How bad is the OSPF vulnerability exposed by Black Hat?


I was asked a few weeks ago by our field engineers to provide a fix for the OSPF vulnerability exposed by Black Hat last month. Prima facie there appeared nothing new in this attack as everyone knows that OSPF (or ISIS) networks can be brought down by insider attacks. This isnt the first time that OSPF vulnerability has been announced at Black Hat. Way back in 2011 Gabi  Nakibly, the researcher at Israel’s Electronic Warfare Research and Simulation Center, had demonstrated how OSPF could be brought down using insider attacks.  Folks were not impressed, as anybody who had access to one of the routers could launch attacks on the routing infrastructure. So it was with certain skepticism that i started looking at yet another OSPF vulnerability exposed by Gabi, again at Black Hat. Its only when i started delving deep into the attack vector that the real scale of the attack dawned on me. This attack evades OSPF’s natural fight back mechanism against malacious LSAs which makes it a bit more insidious than the other attacks reported so far.

I exchanged a few emails with Gabi when i heard about his latest exposé. I wanted to understand how this attack was really different from the numerous other insider attacks that have been published in the past. Insider attacks are not very interesting, really. Well, if you were careless enough to let somebody access your trusted router, or somebody was smart enough to masquerade as one of your routers and was able to inject malicious LSAs then the least that you can expect is a little turbulence in your routing infrastructure. However, this attack stands apart from the others as we shall soon see.

OSPF (and ISIS too) has a natural fight back mechanism against any malacious LSA that has been injected in a network. When an OSPF router receives an LSA that lists that router as the originating router (referred to as a self-originated LSA) it looks at the contents of the LSA (just in case you didnt realize this). If the received LSA looks newer than the LSA that this router had last originated, the router advances the LSA’s LS sequence number one past the received LS sequence number and originates a new instance of this LSA. In case its not interested in this LSA, it flushes the LSA by originating a new LSA with age set to MaxAge.

All other routers in the network now update their LS database with this new instance and the malacious LSA effectively gets purged from the network. Viola,its that simple!

As a result of this, the attacker can only flood malacious LSAs inside the network till the router that the malacious LSA purports to come from (victim router) receives a copy. As soon as this router floods an updated copy, it doesnt take long for other routers in the network to update their LS DB as well – the flooding process is very efficient in disseminating information since network diameters are typically not huge, and yes, packets travel with the speed of light. Did you know that?

In the attack that Gabi described, the victim router does not recognize the malacious LSA as its own and thus never attempts at refreshing it. As a result the malicious LSA remains stealthily hidden in the routing domain and can go undetected for a really long time. Thus by controlling a single router inside an AS (the one that will flood the malacious LSA), an attacker can gain control over the entire routing domain. In fact, an attacker need not even gain control of an entire router inside the AS.  Its enough if it can somehow inject the malacious LSAs over a link such that one of the OSPF routers in the network accept this. In the media release, Black Hat claimed ” The new attack allows an attacker that owns just a single router within an AS to effectively own the routing tables of ALL the routers in that AS without actually owning the routers themselves. This may be utilized to induce routing loops, network cuts or longer routes in order to facilitate DoS of the routing domain or to gain access to information flows which otherwise the attacker had no access to.

So what is this attack?

Lets start by looking at what the LS header looks like.

LS Header

In this attack we are only interested in the two fields, the Link State ID and the Advertising Router, in the LS Header. In the context of a Router LSA, the Link State ID identifies the router whose links are listed in the LSA. Its always populated with the router ID of that router.  The Advertising Router field identifies the router that initially advertised (originated) the LSA. The OSPF spec dictates that only a router itself can originate its own LSA (i.e. no router is expected to originate a LSA on behalf of other routers), therefore in Router LSAs the two fields – ‘Link State ID’ and ‘Advertising Router’ – must have the exact same value. However, the OSPF spec does not specify a check to verify this equality on Router LSA reception.

Unlike several other IETF standards, the OSPF spec is very detailed, leaving little room for any ambiguity in interpreting and implementing the standard. This is usually good as it results in interoperable implementations where everybody does the right thing. The flip side however is that since everybody follows the spec to the tittle, a potential bug or an omission in the standard, would very likely affect several vendor implementations.

This attack exploits a potential omission (or a bug if you will) in the standard where it does not mandate that the receiving router verifies that the Link State ID and the Advertising Router fields in the Router LSA are the exact same value.

This attack sends malacious Router LSAs with two different values in the LS header. The Link State ID carries the Router ID of the router that is being attacked (the victim) and the Advertising Router is set to some different (any) value.

When the victim receives the malacious Router LSA, it does not refresh this LSA as it doesnt recognize this as its own self generated LSA. This is because the OSPF spec clearly says in Sec 13.4 that “A self-originated LSA is detected when either 1) The LSA’s Advertising Router is equal to the router’s own Router ID or 2) the LSA is a network LSA .. “.

