NP - Unit 3 - Forwarding Datagram and ICMP

UNIT-III
FORWARDING IP DATAGRAMS AND ICMP

Internet Protocol:
Forwarding IP Datagrams

Forwarding in an internet
• Routing was used with packet switching systems
• Forwarding refer to the process of choosing the path for a packet.
• Router performs forwarding.
• switched Ethernet-the switches are responsible for forwarding
Ethernet frames among computers.
• The major differences are that in place of frames the Internet accepts
and delivers IP datagrams, and in place of Ethernet addresses, the
Internet uses IP addresses. Therefore, throughout the chapter, we will
restrict the discussion to IP forwarding.
• The information IP software uses to make forwarding decisions is
known as a Forwarding Information Base(FIB).

• Each router has direct connections to two or more networks.
• Both hosts and routers participate in forwarding an IP datagram to its
destination.
• In the figure, two routers connect to the same network as the host.
Some internet destinations lie beyond router R1 and other
destinations lie beyond R2. The host must decide which router to use
for a given datagram.
• A router performs transit forwarding, which means the router will
accept incoming datagrams from any of the networks to which the
router attaches, and will forward each datagram on toward its
destination.

Direct and Indirect delivery
• forwarding into two forms:
• direct delivery
• indirect delivery.
• Direct delivery-the transmission of a datagram from one machine
across a single physical network directly to another(both connected
directly).
• Indirect delivery occurs when the destination of a datagram is not on
a directly attached network.

Transmission across a single network
• Transmission of an IP datagram between two machines on a single
physical network does not involve routers.
• The sender encapsulates the datagram in a physical frame, binds the
next-hop address to a physical hardware address, and sends the
resulting frame directly to the destination.
• To determine if a destination lies on one of the directly connected
networks, IP software extracts the network portion of the destination
IP address and compares the network ID to the network ID of its own
IP address(es).

Indirect delivery
• Indirect delivery is more difficult than direct delivery because the
sending machine must identify an initial router to handle the
datagram.
• The router must then forward the datagram on toward the
destination network.
• Routers in a TCP/IP internet form a cooperative, interconnected
structure. Datagrams pass from router to router until they reach a
router that can deliver the datagram directly.
• Finally,
• A host only knows about directly-connected networks; a host relies on routers
to transfer datagrams to remote destinations.
• Each router knows how to reach all possible destinations in the internet;
given a datagram, a router can forward it correctly.

Table driven IP forwarding
• IP performs datagram forwarding. The IP forwarding algorithm
employs a data structure that stores information about possible
destinations and how to reach them.
• The data structure is known formally as an Internet Protocol
forwarding table or IP forwarding table, and informally as simply a
forwarding table.
• Both hosts and routers have a forwarding table.
• If every forwarding table contained information about every possible
destination in an internet, it would be impossible to keep the tables
current.
• Because it allows forwarding to be based on network prefixes, the IP
addressing scheme controls the size of forwarding tables.

Next-Hop forwarding
• A forwarding table contains a set of pairs (N, R), where N is the
network prefix for a network in the internet and R is the IP address of
the “next” router along the path to network N.
• Router R is called the next hop, and the idea of using a forwarding
table to store a next hop for each destination is called next-hop
forwarding.
• the network portion of a destination IP address instead of the
complete address keeps forwarding tables small. It helps hide
information, keeping the details of specific hosts.

NP - Unit 3 - Forwarding Datagram and ICMP

Default Routes And A Host Example
• If no entry in the table matches the destination address, the
forwarding software takes a second step of checking for a default
route.
• We say that the next hop specified in a default route is a default
router.
• A default route can be incorporated into a forwarding table.
• A default route is especially useful when many destinations lie
beyond a single router.
• Default routing works especially well for typical host computers that
obtain service from an ISP.

Host-Specific Routes
• Most IP forwarding software allows a host-specific route to be
specified as a special case.
• Having host-specific routes gives a network administrator more
control.
• The ability to specify a route for individual machines turns out to have
several possible uses:
• Control over network use
• Testing a new network.
• Security

Longest-Prefix Match Paradigm
• longest-prefix match - to describe the idea of examining the most
specific routes first.

