batman-adv: fragment size adjustments

Hi,

here is the second version of both fragment size patches. There is no actual change - just rebased on top of master.

I'll recommend the commit message (from Linus!) of the first patch for more details.

v2:

* rebased on top of master

Kind regards,
Sven

Matthias Schiffer (1):
batman-adv: decrease maximum fragment size

Sven Eckelmann (1):
batman-adv: Keep fragments equally sized

 net/batman-adv/fragmentation.c | 20 +++++++++++++-------
 net/batman-adv/main.h          |  2 +-
 2 files changed, 14 insertions(+), 8 deletions(-)

From: Matthias Schiffer <[email protected]>

With this patch the maximum fragment size is reduced from 1400 to 1280 bytes.

Fragmentation v2 correctly uses the smaller of 1400 and the interface MTU, thus generally supporting interfaces with an MTU < 1400 bytes, too.

However, currently "Fragmentation v2" does not support re-fragmentation. Which means that once a packet is split into two packets of 1400 + x bytes for instance and the next hop provides an interface with an even smaller MTU of 1280 bytes, then the larger fragment is lost.

A maximum fragment size of 1280 bytes is a safer option as this is the minimum MTU required by IPv6, making interfaces with an MTU < 1280 rather exotic.

Regarding performance, this should have no negative impact on unicast
traffic: Having some more bytes in the smaller and some less in the larger does not change the sum of both fragments.

Concerning TT, choosing 1280 bytes fragments might result in more TT messages than necessary when a large network is bridged into batman-adv. However, the TT overhead in general is marginal due to its reactive nature, therefore such a performance impact on TT should not be noticeable for a user.

---
v2:
- rebased on master

 net/batman-adv/main.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 57a8103..81c6a38 100644
@@ -168,7 +168,7 @@ enum batadv_uev_type {
 /* Maximum number of fragments for one packet */
 #define BATADV_FRAG_MAX_FRAGMENTS 16
 /* Maxumim size of each fragment */
-#define BATADV_FRAG_MAX_FRAG_SIZE 1400
+#define BATADV_FRAG_MAX_FRAG_SIZE 1280
 /* Time to keep fragments while waiting for rest of the fragments */
 #define BATADV_FRAG_TIMEOUT 10000
 

2.11.0

The batman-adv fragmentation packets have the design problem that they cannot be refragmented. This often leads to problems when networks are incorrectly configured and don't use a common MTU.

The sender could for example fragment a 1500 byte packet to fit in a 1280 bytes large MTU. This would create a 1280 large packet and a 284 bytes large packet. But the next hop is then not able to transport 1280 bytes to its next hop. The 1280 byte large packet will be dropped but the 284 bytes large packet will still be forwarded to its destination.

This can partly being avoided by splitting packets more equally. In this example, the two 782 bytes large packets could both potentially reach its destination.

---
v2:
- rebased on master

 net/batman-adv/fragmentation.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

index 11a23fd..8f964be 100644
@@ -404,7 +404,7 @@ bool batadv_frag_skb_fwd(struct sk_buff *skb,
  * batadv_frag_create - create a fragment from skb
  * @skb: skb to create fragment from
  * @frag_head: header to use in new fragment
- * @mtu: size of new fragment
+ * @fragment_size: size of new fragment
  *
  * Split the passed skb into two fragments: A new one with size matching the
  * passed mtu and the old one with the rest. The new skb contains data from the

@@ -414,11 +414,11 @@ bool batadv_frag_skb_fwd(struct sk_buff *skb,
  */
 static struct sk_buff *batadv_frag_create(struct sk_buff *skb,
 					  struct batadv_frag_packet *frag_head,
-					  unsigned int mtu)
+					  unsigned int fragment_size)
 {
 	struct sk_buff *skb_fragment;
 	unsigned int header_size = sizeof(*frag_head);
-	unsigned int fragment_size = mtu - header_size;
+	unsigned int mtu = fragment_size + header_size;
 
 	skb_fragment = netdev_alloc_skb(NULL, mtu + ETH_HLEN);
 	if (!skb_fragment)

@@ -456,7 +456,7 @@ int batadv_frag_send_packet(struct sk_buff *skb,
 	struct sk_buff *skb_fragment;
 	unsigned int mtu = neigh_node->if_incoming->net_dev->mtu;
 	unsigned int header_size = sizeof(frag_header);
-	unsigned int max_fragment_size, max_packet_size;
+	unsigned int max_fragment_size, num_fragments;
 	int ret;
 
 	/* To avoid merge and refragmentation at next-hops we never send

@@ -464,10 +464,15 @@ int batadv_frag_send_packet(struct sk_buff *skb,
 	 */
 	mtu = min_t(unsigned int, mtu, BATADV_FRAG_MAX_FRAG_SIZE);
 	max_fragment_size = mtu - header_size;
-	max_packet_size = max_fragment_size * BATADV_FRAG_MAX_FRAGMENTS;
+
+	if (skb->len == 0 || max_fragment_size == 0)
+		return -EINVAL;
+
+	num_fragments = (skb->len - 1) / max_fragment_size + 1;
+	max_fragment_size = (skb->len - 1) / num_fragments + 1;
 
 	/* Don't even try to fragment, if we need more than 16 fragments */
-	if (skb->len > max_packet_size) {
+	if (num_fragments > BATADV_FRAG_MAX_FRAGMENTS) {
 		ret = -EAGAIN;
 		goto free_skb;
 	}

@@ -507,7 +512,8 @@ int batadv_frag_send_packet(struct sk_buff *skb,
 			goto put_primary_if;
 		}
 
-		skb_fragment = batadv_frag_create(skb, &frag_header, mtu);
+		skb_fragment = batadv_frag_create(skb, &frag_header,
+						  max_fragment_size);
 		if (!skb_fragment) {
 			ret = -ENOMEM;
 			goto put_primary_if;

2.11.0

On Mittwoch, 22. Februar 2017 17:24:47 CET Sven Eckelmann wrote:
[...]

Patches applied in eb60b63140af [1] and 3caa5d14206c [2].

Thanks,
Sven

[1] https://git.open-mesh.org/batman-adv.git/commit/eb60b63140af5ec01ea0916837c2816cad10d6c1
[2] https://git.open-mesh.org/batman-adv.git/commit/3caa5d14206ce8d4bd48bc931f213dec47ea1566