INTERNET-DRAFT Karim El Malki, Ericsson
Hesham Soliman, Flarion
Expires: December 2003
May 2003
Simultaneous Bindings for Mobile IPv6 Fast Handovers
<draft-elmalki-mobileip-bicasting-v6-03.txt>
Status of this memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026.
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This document is an individual submission to the IETF.
Abstract
Fast Handover for Mobile IPv6 [1] and describes protocols that
minimise the amount of service disruption when performing layer-3
handovers. This draft extends the Fast Handover protocol with a
simultaneous bindings function and the BETH capabilities with a
bicasting function to minimise packet loss at the MN. Traffic for the
MN is therefore bicast or n-cast for a short period to its current
location and to one or more locations where the MN is expected to
move to shortly. This removes the timing ambiguity regarding when to
start sending traffic for the MN to its new point of attachment
following a Fast Handover and allows the decoupling of layer-2 and
layer-3 handovers. It also saves the MN periods of service disruption
in the case of ping-pong movement.
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TABLE OF CONTENTS
1. Introduction.....................................................2
1.1 Terminology.....................................................3
2. Simultaneous Bindings............................................3
3. Fast Handovers in Mobile IPv6....................................4
4. Link layer assisted handovers....................................5
5. Decoupling L3 Handovers from L2 handovers using Simultaneous
Bindings 6
6. Avoiding service disruption due to ping-pong movement............7
7. Changes to the Fast Handover and BETH Operations.................7
7.1 MN Operation....................................................7
7.1 HA/MAP/AR Operation.............................................8
8. Simultaneous Bindings Flag in Fast Binding Update (F-BU) message.8
9. Simultaneous Bindings suboption for Fast Binding Acknowledgement.9
(F-BA) message......................................................9
10. Multiple copies of packets received at AR.......................9
11. Reception of multiple copies in the MN.........................10
12. References.....................................................10
13. AuthorsÆ Addresses.............................................11
14. Full Copyright Statement.......................................11
1. Introduction
Fast Handover for Mobile IPv6 (FMIPv6) describes a protocol to
minimise the amount of service disruption when performing layer-3
handovers. This draft extends the Fast Handover protocol with a
simultaneous bindings function to minimise packet loss at the MN.
Traffic for the MN is therefore bicast or n-cast for a short period
to its current location and to one or more locations where the MN is
expected to move to shortly. This removes the timing ambiguity
regarding when to start sending traffic for the MN to its new point
of attachment following a Fast Handover and allows the decoupling of
layer-2 and layer-3 handovers. It also saves the MN periods of
service disruption in the case of ping-pong movement. Appendix A
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contains some calculations illustrating how to achieve zero service
disruption at L3 using FMIPv6 and bicasting.
1.1 Terminology
This section presents a few terms used throughout the document.
PAR û Previous Access Router.
NAR - New Access Router.
L2 handover - Movement of a MN's point of Layer 2 (L2)
connection from one wireless access point to another.
L3 handover - Movement of a MN between ARs which involves
changing the on-link care-of address at Layer 3 (L3).
L2 trigger - Information from L2 that informs L3 of particular
events before and after L2 handover. The descriptions of L2
triggers in this document are not specific to any particular
L2, but rather represent generalizations of L2 information
available from a wide variety of L2 protocols.
Bicasting/n-casting - The splitting of a stream of packets
destined for a MN into two or more streams, and the
simultaneous transmission of the streams to PAR and one or
more NARs. N/casting is a technique used to reduce packet
loss during handover.
ping-ponging - Rapid back and forth movement between two
wireless access points due to failure of L2 handover. Ping-
ponging can occur if radio conditions for both the old and
new access points are about equivalent and less than optimal
for establishing a good, low error L2 connection.
2. Simultaneous Bindings
Simultaneous bindings were built into the Mobile IPv4 protocol [2].
