LDP
Label Distribution Protocol (LDP) is a protocol in which routers capable of Multiprotocol Label Switching (MPLS) exchange label mapping information. Two routers with an established session are called LDP peers and the exchange of information is bi-directional. LDP is used to build and maintain LSP databases that are used to forward traffic through MPLS networks.
LDP discovery runs on UDP port 646 and the session is built on TCP port 646. During the discovery phase hello packets are sent on UDP port 646 to the ‘all routers on this subnet’ group multicast address (224.0.0.2).
LDP is defined by the IETF (RFC 5036).
Configuration
Following an example LDP configuration with one instance attached to a network interface function.
{
"interfaces": {
"network": [
{
"interface": "eth1",
"address": "10.0.1.2/24",
"gateway": "10.0.1.1",
"ldp-instance-id": 1,
}
]
},
"ldp": [
{
"instance-id": 1,
"lsr-id": "10.10.10.11"
}
]
}
{ "ldp": {} }
Attribute |
Description |
Default |
|---|---|---|
instance-id |
LDP instance identifier |
|
keepalive-time |
LDP session keepalive time in seconds |
15 |
hold-time |
LDP hello hold time in seconds |
15 |
hostname |
LDP hostname |
bngblaster |
lsr-id |
LDP LSR identifier |
10.10.10.10 |
teardown-time |
LDP teardown time in seconds |
5 |
ipv6-transport-address |
LDP transport IPv6 address |
|
ipv4-transport-address |
LDP transport IPv6 address |
lsr-id |
no-ipv4-transport |
Disable/discard IPv4 LDP hello messages |
|
prefer-ipv4-transport |
Prefer IPv4 transport even if IPv6 is enabled |
false |
raw-update-file |
LDP RAW update file |
The keepalive-time defines the local LDP session keepalive timeout. Each LDP peer must calculate the effective keepalive timeout by using the smaller of its locally defined and received timeout in the PDU. The value chosen indicates the maximum number of seconds that may elapse between the receipt of successive PDUs from the LDP peer on the session TCP connection. The keepalive timeout is reset each time a PDU arrives. The BNG Blaster will send keepalive messages at an interval calculated by using the effective keepalive time divided by 3. Assuming an effective keepalive time of 15 seconds results in a keepalive interval of 5 seconds.
Setting a valid ipv6-transport-address enables LDP IPv6 hello and transport. According to RFC7552, IPv6 is preferred over IPv4 which can be changed with prefer-ipv4-transport.
Limitations
The following LDP functionalities are currently not supported:
Targeted LDP
LDP TCP authentication
Multiple links between LDP instance and DUT (ECMP)
LDP Adjacencies
When the BNG Blaster receives an LDP discovery hello message, an LDP adjacency is set up between the two peers.
$ sudo bngblaster-cli run.sock ldp-adjacencies
{
"status": "ok",
"code": 200,
"ldp-adjacencies": [
{
"ldp-instance-id": 1,
"interface": "eth0",
"state": "up"
}
]
}
LDP Sessions
LDP peers exchange messages over a TCP session which is initiated by the peer with the larger transport IP address (active peer).
The LDP transport IP address can be explicitly configured for the LDP instance using the option ipv4-transport-address. The lsr-id is used as a transport IP address if not explicitly configured.
Note
It is currently not supported to setup multiple links between a single LDP instance and the device under test (ECMP).
$ sudo bngblaster-cli run.sock ldp-sessions
{
"status": "ok",
"code": 200,
"ldp-sessions": [
{
"ldp-instance-id": 1,
"interface": "eth0",
"local-address": "10.2.3.1",
"local-identifier": "10.2.3.1:0",
"peer-address": "10.2.3.2",
"peer-identifier": "10.2.3.2:0",
"state": "operational",
"raw-update-state": "done",
"raw-update-file": "out.ldp",
"stats": {
"pdu-rx": 23,
"pdu-tx": 32,
"messages-rx": 24,
"messages-tx": 34,
"keepalive-rx": 21,
"keepalive-tx": 21
}
}
]
}
LDP Traffic Streams
Traffic streams send from network interface functions (downstream) can be configured to dynamically resolve the outer MPLS label using the learned label mappings.
