The Following Is Normative Text Proposed As an Amendment to P802.11S/D0.03

September2006doc.: IEEE 802.11-06/1481r0

IEEE P802.11
Wireless LANs

UpdatedRA-OLSR Texts for Clause 11A.5
Date: 2006-09-19
Author(s):
Name / Company / Address / Phone / email
Kyeongsoo (Joseph) Kim / STMicroelectronics, Inc. / 1060 East Brokaw Road, MS 212, San Jose, CA95131 / +1-408-451-8137 /
Azman-Osman Lim / National Institute of Information and Communications Technology (NICT) / 3-5 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0289, Japan / +81-774-98-6868 /
Youiti Kado / National Institute of Information and Communications Technology (NICT) / 3-5 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0289, Japan / +81-774-98-6900 /
Bing Zhang / National Institute of Information and Communications Technology (NICT) / 3-5 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0289, Japan / +81-774-98-6820 /
Masanori Nozaki / Oki Electric Industry Co., Ltd. / 2-5-7 Honmachi, Chuo-ku, Osaka 541-0053, Japan / +81-6-6260-0700 /
Oyunchimeg Shagdar / ATR Adaptive Communication Research Laboratories / 2-2-2 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0288, Japan / +81-774-95-1532 /
Mahdad N. Shirazi / ATR Adaptive Communication Research Laboratories / 2-2-2 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0288, Japan / +81-774-95-1506 /
Suhua Tang / ATR Adaptive Communication Research Laboratories / 2-2-2 Hikaridai, Seika-cho, Soraku-gun, Kyoto 619-0288, Japan / +81-774-95-1544 /

Background

This contribution providesupdated texts for RA-OLSR protocol based on the new action frame format and information elements specifications given in a separate contribution [2].

The following is normative text proposed as an amendment to P802.11s/D0.03.

Replace the whole texts in clause 11A.5 withthose given in the followinig pages:

11A.5Radio Aware OLSR Path Selection Protocol (Optional)

11A.5.1Introduction

Radio Aware Optimized Link State Routing (RA-OLSR) protocol is a proactive, link-state wireless mesh path selection protocol based on Optimized Link State Routing (OLSR)protocol [IETF RFC 3626] with extensions from Fisheye State Routing (FSR) protocol and uses radio-aware metrics in forwarding path and multipoint relay (MPR) set calculation. RA-OLSR enables the discovery and maintenance of optimal routes based on a predefined metric, given that each node has a mechanism to determine the metric cost of a link to each of its neighbors. In order to propagate the metric link cost information between nodes, a metric field is used in RA-OLSR control messages. In disseminating topology information over the network, RA-OLSR adopts the following approaches in order to reduce the related control overhead:

• It uses only a subset of nodes in the network, called MPRs, in flooding process;
• It optionally controls (and thereby reduces) the message exchange frequencies based on the Fisheye scopes.

The current RA-OLSR protocol specifications also include an association discovery and maintenance protocol to support non-mesh STAs both internal (associated with MAPs) and external (with MPPs): When a non-mesh STA is a source or a destination of an IEEE 802 data link,RA-OLSR protocol sets up a routing path to the MAP or the MPPassociated with that STA by complementing routing information among MPs with STA association information.

11A.5.2Overview

RA-OLSR is an optimization over a classical link-state routing protocol, tailored for WLAN mesh networks; as such, it inherits all the benefits of link-state routing protocols, including the immediate availability of routeswhen needed, which greatly reduces the initial route setup delay.

The optimization in RA-OLSR mainly focuses on the minimizationof flooding overhead: First,in RA-OLSRonly a selected subset of 1-hop neighbor MPs (i.e., MPRs)is used by an MP in retransmitting control messages. The MPRsare selected such that a broadcast message, retransmitted by these MPRs, can reach all 2-hop neighbor MPsof the selecting MP (i.e., MPR selector). The information required to perform MPR selection is acquired through the periodic exchange of HELLO messages. In addition, RA-OLSRcan also optionally control the message exchange frequencies based on the fisheye scopes to further reduce the amount of control messages exchanges. These techniques significantly reduce the number of retransmissions required to flood a message to all MPs in the network. Second, RA-OLSR requires only partial link state to be flooded in order to provide shortest path routes. The minimal set of link state information required is that all the nodes selected as MPRs must declare the links to their MPR selectors. Additional topological information, if present, may be utilized, e.g., for redundancy purposes.