This means that OSPF’s natural fight back mechanism is NOT triggered by the victim router as long as the field ‘Advertising Router’ of a LSA is NOT equal to the victim’s Router ID. This is true even if the ‘Link State ID’ of that LSA is equal to the victim’s Router ID. Going further it means no LSA refresh is triggered even if the malacious LSA claims to describe the links of the victim router!

When this LSA is flooded all the routers accept and install this LSA in their LS database. This exists along side the valid LSA originated by the victim router. Thus each router in the network now has two Router LSAs for the victim router – the first that was genuinely originated by the victim router and the second that has been inserted by the attacker.

When computing the shortest path first algorithm, the OSPF spec in Sec 16.1 requires implementations to pick up the LSA from the LS DB by doing a lookup “based on the Vertex ID“. The Vertex ID refers to the Link State ID field in the Router LSAs. This means that when computing SPF, routers only identify the LSAs based on their Link State ID. This creates an ambiguity on which LSA will be picked up from the LS database. Will it be the genuine one originated by the victim router or will it be the malacious LSA injected by the attacker? The answer depends on how the data structures for LS DB lookup have been implemented in the vendor’s routers. Ones that pick up the wrong LSA will be susceptible to the attack. The ones that dont, would be oblivious to the malacious LSA sitting in their LSA DBs.

Most router implementations are vulnerable to this attack since nobody expects the scenario where multiple LSAs with the same Link State ID will exist in the LS DB. It turns out that at least 3 major router vendors (Cisco, Juniper and Alcatel-Lucent) have already released advisories and announced fixes/patches that fixes this issue. The fix for 7210 would be out soon ..

Once again, the attacker does not need to have an OSPF adjacency to inject the forged LSAs.

Doing this is not as difficult as we might think it is. There is no need for the attacker to access the LS DB sequence number – all it needs to do is to send an LSA with a reasonably high sequence number, say something like MAX_SEQUENCE – 1 to get this LSA accepted.

The attack can also be performed without complete information about the OSPF topology. But, this is highly dependent on the attack scenario and what piece of false information the attacker wishes to advertise on behalf of the victim. For example, if the attacker wishes to disconnect the victim router from the OSPF topology then merely sending an empty LSA without knowing the OSPF topology in advance would also work. In the worst case, the attacker can also get partial information on the OSPF topology by using trace routes, etc. This way the attacker can construct LSAs that look very close to what has been originally advertised by the victim router, making it all the more difficult to suspect that such LSAs exist in the network.

DNS poisoned for LinkedIn. Affects us? Sure, it does.


If you were unable to access LinkedIn for almost the entire day earlier this week, then you can take solace in the fact that you were not the only one, not able to. Almost half the world shared your misery where all attempts to access LinkedIn (and several other websites) went awry. This purportedly happened because  a bunch of hackers decided to poison the DNS entries for LinkedIn and some other well known websites ( being another).

Before we delve into the sordid details of this particular incident lets quickly take a look at how DNS works.

Whenever we access, our computer must resolve this human-readable address “” into a computer-readable IP address like “″ thats hosting this website. It does this by requesting a DNS server to return an IP address that can be used. The DNS server responds with one or more IP addresses with which you can reach Your computer then connects to that IP address.

So where is this DNS server located that i just spoke about?

This DNS server lies with your Internet service provider, which caches information from other DNS servers.  The router that we have at home also functions as a DNS server, which caches information from the ISP’s DNS servers — this is done  so that we dont have to perform a DNS lookup each time we have to access a website for which we have already resolved the IP address.

Now that we know the basics, lets see what DNS poisoning is?

A DNS cache is said to be poisoned if it contains an invalid entry. For example, if an attack “somehow” gains control of a DNS server and changes some of the information on it — it could for instance say that actually points to an IP address the attacker owns — that DNS server whenever requested to resolve would tell its users to look for at the wrong address. The attacker’s address could potentially contain some sort of malicious phishing website, which could resemble the original or could simply be used to drop all traffic. The latter is done when ISPs want to block all access to a particular website. China typically does it for lot of websites — its called the Great Firewall of China.  There are multiple techniques which China employs to implement their censorship and one of them is DNS poisoning (more here).


DNS poisoning spreads like wild fire because of how it works.  Clearly Internet service providers cannot hold information about all websites in their DNS caches – they get their DNS information from other DNS servers. Now assume, that they are getting their DNS information from a compromised server. The poisoned DNS entry or entries will spread to the Internet service providers and get cached there. It will then spread to other ISPs that get information from this DNS server. And it wouldnt stop at this, it would spread to routers at campuses, homes and the DNS caches on individual user computers. So everybody who requests for the DNS resolution of the hijacked website will receive an incorrect response and will forward traffic to the address specified by the attacker. and a number of other organizations have registered their domain names with Network Solutions. For some inexplicable reason their DNS nameservers were replaced with nameservers at The nameservers at were configured to reply to DNS requests for the affected domains with IP addresses in the range This address range is belongs to confluence networks, so all traffic bound to LinkedIn was re-routed to a networks hosted by confluence networks.

