Download Routing - King Fahd University of Petroleum and Minerals

Short Course on TCP/IP
IP Routing
Dr. Nasir Darwish
[email protected]
Information and Computer Science Department
King Fahd University of Petroleum and Minerals
Dhahran, Saudi Arabia
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
IP Address Classes
Class
A
B
C
Net Part
8
16
24
Host Part
24
16
8
First Byte Value Subnet Mask
1 - 127
128 - 191
192 - 223
255.0.0.0
255.255.0.0
255.255.255.0
IP Routing: Next Hop?
Which router (hop) to forward the IP packet to next?
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)

A
R0
200.1.1.1
200.1.1.1
196.1.1.1
data
R0
R1
B
R1
Host A
A/R0 (R1/B) : From/To Data Link layer addresses
Note: IP addresses are unchanged
200.1.1.1
196.1.1.1
196.1.1.1
Host B
data
Routing Table at a Host
202.10.10.0
201.12.5.0
173.4.0.0
201.12.5.1
202.10.10.3
173.4.0.1
200.1.1.10
200.1.1.11
200.1.1.12
200.1.1.1
other hosts
Host A
Dest. Network
200.1.1.0
200.12.5.0
202.10.10.0
Subnet Mask
255.255.255.0
255.255.255.0
255.255.255.0
173.4.0.0
255.255.0.0
Next Hop
Directly Conn.
200.1.1.10
200.1.1.11
200.1.1.12
Routing Table at a Router
202.10.10.0
201.12.5.0
P2
P1
201.12.5.1
R0
200.1.1.10
173.4.0.0
202.10.10.3
173.4.0.1
200.1.1.11
200.1.1.12
Routing Table for R0
200.1.1.0
Dest. Network
Subnet Mask
Next Hop
200.1.1.0
201.12.5.0
202.10.10.0
255.255.255.0
255.255.255.0
255.255.255.0
Directly Conn.
Directly Conn.
173.4.0.0
255.255.0.0
200.1.1.11
200.1.1.12
Interface
P1
P2
P1
P1
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-andforward 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.
1.
Decrement TTL and drop packet if TTL reaches 0, send ICMP message to
source host; otherwise, update the header checksum.
2.
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 datalink 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.
3.
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.
4.
TCP/IP 5-Layer Model
Application
Application
TCP
Transport
Transport
IP
Network
Network
Network
Data Link
Data Link
Data Link
Physical
Physical
Physical
Computer A
IP Router
Computer B
IP-Layer Operation
B
Y
X
C
A
X
Y
Application
Application
TCP is end-to-end layer
TCP
TCP
IP
IP
IP
IP
IP
Data Link
Data Link
Data Link
Data Link
Data Link
Physical
Physical
Physical
Physical
Physical
A
B
C
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.
Static Route Example
Router
Only a single network
connection with no need for
routing updates
Point-to-point or circuitswitched connection
Router
local network
Fixed route to address reflects administrator’s knowledge.
Default Route Example
Company X
Router
A
B
Router
Internet
Router
Router
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
Adapting to Topology Change
Router
Router
Router
Router
An alternate route can substitute for a failed route.
Dynamic Routing Operations
Network Layer
Routing Protocol
Routing
Table
Router
Routing Protocol
A router passes
routing information
to its neighbors
Routing
Table
Router
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.
Representing Distance with Metrics

The metrics most commonly used by
routers are:






Bandwidth
Delay
Load
Reliability
Hop Count
Cost
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).
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
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 directneighbor routers.
In this way, the algorithm accumulates network distances so it can
maintain a database of internetwork topology information.
Distance Vector Network Discovery
A
B
W
X
Router
C
Y
Router
Z
Router
Routing Table
Routing Table
Routing Table
W
0
X
0
Y
0
X
0
Y
0
Z
0
Y
1
Z
1
X
1
Z
2
W
1
W
2
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.
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
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
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.
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, Metric=2
After link is broken, what if Router D gets a scheduled update from A
with the old info.
Network: N Next Hop:A Metric: 3
D sends traffic for Network N to A which sends
to D which sends to A ---
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
Break