Adding upstream version 4.2.2.upstream/4.2.2

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 873 insertions, 0 deletions

diff --git a/epan/dissectors/packet-vj-comp.c b/epan/dissectors/packet-vj-comp.c
new file mode 100644
index 00000000..ad01793f
--- /dev/null
+++ b/epan/dissectors/packet-vj-comp.c
@@ -0,0 +1,873 @@
+/* packet-vj-comp.c
+ * Routines for decompression of PPP Van Jacobson compression
+ * RFC 1144
+ *
+ * Wireshark - Network traffic analyzer
+ * By Gerald Combs <gerald@wireshark.org>
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+/* The routines in this file were created by reading the description of
+ * RFC 1144 available here: ftp://ftp.rfc-editor.org/in-notes/rfc1144.pdf
+ * ONLY the description of the protocol in section 3.2 was used.
+ * Notably, the sample implementation in Appendix A was NOT read by this file's
+ * author, due to the questionable legality of using it in Wireshark.
+ * For details on this issue, see:
+ * https://gitlab.com/wireshark/wireshark/-/issues/12138
+ */
+/* Currently hard-coded to assume TCP over IPv4.
+ * Nothing in the standard explicitly prevents an IPv6 implementation...
+ */
+
+#include "config.h"
+
+#include <glib.h>
+#include <epan/tvbuff.h>
+#include <epan/conversation.h>
+#include <epan/in_cksum.h>
+#include <epan/proto.h>
+#include <epan/expert.h>
+#include <epan/packet.h>
+#include <epan/ipproto.h>
+#include <epan/ppptypes.h>
+#include <wsutil/str_util.h>
+#include "packet-ip.h"
+#include "packet-ppp.h"
+
+/* Shorthand macros for reading/writing 16/32 bit values from
+ * possibly-unaligned indexes into a guint8[]
+ */
+#define GET_16(p,i) (guint16)(((p)[(i)] << 8) | ((p)[(i)+1]))
+#define GET_32(p,i) (guint32)(((p)[(i)] << 24) | ((p)[(i)+1] << 16) | ((p)[(i)+2] << 8) | ((p)[(i)+3]))
+#define PUT_16(p,i,v) G_STMT_START { \
+    (p)[(i)]   = ((v) & 0xFF00) >> 8; \
+    (p)[(i)+1] = ((v) & 0x00FF); \
+} G_STMT_END
+#define PUT_32(p,i,v) G_STMT_START { \
+    (p)[(i)]   = ((v) & 0xFF000000) >> 24; \
+    (p)[(i)+1] = ((v) & 0x00FF0000) >> 16; \
+    (p)[(i)+2] = ((v) & 0x0000FF00) >> 8; \
+    (p)[(i)+3] = ((v) & 0x000000FF); \
+} G_STMT_END
+
+/* Store the last connection number we've seen.
+ * Only used on the first pass, in case the connection number itself
+ * gets compressed out.
+ */
+#define CNUM_INVALID G_MAXUINT16
+static guint16 last_cnum = CNUM_INVALID;
+
+/* Location in an IPv4 packet of the IP Next Protocol field
+ * (which VJC replaces with the connection ID in uncompressed packets)
+ */
+#define VJC_CONNID_OFFSET 9
+
+/* Minimum TCP header length. We get compression data from the TCP header,
+ * and also store it for future use.
+ */
+#define VJC_TCP_HDR_LEN 20
+
+/* Structure for tracking the changeable parts of a packet header */
+typedef struct vjc_hdr_s {
+    guint16 tcp_chksum;
+    guint16 urg;
+    guint16 win;
+    guint32 seq;
+    guint32 ack;
+    guint32 ip_id;
+    gboolean psh;
+} vjc_hdr_t;
+
+/* The structure used in a wireshark "conversation" */
+typedef struct vjc_conv_s {
+    guint32     last_frame;     // On first pass, where to get the previous info
+    guint32     last_frame_len; // On first pass, length of prev. frame (for SAWU/SWU)
+    guint8     *frame_headers;  // Full copy of the IP header
+    guint8      header_len;     // Length of the stored IP header
+    wmem_map_t *vals;           // Hash of frame_number => vjc_hdr_t*
+} vjc_conv_t;
+
+static dissector_handle_t vjcu_handle;
+static dissector_handle_t vjcc_handle;
+static dissector_handle_t ip_handle;
+
+void proto_register_vjc(void);
+void proto_reg_handoff_vjc(void);
+
+static int proto_vjc = -1;
+
+static gint ett_vjc = -1;
+static gint ett_vjc_change_mask = -1;
+
+static expert_field ei_vjc_sawu = EI_INIT;
+static expert_field ei_vjc_swu = EI_INIT;
+static expert_field ei_vjc_no_cnum = EI_INIT;
+static expert_field ei_vjc_no_conversation = EI_INIT;
+static expert_field ei_vjc_no_direction = EI_INIT;
+static expert_field ei_vjc_no_conv_data = EI_INIT;
+static expert_field ei_vjc_undecoded = EI_INIT;
+static expert_field ei_vjc_bad_data = EI_INIT;
