CSS432 Routing Textbook Ch3.3

1
CSS432 RoutingTextbook Ch3.3

• Professor Munehiro Fukuda

2
What Is Routing?

• Forwarding vs Routing
• forwarding
• To map a network to an outgoing interface and
  some MAC information in a forwarding table.
• To send a packet to an interface as consulting a
  local and static forwarding table
• OSI Layer 2 data link level
• Implemented in specialized hardware (switch)
• routing
• To build a dynamic routing table
• To update table contents in a dynamic and
  distributed fashion
• OSI Layer 3 network level (internet)
• Using complex distributed algorithms

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Overview
At Node A

• Network as a Graph
• Goal
• Find lowest cost path between two nodes
• Static approach has shortcomings
• Hardware failures
• Static network topology
• Static band width
• Distributed, dynamic routing algorithms
• Distance vector routing (RIP)
• Link state routing (OSPF)

4
Distance Vector

• Each node maintains a set of triples
• (Destination, Cost, NextHop)

An initial distance vector at node A
Destination Cost Next hop
B 1 B
C 1 C
D 8 -
E 1 E
F 1 F
G 8 -
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Distance Vector

• 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)
• From B (A, 1), (C, 1)
• From C (A, 1), (B, 1), (D, 1)
• From E (A, 1)
• From F (A, 1), (G, 1)
• Update local table if receive a better route
• From B (C,1)
• (C, 1, C) lt (C, 2, B)
• From C (D, 1)
• (D, 8, - ) gt (D, 2, C)
• From F (G, 1)
• (G, 8, - ) gt (G, 2, F)
• Refresh existing routes delete if they are
  expired

Destination Cost Next hop
B 1 B
C 1 C
D 2 C
E 1 E
F 1 F
G 2 F
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Routing Loop

• Failure-recovering scenario
• F detects the link to G has failed
• F sets distance to G to 8 and sends an update to
  A
• A sets distance to G to 8
• A receives periodic update from C with a 2-hop
  path to G
• A sets distance to G to 3 and sends update to F
• F sets distance to G in 4 hops via A
• Count-to-infinity problem
• The link from A to E fails
• A advertises distance of infinity to E
• C advertise a distance of 2 to E
• B decides it can reach E in 3 hops
• B advertises this to A
• A decides it can read E in 4 hops
• A advertises this to C
• C decides that it can reach E in 5 hops

To G in 2
To G in 3
To G in 1
To G in 4
8
B
(5) To E in 4
(2) To E in 8
(3) To E in 3
(4) To E in 8
(1) To E in 2
A
C
(6) To E in 5
8
E
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Loop-Breaking Heuristics

• Set infinity to 16
• Scheme Stop an infinity loop in 16.
• Problem No more 16 hops
• Split horizon
• Scheme Dont send a neighbor the routing
  information learned from this neighbor.
• Ex. B includes (E, 2, A) and thus doesnt send
  (E, 3).
• Split horizon with poison reverse
• Scheme Send the routing information learned from
  this neighbor as setting hop count to 8.
• Ex. B includes (E, 2, A) and thus sends (E, 8, A)
• Problem Its slow convergence speed

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Routing Information Protocol (RIP)
frame header
datagram heaader
UDP header
RIP Message

• Cmd 1-6
• 1 request
• 2 reply
• Port 520
• Used by routed
• Advertisement 30secs
• Table entry timeout 3 mins.
• Deleted in 60secs
• Unix commands
• Ripquery (BSD)
• Tcpdump (available in Linux, too)
• Snoop (Solaris)

Cmd
Ver
Routing domain
Addr family (net addr)
Route tag
Address of net 1
Subnet mask
Next hop address (1-16)
Distance to net 1
Addr family (net addr)
Route tag
Address of net 2
Subnet mask
Next hop address
Distance to net 2 (1-16)
25 entries
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Link State

• Strategy
• Reliable dissemination of link-state information
  to all nodes over a system.
• Calculation of routes from the sum of all the
  accumulated link-state knowledge.
• Link State Packet (LSP)
• ID of the node that created the LSP
• A cost of link to each directly connected
  neighbor
• A sequence number (SEQNO)
• A time-to-live (TTL) for this packet

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Link State (cont)

• Reliable flooding
• Store most recent LSP from each node
• Forward LSP to all nodes but one that sent it
• Generate new LSP periodically
• Increment SEQNO
• Start SEQNO at 0 when reboot
• Decrement TTL of each stored LSP
• Discard when TTL0

X
A
D
C
B
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Dijkstras Shortest-Path Algorithm

• put (myself, 0, -) in the confirmed list
• Next myself
• while( true )
• for each edge (X, distance, Next) where X is Ns
  neighbor
• if neither confirmed or tentative list has (X,
  distance, Y) where Y ! Next, put (X, distance,
  Next) in the confirmed list
• if the tentative list has (X, distance, Y) where
  Y ! Next, and (X, distance, Y) gt (X, distance,
  Next)
• Replace (X, distance, Y) with (X, distance, Next)
• If the tentative list is empty,
• exit
• else
• move the shortest edge (A, distance, B) from the
  tentative to the confirmed list.
• Next A

