RFC 3626
Optimized Link State Routing Protocol (OLSR)
4. Information Repositories
Through the exchange of OLSR control messages, each node accumulates information about the network. This information is stored according to the descriptions in this section.4.1. Multiple Interface Association Information Base
For each destination in the network, "Interface Association Tuples" (I_iface_addr, I_main_addr, I_time) are recorded. I_iface_addr is an interface address of a node, I_main_addr is the main address of this node. I_time specifies the time at which this tuple expires and *MUST* be removed. In a node, the set of Interface Association Tuples is denoted the "Interface Association Set".4.2. Link Sensing: Local Link Information Base
The local link information base stores information about links to neighbors.4.2.1. Link Set
A node records a set of "Link Tuples" (L_local_iface_addr, L_neighbor_iface_addr, L_SYM_time, L_ASYM_time, L_time). L_local_iface_addr is the interface address of the local node (i.e., one endpoint of the link), L_neighbor_iface_addr is the interface address of the neighbor node (i.e., the other endpoint of the link), L_SYM_time is the time until which the link is considered symmetric, L_ASYM_time is the time until which the neighbor interface is considered heard, and L_time specifies the time at which this record expires and *MUST* be removed. When L_SYM_time and L_ASYM_time are expired, the link is considered lost. This information is used when declaring the neighbor interfaces in the HELLO messages. L_SYM_time is used to decide the Link Type declared for the neighbor interface. If L_SYM_time is not expired, the link MUST be declared symmetric. If L_SYM_time is expired, the link MUST be declared asymmetric. If both L_SYM_time and L_ASYM_time are expired, the link MUST be declared lost. In a node, the set of Link Tuples are denoted the "Link Set".
4.3. Neighbor Detection: Neighborhood Information Base
The neighborhood information base stores information about neighbors, 2-hop neighbors, MPRs and MPR selectors.4.3.1. Neighbor Set
A node records a set of "neighbor tuples" (N_neighbor_main_addr, N_status, N_willingness), describing neighbors. N_neighbor_main_addr is the main address of a neighbor, N_status specifies if the node is NOT_SYM or SYM. N_willingness in an integer between 0 and 7, and specifies the node's willingness to carry traffic on behalf of other nodes.4.3.2. 2-hop Neighbor Set
A node records a set of "2-hop tuples" (N_neighbor_main_addr, N_2hop_addr, N_time), describing symmetric (and, since MPR links by definition are also symmetric, thereby also MPR) links between its neighbors and the symmetric 2-hop neighborhood. N_neighbor_main_addr is the main address of a neighbor, N_2hop_addr is the main address of a 2-hop neighbor with a symmetric link to N_neighbor_main_addr, and N_time specifies the time at which the tuple expires and *MUST* be removed. In a node, the set of 2-hop tuples are denoted the "2-hop Neighbor Set".4.3.3. MPR Set
A node maintains a set of neighbors which are selected as MPR. Their main addresses are listed in the MPR Set.4.3.4. MPR Selector Set
A node records a set of MPR-selector tuples (MS_main_addr, MS_time), describing the neighbors which have selected this node as a MPR. MS_main_addr is the main address of a node, which has selected this node as MPR. MS_time specifies the time at which the tuple expires and *MUST* be removed. In a node, the set of MPR-selector tuples are denoted the "MPR Selector Set".
4.4. Topology Information Base
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) is recorded. T_dest_addr is the main address of a node, which may be reached in one hop from the node with the main address T_last_addr. Typically, T_last_addr is a MPR of T_dest_addr. T_seq is a sequence number, and T_time specifies the time at which this tuple expires and *MUST* be removed. In a node, the set of Topology Tuples are denoted the "Topology Set".5. Main Addresses and Multiple Interfaces
For single OLSR interface nodes, the relationship between an OLSR interface address and the corresponding main address is trivial: the main address is the OLSR interface address. For multiple OLSR interface nodes, the relationship between OLSR interface addresses and main addresses is defined through the exchange of Multiple Interface Declaration (MID) messages. This section describes how MID messages are exchanged and processed. Each node with multiple interfaces MUST announce, periodically, information describing its interface configuration to other nodes in the network. This is accomplished through flooding a Multiple Interface Declaration message to all nodes in the network through the MPR flooding mechanism. Each node in the network maintains interface information about the other nodes in the network. This information acquired from MID messages, emitted by nodes with multiple interfaces participating in the MANET, and is used for routing table calculations. Specifically, multiple interface declaration associates multiple interfaces to a node (and to a main address) through populating the multiple interface association base in each node.
