IP Routing

1
IP Routing
Short Course on TCP/IP

• Dr. Nasir Darwish
• darwish_at_kfupm.edu.sa
• Information and Computer Science Department
• King Fahd University of Petroleum and Minerals
• Dhahran, Saudi Arabia

2
IP Network Addressing
IP is Internetwork Protocol. Each Host (and
Router) network interface uses a 32-bit
IP-address, part of which is a network
address. The network part of the address is same
for all hosts on the same physical network (e.g.
LAN).
32 bits
8, 16, or 24 bits from InterNIC (or locally
chosen)
Varies with subnet mask
Network ID Host ID
Router
Physical LAN
3
IP Address Classes

• Class Net Part Host Part First Byte
  Value Subnet Mask
• A 8 24 1 - 127 255.0.0.0
• B 16 16 128 - 191 255.255.0.0
• C 24 8 192 - 223 255.255.255.0

4
IP Routing Next Hop?
Which router (hop) to forward the IP packet to
next?
5
Router Basic Function

• The router matches the IP destination address of
  an incoming packet with an entry in his routing
  table
• The router then encapsulates the IP packet in an
  outgoing frame addressed to the hardware address
  of next router (or to destination host)

200.1.1.1
196.1.1.1
data
A
R0
200.1.1.1
196.1.1.1
data
R1
B
200.1.1.1
196.1.1.1
R0
R1
Host B
Host A
A/R0 (R1/B) From/To Data Link layer
addresses Note IP addresses are unchanged
6
Routing Table at a Host
202.10.10.0
173.4.0.0
201.12.5.0
202.10.10.3
173.4.0.1
201.12.5.1
200.1.1.10
200.1.1.11
200.1.1.12
200.1.1.1

• Dest. Network Subnet Mask Next Hop
  200.1.1.0 255.255.255.0 Directly Conn.
• 200.12.5.0 255.255.255.0 200.1.1.10
• 202.10.10.0 255.255.255.0 200.1.1.11
• 173.4.0.0 255.255.0.0 200.1.1.12

other hosts
Host A
7
Routing Table at a Router
202.10.10.0
173.4.0.0
201.12.5.0
202.10.10.3
173.4.0.1
P2
201.12.5.1
R0
200.1.1.10
200.1.1.11
P1
200.1.1.12
Routing Table for R0
200.1.1.0

• Dest. Network Subnet Mask Next Hop Interface
• 200.1.1.0 255.255.255.0 Directly Conn. P1
• 201.12.5.0 255.255.255.0 Directly Conn. P2
• 202.10.10.0 255.255.255.0 200.1.1.11 P1
• 173.4.0.0 255.255.0.0 200.1.1.12 P1

8
Processing Steps for Incoming Packet

• Incoming packets wait in a queue (memory buffer).
  That is why a router is a store-and-forward of
  packets in contrast to a bridge which is a
  store-and-forward of frames.
• The header is checked for errors by recomputing
  checksum and matching it with the received
  checksum. Next, verify that various field values
  in the IP header are valid. If not OK, drop the
  packet and send ICMP message to source host.
• Decrement TTL and drop packet if TTL reaches 0,
  send ICMP message to source host otherwise,
  update the header checksum.
• The router compares the routing table Dest.
  Network entry with NetID part of IP-DA (Is IP-DA
  and Mask Dest. Network?) until a matching entry
  is found. Then the router uses the information in
  the matching row to determine the data-link layer
  (DL) destination address for the outgoing frame
  -- i.e., It may use ARP to find the DL address of
  next router (or destination host) from its IP
  address.
• The packet is then encapsulated in an outgoing
  frame whose DL-DA is that found in step 3 DL-SA
  is that of the outgoing interface.

9
TCP/IP 5-Layer Model
Application Transport Network Data Link Physical
Application Transport Network Data Link
Physical
TCP
Network Data Link Physical
IP
Computer B
Computer A
IP Router

10
IP-Layer Operation
B
Y
X
C
A
X
Y
Application TCP IP Data Link Physical
Application TCP IP Data Link Physical
TCP is end-to-end layer
IP Data Link Physical
IP Data Link Physical
IP Data Link Physical
A
B
C
11
Static versus Dynamic Routes

• Static Route uses a protocol route that a network
  administrator enters into the router.
• Dynamic Route uses a route that a network routing
  protocol adjusts automatically for topology or
  traffic changes.

12
Static Route Example
Point-to-point or circuit-switched connection
Only a single network connection with no need for
routing updates
local network

• Fixed route to address reflects administrators
  knowledge.

