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Forwarding vs. Routing
• Forwarding vs Routing
– forwarding:
• To select an output port based on destination address and
routing table
– routing:
• Process by which routing table is built
• Based on graph algorithms
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Goal
• Determine “good” path (sequence of routers) thru network
from source to destination
• “good” path:
– typically means minimum cost path
– other definitions possible (available path)
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Graph Abstraction
Graph abstraction for routing
algorithms:
• Graph nodes are routers
• Graph edges are physical links
– link cost
A
6
1
3
4
C
2
1
B
9
F
E
1
D
• Delay
• $ cost, or
• Congestion level
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Factors
• Factors
– Static factor
• Topology (does not change frequently)
– Dynamic
• load (link cost): changes with network traffic
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Next-Hop Forwarding
• Performed by router
• Uses table of routes
• Tables gives next hop
Table for Router R2
Destination
Next Hop
A
interface 1
B
interface 1
C
interface 2
D
interface 2
E
computer E
F
computer F
E
A
R1
R2
R3
B
F
D
Interface 1
C
Interface 2
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Source of Routing Table Information
• Manual
– Table created by hand
– Useful in small networks
– Useful if routes never change
• Automatic routing
– Software creates/updates routing table
– Needed in large networks
– Changes routes when failure occurs
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Relationship of Routing To Graph Theory
1
2
•Node models router
•Edge model connection
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Table for Node 1: Table for Node 2: Table for Node 3: Table for Node 4:
Dest NextHop
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-
Dest NextHop
1
(2, 3)
-
Dest NextHop
Dest NextHop
1
(3, 1)
1
(4, 3)
2
(3, 2)
2
(4, 2)
-
3
(4, 3)
4
-
2
(1, 3)
2
3
(1, 3)
3
(2, 3)
3
4
(1, 3)
4
(2, 4)
4
(3, 4)
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Shortest Path Computation
 Algorithms from graph theory
 No central authority (distributed computation)
 A router (automatic routing)
- Must learn route to each destination
- Only communicates with directly attached neighbors
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Algorithms For Computing Shortest Paths
• Distance - sum of weights along the path to the
destination
Two Algorithms:
 Distance Vector (DV)
- Routers exchange information in their routing tables
 Link-state
- Routers exchange link status information
 Both used in practice
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Overview of Distance Vector Algorithm
 Periodic, two-way exchange between neighbors
 During exchange, router sends
- List of pairs
- Each pair gives (destination, distance)
 Receiver
- Compares each item in list to local routes
- Changes routes if better path exists
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Distance Vector Intuition
• Let
-
N be neighbor that sent the routing message
V be destination in a pair
D be distance in a pair
C be D plus the cost to reach the sender
 If no local route to V or local route has cost > C
– install a route with next hop N and cost C
 Else
 ignore pair
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Example of Distance Vector Routing
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1
2
3
4
6
9
8
5
•
•
•
•
•
11
3
2
6
5
7
Consider transmission of one DV message
Node 2 sends to 3, 5, and 6
Node 6 installs cost 8 to route to 2
Later 3 sends update to 6
6 changes route to make 3 the next hop for destination 2
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More Detail on Distance Vector
• Each node maintains a set of triples
– (Destination, Cost, NextHop)
• Exchange updates directly connected neighbors
– periodically (on the order of several seconds)
– whenever table changes (called triggered update)
• Each update is a list of pairs:
– (Destination, Cost)
• Update local table if receive a “better” route
– smaller cost
– came from next-hop
• Refresh existing routes; delete if they time out
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Example
• Cost = HopCount
B
C
A
D
E
F
Final Routing Table for B
Destination Cost NextHop
A
1
A
C
1
C
D
2
C
E
2
A
F
2
A
G
3
A
G
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Distance Vector Algorithm
Given:
a local routing table, a weight for each link that connects to another router, and an incoming routing message
Compute:
an updated routing table
Method:
Maintain a distance field in each routing table entry;
Initialize routing table with a single entry that has the destination equal to the local router, the next-hop unused,
and the distance to zero;
Repeat forever {
wait for the next routing message to arrive over the network from a neighbor; Let N be the sending router;
for each entry in the message {
Let V be the destination in the entry and let D be the distance;
Compute C as D plus the weight assigned to the link over which the message arrived;
Examine and update the local routing table:
if (no route exists to V)
add an entry to the local routing table for destination V with next-hop N and distance C;
else if (a route exists that has next-hop N)
replace the distance in existing route with C;
else if (a route exists with distance greater than C)
change the next-hop to N and distance to C;
}
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Routing Loop Example that terminates (DV)
B
C
A
D
E
F
G
• Suppose, F detects that link to G has failed
• 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
Note that the system becomes stable at end
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Routing Loop Example
B
C
A
D
E
F
G
• Suppose, link from A to E fails
• A advertises distance of infinity to E, but B and C advertise a distance
of 2 to E
• Depending on the exact timing of events, the following might happen:
– Upon hearing that E can be reached in 2 hops from C, B decides it can
reach E in 3 hops; advertises this to A
– A decides it can reach E in 4 hops; advertises this to C
– C decides that it can reach E in 5 hops; and so on.
