diff --git a/crates/tor-cell/src/relaycell.rs b/crates/tor-cell/src/relaycell.rs
index a432bb209..d93e8bdf3 100644
--- a/crates/tor-cell/src/relaycell.rs
+++ b/crates/tor-cell/src/relaycell.rs
@@ -10,9 +10,9 @@ use caret::caret_int;
 use rand::{CryptoRng, Rng};
 
 pub mod extend;
-pub mod msg;
 #[cfg(feature = "onion-service")]
-pub mod onion_service;
+pub mod hs;
+pub mod msg;
 pub mod restrict;
 #[cfg(feature = "experimental-udp")]
 pub mod udp;
diff --git a/crates/tor-cell/src/relaycell/hs.rs b/crates/tor-cell/src/relaycell/hs.rs
new file mode 100644
index 000000000..f05cf7a15
--- /dev/null
+++ b/crates/tor-cell/src/relaycell/hs.rs
@@ -0,0 +1,244 @@
+//! Encoding and decoding for relay messages related to onion services.
+
+#![allow(dead_code, unused_variables)] // TODO hs: remove.
+
+// TODO hs: we'll need accessors for the useful fields in all these types.
+
+use super::msg::{self, Body};
+use caret::caret_int;
+use tor_bytes::{EncodeError, EncodeResult, Error as BytesError, Result};
+use tor_bytes::{Reader, Writer};
+use tor_hscrypto::RendCookie;
+use tor_llcrypto::pk::rsa::RsaIdentity;
+
+pub mod est_intro;
+mod ext;
+
+pub use ext::UnrecognizedExt;
+
+caret_int! {
+    /// The type of the introduction point auth key
+    pub struct AuthKeyType(u8) {
+        /// Ed25519; SHA3-256
+        ED25519_SHA3_256 = 2,
+    }
+}
+
+/// A message sent from client to rendezvous point.
+#[derive(Debug, Clone)]
+pub struct EstablishRendezvous {
+    /// A rendezvous cookie is an arbitrary 20-byte value,
+    /// chosen randomly by the client.
+    cookie: [u8; EstablishRendezvous::COOKIE_LEN], // TODO hs: Make this a RendCookie.
+}
+impl EstablishRendezvous {
+    /// The only acceptable length of a rendezvous cookie.
+    pub const COOKIE_LEN: usize = 20;
+
+    /// Construct a new establish rendezvous cell.
+    pub fn new(cookie: [u8; Self::COOKIE_LEN]) -> Self {
+        Self { cookie }
+    }
+}
+impl msg::Body for EstablishRendezvous {
+    fn decode_from_reader(r: &mut Reader<'_>) -> Result<Self> {
+        let cookie = r.extract()?;
+        r.take_rest();
+        Ok(Self { cookie })
+    }
+    fn encode_onto<W: Writer + ?Sized>(self, w: &mut W) -> EncodeResult<()> {
+        w.write(&self.cookie)
+    }
+}
+
+#[derive(Debug, Clone)]
+/// A message sent from client to introduction point.
+pub struct Introduce1(Introduce);
+
+impl msg::Body for Introduce1 {
+    fn decode_from_reader(r: &mut Reader<'_>) -> Result<Self> {
+        Ok(Self(Introduce::decode_from_reader(r)?))
+    }
+    fn encode_onto<W: Writer + ?Sized>(self, w: &mut W) -> EncodeResult<()> {
+        self.0.encode_onto(w)
+    }
+}
+
+impl Introduce1 {
+    /// All arguments constructor
+    pub fn new(auth_key_type: AuthKeyType, auth_key: Vec<u8>, encrypted: Vec<u8>) -> Self {
+        Self(Introduce::new(auth_key_type, auth_key, encrypted))
+    }
+}
+
+#[derive(Debug, Clone)]
+/// A message sent from introduction point to hidden service host.
+pub struct Introduce2(Introduce);
+
+impl msg::Body for Introduce2 {
+    fn decode_from_reader(r: &mut Reader<'_>) -> Result<Self> {
+        Ok(Self(Introduce::decode_from_reader(r)?))
+    }
+    fn encode_onto<W: Writer + ?Sized>(self, w: &mut W) -> EncodeResult<()> {
+        self.0.encode_onto(w)
+    }
+}
+
+impl Introduce2 {
+    /// All arguments constructor
+    pub fn new(auth_key_type: AuthKeyType, auth_key: Vec<u8>, encrypted: Vec<u8>) -> Self {
+        Self(Introduce::new(auth_key_type, auth_key, encrypted))
+    }
+}
+
+#[derive(Debug, Clone)]
+/// A message body shared by Introduce1 and Introduce2
+struct Introduce {
+    /// Introduction point auth key type and the type of
+    /// the MAC used in `handshake_auth`.
+    auth_key_type: AuthKeyType,
+    /// The public introduction point auth key.
+    auth_key: Vec<u8>,
+    /// Up to end of relay payload.
+    encrypted: Vec<u8>,
+}
+
+impl Introduce {
+    /// All arguments constructor
+    fn new(auth_key_type: AuthKeyType, auth_key: Vec<u8>, encrypted: Vec<u8>) -> Self {
+        Self {
+            auth_key_type,
+            auth_key,
+            encrypted,
+        }
+    }
+    /// Decode an Introduce message body from the given reader
+    fn decode_from_reader(r: &mut Reader<'_>) -> Result<Self> {
+        let legacy_key_id: RsaIdentity = r.extract()?;
+        if !legacy_key_id.is_zero() {
+            return Err(BytesError::InvalidMessage(
+                "legacy key id in Introduce1.".into(),
+            ));
+        }
+        let auth_key_type = r.take_u8()?.into();
+        let auth_key_len = r.take_u16()?;
+        let auth_key = r.take(auth_key_len as usize)?.into();
+        let n_ext = r.take_u8()?;
+        for _ in 0..n_ext {
+            let _ext_type = r.take_u8()?;
+            r.read_nested_u8len(|r| {
+                r.take_rest();
+                Ok(())
+            })?;
+        }
+        let encrypted = r.take_rest().into();
+        Ok(Self {
+            auth_key_type,
+            auth_key,
+            encrypted,
+        })
+    }
+    /// Encode an Introduce message body onto the given writer
+    fn encode_onto<W: Writer + ?Sized>(self, w: &mut W) -> EncodeResult<()> {
+        w.write_all(&[0_u8; 20]);
+        w.write_u8(self.auth_key_type.get());
+        w.write_u16(u16::try_from(self.auth_key.len()).map_err(|_| EncodeError::BadLengthValue)?);
+        w.write_all(&self.auth_key[..]);
+        // No Introduce1 extension for now.
