gnrc_sixlowpan_frag: Initial import of a fragment size hint feature

This allows for sending of fragments smaller than the restrictions set
by the link-layer PDU. E.g. to put some slack for IPHC into the first
fragment (see https://tools.ietf.org/html/draft-ietf-6lo-fragment-recovery-02#section-4.1).

makefiles/pseudomodules.inc.mk

@@ -28,6 +28,7 @@ PSEUDOMODULES += gnrc_netif_dedup
 PSEUDOMODULES += gnrc_sixloenc
 PSEUDOMODULES += gnrc_sixlowpan_border_router_default
 PSEUDOMODULES += gnrc_sixlowpan_default
+PSEUDOMODULES += gnrc_sixlowpan_frag_hint
 PSEUDOMODULES += gnrc_sixlowpan_iphc_nhc
 PSEUDOMODULES += gnrc_sixlowpan_nd_border_router
 PSEUDOMODULES += gnrc_sixlowpan_router

sys/include/net/gnrc/sixlowpan/frag.h

@@ -32,6 +32,9 @@
 #include "msg.h"
 #include "net/gnrc/pkt.h"
 #include "net/gnrc/netif/hdr.h"
+#ifdef MODULE_GNRC_SIXLOWPAN_FRAG_HINT
+#include "net/gnrc/sixlowpan/frag/hint.h"
+#endif  /* MODULE_GNRC_SIXLOWPAN_FRAG_HINT */
 #include "net/gnrc/sixlowpan/internal.h"
 #include "net/ieee802154.h"
 #include "net/sixlowpan.h"
@@ -94,6 +97,13 @@ typedef struct {
     uint16_t tag;           /**< Tag used for the fragment */
     uint16_t offset;        /**< Offset of the Nth fragment from the beginning of the
                              *   payload datagram */
+#ifdef MODULE_GNRC_SIXLOWPAN_FRAG_HINT
+    /**
+     * @brief   Hint for the size (smaller than link-layer PDU) of the
+     *          *first fragment*
+     */
+    gnrc_sixlowpan_frag_hint_t hint;
+#endif /* MODULE_GNRC_SIXLOWPAN_FRAG_HINT */
 } gnrc_sixlowpan_msg_frag_t;
 
 /**

sys/include/net/gnrc/sixlowpan/frag/hint.h

@@ -0,0 +1,59 @@
+/*
+ * Copyright (C) 2019 Freie Universität Berlin
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @defgroup    net_gnrc_sixlowpan_frag_hint    Fragment size hint
+ * @ingroup     net_gnrc_sixlowpan_frag
+ * @brief       Provides a hint for smaller fragment sizes than the link-layer
+ *              PDU for the *first fragment*.
+ * @{
+ *
+ * @file
+ * @brief   Definitions to provide a hint on the final fragment size
+ *
+ * @author  Martine Lenders <[email protected]>
+ */
+#ifndef NET_GNRC_SIXLOWPAN_FRAG_HINT_H
+#define NET_GNRC_SIXLOWPAN_FRAG_HINT_H
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/**
+ * @brief   A fragment size hint
+ */
+typedef struct {
+    /**
+     * @brief   Provides a hint of the size for the next fragment to send
+     *
+     * Set to 0 for fitting to the maximum fragment size of the interface the
+     * datagram is sent over.
+     *
+     * @see gnrc_netif_6lo_t::max_frag_size
+     */
+    uint16_t fragsz;
+    /**
+     * @brief   The size of the data bound by gnrc_sixlowpan_frag_hint_t::fragsz
+     *          uncompressed
+     *
+     * This is only evaluated when gnrc_sixlowpan_frag_hint_t::fragsz is greater
+     * than 0.
+     * Required to calculate the proper offset for the next fragment.
+     */
+    uint16_t fragsz_uncomp;
+} gnrc_sixlowpan_frag_hint_t;
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* NET_GNRC_SIXLOWPAN_FRAG_HINT_H */
+/** @} */

sys/net/gnrc/network_layer/sixlowpan/frag/gnrc_sixlowpan_frag.c

@@ -50,13 +50,48 @@ static inline size_t _min(size_t a, size_t b)
     return (a < b) ? a : b;
 }
 