5.1. MID Message Format
The proposed format of a MID message is as follows: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | OLSR Interface Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | OLSR Interface Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This is sent as the data-portion of the general packet format described in section 3.4, with the "Message Type" set to MID_MESSAGE. The time to live SHOULD be set to 255 (maximum value) to diffuse the message into the entire network and Vtime set accordingly to the value of MID_HOLD_TIME, as specified in section 18.3. OLSR Interface Address This field contains the address of an OLSR interface of the node, excluding the nodes main address (which already indicated in the originator address). All interface addresses other than the main address of the originator node are put in the MID message. If the maximum allowed message size (as imposed by the network) is reached while there are still interface addresses which have not been inserted into the MIDmessage, more MID messages are generated until the entire interface addresses set has been sent.5.2. MID Message Generation
A MID message is sent by a node in the network to declare its multiple interfaces (if any). I.e., the MID message contains the list of interface addresses which are associated to its main address. The list of addresses can be partial in each MID message (e.g., due to message size limitations, imposed by the network), but parsing of all MID messages describing the interface set from a node MUST be complete within a certain refreshing period (MID_INTERVAL). The information diffused in the network by these MID messages will help each node to calculate its routing table. A node which has only a single interface address participating in the MANET (i.e., running OLSR), MUST NOT generate any MID message.
A node with several interfaces, where only one is participating in the MANET and running OLSR (e.g., a node is connected to a wired network as well as to a MANET) MUST NOT generate any MID messages. A node with several interfaces, where more than one is participating in the MANET and running OLSR MUST generate MID messages as specified.5.3. MID Message Forwarding
MID messages are broadcast and retransmitted by the MPRs in order to diffuse the messages in the entire network. The "default forwarding algorithm" (described in section 3.4) MUST be used for forwarding of MID messages.5.4. MID Message Processing
The tuples in the multiple interface association set are recorded with the information that is exchanged through MID messages. Upon receiving a MID message, the "validity time" MUST be computed from the Vtime field of the message header (as described in section 3.3.2). The Multiple Interface Association Information Base SHOULD then be updated as follows: 1 If the sender interface (NB: not originator) of this message is not in the symmetric 1-hop neighborhood of this node, the message MUST be discarded. 2 For each interface address listed in the MID message: 2.1 If there exist some tuple in the interface association set where: I_iface_addr == interface address, AND I_main_addr == originator address, then the holding time of that tuple is set to: I_time = current time + validity time. 2.2 Otherwise, a new tuple is recorded in the interface association set where: I_iface_addr = interface address, I_main_addr = originator address,
I_time = current time + validity time.
5.5. Resolving a Main Address from an Interface Address
In general, the only part of OLSR requiring use of "interface
addresses" is link sensing. The remaining parts of OLSR operate on
nodes, uniquely identified by their "main addresses" (effectively,
the main address of a node is its "node id" - which for convenience
corresponds to the address of one of its interfaces). In a network
with only single interface nodes, the main address of a node will, by
definition, be equal to the interface address of the node. In
networks with multiple interface nodes operating within a common OLSR
area, it is required to be able to map any interface address to the
corresponding main address.
The exchange of MID messages provides a way in which interface
information is acquired by nodes in the network. This permits
identification of a node's "main address", given one of its interface
addresses.
Given an interface address:
1 if there exists some tuple in the interface association set
where:
I_iface_addr == interface address
then the result of the main address search is the originator
address I_main_addr of the tuple.
2 Otherwise, the result of the main address search is the
interface address itself.
6. HELLO Message Format and Generation
A common mechanism is employed for populating the local link
information base and the neighborhood information base, namely
periodic exchange of HELLO messages. Thus this section describes the
general HELLO message mechanism, followed by a description of link
sensing and topology detection, respectively.
6.1. HELLO Message Format
To accommodate for link sensing, neighborhood detection and MPR
selection signalling, as well as to accommodate for future
extensions, an approach similar to the overall packet format is
taken. Thus the proposed format of a HELLO message is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved | Htime | Willingness |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Code | Reserved | Link Message Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Interface Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Interface Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: . . . :
: :
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Code | Reserved | Link Message Size |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Interface Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Neighbor Interface Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
: :
: :
(etc.)
This is sent as the data-portion of the general packet format
described in section 3.4, with the "Message Type" set to
HELLO_MESSAGE, the TTL field set to 1 (one) and Vtime set accordingly
to the value of NEIGHB_HOLD_TIME, specified in section 18.3.
Reserved
This field must be set to "0000000000000" to be in compliance
with this specification.
HTime
This field specifies the HELLO emission interval used by the
node on this particular interface, i.e., the time before the
transmission of the next HELLO (this information may be used in
advanced link sensing, see section 14). The HELLO emission
interval is represented by its mantissa (four highest bits of
Htime field) and by its exponent (four lowest bits of Htime
field). In other words:
HELLO emission interval=C*(1+a/16)*2^b [in seconds]
where a is the integer represented by the four highest bits of
Htime field and b the integer represented by the four lowest
bits of Htime field. The proposed value of the scaling factor
C is specified in section 18.
Willingness
This field specifies the willingness of a node to carry and
forward traffic for other nodes.
A node with willingness WILL_NEVER (see section 18.8, for
willingness constants) MUST never be selected as MPR by any
node. A node with willingness WILL_ALWAYS MUST always be
selected as MPR. By default, a node SHOULD advertise a
willingness of WILL_DEFAULT.
Link Code
This field specifies information about the link between the
interface of the sender and the following list of neighbor
interfaces. It also specifies information about the status of
the neighbor.
Link codes, not known by a node, are silently discarded.
Link Message Size
The size of the link message, counted in bytes and measured
from the beginning of the "Link Code" field and until the next
"Link Code" field (or - if there are no more link types - the
end of the message).