To enable multiple simultaneous bindings using Mobile IPv4 the MN
simply sends the first normal Registration Request for a care-of
address and then sends other Registration messages for another set of
care-of addresses having the æSÆ bit set. The receiver of the
Registration Requests (e.g. the HA) will then maintain all these
care-of address bindings for the MN contemporarily rather than only
servicing the MN at the care-of address in its most recent
Registration Request (which would be the case had the æSÆ bit not
been set). This results in bicasting or n-casting of packets to all
the care-of addresses. This draft extends the Mobile IPv6 protocol
with similar functionality and describes a new Simultaneous Bindings
flag for the Fast Binding Update in [1].
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Multiple simultaneous bindings and bicasting can be an important tool
to decouple L3 handovers from L2 handovers and to reduce packet loss.
In [1] this mechanism instructs the recipient of the F-BU with the
simultaneous bindings flag to make multiple copies of packets
destined to the MN and send them to multiple MN care-of addresses
before the MN actually moves there. This allows a smoothing of the L3
handover, meaning that packet loss is minimised or even eliminated.
Simultaneous bindings are also useful to prevent service disruption
due to ping-ponging as described later.
3. Fast Handovers in Mobile IPv6
The mechanism to obtain fast L3 handovers for Mobile IPv6 is
described in [1] and illustrated in Figure 1. This mechanism involves
the use of L2 triggers which allow the L3 handover to be anticipated
rather than being performed after the L2 handover completion as
normal. Fast Handovers are required to ensure that the layer 3
(Mobile IP) handover delay is minimised, thus also minimising and
possibly eliminating the period of service disruption which normally
occurs when a MN moves between two ARs. This period of service
disruption usually occurs due to the time required by the MN to
update its HA after it moves between ARs. During this time period the
MN cannot resume or continue communications. Following is a short
summary of the Fast Handover mechanism described in [1].
+-----------+ 1a. HI +-----+
| | ---------------->| NAR |
| PAR | 1b. HAck | |
+-----------+ <----------------+-----+
^ | ^
(2a. RtSolPr) | | 2b |
| | Pr | 3. Fast BU (F-BU)
| | RtAdv | 4. Fast BA (F-BACK)
| v v
+------------+
| MN |
+------------+ - - - - - ->
Movement
Figure 1 û Fast MIPv6 Handover Protocol
While the MN is connected to its Previous Access Router (PAR) and is
about to move to a New Access Router (NAR), the Fast Handovers in
Mobile IPv6 requires:
- the MN to obtain a new care-of address at the NAR while connected
to the PAR the MN to send a BU to its old anchor point (e.g. PAR) to
update its binding cache with the MNÆs new care-of address.
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- the old anchor point (e.g. PAR) to start forwarding packets
destined for the MN to NAR.
The MN or PAR may initiate the Fast Handover procedure by using
wireless link-layer information or link-layer ôtriggersö which inform
that the MN will soon be handed off between two wireless access
points respectively attached to PAR and NAR. If the ôtriggerö is
received at the MN, the MN will initiate the layer-3 handover process
by sending a Proxy Router Solicitation message to PAR. Instead if the
ôtriggerö is received at PAR then it will transmit a Proxy Router
Advertisement to the appropriate MN, without the need for
solicitations.
The MN obtains a new care-of address while connected to PAR by means
of router advertisements containing information from the NAR (Proxy
Router Advertisement, PrRtAdv, which may be sent due to a Proxy
Router Solicitation, RtSolPr). The PAR will validate the MNÆs new
COA by sending a Handover Initiate (HI) message to the NAR. Based on
the response generated in the Handover Acknowledge (HAck) message,
the PAR will either generate a tunnel to the MNÆs new COA (if the
address was valid) or generate a tunnel to the NARÆs address (if the
address was already in use on the new subnet). If the address was
already in use on the new subnet, the NAR will generate a host route
for the MN using its old COA. The new COA sent in the HI message is
formed by appending the MNÆs ôcurrentö interface identifier to the
NARÆs prefix. Note that the HI/HAck exchange is decoupled from the
handover procedure and should be performed in advance so as not to
add latency to the protocol exchange.