The traffic stream configuration options ldp-ipv4-lookup-address and ldp-ipv6-lookup-address specifies the lookup IPv4 or IPv6 address. This means that traffic will not start until this address is found in the corresponding label database of the sending network interface function.
{
"streams": [
{
"name": "S1",
"type": "ipv4",
"direction": "downstream",
"priority": 128,
"network-interface": "eth1",
"destination-ipv4-address": "10.0.0.1",
"ldp-ipv4-lookup-address": "13.37.0.1",
"pps": 1
}
]
}
$ sudo bngblaster-cli run.sock ldp-database instance 1
{
"status": "ok",
"code": 200,
"ldp-database": [
{
"direction": "ipv4",
"prefix": "10.0.0.0/24",
"label": 3,
"source-identifier": "10.2.3.1:0"
},
{
"direction": "ipv4",
"prefix": "13.37.0.0/32",
"label": 10000,
"source-identifier": "10.2.3.1:0"
},
{
"direction": "ipv4",
"prefix": "13.37.0.1/32",
"label": 10001,
"source-identifier": "10.2.3.1:0"
}
]
}
The ldp-ipv4-lookup-address and ldp-ipv6-lookup-address are mutually exclusive and must exactly match the prefix address as shown in the LDP database. This means that if the prefix is 10.0.0.0/24, the lookup address should be 10.0.0.0.
Note
There is currently no longest prefix match supported, meaning that the actual prefix length is ignored!
RAW Update Files
The BNG Blaster can inject LDP PDU from a pre-compiled RAW update file into the defined sessions. A RAW update file is not more than a pre-compiled binary stream of LDP PDU.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version | PDU Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LDP Identifier |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. LDP Messages
.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version | PDU Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LDP Identifier |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
. LDP Messages
.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Those files can be created using the included LDP RAW update generator
script ldpupdate or manually using libraries like scapy.
The configured raw-update-file under the LDP instance is loaded
during BNG Blaster startup phase and send it as soon as the session is
established.
The ldp-raw-update command allows to send further updates during
the session lifetime.
$ sudo bngblaster-cli run.sock ldp-raw-update file update1.ldp
This allows loading label mappings after the LDP session has started and manually trigger a series of changes using incremental updates files.
All LDP RAW update files are loaded once and can then be used for multiple sessions. Meaning if two or more sessions reference the same file identified by file name, this file is loaded once into memory and used by multiple sessions.
LDP RAW Update Generator
The LDP RAW update generator is a simple tool to generate LDP RAW update streams for use with the BNG Blaster.
$ ldpupdate --help
usage: ldpupdate [-h] -l ADDRESS [-i N] [-w] [-a ADDRESS] [-A N] [-p PREFIX] [-P N] [-m LABEL] [-M N] [-f FILE] [--append] [--pcap FILE] [--log-level {warning,info,debug}]
The LDP RAW update generator is a simple tool to generate LDP RAW update streams for use with the BNG Blaster.
optional arguments:
-h, --help show this help message and exit
-l ADDRESS, --lsr-id ADDRESS
LSR identifier
-i N, --message-id-base N
message identifier base
-w, --withdraw withdraw
-a ADDRESS, --address-base ADDRESS
address message base
-A N, --address-num N
address message count
-p PREFIX, --prefix-base PREFIX
label mapping base prefix
-P N, --prefix-num N label mapping prefix count
-m LABEL, --label-base LABEL
label base
-M N, --label-num N label count
-f FILE, --file FILE output file
--append append to file if exist
--pcap FILE write LDP updates to PCAP file
--log-level {warning,info,debug}
logging Level
The python LDP RAW update generator is a python script that uses scapy to build LDP PDU. Therefore this tool can be easily modified, extend or used as a blueprint for your own tools to generate valid LDP update streams.