Neighbor Interface Address
The address of an interface of a neighbor node.
6.1.1. Link Code as Link Type and Neighbor Type
This document only specifies processing of Link Codes < 16.
If the Link Code value is less than or equal to 15, then it MUST be
interpreted as holding two different fields, of two bits each:
7 6 5 4 3 2 1 0
+-------+-------+-------+-------+-------+-------+-------+-------+
| 0 | 0 | 0 | 0 | Neighbor Type | Link Type |
+-------+-------+-------+-------+-------+-------+-------+-------+
The following four "Link Types" are REQUIRED by OLSR:
- UNSPEC_LINK - indicating that no specific information about
the links is given.
- ASYM_LINK - indicating that the links are asymmetric (i.e.,
the neighbor interface is "heard").
- SYM_LINK - indicating that the links are symmetric with the
interface.
- LOST_LINK - indicating that the links have been lost.
The following three "Neighbor Types" are REQUIRED by OLSR:
- SYM_NEIGH - indicating that the neighbors have at least one
symmetrical link with this node.
- MPR_NEIGH - indicating that the neighbors have at least one
symmetrical link AND have been selected as MPR by the sender.
- NOT_NEIGH - indicating that the nodes are either no longer or
have not yet become symmetric neighbors.
Note that an implementation should be careful in confusing neither
Link Type with Neighbor Type nor the constants (confusing SYM_NEIGH
with SYM_LINK for instance).
A link code advertising:
Link Type == SYM_LINK AND
Neighbor Type == NOT_NEIGH
is invalid, and any links advertised as such MUST be silently
discarded without any processing.
Likewise a Neighbor Type field advertising a numerical value which is
not one of the constants SYM_NEIGH, MPR_NEIGH, NOT_NEIGH, is invalid,
and any links advertised as such MUST be silently discarded without
any processing.
6.2. HELLO Message Generation
This involves transmitting the Link Set, the Neighbor Set and the MPR
Set. In principle, a HELLO message serves three independent tasks:
- link sensing
- neighbor detection
- MPR selection signaling
Three tasks are all are based on periodic information exchange within
a nodes neighborhood, and serve the common purpose of "local topology
discovery". A HELLO message is therefore generated based on the
information stored in the Local Link Set, the Neighbor Set and the
MPR Set from the local link information base.
A node must perform link sensing on each interface, in order to
detect links between the interface and neighbor interfaces.
Furthermore, a node must advertise its entire symmetric 1-hop
neighborhood on each interface in order to perform neighbor
detection. Hence, for a given interface, a HELLO message will
contain a list of links on that interface (with associated link
types), as well as a list of the entire neighborhood (with an
associated neighbor types).
The Vtime field is set such that it corresponds to the value of the
node's NEIGHB_HOLD_TIME parameter. The Htime field is set such that
it corresponds to the value of the node's HELLO_INTERVAL parameter
(see section 18.3).
The Willingness field is set such that it corresponds to the node's
willingness to forward traffic on behalf of other nodes (see section
18.8). A node MUST advertise the same willingness on all interfaces.
The lists of addresses declared in a HELLO message is a list of
neighbor interface addresses computed as follows:
For each tuple in the Link Set, where L_local_iface_addr is the
interface where the HELLO is to be transmitted, and where L_time >=
current time (i.e., not expired), L_neighbor_iface_addr is advertised
with:
1 The Link Type set according to the following:
1.1 if L_SYM_time >= current time (not expired)
Link Type = SYM_LINK
1.2 Otherwise, if L_ASYM_time >= current time (not expired)
AND
L_SYM_time < current time (expired)
Link Type = ASYM_LINK
1.3 Otherwise, if L_ASYM_time < current time (expired) AND
L_SYM_time < current time (expired)
Link Type = LOST_LINK
2 The Neighbor Type is set according to the following:
2.1 If the main address, corresponding to
L_neighbor_iface_addr, is included in the MPR set:
Neighbor Type = MPR_NEIGH
2.2 Otherwise, if the main address, corresponding to
L_neighbor_iface_addr, is included in the neighbor set:
2.2.1
if N_status == SYM
Neighbor Type = SYM_NEIGH
2.2.2
Otherwise, if N_status == NOT_SYM
Neighbor Type = NOT_NEIGH
For each tuple in the Neighbor Set, for which no
L_neighbor_iface_addr from an associated link tuple has been
advertised by the previous algorithm, N_neighbor_main_addr is
advertised with:
- Link Type = UNSPEC_LINK,
- Neighbor Type set as described in step 2 above
For a node with a single OLSR interface, the main address is simply
the address of the OLSR interface, i.e., for a node with a single
OLSR interface the main address, corresponding to
L_neighbor_iface_addr is simply L_neighbor_iface_addr.
A HELLO message can be partial (e.g., due to message size
limitations, imposed by the network), the rule being the following,
on each interface: each link and each neighbor node MUST be cited at
least once within a predetermined refreshing period,
REFRESH_INTERVAL. To keep track of fast connectivity changes, a
HELLO message must be sent at least every HELLO_INTERVAL period,
smaller than or equal to REFRESH_INTERVAL.