4. Link layer assisted Fast handovers
The following figure is taken from [1].
+------+ HRqst +------+
| PAR |<-------->| NAR |
+------+ HRply +------+
^ ^
old L2 | | new L2
+-------+ +-----+
| |
| |
V V
+------+ movement
| MN | --------->
+------+
Figure 2 û Link layer assisted Handovers
Figure 2 describes a way to implement fast handovers without
involving messages from the MN. Upon receipt of L2 triggers, which
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communicate the upcoming movement of a MN between two ARs, the PAR
will establish a bidirectional tunnel between itself and the NAR (or
vice versa). As soon as the PAR detects that the MN has disconnected
as described in [1], the PAR will start forwarding traffic it
receives for the MN over to NAR. In the opposite direction the NAR
will reverse tunnel traffic sent by the MN on its new link back to
PAR.
5. Decoupling L3 Handovers from L2 handovers using Simultaneous Bindings
The mechanisms described in [1] allow the anticipation of the layer 3
handover such that data traffic can be redirected to the MNÆs new
location before it moves there. However it is not simple to determine
the correct time to start forwarding between PAR and NAR, which has
an impact on how smooth the handover will be. Packet loss will occur
if this is performed too late or too early with respect to the time
in which the MN detaches from PAR and attaches to NAR. Also, some
measure is needed to support the case in which the MN moves quickly
back-and-forth between ARs (ping-pong).
In many wireless networks it is not possible to know in advance
precisely when a MN will detach from the wireless link to PAR and
attach to the one connected to NAR. Therefore determining the time
when to start forwarding packets between PAR and NAR is not possible.
Certain wireless technologies involve layer-2 messages which instruct
the MN to handover immediately or simply identify that the MN has
already detached/attached. Even if the ARs could extract this
information, there may not be sufficient time for the PAR to detect
the MNÆs detachment and start getting packets tunnelled over to NAR
before the MN attached to NAR. This is because wireless layer-2
handover times are quite small (i.e. range from 10Æs to 100Æs ms).
Thus a period of service disruption may occur due to this timing
uncertainty unless further enhancements are made to the handover
mechanism. If the L3 handover is anticipated and the PAR starts
forwarding data to NAR upon receipt of the Fast BU in [1] or upon
receipt of the L2-LD trigger in the link-layer assisted case, then
the period of service disruption will be according to the following:
A = L3 handover anticipation (time difference between the start of
the
L3 fast handover and the moment in which the L2 handover occurs)
h = L2 handover time (disconnection time due to L2 handover)
Approximate period before MN receives packets again = A + h
It is therefore necessary to decouple layer-3 handover timing from
layer-2 handover timing. One solution is to bicast or n-cast packets
destined to the MN for a short period from the old anchor point (e.g.
PAR) to one or more potential future MN locations (e.g. NAR/s) before
the MN actually moves there. This means that the handover procedure
described previously would be enhanced by having the old anchor point
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(e.g. PAR) send one copy of packets to the MNÆs old on-link care-of
address and another copy of the packets to the MNÆs new care-of
address (or addresses) connected to NAR. The MN is thus able to
receive traffic independently of the exact layer-2 handover timing
during the handover period.
6. Avoiding service disruption due to ping-pong movement
It is possible that the layer-2 handover procedure may fail or
terminate abruptly in wireless systems. Therefore a MN which expects
to move between PAR and NAR may unexpectedly never complete the
layer-2 handover and find itself connected to PAR. Another undesired
effect is that the MN could ping-pong between ARs due to layer-2
mobility issues. Both these cases would leave the MN unable to resume
communication and have to transmit a new F-BU in [1] or wait for a
new bidirectional tunnel setup in the link-layer assisted case before
resuming communications.
