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Internet Control Message Protocol

ICMP header format Offset Octet 0 1 2 3 Octet Bit 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 0 0 Type Code Checksum 4 32 Rest of Header
Checksum: 16 bits
Internet checksum for error checking, calculated from the ICMP header and data with value 0 substituted for this field.
deprecatedSource quench (congestion control) 5 – Redirect Message 0Redirect Datagram for the Network 1Redirect Datagram for the Host 2Redirect Datagram for the ToS & network 3Redirect Datagram for the ToS & host 6deprecatedInformation Request 16 – Information Reply 0deprecatedAddress Mask Reply 19deprecatedInformation Request 31deprecatedWhere-Are-You (originally meant for IPv6) 34deprecatedMobile Registration Reply 37deprecatedSKIP Algorithm Discovery Protocol, Simple Key-Management for Internet Protocol 40Photuris, Security failures 41ExperimentalRFC3692-style Experiment 1 254Code: 8 bits
This specifies the type of error, and can be any of the following:
CodeDescription
0 Network unreachable error.
1 Host unreachable error.
2 Protocol unreachable error (the designated transport protocol is not supported).
3 Port unreachable error (the designated protocol is unable to inform the host of the incoming message).
4 The datagram is too big. Packet fragmentation is required but the 'don't fragment' (DF) flag is on.
5 Source route failed error.
6 Destination network unknown error.
7 Destination host unknown error.
8 Source host isolated error.
9 The destination network is administratively prohibited.
10 The destination host is administratively prohibited.
11 The network is unreachable for Type Of Service.
12 The host is unreachable for Type Of Service.
13 Communication administratively prohibited (administrative filtering prevents packet from being forwarded).
14 Host precedence violation (indicates the requested precedence is not permitted for the combination of host or network and port).
15 Precedence cutoff in effect (precedence of datagram is below the level set by the network administrators).
Next-hop MTU: 16 bits
Optional. Contains the MTU of the next-hop network if a code 4 error occurs.
Version: 4 bits; Version == 2
Extension header version.
Extension object header Offset Octet 0 1 2 3 Octet Bit 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 0 0 Length Class-Num C-Type 4 32 (Object payload) 8 64 ⋮ ⋮
  1. 1 2 RFC 1716 and 1009. Updated by RFC 792. Internet Standard 5. Updates Forouzan, Behrouz A. (2007). Data Communications And Networking (Fourth ed.). Boston: McGraw-Hill. pp. 621–630. ISBN 978-0-07-296775-3.
  2. ↑ RFC 2463. Updates "The OSI Model's Seven Layers Defined and Functions Explained". Microsoft Support. Retrieved 2014-12-28.
  3. ↑ RFC 1071. Informational. Updated by Kurose, J.F; Ross, K.W. (2006). Computer Networking: A Top-Down Approach. World student series. Addison-Wesley. ISBN 9780321418494.
  4. 1 2 RFC 792, 1122 and 1812.
  5. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 RFC 1788. Updates RFC 4065. Experimental.
  6. 1 2 RFC 4884.
  7. 1 2 "When Are ICMP Redirects Sent?". Cisco Systems. 2008-06-28. Retrieved 2013-08-15.
  8. ↑ "Cisco IOS IP Command Reference, Volume 1 of 4: Addressing and Services, Release 12.3 - IP Addressing and Services Commands: ip mask-reply through ip web-cache". Cisco Systems. Archived from the original on 2013-01-02. Retrieved 2013-01-07.
  9. ↑ RFC 1063.
  10. 1 2 RFC 792 and 4443. Updated by
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