[DNA] Ideas for ethernet section of draft-ietf-dna-link-information

[Greg Daley <greg.daley@eng.monash.edu.au>]

Dear WG,

At the last IETF, it was made an action point to follow up
getting ethernet related text for the link information draft.

Due to time delays and people being busy, this hasn't been
incorporated into the draft before the deadline.

So I thought I'd have a go at it.

Below is a rough version of some text which may be applicable to
link hints in 802.3/802.1d.

Everyone is asked to read and comment on the text.

People with expertise in MAC implementation and especially bridging
are welcome to review/comment/trash this.

Greg

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The following text requires two references:

ieee 802.3 (2002)  [802.3]
ieee 802.1d (2004) [802.1d]

please note that this 802.3 2002 contains phy specs for
ethernet up to 1G.

please note that this 802.1d 2004 contains RSTP.


Text follows:
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Ethernet is the most commonly deployed Local Area Network
technology in use today.
As deployed today, it is specified by both a
physical layer/medium access control (MAC) layer specification [802.3],
and a bridging specification used for connecting different
ethernet LAN segments together [802.1d].

In Ethernet networks, hosts are connected by wires or by optic fibre
to a switch (bridge), a bus (e.g. co-axial cable), a repeater (hub), or
directly to another ethernet device.  Interfaces are symmetric, in
that while many different physical laters may be present, medium access 
control is uniform for all devices.

Ethernet specifies its own link monitoring mechanism, which indicates
when both the local physical and MAC layer are capable of transferring
frames.
The Link Integrity Test operation is used to identify when packets are
able to be received on an ethernet segment.
It is applicable to both wired and optical physical layers, although
details vary between technologies (link pulses in copper, light levels
in fibre).

The status of the link as determined by the Link Integrity Test is
stored in the variable 'link_status'.  Changes to the value of
link_status (for example due to Link Integrity Test failure) will
generate link indications if the technology dependent interface is 
implemented on an ethernet device [802.3].

The link_status has possible values of FAIL, READY and OK.
When an interface is in FAIL state, Link Integrity Tests have failed.
Where status is READY, the link segment has passed integrity tests,
but autonegotiation has not completed.  OK state indicates that the
medium is able to send and receive packets.

Upon transition to a particular state the Physical Medium Attachment
subsystems generates a PMA_LINK.indicate(link_status).  Indications
of OK state may be used as Link-Up indications for DNA.
PMA_LINK.indicate(FAIL) may be used as a Link-Down indication reliably
by DNA[802.3].

Ethernet networks are commonly bridged and hosts are unable to tell
a-priori whether the LAN segment they connect to is bridged.
In any case where bridges are in operation, they rely upon a spanning
tree protocol, to ensure there are no bridging loops.
For these technologies, even though the link-layer is available,
no forwarding will occur until the network is satisfied that
the host is not another bridge port.
Until this time, no packets will be received except for Bridge Protocol
Data Unit (BPDU) configuration frames.

Reception of a PMA_LINK.indicate(OK) is therefore insufficient in most
cases to ensure that packets can be sent across the bridged network.
After reception of an OK indication, a host may be able to monitor
received BPDU frames to determine the delay required before packets
may be bridged.  with the Spanning Tree Protocol (STP) this is twice
the ForwardDelay parameter specified in the frame (default 2 * 15
seconds)[802.1d].

If the switch is performing Rapid Spanning Tree Protocols (RSTP), it
instead waits "Bridge Max Age" (advertised in the BPDU's Max Age
field, default: 20 seconds), before forwarding.  For ports
which are known to be point-to-point through autonegotiation, this
delay is abbreviated to 3 seconds after autonegotiation completes[802.1d].

Upon learning that an adjacent port is running STP or RSTP, the
host may send a Link-Up indication with a 'forwarding available'
parameter upon expiry of calculated delays to indicate that general
packet transfer is available across the LAN.

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