+        w.write_u8(0_u8);
+        w.write_all(&self.encrypted[..]);
+        Ok(())
+    }
+}
+
+/// A message sent from an onion service to a rendezvous point, telling it to
+/// make a connection to the client.
+#[derive(Debug, Clone)]
+pub struct Rendezvous1 {
+    /// The cookie originally sent by the client in its ESTABLISH_REND message.
+    cookie: RendCookie,
+    /// The message to send the client.
+    message: Vec<u8>,
+}
+
+impl Body for Rendezvous1 {
+    fn decode_from_reader(r: &mut Reader<'_>) -> Result<Self> {
+        todo!()
+    }
+
+    fn encode_onto<W: Writer + ?Sized>(self, w: &mut W) -> EncodeResult<()> {
+        todo!()
+    }
+}
+
+/// A message sent from the rendezvous point to the client, telling it about the
+/// onion service's message.
+#[derive(Debug, Clone)]
+pub struct Rendezvous2 {
+    /// The message from the onion service.
+    message: Vec<u8>,
+}
+
+impl Body for Rendezvous2 {
+    fn decode_from_reader(r: &mut Reader<'_>) -> Result<Self> {
+        todo!()
+    }
+
+    fn encode_onto<W: Writer + ?Sized>(self, w: &mut W) -> EncodeResult<()> {
+        todo!()
+    }
+}
+
+/// Reply sent from the introduction point to the onion service, telling it that
+/// an introduction point is now established.
+#[derive(Debug, Clone)]
+pub struct IntroEstablished {
+    /// The extensions included in this cell.
+    //
+    // TODO hs: we should extract this with any DOS related extensions, depending on what we
+    // decide to do with extension in general.
+    extensions: Vec<IntroEstExtension>,
+}
+
+impl Body for IntroEstablished {
+    fn decode_from_reader(r: &mut Reader<'_>) -> Result<Self> {
+        todo!()
+    }
+
+    fn encode_onto<W: Writer + ?Sized>(self, w: &mut W) -> EncodeResult<()> {
+        todo!()
+    }
+}
+
+/// An extension included in an [`IntroEstablished`] message.
+#[derive(Debug, Clone)]
+#[non_exhaustive]
+pub enum IntroEstExtension {
+    /// An unrecognized extension.
+    Unrecognized(Vec<u8>),
+}
+
+/// A reply from the introduction point to the client, telling it that its
+/// introduce1 was received.
+#[derive(Clone, Debug)]
+pub struct IntroduceAck {
+    // TODO hs: use a caret enum for this.
+    /// The status reported for the Introduce1 message.
+    status_code: u16,
+    // TODO hs: add extensions.
+}
+
+impl Body for IntroduceAck {
+    fn decode_from_reader(r: &mut Reader<'_>) -> Result<Self> {
+        todo!()
+    }
+
+    fn encode_onto<W: Writer + ?Sized>(self, w: &mut W) -> EncodeResult<()> {
+        todo!()
+    }
+}
+
+super::msg::empty_body! {
+    /// Acknowledges an EstablishRendezvous message.
+    pub struct RendezvousEstablished {}
+}
diff --git a/crates/tor-cell/src/relaycell/hs/est_intro.rs b/crates/tor-cell/src/relaycell/hs/est_intro.rs
new file mode 100644
index 000000000..2f9a0ee62
--- /dev/null
+++ b/crates/tor-cell/src/relaycell/hs/est_intro.rs
@@ -0,0 +1,203 @@
+//! Define the ESTABLISH_INTRO message and related types.
+
+use caret::caret_int;
+use tor_bytes::{EncodeError, EncodeResult, Readable, Reader, Result, Writeable, Writer};
+use tor_units::BoundedInt32;
+
+use crate::relaycell::{hs::ext::*, hs::AuthKeyType, msg};
+
+caret_int! {
+    /// The introduction protocol extension type
+    #[derive(Ord, PartialOrd)]
+    pub struct EstIntroExtType(u8) {
+        /// The extension used to send DoS parameters
+        DOS_PARAMS = 1,
+    }
+}
+
+caret_int! {
+    /// The recognized parameter types in an establish intro
+    /// DoS extension.
+    pub struct EstIntroExtDosParamType(u8) {
+        /// The rate per second of INTRODUCE2 cell relayed
+        /// to the service.
+        DOS_INTRODUCE2_RATE_PER_SEC = 1,
+        /// The burst per second of INTRODUCE2 cell relayed
+        /// to the service
+        DOS_INTRODUCE2_BURST_PER_SEC = 2,
+    }
+}
+
+/// An establish Introduction protocol extension
+#[derive(Debug, Clone)]
+pub struct DosParams {
+    /// An optional parameter indicates the rate per second of
+    /// INTRODUCE2 cell relayed to the service.
+    ///
+    /// Min: 0, Max: 2147483647
+    rate_per_sec: Option<BoundedInt32<0, { i32::MAX }>>,
+    /// An optional parameter indicates the burst per second of
+    /// INTRODUCE2 cell relayed to the service
+    ///
+    /// Min: 0, Max: 2147483647
+    burst_per_sec: Option<BoundedInt32<0, { i32::MAX }>>,
+}
+
+impl DosParams {
+    /// Create a new establish intro DoS extension.