+static expert_field ei_vjc_error = EI_INIT;
+
+#define VJC_FLAG_R 0x80
+#define VJC_FLAG_C 0x40
+#define VJC_FLAG_I 0x20
+#define VJC_FLAG_P 0x10
+#define VJC_FLAG_S 0x08
+#define VJC_FLAG_A 0x04
+#define VJC_FLAG_W 0x02
+#define VJC_FLAG_U 0x01
+
+#define VJC_FLAGS_SAWU 0x0F
+#define VJC_FLAGS_SWU 0x0B
+
+static int hf_vjc_comp = -1;
+static int hf_vjc_cnum = -1;
+static int hf_vjc_change_mask = -1;
+static int hf_vjc_change_mask_r = -1;
+static int hf_vjc_change_mask_c = -1;
+static int hf_vjc_change_mask_i = -1;
+static int hf_vjc_change_mask_p = -1;
+static int hf_vjc_change_mask_s = -1;
+static int hf_vjc_change_mask_a = -1;
+static int hf_vjc_change_mask_w = -1;
+static int hf_vjc_change_mask_u = -1;
+static int hf_vjc_chksum = -1;
+static int hf_vjc_urg = -1;
+static int hf_vjc_d_win = -1;
+static int hf_vjc_d_ack = -1;
+static int hf_vjc_d_seq = -1;
+static int hf_vjc_d_ipid = -1;
+static int hf_vjc_tcpdata = -1;
+
+static int * const vjc_change_mask_fields[] = {
+    &hf_vjc_change_mask_r,
+    &hf_vjc_change_mask_c,
+    &hf_vjc_change_mask_i,
+    &hf_vjc_change_mask_p,
+    &hf_vjc_change_mask_s,
+    &hf_vjc_change_mask_a,
+    &hf_vjc_change_mask_w,
+    &hf_vjc_change_mask_u,
+    NULL
+};
+
+/* Initialization routine. Called at start of dissection.
+ * Registered in proto_register_vjc() below.
+ */
+static void
+vjc_init_protocol(void)
+{
+    last_cnum = CNUM_INVALID;
+}
+
+/* Cleanup routine. Called at close of file.
+ * Registered in proto_register_vjc() below.
+ */
+static void
+vjc_cleanup_protocol(void)
+{
+    last_cnum = CNUM_INVALID;
+}
+
+/* Find (or optionally create) a VJC conversation. */
+static conversation_t *
+vjc_find_conversation(packet_info *pinfo, guint32 vjc_cnum, gboolean create)
+{
+    /* PPP gives us almost nothing to hook a conversation on; just whether
+     * the packet is considered to be P2P_DIR_RECV or P2P_DIR_SENT.
+     * Ideally we should also be distinguishing conversations based on the
+     * capture interface, VLAN ID, MPLS tags, etc., etc. but that's beyond
+     * the scope of this dissector, and a perennial problem in Wireshark anyway.
+     * See <https://gitlab.com/wireshark/wireshark/-/issues/4561>
+     */
+    conversation_t *conv = (conversation_t *)NULL;
+    switch (pinfo->p2p_dir) {
+        case P2P_DIR_RECV:
+            vjc_cnum |= 0x0100;
+            break;
+        case P2P_DIR_SENT:
+            vjc_cnum |= 0x0200;
+            break;
+        default:
+            return conv;
+    }
+
+    conv = find_conversation_by_id(pinfo->num, CONVERSATION_NONE, vjc_cnum);
+    if (!conv && create) {
+        conv = conversation_new_by_id(pinfo->num, CONVERSATION_NONE, vjc_cnum);
+    }
+
+    return conv;
+}
+
+/* RFC 1144 section 3.2.2 says that "deltas" are sent for many values in the
+ * header. If the initial byte is 0, that means the following 2 bytes are the
+ * 16-bit value of the delta. Otherwise, the initial byte is the 8-bit value.
+ */
+static guint32
+vjc_delta_uint(proto_tree *tree, int hf, tvbuff_t *tvb, guint *offset)
+{
+    guint32 ret_val;
+    if (0 != tvb_get_guint8(tvb, *offset)) {
+        proto_tree_add_item_ret_uint(tree, hf, tvb, *offset, 1,
+                ENC_BIG_ENDIAN, &ret_val);
+        (*offset)++;
+    }
+    else {
+        (*offset)++;
+        proto_tree_add_item_ret_uint(tree, hf, tvb, *offset, 2,
+                ENC_BIG_ENDIAN, &ret_val);
+        *offset += 2;
+    }
+    return ret_val;
+}
+
+/* Same thing but signed, since the TCP window delta can be negative */
+static gint32
+vjc_delta_int(proto_tree *tree, int hf, tvbuff_t *tvb, guint *offset)
+{
+    gint32 ret_val;
+    if (0 != tvb_get_gint8(tvb, *offset)) {
+        proto_tree_add_item_ret_int(tree, hf, tvb, *offset, 1,
+                ENC_BIG_ENDIAN, &ret_val);
+        (*offset)++;
+    }
+    else {
+        (*offset)++;
+        proto_tree_add_item_ret_int(tree, hf, tvb, *offset, 2,
+                ENC_BIG_ENDIAN, &ret_val);
+        *offset += 2;
+    }
+    return ret_val;
+}
+
+/* Main dissection routine for uncompressed VJC packets.
