Source document:

https://mixmin.net/yamn_packet.html

Yamn (Yet Another Mix Network) is a very close relative to Mixmaster.

The keyrings, stats and payload format are similar enough that the
Echolot Pinger can be used with only minor modification to some Regular
Expressions.

Payload Format

The term payload describes the entire Yamn payload at it's highest
level. The Headers content is described in greater detail in subsequent
sections. During client-side message construction, all headers
(excluding the first one) and the Body are wrapped in multiple layers of
encryption. These layers are stripped, one-by-one at each hop until the
plain content is exposed for delivery to the actual recipient.

Each of the 10 headers consists of 256 Bytes split into 4 fields. The
first 3 fields contain instructions on how to decrypt the content of the
4th field, the Sealed Header.The client generates a NaCl key pair for
each hop in the chain. The SK is used to encrypt the Sealed Header and
is then discarded. The PK is transmitted within the header. The KeyID
informs the recipient Remailer of what SK to use for decryption.

NaCl Sealed Header Format

The Sealed Header contains sensitive content, such as how to decrypt
other payload components and further delivery instructions.

Packet Info - Intermediate Hop

Sealed headers that are specific to Intermediate type hops.
IV's are constructed from 12 random bytes and a 4 Byte counter in
Little-Endian format. The top-most header uses sequence 0, the
bottom-most header uses sequence 8 and the Body, sequence 9. The format
is: RRRRNNNNRRRRRRRR.