Randomize packet numbers more, not less (#964)

* Randomize packet numbers more, not less

Closes #864, #850.

* More precisely define the range (not that it matters)

* Remove sentence about encodings

draft-ietf-quic-transport.md

@@ -743,14 +743,22 @@ have special rules for populating the packet number field.
 
 ### Initial Packet Number {#initial-packet-number}
 
-The initial value for packet number MUST be selected from an uniform random
-distribution between 0 and 2^31-1.  That is, the lower 31 bits of the packet
-number are randomized.  {{?RFC4086}} provides guidance on the generation of
-random values.
-
-The first set of packets sent by an endpoint MUST include the low 32-bits of the
-packet number.  Once any packet has been acknowledged, subsequent packets can
-use a shorter packet number encoding.
+The initial value for packet number MUST be selected randomly from a range
+between 0 and 2^32 - 1025 (inclusive).  This value is selected so that Initial
+and Handshake packets exercise as many possible values for the Packet Number
+field as possible.
+
+Limiting the range allows both for loss of packets and for any stateless
+exchanges.  Packet numbers are incremented for subsequent packets, but packet
+loss and stateless handling can both mean that the first packet sent by an
+endpoint isn't necessarily the first packet received by its peer.  The first
+packet received by a peer cannot be 2^32 or greater or the recipient will
+incorrectly assume a packet number that is 2^32 values lower and discard the
+packet.
+
+Use of a secure random number generator {{?RFC4086}} is not necessary for
+generating the initial packet number, nor is it necessary that the value be
+uniformly distributed.
 
 
 ## Handling Packets from Different Versions {#version-specific}