ECN

Wed Nov 13 16:24:01 UTC 2019

This sounds like a bug on Cloudflare’s end (cause trying to do anycast TCP is... out of spec to say the least), not a bug in ECN/ECMP.

> On Nov 13, 2019, at 11:07, Toke Høiland-Jørgensen via NANOG <nanog at nanog.org> wrote:
> 
> 
>> 
>> Hello
>> 
>> I have a customer that believes my network has a ECN problem. We do
>> not, we just move packets. But how do I prove it?
>> 
>> Is there a tool that checks for ECN trouble? Ideally something I could
>> run on the NLNOG Ring network.
>> 
>> I believe it likely that it is the destination that has the problem.
> 
> Hi Baldur
> 
> I believe I may be that customer :)
> 
> First of all, thank you for looking into the issue! We've been having
> great fun over on the ecn-sane mailing list trying to figure out what's
> going on. I'll summarise below, but see this thread for the discussion
> and debugging details:
> https://lists.bufferbloat.net/pipermail/ecn-sane/2019-November/000527.html
> 
> The short version is that the problem appears to come from a combination
> of the ECMP routing in your network, and Cloudflare's heavy use of
> anycast. Specifically, a router in your network appears to be doing ECMP
> by hashing on the packet header, *including the ECN bits*. This breaks
> TCP connections with ECN because the TCP SYN (with no ECN bits set) end
> up taking a different path than the rest of the flow (which is marked as
> ECT(0)). When the destination is anycasted, this means that the data
> packets go to a different server than the SYN did. This second server
> doesn't recognise the connection, and so replies with a TCP RST. To fix
> this, simply exclude the ECN bits (or the whole TOS byte) from your
> router's ECMP hash.
> 
> For a longer exposition, see below. You should be able to verify this
> from somewhere else in the network, but if there's anything else you
> want me to test, do let me know. Also, would you mind sharing the router
> make and model that does this? We're trying to collect real-world
> examples of network problems caused by ECN and this is definitely an
> interesting example.
> 
> -Toke
> 
> 
> 
> The long version:
> 
> From my end I can see that I have two paths to Cloudflare; which is
> taken appears to be based on a hash of the packet header, as can be seen
> by varying the source port:
> 
> $ traceroute -q 1 --sport=10000 104.24.125.13
> traceroute to 104.24.125.13 (104.24.125.13), 30 hops max, 60 byte packets
> 1  _gateway (10.42.3.1)  0.357 ms
> 2  albertslund-edge1-lo.net.gigabit.dk (185.24.171.254)  4.707 ms
> 3  customer-185-24-168-46.ip4.gigabit.dk (185.24.168.46)  1.283 ms
> 4  te0-1-1-5.rcr21.cph01.atlas.cogentco.com (149.6.137.49)  1.667 ms
> 5  netnod-ix-cph-blue-9000.cloudflare.com (212.237.192.246)  1.406 ms
> 6  104.24.125.13 (104.24.125.13)  1.322 ms
> 
> $ traceroute -q 1 --sport=10001 104.24.125.13
> traceroute to 104.24.125.13 (104.24.125.13), 30 hops max, 60 byte packets
> 1  _gateway (10.42.3.1)  0.293 ms
> 2  albertslund-edge1-lo.net.gigabit.dk (185.24.171.254)  3.430 ms
> 3  customer-185-24-168-38.ip4.gigabit.dk (185.24.168.38)  1.194 ms
> 4  10ge1-2.core1.cph1.he.net (216.66.83.101)  1.297 ms
> 5  be2306.ccr42.ham01.atlas.cogentco.com (130.117.3.237)  6.805 ms
> 6  149.6.142.130 (149.6.142.130)  6.925 ms
> 7  104.24.125.13 (104.24.125.13)  1.501 ms
> 
> 
> This is fine in itself. However, the problem stems from the fact that