+ * Registered in proto_reg_handoff_vjc() below.
+ */
+static int
+dissect_vjc_uncomp(tvbuff_t *tvb, packet_info *pinfo, proto_tree *tree, void* data)
+{
+    /* A Van Jacobson uncompressed packet contains a standard TCP/IP header, with
+     * the IP next protocol ID replaced with the connection number.
+     * It's meant to signify a new TCP connection, or refresh an existing one,
+     * which will have subsequent compressed packets.
+     */
+    proto_tree     *subtree     = NULL;
+    proto_item     *ti          = NULL;
+    guint8          ip_ver      = 0;
+    guint8          ip_len      = 0;
+    guint           tcp_len     = 0;
+    guint32         vjc_cnum    = 0;
+    tvbuff_t       *tcpip_tvb   = NULL;
+    tvbuff_t       *sub_tvb     = NULL;
+    conversation_t *conv        = NULL;
+    vjc_hdr_t      *this_hdr    = NULL;
+    vjc_conv_t     *pkt_data    = NULL;
+    guint8         *pdata       = NULL;
+    static guint8   real_proto  = IP_PROTO_TCP;
+
+    ti = proto_tree_add_item(tree, proto_vjc, tvb, 0, -1, ENC_NA);
+    subtree = proto_item_add_subtree(ti, ett_vjc);
+    proto_item_set_text(subtree, "PPP Van Jacobson uncompressed TCP/IP");
+
+    /* Start with some sanity checks */
+    if (VJC_CONNID_OFFSET+1 > tvb_captured_length(tvb)) {
+        proto_tree_add_expert_format(subtree, pinfo, &ei_vjc_bad_data, tvb, 0, -1,
+                "Packet truncated before Connection ID field");
+        return tvb_captured_length(tvb);
+    }
+    ip_ver = (tvb_get_guint8(tvb, 0) & 0xF0) >> 4;
+    ip_len = (tvb_get_guint8(tvb, 0) & 0x0F) << 2;
+    tcp_len = ip_len + VJC_TCP_HDR_LEN;
+    if (4 != ip_ver) {
+        proto_tree_add_expert_format(subtree, pinfo, &ei_vjc_bad_data, tvb, 0, 1,
+                "IPv%d unsupported for VJC compression", ip_ver);
+        return tvb_captured_length(tvb);
+    }
+
+    /* So far so good, continue the dissection */
+    ti = proto_tree_add_boolean(subtree, hf_vjc_comp, tvb, 0, 0, FALSE);
+    proto_item_set_generated(ti);
+
+    proto_tree_add_item_ret_uint(subtree, hf_vjc_cnum, tvb, VJC_CONNID_OFFSET, 1,
+            ENC_BIG_ENDIAN, &vjc_cnum);
+
+    /* Build a composite TVB containing the original TCP/IP data.
+     * This is easy for uncompressed VJC packets because only one byte
+     * is different from the on-the-wire data.
+     */
+    sub_tvb = tvb_new_child_real_data(tvb, &real_proto, 1, 1);
+    tvb_set_free_cb(sub_tvb, NULL);
+
+    tcpip_tvb = tvb_new_composite();
+    tvb_composite_append(tcpip_tvb, tvb_new_subset_length(tvb, 0, VJC_CONNID_OFFSET));
+    tvb_composite_append(tcpip_tvb, sub_tvb);
+    if (0 < tvb_captured_length_remaining(tvb, VJC_CONNID_OFFSET+1)) {
+        tvb_composite_append(tcpip_tvb, tvb_new_subset_length(tvb, VJC_CONNID_OFFSET+1, -1));
+    }
+    tvb_composite_finalize(tcpip_tvb);
+
+    add_new_data_source(pinfo, tcpip_tvb, "Original TCP/IP data");
+
+    if (!(pinfo->p2p_dir == P2P_DIR_RECV || pinfo->p2p_dir == P2P_DIR_SENT)) {
+        /* We can't make a proper conversation if we don't know the endpoints */
+        proto_tree_add_expert(subtree, pinfo, &ei_vjc_no_direction, tvb, 0, 0);
+    }
+    else if (tcp_len > tvb_captured_length(tvb)) {
+        /* Not enough data. We can still pass this packet onward (though probably
+         * to no benefit), but can't base future decompression off of it.
+         */
+        proto_tree_add_expert_format(subtree, pinfo, &ei_vjc_bad_data, tvb, 0, -1,
+                "Packet truncated before end of TCP/IP headers");
+    }
+    else if (!pinfo->fd->visited) {
+        /* If this is our first time visiting this packet, set things up for
+         * decompressing future packets.
