Subject: [DNA] (forward) Review of Link-information

From: Greg Daley <[email-masked]>
Date: Tue, 08 Mar 2005 08:00:48 +0000 (GMT)

Hi Alper, WG, 

Here's the review of link-information from
Bernard.

I'll work with Alper over the next day or two
to get this into a list of trackable issues.

The review follows after --

Greg
--

Here you go.  My comments denoted with [BA].

-------------------------------------------------------------------------
 Review of draft-ietf-dna-link-information-01.txt

 [BA] Overall, my take is that this document does not clearly
distinguish
 Link Establishment/Termination events from "Up"/"Down" link state
changes.
 As I understand it, DNA cares mostly about Link Establishment events,
not
 about whether a link is encountering low or high frame loss at a given
 instant.  In DNA, bi-directional reachability at the IP layer
 determines whether a link is suitable for use, so that link quality
 monitoring is not required.

 It is important for the document to clarify the usage of terms, since
the
 definition of the "Up" and "Down" link states can be somewhat murky in
 wireless networks. Please see: "The Mistaken Axioms of Wireless Network
 Research":
 http://www.pdos.lcs.mit.edu/decouto/papers/kotz03.pdf

 1.  Introduction

 "   The changes on the underlying link-layer status can be relayed to
IP
    in the form of link-layer event notifications.  Establishment and
    tear down of a link-layer connection are two basic events types."

 [BA] I think you are referring to explicit messages resulting in
 establishment (e.g. PPP IPCP) or teardown (PPP LCP-terminate) of the
 link-layer.  However, in wireless technologies the mobile node can
wander
 out of range or suffer from high frame loss for other reasons;  in
these
 cases there can be links in intermediate states between "up" (low loss)
 and "down" (high loss).  The distinction between link
 establishment/teardown events and the link state is an important one.

   "Additional information can be conveyed in addition to the event
type,
    such as the identifier of the network attachment point, or
    network-layer configuration parameters obtained via the link-layer
    attachment process."

 [BA] By identifier I assume you mean MAC layer address, no?  Of are you
 referring perhaps to "network identifiers" such as SSID?  I would add
 "if available" to the end of the sentence, since some link technologies
 do not provide network-layer configuration.

    For example, the
    notification indicating that the node has established a new
    link-layer connection can be used for immediately probing the
network
    for a possible configuration change.  In the absence of such a
    notification from the link-layer, IP has to wait for indications
that
    are not immediately available, such as receipt of next scheduled
    router advertisement, unreachability of the default gateway, etc.

 [BA] This seems to imply that assignment of a new address via PPP IPCP
 (an event) would need to be "confirmed" by DNA.  The current DNAv4 spec
 assumes that this is not necessary;  essentially the bi-directional
 reachability established via PPP IPCP is "good enough" to not require
 an additional demonstration of default gateway reachability.

 "  Two basic link-layer events are considered potentially useful to DNA
    process: link up and link down.  Both of these events are
    deterministic, and their notifications are provided to IP-layer
after
    the events successfully conclude."

 [BA] The "Up" and "Down" link states are only determinstic on wired
 networks.  On wireless networks, frame loss can be intermediate between
 the "up" and "down" states, so that link state indications may not be
 reliable.

    Node's establishment of a link-layer connection with an attachment
    point that signifies the availability of IP service (i.e., being
able
    to send and receive IP packets) between the two is considered a link
    up event.

 [BA] I think we need to distinguish changes in link state due to frame
 loss from "Link Establishment" and "Link Termination" events.  As used
 in this document I am not clear whether "Link Up" refers to the state
 of the link or whether we are really talking about a "Link
Establishment"
 event. For example, a link can be established, and then experience high
 frame loss (e.g. mobile node wanders out of range).  Does this then
 constitute a "Link Down" event, followed by a "Link Up" event when
 frame loss becomes low again?  For the purposes of DNA, don't we
 only need to care about "Link Establishment" events (not even
 "Link Termination")?

    [TO-DO: How about ad-hoc networks? Attached neighbors may be
    considered attachment points].

 [BA] Adhoc networks are tricky because each adjacency is a "link" which
 may be in an intermediate state between "up" and "down".  Also, there
 may not be a clearly delineated point at which IP traffic can be sent.
 For example, in 802.11 adhoc it is possible to send data frames with
 "For DS" and "To DS" both set to zero, even in state 1 (unassociated,
 unauthenticated).  Given this, when is the link "established"?  One
 view could be that it is only established when the destination is
 enabled to forward packets to other nodes.

    By the time the notification is
    delivered, the link-layer of the node must be ready to accept IP
    packets from the IP and the physical-layers.

 [BA] This sentence and other text related to specific link technologies
 leads me to believe that "Link Up" as used in this document refers to a
 "Link Establishment" event, rather than the link state.  Is that right?

    Link down event signifies the discontinuation of the IP service
    between the node and the attachment point.  When the link-layer
    connection is physically or logically torn down and it can no longer
    carry IP packets, this is considered to be a link down event.