But what caused the name servers to be replaced?

According to Network Solutions (NS), they were hit by a distributed denial-of-service (DDOS) attack on night of 19/06. This is certainly is plausible since Network Solutions, being the original registrar for .com, .net, and .org domain names, is certainly an attractive target for attackers. Most of you would remember the (in)famous August 2009 NS server breach which allegedly led to the exposure of names, addresses, and credit card numbers of more than 500,000 people who made purchases on web sites hosted by the NS.

A spokesperson from Network Solutions had the following to say regarding the DNS poisoning issue:

“In the process of resolving a Distributed Denial of Service (DDoS) incident on Wednesday night, the websites of a small number of Network Solutions customers were inadvertently affected for up to several hours.”

They have reassured customers that no confidential data has been compromised as a result of the incident.

The jury meanwhile, is still out on whether this was a configuration error or a coordinated DNS attack on Network Solutions.

Regardless of what it was, the fact is that enormous amount of  LinkedIn traffic was redirected to some other network. This is should make all of us very nervous since LinkedIn does not use Secure Socket Layer (SSL), which means that all communication between you and LinkedIn goes in plaintext — leaving you vulnerable to eavesdropping and man-in-the-middle attacks. If an attacker is able to intercept all data being sent between a browser and a web server they can see and use that information. In this event all traffic bound to LinkedIn was diverted to IP addresses owned by Confluence Networks.

This isnt the first time LinkedIn has compromised the security of its users. Earlier in June 2012, nearly 6.5 million encrypted passwords were compromised when they were dumped onto a Russian hacker forum. Its around this time that a team of mobile security researchers discovered that LinkedIn’s mobile app for iOS was transmitting information about calendar entries made on that app, including sensitive information like meeting locations and passwords, back to LinkedIn’s servers without users’ knowledge.

Not only is this a clear violation of their user’s privacy (which is a different discussion btw) but is also extremely dangerous if this data transfer is not being done securely, as this would leave LinkedIn users very vulnerable to eavesdropping attacks.

So when the DNS entry for LinkedIn was poisoned we know that all our confidential information was diverted to unknown servers that can mine that data in whatever manner they find most amusing. I just hope that you didnt have any confidential data plugged into your LinkedIn iOS app, as somebody somewhere may just be reading all that as you read this blog post.

Why do we need specialized switches in Data Centers?

data center

Whats the big deal about Data centers and why do they need special routers and switches anyway? Why cant they use the existing switches that folks use in their back offices or service providers in their networks. What’s so special, really, about a bunch of servers that need Internet connectivity, huh?

Working in the metro Ethernet space all my life I wasn’t sure if I really understood the hype and the reason why Data centers required specialized HW.

It’s only once I started reading about Data centers and how they work and what they’re supposed to do that I was able to appreciate their need for specialized HW – and why the existing products may not be cut for them.

In the world of Wall Street, milliseconds can mean billions of dollars. Really, am not kidding here. Packets carrying Wall Street transactions get delivered to the switch and are then forwarded to the server in the Data Center. There they ride up the protocol stack to the application that executes the trade. The commit message then has to go back down the stack and then be sent over the wire to the switch. The switch does a lookup in its forwarding tables and sends it out on an egress destination port.

One of the things that would differentiate one Data Center switch from the other would be the time it takes for the switch to process the incoming packet, the amount and the nature of queuing that happens (which directly affects the latency), the serialization delays at the ingress and egress, and other factors that can contribute to adding a few microseconds to each packet processing (or transaction in Wall Street speak).

So is adding a few microseconds really a big deal?

Oh Yes, you bet it is – especially in the big bad world of Wall Street.

wall street

You only need to google for “high-frequency trading” and you would understand why it’s the suddenly become one of the most talked about thing in the Wall Street.

Lets see how shaving off a few microseconds can help?

A Mutual Fund house places an order to purchase 100000 shares of a company ABC that’s currently going at $10. NASDAQ (or some other exchange) could offer a few selected high frequency traders a peek into the incoming orders for 30 milliseconds or so (this is illegal, but there are loopholes in the system). The high frequency trader, knows that a purchase order for 100000 shares of ABC is coming and immediately picks up all the available shares at $10. After a few seconds, the Mutual Fund house order hits the market place and the high frequency trader sells their shares at $10.50, pocketing $50000 from a single transaction. Now, multiply this by the average number of transactions that typically take place in an Exchange and you would you arrive at the staggering amount of $$$ that’s at stake here.

Its thus imperative that the high speed computers that are doing all the number crunching have supporting network infrastructure that can help them in making the kill. Lower network latency and increased throughput means faster and better profits for the trading companies.

Firms using high frequency trading earned over $21 billion in profits last year. The TABB Group estimates that a 5 millisecond delay in transmitting an automatic trade can cost a broker 1% of its flow; which could be worth $4 million in revenues per millisecond. According to Reuters, trading a stock is now far faster than a blink of an eye or the speed of a lightning strike.

