Distance Vector Routing Dynamic Programming

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Distance Vector Routing Dynamic Programming

• Limited state information. Just the next hop and
  cost.

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Distance Vector Routing Dynamic Programming

• Suppose a new node comes on line.

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Distance Vector Routing Dynamic Programming

• Suppose a new node comes on line.
• Suppose I first talks to A.

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Distance Vector Routing Dynamic Programming

• Suppose a new node comes on line.
• Suppose I first talks to A.
• Next I talks to D.

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Distance Vector - Algorithm

• Start with all destinations with infinite
  distance, except for the actual node, which is
  distance 0.
• Every 30 seconds (RIP), or when a change occurs
  in the table, send table to neighbors.
• If the distance to a prefix advertised by a
  neighbor is less plus the distance to the
  neighbor is less than known distance, reduce
  distance to prefix and route packets with that
  destination prefix to that neighbor.

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Count to Infinity Problem
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Approaches to Mitigate Count Infinity
Why is count to infinity a problem? It generates
tons of routing updates too much traffic The
network should report that a route is
unreachable. Put upper bound an upper bound the
the diameter of the network. But what is the
network grows (as it did). Split horizon. A
router does not report a distance to the neighbor
it learned the distance from. Split horizon with
poison reverse. If A advertises the best cost to
E to B, then B advertises a cost of infinity to E
back to A. This only works for loops that
involve two nodes. With larger loops, the
mitigation is more difficult and these remedies
reduce the rate of convergence. The way to fix
it is to use link state routing.
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Link State Routing (IS-IS and OSPF)

• Each router learns the entire network. (Compare
  to distance vector)
• If the entire network is known, the shortest cost
  routing can be computed.
• Each router advertises to its neighbors who it is
  connected to.
• Each router floods any advertisement it receives.

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Link State Routing

• HELLO Determining who the router is connected
  to.
• Reliable Flooding of LSA (link state
  advertisement) and keeping/getting up-to-date
  information.
• Calculate shortest path.

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HELLO

• Periodically, a node sends a HELLO LSP (Link
  State Packet) to its neighbors.
• The neighbor responds with a HELLO reply.
• This way the router can determine which router it
  is connected to.
• The default period is 10 30sec.

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Reliable Flooding

• Each LSP contains
• The ID of the node that created the LSP.
• The list of the neighbors directly connected to
  that node with the cost of each link.
• A sequence number
• A time to live
• Each LSP reception is ACKed.

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Sequence Number

• The node that generated the LSA increments the
  sequence number for each LSA it sends out.
• The sequence number is 32 bits long, so wrap
  around is not possible.
• When router receives an LSA, it checks if the seq
  num of the received LSA is larger than the LSA in
  memory.
• If the LSA has a larger seq. num, it is stored,
  the old one discarded and the new LSA is flooded
  to all neighbors, except the one that sent it.
• If the LSA has a smaller or equal seq. no, it is
  discarded.

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Time to Live (TTL)

• Each second a LSA sits in memory, its TTL is
  decremented.
• Each time the LSA is transmitted, its TTL is
  decremented.
• When the TTL reaches zero, the LSA is discarded
  and the router floods the LSA with TTL0 to tell
  other routers to delete this information.

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• A LSA is generated periodically (on the order of
  hours) or when a change is detected (via the
  HELLO).
• When a node reboots, it doesnt know its seq. no.
  
• It floods its link state with seq. no. 0.
• It sends a Link State Request to its neighbors
• The neighbors respond with the most up to date
  LSA they have. These LSA may contain the LSA of
  the before it crashed. In this case, the node
  updates its seq. no.
• Remember that every time a router gets a new
  info, it floods the information.

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Security Issues

• An attacker could send fake LSAs.
• Must filter out LSA from any other source but the
  neighbor and use authentication.
• If a router is compromised, it could advertise a
  low cost to all nodes. Then all nodes will send
  packets to this node and the network would stop
  working.

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metrics

• ARPANET the link cost included the size of the
  queue (a measure of congestion).
• This does not work..
• Static cost just use delay (Juniper seems to do
  this)
• Dynamic cost must make sure that link costs do
  not vary too much.
• The cost can is restricted to only change slowly
  over time.
• The cost between links cannot vary much. There
  cannot be more than a factor of 7 difference
  between the most and least expensive link.
• The cost of a link can only vary by a factor of
  3.
• Cost only depends on utilization at moderate or
  high loads.
• Only send updates when the cost crosses a
  threshold.
• Is this stable?

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TOS Type of Service

• A data packet can identify the type of service it
  wants.
• The router can provide different routing
  according to the TOS..
• To support this, OSPF allows the link cost to
  depend on the TOS.
• This is not widely deployed.

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OSPF Header
Type 1hello 2database description 3link
status request 4link state update 5link state
ack
version
type
length
Source address
Address of the sender
Area ID
Checksum
Authentication type
Authentication
Which area the packet originated
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OSPF LSA
Like TTL
LS Age
Options
Type1
Link State ID
The same
Advertising Router
Seq. No
Checksum
Length
Number of Links
Link ID
Link Data
Metric
Link Type
Num TOS
Optional TOS
More Links
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