+    pub fn new(rate_per_sec: Option<i32>, burst_per_sec: Option<i32>) -> crate::Result<Self> {
+        let normalize = |supplied: Option<i32>| -> crate::Result<_> {
+            supplied
+                .map(|val| {
+                    BoundedInt32::checked_new(val).map_err(|_| {
+                        crate::err::Error::CantEncode(
+                            "EST_INTRO_DOS_EXT parameter value out of bound.",
+                        )
+                    })
+                })
+                .transpose()
+        };
+        Ok(Self {
+            rate_per_sec: normalize(rate_per_sec)?,
+            burst_per_sec: normalize(burst_per_sec)?,
+        })
+    }
+}
+
+impl Ext for DosParams {
+    type Id = EstIntroExtType;
+    fn type_id(&self) -> EstIntroExtType {
+        EstIntroExtType::DOS_PARAMS
+    }
+    fn take_body_from(b: &mut Reader<'_>) -> Result<Self> {
+        let n_prams = b.take_u8()?;
+        let mut rate_per_sec = None;
+        let mut burst_per_sec = None;
+        for _i in 0..n_prams {
+            let param_to_store = match b.take_u8()?.into() {
+                EstIntroExtDosParamType::DOS_INTRODUCE2_RATE_PER_SEC => Some(&mut rate_per_sec),
+                EstIntroExtDosParamType::DOS_INTRODUCE2_BURST_PER_SEC => Some(&mut burst_per_sec),
+                _ => None,
+            };
+            if let Some(param) = param_to_store {
+                if let Ok(rate) = i32::try_from(b.take_u64()?) {
+                    *param = BoundedInt32::checked_new(rate).ok();
+                }
+            }
+        }
+        Ok(Self {
+            rate_per_sec,
+            burst_per_sec,
+        })
+    }
+    fn write_body_onto<B: Writer + ?Sized>(&self, b: &mut B) -> EncodeResult<()> {
+        let mut params = vec![];
+        let mut push_params = |ty, value| {
+            if let Some(value) = value {
+                params.push((ty, value));
+            }
+        };
+        push_params(
+            EstIntroExtDosParamType::DOS_INTRODUCE2_RATE_PER_SEC,
+            self.rate_per_sec,
+        );
+        push_params(
+            EstIntroExtDosParamType::DOS_INTRODUCE2_BURST_PER_SEC,
+            self.burst_per_sec,
+        );
+        b.write_u8(u8::try_from(params.len()).map_err(|_| EncodeError::BadLengthValue)?);
+        for (t, v) in params {
+            b.write_u8(t.get());
+            b.write_u64(v.get() as u64);
+        }
+        Ok(())
+    }
+}
+
+decl_extension_group! {
+    /// An extension to an EstablishIntro cell.
+    #[derive(Debug,Clone)]
+    enum EstablishIntroExt [ EstIntroExtType ] {
+        DosParams,
+    }
+}
+
+/// A hidden services establishes a new introduction point,
+/// by sending an EstablishIntro message.
+#[derive(Debug, Clone)]
+pub struct EstablishIntro {
+    /// Introduction point auth key type and the type of
+    /// the MAC used in `handshake_auth`.
+    auth_key_type: AuthKeyType,
+    /// The public introduction point auth key.
+    auth_key: Vec<u8>,
+    /// A list of extensions on this cell.
+    extensions: ExtList<EstablishIntroExt>,
+    /// the MAC of all earlier fields in the cell.
+    handshake_auth: [u8; 32],
+    /// A signature using `auth_key` of all contents
+    /// of the cell.
+    sig: Vec<u8>,
+}
+
+impl msg::Body for EstablishIntro {
+    fn decode_from_reader(r: &mut Reader<'_>) -> Result<Self> {
+        let auth_key_type = r.take_u8()?.into();
+        let auth_key_len = r.take_u16()?;
+        let auth_key = r.take(auth_key_len as usize)?.into();
+        let extensions = r.extract()?;
+        let handshake_auth = r.extract()?;
+        let sig_len = r.take_u16()?;
+        let sig = r.take(sig_len as usize)?.into();
+        Ok(EstablishIntro {
+            auth_key_type,
+            auth_key,
+            extensions,
+            handshake_auth,
+            sig,
+        })
+    }
+    fn encode_onto<W: Writer + ?Sized>(self, w: &mut W) -> EncodeResult<()> {
+        w.write_u8(self.auth_key_type.get());
+        w.write_u16(u16::try_from(self.auth_key.len()).map_err(|_| EncodeError::BadLengthValue)?);
+        w.write_all(&self.auth_key[..]);
+        w.write(&self.extensions)?;
+        w.write_all(&self.handshake_auth[..]);
+        w.write_u16(u16::try_from(self.sig.len()).map_err(|_| EncodeError::BadLengthValue)?);
+        w.write_all(&self.sig[..]);
+        Ok(())
+    }
+}
+
+impl EstablishIntro {
+    /// All arguments constructor
+    pub fn new(
+        auth_key_type: AuthKeyType,
+        auth_key: Vec<u8>,
+        handshake_auth: [u8; 32],
+        sig: Vec<u8>,
+    ) -> Self {
+        Self {
+            auth_key_type,
+            auth_key,
+            handshake_auth,
+            sig,
+            extensions: Default::default(),
+        }
+    }
+
+    /// Set EST_INTRO_DOS_EXT with given `extension_dos`.
+    pub fn set_extension_dos(&mut self, extension_dos: DosParams) {
+        self.extensions.replace_by_type(extension_dos.into());
+    }
+
+    /// Add an extension of some other type.
+    pub fn set_extension_other(&mut self, other: UnrecognizedExt<EstIntroExtType>) {
+        self.extensions.replace_by_type(other.into());
+    }
+
+    // TODO hs: we'll need accessors.
+    //
+    // TODO hs: we will need some way to ensure that the mac is valid and well-signed.  Possibly
+    // we should look into using a SignatureGated (if we can reasonably do so?)
+}
diff --git a/crates/tor-cell/src/relaycell/hs/ext.rs b/crates/tor-cell/src/relaycell/hs/ext.rs
new file mode 100644
index 000000000..e2fcdd1c6
--- /dev/null
+++ b/crates/tor-cell/src/relaycell/hs/ext.rs
@@ -0,0 +1,219 @@
+//! Helpers to manage lists of HS cell extensions.
+//
+// TODO: We might generalize this even more in the future to handle other
+// similar lists in our cell protocol.
+
+use tor_bytes::{EncodeError, EncodeResult, Readable, Reader, Result, Writeable, Writer};
+
+/// A list of extensions, represented in a common format used by many HS-related
+/// message.
