diff --git a/net/homa/homa_wire.h b/net/homa/homa_wire.h
new file mode 100644
index 000000000000..c0d4247ca196
--- /dev/null
+++ b/net/homa/homa_wire.h
@@ -0,0 +1,355 @@
+/* SPDX-License-Identifier: BSD-2-Clause */
+
+/* This file defines the on-the-wire format of Homa packets. */
+
+#ifndef _HOMA_WIRE_H
+#define _HOMA_WIRE_H
+
+#include <linux/skbuff.h>
+
+/**
+ * enum homa_packet_type - Defines the possible types of Homa packets.
+ *
+ * See the xxx_header structs below for more information about each type.
+ */
+enum homa_packet_type {
+	DATA               = 0x10,
+	RESEND             = 0x12,
+	UNKNOWN            = 0x13,
+	BUSY               = 0x14,
+	NEED_ACK           = 0x17,
+	ACK                = 0x18,
+	BOGUS              = 0x19,      /* Used only in unit tests. */
+	/* If you add a new type here, you must also do the following:
+	 * 1. Change BOGUS so it is the highest opcode
+	 * 2. Add support for the new opcode in homa_print_packet,
+	 *    homa_print_packet_short, homa_symbol_for_type, and mock_skb_new.
+	 * 3. Add the header length to header_lengths in homa_plumbing.c.
+	 */
+};
+
+/** define HOMA_IPV6_HEADER_LENGTH - Size of IP header (V6). */
+#define HOMA_IPV6_HEADER_LENGTH 40
+
+/** define HOMA_IPV4_HEADER_LENGTH - Size of IP header (V4). */
+#define HOMA_IPV4_HEADER_LENGTH 20
+
+/**
+ * define HOMA_SKB_EXTRA - How many bytes of additional space to allow at the
+ * beginning of each sk_buff, before the IP header. This includes room for a
+ * VLAN header and also includes some extra space, "just to be safe" (not
+ * really sure if this is needed).
+ */
+#define HOMA_SKB_EXTRA 40
+
+/**
+ * define HOMA_ETH_OVERHEAD - Number of bytes per Ethernet packet for Ethernet
+ * header, CRC, preamble, and inter-packet gap.
+ */
+#define HOMA_ETH_OVERHEAD 42
+
+/**
+ * define HOMA_MIN_PKT_LENGTH - Every Homa packet must be padded to at least
+ * this length to meet Ethernet frame size limitations. This number includes
+ * Homa headers and data, but not IP or Ethernet headers.
+ */
+#define HOMA_MIN_PKT_LENGTH 26
+
+/**
+ * define HOMA_MAX_HEADER - Number of bytes in the largest Homa header.
+ */
+#define HOMA_MAX_HEADER 90
+
+/**
+ * struct homa_common_hdr - Wire format for the first bytes in every Homa
+ * packet. This must (mostly) match the format of a TCP header to enable
+ * Homa packets to actually be transmitted as TCP packets (and thereby
+ * take advantage of TSO and other features).
+ */
+struct homa_common_hdr {
+	/**
+	 * @sport: Port on source machine from which packet was sent.
+	 * Must be in the same position as in a TCP header.
+	 */
+	__be16 sport;
+
+	/**
+	 * @dport: Port on destination that is to receive packet. Must be
+	 * in the same position as in a TCP header.
+	 */
+	__be16 dport;
+
+	/**
+	 * @sequence: corresponds to the sequence number field in TCP headers;
+	 * used in DATA packets to hold the offset in the message of the first
+	 * byte of data. This value will only be correct in the first segment
+	 * of a GSO packet.
+	 */
+	__be32 sequence;
+
+	/* The fields below correspond to the acknowledgment field in TCP
+	 * headers; not used by Homa, except for the low-order 8 bits, which
+	 * specify the Homa packet type (one of the values in the
+	 * homa_packet_type enum).
+	 */
+	__be16 ack1;
+	__u8 ack2;
+	__u8 type;
+
+	/**
+	 * @doff: High order 4 bits holds the number of 4-byte chunks in a
+	 * homa_data_hdr (low-order bits unused). Used only for DATA packets;
+	 * must be in the same position as the data offset in a TCP header.
+	 * Used by TSO to determine where the replicated header portion ends.
+	 */
+	__u8 doff;
+
+	__u8 reserved1;
+
+	/**
+	 * @window: Corresponds to the window field in TCP headers. Not used
+	 * by HOMA.
+	 */
+	__be16 window;
+
+	/**
+	 * @checksum: not used by Homa, but must occupy the same bytes as
+	 * the checksum in a TCP header (TSO may modify this?).
