From b3c6273efaa15976cc1290f00d5abde4991e184e Mon Sep 17 00:00:00 2001
From: "Ondrej Zajicek (work)" <santiago@crfreenet.org>
Date: Fri, 9 Jun 2017 13:00:20 +0200
Subject: [PATCH] Babel: Implement IPv6 prefix compression on outgoing updates
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Previously, the Babel protocol would never use prefix compression on outgoing
updates (but would parse it on incoming ones). This adds compression of IPv6
addresses of outgoing updates.

The compression only works to the extent that the FIB is walked in lexicographic
order; i.e. a prefix is only compressed if it shares bytes with the previous
prefix in the same packet.

Thanks to Toke Høiland-Jørgensen <toke@toke.dk> for the patch.
---
 proto/babel/packets.c | 34 +++++++++++++++++++++++++++++++++-
 1 file changed, 33 insertions(+), 1 deletion(-)

diff --git a/proto/babel/packets.c b/proto/babel/packets.c
index b52c2ddf..72ac4f29 100644
--- a/proto/babel/packets.c
+++ b/proto/babel/packets.c
@@ -133,6 +133,8 @@ struct babel_write_state {
   u8 router_id_seen;
   ip_addr next_hop_ip4;
   ip_addr next_hop_ip6;
+  u8 def_ip6_prefix[16];	/* Implicit IPv6 prefix in network order */
+  u8 def_ip6_pxlen;
 };
 
 
@@ -150,6 +152,14 @@ struct babel_write_state {
 
 #define NET_SIZE(n) BYTES(net_pxlen(n))
 
+static inline uint
+bytes_equal(u8 *b1, u8 *b2, uint maxlen)
+{
+  uint i;
+  for (i = 0; (i < maxlen) && (*b1 == *b2); i++, b1++, b2++)
+    ;
+  return i;
+}
 
 static inline u16
 get_time16(const void *p)
@@ -696,7 +706,29 @@ babel_write_update(struct babel_tlv *hdr, union babel_msg *m,
   {
     tlv->ae = BABEL_AE_IP6;
     tlv->plen = net6_pxlen(&msg->net);
-    put_ip6_px(tlv->addr, &msg->net);
+
+    /* Address compression - omit initial matching bytes */
+    u8 buf[16], omit;
+    put_ip6(buf, net6_prefix(&msg->net));
+    omit = bytes_equal(buf, state->def_ip6_prefix,
+		       MIN(tlv->plen, state->def_ip6_pxlen) / 8);
+
+    if (omit > 0)
+    {
+      memcpy(tlv->addr, buf + omit, NET_SIZE(&msg->net) - omit);
+
+      tlv->omitted = omit;
+      tlv->length -= omit;
+      len -= omit;
+    }
+    else
+    {
+      put_ip6_px(tlv->addr, &msg->net);
+      tlv->flags |= BABEL_FLAG_DEF_PREFIX;
+
+      put_ip6(state->def_ip6_prefix, net6_prefix(&msg->net));
+      state->def_ip6_pxlen = tlv->plen;
+    }
   }
 
   put_time16(&tlv->interval, msg->interval);