Internet Engineering Task Force Gorry Fairhurst
INTERNET DRAFT University of Aberdeen
Lloyd Wood
Cisco Systems Ltd
draft-ietf-pilc-link-arq-issues-01.txt March 2001
Expires: September 2001
Link ARQ issues for IP traffic
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as "work in progress".
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
ABSTRACT
This document discusses the use of automatic repeat request (ARQ)
procedures as an option in the design of a link protocol, and the
impact of the use of ARQ on IP traffic travelling over the link.
ARQ is described in a general way that includes its use over a wide
range of underlying physical media, including cellular wireless,
wireless LANs, RF links, and other types of bearer channel. This
document is intended to outline matters for consideration by, and to
provide guidance to, designers and implementers of future link ARQ
mechanisms.
The material presented here is intended to be combined with the PILC
'Advice for Internet Subnetwork Designers' draft [DRAFTKARN00] to
provide a single document giving guidance to subnetwork designers.
PILC WORKING GROUP INTERNET DRAFT expires September 2001 1 Link ARQ issues for IP traffic March 2001
CONTENTS
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1 Link ARQ 3
1.2 Causes of packet loss on a link . . . . . . . . . . . . .3
1.3 Why use ARQ? 4
1.4 Commonly-used ARQ techniques . . . . . . . . . . . . . . 5
1.4.1 Stop-and-wait ARQ 5
1.4.2 Sliding-window ARQ. . . . . . . . . . . . . . . . .5
1.5 Causes of delay across a link 6
2. ARQ persistence. . . . . . . . . . . . . . . . . . . . . . . 6
2.1 Perfectly-persistent (reliable) ARQ protocols 7
2.2 High-persistence (highly-reliable) ARQ protocols. . . . .8
2.3 Low-persistence (partially-reliable) ARQ protocols 9
2.4 Choosing your persistency. . . . . . . . . . . . . . . .10
3. Packet ordering 11
3.1 Using link ARQ to support multiple flows. . . . . . . . 12
3.2 Differentiation of link service classes 13
4. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . .14
5. Security implications 16
6. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . 16
7. References 16
Authors' addresses. . . . . . . . . . . . . . . . . . . . . . .19
Full copyright statement 19
1. INTRODUCTION
Over the years, traffic using the Internet Protocol (IP [RFC791])
has been carried over a wide variety of links, of vastly different
capacities, delays and loss characteristics. IP traffic can be
expected to be carried over a very wide variety of new and existing
link designs in the future.
This document is intended for three distinct groups of readers:
a. Link designers wishing to configure (or tune) a link for the IP
traffic that it will carry, using standard link-layer mechanisms
such as the ISO High-level Data Link Control (HDLC) [ISO4335] or
its derivatives.
b. Link implementers who may wish to design new link mechanisms that
perform well for IP traffic when carrying IP.
c. The community of people using or developing TCP, UDP and related
protocols, who may wish to be aware of the ways in which links
can operate.
The primary audiences are intended to be groups a. and b. Group c.
should not need to be aware of the exact details of an ARQ scheme
across a single link, and should not have to consider it in their
PILC WORKING GROUP INTERNET DRAFT expires September 2001 2 Link ARQ issues for IP traffic March 2001
implementations; much of the Internet runs across links that do not
use any form of ARQ.
However, the TCP/IP community does need to be aware that the IP
protocol operates over a diverse range of underlying subnetworks.
This document may help to raise that awareness.
Some familiarity with TCP [RFC2581, RFC2488, STE94] is assumed here.
There are a number of variants of TCP, with differing levels of
sophistication in their procedures for handling loss recovery and
congestion avoidance. Far from being static, the TCP protocol is
itself subject to ongoing gradual refinement and evolution, e.g.
[RFC2760].
This document should be read in conjunction with other documents
produced by the Performance Implications of Link Characteristics
(PILC) IETF workgroup [DRAFTDAW00, DRAFTKARN00, DRAFTBOR00].
1.1 LINK ARQ
At the link layer, ARQ operates on blocks of data, known as frames,
and attempts to deliver frames from the sender to the receiver over
a physical link. Frames often have a small maximum size for
convenience of processing by media-access control (MAC) and link-
layer protocols. This contrasts with the variable lengths of IP
datagrams, or 'packets'.
The ARQ link mechanism provides an integrity check and a
retransmission process to retransmit lost or errored frames. Link
ARQ protocols operate within the link layer of the OSI reference
model [DRAFTKARN00].
A frame may contain one or more IP datagrams, although transparent
implicit fragmentation is often used so that each frame will contain
only a fraction of an IP datagram [KEN88].
Explicit network-layer IP fragmentation is undesirable for a variety
of reasons, and should be avoided [KEN88]. Its use can be minimised
with use of Path MTU discovery for TCP connections [RFC1191,
RFC1435].
