Re: [DNA] (For reference) text with unconfirmed modifications to L2indications Ethernet/802.3 section

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

Hi Dan,

I'll take your suggestion.

There's a little iteration to be done from Tom's guidance,
but I'm agreeing with his text suggestions, so I'll send
the text which has been merged.

Greg

Romascanu, Dan (Dan) wrote:
> Greg,
> 
> 
> One comment:
> 
> 
>>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.
>>
> 
> 
> This is not accurate. LLDP may tell you that on the other end of the
> link there is a bridge or repeater, but not whether frame forwarding is
> immediately available. I suggest to wipe out this paragraph. 
> 
> Could you send me a 'clean' version, without editorial brackets to
> forward to the Ethernet and Bridge MIB lists for further scrutiny? 
> 
> Dan
> 
> 
> 
> 
>  
> 
> 
>>-----Original Message-----
>>From: owner-dna@ecselists.eng.monash.edu.au 
>>[mailto:owner-dna@ecselists.eng.monash.edu.au] On Behalf Of Greg Daley
>>Sent: Tuesday, June 07, 2005 8:50 AM
>>To: dna@eng.monash.edu.au
>>Subject: [DNA] (For reference) text with unconfirmed 
>>modifications to L2 indications Ethernet/802.3 section
>>
>>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.
>>
>>
> 
>