RMT Working Group Generic Router Assist (GRA) Brad Cain
INTERNET-DRAFT for Multicast Transport Protocols Cereva Networks
Tony Speakman
Expires January 2002 cisco
Don Towsley
UMASS
20 July 2001
Generic Router Assist (GRA) Building Block
Motivation and Architecture
<draft-ietf-rmt-gra-arch-02.txt>
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
Generic Router Assist (GRA) is a network-based service that enables
end-to-end multicast transport protocols to take advantage of infor-
mation distributed across the network elements in a given multicast
distribution tree. The service consists of a canonical set of simple
functions which network elements may apply to selected packets in the
transport session as they traverse the distribution tree. The choice
of function and the packet parameters to which it is applied can be
defined and customized for a given transport session in a highly con-
trolled fashion that still permits enough flexibility for GRA to be
used to address a wide range of multicast transport problems not
Cain/Speakman/Towsley [Page 1]
INTERNET-DRAFT Generic Router Assist July 2001
amenable to end-to-end solution. This document provides the motiva-
tion and an architecture for GRA.
Cain/Speakman/Towsley [Page 2]
INTERNET-DRAFT Generic Router Assist July 2001
Table of Contents
1. Introduction ............................................... 4
2. Scope of Generic Router Assist ............................. 6
3. Canonical Services and Functional Models/Examples .......... 11
4. Implementation Considerations .............................. 19
Abbreviations .................................................. 22
References ..................................................... 23
Revision History ............................................... 24
Authors' Addresses ............................................. 24
Cain/Speakman/Towsley [Page 3]
INTERNET-DRAFT Generic Router Assist July 2001
1. Introduction
The development of scalable end-to-end multicast protocols poses a
tremendous challenge to network protocol designers. For example the
development of reliable multicast protocols has received considerable
attention in recent years. Most protocols are based on an end-to-end
solution [SRM, RMTP, TKP] and have found the problem of scaling to
1000s or even 100s of receivers daunting. The primary obstacles to
the development of scalable protocols have been feedback implosion
and transmission isolation. The first of these concerns the diffi-
culty of a large multicast application to limit feedback from
receivers to a data source or to each other. The second concerns the
difficulty of limiting the transmission of data to the subset of a
multicast group that requires it.
Several proposals have been made to add functionality to routers for
the purpose of improving the performance of multicast applications,
particularly reliable multicast. Papadopoulos and Parulkar [LSM]
introduced additional forwarding functionality to a router which
would allow each router to identify a "special outgoing interface"
over which to transmit a particular class of packets. They showed how
this "turning point functionality" could be used to improve the per-
formance of reliable multicast protocols. Levine and Garcia-Luna-
Aceves [LABEL] proposed the addition of "routing labels" to routing
tables which could be used to direct packets over specific inter-
faces. One of these, called a distance label, was shown to be quite
useful in reliable multicast for directing requests for repairs to
nearby repair servers. The third and, perhaps most relevant proposal
is the PGM protocol [PGM]. Briefly, PGM is a reliable multicast pro-
tocol which uses negative acknowledgements (NAKs). The PGM protocol
is an end-to-end transport protocol that contains a router component
which performs NAK elimination and retransmission subcasting func-
tionality. This proposal, like others [GMTS, BFS], is primarily
motivated by PGM and the recognition of the benefits of exporting a
set of flexible, simple router-based functionality for the purpose of
multicast protocol design. Such functionality can significantly sim-
plify the design of a large class of scalable multicast transport
protocols.
In this draft, we present Generic Router Assist (GRA) functionality
intended to help protocol designers deal with the two problems of
feedback implosion and transmission isolation. This functionality is
designed to assist in the scaling of receiver feedback information
and in providing subcasting for large multicast groups. It consists
of simple filtering functions that reside within routers.
Signaling protocols are used by hosts to set up and invoke this func-
tionality. Briefly, a data source first initializes one or more
Cain/Speakman/Towsley [Page 4]
INTERNET-DRAFT Generic Router Assist July 2001
desired services on its multicast tree using GRA setup messages. The
GRA-capable routers on the tree then selectively eliminate feedback
from receivers and/or isolate transmissions through the use of
filters set by either the sender or the receivers. For robustness,
periodic transmissions of setup messages on the multicast tree are
used to refresh GRA state in the face of routing changes and other
possible errors. It should be stressed that GRA services are only
invoked for certain packets; data packets are usually not treated any
differently and will not cause any additional processing in routers.
