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TCP-IP Digest Vol. 2 No. 09
TCP/IP Digest Wednesday, 15 June 1983 Volume 2 : Issue 9
Today's Topics:
Pointer to Naming, Addressing, & Routing Memo
Routing of Routing Messages -vs- Data Messages
Fix for BBN VAX TCP Problem
Comments on SMTP Design Flaw + Discussion
Request for TCP/IP Implementations on Micros
Wanted: Spooled FTP for BBN UNIX TCP
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TCP/IP Digest --- The InterNet Digest
LIMITED DISTRIBUTION
For Research Use Only --- Not for Public Distribution
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Date: 7 Jun 1983 1419-PDT
From: POSTEL@usc-isif
Subject: IEN-19: Names, Addresses, and Routes
To: TCP-IP@brl
The memo by John Shoch titled "Inter-Network Naming, Addressing, and
Routing", issued as IEN 19 was produced using some fancy formating
and printing facilities at Xerox PARC and was distributed only in hardcopy.
The later (and improved) version of this memo with the same title was
presented at COMPCON Fall 1987. The order number from the IEEE
Computer Society is CH1388-8/78-0000-0072.
--jon.
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Date: 7 Jun 1983 1435-PDT
From: POSTEL@usc-isif
Subject: Routing Messages vs. Data Messages
To: tcp-ip@brl
Ed Taft's comments on routing information exchange being distinct from the
actual routes that data traffic moves on are well taken. One aspect of the
development of the Exterior Gateway Protocol (RFC 827) is to allow testing
of different routing procedures in subsets of gateways without interfering
with the actual transmission of data datagrams. The logical systems of
gateways in EGP are independent of the actual network topology.
--jon.
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Date: 8 Jun 1983 9:32:11 EDT (Wednesday)
From: Dennis Rockwell <drockwel@bbn-vax>
Subject: TCP problem
To: Rob Gurwitz <gurwitz@BBN-UNIX>, dpk@brl-vgr
[ This message was forwarded to the Digest by Doug Kingston, and details
a particularly insidious bug in the BBN VAX TCP. Hopefully, if any site
using that code has not heard of this problem yet, this message will
have served a purpose. Note that Berkeley 4.1a and 4.1c code does
not have this bug. -Mike ]
The fix for this bug is as follows: (I'm afraid I have no
familiarity whatsoever with the TCP variants, but the
code should be fairly easy to find)
In the routine rcv_text (in tcp_procs.c in our code), the routine
that does the resequencing, there is the following code:
/* new overlaps at beginning of successor frags */
q = savq->t_next;
while ((q != (struct th *)tp) && (t->t_len != 0) &&
SEQ_LEQ(q->t_seq, last))
/* complete cover */
if (SEQ_LEQ(t_end(q), last)) {
p = q->t_next;
tcp_deq(q);
m_freem(dtom(q));
q = p;
} else { /* overlap */
In your code, the first SEQ_LEQ is probably a SEQ_LT. This is the bug.
There is an analogous bug in ip_input.c:
/* new overlaps at beginning of successor frags */
q = savq->ip_next;
while ((q != (struct ip *)fp) &&
(ip->ip_len != 0) &&
(q->ip_off <= ip->ip_end))
/* complete cover */
if (q->ip_end <= ip->ip_end) {
p = q->ip_next;
ip_deq(q);
m_freem(dtom(q));
q = p;
Again, in your code the first <= is probably a <.
This fix was made just last week, and is slowly percolating out to the
other local BBN sites. We are trying to spread this fix around, but
that's difficult. Please tell everybody you know who is running this
code about the fix.
Good luck! Feel free to write or call me at (617) 497-2643.
------------------------------
Date: 8 Jun 83 5:39:10 EDT (Wed)
From: Doug Kingston <dpk@BRL-VGR>
To: tcp-ip@BRL-VGR
Subject: SMTP Design Flaw
SMTP as it currently stands has a serious flaw from the
point of view of fault-tolerant software. The crux of the problem
is that the reply codes reuse certain values in different contexts.
This opens the possibility of of mis-interpreting replies in the
case where the user-smtp or the transmission channel is generating
spurious data. A similiar problem exists if the smtp server
generates spurious responses.
If both sides are bug-free, there will be no problems.
But, the world of software is seldom if ever bug-free. A prudent
implementer plans for the inevitable error to be made by the
hardware, his own software, or someone else's software. Spurious
commands or spurious responses can cause the server/user pair to
get out of sync without either being able to conclusively detect
the situation.
The worst problem exists for the 250 reply which is the
affirmative reply from HELO, MAIL, RCPT, "final dot", and RSET
to name only the most commonly used commands. The 500, 501, and
503 commands are also dangerously overloaded.
Senarios:
RCPT -->
unknown to sender junk -->
<-- 250
DATA -->
<-- 500 (actually reply to junk)
Result: Mail is returned wrongfully.
