RFC2892 - The Cisco SRP MAC Layer Protocol

Network Working Group D. Tsiang

Request for Comments: 2892 G. Suwala

Category: Informational Cisco Systems

August 2000

The Cisco SRP MAC Layer Protocol

Status of this Memo

This memo provides information for the Internet community. It does

not specify an Internet standard of any kind. Distribution of this

memo is unlimited.

Abstract

This document specifies the MAC layer protocol, "Spatial Reuse

Protocol" (SRP) for use with ring based media. This is a second

version of the protocol (V2).

The primary requirements for SRP are as follows:

- Efficient use of bandwidth using:

spatial reuse of bandwidth

local reuse of bandwidth

minimal protocol overhead

- Support for priority traffic

- Scalability across a large number of nodes or stations attached to

a ring

- "Plug and play" design without a software based station management

transfer (SMT) protocol or ring master negotiation as seen in

other ring based MAC protocols [1][2]

- Fairness among nodes using the ring

- Support for ring based redundancy (error detection, ring wrap,

etc.) similar to that found in SONET BLSR specifications.

- Independence of physical layer (layer 1) media type.

This document defines the terminology used with SRP, packet formats,

the protocol format, protocol operation and associated protocol

finite state machines.

Table of Contents

1. Differences between SRP V1 and V2 ....................... 3

2. Terms and Taxonomy ...................................... 4

2.1. Ring Terminology .................................. 4

2.2. Spatial Reuse ..................................... 5

2.3. Fairness .......................................... 6

2.4. Transit Buffer .................................... 7

3. SRP Overview ............................................ 8

3.1. Receive Operation Overview ........................ 8

3.2. Transmit Operation Overview ....................... 8

3.3. SRP Fairness Algorithm (SRP-fa) Overview .......... 9

3.4. Intelligent Protection Switching (IPS) Protocol

Overview .......................................... 9

4. Packet Formats .......................................... 13

4.1. Overall Packet Format ............................. 13

4.2. Generic Packet Header Format ...................... 14

4.2.1. Time To Live (TTL) ......................... 14

4.2.2. Ring Identifier (R) ........................ 15

4.2.3. Priority Field (PRI) ....................... 15

4.2.4. MODE ....................................... 15

4.2.5. Parity Bit (P-bit) ......................... 16

4.2.6. Destination Address ........................ 16

4.2.7. Source Address ............................. 16

4.2.8. Protocol Type .............................. 16

4.3. SRP Cell Format ................................... 16

4.4. SRP Usage Packet Format ........................... 17

4.5. SRP Control Packet Format ......................... 18

4.5.1. Control Ver ................................ 19

4.5.2. Control Type ............................... 19

4.5.3. Control TTL ................................ 19

4.5.4. Control Checksum ........................... 19

4.5.5. Payload .................................... 20

4.5.6. Addressing ................................. 20

4.6. Topology Discovery ................................ 20

4.6.1. Topology Length ............................ 22

4.6.2. Topology Originator ........................ 22

4.6.3. MAC bindings ............................... 22

4.6.4. MAC Type Format ............................ 22

4.7. Intelligent Protection Switching (IPS) ............ 23

4.7.1. Originator MAC Address ..................... 23

4.7.2. IPS Octet .................................. 24

4.8. Circulating packet detection (stripping) .......... 24

5. Packet acceptance and stripping ......................... 25

5.1. Transmission and forwarding with priority ......... 27

5.2. Wrapping of Data .................................. 28

6. SRP-fa Rules Of Operation ............................... 28

6.1. SRP-fa pseudo-code ................................ 30

6.2. Threshold settings ................................ 32

7. SRP Synchronization ..................................... 32

7.1. SRP Synchronization Examples ...................... 33

8. IPS Protocol Description ................................ 34

8.1. The IPS Request Types ............................. 35

8.2. SRP IPS Protocol States ........................... 36

8.2.1. Idle ....................................... 36

8.2.2. Pass-through ............................... 36

8.2.3. Wrapped .................................... 36

8.3. IPS Protocol Rules ................................ 36

8.3.1. SRP IPS Packet Transfer Mechanism .......... 36

8.3.2. SRP IPS Signaling and Wrapping Mechanism ... 37

8.4. SRP IPS Protocol Rules ............................ 38

8.5. State Transitions ................................. 41

8.6. Failure Examples .................................. 41

8.6.1. Signal Failure - Single Fiber Cut Scenario . 41

8.6.2. Signal Failure - Bidirectional Fiber Cut

Scenario ................................... 43

8.6.3. Failed Node Scenario ....................... 45

8.6.4. Bidirectional Fiber Cut and Node Addition

Scenarios .......................................... 47

9. SRP over SONET/SDH ...................................... 48

10. Pass-thru mode .......................................... 49

11. References .............................................. 50

12. Security Considerations ................................. 50

13. IPR Notice .. ........................................... 50

14. Acknowledgments ......................................... 50

15. Authors' Addresses ...................................... 51

16. Full Copyright Statement ................................ 52

1. Differences between SRP V1 and V2

This document pertains to SRP V2. SRP V1 was a previously published

draft specification. The following lists V2 feature differences from

V1:

- RedUCtion of the header format from 4 bytes to 2 bytes.

- Replacement of the keepalive packet with a new control packet that

carries usage information in addition to providing a keepalive

function.

- Change bit value of inner ring to be 1 and outer to be 0.

- Reduction in the number of TTL bits from 11 to 8.

- Removal of the DS bit.

- Change ordering of CRC transmission to be most significant octet

first (was least significant octet in V1). The SRP CRC is now the

same as in [5].

- Addition of the SRP cell mode to carry ATM cells over SRP.

- Changes to the SRP-fa to increase the usage field width and to

remove the necessity of adding a fixed constant when propagating

usage messages.

2. Terms and Taxonomy

2.1. Ring Terminology

SRP uses a bidirectional ring. This can be seen as two symmetric

counter-rotating rings. Most of the protocol finite state machines

(FSMs) are duplicated for the two rings.

The bidirectional ring allows for ring-wrapping in case of media or

station failure, as in FDDI [1] or SONET/SDH [3]. The wrapping is

controlled by the Intelligent Protection Switching (IPS) protocol.

To distinguish between the two rings, one is referred to as the

"inner" ring, the other the "outer" ring. The SRP protocol operates

by sending data traffic in one direction (known as "downstream") and

it's corresponding control information in the opposite direction

(known as "upstream") on the opposite ring. Figure 1 highlights this

graphically.

FIGURE 1. Ring Terminology

{outer_data

----- inner_ctl}

----------------> N -----------------

--------------- 1 <--------------

{inner_data -----

outer_ctl}

----- -----

N N

6 2

----- -----

^ ^

o i

u n

t n

e e

r r

v v

----- -----

N N

5 3

----- -----

-----

--------------> N ---------------

----------------- 4 <----------------

-----

2.2. Spatial Reuse

Spatial Reuse is a concept used in rings to increase the overall

aggregate bandwidth of the ring. This is possible because unicast

traffic is only passed along ring spans between source and

destination nodes rather than the whole ring as in earlier ring based

protocols such as token ring and FDDI.

Figure 2 below outlines how spatial reuse works. In this example,

node 1 is sending traffic to node 4, node 2 to node 3 and node 5 to

node 6. Having the destination node strip unicast data from the ring

allows other nodes on the ring who are downstream to have full Access

to the ring bandwidth. In the example given this means node 5 has

full bandwidth access to node 6 while other traffic is being

simultaneously transmitted on other parts of the ring.