In wireless mesh networks, unlike traditional mobile ad hoc networks where all mobile nodes are directly participating in routing procedures, legacy STAs that do not support WLAN mesh services are indirectly participating in routing procedures through their associated MAPs or MPPs. To support theseSTAs, RA-OLSR provides information repositories for STA association at MAPs/MPPs — Local Association Base (LAB) and Global Association Base (GAB) — with efficient advertisement and management schemes, which complements a routing table at each MP during path selection process. This is detailed in clause 11A.5.13.

RA-OLSR may optimize the reactivity to topological changes by reducing the maximum time interval for periodic control message transmissions. Furthermore, as RA-OLSR continuously maintains routes to all destinations in the network, the protocol is beneficial for traffic patterns where a large subset of MPs are communicating with another large subset of MPs, and where the [source, destination] pairs are changing over time. The protocol is particularly suited for large and dense networks, as the optimization done using MPRs works well in this context. The larger and denser is a network, the more optimization can RA-OLSR achieves than the classic link-state algorithm.

RA-OLSR is designed to work in a completely distributed manner and does not depend on any central entity. The protocol does not require reliable transmission of control messages: Each MP sends control messages periodically, and can therefore sustain a reasonable loss of some of such messages. Such losses occur frequently in radio networks due to collisions or other transmission problems.

Also, RA-OLSR does not require sequenced delivery of messages. Each control message contains a sequence number which is incremented for each message. Thus the recipient of a control message can, if required, easily identify which information is more recent — even if messages have been re-ordered while in transmission.

Given a network with only single-interface MPs, anMP may deduct the neighbor set directly from the information exchanged as part of link sensing: the “Main Address” of a single interface MP is, by definition, the address of the only interface on that MP.In a network with multiple-interface MPs, additional information is required in order to map interface addresses to main addresses (and, thereby, to MPs). This additional information is acquired through multiple interface declaration (MID) messages, described in clause 11A.5.5.

11A.5.2.1Terminology

Terminology / Description
Main Address / Primary MAC address of the MP, if it has more than one radio interface
Originator Address / The main address of a NODE, which sent a given message
Sender Interface Address / The address of the interface, over which the message was last transmitted
Receiving Interface / The interface, over which the message was received
Associated Station / A station which is associated with a MAP
Local Association Base (LAB) / The table of the associated stations to a given MAP
Global Association Base (GAB) / The information base which maintains the list of the associated stations to all the MAPs in the WLAN Mesh (in other terms, all the Local Association Base of all the MAPs of the WLAN Mesh)
Association Tuple / The information about associated stations is maintained in entries called “association tuples”, either “Local Association Tuple”, in the LAB, or “Global Association Tuple”, in the GAB
Checksum / The value obtained by applying a hash function to the information in the LAB / GAB (or subsets of these Association Bases)

11A.5.3Message Processing and Forwarding

One or more RA-OLSR messages (i.e., IEs; see clause 7.3.2.59 for details) are carried in an RA-OLSR frame (see clause 7.4.5.14 for its format). Here we describe a general rule for processing and forwarding messages included in an RA-OLSR frame.

11A.5.3.1Message Processing and Flooding

Upon receiving an RA-OLSR management frame, an MP examines each of the included messages (i.e., IEs). Based on the “ID”value, the MP can determine the further processing of the message.

An MP may receive the same message several times in a wireless mesh network. Therefore, each MP maintains a “Duplicate Set” where the MP records information about the most recently received messages,by which any duplicate processing of those messagescan be avoided. For such a message, an MP records a “Duplicate Tuple” as follows:

(D_addr, D_seq_num, D_retransmitted, D_iface_list, D_time)

Field / Description
D_addr / The originator address of the message
D_seq_num / The message sequence number of the message
D_retransmitted / A boolean indicating if the message has already been retransmitted
D_iface_list / A list of the addresses of the interfaces on which the message has been received
D_time / Expiration time of the tuple when it must be removed

In an MP, the set of Duplicate Tuples are denoted the “Duplicate set”.

Thus, upon receiving an RA-OLSR management frame, an MPmust perform the following tasks for each encapsulated message:

1. If the RA-OLSR frame contains no messages, the framemust be silently discarded.
2. If the TTL of the message is less than or equal to zero, or if the message was sent by the receiving MP (i.e., the Originator Address of the message is the main address of the receiving MP), then the message must be silently dropped.
3. Processing condition:

a)If there exists a tuple in the duplicate set, where:

D_addr == Originator Address, AND

D_seq_num == Message Sequence Number.

