Subject: [Diffserv] rfc2598bis comments

From: Dan Grossman <[email-masked]>
Date: Thu, 22 Feb 2001 14:05:46 -0500

Looks good.  Here are some comments, many of them nits:

1) Section 1, 3rd para: remove "speed-of-light" before propagation delays.  
Prop delay has sources other than propagation through the medium (like 
regenerators) and not all transmission is optical.  In fact, it is better to 
talk about fixed delay rather than propagation delay.  Also, the minimum delay 
is the fixed delay.  So better to use the term fixed delay throughout.

2) 2.1, second para: change "We define the departure time of a packet..." to 
"We adopt the convention that the departure time of a packet is defined..."  
Meaning that the equations could be made to work the other way.  Also, add:  
"The last bit of the packet is the last bit of the Layer 2 trailer, if 
present." This approach has problems, but since we're defining this in terms 
of an interface rather than an OSI-like Service Access Point, it will have to 
do.  It has to be this way because a_j is taken as the time that the packet is 
availabe for forwarding, and a packet is not available for forwarding in most 
layer 2 protocols until after the CRC has been computed.

3) 2.1, fourth para:  How can a scheduler run early?

4) 2.1 fifth para change "color-blind" to "aggregate behavior" and 
"packet-identity aware" to "individual flow behavior".

5) 2.2, definition of l_j, L_j (and R):  Isn't excluding layer 2 overhead a 
problem, since IP packets cannot be serviced without also servicing the layer 
2 overhead?  Especially for relatively small IP packets and layer 2 protocols 
that have variable overhead? 

6) eq. 1 and eq.3, don't these require:  "For all j"

7) In the paragraph at the bottom of page 6, and in the definitons of F_j and 
D_j, where the text says "packet that arrived".... arrived _where_?  I think 
the answer is "at any input interface to that DS-node", but not quite sure.

8) 2.4, "a device may advertise..."  You probably want some other words, like 
"E_p for a device may be characterised as having two separate components".  
"Advertise" implies either routing metrics or *Business Things We Don't 
Discuss In Diffserv*

9) In security considerations, add the following (as agreed on the list 
earlier and documented in newterms draft):
"In addition, traffic conditioning at the ingress to a DS-domain MUST ensure 
that only packets having DSCPs that correspond to an EF PHB when they enter 
the DS-domain are marked with a DSCP that corresponds to EF inside the 
DS-domain.  Such behavior is as required by RFC 2475.  It protects against 
denial-of-service and theft-of-service attacks which exploit DSCPs that are 
not identified in any TCS provisioned at an ingress interface, but which map 
to EF inside the DS-domain."  

10) Appendix: One thing that's not addressed is the case where there is more 
than one instance of the EF PHB at an interface.  Need to do such a thing was 
a result of the VW discussions leading up to Adelaide.  Particularly with PQ, 
the analysis of E_a (and possibly E_p) really only applies to most constrained 
EF instance.

11) Appendix:  Wouldn't an earliest deadline first scheduling discipline (or 
an approximation like RPQ) also meet the requirements of eq.1 - eq.4, with low 
E_a and low E_p?

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