Microsoft PowerPoint - csc521-09.pptx
Internetworking With TCP/IP, Vol I Sixth Edition, Douglas E. Comer
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Outline
1 Introduction 2 The Internet Control Message Protocol 3 Error Reporting Vs. Error Correction 4 ICMP Message Delivery 5 Conceptual Layering 6 ICMP Message Format 7 Example ICMP Message Types Used With IPv4 And IPv6 8 Testing Destination Reachability And Status (Ping) 9 Echo Request And Reply Message Format 10 Checksum Computation And The IPv6 Pseudo-Header 11 Reports Of Unreachable Destinations 12 ICMP Error Reports Regarding Fragmentation 13 Route Change Requests From Routers 14 Detecting Circular Or Excessively Long Routes 15 Reporting Other Problems 16 Older ICMP Messages Used At Startup 17 Summary
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Error Detection and Reporting Mechanisms • Errors in Packet Switching Networks
– Temporary or permanent disconnection – Hardware failures – Router overrun – Routing loops Need mechanisms to detect and correct
• Error handling – IP header checksum to detect transmission errors – Error reporting mechanism to distinguish between events such as lost
datagrams and incorrect addresses – Higher level protocols (i.e., TCP) must handle all other problems
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Error Reporting Mechanism – Internet Control Message Protocol (ICMP)
• Required and integral part of IP – Used primarily by routers to report delivery or routing problems to original
source – Also includes informational (non-error) functionality – Uses IP to carry control messages – No error messages sent about error messages
• Purpose – The Internet Control Message Protocol allows a router to send error or
control messages to the source of a datagram, typically a host. ICMP provides communication between the Internet Protocol software on one machine and the Internet Protocol software on another.
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Important Restrictions of ICMP • ICMP only reports problems to original source
– Provide interaction between a router and the source of trouble
• Error Reporting vs. Error Correction: ICMP does not – Maintain state information (each packet is handled independently)
• Thus: 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
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ICMP Encapsulation
• ICMP message travels in IP datagram – ICMP message has header and data area – Complete ICMP message is treated as data in IP datagram – Complete IP datagram is treated as data in physical network frame
ICMP Message Format • Two sets of ICMP messages
– a set for IPv4 – a larger set for IPv6
• In both versions of IP – Each ICMP message has its own format – All ICMP messages begin with the same three fields
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• Multiple message types – Each message has its own format
• Messages – Begin with 1-octet TYPE field that identifies which of the basic ICMP
message types follows – Some messages have a 1-octet CODE field that further classifies the
message – Example Encapsulation In Ethernet
• ICMP header follows IP header, and contains eight bytes • ICMP type field specifies echo request message (08)
– TYPE specifies destination unreachable – CODE specifies whether host or network was unreachable
• ICMP sequence number is zero
ICMP Message Format
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• Example: ICMP Echo Request – Sent by ping program to test destination reachability and status – Request: Type=8 – Reply: Type=0
Example ICMP Message
Checksum Computation And The IPv6 Pseudo-Header
• Pseudo-header and the use of dashed lines – is merely used for the checksum computation and is never placed in a
packet
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• Example: Destination Unreachable – Used by a router to report that
datagram could not be delivered – Code specifies details
Example ICMP Message
Reports Of Unreachable Destinations • A router
– Drops (i.e., discards) the datagram whenever an error occurs in a datagram – sends an ICMP destination unreachable message back to the source
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ICMP Error Reports Regarding Fragmentation • Both IPv4 and IPv6 allow a router to report an error when a
datagram is too large for a network over which it must travel and cannot be fragmented – Type=3: Destination Unreachable – Code=4: Fragmentation Needed and DF Set
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• Example: Time Exceeded – At least one fragment failed to arrive, or TTL field in IP header
reached zero – Code=0: Time-to-live count exceeded – Code=1: Fragment reassembly time exceeded
Example ICMP Message
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• Include datagram that caused problem in the error message – Efficient (sender must determine how to correct problem) – Eliminates need to construct detailed message
• Problem: entire datagram may be too large • Solution: send IP header plus 64 bits of data area (sufficient in
most cases)
ICMP Trick
Route Change Requests From Routers • Host’s Forwarding table
– A host initializes its forwarding table at system startup – System administrators seldom change the table during normal operation – The host relies on routers to update its forwarding table as needed
• Router’s Forwarding table – Routers exchange routing information periodically
A router only sends a redirect message if the host sends a datagram along a non-preferred route
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• Example: Redirect – Used by a router to report incorrect route – Suggesting the source host to redirect the attached IP datagram
Example ICMP Message
Code Meaning 0 Redirect datagrams for the Net (now obsolete) 1 Redirect datagrams for the Host 2 Redirect datagrams for the Type of Service and Net 3 Redirect datagrams for the Type of Service and Host
Older ICMP Messages Used At Startup • DHCP
– Originally defined in – a set of messages that a host used at startup to determine its IP address,
the address of a router, and the address mask used on the network • Router Discovery message
– At startup, an IPv6 host multicasts an ICMPv6 Router Discovery message to learn about routers on the local network
• Two conceptual differences between router discovery and DHCP – With router discovery, there is never a third-party error; with DHCP, such
errors are possible – ICMP router discovery uses a soft state technique with timers to prevent
hosts from retaining a forwarding table entry after a router crashes
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• ICMP – Required part of IP – Used to report errors to original source – Reporting only: no interaction or error correction
• ICMP encapsulated in IP for delivery – Several ICMP message types, each with its own format ICMP message
begins with 1-octet TYPE field • Examples
– Echo Request/Reply, Destination Unreachable, Time Exceeded, Redirect, NDP/DHCP, etc.
Summary