FRF.16.1 defines a MLFR Bundle Link Integrity Protocol which verifies the serviceability of a member link. If a problem is found on the member link the link integrity protocol identifies the problem, flags the link as unusable, and adjusts the Bundle’s available bandwidth. For MLFR Bundles the link integrity protocol is always enabled.
For each member link of a bundle the link integrity protocol does the following:
Confirms frame processing capabilities of each member link.
Verifies membership of a link to a specific remote bundle.
Reports to the remote end of the member link the bundle to which the link belongs
Detects loopbacks on the member link. This is always enabled on the 7750 SR. The near-end monitors the magic number Information Element (IE) sent by the far-end and if its value matches the one it transmitted in ten consecutive control messages, it sends a remove_link message to the far-end and brings the link down. The near-end attempts to add the link until it succeeds.
Estimate propagation delay on the member link. The differential delay is calculated as follows in the 7750 SR implementation. Every time the near-end sends an add_link or Hello message to the far-end, it includes the Timestamp Information Element (IE) with the local time the packet was sent. FRF16.1 standard requires that the remote equipment includes the timestamp IE and copies the received timestamp value unchanged if the sender included this IE. When the far-end node sends back the ACK for these messages, the near-end calculates the round trip time. The 7750 SR implementation maintains a history of the last ‟N” round-trip-times that were received. It takes the fastest of these samples for each member link to find out the member link with the fastest RTT. Then for each link it calculates the difference between the fastest links RTT, and the RTT for the current link. The user has the option to coordinate link removal between the local and remote equipment. Note, however, that in the SR 7750 implementation, the addition of a link is hitless but the removing a link is not.
Specifically, the MLFR Bundle Link Integrity Protocol defines the following control messages:
ADD_LINK
ADD_LINK_ACK
ADD_LINK_REJ
HELLO
HELLO_ACK
REMOVE_LINK
REMOVE_LINK_ACK
The control messages are encapsulated in a single-fragment frame where the C-bit, the B-bit, and the E-bit are all set. The details of the message format are given in FRF.16.1. Table 1 lists the user configured control parameters with values as specified in FRF.16.1.
| Parameter | Default value | Minimum value | Maximum value |
|---|---|---|---|
Timer T_HELLO |
10 seconds |
1 second |
180 seconds |
Timer T_ACK |
4 seconds |
1 second |
10 |
Count N_MAX_RETRY |
2 |
1 |
5 |
T_HELLO Timer - this timer controls the rate at which hello messages are sent. Following a period of T_HELLO duration, a HELLO message is transmitted onto the Bundle Link.
Note that T_HELLO Timer is also used, during the Bundle Link adding process, as an additional delay before re-sending an ADD_LINK message to the peer Bundle Link when this peer Bundle Link does not answer as expected.
T_ACK Timer - this timer defines the maximum period to wait for a response to any message sent onto the Bundle Link before attempting to retransmit a message onto the Bundle Link.
N_RETRY - this counter specifies the number of times a retransmission onto a Bundle Link is attempted before an error is declared and the appropriate action taken.