GRA is not intended to provide sophisticated services which are dif-
ficult or impossible to implement in routers. GRA functionality is
implemented at the IP layer and provides unreliable, best-effort ser-
vices. Transport protocols which make use of GRA must be robust in
the face of failures and the absence of GRA-capable routers in the
network.
Before describing the details of GRA, we present a simple example in
the context of a PGM-like reliable multicast protocol.
Consider a NAK-based reliable multicast protocol which places the
responsibility of packet loss detection on each receiver. Each time
that a receiver detects a loss (based on a gap in the sequence
numbers of the packets that it receives), it unicasts a request for a
repair (NAK) to the sender. Upon receipt of a NAK for a specific
packet, the sender retransmits the packet to all receivers.
This protocol faces considerable challenges in dealing with multiple
NAKs for the same packet. First, there is the problem of the sender
having to process many NAKs. Second, there is the problem of limiting
the number of retransmissions to the same packet. GRA can be used to
address these two problems. Prior to the transfer of any data, the
application sets up a NAK elimination service at each GRA-capable
router using a setup message. This service is set up to eliminate
NAKs generated for the same packet. In addition, the service main-
tains information regarding the interfaces over which it has received
NAKs so that it can subcast the retransmission on the portion of the
multicast tree that contains receivers requiring a retransmission of
the packet.
In Figure 1, we show how GRA can be used to eliminate feedback infor-
mation in a reliable multicast transport protocol. In this figure, a
multicast source (Src 1) transmits to two receivers (Rec 1 and Rec
2). The data packets from Src 1 are treated as regular multicast
packets and forwarded accordingly. On the link between router R1 and
router R2, a data packet is lost. Assuming a NAK based reliable mul-
ticast protocol, this loss causes the receivers to each send a NAK to
the source for the packet that was lost. In the example, receivers
Cain/Speakman/Towsley [Page 5]
INTERNET-DRAFT Generic Router Assist July 2001
include a GRA header in NAKs sent to the source. Router R2 treats
these NAKs in a special manner, elminating the redundant NAKs to the
source. Therefore, only one NAK arrives at the source. We see from
this example that only certain types of packets require additional
processing at GRA routers and that the majority of end-to-end packets
are forwarded according to normal multicast forwarding rules (i.e.
without additional router processing).
Src 1
^ | |
| | | data packet
| | |
R1 |
^ | |
| | V
R2 eliminates | | X data packet dropped
duplicate +--->R2 <---+
NAK | --- --- |
| | | |
| | | |
| | | |
R3 R4
GRA ^ | | ^ GRA
NAK | | | | NAK
| | | |
Rec 1 Rec 2
Figure 1. Example of network support for transport protocols.
GRA may be used on all types of multicast forwarding trees. However,
GRA state is per-transport-session state and so requires per-
transport-session state in routers in addition to the underlying mul-
ticast routing state.
2. Scope of Generic Router Assist
The types of services implemented with GRA are bounded by constraints
and limitations of routing devices. In this section we explicitly
describe the limitations and constraints of routing devices and
explain some reasonable services to implement in routers.
We specifically describe router limitations in order to limit the
scope of GRA. A small set of GRA services in routers can assist in
scaling many of the problems in end-to-end multicast. Previous
proposals[ACTIVE] have proposed complex elements that reside in
routers to provide sophisticated capabilities. These are probably
Cain/Speakman/Towsley [Page 6]
INTERNET-DRAFT Generic Router Assist July 2001
unreasonable for current generation routers. The approach of GRA is
to provide a simple fixed set of services which give the maximum
benefit with the least cost.
2.1. Service Properties
GRA services are performed on a subset of packets sent between end-
to-end transport protocols on the multicast distribution tree between
a GRA Source and the set of GRA receivers. Only routers on the dis-
tribution tree for a particular GRA source act upon GRA packets. The
advantages of GRA type services can only be realized when the actions
are performed on packets that are directly on the forwarding tree of
a multicast group.
In order to describe the requirements of GRA, we first describe the
properties of appropriate services.
Fixed: by fixed services we mean those which are statically part
of router software or hardware. We DO NOT mean dynamically load-
able modules. A fixed set of simple services will probably suf-
fice for most of the scaling issues in transport protocols.
