Cisco Cisco Packet Data Interworking Function (PDIF)
Evolved Packet Data Gateway Overview
▀ Features and Functionality
▄ ePDG Administration Guide, StarOS Release 17
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P-GW selection
The ePDG selects P-GW node based one of the logic:
eDNS
DNS over TCP
P-GW re-selection on session timeout
PGW re-selection on call attempt failure due to PGW reject
eDNS
The ePDG supports extended DNS client to handle DNS response larger than 512 bytes.
RFC 1035 limits the size of DNS responses over UDP to 512 bytes. If P-GW discovery is done via DNS, there is a
chance of 512 byte limit is hit as there are multiple P-GWs supporting an APN consequently having multiple responses
to the DNS query, resulting in truncation of the RRs.
chance of 512 byte limit is hit as there are multiple P-GWs supporting an APN consequently having multiple responses
to the DNS query, resulting in truncation of the RRs.
Extended the DNS (RFC 2671) allows the client to advertise a bigger re-assemble buffer size to the DNS server so that
the server can send a response bigger than 512 bytes. An interim solution to the truncation issue is to arrange the RRs
hierarchically so that the limit is never hit.
the server can send a response bigger than 512 bytes. An interim solution to the truncation issue is to arrange the RRs
hierarchically so that the limit is never hit.
DNS over TCP
By default DNS client communicates with the server over UDP port. The client can support eDNS, DNS responses up to
4 K Bytes in size from the server. If FQDN resolves too many RRs, the 4 KB limit could be exhausted.
4 K Bytes in size from the server. If FQDN resolves too many RRs, the 4 KB limit could be exhausted.
Use the following approach to resolve this issue:
Use TCP port when the server needs to send bigger responses (up to 64 KB), this needs to be driven by the client. When
the server indicates that it is not able to send all the answers to a query by setting the truncation bit in the response
header. The client on seeing this would switch to TCP port and re-sends the same query. The client continues to use
UDP port for new requests.
the server indicates that it is not able to send all the answers to a query by setting the truncation bit in the response
header. The client on seeing this would switch to TCP port and re-sends the same query. The client continues to use
UDP port for new requests.
P-GW re-selection on session timeout
During dynamic P-GW node selection by ePDG, if the selected P-GW is unreachable, the ePDG will select the next P-
GW entry from the P-GW candidate list returned during the S-NAPTR procedure to set up the PDN connection.
GW entry from the P-GW candidate list returned during the S-NAPTR procedure to set up the PDN connection.
PGW re-selection on call attempt failure due to PGW reject
ePDG attempts to select alternate PGW when the first PGW has rejected the call with the below error causes. Maximum
alternate PGW selection attempts(0-64) can be configured per APN profile using CLI, default is 3.
alternate PGW selection attempts(0-64) can be configured per APN profile using CLI, default is 3.
EGTP_CAUSE_ALL_DYNAMIC_ADDR_OCCUPIED (0x54)
EGTP_CAUSE_NO_RESOURCES_AVAILABLE(73)
EGTP_CAUSE_SERVICE_DENIED (0x59),
EGTP_CAUSE_PEER_NOT_RESPONDING-(100)
EGTP_CAUSE_SERVICE_NOT_SUPPORTED (0x44)
S2b GTPv2 support
ePDG supports PDN connection, session establishment and release, along with support for dedicated bearer creation,
deletion and modification that is initiated by the P-GW.
deletion and modification that is initiated by the P-GW.