+         */
+        last_cnum = vjc_cnum;
+        conv = vjc_find_conversation(pinfo, vjc_cnum, TRUE);
+        pkt_data = (vjc_conv_t *)conversation_get_proto_data(conv, proto_vjc);
+        if (NULL == pkt_data) {
+            pkt_data = wmem_new0(wmem_file_scope(), vjc_conv_t);
+            pkt_data->vals = wmem_map_new(wmem_file_scope(), g_direct_hash, g_direct_equal);
+            conversation_add_proto_data(conv, proto_vjc, (void *)pkt_data);
+        }
+        pdata = // shorthand
+            pkt_data->frame_headers =
+            (guint8 *)tvb_memdup(wmem_file_scope(), tcpip_tvb, 0, tcp_len);
+
+        pkt_data->last_frame = pinfo->num;
+        pkt_data->header_len = tcp_len;
+
+        // This value is used for re-calculating seq/ack numbers
+        pkt_data->last_frame_len = tvb_reported_length(tvb) - ip_len;
+
+        this_hdr = wmem_new0(wmem_file_scope(), vjc_hdr_t);
+        this_hdr->ip_id = GET_16(pdata, 4);
+        this_hdr->seq = GET_32(pdata, ip_len + 4);
+        this_hdr->ack = GET_32(pdata, ip_len + 8);
+        this_hdr->psh = (pdata[ip_len + 13] & 0x08) == 0x08;
+        this_hdr->win = GET_16(pdata, ip_len + 14);
+        this_hdr->tcp_chksum = GET_16(pdata, ip_len + 16);
+        this_hdr->urg = GET_16(pdata, ip_len + 18);
+        wmem_map_insert(pkt_data->vals, GUINT_TO_POINTER(pinfo->num), this_hdr);
+    }
+    else {
+        /* We've already visited this packet, we should have all the info we need. */
+    }
+
+    return call_dissector_with_data(ip_handle, tcpip_tvb, pinfo, tree, data);
+}
+
+/* Main dissection routine for compressed VJC packets.
+ * Registered in proto_reg_handoff_vjc() below.
+ */
+static int
+dissect_vjc_comp(tvbuff_t *tvb, packet_info *pinfo _U_, proto_tree *tree, void* data _U_)
+{
+    /* A Van Jacobson compressed packet contains a change mask, which indicates
+     * possible fields that may be present.
+     */
+    proto_tree     *subtree     = NULL;
+    proto_item     *ti          = NULL;
+    guint           hdr_len     = 3;    // See below
+    gboolean        hdr_error   = FALSE;
+    guint           ip_len      = 0;
+    guint           pkt_len     = 0;
+    guint           d_ipid      = 0;
+    guint           d_seq       = 0;
+    guint           d_ack       = 0;
+    gint            d_win       = 0;
+    guint8          flags       = 0;
+    guint           offset      = 0;
+    guint32         urg         = 0;
+    guint32         ip_chksum   = 0;
+    guint32         tcp_chksum  = 0;
+    guint32         vjc_cnum    = 0;
+    conversation_t *conv        = NULL;
+    vjc_hdr_t      *this_hdr    = NULL;
+    vjc_hdr_t      *last_hdr    = NULL;
+    vjc_conv_t     *pkt_data    = NULL;
+    guint8         *pdata       = NULL;
+    tvbuff_t       *tcpip_tvb   = NULL;
+    tvbuff_t       *sub_tvb     = NULL;
+
+    /* Calculate the length of the VJC header,
+     * accounting for extensions in the delta fields.
+     * We start with a value of 3, because we'll always have
+     * an 8-bit change mask and a 16-bit TCP checksum.
+     */
+#define TEST_HDR_LEN \
+    if (hdr_len > tvb_captured_length(tvb)) { hdr_error = TRUE; goto done_header_len; }
+
+    TEST_HDR_LEN;
+    flags = tvb_get_guint8(tvb, offset);
+    if (flags & VJC_FLAG_C) {
+        // have connection number
+        hdr_len++;
+        TEST_HDR_LEN;
+    }
+    if ((flags & VJC_FLAGS_SAWU) == VJC_FLAGS_SAWU) {
+        /* Special case for "unidirectional data transfer".
+         * No change to header size; d_ack = 0, and
+         * we're to calculate d_seq ourselves.
+         */
+    }
+    else if ((flags & VJC_FLAGS_SAWU) == VJC_FLAGS_SWU) {
+        /* Special case for "echoed interactive traffic".
+         * No change to header size; we're to calculate d_seq and d_ack.
+         */
+    }
+    else {
+        /* Not a special case, determine the header size by
+         * testing the SAWU flags individually.
+         */
+        if (flags & VJC_FLAG_U) {
+            // have urgent pointer
+            hdr_len += 2;
+            TEST_HDR_LEN;
+        }
+        if (flags & VJC_FLAG_W) {
+            // have d_win
+            if (0 == tvb_get_gint8(tvb, offset + hdr_len))
+                hdr_len += 3;
+            else
+                hdr_len++;
+            TEST_HDR_LEN;
+        }
+        if (flags & VJC_FLAG_A) {
+            // have d_ack
+            if (0 == tvb_get_guint8(tvb, offset + hdr_len))
+                hdr_len += 3;
+            else
+                hdr_len++;
+            TEST_HDR_LEN;
+        }
+        if (flags & VJC_FLAG_S) {
+            // have d_seq
+            if (0 == tvb_get_guint8(tvb, offset + hdr_len))
+                hdr_len += 3;
+            else
+                hdr_len++;
+            TEST_HDR_LEN;
+        }
+    }
+    if (flags & VJC_FLAG_I) {
+        // have IP ID
+        if (0 == tvb_get_guint8(tvb, offset + hdr_len))
+            hdr_len += 3;
+        else
+            hdr_len++;
+        TEST_HDR_LEN;
+    }
+
+    /* Now that we have the header length, use it when assigning the
+     * protocol item.