Forwarding Tables And IP Addresses
• It is important to understand that except
for decrementing the hop limit (TTL in
IPv4) and recomputing the checksum,
IP forwarding does not alter the
original datagram.
• In the algorithm, the IP address selected by the IP forwarding
algorithm is called a next-hop address because it tells where the
datagram must be sent next.

Handling Incoming Datagrams
• host software
• router software
• four reasons why a host should refrain from performing any
forwarding.

• Forwarding In The Presence Of Broadcast And Multicast
• Software Routers And Sequential Lookup
• Establishing Forwarding Tables

Internet Protocol: Error And
Control Messages (ICMP)

Introduction
• Connectionless system.
• When a datagram arrives, a router forwards or delivers the datagram
and then goes on to the next datagram; the router does not
coordinate with the original sender of a datagram.
• Failures of network and processor hardware, IP cannot deliver a
datagram if the destination machine is temporarily or permanently
disconnected from the network, if the hop limit expires before a
datagram reaches its destination, or if an intermediate router
becomes so overloaded that it must discard a datagram.

ICMP - Internet Control Message Protocol
• allows routers to send error or control messages back to the source of a
datagram that caused a problem.
• ICMP is primarily intended to inform a source when a datagram sent by the
source experiences problems.
• ICMP messages are not usually delivered to applications.
• ICMP messages are sent to Internet Protocol software on the source
computer. That is, when an ICMP error message arrives on a computer, the
ICMP software module on the computer handles the message.
• The chief advantage of allowing hosts to use ICMP is that it provides a
single mechanism used for all control and information messages.

Error Reporting Vs. Error Correction
• ICMP is an error reporting mechanism
• It provides a way for routers that encounter an error to report the
error to the original source, but ICMP does not interact with the host
nor does ICMP attempt to correct the error.
• When a datagram causes an error, ICMP can only report the error
condition back to the original source of the datagram; the source
must relate the error to an individual application program or take
other action to correct the problem.

ICMP Message Delivery
• Because each ICMP message travels in an IP datagram, two levels of
encapsulation are required.
• IPv4 uses the PROTOCOL field in the datagram header as a type field.
• When an ICMP message is carried in the payload area of an IPv4 datagram,
the PROTOCOL field is set to 1.
• IPv6 uses the NEXT HEADER field to specify the type of the item being
carried.
• When an ICMP message is carried in the payload area of an IPv6 datagram,
the NEXT HEADER field of the header that is previous to the ICMP message
contains 58.

ICMP Message Format
• Conceptual Layering
• TYPE
• CODE
• CHECKSUM

Example ICMP Message Types Used With IPv4

Example ICMPv6 message types and the
meaning of each.

Testing Destination Reachability And Status
(Ping)
• Ping sends an ICMP Echo Request message to a remote computer.
• Any computer that receives an ICMP Echo Request creates an ICMP
Echo Reply and returns the reply to the original sender.
Echo Request And Reply Message Format:
• IPv4 : TYPE 8 – request, 0-reply
• IPv6: TYPE request -128 reply-129

Checksum Computation And The IPv6
Pseudo-Header

Reports Of Unreachable Destinations
• Whenever an error prevents a router from forwarding or delivering a
datagram, the router sends an ICMP destination unreachable
message back to the source and then drops(i.e., discards) the
datagram.
• Both IPv4 and IPv6 use the same format for destination unreachable
messages.

The CODE values for an ICMP destination
unreachable message

ICMP Error Reports Regarding Fragmentation

Route Change Requests From Routers
• Routers are assumed to know correct routes; hosts begin with
minimal routing information and learn new routes from routers

Detecting Circular Or Excessively Long Routes
• ICMP time exceeded message

Reporting Other Problems
• parameter problem message

Older ICMP Messages Used At Startup
• ICMPv6 Router Discovery message

NP - Unit 3 - Forwarding Datagram and ICMP

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NP - Unit 3 - Forwarding Datagram and ICMP