Long Header

The long header is 32 bytes. It is used before a session is created, for Token Request, SessionRequest, SessionCreated, and Retry. It is also used for out-of-session Peer Test and Hole Punch messages.

Before header encryption:

+----+----+----+----+----+----+----+----+
|      Destination Connection ID        |
+----+----+----+----+----+----+----+----+
|   Packet Number   |type| ver| id |flag|
+----+----+----+----+----+----+----+----+
|        Source Connection ID           |
+----+----+----+----+----+----+----+----+
|                 Token                 |
+----+----+----+----+----+----+----+----+

Destination ID :: 8 bytes, unsigned big endian integer

Packet Number :: 4 bytes, unsigned big endian integer

type :: The message type = 0, 1, 7, 9, 10, or 11

ver :: The protocol version, equal to 2

id :: 1 byte, the network ID (currently 2, except for test networks)

flag :: 1 byte, unused, set to 0 for future compatibility

Source ID :: 8 bytes, unsigned big endian integer

Token :: 8 bytes, unsigned big endian integer

Short Header

The short header is 16 bytes. It is used for Session Created and for Data messages. Uauthenticated messages such as Session Request, Retry, and Peer Test will always use the long header.

16 bytes is required, because the receiver must decrypt the first 16 bytes to get the message type, and then must decrypt an additional 16 bytes if it's actually a long header, as indicated by the message type.

For Session Confirmed, before header encryption:

+----+----+----+----+----+----+----+----+
|      Destination Connection ID        |
+----+----+----+----+----+----+----+----+
|   Packet Number   |type|frag|  flags  |
+----+----+----+----+----+----+----+----+

Destination ID :: 8 bytes, unsigned big endian integer

Packet Number :: 4 bytes, all zeros

type :: The message type = 2

frag :: 1 byte fragment info:
       bit order: 76543210 (bit 7 is MSB)
       bits 7-4: fragment number 0-14, big endian
       bits 3-0: total fragments 1-15, big endian

flags :: 2 bytes, unused, set to 0 for future compatibility

See the Session Confirmed Fragmentation section below for more information about the frag field.

For Data messages, before header encryption:

+----+----+----+----+----+----+----+----+
|      Destination Connection ID        |
+----+----+----+----+----+----+----+----+
|   Packet Number   |type|flag|moreflags|
+----+----+----+----+----+----+----+----+

Destination ID :: 8 bytes, unsigned big endian integer

Packet Number :: 4 bytes, unsigned big endian integer

type :: The message type = 6

flag :: 1 byte flags:
       bit order: 76543210 (bit 7 is MSB)
       bits 7-1: unused, set to 0 for future compatibility
       bits 0: when set to 1, immediate ack requested

moreflags :: 2 bytes, unused, set to 0 for future compatibility

Connection ID Numbering

Connection IDs must be randomly generated. Source and Destination IDs must NOT be identical, so that an on-path attacker cannot capture and send a packet back to the originator that looks valid. Do NOT use a counter to generate connection IDs, so that an on-path attacker cannot generate a packet that looks valid.

Unlike in QUIC, we do not change the connection IDs during or after the handshake, even after a Retry message. The IDs remain constant from the first message (Token Request or Session Request) to the last message (Data with Termination). Additionally, connection IDs do not change during or after path challenge or connection migration.

Also different than QUIC is that connection IDs in the headers are always header-encrypted. See below.

Packet Numbering

If no First Packet Number block is sent in the handshake, packets are numbered within a single session, for each direction, starting from 0, to a max of (2**32 -1). A session must be terminated, and a new session created, well before the max number of packets is sent.

If a First Packet Number block is sent in the handshake, packets are numbered within a single session, for that direction, starting from that packet number. The packet number may wrap around during the session. When a max of 2**32 packets have been sent, wrapping the packet number back to the first packet number, that session is no longer valid. A session must be terminated, and a new session created, well before the max number of packets is sent.

TODO key rotation, reduce max packet number?

Handshake packets that are determined to be lost are retransmitted whole, with the identical header including packet number. The handshake messages Session Request, Session Created, and Session Confirmed MUST be retransmitted with the same packet number and identical encrypted contents, so that the same chained hash will be used to encrypt the response. The Retry message is never transmitted.

Data phase packets that are determined to be lost are never retransmitted whole (except termination, see below). The same applies to the blocks that are contained within lost packets. Instead, the information that might be carried in blocks is sent again in new packets as needed. Data Packets are never retransmitted with the same packet number. Any retransmission of packet contents (whether or not the contents remain the same) must use the next unused packet number.

Retransmitting an unchanged whole packet as-is, with the same packet number, is not allowed for several reasons. For background see QUIC [RFC-9000] section 12.3.

• It's inefficient to store packets for retransmission
• A new packet data looks different to an on-path observer, can't tell it's retransmitted
• A new packet gets an updated ack block sent with it, not the old ack block
• You only retransmit what's necessary. some fragments could have been already retransmitted once and been acked
• You can fit as much as you need into each retransmitted packet if more is pending
• Endpoints that track all individual packets for the purposes of detecting duplicates are at risk of accumulating excessive state. The data required for detecting duplicates can be limited by maintaining a minimum packet number below which all packets are immediately dropped.
• This scheme is much more flexible

New packets are used to carry information that is determined to have been lost. In general, information is sent again when a packet containing that information is determined to be lost, and sending ceases when a packet containing that information is remain the same) acknowledged.

Exception: A data phase packet containing a Termination block may, but is not required to be, retransmitted whole, as-is. See the Session Termination section below.

The following packets contain a random packet number that is ignored:

• Session Request
• Session Created
• Token Request
• Retry
• Peer Test
• Hole Punch

For Alice, outbound packet numbering starts at 0 with Session Confirmed. For Bob, outbound packet numbering starts at 0 with first Data packet, which should be an ACK of the Session Confirmed. The packet numbers in an example standard handshake will be:

Alice                           Bob

SessionRequest (r)    ------------>
<-------------   SessionCreated (r)
SessionConfirmed (0)  ------------>
<-------------             Data (0) (Ack-only)
Data (1)              ------------> (May be sent before Ack is received)
<-------------             Data (1)
Data (2)              ------------>
Data (3)              ------------>
Data (4)              ------------>
<-------------             Data (2)

r = random packet number (ignored)
Token Request, Retry, and Peer Test
also have random packet numbers.

Any retransmission of handshake messages (SessionRequest, SessionCreated, or SessionConfirmed) must be resent unchanged, with the same packet number. Do not use different ephemeral keys or change the payload when retransmitting these messages.

Header Binding

The header (before obfuscation and protection) is always included in the associated data for the AEAD function, to cryptographically bind the header to the data.

Header Encryption

Header encryption has several goals. See the "Additional DPI Discussion" section above for background and assumptions.

• Prevent online DPI from identifying the protocol
• Prevent patterns in a series of messages in the same connection, except for handshake retransmissions
• Prevent patterns in messages of the same type in different connections
• Prevent decryption of handshake headers without knowledge of the introduction key found in the netdb
• Prevent identification of X25519 ephemeral keys without knowledge of the introduction key found in the netdb
• Prevent decryption of data phase packet number and type by any online or offline attacker
• Prevent injection of valid handshake packets by an on-path or off-path observer without knowledge of the introduction key found in the netdb
• Prevent injection of valid data packets by an on-path or off-path observer
• Allow rapid and efficient classification of incoming packets
• Provide "probing" resistance so that there is no response to a bad Session Request, or if there is a Retry response, the response is not identifiable as I2P without knowledge of the introduction key found in the netdb
• The Destination Connection ID is not critical data, and it's ok if it can be decrypted by an observer with knowledge of the introduction key found in the netdb
• The packet number of a data phase packet is an AEAD nonce and is critical data. It must not be decryptable by an observer even with knowledge of the introduction key found in the netdb. See [Nonces].

Headers are encrypted with known keys published in the network database or calculated later. In the handshake phase, this is for DPI resistance only, as the key is public and the key and nonces are reused, so it is effectively just obfuscation. Note that the header encryption is also used to obfuscate the ephemeral keys X (in Session Request) and Y (in Session Created).

See the Inbound Packet Handling section below for additional guidance.

Bytes 0-15 of all headers are encrypted using a header protection scheme by XORing with data calculated from known keys, using ChaCha20, similar to QUIC [RFC-9001] and [Nonces]. This ensures that the encrypted short header and the first part of the long header will appear to be random.

For Session Request and Session Created, bytes 16-31 of the long header and the 32-byte Noise ephemeral key are encrypted using ChaCha20. The unencrypted data is random, so the encrypted data will appear to be random.

For Retry, bytes 16-31 of the long header are encrypted using ChaCha20. The unencrypted data is random, so the encrypted data will appear to be random.

Unlike the QUIC [RFC-9001] header protection scheme, ALL parts of all headers, including destination and source connection IDs, are encrypted. QUIC [RFC-9001] and [Nonces] are primarily focused on encrypting the "critical" part of the header, i.e. the packet number (ChaCha20 nonce). While encrypting the session ID makes incoming packet classification a little more complex, it makes some attacks more difficult. QUIC defines different connection IDs for different phases, and for path challenge and connection migration. Here we use the same connection IDs throughout, as they are encrypted.

There are seven header protection key phases:

• Session Request and Token Request
• Session Created
• Retry
• Session Confirmed
• Data Phase
• Peer Test
• Hole Punch

Header encryption is designed to allow rapid classification of inbound packets, without complex heuristics or fallbacks. This is accomplished by using the same k_header_1 key for almost all inbound messages. Even when the source IP or port of a connection changes due to an actual IP change or NAT behavior, the packet may be rapidly mapped to a session with a single lookup of the connection ID.

Note that Session Created and Retry are the ONLY messages that require fallback processing for k_header_1 to decrypt the Connection ID, because they use the sender's (Bob's) intro key. ALL other messages use the receiver's intro key for k_header_1. The fallback processing need only look up pending outbound connections by source IP/port.

