| 1 | ICMP Messages
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| 2 | What follows is the ICMP message descriptions as they appear in RFC792. The ICMP 'header' appears immediately after the IP header. The format of the header is dependant on the ICMP Type field.
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| 3 |
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| 4 |
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| 5 | Destination Unreachable Message
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| 6 |
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| 7 |
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| 8 | 0 1 2 3
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| 9 | 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
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| 10 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 11 | | Type | Code | Checksum |
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| 12 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 13 | | unused |
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| 14 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 15 | | Internet Header + 64 bits of Original Data Datagram |
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| 16 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 17 |
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| 18 | IP Fields:
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| 19 |
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| 20 | Destination Address
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| 21 |
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| 22 | The source network and address from the original datagram's data.
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| 23 |
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| 24 | ICMP Fields:
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| 25 |
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| 26 | Type
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| 27 |
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| 28 | 3
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| 29 |
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| 30 | Code
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| 31 |
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| 32 | 0 = net unreachable;
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| 33 |
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| 34 | 1 = host unreachable;
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| 35 |
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| 36 | 2 = protocol unreachable;
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| 37 |
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| 38 | 3 = port unreachable;
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| 39 |
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| 40 | 4 = fragmentation needed and DF set;
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| 41 |
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| 42 | 5 = source route failed.
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| 43 |
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| 44 | Checksum
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| 45 |
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| 46 | The checksum is the 16-bit ones's complement of the one's
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| 47 | complement sum of the ICMP message starting with the ICMP Type.
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| 48 | For computing the checksum , the checksum field should be zero.
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| 49 | This checksum may be replaced in the future.
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| 50 |
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| 51 | Internet Header + 64 bits of Data Datagram
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| 52 |
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| 53 | The internet header plus the first 64 bits of the original
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| 54 | datagram's data. This data is used by the host to match the
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| 55 | message to the appropriate process. If a higher level protocol
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| 56 | uses port numbers, they are assumed to be in the first 64 data
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| 57 | bits of the original datagram's data.
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| 58 |
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| 59 | Description
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| 60 |
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| 61 | If, according to the information in the gateway's routing tables,
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| 62 | the network specified in the internet destination field of a
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| 63 | datagram is unreachable, e.g., the distance to the network is
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| 64 | infinity, the gateway may send a destination unreachable message
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| 65 | to the internet source host of the datagram. In addition, in some
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| 66 | networks, the gateway may be able to determine if the internet
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| 67 | destination host is unreachable. Gateways in these networks may
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| 68 | send destination unreachable messages to the source host when the
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| 69 | destination host is unreachable.
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| 70 |
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| 71 | If, in the destination host, the IP module cannot deliver the
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| 72 | datagram because the indicated protocol module or process port is
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| 73 | not active, the destination host may send a destination
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| 74 | unreachable message to the source host.
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| 75 |
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| 76 | Another case is when a datagram must be fragmented to be forwarded
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| 77 | by a gateway yet the Don't Fragment flag is on. In this case the
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| 78 | gateway must discard the datagram and may return a destination
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| 79 | unreachable message.
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| 80 |
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| 81 | Codes 0, 1, 4, and 5 may be received from a gateway. Codes 2 and
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| 82 | 3 may be received from a host.
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| 83 |
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| 84 |
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| 85 | Time Exceeded Message
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| 86 |
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| 87 |
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| 88 | 0 1 2 3
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| 89 | 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
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| 90 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 91 | | Type | Code | Checksum |
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| 92 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 93 | | unused |
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| 94 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 95 | | Internet Header + 64 bits of Original Data Datagram |
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| 96 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 97 |
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| 98 | IP Fields:
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| 99 |
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| 100 | Destination Address
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| 101 |
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| 102 | The source network and address from the original datagram's data.
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| 103 |
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| 104 | ICMP Fields:
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| 105 |
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| 106 | Type
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| 107 |
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| 108 | 11
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| 109 |
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| 110 | Code
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| 111 |
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| 112 | 0 = time to live exceeded in transit;
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| 113 |
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| 114 | 1 = fragment reassembly time exceeded.