+///
+/// The common format is:
+/// ```text
+///      N_EXTENSIONS     [1 byte]
+///      N_EXTENSIONS times:
+///           EXT_FIELD_TYPE [1 byte]
+///           EXT_FIELD_LEN  [1 byte]
+///           EXT_FIELD      [EXT_FIELD_LEN bytes]
+/// ```
+///
+/// It is subject to the additional restraints:
+///
+/// * Each extension type SHOULD be sent only once in a message.
+/// * Parties MUST ignore any occurrences all occurrences of an extension
+///   with a given type after the first such occurrence.
+/// * Extensions SHOULD be sent in numerically ascending order by type.
+#[derive(Clone, Debug, derive_more::Deref, derive_more::DerefMut)]
+pub(super) struct ExtList<T> {
+    /// The extensions themselves.
+    extensions: Vec<T>,
+}
+impl<T> Default for ExtList<T> {
+    fn default() -> Self {
+        Self {
+            extensions: Vec::new(),
+        }
+    }
+}
+/// An kind of extension that can be used with some kind of HS-related message.
+///
+/// Each extendible message will likely define its own enum,
+/// implementing this trait,
+/// representing the possible extensions.
+pub(super) trait ExtGroup: Readable + Writeable {
+    /// An identifier kind used with this sort of extension
+    type Id: From<u8> + Into<u8> + Eq + PartialEq + Ord + Copy;
+    /// The field-type id for this particular extension.
+    fn type_id(&self) -> Self::Id;
+}
+/// A single typed extension that can be used with some kind of HS-related message.
+pub(super) trait Ext: Sized {
+    /// An identifier kind used with this sort of extension.
+    ///
+    /// Typically defined with caret_int.
+    type Id: From<u8> + Into<u8>;
+    /// The field-type id for this particular extension.
+    fn type_id(&self) -> Self::Id;
+    /// Extract the body (not the type or the length) from a single
+    /// extension.
+    fn take_body_from(b: &mut Reader<'_>) -> Result<Self>;
+    /// Write the body (not the type or the length) for a single extension.
+    fn write_body_onto<B: Writer + ?Sized>(&self, b: &mut B) -> EncodeResult<()>;
+}
+impl<T: ExtGroup> Readable for ExtList<T> {
+    fn take_from(b: &mut Reader<'_>) -> Result<Self> {
+        let n_extensions = b.take_u8()?;
+        let extensions: Result<Vec<T>> = (0..n_extensions).map(|_| b.extract::<T>()).collect();
+        Ok(Self {
+            extensions: extensions?,
+        })
+    }
+}
+impl<T: ExtGroup> Writeable for ExtList<T> {
+    fn write_onto<B: Writer + ?Sized>(&self, b: &mut B) -> EncodeResult<()> {
+        let n_extensions = self
+            .extensions
+            .len()
+            .try_into()
+            .map_err(|_| EncodeError::BadLengthValue)?;
+        b.write_u8(n_extensions);
+        let mut exts_sorted: Vec<&T> = self.extensions.iter().collect();
+        exts_sorted.sort_by_key(|ext| ext.type_id());
+        exts_sorted.iter().try_for_each(|ext| ext.write_onto(b))?;
+        Ok(())
+    }
+}
+impl<T: ExtGroup> ExtList<T> {
+    /// Insert `ext` into this list of extensions, replacing any previous
+    /// extension with the same field type ID.
+    pub(super) fn replace_by_type(&mut self, ext: T) {
+        self.retain(|e| e.type_id() != ext.type_id());
+        self.push(ext);
+    }
+
+    /// Find the first entry (if any) in this list of extensions with a given
+    /// type ID.
+    ///
+    /// (We do not enforce uniqueness here, since the spec explicitly says that
+    /// we should ignore all but the first extension of a given type.)
+    pub(super) fn by_type(&self, id: T::Id) -> Option<&T> {
+        self.iter().find(|e| e.type_id() == id)
+    }
+}
+
+/// An unrecognized extension for some HS-related message.
+#[derive(Clone, Debug)]
+pub struct UnrecognizedExt<ID> {
+    /// The field type ID for this extension.
+    pub(super) type_id: ID,
+    /// The body of this extension.
+    pub(super) body: Vec<u8>,
+}
+
+impl<ID> UnrecognizedExt<ID> {
+    /// Return a new unrecognized extension with a given ID and body.
+    ///
+    /// NOTE: nothing actually enforces that this type ID is not
+    /// recognized.
+    ///
+    /// NOTE: This function accepts bodies longer than 255 bytes, but
+    /// it is not possible to encode them.
+    pub fn new(type_id: ID, body: impl Into<Vec<u8>>) -> Self {
+        Self {
+            type_id,
+            body: body.into(),
+        }
+    }
+}
+
+/// Declare an Extension group that takes a given identifier.
+//
+// TODO: This is rather similar to restrict_msg(), isn't it?  Also, We use this
+// pattern of (number, (cmd, length, body)*) a few of times in Tor outside the
+// hs module.  Perhaps we can extend and unify our code here...
+macro_rules! decl_extension_group {
+    {
+        $( #[$meta:meta] )*
+        $v:vis enum $id:ident [ $type_id:ty ] {
+            $(
+                $(#[$cmeta:meta])*
+                $case:ident),*
+            $(,)?
+        }
+    } => {paste::paste!{
+        $( #[$meta] )*
+        $v enum $id {
+            $( $(#[$cmeta])*
+               $case($case),
+            )*
+            /// An extension of a type we do not recognize.
