The Networking Layer Rudra Dutta CSC 453 - Fall 2012

The Networking Layer

Rudra DuttaCSC 453 - Fall 2012

Copyright Rudra Dutta, NCSU, Fall 2013 2

Positioning DLC allows communication between connected computers Cooperation can create connectivity not present in physical

topology – networking Lowest layer aware of end-to-end traffic Functions:

– Forwarding– Routing– Addressing?

We want to understand:– Descriptive– Algorithmic


Concepts Inter-networking

– Assume physical (“sub”)network knows how to deliver Addressing

– Physical network computers already have addresses– Do these make sense in broader context?

Consider postal - house, subdivision, ...– If not, networking layer addresses must be assigned

Forwarding and Routing Switching granularity and context

– Circuits (L1), packets (L3)– Hierarchical switching

Forwarding Information Base


Concepts - Forwarding / Routing Forwarding

– The act of forwarding PDUs Where to forward?

– Likely to be partially determined by data source/destination Must be embedded in data

– Likely to be partially determined by network factors Must be stored at node

– Forwarding will combine these two types of information Routing

– Path-finding For any and all traffic, determination of path it takes from source to

destination– At a node, must translate into:

Distilling “network factors” above into easy-to-refer information for forwarding - mapping from data to next-hop


System Components


Concepts - Forwarding Information Base A “lookup table” for next-hop router

– Packet arrives with embedded context (full or partial)– Context information is extracted and looked up in FIB– Packet forwarded to next-hop intermediate node

DLC

PHY

NET

DLC

PHY

DLC

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A R D

DLC

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Forwarding

Next-hop

FIBContext

Copyright Fall 2013, Rudra Dutta, NCSU 7

Overlay Networking Operation of L3 can be performed by L7

– Overlay forwarding – integrate with application logic Receive-process-decide-output cycle

– May fall behind Discard data loss

DLC

PHY

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DLC

PHY

DLC

PHY

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DLC

PHY

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L7 “NET”

TRANSPORTForwarding

Host

TRANSPORT

APP

APPLICATION

Next-hop

FIBContext


Connection-less Forwarding

H1 sends packet 1 to A A consults FIB forward to C C consults FIB forward to E E consults FIB forward to F F uses LAN to forward to H2

If FIB changes, a following packet may traverse different path

A’s FIB A’s FIB C’s FIB E’s FIB


Routing - Fundamentals Different concerns

– Correctness and optimality– Robustness, stability, …– More complicated “policy” concerns– Optimality – who decides?

Once design goals have been decided,– Algorithm must be designed to attain them– Some fundamental approaches to routing exist

Routing may be fixed, or fixed alternate May be adaptive, or dynamic

– Adapting to what? How is information shared?


Space of Routing Choices Static vs. dynamic or adaptive For a node, next-hop determined by

– Destination only– Source and destination– Particular flow of traffic– Other characteristics

May be bifurcated (split) or non-bifurcated Next-hop may be unique or multiple

– Deterministically or randomly picked


Flooding Simple – forward to everybody else Generates unbounded copies Hop count can be introduced to bound Can be more intelligent if packets can be

recognized Selective flooding – shortlist of links Highly robust Serves as benchmark Useful as a “phase” in other strategies

Flooding


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Link State Protocols Each router distributes information it has to every other

router Information is in the form of LSA’s

– (myID, neighborID, link cost) Distribution is by controlled flooding

– Distribute along each link but receiving one Now, each router has complete information OSPF is a prevalent example

– SPF with LSA to find arc costs


LSA Routing AlgorithmEach router does the following1. “Meets the (immediately adjacent) neighbors” and

learns their IDs2. Builds an LSA containing IDs and distance to each of

its neighbors3. Transmits the LSA to all other routers

– How? Most complex and critical piece of link-state routing algorithm

4. Stores the most recent LSA from every other router in the network

– Not just from its immediate neighbors!5. Creates a “map” of the network topology from LSAs6. Computes routes (to store in forwarding table) from its

local map of the topology


Generating LSAs A router generates LSAs periodically background

refresh rate A router also generates LSAs when its local

environment changes– it has a new neighbor (router comes online)– a link goes down (indicated by absence of "Hello" packets)– the cost of a link to an existing neighbor has changed

Limiting the overhead (network bandwidth) consumed by routing messages, particularly LSAs– set a minimum interval between successive updates