Simple: We include only those services that are reasonable to be
implemented in the forwarding path in routers. These are services
which can be performed with minimum CPU and memory overhead.
Short Term: We provide services for which state and processing
overhead is short lived. GRA makes use of soft-state design prin-
ciples.
2.2. GRA Requirements
When considering the service and architecture of GRA, we adhere to
the following principles:
1. GRA services should be simple and must be fixed. They must not
require excessive processing in routing devices and they must not
buffer packets.
2. GRA services are not substitutes for well-engineered end-to-end
protocol designs. We support the end-to-end design principles of
transport protocols. GRA is an assist service which is designed
to assist protocols in scaling aspects.
3. GRA services will not take on active networking attributes such
as dynamically uploadable modules or programming language propo-
sals.
Cain/Speakman/Towsley [Page 7]
INTERNET-DRAFT Generic Router Assist July 2001
4. GRA services should not directly participate in transport pro-
tocols. GRA should not be required for any transport protocols
nor should any GRA services directly support a particular proto-
col.
5. GRA services should be those which may assist all or a reason-
able subset of transport protocols.
6. GRA services should be used for assisting in control packet
operations. GRA services should not be for the majority of pack-
ets in a multicast group.
2.3. Constraints of networking devices
Current generation routers perform processing of packets and execute
routing and signaling protocols. Routers perform fast packet for-
warding on the forwarding path. This is usually performed by
hardware and software specifically designed for the forwarding of
packets (and may include other functions such as policing, shaping,
etc). This is in contrast to the control plane of a router where
control protocols are run in protocol-specific control components.
Examples of these types of protocols are routing, management and sig-
naling protocols.
We now describe the role and impact of GRA services on these two
planes of a router.
Forwarding path: A router forwarding path usually consists of spe-
cialized hardware and software which is designed specifically for
the purpose of forwarding packets. Newer routers also have abili-
ties to perform other actions such as marking or policing for ser-
vices such as QoS. In general, the forwarding path of routers is
very limited in the amount of state and complexity of processing
that can be performed. The processing complexity and state over-
head of GRA services may limit the extent to which they may be
implemented in the forwarding path.
Control plane: Router control planes run embedded or general
operating systems. On top of these operating systems are imple-
mentations of IP control protocols such as routing, signaling, and
network management. The processing power and memory of a control
plane depends highly on the hardware design of the router. Most
routers use general purpose CISC or RISC processors for running
the control plane operating system and protocols. The control
plane is limited in processing by its hardware and the load gen-
erated from the other processes or tasks running. We expect that
complex or stateful GRA operations will be performed in the con-
trol plane where state and processing power are more readily
Cain/Speakman/Towsley [Page 8]
INTERNET-DRAFT Generic Router Assist July 2001
available. However, we do stress that routers generally have
fairly slow control plane hardware. This is to keep the cost low
and because the processing required for control plane protocols is
usually low.
2.4. State Constraints
As we evaluate particular services which are candidates for inclusion
in GRA, constraints in router state are an important consideration.
We should select services which will not create substantial or long-
lived state.
2.4.1. Session State
Routers which perform multicast forwarding contain per-tree forward-
ing state. For trees rooted at multicast sources, this amounts to
per-source, per-group forwarding entries. This state must be kept in
both the forwarding as well as the control plane. GRA services are
per-session, or per-source. This is simply because GRA services are
per-transport-session.
The session state of GRA imposes additional state on routers. Each
session requires a state block describing the desired services.
Other types of state may also be created during the course of a GRA
session. The GRA session state is the only state required for the
length of a session; other state is set up and torn down in smaller
intervals.
2.4.2. Per-Packet State
The implementation of particular services may require per-packet
state in GRA routers. Services which require per-packet state should
use short-lived timers to tear down this state so as to avoid an
explosion in the amount of state at routers. An example of a service
which causes per-packet state is NAK elimination. The use of small
timers should minimize problems in state growth.
2.4.3. Buffering
GRA services must not buffer packets. GRA services may drop or
modify packets in transit, but they will never buffer packets.
The buffering of packets in routing devices is generally unacceptable
due to the unpredictable behavior of such a service. In addition
this is, in general, an unreasonable service for routers to support
without a significant payback in end-to-end scaling.
