Cisco Cisco Packet Data Gateway (PDG)
For more information on ICSR, see the System Administration Guide.
Important
ePDG 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.
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.
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.
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.
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.
responses up to 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.
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.
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.
the next P-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.
Maximum alternate PGW selection attempts(0-64) can be configured per APN profile using CLI, default is
3.
• EGTP_CAUSE_ALL_DYNAMIC_ADDR_OCCUPIED (0x54)
ePDG Administration Guide, StarOS Release 19
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Evolved Packet Data Gateway Overview
ePDG P-GW selection