> the ECN bits in the IP header are also included in the ECMP hash (-t
> sets the TOS byte; -t 1 ends up as ECT(0) on the wire and -t 2 is
> ECT(1)):
> 
> $ traceroute -q 1 --sport=10000 104.24.125.13 -t 1
> traceroute to 104.24.125.13 (104.24.125.13), 30 hops max, 60 byte packets
> 1  _gateway (10.42.3.1)  0.336 ms
> 2  albertslund-edge1-lo.net.gigabit.dk (185.24.171.254)  6.964 ms
> 3  customer-185-24-168-46.ip4.gigabit.dk (185.24.168.46)  1.056 ms
> 4  te0-1-1-5.rcr21.cph01.atlas.cogentco.com (149.6.137.49)  1.512 ms
> 5  netnod-ix-cph-blue-9000.cloudflare.com (212.237.192.246)  1.313 ms
> 6  104.24.125.13 (104.24.125.13)  1.210 ms
> 
> $ traceroute -q 1 --sport=10000 104.24.125.13 -t 2
> traceroute to 104.24.125.13 (104.24.125.13), 30 hops max, 60 byte packets
> 1  _gateway (10.42.3.1)  0.339 ms
> 2  albertslund-edge1-lo.net.gigabit.dk (185.24.171.254)  2.565 ms
> 3  customer-185-24-168-38.ip4.gigabit.dk (185.24.168.38)  1.301 ms
> 4  10ge1-2.core1.cph1.he.net (216.66.83.101)  1.339 ms
> 5  be2306.ccr42.ham01.atlas.cogentco.com (130.117.3.237)  6.570 ms
> 6  149.6.142.130 (149.6.142.130)  6.888 ms
> 7  104.24.125.13 (104.24.125.13)  1.785 ms
> 
> 
> So why is this a problem? The TCP SYN packet first needs to negotiate
> ECN, so it is sent without any ECN bits set in the header; after
> negotiation succeeds, the data packets will be marked as ECT(0). But
> because that becomes part of the ECMP hash, those packets will take
> another path. And since the destination is anycasted, that means they
> will also end up at a different endpoint. This second endpoint won't
> recognise the connection, and reply with a TCP RST. This is clearly
> visible in tcpdump; notice the different TOS values, and that the RST
> packet has a different TTL than the SYN-ACK:
> 
> 12:21:47.816359 IP (tos 0x0, ttl 64, id 25687, offset 0, flags [DF], proto TCP (6), length 60)
>    10.42.3.130.34420 > 104.24.125.13.80: Flags [SEW], cksum 0xf2ff (incorrect -> 0x0853), seq 3345293502, win 64240, options [mss 1460,sackOK,TS val 4248691972 ecr 0,nop,wscale 7], length 0
> 12:21:47.823395 IP (tos 0x0, ttl 58, id 0, offset 0, flags [DF], proto TCP (6), length 52)
>    104.24.125.13.80 > 10.42.3.130.34420: Flags [S.E], cksum 0x9f4a (correct), seq 1936951409, ack 3345293503, win 29200, options [mss 1400,nop,nop,sackOK,nop,wscale 10], length 0
> 12:21:47.823479 IP (tos 0x0, ttl 64, id 25688, offset 0, flags [DF], proto TCP (6), length 40)
>    10.42.3.130.34420 > 104.24.125.13.80: Flags [.], cksum 0xf2eb (incorrect -> 0x503e), seq 1, ack 1, win 502, length 0
> 12:21:47.823665 IP (tos 0x2,ECT(0), ttl 64, id 25689, offset 0, flags [DF], proto TCP (6), length 117)
>    10.42.3.130.34420 > 104.24.125.13.80: Flags [P.], cksum 0xf338 (incorrect -> 0xc1d4), seq 1:78, ack 1, win 502, length 77: HTTP, length: 77
>    GET / HTTP/1.1
>    Host: 104.24.125.13
>    User-Agent: curl/7.66.0
>    Accept: */*
>    
> 12:21:47.825485 IP (tos 0x2,ECT(0), ttl 60, id 0, offset 0, flags [DF], proto TCP (6), length 40)
>    104.24.125.13.80 > 10.42.3.130.34420: Flags [R], cksum 0x3a65 (correct), seq 1936951410, win 0, length 0
> 
> 
> The fix is to stop hashing on the ECN bits when doing ECMP. You could
> keep hashing on the diffserv part of the TOS field if you want, but I
> think it would also be fine to just exclude the TOS field entirely from
> the hash.