Notice that for limiting the impact from loss of control messages, it is desirable that a message (plus the generic packet header) can fit into a single MAC frame.6.3. HELLO Message Forwarding
Each HELLO message generated is broadcast by the node on one interface to its neighbors (i.e. the interface for which the HELLO was generated). HELLO messages MUST never be forwarded.6.4. HELLO Message Processing
A node processes incoming HELLO messages for the purpose of conducting link sensing (detailed in section 7), neighbor detection and MPR selector set population (detailed in section 8)7. Link Sensing
Link sensing populates the local link information base. Link sensing is exclusively concerned with OLSR interface addresses and the ability to exchange packets between such OLSR interfaces. The mechanism for link sensing is the periodic exchange of HELLO messages.7.1. Populating the Link Set
The Link Set is populated with information on links to neighbor nodes. The process of populating this set is denoted "link sensing" and is performed using HELLO message exchange, updating a local link information base in each node. Each node should detect the links between itself and neighbor nodes. Uncertainties over radio propagation may make some links unidirectional. Consequently, all links MUST be checked in both directions in order to be considered valid. A "link" is described by a pair of interfaces: a local and a remote interface. For the purpose of link sensing, each neighbor node (more specifically, the link to each neighbor) has an associated status of either "symmetric" or "asymmetric". "Symmetric" indicates, that the link to that neighbor node has been verified to be bi-directional, i.e., it is possible to transmit data in both directions. "Asymmetric" indicates that HELLO messages from the node have been
heard (i.e., communication from the neighbor node is possible), however it is not confirmed that this node is also able to receive messages (i.e., communication to the neighbor node is not confirmed). The information, acquired through and used by the link sensing, is accumulated in the link set.7.1.1. HELLO Message Processing
The "Originator Address" of a HELLO message is the main address of the node, which has emitted the message. Upon receiving a HELLO message, a node SHOULD update its Link Set. Notice, that a HELLO message MUST neither be forwarded nor be recorded in the duplicate set. Upon receiving a HELLO message, the "validity time" MUST be computed from the Vtime field of the message header (see section 3.3.2). Then, the Link Set SHOULD be updated as follows: 1 Upon receiving a HELLO message, if there exists no link tuple with L_neighbor_iface_addr == Source Address a new tuple is created with L_neighbor_iface_addr = Source Address L_local_iface_addr = Address of the interface which received the HELLO message L_SYM_time = current time - 1 (expired) L_time = current time + validity time 2 The tuple (existing or new) with: L_neighbor_iface_addr == Source Address is then modified as follows: 2.1 L_ASYM_time = current time + validity time; 2.2 if the node finds the address of the interface which received the HELLO message among the addresses listed in the link message then the tuple is modified as follows:
2.2.1
if Link Type is equal to LOST_LINK then
L_SYM_time = current time - 1 (i.e., expired)
2.2.2
else if Link Type is equal to SYM_LINK or ASYM_LINK
then
L_SYM_time = current time + validity time,
L_time = L_SYM_time + NEIGHB_HOLD_TIME
2.3 L_time = max(L_time, L_ASYM_time)
The above rule for setting L_time is the following: a link losing its
symmetry SHOULD still be advertised during at least the duration of
the "validity time" advertised in the generated HELLO. This allows
neighbors to detect the link breakage.
8. Neighbor Detection
Neighbor detection populates the neighborhood information base and
concerns itself with nodes and node main addresses. The relationship
between OLSR interface addresses and main addresses is described in
section 5.
The mechanism for neighbor detection is the periodic exchange of
HELLO messages.
8.1. Populating the Neighbor Set
A node maintains a set of neighbor tuples, based on the link tuples.
This information is updated according to changes in the Link Set.
The Link Set keeps the information about the links, while the
Neighbor Set keeps the information about the neighbors. There is a
clear association between those two sets, since a node is a neighbor
of another node if and only if there is at least one link between the
two nodes.
In any case, the formal correspondence between links and neighbors is
defined as follows:
The "associated neighbor tuple" of a link tuple, is, if it
exists, the neighbor tuple where:
N_neighbor_main_addr == main address of
L_neighbor_iface_addr
The "associated link tuples" of a neighbor tuple, are all the
link tuples, where:
N_neighbor_main_addr == main address of
L_neighbor_iface_addr
The Neighbor Set MUST be populated by maintaining the proper
correspondence between link tuples and associated neighbor tuples, as
follows:
Creation
Each time a link appears, that is, each time a link tuple is
created, the associated neighbor tuple MUST be created, if it
doesn't already exist, with the following values:
N_neighbor_main_addr = main address of
L_neighbor_iface_addr
(from the link tuple)
In any case, the N_status MUST then be computed as described
in the next step
Update
Each time a link changes, that is, each time the information
of a link tuple is modified, the node MUST ensure that the
N_status of the associated neighbor tuple respects the
property:
If the neighbor has any associated link tuple which
indicates a symmetric link (i.e., with L_SYM_time >=
current time), then
N_status is set to SYM
else N_status is set to NOT_SYM
Removal
Each time a link is deleted, that is, each time a link tuple
is removed, the associated neighbor tuple MUST be removed if
it has no longer any associated link tuples.
