Subject: Re: [XFRM]: Improve MTU estimation

From: Patrick McHardy <[email-masked]>
Date: Fri, 04 Aug 2006 13:51:24 +0200

Patrick McHardy wrote:
> Now I get it, thanks :) I missed that the IP header isn't part of the
> length when it is aligned. So the worst-case increases by block-size
> - 4 (- 8 for IPv6). How does this look?

Forgot the patch ..

diff --git a/include/net/xfrm.h b/include/net/xfrm.h
index 9c5ee9f..ea1b028 100644
--- a/include/net/xfrm.h
+++ b/include/net/xfrm.h
@@ -262,8 +262,7 @@ struct xfrm_type
 	void			(*destructor)(struct xfrm_state *);
 	int			(*input)(struct xfrm_state *, struct sk_buff *skb);
 	int			(*output)(struct xfrm_state *, struct sk_buff *pskb);
-	/* Estimate maximal size of result of transformation of a dgram */
-	u32			(*get_max_size)(struct xfrm_state *, int size);
+	u32			(*get_mtu)(struct xfrm_state *, int size);
 };
 
 extern int xfrm_register_type(struct xfrm_type *type, unsigned short family);
diff --git a/net/ipv4/esp4.c b/net/ipv4/esp4.c
index fc2f8ce..1ce06cc 100644
--- a/net/ipv4/esp4.c
+++ b/net/ipv4/esp4.c
@@ -251,21 +251,23 @@ out:
 	return -EINVAL;
 }
 
-static u32 esp4_get_max_size(struct xfrm_state *x, int mtu)
+static u32 esp4_get_mtu(struct xfrm_state *x, int mtu)
 {
 	struct esp_data *esp = x->data;
 	u32 blksize = ALIGN(crypto_tfm_alg_blocksize(esp->conf.tfm), 4);
+	u32 align;
 
-	if (x->props.mode) {
-		mtu = ALIGN(mtu + 2, blksize);
-	} else {
-		/* The worst case. */
-		mtu = ALIGN(mtu + 2, 4) + blksize - 4;
-	}
-	if (esp->conf.padlen)
-		mtu = ALIGN(mtu, esp->conf.padlen);
+	align = blksize;
+	if (esp->conf.padlen > align)
+		align = esp->conf.padlen;
 
-	return mtu + x->props.header_len + esp->auth.icv_trunc_len;
+	mtu -= x->props.header_len + esp->auth.icv_trunc_len;
+	mtu &= ~(align - 1);
+	mtu -= 2;
+	if (!x->props.mode)
+		mtu -= blksize - 4;
+
+	return mtu;
 }
 
 static void esp4_err(struct sk_buff *skb, u32 info)
@@ -307,6 +309,7 @@ static void esp_destroy(struct xfrm_stat
 static int esp_init_state(struct xfrm_state *x)
 {
 	struct esp_data *esp = NULL;
+	u32 align;
 
 	/* null auth and encryption can have zero length keys */
 	if (x->aalg) {
@@ -385,7 +388,10 @@ static int esp_init_state(struct xfrm_st
 		}
 	}
 	x->data = esp;
-	x->props.trailer_len = esp4_get_max_size(x, 0) - x->props.header_len;
+	align = ALIGN(crypto_tfm_alg_blocksize(esp->conf.tfm), 4);
+	if (esp->conf.padlen)
+		align = ALIGN(align, esp->conf.padlen);
+	x->props.trailer_len = align - 1 + esp->auth.icv_trunc_len;
 	return 0;
 
 error:
@@ -402,7 +408,7 @@ static struct xfrm_type esp_type =
 	.proto	     	= IPPROTO_ESP,
 	.init_state	= esp_init_state,
 	.destructor	= esp_destroy,
-	.get_max_size	= esp4_get_max_size,
+	.get_mtu	= esp4_get_mtu,
 	.input		= esp_input,
 	.output		= esp_output
 };
diff --git a/net/ipv6/esp6.c b/net/ipv6/esp6.c
index a278d5e..0c9c159 100644
--- a/net/ipv6/esp6.c
+++ b/net/ipv6/esp6.c
@@ -222,22 +222,23 @@ out:
 	return ret;
 }
 
-static u32 esp6_get_max_size(struct xfrm_state *x, int mtu)
+static u32 esp6_get_mtu(struct xfrm_state *x, int mtu)
 {
 	struct esp_data *esp = x->data;
 	u32 blksize = ALIGN(crypto_tfm_alg_blocksize(esp->conf.tfm), 4);
-
-	if (x->props.mode) {
-		mtu = ALIGN(mtu + 2, blksize);
-	} else {
-		/* The worst case. */
-		u32 padsize = ((blksize - 1) & 7) + 1;
-		mtu = ALIGN(mtu + 2, padsize) + blksize - padsize;
-	}
-	if (esp->conf.padlen)
-		mtu = ALIGN(mtu, esp->conf.padlen);
-
-	return mtu + x->props.header_len + esp->auth.icv_trunc_len;
+	u32 align;
+
+	align = blksize;
+	if (esp->conf.padlen > align)
+		align = esp->conf.padlen;
+	
+	mtu -= x->props.header_len + esp->auth.icv_trunc_len;
+	mtu &= ~(align - 1);
+	mtu -= 2;
+	if (!x->props.mode && blksize >= 8)
+		mtu -= blksize - 8;
+
+	return mtu;
 }
 
 static void esp6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
@@ -363,7 +364,7 @@ static struct xfrm_type esp6_type =
 	.proto	     	= IPPROTO_ESP,
 	.init_state	= esp6_init_state,
 	.destructor	= esp6_destroy,
-	.get_max_size	= esp6_get_max_size,
+	.get_mtu	= esp6_get_mtu,
 	.input		= esp6_input,
 	.output		= esp6_output
 };
diff --git a/net/xfrm/xfrm_state.c b/net/xfrm/xfrm_state.c
index 0021aad..7b0b100 100644
--- a/net/xfrm/xfrm_state.c
+++ b/net/xfrm/xfrm_state.c
@@ -1129,37 +1129,20 @@ void xfrm_state_delete_tunnel(struct xfr
 }
 EXPORT_SYMBOL(xfrm_state_delete_tunnel);
 
-/*
- * This function is NOT optimal.  For example, with ESP it will give an
- * MTU that's usually two bytes short of being optimal.  However, it will
- * usually give an answer that's a multiple of 4 provided the input is
- * also a multiple of 4.
- */
 int xfrm_state_mtu(struct xfrm_state *x, int mtu)
 {
-	int res = mtu;
-
-	res -= x->props.header_len;
-
-	for (;;) {
-		int m = res;
-
-		if (m < 68)
-			return 68;
-
-		spin_lock_bh(&x->lock);
-		if (x->km.state == XFRM_STATE_VALID &&
-		    x->type && x->type->get_max_size)
-			m = x->type->get_max_size(x, m);
-		else
-			m += x->props.header_len;
-		spin_unlock_bh(&x->lock);
+	int res;
 
-		if (m <= mtu)
-			break;
-		res -= (m - mtu);
-	}
+	spin_lock_bh(&x->lock);
+	if (x->km.state == XFRM_STATE_VALID &&
+	    x->type && x->type->get_mtu)
+		res = x->type->get_mtu(x, mtu);
+	else
+		res = mtu - x->props.header_len;
+	spin_unlock_bh(&x->lock);
 
+	if (res < 68)
+		res = 68;
 	return res;
 }