+     */
+#undef TEST_HDR_LEN
+done_header_len:
+    ti = proto_tree_add_item(tree, proto_vjc, tvb, 0,
+            MIN(hdr_len, tvb_captured_length(tvb)), ENC_NA);
+    subtree = proto_item_add_subtree(ti, ett_vjc);
+    proto_item_set_text(subtree, "PPP Van Jacobson compressed TCP/IP");
+    if (hdr_error) {
+        proto_tree_add_expert_format(subtree, pinfo, &ei_vjc_bad_data, tvb, 0, -1,
+                "Packet truncated, compression header incomplete");
+        return tvb_captured_length(tvb);
+    }
+
+    ti = proto_tree_add_boolean(subtree, hf_vjc_comp, tvb, 0, 0, TRUE);
+    proto_item_set_generated(ti);
+
+   proto_tree_add_bitmask(subtree, tvb, 0, hf_vjc_change_mask,
+            ett_vjc_change_mask, vjc_change_mask_fields, ENC_NA);
+    if ((flags & VJC_FLAGS_SAWU) == VJC_FLAGS_SAWU) {
+        proto_tree_add_expert(subtree, pinfo, &ei_vjc_sawu, tvb, 0, 1);
+    }
+    else if ((flags & VJC_FLAGS_SAWU) == VJC_FLAGS_SWU) {
+        proto_tree_add_expert(subtree, pinfo, &ei_vjc_swu, tvb, 0, 1);
+    }
+
+    offset++;
+
+    if (flags & VJC_FLAG_C) {
+        proto_tree_add_item_ret_uint(subtree, hf_vjc_cnum, tvb, offset, 1,
+                ENC_BIG_ENDIAN, &vjc_cnum);
+        last_cnum = vjc_cnum;
+        offset++;
+    }
+    else {
+        vjc_cnum = last_cnum;
+        if (vjc_cnum != CNUM_INVALID) {
+            ti = proto_tree_add_uint(subtree, hf_vjc_cnum, tvb, offset, 0, vjc_cnum);
+            proto_item_set_generated(ti);
+        }
+        else {
+            proto_tree_add_expert(subtree, pinfo, &ei_vjc_no_cnum, tvb, 0, 0);
+        }
+    }
+    conv = vjc_find_conversation(pinfo, vjc_cnum, FALSE);
+    if (NULL != conv) {
+        pkt_data = (vjc_conv_t *)conversation_get_proto_data(conv, proto_vjc);
+        // Will be testing that pkt_data exists below
+    }
+    else {
+        proto_tree_add_expert(subtree, pinfo, &ei_vjc_no_conversation,
+                tvb, 1, (flags & VJC_FLAG_C) ? 1 : 0);
+    }
+
+    proto_tree_add_item_ret_uint(subtree, hf_vjc_chksum, tvb, offset, 2,
+            ENC_BIG_ENDIAN, &tcp_chksum);
+    offset += 2;
+
+    if ((flags & VJC_FLAGS_SAWU) == VJC_FLAGS_SAWU) {
+        /* Special case for "unidirectional data transfer".
+         * d_ack is 0, and d_seq changed by the amount of data in the previous packet.
+         */
+        flags &= ~VJC_FLAGS_SAWU;
+        d_ack = 0;
+        if (NULL != pkt_data) {
+            d_seq = pkt_data->last_frame_len;
+        }
+        ti = proto_tree_add_uint(subtree, hf_vjc_d_ack, tvb, offset, 0, d_ack);
+        proto_item_set_generated(ti);
+        ti = proto_tree_add_uint(subtree, hf_vjc_d_seq, tvb, offset, 0, d_seq);
+        proto_item_set_generated(ti);
+    }
+    else if ((flags & VJC_FLAGS_SAWU) == VJC_FLAGS_SWU) {
+        /* Special case for "echoed interactive traffic".
+         * d_seq and d_ack changed by the amount of user data in the
+         * previous packet.
+         */
+        flags &= ~VJC_FLAGS_SAWU;
+        if (NULL != pkt_data) {
+            d_seq = d_ack = pkt_data->last_frame_len;
+        }
+        ti = proto_tree_add_uint(subtree, hf_vjc_d_ack, tvb, offset, 0, d_ack);
+        proto_item_set_generated(ti);
+        ti = proto_tree_add_uint(subtree, hf_vjc_d_seq, tvb, offset, 0, d_seq);
+        proto_item_set_generated(ti);
+    }
+    else {
+        /* Not a special case, read the SAWU flags individually */
+
+        if (flags & VJC_FLAG_U) {
+            /* "The packet’s urgent pointer is sent if URG is set ..."
+            * I assume that means the full 16-bit value here.