These rules ensure that there is exactly one associated neighbor tuple for a link tuple, and that every neighbor tuple has at least one associated link tuple.8.1.1. HELLO Message Processing
The "Originator Address" of a HELLO message is the main address of the node, which has emitted the message. Likewise, the "willingness" MUST be computed from the Willingness field of the HELLO message (see section 6.1). Upon receiving a HELLO message, a node SHOULD first update its Link Set as described before. It SHOULD then update its Neighbor Set as follows: - if the Originator Address is the N_neighbor_main_addr from a neighbor tuple included in the Neighbor Set: then, the neighbor tuple SHOULD be updated as follows: N_willingness = willingness from the HELLO message8.2. Populating the 2-hop Neighbor Set
The 2-hop neighbor set describes the set of nodes which have a symmetric link to a symmetric neighbor. This information set is maintained through periodic exchange of HELLO messages as described in this section.8.2.1. HELLO Message Processing
The "Originator Address" of a HELLO message is the main address of the node, which has emitted the message. Upon receiving a HELLO message from a symmetric neighbor, a node SHOULD update its 2-hop Neighbor Set. Notice, that a HELLO message MUST neither be forwarded nor be recorded in the duplicate set. Upon receiving a HELLO message, the "validity time" MUST be computed from the Vtime field of the message header (see section 3.3.2). If the Originator Address is the main address of a L_neighbor_iface_addr from a link tuple included in the Link Set with L_SYM_time >= current time (not expired) (in other words: if the Originator Address is a symmetric neighbor) then the 2-hop Neighbor Set SHOULD be updated as follows:
1 for each address (henceforth: 2-hop neighbor address), listed
in the HELLO message with Neighbor Type equal to SYM_NEIGH or
MPR_NEIGH:
1.1 if the main address of the 2-hop neighbor address = main
address of the receiving node:
silently discard the 2-hop neighbor address.
(in other words: a node is not its own 2-hop neighbor).
1.2 Otherwise, a 2-hop tuple is created with:
N_neighbor_main_addr = Originator Address;
N_2hop_addr = main address of the
2-hop neighbor;
N_time = current time
+ validity time.
This tuple may replace an older similar tuple with same
N_neighbor_main_addr and N_2hop_addr values.
2 For each 2-hop node listed in the HELLO message with Neighbor
Type equal to NOT_NEIGH, all 2-hop tuples where:
N_neighbor_main_addr == Originator Address AND
N_2hop_addr == main address of the
2-hop neighbor
are deleted.
8.3. Populating the MPR set
MPRs are used to flood control messages from a node into the network
while reducing the number of retransmissions that will occur in a
region. Thus, the concept of MPR is an optimization of a classical
flooding mechanism.
Each node in the network selects, independently, its own set of MPRs
among its symmetric 1-hop neighborhood. The symmetric links with
MPRs are advertised with Link Type MPR_NEIGH instead of SYM_NEIGH in
HELLO messages.
The MPR set MUST be calculated by a node in such a way that it, through the neighbors in the MPR-set, can reach all symmetric strict 2-hop neighbors. (Notice that a node, a, which is a direct neighbor of another node, b, is not also a strict 2-hop neighbor of node b). This means that the union of the symmetric 1-hop neighborhoods of the MPR nodes contains the symmetric strict 2-hop neighborhood. MPR set recalculation should occur when changes are detected in the symmetric neighborhood or in the symmetric strict 2-hop neighborhood. MPRs are computed per interface, the union of the MPR sets of each interface make up the MPR set for the node. While it is not essential that the MPR set is minimal, it is essential that all strict 2-hop neighbors can be reached through the selected MPR nodes. A node SHOULD select an MPR set such that any strict 2-hop neighbor is covered by at least one MPR node. Keeping the MPR set small ensures that the overhead of the protocol is kept at a minimum. The MPR set can coincide with the entire symmetric neighbor set. This could be the case at network initialization (and will correspond to classic link-state routing).8.3.1. MPR Computation
The following specifies a proposed heuristic for selection of MPRs. It constructs an MPR-set that enables a node to reach any node in the symmetrical strict 2-hop neighborhood through relaying by one MPR node with willingness different from WILL_NEVER. The heuristic MUST be applied per interface, I. The MPR set for a node is the union of the MPR sets found for each interface. The following terminology will be used in describing the heuristics: neighbor of an interface a node is a "neighbor of an interface" if the interface (on the local node) has a link to any one interface of the neighbor node. 2-hop neighbors reachable from an interface the list of 2-hop neighbors of the node that can be reached from neighbors of this interface.
MPR set of an interface
a (sub)set of the neighbors of an interface with a
willingness different from WILL_NEVER, selected such that
through these selected nodes, all strict 2-hop neighbors
reachable from that interface are reachable.
N:
N is the subset of neighbors of the node, which are
neighbor of the interface I.
N2:
The set of 2-hop neighbors reachable from the interface
I, excluding:
(i) the nodes only reachable by members of N with
willingness WILL_NEVER
(ii) the node performing the computation
(iii) all the symmetric neighbors: the nodes for which
there exists a symmetric link to this node on some
interface.