+            Unrecognized(UnrecognizedExt<$type_id>)
+        }
+        impl Readable for $id {
+            fn take_from(b: &mut Reader<'_>) -> Result<Self> {
+                let type_id = b.take_u8()?.into();
+                Ok(match type_id {
+                    $(
+                        $type_id::[< $case:snake:upper >] => {
+                            Self::$case( b.read_nested_u8len(|r| $case::take_body_from(r))? )
+                        }
+                    )*
+                    other => {
+                        Self::Unrecognized(UnrecognizedExt {
+                            type_id,
+                            body: b.read_nested_u8len(|r| Ok(r.take_rest().into()))?,
+                        })
+                    }
+                })
+            }
+        }
+        impl Writeable for $id {
+            fn write_onto<B: Writer + ?Sized>(&self, b: &mut B) -> EncodeResult<
+()> {
+                #[allow(unused)]
+                use std::ops::DerefMut;
+                match self {
+                    $(
+                        Self::$case(val) => {
+                            b.write_u8(val.type_id().into());
+                            let mut nested = b.write_nested_u8len();
+                            val.write_body_onto(nested.deref_mut())?;
+                            nested.finish()?;
+                        }
+                    )*
+                    Self::Unrecognized(unrecognized) => {
+                        b.write_u8(unrecognized.type_id.into());
+                        let mut nested = b.write_nested_u8len();
+                        nested.write_all(&unrecognized.body[..]);
+                        nested.finish()?;
+                    }
+                }
+                Ok(())
+            }
+        }
+        impl ExtGroup for $id {
+            type Id = $type_id;
+            fn type_id(&self) -> Self::Id {
+                match self {
+                    $(
+                        Self::$case(val) => val.type_id(),
+                    )*
+                    Self::Unrecognized(unrecognized) => unrecognized.type_id,
+                }
+            }
+        }
+        $(
+        impl From<$case> for $id {
+            fn from(val: $case) -> $id {
+                $id :: $case ( val )
+            }
+        }
+        )*
+        impl From<UnrecognizedExt<$type_id>> for $id {
+            fn from(val: UnrecognizedExt<$type_id>) -> $id {
+                $id :: Unrecognized(val)
+            }
+        }
+}}
+}
+pub(super) use decl_extension_group;
diff --git a/crates/tor-cell/src/relaycell/msg.rs b/crates/tor-cell/src/relaycell/msg.rs
index 86c561c59..ae9e14f65 100644
--- a/crates/tor-cell/src/relaycell/msg.rs
+++ b/crates/tor-cell/src/relaycell/msg.rs
@@ -19,9 +19,9 @@ use bitflags::bitflags;
 
 #[cfg_attr(docsrs, doc(cfg(feature = "onion-service")))]
 #[cfg(feature = "onion-service")]
-pub use super::onion_service::{
-    EstablishIntro, EstablishRendezvous, IntroEstablished, Introduce1, Introduce2, IntroduceAck,
-    Rendezvous1, Rendezvous2, RendezvousEstablished,
+pub use super::hs::{
+    est_intro::EstablishIntro, EstablishRendezvous, IntroEstablished, Introduce1, Introduce2,
+    IntroduceAck, Rendezvous1, Rendezvous2, RendezvousEstablished,
 };
 #[cfg_attr(docsrs, doc(cfg(feature = "experimental-udp")))]
 #[cfg(feature = "experimental-udp")]
diff --git a/crates/tor-cell/src/relaycell/onion_service.rs b/crates/tor-cell/src/relaycell/onion_service.rs
deleted file mode 100644
index da9a495b3..000000000
--- a/crates/tor-cell/src/relaycell/onion_service.rs
+++ /dev/null
@@ -1,459 +0,0 @@
-//! Encoding and decoding for relay messages related to onion services.
-
-#![allow(dead_code, unused_variables)] // TODO hs: remove.
-
-// TODO hs design: We need to discuss how we want to handle extensions in these
-// cells: extensions are used all over the protocol, and it would be nice to be
-// more clever about them.
-//
-// It would be neat if recognized extension types were decoded into recognized
-// structures. On the other hand, it would be good to retain unrecognized
-// extension types, so that other code  can use them in the future without having to
-// add support here.
-
-// TODO hs: we'll need accessors for the useful fields in all these types.
-
-use super::msg::{self, Body};
-use caret::caret_int;
-use tor_bytes::{EncodeError, EncodeResult, Error as BytesError, Readable, Result, Writeable};
-use tor_bytes::{Reader, Writer};
-use tor_hscrypto::RendCookie;
-use tor_llcrypto::pk::rsa::RsaIdentity;
-use tor_units::BoundedInt32;
-
-caret_int! {
-    /// The type of the introduction point auth key
-    pub struct AuthKeyType(u8) {
-        /// Ed25519; SHA3-256
-        ED25519_SHA3_256 = 2,
-    }
-}
-
-caret_int! {
-    /// The introduction protocol extension type
-    pub struct EstIntroExtType(u8) {
-        /// The extension used to send DoS parameters
-        EST_INTRO_DOS_EXT = 1,
-    }
-}
-
-caret_int! {
-    /// The recognized parameter types in an establish intro
-    /// DoS extension.
-    pub struct EstIntroExtDosParamType(u8) {
-        /// The rate per second of INTRODUCE2 cell relayed
-        /// to the service.
-        DOS_INTRODUCE2_RATE_PER_SEC = 1,
-        /// The burst per second of INTRODUCE2 cell relayed
-        /// to the service
-        DOS_INTRODUCE2_BURST_PER_SEC = 2,
-    }
-}
-
-/// An establish Introduction protocol extension
-#[derive(Debug, Clone)]
-pub struct EstIntroExtDoS {
-    /// An optional parameter indicates the rate per second of
-    /// INTRODUCE2 cell relayed to the service.
-    ///
-    /// Min: 0, Max: 2147483647
-    rate_per_sec: Option<BoundedInt32<0, { i32::MAX }>>,
-    /// An optional parameter indicates the burst per second of
-    /// INTRODUCE2 cell relayed to the service
-    ///
-    /// Min: 0, Max: 2147483647
-    burst_per_sec: Option<BoundedInt32<0, { i32::MAX }>>,
-}
-
-impl EstIntroExtDoS {
-    /// Create a new establish intro DoS extension.