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| 115 |
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| 116 | Checksum
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| 117 |
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| 118 | The checksum is the 16-bit ones's complement of the one's
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| 119 | complement sum of the ICMP message starting with the ICMP Type.
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| 120 | For computing the checksum , the checksum field should be zero.
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| 121 | This checksum may be replaced in the future.
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| 122 |
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| 123 | Internet Header + 64 bits of Data Datagram
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| 124 |
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| 125 | The internet header plus the first 64 bits of the original
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| 126 | datagram's data. This data is used by the host to match the
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| 127 | message to the appropriate process. If a higher level protocol
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| 128 | uses port numbers, they are assumed to be in the first 64 data
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| 129 | bits of the original datagram's data.
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| 130 |
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| 131 | Description
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| 132 |
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| 133 | If the gateway processing a datagram finds the time to live field
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| 134 | is zero it must discard the datagram. The gateway may also notify
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| 135 | the source host via the time exceeded message.
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| 136 |
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| 137 | If a host reassembling a fragmented datagram cannot complete the
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| 138 | reassembly due to missing fragments within its time limit it
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| 139 | discards the datagram, and it may send a time exceeded message.
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| 140 |
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| 141 | If fragment zero is not available then no time exceeded need be
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| 142 | sent at all.
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| 143 |
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| 144 | Code 0 may be received from a gateway. Code 1 may be received
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| 145 | from a host.
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| 146 |
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| 147 |
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| 148 | Parameter Problem Message
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| 149 |
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| 150 |
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| 151 | 0 1 2 3
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| 152 | 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
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| 153 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 154 | | Type | Code | Checksum |
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| 155 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 156 | | Pointer | unused |
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| 157 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 158 | | Internet Header + 64 bits of Original Data Datagram |
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| 159 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 160 |
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| 161 | IP Fields:
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| 162 |
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| 163 | Destination Address
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| 164 |
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| 165 | The source network and address from the original datagram's data.
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| 166 |
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| 167 | ICMP Fields:
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| 168 |
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| 169 | Type
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| 170 |
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| 171 | 12
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| 172 |
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| 173 | Code
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| 174 |
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| 175 | 0 = pointer indicates the error.
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| 176 |
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| 177 | Checksum
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| 178 |
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| 179 | The checksum is the 16-bit ones's complement of the one's
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| 180 | complement sum of the ICMP message starting with the ICMP Type.
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| 181 | For computing the checksum , the checksum field should be zero.
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| 182 | This checksum may be replaced in the future.
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| 183 |
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| 184 | Pointer
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| 185 |
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| 186 | If code = 0, identifies the octet where an error was detected.
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| 187 |
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| 188 | Internet Header + 64 bits of Data Datagram
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| 189 |
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| 190 | The internet header plus the first 64 bits of the original
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| 191 | datagram's data. This data is used by the host to match the
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| 192 | message to the appropriate process. If a higher level protocol
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| 193 | uses port numbers, they are assumed to be in the first 64 data
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| 194 | bits of the original datagram's data.
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| 195 |
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| 196 |
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| 197 | Description
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| 198 |
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| 199 | If the gateway or host processing a datagram finds a problem with
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| 200 | the header parameters such that it cannot complete processing the
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| 201 | datagram it must discard the datagram. One potential source of
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| 202 | such a problem is with incorrect arguments in an option. The
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| 203 | gateway or host may also notify the source host via the parameter
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| 204 | problem message. This message is only sent if the error caused
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| 205 | the datagram to be discarded.
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| 206 |
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| 207 | The pointer identifies the octet of the original datagram's header
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| 208 | where the error was detected (it may be in the middle of an
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| 209 | option). For example, 1 indicates something is wrong with the
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| 210 | Type of Service, and (if there are options present) 20 indicates
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| 211 | something is wrong with the type code of the first option.