D(y):
The degree of a 1-hop neighbor node y (where y is a
member of N), is defined as the number of symmetric
neighbors of node y, EXCLUDING all the members of N and
EXCLUDING the node performing the computation.
The proposed heuristic is as follows:
1 Start with an MPR set made of all members of N with
N_willingness equal to WILL_ALWAYS
2 Calculate D(y), where y is a member of N, for all nodes in N.
3 Add to the MPR set those nodes in N, which are the *only*
nodes to provide reachability to a node in N2. For example,
if node b in N2 can be reached only through a symmetric link
to node a in N, then add node a to the MPR set. Remove the
nodes from N2 which are now covered by a node in the MPR set.
4 While there exist nodes in N2 which are not covered by at
least one node in the MPR set:
4.1 For each node in N, calculate the reachability, i.e., the
number of nodes in N2 which are not yet covered by at
least one node in the MPR set, and which are reachable
through this 1-hop neighbor;
4.2 Select as a MPR the node with highest N_willingness among
the nodes in N with non-zero reachability. In case of
multiple choice select the node which provides
reachability to the maximum number of nodes in N2. In
case of multiple nodes providing the same amount of
reachability, select the node as MPR whose D(y) is
greater. Remove the nodes from N2 which are now covered
by a node in the MPR set.
5 A node's MPR set is generated from the union of the MPR sets
for each interface. As an optimization, process each node, y,
in the MPR set in increasing order of N_willingness. If all
nodes in N2 are still covered by at least one node in the MPR
set excluding node y, and if N_willingness of node y is
smaller than WILL_ALWAYS, then node y MAY be removed from the
MPR set.
Other algorithms, as well as improvements over this algorithm, are
possible. For example, assume that in a multiple-interface scenario
there exists more than one link between nodes 'a' and 'b'. If node
'a' has selected node 'b' as MPR for one of its interfaces, then node
'b' can be selected as MPR without additional performance loss by any
other interfaces on node 'a'.
8.4. Populating the MPR Selector Set
The MPR selector set of a node, n, is populated by the main addresses
of the nodes which have selected n as MPR. MPR selection is signaled
through HELLO messages.
8.4.1. HELLO Message Processing
Upon receiving a HELLO message, if a node finds one of its own
interface addresses in the list with a Neighbor Type equal to
MPR_NEIGH, information from the HELLO message must be recorded in the
MPR Selector Set.
The "validity time" MUST be computed from the Vtime field of the
message header (see section 3.3.2). The MPR Selector Set SHOULD then
be updated as follows:
1 If there exists no MPR selector tuple with:
MS_main_addr == Originator Address
then a new tuple is created with:
MS_main_addr = Originator Address
2 The tuple (new or otherwise) with
MS_main_addr == Originator Address
is then modified as follows:
MS_time = current time + validity time.
Deletion of MPR selector tuples occurs in case of expiration of the
timer or in case of link breakage as described in the "Neighborhood
and 2-hop Neighborhood Changes".
8.5. Neighborhood and 2-hop Neighborhood Changes
A change in the neighborhood is detected when:
- The L_SYM_time field of a link tuple expires. This is
considered as a neighbor loss if the link described by the
expired tuple was the last link with a neighbor node (on the
contrary, a link with an interface may break while a link with
another interface of the neighbor node remains without being
observed as a neighborhood change).
- A new link tuple is inserted in the Link Set with a non
expired L_SYM_time or a tuple with expired L_SYM_time is
modified so that L_SYM_time becomes non-expired. This is
considered as a neighbor appearance if there was previously no
link tuple describing a link with the corresponding neighbor
node.
A change in the 2-hop neighborhood is detected when a 2-hop neighbor
tuple expires or is deleted according to section 8.2.
The following processing occurs when changes in the neighborhood or
the 2-hop neighborhood are detected:
- In case of neighbor loss, all 2-hop tuples with
N_neighbor_main_addr == Main Address of the neighbor MUST be
deleted.
- In case of neighbor loss, all MPR selector tuples with
MS_main_addr == Main Address of the neighbor MUST be deleted
- The MPR set MUST be re-calculated when a neighbor appearance
or loss is detected, or when a change in the 2-hop
neighborhood is detected.
- An additional HELLO message MAY be sent when the MPR set
changes.
9. Topology Discovery
The link sensing and neighbor detection part of the protocol
basically offers, to each node, a list of neighbors with which it can
communicate directly and, in combination with the Packet Format and
Forwarding part, an optimized flooding mechanism through MPRs. Based
on this, topology information is disseminated through the network.
The present section describes which part of the information given by
the link sensing and neighbor detection is disseminated to the entire
network and how it is used to construct routes.
Routes are constructed through advertised links and links with
neighbors. A node must at least disseminate links between itself and
the nodes in its MPR-selector set, in order to provide sufficient
information to enable routing.