+            */
+            proto_tree_add_item_ret_uint(subtree, hf_vjc_urg, tvb, offset, 2,
+                    ENC_BIG_ENDIAN, &urg);
+            offset += 2;
+        }
+        else {
+            urg = 0;
+        }
+
+        if (flags & VJC_FLAG_W) {
+            /* "The number sent for the window is also the difference between the current
+            * and previous values. However, either positive or negative changes are
+            * allowed since the window is a 16-bit field."
+            */
+            d_win = vjc_delta_int(subtree, hf_vjc_d_win, tvb, &offset);
+        }
+        else {
+            d_win = 0;
+        }
+
+        /* The rest of the deltas can only be positive. */
+        if (flags & VJC_FLAG_A) {
+            d_ack = vjc_delta_uint(subtree, hf_vjc_d_ack, tvb, &offset);
+        }
+        else {
+            d_ack = 0;
+        }
+
+        if (flags & VJC_FLAG_S) {
+            d_seq = vjc_delta_uint(subtree, hf_vjc_d_seq, tvb, &offset);
+        }
+        else {
+            d_seq = 0;
+        }
+    }
+
+    if (flags & VJC_FLAG_I) {
+        d_ipid = vjc_delta_uint(subtree, hf_vjc_d_ipid, tvb, &offset);
+    }
+    else {
+        /* "However, unlike the rest of the compressed fields, the assumed
+         * change when I is clear is one, not zero." - section 3.2.2
+         */
+        d_ipid = 1;
+        ti = proto_tree_add_uint(subtree, hf_vjc_d_ipid, tvb, offset, 0, d_ipid);
+        proto_item_set_generated(ti);
+    }
+
+    if (!(pinfo->p2p_dir == P2P_DIR_RECV || pinfo->p2p_dir == P2P_DIR_SENT)) {
+        /* We can't make a proper conversation if we don't know the endpoints */
+        proto_tree_add_expert(subtree, pinfo, &ei_vjc_no_direction, tvb, offset,
+                tvb_captured_length_remaining(tvb, offset));
+        return tvb_captured_length(tvb);
+    }
+    if (NULL == conv) {
+        proto_tree_add_expert(subtree, pinfo, &ei_vjc_undecoded, tvb, offset,
+                tvb_captured_length_remaining(tvb, offset));
+        return tvb_captured_length(tvb);
+    }
+    if (NULL == pkt_data) {
+        proto_tree_add_expert(subtree, pinfo, &ei_vjc_no_conv_data, tvb, offset,
+                tvb_captured_length_remaining(tvb, offset));
+        return tvb_captured_length(tvb);
+    }
+
+    if (!pinfo->fd->visited) {
+        /* We haven't visited this packet before.
+         * Form its vjc_hdr_t from the deltas and the info from the previous frame.
+         */
+        last_hdr = (vjc_hdr_t *)wmem_map_lookup(pkt_data->vals,
+                GUINT_TO_POINTER(pkt_data->last_frame));
+
+        if (NULL != last_hdr) {
+            this_hdr = wmem_new0(wmem_file_scope(), vjc_hdr_t);
+            this_hdr->tcp_chksum = (guint16)tcp_chksum;
+            this_hdr->urg = (guint16)urg;
+            this_hdr->win = last_hdr->win + d_win;
+            this_hdr->seq = last_hdr->seq + d_seq;
+            this_hdr->ack = last_hdr->ack + d_ack;
+            this_hdr->ip_id = last_hdr->ip_id + d_ipid;
+            this_hdr->psh = (flags & VJC_FLAG_P) == VJC_FLAG_P;
+            wmem_map_insert(pkt_data->vals, GUINT_TO_POINTER(pinfo->num), this_hdr);
+
+            // This frame is the next frame's last frame
+            pkt_data->last_frame = pinfo->num;
+            pkt_data->last_frame_len = tvb_reported_length_remaining(tvb, offset);
+        }
+        else {
+            proto_tree_add_expert_format(subtree, pinfo, &ei_vjc_error, tvb, 0, 0,
+                    "Dissector error: unable to find headers for prior frame %d",
+                    pkt_data->last_frame);
+            return tvb_captured_length(tvb);
+        }
+        // if last_hdr is null, then this_hdr will stay null and be handled below
+    }
+    else {
+        /* We have visited this packet before.
+         * Get the values we saved the first time.
+         */
+        this_hdr = (vjc_hdr_t *)wmem_map_lookup(pkt_data->vals,
+                GUINT_TO_POINTER(pinfo->num));
+    }
+    if (NULL != this_hdr) {
+        /* pkt_data->frame_headers is our template packet header data.
+         * Apply changes to it as needed.
+         * The changes are intentionally done in the template before copying.