2.3. Fairness

Since the ring is a shared media, some sort of access control is

necessary to ensure fairness and to bound latency. Access control can

be broken into two types which can operate in tandem:

Global access control - controls access so that everyone gets a

fair share of the global bandwidth of the ring.

Local access control - grants additional access beyond that

allocated globally to take advantage of segments of the ring that

are less than fully utilized.

As an example of a case where both global and local access are

required, refer again to Figure 2. Nodes 1, 2, and 5 will get 1/2 of

the bandwidth on a global allocation basis. But from a local

perspective, node 5 should be able to get all of the bandwidth since

its bandwidth does not interfere with the fair shares of nodes 1 and

FIGURE 2. Global and Local Re-Use

. . . . . . . . . . . . . . . . .

. .

----- .

----------------> N ----------------- .

--------------- 1 <-------------- .

----- .

----- ----- .

. .> N N . .. .

. 6 2 . .

. ----- ----- . .

. ^ ^ . .

. o i . .

. u n . .

. t n . .

. e e . .

. r r . .

. v v . .

. ----- ----- . .

. . N N <. . .

5 3 .

----- ----- .

----- .

--------------> N --------------- .

----------------- 4 <---------------- .

----- .

^ .

. .

. . . . .<. . . . . . . . . . . .

2.4. Transit Buffer

To be able to detect when to transmit and receive packets from the

ring, SRP makes use of a transit (sometimes referred as insertion)

buffer as shown in Figure 3 below. High priority packets and low

priority packets can be placed into separate fifo queues.

FIGURE 3. Transit buffer

---- ----

----Rx ----Tx

Buffer Buffer

---- ----

^^ Transit

Buffer

------ vv

===========>------==========>

------

3. SRP Overview

3.1. Receive Operation Overview

Receive Packets entering a node are copied to the receive buffer if a

Destination Address (DA) match is made. If a DA matched packet is

also a unicast, then the packet will be stripped. If a packet does

not DA match or is a multicast and the packet does not Source Address

(SA) match, then the packet is placed into the Transit Buffer (TB)

for forwarding to the next node if the packet passes Time To Live and

Cyclic Redundancy Check (CRC) tests.

3.2. Transmit Operation Overview

Data sent from the node is either forwarded data from the TB or

transmit data originating from the node via the Tx Buffer. High

priority forwarded data always gets sent first. High priority

transmit data may be sent as long as the Low Priority Transit Buffer

(LPTB) is not full.

A set of usage counters monitor the rate at which low priority

transmit data and forwarded data are sent. Low priority data may be

sent as long as the usage counter does not exceed an allowed usage

governed by the SRP-fa rules and the LPTB has not exceeded the low

priority threshold.

3.3. SRP Fairness Algorithm (SRP-fa) Overview

If a node eXPeriences congestion, then it will advertise to upstream

nodes via the opposite ring the value of its transmit usage counter.

The usage counter is run through a low pass filter function to

stabilize the feedback. Upstream nodes will adjust their transmit

rates so as not to exceed the advertised values. Nodes also

propagate the advertised value received to their immediate upstream

neighbor. Nodes receiving advertised values who are also congested

propagate the minimum of their transmit usage and the advertised

usage.

Congestion is detected when the depth of the low priority transit

buffer reaches a congestion threshold.

Usage messages are generated periodically and also act as keepalives

informing the upstream station that a valid data link exists.

3.4. Intelligent Protection Switching (IPS) Protocol Overview

An SRP Ring is composed of two counter-rotating, single fiber rings.

If an equipment or fiber facility failure is detected, traffic going

towards and from the failure direction is wrapped (looped) back to go

in the opposite direction on the other ring (subject to the

protection hierarchy). The wrap around takes place on the nodes

adjacent to the failure, under control of the IPS protocol. The wrap

re-routes the traffic away from the failed span.

An example of the data paths taken before and after a wrap are shown

in Figures 4 and 5. Before the fiber cut, N4 sends to N1 via the

path N4->N5->N6->N1.

If there is a fiber cut between N5 and N6, N5 and N6 will wrap the

inner ring to the outer ring. After the wraps have been set up,

traffic from N4 to N1 initially goes through the non-optimal path

N4->N5->N4->N3->N2->N1->N6->N1.

Subsequently a new ring topology is discovered and a new optimal path

is used N4->N3->N2-N1 as shown in Figure 6. Note that the topology

discovery and the subsequent optimal path selection are not part of

the IPS protocol.

FIGURE 4. Data path before wrap, N4 -> N1

-----

################> N -----------------

# --------------- 1 <--------------

# -----

----- -----

N N

6 2

----- -----

^ ^

# v v

----- -----

N N

5 3

----- -----

# -----

# --------------> N ---------------

################# 4 <----------------

-----

The ring wrap is controlled through SONET BLSR [3][4] style IPS

signaling. It is an objective to perform the wrapping as fast as in

the SONET equipment or faster.

The IPS protocol processes the following request types (in the order

of priority, from highest to lowest):

1. Forced Switch (FS): operator originated, performs a protection

switch on a requested span (wraps at both ends of the span)

2. Signal Fail (SF): automatic, caused by a media Signal Failure

or SRP keep-alive failure - performs a protection switch on a

requested span

FIGURE 5. Data path after the wrap, N4 -> N1

-----

################> N -----------------

# ############### 1 <##############

# # ----- #

# v #

----- -----

N N

6 2

----- -----

^ # wrap ^

### #

_________ #

fiber cut #

--------- #

### #

# v wrap # v

----- -----

N N

5 3

----- -----

# # #

# # ----- #

# ##############> N ###############

################# 4 <----------------

3. Signal Degrade (SD): automatic, caused by a media Signal

Degrade (e.g. excessive Bit Error Rate) - performs a protection

switch on a requested span

4. Manual Switch (MS): operator originated, like Forced Switched

but of a lower priority

5. Wait to Restore (WTR): automatic, entered after the working

channel meets the restoration criteria after SF or SD condition

disappears. IPS waits WTR period before restoring traffic in

order to prevent protection switch oscillations

If a protection (either automatic or operator originated) is

requested for a given span, the node on which the protection has been

requested issues a protection request to the node on the other end of

the span using both the short path (over the failed span, as the

failure may be unidirectional) and the long path (around the ring).

FIGURE 6. Data path after the new topology is discovered

-----

----------------- N -----------------

--------------- 1 <##############

----- #

v #

----- -----

N N

6 2

----- -----

^ wrap ^

-- #

_________ #

fiber cut #

--------- #

-- #

v wrap # v

----- -----

N N

5 3

----- -----

----- #

--------------> N ###############

----------------- 4 <----------------

-----

As the protection requests travel around the ring, the protection

hierarchy is applied. If the requested protection switch is of the

highest priority e.g. Signal Fail request is of higher priority than

the Signal Degrade than this protection switch takes place and the

lower priority switches elsewhere in the ring are taken down, as

appropriate. If a lower priority request is requested, it is not

allowed if a higher priority request is present in the ring. The only

exception is multiple SF and FS switches, which can coexist in the

ring.

All protection switches are performed bidirectionally (wraps at both

ends of a span for both transmit and receive directions, even if a

failure is only unidirectional).

