1. Dan LI CS Department, Tsinghua University
Intra-domain routing
Dan LI
CS Department, Tsinghua University
Today we’ll talk about routing.
2018/11/12

2. Today’s Lecture
Routing Model
Intra-domain Routing
2018/11/12

3. Notion of an Internet Protocol
How is it possible to send bits across incompatible LANs and WANs?
Solution: protocol software running on each host and router smooths out differences between different networks
Implements an internet protocol (i.e., set of rules) that governs how hosts and routers should cooperate when they transfer data from network to network
TCP/IP is protocol (family) for global IP Internet

4. What Does an Internet Protocol Do?
Provides naming scheme
Defines uniform format for host addresses
Each host (and router) is assigned at least one internet address that uniquely identifies it
Provides delivery mechanism
An internet protocol defines a standard transfer unit (packet)
Packet consists of header and payload
Header: contains info such as packet size, source and destination addresses
Payload: contains data bits sent from source host
Encapsulation - key to network messages

5. Transferring Data via Internet
Host A
Host B
client
server
(1)
(8)
data
data
protocol
software
protocol
software
internet packet
(2)
(7)
data PH
data PH
LAN1
adapter
LAN2
adapter
Frame
Router
(3)
(6)
data PH
FH1
data PH
FH2
LAN1
adapter
LAN2
adapter
LAN1
LAN2
LAN2 frame
(4)
data PH
FH1
(5)
data PH
FH2
protocol
software

6. But… How do routers know where to forward frames?
How do routers learn when the network topology changes?
That’s what routing protocol does…

7. Routing
Goal: determine “good” path (sequence of routers) through network from source to destination
“good path”
Typically means minimum cost path
Other definitions also possible
2018/11/12

8. Dynamic Routing IP traffic can be very bursty
Dynamic adjustments in routing typically operate more slowly than fluctuations in traffic load
Dynamically adapting routing to account for traffic load can lead to wild, unstable oscillations of routing system
Usually, IP routing protocols do not dynamically route around network congestion.
2018/11/12

9. Graph Model Represent each router as node
Direct link between routers represented by edge
Symmetric links  undirected graph
Edge “cost” c(x,y) denotes measure of difficulty of using link
delay, $ cost, or congestion level
Task
Determine least cost path from every node to every other node
Path cost d(x,y) = sum of link costs
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10. A Simple Model (Cont.) E C 3 1 F 1 2 6 1 D 3 A 4 B
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11. Routes from Node A Properties Some set of shortest paths forms tree
Table for A
Dest
Cost
Next Hop A B 4 C 6 E D 7 2 F 5 E C 3 1 F 1 2 6 1 D 3 A 4 B
Properties
Some set of shortest paths forms tree
Shortest path spanning tree for each node
Solution not unique
E.g., A-E-F-C-D also has cost 7
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12. Ways to Compute Shortest Paths
Centralized
Collect graph structure in one place
Use standard graph algorithm
Disseminate routing tables
Partially Distributed
Every node collects complete graph structure
Each computes shortest paths from it
Each generates own routing table
“Link-state” algorithm
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13. Ways to Compute Shortest Paths (Cont.)
Fully Distributed
No one has copy of graph
Nodes construct their own tables iteratively
Each sends information about its table to neighbors
“Distance-Vector” algorithm
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14. Internet Routing Internet organized as a two level hierarchy
First level – autonomous systems (AS’s)
AS – region of network under a single administrative domain
AS’s run an intra-domain routing protocols
Distance Vector, e.g., Routing Information Protocol (RIP)
Link State, e.g., Open Shortest Path First (OSPF)
Between AS’s runs inter-domain routing protocols, e.g., Border Gateway Routing (BGP)
De facto standard today, BGP-4
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15. Example
Interior router
BGP router
AS-1
AS-3
AS-2
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16. Today’s Lecture
Routing Model
IntraDomain Routing
2018/11/12

