Trident3 vs Jericho2

William Herrin bill at
Fri Apr 9 17:07:16 UTC 2021

On Fri, Apr 9, 2021 at 6:05 AM Mike Hammett <nanog at> wrote:
> What I've observed is that it's better to have a big buffer device
> when you're mixing port speeds. The more dramatic the port
> speed differences (and the more of them), the more buffer you need.
> If you have all the same port speed, small buffers are fine. If you have
> 100G and 1G ports, you'll need big buffers wherever the transition to
> the smaller port speed is located.

When a network is behaving well (losing few packets to data
corruption), TCP throughput is is impacted by exactly two factors:

1. Packet round trip time
2. The size to which the congestion window has grown when the first
packet is lost

Assuming the sender has data ready, it will (after the initial
negotiation) slam out 10 packets back to back at the local wire speed.
Those 10 packets are the initial congestion window. After sending 10
packets it will wait and wait and wait until it hits a timeout or the
other side responds with an acknowledgement. So those initial packets
start out crammed right at the front of the round trip time with lots
of empty afterwards.

The receiver gets the packets in a similar burst and sends its acks.
As the sender receives acknowledgement for each of the original
packets, it sends two more. This doubling effect is called "slow
start," and it's slow in the sense that the sender doesn't just throw
the entire data set at the wire and hope. So, having received acks for
10 packets, it sends 20 more. These 20 have spread out a little bit,
more or less based on the worst link speed in the path, but they're
still all crammed up in a bunch at the start of the round trip time.

Next round trip time it doubles to 40 packets. Then 80. Then 160. All
crammed up at the start of the round trip time causing them to hit
that one slowest link in the middle all at once. This doubling
continues until one of the buffers in the middle is too small to hold
the trailing part of the burst of packets while the leading part is
sent. With a full buffer, a packet is dropped. Whatever the congestion
window size is when that first packet is dropped, that number times
the round trip time is more or less the throughput you're going to see
on that TCP connection.

The various congestion control algorithms for TCP do different things
after they see that first packet drop. Some knock the congestion
window in half right away. Others back down more cautiously. Some
reduce growth all the way down to 1 packet per round trip time. Others
will allow faster growth as the packets spread out over the whole
round trip time and demonstrate that they don't keep getting lost. But
in general, the throughput you're going to see on that TCP connection
has been decided as soon as you lose that first packet.

So, TCP will almost always get better throughput with more buffers.
The flip side is latency: packets sitting in a buffer extend the time
before the receiver gets them. So if you make a buffer that's 500
milliseconds long and then let a TCP connection fill it up, apps which
work poorly in high latency environments (like games and ssh) will

Bill Herrin

William Herrin
bill at

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