Subject: RE: [mobile-ip] Times for Low-Latency IPv6 Handoff

From: Nakhjiri Madjid-MNAKHJI1 <[email-masked]>
Date: Tue, 5 Jun 2001 16:41:45 -0500

James,

Thanks for sending the analysis and sorry for my late response,
I am still catching up with the list. The analysis made me
go and read the new version of the draft! So now I am even more
behind on the list, sorry if there has been more exchange on this.

I have one question and a few comments.

First, shouldn't you include the propagation delay from the oldAR to
the MN and back into the calculation?
Speed of light is 1 ns/foot=3ns/meter=3ms/km, so
in a CDMA cell of about 3Km in radius, you have 10 ms
propagation delay in each direction, which mean 20 ms total.

Also, I don't know what WCDMA does these days, but don't they add
some padding to the layer 3 signal (maybe that is not too big :) )

Now, my question isn't HI/HACK between oldAR/ newAR included in stateful
case, aren't
those messages in critical path? I saw your comment about Mobile
determined case (that HI/HACK should be included). Doesn't the same 
apply to Network determined stateful?

Again the HI/HACK has both transmission delay and propagation delay.

Thanks,
Madjid Nakhjiri

-----Original Message-----
From: James Kempf [mailto:[email-masked]]
Sent: Monday, May 14, 2001 4:40 PM
To: [email-masked]
Subject: [mobile-ip] Times for Low-Latency IPv6 Handoff

As an interesting exercise, I went through some calculations about how
long an IPv6 low latency handoff would take in some of the scenerios
described in draft-ietf-mobileip-fast-mipv6-01.txt. I looked at the
following cases:

	- Network determined, stateless with the following two subcases:
		- F-BU/F-BACK received before handoff 
		- F-BU/F-BACK received after handoff  
		
	- Network deterimed, stateful 
	
	- Mobile determined, with the following two subcases:
		- F-BU/F-BACK received before handoff
		- F-BU/F-BACK received after handoff
		
I also made the following assumptions:

	- Prior to handoff, the header compressor on the old AR is
	"hot" enough to compress the basic 40 byte IPv6 header down
	to 1 byte, but after handoff, the header compressor is "cold"
	and so the full IPv6 header needs to be sent. Currently, there
	is some debate about whether the compressor could be partially
	restarted on the new AR using context transfer, so this could
	introduce an additional optimization and also a potential 
	area of timing uncertainty involving the context transfer
	time itself.
	
	- The BU/BACK pair completes on either side of the handoff.
	In actuality, it may span the handoff and Section 3.3 may
	come into play if the MN doesn't get a BACK back (so to
	speak) fast enough. Or, an additional BU may need to be sent.
	I view the assumptions made here as being fairly optimistic,
	since over the air signalling after L2 has indicated that
	handover is pending is likely to be subject to a considerably
	higher error rate due to deteriorating radio conditions in
	the old cell and not-yet improved radio conditions in the new.
	
	- The radio link layer is using its lowest bit rate for signalling.
	This assumption is based on either using a multiplexed common
	channel for signalling, in which case the MN would have to share
	the channel with other MNs in the cell, or using a dedicated
	channel but not wanting to charge the user much or nothing at
	all, and therefore not wanting to use a higher bandwidth. Of
	course, this will be radio-specific, but it seems like a good
	assumption for cellular systems. The value I used was 7.5 kbps
	(940 bytes/sec), which is the minimum for a wCDMA dedicated channel
	from Holma and Toskala, "WCDMA for UMTS", Wiley, 2000.
	
	- The wired link ARs use Ethernet (48 bit MAC address link
	layer) and the wireless link is using 15 byte IMSIs (International 
	Mobile Station Identifiers) sent as ASCII digits.
	
	- The MN can neither send or receive traffic on the new AR until
	the old AR has confirmed the new CoA via a F-BACK. 
	
Here's the results.

Network Determined, Stateless
-----------------------------

As near as I can tell, the following messages get exchanged over the
air: PrRtAdv, F-BU, F-BACK.