If the fallback processing by source IP/port fails to find a pending outbound connection, there could be several causes:

• Not an SSU2 message
• A corrupted SSU2 message
• The reply is spoofed or modified by an attacker
• Bob has a symmetric NAT
• Bob changed IP or port during processing of the message
• Bob sent the reply out a different interface

While additional fallback processing is possible to attempt to find the pending outbound connection and decrypt the connection ID using the k_header_1 for that connection, it is probably not necessary. If Bob has issues with his NAT or packet routing, it is probably better to let the connection fail. This design relies on endpoints retaining a stable address for the duration of the handshake.

See the Inbound Packet Handling sesion below for additional guidelines.

See the individual KDF sections below for the derivation of the header encryption keys for that phase.

Header Encryption KDF

// incoming encrypted packet
packet = incoming encrypted packet
len = packet.length

// take the next-to-last 12 bytes of the packet
iv = packet[len-24:len-13]
k_header_1 = header encryption key 1
data = {0, 0, 0, 0, 0, 0, 0, 0}
mask = ChaCha20.encrypt(k_header_1, iv, data)

// encrypt the first part of the header by XORing with the mask
packet[0:7] ^= mask[0:7]

// take the last 12 bytes of the packet
iv = packet[len-12:len-1]
k_header_2 = header encryption key 2
data = {0, 0, 0, 0, 0, 0, 0, 0}
mask = ChaCha20.encrypt(k_header_2, iv, data)

// encrypt the second part of the header by XORing with the mask
packet[8:15] ^= mask[0:7]


// For Session Request and Session Created only:
iv = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}

// encrypt the third part of the header and the ephemeral key
packet[16:63] = ChaCha20.encrypt(k_header_2, iv, packet[16:63])


// For Retry, Token Request, Peer Test, and Hole Punch only:
iv = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}

// encrypt the third part of the header
packet[16:31] = ChaCha20.encrypt(k_header_2, iv, packet[16:31])

This KDF uses the last 24 bytes of the packet as the IV for the two ChaCha20 operations. As all packets end with a 16 byte MAC, this requires that all packet payloads are a minimum of 8 bytes. This requirement is additionally documented in the message sections below.

Header Validation

After decrypting the first 8 bytes of the header, the receiver will know the Destination Connection ID. From there, the receiver knows what header encryption key to use for the remainder of the header, based on the key phase of the session.

Decrypting the next 8 bytes of the header will then reveal the message type and be able to determine if it is a short or long header. If it is a long header, the receiver must validate the version and netid fields. If the version is != 2, or the netid is != the expected value (generally 2, except in test networks), the receiver should drop the message.

ChaCha20/Poly1305

Encrypted and authenticated data format.

Inputs to the encryption/decryption functions:

k :: 32 byte cipher key, as generated from KDF

nonce :: Counter-based nonce, 12 bytes.
         Starts at 0 and incremented for each message.
         First four bytes are always zero.
         Last eight bytes are the counter, little-endian encoded.
         Maximum value is 2**64 - 2.
         Connection must be dropped and restarted after
         it reaches that value.
         The value 2**64 - 1 must never be sent.

ad :: In handshake phase:
      Associated data, 32 bytes.
      The SHA256 hash of all preceding data.
      In data phase:
      The packet header, 16 bytes.

data :: Plaintext data, 0 or more bytes

Output of the encryption function, input to the decryption function:

+----+----+----+----+----+----+----+----+
|                                       |
+                                       +
|       ChaCha20 encrypted data         |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+
|  Poly1305 Message Authentication Code |
+              (MAC)                    +
|             16 bytes                  |
+----+----+----+----+----+----+----+----+

encrypted data :: Same size as plaintext data, 0 - 65519 bytes

MAC :: Poly1305 message authentication code, 16 bytes

For ChaCha20, what is described here corresponds to [RFC-7539], which is also used similarly in TLS [RFC-7905].

Notes

• Since ChaCha20 is a stream cipher, plaintexts need not be padded. Additional keystream bytes are discarded.
• The key for the cipher (256 bits) is agreed upon by means of the SHA256 KDF. The details of the KDF for each message are in separate sections below.

AEAD Error Handling

• In all messages, the AEAD message size is known in advance. On an AEAD authentication failure, recipient must halt further message processing and discard the message.
• Bob should maintain a blacklist of IPs with repeated failures.

KDF for Initial ChainKey

// Define protocol_name.
Set protocol_name = "Noise_XKchaobfse+hs1+hs2+hs3_25519_ChaChaPoly_SHA256"
 (52 bytes, US-ASCII encoded, no NULL termination).

// Define Hash h = 32 bytes
h = SHA256(protocol_name);

Define ck = 32 byte chaining key. Copy the h data to ck.
Set ck = h

// MixHash(null prologue)
h = SHA256(h);

// up until here, can all be precalculated by Alice for all outgoing connections

// Bob's X25519 static keys
// bpk is published in routerinfo
bsk = GENERATE_PRIVATE()
bpk = DERIVE_PUBLIC(bsk)

// Bob static key
// MixHash(bpk)
// || below means append
h = SHA256(h || bpk);

// Bob introduction key
// bik is published in routerinfo
bik = RANDOM(32)

// up until here, can all be precalculated by Bob for all incoming connections

KDF for Session Request

// MixHash(header)
h = SHA256(h || header)

This is the "e" message pattern:

// Alice's X25519 ephemeral keys
aesk = GENERATE_PRIVATE()
aepk = DERIVE_PUBLIC(aesk)

// Alice ephemeral key X
// MixHash(aepk)
h = SHA256(h || aepk);

// h is used as the associated data for the AEAD in Session Request
// Retain the Hash h for the Session Created KDF


End of "e" message pattern.

This is the "es" message pattern:

// DH(e, rs) == DH(s, re)
sharedSecret = DH(aesk, bpk) = DH(bsk, aepk)

// MixKey(DH())
//[chainKey, k] = MixKey(sharedSecret)
// ChaChaPoly parameters to encrypt/decrypt
keydata = HKDF(chainKey, sharedSecret, "", 64)
chainKey = keydata[0:31]

// AEAD parameters
k = keydata[32:64]
n = 0
ad = h
ciphertext = ENCRYPT(k, n, payload, ad)

// retain the chainKey for Session Created KDF


End of "es" message pattern.

// Header encryption keys for this message
// bik = Bob's intro key
k_header_1 = bik
k_header_2 = bik

// Header encryption keys for next message (Session Created)
k_header_1 = bik
k_header_2 = HKDF(chainKey, ZEROLEN, "SessCreateHeader", 32)

// Header encryption keys for next message (Retry)
k_header_1 = bik
k_header_2 = bik

Payload

• DateTime block
• Options block (optional)
• Relay Tag Request block (optional)
• Padding block (optional)

The minimum payload size is 8 bytes. Since the DateTime block is only 7 bytes, at least one other block must be present.

Notes

• The unique X value in the initial ChaCha20 block ensure that the ciphertext is different for every session.
• To provide probing resistance, Bob should not send a Retry message in response to a Session Request message unless the message type, protocol version, and network ID fields in the Session Request message are valid.
• Bob must reject connections where the timestamp value is too far off from the current time. Call the maximum delta time "D". Bob must maintain a local cache of previously-used handshake values and reject duplicates, to prevent replay attacks. Values in the cache must have a lifetime of at least 2*D. The cache values are implementation-dependent, however the 32-byte X value (or its encrypted equivalent) may be used. Reject by sending a Retry message containing a zero token and a termination block.
• Diffie-Hellman ephemeral keys may never be reused, to prevent cryptographic attacks, and reuse will be rejected as a replay attack.
• The "KE" and "auth" options must be compatible, i.e. the shared secret K must be of the appropriate size. If more "auth" options are added, this could implicitly change the meaning of the "KE" flag to use a different KDF or a different truncation size.
• Bob must validate that Alice's ephemeral key is a valid point on the curve here.
• Padding should be limited to a reasonable amount. Bob may reject connections with excessive padding. Bob will specify his padding options in Session Created. Min/max guidelines TBD. Random size from 0 to 31 bytes minimum? (Distribution to be determined, see Appendix A.)
• On most errors, including AEAD, DH, apparent replay, or key validation failure, Bob should halt further message processing and drop the message without responding.
• Bob MAY send a Retry message containing a zero token and a Termination block with a clock skew reason code if the timestamp in the DateTime block is too far skewed.
• DoS Mitigation: DH is a relatively expensive operation. As with the previous NTCP protocol, routers should take all necessary measures to prevent CPU or connection exhaustion. Place limits on maximum active connections and maximum connection setups in progress. Enforce read timeouts (both per-read and total for "slowloris"). Limit repeated or simultaneous connections from the same source. Maintain blacklists for sources that repeatedly fail. Do not respond to AEAD failure. Alternatively, respond with a Retry message before the DH operation and AEAD validation.
• "ver" field: The overall Noise protocol, extensions, and SSU2 protocol including payload specifications, indicating SSU2. This field may be used to indicate support for future changes.
• The network ID field is used to quickly identify cross-network connections. If this field is does not match Bob's network ID, Bob should disconnect and block future connections.
• Bob must drop the message if the Source Connection ID equals the Destination Connection ID.

Payload

• DateTime block
• Address block
• Relay Tag block (optional)
• New Token block (optional)
• First Packet Number block (optional)
• Options block (optional)
• Termination block (not recommended, send in a retry message instead)
• Padding block (optional)

The minimum payload size is 8 bytes. Since the DateTime and Address blocks total more than that, the requirement is met with only those two blocks.

