We recently identified the cause of a problem affecting one of Papertrail’s service providers. Their hosts occasionally could not establish TCP connections with a seemingly-random small set of Internet hosts.

Troubleshooting was difficult because:

Among this tiny pool, there was one common theme: the remote hosts traversed NAT. My investigation eventually led to tcp_tw_recycle, a Linux sysctl flag with far more Google results than it deserves. Here’s why.

Linux TCP header verification

Modern Linux kernels verify that TCP header values meet certain requirements. These include “Protect Against Wrapped Sequence numbers” or PAWS, defined in RFC 1323, and RFC 6191 “Reducing the TIME-WAIT State Using TCP Timestamps.”

With tcp_tw_recycle enabled, a connection’s TCP header timestamp value is retained in cases where it otherwise would not have been kept. From tcp_ipv4.c:

if (tcp_death_row.sysctl_tw_recycle &&
    !tp->rx_opt.ts_recent_stamp && rt->rt_dst == daddr) {
        struct inet_peer *peer = rt_get_peer(rt);
         * VJ's idea. We save last timestamp seen from
         * the destination in peer table, when entering state
         * TIME-WAIT * and initialize rx_opt.ts_recent from it,
         * when trying new connection.
        if (peer != NULL &&
            peer->tcp_ts_stamp + TCP_PAWS_MSL >= get_seconds()) {
                tp->rx_opt.ts_recent_stamp = peer->tcp_ts_stamp;
                tp->rx_opt.ts_recent = peer->tcp_ts;

The problem: the TCP timestamp is only tracked on a per-remote-IP basis, yet some NAT devices don’t rewrite TCP timestamps in the translation process. As a result, the Internet-facing IP of a NAT device may transmit valid packets with unrelated timestamps.

The problem we saw manifests when more than one remote host (for example, two employees on an office network) try to connect to this Linux host within a few minutes of one another. The first connection will succeed, but the second connection attempt (from the same public NAT IP) will fail. The kernel considers its timestamp invalid.

In another function, a comment hints at the difference between tracking timestamps on a per-host basis and doing so on a per-port-pair basis. From tcp_ipv4.c:

/* With PAWS, it is safe from the viewpoint
   of data integrity. Even without PAWS it is safe provided sequence
   spaces do not overlap i.e. at data rates <= 80Mbit/sec.

   Actually, the idea is close to VJ's one, only timestamp cache is
   held not per host, but per port pair and TW bucket is used as state

   If TW bucket has been already destroyed we fall back to VJ's scheme
   and use initial timestamp retrieved from peer table.

Note: I haven’t read anywhere near all of Linux’s TCP header source. If you find an error in this post, let me know.

Root cause

Although that’s the problem, the root cause is poor documentation. The two places that a systems administrator is most likely to consult are the kernel IP sysctl docs, which suggests consulting “technical experts”:

tcp_tw_recycle - BOOLEAN
        Enable fast recycling TIME-WAIT sockets. Default value is 0.
        It should not be changed without advice/request of technical

.. and the tcp.7 man page, which says:

tcp_tw_recycle (Boolean; default: disabled; since Linux 2.4)
        Enable fast recycling of TIME_WAIT sockets.  Enabling this
        option is not recommended since this causes problems when
        working with NAT (Network Address Translation).

Neither of these explain what the option changes or how it interacts with NAT. I’ve submitted a patch for the man page. The changed copy warns of the possible impact and says where to learn more.

Enable fast recycling of TIME_WAIT sockets. Enabling this option is not recommended for devices communicating with the general Internet or using NAT (Network Address Translation). Since some NAT gateways pass through IP timestamp values, one IP can appear to have non-increasing timestamps. See RFC 1323 (PAWS), RFC 6191.

There’s also a 2013 BCP (“Network Address Translation Behavioral Requirements Updates”) which informs future NAT implementors of this consideration.