Hybrid Architectures:Networks Exuding both DTN and MANET Characteristics

Darien HirotsuUC Santa Cruz

Introduction

Gregg Bachmeyer Integrating UMTS and Bluetooth Integrating Infrastructure-based and

Infrastructure-less Networks

Darien Hirotsu Integrating DTN and MANET

Paradigms

A Brief Delay/Disruption Tolerant Network (DTN) Introduction

An Introduction to Delay/Disruption Tolerant Networks (DTN’s)

My apologies to Rance and Philip… I hope I don’t steal your thunder here (I promise to

keep this short) Feel free to interrupt/augment any explanations at any

time

What constitutes a DTN? Traditional IP networks (both wired and even MANET)

suffer in performance when seeing certain qualities:▪ Large delays▪ Frequent disruptions▪ Disconnected network graphs

The DTN architecture attempts to address these issues

An Introduction to Delay/Disruption Tolerant Networks (DTN’s)

Examples where a DTN architecture helps: Sparsely connected sensor networks

Networks with frequent disconnections due to node mobility (interplanetary networks)

An Introduction to Delay/Disruption Tolerant Networks (DTN’s)

The DTN protocol stack provides a bundle layer

This allows for a DTN overlay network to be built over the existing architecture

DTN nodes typically use spray-and-wait between hops

Note that internet routers can be integrated between existing DTN nodes

Why do we need a hybrid MANET and DTN architecture?

Why is there a need for this “hybrid architecture?”

DTN Assumptions: Connectivity is typically achieved without fixed

infrastructure Nodes move independently and freely Nodes are typically NOT CONNECTED due to node

mobility or sparse node density▪ The topology that a node sees is limited to ONLY what is

“locally connected”▪ A route typically does NOT exist between the sender

and receiver▪ As a result,a persistent store and forward model is

needed to allow traffic to be routed and forwarded to the end destination

Why is there a need for this “hybrid architecture?”

MANET Assumptions: Connectivity is typically achieved without fixed

infrastructure Nodes move independently and freely (although

only moderate movement is tolerable) Nodes are inherently CONNECTED via a reasonably

dense or stable topology or redundancy of routes▪ A full topology of the network is maintained by each

node▪ If a route to an end-node does not exist with protocols

like AODV or DSR, packets are not routed or forwarded to the next-hop

▪ If a stale route exists, packets are “black-holed”

Why is there a need for this “hybrid architecture?” There are many scenarios in which a

MANET would benefit from DTN capability Sensors in a densely connected MANET

may experience battery failure causing sparse connectivityXX X

XX X

X X

Failed sensors may cause A to become disconnected from B

If A and B were DTN nodes, A could “save” the data until a path to B became available again

In the MANET, scenario all may be lost

A

B

C

D

Why is there a need for this “hybrid architecture?” There are many scenarios in which a

DTN may benefit from MANET features DTN’s typically make forwarding

decisions based on node availability rather than optimal paths

Suppose C moves at a constant rate with slight pauses between points 1, 2

At point 1, C can forward traffic between A, B

A MANET could p0tentially discover the more optimal path (A-C-B) over the typical one (A-D-E-B)

A

B

1

2

C

D

E

What types of solutions are available to account for this?

Methods to account for DTN and MANET like qualities

Connectivity-focused routing solutions Implement a routing protocol that can build BOTH

optimal end-to-end paths when available but also leverage store-and-forward if needed▪ HYMAD▪ HYbrid MANET-DTN routing protocol

▪ DTN-AODV

“Cooperative” routing solutions Allow nodes to cooperate to use available

bandwidth optimally▪ TIL▪ Transient Information Level based routing

HYMAD Routing Protocol

An Illustration to Show the Need for a Hybrid Architecture

Higher node density makes a strong case for MANET since a path likely exists end-to-end

High node mobility makes a strong case for DTN since the network is likely to become disconnected

A hybrid network that falls in between the extremes could benefit from both paradigms

Also, networks can flip-flop between the two types to have characteristics of both

HYMAD

The goal of HYMAD is to group individual nodes such that only groups need to exchange data between each other

Inter-group routing can be achieved via DTN. Intra-group routing can be achieved via MANET. This simplifies the topology from 12 connected nodes to 3 connected groups.