In fact several high frequency traders house their systems as close as possible to the exchanges to minimize the latency in executing their orders.

There are also other environments where low latency is desirable. Environments such as computer animation studios that may spend 80 to 90 hours rendering a single frame for a 3D movie, or scientific compute server farms (for Computational Fluid Dynamics) that might involve tens of thousands of compute cores. If the network is the bottleneck within those massive computer arrays, the overall performance is affected.

The Data centers thus patently need switches that have extremely extremely low latency in forwarding packets.

So what are the other things that the Data center switches must support – and which may not be available in ordinary switches?

Micro-bursting often happens in Data Centers, wherein the buffers overrun and the switches drop packets. The problem is that these micro-bursts happen often at microsecond intervals, so the switches may not report them. A good Data Center switch will absorb the micro-burst and forward the packets without dropping ’em.

Data centers as we just saw are designed for critical systems that require high availability. This means redundant power, efficient cooling, secure access, ideally no down time, and a whole lot of other things, but most of all, it means no single points of failure.

Every device in a data center should have dual power supplies, and each one of those power supplies should be fed from independent power feeds. The power supplies are sized such that the device operates with only power path. All devices in a data center should have front-to-back airflow, or ideally, airflow that can be configured front to back or back to front. Thermal guidelines for Data Centers is a science by itself and there is more than petabytes of information available on the net on how this needs to be effectively done.

All devices in a data center should support the means to upgrade, replace, or shut down any single chassis at any time without interruption to meet the hard Service Level Agreements (SLAs). In-Service Software Upgrades (ISSU) should ideally be available, but this can be circumvented by properly distributing load to allow meeting the prior requirement. Data center devices should offer robust hardware, even NEBS compliance where required, and robust software to match.

This isnt the most exhaustive list of the things required out of switches deployed in Data Centers, and only serves to give a hint of whats needed there.

Oh and btw, I must finish this post and rush to place an order on NASDAQ before some high frequency trader preempts me and books all the profits!

OpenFlow, Controllers – Whats missing in Routing Protocols today?

openflowThere is a lot of hype around OpenFlow as a technology and as a protocol these days. Few envision this to be the most exciting innovation in the networking industry after the vaccum tubes, diodes and transistors were miniaturized to form integrated circuits.  This is obviously an exaggeration, but you get the drift, right?

The idea in itself is quite radical. It changes the classical IP forwarding model from one where all decisions are distributed to one where there is a centralized beast – the controller – that takes the forwarding decisions and pushes that state to all the devices (could be routers, switches, WiFi access points, remote access devices such as CPEs) in the network.

Before we get into the details, let’s look at the main components – the Management, Control and the Forwarding (Data) plane – of a networking device. The Management plane is used to manage (CLI, loading firmware, etc) and monitor the device through its connection to the network and also coordinates functions between the Control and the Forwarding plane. Examples of protocols processed in the management plane are SNMP, Telnet, HTTP, Secure HTTP (HTTPS), and SSH.

The Forwarding plane is responsible for forwarding frames – it receives frames from an ingress port, processes them, and sends those out on an egress port based on what’s programmed in the forwarding tables. The Control plane gathers and maintains network topology information, and passes it to the forwarding plane so that it knows where to forward the received frames. It’s in here that we run OSPF, LDP, BGP, STP, TRILL, etc – basically, whatever it takes us to program the forwarding tables.

Routing Protocols gather information about all the devices and the routes in the network and populate the Routing Information Base (RIB) with that information. The RIB then selects the best route from all the routing protocols and populates the forwarding tables – and Routing thus becomes Forwarding.

So far, so good.

The question that keeps coming up is whether our routing protocols are good enough? Are ISIS, OSPF, BGP, STP, etc the only protocols that we can use today to map the paths in the network? Are there other, better options – Can we do better than what we have today?

Note that these protocols were designed more than 20 years ago (STP was invented in 1985 and the first version of OSPF in 1989) with the mathematics that goes in behind these protocols even further. The code that we have running in our networks is highly reliable, practical, proven to be scalable – and it works. So, the question before us is – Are there other, alternate, efficient ways to program the network?

Lets start with what’s good in the Routing Protocols today.

They are reliable – We’ve had them since last 20+ years. They have proven themselves to be workable. The code that we use to run them has proven itself to be reliable. There wouldn’t be an Internet if these protocols weren’t working.

They are deterministic in that we know and understand them and are highly predictable – we have experience with them. So we know that when we configure OSPF, what exactly will it end up doing and how exactly will it work – there are no surprises.

Also what’s important about today’s protocols are that they are self healing. In a network where there are multiple paths between the source and the destination, a loss of an interface or a device causes the network to self heal. It will autonomously discover alternate paths and will begin to forward frames along the secondary path. While this may not necessarily be the best path, the frames will get delivered.

