Skip to main content

UE IP Pool Allocation Documentation

IP Address Management for Mobile Devices

Table of Contents​

  1. Overview
  2. IP Allocation Concepts
  3. Configuration
  4. Allocation Process
  5. Advanced Topics
  6. Monitoring
  7. Troubleshooting

Overview​

The PGW-C allocates IP addresses to UE (User Equipment) devices when they establish PDN (Packet Data Network) connections. This is a critical function that enables mobile devices to communicate with external networks.

Why IP Allocation Matters​

Each UE receives a unique IP address from the PGW-C that:

  • Identifies the device on the network
  • Routes traffic to/from the device
  • Enables charging and policy enforcement
  • Persists for the duration of the PDN connection

Supported IP Versions​

IP VersionSupportDescription
IPv4✅ FullStandard IPv4 addresses
IPv6✅ FullIPv6 addresses and prefixes
IPv4v6✅ FullDual-stack (both IPv4 and IPv6)

IP Allocation Concepts​

PDN Type​

When a UE requests a PDN connection, it specifies a PDN Type:

PDN TypeDescriptionAllocated Addresses
IPv4IPv4-only connectionSingle IPv4 address
IPv6IPv6-only connectionIPv6 prefix (e.g., /64)
IPv4v6Dual-stack connectionBoth IPv4 address and IPv6 prefix

Allocation Methods​

PGW-C supports two IP allocation methods:

1. Dynamic Allocation (Most Common):

  • PGW-C selects IP from configured pool
  • Random selection to avoid predictability
  • Collision detection ensures uniqueness

2. Static Allocation:

  • UE requests specific IP in GTP-C message
  • PGW-C validates availability
  • Useful for enterprise devices with fixed IPs

APN-Based Subnet Selection​

Different APNs (Access Point Names) can use different IP pools:

Benefits:

  • Traffic Segregation - Different APNs route to different networks
  • Policy Differentiation - Apply different policies per APN
  • Capacity Planning - Size pools based on expected usage
  • Billing - Track usage by service type

Address Registry​

The Address Registry tracks allocated IPs:

FunctionDescription
RegistrationMaps UE IP → Session Process PID
LookupFind session by UE IP
DeregistrationRelease IP when session ends
Collision DetectionPrevent duplicate allocations

Configuration​

Basic Configuration​

Edit config/runtime.exs:

config :pgw_c,
  ue: %{
    subnet_map: %{
      # APN "internet" uses two subnets
      "internet" => [
        "100.64.1.0/24",    # 254 usable IPs
        "100.64.2.0/24"     # 254 usable IPs
      ],

      # APN "ims" uses one subnet
      "ims" => [
        "100.64.10.0/24"
      ],

      # Default pool for unknown APNs
      default: [
        "42.42.42.0/24"
      ]
    }
  }

Subnet Notation​

CIDR Notation: <network>/<prefix_length>

CIDRUsable IPsExample Range
/24254100.64.1.1 - 100.64.1.254
/23510100.64.0.1 - 100.64.1.254
/221022100.64.0.1 - 100.64.3.254
/204094100.64.0.1 - 100.64.15.254
/1665534100.64.0.1 - 100.64.255.254

Notes:

  • Network address (e.g., 100.64.1.0) is not allocated
  • Broadcast address (e.g., 100.64.1.255) is not allocated
  • PGW-C allocates from <network> + 1 to <broadcast> - 1

Multiple Subnets per APN​

Load Balancing Across Subnets:

config :pgw_c,
  ue: %{
    subnet_map: %{
      "internet" => [
        "100.64.1.0/24",
        "100.64.2.0/24",
        "100.64.3.0/24",
        "100.64.4.0/24"
      ]
    }
  }

Selection Method:

  • PGW-C randomly selects one subnet from the list
  • Provides basic load balancing
  • Each session independently selects a subnet

Benefits:

  • Distribute load across multiple subnets
  • Easier capacity expansion (add new subnets)
  • Flexibility for routing policies

Real-World Example​

config :pgw_c,
  ue: %{
    subnet_map: %{
      # General internet access
      "internet" => [
        "100.64.0.0/20"     # 4094 IPs for general use
      ],