+static inline uint8_t _max_frag_size(gnrc_netif_t *iface,
+                                     gnrc_sixlowpan_msg_frag_t *fragment_msg)
+{
+#ifdef MODULE_GNRC_SIXLOWPAN_FRAG_HINT
+    if (fragment_msg->hint.fragsz > 0) {
+        /* account for rounding down to 8*/
+        return (fragment_msg->hint.fragsz & 0x7)
+               ? (fragment_msg->hint.fragsz + 8U)
+               : fragment_msg->hint.fragsz;
+    }
+#endif /* MODULE_GNRC_SIXLOWPAN_FRAG_HINT */
+    (void)fragment_msg;
+    return iface->sixlo.max_frag_size;
+}
+
+static inline int _payload_diff(gnrc_sixlowpan_msg_frag_t *fragment_msg,
+                                size_t payload_len)
+{
+#ifdef MODULE_GNRC_SIXLOWPAN_FRAG_HINT
+    if (fragment_msg->hint.fragsz > 0) {
+        return fragment_msg->hint.fragsz_uncomp - fragment_msg->hint.fragsz;
+    }
+#endif /* MODULE_GNRC_SIXLOWPAN_FRAG_HINT */
+    return (fragment_msg->datagram_size - payload_len);
+}
+
 static gnrc_pktsnip_t *_build_frag_pkt(gnrc_pktsnip_t *pkt,
                                        gnrc_sixlowpan_msg_frag_t *fragment_msg,
                                        size_t payload_len, size_t size)
 {
     sixlowpan_frag_t *frag_hdr;
     gnrc_netif_hdr_t *netif_hdr = pkt->data, *new_netif_hdr;
     gnrc_pktsnip_t *netif, *frag;
+#ifdef MODULE_GNRC_SIXLOWPAN_FRAG_HINT
+    size_t fragment_size = ((fragment_msg->hint.fragsz > 0) &&
+                            (fragment_msg->offset == 0))
+                         ? size     /* we want the calculated fragment size
+                                     * to include full IPHC header */
+                         : _min(size, payload_len);
+#else   /* MODULE_GNRC_SIXLOWPAN_FRAG_HINT */
+    size_t fragment_size = _min(size, payload_len);
+#endif  /* MODULE_GNRC_SIXLOWPAN_FRAG_HINT */
 
     netif = gnrc_netif_hdr_build(gnrc_netif_hdr_get_src_addr(netif_hdr),
                                  netif_hdr->src_l2addr_len,
@@ -74,8 +109,7 @@ static gnrc_pktsnip_t *_build_frag_pkt(gnrc_pktsnip_t *pkt,
     new_netif_hdr->rssi = netif_hdr->rssi;
     new_netif_hdr->lqi = netif_hdr->lqi;
 
-    frag = gnrc_pktbuf_add(NULL, NULL, _min(size, payload_len),
-                           GNRC_NETTYPE_SIXLOWPAN);
+    frag = gnrc_pktbuf_add(NULL, NULL, fragment_size, GNRC_NETTYPE_SIXLOWPAN);
 
     if (frag == NULL) {
         DEBUG("6lo frag: error allocating first fragment\n");
@@ -102,12 +136,13 @@ static uint16_t _send_1st_fragment(gnrc_netif_t *iface,
     uint8_t *data;
     /* payload_len: actual size of the packet vs
      * datagram_size: size of the uncompressed IPv6 packet */
-    int payload_diff = (fragment_msg->datagram_size - payload_len);
+    int payload_diff = _payload_diff(fragment_msg, payload_len);
     uint16_t local_offset = 0;
     /* virtually add payload_diff to flooring to account for offset (must be divisable by 8)
      * in uncompressed datagram */
-    uint16_t max_frag_size = _floor8(iface->sixlo.max_frag_size + payload_diff -
-                                     sizeof(sixlowpan_frag_t)) - payload_diff;
+    uint16_t max_frag_size = _floor8(_max_frag_size(iface, fragment_msg) +
+                                     payload_diff - sizeof(sixlowpan_frag_t)) -
+                             payload_diff;
 
     DEBUG("6lo frag: determined max_frag_size = %" PRIu16 "\n", max_frag_size);
 
@@ -158,7 +193,8 @@ static uint16_t _send_nth_fragment(gnrc_netif_t *iface,
     uint16_t local_offset = 0, offset_count = 0, offset = fragment_msg->offset;
     /* since dispatches aren't supposed to go into subsequent fragments, we need not account
      * for payload difference as for the first fragment */
-    uint16_t max_frag_size = _floor8(iface->sixlo.max_frag_size - sizeof(sixlowpan_frag_n_t));
+    uint16_t max_frag_size = _floor8(iface->sixlo.max_frag_size -
+                                     sizeof(sixlowpan_frag_n_t));
 
     DEBUG("6lo frag: determined max_frag_size = %" PRIu16 "\n", max_frag_size);
 
@@ -174,8 +210,8 @@ static uint16_t _send_nth_fragment(gnrc_netif_t *iface,
     data = (uint8_t *)(hdr + 1);
     hdr->disp_size.u8[0] |= SIXLOWPAN_FRAG_N_DISP;
     /* don't mention payload diff in offset */
-    hdr->offset = (uint8_t)((offset +
-                             (fragment_msg->datagram_size - payload_len)) >> 3);
+    hdr->offset = (uint8_t)((offset + _payload_diff(fragment_msg,
+                                                    payload_len)) >> 3);
     pkt = pkt->next;    /* don't copy netif header */
 
     while ((pkt != NULL) && (offset_count != offset)) {   /* go to offset */

sys/net/gnrc/network_layer/sixlowpan/gnrc_sixlowpan.c

@@ -134,6 +134,9 @@ void gnrc_sixlowpan_multiplex_by_size(gnrc_pktsnip_t *pkt,
         fragment_msg->tag = gnrc_sixlowpan_frag_next_tag();
         /* Sending the first fragment has an offset==0 */
         fragment_msg->offset = 0;
+#ifdef MODULE_GNRC_SIXLOWPAN_FRAG_HINT
+        fragment_msg->hint.fragsz = 0;
+#endif
 
         gnrc_sixlowpan_frag_send(pkt, fragment_msg, page);
     }