Subject: [tsvwg] Review: draft-ietf-tsvwg-ecn-encap-guidelines-14.txt

From: Ingemar Johansson S <[email-masked]>
Date: Thu, 17 Dec 2020 15:10:29 +0000

Hi Bob, John + others
Very slow with the review of these documents, sorry for that.

I have now reviewed draft-ietf-tsvwg-ecn-encap-guidelines-14.txt

My comments are mostly around the implementation of ECN capability in 4G/5G 
(eNB/gNB). The comments are applicable mainly for downlink data traffic and 
the case where queues build up in the RLC layer.

Section 4 : Feed-Forward-and-Up mode. A 3GPP RAN2 contribution was proposed a 
while ago, the outline was here that a RLC Control PDU (or a MAC Control 
Element) is signaled from the eNB or gNB to the UE (mobile terminal or modem). 
This can be just an indication that the next decapsulated ECT IP packet should 
be CE marked, or it can be an indication to CE mark packets with a given 
probability. At least for now I believe that the mark probability option is to 
prefer as a per packet signaling can become too burdensome. It can be worth to 
mention that this proposal has so far not gained momentum in 3GPP

Section 5 : Feed-Up-and-Forward mode. This is problematic to achieve with the 
present LTE and NR standards (4G and 5G) as the IP headers are encrypted on 
the RLC layer. An approach to mark an IP packet would the require that the RLC 
SDU is decrypted, and the IP header is extracted and marked , then encrypted 
again. This can perhaps be doable with classic ECN but appears to me as quite 
unfeasible to do for instance for L4S where a large fraction of packets may be 
marked.

Section 6 : Feed-Backward mode. This is currently the mode that is the most 
feasible to implement in LTE/NR, given the current 3GPP standardization 
status. The approach requires signaling from the layer where the queue builds 
up (RLC) to the PDCP layer where the next ECT IP packet subject to ciphering 
is marked, thus it essentially becomes tail marking. In Davide Brunello's L4S 
in 5G paper an additional constraint was put on the signaling, which resulted 
in that a mark probability was signaled up to the PDCP layer.

/Ingemar