X-Git-Url: http://www.chiark.greenend.org.uk/ucgi/~ian/git?p=secnet.git;a=blobdiff_plain;f=NOTES;h=f5ebc652045e20818d83188fd6f919e8deb11b53;hp=7ead923be70060d708e287b24d63a7dc2093e39a;hb=5c19b79c7c817a28a1582970b2685c98e05b2de5;hpb=5b5f297f9a9d47ee7e9804d5bdaa552f1953c6b6

diff --git a/NOTES b/NOTES
index 7ead923..f5ebc65 100644
--- a/NOTES
+++ b/NOTES
@@ -198,6 +198,8 @@ The optional additional data after the sender's name consists of some
 initial subset of the following list of items:
  * A 32-bit integer with a set of capability flags, representing the
    abilities of the sender.
+ * In MSG3/MSG4: a 16-bit integer being the sender's MTU, or zero.
+   (In other messages: nothing.)  See below.
  * More data which is yet to be defined and which must be ignored
    by receivers.
 The optional additional data after the receiver's name is not
@@ -220,6 +222,45 @@ No capability flags are currently defined.  Unknown capability flags
 should be treated as late ones.
 
 
+MTU handling
+
+In older versions of secnet, secnet was not capable of fragmentation
+or sending ICMP Frag Needed.  Administrators were expected to configure
+consistent MTUs across the network.
+
+It is still the case in the current version that the MTUs need to be
+configured reasonably coherently across the network: the allocated
+buffer sizes must be sufficient to cope with packets from all other
+peers.
+
+However, provided the buffers are sufficient, all packets will be
+processed properly: a secnet receiving a packet larger than the
+applicable MTU for its delivery will either fragment it, or reject it
+with ICMP Frag Needed.
+
+The MTU additional data field allows secnet to advertise an MTU to the
+peer.  This allows the sending end to handle overlarge packets, before
+they are transmitted across the underlying public network.  This can
+therefore be used to work around underlying network braindamage
+affecting large packets.
+
+If the MTU additional data field is zero or not present, then the peer
+should use locally-configured MTU information (normally, its local
+netlink MTU) instead.
+
+If it is nonzero, the peer may send packets up to the advertised size
+(and if that size is bigger than the peer's administratively
+configured size, the advertiser promises that its buffers can handle
+such a large packet).
+
+A secnet instance should not assume that just because it has
+advertised an mtu which is lower than usual for the vpn, the peer will
+honour it, unless the administrator knows that the peers are
+sufficiently modern to understand the mtu advertisement option.  So
+secnet will still accept packets which exceed the link MTU (whether
+negotiated or assumed).
+
+
 Messages:
 
 1) A->B: *,iA,msg1,A+,B+,nA
@@ -291,3 +332,37 @@ vaguely recent version of secnet.  (In fact, there is no evidence in
 the git history of it ever being sent.)
 
 This message number is reserved.
+
+11) *,*,PROD,A,B
+
+Sent in response to a NAK from B to A.  Requests that B initiates a
+key exchange with A, if B is willing and lacks a transport key for A.
+(If B doesn't have A's address configured, implicitly supplies A's
+public address.)
+
+This is necessary because if one end of a link (B) is restarted while
+a key exchange is in progress, the following bad state can persist:
+the non-restarted end (A) thinks that the key is still valid and keeps
+sending packets, but B either doesn't realise that a key exchange with
+A is necessary or (if A is a mobile site) doesn't know A's public IP
+address.
+
+Normally in these circumstances B would send NAKs to A, causing A to
+initiate a key exchange.  However if A and B were already in the
+middle of a key exchange then A will not want to try another one until
+the first one has timed out ("setup-time" x "setup-retries") and then
+the key exchange retry timeout ("wait-time") has elapsed.
+
+However if B's setup has timed out, B would be willing to participate
+in a key exchange initiated by A, if A could be induced to do so.
+This is the purpose of the PROD packet.
+
+We send no more PRODs than we would want to send data packets, to
+avoid a traffic amplification attack.  We also send them only in state
+WAIT, as in other states we wouldn't respond favourably.  And we only
+honour them if we don't already have a key.
+
+With PROD, the period of broken communication due to a key exchange
+interrupted by a restart is limited to the key exchange total
+retransmission timeout, rather than also including the key exchange
+retry timeout.