********************
RCPT -->
unknown to sender junk1 -->
junk2 --> (we are now off by two...)
<-- 250
RCPT -->
<-- 500 (actually reply to junk1)
<Mail is returned here wrongfully>
DATA -->
<-- 500 (actually reply to junk2)
we detect problem RSET -->
<-- 250 (actually reply to second RCPT !!!)
MAIL -->
<-- 354 (bletch.) (Protocol errror)
RSET -->
<-- 250 (reply to first RSET!!!!!)
MAIL -->
<-- 250 (reply to first MAIL...
RCPT -->
<-- 250 (reply to second RSET...
RCPT -->
<-- 250 (reply to second MAIL...
... until we get to the DATA command again.
Result: Mail is returned wrongfully.
********************
RCPT -->
unknown to sender junk -->
<-- 250
RCPT -->
<-- 500 (response to junk)
<mail is returned here>
RCPT -->
<-- 250 (response to SECOND rcpt command!!!)
********************
RCPT -->
unknown to sender RCPT -->
<-- 250
RCPT -->
<-- 250 (response to Second RCPT !!!)
RCPT -->
<-- 250 (response to Third RCPT !!!)
DATA -->
<-- 250 (response to Fourth RCPT !!!)
***********************
As I see it, there are two things that can be done done to
help out this situation. First, the reply codes should be unique
for each command. This would give the USER SMTP the ability to
detect sequencing errors from syntax or argument errors which
some hosts give on hosts they don't know how to reach, but I wont
drag the offenders out into public here. They know who they are.
Second, and perhaps more radical, is to add a sequence number
each command/reply pair. Each command would be sent with a sequence
number. The reply to that command would have to contain the same
sequence number. This would allow both the server and user SMTP
to detect sequenceing errors. I feel that this would be the most
prudent and reliable mechanism for detecting these faults.
This problem is not just a figment of my imagination. This
very problem has plagued mail between out sites and sites running
BBN based TCP implementations for about a month now. The spurious
input was probably induced by a bug in the BBN TCP's handling
of the sequenceing and/or retransmission or large numbers of incoming
1 character packets due to a bug in my mailer that caused it not to
buffer its output.
Comments and Discussion are Welcomed
-Doug-
------------------------------
Date: 8 Jun 1983 1445-PDT
From: POSTEL at USC-ISIF
Subject: Re: SMTP Design Flaw
To: dpk at BRL-VGR, tcp-ip at BRL-VGR
Doug:
SMTP explicitly requires that it be operated on a reliable channel
(such as provided by TCP). The situation you suggest does not arise
because of "junk" being inserted in the channel unknown to the sender.
There was (still is?) a totally bogus "implementation" of SMTP that sent
several commands in succession without even looking at the replies. I
suggest that it is better to assume a reliable channel than to reimplement
TCP inside of SMTP, and to stomp out bad implementations of SMTP as soon
as we see them.
--jon.
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Date: 8 Jun 83 18:13:50 EDT (Wed)
From: Doug Kingston <dpk@BRL-VGR>
To: POSTEL@Usc-Isif
cc: dpk@BRL-VGR, tcp-ip@BRL-VGR
Subject: Re: SMTP Design Flaw
Jon,
I do not dispute that the problem is induced by a faulty
SMTP or channel. I am arguing for cheap enhancements the buy
us a great deal of fault tolerance. Obviously we're not going to
change SMTP, but I think this should be kept in mind for later
incarnations. Faults will occur. Mail software should make
every effort to detect and deal with errors. We need to give
it a mechanism, and it doesn't have to be complicated.
The problem with bugs is
that is they're so ingenious,
-Doug-
------------------------------
Date: 9 Jun 1983 17:53:43-PST
From: ram.arizona@rand-relay
Subject: TCP/IP on Micros?
To: tcp-ip-request.brl@rand-relay
[ Anybody having information about micro implementations, please feel
free to either contact Ralph directly, or to submit a message to the
digest for all to see. -Mike ]
I am interested in locating information on tcp/ip implementations on
microcomputer systems. Specifically, on the LSI-11/23, 68000, and 8086.
I would like to know the language, operating system, and availability of
such implementations (costs). Thanks for passing on this request.
----Ralph Martinez (ram.arizona@rand-relay)
University of Arizona
------------------------------
Date: 9 Jun 1983 8:36-PDT (Thursday)
From: Jim Rees <jim@uw-beaver>
Subject: Want spooled FTP for BBN Unix TCP
To: tcp-ip@brl, bbn-tcp@bbn-vax
I am running BBN's TCP on a Vax with 4.1bsd. I want a spooled FTP,
something like uucp, to run over the net between Unix machines. Has
anyone already done this? Does anyone have bright ideas?
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END OF TCP-IP DIGEST
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