      # IMS (Voice over LTE)
      "ims" => [
        "100.64.16.0/22"    # 1022 IPs for IMS
      ],

      # Enterprise APN
      "enterprise.corp" => [
        "10.100.0.0/16"     # 65534 IPs for enterprise
      ],

      # IoT devices (low bitrate)
      "iot.m2m" => [
        "100.64.20.0/22"    # 1022 IPs for IoT
      ],

      # Default fallback
      default: [
        "42.42.42.0/24"     # 254 IPs for unknown APNs
      ]
    }
  }

IPv6 Configuration​

config :pgw_c,
  ue: %{
    subnet_map: %{
      "internet" => [
        # IPv4 pools
        "100.64.1.0/24"
      ],
      "internet.ipv6" => [
        # IPv6 pools (prefix delegation)
        "2001:db8:1::/48"
      ],
      default: [
        "42.42.42.0/24"
      ]
    }
  }

IPv6 Prefix Delegation:

  • UE typically receives a /64 prefix
  • Allows UE to assign multiple IPs (e.g., for tethering)
  • Example: UE receives 2001:db8:1:a::/64

Dual-Stack (IPv4v6) Configuration​

config :pgw_c,
  ue: %{
    subnet_map: %{
      "internet" => [
        "100.64.1.0/24",        # IPv4 pool
        "2001:db8:1::/48"       # IPv6 pool (will be used for IPv6 allocation)
      ]
    }
  }

Dual-Stack Allocation:

  • UE requests PDN Type: IPv4v6
  • PGW-C allocates both IPv4 address and IPv6 prefix
  • Both addresses active simultaneously

Allocation Process​

IP allocation occurs during session creation when PGW-C receives a Create Session Request via the S5/S8 interface. See S5/S8 Interface for GTP-C message details and Session Management for session lifecycle.

Step-by-Step: Dynamic IPv4 Allocation​

How It Works​

Dynamic Allocation Process:

  1. Subnet Lookup: System retrieves configured subnets for the requested APN
  2. Random Selection: One subnet is randomly selected from the available list
  3. IP Generation: A random IP is generated within the subnet range
  4. Uniqueness Check: System verifies the IP hasn't been allocated
  5. Retry Logic: If collision detected, retry up to 100 times with new random IP
  6. Registration: Once unique IP found, it's registered to the session

Key Design Points:

  • Max 100 attempts: Prevents infinite loops when pool is nearly exhausted
  • Random selection: Avoids predictable IP assignment patterns for security
  • Atomic operations: Process-based registry ensures no duplicate allocations
  • Fallback to default: If APN not found in config, uses default pool

Collision Handling​

Scenario: Two sessions try to allocate same IP simultaneously

How Collision Prevention Works:

  • Registry processes requests one at a time (serialized)
  • No race conditions possible
  • First request to register an IP succeeds
  • Subsequent requests for same IP are rejected
  • Rejected sessions automatically retry with new random IP

Default Subnet Fallback​

Scenario: UE requests unknown APN

Example Configuration:

# Config
subnet_map: %{
  "internet" => ["100.64.1.0/24"],
  default: ["42.42.42.0/24"]
}

Behavior:

  • UE requests APN: "unknown.apn"
  • System looks for "unknown.apn" in subnet_map
  • Not found, so falls back to default pool
  • Allocates IP from 42.42.42.0/24

Fallback Logic:

  1. First, try to find APN-specific pool in configuration
  2. If not found, use the default pool
  3. If no default configured, allocation fails

Deallocation on Session Termination​

Automatic Cleanup:

  • When session process terminates, registry cleans up
  • IP immediately available for new allocations
  • No manual intervention required

Advanced Topics​

Pool Exhaustion​

Scenario: All IPs in pool are allocated

Pool: 100.64.1.0/24 (254 usable IPs)
Allocated: 254 IPs
New request arrives → Exhaustion

What Happens:

  1. PGW-C attempts 100 random allocations
  2. All attempts find IP already allocated
  3. Returns: {:error, :ue_ip_address_allocation_failed}
  4. Session establishment fails
  5. SGW-C receives error response

Prevention:

# Monitor pool utilization
address_registry_count / total_pool_size > 0.8  # Alert at 80%

# Expand pool before exhaustion
"internet" => [
  "100.64.1.0/24",
  "100.64.2.0/24",  # Add additional subnet
  "100.64.3.0/24"
]

Static IP Allocation​

Use Case: Enterprise device needs fixed IP

GTP-C Message Format:

Create Session Request
├── IMSI: 310260123456789
├── APN: enterprise.corp
├── PDN Address Allocation (IE)
│   └── PDN Type: IPv4
│   └── IPv4 Address: 10.100.0.50  ← UE requests specific IP

OmniPGW Processing:

  1. Extract Requested IP: Parse PDN Address Allocation IE from request
  2. Validate IP: Check if requested IP is in configured pool for this APN
  3. Check Availability: Verify IP is not already allocated to another session
  4. Allocate or Reject:
    • If available: Allocate requested IP to this session
    • If unavailable: Reject session with appropriate cause code

