Download Chapter 11 Interior Routing Protocols

CIS 203
11 : Interior Routing Protocols
Introduction
• Routing protocols essential to operation of an
internet
• Routers forward IP datagrams from one router
to another on path from source to destination
• Router must have idea of topology of internet
• Routing protocols provide this information
Internet Routing Principles
• Routers receive and forward datagrams
• Make routing decisions based on knowledge of
topology and conditions on internet
• Decisions based on some least cost criterion
(chapter 14)
Fixed Routing
• Single permanent route configured for each
source-destination pair
—Routes fixed
—May change when topology changes
—Link cost not based on dynamic data
—Based on estimated traffic volumes or capacity of link
Figure 11.1 A Configuration of
Routers and Networks
Discussion of Example
• 5 networks, 8 routers
• Link cost for output side of each router for each
network
—Next slide shows how fixed cost routing may be
implemented
• Each router has routing table
Routing Table
• One required for each router
• Entry for each network
— Not for each destination
— Routing only needs network portion
• Once datagram reaches router attached to destination
network, that router can deliver to host
• IP address typically has network and host portion
• Each entry shows next node on route
— Not whole route
Routing Tables in Hosts
• May also exist in hosts
—If attached to single network with single router then
not needed
• All traffic must go through that router (called the gateway)
—If multiple routers attached to network, host needs
table saying which to use
Figure 11.2
Example Routing Tables
Adaptive Routing
• As conditions on internet changes, routes may
change
—Failure
• Can route round problems
—Congestion
• Can route round congestion
• Avoid, or at least not add to further congestion
Drawbacks of Adaptive Routing
• More complex routing decisions
— Router processing increases
• Depends on information collected in one place but used
in another
— More information exchanged improves routing decisions but
increases overhead
• May react two fast causing congestion through
oscillation
• May react to slow, being irrelevant
• Can produce pathologies
— Fluttering
— Looping
Fluttering
• Rapid oscillation in routing
• Due to router attempting load balancing or
splitting
—Splitting traffic among a number of routes
—May result in successive packets bound for same
destination taking very different routes (see next
slide)
Figure 11.3 Example of
Fluttering
Problems with Fluttering
• If in one direction only, route characteristics
may differ in the two directions
—Including timing and error characteristics
• Confuses management and troubleshooting applications that
measure these
• Difficulty estimating round trip times
• TCP packets arrive out of order
—Spurious retransmission
—Duplicate acknowledgements
Looping
• Packet forwarded by router eventually returns to
that router
• Algorithms designed to prevent looping
• May occur when changes in connectivity not
propagated fast enough to all other routers
Adaptive Routing Advantages
•
•
•
•
Improve performance as seen by user
Can aid congestion control
Benefits depend on soundness of design
Adaptive routing very complex
—Continual evolution of protocols
Classification of Adaptive
Routing Strategies
• Based on information sources
—Local
• E.g. route each datagram to network with shortest queue
• Balance loads on networks
• May not be heading in correct direction
– Include preferred direction
• Rarely used
—Adjacent nodes
• Distance vector algorithms
—All nodes
• Link-state algorithms
• Both need routing protocol to exchange information
Autonomous Systems (AS)
• Group of routers exchanging information via
common routing protocol
• Set of routers and networks managed by single
organization
• Connected
—Except in time of failure
Interior Routing Protocol (IRP)
• Passes routing information between routers
within AS
• Does not need to be implemented outside AS
—Allows IRP to be tailored
• May be different algorithms and routing
information in different connected AS
• Need minimum information from other
connected AS
—At least one router in each AS must talk
—Use Exterior Routing Protocol (ERP)
Exterior Routing Protocol (ERP)
• Pass less information than IRP
• Router in first system determines route to target
AS
• Routers in target AS then co-operate to deliver
datagram
• ERP does not deal with details within target AS
Figure 11.4 Application of Exterior
and Interior Routing Protocols
Approaches to Routing –
Distance-vector
• Each node (router or host) exchange information with
neighboring nodes
— Neighbors are both directly connected to same network
• First generation routing algorithm for ARPANET
• Node maintains vector of link costs for each directly
attached network and distance and next-hop vectors for
each destination
• Used by Routing Information Protocol (RIP)
• Requires transmission of lots of information by each
router
— Distance vector to all neighbors
— Contains estimated path cost to all networks in configuration
— Changes take long time to propagate
Approaches to Routing –
Link-state
• Designed to overcome drawbacks of distance-vector
• When router initialized, it determines link cost on each interface
• Advertises set of link costs to all other routers in topology
— Not just neighboring routers
• From then on, monitor link costs
— If significant change, router advertises new set of link costs
• Each router can construct topology of entire configuration
— Can calculate shortest path to each destination network
• Router constructs routing table, listing first hop to each destination
• Router does not use distributed routing algorithm
— Use any routing algorithm to determine shortest paths
— In practice, Dijkstra's algorithm
• Open shortest path first (OSPF) protocol uses link-state routing.