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| 212 |
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| 213 | Code 0 may be received from a gateway or a host.
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| 214 |
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| 215 |
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| 216 | Source Quench Message
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| 217 |
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| 218 |
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| 219 | 0 1 2 3
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| 220 | 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
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| 221 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 222 | | Type | Code | Checksum |
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| 223 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 224 | | unused |
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| 225 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 226 | | Internet Header + 64 bits of Original Data Datagram |
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| 227 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 228 |
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| 229 | IP Fields:
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| 230 |
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| 231 | Destination Address
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| 232 |
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| 233 | The source network and address of the original datagram's data.
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| 234 |
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| 235 | ICMP Fields:
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| 236 |
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| 237 | Type
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| 238 |
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| 239 | 4
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| 240 |
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| 241 | Code
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| 242 |
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| 243 | 0
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| 244 |
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| 245 | Checksum
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| 246 |
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| 247 | The checksum is the 16-bit ones's complement of the one's
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| 248 | complement sum of the ICMP message starting with the ICMP Type.
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| 249 | For computing the checksum , the checksum field should be zero.
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| 250 | This checksum may be replaced in the future.
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| 251 |
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| 252 | Internet Header + 64 bits of Data Datagram
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| 253 |
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| 254 | The internet header plus the first 64 bits of the original
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| 255 | datagram's data. This data is used by the host to match the
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| 256 | message to the appropriate process. If a higher level protocol
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| 257 | uses port numbers, they are assumed to be in the first 64 data
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| 258 | bits of the original datagram's data.
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| 259 |
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| 260 | Description
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| 261 |
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| 262 | A gateway may discard internet datagrams if it does not have the
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| 263 | buffer space needed to queue the datagrams for output to the next
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| 264 | network on the route to the destination network. If a gateway
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| 265 | discards a datagram, it may send a source quench message to the
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| 266 | internet source host of the datagram. A destination host may also
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| 267 | send a source quench message if datagrams arrive too fast to be
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| 268 | processed. The source quench message is a request to the host to
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| 269 | cut back the rate at which it is sending traffic to the internet
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| 270 | destination. The gateway may send a source quench message for
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| 271 | every message that it discards. On receipt of a source quench
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| 272 | message, the source host should cut back the rate at which it is
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| 273 | sending traffic to the specified destination until it no longer
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| 274 | receives source quench messages from the gateway. The source host
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| 275 | can then gradually increase the rate at which it sends traffic to
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| 276 | the destination until it again receives source quench messages.
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| 277 |
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| 278 | The gateway or host may send the source quench message when it
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| 279 | approaches its capacity limit rather than waiting until the
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| 280 | capacity is exceeded. This means that the data datagram which
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| 281 | triggered the source quench message may be delivered.
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| 282 |
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| 283 | Code 0 may be received from a gateway or a host.
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| 284 |
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| 285 |
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| 286 | Redirect Message
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| 287 |
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| 288 |
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| 289 | 0 1 2 3
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| 290 | 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
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| 291 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 292 | | Type | Code | Checksum |
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| 293 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 294 | | Gateway Internet Address |
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| 295 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 296 | | Internet Header + 64 bits of Original Data Datagram |
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| 297 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 298 |
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| 299 | IP Fields:
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| 300 |
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| 301 | Destination Address
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| 302 |
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| 303 | The source network and address of the original datagram's data.
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| 304 |
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| 305 | ICMP Fields:
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| 306 |
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| 307 | Type
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| 308 |
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| 309 | 5
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| 310 |
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| 311 | Code
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| 312 |
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| 313 | 0 = Redirect datagrams for the Network.
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| 314 |
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| 315 | 1 = Redirect datagrams for the Host.
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| 316 |
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| 317 | 2 = Redirect datagrams for the Type of Service and Network.
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| 318 |
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| 319 | 3 = Redirect datagrams for the Type of Service and Host.