This may be solved through the use of simultaneous bindings which
allow the MN to maintain layer-3 connectivity with the PAR during the
affected handover period, thus smoothing the handover. This
eliminates the need for continuous transmission of Fast Binding
Updates in [1] or continuous bidirectional tunnel setups in the link-
layer assisted case [1]. It also prevents the period of service
disruption from being extended due to the effect of the above link-
layer issues on L3 handover.
7. Extensions to the Fast Handover Operations
7.1 MN Operation
The MN operation in [1] is affected by the changes introduced by this
document. The addition to [1] is that a MN with an existing active
binding which receives a new router advertisement (PrRtAdv) MUST be
"eager" to establish new bindings. When a MN has at least one
existing binding and receives a new PrRtAdv it MUST send a Fast
Binding Update (F-BU) with the Simultaneous Bindings flag set (ôBö
flag). The new flag is described in section 8. In addition the MN
MUST be able to process the new simultaneous bindings option in the
Fast Binding Acknowledgement message described in section 9. The
lifetime field returned in this option MUST be used by the MN to
identify the lifetime of the simultaneous binding requested. Two BU
lifetime values will be returned: Bicasting lifetime (in the
simultaneous bindings option) and new CoA lifetime (in the BA option)
as described in the following sections. The new CoA lifetime (placed
in the BA option as specified in [3]) runs in parallel with the
Bicasting lifetime. Hence, when the bicasting lifetime ends, the MN
will remove this entry from the Binding Update list and simply keep
one entry for the new CoA with the remaining new CoA lifetime.
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7.2 HA/MAP/AR Operation
The HA [3], MAP [4] and AR [1] are the possible recipients of a F-BU
message. Upon receiving a F-BU message having the ôBö flag set (see
section 8), the HA/MAP/PAR MUST create a new binding cache sub-entry
(linked to the original entry for the old CoA) for the MNÆs new CoA.
This sub-entry contains the same fields as normal binding cache
entries but it MUST NOT replace any existing entries for the MN. The
new sub-entry will have two lifetimes instead of one: the normal new
CoA BU lifetime (sent in the BA) and a Bicasting lifetime set to
SHORT_BINDING_LIFETIME (sent in the BA option). The new CoA lifetime
runs in parallel with the Bicasting lifetime. Until the Bicasting
lifetime expires, the HA/MAP/PAR MUST send a copy of the data
destined for the MN to the old CoA and to the new CoA/s in the linked
binding cache sub-entry or sub-entries. When the Bicasting lifetime
expires, the MAP/HA/PAR MUST remove the bicasting lifetime field and
replace the old binding cache entry for the old CoA with the new CoA
sub-entry. As a result, the HA/MAP/AR will end up with one entry for
the MNÆs new CoA with the remaining new CoA lifetime.
In the link-layer assisted case [1] the F-BU messages are not used.
When a bidirectional tunnel is established for the first time (i.e.
not renewed) between PAR and NAR, the PAR MUST maintain two lifetime
entries for the tunnel: normal tunnel lifetime (already described in
[1]) and a Bicasting lifetime set to SHORT_BINDING_LIFETIME. Until
the Bicasting lifetime expires the PAR MUST copy data destined to the
MN both to its on-link CoA and through the tunnel to NAR. When the
Bicasting lifetime expires, the PAR MUST stop bicasting and only
forward traffic for the MN through the tunnel to nAR until the tunnel
lifetime itself expires.
The default value for SHORT_BINDING_LIFETIME is 2s. This parameter
MUST be configurable as it my vary depending on the type of radio
link in the system.
8. Simultaneous Bindings Flag in Fast Binding Update (F-BU) message
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence # |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|H|L|K|M|B| Reserved | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Mobility Options .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Description of the flag added to the F-BU option already defined in
[1]:
B When set indicates a request for bicasting all
packets to both COAs of the MN (in the source
address field and the alternate-CoA suboption).
