1. Rudra Dutta ECE/CSC 570 - Fall 2010, Section 001, 601
The Networking Layer
Rudra Dutta
ECE/CSC Fall 2010, Section 001, 601

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
Concerns
Forwarding
Routing
Addressing?
Copyright Rudra Dutta, NCSU, Fall, 2010

3. Concepts Inter-networking Addressing Forwarding and Routing
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
There’s way too much information out there, so
Concepts are your best friend
Copyright Rudra Dutta, NCSU, Fall, 2010

4. Concepts - Forwarding / Routing
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
Copyright Rudra Dutta, NCSU, Fall, 2010

5. Concept - Context Context is needed to switch data
Wire - perfect context (no-brains forwarding)
At L3, need context to switch
Meta-data makes it unnecessary to maintain context
Each PDU carries context information (destination, source, ...)
Allows store-and-forward, therefore utilization, but
Now need to undertake significant forwarding effort for each packet
Context makes it unnecessary to reinvent the wheel
First send some “special” traffic packets establishing context
Units of traffic following the context establishment forwarded cheaply, but
To ensure that we do not drop data, must end up wasting bandwidth
The overhead of context maintenance is also present
Copyright Rudra Dutta, NCSU, Fall, 2010

6. 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
Next-hop
FIB
Context
Forwarding
DLC
PHY
NET A R D
Copyright Rudra Dutta, NCSU, Fall, 2010

7. Context-less Forwarding
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8. Context-ful Forwarding
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9. Comparison
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10. 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?
Copyright Rudra Dutta, NCSU, Fall, 2010

11. 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
Copyright Rudra Dutta, NCSU, Fall, 2010

12. 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
Copyright Rudra Dutta, NCSU, Fall, 2010

13. Optimality
Nice characteristic, works for “distance” or “cost” related routing goals
Based on “sink tree” argument
Goals that can be expressed additively
May not work for complicated policies
Copyright Rudra Dutta, NCSU, Fall, 2010

14. “Flow” Routing Source-destination traffic flow is routed
Allows Traffic Engineering
Balance load
Split for policy purposes
Can form “fish network”
Other possibilities: split routing
Copyright Rudra Dutta, NCSU, Fall, 2010

15. 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”
Copyright Rudra Dutta, NCSU, Fall, 2010

16. Dijkstra’s Algorithm 7 µ µ µ µ µ µ µ 3 µ µ µ 5 7 µ 6 µ 4 4 3 3 µ µ 4 42 7 2 5 5 3 1 3 1 3 2 2 5 1 5 8 1 8 5 7 6 5 5 7 2 7 2 5 5 3 1 4 3 1 4 3 3 2 2 5 1 5 8 1 8 4 4
Copyright Rudra Dutta, NCSU, Fall, 2010

17. How to Find Arc Costs? Must know arc costs to neighbors
Depending on cost metric, may be possible to measure
Beyond that, may trust neighbors’ claims
Local, do not have full picture of network
Cannot run Dijkstra or equivalent
Or may attempt to exchange local information
Full model of network can be built
Either way, routing information must be exchanged
Bootstrapping problem - solved by flooding
The exchange mechanism is a distributed algorithm, and often becomes known as a “routing method”
Actually, a “dynamic routing protocol”
More correctly, a “dynamic routing enablement protocol”
Copyright Rudra Dutta, NCSU, Fall, 2010

18. 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
Copyright Rudra Dutta, NCSU, Fall, 2010

19. Hey, I can get you there for $8!
Bellman's Equation 7 3
I can get you to for $10
Hey, I can get you there for $8!
Copyright Rudra Dutta, NCSU, Fall, 2010

20. Distance-Vector Algorithm (Each Router)
Start with a distance vector consisting of the value
"0" for itself
"infinity" for every other destination
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
Copyright Rudra Dutta, NCSU, Fall, 2010

21. DV Routing Example
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22. DV Routing Example (cont'd)
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23. DV Routing Example (cont'd)
Copyright Rudra Dutta, NCSU, Fall, 2010

24. DV Routing Example (cont'd)
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25. 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
how determine link / neighbor is down?
how long does it take to determine this?
Copyright Rudra Dutta, NCSU, Fall, 2010

