============================ Tunnel Message Specification ============================ Category: Design Last updated: 2021-01 Accurate for: 0.9.49 Overview ======== This document specifies the format of tunnel messages. For general information about tunnels see the tunnel documentation [TUNNEL-IMPL]. Message preprocessing ===================== A *tunnel gateway* is the entrance, or first hop, of a tunnel. For an outbound tunnel, the gateway is the creator of the tunnel. For an inbound tunnel, the gateway is at the opposite end from the creator of the tunnel. A gateway *preprocesses* [I2NP] messages by fragmenting and combining them into tunnel messages. While I2NP messages are variable size from 0 to almost 64 KB, tunnel messages are fixed-size, approximately 1 KB. Fixed message size restricts several types of attacks that are possible from observing message size. After the tunnel messages are created, they are encrypted as described in the tunnel documentation [TUNNEL-IMPL]. .. _msg-Tunnel: Tunnel Message (Encrypted) -------------------------- These are the contents of a tunnel data message after encryption. +----+----+----+----+----+----+----+----+ | Tunnel ID | IV | +----+----+----+----+ + | | + +----+----+----+----+ | | | +----+----+----+----+ + | | + Encrypted Data + ~ ~ | | + +-------------------+ | | +----+----+----+----+ Tunnel ID :: [TunnelId] 4 bytes the ID of the next hop, nonzero IV :: 16 bytes the initialization vector Encrypted Data :: 1008 bytes the encrypted tunnel message total size: 1028 Bytes [TunnelId]: /spec/common-structures#type-tunnelid Tunnel Message (Decrypted) -------------------------- These are the contents of a tunnel data message when decrypted. +----+----+----+----+----+----+----+----+ | Tunnel ID | IV | +----+----+----+----+ + | | + +----+----+----+----+ | | Checksum | +----+----+----+----+----+----+----+----+ | nonzero padding... | ~ ~ | | + +----+ | |zero| +----+----+----+----+----+----+----+----+ | | | Delivery Instructions 1 | ~ ~ | | +----+----+----+----+----+----+----+----+ | | + I2NP Message Fragment 1 + | | ~ ~ | | +----+----+----+----+----+----+----+----+ | | | Delivery Instructions 2... | ~ ~ | | +----+----+----+----+----+----+----+----+ | | + I2NP Message Fragment 2... + | | ~ ~ | | + +-------------------+ | | +----+----+----+----+ Tunnel ID :: [TunnelId] 4 bytes the ID of the next hop, nonzero IV :: 16 bytes the initialization vector Checksum :: 4 bytes the first 4 bytes of the SHA256 hash of (the contents of the message (after the zero byte) + IV) Nonzero padding :: 0 or more bytes random nonzero data for padding Zero :: 1 byte the value 0x00 Delivery Instructions :: [TunnelMessageDeliveryInstructions] length varies but is typically 7, 39, 43, or 47 bytes Indicates the fragment and the routing for the fragment Message Fragment :: 1 to 996 bytes, actual maximum depends on delivery instruction size A partial or full I2NP Message total size: 1028 Bytes [TunnelMessageDeliveryInstructions]: /spec/tunnel-message#struct-tunnelmessagedeliveryinstructions [TunnelId]: /spec/common-structures#type-tunnelid Notes ````` * The padding, if any, must be before the instruction/message pairs. There is no provision for padding at the end. * The checksum does NOT cover the padding or the zero byte. Take the message starting at the first delivery instructions, concatenate the IV, and take the Hash of that. .. _struct-TunnelMessageDeliveryInstructions: Tunnel Message Delivery Instructions ==================================== The instructions are encoded with a single control byte, followed by any necessary additional information. The first bit (MSB) in that control byte determines how the remainder of the header is interpreted - if it is not set, the message is either not fragmented or this is the first fragment in the message. If it is set, this is a follow on fragment. This specification is for Delivery Instructions inside Tunnel Messages only. Note that "Delivery Instructions" are also used inside Garlic Cloves [I2NP-GC], where the format is significantly different. See the I2NP documentation [I2NP-GCDI] for details. Do NOT use the following specification for Garlic Clove Delivery Instructions! First Fragment Delivery Instructions ------------------------------------ If the MSB of the first byte is 0, this is an initial I2NP message fragment, or a complete (unfragmented) I2NP message, and the instructions are: +----+----+----+----+----+----+----+----+ |flag| Tunnel ID (opt) | | +----+----+----+----+----+ + | | + + | To Hash (optional) | + + | | + +--------------+ | |dly | Message +----+----+----+----+----+----+----+----+ ID (opt) |extended opts (opt)| size | +----+----+----+----+----+----+----+----+ flag :: 1 byte Bit order: 76543210 bit 7: 0 to specify an initial fragment or an unfragmented message bits 6-5: delivery type 0x0 = LOCAL 0x01 = TUNNEL 0x02 = ROUTER 0x03 = unused, invalid Note: LOCAL is used for inbound tunnels only, unimplemented for outbound tunnels bit 4: delay included? Unimplemented, always 0 If 1, a delay byte is included bit 3: fragmented? If 0, the message is not fragmented, what follows is the entire message If 1, the message is fragmented, and the instructions contain a Message ID bit 2: extended options? Unimplemented, always 0 If 1, extended options are included bits 1-0: reserved, set to 0 for compatibility with future uses Tunnel ID :: [TunnelId] 4 bytes Optional, present if delivery type is TUNNEL The destination tunnel ID, nonzero To Hash :: 32 bytes Optional, present if delivery type is ROUTER or TUNNEL If ROUTER, the SHA256 Hash of the router If TUNNEL, the SHA256 Hash of the gateway router Delay :: 1 byte Optional, present if delay included flag is set In tunnel messages: Unimplemented, never present; original specification: bit 7: type (0 = strict, 1 = randomized) bits 6-0: delay exponent (2^value minutes) Message ID :: 4 bytes Optional, present if this message is the first of 2 or more fragments (i.e. if the fragmented bit is 1) An ID that uniquely identifies all fragments as belonging to a single message (the current implementation uses [I2NPMessageHeader].msg_id) Extended Options :: 2 or more bytes Optional, present if extend options flag is set Unimplemented, never present; original specification: One byte length and then that many bytes size :: 2 bytes The length of the fragment that follows Valid values: 1 to approx. 960 in a tunnel message Total length: Typical length is: 3 bytes for LOCAL delivery (tunnel message); 35 bytes for ROUTER delivery or 39 bytes for TUNNEL delivery (unfragmented tunnel message); 39 bytes for ROUTER delivery or 43 bytes for TUNNEL delivery (first fragment) [I2NPMessageHeader]: /spec/i2np#struct-i2npmessageheader [TunnelId]: /spec/common-structures#type-tunnelid Follow-on Fragment Delivery Instructions ---------------------------------------- If the MSB of the first byte is 1, this is a follow-on fragment, and the instructions are: +----+----+----+----+----+----+----+ |frag| Message ID | size | +----+----+----+----+----+----+----+ frag :: 1 byte Bit order: 76543210 binary 1nnnnnnd bit 7: 1 to indicate this is a follow-on fragment bits 6-1: nnnnnn is the 6 bit fragment number from 1 to 63 bit 0: d is 1 to indicate the last fragment, 0 otherwise Message ID :: 4 bytes Identifies the fragment sequence that this fragment belongs to. This will match the message ID of an initial fragment (a fragment with flag bit 7 set to 0 and flag bit 3 set to 1). size :: 2 bytes the length of the fragment that follows valid values: 1 to 996 total length: 7 bytes JavaDoc: http://docs.i2p-projekt.de/javadoc/net/i2p/router/tunnel/FragmentHandler.html Notes ===== I2NP Message Maximum Size ------------------------- While the maximum I2NP message size is nominally 64 KB, the size is further constrained by the method of fragmenting I2NP messages into multiple 1 KB tunnel messages. The maximum number of fragments is 64, and the initial fragment may not be perfectly aligned at the start of a tunnel message. So the message must nominally fit in 63 fragments. The maximum size of an initial fragment is 956 bytes (assuming TUNNEL delivery mode); the maximum size of a follow-on fragment is 996 bytes. Therefore the maximum size is approximately 956 + (62 * 996) = 62708 bytes, or 61.2 KB. Ordering, Batching, Packing --------------------------- Tunnel messages may be dropped or reordered. The tunnel gateway, who creates tunnel messages, is free to implement any batching, mixing, or reordering strategy to fragment I2NP messages and efficiently pack fragments into tunnel messages. In general, an optimal packing is not possible (the "packing problem"). The gateways may implement various delay and reordering strategies. Cover Traffic ------------- Tunnel messages may contain only padding (i.e. no delivery instructions or message fragments at all) for cover traffic. This is unimplemented. References ========== [I2NP] http://i2p.net/en/docs/protocol/i2np [I2NP-GC] http://i2p.net/spec/i2np#struct-garlicclove [I2NP-GCDI] http://i2p.net/spec/i2np#struct-garlicclovedeliveryinstructions [TUNNEL-IMPL] http://i2p.net/en/docs/tunnels/implementation