Notes

• Alice must validate that Bob's ephemeral key is a valid point on the curve here.
• Padding should be limited to a reasonable amount. Alice may reject connections with excessive padding. Alice will specify her padding options in Session Confirmed. Min/max guidelines TBD. Random size from 0 to 31 bytes minimum? (Distribution to be determined, see Appendix A.)
• On any error, including AEAD, DH, timestamp, apparent replay, or key validation failure, Alice must halt further message processing and close the connection without responding.
• Alice must reject connections where the timestamp value is too far off from the current time. Call the maximum delta time "D". Alice must maintain a local cache of previously-used handshake values and reject duplicates, to prevent replay attacks. Values in the cache must have a lifetime of at least 2*D. The cache values are implementation-dependent, however the 32-byte Y value (or its encrypted equivalent) may be used.
• Alice must drop the message if the source IP and port do not match the destination IP and port of the Session Request.
• Alice must drop the message if the Destination and Source Connection IDs do not match the Source and Destination Connection IDs of the Session Request.
• Bob sends a relay tag block if requested by Alice in the Session Request.

Issues

• Include min/max padding options here?

Payload

• RouterInfo block (must be the first block)
• Options block (optional)
• New Token block (optional)
• Relay Request block (optional)
• Peer Test block (optional)
• First Packet Number block (optional)
• I2NP, First Fragment, or Follow-on Fragment blocks (optional, but probably no room)
• Padding block (optional)

The minimum payload size is 8 bytes. Since the RouterInfo block will be well more than that, the requirement is met with only that block.

Notes

• Bob must perform the usual Router Info validation. Ensure the signature type is supported, verify the signature, verify the timestamp is within bounds, and any other checks necessary. See below for notes on handling fragmented Router Infos.
• Bob must verify that Alice's static key received in the first frame matches the static key in the Router Info. Bob must first search the Router Info for a NTCP or SSU2 Router Address with a matching version (v) option. See Published Router Info and Unpublished Router Info sections below. See below for notes on handling fragmented Router Infos.
• If Bob has an older version of Alice's RouterInfo in his netdb, verify that the static key in the router info is the same in both, if present, and if the older version is less than XXX old (see key rotate time below)
• Bob must validate that Alice's static key is a valid point on the curve here.
• Options should be included, to specify padding parameters.
• On any error, including AEAD, RI, DH, timestamp, or key validation failure, Bob must halt further message processing and close the connection without responding.
• Message 3 part 2 frame content: This format of this frame is the same as the format of data phase frames, except that the length of the frame is sent by Alice in Session Request. See below for the data phase frame format. The frame must contain 1 to 4 blocks in the following order: 1) Alice's Router Info block (required) 2) Options block (optional) 3) I2NP blocks (optional) 4) Padding block (optional) This frame must never contain any other block type. TODO: what about relay and peer test?
• Message 3 part 2 padding block is recommended.
• There may be no space, or only a small amount of space, available for I2NP blocks, depending on the MTU and the Router Info size. Do NOT include I2NP blocks if the Router Info is fragmented. The simplest implementation may be to never include I2NP blocks in the Session Confirmed message, and send all I2NP blocks in subsequent Data messages. See Router Info block section below for maximum block size.

Session Confirmed Fragmentation

The Session Confirmed message must contain the full signed Router Info from Alice so that Bob may perform several required checks:

• The static key "s" in the RI matches the static key in the handshake
• The introduction key "i" in the RI must be extracted and valid, to be used in the data phase
• The RI signature is valid

Unfortunately, the Router Info, even when gzip compressed in the RI block, may exceed the MTU. Therefore, the Session Confirmed may be fragmented across two or more packets. This is the ONLY case in the SSU2 protocol where an AEAD-protected payload is fragmented across two or more packets.

The headers for each packet are constructed as follows:

• ALL headers are short headers with the same packet number 0
• ALL headers contain a "frag" field, with the fragment number and total number of fragments
• The unencrypted header of fragment 0 is the associated data (AD) for the "jumbo" message
• Each header is encrypted using the last 24 bytes of data in THAT packet

Construct the series of packets as follows:

• Create a single RI block (fragment 0 of 1 in the RI block frag field). We do not use RI block fragmentation, that was for an alternate method of solving the same problem.
• Create a "jumbo" payload with the RI block and any other blocks to be included
• Calculate the total data size (not including the header), which is the payload size + 64 bytes for the static key and two MACs
• Calculate the space available in each packet, which is the MTU minus the IP header (20 or 40), minus the UDP header (8), minus the SSU2 short header (16). Total per-packet overhead is 44 (IPv4) or 64 (IPv6).
• Calculate the number of packets.
• Calculate the size of the data in the last packet. It must be greater than or equal to 24 bytes, so that header encryption will work. If it is too small, either add a padding block, OR increase the size of the padding block if already present, OR reduce the size of one of the other packets so that the last packet will be big enough.
• Create the unencrypted header for the first packet, with the total number of fragments in the frag field, and encrypt the "jumbo" payload with Noise, using the header as AD, as usual.
• Split up the encrypted jumbo packet into fragments
• Add an unencrypted header for each fragment 1-n
• Encrypt the header for each fragment 0-n. Each header uses the SAME k_header_1 and k_header_2 as defined above in the Session Confirmed KDF.
• Transmit all fragments

Reassembly process:

When Bob receives any Session Confirmed message, he decrypts the header, inspects the frag field, and determines that the Session Confirmed is fragmented. He does not (and cannot) decrypt the message until all fragments are received and reassembled.

• Preserve the header for fragment 0, as it is used as the Noise AD
• Discard the headers for other fragments before reassembly
• Reassemble the "jumbo" payload, with the header for fragment 0 as AD, and decrypt with Noise
• Validate the RI block as usual
• Proceed to the data phase and send ACK 0, as usual

There is no mechanism for Bob to ack individual fragments. When Bob receives all fragments, reassembles, decrypts, and validates the contents, Bob does a split() as usual, enters the data phase, and sends an ACK of packet number 0.

If Alice does not receive an ACK of packet number 0, she must retransmit all session confirmed packets as-is.

Examples:

For 1500 MTU over IPv6, max payload is 1372, RI block overhead is 5, max (gzip compressed) RI data size is 1367 (assuming no other blocks). With two packets, the overhead of the 2nd packet is 64, so it can hold another 1436 bytes of payload. So two packets is enough for a compressed RI up to 2803 bytes.

The largest compressed RI seen in the current network is about 1400 bytes; therefore, in practice, two fragments should be enough, even with a minimum 1280 MTU. The protocol allows for 15 fragments max.

Security analysis:

The integrity and security of a fragmented Session Confirmed is the same as that of an unfragmented one. Any alteration of any fragment will cause the Noise AEAD to fail after reassembly. The headers of the fragments after fragment 0 are only used to identify the fragment. Even if an on-path attacker had the k_header_2 key used to encrypt the header (unlikely, derived from the handshake), this would not allow the attacker to substitute a valid fragment.

Notes

• The router must drop a message with an AEAD error.

+----+----+----+----+----+----+----+----+
|  Short Header 16 bytes, ChaCha20      |
+  encrypted with intro key and         +
|  derived key, see Data Phase KDF      |
+----+----+----+----+----+----+----+----+
|   ChaCha20 data                       |
+   Encrypted and authenticated data    +
|  length varies                        |
+  k defined in Data Phase KDF          +
|  n = packet number from header        |
+                                       +
|                                       |
+----+----+----+----+----+----+----+----+
|                                       |
+        Poly1305 MAC (16 bytes)        +
|                                       |
+----+----+----+----+----+----+----+----+

Unencrypted data (Poly1305 auth tag not shown):

+----+----+----+----+----+----+----+----+
|      Destination Connection ID        |
+----+----+----+----+----+----+----+----+
|   Packet Number   |type|    flags     |
+----+----+----+----+----+----+----+----+
|     Noise payload (block data)        |
+          (length varies)              +
|                                       |
+----+----+----+----+----+----+----+----+

Destination ID :: As specified in session setup

Packet Number :: 4 byte big endian integer

type :: 6

flags :: 3 bytes, unused, set to 0 for future compatibility

Notes

• The minimum payload size is 8 bytes. This requirement will be met by any ACK, I2NP, First Fragment, or Follow-on Fragment block. If the requirement is not met, a Padding block must be included.
• Each packet number may only be used once. When retransmitting I2NP messages or fragments, a new packet number must be used.

Payload

• DateTime block
• Address block (required for messages 6 and 7, see note below)
• Peer Test block
• Padding block (optional)

The minimum payload size is 8 bytes. Since the Peer Test block totals more than that, the requirement is met with only this block.

In messages 5 and 7, the Peer Test block may be identical to the block from in-session messages 3 and 4, containing the agreement signed by Charlie, or it may be regenerated. Signature is optional.

In message 6, the Peer Test block may be identical to the block from in-session messages 1 and 2, containing the request signed by Alice, or it may be regenerated. Signature is optional.

Connection IDs: The two connection IDs are derived from the test nonce. For messages 5 and 7 sent from Charlie to Alice, the Destination Connection ID is two copies of the 4-byte big-endian test nonce, i.e. ((nonce << 32) | nonce). The Source Connection ID is the inverse of the Destination Connection ID, i.e. ~((nonce << 32) | nonce). For message 6 sent from Alice to Charlie, swap the two connection IDs.

Address block contents:

• In message 5: Not required.
• In message 6: Charlie's IP and port as selected from Charlie's RI.
• In message 7: Alice's actual IP and port message 6 was received from.

Payload

• DateTime block
• Address block
• Options block (optional)
• Termination block (optional, if session is rejected)
• Padding block (optional)

The minimum payload size is 8 bytes. Since the DateTime and Address blocks total more than that, the requirement is met with only those two blocks.

Notes

• To provide probing resistance, a router should not send a Retry message in response to a Session Request or Token Request message unless the message type, protocol version, and network ID fields in the Request message are valid.
• To limit the magnitude of any amplification attack that can be mounted using spoofed source addresses, the Retry message must not contain large amounts of padding. It is recommended that the Retry message be no larger than three times the size of the message it is responding to. Alternatively, use a simple method such as adding a random amount of padding in the range 1-64 bytes.