Cain/Speakman/Towsley [Page 9]
INTERNET-DRAFT Generic Router Assist July 2001
2.5. Processing Constraints
GRA services may require differing amount of computation. As a gen-
eral rule, GRA services should require minimal computation and packet
manipulation.
2.5.1. Computation
Most GRA services should require minimal computation. Services which
require minimal computation are reasonable to implement in routing
devices and minimize security risk. Examples of some appropriate
operations are:
Comparison Operations: comparison operations may be performed on
GRA packets against the GRA session state in the router in order
to determine whether a service should be invoked. Examples of
comparison operations are: equal to, less than, greater than, etc.
Update Operations: When receiving a GRA packet, a router may be
required to update its GRA session state. The operations required
for updating the state should remain simple. Example of simple
operations are addition, subtraction, etc.
In order to limit the computational complexity of GRA services, GRA
operations should remain singular. The combination of predicates and
operations creates problems in both router processing and in GRA
specifications themselves. This restriction will avoid problems
regarding operator and action precedence.
2.5.2. Buffer Operations
One can define services requiring extensive packet manipulation by
GRA routers. This is probably expensive and therefore unreasonable
for routers. GRA services should be invoked without extensive mani-
pulation of packets. Services which update or overwrite fields are
acceptable. Services which require the formation of new packets or
accumulate information into new packets are unacceptable.
2.6. Examples of Reasonable Services
In this section we briefly describe two GRA services and the ways in
which they meet the above requirements. The two example services are
those that provide general functions which do not incur large amounts
of state or processing.
Elimination: Elimination is the selected dropping of redundant
signaling. An example of elimination is NAK elimination in a
reliable multicast protocol. Elimination is a service which
Cain/Speakman/Towsley [Page 10]
INTERNET-DRAFT Generic Router Assist July 2001
requires little computational overhead. It does however, require
per-packet (or per-sequence-block) state. This state can be con-
trolled by the use of short soft state timers.
Subcasting: Subcasting is the forwarding of a multicast packet to
a subset of the multicast tree. Subcasting is useful for a
variety of multicast protocols. Subcasting does not require a
large amount of processing. The state required is an identifier
for the subtree and a list of outgoing interfaces. This state is
similar to multicast forwarding tree state.
3. Canonical Services and Functional Models/Examples
While a variety of mechanisms must come together to enable a specific
GRA service in a distribution tree (session path messages to estab-
lish session parameters and neighbour information, a control protocol
to define, enable, and disable specific filtering services, etc.),
the basic mechanism of GRA consists of router based services that can
be described in the language of filters, keys, and conditional func-
tions (binary predicates and their outcomes).
Using the example of reliable multicast presented in Figure 1., this
section expands the model and terminology of filters to describe the
full flexibility of GRA. The intent here is to generalize the
mechanism fully enough to be able to accommodate currently unspeci-
fied requirements for multicast transport services as they emerge.
Revisiting the example in Figure 1., note that the receipt of a loss
report at a router implies several things. The packet type implies
that a type of filter should be established for the transport ses-
sion, that the filter key is (a sequence number) of a particular
length at a particular offset, that the loss report in hand should be
forwarded, that the interface upon which the loss report was received
should be recorded and associated with the key in the filter, and
that subsequent matching loss reports on any interface should be
eliminated. The filter itself has other implied characteristics. It
eliminates only for a certain interval after forwarding any subse-
quent loss report, and it has an implied lifetime after which it is
locally expired by the router.
Similarly in the case of sub-casting, the receipt of a retransmis-
sion at a router implies several things. The packet type implies
that the packet should be matched against an existing filter type
based on a key of a particular length and at a particular offset,
that the packet itself should be forwarded on all interfaces for
which a loss report associated with the key was recorded, and that
the key's state should be discarded. By implication, unmatched
retransmissions should not be forwarded.
Cain/Speakman/Towsley [Page 11]
INTERNET-DRAFT Generic Router Assist July 2001
From this example some generalities emerge which, once they are
extricated from their specific semantics, can be re-assembled in a
variety of useful ways to provide router-based assistance for a
broader class of transport services.
3.1. General Model
The general model is one in which packets carry one of a tightly con-
strained set of signals that alert the router to apply a pre-defined
filtering service.
The packet-borne signal conveys
a filter type,
an associated action,
a key,
and possible packet operands.