4. Packet Formats

This section describes the packet formats used by SRP. Packets can be

sent over any point to point link layer (e.g. SONET/SDH, ATM, point

to point ETHERNET connections). The maximum transfer unit (MTU) is

9216 octets. The minimum transfer unit for data packets is 55

octets. The maximum limit was designed to accommodate the large IP

MTUs of IP over AAL5. SRP also supports ATM cells. ATM cells over

SRP are 55 octets. The minimum limit corresponds to ATM cells

transported over SRP. The minimum limit does not apply to control

packets which may be smaller.

These limits include everything listed in Figure 7: but are exclusive

of the frame delineation (e.g. for SRP over SONET/SDH, the flags used

for frame delineation are not included in the size limits).

The following packet and cell formats do not include any layer 1

frame delineation. For SRP over POS, there will be an additional

flag that delineates start and end of frame.

4.1. Overall Packet Format

The overall packet format is show below in Figure 7:

FIGURE 7. Overall Packet Format

---------------------------------

SRP Header

---------------------------------

Dest. Addr.

---------------------------------

Source Addr.

---------------------------------

Protocol Type

---------------------------------

Payload

---------------------------------

FCS

---------------------------------

The frame check sequence (FCS) is a 32-bit cyclic redundancy check

(CRC) as specified in RFC-1662 and is the same CRC as used in Packet

Over SONET (POS - specified in RFC-2615). The generator polynomial

is:

CRC-32:

x32 + x26 + x23 + x22 + x16 + x12 + x11 + x10 + x8 + x7 + x5 + x4 +

x2 + x + 1

The FCS is computed over the destination address, source address,

protocol type and payload. It does not include the SRP header.

Note that the packet format after the SRP header is identical to

Ethernet Version 2.

4.2. Generic Packet Header Format

Each packet has a fixed-sized header. The packet header format is

shown in Figure 8.

FIGURE 8. Detailed Packet Header Format

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Time to Live R MOD PRI P

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Destination Address

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ Source Address +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Protocol Type

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ +

Payload

. .

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The fields are described below.

4.2.1. Time To Live (TTL)

This 8 bit field is a hop-count that must be decremented every time a

node forwards a packet. If the TTL reaches zero it is stripped off

the ring. This allows for a total node space of 256 nodes on a ring.

However, due to certain failure conditions (e.g. when the ring is

wrapped) the total number of nodes that are supported by SRP is 128.

When a packet is first sent onto the ring the TTL should be set to at

least twice the total number of nodes on the ring.

4.2.2. Ring Identifier (R)

This single bit field is used to identify which ring this packet is

designated for. The designation is as follows:

TABLE 1. Ring Indicator Values

Outer Ring 0

Inner Ring 1

4.2.3. Priority Field (PRI)

This three bit field indicates the priority level of the SRP packet

(0 through 7). The higher the value the higher the priority. Since

there are only two queues in the transit buffer (HPTB and LPTB) a

packet is treated as either low or high priority once it is on the

ring. Each node determines the threshold value for determining what

is considered a high priority packet and what is considered a low

priority packet. However, the full 8 levels of priority in the SRP

header can be used prior to transmission onto the ring (transmit

queues) as well as after reception from the ring (receive queues).

4.2.4. MODE

This three bit field is used to identify the mode of the packet. The

following modes are defined in Table 2 below.

TABLE 2. MODE Values

Value Description

000 Reserved

001 Reserved

010 Reserved

011 ATM cell

100 Control Message (Pass to host)

101 Control Message (Locally Buffered for host)

110 Usage Message

111 Packet Data

These modes will be further explained in later sections.

4.2.5. Parity Bit (P-bit)

The parity bit is used to indicate the parity value over the 15 bits

of the SRP header to provide additional data integrity over the

header. Odd parity is used (i.e. the number of ones including the

parity bit shall be an odd number).

4.2.6. Destination Address

The destination address is a globally unique 48 bit address assigned

by the IEEE.

4.2.7. Source Address

The source address is a globally unique 48 bit address assigned by

the IEEE.

4.2.8. Protocol Type

The protocol type is a two octet field like that used in EtherType

representation. Current defined values relevant to SRP are defined in

Table 3 below.

TABLE 3. Defined Protocol Types

Value Protocol Type

0x2007 SRP Control

0x0800 IP version 4

0x0806 ARP

4.3. SRP Cell Format

SRP also supports the sending of ATM cells. The detailed cell format

is shown below:

FIGURE 9. SRP Cell Format

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Time to Live R MOD PRI P VPI/VCI

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

VCI PTI C HEC

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +

. .

. ATM Payload .

. ( 48 Bytes ) +-+-+-+-+-+-+-+-+

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Packet nodes would typically ignore (never receive or strip) and

always forward ATM-cells. The idea is that ATM switches and routers

could coexist in a ring. Note that SRP cells do not contain an FCS.

Data integrity is handled at the AAL layer.

4.4. SRP Usage Packet Format

SRP usage packets are sent out periodically to propagate allowed

usage information to upstream nodes. SRP usage packets also perform

a keepalive function. SRP usage packets should be sent approximately

every 106 usec.

If a receive interface has not seen a usage packet within the

keepalive timeout interval it will trigger an L2 keepalive timeout

interrupt/event. The IPS software will subsequently mark that

interface as faulty and initiate a protection switch around that

interface. The keepalive timeout interval should be set to 16 times

the SRP usage packet transmission interval.

FIGURE 10. Usage Packet Format

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Time to Live R MOD PRI P

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Originator MAC Address +

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Reserved Usage

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

A USAGE of all ones indicates a value of NULL.

4.5. SRP Control Packet Format

If the MODE bits are set to 10X (SRP control) then this indicates a

control message. Control messages are always received and stripped by

the adjacent node. They are by definition unicast, and do not need

any addressing information. The destination address field for

control packets should be set to 0's. The source address field for a

control packet should be set to the source address of the

transmitting node.

Two types of controls messages are defined : Pass to host and Locally

buffered. Pass to host messages can be passed to the host software by

whatever means is convenient. This is most often the same path used

to transfer data packets to the host. Locally buffered control

messages are usually reserved for protection messages. These are

normally buffered locally in order to not contend for resources with

data packets. The actual method of handling these messages is up to

the implementor.

The control packet format is shown in Figure 11.

FIGURE 11. Control Packet Format

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Time to Live R MOD PRI P

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Destination Address

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ Source Address +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Protocol Type = 0x2007

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Control Ver Control Type Control Checksum

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Control TTL

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +

. .

. Payload .

. .

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The priority (PRI) value should be set to 0x7 (all one's) when

sending control packets and should be queued to the highest priority

transmit queue available. The Time to Live is not relevant since all

packets will be received and stripped by the nearest downstream

neighbor and can be set to any value (preferably this should be set

to 001).

4.5.1. Control Ver

This one octet field is the version number associated with the

control type field. Initially, all control types will be version 0.

4.5.2. Control Type

This one octet field represents the control message type. Table 4

contains the currently defined control types.

TABLE 4. Control Types

Control Type Description

0x01 Topology Discovery

0x02 IPS message

0x03-

0xFF Reserved

4.5.3. Control TTL

The Control TTL is a control layer hop-count that must be decremented

every time a node forwards a control packet. If a node receives a

control packet with a control TTL <= 1, then it should accept the

packet but not forward it.