1.2 CAUSES OF PACKET LOSS ON A LINK
Not all packets sent to a link are necessarily received successfully
by the receiver at the other end of the link. There are a number of
possible causes of packet loss that may occur as frames travel
across a link. These include:
a. Loss due to channel noise, often characterised by random frame
loss.
PILC WORKING GROUP INTERNET DRAFT expires September 2001 3 Link ARQ issues for IP traffic March 2001
b. Frame loss due to channel interference. This interference can
be random, structured, and in some cases even periodic.
c. A link outage, causing loss due to unavailability of the
physical link for a period of time. This is a common
characteristic of some types of link, e.g. mobile cellular
radio. In the outage, the link loses all or virtually all frames
for a period of time, until the link is restored.
Other forms of packet loss are not related to channel conditions,
but include:
i. Packet discards due to congestion. Queues will eventually
overflow as the arrival rate of new packets to send continues to
exceed the outgoing packet transmission rate over the link.
ii. Loss due to implementation errors, including hardware faults and
software errors.
The levels of loss and patterns of loss experienced are functions of
the design of the link's physical and link layers. These vary
significantly across different link configurations. The performance
of a specific implementation may also vary considerably across the
same link configuration when operated over different types of
physical channel.
1.3 WHY USE ARQ?
Reasons that encourage considering the use of ARQ include:
a. ARQ across a single link has a shorter control loop than the end-
to-end path over which TCP must operate. ARQ can therefore
provide more rapid retransmission for TCP segments lost on the
link, at least for a reasonable number of retries [DRAFTDAW00,
SAL81].
b. Link ARQ can operate on individual frames, which are much smaller
than IP datagrams and may therefore permit more efficient
recovery in terms of repetition of data after occasional losses.
For any specific target error rate there is an optimal trade-off
between header overhead and frame size.
c. A link ARQ procedure may be able to use local knowledge that is
not available to endhosts, including information about the state
of the link and physical layer techniques, in order to optimise
delivery performance for the current link conditions. This
information can include knowledge of the current available
transmission rate, the prevailing error environment, or available
transmit power in wireless links.
PILC WORKING GROUP INTERNET DRAFT expires September 2001 4 Link ARQ issues for IP traffic March 2001
1.4 COMMONLY-USED ARQ TECHNIQUES
Two basic categories of ARQ retransmission process are widely used:
1.4.1 STOP-AND-WAIT ARQ
A stop-and-wait ARQ sender transmits a single frame and then waits
for an acknowledgement from the receiver for that frame, before
either continuing transmission with the next frame, or repeating
transmission of the same frame if the original frame is errored or
lost.
Stop-and-wait ARQ is simple for protocol designers to implement,
e.g. tftp [RFC1350]. However, when stop-and-wait ARQ is used in the
link layer, it is well-suited only to links with low bandwidth-delay
products. This technique is not discussed further in this document.
1.4.2 SLIDING-WINDOW ARQ
A sliding-window link ARQ protocol numbers every frame with a unique
sequence number, according to a modulus. TCP is itself a sliding-
window protocol, so similarities between this link-interface-to-
link-interface protocol and end-to-end TCP may be recognisable. A
sliding-window link protocol is much more complex in implementation
than a simpler stop-and-wait protocol, particularly if per-flow
ordering is preserved.
At any time the link sender may have a number of frames outstanding
and awaiting acknowledgement, up to the space available in its
transmission window. A sufficiently large link sender window
(equivalent to or greater than the number of frames sent, or larger
than the bandwidth*delay capacity of the link) enables continuous
transmission of new frames. A smaller link sender window causes the
sender to pause transmission of new frames until a timeout or a
control frame, such as an acknowledgement, is received. When frames
are lost, a larger window, i.e. more than the link's bandwidth*delay
product, is needed to allow continuous operation while frame
retransmission takes place.
The modulus numbering space determines the size of the frame header
sequence number field. This sequence space needs to be larger than
the link window size, and if using selective frame repeat, larger
than twice the link window size. For continuous operation, the
sequence space should be larger than the bandwidth-persistence
product, so that in-flight frames can be identified uniquely.
As with TCP, which provides sliding-window delivery across an entire
path, rather than a single link, there are a number of variations on
the basic sliding-window implementation, with increased complexity
and sophistication to make them suitable for various conditions.
PILC WORKING GROUP INTERNET DRAFT expires September 2001 5 Link ARQ issues for IP traffic March 2001
1.5 CAUSES OF DELAY ACROSS A LINK
Links and link protocols contribute to the total path delay
experienced between communicating applications on hosts. Delay has
a number of causes, including:
a. Input packet queuing and frame buffering at the link head before
transmission over the link.
b. Waiting for access to the allocated channel when the physical
medium is shared. There may be processing or protocol-induced
delay before transmission takes place [FER99, PAR00].
c. Frame serialisation and transmission processing. These are
functions of frame size and the transmission speed of the link.
d. Physical link propagation time, caused by the speed of light in
the channel medium.
e. Processing, including buffering frame contents for packet
reassembly, at the receiver before onward transmission of the
packet.
