Routing Within An Autonomous System RIP, OSPF

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Routing Within An Autonomous System(RIP, OSPF)

• Chapter 15

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Static vs Dynamic Interior Routes

• Interior Routers
• Two routers within an autonomous system
• Are considered to be interior to one another
• How do they learn about their own networks?
• Small, slowly changing systems
• Establish and modify routes by hand
• Update table when new network added or deleted
• Figurerouting is trivial

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Figure 15.1
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• Disadvantages of manual system
• Cannot accommodate rapid growth
• Cannot accommodate rapid change
• Need automated methods
• Respond to change more easily
• Improve reliability
• Better response to failures

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Figure 15.2
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• Multiple physical paths
• Usually pick one to be primary
• Router(s) fail along primary, must change
• Manually time-consuming error prone
• Even in small internets need automated system
• For automation
• Interior routers must communicate
• Exchange routing information
• Once data established, advertise
• One interior router advertises to other
  autonomous systems via Exterior Gateway Protocol

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• No single interior protocol has emerged
• Varied topologies and technologies
• Tradeoffs between simplicity functionality
• Easy to install configure less functionality
• Multiple protocols have become popular
• Small AS
• Choose a single one use exclusively internally
• Larger AS
• Often choose a small set

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• Interior Gateway Protocol
• Used as generic description
• Refers to any algorithm used by interior routers
• Routers may run BGP to advertise reachability
• Need an IGP to obtain information within the AS

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Figure 15.3
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Routing Information Protocol (RIP)

• One of most widely used IGPs
• Also know as route-d
• Came from Univ of CA Berkeley
• Developed for machines on their LANs
• Relies on physical network broadcast to make
  routing exchanges quickly
• Not designed to use on large WANs
• Versions of RIP adapted for WANs are sold

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• Popularity not only due to technical merits
• Was distributed with popular 4BSD UNIX systems
• Lots of TCP/IP sites used without even
  considering technical aspects
• Once installed it became the basis for local
  routing
• RIP was built and adopted without a standard
• Most implementations derived from Berkeley code
• Interoperability was limited
• Many undocumented details and subtleties
• New versions led to more problems
• Standard appeared in June 1988

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• RIP Operation
• Straightforward distance-vector routing
• Partitions participants into two categories
• Active
• Advertise their routes
• Only routers
• Passive
• Do not advertise
• Host must use passive mode

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• Active RIP routers broadcast every 30 seconds
• Sends routing update message
• Takes information from current routing database
• Update is a set of pairs
• (IP address, integer distance to that network)
• Hop count is used as the distance metric
• One hop directly connected
• Two hops reachable through one other router
• Hops number of networks datagram will encounter
• Hop count for shortest path not always optimal
• 3 Ethernets faster than 2 satellites
• Some RIP implementations allow assignment of
  artificially high hop counts when advertising
  slow networks

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• Active passive routers listen to broadcasts
• All update tables according to DV algorithm
• May take some time for advertisements to
  propagate
• Routers use hysteresis to improve performance
• Does not replace a route with an equal cost route
• Prevents oscillation among equal paths
• Timers are used on all routes
• Solves problem of routes through crashed routers
• Start timer when install route in table
• Restart whenever receive msg advertising that
  route
• Route is invalid if 180 seconds pass without
  another advertisement

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• RIP must handle three types of errors
• Routing loops
• Algorithm does not explicitly detect routing
  loops
• Must either assume participants can be trusted or
  take precautions to prevent
• Instabilities
• Must limit hop count to prevent
• Maximum possible distance value is 16
• If legitimate hop count is higher, must divide
  the internet into sections or use an alternate
  protocol
• Slow convergence
• Routing messages propagate slowly across the
  network
• Can lead to inconsistencies
• Not unique to RIP fundamental problem in any DV
  protocol
• Hop count limit helps but does not eliminate

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Figure 15.4
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• Solving slow convergence
• Good news travel quickly bad news travel
  slowly
• Quick to install good route
• Unreachable only after timeout then learn and
  propagate new route
• Split horizon update
• Router does not propagate information over the
  interface from where the route arrived
• R2 would not advertise route to network 1 to R1
• If R1 loses connectivity, it must quit
  advertising
• After a few rounds of routing updates, all
  routers agree that the network is unreachable
• Does not prevent all routing loops

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• Hold down
• Router ignores information for a period of time
• Typical time is 60 seconds
• Done after receiving msg that network is
  unreachable
• Wait so all machines can get bad news keeps from
  mistakenly accepting an out-of-date message
• All machines must have same idea of hold down
• Otherwise, get routing loops
• Disadvantages
• If routing loop occurs, will be preserved for the
  hold down
• Also preserves incorrect routes during the hold
  down time
• Even when alternatives exist

