07 Mar

Confusing traceroutes and more

And here goes my first post for 2017. The start of this year did not go well as I broke my hand in Jan and that resulted in a lot of time loss. Now I am almost recovered and in much better condition. I just attended HKNOG 4.0 at Hong Kong followed by APRICOT 2017 at Ho Chi Minh, Vietnam. an event and I enjoyed the both. Here’s my presentation from APRICOT 2017.

I recently I came across some of crazy confusing traceroutes as passed by one of my friends. I cannot share that exact traceroute on this blog post but can produce the same effect about which I am posting by doing a trace from one of large network like Telia London PoP to one of the Indian destinations via their looking glass

Example traceroute:

 

 

Here’s trace is as London (AS1299) > London (AS15412) > Mumbai (AS15412) >>>> Somewhere in India (AS9498) > destination (AS132933)

 

So traffic enters India via Reliance and next handed off to Airtel and reaches the destination. Let’s check BGP table view of same PoP for this prefix:

 

So out of both available routes both are 15412 > 18101 > 132933 direct and there are no AS9498 while Airtel (AS9498) does appear in the traceroute. 2nd last hop in the trace is 182.72.29.222 and that indeed belongs to Airtel.

If we trust routing table as well as the fact that usually Airtel and Reliance exchange domestic traffic only and typically we do not see AS15412 pushing traffic via Airtel. This means trace is wrong and it indeed is. Before we get to on why it’s wrong to let’s try to understand how exactly traceroute works.

 

Working of traceroute

The way traceroute works is by using TTL i.e Time to live on packets the tool is sending out. IP headers carry TTL to prevent them for looping forever. So for instance, if router R1 sends some traffic to router R2 and R2 is not learning that route from anywhere while has a default back to R1 then traffic will start looping between R1 and R2. IP routing prevents this by using TTL and IP packets are sent with certain TTL value and as soon as they cross a router, TTL is decreased. When TTL is zero a router is supposed to drop the traffic and not carry them any further. When a router drops traffic it is supposed to reply back with error “TTL exceeded”.

Now the way “traceroute tool” works is by sending packets with increasing TTL one after other. It sends first one with TTL 1. Router directly connected to it gets the packet. It reduces TTL (and 1 – 1 so it becomes zero) and since next TTL is now zero it just drops prefix instead of sending it further. And as a part of dropping it replies back to a system running a trace with “TTL time exceeded error” revealing it’s IP to the tool. Next, another packet will be sent with TTL 2 and it will cross 1st router & would drop on a 2nd router with “TTL time exceeded” revealing it’s IP.

 

 

Back to our problem…

Now, so that was about the working of traceroute. Now going back to the case I was discussing. Think of routing between two networks when routing is not symmetric. With asymmetric routing, I mean that source & destinations may be carried via different paths.

 

Say e.g here A is sending traffic to B via R1-R2 and B is replying back to A via R3. Now if A does a trace to B, R1 & R2 may appear fine but what source IP B uses to convey the message of TTL exceeded can confuse things. When packets reach B with TTL 1, B decrements TTL and drops them. Next to send that “TTL timeout exceeded message” B has two options:

  1. B can reply back from IP address on the interface connected to R2. Remember I am talking about B just using source IP for TTL exceeded error.  Actual reply path, of course, is via R3
  2. B can reply back from IP of address of the interface connected to R3 using the usual logic of how packets go out – use the source IP of the interface of the best path installed in the router

 

What logic B uses has it’s own advantages and disadvantages. If B follows #1 i.e sends TTL exceeded from the same interface which is connected to R2 then it will give very logical traceroute output. But if network R3 is filtering packets based on BCP38, it will just drop the traffic coming from B from R2’s IP. While if B follows #2 it won’t cause any issues with BCP38 but will confuse the traceroute replies as suddenly one hop in trace will appear from entirely another network. That is what exactly has been happening in the trace I shared above. Let’s read trace again.

