diff --git a/CHANGELOG b/CHANGELOG
index e7228c31..3188dc5a 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -13,6 +13,11 @@ HISTORY
 
   ++ New features:
 
+  2010-04-30: Simon Goldschmidt
+  * udp.h/.c, pbuf.h/.c: task #6849: added udp_send(_to/_if) functions that
+    take a precalculated checksum, added pbuf_fill_chksum() to copy data
+    into a pbuf and at the same time calculating the checksum for that data
+
   2010-04-29: Simon Goldschmidt
   * ip_addr.h, etharp.h/.c, autoip.c: Create overridable macros for copying
     2-byte-aligned IP addresses and MAC addresses
diff --git a/src/core/pbuf.c b/src/core/pbuf.c
index fe4b049b..ffe354d2 100644
--- a/src/core/pbuf.c
+++ b/src/core/pbuf.c
@@ -73,6 +73,9 @@
 #if TCP_QUEUE_OOSEQ
 #include "lwip/tcp_impl.h"
 #endif
+#if LWIP_CHECKSUM_ON_COPY
+#include "lwip/inet_chksum.h"
+#endif
 
 #include <string.h>
 
@@ -927,3 +930,44 @@ pbuf_coalesce(struct pbuf *p, pbuf_layer layer)
   pbuf_free(p);
   return q;
 }
+
+#if LWIP_CHECKSUM_ON_COPY
+/**
+ * Copies data into a single pbuf (*not* into a pbuf queue!) and updates
+ * the checksum while copying
+ *
+ * @param p the pbuf to copy data into
+ * @param start_offset offset of p->payload where to copy the data to
+ * @param dataptr data to copy into the pbuf
+ * @param len length of data to copy into the pbuf
+ * @param chksum pointer to the checksum which is updated
+ * @return ERR_OK if successful, another error if the data does not fit
+ *         within the (first) pbuf (no pbuf queues!)
+ */
+err_t
+pbuf_fill_chksum(struct pbuf *p, u16_t start_offset, const void *dataptr,
+                 u16_t len, u16_t *chksum)
+{
+  u32_t acc;
+  u16_t copy_chksum;
+  char *dst_ptr;
+  LWIP_ASSERT("p != NULL", p != NULL);
+  LWIP_ASSERT("dataptr != NULL", dataptr != NULL);
+  LWIP_ASSERT("chksum != NULL", chksum != NULL);
+  LWIP_ASSERT("len != 0", len != 0);
+
+  if ((start_offset >= p->len) || (start_offset + len > p->len)) {
+    return ERR_ARG;
+  }
+
+  dst_ptr = ((char*)p->payload) + start_offset;
+  copy_chksum = LWIP_CHKSUM_COPY(dst_ptr, dataptr, len);
+  if ((start_offset & 1) != 0) {
+    copy_chksum = SWAP_BYTES_IN_WORD(copy_chksum);
+  }
+  acc = *chksum;
+  acc += copy_chksum;
+  *chksum = FOLD_U32T(acc);
+  return ERR_OK;
+}
+#endif /* LWIP_CHECKSUM_ON_COPY */
diff --git a/src/core/udp.c b/src/core/udp.c
index 2ccf0d68..dbb12db6 100644
--- a/src/core/udp.c
+++ b/src/core/udp.c
@@ -336,6 +336,19 @@ udp_send(struct udp_pcb *pcb, struct pbuf *p)
   return udp_sendto(pcb, p, &pcb->remote_ip, pcb->remote_port);
 }
 
