Prop. 159 MTU and RI block updates

i2p2www/spec/proposals/159-ssu2.rst

@@ -128,6 +128,8 @@ Design Goals
 
 - Avoid any year 2038 issues in timestamps, must work until at least 2106.
 
+- Increase minimum MTU from 620 to 1280 for efficiency and ease of implementation.
+
 - Maintain a max I2NP message size of approximately 32K, as in SSU.
   Increase to 64 KB? TBD
 
@@ -3835,6 +3837,7 @@ Notes
   The simplest implementation may be to never include I2NP blocks in
   the Session Confirmed message, and send all I2NP blocks in
   subsequent Data messages.
+  See Router Info block section below for maximum block size.
 
 
 KDF for data phase
@@ -4383,10 +4386,33 @@ Notes:
   Compression is recommended if it allows a Router Info to fit
   in a single message without fragmentation.
 
+- Maximum size of first or only fragment in the Session Confirmed message:
+  MTU - 113 for IPv4 or MTU - 133 for IPv6.
+  Assuming 1500 byte default MTU, and no other blocks in the message,
+  1387 for IPv4 or 1367 for IPv6.
+  97% of current router infos are smaller than 1367 witout gzipping.
+  99.9% of current router infos are smaller than 1367 when gzipped.
+  Assuming 1280 byte minimum MTU, and no other blocks in the message,
+  1167 for IPv4 or 1147 for IPv6.
+  94% of current router infos are smaller than 1147 witout gzipping.
+  97% of current router infos are smaller than 1147 when gzipped.
+
+- Maximum size of a follow-on fragment in a Data message:
+  MTU - TBD for IPv4 or MTU - TBD for IPv6.
+  Assuming 1500 byte default MTU, and no other blocks in the message,
+  TBD for IPv4 or TBD for IPv6.
+  Assuming 1280 byte minimum MTU, and no other blocks in the message,
+  TBD for IPv4 or TBD for IPv6.
+
 - If the Router Info is compressed AND fragmented,
   the data is compressed first and then fragmented.
   The fragments are not individually compressed.
 
+- State machine handling and retransmission for a fragmented router info
+  in the handshake will be quite complex; it would be much easier
+  to simply require the router info to be unfragmented,
+  and avoid outbound SSU2 connections if the local router info is too large.
+
 - Flooding must not be requested unless there are published
   RouterAddresses in the RouterInfo. The receiving router
   must not flood the RouterInfo unless there are published
@@ -5563,20 +5589,24 @@ MTU
 ========
 
 Requirements vary, based on whether the published address is shared with SSU 1.
-Current SSU 1 IPv4 minimum is 620, which is probably too small.
+Current SSU 1 IPv4 minimum is 620, which is definitely too small.
+
+The minimum SSU2 MTU is 1280 for both IPv4 and IPv6,
+which is the same as specified in [RFC-9000]_.
+See below.
+
 
 SSU Address
 ------------
 Shared address with SSU 1, must follow SSU 1 rules.
-IPv4: Default and max is 1484. Min is 620.
+IPv4: Default and max is 1484. Min is 1292.
 (IPv4 MTU + 4) must be a multiple of 16.
 IPv6: Must be published, min is 1280 and the max is 1488.
 IPv6 MTU must be a multiple of 16.
 
 SSU2 Address
 ------------
-Must follow SSU 1 rules.
+IPv4: Default and max is 1500. Min is 1280.
-IPv4: Default and max is 1500. Min is TBD.
 IPv6: Default and max is 1500. Min is 1280.
 No multiple of 16 rules, but should probably be a multiple of 2 at least.