-    pub fn new(rate_per_sec: Option<i32>, burst_per_sec: Option<i32>) -> crate::Result<Self> {
-        let normalize = |supplied: Option<i32>| -> crate::Result<_> {
-            supplied
-                .map(|val| {
-                    BoundedInt32::checked_new(val).map_err(|_| {
-                        crate::err::Error::CantEncode(
-                            "EST_INTRO_DOS_EXT parameter value out of bound.",
-                        )
-                    })
-                })
-                .transpose()
-        };
-        Ok(Self {
-            rate_per_sec: normalize(rate_per_sec)?,
-            burst_per_sec: normalize(burst_per_sec)?,
-        })
-    }
-}
-
-impl Readable for EstIntroExtDoS {
-    fn take_from(b: &mut Reader<'_>) -> Result<Self> {
-        let n_prams = b.take_u8()?;
-        let mut rate_per_sec = None;
-        let mut burst_per_sec = None;
-        for _i in 0..n_prams {
-            let param_to_store = match b.take_u8()?.into() {
-                EstIntroExtDosParamType::DOS_INTRODUCE2_RATE_PER_SEC => Some(&mut rate_per_sec),
-                EstIntroExtDosParamType::DOS_INTRODUCE2_BURST_PER_SEC => Some(&mut burst_per_sec),
-                _ => None,
-            };
-            if let Some(param) = param_to_store {
-                if let Ok(rate) = i32::try_from(b.take_u64()?) {
-                    *param = BoundedInt32::checked_new(rate).ok();
-                }
-            }
-        }
-        Ok(Self {
-            rate_per_sec,
-            burst_per_sec,
-        })
-    }
-}
-
-impl Writeable for EstIntroExtDoS {
-    fn write_onto<B: Writer + ?Sized>(&self, b: &mut B) -> EncodeResult<()> {
-        let mut params = vec![];
-        let mut push_params = |ty, value| {
-            if let Some(value) = value {
-                params.push((ty, value));
-            }
-        };
-        push_params(
-            EstIntroExtDosParamType::DOS_INTRODUCE2_RATE_PER_SEC,
-            self.rate_per_sec,
-        );
-        push_params(
-            EstIntroExtDosParamType::DOS_INTRODUCE2_BURST_PER_SEC,
-            self.burst_per_sec,
-        );
-        b.write_u8(u8::try_from(params.len()).map_err(|_| EncodeError::BadLengthValue)?);
-        for (t, v) in params {
-            b.write_u8(t.get());
-            b.write_u64(v.get() as u64);
-        }
-        Ok(())
-    }
-}
-
-/// A hidden services establishes a new introduction point,
-/// by sending an EstablishIntro message.
-#[derive(Debug, Clone)]
-pub struct EstablishIntro {
-    /// Introduction point auth key type and the type of
-    /// the MAC used in `handshake_auth`.
-    auth_key_type: AuthKeyType,
-    /// The public introduction point auth key.
-    auth_key: Vec<u8>,
-    /// An optional denial-of-service extension.
-    //
-    // TODO hs: we may want to consider making this a vector of extensions instead,
-    // to allow unrecognized extensions?
-    extension_dos: Option<EstIntroExtDoS>,
-    /// the MAC of all earlier fields in the cell.
-    handshake_auth: [u8; 32],
-    /// A signature using `auth_key` of all contents
-    /// of the cell.
-    sig: Vec<u8>,
-}
-
-impl msg::Body for EstablishIntro {
-    fn decode_from_reader(r: &mut Reader<'_>) -> Result<Self> {
-        let auth_key_type = r.take_u8()?.into();
-        let auth_key_len = r.take_u16()?;
-        let auth_key = r.take(auth_key_len as usize)?.into();
-        let n_ext = r.take_u8()?;
-        let mut extension_dos = None;
-        for _ in 0..n_ext {
-            let ext_type: EstIntroExtType = r.take_u8()?.into();
-            r.read_nested_u8len(|r| {
-                if ext_type == EstIntroExtType::EST_INTRO_DOS_EXT {
-                    extension_dos.get_or_insert(r.extract()?);
-                } else {
-                    r.take_rest();
-                }
-                Ok(())
-            })?;
-        }
-        let handshake_auth = r.extract()?;
-        let sig_len = r.take_u16()?;
-        let sig = r.take(sig_len as usize)?.into();
-        Ok(EstablishIntro {
-            auth_key_type,
-            auth_key,
-            extension_dos,
-            handshake_auth,
-            sig,
-        })
-    }
-    fn encode_onto<W: Writer + ?Sized>(self, w: &mut W) -> EncodeResult<()> {
-        w.write_u8(self.auth_key_type.get());
-        w.write_u16(u16::try_from(self.auth_key.len()).map_err(|_| EncodeError::BadLengthValue)?);
-        w.write_all(&self.auth_key[..]);
-
-        let mut extensions: Vec<(EstIntroExtType, Vec<u8>)> = vec![];
-        if let Some(extension_dos) = self.extension_dos {
-            let mut extension = vec![];
-            extension.write(&extension_dos)?;
-            extensions.push((EstIntroExtType::EST_INTRO_DOS_EXT, extension));
-        }
-        w.write_u8(u8::try_from(extensions.len()).map_err(|_| EncodeError::BadLengthValue)?);
-        for (t, v) in extensions {
-            w.write_u8(t.get());
-            let mut w = w.write_nested_u8len();
-            w.write(&v)?;
-            w.finish()?;
-        }
-
-        w.write_all(&self.handshake_auth[..]);
-        w.write_u16(u16::try_from(self.sig.len()).map_err(|_| EncodeError::BadLengthValue)?);
-        w.write_all(&self.sig[..]);
-        Ok(())
-    }
-}
-
-impl EstablishIntro {
-    /// All arguments constructor
-    pub fn new(
-        auth_key_type: AuthKeyType,
-        auth_key: Vec<u8>,
-        handshake_auth: [u8; 32],
-        sig: Vec<u8>,
-    ) -> Self {
-        Self {
-            auth_key_type,
-            auth_key,
-            handshake_auth,
-            sig,
-            extension_dos: None,
-        }
-    }
-
-    /// Set EST_INTRO_DOS_EXT with given `extension_dos`.
-    pub fn set_extension_dos(&mut self, extension_dos: EstIntroExtDoS) {
-        self.extension_dos = Some(extension_dos);
-    }
-
-    // TODO hs: we'll need accessors.
-    //
-    // TODO hs: we will need some way to ensure that the mac is valid and well-signed.  Possibly
-    // we should look into using a SignatureGated (if we can reasonably do so?)
-}
-
-/// A message sent from client to rendezvous point.