+#if LWIP_CHECKSUM_ON_COPY
+/** Same as udp_send() but with checksum
+ */
+err_t
+udp_send_chksum(struct udp_pcb *pcb, struct pbuf *p,
+                u8_t have_chksum, u16_t chksum)
+{
+  /* send to the packet using remote ip and port stored in the pcb */
+  return udp_sendto_chksum(pcb, p, &pcb->remote_ip, pcb->remote_port,
+    have_chksum, chksum);
+}
+#endif /* LWIP_CHECKSUM_ON_COPY */
+
 /**
  * Send data to a specified address using UDP.
  *
@@ -357,6 +370,16 @@ err_t
 udp_sendto(struct udp_pcb *pcb, struct pbuf *p,
   ip_addr_t *dst_ip, u16_t dst_port)
 {
+#if LWIP_CHECKSUM_ON_COPY
+  return udp_sendto_chksum(pcb, p, dst_ip, dst_port, 0, 0);
+}
+
+/** Same as udp_sendto(), but with checksum */
+err_t
+udp_sendto_chksum(struct udp_pcb *pcb, struct pbuf *p, ip_addr_t *dst_ip,
+                  u16_t dst_port, u8_t have_chksum, u16_t chksum)
+{
+#endif /* LWIP_CHECKSUM_ON_COPY */
   struct netif *netif;
 
   LWIP_DEBUGF(UDP_DEBUG | LWIP_DBG_TRACE, ("udp_send\n"));
@@ -375,7 +398,11 @@ udp_sendto(struct udp_pcb *pcb, struct pbuf *p,
     UDP_STATS_INC(udp.rterr);
     return ERR_RTE;
   }
+#if LWIP_CHECKSUM_ON_COPY
+  return udp_sendto_if_chksum(pcb, p, dst_ip, dst_port, netif, have_chksum, chksum);
+#else /* LWIP_CHECKSUM_ON_COPY */
   return udp_sendto_if(pcb, p, dst_ip, dst_port, netif);
+#endif /* LWIP_CHECKSUM_ON_COPY */
 }
 