+         */
+        pkt_len = pkt_data->header_len + tvb_reported_length_remaining(tvb, offset);
+
+        pdata = pkt_data->frame_headers; /* shorthand */
+        ip_len = (pdata[0] & 0x0F) << 2;
+
+        /* IP length */
+        PUT_16(pdata, 2, pkt_len);
+
+        /* IP ID */
+        PUT_16(pdata, 4, this_hdr->ip_id);
+
+        /* IP checksum */
+        PUT_16(pdata, 10, 0x0000);
+        ip_chksum = ip_checksum(pdata, ip_len);
+        PUT_16(pdata, 10, g_htons(ip_chksum));
+
+        /* TCP seq */
+        PUT_32(pdata, ip_len + 4, this_hdr->seq);
+
+        /* TCP ack */
+        PUT_32(pdata, ip_len + 8, this_hdr->ack);
+
+        /* TCP window */
+        PUT_16(pdata, ip_len + 14, this_hdr->win);
+
+        /* TCP push */
+        if (this_hdr->psh) {
+            pdata[ip_len + 13] |= 0x08;
+        }
+        else {
+            pdata[ip_len + 13] &= ~0x08;
+        }
+
+        /* TCP checksum */
+        PUT_16(pdata, ip_len + 16, this_hdr->tcp_chksum);
+
+        /* TCP urg */
+        if (this_hdr->urg) {
+            pdata[ip_len + 13] |= 0x20;
+            PUT_16(pdata, ip_len + 18, this_hdr->urg);
+        }
+        else {
+            pdata[ip_len + 13] &= ~0x20;
+            PUT_16(pdata, ip_len + 18, 0x0000);
+        }
+
+        /* Now that we're done manipulating the packet header, stick it into
+         * a TVB for sub-dissectors to use.
+         */
+        sub_tvb = tvb_new_child_real_data(tvb, pdata,
+                pkt_data->header_len, pkt_data->header_len);
+        tvb_set_free_cb(sub_tvb, NULL);
+
+        // Reuse pkt_len
+        pkt_len = tvb_captured_length_remaining(tvb, offset);
+        if (0 < pkt_len) {
+            tcpip_tvb = tvb_new_composite();
+            tvb_composite_append(tcpip_tvb, sub_tvb);
+            tvb_composite_append(tcpip_tvb, tvb_new_subset_remaining(tvb, offset));
+            tvb_composite_finalize(tcpip_tvb);
+
+            ti = proto_tree_add_item(subtree, hf_vjc_tcpdata, tvb, offset, pkt_len, ENC_NA);
+            proto_item_set_text(ti, "TCP data (%d byte%s)", pkt_len, plurality(pkt_len, "", "s"));
+        }
+        else
+        {
+            tcpip_tvb = sub_tvb;
+        }
+
+        add_new_data_source(pinfo, tcpip_tvb, "Decompressed TCP/IP data");
+        return offset + call_dissector_with_data(ip_handle, tcpip_tvb, pinfo, tree, data);
+    }
+    else {
+        proto_tree_add_expert_format(subtree, pinfo, &ei_vjc_error, tvb, 0, 0,
+                "Dissector error: unable to find headers for current frame %d",
+                pinfo->num);
+    }
+    return tvb_captured_length(tvb);
+}
+
+void
+proto_register_vjc(void)
+{
+    static hf_register_info hf[] = {
+        { &hf_vjc_comp,
+            { "Is compressed", "vjc.compressed", FT_BOOLEAN, BASE_NONE,
+                NULL, 0x0, NULL, HFILL }},
+        { &hf_vjc_cnum,
+            { "Connection number", "vjc.connection_number", FT_UINT8, BASE_DEC,
+                NULL, 0x0, NULL, HFILL }},
+        { &hf_vjc_change_mask,
+            { "Change mask", "vjc.change_mask", FT_UINT8, BASE_HEX,
+                NULL, 0x0, NULL, HFILL }},
+        { &hf_vjc_change_mask_r,
+            { "Reserved", "vjc.change_mask.reserved", FT_BOOLEAN, 8,
+                TFS(&tfs_set_notset), VJC_FLAG_R, "Undefined bit", HFILL }},
+        { &hf_vjc_change_mask_c,
+            { "Connection number flag", "vjc.change_mask.connection_number", FT_BOOLEAN, 8,
+                TFS(&tfs_set_notset), VJC_FLAG_C, "Whether connection number is present", HFILL }},
+        { &hf_vjc_change_mask_i,
+            { "IP ID flag", "vjc.change_mask.ip_id", FT_BOOLEAN, 8,
+                TFS(&tfs_set_notset), VJC_FLAG_I, "Whether IP ID is present", HFILL }},
+        { &hf_vjc_change_mask_p,
+            { "TCP PSH flag", "vjc.change_mask.psh", FT_BOOLEAN, 8,
+                TFS(&tfs_set_notset), VJC_FLAG_P, "Whether to set TCP PSH", HFILL }},
+        { &hf_vjc_change_mask_s,
+            { "TCP Sequence flag", "vjc.change_mask.seq", FT_BOOLEAN, 8,
+                TFS(&tfs_set_notset), VJC_FLAG_S, "Whether TCP SEQ is present", HFILL }},
+        { &hf_vjc_change_mask_a,
+            { "TCP Acknowledgement flag", "vjc.change_mask.ack", FT_BOOLEAN, 8,
+                TFS(&tfs_set_notset), VJC_FLAG_A, "Whether TCP ACK is present", HFILL }},
+        { &hf_vjc_change_mask_w,