HYMAD

Intra-group routing (control plane) Distance vector routing messages to

achieve mesh like connectivity

Nodes within a group exchange distance vector routing messages much like AODV.

HYMAD Inter-group routing (control plane)

Border nodes are elected for the group Border nodes use a flag in the routing messages to discover

each other Spray-and-wait routing of data between border nodes

Nodes A and B are border nodes use a spray-and-wait type routing to forward messages between groups.

AB

HYMAD Data plane operation

Suppose A wants to send a “bundle” of data to B▪ A forwards the message to the border node C

The data from A is first forwarded to C which is the border node. C notes that A is the “custodian” for the bundle. C being a border node has routing knowledge of both Group 1 and 2.

A

B

CGroup 1

Group 2

Group 3

HYMAD Data plane operation

Suppose A wants to send a “bundle” of data to B▪ C forwards the bundle to node D which is a border

node for Group 2

Border nodes maintain a “messages-in-group-list” which contains a list of data needing to be delivered. C compares its list with D and sees that D does not have the bundle from A, so it’s forwarded from C to D.

A

B

C D

Group 1Group 2

Group 3

HYMAD Data plane operation

Suppose A wants to send a “bundle” of data to B▪ D randomly elects E as the “custodian” of the bundle

and forwards it

D sees that the destination B is not within its group. D elects E randomly to be the “custodian” of the DTN bundle from “A” and forwards it accordingly. D adds the bundle to its “messages-in-group-list.”

A

B

C D

Group 1Group 2

Group 3E

HYMAD Data plane operation

Suppose A wants to send a “bundle” of data to B▪ E happens to be a border node as well to Group 3

E references its “messages-in-group-list” and sees F is missing the bundle in A. E forwards the “bundle” accordingly via “spray-and-pray.”

A

B

C D

Group 1Group 2

Group 3EF

HYMAD Data plane operation

Suppose A wants to send a “bundle” of data to B▪ F sees B is in its group and forwards the “bundle” as

needed

F sees that B is a member of its group. F forwards the datagram to B using distance vector like routing.

A

B

C D

Group 1Group 2

Group 3EF

HYMAD Group formation (control plane revisited)

Intra-group disconnections may cause new groups to form▪ Border nodes are re-elected▪ “Messages-in-group” list allows “bundles” to be re-forwarded as

needed Dmax is a group diameter that represents the maximum

number of allowable hops to reach the farthest node of a group (restricts group membership)

When routes are lost due to disconnections, data is stored by the “custodian” rather than dropped

A B

Group 1 may disconnect into different groups should node mobility cause link disconnections

If Dmax=2, node B cannot join the adjacent group since it’s 3 hops away from node A.

HYMAD Benefits of HYMAD:

Outperforms a standard spray-and-wait forwarding scheme in terms of delay and delivery-ratio over a variety of mobility scenarios▪ Random Waypoint (synthetic)▪ Rollernet (trace-based group movement)

Performance in terms of overhead is NOT dependent on Dmax▪ Increasing Dmax increases the group size which

increases the amount of control plane messages within the group

▪ As a tradeoff, this also decreases the amount of control messages between groups

HYMAD Detriments of HYMAD:

Over a wide variety of scenarios, Epidemic routing still outperforms HYMAD in terms of delay and delivery-ratio▪ In Epidemic routing, random pairs of nodes that establish

connectivity exchange information as needed

Performance is strongly tied to timer values of the control plane messages

All nodes need to be DTN nodes ▪ All nodes need to have storage for the batch layer

Assumes that the “custodian” of data can store data for a period long enough for delivery ▪ How does it deal with storage failure?