Note that the control layer hop count is separate from the SRP L2 TTL

which is always set to 1 for control messages.

The originator of the control message should set the initial value of

the control TTL to the SRP L2 TTL normally used for data packets.

4.5.4. Control Checksum

The checksum field is the 16 bit one's complement of the one's

complement sum of all 16 bit Words starting with the control version.

If there are an odd number of octets to be checksummed, the last

octet is padded on the right with zeros to form a 16 bit word for

checksum purposes. The pad is not transmitted as part of the

segment. While computing the checksum, the checksum field itself is

replaced with zeros. This is the same checksum algorithm as that

used for TCP. The checksum does not cover the 32 bit SRP FCS.

4.5.5. Payload

The payload is a variable length field dependent on the control type.

4.5.6. Addressing

All nodes must have a globally unique IEEE 48 bit MAC address. A

multicast bit is defined using canonical addressing conventions i.e.

the multicast bit is the least significant bit of the most

significant octet in the destination address. It is acceptable but

not advisable to change a node's MAC address to one that is known to

be unique within the administrative layer 2 domain (that is the SRP

ring itself along with any networks connected to the SRP ring via a

layer 2 transparent bridge).

FIGURE 12. Multicast bit position

Destination Address

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

----Multicast bit

Note that for SONET media, the network order is MSB of each octet

first, so that as viewed on the line, the multicast bit will be the

8th bit of the destination address sent. (For SRP on Ethernet media,

the multicast bit would be sent first).

4.6. Topology Discovery

Each node performs topology discovery by sending out topology

discovery packets on one or both rings. The node originating a

topology packet marks the packet with the egressing ring id, appends

the node's mac binding to the packet and sets the length field in the

packet before sending out the packet. This packet is a point-to-point

packet which hops around the ring from node to node. Each node

appends its mac address binding, updates the length field and sends

it to the next hop on the ring. If there is a wrap on the ring, the

wrapped node will indicate a wrap when appending its mac binding and

wrap the packet. When the topology packets travel on the wrapped

section with the ring identifier being different from that of the

topology packet itself, the mac address bindings are not added to the

packet.

Eventually the node that generated the topology discovery packet gets

back the packet. The node makes sure that the packet has the same

ingress and egress ring id before excepting the packet. A topology

map is changed only after receiving two topology packets which

indicate the same new topology (to prevent topology changes on

transient conditions).

Note that the topology map only contains the reachable nodes. It does

not correspond to the failure-free ring in case of wraps and ring

segmentations.

FIGURE 13. Topology Packet Format

Topology

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Time to Live R MOD PRI P

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Destination Address

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ Source Address +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Protocol Type = 0x2007

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Control Ver=0 Control Type=1 Control Checksum

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Control TTL Topology Length

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Originator's Globally Unique

+ MAC Address (48 bits) +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

MAC Type

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +

MAC Address (48 bits)

+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Other MAC bindings

+-+-+-+-+-+-+-+-+ +

+ +

Note that the Source address should be set to the source address of

the TRANSMITTING node (which is not necessarily the ORIGINATING

node).

4.6.1. Topology Length

This two octet field represents the length of the topology message in

octets starting with the first MAC Type/MAC Address binding.

4.6.2. Topology Originator

A topology discovery packet is determined to have been originated by

a node if the originator's globally unique MAC address of the packet

is that node's globally unique MAC address (assigned by the IEEE).

Because the mac addresses could be changed at a node, the IEEE MAC

address ensures that a unique identifier is used to determine that

the topology packet has gone around the ring and is to be consumed.

4.6.3. MAC bindings

Each MAC binding shall consist of a MAC Type field followed by the

node's 48 bit MAC address. The first MAC binding shall be the MAC

binding of the originator. Usually the originator's MAC address will

be it's globally unique MAC Address but some implementations may

allow this value to be overridden by the network administrator.

4.6.4. MAC Type Format

This 8 bit field is encoded as follows:

TABLE 5. MAC Type Format

Bit Value

0 Reserved

1 Ring ID (1 or 0)

2 Wrapped Node (1) / Unwrapped Node (0)

3-7 Reserved

Determination of whether a packet's egress and ingress ring ID's are

a match should be done by using the Ring ID found in the MAC Type

field of the last MAC binding as the ingress ring ID rather than the

R bit found in the SRP header. Although they should be the same, it

is better to separate the two functions as some implementations may

not provide the SRP header to upper layer protocols.

The topology information is not required for the IPS protection

mechanism. This information can be used to calculate the number of

nodes in the ring as well as to calculate hop distances to nodes to

determine the shortest path to a node (since there are two counter-

rotating rings).

The implementation of the topology discovery mechanism could be a

periodic activity or on "a need to discover" basis. In the periodic

implementation, each node generates the topology packet periodically

and uses the cached topology map until it gets a new one. In the need

to discover implementation, each node generates a topology discovery

packet whenever they need one e.g., on first entering a ring or

detecting a wrap.

4.7. Intelligent Protection Switching (IPS)

IPS is a method for automatically recovering from various ring

failures and line degradation scenarios. The IPS packet format is

outlined in Figure 14 below.

FIGURE 14. IPS Packet Format

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Time to Live R MOD PRI P

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Destination Address

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

+ Source Address +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Protocol Type = 0x2007

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Control Ver=0 Control Type=2 Control Checksum

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Control TTL

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +

Originator MAC Address

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Ips Octet Rsvd Octet

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The IPS specific fields are detailed below.

4.7.1. Originator MAC Address

This is the MAC address of the originator of the IPS message. It is

not necessarily the same as the SRP Header Source Address as a node

may be simply propagating an IPS message (see the section "SRP IPS

Protocol Rules" Rule P.8 as an example).

4.7.2. IPS Octet

The IPS octet contains specific protection information. The format of

the IPS octet is as follows:

FIGURE 15. IPS Octet Format:

Bits Values (values not listed are reserved)

0-3 IPS Request Type

1101 - Forced Switch (FS)

1011 - Signal Fail (SF)

1000 - Signal Degrade (SD)

0110 - Manual Switch (MS)

0101 - Wait to Restore (WTR)

0000 - No Request (IDLE)

4 Path indicator

0 - short (S)

1 - long (L)

5-7 Status Code

010 - Protection Switch Completed - traffic Wrapped (W)

000 - Idle (I)

The currently defined request types with values, hierarchy and

interpretation are as used in SONET BLSR [3], [4], except as noted.

4.8. Circulating packet detection (stripping)

Packets continue to circulate when transmitted packets fail to get

stripped. Unicast packets are normally stripped by the destination

station or by the source station if the destination station has

failed. Multicast packets are only stripped by the source station. If

both the source and destination stations drop out of the ring while a

unicast packet is in flight, or if the source node drops out while

its multicast packet is in flight, the packet will rotate around the

ring continuously.

The solution to this problem is to have a TTL or Time To Live field

in each packet that is set to at least twice the number of nodes in

the ring. As each node forwards the packet, it decrements the TTL. If

the TTL reaches zero it is stripped off of the ring.

The ring ID is used to qualify all stripping and receive decisions.

This is necessary to handle the case where packets are being wrapped

by some node in the ring. The sending node may see its packet on the

reverse ring prior to reaching its destination so must not source

strip it. The exception is if a node is in wrap. Logically, a node

in wrap "sees" the packet on both rings. However the usual

implementation is to receive the packet on one ring and to transmit

it on the other ring. Therefore, a node that is in the wrap state

ignores the ring ID when making stripping and receiving decisions.