 

Here router right before destination i.e on hop7 is connected to Reliance & Airtel. It’s announcing the prefix covering the destination to Reliance and Reliance is bringing traffic but it’s using Airtel to send traffic out back to London router of Telia. While replying for “TTL exceeded” router 7 is using source IP of Airtel and thus we see the PTR record pointing to Airtel. This can be referred as “Random factoid” behaviour in traceroute. This comes from RFC1812 which suggests “ICMP source must be from the egress iface” and  Richard Steenbergen puts its very nicely in his presentation at NANOG here.

Checko

So that’s all about it for now!

25 Nov

Peering with content networks in India

peering

One of frequent email and contact form message I get my blog is about available content networks in India and where one can peer. There are certain content networks in India and of course most of the content networks have open peering policy and are usually happy with direct inter-connection (we call as “peering“) with the ISP networks (often referred to as “eyeball networks”). Some of these networks have a backbone which connects back to their key datacenter locations on their own circuits via Singapore/Europe, some other have simply placed their caching server where cache fill happens over IP transit.

 

Based on publically known information across community and of course peeringdb, following content players are available in India and known to be open for peering:

  1. Google
  2. Microsoft
  3. Amazon
  4. Limelight

 

A quick list of these with datacenter names and locations as taken from Peeringdb record of these networks.

Organisation ASN City Datacenter Location
Amazon 16509 Mumbai GPX Mumbai Unit A-001, Boomerang Chandivali Farm Road, Near Chandivali Studio, Andheri East Mumbai, Mumbai, 400 051
Amazon 16509 Noida Sify Greenfort – Noida B7, Block A, Sector 132, Noida Expressway, Noida , UP 201304
Amazon 16509 Mumbai Tata Mumbai IDC LVSB, Opposite Kirti College
6th floor, Prabahdevi
Mumbai, MH, 400 028
Google 15169 Chennai Bharti Airtel Santhome Bharti Towers, 101 Santhome High Road, Chennai, 600 028
Google 15169 Mumbai GPX Mumbai Unit A-001, Boomerang Chandivali Farm Road, Near Chandivali Studio, Andheri East Mumbai, Mumbai, 400 051
Google 15169 Noida Sify Greenfort – Noida B7, Block A, Sector 132, Noida Expressway, Noida , UP 201304
Google 15169 Chennai TATA Communications Ltd 14th floor, 2nd block
4, Swami Sivanand Salai, Chennai, TN 600 002
Google 15169 Delhi Tata Delhi VSB, Bangla Sahib Road, New Delhi 110001
Google 15169 Mumbai Tata Mumbai IDC LVSB, Opposite Kirti College
6th floor, Prabhadevi
Mumbai, MH, 400 028
Limelight 55439 / 22822 Chennai Bharti Airtel Santhome Bharti Towers, 101 Santhome High Road, Chennai, 600 028
Limelight 55439 / 22822 Mumbai Netmagic Vikhroli Mehra Industrial Estate
LBS Marg, Vikhroli
Mumbai, 400 079
Microsoft 8075 Mumbai Bharti Airtel Mumbai Plot No, TPS-2, 14/3, 2nd floor
Dattatray Road, Linking Road Extension
Mumbai, 400054
Microsoft 8075 Chennai Bharti Airtel Santhome Bharti Towers, 101 Santhome High Road, Chennai, 600 028
Microsoft 8075 Chennai TATA Communications Ltd 14th floor, 2nd block
4, Swami Sivanand Salai, Chennai, TN 600 002
Microsoft 8075 Delhi Tata Communications Ltd – GK1 Greater Kailash-1
New Delhi, 110048
Microsoft 8075 Mumbai Tata Mumbai IDC LVSB, Opposite Kirti College
6th floor, Prabhadevi
Mumbai, MH, 400 028

 

Besides these Google also has an option of GGC, Akamai has an option of Akamai Caching server, Facebook has the option for caching server which is hosted inside ISP’s network and Netflix has an option for OCAs. Besides these networks there are known nodes of Verizon’s Edgecast in Delhi, Mumbai & Chennai (as per this map), Cloudflare has nodes in Delhi, Mumbai & Chennai (as per this map), PCH & K-root server have a node with Web Werks available on MCH peering fabric and Dyn has a node in Mumbai (as per this map).

Go ahead and peer as after all it all starts with a handshake. 🙂