[DNA] (For reference) text with unconfirmed modifications to L2 indicationsEthernet/802.3 section

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

Hi DNA WG,

After Tom and Dan's reviews of the 802.3 section
for the link information draft, here's a summary
of the text proposals:

These have not been confirmed on-list, but are placed
here for reference in order that the potential changes
can be all seen together.

Greg


-------------------------------------------------------


Section title:   IEEE 802.3 CSMA/CD
------------------------------------

(begin revised)
   IEEE 802.3 CSMA/CD (commonly referred to as 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].
   In order to provide connection of different LANs together into
   a larger network, 802.3 LANs are often bridged together[802.1d].

   In this section, the terms 802.3 and Ethernet are used
   interchangeably.  This section describes some issues in providing
   link-layer indications on Ethernet networks, and shows how bridging
   affects these indications.
(end revised)

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.

(begin revised)
   In order to determine whether the physical medium is ready for
   frame transfer, IEEE 802.3 Ethernet specifies its own link
   monitoring mechanism, which is defined for some, but not all
   classes of media.
   Where available, this 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 twisted pair copper, light levels in fibre).
(end revised)


Subsection: Link Integrity Tests in 802.3 networks.
---------------------------------------------------

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].

(begin revised)
   Such indications do not definitively ensure that packets will
   be able to be received through the bridge domain, though.  Such
   operations are governed by bridging.
(end revised)


Subsection: IEEE 802.1D Bridging and its effects on link indications
--------------------------------------------------------------

(begin revised)
   Ethernet networks are commonly bridged.  Bridging is a mechanisms
   which provides loopless paths across LANs interconnected by
   IEEE 802.1D bridges.


   Due to its necessity in preventing bridge loops, bridging has
   become synonymous with the spanning tree protocol.
   The spanning tree protocol and its variants provide loopless
   forwarding between ethernet-like bridges within a bridging domain.

   The Spanning Tree Protocol (STP) exchanges Bridge Protocol Data Unit
   (BPDU) frames in order to pass information about which
   bridges are connected to others.

   By default, the spanning tree protocol does not know whether
   a particular newly connected piece of ethernet will cause a
   loop.

   Therefore it will block all traffic from and to a newly connected
   ports with the exception of some unbridged management frames.
   The STP will determine if the port can be connected to the network in
   a loop-free environment.

   For these technologies, even though the link-layer appears
   available, no forwarding will occur until it is determined that
   the port can be connected to the network in a loop-free
   environment.
(end revised)

(start added context)
   For host which are providing indications to upper layer protocols,
   even if the host itself does not implement bridging or STP,
   packet delivery across the network can be affected by their
   directly adjacent LAN peer which may be a bridge, or if this is
   a repeater, by the presence of STP bridges upon that LAN.
(end added context)

(start revised)
   Where the host is not running STP itself, no explicit indication
   that forwarding has begun is sent from a switch to its peer devices.
   Therefore, a host may not know when STP operations have completed,
   and when it is safe to inform upper layers to transmit packets.
(end revised)

As described in (Subsection on 802.1AB LLDP) explicit
knowledge may be available through the new Link-Layer
Discovery Protocol as to whether frame forwarding operation
is immediately available.

Where it is not known that forwarding operations are
available, a host needs to assume that STP is being
performed, and may indicate full connectivity only based on
reception of BPDUs or timeouts.

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.

(begin revised)
   If no bridge configuration messages are received within the
   default Bridge_Max_Age interval (20s), then it is likely that
   the peer's port is Disabled for bridging (S8.4.5 of [802.1D]),
   or the device is not a bridge, since at least two BPDU hello
   messages have been lost.

   It is not easy for a non-STP host to distinguish between
   Disabled ports and non-bridge ports with no IP nodes on them,
   as Disabled ports will have no traffic on them, and incur
   100% sender loss.

   Upon this Bridge_Max_Age timeout, a host may (re)-send a
   link-layer indication showing that packets sent within the prior
   interval were likely to have traversed the forwarding path
   (unless the port is disabled).
(end revised)

If a BPDU is received, and the peer is running the original
Spanning Tree Protocol, then host cannot successfully send
packets until at least the peer's ForwardDelay timer has expired
twice.   This timeout defaults to 30 seconds (2 * 15 seconds)
[802.1d].  After this time, the host can send a link-layer up
indication showing that the previous interval after the
PMA_LINK.indicate(OK) event was one of loss.

If the switch is identified as performing Rapid Spanning Tree
Protocol (RSTP), it instead waits Bridge_Max_Age after packet
reception (advertised in the BPDU's Max Age field), 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].
After either of these delays, a link-layer indication updating the
PMA_LINK.indicate(OK) can show that the interval between
indications was a lossy interval.


Subsection: 802.1AB Link-Layer Discovery Protocol.
-------------------------------------------------

The recently defined 802.1AB Link-Layer Discovery Protocol
(LLDP) provides information to directly connected devices
about its MAC peer[802.1AB].

LLDP sends information periodically, and at link status
change time to indicate the configuration parameters of the device.
Devices may either send or receive these messages, or both.

(begin revised)
   The LLDP message may a System Capabilities TLV, which
   describes the MAC and IP layer functions which a device is
   currently using. Where a host receives the
   Systems Capabilities TLV which indicate that no Bridging or
   Repeating is occurring on the link-layer peer, then no delays
   for STP calculation will be applied to packets sent from this
   host, if does not perform STP itself.  This would allow the
   host's link-layer to indicate that to its knowledge the link
   is available for packet transfer.
(end revised)

Additionally, if a host receives a Systems Capabilities TLV
which indicates that the peer is a switch or repeater, the
host's advertisement that it is an (end-host) Station-Only,
may tell the peer switch not to run STP, and immediately
allow forwarding.

Proprietary extensions may also indicate that data forwarding
is already available on such a port.  Discussions of such
optimizations is out-of-scope for this document.

Due to the protocol's newness and lack of deployment, it is
unclear how this protocol will eventually affect DNA in IPv4
or IPv6 networks.


Summary
-------

Link-Layer indications in Ethernet-like networks are
complicated by additional unadvertised delays due to Spanning
Tree calculations.  This may cause re-indication or retraction
of indications previously sent to upper layer protocols.