Possible Results:

  • Success: UE receives exactly the IP address it requested
  • Failure (IP in use): Session rejected - IP already allocated
  • Failure (IP not in pool): Session rejected - IP not in configured range

IPv6 Prefix Delegation​

UE requests IPv6:

Create Session Request
├── PDN Type: IPv6

PGW-C allocates /64 prefix:

Allocated Prefix: 2001:db8:1:a::/64

UE can use:
- 2001:db8:1:a::1
- 2001:db8:1:a::2
- ... (18 quintillion addresses)

Benefits:

  • UE can assign multiple IPs (e.g., tethering)
  • Supports SLAAC (Stateless Address Autoconfiguration)
  • Eliminates NAT requirement

Dual-Stack Allocation​

UE requests IPv4v6:

Create Session Request
├── PDN Type: IPv4v6

PGW-C allocates both:

IPv4: 100.64.1.42
IPv6: 2001:db8:1:a::/64

Traffic Handling:

  • IPv4 traffic uses IPv4 address
  • IPv6 traffic uses IPv6 prefix
  • Both active simultaneously
  • Separate GTP tunnels (or dual-stack tunnel)

Private vs. Public IP Addresses​

Private IP Pools (RFC 1918):

# Not routable on public internet
subnet_map: %{
  "internet" => [
    "10.0.0.0/8",
    "172.16.0.0/12",
    "192.168.0.0/16"
  ]
}

Requires NAT at PGW-U to access internet

Public IP Pools:

# Routable public IPs (example only)
subnet_map: %{
  "internet" => [
    "203.0.113.0/24"  # Public IP block
  ]
}

No NAT required - direct internet routing

Recommendation:

  • Use private IPs (RFC 6598): 100.64.0.0/10 (Carrier-Grade NAT)
  • Reserve public IPs for special services only

Monitoring​

Web UI - IP Pool Management​

OmniPGW provides a real-time web interface for monitoring IP pool allocation and utilization.

Access: http://<omnipgw-ip>:<web-port>/ip_pools

IP Pool Management Interface

Features:

1. Pool Overview

  • Total IPs across all pools
  • Currently allocated addresses
  • Available IPs remaining
  • Real-time utilization percentage

2. Per-APN Pool Status Each configured pool displays:

  • Pool Name - APN identifier (e.g., "default", "ims.something.else", "Internet")
  • APN Label - Configured APN name badge
  • IP Range - CIDR notation showing subnet range
  • Utilization - Visual indicator showing percentage used
  • Allocation Stats:
    • Total: Number of IPs in pool
    • Allocated: Currently assigned IPs
    • Available: Remaining IPs for allocation

3. Real-time Updates

  • Auto-refresh every 2 seconds
  • No page reload required
  • Live utilization tracking

Use Cases:

  • Quick capacity check before maintenance
  • Identify pools approaching exhaustion
  • Verify pool configuration
  • Monitor allocation patterns by APN

Key Metrics​

Address Registry Count:

# Current allocated IPs
address_registry_count

# Pool utilization (requires calculation)
address_registry_count / <total_pool_size> * 100

Example:

Pool: 100.64.1.0/24 (254 IPs)
Allocated: 150 IPs
Utilization: 150 / 254 = 59%

Alerts​

# Alert on high pool utilization
- alert: UEIPPoolUtilizationHigh
  expr: address_registry_count > 200  # For /24 pool
  for: 10m
  annotations:
    summary: "UE IP pool utilization above 80%"
    description: "Current: {{ $value }} / 254 IPs allocated"

# Alert on pool exhaustion
- alert: UEIPPoolExhausted
  expr: address_registry_count >= 254  # For /24 pool
  for: 1m
  annotations:
    summary: "UE IP pool exhausted - no IPs available"

# Alert on allocation failures
- alert: UEIPAllocationFailures
  expr: rate(ue_ip_allocation_failures_total[5m]) > 0
  for: 5m
  annotations:
    summary: "UE IP allocation failures occurring"

Grafana Dashboard​

Panel 1: IP Pool Utilization

# Gauge showing percentage
(address_registry_count / 254) * 100

Panel 2: Allocated IPs Over Time

# Time series
address_registry_count

Panel 3: Allocation Rate

# Rate of new allocations
rate(address_registry_count[5m])

Panel 4: Pool Exhaustion Risk

# Days until exhaustion (based on current rate)
(254 - address_registry_count) / rate(address_registry_count[1h])

Troubleshooting​

Issue 1: Session Establishment Fails (No IP Available)​

Symptoms:

