Re: [Syslog] AD review discuss/comments for draft-ietf-syslog-dtls

RFC 4346 (from which this text comes from) mostly did not use
RFC2119 keywords, but instead informal language like the
lowercase "should not" you quoted. AFAIK it was meant to
express a strict limit, not a recommendation (this is implied
by other text in the spec, and as you noticed, we clarified
this in RFC 5246).

But even though DTLS records are limited to 2^14 bytes, syslog
messages are not! The current spec seems to support 64K (minus
some small number of overhead) just fine -- the message will be
split to multiple DTLS records (max. 2^14 bytes each), but those 
DTLS records are then combined to a single UDP datagram.

Best regards,
Pasi

________________________________________
From: ext t.petch [ietfc@btconnect.com]
Sent: Tuesday, May 25, 2010 12:31 PM
To: Eronen Pasi (Nokia-NRC/Helsinki); turners@ieca.com
Cc: syslog@ietf.org
Subject: Re: [Syslog] AD review discuss/comments for draft-ietf-syslog-dtls

---- Original Message -----
From: <Pasi.Eronen@nokia.com>
To: <turners@ieca.com>; <ietfc@btconnect.com>
Cc: <syslog@ietf.org>
Sent: Monday, May 24, 2010 9:54 AM

I haven't followed this discussion in detail, but it looks like
there's some confusion about the basic "units" of transmission. As far
as I can tell, we have four different layers:

- a syslog message (SYSLOG-FRAME in ABNF)
- a DTLS record
- a UDP datagram
- an IP packet

As noted in Section 5.4, "It is possible that multiple syslog messages
be contained in one DTLS record, or that a syslog message be
transferred in multiple DTLS records."

The maximum size of a single DTLS record is 2^14 bytes (this limit
comes from TLS).

<tp>
Where?:-)

TLS provides fragmentation and says that
"length MUST NOT exceed 2^14." [RFC5246 s6.2.1]
so that the upper layers can send larger messages which TLS then fragments for
them.

DTLS provides fragmentation for handshake messages [RFC4347 s3.2.3] but not for
the record layer; rather it says,
" As in TLS  1.1, the length should not exceed 2^14."

should not, no MUST NOT as in [RFC5246]
(and draft-ietf-tls-rfc4347-bis has the same text)

So while 65535 byte messages are not generally acceptable, where the users know
their network and its MTU, then we should let them do what they know best.  I
see the main use of syslog in switched Enterprise LAN where large MTU are a
commonplace.

Tom Petch

</tp>

One DTLS record must fit in one UDP datagram, but one
UDP datagram can contain more than one DTLS record.

The maximum size of UDP datagram is 64K (this limit comes from UDP),
but it can be fragmented to multiple IP packets as needed.

There's one additional restriction that I'm not sure is really
mentioned anywhere: A single syslog message has to fit in a single UDP
datagram. So while it can be split to multiple DTLS records, all those
records have to be in a single UDP datagram (so the syslog layer does
not reassemble syslog message pieces from multiple UDP datagrams --
SYSLOG-FRAME does not have sufficient information to do this
anyway).

In addition to the "hard" size limits (coming from DTLS and UDP),
we probably need a recommendation saying that it's better if you
can avoid IP fragmentation -- but this is precisely the same as normal
syslog-over-UDP (minus the small overhead from DTLS).

Best regards,
Pasi
______________________________________
From: syslog-bounces@ietf.org [syslog-bounces@ietf.org] On Behalf Of ext Sean
Turner [turners@ieca.com]
Sent: Saturday, May 22, 2010 6:16 PM
To: t.petch
Cc: syslog
Subject: Re: [Syslog] AD review discuss/comments for draft-ietf-syslog-dtls

t.petch wrote:
> I see that this I-D had entered 'Revised I-D needed' which I would like to
> progress.
>
> I see several comments about maximum record size, including a suggestion that
we
> should make the 'SHOULD NOT' a 'MUST NOT' exceed 2**14.
>
> I am dead set against this change.  We had a clear requirment, early on, to
> allow 65k messages, and I think it wrong to MUST NOT that requirement. The
text
> in the other I-Ds is a compromise to strke a balance between this and having
> everything fit in 576 byte; I think we have the balance right.

Tom,

My response to Alexey was that this I-D borrows that particular
requirement from RFC4347 and that this I-D shouldn't be upping the
requirement.  If it's okay with you, I'll forward him your response.
The way I read his comment was that he's just asking why - he's not
really requesting a change.

spt
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