We can also say that today’s protocols are scalable.  BGP certainly has proven itself to run at the Internet’s scale with extraordinarily large number of routes. ISIS has as per the local folklore proven to be more scalable than OSPF. Trust me when i say that the scalability aspect is not the limitation of the protocol, but is rather the limitation of perhaps the implementation. More on this here.

And like everything else in the world, there are certain things that are not so good.

Routing Protocols work under the idea that if you have a room full of people and you want them to agree on something then they must speak the same language. This means that if we’re running OSPFv3, then all the devices in the network must run the exact same version of OSPFv3 and must understand the same thing. This means that if you throw in a lot of different devices with varying capabilities in the network then they must all support OSPFv3 if they want to be heard.

Most of the protocols are change resistant, i.e., we find it very difficult to extend OSPFv2 to say introduce newer types of LSAs. We find it difficult to make enhancements to STP to make it better, faster – more scalable, to add more features. Nobody wants to radically change the design of these protocols.

Another argument that’s often discussed is that the metrics used by these protocols are really not good enough. BGP for example considers the entire AS as one hop. In OSPF and ISIS, the metrics are a function of the BW of the link. But is BW really the best way to calculate a metric of an interface to feed in to the computation to select the best path?

When OSPF and all the routing protocols that we use today were designed and built they were never designed to forward data packets while they were still re-converging. They were designed to drop data as that was the right thing to do at that time because the mathematical computation/algorithms took long enough and it was more important to avoid loops by dropping packets.  To cite an example, when OSPF comes up, it installs the routes only after it has exchanged the entire LSDB with its neighbors and has reached a FULL state. Given the volume of ancillary data that OSPF today exchanges via Opaque LSAs this design is an over-kill and folks at IETF are already working on addressing this.

We also have poor multipath ability with our current protocols today. We can load balance between multiple interfaces, but we have problems with the return path which does not necessarily come back the way you wanted. We work around that to some extent by network designs that adapt to that.

Current routing protocols forward data based on destination address only. We send traffic to but we don’t care where it came from. In truth as networks get more complex and applications get more sophisticated, we need a way to route by source as well by destination. We need to be able to do more sophisticated forwarding. Is it just enough to send an envelope by writing somebody’s address on an envelope and putting it in a post box and letting it go in the hope that it gets there? Shouldn’t it say that Hey this message is from the electricity deptt. That can go at a lower priority than say a birthday card from grandma that goes at a higher priority. They all go to the same address but do we want to treat them with the same priority?

So the question is that are our current protocols good enough – The answer is of course Yes, but they do have some weaknesses and that’s the part which has been driving the next generation of networking and a part of which is where OpenFlow comes in ..

If we want to replace the Routing protocols (OSPF, STP, LDP, RSVP-TE, etc) then we need something to replace those with. We’ve seen that Routing protocols have only one purpose for their existence, and that’s to update the forwarding tables in the networking devices. The SW that runs the whole system today is reasonably complex, i.e., SW like OSPF, LDP, BGP, multicast is all sitting inside the SW in an attempt to load the data into the forwarding tables. So a reasonably complex layer of Control Plane is sitting inside each device in the network to load the correct data into the forwarding tables so that correct forwarding decisions are taken.

Now imagine for a moment that we can replace all this Control Plane with some central controller that can update the forwarding tables on all the devices in the network. This is essentially the OpenFlow idea, or the OpenFlow model.

In the OpenFlow model there is an OpenFlow controller that sends the Forwarding table data to the OpenFlow client in each device. The device firmware then loads that into the forwarding path. So now we’ve taken all that complexity around the Control Plane in the networking device and replaced it with a simple client that merely receives and processes data from the Controller. The OpenFlow controller loads data directly into the OpenFlow client which then loads it directly into the FIB. In this situation the only SW in the device is the chip firmware to load the data into the FIB or TCAM memories and to run the simple device management functions, the CLI, to run the flash and monitor the system environmentals. All the complexity around generating the forwarding table has been abstracted away into an external controller. Now its also possible that the device can still maintain the complex Control Plane and have OpenFlow support. OpenFlow in such cases would load data into the FIBs in addition to the RIB that’s maintained by the Control Plane.

The Networking OS would change a little to handle all device operations such as Boot, Flash, Memory Management, OpenFlow protocol handler, SNMP agent, etc. This device will have no OSPF, ISIS,RSVP or Multicast – none of the complex protocols running. Typically, routers spend close to 30+% of CPU cycles doing topology discovery. If this information is already available in some central server, then this frees up significant CPU cycles on all routers in the network. There will also be no code bloat – we will only keep what we need on the devices. Clearly, smaller the code running on the devices, lesser is the bugs, resources required to maintain it – all translating into lower cost.

If we have a controller that’s dumping data into the FIB of a network device then it’s a piece of SW – its an application. It’s a SW program that sits on a computer somewhere. It could be an appliance, a virtual machine (VM) or could reside somewhere on a router. The controller needs to have connectivity to all the networking devices so that it can write out, send the FIB updates to all devices. And it would need to receive data back from the devices. It is envisioned that the controller would build a topology of the network in memory and run some algorithm to decide how the forwarding tables should be programmed in each networking device. Once the algorithm has been executed across the network topology then it could dispatch topology updates to the forwarding tables using OpenFlow.