When link ARQ is used, steps b., c. and d. may be repeated together
at intervals a number of times after a frame is lost, increasing
overall delay for the packet that the frame partially transmits.
It is important to understand that applications at the endhosts are
unaware of the individual delays contributed by each link in the
path, and only see the overall path delay. Application performance
is determined by the cumulative delay of the entire end-to-end
Internet path. This path may include an arbitrary or even widely-
varying number of links, where each link may or may not use ARQ.
2. ARQ PERSISTENCE
ARQ protocols may be characterised by their persistence.
Persistence is the willingness of the protocol to retransmit lost
frames to ensure reliable delivery of the stream of traffic across
the link.
A link's retransmission persistency defines how long the link is
allowed to delay an IP packet, in an attempt to transmit all the
frames carrying the packet and its content over the link, before
giving up and discarding the packet. This persistency is normally
measured in milliseconds, but may, if the link propagation delay is
specified, also be expressed in terms of the maximum number of link
retransmission attempts permitted.
PILC WORKING GROUP INTERNET DRAFT expires September 2001 6 Link ARQ issues for IP traffic March 2001
An example of a reliable link protocol that is perfectly persistent
is the ISO HDLC protocol in the Asynchronous Balanced Mode (ABM)
[ISO4335]. A protocol that only retransmits a fixed number of times
before giving up is less persistent, e.g. Ethernet [FER99] or GSM
RLP [RFC2757]. Here, lower persistence ensures stability and fair
sharing of the shared subnetwork, even when many senders are
attempting retransmissions.
In a broadcast wireless environment, the interval between repeat
attempts will be set to be greater than the link round trip delay,
and frequently increases with the number of retransmission attempts.
This is similar to having increasing backoffs in Ethernet / CSMA to
decrease offered channel load in the presence of impairments.
A wireless receiver may experience lost or errored frames due to
fading, shadowing or interference on its channel from the wireless
sender. In this situation, it is wasteful for the sender to consume
shared capacity when retransmissions are likely to experience the
same conditions; other receivers can benefit from that capacity.
One simple solution is to delay retransmissions to that receiver in
the hope that the channel conditions will improve. A complementary
approach is to adapt the way in which retransmitted information is
sent over the channel, e.g. by adapting modulation or coding. The
interval or delay between retransmission attempts may be varied as a
function of the perceived channel state and the number of previous
retransmission attempts.
Perfect reliability is not a requirement for IP subnetworks
[DRAFTKARN00]. IP networks may discard packets due to a variety of
reasons entirely unrelated to link errors, including lack of queuing
space, congestion management, faults, and route changes. It has
long been widely understood that perfect reliability can be obtained
only at the transport layer [SAL81].
TCP [RFC2488, RFC2581, STE94] and a number of applications using UDP
implement their own end-to-end reliable delivery mechanisms. Many
TCP and UDP applications, e.g. streaming multimedia, benefit from
timely delivery from lower layers with sufficient reliability,
rather than perfect reliability with increased link delays.
2.1 PERFECTLY-PERSISTENT (RELIABLE) ARQ PROTOCOLS
Most well-known link protocols are reliable protocols, which use
perfectly-persistent ARQ to ensure that all frames (and therefore
all packets) are received by the other side of the link. The
perfectly-persistent ARQ protocol will repeat a lost or errored
frame an indefinite number of times until the frame is successfully
received.
If traffic is going no further than across one link, this ensures
reliability between the two link ends without requiring higher-
PILC WORKING GROUP INTERNET DRAFT expires September 2001 7 Link ARQ issues for IP traffic March 2001
layers protocols. This reliability can be counterproductive for
traffic traversing multiple links, as it interacts with protocol
mechanisms at higher layers.
Arguments against perfect persistence for IP traffic include:
a. Variable link delay; the impact of ARQ introduces a degree of
jitter, a function of the link's physical delay and frame
serialisation and transmission times, to all flows sharing the
link performing frame retransmission.
b. High persistence does not provide a clear upper bound on the
maximum retransmission delay for the link. Significant changes
in path delay caused by excessive link retransmissions may lead
to timeout of TCP retransmission timers, although a high variance
in link delay and the resulting overall path delay may also cause
a large TCP Retransmission Timeout (RTO) value to be used
[LUD99a, PAR00]. Future TCP implementations may use techniques
that reduce premature expiry of this timer, e.g. Eifel [LUD00]
and Rapid Packet Loss Detection [SAM98], but further research is
required in this area.
c. Applications needing perfectly-reliable delivery can implement a
form of perfectly-persistent ARQ themselves, or use a reliable
transport protocol within the endhosts. Implementing perfect
persistence at each link along the path between the endhosts is
redundant, and cannot guarantee the same reliability [SAL81]. It
may also affect other end-to-end protocol mechanisms, e.g. using
Explicit Congestion Notification (ECN).