Payload

• DateTime block
• Padding block

The minimum payload size is 8 bytes.

Notes

• To provide probing resistance, a router should not send a Retry message in response to a Token Request message unless the message type, protocol version, and network ID fields in the Token Request message are valid.
• This is NOT a standard Noise message and is not part of the handshake. It is not bound to the Session Request message other than by connection IDs.
• On most errors, including AEAD, or apparent replay Bob should halt further message processing and drop the message without responding.
• Bob must reject connections where the timestamp value is too far off from the current time. Call the maximum delta time "D". Bob must maintain a local cache of previously-used handshake values and reject duplicates, to prevent replay attacks. Values in the cache must have a lifetime of at least 2*D. The cache values are implementation-dependent, however the 32-byte X value (or its encrypted equivalent) may be used.
• Bob MAY send a Retry message containing a zero token and a Termination block with a clock skew reason code if the timestamp in the DateTime block is too far skewed.
• Minimum size: TBD, same rules as for Session Created?

Payload

• DateTime block
• Address block
• Relay Response block
• Padding block (optional)

The minimum payload size is 8 bytes. Since the DateTime and Address blocks total more than that, the requirement is met with only those two blocks.

Connection IDs: The two connection IDs are derived from the relay nonce. The Destination Connection ID is two copies of the 4-byte big-endian relay nonce, i.e. ((nonce << 32) | nonce). The Source Connection ID is the inverse of the Destination Connection ID, i.e. ~((nonce << 32) | nonce).

Alice should ignore the token in the header. The token to be used in the Session Request is in the Relay Response block.

DateTime

For time synchronization:

+----+----+----+----+----+----+----+
| 0  |    4    |     timestamp     |
+----+----+----+----+----+----+----+

blk :: 0
size :: 2 bytes, big endian, value = 4
timestamp :: Unix timestamp, unsigned seconds.
             Wraps around in 2106

Notes:

• Unlike in SSU 1, there is no timestamp in the packet header for the data phase in SSU 2.
• Implementations should periodically send DateTime blocks in the data phase.
• Implementations must round to the nearest second to prevent clock bias in the network.

Options

Pass updated options. Options include: Min and max padding.

Options block will be variable length.

+----+----+----+----+----+----+----+----+
| 1  |  size   |tmin|tmax|rmin|rmax|tdmy|
+----+----+----+----+----+----+----+----+
|tdmy|  rdmy   |  tdelay |  rdelay |    |
~----+----+----+----+----+----+----+    ~
|              more_options             |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

blk :: 1
size :: 2 bytes, big endian, size of options to follow, 12 bytes minimum

tmin, tmax, rmin, rmax :: requested padding limits
    tmin and rmin are for desired resistance to traffic analysis.
    tmax and rmax are for bandwidth limits.
    tmin and tmax are the transmit limits for the router sending this options block.
    rmin and rmax are the receive limits for the router sending this options block.
    Each is a 4.4 fixed-point float representing 0 to 15.9375
    (or think of it as an unsigned 8-bit integer divided by 16.0).
    This is the ratio of padding to data. Examples:
    Value of 0x00 means no padding
    Value of 0x01 means add 6 percent padding
    Value of 0x10 means add 100 percent padding
    Value of 0x80 means add 800 percent (8x) padding
    Alice and Bob will negotiate the minimum and maximum in each direction.
    These are guidelines, there is no enforcement.
    Sender should honor receiver's maximum.
    Sender may or may not honor receiver's minimum, within bandwidth constraints.

tdmy: Max dummy traffic willing to send, 2 bytes big endian, bytes/sec average
rdmy: Requested dummy traffic, 2 bytes big endian, bytes/sec average
tdelay: Max intra-message delay willing to insert, 2 bytes big endian, msec average
rdelay: Requested intra-message delay, 2 bytes big endian, msec average

Padding distribution specified as additional parameters?
Random delay specified as additional parameters?

more_options :: Format TBD

Options Issues:

• Options negotiation is TBD.

RouterInfo

Pass Alice's RouterInfo to Bob. Used in Session Confirmed part 2 payload only. Not to be used in the data phase; use an I2NP DatabaseStore Message instead.

Minimum Size: About 420 bytes, unless the router identity and signature in the router info are compressible, which is unlikely.

NOTE: The Router Info block is never fragmented. The frag field is always 0/1. See the Session Confirmed Fragmentation section above for more information.

+----+----+----+----+----+----+----+----+
| 2  |  size   |flag|frag|              |
+----+----+----+----+----+              +
|                                       |
+       Router Info fragment            +
| (Alice RI in Session Confirmed)       |
+ (Alice, Bob, or third-party           +
|  RI in data phase)                    |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

blk :: 2
size :: 2 bytes, big endian, 2 + fragment size
flag :: 1 byte flags
       bit order: 76543210 (bit 7 is MSB)
       bit 0: 0 for local store, 1 for flood request
       bit 1: 0 for uncompressed, 1 for gzip compressed
       bits 7-2: Unused, set to 0 for future compatibility
frag :: 1 byte fragment info:
       bit order: 76543210 (bit 7 is MSB)
       bits 7-4: fragment number, always 0
       bits 3-0: total fragments, always 1, big endian

routerinfo :: Alice's or Bob's RouterInfo

Notes:

• The Router Info is optionally compressed with gzip, as indicated by flag bit 1. This is different from NTCP2, where it is never compressed, and from a DatabaseStore Message, where it always is compressed. Compression is optional because it usually is of little benefit for small Router Infos, where there is little compressible content, but is very beneficial for large Router Infos with several compressible Router Addresses. Compression is recommended if it allows a Router Info to fit in a single Session Confirmed packet without fragmentation.
• Maximum size of first or only fragment in the Session Confirmed message: MTU - 113 for IPv4 or MTU - 133 for IPv6. Assuming 1500 byte default MTU, and no other blocks in the message, 1387 for IPv4 or 1367 for IPv6. 97% of current router infos are smaller than 1367 witout gzipping. 99.9% of current router infos are smaller than 1367 when gzipped. Assuming 1280 byte minimum MTU, and no other blocks in the message, 1167 for IPv4 or 1147 for IPv6. 94% of current router infos are smaller than 1147 witout gzipping. 97% of current router infos are smaller than 1147 when gzipped.
• The frag byte is now unused, the Router Info block is never fragmented. The frag byte must be set to fragment 0, total fragments 1. See the Session Confirmed Fragmentation section above for more information.
• Flooding must not be requested unless there are published RouterAddresses in the RouterInfo. The receiving router must not flood the RouterInfo unless there are published RouterAddresses in it.
• This protocol does not provide an acknowledgment that the RouterInfo was stored or flooded. If acknowledgment is desired, and the receiver is floodfill, the sender should instead send a standard I2NP DatabaseStoreMessage with a reply token.

I2NP Message

A complete I2NP message with a modified header.

This uses the same 9 bytes for the I2NP header as in [NTCP2] (type, message id, short expiration).

+----+----+----+----+----+----+----+----+
| 3  |  size   |type|    msg id         |
+----+----+----+----+----+----+----+----+
|   short exp       |     message       |
+----+----+----+----+                   +
|                                       |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

blk :: 3
size :: 2 bytes, big endian, size of type + msg id + exp + message to follow
        I2NP message body size is (size - 9).
type :: 1 byte, I2NP msg type, see I2NP spec
msg id :: 4 bytes, big endian, I2NP message ID
short exp :: 4 bytes, big endian, I2NP message expiration, Unix timestamp, unsigned seconds.
             Wraps around in 2106
message :: I2NP message body

Notes:

• This is the same 9-byte I2NP header format used in NTCP2.
• This is exactly the same format as the First Fragment block, but the block type indicates that this is a complete message.
• Maximum size including 9-byte I2NP header is MTU - 63 for IPv4 and MTU - 83 for IPv6.

First Fragment

The first fragment (fragment #0) of an I2NP message with a modified header.

This uses the same 9 bytes for the I2NP header as in [NTCP2] (type, message id, short expiration).

Total number of fragments is not specified.

+----+----+----+----+----+----+----+----+
| 4  |  size   |type|    msg id         |
+----+----+----+----+----+----+----+----+
|   short exp       |                   |
+----+----+----+----+                   +
|          partial message              |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

blk :: 4
size :: 2 bytes, big endian, size of data to follow
        Fragment size is (size - 9).
type :: 1 byte, I2NP msg type, see I2NP spec
msg id :: 4 bytes, big endian, I2NP message ID
short exp :: 4 bytes, big endian, I2NP message expiration, Unix timestamp, unsigned seconds.
             Wraps around in 2106
message :: Partial I2NP message body, bytes 0 - (size - 10)

Notes:

• This is the same 9-byte I2NP header format used in NTCP2.
• This is exactly the same format as the I2NP Message block, but the block type indicates that this is a the first fragment of a message.
• Partial message length must be greater than zero.
• As in SSU 1, it is recommended to send the last fragment first, so that the receiver knows the total number of fragments and can efficiently allocate receive buffers.
• Maximum size including 9-byte I2NP header is MTU - 63 for IPv4 and MTU - 83 for IPv6.

Follow-on Fragment

An additional fragment (fragment number greater than zero) of an I2NP message.