Then the message has alreadybeen processed and must not be processed again.

b)Otherwise, if the MPimplements the message type (i.e., “ID”) of the message, the message must be processed according to the specifications for the message type.

1. Forwarding condition:

a)If there exists a tuple in the duplicate set, where:

D_addr == Originator Address, AND

D_seq_num == Message Sequence Number, AND

the receiving interface (address) is in D_iface_list.

Then the message has already been considered for forwarding and should not be retransmitted again.

b)Otherwise:

1. If the MP implements the message type of the message, the message must be considered for forwarding according to the specifications for the message type.
2. Otherwise, if the MP does not implement the message type of the message, the message should be processed according to the default forwarding algorithm described in clause 11A.5.3.2.
3. Default Forwarding Algorithm

The default forwarding algorithm is the following:

1. If the sender interface address of the message is not detected to be in the symmetric 1-hop neighborhood of the MP, the forwarding algorithm must silently stop here (and the message mustnot be forwarded).
2. If there exists a tuple in the duplicate set where:

D_addr == Originator Address, AND

D_seq_num == Message Sequence Number.

Then the message will be further considered for forwarding if and only if:

D_retransmitted is false, AND

the (address of the) interface which received the message is not included among the addresses in D_iface_list.

1. Otherwise, if such an entry doesn’t exist, the message is further considered for forwarding.
   

If the message is not considered for forwardingafter steps 1 through 3, the processing of this clause stops here (i.e., steps 4 to 8 are ignored). Otherwise, the following algorithm is used:

1. If the sender interface address is an interface address of an MPR selector of this MPAND the TTL of the message is greater than ‘1’, the message must be retransmitted (as described later in steps 6 to 8).
2. If there exists an entry in the duplicate set with the same Originator Address and Message Sequence Number, the entry is updated as follows:

D_time = current time + DUP_HOLD_TIME

The receiving interface MAC address is added to D_iface_list.

D_retransmitted is set to true if and only if the message will be retransmitted according to step 4.

Otherwise a new entry is added to the duplicate set with:

D_addr == Originator MAC Address.

D_seq_num == Message Sequence Number.

D_time = current time + DUP_HOLD_TIME.

D_iface_list contains the receiving interface address.

D_retransmitted is set to true if and only if the message will be retransmitted according to step 4.

If, and only if, according to step 4, the message must be retransmitted then:

1. The TTL of the message is decremented by one.
2. The hop-count of the message is increased by one.
3. The message is broadcast on all interfaces.
4. Considerations on Processing and Forwarding

It should be noted that the processing and the forwarding of messages are two different actions, conditioned by different rules. Processing is related to using the content of the messages, while forwarding is related to retransmitting the same message for other MPs in the network.

Notice that this specification includes a description for both the forwarding and the processing of each known message type (i.e., “ID”).Messages with known message types mustnot be forwarded “blindly” by thealgorithm described in clause 11A.5.3.2. Forwarding (and setting the correct message field in the forwarded, known message) is the responsibility of the algorithm specifying how the message is to be handled and, if necessary, retransmitted. This enables a message type to be specified such that the message can be modified while in transit (e.g., to reflect the route the message has taken). In this regard, it would be possible to bypass the MPR flooding mechanism if for some reasons, classical flooding is required for a message type;in such a case, the forwarding algorithm for that message type will specify simple rebroadcasting ofreceived messages, regardless of MPRs.

By defining a set of message types, which must be recognized by all implementations of RA-OLSR, it will be possible to extend the protocol through introduction of additional message types, while still being able to maintain compatibility with older implementations.

11A.5.3.4Message Emission and Jitter

As a basic implementation requirement, the synchronization of control messages should be avoided. As a consequence, RA-OLSR control messages should be emitted such that they avoid synchronization.

Emission of control traffic from neighbor MPs may— for various reasons (mainly timer interactions with packet processing)— become synchronized such that several neighbor MPs attempt to transmit control traffic simultaneously. This may lead to collisions and hence message loss, possibly the loss of several subsequent messages of the same type.

