From: Stephen Kent <kent@bbn.com> Subject: Re: BGP to doom us all Date: Wed, 2 Apr 2003 18:15:05 -0500 Folks, I was not subscribed to the workshop list when Randy forwarded this message at the beginning of last month. However, I would like to respond to the issues raised in the text. Steve ----

From: Iljitsch van Beijnum <iljitsch@muada.com> To: Avi Freedman <freedman@freedman.net> Cc: <nanog@merit.edu> Subject: Re: BGP to doom us all Date: Sat, 1 Mar 2003 21:09:14 +0100 (CET)

On Sat, 1 Mar 2003, Avi Freedman wrote:

...

Re: S-BGP in particular, I think that the analysis on S-BGP has been... limited. Ironic for a security protocol that I haven't seen any real analysis of the effect on router CPUs when *under attack*. I am not saying "oh, the authentication will drive things way too high". I'm just saying that we don't know because the simulations have used very conservative parameters.

My two cents:

By looking at the packet formats I estimate that the memory requirements for S-BGP are about 4 times those of regular BGP (mind you - just the BGP table, RIB and FIB remain the same). This gets us pretty close to common 32-bit CPU address space limits.

Whie it is true that the size of UPDATE messages grows considerably under S-BGP, the amount of extra space needed in RIBs is a function of the implementation approach that one adopts, and so it is hard to translate the space occupied by RAs in an UPDATE into the extra RIB space that will be used. Obviously, if a router has many peers, then the increase in the Adj and Loc RIB sizes will be magnified, and no fanout number was provided in the comment above. But it does not seem likely that we would really be close to 32-bit address space limits based on our analysis and some (limited) implementation experience.

For receiving updates: you are expected to check the validity of each hop in each AS path in the global routing table using a public key signature verification. A Pentium-class CPU can do a few thousand of those per second, so that adds several minutes of pure CPU time to the initialization of each full feed. This means crypto hardware.

In the papers we published, we explicitly noted that initialization could be a problem, and suggested that initialization after a crash would benefit from a restoral strategy (using NV storage for the Loc RIBs) or from a deferred evaluation strategy. We have thought more about this latter strategy, and a variant of it also is attractive for any router with many peers. The observation is that when you have many peers, most of the UPDATEs that are received will not result in generation of UPDATEs, and thus one need not evaluate the signatures, since they didn't influence operation (i.e., didn't result in the generation of UPDATEs). This is true both in steady state operation as well as during initialization. Thus one could mark each Adj RIB entry to indicate whether it had been verified, or not. If the router is too busy to verify an UPDATE, then so long as that UPDATE does not change the notion of best route for the prefixes it contains, one can defer (perhaps forever) the signature verification process. (We do preform other, quick checks to weed out bad UPADTEs upon receipt.) Using the strategy, we anticipate that one can significantly reduce the number of signature validation operations that need to be performed, with proportionally greater savings at routers with the most peers. Still, we do think that use of crypto hardware is desirable in the long run, since it allows for better protection of the private key used by a router, and it offers greater capacity to deal with floods of UPDATEs during worst case scenarios. If routers need to be equipped with additional memory anyway, then making provision for a crypto chip would be a reasonable update to a processor board when the memory size changes were made. Modern crypto chips are capable of very high (10's of thousands/second) signature processing rates, but one must pay close attention to the fine print, since there most of the figures do not include the time to load a new signature key into the chip.

For sending updates: you must include the identity of the receiver in each update (and then sign the update, unless I remember incorrectly). That means sending out updates to a large number of peers (such as on a public internet exchange) becomes a rather expensive operation, even discounting the crypto.

I'm not sure why this is a "rather expensive operation, even discounting the crypto." Certainly the bigger UPDATEs take up more TCP queue space, and maybe there is some concern over the need to generate a distinct UPDATE per peer. However, there is a provision to place more than one next hop AS # in a route attestation, to reduce the need to generate different UPDATEs for each peer, so long as the UPDATE would otherwise be the same. We have not explored the implications of this in detail, but it does suggest a possible way to reduce the signature generation burden. Also, note that S-BGP uses DSA, which provides very efficient signature generation, which is the crypto operation one performs when generating (vs. consuming) an UPDATE.