-#[derive(Debug, Clone)]
-pub struct EstablishRendezvous {
-    /// A rendezvous cookie is an arbitrary 20-byte value,
-    /// chosen randomly by the client.
-    cookie: [u8; EstablishRendezvous::COOKIE_LEN], // TODO hs: Make this a RendCookie.
-}
-impl EstablishRendezvous {
-    /// The only acceptable length of a rendezvous cookie.
-    pub const COOKIE_LEN: usize = 20;
-
-    /// Construct a new establish rendezvous cell.
-    pub fn new(cookie: [u8; Self::COOKIE_LEN]) -> Self {
-        Self { cookie }
-    }
-}
-impl msg::Body for EstablishRendezvous {
-    fn decode_from_reader(r: &mut Reader<'_>) -> Result<Self> {
-        let cookie = r.extract()?;
-        r.take_rest();
-        Ok(Self { cookie })
-    }
-    fn encode_onto<W: Writer + ?Sized>(self, w: &mut W) -> EncodeResult<()> {
-        w.write(&self.cookie)
-    }
-}
-
-#[derive(Debug, Clone)]
-/// A message sent from client to introduction point.
-pub struct Introduce1(Introduce);
-
-impl msg::Body for Introduce1 {
-    fn decode_from_reader(r: &mut Reader<'_>) -> Result<Self> {
-        Ok(Self(Introduce::decode_from_reader(r)?))
-    }
-    fn encode_onto<W: Writer + ?Sized>(self, w: &mut W) -> EncodeResult<()> {
-        self.0.encode_onto(w)
-    }
-}
-
-impl Introduce1 {
-    /// All arguments constructor
-    pub fn new(auth_key_type: AuthKeyType, auth_key: Vec<u8>, encrypted: Vec<u8>) -> Self {
-        Self(Introduce::new(auth_key_type, auth_key, encrypted))
-    }
-}
-
-#[derive(Debug, Clone)]
-/// A message sent from introduction point to hidden service host.
-pub struct Introduce2(Introduce);
-
-impl msg::Body for Introduce2 {
-    fn decode_from_reader(r: &mut Reader<'_>) -> Result<Self> {
-        Ok(Self(Introduce::decode_from_reader(r)?))
-    }
-    fn encode_onto<W: Writer + ?Sized>(self, w: &mut W) -> EncodeResult<()> {
-        self.0.encode_onto(w)
-    }
-}
-
-impl Introduce2 {
-    /// All arguments constructor
-    pub fn new(auth_key_type: AuthKeyType, auth_key: Vec<u8>, encrypted: Vec<u8>) -> Self {
-        Self(Introduce::new(auth_key_type, auth_key, encrypted))
-    }
-}
-
-#[derive(Debug, Clone)]
-/// A message body shared by Introduce1 and Introduce2
-struct Introduce {
-    /// Introduction point auth key type and the type of
-    /// the MAC used in `handshake_auth`.
-    auth_key_type: AuthKeyType,
-    /// The public introduction point auth key.
-    auth_key: Vec<u8>,
-    /// Up to end of relay payload.
-    encrypted: Vec<u8>,
-}
-
-impl Introduce {
-    /// All arguments constructor
-    fn new(auth_key_type: AuthKeyType, auth_key: Vec<u8>, encrypted: Vec<u8>) -> Self {
-        Self {
-            auth_key_type,
-            auth_key,
-            encrypted,
-        }
-    }
-    /// Decode an Introduce message body from the given reader
-    fn decode_from_reader(r: &mut Reader<'_>) -> Result<Self> {
-        let legacy_key_id: RsaIdentity = r.extract()?;
-        if !legacy_key_id.is_zero() {
-            return Err(BytesError::InvalidMessage(
-                "legacy key id in Introduce1.".into(),
-            ));
-        }
-        let auth_key_type = r.take_u8()?.into();
-        let auth_key_len = r.take_u16()?;
-        let auth_key = r.take(auth_key_len as usize)?.into();
-        let n_ext = r.take_u8()?;
-        for _ in 0..n_ext {
-            let _ext_type = r.take_u8()?;
-            r.read_nested_u8len(|r| {
-                r.take_rest();
-                Ok(())
-            })?;
-        }
-        let encrypted = r.take_rest().into();
-        Ok(Self {
-            auth_key_type,
-            auth_key,
-            encrypted,
-        })
-    }
-    /// Encode an Introduce message body onto the given writer
-    fn encode_onto<W: Writer + ?Sized>(self, w: &mut W) -> EncodeResult<()> {
-        w.write_all(&[0_u8; 20]);
-        w.write_u8(self.auth_key_type.get());
-        w.write_u16(u16::try_from(self.auth_key.len()).map_err(|_| EncodeError::BadLengthValue)?);
-        w.write_all(&self.auth_key[..]);
-        // No Introduce1 extension for now.
-        w.write_u8(0_u8);
-        w.write_all(&self.encrypted[..]);
-        Ok(())
-    }
-}
-
-/// A message sent from an onion service to a rendezvous point, telling it to
-/// make a connection to the client.
-#[derive(Debug, Clone)]
-pub struct Rendezvous1 {
-    /// The cookie originally sent by the client in its ESTABLISH_REND message.
-    cookie: RendCookie,
-    /// The message to send the client.
-    message: Vec<u8>,
-}
-
-impl Body for Rendezvous1 {
-    fn decode_from_reader(r: &mut Reader<'_>) -> Result<Self> {
-        todo!()
-    }
-
-    fn encode_onto<W: Writer + ?Sized>(self, w: &mut W) -> EncodeResult<()> {
-        todo!()
-    }
-}
-
-/// A message sent from the rendezvous point to the client, telling it about the
-/// onion service's message.
-#[derive(Debug, Clone)]
-pub struct Rendezvous2 {
-    /// The message from the onion service.
-    message: Vec<u8>,
-}
-
-impl Body for Rendezvous2 {
-    fn decode_from_reader(r: &mut Reader<'_>) -> Result<Self> {
-        todo!()
-    }
-
-    fn encode_onto<W: Writer + ?Sized>(self, w: &mut W) -> EncodeResult<()> {
-        todo!()
-    }
-}
-
-/// Reply sent from the introduction point to the onion service, telling it that
-/// an introduction point is now established.
