The effectiveness of AS path prepending

▼ In a recent blog post The Effectiveness of AS Path Prepending (1) Russ White asks:

Just about everyone prepends AS’ to shift inbound traffic from one provider to another—but does this really work?

(AS path prepending means making the network path as BGP sees it longer to make a path less attractive so traffic will flow over another, shorter path.)

That's an interesting question, as I've been telling people for a long time that it often works too well. A single prepend can flip more than half your traffic from one link to another. In part because of that, Rolf Winter and I worked on a new Origin Preference Attribute that would provide a finer-grained tool for BGP traffic engineering. (Slides, old Internet-Draft, SAC'12 publication.)

But the answer that Russ cites from research into AS path prepending makes sense:

We observe that the effectiveness of prepending can strongly depend on the location (for around 20% of cases, ASPP has moved no targets, while for another 20% , it moved almost all targets).

The the fact that different networks get such different results when performing AS path prepending can be explained by looking at the type of network and the type of connectivity they have. Suppose a small network X has connections to big ISPs A and B. A and B have direct peering relations with most of the other large ISPs. So to the rest of the world, the two paths to X are the same length:

A X = 2 hops
B X is 2 hops

When X now prepends towards A, it's A X X = 3 hops vs B X = 2 hops so most traffic will now flow through the B X path. Prepending was extremely effective.

But what if instead of X doing prepending, A prepends. Now for traffic to X, it's still:

A A X = 3 hops
B X = 2 hops

But other networks that directly connect to A and have traffic for A itself, it doesn't matter if they see A = 1 hop or A A = 2 hops: the traffic needs to go to A, so the path length isn't considered. Even when network C is a customer of B and C sees a direct path and a path over B:

A A A = 3 hops
B A = 2 hops

C will use the direct path, even though it's longer, because the rule is: paths to customers are prioritized over paths to peers and paths to peers over paths to transit providers.

So: will AS path prepending be effective? As with most things in life, it depends.

Permalink - posted 2021-05-13

Hunting down the stuck BGP routes

▼ Ben Cox (Benjojo) has an interesting post about stuck BGP routes and a flaw in many BGP implementations where they hang when their neighbor stops accepting data over TCP: Hunting down the stuck BGP routes

A stuck BGP route means that a prefix was advertised at some point, and then it's withdrawn but the withdrawal somehow gets lost somewhere, so part of the internet still sees the withdrawn route.

There doesn't seem to be an obvious way to get these prefixes unstuck, although I would try advertising them again and withdrawing them again. The AS in the middle where the prefixes are stuck should probably clear (reset) BGP sessions until the problem is fixed, although I would be tempted to just reboot the whole router just to be safe.

The hanging issue happens when a router gets behind on processing incoming BGP updates. At some point its TCP receive buffer fills up, so it sets its TCP window size to 0. Then the buffer on the sending BGP neighbor starts to fill up with updates and keepalives. When that buffer is full, the sending BGP process can't write data to the TCP socket anymore... and bad things start to happen.

Ben feels that the BGP spec needs to be updated to address this issue. However, this is not a matter of interoperation that needs to be standardized, but simply an implementation issue that each vendor can fix on their own, in my opinion.

Permalink - posted 2021-04-22

Getting the "Music" miniplayer back to actual mini

▼ I just had a moderate-size scare: I typed something into iTunes Music, and the miniplayer now showed playback controls:

That's not what I want. I want it to show the album artwork. That's how I've used iTunes and now Music for many years. If I need to interact with the controls, those will then come up when I place my mouse pointer over the miniplayer window. In the view menu, there is no option to get back to my usual setup.

But it's pretty easy to do this if you know how: just drag the bottom corner of the miniplayer window up until the window is roughly rectangular, and after releasing the mouse button:

This is actually quite a good way to change the layout of the miniplayer. (You can also make the window even smaller or even larger to get different results.) However, it's also extremely un-discoverable.

A free tip, from me to you.

Permalink - posted 2021-04-13