+	 */
+	__be16 checksum;
+
+	__be16 reserved2;
+
+	/**
+	 * @sender_id: the identifier of this RPC as used on the sender (i.e.,
+	 * if the low-order bit is set, then the sender is the server for
+	 * this RPC).
+	 */
+	__be64 sender_id;
+} __packed;
+
+/**
+ * struct homa_ack - Identifies an RPC that can be safely deleted by its
+ * server. After sending the response for an RPC, the server must retain its
+ * state for the RPC until it knows that the client has successfully
+ * received the entire response. An ack indicates this. Clients will
+ * piggyback acks on future data packets, but if a client doesn't send
+ * any data to the server, the server will eventually request an ack
+ * explicitly with a NEED_ACK packet, in which case the client will
+ * return an explicit ACK.
+ */
+struct homa_ack {
+	/**
+	 * @client_id: The client's identifier for the RPC. 0 means this ack
+	 * is invalid.
+	 */
+	__be64 client_id;
+
+	/** @server_port: The server-side port for the RPC. */
+	__be16 server_port;
+} __packed;
+
+/* struct homa_data_hdr - Contains data for part or all of a Homa message.
+ * An incoming packet consists of a homa_data_hdr followed by message data.
+ * An outgoing packet can have this simple format as well, or it can be
+ * structured as a GSO packet with the following format:
+ *
+ *    |-----------------------|
+ *    |                       |
+ *    |     data_header       |
+ *    |                       |
+ *    |---------------------- |
+ *    |                       |
+ *    |                       |
+ *    |     segment data      |
+ *    |                       |
+ *    |                       |
+ *    |-----------------------|
+ *    |      seg_header       |
+ *    |-----------------------|
+ *    |                       |
+ *    |                       |
+ *    |     segment data      |
+ *    |                       |
+ *    |                       |
+ *    |-----------------------|
+ *    |      seg_header       |
+ *    |-----------------------|
+ *    |                       |
+ *    |                       |
+ *    |     segment data      |
+ *    |                       |
+ *    |                       |
+ *    |-----------------------|
+ *
+ * TSO will not adjust @homa_common_hdr.sequence in the segments, so Homa
+ * sprinkles correct offsets (in homa_seg_hdrs) throughout the segment data;
+ * TSO/GSO will include a different homa_seg_hdr in each generated packet.
+ */
+
+struct homa_seg_hdr {
+	/**
+	 * @offset: Offset within message of the first byte of data in
+	 * this segment.
+	 */
+	__be32 offset;
+} __packed;
+
+struct homa_data_hdr {
+	struct homa_common_hdr common;
+
+	/** @message_length: Total #bytes in the message. */
+	__be32 message_length;
+
+	__be32 reserved1;
+
+	/** @ack: If the @client_id field of this is nonzero, provides info
+	 * about an RPC that the recipient can now safely free. Note: in
+	 * TSO packets this will get duplicated in each of the segments;
+	 * in order to avoid repeated attempts to ack the same RPC,
+	 * homa_gro_receive will clear this field in all segments but the
+	 * first.
+	 */
+	struct homa_ack ack;
+
+	__be16 reserved2;
+
+	/**
+	 * @retransmit: 1 means this packet was sent in response to a RESEND
+	 * (it has already been sent previously).
+	 */
+	__u8 retransmit;
+
+	char pad[3];
+
+	/** @seg: First of possibly many segments. */
+	struct homa_seg_hdr seg;
+} __packed;
+_Static_assert(sizeof(struct homa_data_hdr) <= HOMA_MAX_HEADER,
+	       "homa_data_hdr too large for HOMA_MAX_HEADER; must adjust HOMA_MAX_HEADER");
+_Static_assert(sizeof(struct homa_data_hdr) >= HOMA_MIN_PKT_LENGTH,
+	       "homa_data_hdr too small: Homa doesn't currently have code to pad data packets");
+_Static_assert(((sizeof(struct homa_data_hdr) - sizeof(struct homa_seg_hdr)) &
+		0x3) == 0,
+	       " homa_data_hdr length not a multiple of 4 bytes (required for TCP/TSO compatibility");
+
+/**
+ * homa_data_len() - Returns the total number of bytes in a DATA packet
+ * after the homa_data_hdr. Note: if the packet is a GSO packet, the result
+ * may include metadata as well as packet data.
+ * @skb:   Incoming data packet
+ */
+static inline int homa_data_len(struct sk_buff *skb)
+{
+	return skb->len - skb_transport_offset(skb) -
+			sizeof(struct homa_data_hdr);
+}
+
+/**
+ * struct homa_resend_hdr - Wire format for RESEND packets.