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Dijkstras Shortest-Path Algorithm
(D, 14, C) (G, 15, F)
(A, 0, -) (E, 2, E) (B, 5, B) (F, 4, F) (C, 8, B)
(A, 0, -)
(A, 0, -)
(B, 5, B) (C, 10, C) (E, 2, E) (F, 4, F)
(A, 0, -) (E, 2, E) (B, 5, B) (F, 4, F) (C, 8,
B) (D, 14, C)
(G, 15, F)
(B, 5, B) (C, 10, C) (F, 4, F)
(A, 0, -) (E, 2, E)
(A, 0, -) (E, 2, E) (F, 4, F)
(C, 10, C) (B, 5, B)
(A, 0, -) (E, 2, E) (B, 5, B) (F, 4, F) (C, 8,
B) (D, 14, C)
(G, 15, F)
(A, 0, -) (E, 2, E) (F, 4, F)
(C, 10, C) (B, 5, B) (G, 15, F)
(A, 0, -) (E, 2, E) (B, 5, B) (F, 4, F) (C, 8,
B) (D, 14, C) (G, 15, F)
(A, 0, -) (E, 2, E) (B, 5, B) (F, 4, F)
(C, 8, B) (G, 18, B)
(C, 8, B) (G, 15, F)
(A, 0, -) (E, 2, E) (B, 5, B) (F, 4, F)
(A, 0, -) (E, 2, E) (B, 5, B) (F, 4, F) (C, 8, B)
(G, 15, F)
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Open Shortest Path First Protocol (OSPF)

• Header
• Hello (reachability)
• Database description (topology)
• Link status request
• Link status update
• Link status acknowledgment
• Advertisement (header type4)
• LS Age TTL
• Type1 link cost b/w routers
• Link-State ID Advertising Router
• Seq from the same router
• Link ID the other end route ID of link
• Link data used if there are two or more links
  to the same router
• Metric link cost
• Link type P2P, ethernet, etc
• TOS delay-sensitive, etc

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OSPF Contd

• Gated daemon directly uses IP datagram.
• Header Type2 Database description (topology)
  message
• Used when the current topology has changed.
• Sent from an initialized router to another router
  which has a topology information
• LS Sequence number
• Used to determine which message is the latest
• Send a message with a new sequence number and
  metric 8 when a router or a link fails.

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Metrics

• Original ARPANET metric
• measures number of packets enqueued on each link
• took neither latency or bandwidth into
  consideration
• New ARPANET metric
• stamp each incoming packet with its arrival time
  (AT)
• record departure time (DT)
• when link-level ACK arrives, compute
• Delay (DT - AT) Transmit Latency
• if timeout, reset DT to departure time for
  retransmission
• link cost average delay over some time period
• Fine Tuning
• compressed dynamic range
• replaced Delay with link utilization

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Virtual Private Networks and Tunnels
10.0.0.1
20.0.0.1
A
B
Application Level
Router
Dest router
Source router
10.0.0.1
20.0.0.1
Router Level
A
B
To 20.0.0.1
To 215.0.0.1
To 20.0.0.1
To 10.0.0.2
Internet
C
215.0.0.1
Company Branch
Company Branch
Physical Network Level
To 20.0.0.1
A
B
20.0.0.1
10.0.0.1
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Why VPN?

• Security
• The final destination/contents of packet cannot
  be easily intercepted.
• Routers
• Routers with special features such as
  multicasting can form a virtual network.
• No-IP packets
• Packets may be non-IP compatible packets.
• Mobile IPs
• The final destination may be a mobile computer.

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Mobile IP

• Invariant Sending hosts want to use the same IP
  address mapped to a mobile host regardless of its
  location.
• Questions
• How does the home agent intercept a packet that
  is destined for the mobile agent? --- Use ARP
• How does the home agent then deliver the packet
  to the mobile host? Use DHCP and VPN

10.0.0.3
Internet
DHCP server
Home agent
12.0.0.6
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Mobile IP (Contd)
1. ARP request Whats the physical addr
corresponding to 10.0.0.9?
3. Packet request sends a packet destined for
10.0.0.9 to the home agents MAC address
2. ARP response sends back MAC of 10.0.0.3
instead of 10.0.0.9
1. DHCP receives a new IP in the foreign network.
10.0.0.3
Internet
DHCP server
Home agent
12.0.0.6
IP tunneling wraps the packet inside an
IP header destined for the mobile host (12.0.0.7).
2. Care-of-address a mobile host informs
its Home agent of its original and new IPs.
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• Reviews
• RIP distance vector, routing loop and breaking
  heurictics
• OSPF link state, Dijkstras shortest path
  algorithm
• VPN and mobile IP
• Exercises in Chapter 3
• Ex. 46 (RIP)
• Ex. 52 (RIP)
• Ex. 62 (OSPF)
• Ex. 64 (OSPF)