+----+----+----+----+----+----+----+----+
| 5  |  size   |frag|    msg id         |
+----+----+----+----+----+----+----+----+
|                                       |
+                                       +
|          partial message              |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

blk :: 5
size :: 2 bytes, big endian, size of data to follow
        Fragment size is (size - 5).
frag :: Fragment info:
        Bit order: 76543210 (bit 7 is MSB)
        bits 7-1: fragment number 1 - 127 (0 not allowed)
        bit 0: isLast (1 = true)
msg id :: 4 bytes, big endian, I2NP message ID
message :: Partial I2NP message body

Notes:

• Partial message length must be greater than zero.
• As in SSU 1, it is recommended to send the last fragment first, so that the receiver knows the total number of fragments and can efficiently allocate receive buffers.
• As in SSU 1, the maximum fragment number is 127, but the practical limit is 63 or less. Implementations may limit the maximum to what is practical for a maximum I2NP message size of about 64 KB, which is about 55 fragments with a 1280 minimum MTU. See the Max I2NP Message Size section below.
• Maximum partial message size (not including frag and message id) is MTU - 68 for IPv4 and MTU - 88 for IPv6.

Termination

Drop the connection. This must be the last non-padding block in the payload.

+----+----+----+----+----+----+----+----+
| 6  |  size   |    valid data packets  |
+----+----+----+----+----+----+----+----+
    received   | rsn|     addl data     |
+----+----+----+----+                   +
~               .   .   .               ~
+----+----+----+----+----+----+----+----+

blk :: 6
size :: 2 bytes, big endian, value = 9 or more
valid received :: The number of valid packets received
                              (current receive nonce value)
                              0 if error occurs in handshake phase
                              8 bytes, big endian
rsn :: reason, 1 byte:
       0: normal close or unspecified
       1: termination received
       2: idle timeout
       3: router shutdown
       4: data phase AEAD failure
       5: incompatible options
       6: incompatible signature type
       7: clock skew
       8: padding violation
       9: AEAD framing error
       10: payload format error
       11: Session Request error
       12: Session Created error
       13: Session Confirmed error
       14: Timeout
       15: RI signature verification fail
       16: s parameter missing, invalid, or mismatched in RouterInfo
       17: banned
       18: bad token
       19: connection limits
       20: incompatible version
       21: wrong net ID
       22: replaced by new session
addl data :: optional, 0 or more bytes, for future expansion, debugging,
             or reason text.
             Format unspecified and may vary based on reason code.

Notes:

• Not all reasons may actually be used, implementation dependent. Most failures will generally result in the message being dropped, not a termination. See notes in handshake message sections above. Additional reasons listed are for consistency, logging, debugging, or if policy changes.
• It is recommended that an ACK block be included with the Termination block.
• In the data phase, for any reason other than "termination received", the peer should respond with a termination block with the reason "termination received".

RelayRequest

Sent in a Data message in-session, from Alice to Bob. See Relay Process section below.

+----+----+----+----+----+----+----+----+
|  7 |  size   |flag|       nonce       |
+----+----+----+----+----+----+----+----+
|     relay tag     |     timestamp     |
+----+----+----+----+----+----+----+----+
| ver| asz|AlicePort|  Alice IP address |
+----+----+----+----+----+----+----+----+
|              signature                |
+            length varies              +
|         64 bytes for Ed25519          |
~                                       ~
|                 . . .                 |
+----+----+----+----+----+----+----+----+

blk :: 7
size :: 2 bytes, big endian, size of data to follow
flag :: 1 byte flags, Unused, set to 0 for future compatibility

The data below here is covered
by the signature, and Bob forwards it unmodified.

nonce :: 4 bytes, randomly generated by Alice
relay tag :: 4 bytes, the itag from Charlie's RI
timestamp :: Unix timestamp, unsigned seconds.
             Wraps around in 2106
ver ::  1 byte SSU version to be used for the introduction:
       1: SSU 1
       2: SSU 2
asz :: 1 byte endpoint (port + IP) size (6 or 18)
AlicePort :: 2 byte Alice's port number, big endian
Alice IP :: (asz - 2) byte representation of Alice's IP address,
            network byte order
signature :: length varies, 64 bytes for Ed25519.
             Signature of prologue, Bob's hash,
             and signed data above, as signed by
             Alice.

Notes:

• The IP address is always included (unlike in SSU 1) and may be different than the IP used for the session.

Signature:

Alice signs the request and includes it in this block; Bob forwards it in the Relay Intro block to Charlie. Signature algorithm: Sign the following data with the Alice's router signing key:

• prologue: 16 bytes "RelayRequestData", not null-terminated (not included in the message)
• bhash: Bob's 32-byte router hash (not included in the message)
• chash: Charlie's 32-byte router hash (not included in the message)
• nonce: 4 byte nonce
• relay tag: 4 byte relay tag
• timestamp: 4 byte timestamp (seconds)
• ver: 1 byte SSU version
• asz: 1 byte endpoint (port + IP) size (6 or 18)
• AlicePort: 2 byte Alice's port number
• Alice IP: (asz - 2) byte Alice IP address

RelayResponse

Sent in a Data message in-session, from Charlie to Bob or from Bob to Alice, AND in the Hole Punch message from Charlie to Alice. See Relay Process section below.

+----+----+----+----+----+----+----+----+
|  8 |  size   |flag|code|    nonce
+----+----+----+----+----+----+----+----+
     |     timestamp     | ver| csz|Char
+----+----+----+----+----+----+----+----+
 Port|   Charlie IP addr |              |
+----+----+----+----+----+              +
|              signature                |
+            length varies              +
|         64 bytes for Ed25519          |
~                                       ~
|                 . . .                 |
+----+----+----+----+----+----+----+----+
|                 Token                 |
+----+----+----+----+----+----+----+----+

blk :: 8
size :: 2 bytes, 6
flag :: 1 byte flags, Unused, set to 0 for future compatibility
code :: 1 byte status code:
       0: accept
       1: rejected by Bob, reason unspecified
       2: rejected by Bob, Charlie is banned
       3: rejected by Bob, limit exceeded
       4: rejected by Bob, signature failure
       5: rejected by Bob, relay tag not found
       6: rejected by Bob, Alice RI not found
       7-63: other rejected by Bob codes TBD
       64: rejected by Charlie, reason unspecified
       65: rejected by Charlie, unsupported address
       66: rejected by Charlie, limit exceeded
       67: rejected by Charlie, signature failure
       68: rejected by Charlie, Alice is already connected
       69: rejected by Charlie, Alice is banned
       70: rejected by Charlie, Alice is unknown
       71-127: other rejected by Charlie codes TBD
       128: reject, source and reason unspecified
       129-255: other reject codes TBD

The data below is covered by the signature if the code is 0 (accept).
Bob forwards it unmodified.

nonce :: 4 bytes, as received from Bob or Alice

The data below is present only if the code is 0 (accept).

timestamp :: Unix timestamp, unsigned seconds.
             Wraps around in 2106
ver ::  1 byte SSU version to be used for the introduction:
       1: SSU 1
       2: SSU 2
csz :: 1 byte endpoint (port + IP) size (0 or 6 or 18)
       may be 0 for some rejection codes
CharliePort :: 2 byte Charlie's port number, big endian
               not present if csz is 0
Charlie IP :: (csz - 2) byte representation of Charlie's IP address,
              network byte order
              not present if csz is 0
signature :: length varies, 64 bytes for Ed25519.
             Signature of prologue, Bob's hash,
             and signed data above, as signed by
             Charlie.
             Not present if rejected by Bob.
token :: Token generated by Charlie for Alice to use
         in the Session Request.
         Only present if code is 0 (accept)

Notes:

The token must be used immediately by Alice in the Session Request.

Signature:

If Charlie agrees (response code 0) or rejects (response code 64 or higher), Charlie signs the response and includes it in this block; Bob forwards it in the Relay Response block to Alice. Signature algorithm: Sign the following data with the Charlie's router signing key:

• prologue: 16 bytes "RelayAgreementOK", not null-terminated (not included in the message)
• bhash: Bob's 32-byte router hash (not included in the message)
• nonce: 4 byte nonce
• timestamp: 4 byte timestamp (seconds)
• ver: 1 byte SSU version
• csz: 1 byte endpoint (port + IP) size (0 or 6 or 18)
• CharliePort: 2 byte Charlie's port number (not present if csz is 0)
• Charlie IP: (csz - 2) byte Charlie IP address (not present if csz is 0)

If Bob rejects (response code 1-63), Bob signs the response and includes it in this block. Signature algorithm: Sign the following data with the Bob's router signing key:

• prologue: 16 bytes "RelayAgreementOK", not null-terminated (not included in the message)
• bhash: Bob's 32-byte router hash (not included in the message)
• nonce: 4 byte nonce
• timestamp: 4 byte timestamp (seconds)
• ver: 1 byte SSU version
• csz: 1 byte = 0

RelayIntro

Sent in a Data message in-session, from Bob to Charlie. See Relay Process section below.

Must be preceded by a RouterInfo block, or I2NP DatabaseStore message block (or fragment), containing Alice's Router Info, either in the same payload (if there's room), or in a previous message.

+----+----+----+----+----+----+----+----+
|  9 |  size   |flag|                   |
+----+----+----+----+                   +
|                                       |
+                                       +
|         Alice Router Hash             |
+             32 bytes                  +
|                                       |
+                   +----+----+----+----+
|                   |      nonce        |
+----+----+----+----+----+----+----+----+
|     relay tag     |     timestamp     |
+----+----+----+----+----+----+----+----+
| ver| asz|AlicePort|  Alice IP address |
+----+----+----+----+----+----+----+----+
|              signature                |
+            length varies              +
|         64 bytes for Ed25519          |
~                                       ~
|                 . . .                 |
+----+----+----+----+----+----+----+----+

blk :: 9
size :: 2 bytes, big endian, size of data to follow
flag :: 1 byte flags, Unused, set to 0 for future compatibility
hash :: Alice's 32-byte router hash,

The data below here is covered
by the signature, as received from Alice in the Relay Request,
and Bob forwards it unmodified.

nonce :: 4 bytes, as received from Alice
relay tag :: 4 bytes, the itag from Charlie's RI
timestamp :: Unix timestamp, unsigned seconds.
             Wraps around in 2106
ver ::  1 byte SSU version to be used for the introduction:
       1: SSU 1
       2: SSU 2
asz :: 1 byte endpoint (port + IP) size (6 or 18)
AlicePort :: 2 byte Alice's port number, big endian
Alice IP :: (asz - 2) byte representation of Alice's IP address,
            network byte order
signature :: length varies, 64 bytes for Ed25519.
             Signature of prologue, Bob's hash,
             and signed data above, as signed by
             Alice.

