1. PREQ/PREP Create/Process/Forward
Feb, 2008
doc.: IEEE /xxxx
September 2008
PREQ/PREP Create/Process/Forward
Date:
Authors:
L. Chu Etc.
Liwen Chu, STMicroelectronics

2. Path Stability With Rate of Sequence Number (SN) Change
September 2008
Path Stability With Rate of Sequence Number (SN) Change
Sequence number (SN) is used to avoid path loop in HWMP.
Increasing SN by 1 each time a MP sends a PREQ may change the path frequently.
One solution to maintain path stability in the current draft is to use the same source SN for dot11MeshHWMPnetDiameterTraversalTime.
PREQ ID is increased by 1 each time a new PREQ is created.
Why avoiding path instability is important:
Baseline dot11 standard requires that “There shall be no reordering of unicast MSDUs with the same TID value and addressed to the same destination.” (Section 6.1.3)
L. Chu Etc.

3. Current PREQ Acceptance Criteria
September 2008
Current PREQ Acceptance Criteria
The PREQ element shall not be accepted:
The Originator DSN < previous Originator DSN
(DSN = previous DSN) AND (updated path metric is worse than previous path metric)
(the destination address of the PREQ is neither the recipient MAC address nor a MAC address proxied by the recipient) AND (dot11MeshForwarding is set to 0)
PREQ ID is not considered in PREQ acceptance criteria.
L. Chu Etc.

4. Current PREP Creating Criteria
September 2008
Current PREP Creating Criteria
Case A: Original transmission A PREP is transmitted if the MP has received a PREQ fulfilling all of the following conditions:
a. One of the following applies:
The Destination Address of the PREQ is the same as MAC address of the receiving MP
The Destination Address of the PREQ = all 1’s (broadcast) and the PREP flag is set to 1 (”Proactive PREP”)
The Destination Address of the PREQ is currently proxied by the MP
b. One of the following applies:
The Originator DSN of the PREQ (preq.orig_dsn) is greater than the DSN of the last PREQ received from the same originator address (that includes the case that there is no path to the originating MP)
The Metric is better than the path selection metric currently associated with the Originator Address and the Originator DSN of the PREQ (preq.orig_dsn) is equal to the DSN of the last PREQ received from the same originator address
PREQ ID is not considered in PREP creating criteria.
L. Chu Etc.

5. PREQ Create/Process/Forward Procedure Refinement
September 2008
PREQ Create/Process/Forward Procedure Refinement
The PREQ element shall not be accepted if any of the following is true:
The Originator DSN < previous Originator DSN
(DSN = previous DSN) AND (PREQ ID of the originator is not the first time to be received) AND (updated path metric is worse than previous path metric)
(the destination address of the PREQ is neither the recipient MAC address nor a MAC address proxied by the recipient) AND (dot11MeshForwarding is set to 0)
Effect of PREQ receipt (Add the following restriction):
The receiving MP shall not update the active forwarding information for the originator if the updated path metric is worse than the previous path metric
L. Chu Etc.

6. September 2008
PREP Introduction
PREP follows unicast reverse path from the destination MP to the source MP
A destination does not need to increase its destination sequence number each time it sends a PREP in the current HWMP
An intermediate MP uses the destination sequence number and path metric to decide if the received PREP is discarded
The PREP element shall not be accepted (and shall not be processed as described in 11B ) if any of the following is true:
The DSN < previous DSN from this originator
The Time to Live is 1 or less
(the destination address of the PREQ is neither the recipient MAC address nor a MAC address proxied by the recipient) AND (dot11MeshForwarding is set to 0)
Different reverse paths from a destination may have same intermediate MPs
these intermediate MPs will decide if the PREPs for different source MPs from the same destination MP are discarded
L. Chu Etc.