17. Intra-domain Routing Protocols
Based on unreliable datagram delivery
Distance vector
Routing Information Protocol (RIP), based on Bellman-Ford
Every node advertises its cost towards all the other nodes in the network to its neighbors
Link state
Open Shortest Path First (OSPF), based on Dijkstra
Every node advertises its cost towards its neighbors in the network to all the other nodes
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18. Distance Vector: Control Traffic
When the routing table of a node changes, the node sends its table to its neighbors
A node updates its table with information received from its neighbors
Host A
Host B
Host E
Host D
Host C N1 N2 N3 N4 N5 N7 N6
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19. Distance-Vector Method
Initial Table for A
Dest
Cost
Next Hop A B 4 C  – D E 2 F 6 E C 3 1 F 1 2 6 1 D 3 A 4 B
Idea
At any time, have cost/next hop of best known path to destination
Use cost  when no path known
Initially
Only have entries for directly connected nodes
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20. Distance-Vector Updatez
d(z,y)
c(x,z) y x
d(x,y)
Update(x,y,z)
d  c(x,z) + d(z,y) # Cost of path from x to y with first hop z
if d < d(x,y)
# Found better path
return d,z # Updated cost / next hop
else
return d(x,y), nexthop(x,y) # Existing cost / next hop
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21. Algorithm Bellman-Ford algorithm Repeat Until Converge
For every node x
For every neighbor z
For every destination y
d(x,y)  Update(x,y,z)
Until Converge
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22. Start Optimum 1-hop paths E C F D A B Table for A Dst Cst Hop A B 4 CB 4 C  – D E 2 F 6
Table for B
Dst
Cst
Hop A 4 B C  – D 3 E F 1 E C 3 1 F 1 2 6 1 D 3 A 4 B
Table for C
Dst
Cst
Hop A  – B C D 1 E F
Table for D
Dst
Cst
Hop A  – B 3 C 1 D E F
Table for E
Dst
Cst
Hop A 2 B  – C D E F 3
Table for F
Dst
Cst
Hop A 6 B 1 C D  – E 3 F
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23. Iteration #1 Optimum 2-hop paths E C F D A B Table for A Dst Cst Hop AB 4 C 7 F D E 2 5
Table for B
Dst
Cst
Hop A 4 B C 2 F D 3 E 1 E C 3 1 F 1 2 6 1 D 3 A 4 B
Table for C
Dst
Cst
Hop A 7 F B 2 C D 1 E 4
Table for D
Dst
Cst
Hop A 7 B 3 C 1 D E  – F 2
Table for E
Dst
Cst
Hop A 2 B 4 F C D  – E 3
Table for F
Dst
Cst
Hop A 5 B 1 C D 2 E 3 F
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24. Iteration #2 Optimum 3-hop paths E C F D A B Table for A Dst Cst Hop AB 4 C 6 E D 7 2 F 5
Table for B
Dst
Cst
Hop A 4 B C 2 F D 3 E 1 E C 3 1 F 1 2 6 1 D 3 A 4 B
Table for C
Dst
Cst
Hop A 6 F B 2 C D 1 E 4
Table for D
Dst
Cst
Hop A 7 B 3 C 1 D E 5 F 2
Table for E
Dst
Cst
Hop A 2 B 4 F C D 5 E 3
Table for F
Dst
Cst
Hop A 5 B 1 C D 2 E 3 F
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25. Link Cost Changes Link cost changes:
Node detects local link cost change
Updates distance table
If cost change in least cost path, notify neighbors A C 1 4 50 B
Node B D C N A 4 1 B D C N A 1 B D C N A 1 B D C N A 1 B
“good
news
travels
fast”
Node C D C N A 5 B 1 D C N A 5 B 1 D C N A 2 B 1 D C N A 2 B 1
time
Link cost changes here
Algorithm terminates
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26. Count to Infinity ProblemA C 1 4 50 B 60
Node B D C N A 4 1 B D C N A 6 1 B D C N A 6 1 B D C N A 8 1 B
“bad
news
travels
slowly”
Node C D C N A 5 B 1 D C N A 5 B 1 D C N A 7 B 1 D C N A 7 B 1 …
time
Link cost changes here
2018/11/12

