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XFOR: IMC Stops Sending Mail, Fails to Release TCP Connection

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This article was previously published under Q152940

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Symptoms

Microsoft Exchange Internet Mail Connector (IMC) may stop sending mail to a foreign SMTP host and fail to terminate the TCP connection until the MSExchangeIMC service is stopped and restarted.

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Cause

An RFC821 SMTP protocol violation of interrupting the IMC's transmission of SMTP message DATA may cause an IMC thread to become blocked. Depending on how the target host requests the connection be closed (FIN or RST), the connection may remain open.

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Workaround

This SMTP protocol violation can be avoided by configuring the IMC to restrict maximum message size on a "per domain" basis. This option is configured in Exchange Administrator, "<Internet Mail Connector>.Internet Mail.Email Domain.Add.Maximum Size (KB):"

It is prudent to configure this setting when communicating with foreign SMTP hosts that have a maximum inbound message size limit, for the followingreasons:
  • It avoids the issue described in this article.

  • A non-delivery report (NDR) is returned to the originator of the message informing them of the limitation *before* the IMC attempts to send the large message. This results in more efficient IMC performance.

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More information

Proper behavior for conversations between SMTP mail hosts is documented in RFC821. In cases where these symptoms (listed under SYMPTOMS above) have been observed, the target SMTP host interrupts the IMC's DATA transmission with a 552 error (sender's message has exceeded fixed maximum message size).

While this is a valid error message, the fact that it is returned to the IMC in the middle of the transmission of the DATA portion of a message is a protocol violation. In the case of an error during the DATA transmission, it is the target's responsibility to continue accepting the message (discarding received data if necessary), and *after completion,* return the reply code for the DATA command to the sending side.

The following sections from RFC821 detail the specifics for this issue:
4.2, paragraph 1, "... Every command must generate exactly one reply."

4.3, "Command-reply" sequence for DATA diagrammed

4.4, State diagram for DATA - Notes: "Note that the data here is a series of lines sent from the sender to the receiver with no response expected until the last line sent."
Example (successful) SMTP Conversation:

I = IMC (initiator), T= Target (receiver) 
    T: <waiting for connection on TCP port 25>
    I: <open connection to target>
    T: 220 tin.pan.alley.com SMTP service ready
    I: HELO cartoonville.com
    T: 250 tin.pan.alley.com says hello to cartoonville.com
    I: MAIL FROM: <clem@cartoonville.com>
    T: 250 sender OK
    I: RCPT TO: <ditty@tin.pan.alley.com>
    T: 250 Recipient OK
x   I: DATA
    T: 354 Enter mail, end with "." on a line by itself
*   I: **** IMC begins sending a message header and body, and according to
    I: RFC821, 4.4, "Note that the data here is a series of lines sent from
    I: the sender to the receiver with no response expected until the last
    I: line sent." The last line sent is signified by "." on a line by
    I: itself - which is coming up next (note: this DATA content portion is
    I: not usually logged)****
*   I: .
y   T: 250 message sent
    I: QUIT
    T: 221 tin.pan.alley.com closing connection
    I: <Closes connection>
    T: <closes connection>

Notice that once the DATA command (x) has been positively acknowledged, the IMC expects to send the message data uninterrupted (lines between asterisks, inclusive) until it signifies the last line (the last asterisk). Then the following line (y) is the appropriate point at which success or error codes should be returned from the target.

Example network trace summary (failure): 
3076    IMC >>> Target     SMTP
Data - continued from frame 3074, 536 bytes
 (IMC is sending DATA)
3077    Target >>> IMC     TCP
.A...., len:    0, seq:1417586500-1417586500, ack: 136770891, win:  967
 (Target ACKs the DATA)
3078    IMC >>> Target     SMTP
Data - continued from frame 3076, 536 bytes
(sending more)
3079    Target >>> IMC     TCP
.A...., len:    0, seq:1417586500-1417586500, ack: 136771427, win:  967
(ACK'd)
3080    IMC >>> Target     SMTP
Data - continued from frame 3078, 536 bytes
(sending more - IMC is not finished sending DATA yet)
3081    Target >>> IMC     SMTP
Rsp: Requested mail action aborted: exceeded storage allocation, 54 bytes
(frame 3081 is the first protocol violation - this frame contains the 552
error code)
3082    Target >>> IMC     SMTP
Rsp: Service not available, closing transmission channel, 46 bytes
(this frame is another protocol violation - this frame contains a 421 error
code "service unavailable")
3083    Target >>> IMC     TCP
.A...F, len:    0, seq:1417586599-1417586599, ack: 136771427, win:  967
(this is a TCP FIN frame. See below)
3084    IMC >>> Target     TCP
.A...., len:    0, seq: 136771963-136771963, ack:1417586599, win: 8254
(this is a TCP ACK frame, which ACKs through frame 3082)
3085    IMC >>> Target     TCP
.A...., len:    0, seq: 136771963-136771963, ack:1417586600, win: 8254
(this is another TCP ACK frame, ACKing frame 3083, the Target's FIN)

In frame 3080, the IMC is still sending more message DATA. In frame 3081 & 3082 the target interrupts this send, returning SMTP error codes. The target should wait until the IMC completed sending DATA before returning these codes.

It has also been noted that receiving a FIN from the target may not terminate the TCP connection. The procedure for closing a TCP connection is documented in section 3.5 of RFC793. It states:
The user who CLOSEs may continue to RECEIVE until he is told that the other side has CLOSED also.
In the trace above, the target sends a FIN in frame 3083, which informs the IMC that it should not expect more data from the target. This action is not a TCP protocol violation. The IMC ACKs the FIN with frame 3085, and, because it is in the middle of sending message DATA, resumes sending, which is now blocked from the previous SMTP protocol violations (frames 3081 and 3082).

According to RFC793, the method to unconditionally abort a TCP connection is by sending an RST [reset] frame. Note, however, that this defeats any ability to provide a "reason" for the connection termination to the applications using the connection.

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Keywords: KB152940, kbinterop, kb3rdparty

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Article Info
Article ID : 152940
Revision : 6
Created on : 10/28/2006
Published on : 10/28/2006
Exists online : False
Views : 357