• Also second generation routing algorithm for ARPANET
Exterior Router Protocols –
Path-vector
• Dispense with routing metrics
• Provide information about which networks can be
reached by a given router and ASs crossed to get there
— Does not include distance or cost estimate
• Each block of information lists all ASs visited on this
route
— Enables router to perform policy routing
— E.g. avoid path to avoid transiting particular AS
— E.g. link speed, capacity, tendency to become congested, and
overall quality of operation, security
— E.g. minimizing number of transit Ass
Least Cost Algorithms
• Least-cost criterion
—If minimize number of hops, link value 1
—Link value may be inversely proportional to capacity,
proportional to current load, or some combination
—May differ in different two directions
—E.g. if cost equaled length of queue
• Cost of path between two nodes as sum of costs
of links traversed
• For each pair of nodes, find least cost path
• Dijkstra's algorithm
• Bellman-Ford algorithm
Dijkstra's Algorithm
• Find shortest paths from given node to all other
nodes, by developing paths in order of
increasing path length
• Proceeds in stages
—By kth stage, shortest paths to k nodes closest to
(least cost away from) source have been determined
—Set T
—Stage (k + 1), node not in T with shortest path from
source added to T
—As each node added to T, path from source defined
Dijkstra's Algorithm –
Formal (1)
•
•
•
•
•
•
•
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N
= set of nodes in the network
s
= source node
T
= set of nodes so far incorporated
w(i, j) = link cost from node i to node j
w(i, i) = 0
w(i, j) =  if nodes not directly connected
w(i, j)  0 if nodes directly connected
L(n) = cost of least-cost path s to n currently known
— At termination, cost of least-cost path in graph from s to n
Dijkstra's Algorithm –
Formal (2)
[Initialization]
T = {s}
i.e. set of nodes so far incorporated consists of only source node
L(n) = w(s, n) for n ≠ s
i.e. initial path costs to neighboring nodes are link costs
[Get Next Node]
Find neighboring node not in T with least-cost path from s
Incorporate node into T
Also incorporate edge incident on that node and node in T that contributes to the
path. This can be expressed as:
Find x T such that
Lx  
min
jT
L j 
Add x to T; add to T the edge that is incident on x and that contributes
the least cost component to L(x), that is, the last hop in the path.

Dijkstra's Algorithm –
Formal (3)
[Update Least-Cost Paths]
L(n) = min[L(n), L(x) + w(x, n)] for all n T
If the latter term is the minimum, the path from s to n is now
the path from s to x concatenated with the edge from x to n.