26. Properties and Problems
DV algorithm eventually will converge on shortest paths
as long as state of the links / routers remains stable
no requirement for updates to be synchronized
During convergence, non-shortest paths and loops may develop
"good news travels fast, bad news travels slowly"
count-to-infinity problem
Copyright Rudra Dutta, NCSU, Fall, 2010

27. DV Routing Example -- Link Failure
Copyright Rudra Dutta, NCSU, Fall, 2010

28. The Count-to-Infinity Problem
“Good news” – availability of a link / router
“Bad news” – failure of a link / router
Copyright Rudra Dutta, NCSU, Fall, 2010

29. 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
Copyright Rudra Dutta, NCSU, Fall, 2010

30. LSA Routing Algorithm Each router does the following
“Meets the (immediately adjacent) neighbors” and learns their IDs
Builds an LSA containing IDs and distance to each of its neighbors
Transmits the LSA to all other routers
How? Most complex and critical piece of link-state routing algorithm
Stores the most recent LSA from every other router in the network
Not just from its immediate neighbors!
Creates a “map” of the network topology from LSAs
Computes routes (to store in forwarding table) from its local map of the topology
Copyright Rudra Dutta, NCSU, Fall, 2010

31. 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
Copyright Rudra Dutta, NCSU, Fall, 2010

32. 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 once
generally, O (n) packets propagated
Actually, could be more (simultaneity)
Copyright Rudra Dutta, NCSU, Fall, 2010

33. LSA Forwarding
Router A below transmits its LSA across the network shown below, using LSA Flooding. In one flooding “step” an LSA can be transmitted to any and all adjacent neighbors. Over what links is the LSA flooded in step #1? In step #2? Continue answering this question until the final step, whatever that is.
Issue of simultaneity becomes clear when doing this example . A B D E F C
Copyright Rudra Dutta, NCSU, Fall, 2010

34. Why DV / LSA ? Loop-freeness in LSA “Local” character of DV
But not impossible to attain in DV
“Local” character of DV
Less overhead, storage
But does not scale well
Luckily, advantages are in different zones
Small scale and simplicity – DV
Large scale and optimality – LSA
Copyright Rudra Dutta, NCSU, Fall, 2010

35. Hierarchical Routing Divide and conquer
Group destinations into logical localities
Break routing into two phases
Between groups
Groups may not be directly connected
Inside a group
For both phases, use some other strategy
Can be generalized into more levels
Application in Ad Hoc routing
Copyright Rudra Dutta, NCSU, Fall, 2010

36. Broadcast Routing Destination is “all hosts”, not a single one
Special case of multicasting
Flooding is obvious choice for distributing to networks
But assumes each networks has “broadcast” semantics
Otherwise need to unicast to each host, impractical
Multidestination routing
Replicate only when necessary
Spanning tree of routers
Flood on spanning tree routers
Copyright Rudra Dutta, NCSU, Fall, 2010

37. Spanning Tree Tree - a connected graph with no cycles
Spanning tree - a subgraph of original graph that is a tree and included every node
Must have exactly N-1 links
Efficient broadcast
Copyright Rudra Dutta, NCSU, Fall, 2010

38. Broadcast – Reverse Path Forwarding
An attempt to obtain the benefits of a spanning tree with no extra information or overhead
With tree, when a broadcast packet arrives
It is from either from the “parent”
Must be forwarded to all “children”
Like flooding – all interfaces except the one packet came in over
Or from a “child”
Must be discarded, to avoid duplication
But how to establish parent-child relation without overhead of forming spanning tree?
Use sink tree for the source
“If this packet originates at A, comes to me through X”
Forward if X is my next-hop router to A (to all neighbors except X)
Otherwise discard
Copyright Rudra Dutta, NCSU, Fall, 2010

39. Multi-level Forwarding
Special case of Generalized Protocol Layering
“Links” of one network are really “paths” of a different one
“Virtual” or “logical” links
Can occur on multiple levels
Virtual links may be instantiated by physical layer
Optical networks: “lightpaths”
Wireless networks: recent approaches using OFDM
Or may be instantiated as “virtual” circuits
Different networking layers at different levels may have differing routing strategies/goals
May be “obviously sub-optimal” when viewed globally
Copyright Rudra Dutta, NCSU, Fall, 2010

40. 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
Copyright Rudra Dutta, NCSU, Fall, 2010