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| 320 |
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| 321 | Checksum
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| 322 |
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| 323 | The checksum is the 16-bit ones's complement of the one's
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| 324 | complement sum of the ICMP message starting with the ICMP Type.
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| 325 | For computing the checksum , the checksum field should be zero.
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| 326 | This checksum may be replaced in the future.
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| 327 |
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| 328 | Gateway Internet Address
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| 329 |
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| 330 | Address of the gateway to which traffic for the network specified
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| 331 | in the internet destination network field of the original
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| 332 | datagram's data should be sent.
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| 333 |
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| 334 |
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| 335 | Internet Header + 64 bits of Data Datagram
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| 336 |
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| 337 | The internet header plus the first 64 bits of the original
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| 338 | datagram's data. This data is used by the host to match the
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| 339 | message to the appropriate process. If a higher level protocol
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| 340 | uses port numbers, they are assumed to be in the first 64 data
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| 341 | bits of the original datagram's data.
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| 342 |
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| 343 | Description
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| 344 |
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| 345 | The gateway sends a redirect message to a host in the following
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| 346 | situation. A gateway, G1, receives an internet datagram from a
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| 347 | host on a network to which the gateway is attached. The gateway,
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| 348 | G1, checks its routing table and obtains the address of the next
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| 349 | gateway, G2, on the route to the datagram's internet destination
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| 350 | network, X. If G2 and the host identified by the internet source
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| 351 | address of the datagram are on the same network, a redirect
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| 352 | message is sent to the host. The redirect message advises the
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| 353 | host to send its traffic for network X directly to gateway G2 as
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| 354 | this is a shorter path to the destination. The gateway forwards
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| 355 | the original datagram's data to its internet destination.
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| 356 |
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| 357 | For datagrams with the IP source route options and the gateway
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| 358 | address in the destination address field, a redirect message is
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| 359 | not sent even if there is a better route to the ultimate
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| 360 | destination than the next address in the source route.
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| 361 |
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| 362 | Codes 0, 1, 2, and 3 may be received from a gateway.
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| 363 |
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| 364 |
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| 365 |
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| 366 | Echo or Echo Reply Message
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| 367 |
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| 368 |
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| 369 | 0 1 2 3
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| 370 | 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
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| 371 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 372 | | Type | Code | Checksum |
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| 373 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 374 | | Identifier | Sequence Number |
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| 375 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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| 376 | | Data ...
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| 377 | +-+-+-+-+-
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| 378 |
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| 379 | IP Fields:
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| 380 |
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| 381 | Addresses
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| 382 |
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| 383 | The address of the source in an echo message will be the
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| 384 | destination of the echo reply message. To form an echo reply
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| 385 | message, the source and destination addresses are simply reversed,
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| 386 | the type code changed to 0, and the checksum recomputed.
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| 387 |
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| 388 | ICMP Fields:
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| 389 |
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| 390 | Type
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| 391 |
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| 392 | 8 for echo message;
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| 393 |
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| 394 | 0 for echo reply message.
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| 395 |
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| 396 | Code
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| 397 |
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| 398 | 0
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| 399 |
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| 400 | Checksum
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| 401 |
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| 402 | The checksum is the 16-bit ones's complement of the one's
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| 403 | complement sum of the ICMP message starting with the ICMP Type.
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| 404 | For computing the checksum , the checksum field should be zero.
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| 405 | If the total length is odd, the received data is padded with one
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| 406 | octet of zeros for computing the checksum. This checksum may be
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| 407 | replaced in the future.
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| 408 |
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| 409 | Identifier
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| 410 |
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| 411 | If code = 0, an identifier to aid in matching echos and replies,
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| 412 | may be zero.
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| 413 |
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| 414 | Sequence Number
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| 415 |
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| 416 | If code = 0, a sequence number to aid in matching echos and
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| 417 | replies, may be zero.
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| 418 |
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| 419 | Description
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| 420 |
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| 421 | The data received in the echo message must be returned in the echo
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| 422 | reply message.
