TCP over UDP : a new hack to pass through (some) middleboxes

Extending TCP in the presence of middleboxes is a difficult but not impossible task as shown by Multipath TCP RFC 6824. A recent IETF draft proposed by Apple suggests to encapsulate TCP segments inside UDP to prevent modifications performed by middleboxes. Apparently, some measurements indicate that UDP passes better through some types of NAT boxes that regular TCP segments. Since TCP is more widely used than UDP, the draft proposes to encapsulate TCP inside UDP. The proposed encapsulation technique is a bit unusual. A classical encapsulation would put the entire TCP segment after the UDP header. Instead, the TCP-over-UDP draft proposes to rewrite the TCP header as follows

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Source Port          |       Destination Port        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|            Length             |           Checksum            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Data |           | |A|P|R|S|F|                               |
| Offset| Reserved  |0|C|S|S|Y|I|            Window             |
|       |           | |K|H|T|N|N|                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Sequence Number                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Acknowledgment Number                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                      (Optional) Options                       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The first eight bytes correspond to the classical UDP header. Then, the Data Offset is placed with the flags and the window field. Placing the Data Offset after the Checksum ensures that a value larger than 0x5 will appear there, which is required for STUN traversal. Then, the sequence numbers and acknowledgment numbers follow. With this translation, the TCP header has been reordered, but starts with a UDP header. The informed reader will have noticed that the URGent pointer has disappeared. This field is rarely used and some middleboxes reset it. This is not a huge loss for most TCP applications.

Time will tell whether this hack becomes actually implemented and is accepted by the IETF. If so, network debugging will continue to be difficult as TCP segments may be exchanged as UDP segments...