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• Poison reverse
• When connection disappears
• Advertising router keeps entry for few update
  periods
• Puts infinite cost in the broadcasts
• Combine with triggered updates
• Router sends immediate broadcast when get bad
  news
• Not wait for next broadcast time
• Minimizes time it is vulnerable to believing good
  news

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• These techniques solve some problems introduce
  others
• Triggered updates
• Suppose many routers share common network
• Single broadcast changes all tables triggering
  more
• Broadcast avalanche
• Broadcasts
• Take substantial bandwidth themselves
• Loops prevent stopping loops
• Looping messages may prevent routing msgs to
  break loops
• Hold down in WANs
• Period so long, higher level protocol timers may
  expire
• Breaks the connections

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• RIP1 message format
• Messages are of two types
• Routing information messages
• Periodic broadcast of unsolicited response
  messages
• Messages to request information
• Routers or hosts can ask for info with request
  command
• Routers reply using a response command
• Both use same format

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Figure 15.5
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• RIP2 Address Conventions
• Skip..

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• RIP route interpretation and aggregation
• Version 1 contains no provision for subnet mask
• Originally designed for classful addressing
• Extended to allow subnetting
• Important restriction
• Subnet routes can only go in updates sent across
  networks that are part of the subnetted prefix
• Cannot use with variable-length subnet addresses
  or classless
• Due to not having explicit subnet mask
  information
• May have updates for networks in out of prefix
• Router must prepare different update messages

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• RIP2 extensions
• Contains provisions for explicit subnet mask
• Also include explicit next-hop information
• Prevents routing loops
• Prevents slow conversion
• RIP2 message format
• Puts new info in unused octets of address field
• Router can use both versions simultaneously
• Version number in same octet inspect before
  process
• Adds 16-bit ROUTE TAG
• Identify the origin of the route

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Figure 15.6
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• Transmitting RIP messages
• Messages do not have explicit length field
• Nor any explicit count of entries
• Rely on delivery mechanism to tell length
• With TCP/IP
• Rely on UDP to tell receiver the message length
• RIP operates on UDP port 520

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• Disadvantage of RIP hop counts
• RIP restricts routing to a hop-count metric
• RIP restricts size of any internet using it
• Has small hop count value for infinity (16)
• Limits span to at most 15 routers between hosts
• Is not a limit on total number or density of
  routers
• In any case, hop count is a crude measure
• Not always get least delay or highest capacity
  routes
• Makes routes static cannot change due to load

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The Hello Protocol

• IGP that uses routing metric other than hops
• Now obsolete
• Historically, used in original NSFNET backbone
  fuzzball routers
• Uses metric of delay
• Provides two functions
• Synchronizes clocks among a set of machines
• Allows each machine to compute shortest delay
  paths

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• Messages carry timestamp as well as routing info
• Each participating machine maintains table
• Contains best estimate of neighboring machine
  clocks
• Transmit timestamp with each packet
• Receiver computes estimate of delay on the link
  by using the timestamp and its estimate of the
  senders clock
• Periodically poll neighbors to update clock
  estimates
• Standard D-V approach for update
• Send table of destinations estimated delays
• Receivers update tables if cheaper route
  advertised

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Delay Metrics Oscillation

• Is delay a good routing metric?
• Would seem so
• Worked well in the early Internet backbone
• Instability is the reason most protocols do not
  use delay
• Any protocol that changes routes quickly can
  become unstable

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• Hop counts fixed delay is not
• Minor variations in delay measurements occur
• Hardware clock drift
• CPU load during measurement
• Delays by link-level synchronization
• If react quickly to slight variations, get
  two-stage oscillation
• Switch back and forth between alternate paths

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• Heuristics to help avoid oscillation
• Hold down
• Slows down changing
• Round off measurements or use threshold
• Ignore differences less than the threshold
• Use average measurement
• Keep average of recent measurements
• Use K-out-of-N rule
• K of the most recent N measurements must be less
  than the current delay before route can be changed

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• Can still have instability
• Due to comparing delays on paths with different
  characteristics
• Traffic has dramatic effect on delay
• As load increases, delay grows rapidly
• Fall into positive feedback cycle
• Burst of traffic at one place increases delay
• Protocol changes route
• New traffic may cause another change in delay
• Another route change occurs
• Must have mechanism to dampen oscillation

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• Previous heuristics may not help
• They help in simple case for paths with same
  throughput characteristics
• Not good when paths have different delay and
  throughput characteristics
• Compare serial line and satellite link
• First, both paths idle serial line have much
  less delay
• Then, traffic quickly overloads low capacity line
• Satellite delay will be less change to it
• High capacity load not significantly change
  delay
• But, unloaded serial line will now become
  attractive
• Routing will change again and the cycle will
  continue
• Oscillations do occur in practice
• Difficult to manage

