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

["tom.petch" <[e-mail]>]

---- Original Message -----
From: "Tim Evens" <[e-mail]>
Sent: Wednesday, May 05, 2010 10:02 PM


Hopefully this isn't too late for a review comment...It appears to me that DTLS
as a SYSLOG-MSG transport introduces a new variable that isn't present with
traditional TCP and UDP based transports with regards to SYSLOG-MSG fragments.

IP/UDP transport allows for IPv4 based fragmentation, even though it should be
avoided for several reasons, there could be useful and valid application uses
for fragmenting UDP packets.

<tp>

True, but RFC5426 says

"It is RECOMMENDED that syslog senders restrict message sizes such
   that IP datagrams do not exceed the smallest MTU of the network in
   use.  This avoids datagram fragmentation and possible issues
   surrounding fragmentation such as incorrect MTU discovery.

   Fragmentation can be undesirable because it increases the risk of the
   message being lost due to loss of just one datagram fragment.  Syslog
   has no acknowledgement facility, and therefore there is no effective
   way to handle retransmission.  This makes it impossible for syslog to
   utilize packetization layer path MTU discovery [9].  When network MTU
   is not known in advance, the safest assumption is to restrict
   messages to 480 octets for IPv4 and 1180 octets for IPv6."

so my take has always been, forget fragmentation, or use TCP.

I would apply that to a DTLS mapping.

Tom Petch
</tp>

IP/TCP as a transport supports message segments that can span multiple packets.
Per RFC4347 section 4.1.1, "each DTLS record MUST fit within a single datagram."
DTLS introduces a sequence_number field to the record layer, which facilitates
reassembly providing the receiving station queues and reorders.
In this draft, section 5.1 suggests that it's optional for the implementer to
adopt a queue mechanism to resolve reordering. Later in section 5.4 it suggests
that a single SYSLOG-MSG can be transferred in multiple DTLS records.

If it is optional to implement DTLS sequence numbers, how does the originator of
the SYSLOG-MSG know that the collector will not reorder messages? If the
originator knows that the collector will not reorder DTLS fragments, then it can
truncate messages prior to transmission. In lieu of truncation, the originator
could generate multiple SYSLOG-MSG's using structured data that enables the
collector to piece them together.

There doesn't appear to be a SYSLOG-FRAME ID as part of each DTLS
application-data record. If a message of say 1400 bytes is sent in 3
packets/DTLS records, and if packet 2 of the 3 was dropped by the network, how
does the collector know that those packets were to be concatenated as a single
SYSLOG-MSG? Without a SYSLOG-FRAME identifier, the collector would have to
assume invalid based on messages not structured per RFC5424. Also, if there is
no frame ID and sequence reordering isn't implemented, a out-of-sequence DTLS
record could possibly inadvertently be prepended or appended to the wrong
message, causing a valid message to be corrupt. It seems this would be moot if
when using DTLS a SYSLOG-MSG must not span multiple DTLS records.
Thanks,Tim
-----Original Message-----From: "Chris Lonvick" [[e-mail]]Date:
05/03/2010 06:49 AM
To: "tom.petch" , "Joe Salowey" , "Sean Turner" CC: [e-mail]
Subject: Re: [Syslog] AD review comments for draft-ietf-syslog-dtls
Hi,  I'm good with that as well.  Joe, can you edit and resubmit a new ID?

Sean, if this covers all of your edits, when can we expect to see it on  the
IESG agenda, and when can we see IETF LC?
Thanks, Chris
 On Tue, 27 Apr 2010, tom.petch wrote: