HP StorageWorks 12000 - Virtual Library System EVA Gateway Manual page 91

• Resilience dependent on business criticality of replication. Even with the most basic package
(single line unprotected) most Telcos will guarantee 99.9% availability. Above this you can opt
for master and standby paths (only one active at a time) that link to a single NTE (Network Termin-
ation Equipment) at your premises. If one path fails YOU are responsible for moving your equipment
over to the standby path. Finally you can have 2 x NTE at your site with Master and Standby
paths and fail-over can be automated.
• Distance inside a single country for example UK, there is generally no additional charge for
distance (unless very high bandwidth requirements > 1 Gb/sec) but in large countries where a
LATA (Local Access and transport and area) boundary is crossed then additional costs (to use
different Telcos lines in different states) to establish a point to point connection will be incurred.
Inter country or Intercontinental distances require a more consultative engagement with your Telco
Provider.
• Contention/Uncontended Contended links are those shared with other users and bandwidth
cannot always be guaranteed. Even some business services have a certain amount of contention.
For example, BT Etherflow Standard Class of Service offers 80% bandwidth guaranteed with 20%
contented. Whereas BT Ethernet Premium Class of Service allows 100% uncontented bandwidth.
HP recommend an 80% uncontended or above link for D2D replication.
• What is not acceptable is to try and run an HP D2D Replication solution on a Home broadband
type of link where contention rates of 20:1 or 50:1 can be present. Upgrade to business broadband
Class of Service. This is a way of prioritizing traffic on the Telco backbone. Voice over IP for ex-
ample requires a high priority to maintain coherence. Replication is not considered business crit-
ical in this respect and so typically default class of service is acceptable. Above default class there
will be a premium class (Assured Forwarding) where data queues in an MPLS environment the
data are marked with an identifier to allow it to have some level of priority within the Telco's in-
frastructure. Finally data such as VOIP is given a strict priority queue identifier so it always takes
precedence; this is sometimes known as expedited forwarding. Class of Service may also have
an effect on the Latency (see below) of your environment.
• Security. Another reason for choosing an uncontented service is that your replication data is not
mixed with other users. Today's Ethernet connections typically take the form of a VPN (Virtual
private network) so an element of securing exists within the transfer protocol already. A further
enhancement if security is a real issue should be to consider implementing IPSEC (Internet Protocol
security). HP D2D will support this in future releases. The addition of IPSEC security however may
reduce overall replication throughput because of the increased overhead involved in encryption.
• Latencies. These are delays introduced in sending data over a distance or delays involved in
routers, etc., used to direct the data paths. A round trip delay (RTD) in milliseconds is a measure
used to indicate latency.
The longer the latency the longer the replication can take, because the HP D2D will only put so
much information on the line before waiting for a response from the target device. HP has tested
D2D with latencies up to 280 ms with no issues.
A Typical Telco example, British Telecom provides a Core Network performance SLA on Round
Trip Delay, this may vary with class of service. For example:
• Expedite Forwarding 20 ms (RTD)
• Assured Forwarding 23 ms (RTD)
• Default 30 ms (RTD)
• Packet Delivery: This is a measure of the packet loss due to noise or transients, packets are retrans-
mitted which again may slow replication down if the link is particularly bad, but most Telcos offer
an SLA on packet delivery of above 99.8%.
• Most Telcos using the latest Networking technology such as MPLS deliver WAN links via a "cloud"
of VPN connections as if their core infrastructure was a massive router.
HP StorageWorks VLS and D2D Solutions Guide 91