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| 423 |
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| 424 | The identifier and sequence number may be used by the echo sender
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| 425 | to aid in matching the replies with the echo requests. For
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| 426 | example, the identifier might be used like a port in TCP or UDP to
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| 427 | identify a session, and the sequence number might be incremented
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| 428 | on each echo request sent. The echoer returns these same values
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| 429 | in the echo reply.
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| 430 |
|
|---|
| 431 | Code 0 may be received from a gateway or a host.
|
|---|
| 432 |
|
|---|
| 433 |
|
|---|
| 434 |
|
|---|
| 435 | Timestamp or Timestamp Reply Message
|
|---|
| 436 |
|
|---|
| 437 |
|
|---|
| 438 | 0 1 2 3
|
|---|
| 439 | 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
|
|---|
| 440 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|---|
| 441 | | Type | Code | Checksum |
|
|---|
| 442 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|---|
| 443 | | Identifier | Sequence Number |
|
|---|
| 444 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|---|
| 445 | | Originate Timestamp |
|
|---|
| 446 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|---|
| 447 | | Receive Timestamp |
|
|---|
| 448 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|---|
| 449 | | Transmit Timestamp |
|
|---|
| 450 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|---|
| 451 |
|
|---|
| 452 | IP Fields:
|
|---|
| 453 |
|
|---|
| 454 | Addresses
|
|---|
| 455 |
|
|---|
| 456 | The address of the source in a timestamp message will be the
|
|---|
| 457 | destination of the timestamp reply message. To form a timestamp
|
|---|
| 458 | reply message, the source and destination addresses are simply
|
|---|
| 459 | reversed, the type code changed to 14, and the checksum
|
|---|
| 460 | recomputed.
|
|---|
| 461 |
|
|---|
| 462 | ICMP Fields:
|
|---|
| 463 |
|
|---|
| 464 | Type
|
|---|
| 465 |
|
|---|
| 466 | 13 for timestamp message;
|
|---|
| 467 |
|
|---|
| 468 | 14 for timestamp reply message.
|
|---|
| 469 |
|
|---|
| 470 | Code
|
|---|
| 471 |
|
|---|
| 472 | 0
|
|---|
| 473 |
|
|---|
| 474 | Checksum
|
|---|
| 475 |
|
|---|
| 476 | The checksum is the 16-bit ones's complement of the one's
|
|---|
| 477 | complement sum of the ICMP message starting with the ICMP Type.
|
|---|
| 478 | For computing the checksum , the checksum field should be zero.
|
|---|
| 479 | This checksum may be replaced in the future.
|
|---|
| 480 |
|
|---|
| 481 | Identifier
|
|---|
| 482 |
|
|---|
| 483 | If code = 0, an identifier to aid in matching timestamp and
|
|---|
| 484 | replies, may be zero.
|
|---|
| 485 |
|
|---|
| 486 | Sequence Number
|
|---|
| 487 |
|
|---|
| 488 | If code = 0, a sequence number to aid in matching timestamp and
|
|---|
| 489 | replies, may be zero.
|
|---|
| 490 |
|
|---|
| 491 | Description
|
|---|
| 492 |
|
|---|
| 493 | The data received (a timestamp) in the message is returned in the
|
|---|
| 494 | reply together with an additional timestamp. The timestamp is 32
|
|---|
| 495 | bits of milliseconds since midnight UT. One use of these
|
|---|
| 496 | timestamps is described by Mills [5].
|
|---|
| 497 |
|
|---|
| 498 | The Originate Timestamp is the time the sender last touched the
|
|---|
| 499 | message before sending it, the Receive Timestamp is the time the
|
|---|
| 500 | echoer first touched it on receipt, and the Transmit Timestamp is
|
|---|
| 501 | the time the echoer last touched the message on sending it.
|
|---|
| 502 |
|
|---|
| 503 | If the time is not available in miliseconds or cannot be provided
|
|---|
| 504 | with respect to midnight UT then any time can be inserted in a
|
|---|
| 505 | timestamp provided the high order bit of the timestamp is also set
|
|---|
| 506 | to indicate this non-standard value.