The filter type specifies a particular router-based service. The
associated action specifies a particular function to carry out in the
context of the service. The key and any packet operands specify
values upon which to operate in the context of the action.
Canonical filtering services in routers can be defined by
a filter type,
associated supported actions,
predicated on specific conditions,
and three functions gated on that predicate whether TRUE or FALSE:
f(p): how to dispose of the packet,
f(s): how to transform the key's state, and
f(v): how to transform the outgoing interface (OIF) list
associated with the key.
All of these as well as the offsets and lengths of the key and any
packet operands for each supported action constitute the definition
of the filtering service.
3.2. An Example of Elimination and Subcasting for ARQ
Given this model, the handling of retransmissions in PGM can be
described as a predicate eliminating and subcasting filter.
Let IIF be the interface on which a packet is received. Let RETX
denote whether a packet retransmission has been requested either in a
loss report (RQST RETX), as interface state (OIF RETX), or as key
state (KEY RETX). Let ET be the elimination timer for a particular
Cain/Speakman/Towsley [Page 12]
INTERNET-DRAFT Generic Router Assist July 2001
key's state, and LT be the life timer for a particular key's state.
The filtering service in the router supports two actions, one inbound
(RCVR_UPDATE) and one outbound (FORWARD).
3.2.1. RCVR_UPDATE
For RCVR_UPDATE, in addition to a transport session identifier, the
following are defined for the signal in the packet:
ELIM_SUBC is the filter type
RCVR_UPDATE is the action
SQN is the key
RETX is a packet-borne operand
(value of one in the PGM example)
For RCVR_UPDATE, the following are defined for the filtering service
in the router:
In case the key fails to match existing key state:
predicate: NOOP
f(v): OIF RETX = 1 for IIF
f(s): start ET, start LT, KEY RETX = RQST RETX
f(p): reverse forward to upstream neighbour
In case the key matches existing key state:
predicate: NOOP
f(v): OIF RETX = 1 for IIF
f(s): restart LT
f(p): discard
3.2.2. FORWARD
For FORWARD, in addition to a transport session identifier, the fol-
lowing are defined for the signal in the packet:
ELIM_SUBC is the filter type
FORWARD is the action
SQN is the key
For FORWARD, the following are defined for the filtering service in
the router:
In case the key fails to match existing key state:
predicate: NOOP
Cain/Speakman/Towsley [Page 13]
INTERNET-DRAFT Generic Router Assist July 2001
f(v): NOOP
f(s): NOOP
f(p): discard
In case the key matches existing key state:
predicate: for all OIF RETXs, OIF RETX NE 0
In case the predicate is TRUE:
f(v): decrement OIF RETX
f(s): NOOP
f(p): forward on OIF
In case the predicate is FALSE:
f(v): NOOP
f(s): NOOP
f(p): discard
Associated with the filtering service would be an additional house-
keeping function which would discard a key's state if either LT
expired or all the OIF COUNTs were zero.
The point of this and subsequent examples is to demonstrate that this
model for GRA can accommodate a highly functional set of router-based
services which, given a transport-layer- independent implementation,
could be provided by routers for general deployment by transport pro-
tocols engineered to signal the routers in a generic way. We now
present a refinement of the PGM example adding forward error correc-
tion.
3.3. An Example of Elimination and Subcasting with FEC
In this example, a packet operand is used to carry a count of parity
packets requested with the additional implication that the interface
upon which the loss report was received along with the count of par-
ity packets requested should be recorded and associated with the key
in the filter, and that subsequent matching loss reports on any
interface should be eliminated unless they request a larger number of
parity packets than has been requested on any interface.
Similarly upon forwarding parity packets implies that the packet
itself should be forwarded on all interfaces with non-zero counts
recorded as state associated with the key, that the corresponding
count should be decremented by 1 per interface until it reaches 0,
and that the key's state should be discarded once all counts are
zero.
Cain/Speakman/Towsley [Page 14]
INTERNET-DRAFT Generic Router Assist July 2001
The handling of parity NAKs and parity retransmissions in PGM can be
described as a predicate eliminating and subcasting filter augmented
by a packet operand, the number of parity packets requested.
Let IIF be the interface on which a packet is received. Let COUNT be
the number of parity packets requested and recorded either in the
loss report (RQST COUNT), as interface state (OIF COUNT), or as key
state (HIGH COUNT). Let ET be the elimination timer for a particular
key's state, and LT be the life timer for a particular key's state.
