Broadcom NetXtreme BCM57 Series, User Manual

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Broadcom Gigabit Ethernet Teaming Services NetXtreme User Guide
Broadcom
®
April 2017 • 2CS57XX-CDUM514-R Page 114
Outbound Traffic Flow
The Broadcom Intermediate Driver manages the outbound traffic flow for all teaming modes. For outbound
traffic, every packet is first classified into a flow, and then distributed to the selected physical adapter for
transmission. The flow classification involves an efficient hash computation over known protocol fields. The
resulting hash value is used to index into an Outbound Flow Hash Table.The selected Outbound Flow Hash
Entry contains the index of the selected physical adapter responsible for transmitting this flow. The source MAC
address of the packets will then be modified to the MAC address of the selected physical adapter. The modified
packet is then passed to the selected physical adapter for transmission.
The outbound TCP and UDP packets are classified using Layer 3 and Layer 4 header information. This scheme
improves the load distributions for popular Internet protocol services using well-known ports such as HTTP and
FTP. Therefore, BASP performs load balancing on a TCP session basis and not on a packet-by-packet basis.
In the Outbound Flow Hash Entries, statistics counters are also updated after classification. The load-balancing
engine uses these counters to periodically distribute the flows across teamed ports. The outbound code path
has been designed to achieve best possible concurrency where multiple concurrent accesses to the Outbound
Flow Hash Table are allowed.
For protocols other than TCP/IP, the first physical adapter will always be selected for outbound packets. The
exception is Address Resolution Protocol (ARP), which is handled differently to achieve inbound load balancing.
Inbound Traffic Flow (SLB Only)
The Broadcom intermediate driver manages the inbound traffic flow for the SLB teaming mode. Unlike outbound
load balancing, inbound load balancing can only be applied to IP addresses that are located in the same subnet
as the load-balancing server. Inbound load balancing exploits a unique characteristic of Address Resolution
Protocol (RFC0826), in which each IP host uses its own ARP cache to encapsulate the IP Datagram into an
Ethernet frame. BASP carefully manipulates the ARP response to direct each IP host to send the inbound IP
packet to the desired physical adapter. Therefore, inbound load balancing is a plan-ahead scheme based on
statistical history of the inbound flows. New connections from a client to the server will always occur over the
primary physical adapter (because the ARP Reply generated by the operating system protocol stack will always
associate the logical IP address with the MAC address of the primary physical adapter).
Like the outbound case, there is an Inbound Flow Head Hash Table. Each entry inside this table has a singly
linked list and each link (Inbound Flow Entries) represents an IP host located in the same subnet.
When an inbound IP Datagram arrives, the appropriate Inbound Flow Head Entry is located by hashing the
source IP address of the IP Datagram. Two statistics counters stored in the selected entry are also updated.
These counters are used in the same fashion as the outbound counters by the load-balancing engine
periodically to reassign the flows to the physical adapter.
On the inbound code path, the Inbound Flow Head Hash Table is also designed to allow concurrent access. The
link lists of Inbound Flow Entries are only referenced in the event of processing ARP packets and the periodic
load balancing. There is no per packet reference to the Inbound Flow Entries. Even though the link lists are not
bounded; the overhead in processing each non-ARP packet is always a constant. The processing of ARP
packets, both inbound and outbound, however, depends on the number of links inside the corresponding link list.
On the inbound processing path, filtering is also employed to prevent broadcast packets from looping back
through the system from other physical adapters.