|
|---|
| 507 |
|
|---|
| 508 | The identifier and sequence number may be used by the echo sender
|
|---|
| 509 | to aid in matching the replies with the requests. For example,
|
|---|
| 510 | the identifier might be used like a port in TCP or UDP to identify
|
|---|
| 511 | a session, and the sequence number might be incremented on each
|
|---|
| 512 | request sent. The destination returns these same values in the
|
|---|
| 513 | reply.
|
|---|
| 514 |
|
|---|
| 515 | Code 0 may be received from a gateway or a host.
|
|---|
| 516 |
|
|---|
| 517 |
|
|---|
| 518 |
|
|---|
| 519 | Information Request or Information Reply Message
|
|---|
| 520 |
|
|---|
| 521 |
|
|---|
| 522 | 0 1 2 3
|
|---|
| 523 | 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
|
|---|
| 524 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|---|
| 525 | | Type | Code | Checksum |
|
|---|
| 526 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|---|
| 527 | | Identifier | Sequence Number |
|
|---|
| 528 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|
|---|
| 529 |
|
|---|
| 530 | IP Fields:
|
|---|
| 531 |
|
|---|
| 532 | Addresses
|
|---|
| 533 |
|
|---|
| 534 | The address of the source in a information request message will be
|
|---|
| 535 | the destination of the information reply message. To form a
|
|---|
| 536 | information reply message, the source and destination addresses
|
|---|
| 537 | are simply reversed, the type code changed to 16, and the checksum
|
|---|
| 538 | recomputed.
|
|---|
| 539 |
|
|---|
| 540 | ICMP Fields:
|
|---|
| 541 |
|
|---|
| 542 | Type
|
|---|
| 543 |
|
|---|
| 544 | 15 for information request message;
|
|---|
| 545 |
|
|---|
| 546 | 16 for information reply message.
|
|---|
| 547 |
|
|---|
| 548 | Code
|
|---|
| 549 |
|
|---|
| 550 | 0
|
|---|
| 551 |
|
|---|
| 552 | Checksum
|
|---|
| 553 |
|
|---|
| 554 | The checksum is the 16-bit ones's complement of the one's
|
|---|
| 555 | complement sum of the ICMP message starting with the ICMP Type.
|
|---|
| 556 | For computing the checksum , the checksum field should be zero.
|
|---|
| 557 | This checksum may be replaced in the future.
|
|---|
| 558 |
|
|---|
| 559 | Identifier
|
|---|
| 560 |
|
|---|
| 561 | If code = 0, an identifier to aid in matching request and replies,
|
|---|
| 562 | may be zero.
|
|---|
| 563 |
|
|---|
| 564 | Sequence Number
|
|---|
| 565 |
|
|---|
| 566 | If code = 0, a sequence number to aid in matching request and
|
|---|
| 567 | replies, may be zero.
|
|---|
| 568 |
|
|---|
| 569 | Description
|
|---|
| 570 |
|
|---|
| 571 | This message may be sent with the source network in the IP header
|
|---|
| 572 | source and destination address fields zero (which means "this"
|
|---|
| 573 | network). The replying IP module should send the reply with the
|
|---|
| 574 | addresses fully specified. This message is a way for a host to
|
|---|
| 575 | find out the number of the network it is on.
|
|---|
| 576 |
|
|---|
| 577 | The identifier and sequence number may be used by the echo sender
|
|---|
| 578 | to aid in matching the replies with the requests. For example,
|
|---|
| 579 | the identifier might be used like a port in TCP or UDP to identify
|
|---|
| 580 | a session, and the sequence number might be incremented on each
|
|---|
| 581 | request sent. The destination returns these same values in the
|
|---|
| 582 | reply.
|
|---|
| 583 |
|
|---|
| 584 | Code 0 may be received from a gateway or a host.
|
|---|