DTN-AODV

DTN-AODV The goal of AODV with DTN capability is to

extend the reach of AODV by discovering DTN nodes

If DTN nodes (D) are able to speak AODV at the IP layer, DTN nodes can extend the reach of AODV to allow nodes such as originator (O) to reach targets that would be unreachable (T) under typical circumstances.

DTN-AODV

Integrating AODV with DTN: Need to expand the route discovery process

and processing of RREQ/RREP messages▪ Add a flag in the AODV header to tell nodes to

check for the DTN header shown below

DTN-AODV

Integrating AODV with DTN (RREQ):▪ O wants to send data to T▪ O generates a RREQ as per usual

O

TO generates a RREQ and forwards it to adjacent nodes. Node A is a non-DTN AODV speaker, so it does not toggle the DTN flag in the header, but still forwards the RREQ accordingly.

A

DTN-AODV

Integrating AODV with DTN (RREQ):▪ O wants to send data to T▪ B is a DTN speaker so it toggles DTN bit in the header and

adds its DTN header telling AODV nodes it has DTN capability

O

T

A B

B toggles the DTN bit in the AODV header. B also adds the DTN Router Info extension to the RREQ including a hop count to O. The RREQ is then forwarded normally.

DTN-AODV

Integrating AODV with DTN (RREQ):▪ O wants to send data to T▪ D also is a DTN node so it adds another Router Info. Extension

header to the RREQ and adds B to its list of DTN nodes

O

T

A B

C is a non-DTN node, so it forwards the RREQ per normal AODV. D sees the DTN bit toggled already, but it still needs to add its Router Info header to the RREQ. It also adds node B to its list of DTN aware nodes establishing a full DTN overlay topology.

C

D

DTN-AODV

Integrating AODV with DTN (RREP):▪ O wants to send data to T▪ T sees that it’s the target for node O so it generates

an RREPO

T

A B

T generates the RREP and copies all Router Info extension headers from the RREQ onto the RREP and forwards it normally. If node T does not understand the DTN extension, all Router Info extension headers are dropped.

C

D

DTN-AODV

Integrating AODV with DTN (RREP):▪ O wants to send data to T▪ Nodes may also learn about DTN information based

on RREPO

T

A B

Node E can also see RREP packets and also learn about DTN nodes B and D through that process. This ensures robustness for nodes in learning the DTN overlay network.

C

D

E

DTN-AODV

Integrating AODV with DTN (RREP):▪ O wants to send data to T▪ Node O can make in intelligent decision on how to forward

traffic to T based on the DTN information it has received

O

T

A B

Node O processes RREP normally. All intermediate nodes may now make “optimal” forwarding decisions based on the knowledge of both the AODV network and the DTN overlay network. The DTN Router Info. header has a “metric” field which potentially could indicate how trustworthy a DTN route is.

C

D

E

DTN-AODV

Integrating AODV with DTN (RREP):▪ Failure of RREQ▪ The first DTN node in a path for an RREQ is responsible

for ensuring the RREQ reaches the destinationO

T

A B

B is the first DTN node in the path from O to T. It maintains a timer that kicks off after getting the RREQ and listens for an RREP from T. If node T fails and does not send an RREP before B’s timer expires, B sends a special “DTN-Only” RREP to allow nodes to still learn the DTN overlay network.

C

D

E

X

DTN-AODV

Benefits of DTN-AODV Allows for the dynamic establishment of the

DTN overlay network over the AODV network▪ Normally overlay networks requires static

configuration▪ BGP is an example

Allows for the learning of a granular view of the network to provide optimal forwarding decisions▪ Knowing both AODV and DTN nodes allows the mobile

node to intelligently determine an optimal next-hop

DTN-AODV

Benefits of DTN-AODV Evaluation of a similar implementation shows

performance improvement over Epidemic routing in terms of delivery-ratio and delay▪ Implementation (called adaptive routing) was

slightly different but still integrated AODV and DTN capability

Not all nodes have to be DTN speakers▪ Not all nodes even have to understand the DTN

extensions

DTN-AODV

Detriments of DTN-AODV Overhead increases with the number of DTN nodes▪ Since each DTN node adds its Router Info header to the