 /**
@@ -401,6 +428,17 @@ err_t
 udp_sendto_if(struct udp_pcb *pcb, struct pbuf *p,
   ip_addr_t *dst_ip, u16_t dst_port, struct netif *netif)
 {
+#if LWIP_CHECKSUM_ON_COPY
+  return udp_sendto_if_chksum(pcb, p, dst_ip, dst_port, netif, 0, 0);
+}
+
+/** Same as udp_sendto_if(), but with checksum */
+err_t
+udp_sendto_if_chksum(struct udp_pcb *pcb, struct pbuf *p, ip_addr_t *dst_ip,
+                     u16_t dst_port, struct netif *netif, u8_t have_chksum,
+                     u16_t chksum)
+{
+#endif /* LWIP_CHECKSUM_ON_COPY */
   struct udp_hdr *udphdr;
   ip_addr_t *src_ip;
   err_t err;
@@ -501,7 +539,18 @@ udp_sendto_if(struct udp_pcb *pcb, struct pbuf *p,
     /* calculate checksum */
 #if CHECKSUM_GEN_UDP
     udphdr->chksum = inet_chksum_pseudo_partial(q, src_ip, dst_ip,
-                                        IP_PROTO_UDPLITE, q->tot_len, chklen);
+      IP_PROTO_UDPLITE, q->tot_len,
+#if !LWIP_CHECKSUM_ON_COPY
+      chklen);
+#else /* !LWIP_CHECKSUM_ON_COPY */
+      (have_chksum ? UDP_HLEN : chklen));
+    if (have_chksum) {
+      u32_t acc;
+      acc = udphdr->chksum + (u16_t)~(chksum);
+      udphdr->chksum = FOLD_U32T(acc);
+    }
+#endif /* !LWIP_CHECKSUM_ON_COPY */
+
     /* chksum zero must become 0xffff, as zero means 'no checksum' */
     if (udphdr->chksum == 0x0000) {
       udphdr->chksum = 0xffff;
@@ -524,11 +573,25 @@ udp_sendto_if(struct udp_pcb *pcb, struct pbuf *p,
     /* calculate checksum */
 #if CHECKSUM_GEN_UDP
     if ((pcb->flags & UDP_FLAGS_NOCHKSUM) == 0) {
-      udphdr->chksum = inet_chksum_pseudo(q, src_ip, dst_ip, IP_PROTO_UDP, q->tot_len);
-      /* chksum zero must become 0xffff, as zero means 'no checksum' */
-      if (udphdr->chksum == 0x0000) {
-        udphdr->chksum = 0xffff;
+      u16_t udpchksum;
+#if LWIP_CHECKSUM_ON_COPY
+      if (have_chksum) {
+        u32_t acc;
+        udpchksum = inet_chksum_pseudo_partial(q, src_ip, dst_ip, IP_PROTO_UDP,
+          q->tot_len, UDP_HLEN);
+        acc = udpchksum + (u16_t)~(chksum);
+        udpchksum = FOLD_U32T(acc);
+      } else
+#endif /* LWIP_CHECKSUM_ON_COPY */
+      {
+        udpchksum = inet_chksum_pseudo(q, src_ip, dst_ip, IP_PROTO_UDP, q->tot_len);
       }
+
+      /* chksum zero must become 0xffff, as zero means 'no checksum' */
+      if (udpchksum == 0x0000) {
+        udpchksum = 0xffff;
+      }
+      udphdr->chksum = udpchksum;
     }
 #endif /* CHECKSUM_GEN_UDP */
     LWIP_DEBUGF(UDP_DEBUG, ("udp_send: UDP checksum 0x%04"X16_F"\n", udphdr->chksum));
diff --git a/src/include/lwip/pbuf.h b/src/include/lwip/pbuf.h
index 9b4c024a..9ab75afd 100644
--- a/src/include/lwip/pbuf.h
+++ b/src/include/lwip/pbuf.h
@@ -111,6 +111,10 @@ err_t pbuf_copy(struct pbuf *p_to, struct pbuf *p_from);
 u16_t pbuf_copy_partial(struct pbuf *p, void *dataptr, u16_t len, u16_t offset);
 err_t pbuf_take(struct pbuf *buf, const void *dataptr, u16_t len);
 struct pbuf *pbuf_coalesce(struct pbuf *p, pbuf_layer layer);
+#if LWIP_CHECKSUM_ON_COPY
+err_t pbuf_fill_chksum(struct pbuf *p, u16_t start_offset, const void *dataptr,
+                       u16_t len, u16_t *chksum);
+#endif /* LWIP_CHECKSUM_ON_COPY */
 
 #ifdef __cplusplus
 }
diff --git a/src/include/lwip/udp.h b/src/include/lwip/udp.h
index fd3b9d4f..ac1bb769 100644
--- a/src/include/lwip/udp.h
+++ b/src/include/lwip/udp.h
@@ -135,6 +135,18 @@ err_t            udp_sendto     (struct udp_pcb *pcb, struct pbuf *p,
                                  ip_addr_t *dst_ip, u16_t dst_port);
 err_t            udp_send       (struct udp_pcb *pcb, struct pbuf *p);
 
+#if LWIP_CHECKSUM_ON_COPY
+err_t            udp_sendto_if_chksum(struct udp_pcb *pcb, struct pbuf *p,
+                                 ip_addr_t *dst_ip, u16_t dst_port,
+                                 struct netif *netif, u8_t have_chksum,
+                                 u16_t chksum);
+err_t            udp_sendto_chksum(struct udp_pcb *pcb, struct pbuf *p,
+                                 ip_addr_t *dst_ip, u16_t dst_port,
+                                 u8_t have_chksum, u16_t chksum);
+err_t            udp_send_chksum(struct udp_pcb *pcb, struct pbuf *p,
+                                 u8_t have_chksum, u16_t chksum);
+#endif /* LWIP_CHECKSUM_ON_COPY */
+
 #define          udp_flags(pcb) ((pcb)->flags)
 #define          udp_setflags(pcb, f)  ((pcb)->flags = (f))