RFC 2525
Known TCP Implementation Problems
Pages: 61
Informational
Informational
Part 3 of 3 – Pages 40 to 61
2.13.
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,
according to [RFC1122]. However, an ACK should not be delayed an
inordinate amount of time. Specifically, ACKs SHOULD 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 should 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 ack'ing 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
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
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.
2.14.
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 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 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
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.
2.15.
Name of Problem
Failure to send FIN notification promptly
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 by the packet filter).
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)
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 by the packet filter).
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.
2.16.
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.
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
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. How to detect The problem can be detected by inspecting packet traces of a large, interrupted bulk transfer.2.17.
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
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 occurred 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
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 by the packet filter.
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
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.
2.18.
Name of Problem
Options missing from TCP MSS calculation
Classification
Reliability / performance
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)
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 [Hobbit96] 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
^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.
3. Security Considerations
This memo does not discuss any specific security-related TCP
implementation problems, as the working group decided to pursue
documenting those in a separate document. Some of the implementation
problems discussed here, however, can be used for denial-of-service
attacks. Those classified as congestion control present
opportunities to subvert TCPs used for legitimate data transfer into
excessively loading network elements. Those classified as
"performance", "reliability" and "resource management" may be
exploitable for launching surreptitious denial-of-service attacks
against the user of the TCP. Both of these types of attacks can be
extremely difficult to detect because in most respects they look
identical to legitimate network traffic.
4. Acknowledgements
Thanks to numerous correspondents on the tcp-impl mailing list for
their input: Steve Alexander, 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, 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; and to John Polstra, who
analyzed the "Window probe deadlock" problem.
5. References
[Allman97] M. Allman, "Fixing Two BSD TCP Bugs," Technical Report CR-204151, NASA Lewis Research Center, Oct. 1997. http://roland.grc.nasa.gov/~mallman/papers/bug.ps [RFC2414] Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's Initial Window", RFC 2414, September 1998. [RFC1122] Braden, R., Editor, "Requirements for Internet Hosts -- Communication Layers", STD 3, RFC 1122, October 1989. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [Brakmo95] L. Brakmo and L. Peterson, "Performance Problems in BSD4.4 TCP," ACM Computer Communication Review, 25(5):69-86, 1995. [RFC813] Clark, D., "Window and Acknowledgement Strategy in TCP," RFC 813, July 1982. [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. [Hobbit96] Hobbit, Avian Research, netcat, available via anonymous ftp to ftp.avian.org, 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 [Jacobson89] V. Jacobson, C. Leres, and S. McCanne, tcpdump, available via anonymous ftp to ftp.ee.lbl.gov, Jun. 1989.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP Selective Acknowledgement Options", RFC 2018, October 1996. [RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191, November 1990. [RFC896] Nagle, J., "Congestion Control in IP/TCP Internetworks", RFC 896, January 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] Postel, J., Editor, "Transmission Control Protocol," STD 7, RFC 793, September 1981. [RFC2001] Stevens, W., "TCP Slow Start, Congestion Avoidance, Fast Retransmit, and Fast Recovery Algorithms", RFC 2001, January 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, 1995.6. Authors' Addresses
Vern Paxson ACIRI / ICSI 1947 Center Street Suite 600 Berkeley, CA 94704-1198 Phone: +1 510/642-4274 x302 EMail: vern@aciri.org
Mark Allman <mallman@grc.nasa.gov> NASA Glenn Research Center/Sterling Software Lewis Field 21000 Brookpark Road MS 54-2 Cleveland, OH 44135 USA Phone: +1 216/433-6586 Email: mallman@grc.nasa.gov Scott Dawson Real-Time Computing Laboratory EECS Building University of Michigan Ann Arbor, MI 48109-2122 USA Phone: +1 313/763-5363 EMail: sdawson@eecs.umich.edu William C. Fenner Xerox PARC 3333 Coyote Hill Road Palo Alto, CA 94304 USA Phone: +1 650/812-4816 EMail: fenner@parc.xerox.com Jim Griner <jgriner@grc.nasa.gov> NASA Glenn Research Center Lewis Field 21000 Brookpark Road MS 54-2 Cleveland, OH 44135 USA Phone: +1 216/433-5787 EMail: jgriner@grc.nasa.gov
Ian Heavens Spider Software Ltd. 8 John's Place, Leith Edinburgh EH6 7EL UK Phone: +44 131/475-7015 EMail: ian@spider.com Kevin Lahey NASA Ames Research Center/MRJ MS 258-6 Moffett Field, CA 94035 USA Phone: +1 650/604-4334 EMail: kml@nas.nasa.gov Jeff Semke Pittsburgh Supercomputing Center 4400 Fifth Ave Pittsburgh, PA 15213 USA Phone: +1 412/268-4960 EMail: semke@psc.edu Bernie Volz Process Software Corporation 959 Concord Street Framingham, MA 01701 USA Phone: +1 508/879-6994 EMail: volz@process.com
7. Full Copyright Statement
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