27. Poisoned Reverse If C routes through B to get to A:
C tells B its (C’s) distance to A is infinite (so B won’t route to A via C)
Will this completely solve count to infinity problem? A C 1 4 50 B 60
Node B D C N A 4 1 B D C N A 60 1 B D C N A 60 1 B D C N A 51 1 B D C N A 51 1 B
Node C D C N A 5 B 1 D C N A 5 B 1 D C N A 50 B 1 D C N A 50 B 1 D C N A 50 B 1
time
Link cost changes here; B updates D(B, A) = 60 as
C has advertised D(C, A) = ∞
Algorithm terminates
2018/11/12

28. Poison Reverse Failures
Table for A
Dst
Cst
Hop C 7 F
Table for B
Dst
Cst
Hop C 8 A
Table for D
Dst
Cst
Hop C 9 B
Table for F
Dst
Cst
Hop C 1  F C 6 1 B D A 4
Table for A
Dst
Cst
Hop C  –
Table for F
Dst
Cst
Hop C  –
Forced
Update
Forced
Update
Table for A
Dst
Cst
Hop C 13 D
Better
Route
Table for B
Dst
Cst
Hop C 14 A
The poison reverse sometimes fail, here we will show an example.
The cost of the link F-C is 1, but the link goes down and the cost becomes infinity.
With poison reverse, A and F change the cost to C to be infinity.
But A find B also has a path to D with cost 9, and D doesn’t route to C via A, but B.
So A consider it find a better router, and change the cost to be 13, it doesn’t know that the path from D to C also pass F-C.
So the process will count to infinity.
A possible solution is to make “infinity” smaller, but what is upper bound on maximum path length? Should it be larger or smaller, this is a problem.
Forced
Update
Iterations don’t converge
“Count to infinity”
Solution
Make “infinity” smaller
What is upper bound on maximum path length?
Table for D
Dst
Cst
Hop C 15 B
Forced
Update
Table for A
Dst
Cst
Hop C 19 D •
Forced
Update
2018/11/12

29. Routing Information Protocol (RIP)
Earliest IP routing protocol (1982 BSD)
Current standard is version 2 (RFC 1723)
Features
Every link has cost 1
“Infinity” = 16
Limits to networks where everything reachable within 15 hops
Sending Updates
Every router listens for updates on UDP port 520
RIP message can contain entries for up to 25 table entries
2018/11/12

30. RIP Updates Initial Periodic Triggered
When router first starts, asks for copy of table for every neighbor
Uses it to iteratively generate own table
Periodic
Every 30 seconds, router sends copy of its table to each neighbor
Neighbors use to iteratively update their tables
Triggered
When every entry changes, send copy of entry to neighbors
Except for one causing update (split horizon rule)
Neighbors use to update their tables
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31. RIP Staleness / Oscillation Control
Small Infinity
Count to infinity doesn’t take very long
Route Timer
Every route has timeout limit of 180 seconds
Reached when haven’t received update from next hop for 6 periods
If not updated, set to infinity
Soft-state refresh  important concept!!!
Behavior
When router or link fails, can take minutes to stabilize
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32. RIP Table Processing
RIP routing tables managed by application-level process called route-d (daemon)
advertisements sent in UDP packets, periodically repeated
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33. Link State: Control Traffic
Each node floods its local information to every other node in the network
Each node ends up knowing the entire network topology  use Dijkstra to compute the shortest path to every other node
Host A
Host B
Host E
Host D
Host C N1 N2 N3 N4 N5 N7 N6
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34. Link State Protocol Concept
Every node gets complete copy of graph
Every node “floods” network with data about its outgoing links
Every node computes routes to every other node
Using single-source, shortest-path algorithm
Process performed whenever needed
When connections die / reappear
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35. Sending Link States by Flooding
X Wants to Send Information
Sends on all outgoing links
When Node Y Receives Information from Z
Send on all links other than Z X A C B D
(a)
(b)
(c)
(d)
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36. Dijkstra’s Algorithm Given Shortest Path First Algorithm
Graph with source node s and edge costs c(u,v)
Determine least cost path from s to every node v
Shortest Path First Algorithm
Traverse graph in order of least cost from source
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37. Dijkstra’s Algorithm: Concept2 5 3 
Current Path Costs
Last Links E C 3 1 F 1 2 6 1
Source
Node D 3 A 3 B
Done
Unseen
Horizon
Node Sets
Done
Already have least cost path to it
Horizon:
Reachable in 1 hop from node in Done
Unseen:
Cannot reach directly from node in Done
Label
d(v) = path cost
From s to v
Path
Keep track of last link in path
2018/11/12