+ *
+ * A RESEND is sent by the receiver when it believes that message data may
+ * have been lost in transmission (or if it is concerned that the sender may
+ * have crashed). The receiver should resend the specified portion of the
+ * message, even if it already sent it previously.
+ */
+struct homa_resend_hdr {
+	/** @common: Fields common to all packet types. */
+	struct homa_common_hdr common;
+
+	/**
+	 * @offset: Offset within the message of the first byte of data that
+	 * should be retransmitted.
+	 */
+	__be32 offset;
+
+	/**
+	 * @length: Number of bytes of data to retransmit; this could specify
+	 * a range longer than the total message size. Zero is a special case
+	 * used by servers; in this case, there is no need to actually resend
+	 * anything; the purpose of this packet is to trigger an UNKNOWN
+	 * response if the client no longer cares about this RPC.
+	 */
+	__be32 length;
+} __packed;
+_Static_assert(sizeof(struct homa_resend_hdr) <= HOMA_MAX_HEADER,
+	       "homa_resend_hdr too large for HOMA_MAX_HEADER; must adjust HOMA_MAX_HEADER");
+
+/**
+ * struct homa_unknown_hdr - Wire format for UNKNOWN packets.
+ *
+ * An UNKNOWN packet is sent by either server or client when it receives a
+ * packet for an RPC that is unknown to it. When a client receives an
+ * UNKNOWN packet it will typically restart the RPC from the beginning;
+ * when a server receives an UNKNOWN packet it will typically discard its
+ * state for the RPC.
+ */
+struct homa_unknown_hdr {
+	/** @common: Fields common to all packet types. */
+	struct homa_common_hdr common;
+} __packed;
+_Static_assert(sizeof(struct homa_unknown_hdr) <= HOMA_MAX_HEADER,
+	       "homa_unknown_hdr too large for HOMA_MAX_HEADER; must adjust HOMA_MAX_HEADER");
+
+/**
+ * struct busy_hdr - Wire format for BUSY packets.
+ *
+ * These packets tell the recipient that the sender is still alive (even if
+ * it isn't sending data expected by the recipient).
+ */
+struct homa_busy_hdr {
+	/** @common: Fields common to all packet types. */
+	struct homa_common_hdr common;
+} __packed;
+_Static_assert(sizeof(struct homa_busy_hdr) <= HOMA_MAX_HEADER,
+	       "homa_busy_hdr too large for HOMA_MAX_HEADER; must adjust HOMA_MAX_HEADER");
+
+/**
+ * struct homa_need_ack_hdr - Wire format for NEED_ACK packets.
+ *
+ * These packets ask the recipient (a client) to return an ACK message if
+ * the packet's RPC is no longer active.
+ */
+struct homa_need_ack_hdr {
+	/** @common: Fields common to all packet types. */
+	struct homa_common_hdr common;
+} __packed;
+_Static_assert(sizeof(struct homa_need_ack_hdr) <= HOMA_MAX_HEADER,
+	       "homa_need_ack_hdr too large for HOMA_MAX_HEADER; must adjust HOMA_MAX_HEADER");
+
+/**
+ * struct homa_ack_hdr - Wire format for ACK packets.
+ *
+ * These packets are sent from a client to a server to indicate that
+ * a set of RPCs is no longer active on the client, so the server can
+ * free any state it may have for them.
+ */
+struct homa_ack_hdr {
+	/** @common: Fields common to all packet types. */
+	struct homa_common_hdr common;
+
+	/** @num_acks: Number of (leading) elements in @acks that are valid. */
+	__be16 num_acks;
+
+#define HOMA_MAX_ACKS_PER_PKT 5
+	/** @acks: Info about RPCs that are no longer active. */
+	struct homa_ack acks[HOMA_MAX_ACKS_PER_PKT];
+} __packed;
+_Static_assert(sizeof(struct homa_ack_hdr) <= HOMA_MAX_HEADER,
+	       "homa_ack_hdr too large for HOMA_MAX_HEADER; must adjust HOMA_MAX_HEADER");
+
+/**
+ * homa_local_id(): given an RPC identifier from an input packet (which
+ * is network-encoded), return the decoded id we should use for that
+ * RPC on this machine.
+ * @sender_id:  RPC id from an incoming packet, such as h->common.sender_id
+ */
+static inline __u64 homa_local_id(__be64 sender_id)
+{
+	/* If the client bit was set on the sender side, it needs to be
+	 * removed here, and conversely.
+	 */
+	return be64_to_cpu(sender_id) ^ 1;
+}
+
+#endif /* _HOMA_WIRE_H */