OpenFlow is an API and a protocol which decides how to map the FIB entries out of the controller and into the device. In this sense a controller is, if we look back at what we understand today, very similar to Stack Master in Cisco. So if one has 5 switches in a stack then one of them becomes the Stack Master. It takes all of the data about the forwarding table. It’s the one that runs the STP algo, decides what the FIB looks like and sends the FIB data on the stacking backplane to each of the devices so that each has a local FIB (that was decided by the Stack Master).

To better understand the Controllers we need to think of 5 elements as shown in the figure.


At the bottom we have the network with all the devices. The OpenFlow protocol communicates with these devices and the Controller. The Controller has its own model of the network (as shown on the right) and presents the User Interface out to the user so that the config data can come in. Via the User Interface the admin selects the rules, does some configuration, instructs on how it wants the network to look like. The Controller then looks at its model of the network that it has constructed by gathering information from the network and then proceeds with programming the forwarding tables in all the network devices to be able to achieve that successful outcome. OpenFlow is a protocol – its not a SW or a platform – it’s a defined information style that allows for dynamic configuration of the networking devices.

A controller could build a model of the network and have a database and then run SPF, RSVP-TE, etc algorithms across the network to produce the same results as OSPF, RSVP-TE running on live devices. We could build an SPF model inside the controller and run SPF over that model and load the forwarding tables in all devices in the network. This would free up each device in the network from running OSPF, etc.

The controller has real time visibility of the network in terms of the topology, preferences, faults, performance, capacity, etc. This data can be aggregated by the controller and made available to the network applications.  The modern network applications can be made adaptive, with the potential to become more network-efficient and achieve better application performance (e.g., accelerated download rates, higher resolution videos), by leveraging better network provided information.

Theoretically these concepts can be used for saving energy by identifying underused devices and shutting them down when they are not needed.

So for one last time, lets see what OpenFlow is.

OpenFlow is a protocol between networking devices and an external controller, or in other words a standard method to interface between the control and data planes. In today’s network switches, the data forwarding path and the control path execute in the same device. The OpenFlow specification defines a new operational model for these devices that separates these two functions with the packet processing path on the switch but with the control functions such as routing protocols, ACL definition moved from the switch to a separate controller. The OpenFlow specification defines the protocol and messages that are communicated between the controller and network elements to manage their forwarding operation.

Added Later: Network Function Virtualization is not directly SDN. However, if youre interested i have covered it here and here.

Its time we retire Authentication Header (AH) from the IPsec Suite!

Folks who think Authentication Header (AH) is a manna from heavens need to read the Bible again. Thankfully you dont find too many such folks these days. But there are still some who thank Him everyday for blessing their lives with AH. I dread getting stuck with such people in the elevators — actually, i dont think i would like getting stuck with anybody in an elevator, but these are definitely the worst kind to get stuck with.

So lets start from the beginning.

IPsec, for reasons that nobody cares to remember now, decided to come out with two protocols – Encapsulating Security Payload (ESP) and AH, as part of the core architecture. ESP did pretty much what AH did, with the addition of providing encryption services. While both provided data integrity protection, AH went a step further and also secured a few fields from the IP header for you.

There are bigots, and i unfortunately met one a few days ago, who like to argue that AH provides greater security than ESP since AH covers the IP header as well. They parrot this since that’s what most textbooks and wannabe CCIE blogs and websites say. Lets see if securing the IP header really helps us.

When IPsec successfully authenticates the payload, we know that the packet came from someone who knew the authentication key. I would wager that that should be enough to accept the packet. The IP header is just required to route the packet to reach the recipient – its not meant to do anything else. Thats networking 101 really.

IPsec Security Associations are established based on the source and destination addresses and some L4 port information. The receiver matches the incoming packet’s against SPI and inbound selectors associated with the SA. Packet is only accepted if it came from the correct source and destination IP address. If an attacker somehow manages to change the IP header then there are high chances that it will get rejected by IPsec since it will fail the Security Policy Database (SPD) check.  So, what is protecting the header really giving us?

BTW ESP can also protect the IP header if its used in the tunnel mode. So, if someone is really keen on protecting the IP header then ESP in the tunnel mode can also be used. It should however be noted that ESP tunnel mode SA applied to an, say IPv6 flow,  results in at least 50 bytes of additional overhead per packet. This additional overhead may be undesirable for many bandwidth-constrained wireless and/or satellite communications networks, as these types of infrastructure are not over provisioned.

Packet overhead is particularly significant for traffic profiles characterized by small packet payloads (e.g., various voice codecs). If these small packets are afforded the security services of an IPsec tunnel mode SA, the amount of per-packet overhead is increased.

This issue will be alleviated by header compression schemes defined in the IETF.