13
Default Route Example
Company X
B
A
Internet
Routing Table If no entry for destination net Try
Router B (default router)

• Use if next hop is not explicitly listed in the
  routing table

14
Adapting to Topology Change

• An alternate route can substitute for a failed
  route.

15
Dynamic Routing Operations
Network Layer
Routing Protocol
Routing Protocol
A router passes routing information to its
neighbors
Routing Table
Routing Table
16
Dynamic Routing Operations

• Routing protocol maintains and distributes
  routing information.
• The success of dynamic routing depends on two
  basic router functions
• Maintenance of routing table
• Timely distribution of knowledge -- in the form
  of routing updates -- to other routers
• Dynamic routing relies on a routing protocol to
  disseminate knowledge.

17
Representing Distance with Metrics

• The metrics most commonly used by routers are
• Bandwidth
• Delay
• Load
• Reliability
• Hop Count
• Cost

18
Classes of Routing Protocols

• Most routing algorithms can be classified as
  conforming to one of two basic algorithms
  distance vector and link state.
• The distance vector routing approach determines
  the direction (vector) and distance to any link
  in the internetwork.
• The link-state approach re-creates the exact
  topology of the entire internetwork (or at least
  the partition in which the router is situated).

19
One Issue Time to Convergence

• Convergence occurs when all routers use a
  consistent perspective of network topology
• After a topology changes, routers must recompute
  routes, which disrupts routing
• The process and time required for router
  reconvergence varies in routing protocols

20
Distance Vector Concept

• Distance vector based routing algorithm (also
  known as Bellman-Ford algorithms)
• passes periodic copies of a routing table from
  router to router.
• Regular updates between routers communicate
  topology changes.
• Each router receives a routing table from its
  direct neighbor. For example, in the graphic,
  router B receives information from router A.
  Router B adds a distance vector number (such as a
  number of hops) increasing the distance vector,
  then passes the routing table to its other
  neighbor, C. The process continues between
  direct-neighbor routers.
• In this way, the algorithm accumulates network
  distances so it can maintain a database of
  internetwork topology information.

21
Distance Vector Network Discovery
A
B
C
W
Z
X
Y
Routing Table W 0 X 0 Y
1 Z 2
Routing Table Y 0 Z 0 X
1 W 2
22
Distance Vector Network Discovery

• Each router using distance vector routing begins
  by identifying its own neighbors. In the graphic,
  the port to each directly connected network is
  shown as having a distance of 0.
• As the distance vector network discovery process
  proceeds, routers discover the best path to
  destination networks based on accumulated metrics
  from each neighbor.

23
Routing Information Protocol (RIP)

• Included as routed (route-dee) program which is
  part of Berkley's Unix TCP/IP.
• RIP was in widespread use years before it was
  standardized in as RFC 1058 in 1988. Version 2 of
  RIP was completed in 1994.
• Uses Hop Count ( of routers to destination) as a
  distance metric.
• Each router broadcasts its routing table every
  30-sec.
• Routes are not updated beyond a hop count of 15
• RIP messages Use UDP packets with source and
  destination ports set to 520

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RIP Update Mechanism

• When a router receives a neighbor's table, it
  examines it entry by entry
• Adds 1 (or sometimes more if the corresponding
  link is slow) to the metric
• If the destination is new, it is added to the
  local routing table
• If the destination is known before and the update
  provides a smaller metric, the existing entry in
  the local routing table is replaced
• If for existing non-direct entry no router update
  info. Is received within three cycles (90 sec),
  the entry is removed

25
Triggered Updates

• To speed up the process of discovering change
• Whenever a router changes its metric for a route,
  it broadcasts the change. This may lead to more
  triggered updates by other routers
• It is best that whenever an update occurs , that
  the router waits for some short time and send
  updates in batches
• If a destination is marked unreachable it
  includes a hold down rule (time period) during
  which updates are ignored. This prevents
  misinformation from spreading.

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Incorrect Route Info Lead to Loops
A
B
C
D
10 hops
link broken
Network N
Info. at A, B, C (before link is broken)
Network N, Metric2 After link is broken, what if
Router D gets a scheduled update from A with the
old info.
Network N Next HopA Metric 3
D sends traffic for Network N to A which sends to
D which sends to A ---
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Split Horizon Technique

• The router records the interface over which a new
  route is received. The router does not send this
  update back through the interface on which it is
  received
• In the previous example, Router A updates on the
  shown interface will not include entries for
  Network N

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Break