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Internet Command Message Protocol
(ICMP)
CS-431
Dick Steflik
ICMP
• Internet Command Message Protocol (ICMP)
• RFC 792
• Used to communicate IP status and error messages
between host and routers
ICMP
• Used to communicate IP status and error messages
between hosts and routers
• Uses IP to route its messages between hosts
• Must be implemented with IP
• remember, IP is just a packet delivery system
• transmits and routes datagrams from sources to destinations through a
series of interconnected networks
• it has a checksum in the IP header to detect lost bits
– no error detection on the datagram payload though
• but has no native mechanism for source host notification
• This is where ICMP comes in
• its used to report IP errors to the source host
• ICMP data is carried as the payload of an IP datagram
• specifies additional message formats within this area
Basic ICMP Header
• Headers are 32 bits in length; all contain same three fields
• type - 8 bit message type code
• thirteen message type are defined
• code - 8 bit; indicating why message is being sent
• checksum - standard internet checksum
• 16 bit 1’s complement sum of the payload and header
– for purpose of calculation the checksum field is set to zero
ICMP Message types
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0 - Echo Reply
3 - Destination Unreachable
4 - Source Quench
5 - Redirect
8 - Echo
11 - Time Exceeded
12 - Parameter Problem
13 - Timestamp
14 - Timestamp Reply
15 - Information Request
16 - Information Reply
17 - Address Mask Request
18 - Address Mask Reply
Destination Unreachable (3)
• ICMP header (4 bytes) + unused 32 bits (4 bytes) + IP header (24
bytes) + first 64 bits of data (8 bytes) = 40 bytes
• Codes:
• 0 - net unreachable ; 1 - host unreachable
• 2 - protocol unreachable ; 3 - port unreachable
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• sent by destination host IP module
4 - fragmentation needed DF set ; 5 source route failed
6 - destination network unknown ; 7 destination host unknown
8 - source host isolated ; 9 - comm. with destn network prohibited
10 - comm. With dest host prohibited ; 11 - network unreachable for service
12 - host unreachable for service
• Sent to originating host because destination is unreachable
• may be determined by a router
• destination IP may find the indicated protocol unavailable
• Don’t Fragment (DF) bit in the IP header is set but fragmentation is required
to continue forwarding
Source Quench (4)
• Same message format as type 3
• Code : 0
• Sent to a host when an intermediate router or the
destination host with the source host’s transmission rate
• may be sent to a source when a router is saturated
• may be sent by a receiving host if it receive buffers are filling up
• Upon receipt the source host should throttle back on its
transmission rate until the Source Quench goes away.
• Can then increase its transmission rate
Redirect (5)
• Same format as type 3
• Code:
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0 - redirect datagrams for the network
1 - redirect datagrams for the host
2 - redirect datagrams for the type of service and the network
3 - redirect datagrams for the type of service and host
• a router sends a message to a host when it determines a
datagram that originated from the host must be forwarded
to router that can be directly reached
• allows the host to sent future datagrams to the optimal first-hop
router increasing network efficiency
• not used for datagrams that have source routing options
Echo (8)/Echo Reply (0)
• ICMP header (4 bytes) + identifier (2 bytes) + sequence
number (2 bytes) + data (4 bytes)
• identifier - used to match Echoes and Echo Replies
• sequence - used to match Echoes with Echo Replies
• Used to determine if a host is reachable
• a host receiving an echo message
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reverses the IP source and destination addresses
sets the ICMP type field to zero (echo reply)
recomputes the ICMP checksum
identifier, sequence and data are sent back unchanged
Time Exceeded (11)
• Same format as type 3
• Code:
• 0 - time to live exceeded in transit
• 1 - fragment reassembly time exceeded
• Time exceeded message is sent if:
• a router finds a datagram with TTL set to zero
• router discards the datagram and sends message with code field set to 0
• a host does not receive all of the fragments of a datagram before its
local reassemble timer expires
• host discards all fragments and return message with code field set to 1
Parameter Problem (12)
• ICMP Header (4 bytes) + pointer (1 byte) + unused (3 bytes)
+ IP header (24 bytes) + first 64 bits of data (8 bytes)
• pointer - identifies octet where error occurred
• Code:
• 0 - misc parameter problem
• 2 - required option missing
• sent to a host when a router or host processing a datagram
finds a problem with the information in the datagram.
• Only sent if the datagram had to be discarded
• pointer field is zero based
– ex: 1 indicates problem with type of service; 20 indicates first option
Timestamp(13)/Timstamp Reply(14)
• ICMP Header (4 bytes) + identifier (2 bytes) + seq num (2
bytes) + Originate Timestamp (4 bytes) + Receive
Timestamp (4 bytes) + Transmit Timestamp (4 bytes)
• Timestamps are number of msec past midnight UTC
• used to determine the latency between the sender and
receiver
• receiver forms a message by
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reversing the originate and destination addr in the IP header
setting ICMP type code to 14
updating the timestamp fields
recomput the ICMP checksum
Info Request (15)/Info Reply(16)
• Same as type 8, but no data
• Code: 0
• used by a host to determine the network number the host
resides on
• receiver
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reverses the source and destination addresses in the IP header
set the correct network number
set ICMP type to 16
recompute the ICMP checksum
• Obsolete; shouldn’t be used replaced by BOOTP and
RARP
Addr Mask Rqst (17)/Addr Mask Reply(18)
• ICMP Header (4 bytes) + identifier (2 bytes) + seq.num. (2
bytes) + addr mask (4 bytes)
• identifier - used to match requests with replies.
• seq.num. - used to match requests with replies.
• Hosts and routers can request the subnet address mask for
the network they reside on at boot time.
• Host or router broadcasts it on the local network
• a receiving router should return it in a reply message
• This message is defined in RFC 950
ICMPv6
• New version of ICMP to go along with IPv6
• Absorbs many IGMP and ARP functions
• Used for:
• Reporting errors in IP packet processing
• Performing diagnostics
• performing Neighbor Discovery and reporting
IPv6 multicast memberships
• Two types of messages:
• Error messages
• Information messages
Error Message Categories
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Destination Unreachable
Packet too big
Time exceeded
Parameter problem
Informational Messages
• Diagnostic messages
• Multicast group management messages
• Neighbor discovery messages
• Every ICMPv6 message is preceeded by an
IPv6 header and 0 or more IPv6 extension
headers.
• A next header field of 58 nidentifies the
ICMPv6 message (different than IPv4)
Message Format
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0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| Type
| Code
| Checksum
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|-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
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| Message Body
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| +-------------------------------------------------------------+