Transcript document

CSCI 4550/8556
Computer Networks
Comer, Chapter 23:
An Error Reporting Mechanism (ICMP)
Introduction
IP provides best-effort delivery.
Delivery problems can be ignored by IP; datagrams
can be “dropped on the floor.”
The Internet Control Message Protocol (ICMP)
provides an error-reporting mechanism for other
types of errors.
Error Detection
The internet layer (IP) can detect a variety of errors:
checksum errors (but in the header only!)
TTL expiration
no available route to the destination network
inability to deliver to the destination host (e.g., no ARP reply)
The internet layer discards datagrams with problems.
Some (e.g. those with a checksum error) - cannot trigger
error messages, because the source is unknown.
Error Reporting
Some errors can be reported:
The router sends a message back to the source in a
datagram.
The message contains information about the problem.
The message, as expected, is encapsulated in an
IP datagram.
Types of Messages
The Internet Control Message Protocol (ICMP) defines error
and informational messages.
Error messages:
Source quench – tells source to slow down because of congestion
Time exceeded – a datagram’s TTL expired
Destination unreachable – there is no route to the destination
Redirect – router reports that its use is inappropriate
Fragmentation required – a datagram would need to be fragmented
Informational messages:
Echo request/reply – used with the ping program
Address mask request/reply
Router discovery
Summary
The internet layer (IP) provides best-effort delivery
of datagrams.
It may choose to report errors for some problems.
The ICMP protocol provides the mechanism by
which such error messages are communicated.
ICMP Message Transport
ICMP messages are encapsulated in IP datagrams:
ICMP messages are sent in response to incoming
datagrams with problems.
An ICMP message is not sent for an ICMP
message.
ICMP and Reachability
An internet host B is said to be reachable from
another host A if datagrams can be delivered from
A to B.
The ping program tests reachability – it sends a
datagram from A to B that B echoes back to A.
Ping uses the ICMP echo request and echo reply
messages.
The Internet layer includes code to reply to
incoming ICMP echo request messages.
ICMP and Internet Routes
The list of all routers on the path from A to B is called the route from A
to B
traceroute uses UDP to a non-existent port and TTL field to find a route
using an expanding ring search
It sends ICMP echo messages with increasing TTL values.
A router that decrements a TTL value to 0 sends an ICMP time exceeded
message in response, with that router's address as the source address in
the message.
The first message, with a TTL value of 1, gets to the first router, which
discards the message and sends the time exceeded reply.
The next message, with a TTL value of 2, gets through the first router to
the second router.
The procedure continues until a message from the destination is received.
traceroute must accommodate varying network delays.
It must also accommodate dynamically changing routes.
ICMP and Path MTU Discovery
Fragmentation should be avoided, if possible.
How can a source configure outgoing datagrams to avoid
fragmentation?
The source determines the path MTU – the smallest
network MTU on the path from the source to the
destination.
The source probes the path using IP datagrams with the
don't fragment flag set.
A router responds with the ICMP fragmentation required
message if it would be required to fragment a datagram
marked with the don’t fragment flag.
The source sends smaller and smaller probes until the
destination is reached.
ICMP and Router Discovery
A router can fail, causing a “black-hole” or isolating
a host from the internet.
The ICMP router discovery message is used to find
new router.
A host can broadcast a request for router
announcements to auto-configure its default route.
A host can broadcast this request if a router fails.
A router can broadcast advertisement of its
existence when it is first connected.
ICMP Redirect Messages
A default route may cause an extra hop in a path.
A router that forwards a datagram on the interface
where the datagram was received sends an ICMP
redirect message.
The host installs the new route with the correct
router’s address as the next hop.