A potential optimization would be to allow ring ID independent

destination stripping of unicast packets. One problem with this is

that packets may be delivered out of order during a transition to a

wrap condition. For this reason, the ring ID should always be used as

a qualifier for all strip and receive decisions.

5. Packet acceptance and stripping

A series of decisions based on the type of packet (mode), source and

destination addresses are made on the MAC incoming packets. Packets

can either be control or data packets. Control packets are stripped

once the information is extracted. The source and destination

addresses are checked in the case of data packets. The rules for

reception and stripping are given below as well as in the flow chart

in Figure 16.

1. Decrement TTL on receipt of a packet, discard if it gets to

zero; do not forward.

2. Strip unicast packets at the destination station. Accept and

strip "control" packets.

3. Do not process packets other than for TTL and forwarding if

they have the "wrong" ring_id for the direction in which they

are received unless the node is in wrap. If the node is in

wrap then ignore the ring_id.

4. Do not process packets other than for TTL and forwarding if the

mode is not supported by the node (e.g. reserved modes, or ATM

cell mode for packet nodes).

5. Packets accepted by the host because of the destination address

should be discarded at the upper level if there is CRC error.

6. Control messages are point to point between neighbors and

should always be accepted and stripped.

7. Packets whose source address is that of the receiving station

and whose ring_id matches should be stripped. If a node is in

wrap then ignore the ring_id.

FIGURE 16. SRP Receive Flowchart (Packet node)

if (MODE == 4,5)-------------------------------->[to host]--->

if (MODE == 6)---------------------------------->[strip]----->

if (!WRAPPED

& WRONG_RING_ID)---------------------------------------------->

if (MODE == 0,1,2,3)--------------------------------------------->

if (DA MATCH)--------------->if !(SA MATCH)----->[to host]--->

if (unicast)------->[to host]--->

if (SA MATCH)-------------------->[strip]-------------------->

---------------------------><---------------------------

if (ttl < 2)------->[strip]----->

[decrement ttl]

[fwd pkt to tb]

<-----------------------

[back to top]

Notes: Host is responsible for discarding CRC errored packets.

Conditionals (if statements) branch to the right if true

and branch down if false.

5.1. Transmission and forwarding with priority

A node can transmit four types of packets:

1. High priority packets from the high priority transit

buffer.

2. Low priority packets from the low priority transit buffer.

3. High priority packets from the host Tx high priority fifo.

4. Low priority packets from the host Tx low priority fifo.

High priority packets from the transit buffer are always sent first.

High priority packets from the host are sent as long as the low

priority transit buffer is not full. Low priority packets are sent

as long as the transit buffer has not crossed the low priority

threshold and the SRP-fa rules allow it (my_usage < allowed_usage).

If nothing else can be sent, low priority packets from the low

priority transit buffer are sent.

This decision tree is shown in Figure 17.

FIGURE 17. SRP transmit flowchart

if (TB_High has pkt)----------->[send pkt from TB_high]-->

if (TB_Low full)---------------------------------------------->

if (Tx_High has pkt)----------->[send pkt from Tx_high]-->

if (TB_Low > Hi threshold)------------------------------------>

if (my_usage >= allowed_usage)-------------------------------->

if (Tx_Low has pkt)------------>[send pkt from Tx_low]--->

<-----------------------------------------------------

if (TB_Low has pkt)------------>[send pkt from TB_low]--->

<------------------------------------------------

[Go to Top]

Notes: Conditionals (if statements) branch to the right if true

and branch down if false.

5.2. Wrapping of Data

Normally, transmitted data is sent on the same ring to the downstream

neighbor. However, if a node is in the wrapped state, transmitted

data is sent on the opposite ring to the upstream neighbor.

6. SRP-fa Rules Of Operation

The SRP-fa governs access to the ring. The SRP-fa only applies to

low priority traffic. High priority traffic does not follow SRP-fa

rules and may be transmitted at any time as long as there is

sufficient transit buffer space.

The SRP-fa requires three counters which control the traffic

forwarded and sourced on the SRP ring. The counters are my_usage

(tracks the amount of traffic sourced on the ring), forward_rate

(amount of traffic forwarded on to the ring from sources other than

the host) and allowed_usage (the current maximum transmit usage for

that node).

With no congestion all nodes build up allowed usage periodically.

Each node can send up to max_usage. Max_usage is a per node

parameter than limits the maximum amount of low priority traffic a

node can send.

When a node sees congestion it starts to advertise its my_usage which

has been low pass filtered (lp_my_usage).

Congestion is measured by the transit buffer depth crossing a

congestion threshold.

A node that receives a non-null usage message (rcvd_usage) will set

its allowed usage to the value advertised. However, if the source of

the rcvd_usage is the same node that received it then the rcvd_usage

shall be treated as a null value. When comparing the rcvd_usage

source address the ring ID of the usage packet must match the

receiver's ring ID in order to qualify as a valid compare. The

exception is if the receive node is in the wrap state in which case

the usage packet's ring ID is ignored.

Nodes that are not congested and that receive a non-null rcvd_usage

generally propagate rcvd_usage to their upstream neighbor else

propagate a null value of usage (all 1's). The exception is when an

opportunity for local reuse is detected. Additional spatial reuse

(local reuse) is achieved by comparing the forwarded rate (low pass

filtered) to allow_usage. If the forwarded rate is less than the

allowed usage, then a null value is propagated to the upstream

neighbor.

Nodes that are congested propagate the smaller of lp_my_usage and

rcvd_usage.

Convergence is dependent upon number of nodes and distance.

Simulation has shown simulation convergence within 100 msec for rings

of several hundred miles.

6.1. SRP-fa pseudo-code

A more precise definition of the fairness algorithm is shown below:

Variables:

lo_tb_depth low priority transit buffer depth

my_usage count of octets transmitted by host

lp_my_usage my_usage run through a low pass filter

my_usage_ok flag indicating that host is allowed to transmit

allow_usage the fair amount each node is allowed to transmit

fwd_rate count of octets forwarded from upstream

lp_fwd_rate fwd_rate run through a low pass filter

congested node cannot transmit host traffic without the TB buffer

filling beyond its congestion threshold point.

rev_usage the usage value passed along to the upstream neighbor

Constants:

MAX_ALLOWANCE = configurable value for max allowed usage for this node

DECAY_INTERVAL = 8000 octet times @ OC-12, 32,000 octet times @ OC-48

AGECOEFF = 4 // Aging coeff for my_usage and fwd_rate

LP_FWD = 64 // Low pass filter for fwd_rate

LP_MU = 512 // Low pass filter for my usage

LP_ALLOW = 64 // Low pass filter for allow usage auto increment

NULL_RCVD_INFO = All 1's in rcvd_usage field

TB_LO_THRESHOLD // TB depth at which no more lo-prio host traffic

// can be sent

MAX_LRATE = AGECOEFF * DECAY_INTERVAL = 128,000 for OC-48, 32000 for

OC-12

THESE ARE UPDATED EVERY CLOCK CYCLE:

=====================================

my_usage is incremented by 1 for every octet that is

transmitted by the host (does not include data

transmitted from the Transit Buffer).