7. Destination Sequence Number in 11s Draft
September 2008
Destination Sequence Number in 11s Draft
In section 11B.9.4.2, a destination MP increments its sequence number
Immediately before a destination MP originates a PREP in response to a PREQ, it shall update its own sequence number to the maximum of its current sequence number and the destination sequence number in the PREQ.
The intermediate MP may wrongly discard received PREP based on this DSN updating criteria
L. Chu Etc.

8. Updated Destination Sequence Number in 11s Draft
September 2008
Updated Destination Sequence Number in 11s Draft
A destination MP shall update its sequence number based on the following criteria:
A destination MP shall increment its own sequence number the same as the destination sequence number in the RREQ plus 1 if the destination sequence number in the RREQ is larger than or equal to the sequence number in the destination MP.
Otherwise, a destination MP shall not change its sequence number.
Another equivalent description is that:
Immediately before a destination MP originates a PREP in response to a PREQ, it shall update its own sequence number to the maximum of its current sequence number and the destination sequence number in the PREQ plus 1.
L. Chu Etc.

9. Wrongly PREP Discard in Intermediate MP
September 2008
Wrongly PREP Discard in Intermediate MP
MP1 request a path to MP5 with DO = 1, DSN=0
MP3 discards PREP (D:1) since the SN and path metric are the same.
MP1
S:1 D: 0 P:1
S:1D:0 P:1
MP3
S:1D:0 P:1 1 1
MP5 does not increase its destination SN
since DSN in MP5 is 1.
MP2 1
D:1
MP5 1 1 2
MP0
MP4
New MP2 FIB
DST
NHP
DSN
Metric
PID
MP5
MP3 1 2
MP1
Original MP2 FIB
DST
NHP
DSN
Metric
PID
MP5
MP3 1 2
MPi x
MPj
x: Metric between MPi and MPj
New MP3 FIB
DST
NHP
DSN
Metric
PID
MP2 1
MP5
MP1 2
Original MP3 FIB
DST
NHP
DSN
Metric
PID
MP2 1
MP5
S:x D: y P:z
PREQ for MP5 with source SN x, destination SN y
PREQID z
D:x
PREP for MP5 with destination SN x
New MP5 FIB
DST
NHP
DSN
Metric
PID
MP2
MP3 1 2
MP1 3
Original MP5 FIB
DST
NHP
DSN
Metric
PID
MP2
MP3 1 2
L. Chu Etc.

10. Updated Destination Sequence Number in 11s Draft
September 2008
Updated Destination Sequence Number in 11s Draft
The further update should be done in intermediate MP:
When an intermediate MP accepts an PREQ, if the destination MP’s SN in the forwarding table of the intermediate MP is larger than the DSN of the destination MP in PREQ, the DSN in PREQ shall be updated to the destination MP’s SN maintained in this intermediate MP.
L. Chu Etc.

11. September 2008
Backup Slides
L. Chu Etc.

12. Wrongly PREQ Discard in Intermediate MP
September 2008
Wrongly PREQ Discard in Intermediate MP
MP1 requests a path to MP5. x
MPi
MPj
x: Metric between MPi and MPj
PREQ for MP5 with source SN x and PREQ ID y
S:x P:y
PREQ for MP6 with source SN x and PREQ ID y
MP1 requests a path to MP6.
MP2
S:1 P:2
S:1 P:2
S:1 P:1
MP5 1 2 1
MP1
MP4
MP4 discards PREQ (S:1 P:2) Since bullet 2 in PREQ accepting criteria is true 1 1 1
S:1 P:1
MP3
S:1 P:1
MP6
MP3 FIB
DST
NHP
DSN
Metric
PID
MP1 1
MP4 FIB
DST
NHP
DSN
Metric
PID
MP1
MP3 1 2
MP2 FIB
DST
NHP
DSN
Metric
PID
MP1 1 2
MP5 FIB
DST
NHP
DSN
Metric
PID
MP1
MP4 1 3
L. Chu Etc.