I have recently published an IETF draft where i explicitly ask for AH to be retired since there is nothing useful that it does that cant be achieved with ESP with NULL encryption algorithm.

Please note that i have absolutely no complaints with AH and the claims that it makes. It does its job really well. Its just that its completely redundant and the world can certainly do with one less protocol to manage.

Retiring AH doesn’t mean that people have to stop using AH right now. It only means that in the opinion of the community there are now better alternatives. This will discourage new applications and protocols to mandate the use of AH. It however, does not preclude the possibility of new work to IETF that will require or enhance AH. It just means that the authors will have to do a real good job of convincing the community on why that solution is really needed and the reason why ESP with NULL encryption algorithm cannot be used instead.

The IETF draft that i have written aims to dispel several myths  surrounding AH and i show that in each case ESP with NULL encryption algorithm can be used instead, often with better results.

Life of Crypto Keys employed in Routing Protocols

Everyone knows that the cryptographic key used for securing your favorite protocol (OSPF, IS-IS, BGP TCP-AO, PIM-SM, BFD, etc)  must have a limited life time and the keys must be changed frequently. However, most people don’t understand the real reason for doing so. They argue that keys must be regularly changed since they are vulnerable to cryptanalysis attacks. Each time a crypto key is employed it generates a cipher text. In case of routing protocols the cipher text is the authentication data that is carried by the protocol packets. Its alleged that using the same key repetitively allows an attacker to build up a store of cipher texts which can prove sufficient for a successful cryptanalysis of the key value. It is also believed that if a routing protocol is transmitting packets at a high rate then the “long life” may be in order of a few hours. Thus it’s the amount of traffic that has been put on the wire using a specific key for authentication and not necessarily the duration for which the key has been in use that determines how long the key should be employed.

This was true in the Jurassic ages but not any more. The number of times a key can be used is  dependent upon the properties of the cryptographic mode than the algorithms themselves. In a cipher block chaining mode, with a b-bit block, one can safely encrypt to around 2^(b/2) blocks. AES (Advanced Encryption Standard)  used worldwide has a fixed block size of 128, which means that it can be safely used for 2^(64+4) bytes of routing data. If we assume a protocol that sends 1 Gig (!!) worth of control traffic *every* second, even then it is safe enough to be used for around 8700 *years* without changing the key! Hopefully, the system admin will remember to change the crypto key after 8700 years! 😉

So, if the data is secure then why do we really need to change the crypto keys ever?

As a general rule, where strong cryptography is employed, physical, procedural, and logical access protection considerations often have more impact on the key life than do algorithm and key size factors. People need to change the keys when an operator who had access to the keys leaves the company. Using a key chain, a set of keys derived from the same keying material and used one after the other, also does not help as one still has to change all the keys in the key chain when an operator having access to all those keys leaves the company. Additionally, key chains will not help if the routing transport subsystem does not support rolling over to the new keys without bouncing the routing sessions and adjacencies.

Another threat against a long-lived key is that one of the systems storing the key, or one of the users entrusted with the key, could be subverted. So, while there may not be cryptographic motivations of changing the keys, there could be system security motivations for rolling or changing the key.

What complicates this further is that more frequent manual key changes might actually increase the risk of exposure as it is during the time that the keys are being changed that they are likely to be disclosed! In these cases, especially when very strong cryptography is employed, it may be more prudent to have fewer, well controlled manual key distributions rather than more frequent, poorly controlled manual key distributions.

To summarize, operators need to change their crypto keys because of social and political, rather than scientific or engineering driven reasons.

You can read more about this in the IETF draft that i have co-authored here.

Issues with how BFD is currently implemented over LAGs

The BFD standards dont explicitly talk about how BFD should be implemented on Link Aggregation Groups (LAGs). This leaves a lot of room for imagination and vendors have implemented their own proprietary mechanisms to make BFD work on LAGs. Now, there is only this much room for innovation and most vendors have naturally arrived at similar techniques to implement interoperable BFD over LAGs. So, what makes BFD so sticky to implement on LAGs?

BFD being an L3 protocol, is oblivious to the physical link that the BFD packets go out on. Usually, there is only one link associated with an L3 interface, and there is thus no ambiguity on the link that packet needs to go out on. However, when an IP interface is configured over a LAG, there are multiple constituent links that the packet can go out on, and BFD has to decide the link it wants to use for sending the packets out.

A LAG binds together several physical ports between two adjacent nodes so they appear to higher-layer protocols and applications as a single, higher bandwidth “virtual” pipe.

The problem with running BFD over a LAG is that without internal knowledge of the LAG structure it is impossible for BFD to guarantee a detection of anything but a full LAG shutdown within the BFD timeout period. The LAG shutdown is typically initiated by some LAG module. LAG timers are typically multiple times slower than the BFD detection timers (multiple 100ms vs. multiple 10ms of BFD). There is thus a need to bring some sort of determinism in how BFD runs over a LAG. There is also a need to detect member link failures much faster than what Link Aggregation Control Protocol (LACP) allows.