9.1. TC Message Format
The proposed format of a TC message is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ANSN | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertised Neighbor Main Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertised Neighbor Main Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This is sent as the data-portion of the general message format with
the "Message Type" set to TC_MESSAGE. The time to live SHOULD be set
to 255 (maximum value) to diffuse the message into the entire network
and Vtime set accordingly to the value of TOP_HOLD_TIME, as specified
in section 18.3.
Advertised Neighbor Sequence Number (ANSN)
A sequence number is associated with the advertised neighbor
set. Every time a node detects a change in its advertised
neighbor set, it increments this sequence number ("Wraparound"
is handled as described in section 19). This number is sent
in this ANSN field of the TC message to keep track of the most
recent information. When a node receives a TC message, it can
decide on the basis of this Advertised Neighbor Sequence
Number, whether or not the received information about the
advertised neighbors of the originator node is more recent
than what it already has.
Advertised Neighbor Main Address
This field contains the main address of a neighbor node. All
main addresses of the advertised neighbors of the Originator
node are put in the TC message. If the maximum allowed
message size (as imposed by the network) is reached while
there are still advertised neighbor addresses which have not
been inserted into the TC-message, more TC messages will be
generated until the entire advertised neighbor set has been
sent. Extra main addresses of neighbor nodes may be included,
if redundancy is desired.
Reserved
This field is reserved, and MUST be set to "0000000000000000"
for compliance with this document.
9.2. Advertised Neighbor Set
A TC message is sent by a node in the network to declare a set of
links, called advertised link set which MUST include at least the
links to all nodes of its MPR Selector set, i.e., the neighbors which
have selected the sender node as a MPR.
If, for some reason, it is required to distribute redundant TC
information, refer to section 15.
The sequence number (ANSN) associated with the advertised neighbor
set is also sent with the list. The ANSN number MUST be incremented
when links are removed from the advertised neighbor set; the ANSN
number SHOULD be incremented when links are added to the advertised
neighbor set.
9.3. TC Message Generation
In order to build the topology information base, each node, which has been selected as MPR, broadcasts Topology Control (TC) messages. TC messages are flooded to all nodes in the network and take advantage of MPRs. MPRs enable a better scalability in the distribution of topology information [1]. The list of addresses can be partial in each TC message (e.g., due to message size limitations, imposed by the network), but parsing of all TC messages describing the advertised link set of a node MUST be complete within a certain refreshing period (TC_INTERVAL). The information diffused in the network by these TC messages will help each node calculate its routing table. When the advertised link set of a node becomes empty, this node SHOULD still send (empty) TC-messages during the a duration equal to the "validity time" (typically, this will be equal to TOP_HOLD_TIME) of its previously emitted TC-messages, in order to invalidate the previous TC-messages. It SHOULD then stop sending TC-messages until some node is inserted in its advertised link set. A node MAY transmit additional TC-messages to increase its reactiveness to link failures. When a change to the MPR selector set is detected and this change can be attributed to a link failure, a TC-message SHOULD be transmitted after an interval shorter than TC_INTERVAL.9.4. TC Message Forwarding
TC messages are broadcast and retransmitted by the MPRs in order to diffuse the messages in the entire network. TC messages MUST be forwarded according to the "default forwarding algorithm" (described in section 3.4).9.5. TC Message Processing
Upon receiving a TC message, the "validity time" MUST be computed from the Vtime field of the message header (see section 3.3.2). The topology set SHOULD then be updated as follows (using section 19 for comparison of ANSN): 1 If the sender interface (NB: not originator) of this message is not in the symmetric 1-hop neighborhood of this node, the message MUST be discarded.
2 If there exist some tuple in the topology set where:
T_last_addr == originator address AND
T_seq > ANSN,
then further processing of this TC message MUST NOT be
performed and the message MUST be silently discarded (case:
message received out of order).
3 All tuples in the topology set where:
T_last_addr == originator address AND
T_seq < ANSN
MUST be removed from the topology set.
4 For each of the advertised neighbor main address received in
the TC message:
4.1 If there exist some tuple in the topology set where:
T_dest_addr == advertised neighbor main address, AND
T_last_addr == originator address,
then the holding time of that tuple MUST be set to:
T_time = current time + validity time.
4.2 Otherwise, a new tuple MUST be recorded in the topology
set where:
T_dest_addr = advertised neighbor main address,
T_last_addr = originator address,
T_seq = ANSN,
T_time = current time + validity time.
