site: Make return value of transforms be an enum

[secnet.git] / NOTES

diff --git a/NOTES b/NOTES
index 40cbf04ecea71b01c329443248c7c3f110ebf1a0..840f06fc203540eec36d7fce7a1ff2b2bdea8d27 100644 (file)
--- 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.
 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
  * 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


@@ -216,13 +218,62 @@ Capability flag bits must be in one the following two categories:
    applicable.  They may also appear in MSG1, but this is not
    guaranteed.  MSG4 must advertise the same set as MSG2.
 
    applicable.  They may also appear in MSG1, but this is not
    guaranteed.  MSG4 must advertise the same set as MSG2.
 

-No capability flags are currently defined.  Unknown capability flags
-should be treated as late ones.
+Currently, the low 16 bits are allocated for negotiating bulk-crypto
+transforms.  Bits 8 to 15 are used by Secnet as default capability
+numbers for the various kinds of transform closures: bit 8 is for the
+original CBCMAC-based transform, and bit 9 for the new EAX transform;
+bits 10 to 15 are reserved for future expansion.  The the low eight bits
+are reserved for local use, e.g., to allow migration from one set of
+parameters for a particular transform to a different, incompatible set
+of parameters for the same transform.  Bit 31, if advertised by both
+ends, indicates that a mobile end gets priority in case of crossed MSG1.
+The remaining bits have not yet been assigned a purpose.
+
+No early capability bits are currently defined.
+
+
+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:
 
 
 
 Messages:
 

-1) A->B: *,iA,msg1,A+,B+,nA
+1) A->B: i*,iA,msg1,A+,B+,nA

 
 i* must be encoded as 0.  (However, it is permitted for a site to use
 zero as its "index" for another site.)
 
 i* must be encoded as 0.  (However, it is permitted for a site to use
 zero as its "index" for another site.)