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Combining RIP, Hello, and BGP

• Single router may use multiple protocols
• Interior Gateway Protocol
• Gather routing information within AS
• Exterior Gateway Protocol
• Advertise routes to other ASs
• Should be easy to combine the two
• Technical and political obstacles exist

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• IGP protocols are routing protocols
• RIP and HELLO used to update routing tables
• Get info from other routers inside AS
• routed implements RIP
• Advertises information from local routing table
• Updates local table when it receives updates
• RIP trusts routers within the AS to send correct
  data
• Exterior protocols (BGP) do not trust routers
• Do not advertise all possible routes in local
  table
• Keep database of reachability
• Apply policy constraints when sending/receiving
  info

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• Ignoring policy constraints can make some parts
  of the internet unreachable
• Example
• Suppose router running RIP
• Propagates route to Purdue actually has no route
• Other RIP routers will accept and update
• Will pass Purdue traffic to the erroneous router
• Problem if EGP protocol not have policy
  constraints
• Border router pass illegal route to other ASs
• Purdue may become unreachable for parts of the
  internet

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Gated Inter-AS Communication

• gated
• Interface between autonomous systems
• Understands multiple protocols
• Both IGPs and BGP
• Ensures policy constraints are honored
• Can accept RIP msgs and modify local table
  (routed)
• Can advertise routes from within AS using BGP
• Has rules on which networks it may may not
  advertise
• Also has rules on how to report distances to
  those networks
• Links IGP with BGP

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Open SPF Protocol (OSPF)

• Chapter 13 link state algorithm
• Uses SPF to compute shortest paths
• Scales better than distance-vector algorithms
• OSPF is an IGP using link state algorithm
• Designed by Internet Engineering Task Force
• To encourage adoption of link state technology
• Tackles several ambitious goals

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• Open standard
• Anyone can implement without license fees
• Includes type of service routing
• Have multiple routes for a given destination
• Choose by TOS field in IP header
• OSPF first among TCP/IP protocols to have this
• Provides load balancing
• Distributes traffic over multiple, same cost
  routes
• Can partition routers and networks into areas
• Permits growth makes management easier

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• Allows exchanges to be authenticated
• Variety of authentication schemes
• Supports host-specific, subnet-specific, and
  classless routes
• Accommodates multi-access nets (Ethernet)
• Can describe network via virtual network
• Abstracts away from details of physical
  connections
• Provides flexibility for managers
• Allows routers to exchange routing info learned
  from external sites
• Distinguishes where information came from

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Figure 15.7
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Figure 15.8
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• Routers exchange database description msgs
• Used to initialize network topology database
• During exchange
• One router is master other is slave
• Slave acknowledges each description with a
  response
• Topology database may be large
• Can divide into several messages using I and M
  bits
• I 1 is initial message
• M 1 additional messages follow
• Bit S indicates if sent by master (1) or slave
  (0)
• Sequence numbers used to make sure all received

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Figure 15.9
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• Link status request message
• After exchanging DB descriptions, router may
  discover parts of its DB are out of date
• Requests neighbor to send update
• Lists specific links it wants info about
• Neighbor responds with most current information
  about those links

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Figure 15.10
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Figure 15.11

• Links from router to
• - given area
• - specific network
• - single, subnetted IP network
• - networks at other sites

Figure 15.12
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Routing with Partial Information

• Hosts can have partial information
• Rely on routers
• Not all routers have complete information
• Usually single router in AS connects to others
• Suppose site connects to global Internet
• At least one router must have connection to an
  ISP
• Routers inside AS know all destinations within
• Have default route to send all traffic to the ISP

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• Examine routing tables
• Routers at center of Internet know all
• Have complete set of routes to all destinations
• Learn from the routing arbiter system
• They do not use default routing
• If destination address not in their table
• Either address is not valid, or
• Address is valid but currently unreachable
• Routers beyond those at center do not know all
• Have an incomplete set of routes
• Rely on default routes for addresses they do not
  have

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• Using default routes for most routers has
  consequences
• Local routing errors can go undetected
• Machine in AS routes packet to external AS
• Suppose should have gone to local router
• External system will route it back
• Preserves connectivity even if routing incorrect
• Very bad for a WAN
• Routing update messages will be smaller
• A good thing!

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Summary

• Must have automated routing procedures
• IGPs
• Used by 2 routers under control of single mgr
• Uses either DV or link state algorithm (SPF)
• RIP DV implemented by routed
• Uses split horizon, hold-down and poison reverse
• Hello obsolete uses delay versus hop count
• OSPF uses link status algorithm
• gated
• Interface between IGP (RIP) and EGP (BGP)