The filtering service in the router supports two actions, one inbound
(RCVR_UPDATE) and one outbound (FORWARD).
3.3.1. RCVR_UPDATE
For RCVR_UPDATE, in addition to a transport session identifier, the
following are defined for the signal in the packet:
ELIM_SUBC is the filter type
RCVR_UPDATE is the action
SQN is the key
COUNT is a packet-borne operand
For RCVR_UPDATE, the following are defined for the filtering service
in the router:
In case the key fails to match existing key state:
predicate: NOOP
f(v): OIF COUNT for IIF = MAX(RQST COUNT, 0)
f(s): start ET, start LT, HIGH COUNT = RQST COUNT
f(p): reverse forward to upstream neighbour
In case the key matches existing key state:
predicate: ET is running or RQST COUNT LEQ HIGH COUNT
In case the predicate is TRUE:
f(v): OIF COUNT for IIF = MAX(RQST COUNT, OIF COUNT for IIF)
f(s): restart LT
f(p): discard
In case the predicate is FALSE:
f(v): OIF COUNT for IIF = MAX(RQST COUNT, OIF COUNT for IIF)
f(s): restart ET, HIGH COUNT = RQST COUNT
f(p): reverse forward to upstream neighbour
Cain/Speakman/Towsley [Page 15]
INTERNET-DRAFT Generic Router Assist July 2001
3.3.2. FORWARD
For FORWARD, in addition to a transport session identifier, the fol-
lowing are defined for the signal in the packet:
ELIM_SUBC is the filter type
FORWARD is the action
SQN is the key
For FORWARD, the following are defined for the filtering service in
the router:
In case the key fails to match existing key state:
predicate: NOOP
f(v): NOOP
f(s): NOOP
f(p): discard
In case the key matches existing key state:
predicate: for all OIF COUNTs, OIF COUNT NE 0
In case the predicate is TRUE:
f(v): decrement OIF COUNT
f(s): NOOP
f(p): forward on OIF
In case the predicate is FALSE:
f(v): NOOP
f(s): NOOP
f(p): discard
Associated with the filtering service would be an additional house-
keeping function which would discard a key's state if either LT
expired or all the OIF COUNTs were zero.
3.4. An Example of a Surrogate Service for Subcasting
The "turning point functionality" in LMS [LSM] cited above can be
conceived of as enlisting the help of a willing surrogate to provide
funtionality that may not supported natively in the network but may
be supported by distinguished servers at the edge of the network
(such as retransmitters).
This example describes how surrogates can be supported in GRA. A
Cain/Speakman/Towsley [Page 16]
INTERNET-DRAFT Generic Router Assist July 2001
router first establishes state that enables a surrogate. This state
is maintained per TSI and consists of the network-layer unicast
address of the surrogate, the surrogate interface, and the surrogate
current cost. The state is established after receiving advertise-
ments from surrogates. It is assumed that surrogates send such adver-
tisements periodically. The router selects the surrogate that adver-
tises the lowest cost.
When a packet arrives requesting a service provided by the surrogate,
the router forwards the packet as follows: if the packet came from
the surrogate interface or no surrogate is known, it is forwarded
upstream. Otherwise, the packet is unicast to a known surrogate
after recording the address of the incoming interface and the address
of the surrogate interface in the GRA header.
Finally, the surrogate unicasts a response packet to the router to be
multicast on the interface on which the original request arrived.
For each transport session, let SURR_ADDR be the network layer uni-
cast address of the surrogate (initially NULL), let SURR_IF be the
surrogate interface (initially NULL), and let SURR_COST be the cost
of the current surrogate interface (initially infinity). Let IIF be
the interface on which a packet is received.
The filtering service in the router supports three actions:
SURR_UPDATE. FWD_TP, and FWD_OIF.
SURR_UPDATE is the surrogate election action, FWD_TP is the turning
point locator service, and FWD_OIF is the subcast service.
Note that a packet using the service FWD_TP does not carry the turn-
ing point Information (i.e the PKT_IIF and PKT_OIF are NULL).
3.4.1.