Notes:

• For IPv4, Alice's IP address is always 4 bytes, because Alice is trying to connect to Charlie via IPv4. IPv6 is supported, and Alice's IP address may be 16 bytes.
• For IPv4, this message must be sent via an established IPv4 connection, as that's the only way that Bob knows Charlie's IPv4 address to return to Alice in the RelayResponse. IPv6 is supported, and this message may be sent via an established IPv6 connection.
• Any SSU address published with introducers must contain "4" or "6" in the "caps" option.

Signature:

Alice signs the request and Bob forwards it in this block to Charlie. Verification algorithm: Verify the following data with the Alice's router signing key:

• prologue: 16 bytes "RelayRequestData", not null-terminated (not included in the message)
• bhash: Bob's 32-byte router hash (not included in the message)
• chash: Charlie's 32-byte router hash (not included in the message)
• nonce: 4 byte nonce
• relay tag: 4 byte relay tag
• timestamp: 4 byte timestamp (seconds)
• ver: 1 byte SSU version
• asz: 1 byte endpoint (port + IP) size (6 or 18)
• AlicePort: 2 byte Alice's port number
• Alice IP: (asz - 2) byte Alice IP address

PeerTest

Sent either in a Data message in-session, or a Peer Test message out-of-session. See Peer Test Process section below.

For message 2, must be preceded by a RouterInfo block, or I2NP DatabaseStore message block (or fragment), containing Alice's Router Info, either in the same payload (if there's room), or in a previous message.

For message 4, if the relay is accepted (reason code 0), must be preceded by a RouterInfo block, or I2NP DatabaseStore message block (or fragment), containing Charlie's Router Info, either in the same payload (if there's room), or in a previous message.

+----+----+----+----+----+----+----+----+
| 10 |  size   | msg|code|flag|         |
+----+----+----+----+----+----+         +
| Alice router hash (message 2 only)    |
+             or                        +
| Charlie router hash (message 4 only)  |
+ or all zeros if rejected by Bob       +
| Not present in messages 1,3,5,6,7     |
+                             +----+----+
|                             | ver|
+----+----+----+----+----+----+----+----+
   nonce       |     timestamp     | asz|
+----+----+----+----+----+----+----+----+
|AlicePort|  Alice IP address |         |
+----+----+----+----+----+----+         +
|              signature                |
+            length varies              +
|         64 bytes for Ed25519          |
~                                       ~
|                 . . .                 |
+----+----+----+----+----+----+----+----+

blk :: 10
size :: 2 bytes, big endian, size of data to follow
msg :: 1 byte message number 1-7
code :: 1 byte status code:
       0: accept
       1: rejected by Bob, reason unspecified
       2: rejected by Bob, no Charlie available
       3: rejected by Bob, limit exceeded
       4: rejected by Bob, signature failure
       5: rejected by Bob, address unsupported
       6-63: other rejected by Bob codes TBD
       64: rejected by Charlie, reason unspecified
       65: rejected by Charlie, unsupported address
       66: rejected by Charlie, limit exceeded
       67: rejected by Charlie, signature failure
       68: rejected by Charlie, Alice is already connected
       69: rejected by Charlie, Alice is banned
       70: rejected by Charlie, Alice is unknown
       70-127: other rejected by Charlie codes TBD
       128: reject, source and reason unspecified
       129-255: other reject codes TBD
       reject codes only allowed in messages 3 and 4
flag :: 1 byte flags, Unused, set to 0 for future compatibility
hash :: Alice's or Charlie's 32-byte router hash,
        only present in messages 2 and 4.
        All zeros (fake hash) in message 4 if rejected by Bob.

For messages 1-4, the data below here is covered
by the signature, if present, and Bob forwards it unmodified.

ver :: 1 byte SSU version:
       1: SSU 1 (not supported)
       2: SSU 2 (required)
nonce :: 4 byte test nonce, big endian
timestamp :: Unix timestamp, unsigned seconds.
             Wraps around in 2106
asz :: 1 byte endpoint (port + IP) size (6 or 18)
AlicePort :: 2 byte Alice's port number, big endian
Alice IP :: (asz - 2) byte representation of Alice's IP address,
            network byte order
signature :: length varies, 64 bytes for Ed25519.
             Signature of prologue, Bob's hash,
             and signed data above, as signed by
             Alice or Charlie.
             Only present for messages 1-4.
             Optional in message 5-7.

Notes:

• Unlike in SSU 1, message 1 must include Alice's IP address and port.

• Testing of IPv6 addresses is supported, and Alice-Bob and Alice-Charlie communication may be via IPv6, if Bob and Charlie indicate support with a 'B' capability in their published IPv6 address. See Proposal 126 for details.

  Alice sends the request to Bob using an existing session over the transport (IPv4 or IPv6) that she wishes to test. When Bob receives a request from Alice via IPv4, Bob must select a Charlie that advertises an IPv4 address. When Bob receives a request from Alice via IPv6, Bob must select a Charlie that advertises an IPv6 address. The actual Bob-Charlie communication may be via IPv4 or IPv6 (i.e., independent of Alice's address type).

• Messages 1-4 must be contained in a Data message in an existing session.

• Bob must send Alice's RI to Charlie prior to sending message 2.

• Bob must send Charlie's RI to Alice prior to sending message 4, if accepted (reason code 0).

• Messages 5-7 must be contained in a Peer Test message out-of-session.

• Messages 5 and 7 may contain the same signed data as sent in messages 3 and 4, or it may be regenerated with a new timestamp. Signature is optional.

• Message 6 may contain the same signed data as sent in messages 1 and 2, or it may be regenerated with a new timestamp. Signature is optional.

Signatures:

Alice signs the request and includes it in message 1; Bob forwards it in message 2 to Charlie. Charlie signs the response and includes it in message 3; Bob forwards it in message 4 to Alice. Signature algorithm: Sign or verify the following data with the Alice's or Charlie's signing key:

• prologue: 16 bytes "PeerTestValidate", not null-terminated (not included in the message)
• bhash: Bob's 32-byte router hash (not included in the message)
• ahash: Alice's 32-byte router hash (Only used in the signature for messages 3 and 4; not included in message 3 or 4)
• ver: 1 byte SSU version
• nonce: 4 byte test nonce
• timestamp: 4 byte timestamp (seconds)
• asz: 1 byte endpoint (port + IP) size (6 or 18)
• AlicePort: 2 byte Alice's port number
• Alice IP: (asz - 2) byte Alice IP address

NextNonce

TODO only if we rotate keys

+----+----+----+----+----+----+----+----+
| 11 |  size   |      TBD               |
+----+----+----+                        +
|                                       |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

blk :: 11
size :: 2 bytes, big endian, size of data to follow

Ack

4 byte ack through, followed by an ack count and zero or more nack/ack ranges.

This design is adapted and simplified from QUIC. The design goals are as follows:

• We want to efficiently encode a "bitfield", which is a sequence of bits representing acked packets.
• The bitfield is mostly 1's. Both the 1's and the 0's generally come in sequential "clumps".
• The amount of room in the packet available for acks varies.
• The most important bit is the highest numbered one. Lower numbered ones are less important. Below a certain distance from the highest bit, the oldest bits will be "forgotten" and never sent again.

The encoding specified below accomplishes these design goals, by sending the number of the highest bit that is set to 1, together with additional consecutive bits lower than that which are also set to 1. After that, if there is room, one or more "ranges" specifying the number of consectutive 0 bits and consecutive 1 bits lower than that. See QUIC [RFC-9000] section 13.2.3 for more background.

+----+----+----+----+----+----+----+----+
| 12 |  size   |    Ack Through    |acnt|
+----+----+----+----+----+----+----+----+
|  range  |  range  |     .   .   .     |
+----+----+----+----+                   +
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

blk :: 12
size :: 2 bytes, big endian, size of data to follow,
        5 minimum
ack through :: highest packet number acked
acnt :: number of acks lower than ack through also acked,
        0-255
range :: If present,
         1 byte nack count followed by 1 byte ack count,
         0-255 each

Examples:

We want to ACK packet 10 only:

• Ack Through: 10
• acnt: 0
• no ranges are included

We want to ACK packets 8-10 only:

• Ack Through: 10
• acnt: 2
• no ranges are included

We want to ACK 10 9 8 6 5 2 1 0, and NACK 7 4 3. The encoding of the ACK Block is:

• Ack Through: 10
• acnt: 2 (ack 9 8)
• range: 1 2 (nack 7, ack 6 5)
• range: 2 3 (nack 4 3, ack 2 1 0)

Notes:

• Ranges may not be present. Max number of ranges is not specified, may be as many as will fit in the packet.
• Range nack may be zero if acking more than 255 consecutive packets.
• Range ack may be zero if nacking more than 255 consecutive packets.
• Range nack and ack may not both be zero.
• After the last range, packets are neither acked nor nacked. Length of the ack block and how old acks/nacks are handled is up to the sender of the ack block. See ack sections below for discussion.
• The ack through should be the highest packet number received, and any packets higher have not been received. However, in limited situations, it could be lower, such as acking a single packet that "fills in a hole", or a simplified implementation that does not maintain the state of all received packets. Above the highest received, packets are neither acked nor nacked, but after several ack blocks, it may be appropriate to go into fast retransmit mode.
• This format is a simplified version of that in QUIC. It is designed to efficiently encode a large number of ACKs, together with bursts of NACKs.
• ACK blocks are used to acknowledge data phase packets. They are only to be included for in-session data phase packets.