13. Wrongly PREQ Discard in Destination MP
September 2008
Wrongly PREQ Discard in Destination MP
MP1 requests a path to STA1 associated with MAP4. x
MPi
MPj
x: Metric between MPi and MPj
PREQ for STA1 with source SN x and PREQ ID y
S:x P:y
PREQ for STA2 with source SN x and PREQ ID y
PREP for STA1
MP1 requests a path to STA2 associated with MAP4.
S:1 P:2
S:1 P:2
MP2
STA1 1 2
MP1
MAP4
MAP4 discards PREQ (S:1 P:2) since the metric is not better than PREQ (S:1 P:1) 1 1
S:1 P:1
MP3
S:1 P:1
STA2
MP3 FIB
DST
NHP
DSN
Metric
PID
MP1 1
MAP4 FIB
DST
NHP
DSN
Metric
PID
MP1
MP3 1 2
MP2 FIB
DST
NHP
DSN
Metric
PID
MP1 1 2
MP3 FIB
DST
NHP
DSN
Metric
PID
MP1 1
MAP4
MP1 FIB
DST
NHP
DSN
Metric
PID
MAP4
MP3 1 2
MP1 Proxy Table
Proxy Addr
Proxied Addr
MAP4
STA1
L. Chu Etc.

14. Destination Sequence Number in AODV
September 2008
Destination Sequence Number in AODV
In section 6.1, a destination MP increments its sequence number
Immediately before a destination MP originates a PREP in response to a PREQ, it shall update its own sequence number to the maximum of its current sequence number and the destination sequence number in the PREQ
In section 6.6.1, a destination MP increments its sequence number
If the generating node is the destination itself, it MUST increment its own sequence number by one if the sequence number in the RREQ packet is equal to that incremented value. Otherwise, the destination does not change its sequence number before generating the RREP message
These two criteria are contradictory with each other
The description in section is very difficult to understand
L. Chu Etc.

15. Path Instability with PREP Replying Right away
September 2008
Path Instability with PREP Replying Right away
MP0 requests a path to MP5. x
MPi
MPj
x: Metric between MPi and MPj
PREQ for MP5 with source SN x
S:x P:y
PREP for MP5 with destination SN x
D:x
S:1 P:1
MP3
MP1 1 1 1
MP2
MP5 1
D:1 1
S:1 P:1
MP0
S:1 P:1
MP4
S:1 P:1 2
MP2 FIB
DST
NHP
DSN
Metric
PID
MP0 1
MP4 FIB
DST
NHP
DSN
Metric
PID
MP0
MP2 1 2
MP4 FIB
DST
NHP
DSN
Metric
PID
MP0
MP2 1 2
MP5
MP3 FIB
DST
NHP
DSN
Metric
PID
MP0
MP2 1 2
MP5 FIB
DST
NHP
DSN
Metric
PID
MP0
MP4 1 4
L. Chu Etc.

16. Path Instability with PREP Replying Right away (Cont’d)
September 2008
Path Instability with PREP Replying Right away (Cont’d)
MP0 requests a path to MP5.
MPi x
MPj
x: Metric between MPi and MPj
MP3
MP1
D:2
S:1 P:1 1
S:x P:y
D:2
PREQ for MP5 with source SN x 1 1
MP2
MP5
D:x
D:2 1
PREP for MP5 with destination SN x 1
D:1
D:1 2
MP0
MP4
MP2 FIB
DST
NHP
DSN
Metric
PID
MP0 1
MP5
MP4 3
MP2 FIB
DST
NHP
DSN
Metric
PID
MP0 1
MP5
MP3 2
MP0 FIB
DST
NHP
DSN
Metric
PID
MP5
MP2 1 4
MP0 FIB
DST
NHP
DSN
Metric
PID
MP5
MP2 2 3
MP3 FIB
DST
NHP
DSN
Metric
PID
MP0
MP2 1 2
MP5
MP5 FIB
DST
NHP
DSN
Metric
PID
MP0
MP3 1 3
L. Chu Etc.