Subject: [Isms] Tmsm#3: session reuse issues

From: "David Harrington" <[email-masked]>
Date: Fri, 16 Feb 2007 09:43:50 -0500

>From Bert (1/12):
"> - last para of sect 3.3.1 (page 15/16) states:
> 
>     If a session can be reused for a different type of message, but
a
>     receiver is not prepared to accept different message 
> types over the
>     same session, then the message MAY be dropped by the 
> receiver.  This
>     may strongly affect the usefulness of session reuse, and
transport
>     models should define a standard behavior for this circumstance.
> 
>   Mmm... "type of message" ? I only see this term occur once 
> in RFC3411.
>   It basically is the PDU type. But OK, I can live with it.
>   I am worried about the fact that "a transport model needs to
define
>   standard behaviour for this". How/where does a transport model
>   know about the message type (or PDU type) ?? The pduType is passed
>   between Dispatcher and MP model (in the ASIs), but I do not see it
>   passed from the dispatcher to transport subsystem. Is the TM 
>   suppsoed to go dig into the PDU itself? I hope not.

DBH: "Hmmm. I'm not sure how a transport model knows this. 
I am not a fan of session reuse, so my heart won't be broken if we
simply drop this whole thing, and the transport model finds the right
session by its transport address (e.g., port 161 or 162).
I'll add this to the open issues section."

Bert:
"So how did it get included in the first place?
I guess there were some people who see a need for it?
If so, then we need to figure out how the transport would know
about it, or how this is/can be done otehrwise."

Separtely from Andy Bierman (1/29):
">    SNMP Applications typically have no knowledge of whether 
> the session
>    that will be used to carry commands was initially established as
a
>    notification session, or a request-response session, and SHOULD
NOT
>    make any assumptions based on knowing the direction of the
session.
> 
> Is this true?  Why doesn't the session initiator know what type
> of session is being requested?"

DBH response: There was WG consensus to permit transport-session
reuse, including for different types of messages (request/response vs
notification), and the reuse decision is made by the
transport-specific model. Because of this, an application does not
know whether it is the first to request a session with these security
parameters (including direction), and if it is not the first, whether
the session was initially opened as a request/response session or as a
notification session.

Andy:
">    An SNMP engine containing an application that initiates
>    communication, e.g., a Command Generator or Notification 
> Originator,
>    MUST be able to attempt to establish a session for delivery if a
>    session does not yet exist.  If a session cannot be 
> established then
>    the message is discarded.
> 
> Does the application know it is establishing a session?
> Are the sessions just a transport layer mechanism?
> Some explanation of the document I should have read before
> starting this document should be in the Introduction section."

DBH response:
No, the application does not necessarily know if it is establishing a
session; the session is a transport-specific mechanism. The session
might already exist, started by a different application, and might be
reused for this message. The application might specify a session it
has used before (using domain/address/name/model/level), but the
session might have been closed for various reasons, so this message
might cause the establishment of a new session.

Andy:
">    If a session can be reused for a different type of message, but
a
>    receiver is not prepared to accept different message types over
the
>    same session, then the message MAY be dropped by the 
> receiver.  This
>    may strongly affect the usefulness of session reuse, and
transport
>    models should define a standard behavior for this circumstance.
> 
> IMO, this document should define the behavior, and not pass it
> to the transport implementation.  Silently dropping messages
> in a purposely undefined way is not good standards practice."

DBH response: The whole idea of session reuse has never been used with
SNMP before, or any other widely-deployed IETF NM protocol (if there
were any), and I don't know that we understand how session reuse will
be impacted by different security protocols, or how session reuse will
impact certain security protocols, or what the impact to SNMP
interoperability will be. I don't think we have enough understanding
of the impacts of discarding a message sent through a reused session
to define what the normative behavior should be for all transport
models at this point. 

David Harrington
[email-masked] 
[email-masked]
[email-masked]

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