38. Dijkstra’s Algorithm: Initially
Current Path Costs E C 3 1 F 1 2 6 1
Source
Node D 3 A 3 B
Horizon
Done
Unseen
No nodes done
Source in horizon
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39. Dijkstra’s Algorithm: Selection2 5 3 
Current Path Costs E C 3 1 F 1 2 6 1
Source
Node D 3 A 3 B B
Done
Unseen
Horizon
Select node v in horizon with minimum d(v)
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40. Dijkstra’s Algorithm: Selection (Cont.)2 5 3 
Current Path Costs
Horizon E C 3 1 F 1 2 6 1
Source
Node D 3 A 3 B
Done
Unseen
Add selected node to Done
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41. Dijkstra’s Algorithm: Update
Unseen 2 5 3 
Current Path Costs E C 3 4 6
Revised Path Costs 1 F 1 2 6 1
Source
Node D 3 A 3 B
Horizon
Done
Update costs based on paths having last link from newly added Done node
Could change values of nodes in horizon
Could add new nodes to horizon
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42. Link State Characteristics
With consistent LSDBs, all nodes compute consistent loop-free paths
Can still have transient loops B 1 1 X 3 A C 5 2 D
Packet from CA
may loop around BDC
if B knows about failure
and C & D do not
Link State Data Base
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43. OSPF Routing Protocol Open Shortest-path first More prevalent than RIP
Open standard created by IETF
Shortest-path first
Another name for Dijkstra’s algorithm
More prevalent than RIP
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44. OSPF Reliable Flooding
Transmit link state advertisements
Originating router
Typically, minimum IP address for router
Link ID
ID of router at other end of link
Metric
Cost of link
Link-state age
Incremented each second
Packet expires when reaches 3600
Sequence number
Incremented each time sending new link information
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45. OSPF Flooding Operation
Node X Receives LSA from Node Y
With Sequence Number q
Looks for entry with same origin/link ID
Cases
No entry present
Add entry, propagate to all neighbors other than Y
Entry present with sequence number p < q
Update entry, propagate to all neighbors other than Y
Entry present with sequence number p > q
Send entry back to Y , tell Y that it has out-of-date information
Entry present with sequence number p = q
Ignore it
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46. Flooding Issues When should it be performed
Periodically
When status of link changes
Detected by connected node
What happens when router goes down & back up
Sequence number reset to 0
Other routers may have entries with higher sequence numbers
Router will send out LSAs with number 0
Will get back LSAs with last valid sequence number p
Router sets sequence number to p+1 & resends
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47. Adoption of OSPF RIP viewed as outmoded OSPF Advantages
Good when networks small and routers had limited memory & computational power
OSPF Advantages
Fast convergence when configuration changes
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48. Comparison of LS and DV Algorithms
Message complexity
LS: with n nodes, E links, O(nE) messages
DV: exchange between neighbors only
Speed of Convergence
LS: Complex computation
But…can forward before computation
may have oscillations
DV: convergence time varies
may be routing loops
count-to-infinity problem
(faster with triggered updates)
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49. Comparison of LS and DV Algorithms (Cont.)
Space requirements:
LS maintains entire topology
DV maintains only neighbor state
Robustness: what happens if router malfunctions?
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
errors propagate thru network
Other tradeoffs
Making LSP flood reliable
2018/11/12