+            { "TCP Window flag", "vjc.change_mask.win", FT_BOOLEAN, 8,
+                TFS(&tfs_set_notset), VJC_FLAG_W, "Whether TCP Window is present", HFILL }},
+        { &hf_vjc_change_mask_u,
+            { "TCP Urgent flag", "vjc.change_mask.urg", FT_BOOLEAN, 8,
+                TFS(&tfs_set_notset), VJC_FLAG_U, "Whether TCP URG pointer is present", HFILL }},
+        { &hf_vjc_chksum,
+            { "TCP Checksum", "vjc.checksum", FT_UINT16, BASE_HEX,
+                NULL, 0x0, "TCP checksum of original packet", HFILL}},
+        { &hf_vjc_urg,
+            { "Urgent pointer", "vjc.urgent_pointer", FT_UINT16, BASE_DEC,
+                NULL, 0x0, "TCP urgent pointer of original packet", HFILL}},
+        { &hf_vjc_d_win,
+            { "Delta window", "vjc.delta_window", FT_INT16, BASE_DEC,
+                NULL, 0x0, "Change in TCP window size from previous packet", HFILL}},
+        { &hf_vjc_d_ack,
+            { "Delta ack", "vjc.delta_ack", FT_UINT16, BASE_DEC,
+                NULL, 0x0, "Change in TCP acknowledgement number from previous packet", HFILL}},
+        { &hf_vjc_d_seq,
+            { "Delta seq", "vjc.delta_seq", FT_UINT16, BASE_DEC,
+                NULL, 0x0, "Change in TCP sequence number from previous packet", HFILL}},
+        { &hf_vjc_d_ipid,
+            { "Delta IP ID", "vjc.delta_ipid", FT_UINT16, BASE_DEC,
+                NULL, 0x0, "Change in IP Identification number from previous packet", HFILL}},
+        { &hf_vjc_tcpdata,
+            { "TCP data", "vjc.tcp_data", FT_BYTES, BASE_NONE,
+                NULL, 0x0, "Original TCP payload", HFILL}},
+    };
+
+    static gint *ett[] = {
+        &ett_vjc,
+        &ett_vjc_change_mask,
+    };
+
+    expert_module_t* expert_vjc;
+    static ei_register_info ei[] = {
+        { &ei_vjc_sawu,
+            { "vjc.special.sawu", PI_PROTOCOL, PI_CHAT,
+                ".... 1111 = special case for \"unidirectional data transfer\"", EXPFILL }},
+        { &ei_vjc_swu,
+            { "vjc.special.swu", PI_PROTOCOL, PI_CHAT,
+                ".... 1011 = special case for \"echoed interactive traffic\"", EXPFILL }},
+        { &ei_vjc_no_cnum,
+            { "vjc.no_connection_id", PI_PROTOCOL, PI_WARN,
+                "No connection ID and no prior connection (common at capture start)", EXPFILL }},
+        { &ei_vjc_no_conversation,
+            { "vjc.no_connection", PI_PROTOCOL, PI_WARN,
+                "No saved connection found (common at capture start)", EXPFILL }},
+        { &ei_vjc_no_direction,
+            { "vjc.no_direction", PI_UNDECODED, PI_WARN,
+                "Connection has no direction info, cannot decompress", EXPFILL }},
+        { &ei_vjc_no_conv_data,
+            { "vjc.no_connection_data", PI_UNDECODED, PI_WARN,
+                "Could not find saved connection data", EXPFILL }},
+        { &ei_vjc_undecoded,
+            { "vjc.no_decompress", PI_UNDECODED, PI_WARN,
+                "Undecoded data (impossible due to missing information)", EXPFILL }},
+        { &ei_vjc_bad_data,
+            { "vjc.bad_data", PI_PROTOCOL, PI_ERROR,
+                "Non-compliant packet data", EXPFILL }},
+        { &ei_vjc_error,
+            { "vjc.error", PI_MALFORMED, PI_ERROR,
+                "Unrecoverable dissector error", EXPFILL }},
+    };
+
+    proto_vjc = proto_register_protocol("Van Jacobson PPP compression", "VJC", "vjc");
+    proto_register_field_array(proto_vjc, hf, array_length(hf));
+    proto_register_subtree_array(ett, array_length(ett));
+    expert_vjc = expert_register_protocol(proto_vjc);
+    expert_register_field_array(expert_vjc, ei, array_length(ei));
+    vjcc_handle = register_dissector("vjc_compressed", dissect_vjc_comp, proto_vjc);
+    vjcu_handle = register_dissector("vjc_uncompressed", dissect_vjc_uncomp, proto_vjc);
+
+    register_init_routine(&vjc_init_protocol);
+    register_cleanup_routine(&vjc_cleanup_protocol);
+}
+
+void
+proto_reg_handoff_vjc(void)
+{
+    ip_handle = find_dissector("ip");
+
+    dissector_add_uint("ppp.protocol", PPP_VJC_COMP, vjcc_handle);
+    dissector_add_uint("ppp.protocol", PPP_VJC_UNCOMP, vjcu_handle);
+}
+
+/*
+ * Editor modelines  -  https://www.wireshark.org/tools/modelines.html
+ *
+ * Local variables:
+ * c-basic-offset: 4
+ * tab-width: 8
+ * indent-tabs-mode: nil
+ * End:
+ *
+ * vi: set shiftwidth=4 tabstop=8 expandtab:
+ * :indentSize=4:tabSize=8:noTabs=true:
+ */