Sizes of these are:

            hdr   msg   lnkAddrOpt   prxLnkAddrOpt   newCoA	Total
	
PrRtAdv      1     8        9             9            24         51

F-BU 
before       1    10						  11

F-BU
after        40   10						  50

F-BACK
before       1	   9						  10

F-BACK
after        40    9					          49

Total over the air sizes:

F-BU/F-BACK before: 51 + 11 + 10 = 72 bytes
F-BU/F-BACK after:  51 + 50 + 49 = 150 bytes

Times:

F-BU/F-BACK before: 72 bytes / 940 bytes/sec = 0.077 sec = 77 msec.
F-BU/F-BACK after:  150 bytes/ 940 bytes/sec = 0.160 sec = 160 msec.

However, total handoff time will consist of the above plus a
nondeterministic amount of time involved in L2 handoff and the
amount of processing time required on new AR to perform DAD and 
process the HI/HACK pair. Perhaps one could use the maximum
of L2 handover time and HI/HACK processing time as an estimate
to add to the above.

Network Determined, Stateful
----------------------------

Messages exchanged over the air- PrRtAdv prior to handoff.

Byte counts the same as the above. No F-BU/F-BACK needed, so the
only over the air message required is the PrRtAdv.

Total over the air size:

PrRtAdv size: 51 bytes

Time:

51 bytes/ 940 bytes/sec = 0.054 sec = 54 msec.

L2 handover time needs to be added.

Mobile Determined
-----------------

Messages exchanged over the air: RtSolPr, PrRtAdv, F-BU, F-BACK.

Byte counts for PrRtAdv and F-BU/F-BACK the same as above. RtSolPr
sent by MN is done prior to L2 handoff.

           hdr    RtSolPr     MNlnkAddrOpt     newAttPtLnkOpt      Total
      
RtSolPr     1       8              18                9               36

Total over the air sizes:

F-BU/F-BACK before: 36 + 51 + 18 + 10 = 115 bytes
F-BU/F-BACK after:  36 + 51 + 50 + 49 = 186 bytes

Times:

F-BU/F-BACK before: 115 bytes/ 940 bytes/sec. = 0.122 sec = 122 ms
F-BU/F-BACK after:  198 bytes/ 940 bytes/sec. = 0.198 sec = 198 ms

Again, the maximum of L2 handoff time and HI/HACK processing time
would need to be included.

Conclusions
-----------

What conclusions to draw? These time range from respectible (55 ms
for network determined, stateful) to the clearly questionable (198 ms for 
mobile determined, F-BU/F-BACK sent after). The ITU recommends 300 ms
maximum for delay before the user notices with real time voice traffic. 
Depending on the amount of time required for the L2 handover, the HI/HACK 
processing time, and any time required to send buffered traffic to
the MN if buffering is used on the old AR, the mobile determined,
F-BU/F-BACK
sent after case could easily exceed the ITU guideline, as almost 2/3 
of the noticable delay time is taken up by the L3 handoff. The others
require less L3 handoff time, but only the network determined, stateless,
F-BU/F-BACK before and the network determined, stateful are less than
or equal to about 1/4 of the ITU guideline. 

Clearly, header compression on the new AR could help a lot. IPv6 headers
are big, and if there were some way the header compressors on MN and new AR
could be jump started prior to handoff, the 40 byte header overhead could be
eliminated. Alternatively, if the link layer can guarantee that
the F-BU/F-BACK occurs before the L2 handoff, the header overhead won't
occur. Using a higher bandwidth channel could help also.

Please send any comments or corrections (in case my math is wrong).

		jak

---

From: Michael Thomas <[email-masked]>
Date: Tue, 5 Jun 2001 15:13:59 -0700 (PDT)

Er Madjid, my physics book sez that c ~= 3x10^8
m/s.  that makes a km about 1/3x10^5 (in vacuum)
so even allowing for wire/fiber, you seem to be
off by a couple of orders of magnitude.

	   Mike