Examples of ARQ protocols that are perfectly persistent include
ISO/ITU-T LAP-B [ISO7776] and ISO HDLC in the Asynchronously
Balanced Mode (ABM) [ISO4335], e.g. using Multiple Selective Reject
(MSREJ [ISO4335a]). These protocols will retransmit a frame an
unlimited number of times until receipt of the frame is
acknowledged.
2.2 HIGH-PERSISTENCE (HIGHLY-RELIABLE) ARQ PROTOCOLS
High-persistence ARQ protocols attempt only a finite, large, number
of transmissions of each frame before giving up. Arguments against
high persistence are similar to those against perfect persistence,
except that infinite delay and perfectly reliable delivery are
traded off against imperfect reliability and an upper bound on link
delay.
It has been recommended that a single IP datagram should never be
delayed by more than the Maximum Segment Lifetime (MSL) of 120
seconds defined for TCP [RFC1122]. Failure to limit the maximum
datagram lifetime can result in sequence number wrapping at high TCP
transmission rates [RFC1323], where old data segments may be
PILC WORKING GROUP INTERNET DRAFT expires September 2001 8 Link ARQ issues for IP traffic March 2001
confused with newer segments where the sequence number space has
been exhausted and reused in the interim.
Some TCP implementations use timestamps to remove this ambiguity
[RFC1323].
(One case where segment lifetime may be significant is where
alternate paths of different delays exist between hosts. Some TCP
segments may follow a short path, while others follow a much longer
path, e.g. using persistent ARQ over a link outage.)
In practice, the MSL is usually very large compared to the typical
TCP RTO. The calculation of RTO is based on estimated round-trip
path delay. If the number of link retransmissions causes a path
delay larger than the value of RTO, the TCP retransmission timer
will expire, leading to a timeout and retransmission by the TCP
sender.
Although high persistency may benefit bulk flows, the additional
delay (and variations in delay) may be highly undesirable for other
types of flows. Being able to treat flows separately with different
classes of link service is useful, and is discussed in section 3.2.
Examples of highly-persistent ARQ include [BHA97, ECK98, LUD99,
MEY99].
2.3 LOW-PERSISTENCE (PARTIALLY-RELIABLE) ARQ PROTOCOLS
The characteristics of a link using a low-persistence ARQ protocol
may be summarised as:
a. The link is not perfectly reliable and does not provide an
absolute guarantee of delivery; i.e. the transmitter will discard
some frames as it 'gives up' before receiving an acknowledgement
of successful transmission across the link.
b. There is a lowered limit on the maximum added delay that IP
datagrams will experience when using the link. This reduces
interaction with TCP timers or with UDP-based error-control
schemes.
c. The link bounds the time that retransmission for any one frame
can block completed transmission and assembly of other correctly-
received datagrams originally sent before the missing frame.
Limiting delay avoids aggravating contention and interaction
between different packet flows (see also 3.2).
Selection of even a quite high persistence, e.g. allowing tens of
attempts to deliver a single frame, would usually satisfy a bound
for b. for low-delay links. However, a packet may traverse a number
PILC WORKING GROUP INTERNET DRAFT expires September 2001 9 Link ARQ issues for IP traffic March 2001
of successive links on its total end-to-end path. This is an
argument for much lower persistency on individual links.
2.4 CHOOSING YOUR PERSISTENCY
The TCP Maximum RTO is an upper limit on the maximum time that TCP
will wait until it performs a retransmission. Most TCP
implementations will generally have a TCP RTO of typically a few
times the path delay.
Setting a lower link persistency (of the order 2-5 retransmission
attempts) reduces interaction with the RTO timer, and may therefore
reduce the probability of duplicate copies of the same packet being
present in the link transmit buffer under some patterns of loss.
The following table is calculated from the probability of loss,
assuming the 'ideal' case of random loss. It shows the probability
of successful frame transmission using an idealised ARQ procedure,
i.e. one in which the control frames are not corrupted.
Frame loss (random) Persistence (Link RTT) Average frame loss
after link ARQ
1% x3 0.0001%
1% x5 0.00000001%
3% x3 0.003%
3% x5 0.000002%
10% x3 0.1%
10% x5 0.001%
10% x7 0.00001%
30% x3 3%
30% x5 0.2%
30% x7 0.02%
30% x9 0.002%
50% x3 12%
50% x5 3%
50% x7 0.8%
50% x9 0.2%
When interpreting these results, remember that:
a. Link protocols normally employ transparent fragmentation and all
frames containing fragments must be delivered to reassemble an IP
packet.
b. Channel error conditions are seldom characterised by random loss.