However, there is an alternative to S-BGP: soBGP (secure origin BGP, see URL in my sig for links). Unlike S-BGP, soBGP mainly focusses on the origin of a route. At first sight this would make soBGP less secure than S-BGP, but it looks like S-BGP can't protect the entire path under all circumstances anyway. Also, soBGP is a more open protocol, which can easily be extended with additional security checks. And a big plus for soBGP is that unlike S-BGP, where the security information is stored in path attributes (which means you can easily bring down an existing router by sending it S-BGP info, no filtering on unknown attributes AFAIK), this information is exchanged using a new type of message. This makes it possible to offload the security processing to an external box.

S-BGP does detect and reject attempts to propagate unauthorized paths within contiguous regions of ASes that use S-BGP. By this we mean that if an AS tries to modify a path that has been passed to it by an AS executing S-BGP, or to create a bogus path from scratch, other S-BGP ASes will detect the tampering. So I am at a loss to understand the comment "but it looks like S-BGP can't protect the entire path under all circumstances anyway." The latest version of soBGP, according to a discussion with some Cisco folks at the IETF in SF, will address the secure path vulnerability, not just secure origin advertisements. However, soBGP is not an architecture, at this time; it is really a collection of mechanisms. It is more "open" in that some of the hard decisions have not been made. For example, I was told that one could distribute digitally signed topology data via repositories, or via some extension to BGP for inband distribution. I asked which mechanism was the mandatory default, and the answer was neither. This does not ensure interoperability, because if one AS decides to distribute this info only via a repository, and other ASes decide to receive it only inband, the data will not be propagated! There are several "you could do A or you could do B" aspects to the soBGP design, which make it impossible to analyze completely, or to compare with S-BGP, at least for now. I don't understand the assertion about bringing down an S-BGP router based on "sending it S-BGP info ...." If this is a reference to an implementation vulnerability concern, then it needs to be addressed, as do the many other implementation vulnerabilities that are present in routers today and which make them vulnerable to flooding of UPDATEs or other management traffic. Also, S-BGP can be implemented using an ancillary processor associated with a router, an option we proposed several years ago, but which was not well received by most operations folks. In soBGP, if the processing is performed at some distant device, issues about availability of these devices and timeliness of processing arise. One thing that any design should avoid is a loss of sync between security data being propagated and UPDATEs that are to be verified based on that security data.

The problem with fully protecting the path (such as S-BGP wants to do) is that the only way to do this well is to have the source say which paths are good, but this is simply too much information. Without this, it becomes possible for someone who legitimately received the route to propagate it even though the source didn't intend this. Also, giving the source full control makes dynamic rerouting (like 9/11 emergency transit) much, much harder. Obviously we all want routes we know are good, and don't want routes we know are bad. That's wat S-BGP and soBGP can do. But what if we can't determine if routes are good or bad? If you reject the routes you break a lot of connecitvity. If you don't, nobody will bother jumping through all the hoops to make their route authenticity verifiable. I really don't envy the first operator to deploy (S-|so)BGP...

This paragraph seem to be just plain wrong. S-BGP relies on prefix "owners" to announce origin ASes via a repository system, consistent with the fact that these binding change slowly. S-BGP requires each AS along a path to sign a route attestation as the means of authorizing the next hop AS(es) to advertise the prefix(es) in the UPDATE. The reason S-BGP carries route attestation data in UPDATEs is to match the dynamic characteristics of route advertisements. Thus a 9/11 emergency or a Baltimore train tunnel fire can be accommodated, merely by having the affected ASes sign new route attestations and insert them into UPDATEs. This is precisely why we do not include a repository distribution option for this data (as soBGP does), since repository distribution would not be responsive enough for such conditions. Only if access lines from subscribers are lost and the subscribers are rehomed to ISPs with whom they have had no prior relationship would repository distribution of data (address attestations) be involved. We have assumed that there are enough other hurdles to this sort of rehoming that the repository mechanism would be viable. We did have a facility for inband distribution of address attestations in emergencies, and we could revive that feature, if it really is necessary. There is no concept of "good" vs. "bad" routes in S-BGP. Only "authorized" vs. "unauthorized." If by "source" Iljitsch means "origin AS" then there is a disconnect between what BGP does and what is said here, since BGP in no way suggests that an origin AS can control the advertisement of a route beyond the immediate neighbors to which the route is advertised. S-BGP is consistent with the semantics of BGP, as we understand them. if this is an allusion to other path attributes, e.g., community info, then note that S-BGP can provide protection for them too, selectively. Steve