Forwarding LSAs Flooding except to transmitting neighbor

– Infinite number of packets– Hop-limited would still result in exponential number of

packets LSA flooding can use information in LSA

– Each router stores the most recent copy of LSAs from all routers easy to recognize duplicate LSAs

– Each router floods an LSA only once an LSA will travel over each link of the network exactly oncegenerally, O (n) packets propagatedActually, could be more (simultaneity)


“Optimality Criterion”

• Nice characteristic, works for “distance” or “cost” related routing goals• Based on “sink tree” argument• Goals that can be expressed additively• “Sub-path of optimal path must be optimal”


Finding Shortest Paths An example where a graph-theoretic abstraction is

useful– Abstract networking into vertices and arcs– Weighted arcs – least cost approach

Dijkstra’s algorithm– Finds least cost path to all vertices from one– Based on moving vertices from a set with “tentative” distances

to a set with “fixed” distances– Desirable computational properties– Other algorithms exist

Useful for additive measures of path cost– “Shortest” is better understood as “least additive cost”

∞

∞

∞


Dijkstra’s Algorithm Finding shortest distances (and

corresponding paths) from ONE source to ALL destinations– Easily adapted to vice-versa

First, mark source node with tentative distance zero

Mark all other nodes tentatively with distance “infinity” (not reachable)

Do iteratively (until all nodes marked):– Pick the node with the least tentative distance,

and mark it as final distance – call it node X– Pick all outgoing links from this node, for each:

Update the tentative distance of the neighbor IF: (Final distance of X + link cost) < Tentative distance

of neighbor If updated, remember X as previous node of

neighbor

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∞

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At the end, previous nodes can be read off to find complete paths

0


Dijkstra’s Algorithm

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Distance Vector (DV) Routing Asynchronous, iterative distributed computation

– exchange of routing information: (destination, min_distance)– exchange info with neighbors only– computation step: based on Bellman-Ford method

Find for each destination– cost of least cost path– next node (neighbor) on least cost path (not complete path)

Routers do not store or compute the network topology; they only store distance / next hop information

Used by many protocols: RIP, BGP, ISO IDRP, Novel IPX Advertise distances to route prefixes (networks, subnets, hosts),

not routers


Bellman's Equation

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3

I can get you to 54.0.0.0 for $10

Hey, I can get you there for

$8!

For any node A,– Edge costs l(ij) =

– Cost of link from i to j, if exists– ∞, otherwise

Beyond neighborhood, trustneighbors’ claims D(i,j) = mink {l(ik) + D(k,j)} D(i,i) = 0

A forms vector of D(A,j), distributes to neighbors So does every other node Nodes collectively and iteratively learn about better and better

paths

A D(A,54.0.0.0 = min (3+10, 7+8)


Distance-Vector Algorithm (Each Router) Start with a distance vector consisting of the value

– "0" for itself– "infinity" for every other destination

Link cost to neighbors is available, through direct measurement, or administrator configuration

Transmit its distance vector to each of its neighbors– when the link to the neighbor first comes up– whenever the information changes triggered updates– periodically (even if there are no changes)

Saves the most recent distance vector from each neighbor

Calculates its own distance vector using Bellman's equation

Iterate

DV Example


DE

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D(i,j) = mink {l(ik) + D(k,j)}