RREQ, more DTN nodes yields higher protocol overhead

DTN-AODV results in a lot of queueing▪ DTN-AODV results in far more queuing on average per

node compared to Epidemic routing

Route cacheing may cause issues with AODV-only speakers▪ If an AODV-only node receives an RREQ and responds via

route-cache, it will only send AODV routes limiting the full realization of the DTN network

TIL: Transient Information Level Routing

TIL

Created for directional tactical networks using RF and optical links (free-space optical networks)

TIL Directional networks

Sometimes nodes can’t transmit data in the optimal path due to interference or physical conditions

A satellite may not be pointing the optimal direction

DA

B

C

Node A needs to transmit to D, but it can’t send data directly due to the direction of the antennae. Instead, it takes a suboptimal path through B and C.

TIL

The goal of TIL is to have nodes coordinate together and share bandwidth optimally based on a “DTN algorithm” using a called TIL An individual node in a network is typically selfish Since bandwidth is finite, there is some benefit for

nodes to “cooperate” and share to maximize bandwidth

Built primarily for DTN’s, but the paper claims that the solution works for directional MANET’s as well

TIL

Gregg Darien

802.11n

802.11aX

Consider the scenario where Gregg and Darien see an 802.11n and 802.11a access point

Both of us knowing 802.11n is faster, we both join that access point and potentially contend over the available bandwidth

If we played nicer and coordinated, we could work together and join different access points to potentially maximize our perceived service

TIL

Assumes the following: All nodes have perfect knowledge of:▪ Location of nodes in the network▪ Amount of network traffic at each node▪ Connectivity of the network

If all nodes have this complete information, an algorithm can be used by each node to minimize the amount of work needed to move data from source to destination

TIL

The goal is to forward traffic based on the lowest TIL value for all nodes i and j

T(i, j) : amount of traffic needing to be delivered from node i to node j

D(i, j) : distance between node i and node j h(i, j) : amount of traffic being retained via

DTN on the path from node i to j

TIL

Detriments: Impractical▪ Is it even possible for to get this granular information

AND disperse it through a network? All nodes have to be DTN nodes

Benefits: Novel solution▪ Cooperation makes sense when “fighting” over a shared

resource Simulations show the protocol shows correctness

in delivering data over an RF, optical network

Issues in General with Integration of DTN and MANET

Varying simulation tools Different simulators have different capabilities Comparing performance of unlike protocols in unlike

tools is inefficient

Congestion control mechanisms The possibility exists for congestion to occur at DTN

capable nodes Does the bundle layer need additional congestion

control mechanisms?

Security DTN nodes present a natural target for DoS

Finishing Up

Thanks for listening

Why is there a need for this “hybrid architecture?”

Why not use MANET only? MANET routing protocols

use Hellos for neighbor discovery which could be wasteful if nodes are rarely connected

MANET forwarding is based on availability, so if an end-to-end path does not exist, traffic is dropped

This is not optimal if the network graph is typically disconnected or constantly changing

Why not use DTN only? DTN forwarding is often

based on a per-encounter basis (the node that is connected the “most” based frequency of connectivity may become the best hop)

This means the most optimal path through the network is not always utilized (DTN nodes don’t know the full topology)

Without constant topology discovery, suboptimal forwarding/routing is a possibility

DTN Issues

Persistent storage The assumption is a DTN node shall retain any data in the instance of a

system restart

Storage distribution Storage has to be well distributed through the network

▪ Issues with congestion management▪ Denial-of-Service mitigation

Routing is still a challenge Lossy media Unknown intermittent connectivity

Store-and-Forward Traditional routers are only store-and-forward for a limited duration of time Is store-and-forward a better alternative than continuous connectivity or

other alternatives?