  • Create Session Response: Cause "Request rejected"
  • Log: "UE IP address allocation failed"

Possible Causes:

  1. Pool Exhausted

    # Check current allocation
    curl http://<pgw_c_ip>:42069/metrics | grep address_registry_count

  2. Configuration Error

    # Verify subnet configuration
    config :pgw_c,
      ue: %{
        subnet_map: %{
          "internet" => [
            "100.64.1.0/24"  # Ensure valid CIDR
          ]
        }
      }

  3. APN Misconfiguration

    # If APN not found, falls back to default
    # Ensure default pool exists
    subnet_map: %{
      default: ["42.42.42.0/24"]
    }

Resolution:

  • Expand pool: Add more subnets
  • Cleanup stale sessions: Restart PGW-C to release leaked IPs
  • Verify config: Check runtime.exs for typos

Issue 2: IP Address Collision​

Symptoms:

  • Two UEs receive same IP (very rare)
  • Routing issues

Cause:

  • Bug in Address Registry (should not happen)

Debug:

# Check for duplicate IPs in logs
grep "already_registered" /var/log/pgw_c.log

Resolution:

  • Should self-correct (second session retries)
  • If persistent, report bug

Issue 3: Wrong IP Pool Used​

Symptoms:

  • UE receives IP from unexpected subnet
  • APN "internet" gets IP from "ims" pool

Cause:

  • Incorrect subnet_map configuration

Verify:

# Check exact APN string matching
subnet_map: %{
  "internet" => [...],    # Case-sensitive
  "Internet" => [...]     # Different APN!
}

Resolution:

  • Ensure APN names match exactly (case-sensitive)
  • Use default pool for catch-all

Issue 4: IPv6 Allocation Fails​

Symptoms:

  • UE requests IPv6, receives error

Possible Causes:

  1. No IPv6 pool configured

    # Missing IPv6 subnets
    subnet_map: %{
      "internet" => [
        "100.64.1.0/24"  # Only IPv4
      ]
    }

  2. Invalid IPv6 prefix

    # Too small prefix (should be /48 or larger)
    "internet" => [
      "2001:db8::/128"  # Wrong - no room for allocation
    ]

Resolution:

# Add IPv6 pool
subnet_map: %{
  "internet" => [
    "100.64.1.0/24",
    "2001:db8:1::/48"  # IPv6 pool
  ]
}

Issue 5: High Pool Utilization​

Symptoms:

  • Nearing pool exhaustion
  • address_registry_count approaching max

Proactive Measures:

  1. Add Subnets:

    "internet" => [
      "100.64.1.0/24",   # Existing
      "100.64.2.0/24",   # New subnet (adds 254 IPs)
      "100.64.3.0/24"    # New subnet (adds 254 IPs)
    ]

  2. Use Larger Subnets:

    # Replace /24 with /22
    "internet" => [
      "100.64.0.0/22"  # 1022 usable IPs
    ]

  3. Session Cleanup:

    • Monitor stale sessions
    • Ensure proper Delete Session Request handling

Best Practices​

Capacity Planning​

Calculate required pool size:

Expected concurrent users: 10,000
Peak concurrency: 30% (3,000 simultaneous sessions)
Growth buffer: 50%
Required IPs: 3,000 * 1.5 = 4,500 IPs

Subnet: /20 (4,094 usable IPs) - Too small
Subnet: /19 (8,190 usable IPs) - Sufficient

Subnet Selection​

Recommended:

  • Use 100.64.0.0/10 (RFC 6598 - Carrier-Grade NAT)
  • Provides 4 million IPs
  • Reserved for service provider NAT

Avoid:

  • Public IPs (expensive, limited)
  • Common private ranges that conflict with enterprise VPNs

Configuration Layout​

config :pgw_c,
  ue: %{
    subnet_map: %{
      # Primary internet APN - large pool
      "internet" => [
        "100.64.0.0/18"  # 16,382 IPs
      ],

      # IMS - smaller dedicated pool
      "ims" => [
        "100.64.64.0/22"  # 1,022 IPs
      ],

      # Enterprise - medium pool
      "enterprise.corp" => [
        "100.64.68.0/22"  # 1,022 IPs
      ],

      # IoT - large pool for many devices
      "iot.m2m" => [
        "100.64.72.0/20"  # 4,094 IPs
      ],

      # Default - small fallback
      default: [
        "100.64.127.0/24"  # 254 IPs
      ]
    }
  }

Configuration​

Network Planning​

Operations​

Back to Operations Guide