PILC WORKING GROUP INTERNET DRAFT expires September 2001 10 Link ARQ issues for IP traffic March 2001
c. In these calculations, link persistence is expressed in terms of
multiples of the Link Round Trip Time (Link RTT).
Some implementers have chosen a lower persistence, falling back on
the judgement of TCP or a UDP application to retransmit any packets
that are not recovered by the link ARQ protocol. Examples of low-
persistence ARQ protocols include [SAM96, WAR95, CHE00].
3. PACKET ORDERING
A common debate is whether a link should be allowed to forward
packets in an order different to that in which they were originally
received at the link's transmit interface.
IP networks are not required to deliver all datagrams in order,
although generally networks do deliver most datagrams in their
original transmission order. Routers supporting class-based queuing
do reorder packets received, by reordering packets in different
flows -- but these usually retain per-flow ordering.
Policy-based queuing, allowing fairer access to the link, may
reorder packets. There is still much debate on optimal algorithms,
and on optimal queue sizes for particular link speeds. This,
however, is not related to use of link ARQ and applies to any
(potential) bottleneck router.
Although small amounts of reordering are common in IP networks
[BEN00], significant re-ordering within a flow is undesirable, as it
can have several effects:
a. reordering may cause unwanted side effects in poorly-implemented
IP fragment reassembly code.
b. reordering may cause unwanted side effects in poorly-implemented
IP demultiplexing (filter) code.
c. reordering may cause unwanted side effects in poorly-implemented
UDP applications.
d. reordering will increase packet jitter for real-time
applications. This may lead to application data loss if a small
play-out buffer is used by the receiving application.
e. reordering may impact congestion avoidance and may negatively
impact overall throughput on paths.
f. reordering will interleave arrival of TCP segments, leading to
generation of duplicate ACKs (dupacks [STE94, BAL97]). A sequence
of three identical dupacks causes the TCP sender to trigger fast
retransmission and recovery, a form of congestion avoidance,
PILC WORKING GROUP INTERNET DRAFT expires September 2001 11 Link ARQ issues for IP traffic March 2001
since TCP always presumes loss due to congestion [RFC2581,
STE94]. This reduces TCP throughput efficiency as far as the
application is concerned, but it should not impact data
integrity.
Ordering effects must be considered when breaking the end-to-end
paradigm and evaluating transport-level relays such as split-TCP
implementations or Protocol Enhancing Proxies [DRAFTBOR00].
As with total path delay, reordering effects accumulate over
multiple links. Do not assume that your link is the only link
undertaking packet reordering, as some level of reordering may be
introduced by other links along the same path, or by router
processing within the network [BEN00].
TCP and UDP flows are impacted by the cumulative effect of
reordering along the entire path. The link protocol should ideally
eliminate reordering within a flow, or at least ensure that it does
not significantly increase the level of reordering within the flow.
A number of ARQ protocols have allowed out-of-order delivery [BAL95,
SAM96, WAR95].
3.1 USING LINK ARQ TO SUPPORT MULTIPLE FLOWS
Most links can be expected to carry more than one IP traffic flow at
a time. Some high-capacity links are expected to carry a very large
number of simultaneous flows, often from and to a large number of
different hosts. With use of multiple applications at a host,
multiple flows can be considered the norm rather than the exception,
even for last-hop links.
When packets from several flows are simultaneously in transit within
a link ARQ protocol, ARQ may cause a number of additional effects:
a. ARQ introduces jitter to a TCP flow sharing a link with another
flow experiencing loss. In-sequence link-level delivery of
packets may adversely impact other applications sharing the link,
and can increase the duration of the initial slow-start period
for TCP flows for these applications. This is significant for
short-lived TCP flows (e.g. those used by HTTP/1.0 and earlier),
which spend most of their lives in slow-start.
b. High-persistence ARQ may cause premature timeout of another TCP
flow's RTO timer, although modern TCP implementations should not
experience this since their computed RTO values should leave
sufficient margin over path RTTs to cope with such jitter.
c. ARQ introduces jitter to UDP flows. An end-to-end protocol may
not require reliable delivery, particularly if it is delay-
sensitive.
PILC WORKING GROUP INTERNET DRAFT expires September 2001 12 Link ARQ issues for IP traffic March 2001
Reordering of packets belonging to different flows may be desirable
to achieve fair sharing of the link between established bulk data
transfer sessions and sessions that transmit less data but would
benefit from lower link transit delay. Preserving ordering within
each flow, to avoid the effects of reordering described earlier, is
worthwhile.
3.2 DIFFERENTIATION OF LINK SERVICE CLASSES
High ARQ persistency is generally considered unsuitable for many
applications using UDP, where reliable delivery is not always
required and where it may introduce unacceptable jitter, but may
benefit bulk data transfers under certain link conditions. A scheme
that differentiates packet flows into two or more classes, to
provide different service to each class, is therefore desirable.
