPs that do so are said to exhibit "Options
missing from TCP MSS calculation".
Significance
In some implementations, this causes the transmission of strangely
fragmented packets. In some implementations with Path MTU (PMTU)
discovery [RFC1191], this problem can actually result in a total
failure to transmit any data at all, regardless of the environment
(see below).
Arguably, especially since the wide deployment of firewalls, IP
options appear only rarely in normal operations.
Implications
In implementations using PMTU discovery, this problem can result in
packets that are too large for the output interface, and that have
the DF (don't fragment) bit set in the IP header. Thus, the IP
layer on the local machine is not allowed to fragment the packet to
send it out the interface. It instead informs the TCP layer of the
correct MTU size of the interface; the TCP layer again miscomputes
the MSS by failing to take into account the size of IP options; and
the problem repeats, with no data flowing.
Relevant RFCs
RFC 1122 describes the calculation of the effective send MSS. RFC
1191 describes Path MTU discovery.
Trace file demonstrating it
Trace file taking using tcpdump on host C. The first trace
demonstrates the fragmentation that occurs without path MTU
discovery:
13:55:25.488728 A.65528 > C.discard:
P 567833:569273(1440) ack 1 win 17520
<nop,nop,timestamp 3839 1026342>
(frag 20828:1472@0+)
(ttl 62, optlen=8 LSRR{B#} NOP)
Paxson, Editor [Page 52]
ID Known TCP Implementation Problems August 1998
13:55:25.488943 A > C:
(frag 20828:8@1472)
(ttl 62, optlen=8 LSRR{B#} NOP)
13:55:25.489052 C.discard > A.65528:
. ack 566385 win 60816
<nop,nop,timestamp 1026345 3839> (DF)
(ttl 60, id 41266)
Host A repeatedly sends 1440-octet data segments, but these hare
fragmented into two packets, one with 1432 octets of data, and
another with 8 octets of data.
The second trace demonstrates the failure to send any data
segments, sometimes seen with hosts doing path MTU discovery:
13:55:44.332219 A.65527 > C.discard:
S 1018235390:1018235390(0) win 16384
<mss 1460,nop,wscale 0,nop,nop,timestamp 3876 0> (DF)
(ttl 62, id 20912, optlen=8 LSRR{B#} NOP)
13:55:44.333015 C.discard > A.65527:
S 1271629000:1271629000(0) ack 1018235391 win 60816
<mss 1460,nop,wscale 0,nop,nop,timestamp 1026383 3876> (DF)
(ttl 60, id 41427)
13:55:44.333206 C.discard > A.65527:
S 1271629000:1271629000(0) ack 1018235391 win 60816
<mss 1460,nop,wscale 0,nop,nop,timestamp 1026383 3876> (DF)
(ttl 60, id 41427)
This is all of the activity seen on this connection. Eventually
host C will time out attempting to establish the connection.
How to detect
The "netcat" utility is useful for generating source routed
packets:
1% nc C discard
(interactive typing)
^C
2% nc C discard < /dev/zero
^C
3% nc -g B C discard
(interactive typing)
^C
4% nc -g B C discard < /dev/zero
Paxson, Editor [Page 53]
ID Known TCP Implementation Problems August 1998
^C
Lines 1 through 3 should generate appropriate packets, which can be
verified using tcpdump. If the problem is present, line 4 should
generate one of the two kinds of packet traces shown.
How to fix
The implementation should ensure that the effective send MSS
calculation includes a term for the IP and TCP options, as mandated
by RFC 1122.
4. Security Considerations
This version of this memo does not discuss any security-related
implementation problems. Futures versions most likely will, so
security considerations will require revisiting.
5. Acknowledgements
Thanks to numerous correspondents on the tcp-impl mailing list for
their input: Steve Alexander, Mark Allman, Larry Backman, Jerry Chu,
Alan Cox, Kevin Fall, Richard Fox, Jim Gettys, Rick Jones, Allison
Mankin, Neal McBurnett, Perry Metzger, der Mouse, Thomas Narten,
Andras Olah, Steve Parker, Francesco Potorti`, Luigi Rizzo, Allyn
Romanow, Jeff Semke, Al Smith, Jerry Toporek, Joe Touch, and Curtis
Villamizar.
Thanks also to Josh Cohen for the traces documenting the "Failure to
send a RST after Half Duplex Close" problem.
6. References
[Allman97]
M. Allman, "Fixing Two BSD TCP Bugs," Technical Report CR-204151,
NASA Lewis Research Center, October 1997.
http://gigahertz.lerc.nasa.gov/~mallman/papers/bug.ps
[Allman98]
M. Allman, S. Floyd and C. Partridge, "Increasing TCP's Initial
Window," Internet-Draft draft-floyd-incr-init-win-03.txt, May 1998.