– This cycle stops when the distances to E reach a very large number
considered as infinity
• Known as count-to-infinity problem
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Loop-Breaking Heuristics
• Solution 1
– Use a small number as an approximation of infinity (Example: Set infinity to
16)
– Will limit the time to count to infinity
• Solution 2
– 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
– 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
– Known as split horizon
• Solution 3
– Split horizon with poison reverse (stronger variation of split horizon)
– Example: B actually sends that route back to A, but it puts negative
information in the route to ensure that A will not eventually use B to get to E.
• Last two solutions only work for routing loops that involve two nodes
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RIP: Routing Information Protocol
• Uses straightforward implementation of distance-vector
routing algorithm
• It supports multiple address families
• Most widely used routing protocols in IP networks
• Distributed with BSD version of Unix
• A router running RIP:
– sends its advertisement every 30 seconds
– sends an update message whenever an update from another router
causes it to change its routing table
• Usage limited to small networks - those with no paths
longer than 15 hops
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Link-State Routing (LS Routing)
 Overcomes instabilities in DV
 Pairs of routers periodically:
- Test link between them
- Broadcast link status message i.e., send to all nodes (not just
neighbors) information about directly connected links (not entire
routing table); Also known as flooding
 Router
- Receives status messages on links
- Computes new routes
• Uses Dijkstra's algorithm
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Example of Link-State Information
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11
2
3
4
6
9
8
5
3
2
6
5
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• Assume nodes 2 and 3
– Test link between 2 and 3
– Broadcast information
• Each node
– Receives information
– Re-computes routes as needed
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Link State Packet (LSP)
• Link State Packet (LSP)
–
–
–
–
id of the node that created the LSP
cost of link to each directly connected neighbor
sequence number (SEQNO)
time-to-live (TTL) for this packet
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Reliable flooding
• Store most recent LSP from each node
– A smaller sequence number would imply an older LSP and would
be discarded
• Forward LSP to all nodes but one that sent it
– It helps to bring an end to the flooding of an LSP
• Generate new LSP periodically
– increment SEQNO
• Start SEQNO at 0 when reboot
• Decrement TTL of each stored LSP
– discard when TTL=0
• Transmission of LSPs is made reliable using acks and
retransmissions just as in the link-layer protocol
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Dijkstra's Shortest Path Algorithm
• Input
- Graph with weighted edges
- Node, n
 Output
- Set of shortest paths from n to each node
- Cost of each path
 Called Shortest Path First (SPF) algorithm
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Algorithm Intuition
• Start with self as source node
 Move outward
 At each step
- Find node u such that it
 Has not been considered
 Is "closest" to source
- Compute
 Distance from u to each neighbor v
 If distance shorter, make path from u go through v
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Result of Dijkstra’s Algorithm
3
1
11
2
3
4
6
9
8
5
3
2
6
5
7
• Example routes from node 6
–
–
–
–
To 3, next hop = 3, cost = 2
To 2, next hop = 3, cost = 5
To 5, next hop = 3, cost = 11
To 4, next hop = 7, cost = 8
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Dijkstra’s Algorithm
Given: a graph with a nonnegative weight assigned to each edge and a designated source node
Compute: the shortest distance from the source node to each other node and a next-hop routing table
Method:
Initialize set S to contain all nodes except the source node:
Initialize array D so that D[v] is the weight of the edge from the source to v if such an edge exists,
and infinity otherwise;
Initialize entries of R so that R[v] is assigned v if an edge exists from the source to v, and zero
otherwise;
while (set S is not empty) {
choose a node u from S such that D[u] is minimum;
if (D[u] is infinity) {
no path exists to nodes in S; quit; }
delete u from set S;
for each node v such that (u,v) is an edge {
if (v is still in S) {
c = D[u] + weight(u,v);
if (c < D[v]) {
R[v] = u;
D[v] = c; }
}
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}
OSPF: Open Shortest Path First Protocol
• Most widely used link-state routing protocol
• “Open” refers to nonproprietary standard introduced by IETF
• Additional features over basic link-state algorithm
– Authentication of routing messages
• Uses a simple 8-byte password for authentication
• Can deter malicious users and prevent problems due to
misconfiguration
– Additional hierarchy
• Allows a domain to be partitioned into areas so that a router within a
domain does not necessarily need to know how to reach every
network. Only it needs to know to get to the right area
– Load balancing
• OSPF allows multiple routes to the same place to be assigned the
same cost and will cause traffic to be distributed evenly over those
routes.
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Hierarchical OSPF
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Hierarchical OSPF
• Two-level hierarchy: local area and backbone.
• Link-state advertisements do not leave respective areas.
• Nodes in each area have detailed area topology; they only
know direction (shortest path) to networks in other areas.
• Area Border routers “summarize” distances to networks in
the area and advertise them to other Area Border routers.
• Backbone routers run an OSPF routing algorithm limited to
the backbone.
• Boundary routers connect to other ASs (autonomous
systems).