To avoid such synchronizations, the following simple strategy for emitting control messages is recommended. A node should add an amount of jitter to the interval at which messages are generated. The jitter must be a random value for each message generated. Thus, for anMP utilizing jitter:

Actual message interval = MESSAGE_INTERVAL – jitter

where jitter is a random value in [0, MAXJITTER] and MESSAGE_INTERVAL is the value of the message interval specified for the message being emitted (e.g., HELLO_INTERVAL for HELLO messages, TC_INTERVAL for TC-messages, etc.).

Jitter SHOULD also be introduced when forwarding messages.The following simple strategy may be adopted: When a message is to be forwarded by anMP, it should be kept in the MP during a short period of time:

Keep message period = jitter

where jitter is a random value in [0, MAXJITTER]. Notice that when the MP sends a control message, the opportunity to piggyback other messages (before their keeping period is expired) maybe taken to reduce the number of frame transmissions.

11A.5.4Information Repositories

Through the exchange of RA-OLSR control messages, each node accumulates information about the network. This information is stored according to the descriptions in this section.

11A.5.4.1Link Set

A node records a set of “link tuples”:

(L_local_iface_addr, L_neighb_iface_addr, L_time, L_link_metric)

describing links between local and remote interfaces. The tuples in this set are maintained by some other component of 802.11 and populated using Neighbor Table Entry whose state is subordinate link up or Superordinate, link up as described in clause 11A.3.5.1.

Field / Description
L_local_iface_addr / The interface on the local MP
L_neighb_iface_addr / The interface on the remote MP, with which there exists a symmetric link
L_time / The expiration time of this tuple
L_link_metric / The value representing the metric cost of the link. An example is the Airtime cost given in clause 11A.4.2.1

11A.5.4.2Neighbor Set

An MP records a set of “neighbor tuples”:

(N_neighb_main_addr, N_willingness)

describing neighbors.

Field / Description
N_neighb_main_addr / The interface on the local MP
N_willingness / An integer, between 0 and 7, specifying the nodes willingness to carry traffic on behalf of other MPs

11A.5.4.3Interface Association Set

For each destination in the network, “Interface Association Tuples” are recorded:

(I_iface_addr, I_main_addr, I_time)

Field / Description
I_iface_addr / An interface address of a node
I_main_addr / The main address of a node
I_time / The expiration time of the tuple

11A.5.4.42-hop Neighbor Set

A node records a set of “2-hop tuples”:

(N_neighb_main_addr, N_2hop_addr, N_time)

describing symmetric links between its neighbors and the symmetric 2-hop neighborhood.

Field / Description
N_neighb_main_addr / The main address of a neighbor
N_2hop_addr / The main address of a neighbor, which has a symmetric link to N_neighbor_main_addr
N_time / The expiration time of the tuple

11A.5.4.5MPR Set

An MP maintains a set of neighbors which are selected as MPR. Their main addresses are listed in the MPR Set.

11A.5.4.6MPR Selector Set

An MP records a set of “MPR-selector tuples”:

(MS_main_addr, MS_time)

describing the neighbors which have selected this MP as an MPR.

Field / Description
MS_main_addr / The main address of an MPR-selector
MS_time / The expiration time of the tuple

11A.5.4.7Topology Set

Each node in the network maintains topology information about the network. This information is acquired from TC-messages and is used for routing table calculations. Thus, for each destination in the network, at least one “Topology Tuple”:

(T_dest_addr, T_last_addr, T_seq, T_time, T_link_metric)

is recorded.

Field / Description
T_dest_addr / The main address of a node, which may be reached in one hop from the node with main address T_last_addr
T_last_addr / An MP which can reach T_dest_addr in one hop
T_seq / A sequence number
T_time / The expiration time of the tuple
T_link_metric / The value representing the metric cost of the link. If more than one link exists, the minimum cost (i.e., the cost of the link with the best quality) should be used. An example of link metric is the Airtime cost given in clause 11A.4.2.1.

11A.5.4.7.1Local Association Base (LAB)

Each MAP, as a result of the 802.11 association protocol, keeps a set of associated stations, denoted Local Association Base” (LAB) in this document, which holds “Local Association Tuple”, one for each associated station. Additionally, to provide support for a large number of stations, each MAP divides its LAB into blocks of local association tuples: each block is a subset of the LAB, and the LAB is the union of those subsets. The blocks of the LAB are numbered; hence each block is identified by an integer, the “block index”.