An IPv4 Flowlabel Option

A Flow Label Option for IPv4

Motivation This section describes the motivation to add a flow label option to the IPv4 protocol.

The Flow Label Field of IPv6 The Flow Label field (see and ) of the IPv6 header (see ) is a 20-bit 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 reservation states. 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 distant 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 an 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, a possibly expensive reservation may be fully utilized. Flow sharing also helps to keep 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 its packets into 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 high-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 reduced significantly by applying layered transmission (see , for details).

The Limitations of IntServ via IPv4 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 intermediate routers 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 by another association.
No Multi-Flow Connections: To use layered transmission, e.g. a video via UDP, the transmission of every layer would require its 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.

Definition of the Flow Label Option IPv4 (see ) already defines an option header for a 16-bit SATNET stream identifier. Since this identifier would be incompatible with the 20-bit IPv6 flow label, reuse of this existing option header is inappropriate. Therefore, a new one is defined as follows:

Type: 143
Length: 8 octets
Flow 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. See also . 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 the first option allows for easy processing in hardware.