Subject: [netext] Questions on: draft-ietf-netext-logical-interface-support-05.txt

From: <[email-masked]>
Date: Mon, 30 Apr 2012 21:13:12 +0000

A few questions that come to mind:

1. Sec 6.2 discusses inter-technology handovers. What happens in the
scenario of a break-before-make handover? Interface 1 goes down and there
is a brief interval before another interface (2) becomes active. How does
the logical interface behave in such a scenario? Do you still maintain the
existing connections? The LIF may not have any awareness of whether
another physical interface will be activated.

2. Regarding path MTU, the I-D says that in the case where the MN is
attached simultaneously to multiple interfaces, the LIF will choose the
smaller of the MTU values of the physical interfaces.  However the I-D
also says that the LIF has awareness of forwarding packets on the
interface from which a packet was received. If that is the case, why would
the LIF be configured with the lower value MTU? The LIF should be able to
choose the appropriate MTU for sending a packet depending on the interface
over which the packet is sent/received.

3. The statement: "The logical interface adapts to the point-to-point link
model." is not completely clear. What exactly is the LIF adapting to?

4. Sec 3.2 is unclear. Please elaborate on the requirements and
applicability.

-Raj

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From: Sri Gundavelli <[email-masked]>
Date: Mon, 30 Apr 2012 17:25:11 -0700

> another physical interface will be activated.
[Sri] 
1. The stated assumption for the LIF is about overlapping network coverage; there is WLAN coverage and there is LTE coverage and in that scenario how can we do a session handoff across these access technology domains.  This is clearly about about make-before-break. LIF has no magic to anticipate handover scenarios. Applications will break, if the interfaces that the LIF is abstracting are down. After an handover if the L3 network to which the applications are bound continues to be available, applications will survive, else they will break.

> over which the packet is sent/received.

[Sri] Keeping the lower of the MTU values across different interfaces will ensure there is no MTU change notification to the ND stack after each handover. It will keep a stable MTU value.

> model." is not completely clear. What exactly is the LIF adapting to?

[Sri] We have had this discussion in the past on this topic. LIF is adapting to P2P link model. May be we can add more details.

> 4. Sec 3.2 is unclear. Please elaborate on the requirements and

[Sri] Agreed. Will fix this

Thanks for the review.

Sri

> -Raj 
> 
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From: <[email-masked]>
Date: Tue, 1 May 2012 03:06:06 +0000

Inline:

On 4/30/12 7:25 PM, "ext Sri Gundavelli" <[email-masked]> wrote:

>1. The stated assumption for the LIF is about overlapping network
>coverage; there is WLAN coverage and there is LTE coverage and in that
>scenario how can we do a session handoff across these access technology
>domains.  This is clearly about about make-before-break. LIF has no magic
>to anticipate handover scenarios. Applications will break, if the
>interfaces that the LIF is abstracting are down. After an handover if the
>L3 network to which the applications are bound continues to be available,
>applications will survive, else they will break.

Then you should make this clear in the I-D that a logical interface will
support inter-technology handovers only in the case of make-before-break
scenarios. 

>[Sri] Keeping the lower of the MTU values across different interfaces
>will ensure there is no MTU change notification to the ND stack after
>each handover. It will keep a stable MTU value.

But it comes at the cost of sending packets on the interface in a
suboptimal manner. As a result the bandwidth of the air-interface is (may
be) wasted. Since the LI knows which of the interfaces to send the packets
on, why does it need to adapt to the lower value MTU? I do not believe it
is necessary.

>[Sri] We have had this discussion in the past on this topic. LIF is
>adapting to P2P link model. May be we can add more details.

I am still at a loss to understand what you mean when you say "LIF is
adapting to P2P link model". What exactly is happening at the LIF in terms
of behavior?

-Raj

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From: Sri Gundavelli <[email-masked]>
Date: Mon, 30 Apr 2012 20:45:53 -0700

inline …

> support inter-technology handovers only in the case of make-before-break

Sure

> on, why does it need to adapt to the lower value MTU? I do not believe it

The other aspect is constant MTU change at each handoff. So, we can leave both the considerations and let implementations decide.

> adapting to P2P link model". What exactly is happening at the LIF in terms

Every interface that is exposed to the ND stack is reflecting the interface type, shared/p2p. There is even a socket API to get this property and its used by the application/ND stack for different purposes, Now, the question is about LIF, what is the tag that exposed to the upper layer. The LIF operation has no bearing on the type and given that we have pre-established assumption on the link model, we rather expect the LIF be of another P2P link type. That's all to it.

Sri