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1. Forward onions
Forward onions are used to create anonymous circuits that will be used for ordinary connections. A forward onion consists of an onion proper, a responder header in the payload of a DATA cell, and destination host address and port in a the payload of a DATA cell
2. Reply onions
Reply onions are used to create anonymous circuits that will be used for reply connections. A reply onion consists of an onion proper, a responder header that contains an indicator that the onion is a reply onion, the reply onion processor address and port, the reply onion processor header, the destination host information, and finally the key seeds. The reply onion processor header, the destination host information, and the key seeds are all encrypted under the reply onion processors public key
3. Replayable reply onions
Repayable reply onions are used to create anonymous circuits that will be used for replayable reply connections. A replayable reply onion contains an onion proper, with a bit set in the flags field indicating that it is a reply onion, a responder header with a bit set in the flags field indicating that the onion is a reply onion, the address and port of the reply onion processor, a reply onion processor header, the destination host address and port and finally the key seed material. The reply onion processor header, the destination host information, and the key seeds are all encrypted under the reply onion processors public key
4. Rekeying Onions
Rekeying onions are used to distribute new symmetric keys for a replayable reply connection. A rekeying onion consists of 12 layers with layered encryption similar to an ordinary onion. Rekeying onions differ from ordinaryonions in their structure and in that rekeying onions are encrypted using only symmetric cryptography. Public key cryptography is not used for encrypting layers of rekeying onions.
5 Tunneling Onions
Tunneling onions are used to create anonymous circuits that will be "tunneled". A tunneled circuit consists of more hops than will fit into the largest allowable onion size. A tunneled onion consists of two or more onions proper, a responder header, and destination host information. The last layer of all but the last onion contains a bit in the flags field indicating that tunneling will occur at this hop and that the next N DATA cells should be repackaged as CREATE cells and forwarded to the host whose address and port appear in the next hop field, where N is the number of cells required to package an onion