The algorithm terminates when all nodes have been added to T
Figure 11.5 Dijkstra’s Algorithm
Applied to Figure 11.1
Bellman-Ford Algorithm
• Find shortest paths from source node such that
paths contain at most one link
• Find shortest paths such that paths have at
most two links
• And so on
Figure 11.6 Bellman-Ford Algorithm
Applied to Figure 11.1
Bellman-Ford Algorithm –
Formal (1)
•
•
•
•
•
•
s
= source node
w(i, j) = link cost from node i to node j
w(i, i) = 0
w(i, j) =  if nodes are directly connected
w(i, j)  0 if nodes directly connected
h
= maximum number of links in path at
current stage
• Lh(n) =cost of least-cost path from s to n such
that no more than h links
Bellman-Ford Algorithm –
Formal (2)
[Initialization]
L0(n) = , for all n  s
Lh(s) = 0, for all h
[Update]
For each successive h  0:
For each n ≠ s, compute
min
Lh1n 
Lh j  w j, n
j


Connect n with predecessor node j that achieves
minimum
Eliminate any connection of n with different
predecessor node formed during an earlier iteration
Path from s to n terminates with link from j to n
Comparison of Algorithms
• Bellman-Ford
— Link cost to all neighboring nodes to node n [i.e., w(j, n)] plus total
path cost to those neighboring nodes from a particular source node s
[i.e., Lh(j)]
— Each node can maintain set of costs and associated paths for every
other node and exchange information with direct neighbors
— Each node can use Bellman-Ford based only on information from
neighbors and knowledge of its link costs
• Dijkstra
— Each node must know link costs of all links
— Information must be exchanged with all other nodes
• Both converge under static conditions to same solution
• If costs change algorithm will attempt to catch up
• If cost depends on traffic
• Depends on routes chosen
• then feedback condition exists
— Instabilities may result
Distance Vector Routing
• Each node exchange information with neighbors
—Directly connected by same network
• Each node maintains three vectors
—Link cost
—Distance vector
—Next hop vector
• Every 30 seconds, exchange distance vector
with neighbors
• Use this to update distance and next hop vector
Figure 11.7 Distance Vector
Algorithm Applied to Figure 11.1
Distributed Bellman-Ford
• RIP is a distributed version of Bellman-Ford
• Original routing algorithm in ARPANET
• Each simultaneous exchange of vectors between
routers is equivalent to one iteration of step 2
• In fact, asynchronous exchange used
—At start-up, get vectors from neighbors
• Gives initial routing
—By own timer, update every 30 seconds
—Changes are propagated across network
—Routing converges within finite time
• Proportional to number of routers
RIP Details –
Incremental Update
• Updates do not arrive from neighbors within
small time window
• RIP packets use UDP
• Tables updated after receipt of individual
distance vector
—Add any new destination network
—Replace existing routes with small delay ones
—If update from router R, update all routes using R as
next hop
RIP Details –
Topology Change
• If no updates received from a router within 180
seconds, mark route invalid
—Assumes router crash or network connection unstable
—Set distance value to infinity
• Actually 16
Counting to Infinity Problem (1)
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Slow convergence may cause:
All link costs 1
B has distance to network 5 as 2, next hop D
A & C have distance 3 and next hop B
Counting to Infinity Problem (2)
• Suppose router D fails:
— B determines network 5 no longer reachable via D
• Sets distance to 4 based on report from A or C
— At next update, B tells A and C this
— A and C receive this and increment their network 5 distance to 5
• 4 from B plus 1 to reach B
— B receives distance count 5 and assumes network 5 is 6 away
— Repeat until reach infinity (16)
— Takes 8 to 16 minutes to resolve
Figure 11.8
Counting to Infinity Problem
Split Horizon
• Counting to infinity problem caused by
misunderstanding between B and A, and B and
C
—Each thinks it can reach network 5 via the other
• Split Horizon rule says do not send information
about a route back in the direction it came from
—Router sending information is nearer destination than
you
—Erroneous route now eliminated within time out
period (180 seconds)
Poisoned Reverse
• Send updates with hop count of 16 to neighbors
for route learned from those neighbors
