This document is clear and well-written.  It can easily be implemented based on the description.

There are a few additional issues that should be considered with advice to implementers in Section 12 on performance considerations:
1) Invisibility of packet loss - Inner protocols that require packet losses as a signal of congestion (e.g. TCP) will have a challenge due to not being able to see any packet losses since the outer TCP will repair them (unless sending into a full outer TCP socket buffer shows up back to the inner TCP as a packet loss?).
2) Nesting of ECN -  Inner TCP connections will not be able to use effectively ECN on the portion of the path covered by the outer TCP connection.
3) Impact of congestion response on aggregate - The general "TCP in TCP" problem is mentioned, and is mostly appropriate for a single flow.  If an aggregate of flows is sharing the same outer TCP connection, there may be additional concerns about how the congestion response behavior impacts an aggregate of flows, since it may cause a shared delay spike even to low-rate flows rather than distributing losses proportional to per-flow throughput.
4) Additional potential for bufferbloat - Since TCP does not bound latency, some applications in the IPsec-protected aggregate could drive latency of the shared connection up and impact the aggregate of flows that may include real-time applications.  The socket buffer for the outer TCP connection might need to be limited in size to ensure some bounds?

Not addressing these could lead to poor experiences in deployment, if implementations make wrong assumptions or fail to consider them.

In the security considerations section, there are several RFCs on mechanisms to increase robustness to RST attacks and SYN floods that could be mentioned if it's worthwhile.