**Expedited Forwarding and Packet Scale
Rate Guarantee**

**J.-Y. Le Boudec**

**Abstract:**

We consider the definition of the Expedited Forwarding Per-Hop
Behaviour (EF PHB) as given in RFC 2598, and its
impact on worst case end-to-end delay jitter. On one hand, the
definition in RFC 2598 can be used to predict extremely low
end-to-end delay jitter, independent of the network scale. On the
other hand, we find that the worst case delay jitter can be made
arbitrarily large, while each flow traverses at most a specified
number of hops, if we allow networks to become arbitrarily large;
this is in contradiction with the previous statement. We analyze
where the contradiction originates, and find the explanation. It
resides in the fact that the definition in RFC 2598 is not easily
implementable in schedulers we know of, mainly because it is not
formal enough, and also because it does not contain an error term.
We propose a new definition for the EF PHB, called ``Packet Scale
Rate Guarantee", which preserves the spirit of RFC 2598, while
allowing a number of reasonable implementations, and has very
useful properties for per-node and end-to-end network engineering.
We show that this definition implies the rate-latency service
curve guarantee. Then we propose some proven bounds on delay
jitter for networks implementing this new definition, both in
cases without loss and with loss.

**Presentation
slides**