A B C D E

A’s DV - - - - -

B’s DV - 0 - - -

C’s DV - - - - -

D’s DV - - - - -

E’s DV - - - - -

A B C D E

A’s DV - - - - -

B’s DV - - - - -

C’s DV - - 0 - -

D’s DV - - - - -

E’s DV - - - - -

A B C D E

A’s DV - - - - -

B’s DV - - - - -

C’s DV - - - - -

D’s DV - - - - -

E’s DV - - - - 0

A B C D E

A’s DV - - - - -

B’s DV - - - - -

C’s DV - - - - -

D’s DV - - - 0 -

E’s DV - - - - -

A B C D E

A’s DV 0 - - - -

B’s DV - - - - -

C’s DV - - - - -

D’s DV - - - - -

E’s DV - - - - -

DV Example


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A B C D E

A’s DV - - - - -

B’s DV - 0 - - -

C’s DV - - - - -

D’s DV - - - - -

E’s DV - - - - -

A B C D E

A’s DV - - - - -

B’s DV - - - - -

C’s DV - - 0 - -

D’s DV - - - - -

E’s DV - - - - -

A B C D E

A’s DV - - - - -

B’s DV - - - - -

C’s DV - - - - -

D’s DV - - - - -

E’s DV - - - - 0

A B C D E

A’s DV - - - - -

B’s DV - - - - -

C’s DV - - - - -

D’s DV - - - 0 -

E’s DV - - - - -

A B C D E

A’s DV 0 - - - -

B’s DV - - - - -

C’s DV - - - - -

D’s DV - - - - -

E’s DV - - - - -

A B C D E

A’s DV 0 - - - -

B’s DV 1 0 1 8 -

C’s DV - - 0 - -

D’s DV - - - 0 -

E’s DV - - - - -

A B C D E

A’s DV 0 - - - -

B’s DV - 0 - - -

C’s DV 2 1 0 - 1

D’s DV - - - - -

E’s DV - - - - 0

A B C D E

A’s DV 0 - - - -

B’s DV - - - - -

C’s DV - - 0 - -

D’s DV - - - 0 -

E’s DV 5 - 1 2 0

A B C D E

A’s DV - - - - -

B’s DV - 0 - - -

C’s DV - - - - -

D’s DV - 8 - 0 2

E’s DV - - - - 0

A B C D E

A’s DV 0 1 2 - 5

B’s DV - 0 - - -

C’s DV - - 0 - -

D’s DV - - - - -

E’s DV - - - - 0

A B C D E

A’s DV 0 1 2 - 5

B’s DV - - - - -

C’s DV 2 1 0 - 1

D’s DV - 8 - 0 2

E’s DV 5 - 1 2 0

DV Example


DE

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A B1

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A B C D E

A’s DV 0 1 2 - 5

B’s DV 1 0 1 8 -

C’s DV 2 1 0 - 1

D’s DV - 8 - 0 2

E’s DV - - - - -

A B C D E

A’s DV 0 1 2 - 5

B’s DV 1 0 1 8 -

C’s DV 2 1 0 - 1

D’s DV - - - - -

E’s DV 5 - 1 2 0

A B C D E

A’s DV - - - - -

B’s DV 1 0 1 8 -

C’s DV - - - - -

D’s DV - 8 - 0 2

E’s DV 5 - 1 2 0

A B C D E

A’s DV 0 1 2 - 5

B’s DV 1 0 1 8 -

C’s DV 2 1 0 - 1

D’s DV - - - - -

E’s DV 5 - 1 2 0

A B C D E

A’s DV 0 1 2 - 5

B’s DV 1 0 1 8 2

C’s DV 2 1 0 - 1

D’s DV - 8 - 0 2

E’s DV - - - - -

A B C D E

A’s DV 0 1 2 - 5

B’s DV 1 0 1 8 -

C’s DV 2 1 0 3 1

D’s DV - - - - -

E’s DV 5 - 1 2 0

A B C D E

A’s DV 0 1 2 - 5

B’s DV - - - - -

C’s DV 2 1 0 - 1

D’s DV - 8 - 0 2

E’s DV 3 2 1 2 0

A B C D E

A’s DV - - - - -

B’s DV 1 0 1 8 -

C’s DV - - - - -

D’s DV 7 8 3 0 2

E’s DV 5 - 1 2 0

A B C D E

A’s DV 0 1 2 7 3

B’s DV 1 0 1 8 -

C’s DV 2 1 0 - 1

D’s DV - - - - -

E’s DV 5 - 1 2 0

DV Example


DE

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A B1

215

1

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8


A B C D E

A’s DV 0 1 2 7 3

B’s DV 1 0 1 8 2

C’s DV 2 1 0 3 1

D’s DV 7 8 3 0 2

E’s DV - - - - -

A B C D E

A’s DV 0 1 2 7 3

B’s DV 1 0 1 8 2

C’s DV 2 1 0 3 1

D’s DV - - - - -

E’s DV 3 2 1 2 0

A B C D E

A’s DV 0 1 2 7 3

B’s DV - - - - -

C’s DV 2 1 0 3 1

D’s DV 7 8 3 0 2

E’s DV 3 2 1 2 0

A B C D E

A’s DV - - - - -

B’s DV 1 0 1 8 2

C’s DV - - - - -

D’s DV 7 8 3 0 2

E’s DV 3 2 1 2 0

A B C D E

A’s DV 0 1 2 7 3

B’s DV 1 0 1 8 2

C’s DV 2 1 0 3 1

D’s DV - - - - -

E’s DV 3 2 1 2 0

A B C D E

A’s DV 0 1 2 7 3

B’s DV 1 0 1 4 2

C’s DV 2 1 0 3 1

D’s DV 7 8 3 0 2

E’s DV - - - - -

A B C D E

A’s DV 0 1 2 7 3

B’s DV 1 0 1 8 2

C’s DV 2 1 0 3 1

D’s DV - - - - -

E’s DV 3 2 1 2 0

A B C D E

A’s DV 0 1 2 7 3

B’s DV - - - - -

C’s DV 2 1 0 3 1

D’s DV 7 8 3 0 2

E’s DV 3 2 1 2 0

A B C D E

A’s DV - - - - -

B’s DV 1 0 1 8 2

C’s DV - - - - -

D’s DV 5 4 3 0 2

E’s DV 3 2 1 2 0

A B C D E

A’s DV 0 1 2 5 3

B’s DV 1 0 1 8 2

C’s DV 2 1 0 3 1

D’s DV - - - - -

E’s DV 3 2 1 2 0


DV Routing (cont'd) Events causing recalculation of the distance

vector– receive from a neighbor a distance vector containing

new info– when link goes down discard / disregard distance

vector from the neighbor on the other end of that link

On-demand Routing No pre-determined paths – make forwarding

decision at the time packet arrives– Need to constitute consistent and correct path

decisions May save some information about network

– Neighbors Requires query-response on-demand

– RREQ, RREP May utilize additional information to determine