This BU will add another COA to the Binding
Cache.
9. Simultaneous Bindings option for Fast Binding Acknowledgement
(F-BA) message
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| status | Reserved | Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Option Type TBD
Option Len TBD
Status Indicates success (0) or failure (128
and above).
Lifetime The bicasting lifetime for the
simultaneous binding requested in the
F-BU. This value MUST be used by the MN
to record the validity of this binding
in its binding update list
The alignment requirement for this option is 2n+2.
10. Multiple copies of packets received at AR
If the MN has simultaneous active bindings with HA/MAP/AR, it could
(but preferably should not) receive multiple copies of the same
traffic directed to it. The use of simultaneous bindings does not
mean that the MN is receiving packets contemporarily from multiple
sources. This depends on the characteristics of the access (L2)
technology. The bicasting of packets involves sending a copy of the
data to the AR which the MN is moving to (the NAR). Until the MN
actually completes the L2 handover to the NAR and fully establishes
the new L2 link, the NAR MAY receive packets for a MN to which it
does not have a direct link layer connection. If the new AR is aware
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that the MN is performing a handover (due to earlier reception of the
HI message) The AR MAY:
- drop all packets for the MN,
- drop some packets, based on local policies, or
- buffer packets for the MN.
The choice of which action to take may depend on the type of traffic
involved (e.g. real-time or non real-time), but this is outside the
scope of this document. The AR MAY also in parallel attempt to
establish a link-layer connection with the MN. However an AR MUST NOT
send ICMP Destination Unreachable messages if it drops packets or is
unable to deliver the received IP packets due to unavailability of
direct layer connection with the MN. This is because the MN will be
receiving one copy of the packets. Note that the MN may also select
which flows need bicasting by adding a Flow movement option [7] to
the simultaneous binding update.
11. Reception of multiple copies in the MN
In some rare scenarios it may be possible that the MN receives more
than one copy of the same packet. Generally, Internet routing
mechanisms can not guarantee the delivery of a single copy of an IP
packet to a node. However some TCP congestion avoidance
implementations are known to react negatively to the reception of 3
duplicate acknowledgements. The Eifel detection and response
algorithms in [5] and [6] address this problem. When using [5] and
[6] bicasting should not cause any negative performance impacts for
TCP. Alternatively, the MN may simply request bicasting for non-TCP
connections using a Flow movement option as described in [7].
12. References
[1] R. Koodli (Editor) et al, ôFast Handovers for Mobile IPv6ö,
draft-ietf-mobileip-fast-mipv6-06.txt, work in progress, March 2003.
[2] C. Perkins (Editor), ôIP Mobility Support for IPv4ö, RFC 3220,
Jan 2002.
[3] D. Johnson, C. Perkins and J. Arkko, "Mobility Support in IPv6",
draft-ietf-mobileip-ipv6-21.txt, work in progress, Feb 2003.
[4] H. Soliman, C. Castelluccia, K. El Malki and L. Bellier,
ôHierarchical MIPv6 mobility managementö, draft-ietf-mobileip-hmipv6-
07.txt, work in progress, October 2002.
[5] R. Ludwig, ôThe Eifel Detection Algorithm for TCPö, RFC 3522,
April 2003.
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[6] Ludwig, R. and A. Gurtov, ôThe Eifel response algorithm for
TCPö, draft-ietf-tsvwg-tcp-eifel-response-00, work in progress, July
2002.
[7] Soliman, H., ElMalki, K. and C. Castelluccia, ôFlow movement in
MIPv6ö, draft-soliman-mobileip-flow-move-03, work in progress.
13. AuthorsÆ Addresses
The authors may be contacted at the addresses below:
Karim El Malki
Ericsson Radio Systems AB
LM Ericssons Vag. 8
126 25 Stockholm
SWEDEN
Phone: +46 8 7195803
Fax: +46 8 7190170
E-mail: Karim.El-Malki@era.ericsson.se
Hesham Soliman
Flarion
E-mail: H.Soliman@flarion.com
The working group can be contacted via the current chairs:
Basavaraj Patil Phil Roberts
Nokia Corporation Megisto Systems Inc.