-#[derive(Debug, Clone)]
-pub struct IntroEstablished {
-    /// The extensions included in this cell.
-    //
-    // TODO hs: we should extract this with any DOS related extensions, depending on what we
-    // decide to do with extension in general.
-    extensions: Vec<IntroEstExtension>,
-}
-
-impl Body for IntroEstablished {
-    fn decode_from_reader(r: &mut Reader<'_>) -> Result<Self> {
-        todo!()
-    }
-
-    fn encode_onto<W: Writer + ?Sized>(self, w: &mut W) -> EncodeResult<()> {
-        todo!()
-    }
-}
-
-/// An extension included in an [`IntroEstablished`] message.
-#[derive(Debug, Clone)]
-#[non_exhaustive]
-pub enum IntroEstExtension {
-    /// An unrecognized extension.
-    Unrecognized(Vec<u8>),
-}
-
-/// A reply from the introduction point to the client, telling it that its
-/// introduce1 was received.
-#[derive(Clone, Debug)]
-pub struct IntroduceAck {
-    // TODO hs: use a caret enum for this.
-    /// The status reported for the Introduce1 message.
-    status_code: u16,
-    // TODO hs: add extensions.
-}
-
-impl Body for IntroduceAck {
-    fn decode_from_reader(r: &mut Reader<'_>) -> Result<Self> {
-        todo!()
-    }
-
-    fn encode_onto<W: Writer + ?Sized>(self, w: &mut W) -> EncodeResult<()> {
-        todo!()
-    }
-}
-
-super::msg::empty_body! {
-    /// Acknowledges an EstablishRendezvous message.
-    pub struct RendezvousEstablished {}
-}
diff --git a/crates/tor-cell/tests/testvec_relaymsg.rs b/crates/tor-cell/tests/testvec_relaymsg.rs
index 4b5062e67..02bbe4101 100644
--- a/crates/tor-cell/tests/testvec_relaymsg.rs
+++ b/crates/tor-cell/tests/testvec_relaymsg.rs
@@ -15,7 +15,7 @@ use std::net::{IpAddr, Ipv4Addr, SocketAddr};
 use hex_literal::hex;
 
 #[cfg(feature = "onion-service")]
-use tor_cell::relaycell::onion_service;
+use tor_cell::relaycell::hs;
 #[cfg(feature = "experimental-udp")]
 use tor_cell::relaycell::udp;
 
@@ -631,7 +631,7 @@ fn test_establish_rendezvous() {
         cmd,
         // 20 ones
         "0101010101010101010101010101010101010101",
-        &onion_service::EstablishRendezvous::new(cookie).into(),
+        &hs::EstablishRendezvous::new(cookie).into(),
     );
 
     // Extra bytes are ignored
@@ -658,16 +658,13 @@ fn test_establish_rendezvous() {
 #[cfg(feature = "onion-service")]
 #[test]
 fn test_establish_intro() {
-    use tor_cell::relaycell::{
-        msg::AnyRelayMsg,
-        onion_service::{AuthKeyType, EstIntroExtDoS, EstablishIntro},
-    };
+    use tor_cell::relaycell::hs::{est_intro::*, AuthKeyType, UnrecognizedExt};
 
     let cmd = RelayCmd::ESTABLISH_INTRO;
     let auth_key_type = AuthKeyType::ED25519_SHA3_256;
     let auth_key = vec![0, 1, 2, 3];
-    let extension_dos = EstIntroExtDoS::new(Some(1_i32), Some(2_i32))
-        .expect("invalid EST_INTRO_DOS_EXT parameter(s)");
+    let extension_dos =
+        DosParams::new(Some(1_i32), Some(2_i32)).expect("invalid EST_INTRO_DOS_EXT parameter(s)");
     let handshake_auth = [1; 32];
     let sig = vec![0, 1, 2, 3];
     assert_eq!(Into::<u8>::into(cmd), 32);
@@ -700,34 +697,29 @@ fn test_establish_intro() {
     // and one unknown extension
     let auth_key = vec![0, 1, 2, 3];
     let sig = vec![0, 1, 2, 3];
-    let extension_dos = EstIntroExtDoS::new(Some(1_i32), Some(2_i32))
-        .expect("invalid EST_INTRO_DOS_EXT parameter(s)");
-
-    let body = "02 0004 00010203
-         02 01 13 02 01 0000000000000001 02 0000000000000002 02 01 00
-         0101010101010101010101010101010101010101010101010101010101010101
-         0004 00010203";
-    let actual_msg = decode(cmd, &unhex(body)[..]).unwrap();
-    let mut actual_bytes = vec![];
-    let mut expect_bytes = vec![];
-    actual_msg
-        .encode_onto(&mut actual_bytes)
-        .expect("Encode msg onto byte vector");
+    let extension_dos =
+        DosParams::new(Some(1_i32), Some(2_i32)).expect("invalid EST_INTRO_DOS_EXT parameter(s)");
+    let extension_unrecognized = UnrecognizedExt::new(2.into(), vec![0]);
     let mut es_intro = EstablishIntro::new(auth_key_type, auth_key, handshake_auth, sig);
     es_intro.set_extension_dos(extension_dos);
-    let expected_msg: AnyRelayMsg = es_intro.into();
-    expected_msg
-        .encode_onto(&mut expect_bytes)
-        .expect("Encode msg onto byte vector");
-    assert_eq!(actual_bytes, expect_bytes);
+    es_intro.set_extension_other(extension_unrecognized);
+
+    msg(
+        cmd,
+        "02 0004 00010203
+         02 01 13 02 01 0000000000000001 02 0000000000000002 02 01 00
+         0101010101010101010101010101010101010101010101010101010101010101
+         0004 00010203",
+        &es_intro.into(),
+    );
 }
 
 #[cfg(feature = "onion-service")]
 #[test]
 fn test_introduce() {
     use tor_cell::relaycell::{
+        hs::{AuthKeyType, Introduce1},
         msg::AnyRelayMsg,
-        onion_service::{AuthKeyType, Introduce1},
     };
 
     // Testing with Introduce1 only should be sufficient as long as