fwd_rate is incremented by 1 for every octet that enters the

Transit Buffer

if ((my_usage < allow_usage) &&

!((lo_tb_depth > 0) && (fwd_rate < my_usage)) &&

(my_usage < MAX_ALLOWANCE))

// true means OK to send host packets

my_usage_ok = true;

UPDATED WHEN USAGE_PKT IS RECEIVED:

===================================

if (usage_pkt.SA == my_SA) &&

[(usage_pkt.RI == my_RingID) (node_state == wrapped)]

rcvd_usage = NULL_RCVD_INFO;

else

rcvd_usage = usage_pkt.usage;

THE FOLLOWING IS CALCULATED EVERY DECAY_INTERVAL:

==================================================

congested = (lo_tb_depth > TB_LO_THRESHOLD/2)

lp_my_usage = ((LP_MU-1) * lp_my_usage + my_usage) / LP_MU

my_usage is decremented by min(allow_usage/AGECOEFF, my_usage/AGECOEFF)

lp_fwd_rate = ((LP_FWD-1) * lp_fwd_rate + fwd_rate) / LP_FWD

fwd_rate is decremented by fwd_rate/AGECOEFF

(Note: lp values must be calculated prior to decrement of non-lp

values).

if (rcvd_usage != NULL_RCVD_INFO)

allow_usage = rcvd_usage;

else

allow_usage += (MAX_LRATE - allow_usage) / (LP_ALLOW);

if (congested)

{

if (lp_my_usage < rcvd_usage)

rev_usage = lp_my_usage;

else

rev_usage = rcvd_usage;

}

else if ((rcvd_usage != NULL_RCVD_INFO) &&

(lp_fwd_rate > allow_usage)

rev_usage = rcvd_usage;

else

rev_usage = NULL_RCVD_INFO

if (rev_usage > MAX_LRATE)

rev_usage = NULL_RCVD_INFO;

6.2. Threshold settings

The low priority transit buffer (TB_LO_THRESHOLD) is currently sized

to about 4.4 msec or 320 KB at OC12 rates. The TB_HI_THRESHOLD is

set to about 870 usec higher than the TB_LO_THRESHOLD or at 458 KB at

OC12 rates.

The high priority transit buffer needs to hold 2 to 3 MTUs or about

30KB.

7. SRP Synchronization

Each node operates in "free-run" mode. That is, the receive clock is

derived from the incoming receive stream while the transmit clock is

derived from a local oscillator. This eliminates the need for

expensive clock synchronization as required in existing SONET

networks. Differences in clock frequency are accommodated by

inserting a small amount of idle bandwidth at each nodes output.

The clock source for the transmit clock shall be selected to deviate

by no more than 20 ppm from the center frequency. The overall

outgoing rate of the node shall be rate shaped to accommodate the

worst case difference between receive and transmit clocks of adjacent

nodes. This works as follows:

A transit buffer slip count (tb_cnt) keeps track of the amount of

octets inserted into the TB minus the amount of octets transmitted

and is a positive integer.

To account for a startup condition where a packet is being inserted

into an empty TB and the node was otherwise idle the tb_cnt is reset

if the transmit interface is idle. Idle is defined as no data being

sent even though there is opportunity to send (i.e. the transmit

interface is not prohibited from transmitting by the physical layer).

An interval counter defines the sample period over which rate shaping

is performed. This number should be sufficiently large to get an

accurate rate shaping.

A token_bucket counter implements the rate shaping and is a signed

integer. We increment this counter by one of two fixed values called

quantums each sample period. Quantum1 sets the rate at (Line_rate -

Delta) where delta is the clock inaccuracy we want to accommodate.

Quantum2 sets the rate at (Line_rate + Delta). If at the beginning

of a sample period, tb_cnt >= sync_threshold, then we set the rate to

Quantum2. This will allow us to catch up and causes the TB slip count

to eventually go < sync_threshold. If tb_cnt is < sync_threshold

then we set the rate to Quantum1.

When the input rate and output rates are exactly equal, the tb_cnt

will vary between sync_threshold > tb_cnt >= 0. This will vary for

each implementation dependent upon the burst latencies of the design.

The sync_threshold value should be set so that for equal transmit and

receive clock rates, the transmit data rate is always Line_rate-Delta

and will be implementation dependent.

The token_bucket is decremented each time data is transmitted. When

token_bucket reaches a value <= 0, a halt_transmit flag is asserted

which halts further transmission of data (halting occurs on a packet

boundary of course which can cause token_bucket to become a negative

number).

7.1. SRP Synchronization Examples

Assume an interval of 2^^18 or 262144 clock cycles. A Quantum1 value

must be picked such that the data rate will = (LINE_RATE - DELTA). A

Quantum2 value must be picked and used if the tb_cnt shows that the

incoming rate is greater than the outgoing rate and is = (LINE_RATE +

DELTA). Assume that the source of the incoming and outgoing rate

clocks are +/- 100 ppm.

For an OC12c SPE rate of 600 Mbps and a system clock rate of 800 Mbps

(16 bits @ 50 Mhz). The system clock rate is the rate at which the

system transmits bytes to the framer (in most cases the framer

transmit rate is asynchronous from the rate at which the system

transfers data to the framer).

Quantum1/Interval * 800 Mbps = 600 Mbps(1 - Delta)

Quantum1 = Interval * (600/800) * (1 - Delta)

Quantum1 = Interval * (600/800) * (1 - 1e-4) = 196588

Quantum2/Interval * 800 Mbps = 600 Mbps(1 + Delta)

Quantum2 = Interval * (600/800) * (1 + Delta)

Quantum2 = Interval * (600/800) * (1 + 1e-4) = 196628

Note: The actual data rate for OC-12c is 599.04 Mbps.

8. IPS Protocol Description

An SRP ring is composed of two counter-rotating, single fiber rings.

If an equipment or fiber facility failure is detected, traffic going

towards and from the failure direction is wrapped (looped) back to go

in the opposite direction on the other ring. The wrap around takes

place on the nodes adjacent to the failure, under software control.

This way the traffic is re-routed from the failed span.

Nodes communicate between themselves using IPS signaling on both

inner and outer ring.

The IPS octet contains specific protection information. The format of

the IPS octet is as follows:

FIGURE 18. IPS Octet format:

0-3 IPS Request Type

1101 - Forced Switch (FS)

1011 - Signal Fail (SF)

1000 - Signal Degrade (SD)

0110 - Manual Switch (MS)

0101 - Wait to Restore (WTR)

0000 - No Request (IDLE)

4 Path indicator

0 - short (S)

1 - long (L)

5-7 Status Code

010 - Protection Switch Completed -traffic Wrapped (W)

000 - Idle (I)

The IPS control messages are shown in this document as:

{REQUEST_TYPE, SOURCE_ADDRESS, WRAP_STATUS, PATH_INDICATOR}

8.1. The IPS Request Types

The following is a list of the request types, from the highest to the

lowest priority. All requests are signaled using IPS control

messages.

1. Forced Switch (FS - operator originated)

This command performs the ring switch from the working channel

to the protection, wrapping the traffic on the node at which

the command is issued and at the adjacent node to which the

command is destined. Used for example to add another node to

the ring in a controlled fashion.

2. Signal Fail (SF - automatic)

Protection caused by a media "hard failure" or SRP keep- alive

failure. SONET examples of SF triggers are: Loss of Signal

(LOS), Loss of Frame (LOF), Line Bit Error Rate (BER) above a

preselected SF threshold, Line Alarm Indication Signal (AIS).