6000 Connection Drive Suite 120, 20251 Century Blvd
Irving, TX 75039 Germantown MD 20874
USA USA
Phone: +1 972-894-6709 Phone: +1 847-202-9314
EMail: Raj.Patil@nokia.com EMail: proberts@megisto.com
Fax : +1 972-894-5349
14. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in anyway, such as by removing
the copyright notice or references to the Internet Society or other
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Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English. The limited permissions granted above are perpetual and will
not be revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided onan
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
Appendix A - Timing Calculations for bicasting
Example 1
---------
+--------+
+------| MAP/HA |------+
| +--------+ |
| |
v v
+-----+ +-----+
| PAR | | NAR |
+-----+ +-----+
+-----+
| MN |
+-----+ - - - - - >
Movement
This is the case specified by [1] with the extension of using the MAP
from [4].
A = anticipation time (F-BU is sent from MN at time t-A, where t is
the time when the MN actually hands-off at L2)
h = handover time (L2 only)
D1 = MN to MAP delay (through PAR)
D2 = MN to MAP delay (through NAR)
p = F-BU and routing table processing time in the MAP and MN
To achieve zero L3 service disruption it is necessary for the time
period between starting the fast handover and the MN completing the
L2 handover to be greater than or equal to the tiem it take for
traffic to reach the MN at its new link (through NAR). This is
represented by the following formula:
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(A+h)>=((D1+D2)+p)
Assuming that p<<(D1+D2) this can be simplified to:
(A+h)>=(D1+D2)
To achieve maximum performance from simultaneous bindings it is
necessary for the above relation to hold.
The Anticipation time (A) is important and needs to be calculated
appropriately for the link-layer being used. Depending on the L2 this
may need engineering to synchronise the L2 and L3 handovers.
Once the MN has moved to NAR, it will be receiving traffic delayed by
(D2-D1) with respect to when it was attached to PAR. To smooth this
delay variation (jitter), which may be a problem for real-time
services, it may be necessary to implement a smoothing buffer at NAR.
Example 2
---------
+-----+ +-----+
| PAR | -------------->| NAR |
+-----+ +-----+
|
|
|
v
+-----+
| MN |
+-----+ - - - - - >
Movement
When the MAP/HA/PAR are one entity (as considered in [1]), the
following calculations apply.
A = anticipation time (F-BU is sent from MN at time t-A, where t is
the time when the MN actually hands-off at L2)
h = handover time (L2 only)
d = MN to AR delay (assume constant as MN moves ARs)
L = PAR to NAR delay
As previously, the following must be true for the simultaneous
bindings to yield zero L3 disruption:
(A+h)>=(d+L+d)
=> (A+h)>=(2d+L)
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The Anticipation time (A) is important and needs to be calculated
appropriately for the link-layer being used. Depending on the L2 this
may need engineering to synchronise the L2 and L3 handovers.
Once the MN has moved to NAR, it will be receiving traffic delayed by
an amount L with respect to when it was attached to PAR. To smooth
this delay variation (jitter), which may be a problem for real-time
services, it may be necessary to implement a smoothing buffer at NAR.
Example 3
---------
In BETH [1], a similar calculation applies as that in Example 2 but
there is no need for the MN-AR communication. Therefore the following
formula must be true to achieve zero service disruption at L3:
(A+h)>=(L+d)
The Anticipation time (A) is important and needs to be calculated
appropriately for the link-layer being used.
Once the MN has moved to NAR, it will be receiving traffic delayed by
an amount L with respect to when it was attached to PAR. To smooth
this delay variation (jitter), which may be a problem for real-time
services, it may be necessary to implement a smoothing buffer at NAR.
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