Observation of flow behaviour can tell you which flows are
controlled and congestion-sensitive, or uncontrolled and not, so
that you can treat them differently and ensure fairness. However,
this cannot tell you whether a flow is intended as reliable or
unreliable by its application, or what the application requires for
best operation.
Supporting different link services for different classes of flows
therefore needs an explicit indication of the class of each IP
packet.
Some potential schemes for indicating the class of a packet include:
a. Using the Type of Service (ToS) bits in the IP header [RFC791].
The IETF has replaced these globally-defined bits, which were
rarely used, with a much more complex identification scheme for
use with differentiated services (diffserv [RFC2475]). Diffserv
defines classes of service that are mapped to various Per Hop
Behaviours (PHBs) as the packet is processed within the network.
Its uses include policy-based routing, class-based queuing, and
support for other QoS metrics, including packet priority, delay,
reliability, cost, etc.
b. Inspecting the network packet header and viewing the IP protocol
type [RFC791], to gain an idea of the transport protocol used and
thus guess its needs. This is not possible when carrying an
encrypted payload, e.g. using the IP security extensions (IPSec)
with the Encapsulation Security Payload (ESP) [RFC1827].
c. By inspecting the transport packet header information to view the
TCP or UDP headers and port numbers (e.g. [PAR00, BAL95]). This
is not possible when using payload encryption, e.g. IPSec with
ESP [RFC1827], and incurs processing overhead in routers.
PILC WORKING GROUP INTERNET DRAFT expires September 2001 13 Link ARQ issues for IP traffic March 2001
There are, however, some drawbacks to these schemes:
i. The ToS/Differentiated Services Code Point (DSCP) values
[RFC2475] may not be set reliably, and may be overwritten by
intermediate routers along the packet's path. These values may
be set by an ISP, and do not necessarily indicate the level of
reliability required by the end application.
ii. Tunnelling of traffic (e.g. GRE, MPLS, L2TP, IP-in-IP
encapsulation) can aggregate flows of different transport
classes, complicating individual flow classification with
schemes b. and c. and incurring further header processing if
tunnel contents are inspected.
iii. Use of the port number makes assumptions about application
behaviour and requirements. New applications or protocols can
invalidate these assumptions.
iv. In IPv6, locating the transport layer protocol type requires
parsing the entire IPv6 header, adding complexity to header
inspection. Again, this assumes that IPSec encryption is not
used.
Flow management is desirable; clear flow identification increases
the number of tools available for the link designer to use, and
permits more complex ARQ strategies that may otherwise make
misassumptions about traffic requirements and behaviour without flow
identification.
Despite the difficulties in providing a framework for accurate flow
identification, [RFC3002] advocates approach (a), and urges against
adding optimisations to future links that are triggered by
inspecting the contents of specific IP packets.
Links that are unable to distinguish clearly and safely between
delay-sensitive flows, e.g. real-time multimedia, DNS queries or
telnet, and delay-insensitive flows, e.g. bulk ftp transfers or
reliable multicast file transfer, cannot separate link service
classes safely. All flows should therefore experience the same link
behaviour.
In general, if separation of flows according to class is not
practicable, a low persistency is best for link ARQ.
4. CONCLUSIONS
It is currently difficult to make simple statements about arbitrary
patterns of loss that cover a wide range of link speeds and
propagation delays. Some general statements follow, summarising the
preceding discussion. In each case, it is recommended that specific
PILC WORKING GROUP INTERNET DRAFT expires September 2001 14 Link ARQ issues for IP traffic March 2001
details of the link characteristics and mechanisms are also
considered; solutions vary with conditions.
ARQ has been and continues to be used on links that carry IP
traffic. At low error rates, many of the details of ARQ, such as
degree of persistence or resulting out-of-order delivery, become
unimportant. Most frame losses will be resolved in one or two
retransmission attempts, and this is generally unlikely to cause
significant impact to e.g. TCP. However, on shared high-delay
links, the impact of ARQ on other UDP or TCP flows may lead to
unwanted jitter.
In a link outage, interactions between ARQ and multiple flows are
less significant; the ARQ protocol is likely to be equally
unsuccessful in retransmitting frames for all flows. High
persistence may also be desirable to enable prompt recovery once the
channel is restored.
For links with highly-variable error rates, high ARQ persistence may
provide good performance for a single TCP flow. However, lower
persistence may in some cases have merit, particularly when many
flows share and compete for capacity on the same link. The
reasoning here is that the link can perform useful work forwarding
some complete packets, and that blocking all flows by retransmitting
the frames of a single packet with high persistence is undesirable.
Low ARQ persistence is particularly useful where it is difficult or
impossible to classify traffic flows, and hence treat each flow
independently, and where the link capacity can accommodate a large
number of simultaneous flows.
In summary, low persistence is generally desirable; preserving
packet ordering is generally desirable. Extremely high persistence
is generally undesirable.