For SURR_UPDATE, the following are defined for the signal in the
packet:
SURROGATE is the filter type
SURR_UPDATE is the action
ADDR is a packet-borne operand
COST is a packet-borne operand
For SURR_UPDATE, the following are defined for the filtering service
at the router:
predicate: COST < SURR_COST
Cain/Speakman/Towsley [Page 17]
INTERNET-DRAFT Generic Router Assist July 2001
In case the predicate is true:
f(v): NOOP
f(s): SURR_ADDR = ADDR, SURR_IF = IIF, SURR_COST = COST
f(p): reverse forward to upstream neighbor
In case the predicate is false:
f(v): NOOP
f(s): NOOP
f(p): discard
3.4.2.
For FWD_TP, the following are defined for the signal in the packet.
SURROGATE is the filter type
FWD_TP is the action
PKT_IIF, PKT_OIF are packet_borne variables
For FWD_TP, the following are defined for the filtering service at
the router:
predicate: ((SIF != NULL) && (IIF != SIF))
In case the predicate is TRUE
f(v): NOOP
f(s): NOOP
f(p): PKT_IIF = IIF and PKT_OIF = Surrogate IF
(i.e Insert the Turning Point information)
Unicast to SURR_ADDR
In case the predicate is FALSE
f(v): NOOP
f(s): NOOP
f(p): Unicast to upstream GRA neighbour
3.4.3.
For FWD_OIF the following are defined for the signal in the packet:
SURROGATE is the filter type
FWD_OIF is the action
PKT_OIF is a packet-borne operand
For FWD_OIF, the following are defined for the filtering service at
Cain/Speakman/Towsley [Page 18]
INTERNET-DRAFT Generic Router Assist July 2001
the router:
predicate: for all interfaces in PKT_OIF list:
f(v): NOOP
f(s): NOOP
f(p): Multicast the packet on PKT_OIF
to the S,G associated with the TSI
3.5. Summary
The point of these examples is to demonstrate that this model for GRA
can accommodate a highly functional set of router-based services
which, given a transport-layer-independent implementation, could be
provided by routers for general deployment by transport protocols
engineered to signal the routers in a generic way. Now that we have
established the context and a model for GRA, the next section
discusses implementation considerations in the interest of making the
mechanisms of GRA more concrete, and highlighting their practical and
performance consequences to traditional multicast forwarding.
4. Implementation Considerations
4.1. Signalling Protocol - GRA service indicators
A consideration of the implementation issues attending GRA strongly
determines the class of functions that may be realized with this
mechanism. These considerations relate to both security of the
router and performance in the forwarding path. The former will be
dealt with in another section. This section outlines the time and
space scaling consequences of GRA to the forwarding path.
To be a generic network layer service, it's clear that some minimal
indicator is required in the network layer to signal the presence of
GRA signal on a packet. As has been previously noted, remember that
the GRA indicator is typically NOT borne by basic data packets.
These are switched without exception in the usual forwarding path.
In this section, packets bearing a GRA indicator will be referred to
as GRA packets.
A network layer indicator frees forwarding engines from the burden of
having to walk into and parse transport headers on every switched
packet. It's highly efficient to encode the GRA indicator on the
network layer header since that header is typically already within
the grasp of the forwarding engine. In PGM, this indicator is imple-
mented with an IP Router Alert option, but a single bit in the basic
IP header would function just as well (even better, actually, for
legacy routers).
Cain/Speakman/Towsley [Page 19]
INTERNET-DRAFT Generic Router Assist July 2001
Once GRA packets can be detected in the forwarding path, the next
step is to locate and parse the GRA parameters: the filter type, the
action, the key, and the operands from above. These should be
encoded as TLVs located somewhere between the end of the network
layer header and the beginning of the transport layer payload or SDU.
It's tempting in the case of the key and the packet operands to con-
sider encoding their offsets and lengths in a TLV that could be used
to locate the actual values themselves in the transport header or,
more wildly, in the SDU, but this would amount to providing a near
programmatic directive to the network element which is fraught with
risk. So note that, in the example of elimination above, the number
of loss reports requested would, in this model, be encoded both in
its natural place in the transport header, and also as a GRA-specific
operand in a separate GRA TLV.
An alternative is to establish the location and length of keys and
packet operands as attributes of the filtering service defined in the
network element itself so that the only vulnerability to the network
elements would derive from abuse of the setup protocol in the control
plane in place of vulnerability in the forwarding path or data plane.