Note that the SRP keep-alive failure provides end-to-end

coverage and as a result SONET Path triggers are not necessary.

3. Signal Degrade (SD - automatic)

Protection caused by a media "soft failure". SONET example of a

SD is Line BER or Path BER above a preselected SD threshold.

4. Manual Switch (MS - operator originated)

Like the FS, but of lower priority. Can be used for example to

take down the WTR.

5. Wait to Restore (WTR - automatic)

Entered after the working channel meets the restoration

threshold after an SD or SF condition disappears. IPS waits WTR

timeout before restoring traffic in order to prevent protection

switch oscillations.

8.2. SRP IPS Protocol States

Each node in the IPS protocol is in one of the following states for

each of the rings:

8.2.1. Idle

In this mode the node is ready to perform the protection switches and

it sends to both neighboring nodes "idle" IPS messages, which include

"self" in the source address field {IDLE, SELF, I, S}

8.2.2. Pass-through

Node participates in a protection switch by passing the wrapped

traffic and long path signaling through itself. This state is entered

based on received IPS messages. If a long path message with not null

request is received and if the node does not strip the message (see

Protocol Rules for stripping conditions) the node decrements the TTL

and retransmits the message without modification. Sending of the

Idle messages is stopped in the direction in which the message with

not null request is forwarded.

8.2.3. Wrapped

Node participates in a protection switch with a wrap present. This

state is entered based on a protection request issued locally or

based on received IPS messages.

8.3. IPS Protocol Rules

8.3.1. SRP IPS Packet Transfer Mechanism

R T.1:

IPS packets are transferred in a store and forward mode between

adjacent nodes (packets do not travel more than 1 hop between nodes

at a time). Received packet (payload portion) is passed to software

based on interrupts.

R T.2:

All IPS messages are sent to the neighboring nodes periodically on

both inner and outer rings. The timeout period is configurable 1-600

sec (default 1 sec). It is desirable (but not required) that the

timeout is automatically decreased by a factor of 10 for the short

path protection requests.

8.3.2. SRP IPS Signaling and Wrapping Mechanism

R S.1:

IPS signaling is performed using IPS control packets as defined in

Figure 14 "IPS Packet Format".

R S.2:

Node executing a local request signals the protection request on both

short (across the failed span) and long (around the ring) paths after

performing the wrap.

R S.3:

Node executing a short path protection request signals an idle

request with wrapped status on the short (across the failed span)

path and a protection request on the long (around the ring) path

after performing the wrap.

R S.4:

A node which is neither executing a local request nor executing a

short path request signals IDLE messages to its neighbors on the ring

if there is no long path message passing through the node on that

ring.

R S.5:

Protection IPS packets are never wrapped.

R S.6:

If the protocol calls for sending both short and long path requests

on the same span (for example if a node has all fibers disconnected),

only the short path request should be sent.

R S.7:

A node wraps and unwraps only on a local request or on a short path

request. A node never wraps or unwraps as a result of a long path

request. Long path requests are used only to maintain protection

hierarchy. (Since the long path requests do not trigger protection,

there is no need for destination addresses and no need for topology

maps)

In Figure 19, Node A detects SF (local request/ self-detected

request) on the span between Node A and Node B and starts sourcing

{SF, A, W, S} on the outer ring and {SF, A, W, L} on the inner ring.

Node B receives the protection request from Node A (short path

request) and starts sourcing {IDLE, B, W, S} on the inner ring and

{SF, B, W, L} on the outer ring.

FIGURE 19. SRP IPS Signaling

{SF,A,W,S}

-------------------------------

-----X---------------------

fiber

v cut {IDLE,B,W,S} v

----- -----

A B

----- -----

^ {SF,A,W,L} i ^ o {SF,B,W,L}

n u

n t

e e

v r v r

8.4. SRP IPS Protocol Rules

R P.1:

Protection Request Hierarchy is as follows (Highest priority to the

lowest priority). In general a higher priority request preempts a

lower priority request within the ring with exceptions noted as

rules. The 4 bit values below correspond to the REQUEST_TYPE field in

the IPS packet.

1101 - Forced Switch (FS)

1011 - Signal Fail (SF)

1000 - Signal Degrade (SD)

0110 - Manual Switch (MS)

0101 - Wait to Restore (WTR)

0000 - No Request (IDLE): Lowest priority

R P.2:

Requests >= SF can coexist.

R P.3:

Requests < SF can not coexist with other requests.

R P.4:

A node always honors the highest of {short path request, self

detected request} if there is no higher long path message passing

through the node.

R P.5:

When there are more requests of priority < SF, the first request to

complete long path signaling will take priority.

R P.6:

A Node never forwards an IPS packet received by it which was

originally generated by the node itself (it has the node's source

address).

R P.7:

Nodes never forward packets with the PATH_INDICATOR set to SHORT.

R P.8:

When a node receives a long path request and the request is >= to the

highest of {short path request, self detected request}, the node

checks the message to determine if the message is coming from its

neighbor on the short path. If that is the case then it does not

enter pass-thru and it strips the message.

R P.9:

When a node receives a long path request, it strips (terminates) the

request if it is a wrapped node with a request >= than that in the

request; otherwise it passes it through and unwraps.

R P.10:

Each node keeps track of the addresses of the immediate neighbors

(the neighbor node address is gleaned from the short path IPS

messages).

R P.11:

When a wrapped node (which initially detected the failure) discovers

disappearance of the failure, it enters WTR (user-configurable WTR

time-period). WTR can be configured in the 10-600 sec range with a

default value of 60 sec.

R P.12:

When a node is in WTR mode, and detects that the new neighbor (as

identified from the received short path IPS message) is not the same

as the old neighbor (stored at the time of wrap initiation), the node

drops the WTR.

R P.13:

When a node is in WTR mode and long path request Source is not equal

to the neighbor Id on the opposite side (as stored at the time of

wrap initiation), the node drops the WTR.

R P.14:

When a node receives a local protection request of type SD or SF and

it cannot be executed (according to protocol rules) it keeps the

request pending. (The request can be kept pending outside of the

protection protocol implementation).

R P.15:

If a local non-failure request (WTR, MS, FS) clears and if there are

no other requests pending, the node enters idle state.

R P.16:

If there are two failures and two resulting WTR conditions on a

single span, the second WTR to time out brings both the wraps down

(after the WTR time expires a node does not unwrap automatically but

waits till it receives idle messages from its neighbor on the

previously failed span)

R P.17:

If a short path FS request is present on a given side and a SF/SD

condition takes place on the same side, accept and process the SF/SD

condition ignoring the FS. Without this rule a single ended wrap

condition could take place. (Wrap on one end of a span only).

8.5. State Transitions

Figure 20 shows the simplified state transition diagram for the IPS

protocol:

FIGURE 20. Simplified State Transitions Diagram

Local FS,SF,SD,MS req.