Address

2 byte port and 4 or 16 byte IP address. Alice's address, sent to Alice by Bob, or Bob's address, sent to Bob by Alice.

+----+----+----+----+----+----+----+----+
| 13 | 6 or 18 |   Port  | IP Address
+----+----+----+----+----+----+----+----+
     |
+----+

blk :: 13
size :: 2 bytes, big endian, 6 or 18
port :: 2 bytes, big endian
ip :: 4 byte IPv4 or 16 byte IPv6 address,
      big endian (network byte order)

Relay Tag Request

This may be sent by Alice in a Session Request, Session Confirmed, or Data message. Not supported in the Session Created message, as Bob doesn't have Alice's RI yet, and doesn't know if Alice supports relay. Also, if Bob is getting an incoming connection, he probably doesn't need introducers (except perhaps for the other type ipv4/ipv6).

When sent in the Session Request, Bob may respond with a Relay Tag in the Session Created message, or may choose to wait until receiving Alice's RouterInfo in the Session Confirmed to validate Alice's identity before responding in a Data message. If Bob does not wish to relay for Alice, he does not send a Relay Tag block.

+----+----+----+
| 15 |    0    |
+----+----+----+

blk :: 15
size :: 2 bytes, big endian, value = 0

Relay Tag

This may be sent by Bob in a Session Confirmed or Data message, in response to a Relay Tag Request from Alice.

When the Relay Tag Request is sent in the Session Request, Bob may respond with a Relay Tag in the Session Created message, or may choose to wait until receiving Alice's RouterInfo in the Session Confirmed to validate Alice's identity before responding in a Data message. If Bob does not wish to relay for Alice, he does not send a Relay Tag block.

+----+----+----+----+----+----+----+
| 16 |    4    |    relay tag      |
+----+----+----+----+----+----+----+

blk :: 16
size :: 2 bytes, big endian, value = 4
relay tag :: 4 bytes, big endian, nonzero

New Token

For a subsequent connection. Generally included in the Session Created and Session Confirmed messages. May also be sent again in the Data message of a long-lived session if the previous token expires.

+----+----+----+----+----+----+----+----+
| 17 |   12    |     expires       |
+----+----+----+----+----+----+----+----+
                token              |
+----+----+----+----+----+----+----+

blk :: 17
size :: 2 bytes, big endian, value = 12
expires :: Unix timestamp, unsigned seconds.
           Wraps around in 2106
token :: 8 bytes, big endian

Path Challenge

A Ping with arbitrary data to be returned in a Path Response, used as a keep-alive or to validate an IP/Port change.

+----+----+----+----+----+----+----+----+
| 18 |  size   |    Arbitrary Data      |
+----+----+----+                        +
|                                       |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

blk :: 18
size :: 2 bytes, big endian, size of data to follow
data :: Arbitrary data to be returned in a Path Response
        length as selected by sender

Notes:

A minimum data size of 8 bytes, containing random data, is recommended but not required.

Path Response

A Pong with the data received in the Path Challenge, as reply to the Path Challenge, used as a keep-alive or to validate an IP/Port change.

+----+----+----+----+----+----+----+----+
| 19 |  size   |                        |
+----+----+----+                        +
|    Data received in Path Challenge    |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

blk :: 19
size :: 2 bytes, big endian, size of data to follow
data :: As received in a Path Challenge

First Packet Number

Optionally included in the handshake in each direction, to specify the first packet number that will be sent. This provides more security for header encryption, similar to TCP.

Not fully specified, not currently supported.

+----+----+----+----+----+----+----+
| 20 |  size   |  First pkt number |
+----+----+----+----+----+----+----+

blk :: 20
size :: 4
pkt num :: The first packet number to be sent in the data phase

Congestion

This is block is designed to be an extensible method to exchange congestion control information. Congestion control can be complex and may evolve as we get more experience with the protocol in live testing, or after full rollout.

This keeps any congestion information out of the high-usage I2NP, First Fragment, Followon Fragment, and ACK blocks, where there is no space for flags allocated. While there are three bytes of unused flags in the Data packet header, that also provides limited space for extensibility, and weaker encryption protection.

While it is somewhat wasteful to use a 4-byte block for two bits of information, by putting this in a separate block, we can easily extend it with additional data such as current window sizes, measured RTT, or other flags. Experience has shown that flag bits alone is often insufficient and awkward for implementation of advanced congestion control schemes. Trying to add support for any possible congestion control feature in, for example, the ACK block, would waste space and add complexity to the parsing of that block.

Implementations should not assume that the other router supports any particular flag bit or feature included here, unless implementation is required by a future version of this specification.

This block should probably be the last non-padding block in the payload.

+----+----+----+----+
| 21 |  size   |flag|
+----+----+----+----+

blk :: 21
size :: 1 (or more if extended)
flag :: 1 byte flags
       bit order: 76543210 (bit 7 is MSB)
       bit 0: 1 to request immediate ack
       bit 1: 1 for explicit congestion notification (ECN)
       bits 7-2: Unused, set to 0 for future compatibility

Padding

This is for padding inside AEAD payloads. Padding for all messages are inside AEAD payloads.

Padding should roughly adhere to the negotiated parameters. Bob sent his requested tx/rx min/max parameters in Session Created. Alice sent her requested tx/rx min/max parameters in Session Confirmed. Updated options may be sent during the data phase. See options block information above.

If present, this must be the last block in the payload.

+----+----+----+----+----+----+----+----+
|254 |  size   |      padding           |
+----+----+----+                        +
|                                       |
~               .   .   .               ~
|                                       |
+----+----+----+----+----+----+----+----+

blk :: 254
size :: 2 bytes, big endian, size of padding to follow
padding :: random data

Notes:

• Size = 0 is allowed.
• Padding strategies TBD.
• Minimum padding TBD.
• Padding-only payloads are allowed.
• Padding defaults TBD.
• See options block for padding parameter negotiation
• See options block for min/max padding parameters
• Do not exceed the MTU. If more padding is necessary, send multiple messages.
• Router response on violation of negotiated padding is implementation-dependent.
• The padding length is either to be decided on a per-message basis and estimates of the length distribution, or random delays should be added. These countermeasures are to be included to resist DPI, as message sizes would otherwise reveal that I2P traffic is being carried by the transport protocol. The exact padding scheme is an area of future work, Appendix A of [NTCP2] provides more information on the topic.

Peer Address Spoofing

A peer may spoofing its source address to cause an endpoint to send excessive amounts of data to an unwilling host. If the endpoint sends significantly more data than the spoofing peer, connection migration might be used to amplify the volume of data that an attacker can generate toward a victim.

On-Path Address Spoofing

An on-path attacker could cause a spurious connection migration by copying and forwarding a packet with a spoofed address such that it arrives before the original packet. The packet with the spoofed address will be seen to come from a migrating connection, and the original packet will be seen as a duplicate and dropped. After a spurious migration, validation of the source address will fail because the entity at the source address does not have the necessary cryptographic keys to read or respond to the Path Challenge that is sent to it even if it wanted to.

Off-Path Packet Forwarding

An off-path attacker that can observe packets might forward copies of genuine packets to endpoints. If the copied packet arrives before the genuine packet, this will appear as a NAT rebinding. Any genuine packet will be discarded as a duplicate. If the attacker is able to continue forwarding packets, it might be able to cause migration to a path via the attacker. This places the attacker on-path, giving it the ability to observe or drop all subsequent packets.

Privacy Implications

QUIC [RFC-9000] specified changing connection IDs when changing network paths. Using a stable connection ID on multiple network paths would allow a passive observer to correlate activity between those paths. An endpoint that moves between networks might not wish to have their activity correlated by any entity other than their peer. However, QUIC does not encrypt the connection IDs in the header. SSU2 does do that, so the privacy leak would require the passive observer to also have access to the network database to get the introduction key required to decrypt the connection ID. Even with the introduction key, this is not a strong attack, and we do not change connection IDs after migration in SSU2, as this would be a significant complication.

Introducer Selection

In SSU 1, introduction is relatively inexpensive, and Alice generally sends Relay Requests to all introducers. In SSU 2, introduction is more expensive, as a connection must first be established with an introducer. To minimize introduction latency and overhead, the recommended processing steps are as follows:

• Ignore any introducers that are expired based on the iexp value in the address
• If an SSU2 connection is already established to one or more introducers, pick one and send the Relay Request to that introducer only.
• Otherwise, if a Router Info is locally known for one or more introducers, pick one and connect to that introducer only.
• Otherwise, lookup the Router Infos for all introducers, connect to the introducer whose Router Info is received first.

Response Handling

In both SSU 1 and SSU 2, the Relay Response and Hole Punch may be received in either order, or may not be received at all.

In SSU 1, Alice usually receives the Relay Response (1 RTT) before the Hole Punch (1 1/2 RTT). It may not be well-documented in those specifications, but Alice must receive the Relay Response from Bob before continuing, to receive Charlie's IP. If the Hole Punch is received first, Alice will not recognize it, because it contains no data and the source IP is not recognized. After receiving the Relay Response, Alice should wait for EITHER receiving the Hole Punch from Charlie, OR a short delay (recommended 500 ms) before initiating the handshake with Charlie.

In SSU 2, Alice will usually receive the Hole Punch (1 1/2 RTT) before the Relay Response (2 RTT). The SSU 2 Hole Punch is easier to process than in SSU 1, because it is a full message with defined connection IDs (derived from the relay nonce) and contents including Charlie's IP. The Relay Response (Data message) and Hole Punch message contain the identical signed Relay Response block. Therefore, Alice may initiate the handshake with Charlie after EITHER receiving the Hole Punch from Charlie, OR receiving the Relay Response from Bob.