The location of GRA TLVs before, inside, or after the transport layer
header is at the crux of whether GRA is regarded as integral to a
specific layer or as a shim layer between layers. The answer to the
question determines not just where to locate the TLVs but also where
to implement the service in host protocol stacks.
As for forwarding-time implementation of GRA services, we take it as
a requirement for this specification that some services must be
light-weight enough to be candidates for optimized implementations in
hardware or firmware, while others may be sufficiently complex to
warrant software implementations. Soft-state-based services that do
not maintain per-packet state may be in the first class; services
that maintain per-packet state and therefor require a
sorted/searchable data structure may be in the second class.
Beyond these very GRA-specific issues are the more general control
protocol issues of how to interoperate network elements with
disparate GRA capabilities, and all of the specifics of defining,
enabling, and disabling filters themselves. These issues are best
treated once a solid model of the GRA mechanism itself is esta-
blished.
4.2. Control Protocol - Filter definition, enabling, and disabling
Cain/Speakman/Towsley [Page 20]
INTERNET-DRAFT Generic Router Assist July 2001
4.3. Control Protocol - Session path messages and neighbour informa-
tion
Cain/Speakman/Towsley [Page 21]
INTERNET-DRAFT Generic Router Assist July 2001
Abbreviations
IIF Incoming Interface
NAK Negative Acknowledgement
NOOP No Operation
OIF Outgoing Interface
SDU Service Data Unit
SQN Sequence Number
TLV Type Length Value
Cain/Speakman/Towsley [Page 22]
INTERNET-DRAFT Generic Router Assist July 2001
References
[ACTIVE] L.H. Lehman, S.J. Garland, D. L. Tennenhouse. "Active Reli-
able Multicast", Proc. INFOCOM'98, 1998.
[BFS] Koichi Yano, Steven McCanne, "The Breadcrumb Forwarding Service
and the Digital Fountain Rainbow: Toward a TCP-Friendly Reliable Mul-
ticast", UCB/CSD Technical Report No. UCB//CSD-99-1068
[GMTS] Brad Cain, Don Towsley, "Generic Multicast Transport Services
(GMTS)", Nortel Networks Technical Report, December 1998.
[KKT] S. Kasera, J. Kurose, D. Towsley. "A Comparison of Server-Based
and Receiver-Based Local Recovery Approaches for Scalable Reliable
Multicast", Proc. INFOCOM'98, 1998.
[LABEL] B.N. Levine, J.J. Garcia-Luna-Aceves. "Improving Internet
Multicast with Routing Labels", Proc. ICNP-97, pp. 241-250, Oct.
1997.
[LSM] C. Papadopoulos, G. Parulkar. "An Error Control Scheme for
Large-Scale Multicast Applications", Proc. INFOCOM'98.
[PGM] T. Speakman, D. Farinacci, S. Lin, A. Tweedly. "PGM Reliable
Transport Protocol", IETF draft-speakman-pgm-spec-03.txt, June, 1999.
[RMTP] J. Lin, S. Paul. "RMTP: A reliable multicast transport proto-
col", Proc. of IEEE INFOCOM'95, 1995.
[SRM] S. Floyd, V. Jacobson, S. McCanne, C. Lin, L. Zhang. "A Reli-
able Multicast Framework for Light-weight Sessions and Application
Level Framing", IEEE/ACM Trans on Networking, vol. 5, 784-803, Dec.
1997.
[TKP] D. Towsley, J. Kurose, S. Pingali. "A comparison of sender-
initiated and receiver-initiated reliable multicast protocols" IEEE
JSAC, April 1997.
Cain/Speakman/Towsley [Page 23]
INTERNET-DRAFT Generic Router Assist July 2001
Revision History
draft-ietf-rmt-gra-00.txt October 1999
Original draft.
draft-ietf-rmt-gra-01.txt March 2000
Added the example of simple elmination and subcasting.
draft-ietf-rmt-gra-02.txt July 2001
Replaced references to "suppression" and "aggregation" with
"elimination".
Replaced references to "aggregate" with "accumulate".
Changed "should" to "must" in principle #1.
Added the surrogate example.
Authors' Addresses
Brad Cain
bcain@cereva.com
Tony Speakman
speakman@cisco.com
Don Towsley
towsley@cs.umass.edu
Cain/Speakman/Towsley [Page 24]
