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Added 8/Mar/2005

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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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-- Main.GregDaley - 09 Mar 2005

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