The signature verification of the Hole Punch includes the introducer's (Bob's) router hash. If Relay Requests have been sent to more than one introducer, there are several options to validate the signature:

• Try each hash to which a request was sent
• Use different nonces for each introducer, and use that to determine which introducer this Hole Punch was in response to
• Don't re-validate the signature if the contents are identical to that in the Relay Response, if already received
• Don't validate the signature at all

If Charlie is behind a symmetric NAT, his reported port in the Relay Response and Hole Punch may not be accurate. Therefore, Alice should check the UDP source port of the Hole Punch message, and use that if it is different than the reported port.

Introducers

When published as SSU or SSU2 with introducers, the following options are present:

• ih[0-2]=(Base64 hash) A router hash for an introducer. Base 64 encoded using the standard I2P Base 64 alphabet. 32 bytes in binary, 44 bytes as Base 64 encoded
• iexp[0-2]: Expiration of this introducer. Unchanged from SSU 1.
• itag[0-2]: Introducer's tag 1 - (2**32 - 1) Unchanged from SSU 1.

The following options are for SSU only and are not used for SSU2. In SSU2, Alice gets this information from Charlie's RI instead.

• ihost[0-2]
• ikey[0-2]
• itag[0-2]

A router must not publish host or port in the address when publishing introducers. A router must publish 4 and/or 6 caps in the address when publishing introducers to indicate support for IPv4 and/or IPv6. This is the same as the current practice for recent SSU 1 addresses.

Note: If published as SSU, and there is a mix of SSU 1 and SSU2 introducers, the SSU 1 introducers should be at the lower indexes and the SSU2 introducers should be at the higher indexes, for compatibility with older routers.

Identity Hiding

Deniability is not a goal. See overview above.

Each pattern is assigned properties describing the confidentiality supplied to the initiator's static public key, and to the responder's static public key. The underlying assumptions are that ephemeral private keys are secure, and that parties abort the handshake if they receive a static public key from the other party which they don't trust.

This section only considers identity leakage through static public key fields in handshakes. Of course, the identities of Noise participants might be exposed through other means, including payload fields, traffic analysis, or metadata such as IP addresses.

Alice: (8) Encrypted with forward secrecy to an authenticated party.

Bob: (3) Not transmitted, but a passive attacker can check candidates for the responder's private key and determine whether the candidate is correct.

Bob publishes his static public key in the netdb. Alice may not, but must include it in the RI sent to Bob.

Summary

Initial processing of all inbound messages:

• Decrypt the first 8 bytes of the header (the Destination Connection ID) with the intro key
• Lookup the connection by the Destination Connection ID
• If the connection is found and is in the data phase, go to the data phase section
• If the connection is not found, go to the handshake section
• Note: Peer Test and Hole Punch messages may also be looked up by the Destination Connection ID created from the test or relay nonce.

Handshake messages (Session Request/Created/Confirmed, Retry, Token Request) and other out-of-session messages (Peer Test, Hole Punch) processing:

• Decrypt bytes 8-15 of the header (the packet type, version, and net ID) with the intro key. If it is a valid Session Request, Token Request, Peer Test, or Hole Punch, continue
• If not a valid message, lookup a pending outbound connection by the packet source IP/port, treat the packet as a Session Created or Retry. Re-decrypt the first 8 bytes of the header with the correct key, and the bytes 8-15 of the header (the packet type, version, and net ID). If it is a valid Session Created or Retry, continue
• If not a valid message, fail, or queue as a possible out-of-order data phase packet
• For Session Request/Created, Retry, Token Request, Peer Test, and Hole Punch, decrypt bytes 16-31 of the header
• For Session Request/Created, decrypt the ephemeral key
• Validate all header fields, stop if not valid
• mixHash() the header
• For Session Request/Created/Confirmed, decrypt the payload using Noise
• For Retry and data phase, decrypt the payload using ChaChaPoly
• Process the header and payload

Data phase messages processing:

• Decrypt bytes 8-15 of the header (the packet type, version, and net ID) with the correct key
• Decrypt the payload using ChaChaPoly using the header as AD
• Process the header and payload

Details

In SSU 1, inbound packet classification is difficult, because there is no header to indicate session number. Routers must first match the source IP and port to an existing peer state, and if not found, attempt multiple decryptions with different keys to find the appropriate peer state or start a new one. In the event that the source IP or port for an existing session changes, possibly due to NAT behavior, the router may use expensive heuristics to attempt to match the packet to an existing session and recover the contents.

SSU 2 is designed to minimize the inbound packet classification effort while maintaining DPI resistance and other on-path threats. The Connection ID number is included in the header for all message types, and encrypted (obfuscated) using ChaCha20 with a known key and nonce. Additionally, the message type is also included in the header (encrypted with header protection to a known key and then obfuscated with ChaCha20) and may be used for additional classification. In no case is a trial DH or other asymmetric crypto operation necessary to classify a packet.

For almost all messages from all peers, the ChaCha20 key for the Connection ID encryption is the destination router's introduction key as published in the netdb.

The only exceptions are the first messages sent from Bob to Alice (Session Created or Retry) where Alice's introduction key is not yet known to Bob. In these cases, Bob's introduction key is used as the key.

The protocol is designed to minimize packet classification processing that might require additional crypto operations in multiple fallback steps or complex heuristics. Additionally, the vast majority of received packets will not require a (possibly expensive) fallback lookup by source IP/port and a second header decryption. Only Session Created and Retry (and possibly others TBD) will require the fallback processing. If an endpoint changes IP or port after session creation, the connection ID is still used to lookup the session. It is never necessary to use heuristics to find the session, for example by looking for a different session with the same IP but a different port.

Therefore, the recommended processing steps in the receiver loop logic are:

1. Decrypt the first 8 bytes with ChaCha20 using the local introduction key, to recover the Destination Connection ID. If the Connection ID matches a current or pending inbound session:

   1. Using the appropriate key, decrypt the header bytes 8-15 to recover the version, net ID, and message type.
   2. If the message type is Session Confirmed, it is a long header. Verify the net ID and protocol version are valid. Decrypt the bytes 15-31 of the header with ChaCha20 using the local intro key. Then MixHash() the decrypted 32 byte header and decrypt the message with Noise.
   3. If the message type is valid but not Session Confirmed, it is a short header. Verify the net ID and protocol version are valid. decrypt the rest of the message with ChaCha20/Poly1305 using the session key, using the decrypted 16-byte header as the AD.
   4. (optional) If connection ID is a pending inbound session awaiting a Session Confirmed message, but the net ID, protocol, or message type is not valid, it could be a Data message received out-of-order before the Session Confirmed, so the data phase header protection keys are not yet known, and the header bytes 8-15 were incorrectly decrypted. Queue the message, and attempt to decrypt it once the Session Confirmed message is received.
   5. If b) or c) fails, drop the message.

2. If the connection ID does not match a current session: Check the plaintext header at bytes 8-15 are valid (without doing any header protection operation). Verify the net ID and protocol version are valid, and the message type is Session Request, or other message type allowed out-of-session (TBD).

   1. If all is valid and the message type is Session Request, decrypt bytes 16-31 of the header and the 32-byte X value with ChaCha20 using the local intro key.
   • If the token at header bytes 24-31 is accepted, then MixHash() the decrypted 32 byte header and decrypt the message with Noise. Send a Session Created in response.
   • If the token is not accepted, send a Retry message to the source IP/port with a token. Do not attempt to decrypt the message with Noise to avoid DDoS attacks.
   2. If the message type is some other message that is valid out-of-session, presumably with a short header, decrypt the rest of the message with ChaCha20/Poly1305 using the intro key, and using the decrypted 16-byte header as the AD. Process the message.
   3. If a) or b) fails, go to step 3)

3. Look up a pending outbound session by the source IP/port of the packet.

   1. If found, re-decrypt the first 8 bytes with ChaCha20 using Bob's introduction key to recover the Destination Connection ID.
   2. If the connection ID matches the pending session: Using the correct key, decrypt bytes 8-15 of the header to recover the version, net ID, and message type. Verify the net ID and protocol version are valid, and the message type is Session Created or Retry, or other message type allowed out-of-session (TBD).
   • If all is valid and the message type is Session Created, decrypt the next 16 bytes of the header and the 32-byte Y value with ChaCha20 using Bob's intro key. Then MixHash() the decrypted 32 byte header and decrypt the message with Noise. Send a Session Confirmed in response.
   • If all is valid and the message type is Retry, decrypt bytes 16-31 of the header with ChaCha20 using Bob's intro key. Decrypt and validate the message using ChaCha20/Poly1305 using TBD as the key and TBD as the nonce and the decrypted 32-byte header as the AD. Resend a Session Request with the received token in response.
   • If the message type is some other message that is valid out-of-session, presumably with a short header, decrypt the rest of the message with ChaCha20/Poly1305 using the intro key, and using the decrypted 16-byte header as the AD. Process the message.

   3. If a pending outbound session is not found, or the connection ID does not match the pending session, drop the message, unless the port is shared with SSU 1.

4. If running SSU 1 on the same port, attempt to process the message as an SSU 1 packet.

Error Handling

In general, a session (in the handshake or data phase) should never be destroyed after receiving a packet with an unexpected message type. This prevents packet injection attacks. These packets will also commonly be received after retransmission of a handshake packet, when the header decryption keys are no longer valid.

In most cases, simply drop the packet. An implementation may, but is not required to, retransmit the previously-sent packet (handshake message or ACK 0) in response.

After sending Session Created as Bob, unexpected packets are commonly Data packets that cannot be decrypted because the Session Confirmed packets were lost or out-of-order. Queue the packets and attempt to decrypt them after receiving the Session Confirmed packets.

After receiving Session Confirmed as Bob, unexpected packets are commonly retransmitted Session Confirmed packets, because the ACK 0 of the Sesssion Confirmed was lost. The unexpected packets may be dropped. An implementation may, but is not required to, send a Data packet containing an ACK block in response.