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### ECE 4450:427/527 - Computer NetworksSpring 2014

### Internetworking: Discussions

### Recall: IP Routing Table

### Forwarding vs Routing

### Why Does Routing Matter?

### Different Types of Routing

### Routing

### Routing

### Distance Vector: Algorithm

### Distance Vector

### Distance Vector

### Distance Vector: Local View (initial)

### Distance Vector – 1st Update for A

### Distance Vector – 1st Update

### Distance Vector – 2nd Update

### Distance Vector

### Failure of Link F-G

### Failure of Link F-G: Updated Tables

### Drawback

### Solutions

### Routing Information Protocol (RIP)

### Metric/Cost in Real World

### Distance Vector vs. Link-State

### Link-State Routing

### Reliable Flooding

### Reliable Flooding

### Reliable Flooding

### Route Calculation

### Shortest-Path Problem: Statement

### Dijkstra’s Algorithm: Overview

### Dijkstra’s Algorithm: Implementation

### Dijkstra’s Table

### Shortest-Path Tree

### Open Shortest First Path (OSPF)

### Distance Vector vs. Link-State

### Distance Vector vs. Link-State

### Routing in the Internet

### Routing in the Internet

### BGP

Dr. Nghi Tran

Department of Electrical & Computer Engineering

Lecture 6.3: Routing

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- For Internetworking, we shall look at few sub-problems:
- Interconnect links of the same type: Switches
- We consider an important of class switch: Bridges to interconnect Ethernet segments.
- We also look a way to interconnect disparate networks and links: Gateways, or now mostly known as routers. We shall focus on the IP
- Once we are able to interconnect a whole lot of links and networks with switches and routers, we will look at a way to find a suitable path, or route through a network:
- Paths that are efficient, loop free, etc.: Routing

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- Forwarding: data plane
- Directing a data packet to an outgoing link
- Individual router using a forwarding/routing table
- Routing: control plane
- Computing paths the packets will follow
- Routers talking amongst themselves
- Individual router creating a forwarding table
- Routing can be simply understood as a process by which routing table is built

5

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- End-to-end performance
- Quality of the path affects user performance
- Propagation delay, throughput, and packet loss
- Use of network resources
- Balance of the traffic over the routers and links
- Avoiding congestion by directing traffic to lightly-loaded links
- Transient disruptions during changes
- Failures, maintenance, and load balancing
- Limiting packet loss and delay during changes

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- Routing in a GPS device
- Routing in computer networks
- Shortest path
- Smallest delay
- Highest reliability
- Avoid congested nodes

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- Network as a Graph
- The basic problem of routing is to find the lowest-cost path between any two nodes
- The cost of a path equals the sum of the costs of all the edges that make up the path
- Cost: delay, financial cost, probability of failure, etc.

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- For a simple network, we can calculate all shortest paths and load them into some nonvolatile storage on each node.
- Such a static approach has several shortcomings
- It does not deal with node or link failures
- It does not consider the addition of new nodes or links
- It implies that edge costs cannot change
- What is the solution?
- Need a distributed and dynamic protocol
- Two main classes of protocols
- Distance Vector
- Link State

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- Construct a one-dimensional array (vector) of distances to all other nodes, with assumption that each node knows the cost of the link to each of its directly connected neighbors
- Exchange info with immediate neighbors
- Update distances based on received information
- Stop sending updates as soon as no change occurs

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- Initial distances stored at each node (global view) (assume unit cost for each link)

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Initial routing table at node A, assume unit cost for each link

Now, how about node B?

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1st Update: Every node sends to its directly connected neighbors its personal list of distances

1st Update for A

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- The other common name for this class of algorithm is Bellman-Ford, after its inventors.
- As we can see, it usually takes a number of exchanges/updates between neighbors before each node has complete routing table.
- The process of getting consistent routing information to all nodes is called convergence: vary
- Recall that we have two different circumstances under which a node decides to send a routing update
- Periodic update
- Triggered update

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- When a node detects a link failure
- F detects that link to G has failed (how?)
- F sets distance to G to infinity and sends update to A
- A sets distance to G to infinity since it uses F to reach G
- A receives periodic update from C with 2-hop path to G
- A sets distance to G to 3 and sends update to F
- F decides it can reach G in 4 hops via A

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- Suppose the link from A to E goes down
- In the next round of updates, A advertises a distance of infinity to E, but C advertise a distance of 2 to E
- Depending on the exact timing of events, the following might happen
- Node B, upon hearing that E can be reached in 2 hops from C, concludes that it can reach E in 3 hops and advertises this to A
- Node A concludes it can reach E in 4 hops and advertises this to C
- Node C concludes that it can reach E in 5 hops; and so on.
- This cycle stops only when the distances reach some number that is large enough to be considered infinite
- Count-to-infinity problem

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- One technique to improve the time to stabilize routing is called split horizon
- When a node sends a routing update to its neighbors, it does not send those routes it learned from each neighbor back to that neighbor
- For example, if B has the route (E, 2, A) in its table, then it knows it must have learned this route from A, and so whenever B sends a routing update to A, it does not include the route (E, 2) in that update

- In a stronger version of split horizon, called split horizon with poison reverse
- B actually sends that back route to A, but it puts negative information in the route to ensure that A will not eventually use B to get to E
- For example, B sends the route (E, ∞) to A

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- Early routing protocol for IP networks
- Distance-vector algorithm where vertices are networks and not hosts
- Valid hop count (distances) 1-15, with 16 representing infinity
- Limited to fairly small networks

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- Can we just assign a cost of 1 to all links?
- Certainly, there are so many ways to define cost:
- Number of packets queued waiting to be transmitted
- Consider both bandwidth and latency
- In the current real world
- Common approach: a constant/link bandwidth
- It means metric changes rarely if at all and only under the control of network administrator
- Why?
- i) Dynamically changing metrics are too unstable;
- ii) Many networks today lack the great disparity of speeds and latencies

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- Distance vector:
- each node talks only to its directly connected neighbors …
- but it tell them everything it has learned, i.e., distance to all nodes
- Link-state:
- each nodetalks to all other nodes…
- but it tells them only what it knows for sure, i.e., state of its directly connected links

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Strategy: Send to all nodes (not just neighbors) information about directly connected links (not entire routing table) and associated cost.