[RFC1122]
Paxson, Editor [Page 54]
ID Known TCP Implementation Problems August 1998
R. Braden, Editor, "Requirements for Internet Hosts --
Communication Layers," Oct. 1989.
[RFC2119]
S. Bradner, "Key words for use in RFCs to Indicate Requirement
Levels," Mar. 1997.
[Brakmo95]
L. Brakmo and L. Peterson, "Performance Problems in BSD4.4 TCP,"
ACM Computer Communication Review, 25(5):69-86, 1995.
[Dawson97]
S. Dawson, F. Jahanian, and T. Mitton, "Experiments on Six
Commercial TCP Implementations Using a Software Fault Injection
Tool," to appear in Software Practice & Experience, 1997. A
technical report version of this paper can be obtained at
ftp://rtcl.eecs.umich.edu/outgoing/sdawson/CSE-TR-298-96.ps.gz.
[Fall96]
K. Fall and S. Floyd, "Simulation-based Comparisons of Tahoe, Reno,
and SACK TCP," ACM Computer Communication Review, 26(3):5-21, 1996.
[Hoe96]
J. Hoe, "Improving the Start-up Behavior of a Congestion Control
Scheme for TCP," Proc. SIGCOMM '96.
[Jacobson88]
V. Jacobson, "Congestion Avoidance and Control," Proc. SIGCOMM '88.
ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z
[RFC2018]
M. Mathis, J. Mahdavi, S. Floyd, A. Romanow, "TCP Selective
Acknowledgement Options," Oct. 1996.
[RFC1191]
J. Mogul and S. Deering, "Path MTU discovery," Nov. 1990.
[RFC896]
J. Nagle, "Congestion Control in IP/TCP Internetworks," Jan. 1984.
[Paxson97]
V. Paxson, "Automated Packet Trace Analysis of TCP
Implementations," Proc. SIGCOMM '97, available from
ftp://ftp.ee.lbl.gov/papers/vp-tcpanaly-sigcomm97.ps.Z.
[RFC793]
J. Postel, Editor, "Transmission Control Protocol," Sep. 1981.
Paxson, Editor [Page 55]
ID Known TCP Implementation Problems August 1998
[RFC2001]
W. Stevens, "TCP Slow Start, Congestion Avoidance, Fast Retransmit,
and Fast Recovery Algorithms," Jan. 1997.
[Stevens94]
W. Stevens, "TCP/IP Illustrated, Volume 1", Addison-Wesley
Publishing Company, Reading, Massachusetts, 1994.
[Wright95]
G. Wright and W. Stevens, "TCP/IP Illustrated, Volume 2", Addison-
Wesley Publishing Company, Reading Massachusetts, 1995.
7. Authors' Addresses
Vern Paxson <vern@ee.lbl.gov>
Network Research Group
Lawrence Berkeley National Laboratory
Berkeley, CA 94720
USA
Phone: +1 510/486-7504
Mark Allman <mallman@lerc.nasa.gov>
NASA Lewis Research Center/Sterling Software
21000 Brookpark Road
MS 54-2
Cleveland, OH 44135
USA
Phone: +1 216/433-6586
Scott Dawson <sdawson@eecs.umich.edu>
Real-Time Computing Laboratory
EECS Building
University of Michigan
Ann Arbor, MI 48109-2122
USA
Phone: +1 313/763-5363
Jim Griner <jgriner@lerc.nasa.gov>
NASA Lewis Research Center
21000 Brookpark Road
MS 54-2
Cleveland, OH 44135
USA
Phone: +1 216/433-5787
Ian Heavens <ian@spider.com>
Paxson, Editor [Page 56]
ID Known TCP Implementation Problems August 1998
Spider Software Ltd.
8 John's Place, Leith
Edinburgh EH6 7EL
UK
Phone: +44 131/475-7015
Kevin Lahey <kml@nas.nasa.gov>
NASA Ames Research Center/MRJ
MS 258-6
Moffett Field, CA 94035
USA
Phone: +1 650/604-4334
Jeff Semke <semke@psc.edu>
Pittsburgh Supercomputing Center
4400 Fifth Ave
Pittsburgh, PA 15213
USA
Phone: +1 412/268-4960
Bernie Volz <volz@process.com>
Process Software Corporation
959 Concord Street
Framingham, MA 01701
USA
Phone: +1 508/879-6994
Paxson, Editor [Page 57]