Lets look at what implementations currently do to implement BFD on LAGs.

The simplest approach to run BFD on a LAG interface is to ignore the internal structure and treat the LAG as one “big, virtual pipe”.

Because there is no standard, vendors have implemented their own proprietary mechanisms to run BFD over LAG interfaces. Two examples are shown here.

Some implementations send BFD packets only over the “primary” member link of the LAG. Others spray BFD packets over all member links of the LAG. There are issues with both these designs.

In the first design, BFD will remain Up as long as the primary link is alive. If the primary link goes down, and another link is not selected as the primary, before BFD times out (around 30-50ms), then the BFD session on the LAG comes crashing down. Problems arise as BFD, in this design, is oblivious to the presence of other member links in the LAG. If a non-primary link goes down, the BFD session remains unaffected as it can still send and receive BFD packets over the primary link. Since the BFD session is Up, other routers in the network continue sending traffic meant to egress out of this interface. As expected from the LAG, all traffic egressing out of this interface gets load distributed on all LAG member links. Now, there is one link thats down. All traffic sent over that failed link gets dropped, till the LAG manager detects this and removes it from the LAG.

In the second design, BFD packets are sprayed over all the member links of a LAG. This is done naively via round-robin, where each BFD packet is sent using the subsequent member link, in a round-robin fashion. It solves the problem of BFD going down because of the primary link going down, but it still does not solve the problem of traffic getting lost when one of the member link goes down. This is because, when a member link goes down, BFD remains up and traffic continues to go over the link that has failed till a higher layer protocol (usually LACP or the LAG Manager) detects this and removes the offending link from the LAG.

The above two designs defeat the core purpose of a BFD, which is to detect faults between the two forwarding engines. In each design traffic gets lost on a failed link till some protocol other than BFD detects this and removes that link from the LAG. The timers associated with the other protocol are an order of magnitude higher than BFD.

Operators have since long expressed a need to be able to detect the failed links fast so that their traffic doesnt get lost. The idea is to get BFD to take charge of the LAG and make it responsible for maintaining the list of active links in a LAG. This way we can use the BFD fast timers to quickly detect link failures.

One could argue that there are native Ethernet OAM mechanisms (.1ag, .3ah) that can be used to detect link failures in a LAG, and one need not rely on slow protocols like LACP or the LAG manager. The reality is that operators who have deployed BFD in their IP/MPLS networks want a common failure detection mechanism and dont want a mix of different technologies.

To solve the above mentioned issues I have co-authored an IETF document that proposes running BFD on each constituent link of the LAG. We call the BFD sessions running on each link a “micro BFD session”. We call this mode of BFD on LAGs as BLM – BFD on Lag Members.

BLM is an umbrella BFD session that contains information about the LAG (or the aggregated interface) that its running on. It consists of a set of micro BFD sessions that are running on each constituent link of the LAG. And it contains a state that we call the “Concluded State”, which describes the overall state of the LAG (Up, Down, AdminDown).

Each micro BFD session is a regular RFC 5880 and RFC 5881 compliant BFD session. Only Asynchronous mode is supported for the micro BFD sessions as the sole reason for running BFD on each member link is to verify the link connectivity. The Echo function for the micro BFD sessions is not recommended as it requires twice as many packets to achieve a particular Detection time as does the pure Asynchronous mode.

At least one system MUST take the Active role (possibly both). The micro BFD sessions on the member links are independent BFD sessions. They use their own unique, local discriminator values, maintain their own set of state variables and have their own independent state machine. Typically each micro BFD session will have the same timer values, however, nothing precludes the possibility of having different timer values among the different micro BFD sessions belonging to the same LAG.

A session begins with the periodic, slow transmission of BFD Control packets. When bidirectional communication is achieved, the BFD session becomes Up. The LAG manager is informed at this point, and the member link becomes an active link of the LAG.

If the micro session goes Down, the transmission of Control packets goes back to the slow rate. The LAG Manager is informed which removes the member link from the LAG.

Once a session has been declared Down, it cannot come back up until the remote end first signals that it is down (by leaving the Upstate), thus implementing a three-way handshake.

A session MAY be kept administratively down by entering the AdminDown state and sending an explanatory diagnostic code in the Diagnostic field.

In short, its pretty much the same as a standard BFD session.

This solves the issues that i had described in the earlier two designs. The micro BFD sessions will quickly detect a failed link, and will instantly remove it from the LAG. Traffic that was earlier egressing out over the failed link, will now get hashed to a different link in the LAG. This results in zero traffic loss on the LAG.

You can read more about our proposal here (more on how it evolved within IETF here).

Recognizing the need for running BFD on all member links, various vendors support their own proprietary, un-interoperable implementation of BFD over LAGs. We’re hoping that our IETF proposal to standardize this behavior will bring some order to the chaos thats out there and a relief to the providers who are stuck with proprietary solutions.