—If two routers have routes pointing at each other
advertising reverse route with metric 16 breaks loop
immediately
Figure 11.9
RIP Packet Format
RIP Packet Format Notes
• Command: 1=request 2=reply
— Updates are replies whether asked for or not
— Initializing node broadcasts request
— Requests are replied to immediately
• Version: 1 or 2
• Address family: 2 for IP
• IP address: non-zero network portion, zero host portion
— Identifies particular network
• Metric
— Path distance from this router to network
— Typically 1, so metric is hop count
RIP Limitations
• Destinations with metric more than 15 are
unreachable
—If larger metric allowed, convergence becomes
lengthy
• Simple metric leads to sub-optimal routing
tables
—Packets sent over slower links
• Accept RIP updates from any device
—Misconfigured device can disrupt entire configuration
Open Shortest Path First
(OSPF)
• RIP limited in large internets
• OSPF preferred interior routing protocol for
TCP/IP based internets
• Link state routing used
Link State Routing
• When initialized, router determines link cost on each
interface
• Router advertises these costs to all other routers in
topology
• Router monitors its costs
— When changes occurs, costs are re-advertised
• Each router constructs topology and calculates shortest
path to each destination network
• Not distributed version of routing algorithm
• Can use any algorithm
— Dijkstra
Flooding
• Packet sent by source router to every neighbor
• Incoming packet resent to all outgoing links except source link
• Duplicate packets already transmitted are discarded
— Prevent incessant retransmission
• All possible routes tried so packet will get through if route exists
— Highly robust
• At least one packet follows minimum delay route
— Reach all routers quickly
• All nodes connected to source are visited
— All routers get information to build routing table
• High traffic load
Figure 11.10
Flooding Example
OSPF Overview
• Router maintains descriptions of state of local
links
• Transmits updated state information to all
routers it knows about
• Router receiving update must acknowledge
—Lots of traffic generated
• Each router maintains database
—Directed graph
Router Database Graph
• Vertices
—Router
—Network
• Transit
• Stub
• Edges
—Connecting two routers
—Connecting router to network
• Built using link state information from other
routers
Figure 11.11 Sample
Autonomous System
Figure 11.12 Directed Graph of
Autonomous System of Figure 19.7
Link Costs
• Cost of each hop in each direction is called
routing metric
• OSPF provides flexible metric scheme based on
type of service (TOS)
—Normal (TOS) 0
—Minimize monetary cost (TOS 2)
—Maximize reliability (TOS 4)
—Maximize throughput (TOS 8)
—Minimize delay (TOS 16)
• Each router generates 5 spanning trees (and 5
routing tables)
Figure 11.13 The SPF Tree for
Router R6
Areas
• Make large internets more manageable
• Configure as backbone and multiple areas
• Area – Collection of contiguous networks and
hosts plus routers connected to any included
network
• Backbone – contiguous collection of networks
not contained in any area, their attached routers
and routers belonging to multiple areas
Operation of Areas
• Each are runs a separate copy of the link state
algorithm
—Topological database and graph of just that area
—Link state information broadcast to other routers in
area
—Reduces traffic
—Intra-area routing relies solely on local link state
information
Inter-Area Routing
• Path consists of three legs
—Within source area
• Intra-area
—Through backbone
• Has properties of an area
• Uses link state routing algorithm for inter-area routing
—Within destination area
• Intra-area
Figure 11.14
OSPF Packet Header
Packet Format Notes
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Version number: 2 is current
Type: one of 5, see next slide
Packet length: in octets including header
Router id: this packet’s source, 32 bit
Area id: Area to which source router belongs
Authentication type: null, simple password or
encryption
• Authentication data: used by authentication
procedure
OSPF Packet Types
• Hello: used in neighbor discovery
• Database description: Defines set of link state
information present in each router’s database
• Link state request
• Link state update
• Link state acknowledgement
Required Reading
• Stallings chapter 11