- Rely on two key mechanisms:
- Reliable flooding: Make sure all nodes get the above information of other nodes
- Route calculation: Once a node has a copy of the information from every other node, it is able to compute a complete map of the network, and then can decide the best route to each destination

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- Objective: Make sure all nodes get the link-state information of other nodes: Knowledge of directly connected neighbors and associated cost for each node
- Each node creates an update packet called link-state packet (LSP) with the following information:
- ID of the node that created LSP
- Cost of link to each directly connected neighbor
- Sequence number (SEQNO)
- Time-to-live for this packet
- Reliable flooding: Make sure all nodes get LSP from the other nodes.

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- Transmission of LSPs between adjacent routers using ACK and transmission. But how to send an LSP to all nodes?
- We need some more steps to reliably flood an LSP to all nodes: controlled flooding
- A node x receives a copy of LSP originated from y.
- x then needs to check if it has already had a copy. If not, store it. If it has, compare sequence number, keep a newer one.
- If the received LSP is new, x sends a copy of LSP to all neighbors, except the one from which LSP was received (why?).
- Neighbors then turn around and do the same thing: most recent copy of LSP reaches all nodes.

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Reliable Flooding

Flooding of link-state packets.

(a) LSP arrives at node X;

(b) X floods LSP to A and C;

(c) A and C flood LSP to B (but not X);

(d) flooding is complete

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- Now, assume a given node has copies of LSPs from every other node, how to decide/calculate the best route to each destination?
- The solution is based on a well-known algorithm from graph theory – Dijkstra’s shortest-path algorithm

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- Given: network topology with link costs
- c(x,y): link cost from node x to node y
- Infinity if x and y are not direct neighbors
- Compute: least-cost paths to all nodes
- From a given source u to all other nodes
- For each node a: ustores a’s predecessor nodealong path from u to a, & cost.

2

1

3

1

4

u

a

2

1

5

4

3

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- Iterative algorithm for a given source node u:
- After k iterations, know least-cost path to k nodes
- S: set of nodes whose least-cost path known
- Initially, S = {u} where u is the source node
- Add one node to S in each iteration
- D(a): current cost of path from source to node a
- Initially, D(a) = c(u, a) for all nodes a adjacent to u
- … and D(a) = ∞ for all other nodes a
- Continuously update D(a) as shorter paths are learned

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1 Initialization:

2 S = {u}

3 for all nodes {a}

4 If aadjacent to u {

5 D(a) = c(u,a)

6 else D(v) = ∞

7

8 Loop

9 Find bnot in S with the smallest D(b)

10 Add bto S, store D(b) and P(b) (predecessor node)

11 update D(a) for all {a} adjacent to band not in S:

12 D(a) = min{D(a), D(b) + c(b,a)}

13 until all nodes in S

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2

v

y

1

3

1

4

x

z

u

2

v

(u,v)

1

5

w

(u,w)

t

w

4

3

x

(u,w)

s

y

(u,v)

z

(u,v)

s

(u,w)

(u,w)

t

- Shortest-path tree from u

- Routing table at u

link

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- It is the most widely used link-state routing protocols in practice
- Open: Publicly available
- SPF: Alternative name for link-state
- OSPF adds a number of features to the basic link-state
- Authentication: Make sure all nodes can be trusted
- Additional hierarchy: Network domain divided further in to areas: a router might just need to get to a right area
- Load balancing: Allow multiple routes to the same place to be assigned the same cost: traffic distributed evenly to those routers

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- Distance vector: each node talks only to its directly connected neighbors, but it tell them everything it has learned, i.e., distance to all nodes
- Link-state: each nodetalks to all other nodes, but it tells them only what it knows for sure, i.e., state of its directly connected links

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- Who is the winner?

Robustness: what happens if router malfunctions, misbehaves?

LS:

- node can advertise incorrect link cost
- each node computes only its own table

DV:

- DV node can advertise incorrect path cost
- each node’s table used by others , error propagate thru network

Message complexity

- LS: with n nodes, E links, O(nE) msgs sent . A lot of information needs to be stored.
- DV: exchange between neighbors only

Speed of Convergence

- LS: O(n2) algorithm requires O(nE) msgs
- DV: convergence time varies
- may be routing loops
- count-to-infinity problem

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- Routing protocols we have learned so far: idealization, all routers identical, flat network
- Internet: Network of hundreds of thousands of networks: Not possible to directly using those protocols: they do not scale to those kinds of numbers!!!
- We need something else!!!

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- Internet is organized as autonomous systems (AS) each of which is under the control of a single administrative entity
- Autonomous System (AS)
- corresponds to an administrative domain
- examples: University, company, backbone network, as may the network of a single ISP
- AS chooses its own routing protocol, e.g., distance-vector or link-state
- Divide routing problem in two parts:
- Intra-domain: We have already learned
- Inter-domain: Border Gateway Protocol (BGP) (BGP-v4)

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- Few books dedicated to BGP
- Key points:
- Assumes the Internet is an arbitrarily interconnected set of AS‘s
- Impossible to define optimal path
- Advertise only reachability: complete paths as an enumerated lists of ASs to reach a particular network
- Does not belong to either distance-vector or link-state
- For further discussions (high points), see Chapter 4.1.2

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