“desirability”– Geographic routing


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Reactive & proactive protocols Reactive protocols

– Dynamic Source Routing (DSR) [Johnson96]– Ad Hoc On-Demand Distance Vector Routing

(AODV) [Perkins99Wmcsa]– Link reversal algorithm [Gafni81]

Proactive protocols– Optimized Link State Routing (OLSR)

[Jacquet00ietf,Jacquet99Inria]– Destination-Sequenced Distance-Vector (DSDV)

[Perkins94Sigcomm]

Copyright Rudra Dutta, NCSU, Fall 2013

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Example – DSR operation When S wants to send a packet to D, but does

not know a route to D, S initiates a route discovery

Source node S floods Route Request (RREQ)

Each node appends own identifier when forwarding RREQ


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Route discovery in DSR

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Represents a node that has received RREQ for D from S

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B

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Represents transmission of RREQ

Z

YBroadcast transmission

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[S]

[X,Y] Represents list of identifiers appended to RREQCopyright Rudra Dutta, NCSU, Fall 2013

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B

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S EF

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• Node H receives packet RREQ from two neighbors: potential for collision

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[S,E]

[S,C]


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B

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S EF

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• Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once

Z

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[S,C,G]

[S,E,F]


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B

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S EF

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• Nodes J and K both broadcast RREQ to node D• Since nodes J and K are hidden from each other, their transmissions may collide

N

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[S,C,G,K]

[S,E,F,J]


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B

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• Node D does not forward RREQ, because node D is the intended target of the route discovery

M

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[S,E,F,J,M]


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Route reply in DSR

Destination D on receiving the first RREQ, sends a Route Reply (RREP)

Route of RREP – reversing the route in received RREQ

RREP includes the route from S to D on which RREQ was received by node D


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Route reply in DSR

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RREP [S,E,F,J,D]

Represents RREP control message


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Route reply in DSR Reverse route assumes bi-directional links

– To ensure this, node only forward RREQ if its link is bi-directional

If allow unidirectional (asymmetric) links, then may need a route discovery for S from node D – Unless node D already knows a route to node S– If discover route from D to S, Route Reply is

piggybacked on Route Request from D.


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Data delivery in DSR Node S on receiving RREP, caches the route included in

the RREP

When node S sends a data packet to D, the entire route is included in the packet header– hence the name source routing

Intermediate nodes use the source route included in a packet to determine to whom a packet should be forwarded


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Data delivery in DSR

B

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DATA [S,E,F,J,D]

Packet header size grows with route length



Summary Network layer allows separate physical

networks to cooperate Context may or may not be utilized to minimize

forwarding effort Dual concerns of forwarding and routing Various fundamental approaches to routing

– Real strategies can be designed from combination of these approaches

Can incorporate in overlay networking applications

Documents

The Networking Layer Rudra Dutta CSC 453 - Fall 2012