10. Routing Table Calculation
Each node maintains a routing table which allows it to route data, destined for the other nodes in the network. The routing table is based on the information contained in the local link information base and the topology set. Therefore, if any of these sets are changed, the routing table is recalculated to update the route information about each destination in the network. The route entries are recorded in the routing table in the following format: 1. R_dest_addr R_next_addr R_dist R_iface_addr 2. R_dest_addr R_next_addr R_dist R_iface_addr 3. ,, ,, ,, ,, Each entry in the table consists of R_dest_addr, R_next_addr, R_dist, and R_iface_addr. Such entry specifies that the node identified by R_dest_addr is estimated to be R_dist hops away from the local node, that the symmetric neighbor node with interface address R_next_addr is the next hop node in the route to R_dest_addr, and that this symmetric neighbor node is reachable through the local interface with the address R_iface_addr. Entries are recorded in the routing table for each destination in the network for which a route is known. All the destinations, for which a route is broken or only partially known, are not recorded in the table. More precisely, the routing table is updated when a change is detected in either: - the link set, - the neighbor set, - the 2-hop neighbor set, - the topology set, - the Multiple Interface Association Information Base, More precisely, the routing table is recalculated in case of neighbor appearance or loss, when a 2-hop tuple is created or removed, when a topology tuple is created or removed or when multiple interface association information changes. The update of this routing information does not generate or trigger any messages to be transmitted, neither in the network, nor in the 1-hop neighborhood. To construct the routing table of node X, a shortest path algorithm is run on the directed graph containing the arcs X -> Y where Y is any symmetric neighbor of X (with Neighbor Type equal to SYM), the
arcs Y -> Z where Y is a neighbor node with willingness different of
WILL_NEVER and there exists an entry in the 2-hop Neighbor set with Y
as N_neighbor_main_addr and Z as N_2hop_addr, and the arcs U -> V,
where there exists an entry in the topology set with V as T_dest_addr
and U as T_last_addr.
The following procedure is given as an example to calculate (or
recalculate) the routing table:
1 All the entries from the routing table are removed.
2 The new routing entries are added starting with the
symmetric neighbors (h=1) as the destination nodes. Thus, for
each neighbor tuple in the neighbor set where:
N_status = SYM
(there is a symmetric link to the neighbor), and for each
associated link tuple of the neighbor node such that L_time >=
current time, a new routing entry is recorded in the routing
table with:
R_dest_addr = L_neighbor_iface_addr, of the
associated link tuple;
R_next_addr = L_neighbor_iface_addr, of the
associated link tuple;
R_dist = 1;
R_iface_addr = L_local_iface_addr of the
associated link tuple.
If in the above, no R_dest_addr is equal to the main address
of the neighbor, then another new routing entry with MUST be
added, with:
R_dest_addr = main address of the neighbor;
R_next_addr = L_neighbor_iface_addr of one of the
associated link tuple with L_time >=
current time;
R_dist = 1;
R_iface_addr = L_local_iface_addr of the
associated link tuple.
3 for each node in N2, i.e., a 2-hop neighbor which is not a
neighbor node or the node itself, and such that there exist at
least one entry in the 2-hop neighbor set where
N_neighbor_main_addr correspond to a neighbor node with
willingness different of WILL_NEVER, one selects one 2-hop
tuple and creates one entry in the routing table with:
R_dest_addr = the main address of the 2-hop neighbor;
R_next_addr = the R_next_addr of the entry in the
routing table with:
R_dest_addr == N_neighbor_main_addr
of the 2-hop tuple;
R_dist = 2;
R_iface_addr = the R_iface_addr of the entry in the
routing table with:
R_dest_addr == N_neighbor_main_addr
of the 2-hop tuple;
3 The new route entries for the destination nodes h+1 hops away
are recorded in the routing table. The following procedure
MUST be executed for each value of h, starting with h=2 and
incrementing it by 1 each time. The execution will stop if no
new entry is recorded in an iteration.
3.1 For each topology entry in the topology table, if its
T_dest_addr does not correspond to R_dest_addr of any
route entry in the routing table AND its T_last_addr
corresponds to R_dest_addr of a route entry whose R_dist
is equal to h, then a new route entry MUST be recorded in
the routing table (if it does not already exist) where:
R_dest_addr = T_dest_addr;
R_next_addr = R_next_addr of the recorded
route entry where:
R_dest_addr == T_last_addr
R_dist = h+1; and
R_iface_addr = R_iface_addr of the recorded
route entry where:
R_dest_addr == T_last_addr.
3.2 Several topology entries may be used to select a next hop
R_next_addr for reaching the node R_dest_addr. When h=1,
ties should be broken such that nodes with highest
willingness and MPR selectors are preferred as next hop.
4 For each entry in the multiple interface association base
where there exists a routing entry such that:
R_dest_addr == I_main_addr (of the multiple interface
association entry)
AND there is no routing entry such that:
R_dest_addr == I_iface_addr
then a route entry is created in the routing table with:
R_dest_addr = I_iface_addr (of the multiple interface
association entry)
R_next_addr = R_next_addr (of the recorded
route entry)
R_dist = R_dist (of the recorded
route entry)
R_iface_addr = R_iface_addr (of the recorded
route entry).
11. Node Configuration
This section outlines how a node should be configured, in order to
operate in an OLSR MANET.
11.1. Address Assignment
The nodes in the MANET network SHOULD be assigned addresses within a
defined address sequence, i.e., the nodes in the MANET SHOULD be
addressable through a network address and a netmask.
Likewise, the nodes in each associated network SHOULD be assigned addresses from a defined address sequence, distinct from that being used in the MANET.11.2. Routing Configuration
Any MANET node with associated networks or hosts SHOULD be configured such that it has routes set up to the interfaces with associated hosts or network.11.3. Data Packet Forwarding
OLSR itself does not perform packet forwarding. Rather, it maintains the routing table in the underlying operating system, which is assumed to be forwarding packets as specified in RFC1812.
(page 51 continued on part 3)