There is currently insufficient experience to recommend a specific
ARQ scheme for any class of link. It is also important to realise
that link ARQ is just one method of error recovery, and that other
complementary physical-layer techniques may be used instead of, or
together with, ARQ to improve overall link throughput for IP
traffic.
The choice of potential schemes includes adapting the data rate,
adapting the signal bandwidth, adapting the transmission power,
adaptive modulation, and adaptive information redundancy / forward
error control (FEC). All of these schemes can be used to improve
the received signal energy per bit, and hence reduce error, frame
loss and resulting packet loss rates given specific physical channel
conditions.
There is a need for more research to more clearly identify the
importance of and trade-offs between the above issues over various
PILC WORKING GROUP INTERNET DRAFT expires September 2001 15 Link ARQ issues for IP traffic March 2001
types of link. It would be useful if researchers and implementers
clearly indicated the loss model, link capacity and characteristics,
link and end-to-end path delays, details of TCP, and the number (and
details) of flows sharing a link when describing their experiences.
5. SECURITY IMPLICATIONS
No security implications have been identified as directly impacting
IP traffic. However, an unreliable link service may adversely
impact some link-layer key management distribution protocols if
encryption is used over such a link.
Denial-of-service attacks exploiting the behaviour of the link
protocol, e.g. using knowledge of its retransmission behaviour and
propagation delay to cause a particular form of jamming, may be
specific to an individual link scenario.
6. ACKNOWLEDGMENTS
Much of what is described here has been developed from a summary of
a subset of the discussions on the archived IETF PILC mailing list.
We thank the contributors to PILC for vigorous debate.
7. REFERENCES
References of the form RFCnnnn are Internet Request for Comments
(RFC) documents available online at http://www.rfc-editor.org/.
[BAL95] H. Balakrishnan, S. Seshan and R. H. Katz, Improving
Reliable Transport and Handoff Performance in Cellular Wireless
Networks, ACM MOBICOM, Berkeley, 1995.
[BAL97] H. Balakrishnan, V. N. Padmanabhan, S. Seshan and R. H.
Katz, A Comparison of Mechanisms for Improving TCP Performance over
Wireless Links, IEEE/ACM Transactions on Networking, 5(6),
pp. 756-759, 1997.
[BEN00] J.C. Bennett, C. Partridge, and N. Schectman, Packet
Reordering is Not Pathological Network Behaviour, IEEE/ACM
Transactions on Networking, 7(6), pp. 789-798, 2000.
[BHA97] P. Bhagwat, P. Bhattacharya, A. Krishna and S. K. Tripathi,
Using channel state dependent packet scheduling to improve TCP
throughput over wireless LANs, ACM/Baltzer Wireless Networks
Journal, (3)1, 1997.
PILC WORKING GROUP INTERNET DRAFT expires September 2001 16 Link ARQ issues for IP traffic March 2001
[CHE00] H. S. Cheng, G. Fairhurst et al., An Efficient Partial
Retransmission ARQ Strategy with Error Codes by Feedback Channel,
IEE Proceedings - Communications, (147)5, pp. 263-268, 2000.
[DRAFTBOR00] J. Border, M. Kojo, J. Griner et al., Performance
Enhancing Proxies, draft-ietf-pilc-pep-nn.txt, work in progress as
internet-draft.
[DRAFTDAW00] S. Dawkins, G. Montenegro, M. Kojo, V. Magret et al.,
End-to-end Performance Implications of Links with Errors,
draft-ietf-pilc-error-nn.txt, work in progress as internet-draft.
[DRAFTKARN00] P. Karn, A. Falk, J. Touch et al., Advice for Internet
Subnetwork Designers, draft-ietf-pilc-link-design-nn.txt, work in
progress as internet-draft.
[ECK98] D. A. Eckhardt and P. Steenkiste, Improving Wireless LAN
Performance via Adaptive Local Error Control, IEEE ICNP, 1998.
[FER99] A. Ferrero, The Eternal Ethernet, Addison-Wesley, 1999.
[ISO4335] HDLC Procedures: Specification for Consolidation of
Elements of Procedures, ISO 4335 and AD/1, International
Standardization Organization, 1985.
[ISO4335a] HDLC Procedures: Elements of Procedures, Amendment 4:
Multi-Selective Reject Option, ISO 4335/4, International Standards
Organization, 1991.
[ISO7776] Specification for X.25 LAPB-Compatible DTE Data Link
Procedures, ISO 4335/4, International Standards Organization, 1985.
[KEN88] C. A. Kent and J. C. Mogul, Fragmentation Considered
Harmful, Proceedings of ACM SIGCOMM, pp. 390-401, 1988.
[LUD99] R. Ludwig, B. Rathonyi, A. Konrad, K. Oden and A. Joseph,
Multi-Layer Tracing of TCP over a Reliable Wireless Link, ACM
SIGMETRICS, pp. 144-154, 1999.