 [BA] Here you are mixing "Link Termination" events with the "down"
 link state. There may be no explicit teardown event.  A node may
 wander out of range, or may experience multi-path interference, causing
 high loss.

    Among these two events the first one to take place after an
interface
    becomes enabled must be a link up event.  During the time a network
    interface is enabled, it may go through a series of link up and down
    events.  Each time the interface changes its point of attachment, a
    link down event with the previous attachment point must be followed
    by a link up event with the new one.

 [BA] The problem is that in some implementations the "Link Down" event
 may (mistakenly) result in teardown of TCP connections.  As a result,
only
 a series of "Link Establishment" messages are seen.  Also in the above
 paragraph I am not clear if you are talking about the up/down link
states
 or explicit establishment/termination events.

    Furthermore, IP-layer configuration
    parameters obtained during link-layer connection may be exactly what
    the DNA process is trying to discover (e.g., IP address configured
    during PPP link establishment).

 [BA] This confuses me.  Why would DNA need to be invoked in the case
 where the network configuration is set by the link layer?

    3GPP2 networks use the Point-to-Point Protocol (PPP [RFC1661]) as
the
    link-layer protocol between the MS and the PDSN.  Before any IP
    packets may be sent or received, PPP must reach the Network-Layer
    Protocol phase, and the IP Control Protocol (IPCP [RFC1332], IPV6CP
    [RFC2472]) reach the Opened state.  When these states are reached in
    PPP, a link up event notification must be delivered to the IP-layer.

 [BA] It is also true that most hosts interface to GPRS/EDGE/UMTS
networks
 via PPP as well.  So the statement above seems more general, and you
might
 include a subsection on PPP early on, then reference it.

    Since there is no
    standards-mandated correlation between the interface-identifier and
    other IP-layer configuration parameters, this information is deemed
    not useful for DNA (hence it is not provided as auxiliary
    information).

 [BA] Saying it is useful for DNA is not the same as saying it is not
 provided.  Since the information may be used, it does need to be
available
 to the Internet layer.

 2.3  IEEE 802.11/WiFi

    A STA must establish a IEEE
    802.11 link with an AP in order to send and receive IP packets.  In
a
    WiFi network that supports Robust Secure Network (RSN
    [IEEE-802.11i]), successful completion of 4-way handshake between
the
    STA and AP commences the availability of IP service.  The link up
    event notification must be generated upon this event.  In
    non-RSN-based networks, successful association or re-association
    events on the link-layer must cause a link up notification sent to
    the IP-layer.

    As part of the link establishment, Basic Service Set Identification
    (BSSID) and Service Set Identifier (SSID) associated with the AP is
    learned by the STA.  BSSID is a unique identifier of the AP.  Its
    value is set to the MAC address of the AP.

 [BA] I would delete the last sentence; an AP may have multiple MAC
 addresses (e.g. on the wired side and on the wireless side).

    In ad-hoc mode, mobile station (STA) in range may directly
    communicate with others, i.e., without any infrastructure or
    intermediate hop.  The set of communicating STAs is called IBSS for
    Indepedant Basic Service Set.  In an IBSS, only station services are

 Indepedant -> Independent

    available, i.e.  authentication, deauthentication, privacy and MSDU
    delivery.  STAs do not associate with each other, and therefore may
    exchange data frames in state 2 (authenticated and not associated)
or
    even in state 1 (unauthenticated and unassociated) if authentication
    is not used.

 [BA] They can exchange data frames in state 1 only if "To DS" and "From
 DS" bits are clear.  This is not dependent on authentication (e.g.
State
 1, not State 2).

    Although a link up indication can be generated upon
    authentication, one may not be present per latter usage.  If
    authentication is performed, a deauthentication event is used for
    generating the link down indication.  Concerning the link layer
    identification, both the BSSID (which is a random MAC address chosen
    by a STA of the IBSS) and SSID may be used to identify a link, but
    not to make any assumptions on the IP network configuration.

 [BA] This is a reasonable guess at how it should work, but I'm not
clear
 that it is strictly correct.  If the goal is only for two nodes to
 exchange data frames, this can occur in State 1 without authentication.
 So if the packets can be exchanged, the link can be "up".  Without
 explicit link establishment, whether the link is "up" or "down" becomes
 dependent on the frame loss, which is non-deterministic.

 To clear this up, you might ask Bob O'Hara, editor of 802.11ma.

    [IEEE-802.11i]
               Institute of Electrical and Electronics Engineers, "Draft
               Supplement to STANDARD FOR Telecommunications and
               Information Exchange between Systems - LAN/MAN Specific
               Requirements - Part 11: Wireless Medium Access Control
               (MAC) and physical layer (PHY) specifications:
               Specification for Enhanced Security", IEEE Draft 802.11I/
               D8, February 2004.

 [BA] This document is final now.

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