We need better layering

In a recent blog post Avery Pennarun points out an important limitation of TCP/IP (it’s a long and rambling post: the key section is the last one, headed “Making mobile IP actually work”). At the endpoint, TCP connections are identified by the tuple localaddress-localport-remoteaddress-remoteport. This assumes that both addresses remain fixed for the lifetime of the connection. That assumption breaks down if one endpoint is mobile, moving from access point to access point. The tuple combines identifiers from Layer 3 (Network) and Layer 4 (Transport), to form an identifier for a Layer 5 object: this is poor layering. Geoff Huston makes a similar point in a recent post on his blog. As he puts it, an IPv4 address is overloaded with both the unique identity of an endpoint and its network location.

What Pennarun proposes as a solution is to use the QUIC protocol. This is an experimental replacement for TCP and TLS; it has to be layered on top of UDP, because transport-layer protocols like TCP and UDP are so baked into operating systems and network devices like firewalls that adding a new one is a very slow process (for example, support for SCTP is still limited, seventeen years after RFC 2960 first proposed it). The key feature here is that QUIC identifies connections by a 64-bit connection ID that is independent of the underlying local address or port; if the local address changes (due to device mobility or NAT), then QUIC handles the “connection migration”. Geoff Huston makes the same point in a podcast at Packetpushers (at 00:41).

I’m a fan of good layering: to my mind higher-layer protocols shouldn’t rely upon the identifiers of lower layers, such as specific IP addresses. After all, IP addresses are just a way of shuttling packets around: if IP addresses change due to roaming or translation then applications should be able to recover from this. I just don’t see why the end-to-end principle necessarily implies end-to-end addressing: nevertheless the IPv6 world ties itself in loops in order to preserve constant, end-to-end Layer 3 addresses (see my previous post). Instead of trying to hammer the world flat, shouldn’t we design our protocols to fit the world as it actually is?