Measure a link on a path of loss, reordering and a bottleneck (M6 harness)

020-lossy-paths/run.py runs each case in an unprivileged namespace: a
50 Mbit/s tbf bottleneck under netem's delay, jitter and loss, both
ways. A link may send without UDP GSO (pure_link_gso), as netem takes a
burst as one packet; quinn the same (QUIC_GSO=0). transport.c and the
quinn harness give the records handed over in each 100 ms, and at a
steady rate the time each took; TCP CUBIC flows run beside them.

2 files changed, 8 insertions, 2 deletions

diff --git a/link/link.c b/link/link.c
index 0588633..449ea60 100644
--- a/link/link.c
+++ b/link/link.c
@@ -112,7 +112,7 @@ struct PureLink {
     size_t burst_size;
     unsigned burst_count;
     uint64_t burst_pn[BURST];
-    bool gso;
+    bool gso, no_gso;
 };
 
 PureLink *pure_link_open(const char *url, const char *source, uint32_t shape, const PureSealKey *own,
@@ -148,6 +148,8 @@ PureLink *pure_link_open(const char *url, const char *source, uint32_t shape, co
     return l;
 }
 
+void pure_link_gso(PureLink *l, bool on) { l->no_gso = !on; }
+
 void pure_link_peer(PureLink *l, const unsigned char peer[PURE_SEAL_KEY]) {
     l->peered = true;
     l->keyed = pure_link_sender_peer(l->sender, peer);
@@ -279,7 +281,7 @@ bool pure_link_start(PureLink *l, void (*notify)(void *), void *context, char *e
         setsockopt(l->fd, SOL_SOCKET, SO_RCVBUF, &buffer, sizeof(buffer));
     }
     int segment = DATAGRAM;
-    l->gso = ok && !setsockopt(l->fd, SOL_UDP, UDP_SEGMENT, &segment, sizeof(segment));
+    l->gso = ok && !l->no_gso && !setsockopt(l->fd, SOL_UDP, UDP_SEGMENT, &segment, sizeof(segment));
     ok = ok && l->keyed && (l->burst = malloc((size_t)BURST * DATAGRAM)) && (l->taking = malloc(sizeof(*l->taking))) &&
          (l->wake = eventfd(0, EFD_NONBLOCK | EFD_CLOEXEC)) >= 0;
     l->notify = notify;
diff --git a/link/link.h b/link/link.h
index 5237262..afe1430 100644
--- a/link/link.h
+++ b/link/link.h
@@ -22,6 +22,10 @@ bool pure_link_queue(PureLink *, uint64_t seq, const unsigned char *bytes, size_
 uint64_t pure_link_queued(PureLink *);
 /* The last number the receiver saved; any thread. */
 uint64_t pure_link_delivered(PureLink *);
+/* Packets go in bursts with UDP GSO where the system has it (at first);
+ * off, each in a call of its own: for a path whose queues take a burst as
+ * one packet (netem does). Before pure_link_start. */
+void pure_link_gso(PureLink *, bool on);
 /* Send on a thread of the link's own, calling notify when more was saved. */
 bool pure_link_start(PureLink *, void (*notify)(void *), void *context, char *error, size_t size);
 void pure_link_counts(PureLink *, PureLinkCounts *);