An IPv4 Flowlabel OptionUniversity of Duisburg-Essen, Institute for Experimental MathematicsEllernstrasse 2945326 EssenNordrhein-WestfalenGermany+49-201-1837637+49-201-1837673dreibh@iem.uni-due.dehttp://www.iem.uni-due.de/~dreibh/Internet-DraftThis draft defines an IPv4 option containing a flowlabel that is compatible to IPv6.
It is required for simplified usage of IntServ and interoperability with IPv6.This document uses the following terms:
IntServ (Integrated Services): Reservation of network resources (bandwidth) on a per-flow basis. See
,
,
,
,
,
and
for details.
Flow:
An IntServ reservation between two endpoints.
Flow Label: The Flow Label field of the IPv6 header and the IPv4 option header defined
in this draft. It is used for marking a packet to use a specific IntServ
reservation. See for a detailed description.
RSVP: ReSource Reservation ProtocolSCTP: Stream Control Transmission ProtocolTCP: Transmission Control ProtocolQoS: Quality of ServiceUDP: User Datagram ProtocolThe key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL"
in this document are to be interpreted as described in
.
This section describes the motivation to add a flow label option to the
IPv4 protocol.
The Flow Label field of the IPv6 header
(see and ) is a
20-bit pseudo-random number. All packets from the same source address
having the same flow label MUST contain the same destination address.
Therefore, the flow label combined with the source address is a network-
unique identification for a specific packet flow.
The idea behind the flow label is marking specific flows for IntServ. That
is, the routers on the path from source to destination keep e.g.
reservation states for the flows. The flow label provides easy
identification and utilizes efficient lookup, e.g. using a hash function on the
3-tuple (source address, destination address, flow label).
Using the IPv6 flow label, packets can be mapped easily to specific flows,
with the following features:
Transport Layer Protocol Independence:
Since the mapping is directly specified in the IP
header, all possible layer 4 protocols are supported, even protocols to be
specified in a far future.
Support for Network Layer Encryption: The mapping is
independent of payload encryption (e.g. by IPsec).
Support for Fragmentation: If fragmentation of a large IP packet is
necessary, all fragments contain the same flow label. Therefore,
fragmentation does not cause any flow-marking problem.
Flow Sharing: By marking packets with a flow label, it is possible to
share a single flow (IntServ reservation) with several communication
associations from host A to host B. For example, a video stream via UDP and
a HTTP download via TCP could share a single reservation.
For the user, flow sharing has the advantage that if one of its
communication associations temporarily requires lower bandwidth than
expected, other associations sharing the same flow may use the remaining
bandwidth. That is, his possibly expensive reservation is fully utilized.
Flow sharing also helps keeping the total number of reservations a router
has to handle small, reducing their CPU and memory requirements and
therefore cost.
Multi-Flow Connections: One communication association can divide up its
packets to several flows, simply by marking packets with different flow
labels. This technique can be used for layered transmission. That is, a
stream (e.g. a video) is divided up into several parts (called layers). For
example, the first layer (base layer) of a video contains a low-quality
version, the second (1st enhancement layer) the data to generate a
higher-quality version, etc.. Now, the first layer can be mapped to a high-quality
reservation (guaranteed bandwidth, low loss rate) at higher cost, but the
following layers can be mapped to lower-quality reservations (e.g. higher
loss rate) or even best effort at lower cost. Research shows that the total
transmission cost can be highly reduced using layered transmission (see
for details).
Using IntServ with IPv4, there are several problems that can only be solved
with high management effort:
No Transport Layer Protocol Independence:
It is necessary to mark the packets within the
layer 4 protocol header. For example, the TCP, UDP or SCTP port numbers can be
used to mark flows (with limitations, see below). But for new protocols
(e.g. experimental, new standards, proprietary), software updates for *all*
IntServ routers are necessary to recognize the packet flow!
No Support for Network Layer Encryption: Since it is necessary to
read fields of the layer 4 protocol header, it may not be encrypted. Therefore,
e.g. the usage of IPsec is impossible.
Support for Fragmentation: Only the first fragment of a large packet contains
the layer 4 header necessary to map the packet to a flow. Mapping other fragments
would require the hops to remember packet identities and try to map fragments to
packet identities. Due to the management effort and memory requirements, this is
not realistic for high-bandwidth backbone routers; especially when packet
reordering must be considered. Furthermore, load sharing or traffic distribution
would be impossible.
No Flow Sharing: It is usually impossible for two different communication
associations to share the same flow, e.g. if TCP flows are recognized using
port numbers. This makes it necessary to reserve an IntServ flow for each
communication association. This implies an increased number of flow states
for routers to keep and maintain. Furthermore, if one association
temporarily uses a lower bandwidth, the free bandwidth of its flow cannot
easily be borrowed to another association.
No Multi-Flow Connections: To use layered transmission, e.g. a video via
UDP, the transmission of every layer would require own port numbers. In the
case of connection-oriented transmission protocols (e.g. TCP, SCTP), every
layer would even require its own connection setup and management. Depending
on the transport protocol, the number of communication associations and the
number of flows, much more work is necessary compared to IPv6 using flow
labels.
All in all, using IntServ flows with IPv4 requires much more work compared
to IPv6, where simply the flow label can be used. It is therefore useful to
add such a field to IPv4, too. An appropriate place to add such a field is
an IPv4 option header.
IPv4 (see ) already defines an option header for a
16-bit SATNET stream identifier. Since this identifier would be incompatible
to the 20-bit IPv6 flow label, reuse of this existing option header is
inappropriate. Therefore, a new one is defined as follows.
Type: 143Length: 8 octetsFlow Label: The 20-bit flow label. All definitions of
and for the
IPv6 flow label are also valid for this field. A value of zero denotes that
no flow label is used. In this case, the flow label option is in fact
unnecessary.
The Flow Label option SHOULD be copied on fragmentation. It MUST be the first
option of the IP header and therefore MUST NOT appear more than once per IPv4
packet. The Router Alert option SHOULD NOT be used to mark the necessity for
routers to examine the options. Placing the Flow Label option as first
option allows for easy processing in hardware.
Since the new IPv4 flow label is fully compatible to the IPv6 flow label,
the field MAY be translated in the other protocol's one during protocol
translation. That is, a router can translate an IPv6 packet set from an
IPv6-only host to an IPv4-mapped address of an IPv4-only host and the flow
label may simply be copied. The same may also be applied in the backwards
direction.
Note, that copying the flow label during protocol translation is not
mandatory. There may be IntServ reservation reasons for not copying but
setting the flow label to zero. But a router MUST NOT set the flow label to
another value than the copy or 0, since the source is responsible to ensure
that the source address combined with the flow label is network-unique
Security considerations are similar to the IPv6 flow label, see .This document introduces no additional considerations for IANA.