[LUD99a] R. Ludwig, A. Konrad, A. Joseph, and R. Katz, Optimizing
the End-to-End Performance of Reliable Flows over Wireless Links,
ACM MobiCOM, 1999.
[LUD00] R. Ludwig and R. Katz, The Eifel Algorithm: Making TCP
Robust Against Spurious Retransmissions, ACM Computer Communication
Review, (30)1, January 2000.
[MEY99] M. Meyer, TCP Performance over GPRS, IEEE WCNC, 1999.
[PAR00] C. Parsa and J. J. Garcia-Luna-Aceves, Improving TCP
Performance over Wireless Networks at the Link Layer, Mobile
Networks and Applications, (1)5, pp. 57-71, 2000.
PILC WORKING GROUP INTERNET DRAFT expires September 2001 17 Link ARQ issues for IP traffic March 2001
[RFC791] J. Postel, Internet Protocol, RFC 791, 1981.
[RFC1122] R. Braden et al., Requirements for Internet Hosts --
Communication Layers, RFC 1122, 1989.
[RFC1191] J. Mogul and S. Deering, Path MTU Discovery, RFC 1191,
1990.
[RFC1323] V. Jacobson and R. Braden, TCP Extensions for High
Performance, RFC 1323, 1992.
[RFC1350] K. Solins, The TFTP Protocol (Revision 2), IETF RFC 1350,
1992.
[RFC1435] S. Knowles, IESG Advice from Experience with Path MTU
Discovery, RFC 1435, 1993.
[RFC1827] R. Atkinson, IP Encapsulating Security Payload (ESP),
RFC 1827, 1995.
[RFC2475] S. Blake, D. Black, M. Carlson, E. Davies, Z. Wang and W.
Weiss, An Architecture for Differentiated Services, RFC 2475, 1998.
[RFC2488] M. Allman, D. Glover and L. Sanchez, Enhancing TCP Over
Satellite Channels using Standard Mechanisms, RFC 2488, 1999.
[RFC2581] M. Allman, V. Paxson and W. Stevens, TCP Congestion
Control, RFC 2581, 1999.
[RFC2757] S. Dawkins, M. Kojo, V. Magret and N. Vaidya, Long Thin
Networks, RFC 2757, 2000.
[RFC2760] M. Allman, S. Dawkins, D. Glover, J. Griner et al.,
Ongoing TCP Research Related to Satellites, RFC 2760, 1999.
[RFC3002] D. Mitzel, Overview of 2000 IAB Wireless Internetworking
Workshop, RFC 3002, 2000.
[SAL81] J. H. Saltzer, D. P. Reed and D. Clark, End-to- End
Arguments in System Design, Second International Conference on
Distributed Computing Systems, pp. 509-512, 1988. Published with
minor changes in ACM Transactions in Computer Systems (2)4,
pp. 277-288, 1984.
[SAM96] N. Samaraweera and G. Fairhurst, Robust Data Link Protocols
for Connection-less Service over Satellite Links, International
Journal of Satellite Communications, 14(5), pp. 427-437, 1996.
[SAM98] N. Samaraweera and G. Fairhurst, Reinforcement of TCP/IP
Error Recovery for Wireless Communications, ACM CCR, 28(2),
pp. 30-38, 1998.
PILC WORKING GROUP INTERNET DRAFT expires September 2001 18 Link ARQ issues for IP traffic March 2001
[STE94] W. R. Stevens, TCP/IP Illustrated, Volume 1, Addison-Wesley,
1994.
[WAR95] C. Ward et al., A Data Link Control Protocol for LEO
Satellite Networks Providing a Reliable Datagram Service, IEEE/ACM
Transactions on Networking, 3(1), 1995.
AUTHORS' ADDRESSES
Gorry Fairhurst (gorry@erg.abdn.ac.uk)
Department of Engineering, University of Aberdeen,
Aberdeen AB24 3UE, United Kingdom.
Lloyd Wood (lwood@cisco.com)
Cisco Systems Ltd, 4 The Square, Stockley Park,
Uxbridge UB11 1BY, United Kingdom.
FULL COPYRIGHT STATEMENT
Copyright (C) The Internet Society (2000-2001).
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 any way, such as by removing
the copyright notice or references to the Internet Society or other
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 on an
"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.
PILC WORKING GROUP INTERNET DRAFT expires September 2001 19
Datatracker
draft-ietf-pilc-link-arq-issues-01
This is an older version of an Internet-Draft that was ultimately published as RFC 3366.
| Document | Document type |
This is an older version of an Internet-Draft that was ultimately published as RFC 3366.
Expired & archived
|
|
|---|---|---|---|
| Select version | |||
| Compare versions | |||
| Author | |||
| RFC stream |
|
||
| Other formats | |||
| Additional resources | Mailing list discussion |