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Routing for Mobile Hosts
• A router called home agent running on home network of the mobile host works
as a proxy
– Receives and forwards packets for the mobile host
• All agents periodically announces their presence (broadcast)
• When mobile host leaves its home network
– it registers with foreign agent in foreign network and provides the address of its
home agent to foreign agent
– foreign agent communicates with home agent for all packet delivery to mobile host
Sending host
Home agent
(10.0.0.3)
Foreign agent
(12.0.0.6)
Internetwork
Home network
(network 10)
Mobile host
(10.0.0.9)
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How to Make Routing Scale
• Flat versus Hierarchical Addresses
• Inefficient use of Hierarchical Address Space
– A network with two hosts needs two class C addresses (2/255 =
0.78% efficient)
– A network with 256 hosts needs 256 class B addresses (256/65535 =
0.39% efficient)
• Still Too Many Networks
– routing tables do not scale
– route propagation protocols do not scale
• Solution:
– Classless Interdomain Routing (CIDR)
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CIDR
• IP classes disappear at least for inter-domain
routing purpose
• A scalable solution for routing
• Network numbers are <length, value> pairs
– length represents number of bits in network prefix
– value is an actual IP address
• Slash notation is used instead of mask
– a.b.c.d/n (Ex; 165.95.11.101/28)
• Also called supernetting
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Routing in the Internet
• The Global Internet consists of Autonomous Systems (AS)
interconnected with each other
• Types of AS
– Stub AS: small corporation (only carry local traffic)
– Multihomed AS: large corporation (no transit)
– Transit AS: provider (carry both transit and local traffic)
• Two-level routing:
– Intra-AS: administrator is responsible for choice
– Inter-AS: unique standard
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Internet AS Hierarchy
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Internet Structure
Recent Past (Tree structure in 1990)
NSFNET backbone
Stanford
ISU
BARRNET
regional
Berkeley
PARC
MidNet
regional
Westnet
regional
UNM
NCAR
UNL
KU
UA
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Internet Structure
Today’s Multibackbone Internet
Large corporation
“Consumer ” ISP
Peering
point
Backbone service provider
“ Consumer” ISP
Large corporation
Peering
point
“Consumer”ISP
Small
corporation
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Route Propagation
• Know a smarter router
–
–
–
–
hosts know local router
local routers know site routers
site routers know core router
core routers know everything
• Autonomous System (AS)
– corresponds to an administrative domain
– examples: University, company, backbone network
– assign each AS a 16-bit number
• Two-level route propagation hierarchy
– interior gateway protocol (each AS selects its own) for intradomain
routing
– exterior gateway protocol (Internet-wide standard) for interdomain
routing
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Popular Interior Gateway Protocols
• RIP: Route Information Protocol
–
–
–
–
developed for XNS
distributed with Unix
distance-vector algorithm
based on hop-count
• OSPF: Open Shortest Path First
– recent Internet standard
– uses link-state algorithm
– supports load balancing
– supports authentication
• IGRP: Interior Gateway Routing Protocol (Cisco propr.)
– Similar to RIP
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EGP: Exterior Gateway Protocol
• Overview
– designed for tree-structured Internet
– concerned with reachability, not optimal routes
• Protocol messages
– neighbor acquisition: one router requests that another
be its peer; peers exchange reachability information
– neighbor reachability:
• one router periodically tests if the another is still reachable;
exchange HELLO/ACK messages;
– routing updates:
• peers periodically exchange their routing tables (distancevector)
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BGP-4: Border Gateway Protocol
• AS Types
– stub AS: has a single connection to one other AS
• carries local traffic only
– multihomed AS: has connections to more than one AS
• refuses to carry transit traffic
– transit AS: has connections to more than one AS
• carries both transit and local traffic
• Each AS has:
– one or more border routers
– one BGP speaker that advertises:
• local networks
• other reachable networks (transit AS only)
• gives path information
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BGP Example
• Speaker for AS2 advertises reachability to P and Q
– network 128.96, 192.4.153, 192.4.32, and 192.4.3, can be reached
directly from AS2
Customer P
(AS 4)
128.96
192.4.153
Customer Q
(AS 5)
192.4.32
192.4.3
Customer R
(AS 6)
192.12.69
Customer S
(AS 7)
192.4.54
192.4.23
Regional provider A
(AS 2)
Backbone network
(AS 1)
Regional provider B
(AS 3)
• Speaker for backbone advertises
– networks 128.96, 192.4.153, 192.4.32, and 192.4.3 can be reached
along the path (AS1, AS2).
• Speaker can cancel previously advertised paths
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Why different Intra- and Inter-AS
routing ?
• Policy: Inter is concerned with policies (which provider we
must select/avoid, etc). Intra is contained in a single
organization, so, no policy decisions necessary
• Scale: Inter provides an extra level of routing table size and
routing update traffic reduction above the Intra layer
• Performance: Intra is focused on performance metrics; needs
to keep costs low. In Inter it is difficult to propagate
performance metrics efficiently (latency, privacy etc). Besides,
policy related information is more meaningful.
We need BOTH!
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