--------- or Rx{REQ,SRC,W,S} from mate

IDLE -------------------------------------------

<----------------------------------------

--------- Local REQ clears

^ or Rx{IDLE,SRC,I,S}

Rx{IDLE,SRC,I,S} Rx{REQ,SRC,W,L}

v Local FS,SF,SD,MS REQ > Active req. v

--------- or Rx{REQ,SRC,W,S},REQ > Active req. ---------

PASS ------------------------------------> WRAPPED

THRU <------------------------------------

--------- ---------

Forwards Tx{REQ,SELF,W,S} for local REQ

{REQ,SRC,W,L} Tx{IDLE,SELF,W,S} for mate REQ

& Tx{REQ,SELF,W,L}

Legend: Mate = node on the other end of the affected span

REQ = {FS SF SD MS}

8.6. Failure Examples

8.6.1. Signal Failure - Single Fiber Cut Scenario

Sample scenario in a ring of four nodes A, B, C and D, with

unidirectional failure on a fiber from A to B, detected on B. Ring is

in the Idle state (all nodes are Idle) prior to failure.

Signal Fail Scenario

1. Ring in Idle, all nodes transmit (Tx) {IDLE, SELF, I, S} on both

rings (in both directions)

FIGURE 21. An SRP Ring with outer ring fiber cut

fiber cut

---------X-----------------------------

-----------------------------------

v v

----- -----

A B

----- -----

^ ^

o i

u n

t n

e e

r r

v v

----- -----

D C

----- -----

-----------------------------------

---------------------------------------

2. B detects SF on the outer ring, transitions to Wrapped state

(performs a wrap), Tx towards A on the inner ring/short path:

{SF, B, W, S} and on the outer ring/long path: Tx {SF, B, W, L}

3. Node A receives protection request on the short path,

transitions to Wrapped state, Tx towards B on short path:

{IDLE, A, W, S} (message does not go through due to the

failure) and on the long path: Tx {SF, A, W, L}

4. As the nodes D and C receive a switch request, they enter a

pass-through mode (in each direction) which mean they stop

sourcing the Idle messages and start passing the messages

between A an B

5. Steady state is reached

Signal Fail Clears

1. SF on B clears, B does not unwrap, sets WTR timer, Tx {WTR, B,

W, S} on inner and Tx {WTR, B, W, L}

2. Node A receives WTR request on the short path, does not unwrap,

Tx towards B on short path: {IDLE, A, W, S} (message does not

go through due to the failure) and on the long path: Tx {WTR,

A, W, L}

3. Nodes C and D relay long path messages without changing the IPS

octet

4. Steady state is reached

5. WTR times out on B. B transitions to idle state (unwraps) Tx

{IDLE, B, I, S} on both inner and outer rings

6. A receives Rx {IDLE, B, I, S} and transitions to Idle

7. As idle messages reach C and D the nodes enter the idle state

(start sourcing the Idle messages)

8. Steady state it reached

8.6.2. Signal Failure - Bidirectional Fiber Cut Scenario

Sample scenario in a ring of four nodes A, B, C and D, with a

bidirectional failure between A and B. Ring is in the Idle state

(all nodes are Idle) prior to failure.

Signal Fail Scenario

1. Ring in Idle, all nodes transmit (Tx) {IDLE, SELF, I, S} on

both rings (in both directions)

2. A detects SF on the outer ring, transitions to Wrapped state

(performs a wrap), Tx towards B on the inner ring/short path:

{SF, A, W, S} and on the outer ring/long path: Tx {SF, A, W, L}

3. B detects SF on the outer ring, transitions to Wrapped state

(performs a wrap), Tx towards A on the inner ring/short path:

{SF, B, W, S} and on the outer ring/long path: Tx {SF, B, W, L}

FIGURE 22. An SRP Ring with bidirectional fiber cut

fiber cut

---------X-----------------------------

-------X---------------------------

fiber cut

v v

----- -----

A B

----- -----

^ ^

o i

u n

t n

e e

r r

v v

----- -----

D C

----- -----

-----------------------------------

---------------------------------------

4. As the nodes D and C receive a switch request, they enter a

pass-through mode (in each direction) which mean they stop

sourcing the Idle messages and start passing the messages

between A an B

5. Steady state is reached

Signal Fail Clears

1. SF on A clears, A does not unwrap, sets WTR timer, Tx {WTR, A,

W, S} towards B and Tx {WTR, A, W, L} on the long path

2. SF on B clears, B does not unwrap. Since it now has a short

path WTR request present from A it acts upon this request. It

keeps the wrap, Tx {IDLE, B, W, S} towards A and Tx {WTR, B, W,

L} on the long path

3. Nodes C and D relay long path messages without changing the IPS

octet

4. Steady state is reached

5. WTR times out on A. A enters the idle state (drops wraps) and

starts transmitting idle in both rings

6. B sees idle request on short path and enters idle state

7. Remaining nodes in the ring enter the idle state

8. Steady state is reached

8.6.3. Failed Node Scenario

FIGURE 23. An SRP Ring with a failed node

---------------------------------------

-----------------------------------

v v /

----- ----/

A C/ failed

/ node C

----- -/---

^ /^

o i

u n

t n

e e

r r

v v

----- -----

D B

----- -----

-----------------------------------

---------------------------------------

Sample scenario in a ring where node C fails. Ring is in the Idle

state (all nodes are Idle) prior to failure.

Node Failure (or fiber cuts on both sides of the node)

1. Ring in Idle, all nodes transmit (Tx) {IDLE, SELF, I, S} on

both rings (in both directions)

2. Based on the source field of the idle messages, all nodes

identify the neighbors and keep track of them

3. B detects SF on the outer ring, transitions to Wrapped state

(performs a wrap), Tx towards C on the inner ring/short path:

{SF, B, W, S} and on the outer ring/long path: Tx {SF, B, W, L}

4. A detects SF on the inner ring, transitions to Wrapped state

(performs a wrap), Tx towards C on the outer ring/short path:

{SF, A, W, S} and on the inner ring/long path: Tx {SF, A, W, L}

5. As the nodes on the long path between A and B receive a SF

request, they enter a pass-through mode (in each direction),

stop sourcing the Idle messages and start passing the messages

between A an B

6. Steady state is reached

Failed Node and One Span Return to Service

Note: Practically the node will always return to service with one

span coming after the other (with the time delta potentially close to

0). Here, a node is powered up with the fibers connected and fault

free.

1. Node C and a span between A and C return to service (SF between

A and C disappears)

2. Node C, not seeing any faults starts to source idle messages

{IDLE, C, I, S} in both directions.

3. Fault disappears on A and A enters a WTR (briefly)

4. Node A receives idle message from node C. Because the long path

protection request {SF, B, W, L} received over the long span is

not originating from the short path neighbor (C), node A drops

the WTR and enters a PassThrough state passing requests between

C and B

5. Steady state is reached

Second Span Returns to Service

The scenario is like the Bidirectional Fiber Cut fault clearing

scenario.

8.6.4. Bidirectional Fiber Cut and Node Addition Scenarios

FIGURE 24. An SRP Ring with a failed node

wrap

----->--------------------------------

-<--------------------------------

v v

----- ----

A C Added

node

----- -----

^ ^

o i

u n

t n

e e --- wrap

r r ^

v v

----- -----

D B

----- -----

-----------------------------------

---------------------------------------

Sample scenario in a ring where initially nodes A and B are

connected. Subsequently fibers between the nodes A and B are

disconnected and a new node C is inserted.

Bidirectional Fiber Cut

1. Ring in Idle, all nodes transmit (Tx) {IDLE, SELF, I, S} on

both rings (in both directions)

2. Fibers are removed between nodes A and B